J . Phys. Chem. 1989, 93, 4120-4128
4120
k,, can be written as the product of three factors:” k,, = k,l(kT/h)e-AG*’RT
(7)
Here, X,, is the electronic tranmission coefficient, k T / h is a “universal frequency”, and AG* is the free energy of activation for the transfer process, which generally involves reorganization of the solvent cage. Since k T / h 6 X 101’/s, it is evident that our experimental transfer rate constants are 2 orders of magnitude or more lower than the maximum that might be achieved. Since the excited state of the ruthenium complex is MLCT with the excited electron largely localized on one ligand,27 [Ru~’(L)~L-]~, energy-transfer quenching as well as charge-transfer quenching necessarily involves changes in the dipole moment of the encounter pair. which in turn will necessitate relaxation of the solvent cage and a nonzero free energy of activation. In addition, the electronic transmission coefficient requires favorable overlap of two orbital pairs of the donor and acceptor.” Thus we should not be surprised at finding rate constants lower than the maximum. Our data provide evidence that both of these factors contribute to the reduction in the quenching rate constant. First, we note that there is a substantial variation (more than IO-fold) in the energy-transfer rate constant for the same complex-quencher pair, [Ru(bpy),]-02, as the solvent system is varied. It seems more likely that this variation is due to changes in the activation free energy with solvent than to changes in the orbital overlaps within the encounter pair. We have searched unsuccessfully for a correlation between the rate constants and some appropriate solvent property, but it is probably unreasonable to expect a simple one-parameter correlation for a process that may involve bGth electronic (polarizability) and physical (viscosity) relaxation of the solvent medium on the microscopic level. Indeed, current t h e ~ r e t i c aand l ~ ~experimental3’ results indicate that the correlations will be complex. Second, w e note that the rate constant for energy transfer systematically varies with the ligand, regardless of solvent, and
-
( 2 7 ) Glasstone, S.;Laidler. K . J.; Eyring, H . The Theory o f R a t e Proresses; McGraw-Hill: New York, 1941. (28) Myrick, M. L.; Blakley, R. L.; DeArmond, M. K. J . A m . Chem. Soc. 1987, 109, 2841, and references therein. (29) Nadler, W.; Marcus, R . A. J . Chem. Phys. 1987, 86, 3906. (30) Su. S.-G.. Simon, J . D. J . Chem. Phys. 1988, 89, 908.
that the more easily oxidized ligand (bipyridine) always displays a higher rate constant than the less easily oxidized one (bipyrazine), with the ligand of intermediate oxidizability (bipyrimidine) falling in between. This trend is more pronounced for oxygen quenching but is also observed for quenching by anthracene. We see no way for this variability to be attributed to differences in the free energy of activation but can readily ascribe it to changes in the orbital interactions between the quenchers. Even though direct charge transfer does not contribute to the quenching process, the extent of overlap of the electronic wave functions for energy transfer can be increased by the existence of change-transfer wave functions for the encounter complex that are energetically nearby. The overall wave function describing the transfer process will carry contributions from such chargetransfer states that will depend on the energy difference between the states. Thus, charge-transfer states will contribute more for bipyridine ligands than for bipyrimidine, which in turn will have a higher contribution than for bipyrazine.
Conc I usi on s The studies carried out here have demonstrated that quenching of [ R U ( L ) ~ ] ~ ’excited * states occurs at less than diffusion-controlled rates for energy transfer as well as for charge transfer. We find that quenching by oxygen is best attributed to energy transfer with the rate constant being mediated by the presence of a lowlying charge-transfer state. The intrinsic first-order transfer rate constant falls at least 2 orders of magnitude below the maximum possible, apparently because of both activation free energy requirements and reduced electronic transmission coefficients. Acknowdedgment. This research was supported in part by a Senior Research Grant for J.O. from California State University, Fullerton. Most of the experiments were accomplished by C.J.T. as part of an undergraduate research project, which was partially supported by a grant from the Associated Students of CSUF. The [ R ~ ( b p m )complex ~] was kindly supplied by Dr. Patrick Sullivan (UNC/Chapel Hill). We acknowledge fruitful discussions with Professors Jeff Zink and T. J . Meyer. Registry No. [ R ~ ( b p y ) ~ ]151 ~ +58-62-0; , [Ru(bpm)J2+,80263-32-7; [ Ru(bpz),12+.75523-96-5; 02.7782-44-7; 9-anthracenecarboxylic acid, 723-62-6; p-dinitrobenzene, 100-25-4; p-nitrobenzaldehyde, 555-16-8.
