Comparison between 1-methylphenylhydrazine and phenylhydrazine in the S1 state: Structure, energies and lifetime

Based on the theoretical and REMPI spectroscopic study, a comparison between 1-methylphenylhydrazine and phenylhydrazine in the excited S1 state is conducted to reveal the formation and influence of p–p–π conjugation. The ab initio and DFT calculations predict that phenylhydrazine and 1-methylphenylhydrazine respectively have three stable conformers in the S1 state. However, only the conformer with p–p–π conjugation for phenylhydrazine and the conformer with p–π conjugation for 1-methylphenylhydrazine in the S1 state can match the experimental results well. In the one color resonant two photon ionization (1C-R2PI) spectrum of 1-methylphenylhydrazine, the band origin of the S1 ← S0 transition (E1) is determined to be 33292 ± 3 cm−1, which is red-shifted by 318 cm−1 with respect to that of phenylhydrazine, but is just the same as that of N-methylaniline, showing no amino substitution effect on E1. While the S1 ← S0 transition energy of phenylhydrazine is red-shifted by 419 cm−1 with respect to that of aniline, showing a remarkable amino substitution effect on E1. For the S1 state, some benzene ring vibration frequencies of 1-methylphenylhydrazine are similar to those of N-methylaniline, but different from those of phenylhydrazine. With 2C-R2PI technique, the lifetimes of the S100 state are measured to be about 9 ± 6 ns for 1-methylphenylhydrazine and 15 ± 6 ns for phenylhydrazine. The structural difference between phenylhydrazine and 1-methylphenylhydrazine in the S1 state might account for their differences in transitional energy, vibrational frequencies and the lifetime of the S100 state.