Electronic Spectra of Cycl[3.3.2]azine and Related Compounds: Solvent Effect on Vibronic Couplings

Quantitative ab initio calculations are presented for the ultraviolet–visible peaks of cycl[3.2.2]­azine and its mono- and dibenzannulated polycyclic compounds at the multistate CASPT2 (MS-CASPT2) level of theory, with 11 nm deviation from the experimental S₀ → S₁ absorption. The electrophilic substitution reactions of cycl[3.2.2]­azine, benzo­[a]/[g]­annulated cycl[3.2.2]­azines, and 6-dimethylamino[2.2.3]­cyclazine-1-carboxylates with 3-cyano-4-methylthiomaleimide gave the corresponding functionalized cycl[3.2.2]­azine derivatives, which exhibited the absorption maxima around 510–630 nm. The first intense peaks were investigated by means of time-dependent density functional theory (TD-DFT). These peaks were systematically underevaluated by ∼50 nm, within the acceptable accuracies of TD-DFT. Furthermore, we calculated vibronic coupling constants of the electronic excited states of cycl[3.2.2]­azine and simulated absorption spectra both in vacuo and in ethanol. The solvent effect is found to enhance oscillator strengths and vibronic couplings. This is because the solvent effect gives rise to changes in the electron density difference on the phenyl ring, and in turn, the intensified overlap between the electron density difference and the potential derivative in the phenyl ring leads to enhanced vibronic couplings in ethanol.