Dexter energy transfer pathways

Heavy-Atom-Free Covalent Organic Frameworks for Organic Room-Temperature Phosphorescence via Förster and Dexter Energy Transfer Mechanism

Covalent organic frameworks (COFs), with their accessible nanoscale porosity, selectable building blocks, and precisely engineered topology, offer unique benefits in the design of room-temperature phosphorescent (RTP) materials. However, their potential has been limited by phosphorescence quenching caused by interlayer π-π stacking interactions. This paper presents a novel strategy to enhance RTP in heavy-atom-free COFs by employing a donor-acceptor (D-A) system that leverages the Förster resonance energy transfer (FRET) and Dexter energy transfer (DET) mechanisms. Among the materials investigated, the best-performing COF exhibits a phosphorescence lifetime of 4.35 ms at room temperature. Spectral analysis, structural analysis, and theoretical calculations indicate the presence of intralayer FRET processes as well as interlayer DET processes within the D-A COF system. Potential anti-counterfeiting applications are explored by exploiting the unique phosphorescent properties of these materials. Additionally, the inherent permanent porosity of COFs presents new opportunities for future development and application. This strategy offers many promising prospects for advancing the RTP technology in COF materials and broadens their potential applications in various fields.

Using Adiabatic Energy Splitting To Compute Dexter Energy Transfer Couplings

Dexter energy transfer and transport (DET) are of broad interest in energy science, and DET rates depend on electronic couplings between donor and acceptor species. DET couplings are challenging to compute since they originate from both one- and two-particle interactions, and the strength of this interaction drops approximately exponentially with donor-acceptor distances. Using adiabatic energy splitting to compute DET couplings has advantages because adiabatic states can be calculated directly using conventional quantum chemical methods. We describe a minimum energy splitting method to compute the DET coupling by altering molecular geometries to drive the systems into a T1/T2 energy quasi-degenerate-activated DA complex. We explore the accuracy of various quantum chemical approaches to calculate the Dexter couplings.

Photon Upconversion at Organic-Inorganic Interfaces

Photon upconversion is a process that combines low-energy photons to form useful high-energy photons. There are potential applications in photovoltaics, photocatalysis, biological imaging, etc. Semiconductor quantum dots (QDs) are promising for the absorption of these low-energy photons due to the high extinction coefficient of QDs, especially in the near infrared (NIR). This allows the intriguing use of diffuse light sources such as solar irradiation. In this review, we describe the development of this organic-QD upconversion platform based on triplet-triplet annihilation, focusing on the dark exciton in QDs with triplet character. Then we introduce the underlying energy transfer steps, starting from QD triplet photosensitization, triplet exciton transport, triplet-triplet annihilation, and ending with the upconverted emission. Design principles to improve the total upconversion efficiency are presented. We end with limitations in current reports and proposed future directions. This review provides a guide for designing efficient organic-QD upconversion platforms for future applications, including overcoming the Shockley-Queisser limit for more efficient solar energy conversion, NIR-based phototherapy, and diagnostics in vivo.

Electric-field modulated energy transfer in phosphorescent material- and fluorescent material-codoped polymer light-emitting diodes

The excited-state energy transfer widely exists in mixed-material systems and devices. The modulation of an electric field on the energy transfer in photoluminescence has been demonstrated. However, to date, no studies on the electric-field modulation of the excited-state energy transfer in organic optoelectronic devices have been reported. Herein, we investigate the effect of an electric field on the energy transfer in the poly(N-vinylcarbazole) (PVK) thin films doped with iridium(iii)[bis(4,6-difluorophenyl)pyridinato-N,C2']-tetrakis(1-pyrazolyl)borate (Fir6) and 5,6,11,12-tetraphenylnaphthacene (rubrene) (PVK:Fir6:rubrene) and the corresponding light-emitting diodes. Combined with the Onsager model describing electric-field enhanced exciton dissociation, we find that the electric field increases the rate of Dexter energy transfer from Fir6 to rubrene in the films and the diodes. The voltage-dependent color shift in the PVK:Fir6:rubrene light-emitting diodes can be explained by the electric-field enhanced Dexter energy transfer from Fir6 to rubrene. Our findings are important for the control of energy transfer process in organic optoelectronic devices by an electric field for desirable applications.

Porphyrin-Sensitizers and Anthracene-Annihilators Built in Isostructural Frameworks for Investigating Triplet-Triplet Annihilation Upconversion

In this study, four isostructural pillar-layered frameworks were constructed using a porphyrin layer and an anthracene pillar, which served as the sensitizer and annihilator, respectively, in the triplet-triplet annihilation upconversion (TTA-UC) system. Framework 1 demonstrated the highest upconversion quantum yield of 1.01%. Additionally, 1 and 2 also exhibited down-conversion fluorescence resulting from the porphyrin component. A twist intramolecular charge transfer (TICT) state was observed in the bianthracene chromophore of 2, resulting in transient rotation of two anthracene rings and red-shifted emission. Both computational studies and experiments confirmed the transition from a locally excited state to a TICT state upon the inclusion of polar guest molecules into the framework.

