Triplet Formation Research Articles

The ENA Source Attribute Helper: An API for improved biological source data

Metadata management for sequence data is essential for the accurate description of Earth’s biodiversity. Within metadata attributes, those that reference the biological sources of sequences and samples and allow linking to the specimen or sample of origin are fundamental for facilitating connections between molecular biology, taxonomy, systematic biology and biodiversity research, increasing the discoverability and usability of data by researchers worldwide. Sequence data is publicly archived at the International Nucleotide Sequence Database Collaboration (INSDC) that includes the National Centre for Biotechnology Information (NCBI), the DNA Data Bank of Japan (DDBJ) and the European Nucleotide Archive (ENA). Sequences stored at INSDC have associated a considerable range of metadata, including attributes related to its biological source, such as references to natural history collections or culture collections. But, these source attributes are not always submitted or may be incomplete, limiting the association of the sequence records to the original source material, hampering further data connections (e.g., biological data associated with the voucher or species distribution data). Therefore, we have developed the ENA Source Attribute Helper API, a tool that aims to assist users on the submission of accurate attributes referring to the biological source of samples and sequence data. This tool was developed within the scope of BiCIKL (Biodiversity Community Integrated Knowledge Library) (Penev et al. 2022), a Horizon 2020 project which targets building a wide, biodiversity related community for connecting data along the different axes of biodiversity research. The first version of the tool was designed to support correct annotation of the attributes that identify the source material from which the sample or sequence were obtained, namely /specimen_voucher, /culture_collection, and /biomaterial (INSDC 2021). These attributes follow a Darwin Core Triplet format (Wieczorek et al. 2012), composed of institution code, collection code and the specimen, culture, or material identifier, accordingly. Since the submission of the biological source attributes to the INSDC may be performed both when data is initially uploaded or on following updates using a variety of tools, we developed the API as an open source tool that is publicly accessible and may be used as a free-standing service. The API is built using Representational State Transfer (REST) API Architecture and it is designed to use the data available in the NCBI BioCollections (Sharma et al. 2018). NCBI Biocollections is a curated database of metadata for natural history collections, associated with records in INSDC, that includes the institution and collection codes. The API main functions include the querying of the metadata (the API presents both exact matches and similar matches) for the institutions and collections based on the user input, validation of institution and collection codes in the attribute strings provided by the user, and the construction of the attribute string based on the user-provided information. The API does not include the search or validation of the voucher specimen codes. The API is designed in a way that it can be extended easily for any future enhancements and initially expected to promote and support the submission and any subsequent curation of better structured and more richly described source data. We expect this tool to contribute to better connected biodiversity data and hence provide a stronger foundation to strengthen the value of natural history collections, taxonomic expertise, and biodiversity knowledge.

(Invited) Photon Upconversion through Dynamic Exciton at Organic Semiconductor Interface

Energy of photons, i.e. the wavelength of light, can be upgraded through interactions with materials—a process called photon upconversion (UC). Although UC in organic solids is important for various applications, such as in photovoltaics and bioimaging, conventional UC systems, based on intersystem crossing (ISC), suffer from low efficiency. Herein, we report a novel UC mechanism at heterojunctions of organic semiconductors in bilayer structures. The UC occurs by triplet formation during the charge separation and recombination through dynamic exciton: charge transfer (CT) state at the interface. This can efficiently convert the incident photons to triplets without relying on the ISC, whose rate is typically accelerated by the heavy-atom effect. As a result, a solid-state UC system is achieved with an external efficiency of two orders of magnitude higher than those of the conventional systems. Using this result, efficient UC, from near-infrared to visible light, cab be realized on flexible organic thin films under a weak light-emitting diode-induced excitation, observable by naked eyes.Reference: S. Izawa and M. Hiramoto, Nat. Photon., DOI: org/10.1038/s41566-021-00904-w.

Delayed Fluorescence by Triplet-Triplet Annihilation from Columnar Liquid Crystal Films.

