Toward Quantitative Prediction of Fluorescence Quantum Efficiency by Combining Direct Vibrational Conversion and Surface Crossing: BODIPYs as an Example

Quasi-intrinsic thiobase derivatives as potential targeted photosensitizers in two-photon photodynamic therapy

In this study, a set of potential quasi-intrinsic photosensitizers for two-photon photodynamic therapy (PDT) are proposed based on the unnatural 2-amino-8-(1'-β-ᴅ-2'-deoxyribofuranosyl)-imidazo[1,2-ɑ]-1,3,5-triazin-4(8H)-one (P), which is paired with the 6-amino-5-nitro-3-(1'-β-ᴅ-2'-deoxyribofuranosyl)-2(1H)-pyridone (Z) and can specifically recognize breast and liver cancer cells. Herein, the effects of sulfur substitution and electron-donating/electron-withdrawing groups on the photophysical properties in aqueous solution are systematically investigated. The one- and two-photon absorption spectra evidence that the modifications could result in red-shifted absorption wavelength and large two-photon absorption cross-section, which contributes to selective excitation and provides effective PDT for deep-seated tissues. To ensure the efficient triplet state population, the singlet-triplet energy gaps and spin-orbit coupling constants were examined, which is responsible for a rapid intersystem crossing rate. Furthermore, these thiobase derivatives are characterized by the long-lived T1 state and the large energy gap for radiationless transition to ensure the generation of cytotoxic singlet oxygen.

Hydrosulfonylation of Alkynes for Stereodivergent Synthesis of Vinyl Sulfones: Synthetic Strategy and Mechanistic Insights

Direct synthesis of thermodynamically less favorable (Z)-vinyl sulfones presents a notable challenge in organic synthesis. In addition, the development of a stereodivergent synthesis for (E)- and (Z)-vinyl sulfones is crucial but remains elusive. In this study, we present a hydrosulfonylation of aryl-substituted alkynes, achieving a stereodivergent synthesis of (E)- and (Z)-vinyl sulfones by leveraging both thermodynamic and kinetic controls. Notably, the synthesis of challenging (Z)-vinyl sulfones was achieved through a kinetically controlled process without the need for a catalyst. To synthesize (E)-vinyl sulfones, unconventional visible light-mediated isomerization was employed as a means of facilitating the transition to the thermodynamically favored form. The present study encompasses a comprehensive experimental and computational investigation, which provides valuable insights into the reaction mechanism. This investigation reveals two plausible isomerization pathways: a novel double spin-flip mechanism and a hydrogen atom transfer process in the presence of eosin Y. This study not only advances our understanding of isomerization mechanisms beyond conventional energy-transfer routes but also offers a robust and switchable strategy for synthesizing (E)- and (Z)-vinyl sulfones, thereby providing a versatile avenue for the creation of valuable compounds in the fields of organic synthesis and medicinal chemistry.

Mechanistic insights into diversified photoluminescence behaviours of BF2 complexes of N-benzoyl 2-aminobenzothiazoles

Many BF2 complexes of heteroaromatics are well known for their dual-state emission (DSE) properties. However, AIE and ACQ effects have also been observed in certain cases. To date, no rational explanations have been proposed for these uncommon photoluminescence (PL) behaviours. The current research prepared four BF2 complexes of N-benzoyl 2-aminobenzothiazoles with diversified photoluminescence (PL) properties as model compounds and utilized quantum chemical calculation tools to address this issue. Theoretical calculations revealed that the electron-donating groups (EDGs) at the para-position of the exocyclic phenyl ring exert significant influence on their ground-state electronic structures and vertical excitation features. Potential energy curve (PEC) analysis showed that the exocyclic phenyl ring and NMe2 could not function as effective rotors due to elevated energy barriers. Only the NPh2 of BFBB-3 could spontaneously rotate ∼60° to induce the formation of an emissive twisted intramolecular charge transfer (TICT) state. The two-channel model involving both vibronic relaxation and S0/S1 surface crossing revealed that the drastic narrowing of the S1/S0 energy gap in the region approaching minimun energy conical intersection (MECI) led to the generation of a dark state in BFBB-1. The small energy barrier to access the dark-state region makes the resulting fast internal conversion a competitive channel for excited-state deactivation. In contrast, the presence of EDGs in BFBB-2 and 4 inhibits this pathway, thereby resulting in intense fluorescence emissions in solution. In addition, crystallographic analysis illustrated that the F atoms perpendicular to the polyheterocycle promoted a slipped face-to-face packing mode and enhanced intermolecular interactions. The efficiencies of their solid-state emissions are mainly affected by the degree of π-π overlaps.

