Knowledge Bank

The luminescence of aromatie hydrocarbons and nitrogen heterocyclies usually consists of progressions of totally symmetric vibrations. Those vibrations represent, within a Born-Oppenheimer attitude, those nuclear coordinates that undergo displacements in one state relative to the other. In p-$dibromobenzene1$ and to a lesser extent in $pyrazine2$ we found that the emission consisted of progressions of non-totally symmetric vibrations, particularly in out-of-plane ring modes. Presumably this redistribution of intensity is caused by a change in molecular symmetry on excitation. We will present spectral studies of substituted aromatics and N-heterocyclis to demonstrate that symmetry changes of this sort are not uncommon in condensed media. In addition we will show that perhaps the symmetry of the electronically excited states of many molecules is to a great extent predictable by the inclusion of otherwise neglected nuclear position dependent terms in the electronic Hamiltonian. This becomes especially clear when the molecules are known to have nearby states such that a Born-Oppenheimer approximation is hardly expected to be appropriate.