Triplet State Research Articles

Chirality Assisted Triplet Electron Pairing.

Redox processes that involve pairs of electrons are common in nature. Some of these reactions involve oxygen molecules. The understanding of the efficiency of the oxygen reduction reaction (ORR), for example, is a challenge since the reaction is spin forbidden and requires the transfer of two pairs of electrons. Past experimental and theoretical studies demonstrated that by controlling the spin of the transferred electrons, it is possible to overcome the barrier resulting from the spin mismatch between the reactants and the products. In other works, it was suggested that the reaction is enhanced if the two electrons in each pair have phase relation, namely, they possess the property of a triplet state. Since in nature electrons are transferred through chiral systems, we probed if chirality affects the formation of paired electrons with the same spin, namely, a triplet like state. The model calculations demonstrate that chirality enhances the probability of the formation of electron pairing in the triplet states, even at room temperature. This enhancement originates from breaking the spin degeneracy, enabled by chirality and interaction of the spins with vibrations.

Diplatinum Single-Molecular Photocatalyst Capable of Driving Hydrogen Production from Water via Singlet-to-Triplet Transitions.

Solar-driven hydrogen production is regarded as one of the most ideal methods to achieve a sustainable society. In order to artificially establish efficient photosynthetic systems, efforts have been made to develop single-molecular photocatalysts capable of serving both as a photosensitizer (PS) and a catalyst (Cat) in hydrogen evolution reaction (HER). Although examples of such hybrid molecular photocatalysts have been demonstrated in the literature, their solar energy conversion efficiencies still remain quite limited. Here we demonstrate that a new dinuclear platinum(II) complex Pt2(bpia)Cl3(bpia = bis(2-pyridylimidoyl)amido) serves as a single-molecular photocatalyst for HER with its performance significantly higher than that of the PtCl(tpy)- and PtCl2(bpy)-type photocatalysts developed in our group (tpy = 2,2': 6',2''-terpyridine, bpy = 2,2'-bipyridine). The outstanding feature is that Pt2(bpia)Cl3 can produce H2 even by irradiating the lower-energy light above 500 nm, which is rationalized due to the direct population of triplet states via singlet-to-triplet transitions (i.e., S-T transitions) accelerated by the diplatinum core.To the best of our knowledge, Pt2(bpia)Cl3 is the first example of a single-molecular photocatalyst enabling hydrogen production from water via the S-T transitions using lower-energy light (> 580 nm).

Excited state dynamics of a Bodipy derivative with a twisted molecular structure: Combined experimental and theoretical studies.

The photophysical properties of a boron dipyrromethene (Bodipy, BDP) derivative (BDP-SA) in which one F atom at the BDP core was replaced by an O atom and condensed with salicylaldehyde were investigated. This compound has a twisted molecular structure and unusually low fluorescence quantum yield (1% in toluene). No intersystem crossing was observed with a nanosecond transient absorption study. The triplet state lifetime of BDP-SA was determined to be 115 μs by photosensitizing. Femtosecond transient absorption shows a structure relaxation of ∼1.5ps for the S1 excited state. Theoretical studies show conical intersections, which are responsible for the efficient non-radiative decay of the S1 state, which has extremely weak fluorescence.

A Platinum Complex‐Based Dimerized Electron Acceptor for Efficient Organic Solar Cells

AbstractMetal–organic complexes have demonstrated excellent performance in organic light‐emitting diodes, yet their potential in organic solar cells (OSCs) remains underexplored. In this study, a novel metal–organic complex, Pt‐Y, which features a platinum core connected to Y‐acceptor arms, for application in OSCs is designed and synthesized. The dimerized Pt‐Y acceptor is prepared through straightforward reactions, with the key precursor linking the platinum metal and Y‐acceptors synthesized through Sonogashira coupling. Steady‐state and transient photoluminescence measurements revealed that Pt‐Y exhibits distinct singlet and triplet states with microsecond lifetimes—significantly longer than the nanosecond lifetimes of Y‐acceptors without the metal. Incorporating Pt‐Y as a third component in ternary OSCs has resulted in a remarkable efficiency of 19.2%. Further morphological analysis and transient absorption measurements indicate that the dimerized Pt‐Y displays excellent miscibility with the Y‐acceptor, leading to minimal phase separation and the formation of fibrillar structures. These structures enhance charge separation and transport while reducing charge recombination. This work presents a facile approach to developing metal–organic complexes with exceptional photovoltaic performance in OSCs.

