Dual Fluorescence and Excited-state Double Proton Transfer in the Dimer of 3,5-Dialkyl-4-hydroxybenzylidenemalononitrile

Dual fluorescence of the dimer of 3,5-dialkyl-4-hydroxybenzylidenemalononitrile (HO-BMN) was investigated by steady-state and nanosecond fluorescence spectroscopies. The dimerization of the compounds in 3-methylpentane (MP) solution was confirmed by concentration dependence of absorption and fluorescence spectra at low temperature (<150 K). While the MP solutions of the compounds are almost non-fluorescent at room temperature, the dimer exhibits a short-lived fluorescence (τ=4–9 ns) at 450–520 nm and a long-lived fluorescence (τ=21–34 ns) at 490–550 nm. The short-lived fluorescence was ascribed to a sandwich type dimer (D*) with a center of symmetry. An orientational relaxation leading to the double proton transfer reaction seems to occur in the excited state of the dimer. The long-lived fluorescence in the dimer was tentatively ascribed to the excited species (T*) generated by the double proton transfer reaction in D*. The excited-state proton transfer is discussed in terms of the effect of deuterium substitution of a 4-hydroxyl hydrogen upon fluorescence lifetimes of the dimer.