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

Phenetole derivatives with dual ethoxy substituents exhibit rich conformational diversity and complex vibronic characteristics, making them important model compounds for understanding substituent effects on molecular structure and spectroscopy. In this work, we systematically investigated the stable rotamers, vibronic spectra, and cationic ground-state features of p-diethoxybenzene (PDEB) using resonance-enhanced multiphoton ionization (REMPI), UV-UV hole-burning (HB), and mass-analyzed threshold ionization (MATI) spectroscopies, combined with density functional theory (DFT) calculations. The ground-state potential energy surface (PES) of PDEB was calculated at the B3LYP/6-311++G(d,p) level, identifying eight rotamers with distinct statistical weights and relative energies. Hole-burning spectroscopy resolved two dominant rotamers (cis/up-up and trans/up-down) in the supersonic molecular beam, with their S₁←S₀ transition origins determined as 33,824 cm^-1 and 33,613 cm^-1, respectively. Franck-Condon simulations of the vibronic transitions showed excellent agreement with the experimental REMPI spectra, enabling precise assignment of substituent and benzene ring vibrational modes. MATI experiments yielded accurate adiabatic ionization energies (AIEs) of the cis and trans rotamers as 59,629 ± 5 cm^-1 and 59,432 ± 5 cm^-1, respectively, and identified active cationic vibrational modes in the D₀ state. Geometric parameters of PDEB in the S₀, S₁, and D₀ states were calculated at the B3PW91/aug-cc-pVTZ, TD-B3PW91/aug-cc-pVTZ, and UB3PW91/aug-cc-pVTZ levels, revealing structural evolution during electronic excitation and ionization. The effects of ethoxy substituent orientation on molecular energy, vibrational frequencies, and ionization energy are discussed, and differences in spectral characteristics between PDEB and its meta isomer (MDEB) are compared. This work provides a comprehensive spectral and structural database for p-diethoxybenzene and deepens the understanding of structure-property relationships in diethoxybenzene isomers.