Delayed fluorescence (DF) by triplet–triplet annihilation (TTA) is observed in solutions of a benzoperylene-imidoester mesogen that shows a hexagonal columnar mesophase at room temperature in the neat state. A similar benzoperylene-imide with a slightly smaller HOMO–LUMO gap, that also is hexagonal columnar liquid crystalline at room temperature, does not show DF in solution, and mixtures of the two mesogens show no DF in solution either, because of collisional quenching of the excited triplet states on the imidoester by the imide. In contrast, DF by TTA from the imide but not from the imidoester is observed in condensed films of such mixtures, even though neat films of either single material are not displaying DF. In contrast to the DF from the monomeric imidoester in solution, DF of the imide occurs from dimeric aggregates in the blend films, assisted by the imidoester. Thus, the close contact of intimately stacked molecules of the two different species in the columnar mesophase leads to a unique mesophase-assisted aggregate DF. This constitutes the first observation of DF by TTA from the columnar liquid crystalline state. If the imide is dispersed in films of polybromostyrene, which provides an external heavy-atom effect facilitating triplet formation, DF is also observed. Organic light-emitting diodes (OLEDs) devices incorporating these liquid crystal molecules demonstrated high external quantum efficiency (EQE). On the basis of the literature and to the best of our knowledge, the EQE reported is the highest among nondoped solution-processed OLED devices using a columnar liquid crystal molecule as the emitting layer.

Revisiting the Dynamics of Triplet Formation in Anthraquinones.

Triplet formation pathways in 9,10-anthraquinone (AQ) and its hydroxy derivative, 1-hydroxyanthraquinone (HAQ), are studied theoretically. Dynamics simulations on the model singlet-triplet potential energy surfaces within the linear vibronic coupling framework are performed to elucidate possible internal conversion (IC) and intersystem crossing (ISC) pathways in these molecules. An ultrafast IC decay from the "bright" S4 to S1 followed by efficient ISC via S1-T4 and S1-T5 pathways fosters a high triplet quantum yield (ΦT = 0.90) in AQ. In HAQ, a new nonradiative channel of "barrierless" excited-state intramolecular proton transfer (ESIPT) opens up and competes with the IC decay to S1 upon photoexcitation to the "bright" S2. Extremely fast ESIPT on S2 reduces the efficiency of triplet formation via possible ISC pathways involving S1 and S2, resulting in a low ΦT (=0.17).

Modulation of Intersystem Crossing by Chemical Composition and Solvent Effects: Benzophenone, Anthrone and Fluorenone

AbstractCombined density functional theory and multireference configuration interaction methods including spin‐orbit coupling and spin‐vibronic interactions have been used to elucidate the photophysical pathways of benzophenone (BP), anthrone (A) and fluorenone (FL). Our results reveal that the slower S1(1nπ*) T1(3ππ*) passage of FL in the gas phase, as compared to BP and A, originates from the different electronic structures of the T1(3ππ*) states in these compounds and is not related to the planarity of the nuclear arrangement. Temperature effects on the rate constants of intersystem‐crossing (ISC), reverse ISC (rISC) and fluorescence have been investigated for FL in solvents of different polarity. Experimentally observed trends for rate constants are well reproduced. A nearly temperature‐ and solvent‐independent, slower down‐hill ISC component is attributed to the El‐Sayed‐forbidden S1(1ππ*) T1(3ππ*) transition in these environments. The faster El‐Sayed‐allowed S1(1ππ*) T2(3nπ*) ISC requires thermal activation in tetrahydrofuran and acetonitrile solution so that fluorescence may compete against triplet formation. In cyclohexane solution, S1(1ππ*) and T2(3nπ*) are nearly degenerate according to our calculations. Hence S1 T2 ISC and S1 T2 rISC have similar rate constants. If T2 T1 internal conversion could be slowed down by deuteration or perfluorination, for example, FL‐derivatives might show delayed fluorescence.

