Luminescence of the Carboxyl Group

A luminescence is observed from formic and acetic acids, their salts and esters. The yield of this luminescence is low (Φ < 0.01). On the basis of much circumstantial evidence, we assign this emission as a Γπ,π*3 → 1Γ1 process located on the carboxyl group. Theoretical considerations are in reasonable accord with this conclusion when it is assumed that the Γπ,π*3 state geometry is considerably different from that of Γ11. Absorption spectroscopy indicates the presence of two low-energy transitions in the formate ion; these absorption bands are quite weak and are assigned as two almost-degenerate Γn,π*1 → 1Γ1 transitions of the formate ion.