Exploring the photobehavior of nanocaged monomers and H- and J-aggregates of a proton-transfer dye within NaX and NaY zeolites

Abstract

We report on steady-state absorption and emission and time-resolved emission studies of (E)-2-(2-hydroxybenzyliden)amino-4-nitrophenol (HBA-4NP) interacting within NaX and NaY zeolites in dichloromethane (DCM) suspensions. In pure DCM, the enol (E) structure, which is the only one at S0, undergoes an excited-state intramolecular proton-transfer (ESIPT) reaction at S1 to produce a keto (K) type phototautomer emitting a largely Stokes shifted emission band with a short lifetime (14 ps) due to a twisting motion enhancing the radiationless decay. Upon interaction with NaX and NaY frameworks, different loading yields (NaX: 20%, NaY: 90%) were obtained due to the different amounts of aluminum atoms in the related frameworks and different amounts of sodium cations within their cages. The composites contain caged HBA-4NP E structures in the forms of monomers and H- and J-aggregates. The spectral broadening and shift reflect the emissions of the tautomers of the different electronic species which reflect the confinement effect of the zeolite supercages on their relaxation. The fluorescence lifetimes of K produced from caged monomers within NaX and NaY are remarkably very long (about 6 ns) when compared to that in solution (14 ps) due to the confinement effect on the radiationless pathways. For the composites made from diluted DCM solutions, those of H-aggregates are around 100 ps, while those of J-types are around 1 ns, respectively. Increasing the loading leads to a strong shortening in the K monomers and aggregate emission lifetimes due to enhanced guest:guest interactions within the same cage or between guests located in neighboring cages. Our results show the first observation of three absorbing and emitting structures (monomers and H- and J-aggregates) of intramolecular H-bonded molecules within zeolites, able to undergo ESIPT reactions to produce a broad emission with lifetimes ranging from tens of picoseconds to several nanoseconds. The data give new insights into the zeolite:dye complexes nature and thus can help in the design of new nanophotonics devices (nanosensors, nanolasers) based on this kind of material and in a better understanding of nanocatalysis and drug delivery using zeolites as supports and vehicles. © 2014 American Chemical Society.