Study of Vibronic and Cationic Features of m-Diethoxybenzene via REMPI, Hole-Burning, and MATI Spectroscopy

Phenetole derivatives are widely used in various fields, and the presence of the ethoxy group, with its longer alkyl chain, suggests the possibility of multiple rotamers. In particular, diethoxybenzene derivatives, containing two ethoxy groups, exhibit an even greater number of rotameric forms. In this study, we report the first investigation of the stable structures, vibronic spectra, and cationic spectra of different rotamers of m-diethoxybenzene (MDEB). Resonance-enhanced multiphoton ionization (REMPI) spectra of the rotamers were identified via hole-burning (HB) experiments, while mass-analyzed threshold ionization (MATI) spectra provided precise adiabatic ionization energies (IEs) of the observed rotamers, as well as the active vibrations of their corresponding cations. Density functional theory (DFT) calculations predicted thirteen rotamers of MDEB, but only two rotamers were observed in the supersonic molecular beam. The band origins of the S₁ ← S₀ transition and the adiabatic IEs of the down-up and down-down rotamers of MDEB were determined to be 36,091 ± 2 cm^-1 and 36,165 ± 2 cm^-1 and 62,419 ± 5 cm^-1 and 63,378 ± 5 cm^-1, respectively. Franck-Condon spectral simulations for the S₁ ← S₀ and D₀ ← S₁ transitions were performed based on DFT calculations, and the theoretical results showed good agreement with the experimental data. Vibrational features observed in the S₁ and D₀ states were assigned by comparing the experimentally measured spectra with the simulated spectra as well as the previously reported vibrational spectra of structurally similar molecules. Finally, several key findings and molecular characteristics are discussed in detail.