9554
J. Phys. Chem. C 2010, 114, 9554–9562
Confined Fast and Ultrafast Dynamics of a Photochromic Proton-Transfer Dye within a Zeolite Nanocage Michał Gil,† Marcin Ziółek,†,‡ Juan Angel Organero,† and Abderrazzak Douhal*,† Departamento de Quı´mica Fı´sica, Seccio´n de Quı´micas, Facultad de Ciencias del Medio Ambiente, and Inamol, UniVersidad de Castilla-La Mancha, AVda. Carlos III, S.N., 45071 Toledo, Spain, and Quantum Electronics Laboratory, Faculty of Physics, Adam Mickiewicz UniVersity, Umultowska 85, 61-614 Poznan, Poland ReceiVed: October 22, 2009; ReVised Manuscript ReceiVed: February 22, 2010
We report on studies of salicylaldehyde azine (SAA) dissolved in dichloromethane solution and within the cages of the faujasite zeolite (NaX) using steady-state and femtosecond to nanosecond time-resolved spectroscopy. In solution, an excited-state intramolecular proton-transfer reaction takes place in less than 80 fs, leading to a keto-type tautomer. In contrast within NaX zeolite, a zwitterionic (Z) form is present both at S0 and S1 states, and a large hypsochromic shift of the stationary emission spectrum is observed. The increase in fluorescence lifetime upon encapsulation (from 54 ps to 0.2-2.8 ns) is mainly due to hindrance in twisting motion of the confined Z structure imposed by the nanocage. A significant dependence of the lifetimes on the guest concentration inside the zeolite indicates an interaction between neighboring guest molecules leading to a quenching of the fluorescence. The analysis of emission decays using stretched-exponential model suggests that the excited-state interactions between neighboring dyes play a key role in the deactivation of the trapped Z fluorophores. For the ultrafast relaxation dynamics of the SAA/NaX composite, intramolecular vibrationalenergy redistribution and vibrational cooling process occur in longer times (up to 360 fs and 5 ps, respectively). Additionally, the presence of nonfluorescent twisted (n,π*) state is suggested to form in 6-10 ps. We believe that our results are important for a better understanding of the photocycle of azine-based photochromic material when interacting with nanomaterials. 1. Introduction 1,2
Salicylaldehyde azine (SAA, Scheme 1) belongs to the photochromic aromatic Schiff base molecules with an intramolecular hydrogen (H) bond.3–9 The photochromism of such molecules attracts much interest because of possible applications, for example, in molecular memories and switches.10 The typical photochromic cycle of such molecules is the following: after excitation of the enol tautomer an excited state intramolecular proton transfer (ESITP) reaction takes place leading to an excited cis-keto tautomer exhibiting a characteristic largely Stokes shifted fluorescence band. Then, after electronic changes and relaxation at the S1 state, a long-lived photochromic tautomer in the ground state is generated.3,4,6,11,12 The photochromic cycle of SAA in solution has been recently investigated by time-resolved UV-visible absorption (fs regime) and emission (ps regime) techniques.12,13 The time constant of the ESIPT reaction was found to be shorter than 50 fs in acetonitrile and the lifetime of the excited cis-keto tautomer varies from 20 to 100 ps in homogeneous solvents of low viscosity. One of the possibilities to increase the lifetime of the photochromic form of Schiff bases is their inclusion in microporous and mesoporous molecular sieves. Such hosts are found to be convenient media to control the photochemical properties of the organic species since the molecules adsorbed in their cavities and channels very often exhibit different photophysics and photochemistry from those in solutions.14–16 * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +34 925 265717. Fax: +34 926 268840. † Universidad de Castilla-La Mancha. ‡ Adam Mickiewicz University.
SCHEME 1: Molecular Structures of Different Forms of Salicylaldehyde Azine (SAA)
Recently, we showed that inclusion of Sudan I (a dye capable of ESIPT) into aluminosilica faujasite zeolites and MCM-41 modifies the photodynamics of the formed caged tautomers.17,18 Previous studies of salicylideneaniline (SA), a parent molecule for SAA, in NaY zeolites have shown that its enol form is not
10.1021/jp9101042 2010 American Chemical Society Published on Web 04/16/2010
Photochromic Proton-Transfer Dye within a Zeolite Nanocage
J. Phys. Chem. C, Vol. 114, No. 21, 2010 9555
preferentially stabilized within the cages in favor of a protontransferred structure with a zwitterionic character and to which we will refer as Z structure (Scheme 1).19,20 Additionally, a very long-living structure is formed upon UV-irradiation,20 similarly to other Schiff bases (N-(5-chlorosalicylidene)aniline and N,N′bis(salicylidene)-p-phenylenediamine, (BSP)) in mesoporous MCM-41 material.21,22 However, none of these studies used femtosecond-nanosecond time-resolved techniques to explore the dynamics of the excited states and the transient species formed within the nanocages. Therefore, the main aim of the present study is a detailed analysis of the spectral (absorption and emission) properties of SAA in NaX zeolite suspension, both stationary and in the time scale of femtoseconds to nanoseconds. The results show that the NaX zeolite environment stabilizes the zwitterionic structure of the dye in the S0 state and the host decreases the rate of excited state relaxation of this tautomer in comparison with those of keto-type form in solution. Clearly, the times involved in the photocycle of this photochromic molecule are sensitive to nanocaging. 2. Experimental Section SAA purchased from ABCR GmbH (97%) was recrystallized from acetonitrile:methanol (1:1) mixture before use. Dichloromethane (DCM, spectroscopic grade, 99.9%, Sigma-Aldrich) was used as received. NaX zeolite was from Sigma-Aldrich. SAA/zeolite composite was prepared by adding a 200 mg of a dried zeolite into 15 mL of SAA/DCM solution (1 g/L). Typically, the suspension was stirred at room temperature during 24 h although a characteristic yellow color appeared almost directly upon mixing. The centrifuged residue was washed four times with pure DCM to remove the dye from the external surface of the zeolite crystals and then dried in vacuum at room temperature. The steady-state fluorescence and absorption/diffuse transmittance spectra have been measured using FluoroMax-4 (JobinYvone) and JASCO V-670 equipped with a 60 mm integrating sphere ISN-723, respectively. The picosecond (ps) emission decays were measured using a time-correlated single-photon counting system.23 The sample was excited by a 40 ps pulsed diode laser centered at 371 or 433 nm (
