Phosphorescence emission and polarization of harmane

Abstract

Phosphorescence and excitation spectra, phosphorescence and excitation polarization spectra and phosphorescence lifetimes were measured in methylcyclohexane (MC) and EPA glasses at 77 K for harmane (1-methyl-9H-pyrido[3,4-b]indole).

It has been found that the 0—0 band of the phosphorescence emission is polarized out of plane, using the excitations corresponding to absorptions into the first and second excited ππ^* singlet states. This fact and the order of the lifetimes measured indicate an emitting triplet that is ³(ππ^*) in both solvents. The origin of this polarization is very likely ¹(nπ^*) → ³(ππ^*) mixing as a result of spin–orbit coupling. However, the vibrational structure and changing polarization outside the OO band indicate that second-order effects involving spin—orbit coupling and vibronic interactions are important and the mixing scheme appears to be

The “proximity effect” can qualitatively explain the low intensity of phosphorescence and the stronger vibronic interaction between the nπ^* and ππ^* states of harmane in a hydrocarbon solvent (MC).