Understanding and Controlling Intersystem Crossing in Molecules

Ultrafast Intersystem Crossing in Benzanthrone: Effect of Hydrogen Bonding and Viscosity

Understanding the intricate factors governing intersystem crossing (ISC) in aromatic carbonyl compounds remains a long-standing interest among researchers. This study unveils the crucial roles of vibration in influencing the ISC of a typical aromatic carbonyl chromophore, benzanthrone, and how hydrogen bonding and solvent viscosity affect these vibrations and, thus, the associated ISC kinetics. We demonstrate that for benzanthrone, the ISC is exceedingly facile in an aprotic solvent, while in protic solvents, the ISC is significantly suppressed through the formation of the hydrogen-bonded state. Moreover, in a high-viscosity medium, ISC is further retarded due to restrictions of volume-changing motions, which may assist ISC. Theoretical calculations revealed that the C═O bond vibration and specific out-of-plane vibrations accompanying a volume change could be the probable coordinates for ISC. These findings provide valuable insights for tailoring the excited-state behavior of carbonyl-functionalized materials for diverse applications in photocatalysis, organic electronics, and biomedicine.

Simulating the full spin manifold of triplet-pair states in a series of covalently linked TIPS-pentacenes

Combined density functional theory and multireference configuration interaction methods have been used to elucidate singlet fission (SF) pathways and mechanisms in three regioisomers of side-on linked pentacene dimers. In addition to the optically bright singlets (S1 $$ {}_1 $$ and S2 $$ {}_2 $$ ) and singly excited triplets (T1 $$ {}_1 $$ and T2 $$ {}_2 $$ ), the full spin manifold of multiexcitonic triplet-pair states (1 $$ {}^1 $$ ME,3 $$ {}^3 $$ ME,5 $$ {}^5 $$ ME) has been considered. In the ortho- and para-regioisomers, the1 $$ {}^1 $$ ME and S1 $$ {}_1 $$ potentials intersect upon geometry relaxation of the S1 $$ {}_1 $$ excitation. In the meta-regioisomer, the crossing occurs upon delocalization of the optically bright excitation. The energetic accessibility of these conical intersections and the absence of low-lying charge-transfer states suggests a direct SF mechanism, assisted by charge-resonance effects in the1 $$ {}^1 $$ ME state. While the5 $$ {}^5 $$ ME state does not appear to play a role in the SF mechanism of the ortho- and para-regioisomers, its participation in the disentanglement of the triplet pair is conceivable in the meta-regioisomer.

Triplet dynamics reveal loss pathways in multi-resonance thermally activated delayed fluorescence emitters

Multi-resonance thermally activated delayed fluorescence (MR-TADF) materials are of interest for light-emitting applications due to their narrow emission bandwidths and high photoluminescence quantum yields. Whilst there have been numerous examples of multi-resonance molecules exhibiting efficient TADF, the photophysics and mechanism of TADF in multi-resonance emitters have not been investigated to the same extent as the more conventional spatially separated donor-acceptor TADF materials, limiting the development of MR-TADF devices. Here we study the photophysics of a multi-resonance TADF material, OQAO(mes)2, using transient absorption spectroscopy to spectrally resolve the triplet population(s). We identify multiple triplet populations with distinct spectral contributions, and resolve the dynamics between them. Unlike conventional donor-acceptor TADF materials that have previously been studied, we find these triplet states are not formed in equilibrium, instead exhibiting a slow evolution from a high-energy triplet to a low-energy triplet. Delayed fluorescence predominantly reflects the lifetime of the high-energy triplet state, indicating that the formation of the low-energy triplet is a loss pathway for TADF. We also find that greater amounts of the low-energy triplet are formed in a higher dielectric environment, which leads to less delayed fluorescence. These triplet dynamics have significant implications for TADF in devices, as depending on the identity of the triplet formed by electrical excitation, there will either be a significant barrier to TADF, or a competing nonradiative decay pathway.

Investigation of the Relationship between Quantum Yield, Charge-Transfer State, and Structure of the Ligands in Red-Emitting Heteroleptic Iridium(III) Complexes

Iridium(III) organometallic complexes have been a key component in commercialization of organic light-emitting diodes, but the direct relationship between their structural features and photophysical properties has not yet been fully established. Here, combined experimental and theoretical studies are carried out to elucidate the main factors governing the quantum efficiency of red phosphorescent emitters by using two heteroleptic iridium(III) complexes with high geometrical similarity. It is found that two red-emitting heteroleptic iridium complexes differing only in the steric direction of phenylquinoline (pq) and phenylisoquinoline (piq) ligands, annotated Red-pq and Red-piq, show clearly different degrees of distortion of the ligand geometry in the excited state, which leads to the higher quantum yield of Red-piq than that of Red-pq. This larger distortion of the piq ligand causes more suppressed nonradiative decay of Red-piq than that of Red-pq which is the important factor governing the higher quantum yield of Red-piq.

Tracking ultrafast non-adiabatic dissociation dynamics of the deuterated water dication molecule

We applied reaction microscopy to elucidate fast non-adiabatic dissociation dynamics of deuterated water molecules after direct photo-double ionization at 61 eV with synchrotron radiation. For the very rare D+ + O+ + D breakup channel, the particle momenta, angular, and energy distributions of electrons and ions, measured in coincidence, reveal distinct electronic dication states and their dissociation pathways via spin-orbit coupling and charge transfer at crossings and seams on the potential energy surfaces. Notably, we could distinguish between direct and fast sequential dissociation scenarios. For the latter case, our measurements reveal the geometry and orientation of the deuterated water molecule with respect to the polarization vector that leads to this rare 3-body molecular breakup channel. Aided by multi-reference configuration-interaction calculations, the dissociation dynamics could be traced on the relevant potential energy surfaces and particularly their crossings and seams. This approach also unraveled the ultrafast time scales governing these processes.

Aggregation Enhanced Thermally Activated Delayed Fluorescence through Spin-Orbit Coupling Regulation

Integrating aggregation-induced emission (AIE) into thermally activated delayed fluorescence (TADF) emitters holds great promise for the advancement of highly efficient organic light emitting diodes (OLEDs). Despite recent advancements, a thorough comprehension of the underlying mechanisms remains imperative for the practical application of such materials. In this work, we introduce a novel approach aimed at modulating the TADF process by manipulating dynamic processes in excited states through aggregation effect. Our findings reveal that aggregation not only enhances both prompt and delayed fluorescence simultaneously but also imposes constraints on molecular reorientation. This constraint reinforces spin-orbit coupling and reduces the energy gap between singlets and triplets. These insights deepen our understanding of the fundamental mechanisms governing the aggregation effect on TADF materials and provide valuable guidance for the design of high-efficiency photoluminescent materials.

Purely Organic Room-Temperature Phosphorescence Molecule for High-Performance Non-Doped Organic Light-Emitting Diodes

Purely organic molecules with room-temperature phosphorescence (RTP) are potential luminescent materials with high exciton utilization for organic light-emitting diodes (OLEDs), but those exhibiting superb electroluminescence (EL) performances are rarely explored, mainly due to their long phosphorescence lifetimes. Herein, a robust purely organic RTP molecule, 3,6-bis(5-phenylindolo[3,2-a]carbazol-12(5H)-yl)-xanthen-9-one (3,2-PIC-XT), is developed. The neat film of 3,2-PIC-XT shows strong green RTP with a very short lifetime (2.9 μs) and a high photoluminescence quantum yield (72 %), and behaviors balanced bipolar charge transport. The RTP nature of 3,2-PIC-XT is validated by steady-state and transient absorption and emission spectroscopies, and the working mechanism is deciphered by theoretical simulation. Non-doped multilayer OLEDs using thin neat films of 3,2-PIC-XT furnish an outstanding external quantum efficiency (EQE) of 24.91 % with an extremely low roll-off (1.6 %) at 1000 cd m-2. High-performance non-doped top-emitting and tandem OLEDs are also achieved, providing remarkable EQEs of 24.53 % and 42.50 %, respectively. Delightfully, non-doped simplified OLEDs employing thick neat films of 3,2-PIC-XT are also realized, furnishing an excellent EQE of 17.79 % and greatly enhanced operational lifetime. The temperature-dependent and transient EL spectroscopies demonstrate the electrophosphorescence attribute of 3,2-PIC-XT. These non-doped OLEDs are the best devices based on purely organic RTP materials reported so far.

