Size- and solvent-dependent kinetics for cis–trans isomerization in donor–acceptor systems |
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Authors: | Hans-Christian Becker Kristine Kils |
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Institution: | aDepartment of Photochemistry and Molecular Sciences, Box 523, Uppsala University, SE-751 20 Uppsala, Sweden;bDepartment of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark |
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Abstract: | We have investigated, using time-resolved and steady-state optical spectroscopy, the cis–trans isomerization dynamics in a series of charge transfer, donor–acceptor compounds. The number of donor (dithiafulvene) and acceptor (p-nitrophenyl) moieties as well as their spatial arrangement around a central ethynylethene core has been varied in a systematic way. All compounds in the series are weakly fluorescent. We show that the fluorescence spectrum red-shifts within a few picoseconds, a shift which occurs concurrently with a blue-shift of the transient absorption spectrum. The kinetics following the initial relaxation are in all cases multi-exponential, and the time constants correlate with molecular size and solvent viscosity. We interpret the data as a result of conformational change where the conjugation through the central double bond is broken upon excitation into the charge-transfer transition, and the time for rotation around this bond is dependent on the molecular interactions between solute and solvent. |
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Keywords: | Charge transfer Isomerization Solvent dependence Time-resolved emission and transient absorption spectroscopy |
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