Tunable afterglow for mechanical self-monitoring 3D printing structures

Self-monitoring materials have promising applications in structural health monitoring. However, developing organic afterglow materials for self-monitoring is a highly intriguing yet challenging task. Herein, we design two organic molecules with a twisted donor-acceptor-acceptor' configuration and achieve dual-emissive afterglow with tunable lifetimes (86.1-287.7 ms) by doping into various matrices. Based on a photosensitive resin, a series of complex structures are prepared using 3D printing technology. They exhibit tunable afterglow lifetime and Young's Modulus by manipulating the photocuring time and humidity level. With sufficient photocuring or in dry conditions, a long-lived bright green afterglow without apparent deformation under external loading is realized. We demonstrate that the mechanical properties of complex 3D printing structures can be well monitored by controlling the photocuring time and humidity, and quantitively manifested by afterglow lifetimes. This work casts opportunities for constructing flexible 3D printing devices that can achieve sensing and real-time mechanical detection.

Tunneling-Driven Marcus-Inverted Triplet Energy Transfer in a Two-Dimensional Perovskite

Quantum tunneling, a phenomenon that allows particles to pass through potential barriers, can play a critical role in energy transfer processes. Here, we demonstrate that the proper design of organic-inorganic interfaces in two-dimensional (2D) hybrid perovskites allows for efficient triplet energy transfer (TET), where quantum tunneling of the excitons is the key driving force. By employing temperature-dependent and time-resolved photoluminescence and pump-probe spectroscopy techniques, we establish that triplet excitons can transfer from the inorganic lead-iodide sublattices to the pyrene ligands with rapid and weakly temperature-dependent characteristic times of approximately 50 ps. The energy transfer rates obtained based on the Marcus theory and first-principles calculations show good agreement with the experiments, indicating that the efficient tunneling of triplet excitons within the Marcus-inverted regime is facilitated by high-frequency molecular vibrations. These findings offer valuable insights into how one can effectively manipulate the energy landscape in 2D hybrid perovskites for energy transfer and the creation of diverse excitonic states.

Oriented Triplet Excitons as Long-Lived Electron Spin Qutrits in a Molecular Donor-Acceptor Single Cocrystal

The photogeneration of multiple unpaired electron spins within molecules is a promising route to applications in quantum information science because they can be initialized into well-defined, multilevel quantum states (S > 1/2) and reproducibly fabricated by chemical synthesis. However, coherent manipulation of these spin states is difficult to realize in typical molecular systems due to the lack of selective addressability and short coherence times of the spin transitions. Here, these challenges are addressed by using donor-acceptor single cocrystals composed of pyrene and naphthalene dianhydride to host spatially oriented triplet excitons, which exhibit promising photogenerated qutrit properties. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy demonstrates that spatially orienting triplet excitons in a single crystal platform imparts narrow, well-resolved, tunable resonances in the triplet EPR spectrum, allowing selective addressability of the spin sublevel transitions. Pulse-EPR spectroscopy reveals that at temperatures above 30 K, spin decoherence of these triplet excitons is driven by exciton diffusion. However, coherence is limited by electronic spin dipolar coupling below 30 K, where T2 varies nonlinearly with the optical excitation density due to exciton annihilation. Overall, an optimized coherence time of T2 = 7.1 μs at 20 K is achieved. These results provide important insights into designing solid-state molecular excitonic materials with improved spin qutrit properties.

A supramolecular assembly-based strategy towards the generation and amplification of photon up-conversion and circularly polarized luminescence

For the molecular properties in which energy transfer/migration is determinantal, such as triplet-triplet annihilation-based photon up-conversion (TTAUC), the overall performance is largely affected by the intermolecular distance and relative molecular orientations. In such scenarios, tools that may steer the intermolecular interactions and provide control over molecular organisation in the bulk, become most valuable. Often these non-covalent interactions, found predominantly in supramolecular assemblies, enable pre-programming of the molecular network in the assembled structures. In other words, by employing supramolecular chemistry principles, an arrangement where molecular units are arranged in a desired fashion, very much like a Lego toy, could be achieved. This leads to enhanced energy transfer from one molecule to other. In recent past, chiral luminescent systems have attracted huge attention for producing circularly polarized luminescence (CPL). In such systems, chirality is a necessary requirement. Chirality induction/transfer through supramolecular interactions has been known for a long time. It was realized recently that it may help in the generation and amplification of CPL signals as well. In this review article we have discussed the applicability of self-/co-assembly processes for achieving maximum TTA-UC and CPL in various molecular systems.

Photoswitchable and long-lived seven-membered cyclic singlet diradicals for the bioorthogonal photoclick reaction

Annularly 1,3-localized singlet diradicals are energetic and homolytic intermediates, but commonly too short-lived for widespread utilization. Herein, we describe a direct observation of a long-lived and seven-membered singlet diradical, oxepine-3,6-dione-2,7-diyl (OXPID), via spectroscopic experiments and also theoretical evidence from computational studies, which is generated via photo-induced ring-expansion of 2,3-diaryl-1,4-naphthoquinone epoxide (DNQO). The photo-generated OXPID reverts to the thermally stable σ-bonded DNQO with t1/2 in the μs level, thus constituting a novel class of T-type molecular photoswitches with high light-energy conversion efficiency (η = 7.8-33%). Meanwhile, the OXPID is equilibrated to a seven-membered cyclic 1,3-dipole as an electronic tautomer that can be captured by ring-strained dipolarophiles with an ultrafast cycloaddition rate (k2CA up to 109 M-1 s-1). The T-type photoswitchable DNQO is then exploited to be a highly selective and recyclable photoclick reagent, enabling spatiotemporal-resolved bioorthogonal ligation on living cell membranes via a tailored DNQO-Cy3 probe.