Time-resolved phosphorescence analysis of singlet oxygen generation and Radachlorin/Ce6 triplet state quenching in solutions with albumin

Time-resolved phosphorescence analysis of singlet oxygen generation and Radachlorin/Ce6 triplet state quenching in solutions with albumin

Photochemical regulation of microcystin synthesis and release in cyanobacteria Microcystis aeruginosa by triplet state dissolved organic matter.

The increasing frequency of cyanobacterial blooms, particularly those induced by Microcystis aeruginosa (M. aeruginosa), poses severe economic, ecological and health challenges due to the production of microcystins (MCs). Environmental parameters such as light and nutrient availability influence MCs production, while the role of dissolved organic matter (DOM) photochemical processes in regulating these remains unclear. This study investigates the effects of light-induced triplet dissolved organic matter (3DOM⁎), derived from various DOM origins, on the photosynthesis, MC synthesis and release by M. aeruginosa. Using DOM photochemical experiments and spectroscopic analysis for humic acid, fulvic acid and extracellular organic matter, the regulatory mechanisms underlying gene expression related to MCs production (mcyD and mcyH) using real-time quantitative PCR was elucidated. Our findings indicate that 3DOM⁎ induces an oxidative stress in M. aeruginosa, reducing photosynthetic efficiency and modulating gene expression, thereby regulating MCs synthesis and release. Moreover, the optical properties of DOM from various sources exhibited distinct differential impacts on M. aeruginosa, highlighting the complex influence of DOM photochemistry on aquatic ecosystems. This research offers novel insights into the effects of DOM photochemical processes on MCs regulation and proposes strategies for managing cyanobacterial blooms and mitigating MCs contamination, contributing to significantly improved water quality management.

Dual Charge-Transfer Emission in Chalcone-based Donor-π-Acceptor System and The Modulation of Down-Conversion of Triplet Exciton with the Polarity of the Medium.

Thermally activated delayed fluorescence (TADF) has recently emerged as a promising process with significant potential to advance organic light-emitting diodes (OLEDs) for display applications. The donor-acceptor system is a well-known molecular arrangement exhibiting TADF properties. However, our investigation into the chalcone-based donor-π-acceptor (D-π-A) system (SKG1) reveals that the en-one bridging unit in chalcone plays a crucial role in the reverse intersystem crossing (rISC) process and may be responsible for the existence of two conformational isomers. In stark contrast with the conventional endothermic TADF process, the designed molecule follows a down-converted cold rISC pathway that also from a higher-lying triplet (Tn) state to the lowest singlet (S1) state (in toluene) with remarkably short delayed fluorescence lifetime of 350 ns. Additionally, this rISC process is found to be sensitive to the polarity of the medium. The UV-vis-NIR transient absorption spectroscopy reveals an ultrafast intersystem crossing (ISC) process within <100ps and the involvement of higher lying triplet state in rISC process. This comprehensive research deepens the understanding of the rISC mechanism and paves the way for developing next-generation OLED materials using D-π-A-based delayed emitters.

A Highly Correlated, Multireference Study of the Lowest Lying Singlet and Triplet States of the Four Thiophene Diradicals.

