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.

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.

Rotational isomers (rotamers) of substituted aromatic molecules exhibit distinct physicochemical properties that are fundamental to understanding their reactivity and biological functions. However, resolving individual rotamers spectroscopically remains challenging due to their similar transition energies and overlapping spectral features. Herein, we report the conformer-specific identification and characterization of three stable rotamers of m-ethoxyphenol using a combination of resonance-enhanced multiphoton ionization (REMPI), hole-burning (HB) spectroscopy, and mass-analyzed threshold ionization (MATI) techniques, complemented by high-level quantum chemical calculations at the B3PW91/aug-cc-pVTZ and G4 levels of theory. The S₁ ← S₀ electronic origins of rotamers I, IV, and III were determined to be 35,966 ± 2, 36,031 ± 2, and 36,198 ± 2 cm⁻¹, respectively, while their corresponding adiabatic ionization energies (IEs) were precisely measured as 64,574 ± 5, 64,122 ± 5, and 64,994 ± 5 cm⁻¹. The vibrational spectra of both the S₁ excited state and the D₀ cationic ground state were assigned, with most active modes corresponding to in-plane benzene ring vibrations. Structural analysis reveals that the benzene ring undergoes slight expansion upon S₁ ← S₀ excitation and contraction upon D₀ ← S₁ ionization, while the overall molecular geometry remains remarkably similar across all three electronic states, a key factor underlying the excellent agreement between experimental and simulated Franck–Condon spectra. Comparison with m-methoxyphenol demonstrates that the stronger electron-donating ability of the ethoxy group leads to systematically lower excitation and ionization energies. The distinct spectroscopic fingerprints established herein provide a definitive reference for identifying specific m-ethoxyphenol rotamers in future studies of this molecule and its complexes.

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.

We report a comprehensive spectroscopic investigation of p-diethynylbenzene (pDEB) using two-color resonance-enhanced multiphoton ionization (REMPI) and mass-analyzed threshold ionization (MATI) spectroscopy, complemented by density functional theory (DFT) calculations. The S₁ ← S₀ electronic origin is observed at 34,255 ± 2 cm⁻¹. The adiabatic ionization energy (IE) is determined to be 69,095 ± 5 cm⁻¹ from two-color MATI spectra recorded via the S₁ origin. Notably, a narrow peak is observed at 32 cm⁻¹ above the IE in the two-color MATI spectrum, which is assigned to a one-color, two-photon accidental resonance arising from the near-resonant condition where the S₁ ← S₀ transition energy (34,255 cm⁻¹) is close to half of the IE (69,099/2 = 34,549.5 cm⁻¹). This observation is consistent with similar reports for p-chlorofluorobenzene and p-difluorobenzene. Franck–Condon simulations show good agreement with the experimental spectra. The present results provide a spectroscopic basis for identifying pDEB and distinguishing it from its ortho and meta isomers.