J. Phys. Chem. C 2010, 114, 6311–6317
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Interrogating Confined Proton-Transfer Reaction Dynamics within Mesoporous Nanotubes Michał Gil,† Cristina Martin,† Juan Angel Organero,† Maria Teresa Navarro,‡ Avelino Corma,‡ and Abderrazzak Douhal*,† Departamento de Quı´mica Fı´sica, Seccio´n de Quı´micas, Facultad del Medio Ambiente, UniVersidad de Castilla-La Mancha, AVenida Carlos III s/n, 45071 Toledo, Spain, and Instituto de Tecnologı´a Quı´mica, UPV-CSIC, AVenida de los Naranjos s/n, 46022 Valencia, Spain ReceiVed: December 17, 2009; ReVised Manuscript ReceiVed: March 9, 2010
We report on steady-state and time (ns to fs regime) resolved studies of H-bonding interactions and protontransfer reaction dynamics of silica-based mesoporous material MCM-41 with an H-bond donor and acceptor guest aromatic molecule (7-hydroxyquinoline, 7HQ). We observed the ground state reaction which leads to the formation of intermediates and products of the confined molecular probe. We compare this behavior with the observed one for the dye adsorbed on the surface of silica particles, lacking the nanotubes. The result clearly shows that the formation of keto (or zwitterionic) tautomers at the ground state is enhanced by the confinement provided by the channels of MCM-41. Introduction of hydrophobic groups (by silylation of the OH groups in regular MCM-41 host) changes the ground state tautomeric equilibria and the emission behavior. A new lifetime (3.19 ns, suggested being due to a more stabilized anion of the guest) was observed in addition to the ones due to confined bound enol (0.26 ns), anion (1.5 ns), and zwitterionic (5.5 ns) structures. Both steady-state and ps-data show the importance of solvation of 7HQ structures inside MCM-41, when compared with the solid-state result. We investigated the intermolecular proton-transfer reaction dynamics in the confined structures using femtosecond-resolved emission spectroscopy, and we got the reaction times needed to produce the anion intermediates (0.3 ps) and zwitterion products (3 ps) upon electronic excitation of bound enol forms of the guest, in addition to the cooling times of the final zwitterionic form. We believe that our results might be useful for designing new nanophotonics sensors based on mesoporous materials, and open the window for further studies to better understand the chemical reactivity of silica-based nanohosts, at a short time scale. 1. Introduction The fundamental role of intermolecular proton transfer processes attracts considerable attention from researchers as the reactions involving them are among the most important ones in chemistry and biology.1 Spectroscopic studies of these processes often use fluorescent molecules having protondonating and proton-accepting groups. 7-Hydroxyquinoline (7HQ) is such a bifunctional dye undergoing solvent-assisted excited state proton transfer (ESPT).2 Numerous works,3-7 including very recent ones,8 investigated this reaction in solutions. However, tautomeric equilibria of 7HQ and photobehavior of its prototropic forms are also sensitive to confining environments.9-11 For example, 7HQ caged within β-cyclodextrin at pH 7 exists predominantly as enol (E, Scheme 1) form while in pure water an additional large contribution of zwitterion (Z) is present, beside the minor populations of cation and anion (A).2,9 In turn, the photodynamics of 7HQ encapsulated in reverse micelles strongly depend on the water content (defined as the ratio W0 ) [water]/[surfactant]). The kinetics are much faster at W0 > 5 but not as fast as in pure water.10 Clearly, solvent reorganization around the photoexcited dye, prerequisite of an efficient ESPT process, is affected by a restricting environment. Recently, we have reported on several confined systems within chemical and biological hosts, and discussed the observed spectral and dynamical behavior in terms of confinement.12 * To whom correspondence should be addressed.
[email protected]. Fax: +34-925-268840. † Universidad de Castilla-La Mancha. ‡ Instituto de Tecnologı´a Quı´mica.
E-mail:
SCHEME 1: (A) Molecular Structures of 7-Hydroxyquinoline (7HQ) in Enol (E), Anion (A), Keto (K), and Zwitterion (Z) forms and (B) Schematic Representation of Used Regular MCM-41-R and Its Methyl Silylated Derivative, Me-MCM-41
One of the most promising families of confining hosts are those provided by structured mesoporous silicates.13,14 Recently, we showed that these materials are largely changing the spectral and dynamical properties of the guest.15,16 Thus, it is of great interest to study how relatively immobile hydrogen bonding
10.1021/jp911942d 2010 American Chemical Society Published on Web 03/19/2010
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J. Phys. Chem. C, Vol. 114, No. 14, 2010
groups (-OH, -O-) in mesoporous molecular sieves are interacting with tautomeric structures of 7HQ. MCM-41 is a mesoporous silicate material, whose structure is constituted by parallel hexagonal channels having a diameter of 40 Å. The surface contains numerous hydroxyls, while the silanol groups building channels framework are suitable for chemical modifications.17 Therefore, we have investigated the spectral and photodynamical (in the time scale of femto- to nanosecond) properties of 7HQ embedded within a polar MCM-41 material which contains a large number of -OH groups (MCM-41-R), in aprotic solvent (or in dry solid). Additionally, we studied the dye within modified (silylated) MCM-41, which is a less hydrophilic material.18 The results have been compared with those obtained with silica particles as a host. The studies show that the nanohost environment stabilizes the zwitterion in the S0 state and the ESPT process involving the anionic structure of the dye is faster than that in protic solutions.
Gil et al.
Figure 1. Absorption spectrum (-O-, green) of 7HQ in dichloromethane (DCM) solution and diffuse transmittance spectra of 7HQ within MCM41-R (;, black), Me-MCM-41 (- - -, red) and on silica particles (-b-, blue). For the diffuse transmittance, the Kubelka-Munk remittance function is used: F(R) ) ((1 - R)2)/2R, where R is the diffuse reflectance intensity from the sample.
2. Experimental Section A purely siliceous MCM-41 with an average pore diameter of 40 Å was used directly or upon modification (silylation) with -Si(CH3)3 groups partly replacing -OH ones (see the Supporting Information for synthesis and characterization of these materials). The composite samples of 7-hydroxyquinoline (7HQ, 99%, Across) within MCM-41s were prepared by adding 100 mg of dried MCM-41 to 15 mL of dichloromethane (DCM, anhydrous spectral, 99.9%, Aldrich) solution and stirring at room temperature for 24 h. The obtained material was washed several times with DCM. The washing removes most of the dye loosely adsorbed on the external surface of MCM-41 grains, but some externally adsorbed molecules still may be present. However, their contribution to the measured signals should be small, as the internal surface of MCM-41 is much larger than the external one. To examine this possibility of absorption, we also used silica particles as a host (Merck, particle size 0.063-0.200 mm, used after consecutive washing with n-hexane, tetrahydrofurane, and dichloromethane), having silanol groups but lacking the nanochannel structure of MCM-41. Finally, the samples have been dried under vacuum. The steady-state fluorescence and absorption/diffuse transmittance spectra have been measured with FluoroMax-4 (JobinYvone) and JASCO V-670 equipped with a 60-mm integrating sphere ISN-723, respectively. The fluorescence lifetimes were measured with a time-correlated single-photon counting system (TCSPC).19 The sample was excited by a 40-ps pulsed diode laser centered at 371 nm (
