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

The para -hydrogen ( p -H 2 ) 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 (C 21 H 12 ), a bowl-shaped polycyclic aromatic hydrocarbon and a fragment of C 60 , isolated in solid p -H 2 . 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 C 21 H 12 reported previously by Kunishige et al. We found a rather small matrix shift of 55 cm -1 for the S 1 -S 0 electronic transition origin located at 27 888 cm -1 . Vibrational wavenumbers associated with the S 1 state of C 21 H 12 inferred 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 S 1 state were identified as compared with those in the reported gas-phase work. The potential of p -H 2 matrix-isolation spectroscopy to provide electronic excitation spectra suitable for comparison to astronomical observations is discussed by comparing the spectra of C 21 H 12 isolated in solid p -H 2 and in solid Ne, a matrix host commonly employed in astrochemistry.