THE INFLUENCE OF PLANARITY AND RIGIDITY ON THE ABSORPTION AND FLUORESCENCE PARAMETERS AND INTERSYSTEM CROSSING RATE-CONSTANT IN AROMATIC-MOLECULES

Fluorescence properties of 23 specially chosen aromatic molecules, different in degrees of planarity and rigidity but family-related in pi-structure, are experimentally studied and analyzed. The quantum yields of fluorenscence, gamma, and decay times, tau(f), of deaerated and nondeaerated cyclohexane solutions are measured. The oscillator strengths, f(e), the fluorescence rate constants, K-f, natural lifetimes, tau(0)(T), and intersystem crossing rate constants, K-st, are calculated. Investigations showed differences in behavior of fluorescence parameters from the nonplanar molecule to the planar and more rigid type in the following ways: the values of symmetry line wavenumber, nu(00) (frequency of S-0-->S-pi pi*(1) transition), and Stokes shift, Delta nu(st), decrease. The oscillator strength, with consequences for the fluorescence rate constant, normally decreases. The changes in the quantum yield of fluorescence depend upon changes in the K-f and K-st values. Furthermore, the intersystem crossing rate constant generally decreases, but there are some important exceptions. For example, the K-st value of the nonplanar molecule (9,10-diphenylanthracene) is less than the K-st value of the planar and more rigid molecule (anthracene). The results obtained are important for further understanding of the influence of structural factors in aromatic molecules on the intramolecular transformation of light energy absorbed and can be useful in the quest for effective fluorescent dyes for use in dye-laser technology.