Promotion of intramolecular charge transfer in dimethylamino derivatives: twisting versus acceptor-group rehybridization

Ab initio electronic structure calculations have been performed to characterize the charge-transfer process in 4-dimethylamino-benzonitrile (DMABN) and in 4-dimethylamino-benzethyne (DMBAE). The HF, CIS, CASSCF and CASPT2 methods have been employed. Geometry optimization of the charge-transfer (CT) state at the CIS level predicts two local minima on the potential energy surface of the S1 state: one with twisted dimethylamino group and the second with in-plane bent CN (or CCH) group due to rehybridization of the cyano (or acetyleno) carbon atom(s). The two CT states differ in the electron-donor and electron-acceptor character of particular molecular moieties. The results suggest that the mechanisms may compete with each other in the promotion of the CT emission in DMABN, while in DMABE the rehybridization of the CCH group is strongly preferred over the twisting of the dimethylamino group.