Photodynamics of a Proton-Transfer Dye in Solutions and Confined Within NaX and NaY Zeolites.

We report on steady-state, picosecond and femtosecond time-resolved emission studies of 2-(2'-hydroxyphenyl)benzoxazole (HBO) in solutions and interacting with NaX and NaY zeolites. In solutions, an ultrafast (less than 150 fs) excited-state intramolecular proton-transfer (ESIPT) reaction takes place in syn-enol form, and leads to keto-type tautomer. We also observed a torsional motion in the keto form (~20 ps in dichloromethane, DCM). For NaX and NaY DCM suspensions, anionic forms interacting with the zeolites at S0 and S1 states are generated. They show two fluorescence lifetimes in both zeolites (720 ps and 2.4 ns for NaY and 960 ps and 2.7 ns for NaX), while those of the enol bonded to the zeolite framework and of the free keto forms are ~100 and 250 ps, respectively. The ultrafast dynamics of the anion in alkaline solutions reveals two deactivation pathways: an intramolecular charge transfer (ICT, 1.2 ps) and a twisting motion, affected by the viscosity of the solvent (12 and 20 ps for MeOH and ethylene glycol). When HBO is interacting with NaX and NaY the twisting motion is cancelled, while the ICT becomes slower as a result of a combination of several environment effects. HBO anions within the faujasite framework show also a ~ 30 ps decay associated to a non-fluorescent (n, π*) state. Our results demonstrate how intermolecular H-bonds, the confinement and the electrostatic interactions of HBO with the used materials, affect its ground as well as its excited state properties. Our findings add new knowledge on the interactions of silica-based nanomaterials containing the H-bonding guests.