1ST EXCITED-STATES OF PARA-(N,N-DIMETHYLAMINO)BENZONITRILE - A MOLECULAR-ORBITAL ANALYSIS

Following the molecular orbitals of p-(NN-dimethylamino)benzonitrile (DMABN) along the most important reaction pathway from the pi* <-- pi transition to the locally excited (LE) state and from there to the twisted intramolecular charge transfer (TICT) state, we gain substantial insight into the mechanism of the TICT phenomenon. In the ground state geometry the pi* orbital centred on the cyano group is too high in energy to be directly attainable in the S1 <-- S0 Franck-Condon (FC) excitation. As the FC geometry relaxes by stretching the C=N bond the energy of this orbital decreases rapidly. As a result its contribution to the developing LE state becomes important. Starting from the LE state, torsion of the dialkylamino group causes a change in the character of the ground state highest occupied molecular orbital (HOMO) from pi to an amino-group-centred, in-plane orbital which means that a TICT state has been formed. The resulting TICT emission probability is consistent with the experimentally observed thermal activation of the emission rate.