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 aromati c 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 me asured. The oscillator strength, f(e), fluorescence rate constant, k(f), na tural lifetime, tau (t)(D), and intersystem crossing rate constant, k(ST) w ere calculated for each compound. The orbital nature of the lowest excited singlet state and direction of polarization of the S-0 --> S-1 transitions are determined using the PPP-CI method for each molecule. The investigation shows that substitution of a heavy atom(s) (Cl, S, Br, I etc.) into an aro matic or heteroaromatic molecule may produce different changes in all the f luorescence parameters (sometimes dramatically) and not necessarily lead to the quenching of fluorescence. Substitution of a heavy atom(s) may increas e the value of the spin-orbit operator, (H) over cap (SO), if the S-0 --> S -1 excitation is localized to some extent on a carbon atom bonded to a heav y atom(s) or on the heavy atom itself (O or S). Such substitution may chang e the symmetry of a molecule and hence the values of the < Psi (S1)/(H) ove r cap (SO)\ Psi ' (Ti)> matrix elements would change tin molecules of highe r symmetry groups not all T-i states are able to mix with the perturbing S- 1 state). Such substitution may change the arrangement of T-i states below the S-1, state and hence, the Franck-Condon factors would change. Such subs titution mag. also change the value of the < Psi (S0)/M-i/Psi (S1)> matrix element and, consequently, the oscillator strength of the S-0 --> S-1 trans ition would change. A combination of all these possible changes determines the value of k(f) and k(ST) and, consequently, determines the value of gamm a and tau (f). It is observed that in many cases, the value of the spin-orb it operator is related to the dipole moment operator, e.g. if the introduct ion of a heavy atom increases k(ST) then, as a rule, it decreases f(e)((1)A --> L-1(a)). (C) 2001 Elsevier Science B.V. All rights reserved.