Ultrafast Decay of the Excited Singlet States of Thioxanthone by Internal Conversion and Intersystem Crossing

The experimental ultrafast photophysics of thioxanthone in several aprotic organic solvents at room temperature is presented, measured using femtosecond transient absorption together with high‐level ab initio CASPT2 calculations of the singlet‐ and triplet‐state manifolds in the gas phase, including computed state minima and conical intersections, transition energies, oscillator strengths, and spin–orbit coupling terms. The initially populated singlet ππ* state is shown to decay through internal conversion and intersystem crossing processes via intermediate nπ* singlet and triplet states, respectively. Two easily accessible conical intersections explain the favorable internal conversion rates and low fluorescence quantum yields in nonpolar media. The presence of a singlet–triplet crossing near the singlet ππ* minimum and the large spin–orbit coupling terms also rationalize the high intersystem crossing rates. A phenomenological kinetic scheme is proposed that accounts for the decrease in internal conversion and intersystem crossing (i.e. the very large experimental crescendo of the fluorescence quantum yield) with the increase of solvent polarity.     

Inclusion Complexes of Styryl Dyes with Cucurbiturils: Ultrafast Relaxation of Electronically Excited States

High Energy Chemistry - Published data on the ultrafast (in the picosecond range) relaxation of electronically excited inclusion complexes between macrocyclic cavitands consisting of n glycoluril...     

High‐Yield Excited Triplet States in Pentacene Self‐Assembled Monolayers on Gold Nanoparticles through Singlet Exciton Fission

One of the major drawbacks of organic‐dye‐modified self‐assembled monolayers on metal nanoparticles when employed for efficient use of light energy is the fact that singlet excited states on dye molecules can be easily deactivated by means of energy transfer to the metal surface. In this study, a series of 6,13‐bis(triisopropylsilylethynyl)pentacene–alkanethiolate monolayer protected gold nanoparticles with different particle sizes and alkane chain lengths were successfully synthesized and were employed for the efficient generation of excited triplet states of the pentacene derivatives by singlet fission. Time‐resolved transient absorption measurements revealed the formation of excited triplet states in high yield (172±26 %) by suppressing energy transfer to the gold surface.     

Observation and Identification of Metastable Excited States in Ultrafast Laser-Ionized Pyridine

We report on the fragmentation of ionized pyridine (C₅H₅N) molecules by focused 50 fs, 800 nm laser pulses. Such ionization produces several metastable ionic states that fragment within the field-free drift region of a reflectron-type time of flight mass spectrometer, with one particular metastable dissociation being the leading fragmentation process. Because the time of flight is no longer dependent in a simple way on the mass of the ion, the metastable decay is manifested as an unfocused peak on the mass spectrum that appears at a time of flight not corresponding to an integer mass. A previously-developed method is used to identify the precursor and final masses of these ions. The metastable process that creates the most prevalent peak is shown to be C₅H₅N⁺ → C₄H₄⁺ + HCN. Simulations confirm this result and place restrictions on the processes for several other observed metastable reactions.     

Singlet Excited States and the Light-Harvesting Function of Carotenoids in Bacterial Photosynthesis

Abstract— The recent results of stationary-state and time-resolved absorption, fluorescence and Raman spectroscopies of some typical carotenoids are summarized. Theoretical analyses of carotenoid singlet states and of carotenoid-to-bacteriochlorophyll singlet-energy transfer are also included. On the bases of the energies, the lifetimes and other properties of singlet excited states of the carotenoids in solution and bound to the light-harvesting complexes, the energetics and the dynamics of the light-harvesting function in purple photosynthetic bacteria are discussed with emphasis on the 2A_g and B_u⁺ states.     

