Lowest Singlet Research Articles

Organophotocatalytic or Electrophotocatalytic Reduction and Functionalization Reactions with a Thioxanthone-TfOH Complex Catalyst

AbstractThioxanthone has long been a prominent catalyst in the field of photocatalysis, owing to its high triplet energy and long triplet lifetime that render it suitable for energy transfer reactions. However, its low oxidation potential and short singlet lifetime have posed challenges when employing it for electron-transfer reactions. This account summarizes our efforts in developing a potent and long-lived thioxanthone-TfOH complex (9-HTXTF) catalyst, and its application in energy-demanding redox transformations such as organophotocatalytic or electrophotocatalytic reduction and functionalization reactions.1 Introduction2 Discovery and Properties of the Catalyst 9-HTXTF3 Organophotocatalysis4 Electrophotocatalysis5 Conclusion

Multistate Dynamics and Kinetics of CO2 Activation by Ta+ in the Gas Phase: Insights into Single-Atom Catalysis.

The activation of carbon dioxide (CO2) by a transition-metal cation in the gas phase is a unique model system for understanding single-atom catalysis. The mechanism of such reactions is often attributed to a "two-state reactivity" model in which the high-energy barrier of a spin state correlating with ground-state reactants is avoided by intersystem crossing (ISC) to a different spin state with a lower barrier. However, such a "spin-forbidden" mechanism, along with the corresponding dynamics, has seldom been rigorously examined theoretically, due to the lack of global potential energy surfaces (PESs). In this work, we report full-dimensional PESs of the lowest-lying quintet, triplet, and singlet states of the TaCO2+ system, machine-learned from first-principles data. These PESs and the corresponding spin-orbit couplings enable us to provide an extensive theoretical characterization of the dynamics and kinetics of the reaction between the tantalum cation (Ta+) and CO2, which have recently been investigated experimentally at high collision energies using crossed beams and velocity map imaging, as well as at thermal energies using a selected-ion flow tube apparatus. The multistate quasi-classical trajectory simulations with surface hopping reproduce most of the measured product translational and angular distributions, shedding valuable light on the nonadiabatic reaction dynamics. The calculated rate coefficients from 200 to 600 K are also in good agreement with the latest experimental measurements. More importantly, these calculations revealed that the reaction is controlled by intersystem crossing, rather than potential barriers.

Computational Investigations of the Detailed Mechanism of Reverse Intersystem Crossing in Inverted Singlet-Triplet Gap Molecules.

Inverted singlet-triplet gap (INVEST) materials have promising photophysical properties for optoelectronic applications due to an inversion of their lowest singlet (S1) and triplet (T1) excited states. This results in an exothermic reverse intersystem crossing (rISC) process that potentially enhances triplet harvesting, compared to thermally activated delayed fluorescence (TADF) emitters with endothermic rISCs. However, the processes and phenomena that facilitate conversion between excited states for INVEST materials are underexplored. We investigate the complex potential energy surfaces (PESs) of the excited states of three heavily studied azaphenalene INVEST compounds, namely, cyclazine, pentazine, and heptazine using two state-of-the-art computational methodologies, namely, RMS-CASPT2 and SCS-ADC(2) methods. Our findings suggest that ISC and rISC processes take place directly between the S1 and T1 electronic states in all three compounds through a minimum-energy crossing point (MECP) with an activation energy barrier between 0.11 to 0.58 eV above the S1 state for ISC and between 0.06 and 0.36 eV above the T1 state for rISC. We predict that higher-lying triplet states are not populated, since the crossing point structures to these states are not energetically accessible. Furthermore, the conical intersection (CI) between the ground and S1 states is high in energy for all compounds (between 0.4 to 2.0 eV) which makes nonradiative decay back to the ground state a relatively slow process. We demonstrate that the spin-orbit coupling (SOC) driving the S1-T1 conversion is enhanced by vibronic coupling with higher-lying singlet and triplet states possessing vibrational modes of proper symmetry. We also rationalize that the experimentally observed anti-Kasha emission of cyclazine is due to the energetically inaccessible CI between the bright S2 and the dark S1 states, hindering internal conversion. Finally, we show that SCS-ADC(2) is able to qualitatively reproduce excited state features, but consistently overpredict relative energies of excited state structural minima compared to RMS-CASPT2. The identification of these excited state features elaborates design rules for new INVEST emitters with improved emission quantum yields.

