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.
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