Electronic spectra and reversible photoisomerization of protonated naphthalenes in solid neon

Alpha- and beta-protonated naphthalenes (α- and β-HN(+)) were investigated by electronic absorption and fluorescence spectroscopies in 6 K neon matrixes using a mass-selected C(10)H(9)(+) ion beam. The absorption spectra reveal S(1)/S(2) ← S(0) transitions with onsets at 502.1 and 396.1 nm for α-HN(+), and 534.5 and 322.3 nm in the case of β-HN(+). Wavelength-dispersed fluorescence was detected for α-HN(+), starting at 504.4 nm. Light-induced α-HN(+) → β-HN(+) isomerization was observed upon S(2) ← S(0) excitation of α-HN(+), whereas β-HN(+) relaxed back into the more stable alpha form either upon excitation to S(1) or via thermal population of the ground state vibrational levels near the top of the energy barrier between the two isomers. The intramolecular proton transfer leading to the α-HN(+) ↔ β-HN(+) photoisomerization is fully reversible. The observations are explained with the support of theoretical calculations on the ground- and excited states of the isomers, vertical excitation and adiabatic energies, minimum-energy pathways along the relevant reaction coordinates, and conical intersections between the electronic states.