Nonorthogonal Multireference Wave Function Description of Triplet-Triplet Energy Transfer Couplings

In this study, the use of self-consistent field quasi-diabats is investigated for calculation of triplet energy transfer diabatic coupling elements. It is proposed that self-consistent field quasi-diabats are particularly useful for studying energy transfer (EnT) processes because orbital relaxation in response to changes in electron configuration is implicitly built into the model. The conceptual model that is developed allows for the simultaneous evaluation of direct and charge-transfer mechanisms to establish the importance of the different possible EnT mechanisms. The method's performance is evaluated using two model systems: the ethylene dimer and ethylene with the methaniminium cation. While states that mediate the charge-transfer mechanism were found to be higher in energy than the states involved in the direct mechanism, the coupling elements that control the kinetics were found to be significantly larger in the charge-transfer mechanism. Subsequently, we discuss the advantage of the approach in the context of practical difficulties with the use of established approaches.

Blue organic light-emitting diode with a turn-on voltage of 1.47 V

Among the three primary colors, blue emission in organic light-emitting diodes (OLEDs) are highly important but very difficult to develop. OLEDs have already been commercialized; however, blue OLEDs have the problem of requiring a high applied voltage due to the high-energy of blue emission. Herein, an ultralow voltage turn-on at 1.47 V for blue emission with a peak wavelength at 462 nm (2.68 eV) is demonstrated in an OLED device with a typical blue-fluorescent emitter that is widely utilized in a commercial display. This OLED reaches 100 cd/m2, which is equivalent to the luminance of a typical commercial display, at 1.97 V. Blue emission from the OLED is achieved by the selective excitation of the low-energy triplet states at a low applied voltage by using the charge transfer (CT) state as a precursor and triplet-triplet annihilation, which forms one emissive singlet from two triplet excitons.

Endogenous Photosensitizers in Human Skin

Endogenous photosensitizers play a critical role in both beneficial and harmful light-induced transformations in biological systems. Understanding their mode of action is essential for advancing fields such as photomedicine, photoredox catalysis, environmental science, and the development of sun care products. This review offers a comprehensive analysis of endogenous photosensitizers in human skin, investigating the connections between their electronic excitation and the subsequent activation or damage of organic biomolecules. We gather the physicochemical and photochemical properties of key endogenous photosensitizers and examine the relationships between their chemical reactivity, location within the skin, and the primary biochemical events following solar radiation exposure, along with their influence on skin physiology and pathology. An important take-home message of this review is that photosensitization allows visible light and UV-A radiation to have large effects on skin. The analysis presented here unveils potential causes for the continuous increase in global skin cancer cases and emphasizes the limitations of current sun protection approaches.

Exciplex Emission and Förster Resonance Energy Transfer in Polycyclic Aromatic Hydrocarbon-Based Bischromophoric Cyclophanes and Homo[2]catenanes

Energy transfer and exciplex emission are not only crucial photophysical processes in many living organisms but also important for the development of smart photonic materials. We report, herein, the rationally designed synthesis and characterization of two highly charged bischromophoric homo[2]catenanes and one cyclophane incorporating a combination of polycyclic aromatic hydrocarbons, i.e., anthracene, pyrene, and perylene, which are intrinsically capable of supporting energy transfer and exciplex formation. The possible coconformations of the homo[2]catenanes, on account of their dynamic behavior, have been probed by Density Functional Theory calculations. The unique photophysical properties of these exotic molecules have been explored by steady-state and time-resolved absorption and fluorescence spectroscopies. The tetracationic pyrene-perylene cyclophane system exhibits emission emanating from a highly efficient Förster resonance energy transfer (FRET) mechanism which occurs in 48 ps, while the octacationic homo[2]catenane displays a weak exciplex photoluminescence following extremely fast (<0.3 ps) exciplex formation. The in-depth fundamental understanding of these photophysical processes involved in the fluorescence of bischromophoric cyclophanes and homo[2]catenanes paves the way for their use in future bioapplications and photonic devices.

Waveguided energy transfer in pseudo-two-dimensional systems

Resonance energy transfer (RET) is an important and ubiquitous process whereby energy is transferred from a donor chromophore to an acceptor chromophore without contact via Coulombic coupling. There have been a number of recent advances exploiting the quantum electrodynamics (QED) framework for RET. Here, we extend the QED RET theory to investigate whether real photon exchange can allow for excitation transfer over very long distances if the exchanged photon is waveguided. To study this problem, we consider RET in two spatial dimensions. We derive the RET matrix element using QED in two dimensions, consider an even greater confinement by deriving the RET matrix element for a two-dimensional waveguide using ray theory, and compare the resulting RET elements in 3D and 2D and for the 2D waveguide. We see greatly enhanced RET rates over long distances for both the 2D and 2D waveguide systems and see a great preference for transverse photon mediated transfer in the 2D waveguide system.

Current trends in carbon-based quantum dots development from solid wastes and their applications

Urbanization and a massive population boom have immensely increased the solid wastes (SWs) generation and are expected to reach 3.40 billion tons by 2050. In many developed and emerging nations, SWs are prevalent in both major and small cities. As a result, in the current context, the reusability of SWs through various applications has taken on added importance. Carbon-based quantum dots (Cb-QDs) and their many variants are synthesized from SWs in a straightforward and practical method. Cb-QDs are a new type of semiconductor that has attracted the interest of researchers due to their wide range of applications, which include everything from energy storage, chemical sensing, to drug delivery. This review is primarily focused on the conversion of SWs into useful materials, which is an essential aspect of waste management for pollution reduction. In this context, the goal of the current review is to investigate the sustainable synthesis routes of carbon quantum dots (CQDs), graphene quantum dots (GQDs), and graphene oxide quantum dots (GOQDs) from various types SWs. The applications of CQDs, GQDs, and GOQDs in the different areas are also been discussed. Finally, the challenges in implementing the existing synthesis methods and future research directions are highlighted.

