Bis(4'-dibutylaminostyryl)benzene: spectroscopic behavior upon protonation or methylation.

We have investigated the UV/Vis absorption and emission of 1,4-bis(4'-dibutylaminostyryl)benzene (1) upon protonation with trifluoroacetic acid in dichloromethane and in acetonitrile. We find that 1 does not display significantly dynamic acidity in the excited state, that is, it is not a photoacid. Three protonation states of 1 were investigated, all of which, neutral, singly protonated, and bis-protonated, are fluorescent. As an isolable model for the mono-protonated species, a methylated derivative of 1 was prepared by reaction with methyl triflate. This species displays redshifted emission but similar absorption as the non-quaternized parent, 1. The strong observed emission of all three protonated states of 1 suggests that the kinetic photoacidity of 1* is low or absent. We assume that the lifetime of the excited state is too short to allow solvent reorganization and therefore we cannot make a statement on the thermodynamic photoacidity of the protonated forms of 1.