Molecular mechanisms of light harvesting in the minor antenna CP29 in near-native membrane lipidic environment.

CP29, a chlorophyll a/b-xanthophyll binding protein, bridges energy transfer between the major LHCII antenna complexes and photosystem II reaction centers. It hosts one of the two identified quenching sites, making it crucial for regulated photoprotection mechanisms. Until now, the photophysics of CP29 has been studied on the purified protein in detergent solutions since spectrally overlapping signals affect in vivo measurements. However, the protein in detergent assumes non-native conformations compared to its physiological state in the thylakoid membrane. Here, we report a detailed photophysical study on CP29 inserted in discoidal lipid bilayers, known as nanodiscs, which mimic the native membrane environment. Using picosecond time-resolved fluorescence and femtosecond transient absorption (TA), we observed shortening of the Chl fluorescence lifetime with a decrease of the carotenoid triplet formation yield for CP29 in nanodiscs as compared to the protein in detergent. Global analysis of TA data suggests a 1Chl* quenching mechanism dependent on excitation energy transfer to a carotenoid dark state, likely the proposed S*, which is believed to be formed due to a carotenoid conformational change affecting the S1 state. We suggest that the accessibility of the S* state in different local environments plays a key role in determining the quenching of Chl excited states. In vivo, non-photochemical quenching is activated by de-epoxidation of violaxanthin into zeaxanthin. CP29-zeaxanthin in nanodiscs further shortens the Chl lifetime, which underlines the critical role of zeaxanthin in modulating photoprotection activity.

Geminate and Nongeminate Pathways for Triplet Exciton Formation in Organic Solar Cells

AbstractOrganic solar cells (OSCs) have recently shown a rapid improvement in their performance, bringing power conversion efficiencies to above 18%. However, the open‐circuit voltage of OSCs remains low relative to their optical gap and this currently limits efficiency. Recombination to spin‐triplet excitons is a key contributing factor, and is widely, but not universally, observed in donor–acceptor blends using both fullerene and nonfullerenes as electron acceptors. Here, an experimental framework that combines time‐resolved optical and magnetic resonance spectroscopies to detect triplet excitons and identify their formation mechanisms, is reported. The methodology is applied to two well‐studied polymer:fullerene systems, PM6:PC60BM and PTB7‐Th:PC60BM. In contrast to the more efficient nonfullerene acceptor systems that show only triplet states formed via nongeminate recombination, the fullerene systems also show significant triplet formation via geminate processes. This requires that geminate electron–hole pairs be trapped long enough to allow intersystem crossing. It is proposed that this is a general feature of fullerene acceptor systems, where isolated fullerenes are known to intercalate within the alkyl sidechains of the donor polymers. Thus, the study demonstrates that engineering good donor and acceptor domain purity is key for suppressing losses via triplet excitons in OSCs.

Vibration-Assisted Intersystem Crossing in the Ultrafast Excited-State Relaxation Dynamics of Halocoumarins.

Due to its numerous applications, triplet formation and resulting phosphorescence remain a frontier area of research for over eight decades. Facile intersystem crossing (ISC) is the primary requirement for triplet formation and observation of phosphorescence. The incorporation of a heavy atom in molecules is one of the common approaches employed to facilitate ISC. A detailed study of the excited state dynamics that governs ISC is necessary to understand the mechanism of heavy atom effect (HAE). Incorporation of iodine at the 3 position of coumarin-1 reduces fluorescence quantum yield (ϕf) drastically as expected, whereas bromine substitution at the same position increased the ϕf. Such a contrasting effect of the two heavy atoms suggests that there are other features yet to be discovered to fully understand the HAE. Detailed steady state and femtosecond transient absorption studies along with theoretical calculations suggest that the C3-X (X = Br, I) bond vibration plays an important role in the ISC process. The study reveals that while in the case of the iodo-derivative there is no energy barrier in the singlet triplet crossing path, there is a barrier in the case of the bromo-derivative, which slows the ISC process. Such an unexpected phenomenon is not limited to halocoumarins as this rationalizes the photobehavior of 1-bromo-/iodo-substituted naphthalenes as well.