Photophysical Studies of a Zr(IV) Complex with Two Pyrrolide-Based Tetradentate Schiff Base Ligands.

The reaction of two equivalents of N,N'-bis(2-pyrrolylmethylidene)-1,2-phenylenediamine (H2bppda) with tetrabenzylzirconium provided the air- and moisture-stable eight-coordinate complex Zr(bppda)2. Temperature-dependent steady-state and time-resolved emission spectroscopy established weak photoluminescence (ΦPL = 0.4% at 293 K) by a combination of prompt fluorescence and thermally activated delayed fluorescence (TADF) upon visible light excitation at and around room temperature. TADF emission is strongly quenched by 3O2 and shows highly temperature-sensitive emission lifetimes of hundreds of microseconds. The lifetime of the lowest energy singlet excited state, S1, was established by transient absorption spectroscopy and shows rapid deactivation (τ = 142 ps) by prompt fluorescence and intersystem crossing to the triplet state, T1. Time-dependent density functional theory (TD-DFT) calculations predict moderate ligand-to-metal charge transfer (LMCT) contributions of 25-30% for the S1 and T1 states. A comparison of Zr(bppda)2 to related zirconium pyridine dipyrrolide complexes, Zr(PDP)2, revealed important electronic structure changes due to the eight-coordinate ligand environment in Zr(bppda)2, which were correlated to differences in the photophysical properties between the two compound classes.

Single component long-lived dynamic room temperature phosphorescence with large-scale preparation and visible light excitation

A pure organic room temperature phosphorescence (PORTP) luminogen named as DCP was successfully prepared and showed long room temperature phosphorescence (RTP) and afterglow lifetimes (3 ms and 4 s in sequence). Meanwhile, DCP showed high fluorescence (0.14) and RTP (0.39) quantum yields in crystalline state, simultaneously presented visible light excitation and dynamic RTP. By host-guest doping and optimization, Urea/DCP doping system emitted blue RTP, with RTP and afterglow lifetimes of 341 ms of 5 s in turn. Crystal analysis and theoretical calculations indicated H stacking, large spin orbit coupling constant, and small energy gap between the lowest singlet and triplet states result in long RTP lifetime and high quantum yield for DCP. Based on long phosphorescence emission, an advanced anti-counterfeiting pattern was constructed. As far as we know, it is the first report that such small molecules containing 3, 5-dicyanopyridine show long-lived RTP in both crystalline state and doping system. In this paper, not only ultralong and blue PORTP materials with the advantage of large-scale preparation, but also single component dynamic RTP, and RTP of visible light excitation were achieved.

Static and resonant properties of decorated square kagomé lattice KCu[formula omitted](TeO[formula omitted])(SO[formula omitted])[formula omitted]Cl