Surface hopping modeling of charge and energy transfer in active environments

An active environment is any atomic or molecular system changing a chromophore's nonadiabatic dynamics compared to the isolated molecule. The action of the environment on the chromophore occurs by changing the potential energy landscape and triggering new energy and charge flows unavailable in the vacuum. Surface hopping is a mixed quantum-classical approach whose extreme flexibility has made it the primary platform for implementing novel methodologies to investigate the nonadiabatic dynamics of a chromophore in active environments. This Perspective paper surveys the latest developments in the field, focusing on charge and energy transfer processes.

Properties and applications of photoexcited chromophore–radical systems

Photoexcited organic chromophore-radical systems hold great promise for a range of technological applications in molecular spintronics, including quantum information technology and artificial photosynthesis. However, further development of such systems will depend on the ability to control the magnetic properties of these materials, which requires a profound understanding of the underlying excited-state dynamics. In this Review, we discuss photogenerated triplet-doublet systems and their potential to be used for applications in molecular spintronics. We outline the theoretical description of the spin system in the different coupling regimes and the invoked excited-state mechanisms governing the generation and transfer of spin polarization. The main characterization techniques used to evaluate the optical and magnetic properties of chromophore-radical systems are discussed. We conclude by giving an overview of previously investigated covalently linked triplet-radical systems, and highlight the need for further systematic investigations to improve our understanding of the magnetic interactions in such systems.

Combining tetraphenylethene (TPE) derivative cations with Eu3+-β-diketone complex anions for tunable luminescence

Tetraphenylethene (TPE) derivative cations (TPE⁺) and Eu³⁺-β-diketone complex anions (Eu(ABM)₄^-) were combined to construct a novel dual energy transfer system (TPE⁺ to Eu³⁺ and ABM to Eu³⁺). Our system exhibits tunable luminescence in DMF/water mixtures under different f_w conditions owing to the AIE and ACQ properties of TPE⁺ and ABM, respectively. Its luminescence can be also regulated by adding P-containing oxysalts or polyacrylic acids.

Sensitized 1-Acyl-7-nitroindolines with Enhanced Two-Photon Cross Sections for Release of Neurotransmitters

Precise photochemical control, using two-photon excitation (2PE), of the timing and location of activation of glutamate is useful for studying the molecular and cellular physiology of the brain. Antenna-based light harvesting strategies represent a general method to increase the sensitivity to 2PE of otherwise insensitive photoremovable protecting groups (PPGs). This was applied to the most commonly used form of "caged" glutamate, MNI-Glu. Computational investigation showed that a four- or six-carbon linker attached between the 4-position of thioxanthone (THX) and the 4-position of the 5-methyl derivative of MNI-Glu (MMNI-Glu) would position the antenna and PPG close to one another to enable Dexter energy transfer. Nine THX-MMNI-Glu conjugates were prepared and their photochemical properties determined. Installation of the THX antenna resulted in a red shift of the absorption (λ_max = 385-405 nm) along with increased quantum yield compared to the parent compound MNI-Glu (λ_max = 347 nm). The THX-MMNI-Glu conjugate with a four-carbon linker and attachment to the 4-position of THX underwent photolysis via 1PE at 405 and 430 nm and via 2PE at 770 and 860 nm, yielding glutamate. The two-photon uncaging action cross section (δ_u) was 0.11 and 0.29 GM at 770 and 860, respectively, which was greater than for MNI-Glu (0.06 and 0.072 GM at 720 and 770 nm, respectively). The THX sensitizer harvested the light via 2PE and transferred its resulting triplet energy to MMNI-Glu. Release of glutamate through 2PE at 860 nm from the compound (100 μM) activated iGluSnFR, a genetically encoded, fluorescent glutamate sensor, on the surface of cells in culture, portending its usefulness in studies of neurophysiology in acute brain slice.

Molecular Wires for Efficient Long-Distance Triplet Energy Transfer

We propose design rules for building organic molecular bridges that enable coherent long-distance triplet-exciton transfer. Using these rules, we describe example polychromophoric structures with low inner-sphere exciton reorganization energies, low static and dynamic disorder, and enhanced π-stacking interactions between nearest-neighbor chromophores. These features lead to triplet-exciton eigenstates that are delocalized over several units at room temperature. The use of such bridges in donor-bridge-acceptor assemblies enables fast triplet-exciton transport over very long distances that is rate-limited by the donor-bridge injection and bridge-acceptor trapping rates.