Photophysical Properties of Pyrene in Zeolites X. Liu,’ K.-K. Iu, and J. K. Thomas*%’ Chemistry Department, University of Notre Dame, Notre Dame, Indiana 46556 (Received: January 1, 1988, In Final Form: Nocember 28, 1988) Photophysical properties of pyrene molecules on zeolite A and in zeolites X and Y were investigated. The lo^ value of the 111/1 ratio (0.45 for zeolite X and 0.74 for zeolite Y) indicates a very polar medium for pyrene in both zeolites, zeolite X being more polar than zeolite Y . For pyrene dimer formation, a higher concentration of pyrene is required for zeolite X than for zeolite Y , reflecting the subtle difference of the entry apertures between the samples due to the different framework Si/AI ratios. O2quenching of the pyrene excited state is very efficient in all cases and follows Stern-Volmer kinetics. The O2 quenching rate constants for excited pyrene monomer ((1.97 0.06) X I O ” , (2.43 f 0.10) X lo”, and (2.24 f 0.10) X IO” M-’ s-l for zeolites A, X, and Y, respectively) are obtained from the quenching kinetics for pyrene-zeolites by assuming a concentration of O2 in the zeolite similar to that in the gas phase. However, the quenching of excited pyrene by exchanged Cu2+cations in the same zeolite follows a Perrin-electron tunneling mechanisms. The radius of the “active sphere” where the excited pyrene is instantaneously deactivated by the Cu2+is 13.6 f 0.2 A.
*
Introduction
The ever-increasing interest in photochemistry in organized and constrained media2-4 has recently produced photochemistry in zeolite Marked effects of the constraining zeolite Present address: Department of Chemistry, Jilin University, Changchun. China. * A u t h o r to whom correspondence should be addressed.
0022-3654/89/2093-4120$01.50/0
medium on various photochemical and photophysical processes, such as cage effects’,’ and fluorescence fine-structure measure(1) The authors thank the NSF for support of this work. (2) Fendler. J . H. Membrane Mimetic Chemistry; Wiley: New York, 1983. (3) Kalyanasundaram. K. Photorhemistry in Microheterogeneous Media; Academic Press: Neh York, 1987.
C 1989 American Chemical Society
Photophysical Properties of Pyrene in Zeolites
Zeolite A
The Journal of Physical Chemistry, Vol. 93, No. 10, 1989 4121
Zeolite X ,Y
Figure 1. Framework structure of zeolites X, Y, and A.
ments,6 have been reported. The microenvironment of a probe molecule in a zeolite cage such as a supercage in zeolites X and Y, as shown in Figure 1, gives information of the local polarity of the internal (or external) surfaces and interactions experienced by the molecule with its surroundings (active sites in catalysis) nearby and at a distance. This information leads to a further understanding of the catalytic reactions occurring in a zeolite catalyst. The present work examines in detail surface polarities, excimer (dimer) formations, and two photoinduced reactions of pyrene in various zeolites, the quenching of excited pyrene by 02,a reaction requiring contact of reactants, and the quenching of excited pyrene by Cu2+, an electron tunneling reaction.
Experimental Section Zeolites. Zeolites X and Y were supplied from Exxon (%/A1 ratios are 1.18 for X and 2.43 for Y). Zeolite A was ordered from Aldrich (%/AI ratio is 1). The zeolites were used without further purification. Cu2+-containing zeolite X was prepared by ion exchange of zeolite X with an aqueous solution of CuCI2.2H20 (Fisher Product). The content of Cu2+ in zeolite X was varied by using different concentrations of CuCI2.2H20aqueous solution. All Cu2+ cations were introduced into the zeolite X during ion precipexchange. This was checked by CuS (Ksp = 6 X itation by adding a concentrated aqueous solution of Na2S into the supernatant. Pyrene-Containing Zeolite Preparation. Pyrene crystals (Aldrich Product, 99%) were recrystallized three times from methanol solution to remove impurities. Except for the O2 quenching studies (vide infra), the zeolite samples were prepared as the following: A known amount of recrystallized pyrene was dissolved in cyclohexane (Aldrich Product HPLC grade) to give a pyrene cyclohexane solution. Zeolites and copper(I1)-exchanged zeolites were dehydrated at 300-500 OC for 3 h and then added into the pyrene-cyclohexane solution while stirring, and the suspensions were kept in the dark overnight. After filtration, the pyrene-containing zeolites were washed