Triplet-triplet Energy Transfer Research Articles

Impact of Surface Trap States on Electron and Energy Transfer in CdSe Quantum Dots Studied by Femtosecond Transient Absorption Spectroscopy.

The presence of surface trap states (STSs) is one of the key factors to affect the electronic and optical properties of quantum dots (QDs), however, the exact mechanism of how STSs influence QDs remains unclear. Herein, we demonstrated the impact of STSs on electron transfer in CdSe QDs and triplet-triplet energy transfer (TTET) from CdSe to surface acceptor using femtosecond transient absorption spectroscopy. Three types of colloidal CdSe QDs, each containing various degrees of STSs as evidenced by photoluminescence and X-ray photoelectron spectroscopy, were employed. Time-resolved emission and transient absorption spectra revealed that STSs can suppress band-edge emission effectively, resulting in a remarkable decrease in the lifetime of photoelectrons in QDs from 17.1 ns to 4.9 ns. Moreover, the investigation of TTET process revealed that STSs can suppress the generation of triplet exciton and effectively inhibit band-edge emission, leading to a significant decrease in TTET from CdSe QDs to the surface acceptor. This work presented evidence for STSs influence in shaping the optoelectronic properties of QDs, making it a valuable point of reference for understanding and manipulating STSs in diverse QDs-based optoelectronic applications involving electron and energy transfer.

Photoinduced transposed Paternò-Büchi reaction for effective synthesis of high-performance jet fuel

High-energy-density fuels are important for volume-limited aerospace vehicles, but the increase of the fuel energy density always leads to poor cryogenic performance. Herein, we investigated the transposed Paternò-Büchi reaction of biomass cyclic ketone and cyclic alkene to synthesize the new kind of alkyl-substituted polycyclic hydrocarbon fuels with high energy density and good cryogenic performance. The triplet-energy-quenching results and phosphorescent emission spectra reveal the sensitization mechanism of the reaction, including photosensitizer excitation, triplet-triplet energy transfer, cyclization and relaxation, and the possible reaction path was revealed by the DFT calculations. The reaction conditions of photosensitizer type and addition, molar ratio of substrates, reaction temperature and incident light intensity were optimized, with the target product yield achieving 65.5%. Moreover, the reaction dynamics of the reaction rate versus the light intensity is established. After hydrogenation deoxygenation reaction, three fuels with high density of 0.864-0.938 g·ml−1 and low freezing point of <−55 °C are obtained. This work provides a benign and effective approach to synthesize high-performance fuels.

Near-infrared light-triggered prodrug photolysis by one-step energy transfer

Prodrug photolysis enables spatiotemporal control of drug release at the desired lesions. For photoactivated therapy, near-infrared (NIR) light is preferable due to its deep tissue penetration and low phototoxicity. However, most of the photocleavable groups cannot be directly activated by NIR light. Here, we report a upconversion-like process via only one step of energy transfer for NIR light-triggered prodrug photolysis. We utilize a photosensitizer (PS) that can be activated via singlet-triplet (S-T) absorption and achieve photolysis of boron-dipyrromethene (BODIPY)-based prodrugs via triplet-triplet energy transfer. Using the strategy, NIR light can achieve green light-responsive photolysis with a single-photon process. A wide range of drugs and bioactive molecules are designed and demonstrated to be released under low-irradiance NIR light (100 mW/cm2, 5 min) with high yields (up to 87%). Moreover, a micellar nanosystem encapsulating both PS and prodrug is developed to demonstrate the practicality of our strategy in normoxia aqueous environment for cancer therapy. This study may advance the development of photocleavable prodrugs and photoresponsive drug delivery systems for photo-activated therapy.

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.

Local and Large-Scale Conformational Dynamics in Unfolded Proteins and IDPs. II. Effect of Temperature and Internal Friction.