Carbonyl mediated fluorescence in aceno[n]helicenones and fluoreno[n]helicenes

Helicenes are very attractive chiral non-planar polycyclic aromatic hydrocarbons possessing strong chiroptical properties. However, most of the helicenes absorb light mainly in the ultraviolet region, with only a small segment in the blue part of the visible spectrum. Furthermore, carbo[n]helicenes exhibit only weak luminescence that limits their utilization. Herein, we demonstrate that peripheral decoration of the helicene backbone with an aryl-carbonyl group shifts the absorption to the visible region and simultaneously improves their fluorescence quantum yields. We thus show that the carbonyl group, commonly considered as detrimental to emission, has the capability of improving optical and photophysical properties. Two different families, aceno[n]helicenones and fluoreno[n]helicenes, are presented with comprehensive spectrochemical characterization. TD-DFT calculations were implemented to clarify their electronic profiles. We show that increasing the helical length in aceno[n]helicenes increases absorption onset, g abs and g lum. Extension of the peripheral aromatic part in fluoreno[n]helicenes leads to a blue shift in both absorption and emission.

Photogeneration of α-Bimetalloid Radicals via Selective Activation of Multifunctional C1 Units

Light-driven strategies that enable the chemoselective activation of a specific bond in multifunctional systems are comparatively underexplored in comparison to transition-metal-based technologies, yet desirable when considering the controlled exploration of chemical space. With the current drive to discover next-generation therapeutics, reaction design that enables the strategic incorporation of an sp3 carbon center, containing multiple synthetic handles for the subsequent exploration of chemical space would be highly enabling. Here, we describe the photoactivation of ambiphilic C1 units to generate α-bimetalloid radicals using only a Lewis base and light source to directly activate the C-I bond. Interception of these transient radicals with various SOMOphiles enables the rapid synthesis of organic scaffolds containing synthetic handles (B, Si, and Ge) for subsequent orthogonal activation. In-depth theoretical and mechanistic studies reveal the prominent role of 2,6-lutidine in forming a photoactive charge transfer complex and in stabilizing in situ generated iodine radicals, as well as the influential role of the boron p-orbital in the activation/weakening of the C-I bond. This simple and efficient methodology enabled expedient access to functionalized 3D frameworks that can be further derivatized using available technologies for C-B and C-Si bond activation.

Dissecting Exciton Dynamics in pH-Activatable Long-Wavelength Photosensitizers for Traceable Photodynamic Therapy

Tumor-specific activatable long-wavelength (LW) photosensitizers (PSs) show promise in overcoming the limitations of traditional photodynamic therapy (PDT), such as systemic phototoxicity and shallow tissue penetration. However, their insufficient LW light absorption and low singlet oxygen quantum yield (Φ 1O2) usually require high laser power density to produce thermal energy and synergistically enhance PDT. The strong photothermal radiation causing acute pain significantly reduces patient compliance and hinders the broader clinical application of LW PDT. Through the exciton dynamics dissection strategy, we have developed a series of pH-activatable cyanine-based LW PSs (LET-R, R = H, Cl, Br, I), among which the activated LET-I exhibits strong light absorption at 808 nm and a remarkable 3.2-fold enhancement in Φ 1O2 compared to indocyanine green. Transient spectroscopic analysis and theoretical calculations confirmed its significantly promoted intersystem crossing and simultaneously enhanced LW fluorescence emission characteristics. These features enable the activatable fluorescence and photoacoustic dual-modal imaging-escorted complete photodynamic eradication of tumors by the folic acid (FA)-modified LET-I probe (LET-I-FA), under the ultralow 808 nm laser power density (0.2 W cm-2) for irradiation, without the need for photothermal energy synergy. This research presents a novel strategy of dissecting exciton dynamics to screen activatable LW PSs for traceable PDT.

The N+ Formation Mechanism of Vibrationally Selected N2O+ Ions in the C2Σ+ State: A TPEPICO Imaging Study

The N-NO bond fission of N2O+(C2Σ+) ions can produce two major fragment ions, NO+ or N+. In contrast to the dominant NO+ fragment ion, the N+ formation mechanism remains unclear to date. Here, dissociative photoionization of N2O via the C2Σ+ ionic state has been reinvestigated using a combined approach of threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging and quantum chemical calculations. Accompanying the N+(3P) formation, the NO(X2Π) neutral fragment with low and high vi-rotational distributions was identified, based on the N+ speed and angular distributions derived from the TPEPICO images. In particular, the excitation of the symmetric stretching ν1+ mode promotes the formation of high rotational components, while the asymmetric stretching ν3+ mode shows the exact opposite effect. According to our calculated multistate potential energy surfaces, intersystem crossing from C2Σ+ to 14Π exclusively provides feasible decomposition pathways to produce the N+ fragment. In a slightly bent geometry, spin-orbit couplings between C2Σ+ and two substates of 14Π, 14A' or 14A″, play a crucial role in the N+ formation from vibrationally selected N2O+(C2Σ+) ions. The mechanism also provides new insights into the charge transfer reaction of N+ + NO → N + NO+.

XMECP: Reaching State-of-the-Art MECP Optimization in Multiscale Complex Systems

The Python-based program, XMECP, is developed for realizing robust, efficient, and state-of-the-art minimum energy crossing point (MECP) optimization in multiscale complex systems. This article introduces the basic capabilities of the XMECP program by theoretically investigating the MECP mechanism of several example systems including (1) the photosensitization mechanism of benzophenone, (2) photoinduced proton-coupled electron transfer in the cytosine-guanine base pair in DNA, (3) the spin-flip process in oxygen activation catalyzed by an iron-containing 2-oxoglutarate-dependent oxygenase (Fe/2OGX), and (4) the photochemical pathway of flavoprotein adjusted by the intensity of an external electric field. MECPs related to multistate reaction and multistate reactivity in large-scale complex biochemical systems can be well-treated by workflows suggested by the XMECP program. The branching plane updating the MECP optimization algorithm is strongly recommended as it provides derivative coupling vector (DCV) with explicit calculation and can equivalently evaluate contributions from non-QM residues to DCV, which can be nonadiabatic coupling or spin-orbit coupling in different cases. In the discussed QM/MM examples, we also found that the influence on the QM region by DCV can occur through noncovalent interactions and decay with distance. In the example of DNA base pairs, the nonadiabatic coupling occurs across the π-π stacking structure formed in the double-helix system. In contrast to general intuition, in the example of Fe/2OGX, the central ferrous and oxygen part contribute little to the spin-orbit coupling; however, a nearby arginine residue, which is treated by molecular mechanics in the QM/MM method, contributes significantly via two hydrogen bonds formed with α-ketoglutarate (α-KG). This indicates that the arginine residue plays a significant role in oxygen activation, driving the initial triplet state toward the productive quintet state, which is more than the previous knowledge that the arginine residue can bind α-KG at the reaction site by hydrogen bonds.

