Electronic Spectroscopy of Protonated and Hydrogenated Coronene in
Solid Para-Hydrogen
Weber, I.,1* Lee, Y.-P.2
1iweber@nycu.edu.tw, Department of Applied Chemistry, National Yang-Ming Chiao-Tung
University, Hsinchu 300093, Taiwan.
2
Department of Applied Chemistry, National Yang-Ming Chiao-Tung University, Hsinchu
300093, Taiwan and Center for Emergent Functional Matter Science, National Yang-Ming
Chiao-Tung University, Hsinchu 300093, Taiwan.
Although well over 500 diffuse interstellar bands (DIB) have been identified up to today, their+carrier molecules remain mostly elusive: only the buckminsterfullerene cation C60
has been1
confirmed as the carrier of five DIB in the near IR. The most promising candidates considered
include polycyclic aromatic hydrocarbons (PAH) and their cationic, protonated andhydrogenated derivatives; however, low-temperature gas-phase electronic absorption spectra ofthese molecules are rarely available.
Solid para-hydrogen (para-H2) has frequently been employed as a matrix host to record IR
absorption spectra of protonated and hydrogenated PAH, which can be conveniently preparedby H+-transfer reaction with H3+ generated by electron bombardment during deposition. As aquantum solid, para-H2 is soft and can thus provide a homogenous environment leading to onlysmall matrix shifts. Over the past years, we have studied the dispersed fluorescence andfluorescence excitation spectra of several neutral, protonated, and hydrogenated PAH andPANH (NC < 42) and found consistent but comparably small red shifts (40–110 cm−1) withrespect to literature gas-phase spectra.
Here, we present the dispersed fluorescence and fluorescence excitation spectra of coronene(C24H12) and its most stable protonated (1-H+C24H12) and hydrogenated (1-HC24H12) isomers.
Upon deposition of C24H12 and para-H2 under electron bombardment, we observed two distinct
emission systems with strong origin bands at ~14335 and ~12820 cm−1, which we assign to
1-H+C24H12 and 1-HC24H12, respectively, based on lowest-energy transition energies predictedby TD-DFT calculations (716.51 nm for 1-H+C24H12 and 753.45 nm for 1-HC24H12) and thereasonable agreement of the experimental spectra with simulated emission spectra. For theS1–S0 transition of 1-H+C24H12, our data indicates a red shift of ~40 cm−1 relative to the gasphase 2 due to the para-H2 environment, consistent with our results for other PAH. Protonationand hydrogenation lead to a significant redshift compared to the parent C24H12 (S1–S0 at~23720 cm−1 in para-H2) and a shortening in radiative lifetimes. Detailed assignments for thedispersed fluorescence and fluorescence excitation spectra are derived, illustrating the impactof symmetry breaking due to hydrogenation and protonation, and a comparison to available
literature data from cryogenic ion-trap2 and Ne matrix isolation 3,4 experiments is provided. The
dispersed fluorescence spectrum of 1-HC24H12 has not been reported previously.
Campbell, E.K.; Holz, M.; Gerlich, D.; Maier, J. P. Laboratory confirmation of C60+ as the carrier of two
diffuse interstellar bands. Nature 2015, 523, 322.
2
Rice, C. A.; Hardy, F. X.; Gause, O.; Maier, J. P. (1) 1A'←X 1A' electronic transition of protonated coronene at
15 K. J. Chem. Phys. Lett. 2014, 5, 942.
3
Garkusha, I.; Fulara, J.; Sarre, P. J.; Maier, J. P. Electronic absorption spectra of protonated pyrene andcoronene in Ne matrices. J. Phys. Chem. A 2011, 115, 10972.
4
Garkusha, I.; Fulara, J.; Maier, J. P. Fluorescence of protonated pyrene and coronene in neon matrices. J. Mol.
Struct. 2012, 1025, 147.
1