Internal dynamics of proteins are essential for protein folding and function. Dynamics in unfolded proteins are of particular interest since they are the basis for many cellular processes like folding, misfolding, aggregation, and amyloid formation and also determine the properties of intrinsically disordered proteins (IDPs). It is still an open question of what governs motions in unfolded proteins and whether they encounter major energy barriers. Here we use triplet-triplet energy transfer (TTET) in unfolded homopolypeptide chains and IDPs to characterize the barriers for local and long-range loop formation. The results show that the formation of short loops encounters major energy barriers with activation energies (Ea) up to 18 kJ/mol (corrected for effects of temperature on water viscosity) with very little dependence on amino acid sequence. For poly(Gly-Ser) and polySer chains the barrier decreases with increasing loop size and reaches a limiting value of 4.6 ± 0.4 kJ/mol for long and flexible chains. This observation is in accordance with the concept of internal friction encountered by chain motions due to steric effects, which is high for local motions and decreases with increasing loop size. Comparison with the results from the viscosity dependence of loop formation shows a negative correlation between Ea and the sensitivity of the reaction to solvent viscosity (α) in accordance with the Grote-Hynes theory of memory friction. The Arrhenius pre-exponential factor (A) also decreases with increasing loop size, indicating increased entropic costs for loop formation. Long-range loop formation in the investigated sequences derived from IDPs shows increased Ea and A compared with poly(Gly-Ser) and polySer chains. This increase is exclusively due to steric effects that cause additional internal friction, whereas intramolecular hydrogen bonds, dispersion forces, and charge interactions do not affect the activation parameters.

Local and Large-Scale Conformational Dynamics in Unfolded Proteins and IDPs. I. Effect of Solvent Viscosity and Macromolecular Crowding.

Protein/solvent interactions largely influence protein dynamics, particularly motions in unfolded and intrinsically disordered proteins (IDPs). Here, we apply triplet-triplet energy transfer (TTET) to investigate the coupling of internal protein motions to solvent motions by determining the effect of solvent viscosity (η) and macromolecular crowding on the rate constants of loop formation (kc) in several unfolded polypeptide chains including IDPs. The results show that the viscosity dependence of loop formation depends on amino acid sequence, loop length, and co-solute size. Below a critical size (rc), co-solutes exert a maximum effect, indicating that under these conditions microviscosity experienced by chain motions matches macroviscosity of the solvent. rc depends on chain stiffness and reflects the length scale of the chain motions, i.e., it is related to the persistence length. Above rc, the effect of solvent viscosity decreases with increasing co-solute size. For co-solutes typically used to mimic cellular environments, a scaling of kc ∝ η-0.1 is observed, suggesting that dynamics in unfolded proteins are only marginally modulated in cells. The effect of solvent viscosity on kc in the small co-solute limit (below rc) increases with increasing chain length and chain flexibility. Formation of long and very flexible loops exhibits a kc ∝ η-1 viscosity dependence, indicating full solvent coupling. Shorter and less flexible loops show weaker solvent coupling with values as low as kc ∝ η-0.75±0.02. Coupling of formation of short loops to solvent motions is very little affected by amino acid sequence, but solvent coupling of long-range loop formation is decreased by side chain sterics.

Efficient, Near‐Infrared Light‐Induced Photoclick Reaction Enabled by Upconversion Nanoparticles

AbstractPhotoclick reactions combine the selectivity of classical click chemistry with the high precision and spatiotemporal control afforded by light, finding diverse utility in surface customization, polymer conjugation, photocross‐linking, protein labeling, and bioimaging. Nonetheless, UV light, pivotal in prevailing photoclick reactions, poses issues, especially in biological contexts, due to its limited tissue penetration and cell‐toxic nature. Herein, a reliable and versatile strategy of activating the photoclick reactions of 9,10‐phenanthrenequinones (PQs) with electron‐rich alkenes (ERAs) with near infrared (NIR) light transduced by spectrally and structurally customized upconversion nanoparticles (UCNPs) is introduced. Under NIR irradiation, the UCNPs become UV/blue nanoemitters uniformly distributed in the reaction system. Enabled by the customized UCNPs, 800 or 980 nm light effectively activates the photocycloaddition reactions via radiative energy transfer in both general and triplet–triplet energy transfer (TTET)‐mediated PQ‐ERA systems. In particular, the novel sandwich structure UCNPs achieve the click reaction with up to 76% production yield in 10 min under NIR light irradiation. Meanwhile, the tricky side effect of photoclick product absorption‐induced quenching is successfully circumvented from the fine‐tuning of the upconversion spectrum. Moreover, through‐tissue irradiation experiments, the authors show that the UCNP‐PQ‐ERA reaction unlocks the full potential of photoclick reactions for in vivo applications.

Amplified Triplet Emission of Organic Periphery Groups by Exothermic Triplet-Triplet Energy Transfer from the 3MLCT State of an Ir(pmi)3 Core Complex to the 3LC State of Geometrically Confined Carbazole/Naphthyl Tethers.

