Long-Lived Isomers of C11H9+: New Experimental Insights from the PIRENEA Setup

The dehydrogenated cation of methylated benzene is known to exist in two isomeric forms: benzylium and tropylium. Structurally similar forms have been proposed for the –H cations of methylated polycyclic aromatic hydrocarbons, but their spectroscopic characterization remains limited, and their photophysical properties are still poorly understood. Previous studies identified 2-naphthylmethylium and benzyltropylium as specific long-lived isomers of the –H fragment of methylnaphthalene cations. Here, we investigate the photodissociation spectroscopy and photoprocessing of gas-phase C₁₁H₉⁺ ions in the visible range. Experiments are conducted using the versatile laboratory astrophysics setup PIRENEA, which enables studies over long timescales (∼1000 s) and allows photoprocessing to be combined with ion–molecule reaction experiments. We confirm the presence of 2-naphthylmethylium and benzyltropylium and additionally identify 1-naphthylmethylium, previously undetected in experiments. Moreover, we present the first complete quantitative analysis of the relative abundances of these isomers and provide clear evidence of interconversion among the three long-lived species. These isomerization processes occur below the dissociation threshold, a finding supported by molecular dynamics simulations. The photophysical properties of C₁₁H₉⁺ isomers─including isomerization and fluorescence─make them intriguing candidates for consideration in astrophysical environments exposed to mild UV irradiation (hν < 7 eV). Moreover, their detection via rotational spectroscopy in such regions is facilitated by their closed-shell electronic structure.