Acenaphthene Derivatives as Signatures of C$_{11}$H$_9^+$ Reactivity with Methylated Naphthalenes
C$_{11}$H$_9^+$ ion is the dominant fragment cation formed from methyl-naphthalene (MeNp) and dimethyl-naphthalene (diMeNp). Using the multiplex capabilities of PIRENEA, a setup dedicated to laboratory astrophysics, we studied the reactivity of the benzylium-like isomers of C$_{11}$H$_9^+$ with diMeNp under isolated conditions relevant to radiative association. Two reaction products are observed, C$_{12}$H$_{11}^+$ -- also formed in the reaction with MeNp -- and C$_{13}$H$_{13}^+$, with branching ratios that depend on the specific diMeNp isomer. The reaction products were subsequently exposed to UV-visible irradiation to gain insight into their structures. The acenaphthylene radical cation, C$_{12}$H$_{8}^{\bullet +}$, was identified as the most stable photofragment. We show that this experimental approach, supported by density functional theory calculations and molecular dynamics simulations, provides new constraints on the chemistry of benzylium-type species. We highlight the role that long-lived ion-molecule complexes can have in promoting C-C coupling and the formation of a pentagonal cycle. Moreover, the chemistry uncovered here highlights new pathways for the formation of pentagonal rings during PAH growth under low-pressure and cold conditions. In particular, it can lead to efficient formation of acenaphthylene-like species, recently detected in the TMC-1 cold cloud.