Dependence of the fluorescence parameters and the intersystem crossing rate constant on the orbital nature of the s 1 state of catacondensed aromatics

Abstract

Fluorescence properties of nine specially chosen aromatic compounds are studied experimentally at room temperature (293 K). The compounds, which are from the catacondensed class are arranged in such a way that odd numbered compounds reveal the 1 L b→ 1 A fluorescence, while even numbered compounds show the 1 L a→ 1 A fluorescence. All compounds are family related in π-structure and are of the same degree of planarity and rigidity. The quantum yield of fluorescence γ, and the decay times τ f, of nondeaerated and deaerated cyclohexane solutions are measured. The oscillator strength f e, the fluorescence rate constant K f, natural lifetimes τ 0 T and intersystem crossing rate constant K ST, are calculated. The lowest 1 L b and 1 L a singlet and 3 L a triplet (77 K) levels are determined. Investigations showed that transition from a catacondensed molecular which shows the 1 L a→ 1 A fluorescence to a family related in π-structure molecular which reveals the 1 L b→ 1 A fluorescence is accompanied by certain changes of all fluorescence parameters: γ is usually decreasing, τ f is increasing, K f is decreasing, FWRE of the fluorescence spectrum is decreasing and, moreover, K ST is also decreasing, sometimes very significantly. For example, K ST of phenanthrene is 28.5 times smaller than K ST of anthracene through S α-T 1 interval of phenanthrene is less than S p-T 1 interval of anthracene (7100 and 11660 cm −1 respectively). The systems of singlet and triplet levels of the compounds studied are simulated (PPP-CI) and triplet levels which mix with S 1 state are determined. The low K ST value of perylene is discussed and explained. To explain the observed phenomenon of decreasing values of K ST, the assumption is made that the matrix elements of the spin-orbit coupling between S α and T i state are much smaller than those between S p and T i states. The results obtained are important for further understanding of the influence of the orbital nature of S 1 state on the intramolecular transformation of light energy absorbed by a molecule.