Modulating excited state properties of thermally activated delayed fluorescence molecules by hybrid long-range and short-range charge transfer strategy

Balancing the rapid radiative decay process and the fast reverse intersystem crossing (RISC) process of thermally activated delayed fluorescence (TADF) molecule remains a great challenge and efficient molecular design strategies are highly desired. Herein, from a theoretical perspective, excited state properties of three reported TADF molecules (1TICz, 1BOICz and 2BOICz) are investigated based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations coupled with the thermal vibration correlation function (TVCF) method. Results indicate that, by introducing the multi-resonance (MR) acceptor, 1BOICz possesses hybrid long-range and short-range charge transfer features, balanced small energy gap (ΔEST) and large oscillator strength (f) is obtained. Furthermore, by incorporating double equivalent MR acceptors in 2BOICz, largely enhanced f with slightly changed ΔEST is achieved, inner mechanism for remarkable photophysical property is illustrated. Keep this strategy, seven new TADF molecules (2pDBA-bICz-1, 2pDBA-bICz-2, 2OSBA-bICz, 2DQAO-bICz, 2QAO-bICz, 2SQAO-bICz and 2OQAO-bICz) are theoretically designed, detailed physical parameters are analyzed and excited state energy consumption process is studied. Strong electrophilicity on acceptor is determined and the strength of nucleophilic sites on the bridge-phenyl of 2DQAO-bICz, 2QAO-bICz, 2SQAO-bICz and 2OQAO-bICz is increased, this promotes the short-range charge transfer property. In addition, the excitation processes for all studied molecules are dominated by long-range charge transfer from donor to acceptors, and supplemented by the short-range charge transfer on the bridge-phenyl with MR effect. Compromise energy gap and oscillator strength as well as large spin orbit coupling (SOC) constant are obtained for designed molecules. Thus, by regulating the long-range and short-range charge transfer ratios, excited state properties are successfully modulated and new efficient TADF molecules are proposed. Our research aims to provide deeper insight into long-range and short-range charge transfer features in balancing small ΔEST and large f, which could facilitate the development of novel efficient TADF molecules.

Theoretical design and performance prediction of deep red/near-infrared thermally activated delayed fluorescence molecules with through space charge transfer

Thermally activated delayed fluorescence (TADF) molecules with through-space charge transfer (TSCT) have attracted much attention in recent years because of their ability to simultaneously reduce the energy difference (ΔEST) and enlarge the spin-orbit coupling (SOC). In this paper, 40 molecules are theoretically designed by changing the different substitution positions of the donors and acceptors, and systematically investigated based on the first-principles calculations and excited-state dynamics study. It is found that the emission wavelengths of v-shaped molecules with intramolecular TSCT are larger than those of the molecules without TSCT. Therefore, the intramolecular TSCT can induce the red-shift of the emission and realize the deep-red/near-infrared emission. Besides intramolecular TSCT can simultaneously increase the SOC as well as the oscillator strength and reduce the ΔEST. In addition, PXZ or PTZ can also favor the realization of smaller ΔEST and red-shift emission. Our calculations suggest that intramolecular TSCT and suitable donors (-PXZ or -PTZ) are an effective strategy for the design of efficient deep red/near-infrared TADF emitters.

Design and Application of Fluorescent Probes to Detect Cellular Physical Microenvironments

The microenvironment is indispensable for functionality of various biomacromolecules, subcellular compartments, living cells, and organisms. In particular, physical properties within the biological microenvironment could exert profound effects on both the cellular physiology and pathology, with parameters including the polarity, viscosity, pH, and other relevant factors. There is a significant demand to directly visualize and quantitatively measure the fluctuation in the cellular microenvironment with spatiotemporal resolution. To satisfy this need, analytical methods based on fluorescence probes offer great opportunities due to the facile, sensitive, and dynamic detection that these molecules could enable in varying biological settings from in vitro samples to live animal models. Herein, we focus on various types of small molecule fluorescent probes for the detection and measurement of physical parameters of the microenvironment, including pH, polarity, viscosity, mechanical force, temperature, and electron potential. For each parameter, we primarily describe the chemical mechanisms underlying how physical properties are correlated with changes of various fluorescent signals. This review provides both an overview and a perspective for the development of small molecule fluorescent probes to visualize the dynamic changes in the cellular environment, to expand the knowledge for biological process, and to enrich diagnostic tools for human diseases.

Understanding the effects of sulfur di-oxidation and side chain engineering on absorption and fluorescence of oligothiophene: A theoretical study

Oligothiophene and its derivatives have broad applications in organic electronics because of its stability, easy functionalization, and broad color adjustability, and excellent charge carrier mobility. However, the effects of sulfur di-oxidation and side alkyl chains on the absorption and fluorescence of oligothiophene are still not well understood. In this article, we have applied density functional theory (DFT) and time-dependent DFT (TDDFT) to study a series of quinquethiophene compounds functionalized with S,S-dioxide and side alkyl chains, which were experimentally synthesized. Through benchmark calculations, we have found a reliable computational method, and successfully reproduced experimental UV-Vis absorption and fluorescence emission spectra well. Furthermore, the calculated reorganization energy of these molecules could explain the energy differences between absorption and emission spectra. Last but not lease, we also have calculated the fluorescence quantum yield efficiency (Фfl) of two compounds with good planarity in this series, and the trend of calculated values is consistent with experiment. Our work gives an insight to the effects of sulfur di-oxidation and side chain engineering on absorption and fluorescence of oligothiophene.

Modeling the Fluorescence Quantum Yields of Aromatic Compounds: Benchmarking the Machinery to Compute Intersystem Crossing Rates

The from-first-principles calculation of fluorescence quantum yields (FQYs) and lifetimes of organic dyes remains very challenging. In this article, we extensively test the machinery to calculate FQYs. Specifically, we perform an extensive analysis on the parameters influencing the intersystem crossing (ISC), internal conversion (IC), and fluorescence rate constants calculations. The impact of (i) the electronic structure (chosen exchange-correlation functional and spin-orbit Hamiltonian), (ii) the vibronic parameters (coordinate system, broadening function, and dipole expansion), and (iii) the excited-state kinetic models are systematically assessed for a series of seven rigid aromatic molecules. Our studies provide more insights into the choice of parameters and the expected accuracy for the computational protocols aiming to deliver FQY values. Some challenges are highlighted, such as, on the one hand, the difficulty to benchmark against the experimental nonradiative rate constants, for which the separation between the IC and ISC contributions is often not provided in the literature and, on the other hand, the need to go beyond the harmonic approximation for the calculation of the IC rates.

