Model studies of spectral and photophysical characteristics of a series of donor-acceptor-polyenes of different length

The photophysical behaviour of a series of donor-acceptor-polyenes of different length: 4-dimethylamino-4′-cyanostilbene (DCS), 4-dimethylamino-4′-cyano-1,4-diphenylbutadiene (DCB) and 4-dimethylamino-4′-cyano-1,4-diphenylhexatriene (DCH) has been studied by means of steady state and time resolved fluorescence and low temperature measurements. Quantum chemical calculations were performed for comparison. For all three dyes, the fluorescence quantum yields show a maximum in the most polar solvents. A pronounced solvatochromic behaviour is only observed for the emission spectra whereas the absorption spectra remain nearly unaffected. The nonradiative decay decreases in the order DCS > DCB > DCH. Similarly, the radiative rate constant decreases in this order, contrary to the usual behaviour for a lengthening of the π-system. The chain-length dependence of the dipole moments of the relaxed excited state μe is better described by the Onsager radius derived from molecular length rather than from the density approach. Both calculations and experiments support the conclusion that the emissive state is not a TICT state.