Unexpected longer T1 lifetime of 6-sulfur guanine than 6-selenium guanine: the solvent effect of hydrogen bonds to brake the triplet decay

The decay of the T1 state to the ground state is an essential property of photosensitizers because it decides the lifetime of excited states and, thus, the time window for sensitization. The sulfur/selenium substitution of carbonyl groups can red-shift absorption spectra and enhance the triplet yield because of the large spin-orbit coupling, modifying nucleobases to potential photosensitizers for various applications. However, replacing sulfur with selenium will also cause a much shorter T1 lifetime. Experimental studies found that the triplet decay rate of 6-seleno guanine (6SeGua) is 835 times faster than that of 6-thio guanine (6tGua) in aqueous solution. In this work, we reveal the mechanism of the T1 decay difference between 6SeGua and 6tGua by computing the activation energy and spin-orbit coupling for rate calculation. The solvent effect of water is treated with explicit microsolvation and implicit solvent models. We find that the hydrogen bond between the sulfur atom of 6tGua and the water molecule can brake the triplet decay, which is weaker in 6SeGua. This difference is crucial to explain the relatively long T1 lifetime of 6tGua in an aqueous solution. This insight emphasizes the role of solvents in modulating the excited state dynamics and the efficiency of photosensitizers, particularly in aqueous environments.

The Role of Aqueous Solvation on the Intersystem Crossing of Nitrophenols

The photochemistry of nitrophenols is a source of smog as nitrous acid is formed from their photolysis. Nevertheless, computational studies of the photochemistry of these widespread toxic molecules are scarce. In this work, the initial photodeactivation of ortho-nitrophenol and para-nitrophenol is modeled, both in gas phase and in aqueous solution to simulate atmospheric and aerosol environments. A large number of excited states, six for ortho-nitrophenol and 11 for para-nitrophenol, have been included and were all populated during the decay. Moreover, periodic time-dependent density functional theory (TDDFT) is used for both the explicitly included solvent and the solute. A comparison to periodic QM/MM (TDDFT/MM), with electrostatic embedding, is made, showing notable differences between the decays of solvated nitrophenols simulated with QM/MM and full (TD)DFT. A reduced intersystem crossing in aqueous solution could be observed thanks to the surface hopping approach using explicit, periodic TDDFT solvation including spin-orbit couplings.

Reverse intersystem crossing mechanisms in doped triangulenes

Thermally activated delayed fluorescence (TADF) has emerged as one of the most promising strategies in the quest for organic light emitting diodes with optimal performance. This computational study dissects the mechanistic intricacies of the central photophysical step, reverse intersystem crossing (rISC) in N and B doped triangulenes as potential multi-resonance TADF compounds. Optimal molecular patterns conducive to efficient rISC, encompassing dopant atom size, number, and distribution, are identified. Additionally, we assess various electronic structure methods for characterizing TADF-relevant molecular systems. The findings identify the distinct role of the direct and mediated mechanisms in rISC, and provide insights into the design of advanced TADF chromophores for next-generation OLED technology.

Tuning the Absorption, Fluorescence, Intramolecular Charge Transfer, and Intersystem Crossing in Spiro[fluorene]acridinone

In this work, we prepared a series of electron donor-acceptor systems based on spiro[fluorene-9,7'-dibenzo[c,h]acridine]-5'-one (SFDBAO). Our SFDBAOs consist of orthogonally positioned fluorenes and aromatic ketones. By fine-tuning the substitution of electron-donating pyrenes, the complex interplay among different excited-state decay channels and the overall impact of solvents on these decay channels were uncovered. Placing pyrene, for example, at the aromatic ketones resulted in a profound solvatochromism in the form of a bright charge-transfer (CT) emission spanning from yellow to red-NIR. In contrast, a dark non-emissive CT was noted upon pyrene substitution at the fluorenes. In apolar solvents, efficient triplet-excited state generation was observed for all SFDBAOs. Either charge transfer was concluded to mediate the intersystem crossing (ISC) in the case of pyrene substitution or the El-Sayed rule was applicable when lacking pyrene substitution as in the case of SFABAO. In polar solvents, charge separation is the sole decay upon pyrene substitution. Moreover, competition between ISC and CT lowered the triplet-excited state generation in SFDBAO.

Ground-State Orbital Descriptors for Accelerated Development of Organic Room-Temperature Phosphorescent Materials

Organic materials with room-temperature phosphorescence (RTP) are in high demand for optoelectronics and bioelectronics. Developing RTP materials highly relies on expert experience and costly excited-state calculations. It is a challenge to find a tool for effectively screening RTP materials. Herein we first establish ground-state orbital descriptors (πFMOs ) derived from the π-electron component of the frontier molecular orbitals to characterize the RTP lifetime (τp ), achieving a balance in screening efficiency and accuracy. Using the πFMOs , a data-driven machine learning model gains a high accuracy in classifying long τp , filtering out 836 candidates with long-lived RTP from a virtual library of 19,295 molecules. With the aid of the excited-state calculations, 287 compounds are predicted with high RTP efficiency. Impressively, experiments further confirm the reliability of this workflow, opening a novel avenue for designing high-performance RTP materials for potential applications.

Spin-Vibronic Intersystem Crossing and Molecular Packing Effects in Heavy Atom Free Organic Phosphor

We present a detailed investigation into the excited state properties of a planar D3h symmetric azatriangulenetrione, HTANGO, which has received significant interest due to its high solid-state phosphorescence quantum yield and therefore potential as an organic room temperature phosphorescent (ORTP) dye. Using a model linear vibronic coupling Hamiltonian in combination with quantum dynamics simulations, we observe that intersystem crossing (ISC) in HTANGO occurs with a rate of ∼1010 s-1, comparable to benzophenone, an archetypal molecule for fast ISC in heavy metal free molecules. Our simulations demonstrate that the mechanism for fast ISC is associated with the high density of excited triplet states which lie in close proximity to the lowest singlet states, offering multiple channels into the triplet manifold facilitating rapid population transfer. Finally, to rationalize the solid-state emission properties, we use quantum chemistry to investigate the excited state surfaces of the HTANGO dimer, highlighting the influence and importance of the rotational alignment between the two HTANGO molecules in the solid state and how this contributes to high phosphorescence quantum yield.

Energy transfer photocatalysis: exciting modes of reactivity

Excited (triplet) states offer a myriad of attractive synthetic pathways, including cycloadditions, selective homolytic bond cleavages and strain-release chemistry, isomerizations, deracemizations, or the fusion with metal catalysis. Recent years have seen enormous advantages in enabling these reactivity modes through visible-light-mediated triplet-triplet energy transfer catalysis (TTEnT). This tutorial review provides an overview of this emerging strategy for synthesizing sought-after organic motifs in a mild, selective, and sustainable manner. Building on the photophysical foundations of energy transfer, this review also discusses catalyst design, as well as the challenges and opportunities of energy transfer catalysis.

Modulation of Singlet-Triplet Gap in Atomically Precise Silver Cluster-Assembled Material

Silver cluster-based solids have garnered considerable attention owing to their tunable luminescence behavior. While surface modification has enabled the construction of stable silver clusters, controlling interactions among clusters at the molecular level has been challenging due to their tendency to aggregate. Judicious choice of stabilizing ligands becomes pivotal in crafting a desired assembly. However, detailed photophysical behavior as a function of their cluster packing remained unexplored. Here, we modulate the packing pattern of Ag12 clusters by varying the nitrogen-based ligand. CAM-1 formed through coordination of the tritopic linker molecule and NC-1 with monodentate pyridine ligand; established via non-covalent interactions. Both the assemblies show ligand-to-metal-metal charge transfer (LMMCT) based cluster-centered emission band(s). Temperature-dependent photoluminescence spectra exhibit blue shifts at higher temperatures, which is attributed to the extent of the thermal reverse population of the S1 state from the closely spaced T1 state. The difference in the energy gap (ΔEST ) dictated by their assemblies played a pivotal role in the way that Ag12 cluster assembly in CAM-1 manifests a wider ΔEST and thus requires higher temperatures for reverse intersystem crossing (RISC) than assembly of NC-1. Such assembly-defined photoluminescence properties underscore the potential toolkit to design new cluster- assemblies with tailored optoelectronic properties.