with fresh cyclohexane three times. The liquids were collected for pyrene analysis. The pyrene concentrations introduced by this procedure were calculated by subtraction of the pyrene left in the liquid from the total amount of pyrene added. The amount of pyrene in the liquid was analyzed by UV-vis spectrophotometry. The O2quenching rate for excited pyrene is extremely sensitive to small amounts of residue molecules filling the pores (e.g., water, cyclohexane).8 For the 0 2 quenching studies, the zeolites were dehydrated at high temperatures and for long times (550 OC, 24 h). The pyrene was loaded into the zeolite with a very dry, low boiling point solvent, anhydrous n-pentane (Aldrich Product). The pyrene concentrations were 1.5 X 10" mol/g for both zeolites X and Y and 5 X lo-' mol/g for zeolite A in the quenching of excited pyrene monomer. For excited pyrene dimer studies, pyrene mol/g for time-resolved expericoncentrations were 1.8 X ments and 1.05 X IOy4 mol/g for steady-state experiments. The (4) Thomas, J . K . Chemistry of Excitation at Interfaces; American Chemical Society: Washington, DC, 1984; ACS Monograph No. 182. ( 5 ) Turro, N. J . Pure Appl. Chem. 1986, 59, 1219-28. (6) Suib, S. L.; Kostrapopos, A. J . Am. Chem. SOC.1984,106,7705-7710. (7) Scaiano, J. C.; Casal, H. L.; Netto Ferreira, J. C. ACS Symp. Ser. 1987, 278, 21 1-22. ( 8 ) lu, K . K.; Thomas, J. K., to be published.
L/ 1
350
470 Wavelfxqth (nm)
590
350
410 Wavelength
470
530
inmi
Figure 2. Fluorescence spectra of excited pyrene in zeolites: (A) zeolite X; (B) zeolite Y .
mixture of zeolite and pyrene pentane solution was kept in the dark for at least 4 h, and then the supernatant was checked by UV-vis spectrophotometry to ensure that no pyrene was left behind in the solvent. The sample was vacuum dried ( Torr) and kept at 140 OC for a half-hour before the O2quenching experiment was performed. All experiments were performed at room temperature (22 "C). Characterization of Zeolites and Pyrene-Containing Zeolites. All of the zeolites used were examined by X-ray diffractometry and infrared spectroscopy. N o structural changes were observed in any of them after sample preparation. The X-ray diffraction patterns were recorded on a Diano X-ray diffractometer operating at 45 kV and 30 mA and at a scanning speed of lo/min. The infrared spectra were measured by using an IBM FT-IR/32 spectrometer equipped with an IBM microcomputer with a resolution of 2 cm-'. The KBr wafer technique was used. The Cu2+-containing zeolites X were examined by an electron spin resonance (ESR) spectrometer (Varian, E-Line Century Series ESR spectrometer). The ESR signal intensity increased with increasing Cu2+,and the spectra showed the ordinary Cu2+ESR signals with gl,> g , (gll= 2.35, g, = 2.07). The location of the Cu2+cations inside the sodalite cage was ascertained by reverse ion exchange of Cu2+-containing zeolites X with an aqueous solution of NaC1,9 which could not remove Cu2+. Dehydration causes migration of the Cu2+cations (normally from big cages to small cages).1° The solvent cyclohexane does have some effects on the symmetry of Cu2+ cations in the zeolites as indicated by the change of ESR signals. From the ESR studies, it is suggested that the most likely positions for Cu2+ are sites SI' and SI1 or SII'." Photophysical Properties Measurements. All steady-state emission or excitation data were obtained with a Perkin-Elmer MPF-44B spectrofluorimeter. For time-resolved laser spectroscopy experiments, a short-pulse PRA (Model LN100) Nitromate nitrogen laser with a wavelength of 337.1 nm, pulse width of 300 ps, and pulse energy of 60 pJ was used as an excitation source. A multichannel plate photomultiplier (Hamamatsu R1664U), coupled with a monochromator and a condensing lens system, was used to collect the 400-nm (monomer) or 475-nm (dimer) emission of excited pyrene. The analog signal from the photomultiplier was subsequently transferred to an IBM PC-AT compatible computer (Zenith 2-200 equipped with a math coprocessor) through interfacing with a 500-MHz waveform digitizer (Tetronix 7912 AD) for further data processing. The whole detection system respohse was less than 1 ns. On the basis of Bevington's description,12 we applied the (9) Liu, X.; Thomas, J. K., submitted for publication in Chem. Phys. Lett. ( I O ) Mortier, W. J.; Schoonheydt, R. A. Prog. Solid State Chem. 1985, 16.