To investigate the excited-state properties of metal-organic bichromophores, including energy transfer mechanisms, a series of new homoleptic N-heterocyclic carbene (NHC)-based iridium(III) complexes were prepared by incorporating a peripheral naphthalene (Np) (Ir(Nppmi)3: fac-/mer-Ir(1-Nppmi)3 and fac-/mer-Ir(2-Nppmi)3) or carbazole (Cz) (Ir(Czpmi)3: fac-/mer-Ir(o-Czpmi)3, fac-/mer-Ir(m-Czpmi)3, and fac-/mer-Ir(p-Czpmi)3) unit to the phenyl moiety of the phenylimidazole (pmi) ligand. Through a series of photophysical analyses and femtosecond time-resolved absorption (fs-TA) spectroscopy, it was discovered that the phosphorescence of the Ir core, (Ir(pmi)3), was considerably quenched, while intense phosphorescence peaks arising from the excited triplet Np (3Np*)/Cz (3Cz*) species were primarily observed at room temperature (r.t.) and low temperature. Such amplified phosphorescence of the tethered organic Np and Cz units originated from triplet-triplet energy transfer (TTET) from the high-lying metal-to-ligand charge transfer (3MLCT) state of the Ir(pmi)3 core to the ligand-centered triplet state (3LC) of the peripheral Np and Cz units. This result indicates that the exothermic intramolecular energy transfer (IET) in the excited triplet state realizes the efficient phosphorescent emission of geometrically confined organic tethers.

Triplet-Triplet Energy Transfer in Bis-Cyclometalated Iridium Complexes with Pyrene-Substituted Isocyanides.

Nonlinear optical (NLO) materials are able to modulate responses of electromagnetic radiation, leading to phenomena critical to modern telecommunications technologies. The last two decades have seen significant advances in the area of molecular nonlinear chromophores, particularly with respect to reverse-saturable absorption (RSA). Here, we introduce a strategy for intense excited-state absorption (ESA) that involves bis-cyclometalated iridium complexes with isocyanide ancillary ligands decorated with pyrene triplet acceptors. Upon excitation, the complexes undergo rapid triplet-triplet energy transfer (TTET) to the acceptor excited states. This report describes five bis-cyclometalated iridium complexes using two different pyrene-substituted isocyanides with the general formula [Ir(C^N)2(CNAr)2]PF6 (C^N = cyclometalating ligand, CNAr = isocyanide ancillary ligand: CNArpyr = 2,6-dimethyl-4-(1-pyrenyl)phenyl isocyanide, CNpyr = 1-pyrenyl isocyanide). The synthesized complexes were thoroughly characterized via 1H and 13C{1H} NMR spectroscopy, Fourier-transform Infrared spectroscopy, and electrospray ionization mass spectrometry. The excited states were evaluated with UV-vis absorption, steady-state and time-resolved photoluminescence, and transient absorption spectroscopy. Phosphorescence is completely quenched at room temperature, but in the solvent glass matrix at 77 K, there is luminescence originating from a π → π* triplet state on the pyrene moiety, abbreviated herein as 3pyrene. All five complexes display intense and long-lived ESA originating from the 3pyrene state. The localization of the ground-state absorption on the cyclometalating ligands and the excited-state absorption on the pyrene moiety allows for independent tuning of ground-state absorption (GSA) and ESA to optimize RSA and other NLO attributes.

Imidazophenothiazine-Based Thermally Activated Delayed Fluorescence Materials with Ultra-Long-Lived Excited States for Energy Transfer Photocatalysis.

Triplet-triplet energy transfer (EnT) is a powerful activation pathway in photocatalysis that unlocks new organic transformations and improves the sustainability of organic synthesis. Many current examples, however, still rely on platinum-group metal complexes as photosensitizers, with associated high costs and environmental impacts. Photosensitizers that exhibit thermally activated delayed fluorescence (TADF) are attractive fully organic alternatives in EnT photocatalysis. However, TADF photocatalysts incorporating heavy atoms remain rare, despite their utility in inducing efficient spin-orbit-coupling, intersystem-crossing, and consequently a high triplet population. Here, we describe the synthesis of imidazo-phenothiazine (IPTZ), a sulfur-containing heterocycle with a locked planar structure and a shallow LUMO level. This acceptor is used to prepare seven TADF-active photocatalysts with triplet energies up to 63.9 kcal mol-1. We show that sulfur incorporation improves spin-orbit coupling and increases triplet lifetimes up to 3.64 ms, while also allowing for tuning of photophysical properties via oxidation at the sulfur atom. These IPTZ materials are applied as photocatalysts in five seminal EnT reactions: [2 + 2] cycloaddition, the disulfide-ene reaction, and Ni-mediated C-O and C-N cross-coupling to afford etherification, esterification, and amination products, outcompeting the industry-standard TADF photocatalyst 2CzPN in four of the five studied scenarios. Detailed photophysical and theoretical studies are used to understand structure-activity relationships and to demonstrate the key role of the heavy atom effect in the design of TADF materials with superior photocatalytic performance.

