title: Size- and solvent-dependent kinetics for cis-trans isomerization in donor-acceptor systems.
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.
authors.affiliation: Department of Photochemistry and Molecular Sciences, Box 523, Uppsala University, SE-751 20 Uppsala, Sweden.
authors.forename: Hans-Christian
authors.initials: HC
authors.lastname: Becker
authors.forename: Kristine
authors.initials: K
authors.lastname: Kilså
mesh_terms: D003517:Cyclopentanes / Q000737:chemistry*; D004579:Electron Transport; D007536:Isomerism; D007700:Kinetics; D015394:Molecular Structure; D012997:Solvents / Q000737:chemistry*
chemical_list: D003517:Cyclopentanes; D012997:Solvents; C017332:fulvene