Time resolved fluorescence and solvatochromism in donor-substituted bianthrones

The influence of donor substitution of the bianthrone system is investigated through the solvatochromism of the absorption and fluorescence spectra as well as time resolved fluorescence measurements. Both the absorption and fluorescence spectra show a strong solvatochromic shift indicative of an increased dipole moment in the excited state resulting from a charge transfer from the donor to the ring system. The fluorescence lifetime and yield are also seen to be strongly solvent dependent, both decreasing with increasing polarity which is indicative of the increased favouring of nonradiative routes with increasing polarity. A strong dependence of the lifetime on temperature in a glassy solvent mixture leads to the assignment of this nonradiative channel to the twisted form of the bianthrone moiety which is stabilised by the charge transfer process.