The energies and geometries of the lowest lying singlet and triplet states of the four diradicals formed by removing two H atoms from thiophene have been characterized. We utilized the highly correlated, multireference methods configuration interaction with single and double excitations with and without the Pople correction for size-extensivity (MR-CISD+Q and MR-CISD) and averaged quadratic coupled cluster theory (MR-AQCC). CAS (8,7) and CAS (10,8) active spaces involving σ, σ*, π, and π* orbitals were employed along with the cc-pVDZ and cc-pVTZ basis sets. The larger active space included the two electrons in the nonbonding sp2 hybrid orbital on sulfur. We find that all didehydro isomers exist as planar, stable ground state singlets. The singlet-triplet (S-T) adiabatic gaps range from 15 to 25 kcal/mol while the vertical splittings are 21-35 kcal/mol. The 2,3 isomer has the lowest absolute ground state singlet energy and the largest adiabatic and vertical S-T splitting. The ground states of the 2,3-, and 2,5-didehydrothiophene isomers are predicted to exhibit the smallest and largest diradical character, respectively, based on their electronic structures, spin densities and bonding analysis. To our knowledge, no experimental excitation energies of any of the didehydrothiophene isomers are available, and our computed MR-AQCC/cc-pVTZ data are believed to be among the most accurate computed results. This extensive study shows a competitive performance between MR-AQCC and MR-CISD+Q.

Transient Triplet Metallopnictinidenes M-Pn (M = PdII, PtII; Pn = P, As, Sb): Characterization and Dimerization.

Nitrenes (R-N) have been subject to a large body of experimental and theoretical studies. The fundamental reactivity of this important class of transient intermediates has been attributed to their electronic structures, particularly the accessibility of triplet vs singlet states. In contrast, electronic structure trends along the heavier pnictinidene analogues (R-Pn; Pn = P-Bi) are much less systematically explored. We here report the synthesis of a series of metallodipnictenes, {M-Pn═Pn-M} (M = PdII, PtII; Pn = P, As, Sb, Bi) and the characterization of the transient metallopnictinidene intermediates, {M-Pn} for Pn = P, As, Sb. Structural, spectroscopic, and computational analysis revealed spin triplet ground states for the metallopnictinidenes with characteristic electronic structure trends along the series. In comparison to the nitrene, the heavier pnictinidenes exhibit lower-lying ground state SOMOs and singlet excited states, thus suggesting increased electrophilic reactivity. Furthermore, the splitting of the triplet magnetic microstates is beyond the phosphinidenes {M-P} dominated by heavy pnictogen atom induced spin-orbit coupling.

Aromatic ring compounds with different conjugation degrees in a boronic acid matrix to realize multicolor phosphorescence for time division colorful multiplexing.

Long lifetime multicolor phosphorescence materials possess excellent optical properties and have important application prospects in the fields of advanced anti-counterfeiting and information encryption. However, realizing long lifetime and color-tunable room temperature phosphorescent (RTP) carbon dot (CD) materials has proved challenging. In this study, the organic precursor molecules 2-phenethylamine (2-Ph), 9-aminophenanthrene (9-Ph) and 1-aminopyrene (1-Py) with different degrees of conjugation were selected to synthesize RTP CD composites: 2-Ph@BA, 9-Ph@BA and 1-Py@BA were synthesized by mixing with a boric acid (BA) matrix under high temperature pyrolysis. During high temperature pyrolysis, CDs form effective covalent and hydrogen bonds with the BA matrix to promote RTP activity and obtain a stable triplet state, which suppresses nonradiative transitions and leads to long lifetime RTP. Besides, as the conjugation of the organic precursor molecules 2-Ph, 9-Ph and 1-Py gradually increases, it results in a gradual decrease of the triplet state energy T1. Thus, 2-Ph@BA, 9-Ph@BA and 1-Py@BA RTP emission showed a significant redshift, leading to green, yellow and red phosphorescence colors, respectively. Finally, RTP CD composites are used in information security and time division colorful multiplexing based on the synthesized samples with different phosphorescence colors and lifetimes.