单重态激子裂分的研究进展

单重态激子裂分指的是在有机分子中一个单重态激子与相邻的基态发色团相互作用形成两个三重态激子的过程。利用这种多激子效应制成的光伏器件有望突破肖克利-奎瑟限制,使光电转换的理论效率由30%提高到44.4%。近年来各国科学家在裂分材料的设计合成和器件化应用方面取得了一定的进展,但是对于激子裂分物理本质的认知仍然存在争议和分歧。本文较为系统地介绍了激子裂分材料的最新进展和本研究组的相关工作。简要回顾了激子裂分的发展历史,从概念、裂分的发生条件和作用机制三方面介绍了激子裂分过程,综述了具有分子间和分子内裂分性质的材料的最新研究成果。在系统归纳激子裂分研究现状的基础上对单重态激子裂分的发展趋势和应用探索指出了可能的方向。     

Quenching of the Singlet and Triplet Excited States of Pterin by Amino Acids

Unconjugated oxidized pterins accumulate in the skin of patients suffering from vitiligo and, under UVA irradiation, photosensitize the oxidation of amino acids. In this work, we study the interaction of the singlet and triplet excited states of pterin (Ptr), the parent compound of oxidized pterins, with four oxidizable amino acids: tryptophan (Trp), tyrosine (Tyr), histidine (His) and methionine (Met). Steady‐state and time‐resolved fluorescence measurements and laser flash photolysis experiments were performed to investigate the quenching of the Ptr excited states by the amino acids in aqueous solution. The singlet excited states of Ptr are quenched by Met mainly via a dynamic process and by Trp via a combination of dynamic and static processes. His does not quench singlet excited states of Ptr, and quenching by Tyr could not be investigated due to the low solubility of this amino acid. The triplet excited states of Ptr are quenched by the four studied amino acids, and the corresponding bimolecular quenching rate constants are in the range of diffusion controlled limit. The assessment of the results in the context of the Ptr‐photosensitization of amino acids suggests that triplet excited state of Ptr is the species that initiates the photochemical processes.     

Triplet Excited States and Singlet Oxygen Production by Analogs of Red Wine Pyranoanthocyanins

During the maturation of red wines, the anthocyanins of grapes are transformed into pyranoanthocyanins, which possess a pyranoflavylium cation as their basic chromophore. Photophysical properties of the singlet and triplet excited states of a series of synthetic pyranoflavylium cations were determined at room temperature in acetonitrile solution acidified with 0.10 mol dm^?3 trifluoroacetic acid (TFA, to inhibit competitive excited state proton transfer) and at 77 K in a rigid TFA‐acidified isopropanol glass. In solution, the triplet states of these pyranoflavylium cations are efficiently quenched by molecular oxygen, resulting in sensitized formation of singlet oxygen, as confirmed by direct detection of the triplet‐state decay by laser flash photolysis and of singlet oxygen monomol emission in the near infrared. The strong visible light absorption, the relatively small singlet‐triplet energy differences, the excited state redox potentials and the reasonably long lifetimes of pyranoflavylium triplet states in the absence of molecular oxygen suggest that they might be useful as triplet sensitizers and/or as cationic redox initiators in polar aprotic solvents like acetonitrile.     

Geometry of Transition Metal Complexes with Bipyridine in the Excited States

A technique for estimating the changes in the geometrical structural parameters of complex compounds during charge-transfer electron excitations is proposed. The method combines the experimental Raman resonance data, the wave packet dynamics procedure, and quantum-chemical calculations of the geometry and vibrational spectra of relatively small fragments of the complex ion. The technique is used to determine the changes in the geometrical structure of 2, -bipyridine during metal–ligand charge transfer excitations in Ru(II) complexes.     

Hydrogen-Bonding Interactions between Harmane and Pyridine in the Ground and Lowest Excited Singlet States

Abstract— A spectroscopic (UV-visible, Fourier transform IR, steady-state and time-resolved fluorescence) study of hydrogen-bonding interactions between harmane (1-meth-yl-9H-pyrido/3,4- b /indole) and pyridine in the ground and lowest excited singlet state is reported. In low polar and weakly or nonhydrogen-bonding solvents, such as cy-clohexane, chloroform, carbon tetrachloride, toluene and benzene, the analysis of the spectroscopic data indicates that harmane and pyridine form 1:1 stoichiometric hydrogen-bonded complexes in both the ground and singlet excited states. The formation constants of the complexes are greater in the excited than in the ground state. Hydrogen-bonding interaction in the excited state is essential for the quenching of the fluorescence of harmane by pyridine. The stabilities of the hydrogen-bonded complexes between harmane and pyridine diminish as the polarity and hydrogen-bonding ability of the solvent increase.     

