Photophysical properties of 3,5-diacetyl-1,4-dihydrolutidine in solution

Formaldehyde, which is a generally occurring pollutant in the atmosphere and in building materials, can easily be determined by colorimetry or fluorimetry after derivatization to 3,5-diacetyl-1,4-dihydrolutidine (DDL) according to the Hantzsch reaction. Owing to the fact that the photophysical properties of DDL are to date widely unknown, the absorption and fluorescence behaviours as well as their changes under the influence of heat or irradiation have been investigated.From solvent-dependent measurements an increase in the dipole moment upon excitation is evident. In solvents of comparable polarity the spectra are more red shifted in protic media, which is probably due to hydrogen bonding. The fluorescence quantum yield was determined as ff(20 °C) = 0.005 in aqueous solution and was found to be strongly dependent on temperature. The activation energy EA for non-radiative decay was determined according to an Arrhenius-type relationship. DDL decomposes on exposure to heat or light. The thermal degradation shows deviations from first-order kinetics and follows the time equation for an autocatalytic reaction. The quantum yield of the photochemical degradation was determined as F = (2-3) × 10-³.