The magnetic subsystem of nabokoite, KCu7(TeO4)(SO4)5Cl, is constituted by copper ions forming a buckled square kagomé lattice decorated by quasi-isolated ions. This combination determines peculiar physical properties of this compound evidenced in electron spin resonance (ESR) spectroscopy, dielectric permittivity ɛ, magnetization M and specific heat Cp measurements. At lowering temperature, the magnetic susceptibility χ=M/H passes through a broad hump inherent for low-dimensional magnetic systems at about 150 K and a sharp peak at antiferromagnetic phase transition at TN=3.2 K. The Cp(T,H) curves demonstrate additional peak-like anomaly at Tpeak=5.7 K robust to magnetic field. The latter can be ascribed to low-lying singlet excitations filling the singlet-triplet gap in magnetic excitation spectrum of the square kagomé lattice (Richter et al., 2022). ESR spectroscopy provides indications that antiferromagnetic structure below TN is non-collinear. Separate issue is the observation of antiferroelectric-type behavior in ɛ at low temperatures, which tentatively reduces the symmetry and partially lifts frustration of magnetic interactions of decorating copper ions with buckled square kagomé lattice. These complex thermodynamic and resonant properties signal the presence of two weakly coupled magnetic subsystems in nabokoite, namely a spin-liquid in square kagomé lattice layers and an antiferromagnet represented by decorating ions.

Rational design by DFT calculations of new metal complexes for photodynamic therapy based on an H2O2-Responsive Ir(III) photosensitizer activatable in physiological environment

A series of complexes based on the structure of an H2O2-responsive Ir(III) phenylpyridine(ppy)-containing complex were designed. The effect of changing the conjugation in the ppy ligands as well as of replacing the Ir(III) metal centre with the isoelectronic Os(II) were evaluated by means of DFT and its time-dependent formulation. The computational exploration of the photophysical properties of all the designed complexes suggest that the character of the low-lying singlet state represent the main factor affecting the region of absorption. In particular, for some of the Ir(III)-based complexes the change of the band character from mixed MLCT/LLCT to almost exclusively LLCT is essential for red-shifting the maximum absorption wavelength together with the destabilization of HOMO thus reducing the HOMO-LUMO energy gap; similarly, the replacement of Ir(III) with Os(II) converts the character of the band to mainly MLCT. The computed spin–orbit coupling constants and the triplet states’ energy confirm that all the designed complexes could, in principle, be able to generate the cytotoxic singlet oxygen under one-photon excitation by energy transfer from the triplet state of the photosensitizer to molecular oxygen, while the production of other types of ROS is precluded, similarly to the reference complex.

3,4-Dimethylenecyclobutene: A Building Block for Design of Macrocycles with Excited State Aromatic Low-Lying High-Spin States.

3,4-Dimethylenecyclobutene (DMCB) is an unusual isomer of benzene. Motivated by recent synthetic progress to substituted derivatives of this scaffold, we carried out a theoretical and computational analysis with a particular focus on the extent of (anti)aromatic character in the lowest excited states of different multiplicities. We found that the parent DMCB is non-aromatic in its singlet ground state (S0), lowest triplet state (T1), and lowest singlet excited state (S1), while it is aromatic in its lowest quintet state (Q1) as this state is represented by a triplet multiplicity cyclobutadiene (CBD) ring and two uncoupled same-spin methylene radicals. Interestingly, the Q1 state, despite having four unpaired electrons, is placed merely 4.8 eV above S0, and there is a corresponding singlet tetraradical 0.16 eV above. The DMCB is potentially a highly useful structural motif for the design of larger molecular entities with interesting optoelectronic properties. Here, we designed macrocycles composed of fused DMCB units, and according to our computations, two of these have low-lying nonet states (i. e., octaradical states) at energies merely 2.40 and 0.37 eV above their S0 states as a result of local Hückel- and Baird-aromatic character of individual 6π- and 4π-electron monocycles.

Electroweak phase transition in singlet extensions of the standard model with dimension-six operators