Synthesis, and evaluation of photophysical properties of a potential DPP-derived photosensitizer for photodynamic therapy with D-A-D architecture

The study of a macromolecule derived from DPP and triphenylamine, (DPP-BisTPA) by computational chemistry, its synthesis by direct arylation, optical characterization (UV-Vis and fluorescence) and electrochemistry (cyclic voltammetry), as well as its evaluation as a generator of reactive oxygen species indirectly, through the degradation of uric acid. The results obtained by DFT using B3LYP/6-31G (d, p) and TD-DFT using CAM-B3LYP/6-31G (d, p) reveal values of energy levels of the first singlet and triplet excited state that indicate a possible intersystem crossover and the possible generation of reactive oxygen species by a type I mechanism. The compound presents an absorption region within the phototherapeutic window. The electrochemical bandgap is 1.64 eV which suggests a behavior as a semiconductor. DPP-BisTPa were processed as hemispherical nanoparticles with a size around 100 nm, and NPOs were evaluated as a photosensitizer with a ROS generation yield of 4% using a photodynamic therapy flashlight as the light source.

A Rational Way to Control the Triplet State Wave Function Confinement of Organic Chromophores: Effect of the Connection Sites and Spin Density Distribution-Guided Molecular Structure Design Principles in Bodipy Dimers

To study the intersystem crossing (ISC) and the spatial confinement of the triplet excited states of organic chromophores, we prepared a series of Bodipy dimers. We found that the connection position of the two units has a significant effect on the absorption and fluorescence. Singlet oxygen quantum yields of 3.8-12.4% were observed for the dimers, which are independent of solvent polarity. Nanosecond transient absorption spectra indicate the population of long-lived triplet excited states with lifetimes (τT) of 45-454 μs. Pulsed laser-excited time-resolved electron paramagnetic resonance (TREPR) spectra show that the T1 triplet states are essentially delocalized, which is different from the case for the previously reported Bodipy dimers. The TREPR spectra of the triplet states imply that the delocalization over the whole dimer essentially depends on the electron density of the carbon atoms at the connection sites. This property may become a universal rule for controlling the T1 state confinement in multichromophore organic molecules.

Microemulsion-Assisted Self-Assembly of Indium Porphyrin Photosensitizers with Enhanced Photodynamic Therapy

Designing and constructing supramolecular photosensitizer nanosystems with highly efficient photodynamic therapy (PDT) is vital in the nanomedical field. Despite recent advances in forming well-defined superstructures, the relationship between molecular arrangement in nanostructures and photodynamic properties has rarely been involved, which is crucial for developing stable photosensitizers for highly efficient PDT. In this work, through a microemulsion-assisted self-assembly approach, indium porphyrin (InTPP) was used to fabricate a series of morphology-controlled self-assemblies, including nanorods, nanospheres, nanoplates, and nanoparticles. They possessed structure-dependent 1O2 generation efficiency. Compared with the other three nanostructures, InTPP nanorods featuring strong π-π stacking, J-aggregation, and high crystallinity proved to be much more efficient at singlet oxygen (1O2) production. Also, theoretical modeling and photophysical experiments verified that the intermolecular π-π stacking in the nanorods could cause a decreased singlet-triplet energy gap (ΔEST) compared with the monomer. This played a key role in enhancing intersystem crossing and facilitating 1O2 generation. Both in vitro and in vivo experiments demonstrated that the InTPP nanorods could trigger cell apoptosis and tumor ablation upon laser irradiation (635 nm, 0.1 W/cm2) and exhibited negligible dark toxicity and high phototoxicity. Thus, the supramolecular self-assembly strategy provides an avenue for designing high-performance photosensitizer nanosystems for photodynamic therapy and beyond.

Heavy Atom Effect in Halogenated mCP and Its Influence on the Efficiency of the Thermally Activated Delayed Fluorescence of Dopant Molecules

In this study, we explore the impact of halogen functionalization on the photophysical properties of the commonly used organic light-emitting diode (OLED) host material, 1,3-bis(N-carbazolyl)benzene (mCP). Derivatives with different numbers and types of halogen substituents on mCP were synthesized. By measuring steady-state and transient photoluminescence at 6 K, we study the impact of the type, number, and position of the halogens on the intersystem crossing and phosphorescence rates of the compounds. In particular, the functionalization of mCP with 5 bromine atoms results in a significant increase of the intersystem crossing rate by a factor of 300 to a value of (1.5 ± 0.1) × 1010 s-1, and the phosphorescence rate increases by 2 orders of magnitude. We find that the singlet radiative decay rate is not significantly modified in any of the studied compounds. In the second part of the paper, we describe the influence of these compounds on the reverse intersystem crossing of the 7,10-bis(4-(diphenylamino)phenyl)-2,3-dicyanopyrazino-phenanthrene (TPA-DCPP), a TADF guest, via the external heavy atom effect. Their use results in an increase of the reverse intersystem crossing (RISC) rate from (8.1 ± 0.8) × 103 s-1 for mCP to (2.7 ± 0.1) × 104 s-1 for mCP with 5 bromine atoms. The effect is even more pronounced for the mCP analogue containing a single iodine atom, which gives a RISC rate of (3.3 ± 0.1) × 104 s-1. Time-dependent DFT calculations reveal the importance of the use of long-range corrected functionals to predict the effect of halogenation on the optical properties of the mCP, and the relativistic approximation (ZORA) is used to provide insight into the strength of the spin-orbit coupling matrix element between the lowest-lying excited singlet and triplet states in the different mCP compounds.

Solvent-polarity dependence of ultrafast excited-state dynamics of trans-4-nitrostilbene

Ultrafast excited-state dynamics of the simplest nitrostilbenes, namely trans-4-nitrostilbene (t-NSB), was studied in solvents of various polarities with ultrafast broadband time-resolved fluorescence and transient absorption spectroscopies, and by quantum-chemical computations. The results revealed that the initially excited S1(ππ*) state deactivation dynamics is strongly influenced by the solvent polarity. Specifically, the t-NSB S1-state lifetime decreases by three orders of magnitude from ∼60 ps in high-polarity solvents to ∼60 fs in nonpolar solvents. The strong solvent-polarity dependence arises from the differences in dipole moments among the S1 and relevant states, including the major intersystem crossing (ISC) receiver triplet states, and therefore, the solvent polarity can modulate their relative energies and ISC rates. In nonpolar solvents, the sub-100 fs lifetime is due to a combination of efficient ISC and internal conversion. In medium-polarity solvents, the S1-state population decays via a competing ISC relaxation mechanism in a biphasic manner, and the ISC rates are found to obey the inverse energy gap law of the strong coupling case. In high-polarity solvents, the S1 state is stabilized to a much lower energy such that ISC becomes energetically infeasible, and the S1 state decays via barrier crossing along the torsion angle of the central ethylenic bond to the nonfluorescent perpendicular configuration. Regardless of the initial S1-state deactivation pathways in various solvents, the excited-state population is ultimately trapped in the metastable T1-state perpendicular configuration, at which a slower ISC occurs to bring the system to the ground state and bifurcate into either trans or cis form of NSB.

Molecularly Engineered Room-Temperature Phosphorescence for Biomedical Application: From the Visible toward Second Near-Infrared Window

Phosphorescence, characterized by luminescent lifetimes significantly longer than that of biological autofluorescence under ambient environment, is of great value for biomedical applications. Academic evidence of fluorescence imaging indicates that virtually all imaging metrics (sensitivity, resolution, and penetration depths) are improved when progressing into longer wavelength regions, especially the recently reported second near-infrared (NIR-II, 1000-1700 nm) window. Although the emission wavelength of probes does matter, it is not clear whether the guideline of "the longer the wavelength, the better the imaging effect" is still suitable for developing phosphorescent probes. For tissue-specific bioimaging, long-lived probes, even if they emit visible phosphorescence, enable accurate visualization of large deep tissues. For studies dealing with bioimaging of tiny biological architectures or dynamic physiopathological activities, the prerequisite is rigorous planning of long-wavelength phosphorescence, being aware of the cooperative contribution of long wavelengths and long lifetimes for improving the spatiotemporal resolution, penetration depth, and sensitivity of bioimaging. In this Review, emerging molecular engineering methods of room-temperature phosphorescence are discussed through the lens of photophysical mechanisms. We highlight the roles of phosphorescence with emission from visible to NIR-II windows toward bioapplications. To appreciate such advances, challenges and prospects in rapidly growing studies of room-temperature phosphorescence are described.