First-Principles Calculations of Excited-State Decay Rate Constants in Organic Fluorophores

In this Perspective, we discuss recent advances made to evaluate from first-principles the excited-state decay rate constants of organic fluorophores, focusing on the so-called static strategy. In this strategy, one essentially takes advantage of Fermi's golden rule (FGR) to evaluate rate constants at key points of the potential energy surfaces, a procedure that can be refined in a variety of ways. In this way, the radiative rate constant can be straightforwardly obtained by integrating the fluorescence line shape, itself determined from vibronic calculations. Likewise, FGR allows for a consistent calculation of the internal conversion (related to the non-adiabatic couplings) in the weak-coupling regime and intersystem crossing rates, therefore giving access to estimates of the emission yields when no complex photophysical phenomenon is at play. Beyond outlining the underlying theories, we summarize here the results of benchmarks performed for various types of rates, highlighting that both the quality of the vibronic calculations and the accuracy of the relative energies are crucial to reaching semiquantitative estimates. Finally, we illustrate the successes and challenges in determining the fluorescence quantum yields using a series of organic fluorophores.

Theoretical Insight into the Effect of Phosphorus Oxygenation on Nonradiative Decays: Comparative Analysis of P-Bridged Stilbene Analogs

Incorporation of the phosphorus element into a π-conjugated skeleton offers valuable prospects for adjusting the electronic structure of the resulting functional π-electron systems. Trivalent phosphorus has the potential to decrease the LUMO level through σ*-π* interaction, which is further enhanced by its oxygenation to the pentavalent P center. This study shows that utilizing our computational analysis to examine excited-state dynamics based on radiative/nonradiative rate constants and fluorescence quantum yield (ΦF) is effective for analyzing the photophysical properties of P-containing organic dyes. We theoretically investigate how the trivalent phosphanyl group and pentavalent phosphine oxide moieties affect radiative and nonradiative decay processes. We evaluate four variations of P-bridged stilbene analogs. Our analysis reveals that the primary decay pathway for photoexcited bis-phosphanyl-bridged stilbene is the intersystem crossing (ISC) to the triplet state and nonradiative. The oxidation of the phosphine moiety, however, suppresses the ISC due to the relative destabilization of the triplet states. The calculated rate constants match an increase in experimental ΦF from 0.07 to 0.98, as simulated from 0.23 to 0.94. The reduced HOMO-LUMO gap supports a red shift in the fluorescence spectra relative to the phosphine analog. The thiophene-fused variant with the nonoxidized trivalent P center exhibits intense emission with a high ΦF, 0.95. Our prediction indicates that the ISC transfer is obstructed owing to the relatively destabilized triplet state induced by the thiophene substitution. Conversely, the thiophene-fused analog with the phosphine oxide moieties triggers a high-rate internal conversion mediated by conical intersection, leading to a decreased ΦF.

Resolving the Photophysics of Nitrogen-Embedded Multiple Resonance Emitters: Origin of Color Purity and Emitting Efficiency

The emerging nitrogen-embedded multiple resonance (MR) emitters with an indolo[3,2,1-jk] carbazole (ICz) unit have exhibited promising performance for high-resolution organic light-emitting diode (OLED) devices, while the underlying photophysics has been rarely reported. In this work, the optical spectra, color purity, and emitting efficiency of ICz-based MR emitters were investigated by using electronic structure and thermal vibration correlation function (TVCF) calculations. Unlike B-N MR emitters, the high color purity of investigated ICz-based MR emitters was mainly contributed by considerable structural rigidity, which also greatly affects the radiative decay rate and fluorescence quantum yield of the S1 state. For the majority of investigated emitters, potential reverse intersystem crossing (RISC) channels (T1 → S1 and T2 → S1) are limited by thermally inaccessible ΔEST* or insufficient spin-orbital coupling (SOC), which can be distinguished by the calculated temperature-dependent RISC rate pattern. We provided a systematic photophysical picture for ICz-based MR emitters that might be interesting for the OLED design and application community.

Extensive Analysis of the Parameters Influencing Radiative Rates Obtained through Vibronic Calculations

Defining a theoretical model systematically delivering accurate ab initio predictions of the fluorescence quantum yields of organic dyes is highly desirable for designing improved fluorophores in a systematic rather than trial-and-error way. To this end, the first required step is to obtain reliable radiative rates (kr), as low kr typically precludes effective emission. In the present contribution, using a series of 10 substituted phenyls with known experimental kr, we analyze the impact of the computational protocol on the kr determined through the thermal vibration correlation function (TVCF) approach on the basis of time-dependent density functional theory (TD-DFT) calculations of the energies, structures, and vibrational parameters. Both the electronic structure (selected exchange-correlation functional, application or not of the Tamm-Dancoff approximation) and the vibronic parameters (line-shape formalism, coordinate system, potential energy surface model, and dipole expansion) are tackled. Considering all possible combinations yields more than 3500 cases, allowing to extract statistically-relevant information regarding the impact of each computational parameter on the magnitude of the estimated kr. It turns out that the selected vibronic model can have a significant impact on the computed kr, especially the potential energy surface model. This effect is of the same order of magnitude as the difference noted between B3LYP and CAM-B3LYP estimates. For the treated compounds, all evaluated functionals do deliver reasonable trends, fitting the experimental values.

