Photochemistry of Benzylallene: Ring-Closing Reactions to Form Naphthalene

Abstract

Conformer-specific, vibrationally resolved electronic spectroscopy of benzylallene (4-phenyl-1,2-butadiene) is presented along with a detailed analysis of the products formed via its ultraviolet photoexcitation. Benzylallene is the minor product of the recombination of benzyl and propargyl radicals. The mass-selective resonant two-photon ionization spectrum of benzylallene was recorded under jet-cooled conditions, with its S₀–S₁ origin at 37 483 cm^–1. UV–UV holeburning spectroscopy was used to show that only one conformer was present in the expansion. Rotational band contour analysis provided rotational constants and transition dipole moment direction consistent with a conformation in which the allene side chain is in the <i>anti</i> position, pointing away from the phenyl ring. The photochemistry of benzylallene was studied in a pump–probe geometry in which photoexcitation occurred by counter-propagating the expansion with a photoexcitation laser. The laser was timed to interact with the gas pulse in a short tube that extended the collisional region of the expansion. The products were cooled during expansion of the gas mixture into vacuum, before being interrogated using mass-selective resonant two-photon ionization. The UV–vis spectra of the photochemical products were compared to literature spectra for identification. Several wavelengths were chosen for photoexcitation, ranging from the S₀–S₁ origin transition (266.79 nm) to 193 nm. Comparison of the product spectral intensities as a function of photoexcitation wavelength provides information on the wavelength dependence of the product yields. Photoexcitation at 266.79 nm yielded five products (benzyl radical, benzylallenyl radical, 1-phenyl-1,3-butadiene, 1,2-dihydronaphthalene, and naphthalene), with naphthalene and benzylallenyl radicals dominant. At 193 nm, the benzylallenyl radical signal was greatly reduced in intensity, while three additional C₁₀H₈ isomeric products were observed. An extensive set of calculations of key stationary points on the ground state C₁₀H₁₀ and C₁₀H₉ potential energy surfaces were carried out at the DFT B3LYP/6-311G­(d,p) level of theory. Mechanisms for formation of the observed products are proposed based on these potential energy surfaces, constrained by the results of cursory studies of the photochemistry of 1-phenyl-1,3-butadiene and 4-phenyl-1-butyne. A role for tunneling on the excited state surface in the formation of naphthalene is suggested by studies of partially deuterated benzylallene, which blocked naphthalene formation