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Singlet fission initiating organic photosensitizations.

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

SUBMITTER: Tsuneda T

PROVIDER: S-EPMC10774408 | biostudies-literature | 2024 Jan

REPOSITORIES: biostudies-literature

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Singlet fission initiating organic photosensitizations.

Tsuneda Takao T Taketsugu Tetsuya T

Scientific reports 20240108 1

The feasibility of singlet fission (SF) in organic photosensitizers is investigated through spin-flip long-range corrected time-dependent density functional theory. This study focuses on four major organic photosensitizer molecules: benzophenone, boron-dipyrromethene, methylene blue, and rose bengal. Calculations demonstrate that all these molecules possess moderate [Formula: see text]-stacking energies and closely-lying singlet (S) and quintet (triplet-triplet, TT) excitations, satisfying the e ...[more]

PMID: 38191637

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Project description:The role of singlet fission (SF) in the triplet-state generation mechanism of 1,3,5,7-tetramethyl-boron-dipyrromethene derivatives is revealed by exploring the cause for the solvent dependence of the generation rate. Comparing the adsorption energy calculations of solvent molecules, i.e., cyclohexane, chloroform and acetonitrile molecules, to the derivatives with the [Formula: see text]-stacking energies of these derivatives surprisingly show that the hierarchy of the solvation energies and [Formula: see text]-stacking energies strongly correlates with the experimentally-suggested solvent dependence of the triplet-state generation of these derivatives for five and more adsorbing solvent molecules. Following this finding, the excitation spectra of these derivatives in acetonitrile solvent are explored using the proprietary spin-flip long-range corrected time-dependent density functional theory. It is, consequently, confirmed that the [Formula: see text]-stacking activates the second lowest singlet excitation to trigger the spin-allowed transition to the singlet doubly-excited tetraradical (TT)[Formula: see text] state, which generates the long-lived quintet (TT)[Formula: see text] state causing the SF. However, it is also found that the [Formula: see text]-stacking also get a slow intersystem crossing active around the intersections of the lowest singlet excitations with the lowest triplet T[Formula: see text] excitations in parallel with the SF due to the charge transfer characters of the lowest singlet excitations. These results suggest that SF initiates the triplet-state generations through near-degenerate low-lying singlet and (TT) excitations with a considerable singlet-triplet energy gap after the [Formula: see text]-stacking of chromophores stronger than but not far from the solvation. Since these derivatives are organic photosensitizers, this study proposes that SF should be taken into consideration in developing novel heavy atom-free organic photosensitizers, which will contribute to a variety of research fields such as medical care, photobiology, energy science, and synthetic chemistry.

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Singlet fission initiating organic photosensitizations.

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Singlet fission initiating organic photosensitizations.

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