Vibronic and cation spectroscopy of structural isomers p- and m-diaminobenzene and the amino substitution effect

We applied the resonant two-photon ionization and mass-analyzed threshold ionization techniques to record the vibronic and cation spectra of p-diaminobenzene (PDAB) and m-diaminobenzene (MDAB). The band origins of the S1 ← S0 electronic transition of PDAB and MDAB were found to be 29 805 ± 2 and 33 395 ± 2 cm−1, whereas the adiabatic ionization energies were determined to be 54 624 ± 5 and 58 321 ± 5 cm−1, respectively. Most of the observed active vibrations of these two structural isomers in the electronically excited S1 and cationic ground D0 states result from in-plane ring deformation and bending vibrations. Comparing the present data with those of aniline suggests that the interaction of the amino substituent and aromatic ring in the upper (S1 or D0) state is stronger than that in the S0 state. In addition, the frequency of observed active vibrations may depend on the nature, relative location, vibrational pattern, and the extent of the amino group participated in the normal vibration.