MS-CASPT2 Assignment of the UV/Vis Absorption Spectrum of Diazoquinones Undergoing the Photoinduced Wolff Rearrangement

The absorption spectra of o-diazobenzoquinone (DBQ), o-diazonaphthoquinone (DNQ) and o-diazonaphthoquinone-5-sulfonic acid (DNQSH) are computed at the MS-CASPT2 level to assign the experimental UV/vis spectra. These compounds undergo the photoinduced Wolff rearrangement, which is applied in the fabrication of photoresists. The lowest energy broadband around 400 nm corresponds to excitation to the lowest pi pi* state and has a vibrational structure mainly due to the activity of a ring bond inversion and a diazo bending mode. The remaining bands of the spectra arise from pi pi* states, while the states involving the oxygen lone pairs and the in-plane pi orbitals of the diazo group have low oscillator strengths, The lowest pi pi* state has a nonplanar minimum characterized by an out-of-plane bending of the diazo group and a stretching of the C-N bond. While recent experiments point to an ultrafast, concerted reaction, our results suggest that the process may be asynchronous, and the initial phase dominated by nitrogen elimination.