Sweet light o' mine: Photothermal and photodynamic inactivation of tenacious pathogens using conjugated polymers

Each year a rising number of infections can not be successfully treated owing to the increasing pandemic of antibiotic resistant pathogens. The global shortage of innovative antibiotics fuels the emergence and spread of drug resistant microbes. Basic research, development, and applications of alternative therapies are urgently needed. Since the 90´s, light-mediated therapies have promised to be the next frontier combating multidrug-resistance microbes. These platforms have demonstrated to be a reliable, rapid, and efficient alternative to eliminate tenacious pathogens while avoiding the emergence of resistance mechanisms. Among the materials showing antimicrobial activity triggered by light, conjugated polymers (CPs) have risen as the most promising option to tackle this complex situation. These materials present outstanding characteristics such as high absorption coefficients, great photostability, easy processability, low cytotoxicity, among others, turning them into a powerful class of photosensitizer (PS)/photothermal agent (PTA) materials. Herein, we summarize and discuss the advances in the field of CPs with applications in photodynamic inactivation and photothermal therapy towards bacteria elimination. Additionally, a section of current challenges and needs in terms of well-defined benchmark experiments and conditions to evaluate the efficiency of phototherapies is presented.

Room-temperature luminescence from Pd(II) and Pt(II) complexes: from mechanochromic crystals to flexible polymer matrices

A series of Pd(II) (PdLOMe, PdLOHex) and Pt(II) (PtLOMe, PtLOHex) complexes bearing tetradentate ligands as dianionic luminophores were synthesized. Hence, the cyclometallating chelators were alternatively decorated with two n-hexyloxy (LOHex) or two methoxy (LOMe) moieties to promote crystallization and processability. The new compounds were unambiguously characterized by means of multiple NMR spectroscopies and mass spectrometry as well as by single crystal X-ray diffractometric analysis (PtLOMe and PdLOMe). Steady state and time-resolved photoluminescence spectroscopic studies were carried out in crystalline phases, in fluid solutions at room temperature, in frozen glassy matrices at 77 K and in a flexible polymeric matrix (PMMA). PtLOMe presents an intriguing mechanochromism resulting from the responsive metal-metal interactions involving adjacent monomeric units. Incorporation of the Pd(II) complexes into the polymeric matrix boosts their photophysical properties by stiffening of the coordination environment while reducing non-radiative deactivation pathways mediated by dissociative metal-centred states, which also become thermally inaccessible at 77 K.

A New Frontier in Exciton Transport: Transient Delocalization

Efficient exciton transport is crucial to the application of organic semiconductors (OSCs) in light-harvesting devices. While the physics of exciton transport in highly disordered media is well-explored, the description of transport in structurally and energetically ordered OSCs is less established, despite such materials being favorable for devices. In this Perspective we describe and highlight recent research pointing toward a highly efficient exciton transport mechanism which occurs in ordered OSCs, transient delocalization. Here, exciton-phonon couplings play a critical role in allowing localized exciton states to temporarily access higher-energy delocalized states whereupon they move large distances. The mechanism shows great promise for facilitating long-range exciton transport and may allow for improved device efficiencies and new device architectures. However, many fundamental questions on transient delocalization remain to be answered. These questions and suggested next steps are summarized.

Water-soluble ruthenium complex-pyrene dyads with extended triplet lifetimes for efficient energy transfer applications

Long triplet lifetimes of excited photosensitizers are essential for efficient energy transfer reactions in water, given that the concentrations of dissolved oxygen and suitable acceptors in aqueous media are typically much lower than in organic solvents. Herein, we report the design, synthesis and photochemical characterization of two structurally related water-soluble ruthenium complex-based dyads decorated with a covalently attached pyrene chromophore. The triplet energy of the latter is slightly below that of the metal complex enabling a so-called triplet reservoir and excited-state lifetime extensions of up to two orders of magnitude. The diimine co-ligands, which can be modified easily, have a major impact on both the ultrafast intramolecular energy transfer (iEnT) kinetics upon excitation with visible light and the lifetime of the resulting long-lived pyrene triplet. The phenanthroline-containing dyad shows fast triplet pyrene formation (25 ps) and an exceptionally long triplet lifetime beyond 50 microseconds in neat water. The iEnT process via the Dexter mechanism is slower by a factor of two when bipyridine co-ligands are employed, which is rationalized by a poor orbital overlap. Both dyads are very efficient sensitizers for the formation of singlet oxygen in air-saturated water as well as for the bimolecular generation of anthracene triplets that are key intermediates in upconversion mechanisms. This is demonstrated by the 5-hydroxymethylfurfural oxidation, which yields completely different main products depending on the pH value of the aqueous solution, as an initial application-related experiment and by time-resolved spectroscopy. Our findings are important in the greater contexts of photocatalysis and energy conversion in the "green" solvent water.

Efficient Triplet-Triplet Annihilation Upconversion Sensitized by a Chromium(III) Complex via an Underexplored Energy Transfer Mechanism

Sensitized triplet-triplet annihilation upconversion (sTTA-UC) mainly relies on precious metal complexes thanks to their high intersystem crossing (ISC) efficiencies, excited state energies, and lifetimes, while complexes of abundant first-row transition metals are only rarely utilized and with often moderate UC quantum yields. [Cr(bpmp)2 ]3+ (bpmp=2,6-bis(2-pyridylmethyl)pyridine) containing earth-abundant chromium possesses an absorption band suitable for green light excitation, a doublet excited state energy matching the triplet energy of 9,10-diphenyl anthracene (DPA), a close to millisecond excited state lifetime, and high photostability. Combined ISC and doublet-triplet energy transfer from excited [Cr(bpmp)2 ]3+ to DPA gives 3 DPA with close-to-unity quantum yield. TTA of 3 DPA furnishes green-to-blue UC with a quantum yield of 12.0 % (close to the theoretical maximum). Sterically less-hindered anthracenes undergo a [4+4] cycloaddition with [Cr(bpmp)2 ]3+ and green light.