Two-body hidden charm decays of D wave charmonia

The experimental observations of ψ2(3823) and ψ3(3842) make D wave charmonia family abundant. In the present work, we investigate the hidden charm decay processes of spin triplets of the D-wave charmonia with the meson loop mechanism. The model parameter αΛ is determined by reproducing the branching fraction of ψ(3770)→J/ψη. With this range of model parameter values, the branching fractions (partial widths) of ψ(3770)→ηcω, ψ2(3823)/ψ3(3842)→J/ψη, ψ2(3823)/ψ3(3842)→ηcω are estimated. Our estimations find that the partial width of ψ2(3823)→J/ψη is 29.64-4.63+4.01keV, and the partial width ratio of ψ2(3823)→J/ψη relative to ψ2(3823)→γχc1 is about 10%, which could be tested by further precise measurements from the BESIII, Belle II and LHCb Collaborations.

Ph3PC – A Monosubstituted C(0) Atom in Its Triplet State

This study introduces a novel class of carbon‐centered diradicals: a monosubstituted C‐atom stabilized by a phosphine. The diradical Ph3P→C was photochemically generated from a diazophosphorus ylide precursor (Ph3PCN2) and characterized by EPR and isotope‐sensitive ENDOR spectroscopy at low temperatures. Ph3P→C features an axial zero‐field splitting parameter D = 0.543 cm−1 with a vanishingly small rhombicity |E|/D = 0.002. Time‐ and temperature‐dependent measurements confirm a triplet ground state with a lifetime of approximately 10 min at 127 K in toluene‐d8. Multireference electronic structure calculations predict a clear triplet ground state with a singlet‐triplet gap greater than 20 kcal/mol. In contrast to divalent C(0) compounds, such as Ph3P→C←PPh3, in which carbon needs excitation into a highly‐excited closed‐shell 2s02p4 configuration, Ph3P→C can be explained by direct involvement of carbon in its natural 3P state arising from the 2s22p2 configuration.

Ph3PC - A Monosubstituted C(0) Atom in Its Triplet State.

This study introduces a novel class of carbon-centered diradicals: a monosubstituted C-atom stabilized by a phosphine. The diradical Ph3P→C was photochemically generated from a diazophosphorus ylide precursor (Ph3PCN2) and characterized by EPR and isotope-sensitive ENDOR spectroscopy at low temperatures. Ph3P→C features an axial zero-field splitting parameter D = 0.543 cm-1 with a vanishingly small rhombicity |E|/D = 0.002. Time- and temperature-dependent measurements confirm a triplet ground state with a lifetime of approximately 10 min at 127 K in toluene-d8. Multireference electronic structure calculations predict a clear triplet ground state with a singlet-triplet gap greater than 20 kcal/mol. In contrast to divalent C(0) compounds, such as Ph3P→C←PPh3, in which carbon needs excitation into a highly-excited closed-shell 2s02p4 configuration, Ph3P→C can be explained by direct involvement of carbon in its natural 3P state arising from the 2s22p2 configuration.

Spin-Orbit Coupling and Admixture Coefficients in SA-CASSCF and MS-CASPT2, and Triplet Excitation Yield Simulated via Trajectory Surface Hopping and Calibrated SA-CASSCF in 1,2-Dioxetane Derivatives.

