Edge excitation red-shift and excited-state proton association in acridine

Abstract

Acridine in methanol shows an edge excitation red-shift. On shorter-wavelength excitation the emission from acridine is observed, while on red edge excitation the emission corresponds to acridinium. At room temperature the shift is abrupt with no effect of viscosity and solvent deuteration; however, at 80 K a slightly less abrupt shift is observed. The excited-state proton association is found to be wavelength-dependent. It seems that at room temperature with red edge excitation, protonation in the excited state is fast compared to the lifetime of the excited state, while the excess energy with shorter wavelength excitation may lead to non-promoting modes such that the proton transfer does not take place.