Topology-Controlled Photophysics of Ionized C14H10 Isomers.

The optical signatures of polycyclic aromatic hydrocarbons (PAHs) are highly sensitive to their molecular geometry, charge state, and intermolecular interactions. A comparison between the linear anthracene and angular phenanthrene ions highlights how topological effects govern the degree of electronic delocalization in the excited state, resulting in spectroscopic signatures of vibronic coupling, varying oscillator strengths, and nonradiative relaxation pathways. This work presents the UV-vis spectra of the cationic, protonated, and anionic C14H10 isomers utilizing He-tagging action spectroscopy via helium nanodroplet isolation. The linear anthracene cations retain a well-resolved vibronic structure, characteristic of delocalized π-systems, whereas the angular phenanthrene cation produces broader bands typical of enhanced vibronic coupling and a more localized charge distribution. Hydrogenation disrupts conjugation and quenches the vibronic progression most strongly in anthracene. Notably, the broad, weakly structured spectrum of protonated phenanthrene is consistent with the pronounced excited-state twisting predicted by previous calculations. Anionic C14H10, on the other hand, displays weak continua, aligning with low oscillator strengths and rapid autodetachment. Dimerization generally broadens and weakens the absorption features, particularly for phenanthrene, consistent with the coexistence of several low-lying π-stacked isomers and enhanced vibronic coupling. Comparison with matrix isolation and gas-phase spectra supports the high fidelity of the helium nanodroplet approach.