A detailed photophysical analysis of 4-hydroxy-5-phenylpyrido[3,2,1-jk]carbazol-6-one (HPPCO) is presented. When exposed to UV light, the compound produced deep blue to green luminescence, depending on the solvent. The luminescence peak shifts with the Gutmann donor number (DN) of the solvent and the proton substitution affects luminescence; a correlation between quantum yield and decay time indicated that proton transfer plays a key role in the observed solvatochromism. The ground-state deprotonation of HPPCO was apparently evidenced from the absorption and/or the excitation spectra in the solvents with large DN values. DFT and ZINDO calculations on the structural and optical properties have shown that deprotonation increases the contribution of oxygen atoms to the HOMO, thereby lowering the transition energy from the HOMO to the LUMO. Because the luminescence properties of HPPCO depend on proton transfer, it may be used to detect and quantitate amines in solution. The sensitivity of the luminescence to various amines was ∼10(5) M(-1) and was more effective in ethanol than in methanol.
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