The significance of the electroweak phase transition is undeniable, and although initially it was believed that it was second order, it is now believed that it is a first-order transition. However, it is not a strong first-order phase transition in the context of the standard model, and the remedy to this issue is to use the Higgs portal and directly couple the Higgs to a hidden scalar sector. This can result in a strong electroweak phase transition, while the couplings to a hidden scalar are constrained by several phenomenological constraints, such as the sphaleron rate criterion and the branching ratio of the Higgs to invisible channels. In this work, we consider the standard singlet extensions of the standard model, including dimension-six non-renormalizable operators that couple a real singlet scalar field with the Higgs doublet. As a result, we examine the effects of those Higgs-singlet couplings on the electroweak phase transition. The effective theory, where the nonrenormalizable couplings originate from, is considered to be active beyond 15 TeV. As we show, the Universe experiences a two-step electroweak phase transition, a primary phase transition in the singlet sector at a high temperature, and then a subsequent first-order phase transition from the singlet vacuum to the electroweak vacuum. The singlet’s phase transition can either be second order or first order, depending on the singlet mass and its couplings to the Higgs. In particular, we show that the dimension-six operator assists in generating a strong electroweak phase transition in regions of the parameter space that were excluded in the previous singlet extensions of the standard model. This is further apparent for low singlet masses mS<mH/2, which are rarely taken into account in the literature due to the invisible branching ratio of the Higgs boson. In some limited cases in the parameter space, the electroweak phase transition is weakened by the presence of the higher order operator. Published by the American Physical Society 2024

Direct observation of room-temperature high-level triplet–triplet annihilation process in 1%rubrene-doped OLEDs with a CzDBA sensitizer

Using the magneto-electroluminescence as a sensitive and fingerprint probing tool, a “hot exciton” channel, named the high-level triplet–triplet annihilation (HL-TTA or T2T2A, T2 + T2 → Sn → S1 → S0 + hv) process from the high-lying triplet (T2) to the lowest singlet states, is observed in 1%rubrene-doped organic light-emitting diodes with a thermally activated delayed fluorescence sensitizer of 9,10-bis(4-(9H-carbazol-9-yl)-2,6-dimethylphenyl)-9,10-diboraanthracene (CzDBA) at room temperature. The addition of a sensitizer simultaneously promotes the Dexter energy transfer channels of host-sensitizer and sensitizer-guest triplets, thereby increasing the amounts of T2 states and inducing the occurrence of the HL-TTA process. Additionally, the HL-TTA enhances with the increase in the bias current and decreases with lowering the working temperature, which is consistent with the current dependence of the conventional low-level TTA (T1T1A, T1 + T1 → S1 + S0) process but contrary to its temperature dependence. More interestingly, the high concentration of CzDBA induces the H-type aggregation of rubrene molecules, promoting the singlet fission process but suppressing the HL-TTA process. These findings enrich the physical understanding of hot exciton channels and provide ideas for the preparation of high-performance devices.

Understanding of complex spin up-conversion processes in charge-transfer-type organic molecules

Despite significant progress made over the past decade in thermally activated delayed fluorescence (TADF) molecules as a material paradigm for enhancing the performance of organic light-emitting diodes, the underlying spin-flip mechanism in these charge-transfer (CT)-type molecular systems remains an enigma, even since its initial report in 2012. While the initial and final electronic states involved in spin-flip between the lowest singlet and lowest triplet excited states are well understood, the exact dynamic processes and the role of intermediate high-lying triplet (T) states are still not fully comprehended. In this context, we propose a comprehensive model to describe the spin-flip processes applicable for a typical CT-type molecule, revealing the origin of the high-lying T state in a partial molecular framework in CT-type molecules. This work provides experimental and theoretical insights into the understanding of intersystem crossing for CT-type molecules, facilitating more precise control over spin-flip rates and thus advancing toward developing the next-generation platform for purely organic luminescent candidates.

Thermally Activated Delayed Fluorescent Binuclear Copper(I) Alkynyl Complexes with Cuprophilic Interactions.

