Femtosecond dynamics and photoconversion of a H-bonded dye within mesoporous silicate materials

In this study, we report on the steady-state UV-visible absorption, reflectance, and emission spectra and femtosecond dynamics of 7-hydroxyquinoline (7HQ) interacting with purely siliceous (R-MCM41) and Al-doped (Al-MCM41) mesoporous silicate material in dichloromethane solution, as well as its photoconversion upon 320-nm irradiation. The results show that changes in the electrostatic field inside the nanochannels induced by Al ions significantly affect the ground- and excited-state photobehavior of the 7HQ structures H-bonded to the framework. For the Al-MCM41 sample, the obtained decay times (0.47-0.60 and 5.8-6.6 ps and a rise time of 3.3 ps) in the ultrafast dynamics are assigned to the formation and relaxation processes of anionic (A) and zwitterionic (Z) structures of 7HQ. These times are longer than those observed for the regular MCM-41 sample (0.26-0.32 and ∼3.5 ps and a rise time of 0.75 ps). The difference is explained in terms of different nonspecific (electrostatic) interactions of 7HQ with the nanohosts. In contrast, we found that the influence of the channel diameter (in the 25-40-Å range) is weak. Additionally, the 320-nm irradiation study showed that the main conversion route leads excited E to form A, which is stabilized in the ground state, and lowers the transformation into the Z and neutral keto (K) populations. For 7HQ/R-MCM41, the rate constants of the E → A → (Z, K) photoconversions were found to be 8.8 (±0.23) × 10 and 3.2 (±0.18) × 10 min, respectively. The K form is not produced when unstructured silica particles were used, indicating the effect of nanoconfinement on its formation and stability. We observed the same trend in the photoconversion of E to A and Z (K) in a solid-state sample, suggesting that the solvent does not play a key role in their formation. Our results demonstrate how the confinement and interactions of 7HQ with the MCM-41 nanomaterial affect the steady-state and time-resolved emission spectroscopy of a dye, as well as the equilibrium, stability, and photoconversion of its tautomeric and ionic structures. Such a rich photobehavior might be used in designing H-bonded guest:MCM-41 photoactive materials, such as light-emitting diodes, memories, and gates to cite a few possible applications in nanophotonics. © 2011 American Chemical Society.