Excited state charge-transfer effect on the phosphorescence emissions of 4-aminopyridine at 77 K

Abstract

Absorption and phosphorescence spectral characteristics of 4-aminopyridine (AMP) have been studied in polar and non-polar solvents at 77 K. Absorption data provide evidence that the excited singlet π, π * states of the molecule possess substantial charge-transfer (CT) character. The 4-AMP molecule is nonfluorescent in all the solvents used in the investigation but phosphoresces strongly in ethanol glass at 77 K. The phosphorescence emissions have a lifetime and negative polarization characteristic of π,π * origin. From the observations that in acidic ethanol glassy medium the phosphorescence quantum yield somewhat decreases and the lifetime moderately increases as compared to that in neutral ethanol glass but the negative polarization persists, it has been concluded that all the coupling schemes π 1 , π * (CT) SO ¯ π 3 , π * , π 1 , π * SO ¯ n 3 , π * vib ¯ π 3 , π * and π 1 , π * vib ¯ n 1 , π * SO ¯ π 3 , π * are responsible for the phosphorescence characteristics of 4-AMP molecule. The substantial reductions in the values of the phosphorescence quantum yield and lifetime of 4-AMP in non-polar rigid matrix at 77 K have been attributed to enhanced nonradiative process arising from increased vibronic interactions between the closely spaced 3 n,π * and 3 π,π * states of the molecule in such environment.