TADF-Based X-ray Screens with Simultaneously Efficient Singlet and Triplet Energy Transfer for High Spatial Imaging Resolution.

X-ray imaging scintillators play a crucial role in medical examinations and safety inspections, making them an essential technology in our modern lives. However, commercially available high-performance scintillators are fabricated exclusively from ceramic materials that require harsh preparation conditions and are costly to produce. Organic scintillators have emerged as a promising alternative due to their transparency and ease of fabrication at a low cost. Unfortunately, organic scintillators suffer from inefficient exciton utilization efficiency, leading to poor performance in X-ray imaging screens and hindering their commercialization. In this study, we explore the use of thermally activated delayed fluorescence (TADF) chromophores (4CzIPN-I and 4CzTPN) to enhance the absorption of ionizing radiation in X-ray imaging screens by an order of magnitude. By leveraging the unique features of TADF chromophores through simultaneously singlet-singlet and triplet-triplet efficient energy transfers at the interface between two different TADF systems, we demonstrate an impressive X-ray sensitivity and radioluminescence intensity. Our time-resolved experiments and density functional theory (DFT) calculations provide further evidence for the effectiveness of this approach. The resulting X-ray imaging screens based on this efficient interfacial energy transfer process in TADF systems exhibit outstanding X-ray imaging resolution of 20 line pairs/mm, the highest resolution reported thus far for organic scintillators. This resolution is at least two times higher than that achieved by commonly used commercial inorganic scintillators in the X-ray imaging market. These findings introduce a new component for greatly improving the performance of organic X-ray imaging scintillators, supporting a wide range of emerging X-ray applications with exceptional spatial resolution.

Inorganic Chemistry in Dalian University of Technology: Fundamental Science and Solution for Renewable Energy, Environment, Health, and Materials

Inorganic Chemistry in Dalian University of Technology: Fundamental Science and Solution for Renewable Energy, Environment, Health, and Materials

2D Nano-Sonosensitizers Facilitate Energy Transfer to Enhance Sonodynamic Therapy.

Although sonodynamic therapy (SDT) has shown promise for cancer treatment, the lack of efficient sonosensitizers (SSs) has limited the clinical application of SDT. Here, a new strategy is reported for designing efficient nano-sonosensitizers based on 2D nanoscale metal-organic layers (MOLs). Composed of Hf-oxo secondary building units (SBUs) and iridium-based linkers, the MOL is anchored with 5,10,15,20-tetra(p-benzoato)porphyrin (TBP) sensitizers on the SBUs to afford TBP@MOL. TBP@MOL shows 14.1- and 7.4-fold higher singlet oxygen (1 O2 ) generation than free TBP ligands and Hf-TBP, a 3D nanoscale metal-organic framework, respectively. The 1 O2 generation of TBP@MOL is enhanced by isolating TBP SSs on the SBUs of the MOL, which preventsaggregation-induced quenching of the excited sensitizers, and by triplet-triplet Dexter energy transfer between excited iridium-based linkers and TBP SSs, which more efficiently harnesses broad-spectrum sonoluminescence. Anchoring TBP on the MOL surface also enhances the energy transfer between the excited sensitizer and ground-state triplet oxygen to increase 1 O2 generation efficacy. In mouse models of colorectal and breast cancer, TBP@MOL demonstrates significantly higher SDT efficacy than Hf-TBP and TBP. This work uncovers a new strategy to design effective nano-sonosensitizers by facilitating energy transfer to efficiently capture broad-spectrum sonoluminescence and enhance 1 O2 generation.

Visible-Light-Initiated Sequential Trifluoromethylation/Remote C(sp3)-H Alkynylation of Vinyl Azides by Radical Relay.

A visible-light-initiated trifluoromethylation/remote aliphatic C-H alkynylation of α-alkyl-substituted vinyl azides using acetylenic triflones as both the trifluoromethyl and alkyne donors is described. The reaction occurred under environmentally benign and radical initiator-free reaction conditions, affording γ-alkynylated trifluoromethyl ketone derivatives with a broad scope of substituents. Mechanistic studies suggested that the reaction is initiated via a triplet-triplet energy transfer between the 4CzIPN catalyst and acetylenic triflone, followed by fragmentation to generate a trifluoromethyl radical and an alkynyl radical.

