Intramolecular processes and fluorescence properties of 32 specially chosen aromatic molecules, different in degrees of planarity and rigidity,
but family related in -structure are experimentally and theoretically (using the PPP-method) studied at room temperature (293 K). The quantum
yields of fluorescence , and decay times τf, of deaerated and non-deaerated cyclohexane solutions are measured. The oscillator strength fe, the
fluorescence rate constants kf, the natural lifetimes τT
o , and the intersystem crossing rate constants kST, are calculated. The investigations showed the
following differences in the behaviour of the fluorescence parameters in transition from the non-planar molecule to the planar and more rigid-type:
the value of the symmetry line wave-number υ00 (the frequency of the S0→S1 * transition) and the Stokes shift υST, decrease. The oscillator
strength and fluorescence constant, normally decrease. The change in the quantum yield of the fluorescence depends upon the changes in the kf
and kST values. Furthermore, the intersystem crossing rate constant generally decreases, sometimes very significantly. However, there are some
interesting exceptions. For example, the kST value of the non-planar molecule of 9,10-diphenylanthracene is less than the kST value of the planar
and very rigid molecule of anthracene. This important phenomenon is explained. The differences of the intramolecular processes of planar and
non-planar molecules are discussed. The photochemical stability of planar and non-planar molecules is studied. The effect of planarity, rigidity
and molecular symmetry upon laser properties is also traced.