Ultrafast photoinduced charge transfer in fluorinated derivatives of DMABN

Dimethylaminobenzonitrile (DMABN) is the prototype system of molecules that form a Twisted Intramolecular Charge Transfer (TICT) state after optical excitation. TICT states are revealed by their often considerably, solvent-polarity-induced, red-shifted emission with mostly forbidden character due to the twisted conformation of the emitting species. The energetics of TICT states are governed by the donor and acceptor strengths of the moieties on both sides of the twisted bond. In principle, with sufficiently strong donors and acceptors, the TICT state energy can be lowered very far below that of the spectroscopically relevant state in absorption, that is, the locally excited (LE) state. Fluorine is a strong acceptor, which exerts relatively little sterical hindrance. Its introduction into DMABN is expected to lower the TICT state energy considerably and thus to increase its formation rate. This chapter employs time resolved absorption spectroscopy in order to check the charge transfer nature of the emitting state and tentatively measure its formation rate.