Rationalizing the Fluorescence Behavior of Core-Substituted Naphthalene Diimides

We study the internal conversion (IC) and intersystem crossing (ISC) pathways of low-lying excited electronic states of three core-substituted naphthalene diimides (bNDI, yNDI, and gNDI) using wavepacket simulations within the linear vibronic coupling method. Our wavepacket simulations reproduce the experimental electronic absorption spectra very well. All molecules decay rapidly to S2 upon populating a higher dipole-allowed singlet excited-state. The S2 → S1 IC dynamics and singlet-triplet energy gap, spin-orbit coupling strength trends suggest a favorable S2 → T4 ISC in gNDI. The efficient ultrafast T4 formation and its decay to lower triplet states make gNDI nonfluorescent. Such triplet formation pathways are not operative in both bNDI and yNDI; hence, these molecules emit fluorescence from S1 after a slower S2 → S1 IC.

Photoinduced Charge Transfer and Recombination Dynamics in Star Nonfullerene Organic Solar Cells.

Nonfullerene acceptors (NFAs) are regarded as star candidates for efficient organic solar cells with power conversion efficiency (PCE) over 18%. In contrast to the rapid development of NFA materials, however, the underlying excited-state dynamics which fundamentally govern the device performance remains unclear. In this Perspective, we discuss recent advances and provide our insights on photoinduced charge transfer and combination dynamics in NFA-based organic solar cells (OSCs), including the biphasic hole-transfer process and its correlation with morphology, the role of driving force and Marcus normal region behavior on interfacial hole-transfer properties, and charge recombination energy loss by NFA triplet formation. We also discuss our understanding of how to control the charge-transfer and recombination processes by phase morphology and molecular design to improve OSC performance. Finally, we suggest a few research directions, including the interfacial charge transfer and separation mechanism, the origin of low fill factor, and complex excited-state dynamics in multicomponent OSCs.

Spectroscopic and theoretical insights into hydrogen bonding clusters, triplet species, and the relaxation mechanism of the light (1ππ*) excited state of 4-methyl-1,2,4-triazole-3-thione

Microsolvated hydrogen bonding 4-methyl-1,2,4-triazole-3-thione clusters were characterized experimentally and theoretically in solvents. The mechanism of the excited-state decay and the long-term triplet formation was proposed by CASSCF calculation.

Quantifying triplet formation in conjugated polymer/non-fullerene acceptor blends

Quantification of triplet state populations in non-fullerene acceptor blends has allowed us to correlate triplet and charge carrier concentrations, with important ramifications for organic photovoltaics.

Internal Electric Fields Control Triplet Formation in Halide Perovskite-Sensitized Photon Upconverters

Internal Electric Fields Control Triplet Formation in Halide Perovskite-Sensitized Photon Upconverters

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.

Rubrene Nanoaggregate-Integrated CH3NH3PbI3 Bilayer Film: Role of Singlet Fission and Photon Upconversion

The potential to improve the efficiency of photovoltaic devices through singlet exciton fission (SF) has raised interest of researchers during the last decade. Organic small molecules of rubrene were fabricated into spherical nanoaggregates (NAs) of diameter ∼250 nm by the re-precipitation method. J-type aggregation inside the NA with pivotal electronic coupling occurs in NAs. Observed strong fluorescence quenching indicates the presence of competing pathways in the NAs: hot and thermally activated SF, triplet–triplet annihilation (TTA), and energy transfer between inhomogeneously broadened NA ensembles. SF which proceeds directly from upper excited electronic states within 340 fs (vs 200 fs in crystals; 30 fs in films) results in a triplet quantum yield of 162%. With an aim to apply rubrene in photovoltaics, a rubrene NA-integrated film was prepared on top of the CH3NH3PbI3 layer. Triplet formation within 25 ps in the NA layer was well-documented. Subsequently, the triplet excitons are accumulated and further upconverted via TTA to populate the conduction band of the perovskite within 39 ps (80%) and ≫10 ns (20%). Our findings are crucial for understanding photophysical mechanisms of rubrene/perovskite interaction after light absorption by rubrene.