Self-isomerization of nearly planar superatoms formed by actinide embedded gold clusters

First-principles calculations show a self-isomerization process of the nearly planar superatom, in which the maximum energy difference between different extreme points is below 0.1 eV and a crossing between singlet and triplet states is also involved. Further UV-Vis spectra reveal a correlation between the spectra and structures caused by self-isomerization.

Controlling the fluorescence quantum yields of benzothiazole-difluoroborates by optimal substitution

Precise tuning of the fluorescence quantum yield, vital for countless applications of fluorophores, remains exceptionally challenging due to numerous factors affecting energy dissipation phenomena often leading to its counterintuitive behavior. In contrast to the absorption and emission wavelength which can be precisely shifted to the desired range by simple structural changes, no general strategy exists for controllable modification of the fluorescence quantum yield. The rigidification of the molecular skeleton is known to usually enhance the emission and can be practically realized via the limiting molecular vibrations by aggregation. However, the subtle balance between the abundant possible radiative and non-radiative decay pathways makes the final picture exceptionally sophisticated. In the present study, a series of nine fluorophores obtained by peripheral substitution with two relatively mild electron donating and electron withdrawing groups are reported. The obtained fluorescence quantum yields range from dark to ultra-bright and the extreme values are obtained for the isomeric molecules. These severe changes in emission efficiency have been shown to arise from the complex relationship between the Franck-Condon excited state and conical intersection position. The experimental findings are rationalized by the advanced quantum chemical calculations delivering good correlation between the measured emission parameters and theoretical radiative and internal conversion rate constants. Therefore, the described substituent exchange provides a method to rigorously adjust the properties of molecular probes structurally similar to thioflavin T.

Organic Phosphorescence Lasing Based on a Thermally Activated Delayed Fluorescence Emitter

Organic phosphorescence materials provide an opportunity to use triplets for lasing. However, population inversion based on phosphorescence is hard to establish, owing to low luminescent quantum efficiency and intensive optical loss. By comparison, thermally activated delayed fluorescence emitters exhibit excellent optical gain with the aid of the reverse intersystem crossing (RISC) process. In this work, we designed a multifunctional gain material, not only serving as a thermally activated delayed fluorescence (TADF) emitter with excellent optical gain but also working as a phosphorescence source with high utilization of triplets. The lone pair of electrons in oxygen substitutions promotes a fast spin-flip and high delayed fluorescence quantum yield (Φ_DF = 55%), enabling TADF amplified spontaneous emissions (ASE) of CH₂Cl₂ solution. Single-crystalline nanowires of H-aggregates effectively lower triplet energy levels with high phosphorescence quantum yield (Φ_P = 27%), demonstrating Fabry-Perot mode phosphorescence lasing at 630 nm.

Insight into Isomeric Effect on the Photoluminescence and Mechanoluminescence of Cyanostilbene Derivatives

Molecular structures, packings, and intermolecular interactions significantly affect the photophysical properties of organic luminogens. In this work, the photoluminescence (PL) and mechanoluminescence (ML) of two pairs of isomers, 1/2 and 3/4, were systematically explored. The fluorescence of crystals 1c and 4c is much brighter than that of their isomers 2c and 3c, respectively. Only 1c is ML-active among all four molecules. Single-crystal structural analysis revealed that isomerization of a substituent group affected their molecular packing and intermolecular interactions. Stronger intermolecular interaction and intact three-dimensional hydrogen-bonded networks were formed only in crystal 1c, which were essential for preventing slippage of molecular layers and generating ML; the other molecules were either lacking π-π interactions or C-H···π interactions. Theoretical calculation suggested that the energy barrier between the Franck-Condon (FC) structure and minimum energy crossing point (MECP) structure of 2/3 was much lower than that of 1/4. Nonradiative decay channels of molecules 2 and 3 were thus more easily activated, which led to their lower quantum yield.

Tuning Hybridized Local and Charge-Transfer Mixing for Efficient Hot-Exciton Emission with Improved Color Purity

Delayed fluorescence (DF) emitters with high color purity are of high interest for applications in high-resolution displays. However, the charge transfer required by high emitting efficiency usually conflicts with the expected color purity. In this work, we investigated the S₁/S₀ conformational relaxation, spin-orbital coupling (SOC), and vibronic coupling of hot-exciton emitters while hybrid local and charge transfer (HLCT) state tuning was achieved by a structural meta-effect. The meta-linkage leads to suppressed S₁/S₀ conformational relaxation and weakened vibronic coupling, while the unsacrificed emitting efficiency is largely ensured by multiple rISC channels (T_n → S_m) with thermally accessible triplet-singlet energy gap (ΔE_ST) and effective SOC. We demonstrated that the unique excited-state mechanism provides opportunities to improve the emitting color purity of hot-exciton emitters without sacrificing emitting efficiency by HLCT state tuning with simple chemical structural modification, for which hot-exciton emitters might play a more important role for high-resolution organic light-emitting diode displays.

