Electronic Spectroscopy of 1‑(Phenylethynyl)naphthalene

Abstract

Recently 1-(phenylethynyl)­naphthalene (1-PEN) was suggested to be the primary dimerization product of phenylpropargyl radicals and therefore an important polycyclic hydrocarbon in combustion processes. Here we describe a spectroscopic investigation of a genuine 1-PEN sample by several complementary techniques, infrared spectroscopy, multiphoton ionization (MPI), and threshold photoelectron spectroscopy. The infrared spectrum recorded in a gas cell confirms that 1-PEN is indeed the previously observed dimerization product of phenylpropargyl. The origin of the transition into the electronically excited S₁ state lies at 30823 cm^–1, as found by MPI. Considerable vibrational activity is observed, and a number of low-wavenumber bands are assigned to a progression in the torsional motion. Values of 6 cm^–1 (S₀) and 17 cm^–1 (S₁) were derived for the fundamental of the torsion. In the investigated energy range the excited state lifetimes are in the nanosecond range. Spectra of the 1-PEN/Ar cluster exhibit a red shift of the electronic origin of 22 cm^–1, in good agreement with other aromatic molecules. A threshold photoelectron spectrum recorded using synchrotron radiation yields an ionization energy of 7.58 eV for 1-PEN. An excited electronic state of the cation is found at 7.76 eV, and dissociative photoionization does not set in below 15 eV