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J. Phys. Chem. B 2009, 113, 966–969
Fluorescence Properties of Hemicyanine in the Nanoporous Materials with Varying Pore Sizes Taekyu Shim,†,‡ Myoung Hee Lee,‡,§ Doseok Kim,*,†,‡ Hyun Sung Kim,‡,§ and Kyung Byung Yoon‡,§ Departments of Physics and Chemistry and Interdisciplinary Program of Integrated Biotechnology, Sogang UniVersity, Seoul 121-742, Korea ReceiVed: August 12, 2008; ReVised Manuscript ReceiVed: NoVember 25, 2008
Hemicyanine dye molecules were put into the nanoporous materials (silicalite-1, zeolite-β, and MCM-41) with different pore sizes. Fluorescence anisotropy measured from these systems indicated that the incorporated dye molecules were well-aligned along the straight channel of the zeolite pores, except for dye molecules in the largest pores of MCM-41. The fluorescence decay lifetimes were 2.1, 1.7, and 1.1 ns, for the dyes in silicalite-1, zeolite-β, and MCM-41, respectively. Significant increase in the fluorescence intensity and the fluorescence decay lifetime was observed with the decrease of the pore size. This was due to intramolecular rotational motion of the dye molecules more hindered in smaller pores, demonstrating the fluorescence properties of the dye molecule can be controlled at will by the choice of zeolite matrix. Introduction Incorporation of dye molecules into zeolite or other molecular sieves has received increasing attention in recent years due to its application potential in microlasers, optical sensors, optical switches, and artificial light-harvesting antenna systems.1-5 There are many interesting issues including the fundamental photophysical properties of this dye-in-nanopore system, as have been investigated with various systems of different channel diameters such as ZSM-5,6 AlPO4-5,7 zeolite-L,8 zeolite-Y,9 and MCM-41.10 These reports found out the incorporation mechanism of dye molecules into nanoporous channel, and the simulation study proposed the detailed arrangement of embedded dye molecules.11 Absorption and emission spectra of the embedded dye molecules were compared with those from dyes in common solvents.12 To explain the change of the optical properties under different environments, systematic change of zeolite material parameters would be helpful. However, apart from the reports on the spectral changes for different nanoporous matrices (MCM-41, zeolite-Y, and zeolite-L), there have not been many systematic studies as yet.13,14 Regarding this issue, dyes in zeolites having narrow pores are especially interesting as they would be well-isolated from the other dyes, and the electronic structure resulting from this unique environment would be reflected in their optical properties. Thus, characterization of the photophysical properties of the dye molecules in different zeolites and mesoporous material would provide insights for fundamental understanding of this system. For this study, hemicyanine was chosen to be inserted into the zeolites and mesoporous silica, as this molecule and its derivatives have been subjects of extensive investigation for many years. Due to the strong light absorption and emission in the visible range and the large molecular hyperpolarizability, it has been widely used as a frequency converter,15,16 fluorescence marker,17 and membrane-voltage-sensing probe.18,19 The fluo* Corresponding author. † Department of Physics. ‡ Interdisciplinary Program of Integrated Biotechnology. § Department of Chemistry.
Figure 1. (a) Molecular structure of hemicyanine. (b) Schematic structures of the zeolites and mesoporous silica used.
