Size- and solvent-dependent kinetics for cis–trans isomerization in donor–acceptor systems

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.

Graphical abstractSpectral shifts on the ultrafast timescale show several charge transfer states and isomerization kinetics that correlate with molecular size and solvent viscosity.Figure optionsDownload full-size imageDownload as PowerPoint slide