The energy gaps, spin-orbit coupling (SOC), and admixture coefficients over a series of the configurations are evaluated by the SA-CASSCF/6-31G, SA-CASSCF/6-31G*, SA-CASSCF/ANO-RCC-VDZP, and MS-CASPT2/ANO-RCC-VDZP to reveal the extent of the inaccuracy of the SA-CASSCF. By comparing the mean absolute errors for the energy gaps and the admixture coefficient magnitudes (ACMs) measured between the SA-CASSCF/6-31G, SA-CASSCF/6-31G*, or SA-CASSCF/ANO-RCC-VDZP and the MS-CASPT2/ANO-RCC-VDZP, the SA-CASSCF/6-31G is selected as the electronic structure method in the nonadiabatic molecular dynamics simulation. The major components of the ACMs of the SA-CASSCF/6-31G and MS-CASPT2/ANO-RCC-VDZP are identified and compared; we find that the ACMs are underestimated by the SA-CASSCF/6-31G, which is verified by the reasonable triplet quantum yield simulated by the trajectory surface hopping and the calibrated SA-CASSCF/6-31G. The magnitude of the singlet-triplet mixing positively correlates to the hopping probability between the mixed singlet and triplet states, which is confirmed by the computed S-T transition probability.

Building a Highly Stable Red/Near Infrared Afterglow Library with Highly Branched Structures

AbstractAchieving organic red/near infrared (NIR) phosphorescence at high temperatures is theoretically challenging because of the severe nonradiative transitions of excited triplet states with low energy gaps. This study realizes bright and persistent red/NIR afterglow with excellent high‐temperature resistance up to 413 K via highly efficient (≈100%) phosphorescence resonance energy transfer (PRET) from rationally designed branched phosphorescence luminogens as energy donors to red/NIR dyes as acceptors, coupled with optimized aggregated structures. According to systematic investigations, the abundant internal cavities formed by the highly branched luminogens in solid states ensure dye loading and space limitation, which can considerably suppress nonradiative transitions at high temperatures, promoting a persistent red/NIR afterglow with excellent stability. Moreover, 16 types of host–guest systems with varied topological structures of branched luminogens and different red/NIR dyes of various sizes confirm the universality of the strategy. This study provides an efficient approach to achieve a highly stable red/NIR afterglow.

Enhancing the Efficiency of Polariton OLEDs in and Beyond the Single‐Excitation Subspace

AbstractOrganic light‐emitting diodes (OLEDs) have redefined lighting with their environment‐friendliness and flexibility. However, only 25% of the electronic states of organic molecules can emit light upon electrical excitation, limiting the overall efficiency of OLEDs. Strong light–matter coupling, achieved by confining light within OLEDs using mirrors, creates hybrid light‐matter states known as polaritons, which could “activate” the remaining 75% electronic triplet states. Here, triplet‐to‐polariton transition is studied and rates for both reverse inter‐system crossing and triplet‐triplet annihilation are derived. In addition, how the harmful singlet‐singlet annihilation could be reduced with strong coupling is explored.

Complexes of d‐ and f‐metal Ions with Redox‐Active and Highly Symmetric Truxenone Ligand: Effect of Cations and Coordination on Distortions of Radical Anions and Singlet–Triplet Transition in Dianions

Truxenone (Tr) with high C3h symmetry forms crystalline radical anion and dianion salts. The {Bu3MeP+}(Tr·−)(1) and {Cp*2Co+)}(Tr·‐).2C6H4Cl2(2) show intense low‐energy absorption up to 2000nm. Calculations predict Jahn–Teller distortions for Tr·‐, which are enhanced by the asymmetric approach of cations to Tr·‐, leading to C=O bond length alternation and EPR signal asymmetry in 2. Two cesium ions form short contacts of 3.07–3.35Å with oxygen atoms of Tr2‐ in {Cryptand[2.2.2](Cs+)}2(Tr2‐)(3), linking the dianions into a 1D polymer. Calculations suggest a triplet state for the dianions, but the unsymmetric approach of Cs+ to the oxygen atoms of Tr2‐ stabilizes the singlet ground state. The triplet state is populated above 100K. An estimated singlet–triplet energy gap is 344±7K. Radical anions of Tr·‐ coordinate with one d‐ or f‐metal, forming {Cp*2Co+}{TbIII(TMHD)3×Tr}‐.1.5C7H8(4) and {Cp*2Co+}{MnII(acac)2×Tr}‐.2.4C6H4Cl2(5). Strong antiferromagnetic TbIII–Tr·‐ coupling (J =−7.6cm−1) aligns spins of TbIII and Tr·‐ antiparallel in 4. The χMT value of 3.32 emu×K/mol at 300K for 5 with high‐spin MnII is lower‐than‐expected. That indicates a spin state of S=2 owing to the antiparallel alignment of MnII and Tr·‐ spins. Strong MnII–Tr·‐ coupling arising from the spin density localization on the oxygen coordinated to MnII. Complexes with metals demonstrate low‐energy absorption with maxima at 1504–1740nm.

