Protonated aromatic

This project aims at the characterization of structure, dynamics, and electronic properties of protonated molecules, in particular aromatic ones. While structure elucidation will be carried out using infrared (IR) spectroscopy, electronic structure as well as fragmentation and relaxation dynamics induced by electronic excitation will be using ultraviolet (UV) spectroscopy involving nanosecond, fast picosecond and ultrafast femtosecond lasers. Protonated molecules can be detected by in situ mass spectrometry techniques but not from outside through their optical properties due to the complete absence of information on their spectroscopy. The knowledge of their spectroscopy and optical properties would therfore allow their detection in inaccessible universe region.

 
State-of-the-art in the field

 
Protonated hydrocarbon molecules, in the following denoted AH+, constitute a fundamental class of organic molecules. They play a role as short-lived intermediates in a broad range of environments, ranging from astrochemistry, jet engine gas exhaust  and organic chemistry to biophysics. For example, AH+ are widely accepted as intermediates in electrophilic aromatic substitution reactions (s complexes), the most important reaction mechanism of aromatic molecules In addition, AH+ ions have been detected in various hydrocarbon plasmas (e.g., flame combustion) and are invoked to be present in interstellar space (responsible of the unidentified IR emission bands observed in various interstellar media). In addition, the effects of protonation of aromatic biomolecular building blocks is an interesting issue for models rationalizing the UV photostability of biological macromolecules, such as proteins and DNA.

 Despite their importance, surprisingly very little is known (only one paper in 1977) about the geometric and electronic structure, reactivity, and dynamics of even simple isolated AH⁺ ions, mainly because of the difficulties encountered in the production of high concentrations of these reactive species in gas phase, which are required to probe their properties by spectroscopy. Recent advances in the development of efficient ion sources and sensitive IR spectroscopic detection and ion trapping techniques have allowed substantial progress in the characterization of the geometric structure of isolated and microsolvated AH⁺ ions in the gas phase.. At the same time careful ion bunching of an electrospray source has allowed the first excited state lifetimes measurements.

	The PAH are obviously the systems that we should study. Indeed PAH are compounds of broad interest in fields ranging from combustion and environmental studies to interstellar carbon chemistry. Their protonated forms may also be present in numerous environments (atmosphere, interstellar media). How different are the UV/visible spectra of protonated PAH from that of radical PAH cations which are open-shell structure, whereas the protonated species are closed-shell structures? As attachment of highly abundant H atoms to PAH+ was measured to be fast, the efficient formation of (PAH)H+ in the interstellar medium has been hypothesized So far, no comparison of the DIBs with electronic spectra of (PAH)H+ could be made, as the required laboratory spectra are lacking. However, by comparison with the isoelectronic, closed-shell neutral PAH molecules, small (PAH)H+ molecules were not expected to have low-lying electronic states giving rise to absorption in the visible range. We very recently investigated the photodissociation spectrum of protonated naphthalene, which is the first electronic spectrum of an isolated (PAH)H+ ever observed. The studied transition indeed occurs in the visible region (~500 nm), demonstrating that small and also larger (PAH)H+ molecules are potential Diffuse Interstellar Band carriers. For larger (PAH)H+, the question of their stability and of their fragmentation mechanisms is totally open. One can expect their study to be more difficult since the electronic excitation will  probably be in the visible.