Molecular Motion and Nonradiative Decay: Towards Efficient Photothermal and Photoacoustic Systems

Nonradiative decay invariably competes with radiative decay during the deexcitation process of matter. In the community of luminescence research, nonradiative decay has been deemed less attractive than radiative decay. However, all things in their being are good for something and so is nonradiative decay. As the molecular motion-facilitated nonradiative decay (MMFND) effect is inevitable in photophysical processes, it provides a new avenue to convert the harvested light energy into exploitable forms by harnessing molecular motion. In many cases, active molecular motion enables thermal deactivation from excited states. In this Minireview, recent advances in photothermal and photoacoustic systems with MMFND character are summarized. We believe that this presentation of the rational engineering of molecular motion for efficient photothermal generation will deepen the understanding of the relationship between molecular motion and nonradiative decay and navigate people to rethink the positive aspects of nonradiative decay for the establishment of new light-controllable techniques.

Evaluation of molecular photophysical and photochemical properties using linear response time-dependent density functional theory with classical embedding: Successes and challenges

Time-dependent density functional theory (TDDFT) based approaches have been developed in recent years to model the excited-state properties and transition processes of the molecules in the gas-phase and in a condensed medium, such as in a solution and protein microenvironment or near semiconductor and metal surfaces. In the latter case, usually, classical embedding models have been adopted to account for the molecular environmental effects, leading to the multi-scale approaches of TDDFT/polarizable continuum model (PCM) and TDDFT/molecular mechanics (MM), where a molecular system of interest is designated as the quantum mechanical region and treated with TDDFT, while the environment is usually described using either a PCM or (non-polarizable or polarizable) MM force fields. In this Perspective, we briefly review these TDDFT-related multi-scale models with a specific emphasis on the implementation of analytical energy derivatives, such as the energy gradient and Hessian, the nonadiabatic coupling, the spin–orbit coupling, and the transition dipole moment as well as their nuclear derivatives for various radiative and radiativeless transition processes among electronic states. Three variations of the TDDFT method, the Tamm–Dancoff approximation to TDDFT, spin–flip DFT, and spin-adiabatic TDDFT, are discussed. Moreover, using a model system (pyridine–Ag20 complex), we emphasize that caution is needed to properly account for system–environment interactions within the TDDFT/MM models. Specifically, one should appropriately damp the electrostatic embedding potential from MM atoms and carefully tune the van der Waals interaction potential between the system and the environment. We also highlight the lack of proper treatment of charge transfer between the quantum mechanics and MM regions as well as the need for accelerated TDDFT modelings and interpretability, which calls for new method developments.

Through-Space Interaction of Tetraphenylethylene: What, Where, and How

Electronic conjugation through covalent bonds is generally considered as the basis for the electronic transition of organic luminescent materials. Tetraphenylethylene (TPE), an efficient fluorophore with aggregation-induced emission character, fluoresces blue emission in the aggregate state, and such photoluminescence is always ascribed to the through-bond conjugation (TBC) among the four phenyl rings and the central C═C bond. However, in this work, systematic spectroscopic studies and DFT theoretical simulation reveal that the intramolecular through-space interaction (TSI) between two vicinal phenyl rings generates the bright blue emission in TPE but not the TBC effect. Furthermore, the evaluation of excited-state decay dynamics suggests the significance of photoinduced isomerization in the nonradiative decay of TPE in the solution state. More importantly, different from the traditional qualitative description for TSI, the quantitative elucidation of the TSI is realized through the atoms-in-molecules analysis; meanwhile, a theoretical solid-state model for TPE and other multirotor systems for studying the electronic configuration is preliminarily established. The mechanistic model of TSI delineated in this work provides a new strategy to design luminescent materials beyond the traditional theory of TBC and expands the quantum understanding of molecular behavior to the aggregate level.

Vibronic Coupling Effect on the Vibrationally Resolved Electronic Spectra and Intersystem Crossing Rates of a TADF Emitter: 7-PhQAD

Assessing and improving the performance of organic light-emitting diode (OLED) materials require quantitative prediction of rate coefficients for the intersystem crossing (ISC) and reverse ISC (RISC) processes, which are determined not only by the energy gap and the direct spin-orbit coupling (SOC) between the first singlet and triplet excited-states at a thermal equilibrium position of the initial electronic state but also by the non-Condon effects such as the Herzberg-Teller-like vibronic coupling (HTVC) and the spin-vibronic coupling (SVC). Here we apply the time-dependent correlation function approaches to quantitatively calculate the vibrationally resolved absorption and fluorescence spectra and ISC/RISC rates of a newly synthesized multiple-resonance-type (MR-type) thermally activated delayed fluorescence (TADF) emitter, 7-phenylquinolino[3,2,1-de]acridine-5,9-dione (7-PhQAD), with the inclusion of the Franck-Condon (FC), HTVC, and Duschinsky rotation (DR) effects. The SVC effect on the rates has also been approximately evaluated. We find that the experimentally measured ISC rates of 7-PhQAD originate predominantly from the vibronic coupling, consistent with the previous reports on other MR-type TADF emitters. The SVC effect on ISC rates is about 10 times larger than the HTVC effect, and the latter increases the ISC rates by more than 1 order of magnitude while it slightly affects the vibrationally resolved absorption and fluorescence spectra. The discrepancy between the theoretical and experimental results is attributed to inaccurately describing excited-states calculated by the time-dependent density functional theory as well as to not fully accounting for the complex experimental conditions. This work provides a demonstration of what proportion of ISC and RISC rate coefficients of a MR-type TADF emitter can be covered by the HTVC effect, and it opens design routes that go beyond the FC approximation for the future development of high-performance OLED devices.