Photoluminescent Properties of Tb-UiO-66 Metal-Organic Framework Analogues.

Three new analogues of Tb-UiO-66 with various functional groups (-F, -Br, -NH2) on the terephthalic acid linker of the metal-organic framework (MOF) are synthesized and characterized. The photoluminescent properties of these analogues, as well as Tb-UiO-66 and Tb-UiO-66-(OH)2, are studied and correlated to the calculated energies for the triplet (T1) states of each linker. The results show that the addition of electron withdrawing groups, such as -F and -Br, lead to higher T1 energies, resulting in quantum yields in the range of 6-31%. The addition of electron donating groups, on the other hand, lowers the T1 energy of the organic linker and inhibits energy transfer such that emission is not observed.

Unveiling the Triplet-State Interaction Mechanism Between 4-Carboxybenzophenone and 2-Naphthalene Sulfonate—A Laser Flash Photolysis Study

This communication aims to comprehensively elucidate the intricate mechanism governing the interaction between the excited triplet state of 4-Carboxybenzophenone (CB*) and the anionic form of 2-Naphthalene Sulfonate (NpSO3−), employing the 337 nm Nanosecond Laser Flash Photolysis technique for this investigation. When the CB is selectively excited by a 337 nm laser, two primary processes become possible: (i) energy transfer from 3CB* to NpSO3− and (ii) electron transfer from NpSO3− to 3CB*. The dynamics of these interactions are explored through experimental observations of transient absorption spectra and the analysis of respective kinetic traces. The primary process dominating in the 3(CB...NpSO3−)* system is identified as triplet energy transfer from excited 3CB* to 3(NpSO3−), as demonstrated by characteristic spectral features observed at 410–420 nm. Comparisons are made with a similar system studied by Yamaji and co-workers, 3(BP•−...NpO•)*, revealing differences in the priority of primary process occurrences. These findings contribute to a deeper understanding of the intricate interactions between excited molecules and ground-state donors, aiding in the comprehension of mechanisms governing these reactions.

DNA Triplet Energies by Free Energy Perturbation Theory.

Determining the energetics of triplet electronic states of nucleobases in the biological macromolecular environment of nucleic acids is essential for an accurate description of the mechanism of photosensitization and the design of drugs for cancer treatment. In this work, we aim at developing a methodological approach to obtain accurate free energies of triplets in DNA beyond the state of the art, able to reproduce the decrease of triplet energies measured experimentally for T in DNA (270 kJ/mol) vs in the isolated nucleotide in aqueous solution (310 kJ/mol). For such purposes, we adapt the free energy perturbation method to compute the free energy related to the transformation of a pure singlet state into a pure triplet state via "alchemical" intermediates with mixed singlet-triplet nature. By this means, standard deviation errors are only a few kJ/mol, contrary to the large errors of tenths of kJ/mol obtained by averaging the singlet and triplet energies derived from molecular dynamics simulations. The reduced statistical errors obtained by the free energy perturbation approach allow us to rationalize with confidence the triplet stabilization observed experimentally when comparing the thymine nucleotide and thymine in DNA. Spin polarization rather than excimer interactions between the π-stacked nucleobases originates the lower values of the triplet energies in DNA. The developed approach implemented in QM3 shall be useful for determining free energies of triplets and other states like ionic or charge separation states in any other macromolecular system with impact in biomedicine and materials science.