Copper(I)-based thermally activated delayed fluorescence (TADF) emitters have been conceived to be promising candidates for display and lighting applications because of their multifarious structures and strong photoluminescence. Herein a string of binuclear Cu(I) complexes bearing pronounced cuprophilic interactions have been designed and synthesized. [Cu2(dppb)2(μ2-η1-C≡C-Ph)2] (1 a) and [Cu2(dppb)2(μ2-η1-C≡C-PPXZ)2] (1 b) display photoluminescence quantum yields of up to 67 % in doped films and solid states via TADF and exhibit reversible bicolor luminescence switching upon mechanical stimuli. Computational studies manifest that the metal-to-ligand charge transfer predominant transitions ensure a small energy splitting (ΔEST) between the lowest singlet (S1) and triplet (T1) excited states and cuprophilic interactions promote the spin-orbit coupling (SOC), favoring the reverse intersystem crossing (RISC) process. This study provides a new strategy for the construction of stimuli-responsive metal-based TADF materials.

γ-glutamyl transpeptidase activatable NIR photosensitizer for visualization and selective killing of liver cancer cells

Photodynamic therapy (PDT) is a treatment model that involves using light sources to excite photosensitizers (PSs), generating reactive oxygen species. This method is considered a potential treatment for tumors. However, the non-selective toxicity of traditional PSs to both tumors and normal tissues hinders the advancement of PDT. This lack of selectivity is a major obstacle in the development of effective PDT. The activatable PS can only function in a specific environment, which enhances the selectivity and safety of PDT. In this paper, we focus on GGT (γ-glutamyltranspeptidase) overexpressed in liver cancer as the biological target. We designed and synthesized the activatable PS BrCy-Glu by modifying the substrate of GGT (l-glutamic acid) onto the brominated hemicyanine PS (BrCy-OH). The intramolecular charge transfer (ICT) effect of BrCy-Glu is inhibited, which leads to low fluorescence quantum yield and singlet oxygen yield. However, in the presence of GGT, it can undergo selective hydrolysis and be converted into BrCy-OH through a series of elimination reactions. This process is accompanied by a significant increase in fluorescence quantum yield and singlet oxygen yield. In vitro studies demonstrate that BrCy-Glu is suitable for precise fluorescence identification of liver cancer and selective eradication of cancer cells under light conditions.

Tuning Interactions to Control Molecular Down Conversion in [2.2]Paracyclophane Bridged Oligo‐Tetracenes

AbstractIn tetracene, the energies of the lowest singlet excited state and twice that of the lowest triplet excited state are similar, allowing both down‐conversion (i.e., singlet fission, SF) and up‐conversion (i.e., triplet‐triplet annihilation up‐conversion, TTA‐UC) processes. Through‐space and through‐bond contributions to the inter‐tetracene coupling in purposefully designed oligomers play a crucial role in determining which of the two processes dominates. In this work, the focus is exclusively on SF in newly synthesized oligo‐tetracenes linked by conjugated [2.2]paracyclophane (PCP) building blocks. By choosing different PCP substitution patterns and by varying the degree of substitution the inter‐tetracene couplings are addressed. An independent variable is connecting the tetracences to the PCP at different positions to alter the through‐bond and through‐space coupling of the resulting oligo‐tetracenes. The novel oligo‐tetracenes are investigated by means of steady‐state and time‐resolved absorption and fluorescence spectroscopies with respect to the initial events of SF, that is, the transformation of a singlet excited state into a correlated triplet pair state. Briefly, through‐space couplings are profoundly weaker than through‐bond couplings that enable the correlated triplet pair state formation. If interactions are through‐space, correlated triplet pair state formation is turned off, while it is turned on if through‐bond interactions are operative.

Decarbonylation Products of Binuclear Methylphosphinidene Complexes of Cyclopentadienyliron Carbonyls: Triplet and Quintet Structures Are Favored Energetically over Singlet Structures with Iron-Iron Multiple Bonding.

