Fluorescence and phosphorescence of harmol and harmalol at 77 K

We have investigated the fluorescence and phosphorescence spectra of harmol and harmalol in the solid phase at 77 K. Three molecular species, namely, the cation, the neutral molecule and the molecular anion are characterized for harmol in both the fluorescence and phosphorescence spectra. We have also identified three molecular species, namely, the cation, the zwitterion and the molecular anion in the fluorescence spectra of harmalol but only the cation and the molecular anion in the phosphorescence spectra.

The excited state pK_a values in the first excited triplet state T₁ have been estimated from the Förster equation. It is found that in the first excited triplet state the ring nitrogen atom is more basic and the hydroxy group is more acidic than in the ground state.