Two-Pronged Dormant Photosensitizer-Antibiotic Bacterial Inactivation: Mechanism, Dosage, and Cellular Evolution Visualized at the Single-Cell Level

Innovative therapeutic approaches are required to battle the rise of antibiotic-resistant bacterial strains. Tapping on reactive oxygen species (ROS) generation in bacteria induced by bactericidal antibiotics, here we report a two-pronged strategy for bacterial inactivation relying on the synergistic combination of a bactericidal antibiotic and newly designed dormant photosensitizers (DoPSs) that activate in the presence of ROS. Intramolecular quenching renders DoPS inert in the presence of light. ROS trapping by DoPS aborts the quenching mechanism unmasking, in equal proportions, singlet oxygen (1O2) sensitization and fluorescence emission. Juxtaposed antioxidant-prooxidant activity built within our DoPS enables (i) initial activation of a few molecules by ROS and (ii) subsequent rapid activation of all DoPS in a bacterium via a domino effect mediated by photogenerated 1O2. Bulk colony forming unit studies employing the minimum inhibitory concentration of the antibiotic illustrate rapid and selective inactivation of Escherichia coli and Pseudomonas aeruginosa only in the presence of light, antibiotic, and DoPS. Single-cell, real-time imaging studies on E. coli reveal an autocatalytic progression of DoPS activation from focal points, providing a unique amplification system for sensing. Single-cell analysis further illustrates the impact of DoPS cellular loading on the rate of DoPS activation and cell death times and on the 1O2 dosing necessary for cell death to occur. Our two-pronged therapy discriminates based on cell metabolites and has the potential to result in lower toxicity, pave the way to reduced drug resistance, and provide insightful mechanistic information about bacterial membrane response to 1O2.

Analytical nonadiabatic coupling and state-specific energy gradient for the crystal field Hamiltonian describing lanthanide single-ion magnets

Paramagnetic molecules with a metal ion as an electron spin center are promising building blocks for molecular qubits and high-density memory arrays. However, fast spin relaxation and decoherence in these molecules lead to a rapid loss of magnetization and quantum information. Nonadiabatic coupling (NAC), closely related to spin-vibrational coupling, is the main source of spin relaxation and decoherence in paramagnetic molecules at higher temperatures. Predicting these couplings using numerical differentiation requires a large number of computationally intensive ab initio or crystal field electronic structure calculations. To reduce computational cost and improve accuracy, we derive and implement analytical NAC and state-specific energy gradient for the ab initio parametrized crystal field Hamiltonian describing single-ion molecular magnets. Our implementation requires only a single crystal field calculation. In addition, the accurate NACs and state-specific energy gradients can be used to model spin relaxation using sophisticated nonadiabatic molecular dynamics, which avoids the harmonic approximation for molecular vibrations. To test our implementation, we calculate the NAC values for three lanthanide complexes. The predicted values support the relaxation mechanisms reported in previous studies.

Unveiling the intersystem crossing dynamics in N-annulated perylene bisimides

The efficient population of the triplet excited states in heavy metal-free organic chromophores has been one of the long-standing research problems to molecular photochemists. The negligible spin-orbit coupling matrix elements in the purely organic chromophores and the large singlet-triplet energy gap (ΔES-T) pose a hurdle for ultrafast intersystem crossing (ISC). Herein we report the unprecedented population of triplet manifold in a series of nitrogen-annulated perylene bisimide chromophores (NPBI and Br-NPBI). NPBI is found to have a moderate fluorescence quantum yield (Φf = 68 ± 5%), whereas Br-NPBI showcased a low fluorescence quantum yield (Φf = 2.0 ± 0.6%) in toluene. The femtosecond transient absorption measurements of Br-NPBI revealed ultrafast ISC (kISC = 1.97 × 1010 s-1) from the initially populated singlet excited state to the long-lived triplet excited states. The triplet quantum yields (ΦT = 95.2 ± 4.6% for Br-NPBI, ΦT = 18.7 ± 2.3% for NPBI) calculated from nanosecond transient absorption spectroscopy measurements showed the enhancement in triplet population upon bromine substitution. The quantum chemical calculations revealed the explicit role of nitrogen annulation in tuning the excited state energy levels to favor the ISC. The near degeneracy between the singlet and triplet excited states observed in NPBI and Br-NPBI (ΔES-T = -0.01 eV for NPBI, ΔES-T = 0.03 eV for Br-NPBI) facilitates the spin flipping in the molecules. Nitrogen annulation emerges as a design strategy to open up the ISC pathway and the rate of which can be further enhanced by the substitution of a heavier element.

Ultrafast Excited-State Proton Transfer in 4-Nitrocatechol: Implications for the Photochemistry of Nitrophenols

Nitrophenols are a class of environmental contaminants that exhibit strong absorption at atmospherically relevant wavelengths, prompting many studies of their photochemical degradation rates and mechanisms. Despite the importance of photochemical reactions of nitrophenols in the environment, the ultrafast processes in electronically excited nitrophenols are not well understood. Here, we present an experimental study of ultrafast electron dynamics in 4-nitrocatechol (4NC), a common product of biomass burning and fossil fuel combustion. The experiments are accompanied by time-dependent quantum mechanical calculations to help assign the observed transitions in static and transient absorption spectra and to estimate the rates of singlet-to-triplet intersystem crossing. Our results suggest that electronic triplet states are not efficiently populated upon 340 nm excitation, as efficient proton transfer occurs in the excited state on a time scale of a few picoseconds in water and tens of picoseconds in 2-propanol. This suggests that triplet states do not play a significant role in the photochemical reactions of 4NC in the environment and, by extension, in nitrophenols in general. Instead, consideration should be given to the idea that this class of molecules may serve as strong photoacids.

Polarity-Tunable Luminescence and Intersystem Crossing of a Zinc(II) Diimine Dithiolate Complex

Combined density functional theory and multireference configuration interaction methods including spin-orbit interactions have been employed to investigate the photophysical properties and deactivation pathways of a zinc diimine dithiolate complex involving the phenanthroline derivative bathocuproine and the dianionic dithiosquarate as chelating ligands. Zn(batho)(dtsq) is one of the few luminescent zinc complexes for which triplet emission had been reported in the solid state [Gronlund, P. Inorg. Chim. Acta 1995, 234, 13-18]. Because of the high dipole moment of the complex in the electronic ground state, ligand-to-ligand charge-transfer (LLCT) states experience strong hypsochromic shifts in polar media, while ligand-centered (LC) states are nearly unaffected. Rate constants for the thermally activated upconversion of the TLLCT population to the SLLCT state are promising due to a small singlet-triplet energy gap and the participation of the sulfur in the electronic excitation, but the TLLCT state is not the lowest-lying excited triplet state in ethanol solution. In addition to the TLLCT electronic structure, TLC(batho)' and TLC(dtsq) ππ* excitations form minima on the T1 potential energy surface. The SLLCT luminescence is expected to be quenched at the nanosecond time scale by the dark TLC(dtsq)ππ* state. Moreover, a TLC(dtsq)σπ* state has been identified, which leads to degradation of the compound. In mildly polar media, the dark triplet LC states are energetically inaccessible and the lowest excited singlet and triplet states clearly exhibit an LLCT character. However, their mutual spin-orbit coupling is reduced to the extent that reverse intersystem crossing is not very likely at room temperature. While Zn(diimine)(dithiolate) complexes continue to be perceived as an interesting substance class with potential application as emitters in electroluminescent devices, the particular Zn(batho)(dtsq) complex is not considered suitable for that purpose.