The structures, energetics, and energetically preferred spin states of methylphosphinidene-bridged binuclear cyclopentadienyliron carbonyl complexes MePFe2(CO)nCp2 (n = 4, 3, 2, and 1) related to the experimentally known (μ-RP)Fe2(μ-CO)(CO)2Cp2 (R = cyclohexyl, phenyl, mesityl, and 2,4,6-tBu3C6H2) complexes have been investigated by density functional theory. Singlet structures having a pyramidal pseudotetrahedral phosphorus environment with 18-electron iron configurations are energetically preferred in the tricarbonyl and tetracarbonyl systems MePFe2(CO)nCp2 (n = 4 and 3) with the lowest energy structures of the tricarbonyl very closely resembling the experimentally determined structures. For the more unsaturated dicarbonyl and monocarbonyl systems MePFe2(CO)nCp2 (n = 2 and 1), higher spin state triplet and quintet structures are energetically preferred over singlet structures. These more highly unsaturated structures can be derived from the lowest energy singlet MePFe2(CO)nCp2 (n = 4, 3) by the removal of carbonyl groups. The iron atoms giving up carbonyl groups in their 16- and 14-electron configurations bear the spin density of the unpaired electrons in the higher spin states. The lowest energy singlet structure of the monocarbonyl MePFe2(CO)Cp2, although a relatively high energy isomer, is unusual among the collection of MePFe2(CO)nCp2 (n = 4, 3, 2, and 1) structures by having both the formal Fe=Fe double bond and the four-electron donor MeP unit with the planar phosphorus coordination required to allow each of its iron atoms to attain the favored 18-electron configuration.

Dual Emissive Zn(II) Naphthalocyanines: Synthesis, Structural and Photophysical Characterization with Theory-Supported Insights towards Soluble Coordination Compounds with Visible and Near-Infrared Emission.

Metal phthalocyaninates and their higher homologues are recognized as deep-red luminophores emitting from their lowest excited singlet state. Herein, we report on the design, synthesis, and in-depth characterization of a new class of dual-emissive (visible and NIR) metal naphthalocyaninates. A 4-N,N-dimethylaminophen-4-yl-substituted naphthalocyaninato zinc(II) complex (Zn-NMe2Nc) and the derived water-soluble coordination compound (Zn-NMe3Nc) exhibit a near-infrared fluorescence from the lowest ligand-centered state, along with a unique push-pull-supported luminescence in the visible region of the electromagnetic spectrum. An unprecedentedly broad structural (2D-NMR spectroscopy and mass spectrometry) as well as photophysical characterization (steady-state state and time-resolved photoluminescence spectroscopy) is presented. The unique dual emission was assigned to two independent sets of singlet states related to the intrinsic Q-band of the macrocycle and to the push-pull substituents in the molecular periphery, respectively, as predicted by TD-DFT calculations. In general, the elusive chemical aspects of these macrocyclic compounds are addressed, involving both reaction conditions, thorough purification, and in-depth characterization. Besides the fundamental aspects that are investigated herein, the photoacoustic properties were exemplarily examined using phantom gels to assess their tomographic imaging capabilities. Finally, the robust luminescence in the visible range arising from the push-pull character of the peripheral moieties demonstrated a notable independence from aggregation and was exemplarily implemented for optical imaging (FLIM) through time-resolved multiphoton micro(spectro)scopy.

HLCT-Type Acceptor Molecule-Based Exciplex System for Highly Efficient Solution-Processable OLEDs with Suppressed Efficiency Roll-Offs.

