Photo-induced intramolecular charge transfer and internal conversion in molecules with a small energy gap between S1 and S2. Dynamics and structure

A discussion is presented of the fast excited state intramolecular charge transfer (ICT) and dual fluorescence observed with several 4-aminobenzonitriles. The occurrence or absence of this ICT reaction is shown to be determined by the magnitude of the energy gap ΔE(S1,S2) between the two lowest excited states. The strong decrease of the ICT efficiency in the series P8C to P3C of 4-aminobenzonitriles in which the amino nitrogen atom is part of a three- to eight-membered heterocyclic ring, is attributed to the increase with decreasing ring size of the barrier for the configurational change of the amino nitrogen from pyramidal to planar, which is considered to be an important reaction coordinate in the ICT process. Dual fluorescence does not occur with P3C nor with NC in diethyl ether. Application of a pressure of 6000 bar leads to a threefold increase of the CT/LE fluorescence quantum yield ratio for the 4(dialkylamino)benzonitrile DMABN (methyl) in diethyl ether, whereas this ratio only slightly increases for DDABN (n-decyl) in n-hexane. This difference is attributed to the increase of the solvent dielectric constant with pressure, which effect is much larger for diethyl ether than for n-hexane. These results mean that a large amplitude motion does not occur during the ICT reaction of DMABN and DDABN, in accordance with the model of planar intramolecular charge transfer for dual fluorescence. Dual fluorescence is observed with the rigidized aminobenzonitrile NMC7, showing that a full perpendicular twist of the amino group is not essential for the CT state. I -(Dimethylamino) naphthalene ( I DMAN) undergoes a fast thermally activated internal conversion (IC), for which the activation energy increases with solvent polarity. This IC reaction depends on the magnitude of the ΔE(S1,S2) energy gap and is similar in this respect to the ICT reaction of DMABN.