Effects of solvent induced modulation of energy gaps on electronic relaxation of excited hydrogen bonded complexes of some aromatic carbonyl compounds Original Research Article

A detailed study of the influence of solvent polarity and temperature dependence (T ≤ 300 K) on the radiationless transitions of hydrogen bonded complexes of 2-naphthaldehyde (1), 2-acetonaphthone (2), methyl 2-naphthoate (3) and 1,2-dihydro-3H-benz[e]inden-3-one (4) is presented. The hydrogen bonded complexes are strongly fluorescent. The energy gaps between S1 and S0 and between S1 and T1 could be varied by using various 1,4-dioxane/water mixtures as the solvent. In the case of the complex of 1 intersystem crossing and internal conversion from S1 have both been found to proceed through a direct process as well as by way of a proces involving thermal excitation to S2. The conversion of S1 to T1 proceeds only through thermal excitation to S2 in the case of 2 and 4, whereas in the case of 3 a contribution from a thermally activated process could not be detected. An inverse exponential energy gap law has been found for the temperature independent intersystem crossing from S1 in the case of the complexes of 1 and 3. This is shown theoretically to be in accordance with a nuclear tunneling process. The tunneling appears to proceed along the C-O stretching mode. The internal conversion from the state S1 of the complex of 3 satisfies the regular exponential energy gap law.