Progress on Exploring the Luminescent Properties of Organic Molecular Aggregates by Multiscale Modeling

Luminescent molecular aggregates have attracted worldwide attention because of their potential applications in many fields. The luminescent properties of organic aggregates are complicated and highly morphology-dependent, unraveling the intrinsic mechanism behind is urgent. This review summarizes recent works on investigating the structure-property relationships of organic molecular aggregates at different environments, including crystal, cocrystal, amorphous aggregate, and doped systems by multiscale modeling protocol. We aim to explore the influence of intermolecular non-covalent interactions on molecular packing and their photophysical properties and then pave the effective way to design, synthesize, and develop advanced organic luminescent materials.

Accurate predictions of the electronic excited states of BODIPY based dye sensitizers using spin-component-scaled double-hybrid functionals: a TD-DFT benchmark study

The vertical excitation energies of 13 BODIPY based dye sensitizers are benchmarked by means of TD-DFT, using 36 functionals from different DFT rungs. Most TD-DFT results were found to overestimate the excitation energies, and show mean absolute error (MAE) values in the range 0.2–0.5 eV. The dispersion-corrected, spin-component-scaled, double-hybrid (DSD) functionals DSD-BLYP and DSD-PBEP86 were found to have the smallest MAE values of 0.083 eV and 0.106 eV, respectively, which is close to the range of average errors found in the more expensive coupled-cluster methods. Moreover, DSD-BLYP and DSD-PBEP86 functionals show excellent consistency and quality of results (standard deviation = 0.048 eV and 0.069 eV respectively). However, the range separated hybrid (RSH) and the range separated double hybrid (RSDH) functionals were found to provide the best predictability (linear determination coefficient R² > 0.97 eV).

The excitation energies of 13 BODIPY dye sensitizers are benchmarked by means of TD-DFT, using 36 functionals. Spin-component-scaled double-hybrid (DSD) functionals are found to show the best performance.

Suppression of reversible photocyclization reaction induced fluorescence enhancement: a theoretical study

The fluorescence intensity of SIP-2 and DPI under different environments are mainly related to the competition between the PC reaction and aggregation induced restriction of phenyl ring rotational motions.

The Kröhnke synthesis of benzo[a]indolizines revisited: towards small, red light emitters

Benzo[a]indolizines with an ordered arrangement of various electron-withdrawing substituents (NO2, CF3, CN, CO2R and COPh) were prepared directly from pyridinium salts and chloronitroarenes, allowing for refined control of the photophysical...

Computational Study of Photochemical Relaxation Pathways of Platinum(II) Complexes

A series of functional platinum(II) complexes (Pt1-Pt3), which present high activity in four-photon absorption, in vivo imaging, and precise cancer therapy, as previously reported by the experimental work of Zhang et al. (Inorg. Chem. 2021, 60, 2362-2371), are computationally investigated in the article. We find that after the complex goes through four-photon absorption to the S₁ state, it undergoes intersystem crossing to the T₂ state and eventually reaches the T₁ state through internal conversion. On the T₁ state, both radiative and nonradiative decay to S₀ exit. The radiative decay forms the basis for the phosphorescence imaging in tissues as reported in the original paper. In addition, the nonradiative decay can simultaneously generate cytotoxic singlet oxygen by the excited energy transfer process, also known as triplet oxygen's quenching of triplet states. We conclude that the phosphorescence property as well as the photosensitizer character jointly bring high activity of in vivo imaging and photodynamic therapy to these complexes.

Radiative lifetime of a BODIPY dye as calculated by TDDFT and EOM-CCSD methods: solvent and vibronic effects

The radiative emission lifetime and associated S₁ excited state properties of a BODIPY dye are investigated with TDDFT and EOM-CCSD calculations. The effects of a solvent are described with the polarizable continuum model using the linear response (LR) approach as well as state-specific methods. The Franck-Condon (FC), Herzberg-Teller (HT) and Duschinsky vibronic effects are evaluated for the absorption and emission spectra, and for the radiative lifetime. The transition energies, spectra shapes and radiative lifetime are assessed with respect to experimental results. It is found that the TDDFT transition energies are overestimated by about 0.4-0.5 eV, whereas EOM-CCSD improves the vertical emission energy by about 0.1 eV in comparison to TDDFT. The solvatochromic and Stokes shifts are better reproduced by the state-specific solvation methods, which show that these methods are more suited than the LR model to describe the solvent effects on the BODIPY dye. The vibronic effects lead to an increase of the radiative lifetime of about 0.4 to 1.0 ns depending on the theoretical approach, which highlights the importance of such effects. Moreover, the HT effects are negligible on both the spectra and lifetime, which demonstrates that the FC approximation is accurate for the BODIPY dye. Finally, the comparison with experimental data shows that the radiative lifetimes predicted by EOM-CCSD and TDDFT have comparable accuracy.

