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 O1-H1...O2=C2 and C2-H2...O4=C1 induces bond length lengthening of the groups related to the hydrogen bond, while that of the C2-H2 group is shortened. It is demonstrated that the red-shift hydrogen bond O1-H1...O2=C2 and blue-shift hydrogen bond C2-H2...O4=C1 are both weakened when excited to the S1 state. Moreover, it is found that the groups related to the formation of red-shift hydrogen bond O1-H1...O2=C2 are both strengthened in the S1 state, while the groups related to the blue-shift hydrogen bond C2-H2...O4=C1 are both weakened. This will provide information for the photochemistry and photophysical study of red- and blue-shift hydrogen bond. <alternatives> [...] </alternatives>
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