The impact of spin-orbit coupling on fine-structure and spin polarisation in photoexcited porphyrin triplet states

The photoexcited triplet states of porphyrins show great promise for applications in the fields of opto-electronics, photonics, molecular wires, and spintronics. The magnetic properties of porphyrin triplet states are most conveniently studied by time-resolved continuous wave and pulse electron spin resonance (ESR). This family of techniques is singularly able to probe small yet essential details of triplet states: zero-field splittings, g-anisotropy, spin polarisation, and hyperfine interactions. These characteristics are linked to spin-orbit coupling (SOC) which is known to have a strong influence on photophysical properties such as intersystem crossing rates. The present study explores SOC effects induced by the presence of Pd2+ in various porphyrin architectures. In particular, the impact of this relativistic interaction on triplet state fine-structure and spin polarisation is investigated. These properties are probed using time-resolved ESR complemented by electron-nuclear double resonance. The findings of this study could influence the future design of molecular spintronic devices. The Pd2+ ion may be incorporated into porphyrin molecular wires as a way of controlling spin polarisation.

Hydrogen-migration governed dynamic magnetic coupling characteristics in nitrogen-vacancy-hydrogen nanodiamonds

The nitrogen-vacancy center doped with hydrogen (NVH) is one of the most common defects in diamonds, and the doping of hydrogen is known to enable mobility among three equivalent C-radicals in the defect, which noticeably affects the spin coupling among the radicals. Here, we for the first time uncover the dynamic nature of magnetic coupling induced by H-migration in the NVH center of nanodiamonds, using spin-polarized density functional theory calculations and enhanced sampling metadynamics simulations. The mobility of doping H enables the interior NVH region to become a variable magnetic space (antiferromagnetic/AFM versus ferromagnetic/FM). That is, the dynamic H has three frequently reachable binding C sites where H enables the center to exhibit variable AFM coupling (high up to J = -1282 cm-1) and that in other H-reachable regions including N sites, it enables the center to exhibit FM coupling (high up to J = 598 cm-1). The magnetic switching (AFM ↔ FM) and strength fluctuation strongly depend on the H-position which can adjust the ratio of the C radical orbitals in their mixing orbitals for a special three-electron three-center covalent C⋯H⋯C H-bonding and radical orbital distributions. Clearly, this work provides insights into the dynamic switching of magnetic coupling in such multi-radical centers of defect nanodiamonds.

Triplet Generation Through Singlet Fission in Metal-Organic Framework: An Alternative Route to Inefficient Singlet-Triplet Intersystem Crossing

High quantum yield triplets, populated by initially prepared excited singlets, are desired for various energy conversion schemes in solid working compositions like porous MOFs. However, a large disparity in the distribution of the excitonic center of mass, singlet-triplet intersystem crossing (ISC) in such assemblies is inhibited, so much so that a carboxy-coordinated zirconium heavy metal ion cannot effectively facilitate the ISC through spin-orbit coupling. Circumventing this sluggish ISC, singlet fission (SF) is explored as a viable route to generating triplets in solution-stable MOFs. Efficient SF is achieved through a high degree of interchromophoric coupling that facilitates electron super-exchange to generate triplet pairs. Here we show that a predesigned chromophoric linker with extremely poor ISC efficiency (kISC ) butE S 1 ≥ 2 E T 1 ${{E}_{{S}_{1}}\ge {2E}_{{T}_{1}}}$ form triplets in MOF in contrast to the frameworks that are built from linkers with sizable kISC butE S 1 ≤ 2 E T 1 ${{E}_{{S}_{1}}\le {2E}_{{T}_{1}}}$ . This work opens a new photophysical and photochemical avenue in MOF chemistry and utility in energy conversion schemes.

Photoinduced Ring-Opening and Phototoxicity of an Indolin-3-one Derivative

The study of a fluorescent indolin-3-one derivative is reported that, as opposed to its previously described congeners, selectively undergoes photoactivated ring-opening in apolar solvents. The excited state involved in this photoisomerization was partially deactivated by the formation of singlet oxygen. Cell studies revealed lipid droplet accumulation and efficient light-induced cytotoxicity.

Fast and Tunable Phosphorescence from Organic Ionic Crystals

Crystalline diphosphonium iodides [MeR2 P-spacer-R2 Me]I with phenylene (1, 2), naphthalene (3, 4), biphenyl (5) and anthracene (6) as aromatic spacers, are photoemissive under ambient conditions. The emission colors (λem values from 550 to 880 nm) and intensities (Φem reaching 0.75) are defined by the composition and substitution geometry of the central conjugated chromophore motif, and the anion-π interactions. Time-resolved and variable-temperature luminescence studies suggest phosphorescence for all the titled compounds, which demonstrate observed lifetimes of 0.46-92.23 μs at 297 K. Radiative rate constants kr as high as 2.8×105  s-1 deduced for salts 1-3 were assigned to strong spin-orbit coupling enhanced by an external heavy atom effect arising from the anion-π charge-transfer character of the triplet excited state. These rates of anomalously fast metal-free phosphorescence are comparable to those of transition metal complexes and organic luminophores that utilize triplet excitons via a thermally activated delayed fluorescence mechanism, making such ionic luminophores a new paradigm for the design of photofunctional and responsive molecular materials.

Theoretical investigation on the functional group modulation of UV-Vis absorption profiles of triphenylamine derivatives

Understanding the functional group modulation of electronic structure and excitation is pivotal to the design of organic small molecules (OSMs) for photoelectric applications. In this study, we employed density functional theory (DFT) and time-dependent DFT (TDDFT) calculations to explore the unique absorption character of four triphenylamine photosensitizers. The various conformations were investigated given the multiple single bonds in the compounds, and the resemblance in the electronic structure of different conformations is affirmed because the coplanarity and consequent long-range conjugation is maintained regardless of the orientation of the flexible blocks. Six functionals were evaluated, and MN15 was found to successfully reproduce the intense secondary absorption peak for the double 3,4-ethylenedioxythiophene (EDOT) modified sensitizer over B3LYP, PBE0, M062X, CAM-B3LYP, and ωB97XD. The introduction of EDOT gives rise to a new excited state S4, which is a local excitation constrained in the EDOT substituent triphenylamine block. This new excited state S4, in combination with inherent S2 and S3 derived from prototype molecule TPA-Pyc, jointly contributes to the hump of the secondary absorption peak of ETE-Pyc and finally affects the light-harvesting ability of the dye-sensitized TiO2 photoanode. The current findings provide guidance toward the rational design of OSMs with good light-harvest ability.

N-Heteroacenes as an Organic Gain Medium for Room-Temperature Masers

The development of future quantum devices such as the maser, i.e., the microwave analog of the laser, could be well-served by the exploration of chemically tunable organic materials. Current iterations of room-temperature organic solid-state masers are composed of an inert host material that is doped with a spin-active molecule. In this work, we systematically modulated the structure of three nitrogen-substituted tetracene derivatives to augment their photoexcited spin dynamics and then evaluated their potential as novel maser gain media by optical, computational, and electronic paramagnetic resonance (EPR) spectroscopy. To facilitate these investigations, we adopted an organic glass former, 1,3,5-tri(1-naphthyl)benzene to act as a universal host. These chemical modifications impacted the rates of intersystem crossing, triplet spin polarization, triplet decay, and spin-lattice relaxation, leading to significant consequences on the conditions required to surpass the maser threshold.