Accessing the triplet state of perylenediimide by radical-enhanced intersystem crossing

Owing to their exceptional photophysical properties and high photostability, perylene diimide (PDI) chromophores have found various applications as building blocks of materials for organic electronics. In many light-induced processes in PDI derivatives, chromophore excited states with high spin multiplicities, such as triplet or quintet states, have been revealed as key intermediates. The exploration of their properties and formation conditions is thus expected to provide invaluable insight into their underlying photophysics and promises to reveal strategies for increasing the performance of optoelectronic devices. However, accessing these high-multiplicity excited states of PDI to increase our mechanistic understanding remains a difficult task, due to the fact that the lowest excited singlet state of PDI decays with near-unity quantum yield to its ground state. Here we make use of radical-enhanced intersystem crossing (EISC) to generate the PDI triplet state in high yield. One or two 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) stable radicals were covalently attached to the imide position of PDI chromophores with and without p-tert-butylphenoxy core substituents. By combining femtosecond UV-vis transient absorption and transient electron paramagnetic resonance spectroscopies, we demonstrate strong magnetic exchange coupling between the PDI triplet state and TEMPO, resulting in the formation of excited quartet or quintet states. Important differences in the S₁ state deactivation rate constants and triplet yields are observed for compounds bearing PDI moieties with different core substitution patterns. We show that these differences can be rationalized by considering the varying importance of competitive excited state decay processes, such as electron and excitation energy transfer. The comparison of the results obtained for different PDI–TEMPO derivatives leads us to propose design guidelines for optimizing the efficiency of triplet sensitization in molecular assemblies by EISC.

The triplet state of PDI can be sensitized efficiently by radical-enhanced intersystem crossing. A detailed study of several related structures allows us to propose new strategies to optimize triplet formation in materials for optoelectronic devices.

Triplet-Energy Quenching Functions of Antioxidant Molecules

UV-like DNA damage is created in the dark by chemiexcitation, in which UV-activated enzymes generate reactive oxygen and nitrogen species that create a dioxetane on melanin. Thermal cleavage creates an electronically excited triplet-state carbonyl whose high energy transfers to DNA. Screening natural compounds for the ability to quench this energy identified polyenes, polyphenols, mycosporine-like amino acids, and related compounds better known as antioxidants. To eliminate false positives such as ROS and RNS scavengers, we then used the generator of triplet-state acetone, tetramethyl-1,2-dioxetane (TMD), to excite the triplet-energy reporter 9,10-dibromoanthracene-2-sulfonate (DBAS). Quenching measured as reduction in DBAS luminescence revealed three clusters of 50% inhibitory concentration, ~50 μM, 200–500 μM, and >600 μM, with the former including sorbate, ferulic acid, and resveratrol. Representative triplet-state quenchers prevented chemiexcitation-induced “dark” cyclobutane pyrimidine dimers (dCPD) in DNA and in UVA-irradiated melanocytes. We conclude that (i) the delocalized pi electron cloud that stabilizes the electron-donating activity of many common antioxidants allows the same molecule to prevent an electronically excited species from transferring its triplet-state energy to targets such as DNA and (ii) the most effective class of triplet-state quenchers appear to operate by energy diversion instead of electron donation and dissipate that energy by isomerization.

Elucidating Inner Workings of Naturally Sourced Organic Optoelectronic Materials with Ultrafast Spectroscopy

Recent advances in sustainable optoelectronics including photovoltaics, light-emitting diodes, transistors, and semiconductors have been enabled by π-conjugated organic molecules. A fundamental understanding of light-matter interactions involving these materials can be realized by time-resolved electronic and vibrational spectroscopies. In this Minireview, the photoinduced mechanisms including charge/energy transfer, electronic (de)localization, and excited-state proton transfer are correlated with functional properties encompassing optical absorption, fluorescence quantum yield, conductivity, and photostability. Four naturally derived molecules (xylindein, dimethylxylindein, alizarin, indigo) with ultrafast spectral insights showcase efficient energy dissipation involving H-bonding networks and proton motions, which yield high photostability. Rational design principles derived from such investigations could increase the efficiency for light harvesting, triplet formation, and photosensitivity for improved and versatile optoelectronic performance.

Revealing the Contest between Triplet-Triplet Exchange and Triplet-Triplet Energy Transfer Coupling in Correlated Triplet Pair States in Singlet Fission

Understanding the separation of the correlated triplet pair state ¹(TT) intermediate is critical for leveraging singlet fission to improve solar cell efficiency. This separation mechanism is dominated by two key interactions: (i) the exchange interaction (K) between the triplets which leads to the spin splitting of the biexciton state into ¹(TT),³(TT) and ⁵(TT) states, and (ii) the triplet-triplet energy transfer integral (t) which enables the formation of the spatially separated (but still spin entangled) state ¹(T···T). We develop a simple ab initio technique to compute both the biexciton exchange (K) and biexciton transfer coupling. Our key findings reveal new conditions for successful correlated triplet pair state dissociation. The biexciton exchange interaction needs to be ferromagnetic or negligible to the triplet energy transfer for favorable dissociation. We also explore the effect of chromophore packing to reveal geometries where these conditions are achieved for tetracene.

