Electronic spectroscopy of mass-selected protonated polycyclic aromatic hydrocarbons in neon matrices

This thesis describes the results of experimental studies on production and spectroscopic characterization of protonated polycyclic aromatic hydrocarbons, which represent a fundamental class of organic ions. They are of interest from an astrophysical as well as a fundamental point of view. It is believed that this class of PAH derivatives may be responsible for some of the Diffuse Interstellar and Unidentified Infrared emission bands and even for some of the Red Rectangle emission features. However, experimental data that are of direct relevance for the identification of interstellar or circumstellar protonated PAHs are rather scarce. Electronic spectra of isolated protonated PAHs in the gas phase or in cryogenic matrices are missing in the literature, mainly due to the experimental challenges in their production and spectral detection. The main part of this work was devoted therefore to the production and spectroscopic characterization of protonated PAHs.
Protonated PAHs have been produced in a hot-cathode discharge source and investigated by means of electronic absorption and fluorescence spectroscopy in 6 K neon matrices using a mass-selected ion beam. These molecules have been synthesized by two methods: ionization of dihydro-PAH precursors or through proton transfer reactions of parent PAHs with suitable proton donors such as simple alcohols.
The present investigations on protonated PAHs trapped in neon matrices provide a direct spectroscopic characterization of several structural isomers of these cations. Thus, for example, electronic transitions of protonated benzene and alpha-protonated fulvene, of alpha- and beta-protonated naphthalenes and 2-indenylmethylium have been detected and subsequently identified. For these, smaller protonated PAHs and their structural isomers a photoinduced isomerization processes have been observed and stimulated their extended studies. As for the naphthalenes, the intermolecular proton transfer leading to the alpha – beta photoisomerization is of reversible character. Besides these, electronic absorption and fluorescence spectra of several larger protonated PAHs – protonated anthracene, phenanthrene, pyrene, corannulene and coronene – have been recorded. Their interpretation is based on the experimental observations and supported by computational data. For the majority of protonated species the correspondent neutrals have been also detected and characterized.
Protonated pyrene and coronene, which are often considered as a model of PAHs in the discussions of their role in the ISM, are the largest species studied during this work. Three electronic transitions of the most stable isomer of protonated pyrene and four of protonated coronene have been observed for the first time. The strongest, S1– S0 transitions, are in the visible region, with onset at 487.5 nm for protonated pyrene and 695.6 nm for protonated coronene. The fluorescence spectra have been also recorded. The strongest, visible absorption and fluorescence bands of cations were compared with astronomical spectra for evaluating their relevance.
Despite the great progress made in field of high-resolution gas-phase spectroscopy, matrix isolation technique in combination with mass selection remains a powerful tool for producing and investigating spectroscopic properties of free radicals and ions. It has proven to be successfully applicable to a large variety of species, differing in physical and chemical properties. The results of the present matrix studies provide information on the structure of protonated PAHs and other PAH-related species, their ground and excited states electronic properties, wavelengths and relative intensities of the bands of these species, which can be used as a starting point for gas-phase measurements and for evaluating their astrophysical relevance.