The Excited State Relaxation Path of N,N-Diethyl-5-cyanopyridine and N,N-Diethylbenzaldehyde

The molecules which possess, as electron donors, dialkylamino groups linked by a single bond with the acceptor subunits undergo fast excited state intramolecular charge transfer (ICT), emitting dual fluorescence in polar solvents. The short and the long-wave length fluorescence bands are traditionally assigned as Fb and Fa, respectively. The nature of the excited state from which the Fa emission originates, as well as the mechanism of energy degradation have been the subject of many controversies. As the objects of this study, 2-(N,N-di ethyl)-5-cyanopyridine (PEC) and p-N,N-diethylaminoacetophenone (DEABA) were selected. For these molecules different sequence of the two low-lying pi pi* La and Lb states was postulated, which can help to verify the excited state mechanisms of the charge separation. The experimental results obtained by stationary and time-resolved spectroscopy show that the anomalous Fa emission originates from the ICT state generated by the mutual twist of the donor and acceptor group (TICT model).