Knowledge Bank

The N-heterocyclic acridine exhibits two different fluorescences, depending on the nature of the solvent. In a hydroxylic solvent (EPA) glass, acridine exhibits a blue $\Pi$,$\Pi \ast$ fluorescence, resembling that of the parent polyacene, anthracene. However, in a hydrocarbon glass, acridine shows a broad, structureless, green fluorescence. At $77\circ K$, the lifetime of this emission is $2.5\mu$sec in hydrocarbon glass. This emission is assigned as the n,$\Pi \ast$ fluorescence. The lowest absorption spectra of acridine also show characteristic differences in the two kinds of solvents. The excitation spectrum of each fluorescence agrees with the corresponding absorption spectrum. Because of its long lifetime, the $\Pi$,$\Pi \ast$ fluorescence is not observed readily in a fluid medium at room, temperature. In a low temperature glass, intersystem crossing competes favorably with the n,$\Pi \ast$ fluorescence. Hence, the phosphorescence quantum yield of acridine is much higher in a hydrocarbon than in a hydroxylic solvent.