Spectroscopic Characterization of Structural Isomers of Naphthalene: (E)- and (Z)-Phenylvinylacetylene

Near-pure samples of (E)-phenylvinylacetylene ((E)-PVA) and (Z)-phenylvinylacetylene ((Z)-PVA) were synthesized, and their ultraviolet spectroscopy was studied under jet-cooled conditions. The fluorescence excitation and UV−UV holeburning (UVHB) spectra of both isomers were recorded. The S₀−S₁ origin of (E)-PVA occurs at 33 578 cm⁻¹, whereas that for (Z)-PVA occurs at 33 838 cm⁻¹, 260 cm⁻¹ above that for (E)-PVA. The present study focuses primary attention on the vibronic spectroscopy of (E)-PVA. Single vibronic level fluorescence spectra of many prominent bands in the first 1200 cm⁻¹ of the S₀−S₁ excitation spectrum of (E)-PVA were recorded, including several hot bands involving low-frequency out-of-plane vibrations. Much of the ground-state vibronic structure observed in these spectra was assigned by comparison with styrene and trans-β-methylstyrene, assisted by calculations at the DFT B3LYP/6-311++G(d,p) level of theory. Both S₀ and S₁ states of (E)-PVA are shown to be planar, with intensity appearing only in even overtones of out-of-plane vibrations. Due to its longer conjugated side chain compared with that of its parent styrene, (E)-PVA supports extensive Duschinsky mixing among the four lowest-frequency out-of-plane modes (ν₄₅−ν₄₈), increasing the complexity of this mixing relative to that of styrene. Identification of the v′′ = 0−3 levels of ν₄₈, the lowest frequency torsion, provided a means of determining the 1D torsional potential for hindered rotation about the C_ph−C_vinyl bond. Vibronic transitions due to (Z)-PVA were first identified as small vibronic bands that did not appear in the UVHB spectrum recorded with the hole-burn laser fixed on the S₀−S₁ origin of (E)-PVA. The LIF and UVHB spectra of a synthesized sample of (Z)-PVA confirmed this assignment.