Tuning the triplet population of arylselanyl-BODIPY photosensitizers through substituents engineering for triplet-triplet annihilation photon upconversion with perylene

Organic triplet-triplet annihilation photon upconversion (TTA-UC) systems have attracted considerable attention owing to their promising applicability in solar energy harvesting, optoelectronic devices, photocatalysts, and bio-imaging. In this study, a series of BODIPYs prepared by incorporating substituted/ unsubstituted arylselenium groups, 1 (R = H), 2 (R = OMe), and 3 (R = F), were evaluated as triplet photosensitizers for TTA-UC. Direct Se-C bond formation on the BODIPY core provided a facile intersystem crossing (ISC) channel from the excited singlet state to the metastable triplet state, being the most effective in 2, as inferred from the singlet oxygen generation experiments, possibly because of the n-electron arising from the OMe group. The UC behavior of systems comprising the sensitizers and perylene as the acceptor in deaerated toluene was investigated using a 524 nm-wavelength laser to detect upconverted emission at 449 nm; thus, the UC yield decreased in the order of 21% for 3 &gt; 16% for 2 &gt; 12% for 1. This trend is consistent with the Stern-Volmer constants calculated from the quenched triplet state lifetimes of the sensitizers as a function of the concentration of the perylene quencher. This suggests that the UC efficiency was mainly governed by the intermolecular triplet-triplet energy transfer (TTET) process between arylselanyl-BODIPY photosensitizers and the perylene acceptor. This result was rationalized by the efficient population of the long-lived triplet excited state of the sensitizer, which is advantageous for diffusion-controlled TTA-UC behavior.

Practical Aspects in the Study of Biological Photosensitization Including Reaction Mechanisms and Product Analyses: A Do's and Don'ts Guide†.

The interaction of light with natural matter leads to a plethora of photosensitized reactions. These reactions cause the degradation of biomolecules, such as DNA, lipids, proteins, being therefore detrimental to the living organisms, or they can also be beneficial by allowing the treatment of several diseases by photomedicine. Based on the molecular mechanistic understanding of the photosensitization reactions, we propose to classify them in four processes: oxygen-dependent (type I and type II processes) and oxygen-independent [triplet-triplet energy transfer (TTET) and photoadduct formation]. In here, these processes are discussed by considering a wide variety of approaches including time-resolved and steady-state techniques, together with solvent, quencher, and scavenger effects. The main aim of this survey is to provide a description of general techniques and approaches that can be used to investigate photosensitization reactions of biomolecules together with basic recommendations on good practices. Illustration of the suitability of these approaches is provided by the measurement of key biomarkers of singlet oxygen and one-electron oxidation reactions in both isolated and cellular DNA. Our work is an educational review that is mostly addressed to students and beginners.

Multi-wavelength excited triplet-triplet upconversion microcrystals based on hot-band excitation for optical information encryption.

Multi-wavelength hot-band excitation, forbidden in the conventional Stokes fluorescence mechanism, is found to be available with cascading triplet-triplet annihilation upconversion (TTA-UC). Selective excitation of Pt(II)octaethylporphyrin (PtOEP) by diode lasers with wavelengths of 532 nm, 589 nm, 635 nm, 655 nm, and 671 nm respectively can all induce 9,10-diphenylanthracene (DPA) to emit blue upconversion, with the maximum anti-Stokes shift of 0.95 eV in the microcrystals exposed to air. Whether the zero-vibrational energy level excitation or the hot-vibrational energy level excitation in the ground state, the PtOEP/DPA pair showed triplet-triplet energy transfer (TTET) efficiencies approaching ∼95%. The doped microcrystal samples without encapsulation can emit blue upconversion from green/yellow/red excitation with stability for ∼20 days under atmospheric conditions, demonstrating their potential applications in multiple information encryption.

Isoacridone dyes with parallel reactivity from both singlet and triplet excited states for biphotonic catalysis and upconversion.

