Femtosecond dynamics of the S2 and S1 fluorescence of ionic styryl dyes in polar solvents

Femtosecond fluorescence upconversion and picosecond time-correlated single-photon counting fluorescence experiments for bridged and unbridged ionic styryl dye compounds in polar solvents are reported. The measured fluorescence transients reveal S2 → S1 internal conversion (IC) with a typical time of 300 fs, independent of bridged or unbridged structure. The lifetime of the relaxed emissive S1 state differs considerably for the bridged and unbridged structures: when the styryl-group single bonds are unbridged, the S1-state lifetime is only about 20 ps and the non-radiative decay to the ground state is very effective. When both single bonds of the styryl dye are chemically bridged and only the double bond is free to rotate, the fast decay is suppressed and the lifetime becomes about 2 ns. In addition, the fluorescence of the ionic styryl dyes shows picosecond transient behavior that is attributed to vibrational cooling in the excited S1 state. Finally, a qualitative discussion is given of the influence of the donor–acceptor strength of the styryl dye compounds on the polymethine and stilbenoid character of the S1 state and how this affects the effectiveness of the single- and double-bond twisting relaxation pathways for this state.