Article pubs.acs.org/JPCC
Exploring the Photobehavior of Nanocaged Monomers and H- and J‑Aggregates of a Proton-Transfer Dye within NaX and NaY Zeolites Noemí Alarcos,† Juan Angel Organero,† Félix Sánchez,‡ and Abderrazzak Douhal*,† †
Departamento de Química Física, Facultad de Ciencias Ambientales y Bioquímica, and INAMOL, Universidad de Castilla-La Mancha, Avenida Carlos III, S.N., 45071 Toledo, Spain ‡ Instituto de Química Orgánica, CSIC, Juan de la Cierva, 3, 28006 Madrid, Spain S Supporting Information *
ABSTRACT: We report on steady-state absorption and emission and timeresolved 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 Jaggregates. 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.
1. INTRODUCTION Zeolites are aluminosilicate materials possessing nanometer void spaces in the form of cages and/or channels.1−6 They are used as host systems for diverse organic molecules in a variety of important industrial applications, such as drying agents, molecular sieves, heterogeneous catalysts, and chemical sensors,7−17 while they allow for the changing and controlling of the photochemical behavior of the confined guests.18,19 One of the most intensively used and studied zeolite groups is the family of the synthetic faujasites X and Y. They form cages of ∼13 Å in diameter interconnected by four windows of 8 Å. Both types (X and Y) differ in Si/Al framework composition [Si/Al(X) = 1.24, Si/Al(Y) = 2.86], which leads to a difference in amount of Na+ cation content and in polarity of internal spaces. The composition, as well as the structure of the faujasite framework, can affect the emission properties of the guest molecule, as has been shown for a variety of molecules, such as nile red, salicylaldehyde azine (SAA), or Sudan I.20−23 Many organic dyes tend to form aggregates even © 2014 American Chemical Society
at low concentrations. This process was observed both in solution and in inorganic frameworks including zeolites, mesoporous silica, and nanocrystalline films. Both H- (faceto-face) and J-type (face-to-edge) aggregates of several dyes have been reported in aluminosilicate materials.24−27 For example, the formation of J-aggregates of pyronine within the L zeolite at different loadings was reported, and a shortening of emission lifetimes at larger dye loading was observed.24 The influence of the type of cation and coadsorbed solvent on the pyrene excimers formation into the faujasite zeolites has also been reported.26,27 The host materials can influence the aggregation process, leading to a preferential H- or J-aggregate types, or can shift the equilibrium toward either the monomers or the aggregates.28,29 Received: December 22, 2013 Revised: February 12, 2014 Published: February 14, 2014 8217
dx.doi.org/10.1021/jp412544y | J. Phys. Chem. C 2014, 118, 8217−8226
The Journal of Physical Chemistry C
Article
Scheme 1. (A) Molecular Structures of Different Forms of (E)-2-(2-Hydroxybenzyliden)amino-4-nitrophenol (HBA-4NP) and (B) Basic Structural Unit of X and Y Zeolites, Having Pores and Supercage Diameters of 8 and 13 Å, Respectively
and ∼1 ns for H- and J-types, respectively) are shorter than that of monomers (∼6 ns) due to excitonic (face-to-face or face-toedge) interactions in the former. The novelty of our study is the observation of caged monomers and H- and J-type aggregates of a proton-transfer dye within zeolites, able to exhibit protontransfer and excitonic interactions allowing a broad band and picosecond−nanosecond emission which we showed can be controlled by loading in space and time domains.
Photochromic aromatic Schiff bases with an intramolecular hydrogen bond (IHB) belonging to the family of salicylideneaniline (SA) have attracted much interest because of possible applications in molecular memories and switches30 and nonlinear optics.31−33 SA and its derivatives display an excited-state intramolecular proton-transfer (ESIPT) reaction along the IHB of the o-hydroxyl group to the imine nitrogen to form a cis-keto product in the femtosecond (fs) regime and which could isomerize to a weakly fluorescent trans-keto tautomer.34,35 Modified chemical (electronic) structures of SA have been reported, and their studies showed that the activation energy of the cis−trans isomerization in solution depends on the involved electron-donor strength substituent.36 Other studies of SA into faujasite zeolites have reported that its enol form is not preferentially stabilized within the cages in favor of a proton-transferred structure having a zwitterionic character.37,38 Time-resolved spectroscopy provides useful tools to study such systems formed by the interaction of molecules with zeolites. In previous reports from this group, we have shown that inclusion of Sudan I into MX (M = Na+, Li+, Mg2+), NaY zeolites, and MCM-41 materials modifies the photodynamics of the formed caged tautomers. More recently, we reported on how the interaction of salicylaldehyde azine (SAA) within the cages of the NaX zeolite modifies its ground- and excited-state properties.21 The emission lifetimes are significantly longer in NaX than in dichloromethane (DCM) solution and showed a strong dependence on the concentration of the dye within the zeolites, indicating aggregate formation of this dye within the NaX.21 Here, we continue our efforts for a better understanding of zeolite:dye interactions, by reporting on ground and electronically excited states studies of (E)-2-(2-hydroxybenzyliden)amino-4-nitrophenol (HBA-4NP) interacting with NaX and NaY zeolites (Scheme 1) in DCM suspensions. We observed UV−visible absorption/diffuse transmittance, emission and fluorescence lifetimes of caged monomers, and H- and Jaggregated structures when interacting with these hosts. The amount and nature of the aggregates depend on the dye loading (percent of the complexed guest relative to the initial dye solution). For NaY the loading is 90%, while for NaX it is only 20%. While in solution the tautomer emission is largely stokesshifted, having a single 14 ps decay, caged monomers and Hand J-aggregates exhibit ESIPT reactions giving blue-shifted and broad emission bands. The lifetimes of the aggregates (∼100 ps
2. EXPERIMENTAL SECTION The synthesis39,40 of (E)-2-(2-hydroxybenzyliden)amino-4nitrophenol (HBA-4NP) was made by adding 2-hydroxybenzaldehyde (1.22 g, 10 mmol) in dry ethanol (10 mL) to an ice-cooled solution of 2-amino-4-nitrophenol (1.54 g, 10 mmol) in dry ethanol (20 mL) and 100 μL of formic acid. The reaction mixture was stirred for 10 min at 0 °C and 16 h at room temperature. The formed product, HBA-4NP, was filtered, washed with cold methanol, recrystallized from ethanol, and dried in vacuum. We obtained HBA-4NP (2.42 g, 94% yield) with a melting point of 239−240 °C (228−231 °C).41,42 Dichloromethane (DCM, spectroscopic grade, 99.9%) and NaX and NaY zeolites were purchased from Sigma-Aldrich and used as received. The steady-state fluorescence and absorption/ diffuse transmittance spectra of HBA-4NP in DCM suspensions have been recorded using FluoroMax-4 (Jobin-Yvone) and JASCO V-670 equipped with a 60 nm integrating sphere ISN723, respectively. The fluorescence quantum yield of HBA-4NP in DCM was calculated using Quinine Sulfate in 0.1 N H2SO4 solution, as a reference (Φ = 0.51 at 20 °C).43 The nature of the DCM suspensions (of HBA-4NP interacting with NaX and NaY particles) does not allow measurement of their fluorescent quantum yields. Time-resolved emission decays were measured using a time-correlated single-photon counting (TCSPC) system.44 The sample was excited by a 40 ps pulsed diode laser centered at 371 or 433 nm (