Exciplex systems are promising candidates for thermally activated delayed fluorescence (TADF) molecules because of the small energy difference between the lowest singlet and triplet excited states (ΔEST). However, realizing high-efficiency and low-external-quantum-efficiency (EQE) roll-off in solution-processed organic light-emitting diodes (OLEDs) using an exciplex system remains a formidable challenge. In this study, two (HLCT)-type isomers with a spiro skeleton, 2-tBuspoCz-TRZ and 10-tBuspoCz-TRZ, are designed and synthesized as acceptors of exciplexes, where tert-butylspirofluorene indole is regarded as a donor and the triazine unit as an acceptor. Green exciplex emissions are observed for the 2-tBuspoCz-TRZ:TAPC and 10-tBuspoCz-TRZ:TAPC exciplexes, indicating distinct TADF characteristics with a very small ΔEST of 35 ± 5 meV. By using the TADF exciplex system based on the HLCT acceptor as an emitter, solution-processable OLEDs achieve a maximum external quantum efficiency (EQEmax) of 20.8%. Furthermore, a high EQEmax > 25% with a very low-efficiency roll-off (≈3.5% at 1000cd m-2) is obtained for solution-processable phosphorescent devices using HLCT-based exciplexes as the host matrix of phosphors. This study paves the way for a novel strategy for designing acceptor exciplex molecules for effective TADF molecules and host matrices in solution-processable OLEDs.

Photogeneration and quenching of singlet molecular oxygen by bacterial C40 carotenoids with long chain of conjugated double bonds.

Measurement of photosensitized luminescence of singlet oxygen has been applied to studies of singlet oxygen generation and quenching by C40 carotenoids (neurosporene, lycopene, rhodopin, and spirilloxanthin) with long chain of conjugated double bonds (CDB) using hexafluorobenzene as a solvent. It has been found that neurosporene, lycopene, and rhodopin are capable of the low efficient singlet oxygen generation in aerated solutions upon photoexcitation in the spectral region of their main absorption maxima. The quantum yield of this process was estimated to be (1.5-3.0) × 10-2. This value is near the singlet oxygen yields in solutions of ζ-carotene (7 CDB) and phytoene (3 CDB) and many-fold smaller than in solutions of phytofluene (5 CDB) (Ashikhmin et al. Biochemistry (Mosc) 85:773-780, https://doi.org/10.1134/S0006297920070056 , 2020, Biochemistry (Mosc) 87:1169-1178, 2022, https://doi.org/10.1134/S00062979221001082022 ). Photogeneration of singlet oxygen was not observed in spirilloxanthin solutions. A correlation was found between the singlet oxygen yields and the quantum yields and lifetimes of the fluorescence of the carotenoid molecules. All carotenoids were shown to be strong physical quenchers of singlet oxygen. The rate constants of 1O2 quenching by the carotenoids with long chain of CDB (9-13) were close to the rate constant of the diffusion-limited reactions ≈1010M-1s-1, being many-fold greater than the rate constants of 1O2 quenching by the carotenoids with the short chain of CDB (3-7) phytoene, phytofluene, and ζ-carotene studied in prior papers of our group (Ashikhmin et al. 2020, 2022). To our knowledge, the quenching rate constants of rhodopin and spirilloxanthin have been obtained in this paper for the first time. The mechanisms of 1O2 photogeneration by carotenoids in solution and in the LH2 complexes of photosynthetic cells, as well as the efficiencies of their protective action are discussed.

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

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

Singlet fission initiating organic photosensitizations

The feasibility of singlet fission (SF) in organic photosensitizers is investigated through spin-flip long-range corrected time-dependent density functional theory. This study focuses on four major organic photosensitizer molecules: benzophenone, boron-dipyrromethene, methylene blue, and rose bengal. Calculations demonstrate that all these molecules possess moderate pi-stacking energies and closely-lying singlet (S) and quintet (triplet–triplet, TT) excitations, satisfying the essential conditions for SF: (1) Near-degenerate low-lying S and (TT) excitations with a significant S–T energy gap, and (2) Moderate pi-stacking energy of chromophores, slightly higher than solvation energy, enabling dissociation for triplet-state chromophore generation. Moreover, based on the El-Sayed rule, intersystem crossing is found to simultaneously proceed at very slow rates in all these photosensitizers. This is attributed to the fact that the lowest singlet excitation of the monomers partly involves npi ^* transitions alongside the main pi pi ^* transitions. The proposed mechanisms are strongly substantiated by comparisons with experimental studies.