Thermally activated delayed fluorescence materials with aggregation-induced emission properties: a QM/MM study

Organic molecules with thermally activated delayed fluorescence (TADF) and aggregation induced emission (AIE) properties have attracted increasing research interest due to their great potential applications in organic light emitting diodes (OLEDs), especially for those with multicolor mechanochromic luminescence (MCL) features. Theoretical research on the luminescence characteristics of organic TADF emitters based on the aggregation states is highly desired to quantify the relationship between the TADF properties and aggregation states. In this work, we study the 4,4'-(6-(9,9-dimethylacridine-10(9H)-yl)quinoline-2,3-dibenzonitrile (DMAC-CNQ) emitter with TADF and AIE properties, and calculate the photophysical properties in gas, solid and amorphous states by using the quantum mechanics and molecular mechanics (QM/MM) method. Our simulations demonstrate that the aggregation states enhance obviously the reverse intersystem crossing rates and transition dipole moments of the DMAC-CNQ emitter, and suppress the non-radiative rates from the lowest excited singlet state (S₁) to ground state (S₀). Specifically, the molecular stacking of DMAC-CNQ in solid phases can mainly restrict the geometric torsion of the DMAC moiety for decreasing non-radiative decay rates, and the torsion of the CNQ moiety for increasing the reverse intersystem crossing rates. As a result, the calculated fluorescence efficiencies of the DMAC-CNQ emitter in the crystal and amorphous states are 67% and 26% respectively, and in good agreement with the experimental results.

Designing a Red-Emitting Viscosity-Sensitive BODIPY Fluorophore for Intracellular Viscosity Imaging

Viscosity imaging at a microscopic scale can provide important information about biosystems, including the development of serious illnesses. Microviscosity imaging is achievable with viscosity-sensitive fluorophores, the most popular of which are based on the BODIPY group. However, most of the BODIPY probes fluoresce green light, whereas the red luminescence is desired for the imaging of biological samples. Designing a new viscosity probe with suitable spectroscopic properties is a challenging task because it is difficult to preserve viscosity sensitivity after modifying the molecular structure. Here we describe how we developed a new red-emitting, viscosity-sensitive, BODIPY fluorophore BP-PH-2M-NO₂ that is suitable for reliable intracellular viscosity imaging of lipid droplets in MCF-7 breast cancer cells. The design of BP-PH-2M-NO₂ was aided by DFT calculations that allowed a successful prediction of the viscosity sensitivity of fluorophores before synthesis. In summary, we report a new red viscosity probe possessing monoexponential fluorescence decay that makes it attractive for lifetime-based viscosity imaging.

Mild Open-Shell Character of BODIPY and Its Impact on Singlet and Triplet Excitation Energies

This study describes and rationalizes the electronic structure of BODIPY combining a large variety of quantum chemistry methods and computational tools. Examination of the obtained results using state-of-the-art electronic structure analyses provides a new and complete interpretation of the nature of low-lying electronic states in BODIPY and elucidates the limitations of excited-state methods in the computation of T₁ and S₁ energies, that is, systematic under- and overestimation of time-dependent density functional theory energies, respectively, and a large overestimation of the T₁/S₁ energy gap. Our analysis identifies the important role and physical origin of the mild open-shell character in the BODIPY ground state, that is, strong highest occupied and lowest unoccupied molecular orbital exchange interactions. The study provides guidelines for the accurate quantification of the T₁/S₁ gap, which is extremely relevant for the computational investigation of the photophysical properties of BODIPY and its derivatives. These conclusions should be taken into consideration in order to predict and interpret conspicuous photoactivated phenomena such as intersystem crossing, singlet fission, and triplet-triplet annihilation. Moreover, we believe that our study might provide new ideas and strategies for the analysis of other molecular chromophores.

Unraveling the Mechanism for Tuning the Fluorescence of Fluorescein Derivatives: The Role of the Conical Intersection and nπ* State

Although a large number of fluorescein derivatives have been developed and applied in many different fields, the general mechanisms for tuning the fluorescence of fluorescein derivatives still remain uncovered. Herein, we found that the fluorescence quenching of neutral form of fluorescein derivatives in acidic medium resulted from a dark nπ* state, whereas the fluorescence of the anionic form of fluorescein derivatives in the gas phase and alkaline solutions was tuned by minimal energy conical intersection (MECI). The formation of MECI involved significant rotation of benzene ring and flip-flop motion of xanthene moiety, which would be restricted by intermolecular hydrogen bonding and lowering temperature. The energy barrier for reaching MECI depended on the substituents in the benzene moiety in accordance with experimentally observed substituent effects. These unprecedented mechanisms would lead to a recognition of fluorescein derivatives and could provide a correct and instructive design strategy for further developing new fluorescein derivatives.

A static and dynamic NMR study of 10-hydrazino-BODIPY

10-Hydrazino-BODIPY, BoNHNH₂ , presents slow rotation about the C10-NH bond that results in anisochronous ¹ H and ¹³ C NMR signals. The assignment of the different signals has been made using traditional two-dimensional methods as well as spin-spin coupling constants and confirmed by DFT calculations (B3LYP) using the 6-311++G(d,p) basis set. The rotational barrier has been determined in three pairs of proton signals and compared with the calculated barrier.