Rational Design and Synthesis of OEP and TPP Centered Phosphorus(V) Porphyrin-Naphthalene Conjugates: Triplet Formation via Rapid Charge Recombination.

Six new "axial-bonding" type "phosphorus(V) porphyrin-naphthalene" conjugates have been prepared consisting of octaethylporphyrinatophosphorus(V) (POEP+)/tetraphenylporphyrinatophosphorus(V) (PTPP+) and naphthalene (NP). The distance between the porphyrin and NP was systematically varied using polyether bridges. The unique structural topology of the octaethylporphyrinatophosphorus(V) (POEP+) and tetraphenylporphyrinatophosphorus(V) (PTPP+) enabled construction of mono- and disubstituted phosphorus(V) porphyrin-naphthalene conjugates, respectively. The steady-state and transient spectral properties were investigated as a function of redox properties, distance, and molecular topology. Strong electronic interactions between the phosphorus(V) porphyrin and NP in directly bound conjugates were observed. The established energy diagrams predicted reductive electron transfer involving singlet excited phosphorus(V) porphyrin and NP to generate high-energy (∼1.83-2.11 eV) charge-separated states (POEP/PTPP)•-(NP)•+. Femtosecond transient absorption spectral studies revealed rapid deactivation of singlet excited phosphorus(V) porphyrin due to charge separation wherein the estimated forward rate constants were in the range of 109-1010 s-1 and were dependent on the distance between the NP and porphyrins units, as well as the redox potentials of the type of the phosphorus(V) porphyrin. Additionally, due to high exothermicity and low-lying triplet states, the charge recombination process was found to be rapid, leading to populating the triplet states of phosphorus(V) porphyrins.

Efficient solid-state photon upconversion enabled by triplet formation at an organic semiconductor interface

Efficient solid-state photon upconversion enabled by triplet formation at an organic semiconductor interface

Time-transformation technique for generation of optical pulse multiplets in a single mode anomalous dispersion optical fiber.

We present the generation of optical pulse multiplets in the temporal domain from a single pulse using the time-transformation technique for the first time, to the best of our knowledge. The generation of a pair of compressed, well-spaced, and identical Gaussian pulses from a third-order super-Gaussian input pulse when propagated through an anomalous dispersion fiber is reported. Detailed analysis on the effect of variation of input energy, pulse parameters, and optical gain in view of doublet formation leading to the possibility of high effective repetition rate (ERR) at smaller propagating length is reported. The formation of triplets and quadruplets from different input pulse shapes is investigated.

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.

Singlet and Triplet Excited-State Dynamics of a Nonfullerene Electron Acceptor Y6

Understanding the excited-state dynamics of nonfullerene electron acceptors is essential for further improvement of organic solar cells. Herein, we investigated the singlet and triplet excited-state dynamics in Y6, a novel nonfullerene acceptor, using transient absorption spectroscopy. We found that pristine Y6 films show biphasic singlet exciton decay kinetics with decay constants of ~220 ps and ~1200 ps, which is the origin of the large discrepancies in the previously reported exciton lifetimes in the solid state. The majority of the Y6 singlet excitons decayed with the faster (~220 ps) component, whereas a clear photoluminescence with the slower (~1200 ps) component was observed. Y6 singlet excitons undergo fast diffusion in the crystalline domains, resulting in fast singlet–singlet exciton annihilation, after which ultrafast triplet formation, assigned to singlet fission from higher excited singlet states, is observed.