Intermolecular Interactions and Charge Resonance Contributions to Triplet and Singlet Exciton States of Oligoacene Aggregates

Intermolecular interactions modulate the electro-optical properties of molecular materials and the nature of low-lying exciton states. Molecular materials composed by oligoacenes are extensively investigated for their semiconducting and optoelectronic properties. Here, we analyze the exciton states derived from time-dependent density functional theory (TDDFT) calculations for two oligoacene model aggregates: naphthalene and anthracene dimers. To unravel the role of inter-molecular interactions, a set of diabatic states is selected, chosen to coincide with local (LE) and charge-transfer (CT) excitations within a restricted orbital space including two occupied and two unoccupied orbitals for each molecular monomer. We study energy profiles and disentangle inter-state couplings to disclose the (CT) character of singlet and triplet exciton states and assess the influence of inter-molecular orientation by displacing one molecule with respect to the other along the longitudinal translation coordinate. The analysis shows that (CT) contributions are relevant, although comparably less effective for triplet excitons, and induce a non-negligible mixed character to the low-lying exciton states for eclipsed monomers and for small translational displacements. Such (CT) contributions govern the L_a/L_b state inversion occurring for the low-lying singlet exciton states of naphthalene dimer and contribute to the switch from H- to J-aggregate type of the strongly allowed B_b transition of both oligoacene aggregates.     

Hydrogen-bonding and Proton Transfer Interactions between Harmane and Trifluoroethanol in the Ground and Excited Singlet States

A study of the hydrogen-bonding and proton transfer reactions of the ground and excited states of harmane (1-methyl-9H-pyrido/3,4-b/indole) and its N ₉-methyl derivative with 2,2,2-trifluoroethanol in cyclohexane is reported. Spectral measurements (UV–visible, Fourier trans-form IR, steady-state and time-resolved fluorescence) show the formation of fluorescent ground-state hydrogen-bonded complexes. The results have been interpreted assuming a tautomeric equilibrium between a 1:1 hydrogen-bonded complex and its 1:2 proton transfer tautomer (hydrogen-bonding ion pair). Upon excitation to its singlet excited state, the proton transfer tautomer of harmane reacts with an additional 2,2,2-trifluoroethanol molecule to give a zwitterionic exciplex, which fluoresces at longer wavelength.     

Excited States of Weak Interacting Complexes of Formaldehyde and Alkali Metal Ions

The electronically excited states of formaldehyde and its complexes with alkali metal ions are investigated with the time-dependent density functional theory (TD DFT) method. Vertical transition energies for several singlet and triplet excited states, adiabatic transition energies for the first singlet and triplet excited states S₁ and T₁, the adiabatic geometries and vibrational frequencies of the ground state S₀ and the first singlet and triplet excited states S₁ and T₁ for formaldehyde and its complexes are calculated. Better agreement with the experiment than that of the CIS method is obtained for CH₂O at the TD DFT level. The nonlinear C=O?M⁺ interaction in the excited states S₁ and T₁ is weaker than the linear interaction in the ground state. In the S₀ and S₁ states, the C=O bond is elongated by cation complexation and its stretching frequency is red-shifted, but in the T₁ state the C=O bond is shortened and its frequency is blue-shifted.     

Theoretical Study of Electron Density Delocalization in the Excited States of Transition Metal Complexes

This paper presents a theoretical study of electron density delocalization effects over an electron-accepting ligand in metal-to-ligand charge-transfer (MLCT) complexes in the excited states, where the ligand is 4,4'-X₂-2,2'-bpy (X = H, NH₂, CH₃, Ph, Cl, CO₂Et, NO₂, bpy = 2,2'-bipyridine) or terpy (2,2':6',2'-terpyridine). Optimal geometry calculations are performed for neutral ligand molecules and their radical anions modeling the state of the ligands during MLCT excitations. Spin density distribution over atoms in the radical anions is used as a measure of the degree of delocalization. The role of spin density distribution in excitation-induced changes of geometrical parameters of the ligands is considered.