(11) Breck, D. W. Zeolite Molecular Sieues; Wiley: New York, 1974. ( 1 2) Bevington, P.R.Data Reduction and Error Analysis for the Physical Sciences; McGraw-Hill: New York, 1969.
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TABLE I: Different Loading of Pyrene in Zeolites X and Y with and without Cu2+ Cations
no. 1 2 3 4 5 6 7 8 9 10 11 12 13
concn of pyrene mol/g NbvIuC(I
sample
Na-X
Na-X Na-X Cu-Na-X Cu-Na-X Cu-Na-X Na-X Na-X Na-X Na-Y N a-Y N a-Y
Na-Y
2.38 X 10” 4.24 X 10” 5.05 X 5.05 X 10” 5.50 X 10” 1.03 X lo-’ 2.27 x 10-5 5.04 x 10-5 1.05 X lo4 1.65 X lod 6.27 X 10” 9.33 x 10” 1.54 x 10-5
state of pyrene 0.032 monomer 0.057 monomer monomer 0.068 monomer 0.068 monomer 0.074 0.138 monomer 0.305 (dimer) monomer 0.678 (dimer) monomer 1.41 1 (dimer) monomer 0.021 monomer 0.080 (dimer) monomer 0.1 19 (dimer) monomer 0.196 (dimer) monomer
III/I 0.48 0.45 0.48 0.49 0.47 0.49 0.62 0.64 0.78 0.74 0.82 0.82 0.82
“uc means unit cell.
Wavelength
nm
Figure 3. Excitation spectra of pyrene in zeolite X: (A) excitation spectrum of pyrene monomer (emission at 392 nm); (B) excitation spectrum of pyrene dimer (emission at 472 nm).
Marquardt algorithm in an reiterative nonlinear least-square program for analyzing all the time-resolved experimental data. The goodness of the fits were judged by the magnitude of the x2 and the distribution of the unweighted residuals. The zeolite sample was contained either in a ESR tube or in a I-cm2 quartz cuvette for all experiments.
lm le
40- 60
30.
Results The fine structure of pyrene fluorescence reports on the microenvironment that this molecule experiences in a ~ y s t e m . ~ In? ~ particular the ratio of the fluorescence peaks at 391 and 372 nm, the III/I ratio, follows the hydrophobicity of the medium. Typical fluorescence spectra of excited pyrene in zeolites X and Y are shown in Figure 2. Table I shows the III/I ratio of pyrene in zeolites X and Y with and without exchanged Cu2+ cations and at various pyrene concentrations. The low value of the III/I (0.45 for zeolite X and 0.74 for zeolite Y) indicates a very polar medium for pyrene in both zeolites, zeolite X being more polar than zeolite Y. Cu2+, which quenches the pyrene fluorescence, has no effect on the III/I ratio. Increasing the [pyrene] increases the III/I ratio as pyrenes are located closer together in the different zeolite cages. This also leads to excimer formation, which is observed around 475 nm. Dimer formation occurs at much lower pyrene concentrations in zeolite Y compared to zeolite X. This is probably due to the subtle difference of the entry apertures between zeolites X and Y as the framework Si/AI ratio increases from zeolite X to zeolite Y (unit cell dimension decreases with increasing Si/AI ratio due to the different S i 4 (1.62 A) and A 1 4 (1.82 A) bond length). The diffusion of pyrene molecules is much easier in zeolite X than in zeolite Y. The concentration required for dimer formation is in the range of 0.14-0.30 pyrene per unit cell (1.0 X 1O-5-2.0 X 10-5 mol/g) for zeolite X and 0.02-0.08 pyrene per unit cell (1.6 X 1 Ow6-6.0 X 10” mol/g) for zeolite Y, as shown in Table I. The pyrene excimer is formed instantaneously within s in a pulsed laser experiment (Figure 8A). This shows that little or no movement of the excited pyrene and ground-state pyrene is required to produce the excited complex. Excitation spectra of the zeolite with 1.05 X lo4 mol of pyrene/g (containing monomer and dimer; A,, = 392 nm for monomer and A,, = 472 nm for excimer) exhibit a strong absorption at 350 nm which is not present in the monomer spectrum (see Figure 3). These results also indicate that the pyrene complex (dimer) is already formed in the zeolite cage prior to excitation. Microcrystals of pyrene could also be formed on the zeolite external surfaces and give rise to pyrene excimer emission on excitation. However, the X-ray diffraction patterns gave no evidence of microcrystalline pyrene in the present samples. The I R spectrum of the sample with 1.05 X 1O4 mol of pyrene/g shows a weak absorption band at 863 cm-’. Pyrene crystals exhibit strong bands at 839.9 cm-’, while pyrene in ethanol (0.025 mol/L) gives a weak band at 846.0 cm-1.6 The weak IR band at 863 cm-’ is the indication of pyrene perturbed by the zeolite surface. Excitation measurements of the sample containing pyrene dimer also
20 40 I
I
I
I
0
10
20
Time
h
Figure 4. Effects of dehydration under vacuum on the intensities of pyrene monomer and pyrene dimer: (A) change of intensity of pyrene monomer during dehydration; (B) change of intensity of pyrene dimer during dehydration; (C) change of Zexcimer/lmonomer.
