The influence of molecular symmetry and topological factors on the internal heavy atom effect in aromatic and heteroaromatic compounds.

The absorption and fluorescence properties of 26 specially selected aromatic and heteroaromatic compounds, from different classes, are studied quantum chemically and experimentally at room temperature (293 K). Seven of these compounds have not been studied before. The compounds are arranged in seven groups, which illustrate different cases of the internal heavy atom effect. The quantum yield of fluorescence, gamma and fluorescence decay time, tau(f) of deaerated and non-deaerated cyclohexane or ethanol solutions are measured. The oscillator strength, f(e), fluorescence rate constant, k(f), natural lifetime, tau(0)t, and intersystem crossing rate constant, kST, were calculated for each compound. The orbital nature of the lowest excited singlet state and direction of polarization of the S0 --> S1 transitions are determined using the PPP-Cl method for each molecule. The investigation shows that substitution of a heavy atom(s) (Cl, S, Br, I etc.) into an aromatic or heteroaromatic molecule may produce different changes in all the fluorescence parameters (sometimes dramatically) and not necessarily lead to the quenching of fluorescence. Substitution of a heavy atom(s) may increase the value of the spin-orbit operator, Hso, if the S0 --> S1 excitation is localized to some extent on a carbon atom bonded to a heavy atom(s) or on the heavy atom itself (O or S). Such substitution may change the symmetry of a molecule and hence the values of the [psiS1/Hso/psi'T1] matrix elements would change (in molecules of higher symmetry groups not all Ti states are able to mix with the perturbing S1 state). Such substitution may change the arrangement of Ti states below the S1, state and hence, the Franck-Condon factors would change. Such substitution may also change the value of the [psiS0/Mj/psiS1] matrix element and, consequently, the oscillator strength of the S0 --> S1 transition would change. A combination of all these possible changes determines the value of k(f) and kST and, consequently, determines the value of gamma and tau(f). It is observed that in many cases, the value of the spin-orbit operator is related to the dipole moment operator, e.g. if the introduction of a heavy atom increases kST then, as a rule, it decreases f(e)(1A --> 1La).