Effects of intermolecular hydrogen bonding on the excited-state proton transfer properties of the cinnamonitrile–methanol complex

The time-dependent density functional theory (TD-DFT) method was used to study the excited-state proton transfer (ESPT) properties of the hydrogen-bonded cinnamonitrile (3TPAN)–methanol (MeOH) complex (3TPAN–MeOH). The intermolecular hydrogen bonds N1···H11 in both the ground state S0 and the excited state S1 were demonstrated by the optimized geometric structures of the hydrogen-bonded 3TPAN–MeOH complex. While in the excited state S3, a new hydrogen bond H11···O1 was formed after the ESPT took place from the hydrogen-bonded MeOH molecule to the 3TPAN moiety. It was demonstrated that the electronic transitions of the S1 states for both the 3TPAN monomer (including the S3 state) and the hydrogen-bonded 3TPAN–MeOH complex should be of a localized-excited (LE) nature on the 3TPAN molecule, while the S3 state of the hydrogen-bonded 3TPAN–MeOH complex should be of charge transfer (CT) character from the hydrogen-bonded MeOH molecule (through O1···H11) to the 3TPAN moiety. The S3-state proton transfer and charge transfer due to the intermolecular hydrogen-bonding interaction should be the reasons for the remarkable redshift (0.91 eV) of the S3-state electronic energy for the hydrogen-bonded 3TPAN–MeOH complex compared with that of the 3TPAN monomer.