Rotamer-Resolved Vibronic and Cationic Properties of m-Aminostyrene: A Combined 2C-REMPI, Hole-Burning, and MATI Study

m-Aminostyrene (MAS) is a key molecular scaffold with an electron-donating amino group and a conjugating vinyl group, exhibiting significant potential in photonic materials and biological applications due to its rotamerism and photoinduced behavior. Despite its importance, a comprehensive, rotamer-resolved investigation of its vibronic and cationic spectroscopic properties is lacking. Here, we report a high-resolution study on the cis and trans rotamers of jet-cooled MAS using two-color resonant enhanced multi-photon ionization (2C-REMPI), UV-UV hole-burning (HB), and mass-analyzed threshold ionization (MATI) spectroscopies, combined with density functional theory (DFT) calculations. The HB technique unambiguously resolves the vibronic spectra of each rotamer, overcoming the limitations of previous one-color REMPI studies. The excitation energies (S1 ← S0) are determined to be 30,416 cm−1 (cis) and 30,932 cm−1 (trans). The MATI spectra yield precise adiabatic ionization energies (AIEs) of 61,569 cm−1 (cis) and 61,274 cm−1 (trans). A comprehensive assignment of vibrational modes in both the S1 and D0 states is provided, revealing distinct mode activities and frequency shifts between the two rotamers. A propensity for Δν = 0 upon ionization is observed, indicating high geometrical similarity between the S1 and D0 states. This work provides a crucial spectroscopic blueprint for understanding the electronic and vibrational structure of MAS rotamers, with implications for the design of functionalized styrene-based molecular systems.