Spectroscopy and relaxation dynamics of salicylideneaniline derivative aggregates encapsulated in MCM41 and SBA15 pores

Abstract

We report on steady-state and picosecond time-resolved emission studies of (E)-2-(2’-hydroxybenzyliden)amino-4-nitrophenol (HBA-4NP) interacting with MCM41, Al-MCM41 and SBA15. In these materials, the molecule shows spectral changes, with a broadening and shifts of absorption and emission bands, which depend on the concentration used in the preparation of the samples, indicating the aggregates formation. These caged monomers (M), H- and J-aggregates are in enol form at S0 and in keto one at S1, as a result of an excited-state intramolecular proton transfer (ESIPT) reaction. The K lifetime of the caged M is longer (3 ns) than the observed in solution (14 ps), due to the confinement effect on the radiationless pathways. The K lifetimes of J- and H-aggregates are 600 and 100 ps, respectively, and their populations and stability depend on pore size and chemical composition (in terms of Brönsted acidity) of the host. For MCM41 and Al-MCM41 ones, the formation of H- and J-aggregates is higher than using SBA15. This is due to a better encapsulation by the formers (85–95% MCM41/Al-MCM41 vs 60% SBA15). Doping MCM41 by Al allows more J-aggregates as result of strong interaction between the guest molecules and the Al-MCM41 framework. Thus, modulating the topological and structural properties of the mesoporous material leads to the formation of different encapsulated species, which could be of great interest to design nanophotonics devices based on this kind of materials.