Wide-range lifetime-tunable and responsive ultralong organic phosphorescent multi-host/guest system

The rational lifetime-tuning strategy of ultralong organic phosphorescence is extraordinarily important but seldom reported. Herein, a series of multi-host/guest ultralong organic phosphorescence materials with dynamic lifetime-tuning properties were reported. By doping a non-room-temperature phosphorescence emitter into various solid host matrices with continuously reduced triplet energy levels, a wide-range lifetime (from 3.9 ms gradually to 376.9 ms) phosphorescence with unchangeable afterglow colors were realized. Further studies revealed that the host matrices were employed to afford rigid environment and proper energy levels to generate and stabilize the long-live triplet excitons. Meanwhile, these multi-host/guest ultralong organic phosphorescence materials also exhibited excitation-dependent phosphorescence and temperature-controlled afterglow on/off switching properties, according to the virtue of various photophysical and thermal properties of the host matrices. This work provides a guiding strategy to realize lifetime-tuning ultralong organic phosphorescence with lifetime-order encoding characteristic towards widespread applications in time-resolved information displaying, higher-level security protection, and dynamic multi-dimensional anti-counterfeiting.

Shallow distance-dependent triplet energy migration mediated by endothermic charge-transfer

Conventional wisdom posits that spin-triplet energy transfer (TET) is only operative over short distances because Dexter-type electronic coupling for TET rapidly decreases with increasing donor acceptor separation. While coherent mechanisms such as super-exchange can enhance the magnitude of electronic coupling, they are equally attenuated with distance. Here, we report endothermic charge-transfer-mediated TET as an alternative mechanism featuring shallow distance-dependence and experimentally demonstrated it using a linked nanocrystal-polyacene donor acceptor pair. Donor-acceptor electronic coupling is quantitatively controlled through wavefunction leakage out of the core/shell semiconductor nanocrystals, while the charge/energy transfer driving force is conserved. Attenuation of the TET rate as a function of shell thickness clearly follows the trend of hole probability density on nanocrystal surfaces rather than the product of electron and hole densities, consistent with endothermic hole-transfer-mediated TET. The shallow distance-dependence afforded by this mechanism enables efficient TET across distances well beyond the nominal range of Dexter or super-exchange paradigms.

An effective potential for Frenkel excitons

Excitation energy transfer (EET) is a ubiquitous process in life and materials sciences. Here, a new and computationally efficient method of evaluating the electronic EET couplings between interacting chromophores is introduced that is valid in a wide range of intermolecular distances. The proposed approach is based on the effective elimination of electron repulsion integrals from the excitonic Hamiltonian matrix elements via the density-fitting approach and distributed multipole approximation. The excitonic Hamiltonian represented in a basis including charge transfer (CT) states is re-cast in terms of the effective one-electron potential functions (EOPs) and adapted into the effective fragment parameter (EFP) framework. Calculations for model systems indicate that the speedup of at least three orders of magnitude, as compared to the state-of-the-art methods, can be achieved while maintaining the accuracy of the EET couplings even at short intermolecular distances.

Bifurcation of excited state trajectories toward energy transfer or electron transfer directed by wave function symmetry

This work explores the concept that differential wave function overlap between excited states can be engineered within a molecular chromophore. The aim is to control excited state wave function symmetries, so that symmetry matches or mismatches result in differential orbital overlap and define low-energy trajectories or kinetic barriers within the excited state surface, that drive excited state population toward different reaction pathways. Two donor-acceptor assemblies were explored, where visible light absorption prepares excited states of different wave function symmetry. These states could be resolved using transient absorption spectroscopy, thanks to wave function symmetry-specific photoinduced optical transitions. One of these excited states undergoes energy transfer to the acceptor, while another undertakes a back-electron transfer to restate the ground state. This differential behavior is possible thanks to the presence of kinetic barriers that prevent excited state equilibration. This strategy can be exploited to avoid energy dissipation in energy conversion or photoredox catalytic schemes.

Prospects of Coupled Organic-Inorganic Nanostructures for Charge and Energy Transfer Applications

We review the field of organic-inorganic nanocomposites with a focus on materials that exhibit a significant degree of electronic coupling across the hybrid interface. These nanocomposites undergo a variety of charge and energy transfer processes, enabling optoelectronic applications in devices which exploit singlet fission, triplet energy harvesting, photon upconversion or hot charge carrier transfer. We discuss the physical chemistry of the most common organic and inorganic components. Based on those we derive synthesis and assembly strategies and design criteria on material and device level with a focus on photovoltaics, spin memories or optical upconverters. We conclude that future research in the field should be directed towards an improved understanding of the binding motif and molecular orientation at the hybrid interface.

Mechanistic Understanding and Rational Design of Quantum Dot/Mediator Interfaces for Efficient Photon Upconversion

