Infrared and Laser-Induced Fluorescence Spectra of Sumanene Isolated in Solid para-Hydrogen

The para-hydrogen (p-H₂) matrix-isolation technique has been scarcely used to record electronic absorption and emission spectra. It is expected that its small matrix shifts due to diminished molecular interactions and the softness of the lattice might be advantageous to help identify the carriers of the diffuse interstellar bands. In this article, we present infrared, fluorescence excitation, and dispersed fluorescence spectra of sumanene (C₂₁H₁₂), a bowl-shaped polycyclic aromatic hydrocarbon and a fragment of C₆₀, isolated in solid p-H₂. The recorded vibrational wavenumbers from infrared and dispersed fluorescence agree with the scaled harmonic vibrational wavenumbers calculated with the B3PW91/6-311++G(2d,2p) and B3LYP/6-311++G(2d,2p) methods. The recorded fluorescence excitation spectra are consistent with the spectra of jet-cooled gas-phase C₂₁H₁₂reported previously by Kunishige et al. We found a rather small matrix shift of 55 cm^-1for the S₁-S₀electronic transition origin located at 27 888 cm^-1. Vibrational wavenumbers associated with the S₁state of C₂₁H₁₂inferred from the experimental spectrum can be assigned mostly to fundamental normal modes; they are in satisfactory agreement with scaled harmonic vibrational wavenumbers calculated at the TD-B3PW91/6-311++G(2d,2p) level of theory. Significantly more vibrational modes of the S₁state were identified as compared with those in the reported gas-phase work. The potential of p-H₂matrix-isolation spectroscopy to provide electronic excitation spectra suitable for comparison to astronomical observations is discussed by comparing the spectra of C₂₁H₁₂isolated in solid p-H₂and in solid Ne, a matrix host commonly employed in astrochemistry.