Metal-based photosensitizers commonly undergo quantitative intersystem crossing into photoactive triplet excited states. In contrast, organic photosensitizers often feature weak spin-orbit coupling and low intersystem crossing efficiencies, leading to photoactive singlet excited states. By modifying the well-known acridinium dyes, we obtained a new family of organic photocatalysts, the isoacridones, in which both singlet- and triplet-excited states are simultaneously photoactive. These new isoacridone dyes are synthetically readily accessible and show intersystem crossing efficiencies of up to 52%, forming microsecond-lived triplet excited states (T1), storing approximately 1.9 eV of energy. Their photoactive singlet excited states (S1) populated in parallel have only nanosecond lifetimes, but store ∼0.4 eV more energy and act as strong oxidants. Consequently, the new isoacridone dyes are well suited for applications requiring parallel triplet-triplet energy transfer and photoinduced electron transfer elementary steps, which have become increasingly important in modern photocatalysis. In proof-of-principle experiments, the isoacridone dyes were employed for Birch-type arene reductions and C-C couplings via sensitization-initiated electron transfer, substituting the commonly used iridium or ruthenium based photocatalysts. Further, in combination with a pyrene-based annihilator, sensitized triplet-triplet annihilation upconversion was achieved in an all-organic system, where the upconversion quantum yield correlated with the intersystem crossing quantum yield of the photosensitizer. This work seems relevant in the greater contexts of developing new applications that utilize biphotonic photophysical and photochemical behavior within metal-free systems.

The effect of thionation of the carbonyl group on the photophysics of compact spiro rhodamine-naphthalimide electron donor-acceptor dyads: intersystem crossing, charge separation, and electron spin dynamics.

Herein, a spiro rhodamine (Rho)-thionated naphthalimide (NIS) electron donor-acceptor orthogonal dyad (Rho-NIS) was prepared to study the formation of a long-lived charge separation (CS) state via the electron spin control approach. The transient absorption (TA) spectra of Rho-NIS indicated that the intersystem crossing (ISC) occurs within 7-42 ps to produce the 3NIS state via the spin orbit coupling ISC (SOC-ISC). The energy order of 3CS (2.01 eV in n-hexane, HEX) and 3LE states (1.68 eV in HEX) depended on the solvent polarity. The 3NIS state having n-π* character and a lifetime of 0.38 μs was observed for Rho-NIS in toluene (TOL). Alternatively, in acetonitrile (ACN), the long-lived 3CS state (0.21 μs) with a high CS state quantum yield (ΦCS, 97%) was produced with the 3NIS state as the precursor and the CS took 134 ps. On the contrary, in the case of the reference Rho-naphthalimide (NI) Rho-NI dyad without thionation of its carbonyl group, a long-lived CS state (0.94 μs) with a high energy level (ECS = 2.12 eV) was generated even in HEX with a lower ΦCS (49%). In the presence of an acid, the Rho unit in the Rho-NIS adopted an open form (Rho-o) and the 3NIS state was produced within 24-47 ps with the 1Rho-o state as the precursor. Subsequently, slow intramolecular triplet-triplet energy transfer (TTET, 0.11-0.60 μs) produced the 3Rho-o state (9.4-13.6 μs). According to the time-resolved electron paramagnetic resonance (TREPR) spectra of NIS-NH2, the zero-field splitting (ZFS) parameter |D| and E of the triplet state were determined to be 6165 MHz and -1233 MHz, respectively, indicating that its triplet state has significant nπ* character, which was supported by its short triplet state lifetime (6.1 μs).

Unraveling the electronic origin of a special feature in the triplet-minus-singlet spectra of carotenoids in natural photosystems.

The influence of carotenoid triplet states on the Qy electronic transitions of chlorophylls has been observed in experiments on light-harvesting complexes over the past three decades, but the interpretation of the resulting spectral feature in the triplet minus singlet (T-S) absorption spectra of photosystems is still debated, as the physical-chemical explanation of this feature has been elusive. Here, we resolve this debate, by explaining the T-S spectra of pigment complexes over the Qy-band spectral region through a comparative study of chlorophyll-carotenoid model dyads and larger pigment complexes from the main light harvesting complex of higher plants (LHCII). This goal is achieved by combining state-of-the-art time-dependent density functional theory with analysis of the relationship between electronic properties and nuclear structure, and by comparison to the experiment. We find that the special signature in the T-S spectra of both model and natural photosystems is determined by singlet-like triplet excitations that can be described as effective singlet excitations on chlorophylls influenced by a stable electronic triplet on the carotenoid. The comparison with earlier experiments on different light-harvesting complexes confirms our theoretical interpretation of the T-S spectra in the Qy spectral region. Our results indicate an important role for the chlorophyll-carotenoid electronic coupling, which is also responsible for the fast triplet-triplet energy transfer, suggesting a fast trapping of the triplet into the relaxed carotenoid structure. The gained understanding of the interplay between the electronic and nuclear structures is potentially informative for future studies of the mechanism of photoprotection by carotenoids.