The semiconductor-nanocrystal-sensitized, three-component upconversion system has made great strides over the past 5 years. The three components (i.e., triplet photosensitizer, mediator, and emitter) each play critical roles in determining the input and output photon energy and overall quantum efficiency (QE). The nanocrystal photosensitizer converts the absorbed photon into singlet excitons and then triplet excitons via intersystem crossing. The mediator accepts the triplet exciton via either direct Dexter-type triplet energy transfer (TET) or sequential charge transfer (CT) while extending the exciton lifetime. Through a second triplet energy-transfer step from the mediator to the emitter, the latter is populated in its lowest excited triplet state. Triplet-triplet annihilation (TTA) between two triplet emitters generates the emitter in its bright singlet state, which then emits the upconverted photon. Quantum dots (QD) have a tunable band gap, large extinction coefficient, and small singlet-triplet energy losses compared to metal-ligand charge-transfer complexes. This high triplet exciton yield makes QDs good candidates for photosensitizers. In terms of driving triplet energy transfer, the triplet energy of the mediator should be slightly lower than the triplet exciton energy of the QD sensitizer for a downhill energy landscape with minimal energy loss. The same energy cascade is also required for the transfer from the mediator to the emitter. Finally, the triplet energy of the emitter must be slightly larger than one-half of its singlet energy to ensure that TTA is exothermic. Optimization of the sensitizer, mediator, and emitter will lead to an increase in the anti-Stokes shift and the total quantum efficiency. Evaluating each individual step's efficiency and kinetics is necessary for the understanding of the limiting factors in existing systems.This review summarizes chalcogenide QD-based photon upconversion systems with a focus on the mechanistic aspects of triplet energy transfer conducted by the Tang and Lian groups. Via time-resolved spectroscopy, the rates and major loss pathways associated with the two triplet energy-transfer steps were identified. The studies are focused on the near-infrared (NIR) to visible (VIS) PbS-tetracene-based systems as they allow systematic control of the QD, mediator, and emitter. Our results show that the mediator triplet state is mostly formed by direct TET from the QD and the transfer rate is influenced by the density of bound mediator molecules. Charge transfer, a loss pathway, does not produce triplet excitons and can be minimized by adding an inert shell to the QD. This transfer rate decreases exponentially with the distance between the QD and mediator molecule. The second TET rate was found to be much slower than the diffusion-limited collision rate, which results in the triplet lifetime of the mediator being the main factor limiting its efficiency. Finally, the total quantum efficiency can be calculated using these measured quantities including the TET1 and TET2 efficiencies. The agreement between calculated and measured quantum efficiencies suggests a firm understanding of QD-sensitized photon upconversion. We believe the above conclusions are general and should be widely applicable to similar systems, including singlet fission in hybrid organic-nanocrystal materials.

Excitation Energy Transfer and Exchange-Mediated Quartet State Formation in Porphyrin-Trityl Systems

Photogenerated multi‐spin systems hold great promise for a range of technological applications in various fields, including molecular spintronics and artificial photosynthesis. However, the further development of these applications, via targeted design of materials with specific magnetic properties, currently still suffers from a lack of understanding of the factors influencing the underlying excited state dynamics and mechanisms on a molecular level. In particular, systematic studies, making use of different techniques to obtain complementary information, are largely missing. This work investigates the photophysics and magnetic properties of a series of three covalently‐linked porphyrin‐trityl compounds, bridged by a phenyl spacer. By combining the results from femtosecond transient absorption and electron paramagnetic resonance spectroscopies, we determine the efficiencies of the competing excited state reaction pathways and characterise the magnetic properties of the individual spin states, formed by the interaction between the chromophore triplet and the stable radical. The differences observed for the three investigated compounds are rationalised in the context of available theoretical models and the implications of the results of this study for the design of a molecular system with an improved intersystem crossing efficiency are discussed.

Role of Spin-Orbit Coupling in Long Range Energy Transfer in Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are emerging as a promising platform for solar energy conversion applications. Their potential utilization as efficient chromophores in artificial photosynthesis is closely related to the understanding of light-harvesting and energy transfer processes that occur within these molecular scaffolds. Herein, we present the photophysical investigation of Ru(II), Ir(III), and Os(II) polypyridyl complexes incorporated into the backbone of UiO-67. In this work, we systematically study the effect of spin-orbit coupling on dipole-dipole energy transfer in MOFs using steady-state and time-resolved spectroscopic techniques. The results of our work indicate successful triplet-to-singlet energy transfer and a sizable increase in the transfer kinetics and critical distance, as direct consequences of strong spin-orbit couplings. Remarkably, the reported R0 value for OsDCBPY (R0 = 88 ± 10 Å) represents one of the largest Förster distances observed in an MOF. Collectively, this work contributes to the general knowledge of energy transfer in materials and provides groundwork for efficient utilization in artificial photosynthetic assemblies.

Control of electron transfer by protein dynamics in photosynthetic reaction centers

Trehalose and glycerol are low molecular mass sugars/polyols that have found widespread use in the protection of native protein states, in both short- and long-term storage of biological materials, and as a means of understanding protein dynamics. These myriad uses are often attributed to their ability to form an amorphous glassy matrix. In glycerol, the glass is formed only at cryogenic temperatures, while in trehalose, the glass is formed at room temperature, but only upon dehydration of the sample. While much work has been carried out to elucidate a mechanistic view of how each of these matrices interact with proteins to provide stability, rarely have the effects of these two independent systems been directly compared to each other. This review aims to compile decades of research on how different glassy matrices affect two types of photosynthetic proteins: (i) the Type II bacterial reaction center from Rhodobacter sphaeroides and (ii) the Type I Photosystem I reaction center from cyanobacteria. By comparing aggregate data on electron transfer, protein structure, and protein dynamics, it appears that the effects of these two distinct matrices are remarkably similar. Both seem to cause a "tightening" of the solvation shell when in a glassy state, resulting in severely restricted conformational mobility of the protein and associated water molecules. Thus, trehalose appears to be able to mimic, at room temperature, nearly all of the effects on protein dynamics observed in low temperature glycerol glasses.