Electronic spectra of two long-lived photoproducts: double-proton transfer in 7-hydroxyquinoline dimer in a 2-methyltetrahydrofuran glass matrix.

Photoreactions of 7-hydroxyquinoline (7-HQ) in low-temperature (77-100 K) 2-methyltetrahydrofuran glass matrices are investigated using electronic spectroscopy. We have observed fluorescence excitation and fluorescence spectra of two long-lived species generated by irradiation of UV light (230-400 nm). The dominant species responsible for the fluorescence spectrum between 470 and 600 nm was assigned to the S(1)-->S(0) (pipi*) transition of the keto form of cyclic 7-HQ dimer [(7-HQ)(2)] produced by excited-state double-proton transfer, the corresponding S(1)-S(0) fluorescence excitation spectrum of which was detected between 360 and 510 nm. Temperature dependence of the fluorescence excitation spectra showed the occurrence of keto --> enol isomerization in the S(0) state of (7-HQ)(2) due to a back double-proton transfer. A very slow rate for the keto --> enol isomerization implies that the potential barrier height for the back double-proton transfer reacion is substantially high. Theoretical calculations at the MP2/aug-cc-pVDZ level of theory indicate that the enol and keto forms of cyclic (7-HQ)(2) are nonplanar, therefore a large change in the geometry is necessary for the back double-proton transfer. A second long-lived species that emits between 410 and 600 nm has been tentatively assigned to the D(3)(2A'') --> D(0)(1A'') transition of the 7-quinolinoxyl radical on the basis of calculated electronic transition energies for possible candidates obtained by MS-CASPT2/aug-cc-pVDZ level calculations as well as IR study of 7-HQ in argon matrices [Sekine, M.; Nagai, Y.; Sekiya, H.; Nakata, M. J. Phys. Chem. A 2009, 113, 8286]. Photoreaction processes leading to the two long-lived species have been discussed.