rescence lifetime and quantum yield of hemicyanine are known to depend sensitively on the microscopic environment of the molecule, due to its internal rotational motions in the excited state. Molecules that undergo this intramolecular rotation are deexcited to the ground-state nonradiatively, thus providing an additional nonradiative decay pathway.20,21 This intramolecular rotation of the hemicyanine molecules is affected by their environmental conditions such as the viscosity of solvent matrices.22,23 In this work, we investigated the absorption, fluorescence, fluorescence anisotropy, and time-resolved fluorescence from hemicyanine dye molecules incorporated in nanoporous materials (silicalite-1, zeolite-β, and MCM-41) with different pore sizes. The dye molecules in zeolite were found to be clearly well-aligned, along the straight channel of the zeolite pores, except for the case of MCM-41 having larger pores. Significant increase in the fluorescence intensity and the fluorescence decay lifetime of hemicyanine dye molecules was observed with the decrease in the nanoporous material pore size. Experimental Section The dye molecule used in this work is hemicyanine (4-[4(dimethylamino)styryl]-1-n-alkylpyridinium bromide, hereafter denoted HC-n). Figure 1a shows the chemical structure of the hemicyanine molecule, and Figure 1b shows the schematics of
10.1021/jp8073333 CCC: $40.75 2009 American Chemical Society Published on Web 01/08/2009
Hemicyanine in Nanoporous Materials zeolites (silicalite-1, zeolite-β) and mesoporous silica (MCM41) used in this study. The pore diameters deduced from X-ray diffraction pattern and TEM (transmission electron microscopy) were 0.55, 0.73, and 3-6 nm for silicalite-1, zeolite-β, and MCM-41, respectively. These were chosen as they have the identical chemical composition (SiO2) of the pore wall, differing only in their pore sizes. Dye molecules were incorporated into the pores of nanoporous materials from methanol solution as described in ref 24. To allow enough distance between the molecules, the number density was adjusted to be very low (approximately less than four molecules per one straight channel). The absorption spectra of the hemicyanine dyes in nanoporous powder with different pore sizes were obtained by measuring diffuse reflectance with a Varian Carry-5000 spectrophotometer. For measurements of fluorescence anisotropy and timeresolved fluorescence, the HC/zeolites and HC/mesoporous powders were sandwiched between two cover glasses. Pulses of 400 nm and 100 fs obtained from frequency doubling of the femtosecond Ti:sapphire laser output were used as an excitation source. The polarization of the excitation light was adjusted using a Glan-Laser polarizer placed in front of the sample, and the beam entered the sandwiched film at 45° incidence angle. The fluorescence signal collected at 45° from the sample was sent through a polarizer and achromatic waveplate, dispersed by a monochromator (MS3504i, SOLAR TII) and detected by using a photomultiplier tube (PMH-100, Becker & Hickl) and a photon counting system (SPC730, Becker & Hickl). The instrumental response function (IRF) of the system was about 150 ps.
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Figure 2. (a) Diffuse reflectance and fluorescence spectra of hemicyanine in silicalite-1 (solid line), in zeolite-β (dashed line), and in MCM-41 (dotted line). (b) Absorption and fluorescence spectra of hemicyanine in MeOH (solid line), in ethylene glycol (dashed line), and in glycerol (dotted line). F(R) (the scale on the left side of (a) is the so-called remission function for diffuse reflection measurements and is calculated from F(R) ) (1 - R)2/2R, where R is the diffuse reflectance.33,34
Results and Discussion UV-visible and fluorescence spectra (pumped at 400 nm) of HC-6 in nanoporous materials with different pore sizes are shown in Figure 2a. Also shown in Figure 2b for comparison purposes are UV-visible absorption spectra (measured in transmission geometry) of HC-6 in several common solvents of different viscosity (10-5 mol/L). Varying the length of the alkyl chain attached to the pyridine moiety hardly changed the steady-state absorption and fluorescence spectra, and all the results presented in this report were obtained with HC-6. The absorption and fluorescence peaks of the dye-in-nanoporous materials with different pore sizes are listed in Table 1. The absorption maximum was about 500 nm, and the fluorescence maximum was observed at ∼580 nm for HC/silicalite-1 and HC/zeolite-β. Apart from the spectra from HC/MCM-41 (described below), the red shift of absorption spectra and the blue shift of fluorescence compared to the solution cases are clear, resulting in much smaller Stokes shifts (∼2700 cm-1) for HC/silicalite-1 and HC/zeolite-β, as compared to the cases of common solvents (e.g., Stokes shift is 4300 cm-1 for HC in ethylene glycol). This spectral shift reflects the dielectric and electrical properties of the surrounding media and suggests that the environment of silicate channels is less polar (having smaller dielectric constant) than the common solvents, obviously so as the atoms consisting the zeolite pore wall cannot move unlike the solvent molecules. The absorption and fluorescence spectra for HC/silicalite-1 and HC/zeolite-β had shapes and peak widths similar to or smaller than those from common solvents in Figure 2b, and it can be inferred that these systems are free from artifacts such as dye aggregation and the dyes in zeolite are well-isolated as monomers. By contrast, the absorption and fluorescence spectra from HC/MCM-41 were shifted to shorter wavelength and
TABLE 1: Absorption Maxima, Fluorescence Maxima, and Intensity Ratios for Hemicyanine in Several Common Solvents and in Nanoporous Materials with Different Pore Sizes
silicalite-1 zeolite-β MCM-41 glycerol ethylene glycol MeOH a
λabs (nm)
λflu (nm)
500 508 469 483 481 479
580 586 581 606 607 602
viscositya (cP)
pore size, (nm)
IVV/IVH or IHH/IHV
∼0.55 ∼0.73 3-6
2.9 2.9 1.6 2.7
900 3.2 0.54
Viscosity (cP) values for glycerol is from ref 32.