The TDDFT Excitation Energies of the BODIPYs; The DFT and TDDFT Challenge Continues

The derivatives of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) are pivotal ingredients for a large number of functional, stimuli-responsive materials and therapeutic molecules based on their photophysical properties, and there is a urgent need to understand and predict their optical traits prior to investing a large amount of resources in preparing them. Density functional theory (DFT) and time-dependent DFT (TDDFT) computations were performed to calculate the excitation energies of the lowest-energy singlet excited state of a large series of common BODIPY derivatives employing various functional aiming at the best possible combination providing the least deviations from the experimental values. Using the common "fudge" correction, a series of combinations was investigated, and a methodology is proposed offering equal or better performances than what is reported in the literature.

Machine Learning Enables Highly Accurate Predictions of Photophysical Properties of Organic Fluorescent Materials: Emission Wavelengths and Quantum Yields

The development of functional organic fluorescent materials calls for fast and accurate predictions of photophysical parameters for processes such as high-throughput virtual screening, while the task is challenged by the limitations of quantum mechanical calculations. We establish a database covering >4300 solvated organic fluorescent dyes with 3000 distinct compounds and develop a new machine learning approach aimed at efficient and accurate predictions of emission wavelength and photoluminescence quantum yield (PLQY). Our feature engineering has given rise to a functionalized structure descriptor (FSD) and a comprehensive general solvent descriptor (CGSD), whereby a highly black-box computational framework is realized with consistently good accuracy across different dye families, ability of describing substitution effects and solvent effects, efficiency for large-scale predictions, and workability with on-the-fly learning. Evaluations with unseen molecules suggest a remarkable mean absolute error of 0.13 for PLQY and 0.080 eV for emission energy, the latter comparable to time-dependent density functional theory (TD-DFT) calculations. An online prediction platform was constructed based on the ensemble model to make predictions in various solvents. Our statistical learning methodology will complement quantum mechanical calculations as an efficient alternative approach for the prediction of these parameters.

Assessment of local coupled cluster methods for excited states of BODIPY/Aza-BODIPY families

It was previously reported that Laplace transformed local CC2 (LCC2*) provided the best agreement (MAE = 0.145 eV) when comparing vertical excitation energies to experimental λ_max for a benchmark set of 17 BODIPY/Aza-BODIPY molecules. However, these energies did not agree with values obtained from canonical CC2. Here we report LCC2* computations of vertical excitation energies on the same benchmark set of molecules using a newly implemented treatment of the ground state. Comparison with resolution-of-identity approximate coupled cluster to second-order (RI-CC2) results demonstrate that the new LCC2* results agree quantitatively. Furthermore, these values can easily be corrected empirically to also provide excellent agreement with the experiment. We show that the local algebraic diagrammatic construction to second-order (LADC(2)) method exhibits the same differences between implementations as seen for LCC2. The source of the difference is traced to an improved treatment of the ground state in the local methods, which decreases agreement with the experiment (as attributed to a fortuitous cancellation of errors) but significantly improves agreement with RI-CC2. While the absolute vertical excitation energies now show larger deviations, there remains a strong linear correlation between the LCC2* results and the experiment. For the 17 BODIPY/Aza-BODIPY molecules vertical excitation energies are determined using DLPNO-STEOM-CCSD and shown to have excellent agreement with experimental λ_max (MAE = 0.145 eV), which is the best of all the single-reference methods. The vertical excitation energies are determined using LCC2*, empirically corrected LCC2*, and RI-CC2 for a series of eight large BODIPYs and Aza-BODIPYs.

Effects of Secondary Acceptors on Excited-State Properties of Sky-Blue Thermally Activated Delayed Fluorescence Molecules: Luminescence Mechanism and Molecular Design

The development of efficient sky-blue thermally activated delayed fluorescence (TADF) emitters is highly desired. However, the types and amounts of sky-blue TADF are far from meeting the requirements, and effective molecular design strategies are expected. Herein, the photophysical properties and excited-state dynamics of 12 molecules are theoretically studied based on the thermal vibration correlation function method. Distributions of holes and electrons are analyzed by the heat maps. The frontier molecular orbital distribution, adiabatic singlet-triplet energy gap, and reorganization energy are analyzed in detail. Furthermore, the radiative and non-radiative as well as the intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes are studied, and the up-conversion process is illustrated. Our results indicate that different substitution positions and numbers play an important role in the luminescence properties of TADF molecules. The meta-position substitutions restrict the geometry variations, hinder the non-radiative energy consumption process, and promote the radiative process of TADF molecules. Meanwhile, molecules with ortho-position substitutions possess the smallest energy gaps (ΔE_st) and the largest RISC rates. Moreover, molecules with the substitutions of one tBCz group and two PO groups have the smallest ΔE_st and the largest spin orbital coupling. Thus, a wise molecular design strategy, namely, ortho-position substitutions as well as substitutions with one tBCz group and two PO groups, is proposed to facilitate the RISC process. Based on this rule, new efficient TADF molecules are theoretically designed and proposed. Our work reasonably elucidates the experimental measurements, and the effects of different substitution numbers and positions of secondary acceptors on TADF properties are highlighted, which could provide a theoretical perspective for designing efficient sky-blue TADF molecules.

Theoretical perspective on the luminescence mechanism of a hybridized local and charge transfer state emitter with aggregation induced emission: a QM/MM study

Relationship among basic molecular structures and intermolecular interactions as well as AIE–HLCT mechanisms are theoretically revealed.