Move Aside Pentacene: Diazapentacene-Doped para-Terphenyl, a Zero-Field Room-Temperature Maser with Strong Coupling for Cavity Quantum Electrodynamics

Masers can deliver ultralow-noise amplification of microwave signals in medical imaging and deep-space communication, with recent research being rekindled through the discovery of gain media operating at room-temperature, eschewing bulky cryogenics that hindered their use. This work shows the discovery of 6,13-diazapentacene doped in para-terphenyl (DAP:PTP) as a maser gain medium that can operate at room-temperature, without an external magnetic field. With a maser output power of -10 dBm, it is on par with pentacene-doped para-terphenyl in masing power, while possessing compelling advantages such as faster amplification startup times, being pumped by longer wavelength light at 620 nm and greater chemical stability from nitrogen groups. Furthermore, the maser bursts from DAP:PTP allow one to reach the strong coupling regime for cavity quantum electrodynamics, with a high cooperativity of 182. The optical and microwave spin dynamics of DAP:PTP are studied in order to evaluate its capabilities as a maser gain medium, where it features fast intersystem crossing and an advantageously higher triplet quantum yield. The results pave the way for the future discovery of similar maser materials and help designate them as promising candidates for quantum sensors, optoelectronic devices and the study of cavity quantum electrodynamic effects at room-temperature.

Spin-dependent reactivity and spin-flipping dynamics in oxygen atom scattering from graphite

The formation of two-electron chemical bonds requires the alignment of spins. Hence, it is well established for gas-phase reactions that changing a molecule's electronic spin state can dramatically alter its reactivity. For reactions occurring at surfaces, which are of great interest during, among other processes, heterogeneous catalysis, there is an absence of definitive state-to-state experiments capable of observing spin conservation and therefore the role of electronic spin in surface chemistry remains controversial. Here we use an incoming/outgoing correlation ion imaging technique to perform scattering experiments for O(³P) and O(¹D) atoms colliding with a graphite surface, in which the initial spin-state distribution is controlled and the final spin states determined. We demonstrate that O(¹D) is more reactive with graphite than O(³P). We also identify electronically nonadiabatic pathways whereby incident O(¹D) is quenched to O(³P), which departs from the surface. With the help of molecular dynamics simulations carried out on high-dimensional machine-learning-assisted first-principles potential energy surfaces, we obtain a mechanistic understanding for this system: spin-forbidden transitions do occur, but with low probabilities.

Origin of intersystem crossing in highly distorted organic molecules: a case study with red light-absorbing N,N,O,O-boron-chelated Bodipys

To explore the relationship between the twisted π-conjugation framework of aromatic chromophores and the efficacy of intersystem crossing (ISC), we have studied a N,N,O,O-boron-chelated Bodipy derivative possessing a severely distorted molecular structure. Surprisingly, this chromophore is highly fluorescent, showing inefficient ISC (singlet oxygen quantum yield, Φ_Δ = 12%). These features differ from those of helical aromatic hydrocarbons, where the twisted framework promotes ISC. We attribute the inefficient ISC to a large singlet-triplet energy gap (ΔE_S1/T1 = 0.61 eV). This postulate is tested by critical examination of a distorted Bodipy having an anthryl unit at the meso-position, for which Φ_Δ is increased to 40%. The improved ISC yield is rationalized by the presence of a T₂ state, localized on the anthryl unit, with energy close to that of the S₁ state. The electron spin polarization phase pattern of the triplet state is (e, e, e, a, a, a), with the T_z sublevel of the T₁ state overpopulated. The small zero-field splitting D parameter (-1470 MHz) indicates that the electron spin density is delocalized over the twisted framework. It is concluded that twisting of π-conjugation framework does not necessarily induce ISC, but S₁/T_n energy matching may be a generic feature for increasing ISC for a new-generation of heavy atom-free triplet photosensitizers.

Theoretical study of C6F5-corrole molecules functionalized with aromatic groups for Photodynamic Therapy

The singlet oxygen generation by electronically excited molecules in photodynamic therapy (PDT) requires light absorption within a specific wavelength window, and a subsequent intersystem crossing transition to a triplet excited state that is, at least, 0.98 eV higher in energy than the singlet ground state. Tetrapyrrolic macrocycles, such as porphyrin and corrole, have been widely used in oxygen singlet generation for PDT. Suitable functionalization can potentialize these macrocycles as photosensitizers. In this contribution, we use Density Functional Theory (DFT) calculations to determine the structural, electronic and spectroscopic properties of corrole macrocycles bound to different polycyclic aromatic groups in the gas phase, dichloromethane, and water. We also calculate the spin-orbit coupling (SOC) matrix elements of the intersystem crossing channels involving the first excited singlet states and excited triplet states. The results for optical absorption show that the threshold wavelength for optical absorption increases with the polarity of the environment and the number of aromatic rings of the ligands, whereas the oscillator strengths increase with the polarity of the environment but decrease with the number of aromatic rings. It is verified that the triplet excited states involved in the intersystem crossing transitions satisfy the energy requirement for the oxygen singlet generation. The magnitude of spin-orbit coupling (SOC) matrix elements associated with the intersystem crossing are also seen to be dependent on the environment involving the corrole molecules, and on the number of aromatic rings of the ligands connected to the corrole. Further, the binding of the functionalized corrole molecules with biomolecules as the calf thymus DNA and human serum albumin is studied and characterized through molecular docking. These results show that the corrole macrocycles, suitably functionalized with polycyclic aromatic groups, fulfill several criteria to be considered as good PDT photosensitizers.

Quantum and Semiclassical Dynamics of Nonadiabatic Electronic Excitation of C(3P) to C1D) by Hyperthermal Collisions with N2

The dynamics and kinetics of nonadiabatic excitation of C(3P) to C(1D) induced by hyperthermal collisions with N2 molecules are investigated using a quantum mechanical and two semiclassical nonadiabatic methods. The full-dimensional interaction potential energy surfaces and spin-orbit coupling, which facilitates the spin-forbidden process, are represented by a recently constructed diabatic potential energy matrix. The multistate quantum dynamics for selected partial waves found small transition probabilities due to the weak spin-orbit coupling. The spin-flip transition is the most favored near the threshold due to effective curve crossing. Strong oscillations are also found in the probabilities, which are attributable to resonances supported by the deep well in the singlet-state potential. Vibrational state-specified rate coefficients are reported from J-shifted quantum dynamics calculations, and they follow the Arrhenius form. Vibrational excitation in the N2 collision partner is found to increase the excitation rate at low temperatures, but the trend is reversed at high temperatures. The two semiclassical methods qualitatively reproduce the quantum rate coefficients.

Predicting Kinetics and Dynamics of Spin-Dependent Processes

ConspectusPredicting mechanisms and rates of nonadiabatic spin-dependent processes including photoinduced intersystem crossings, thermally activated spin-forbidden reactions, and spin crossovers in metal centers is a very active field of research. These processes play critical roles in transition-metal-based and metalloenzymatic catalysis, molecular magnets, light-harvesting materials, organic light-emitting diodes, photosensitizers for photodynamic therapy, and many other applications. Therefore, accurate modeling of spin-dependent processes in complex systems and on different time scales is important for many problems in chemistry, biochemistry, and materials sciences.Nonadiabatic statistical theory (NAST) and nonadiabatic molecular dynamics (NAMD) are two complementary approaches to modeling the kinetics and dynamics of spin-dependent processes. NAST predicts the probabilities and rate constants of nonradiative transitions between electronic states with different spin multiplicities using molecular properties at only few critical points on the potential energy surfaces (PESs), including the reactant minimum and the minimum energy crossing point (MECP) between two spin states. This makes it possible to obtain molecular properties for NAST calculations using accurate but often computationally expensive electronic structure methods, which is critical for predicting the rate constants of spin-dependent processes. Alternatively, NAST can be used to study spin-dependent processes in very large complex molecular systems using less computationally expensive electronic structure methods. The nuclear quantum effects, such as zero-point vibrational energy, tunneling, and interference between reaction paths can be easily incorporated. However, the statistical and local nature of NAST makes it more suitable for large systems and slow kinetics. In contrast, NAMD explores entire PESs of interacting electronic states, making it ideal for modeling fast barrierless spin-dependent processes. Because the knowledge of large portions of PESs is often needed, the simulations require a very large number of electronic structure calculations, which limits the NAMD applicability to relatively small molecular systems and ultrafast kinetics.In this Account, we discuss our contribution to the development of the NAST and NAMD approaches for predicting the rates and mechanism of spin-dependent processes. First, we briefly describe our NAST and NAMD implementations. The NAST implementation is an extension of the transition state theory to the processes involving two crossing potential energy surfaces of different spin multiplicities. The NAMD approach includes the trajectory surface hopping (TSH) and ab initio multiple spawning (AIMS) methods. Second, we discuss several applications of NAST and NAMD to model spin-dependent processes in different systems. The NAST applicability to large complex systems is demonstrated by the studies of the spin-forbidden isomerization of the active sites of metal-sulfur proteins. Our implementation of the MECP search algorithm within the fully ab initio fragment molecular orbital method allows applying NAST to systems with thousands of atoms, such as the solvated protein rubredoxin. Applications of NAMD to ultrafast spin-dependent processes are represented by the generalized AIMS simulations utilizing the fast GPU-based TeraChem electronic structure program to gain insight into the complex photoexcited state relaxation in 2-cyclopentenone.