appreciably broader, suggesting (1) dye molecules formed aggregates in the wider channels of MCM-41 and (2) the environment provided by the walls of MCM-41 is more inhomogeneous than those of silicalite-1 or zeolite-β.25,26 Figure 3 shows the fluorescence anisotropy for HC/zeolite and HC/mesoporous samples. The intensity ratios of theses samples are listed in Table 1. The subscripts (as in IVV, IVH, IHV, and IHH) represent the polarization direction of the incident and the fluorescence lights, respectively. This polarized fluorescence probes the orientational distribution of emitting dipoles in a sample with respect to the pump-beam polarization. If the rotational motion of the dyes in the medium is much slower than the fluorescence decay lifetime, the intensity ratio IVV/IVH should be 3 for isotropic samples.27 The fluorescence intensity ratio (IVV/IVH or IHH/IHV) was 2.9 for hemicyanine in silicalite-1 and in zeolite-β, indicating HC molecules inside these zeolites are immobile. For silicalite-1, the pore diameter 0.5 nm is about the lateral size of the HC molecule, and certainly HC molecules
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Figure 3. Anisotropic fluorescence spectra for hemicyanine in nanoporous materials with different pore sizes. The subscripts represent the polarization directions of the input pump and output fluorescence, respectively.
Figure 4. Fluorescence decay curves of hemicyanine in ethylene glycol, glycerol, MCM-41, zeolite-β, and silicalite-1, in increasing order of the fluorescence decay lifetime.
TABLE 2: Fitting Parameters for Fluorescence Decay of Hemicyanine Dye in Various Environments A1 (%) silicalite-1 zeolite-β MCM-41 glycerol ethylene glycol MeOH
100 34.3 57.7 32.7 100 34
A2 (%) 65.7 42.3 67.3 66
τ1 (ns) 2.14 0.64 0.41 0.30 0.20 5 ps
τ2 (ns) 2.30 2.11 1.42 39 ps
τav (ns)a 2.14 1.73 1.13 1.05 0.20 27 ps
a
The fluorescence decay curves are fitted with a two-exponential function A1 exp(-t/τ1) + A2 exp(-t/τ2), and the average fluorescence decay lifetime is defined in the usual way as τav ) (A1τ1 + A2τ2)/(A1 + A2).
inside the pore cannot rotate or move. Although pore size of zeolite-β (0.7 nm) is slightly larger than silicalite-1, presumably it would still not allow the dye molecules to rotate freely. On the other hand, the ratio IVV/IVH was reduced to 1.6 for hemicyanine in MCM-41, indicating the dye molecules in MCM-41 can rotate or move relatively easily during the fluorescence decay lifetime of HC. We then investigated the fluorescence decay lifetime of the dye molecules in different media. Figure 4 shows the fluorescence decay curves of hemicyanine in common solvents and in nanoporous materials, measured at the corresponding peaks of the steady-state fluorescence spectra. For common solvents, the increase of the fluorescence decay lifetime in more viscous solvent is obvious from Table 2 and Figure 4. Since the fluorescence lifetime is given by the equation τ ) 1/(kr + knr) (kr and knr are the radiative decay rate and nonradiative decay rate, respectively), the observed increase in the fluorescence decay lifetime is caused by the decrease in the nonradiative decay process. Because the main pathway of the nonradiative decay process in hemicyanine is intramolecular rotation in the excited state,20,21 it indicates that the intramolecular rotational motion is hindered in more viscous solvent.
