A theoretical study on the red- and blue-shift hydrogen bonds of cis-trans formic acid dimer in excited states

The excited states of cis-trans formic acid dimer and its monomers have been investigated by time-dependent density functional theory (TDDFT) method. The formation of intermolecular hydrogen bonds O-1-H-1...O-2=C-2 and C-2-H-2...O-4=C-1 induces bond length lengthening of the groups related to the hydrogen bond, while that of the C-2-H-2 group is shortened. It is demonstrated that the red-shift hydrogen bond O-1-H-1...O-2=C-2 and blue-shift hydrogen bond C-2-H-2...O-4=C-1 are both weakened when excited to the S-1 state. Moreover, it is found that the groups related to the formation of red-shift hydrogen bond O-1-H-1...O-2=C-2 are both strengthened in the S1 state, while the groups related to the blue-shift hydrogen bond C-2-H-2...O-4=C-1 are both weakened. This will provide information for the photochemistry and photophysical study of red- and blue-shift hydrogen bond.