Red edge excitation and proton association in the excited state of acridine

A comprehensive study of acridine spectra with variation of pH, wave length of excitation, deuteration of the solvent, etc., has been made. The excited state protonation of acridine is found extra-ordinarily excitation wavelength sensitive near the red edge of the first absorption band. The proton association takes place very fast (K _PT ~ 10¹⁰ sec^-1) on excitation at the red edge of the first absorption band (ree) and acridinium emission is observed while it is slow on short wavelength excitation (swe). The reaction rate slows down at lower temperature which is indicated by a delay in the initiation of the effect by ~ 8 nm on ree. The acridinium type emission with ree at 80 K shows that proton tunnelling is the chief mechanism of proton transfer. The quantum yields are also found wavelength dependent. Contrary to previous observations acridinium ion also shows a ree shift at 80 K.