do not show microcrystalline spectra. It is therefore concluded that the excimer present in the fluorescence spectra is not due to microcrystalline pyrene but is due to dimers in the supercage. Figure 4 shows effects of dehydration under vacuum on the Zexcimer/lmonomer ratio. The intensity of monomer pyrene decreases and the intensity of excimer (dimer) pyrene increases during dehydration, and as a consequence the lexcimer/Zmonomer increases. Over the first 7 h the linear change of the intensities of monomer and excimer (dimer) pyrene indicates that the increase in intensity of the excimer (dimer) is concomitant with loss of monomer, for example, the additional excimer (dimer) pyrene is formed from the monomer. The process is reversible. On hydration, some excimers (dimers) are separated into monomers. After 7 h under vacuum, the rate of increase of the excimer (dimer) decreases, and finally the Zexcimer/Zmonomer ratio reaches a constant value. The observed phenomenon is different from that for pyrene on amorphous silica, where dehydration causes separation of dimers and hydration causes formation of dimers.I3 The different features of excimer (dimer) formation in zeolite X from those on silica are due to different features of the structures. Compared to zeolites, amorphous silica has big pores (greater by 1 or 2 orders of magnitude), and the pyrene molecules have more freedom to migrate on the silica surface. Dehydration of the silica surface increases the interaction between pyrene molecules and the silica surface, causing separation of any dimers. On hydration, the adsorbed water molecules decreases these interactions and the pyrene molecules cluster, forming dimers (or microcrystals) due to the limited solubility of pyrene on hydrated S O 2 . By contrast, the role water molecules play in zeolite X is (13) Krasnansky, R.; Thomas, J. K., to be published.
The Journal of Physical Chemistry, Vol. 93, No. 10, 1989 4123
Photophysical Properties of Pyrene in Zeolites TABLE 11: Bimolecular Rate Constants for the O2 Quenching of Excited Pyrene Monomer and Dimer
zeolite A" X" Y" Xb
X'
concn of pyrene, IO6 mol/g
IO-' Torr-I s-I
0.5 1.5 1.5 180 180
1.07 i 0.03 1.32 i 0.05 1.22 f 0.05 0.20 f 0.02 0.12 i 0.01
k,d lo-" 1.97 2.43 2.24 0.37 0.22
M-I s-I i 0.06 f 0.10 f 0.10 f 0.04 i 0.02
"02 quenching of excited pyrene monomer. The pyrene concentrations in these cases were low enough to preclude the formation of dimer. quenching of excited pyrene monomer. quenching of excited pyrene dimer. "The experiments were done at room temperature (22 f 1 "C). Assuming the O2 concentrations in zeolites are the same as in the gas phase, the conversion factor is 1 Torr-' s-I = 18 384 M-I s-I. just the opposite to the above. The dimer is formed in the supercage of zeolite X where little if any space is available for the dimer to migrate and little space for adsorption of water molecules. (It is important to note that the pyrene molecules were introduced into zeolites in a cyclohexane or pentane solution.) On hydration, the interaction between the surface surrounding pyrene and water molecules tends to destroy the dimer either through separation of the dimer into monomers, which migrate to the neighboring supercages, or through change of the configuration of the two pyrene molecules in the dimer. The results obtained in dehydration experiments suggest that it is more likely that a configuration change of the dimer is responsible for the increase of the pyrene monomer intensity and the decrease of the pyrene excimer (dimer) intensity. This explanation of dimer formation and separation during dehydration and hydration is substantiated further by O2 quenching experiments of the excimer (dimer). Quenching of Excited Pyrene by 02. ( A ) Low pyrene loading samples, where only monomer emission was observed. The decays of pyrene monomer (400 nm) on zeolite A and in zeolites X and Y under vacuum are shown in Figure 5; the data show deviation from the first-order decay kinetics in all cases. As the O2pressure increased (
