Dispersed fluorescence and delayed ionization of jet-cooled 2-aminopurine: relaxation to a dark state causes weak fluorescence.

Dark state: The photophysics of 9H-2AP (see figure) depends strongly on the solvent and the temperature. In polar, aprotic solvents the fluorescence quantum yield is lower compared to protic ones. Analysis of the diffuse fluorescence spectra point to the existence of a dark singlet n,pi* state which is long-lived (1 microsecond).