A Thermally Activated Delayed Fluorescence Emitter Investigated by Time-Resolved Near-Infrared Spectroscopy

Emitters for organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) require small singlet (S₁ )-triplet (T₁ ) energy gaps as well as fast intersystem crossing (ISC) transitions. These transitions can be mediated by vibronic mixing with higher excited states S_n and T_n (n=2, 3, 4, …). For a prototypical TADF emitter consisting of a triarylamine and a dicyanobenzene moiety (TAA-DCN) it is shown that these higher states can be located energetically by time-resolved near-infrared (NIR) spectroscopy.

High Intrinsic Phosphorescence Efficiency and Density Functional Theory Modeling of Ru(II)-Bipyridine Complexes with π-Aromatic-Rich Cyclometalated Ligands: Attributions of Spin-Orbit Coupling Perturbation and Efficient Configurational Mixing of Singlet Excited States

A series of π-aromatic-rich cyclometalated ruthenium(II)-(2,2'-bipyridine) complexes ([Ru(bpy)₂(π_Ar-CM)]⁺) in which π_Ar-CM is diphenylpyrazine or 1-phenylisoquinoline were prepared. The [Ru(bpy)₂(π_Ar-CM)]⁺ complexes had remarkably high phosphorescence rate constants, k _RAD(p), and the intrinsic phosphorescence efficiencies (ι_em(p) = k _RAD(p)/(ν_em(p))³) of these complexes were found to be twice the magnitudes of simply constructed cyclometalated ruthenium(II) complexes ([Ru(bpy)₂(sc-CM)]⁺), where ν_em(p) is the phosphorescence frequency and sc-CM is 2-phenylpyridine, benzo[h]quinoline, or 2-phenylpyrimidine. Density functional theory (DFT) modeling of the [Ru(bpy)₂(CM)]⁺ complexes indicated numerous singlet metal-to-ligand charge transfers for ¹MLCT-(Ru-bpy) and ¹MLCT-(Ru-CM), excited states in the low-energy absorption band and ¹ππ*-(aromatic ligand) (¹ππ*-L_Ar) excited states in the high-energy band. DFT modeling of these complexes also indicated phosphorescence-emitting state (T_e) configurations with primary MLCT-(Ru-bpy) characteristics. The variation in ι_em(p) for the spin-forbidden T_e (³MLCT-(Ru-bpy)) excited state of the complex system that was examined in this study can be understood through the spin-orbit coupling (SOC)-mediated sum of intensity stealing (∑SOCM-IS) contribution from the primary intensity of the low-energy ¹MLCT states and second-order intensity perturbation from the significant configuration between the low-energy ¹MLCT and high-energy intense ¹ππ*-L_Ar states. In addition, the observation of unusually high ι_em(p) magnitudes for these [Ru(bpy)₂(π_Ar-CM)]⁺ complexes can be attributed to the values for both intensity factors in the ∑SOCM-IS formalism being individually greater than those for [Ru(bpy)₂(sc-CM)]⁺ ions.

Finding Design Principles of OLED Emitters through Theoretical Investigations of Zn(II) Carbene Complexes

In this work, Zn(II) carbene complexes carrying a dianionic 1,2-dithiolbenzene (dtb) or 1,2-diolbenzene (dob) ligand were investigated regarding their suitability as organic light-emitting diode (OLED) emitter. For the optimization of the complexes, density functional-based methods were used and frequency analyses verified the obtained structures as minima. All calculations were carried out including a polarizable continuum model to mimic solvent-solute interactions. Multireference configuration interaction methods were used to determine excitation energies, spin-orbit couplings, and luminescence properties. Rate constants of spin-allowed and spin-forbidden transitions were calculated according to a Fermi golden rule expression. Using carbene ligands with varying σ-donor and π-acceptor strengths, the luminescence is found to be tunable from yellow to orange/red to deep red/near-infrared. The calculated intersystem crossing (ISC) time constants indicate thermally activated delayed fluorescence (TADF) to be the main decay channel. In contrast to many d¹⁰ coinage metal complexes, a parallel orientation of dtb or dob and the carbene ligand is found to be highly favorable. For the complexes with a cyclic (alkyl)(amino) carbene (CAAC) or cyclic (amino)(aryl) carbene (CAArC) ligand, the S₁ and T₁ states have ligand-to-ligand charge-transfer (LLCT) character and are energetically close. The complex with a classical N-heterocyclic carbene (NHC) ligand has S₁ and T₁ states with mixed ligand-to-metal charge-transfer (LMCT)/LLCT character and is a very rare example in which the zinc ion contributes to the excitation.

Competition between ultralong organic phosphorescence and thermally activated delayed fluorescence in dichloro derivatives of 9-benzoylcarbazole

Optoelectronic materials based on metal-free organic molecules represent a promising alternative to traditional inorganic devices. Significant attention has been devoted to the development of the third generation of OLEDs which are based on the temperature-activated delayed fluorescence (TADF) mechanism. In the last few years, several materials displaying ultra-long organic phosphorescence (UOP) have been designed using strategies such as crystal engineering and halogen functionalisation. Both TADF and UOP are controlled by the population of triplet states and the energy gaps between the singlet and triplet manifolds. In this paper, we explore the competition between TADF and UOP in the molecular crystals of three dichloro derivatives of 9H-carbazol-3-yl(phenyl)methanone. We investigate the excited state mechanisms in solution and the crystalline phase and address the effects of exciton transport and temperature on the rates of direct and reverse intersystem crossing under the Marcus-Levich-Jortner model. We also analyse how the presence of isomeric impurities and the stabilisation of charge transfer states affect these processes. Our simulations explain the different mechanisms observed for the three derivatives and highlight the role of intramolecular rotation and crystal packing in determining the energy gaps. This work contributes to a better understanding of the connection between chemical and crystalline structures that will enable the design of efficient materials.

Intersystem Crossing in Acceptor-Donor-Acceptor Type Organic Photovoltaic Molecules Promoted by Symmetry Breaking in Polar Environments

The intramolecular electron push-pulling effect has been widely applied to manipulate the excited states in organic photovoltaic (OPV) molecules toward efficient photocurrent generation in working devices with bias fields. However, the effect of field induced polar environments on the excited-state dynamics remains largely unexplored. Here, we investigate the polar environment effect on excited dynamics in acceptor-donor-acceptor type OPV molecules dissolved in solvents with different polarities. By combining ultrafast transient absorption spectroscopy and quantum chemical computation, we observe the stabilization of excited states induced by symmetry breaking in the polar solvent in the molecules exhibiting strong electron push-pulling effects. The stabilized excited states undergo faster intersystem crossing processes with reduced singlet-triplet energy gaps. The findings suggest that the dynamics of charge generation and recombination may be controlled by manipulating the polar environment and electron push-pulling effect to improve the device performance.