For dyes in nanoporous systems, Figure 4 shows that even the lifetime for HC/MCM-41 is longer than that from HC dissolved in glycerol, the most viscous solvent studied. With very small pore size, the molecular motion of hemicyanine in zeolite pore is expected to be restricted severely. This restriction in intramolecular rotation works to block the nonradiative decay pathway and is considered to be the cause of the increase in fluorescence decay lifetime. The measured instrument response function was used to deconvolve the experimental decay curves with one- or two-component exponential decay function, and the resulting values from the fitting are in Table 2. The average fluorescence decay lifetimes of HC in silicalite-1, zeolite-β, and MCM-41 were 2.14, 1.73, and 1.13 ns, respectively, showing clear increase with decreasing pore sizes. This indicates that the intramolecular rotational motion of the dye molecules are hindered the most in silicalite-1 having the smallest pores. There is a clear increase in fluorescence decay lifetime from HC/ zeolite-β to HC/silicalite-1, even if they showed the same fluorescence anisotropy of 2.9. Unlike the fluorescence anisotropy that reflects the rotation of the dipole direction (that is, the orientation of the entire molecule), the fluorescence decay lifetime is affected by smaller-scale motion of intramolecular rotation about the aniline-ethylene bond.28 Presumably even a small increase of pore diameter from 0.5 to 0.7 nm is enough to facilitate this intramolecular motion. Another notable result is the single-exponential decay for hemicyanine in silicalite-1, unlike the case of zeolite-β and MCM-41. This single-exponential decay together with the fluorescence anisotropy result indicates that silicalite-1 provides a very homogeneous environment; with rigid and inert pore walls to isolate the incorporated guest molecules. This contrasts with the zeolite with lager pore size and mesoporous silica (zeolite-β and MCM-41), or other matrices such as viscous liquid, reverse micelle, or mica studied to date in order to confine the HC molecule spatially to modify the photophysical properties incorporated dye molecules.29-31 It shows that the zeolite showing these particular characteristics can be utilized as a matrix for investigating the properties of a molecule in a controlled and well-defined environment. Conclusion The properties of the hemicyanine dyes in nanoporous materials with different pore sizes were studied by using polarization anisotropy and time-resolved fluorescence measurement. The Stokes shift of the dyes in zeolites was smaller than those in common solvents. The fluorescence decay lifetime of hemicyanine dye increased with the decrease in the pore sizes of the nanoporous material, due to the restriction of intramolecular rotation in dye molecules. Fluorescence anisotropy and time-resolved fluorescence results indicate that the dye
Hemicyanine in Nanoporous Materials molecules incorporated in small straight channels of silicalite-1 are in a very homogeneous environment and suggest that zeolite can be utilized as a well-controlled and well-defined matrix for investigating the properties of incorporated molecules. Acknowledgment. This work was supported by the Seoul Research and Business Development Program (Grant No. 10816), the Quantum Photonic Science Research Center at Hanyang University, the Seoul Science Fellowship, and the Science and Scholarship. References and Notes (1) Vietze, U.; Krauss, O.; Laeri, F.; Ihlein, G.; Schu¨th, F.; Limburg, F. B.; Abraham, M. Phys. ReV. Lett. 1998, 81, 4628. (2) MacCraith, B. D. Chem. Anal. 1998, 150, 195. (3) Wirnsberger, G.; Scott, B. J.; Chemlka, F.; Stuchy, G. D. AdV. Mater. 2000, 12, 1450. (4) Calzaferri, G.; Huber, S.; Maas, H.; Minkowski, C. Angew. Chem., Int. Ed. 2003, 42, 3732. (5) Laeri, F.; Schu¨th, F.; Simon, U.; Wark, M. Host-Guest Systems Based on Nanoporous Crystals; Wiley-VCH: Weinheim, Germany, 2003. (6) Cano, M. L.; Chretien, M. N.; Garcia, H.; Scaiano, J. C. Chem. Phys. Lett. 2001, 345, 409. ¨ .; Loerke, J.; Wu¨stefeld, U.; Marlow, F.; Schu¨th, F. J. (7) Weiss, O Solid State Chem. 2002, 167, 302. (8) Maas, H.; Khatyr, A.; Calzaferri, G. Microporous Mesoporous Mater. 2003, 65, 233. (9) Ganesan, V.; Ramaraj, R. J. Lumin. 2001, 92, 167. (10) Gu, G.; Ong, P. P.; Li, Q. T. J. Phys. D 1999, 32, 2287. (11) Meselski, S.; Lieb, A.; Pauchard, M.; Dreshsler, A.; Gluaus, S.; Debus, C.; Meixner, A. J.; Calzaferri, G. J. Phys. Chem. B 2001, 105, 25. (12) Li, D.; Zhao, W.; Sun, X.; Zhang, J.; Anpo, M.; Zhao, J. Dyes Pigments 2006, 68, 33. (13) Guo, H.; Zhang, X.; Aydin, M.; Xu, W.; Zhu, H. R.; Akins, D. L. J. Mol. Struct. 2004, 689, 153.
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