J. Phys. Chem. 1989, 93, 3683-3688 structure have an important influence on the acid strength. From quantum chemical cluster model calculations and from the electronegativity concept follow a decrease in deprotonation energy and an increase in proton charge with decreasing aluminum content. From this, different authors have deduced that the entire acid strength distribution (weak, medium, strong sites) is shifted toward stronger values. The theoretical predictions which are related to average values of the acid strength are confirmed by the results obtained here. However, contrary to the view in the literature, the increase in the acidity is due to the fact that nonacidic sites become acidic, whereas the acid strength spectrum is not changed significantly. The shift in the wavenumbers of bridging hydroxyls (high-frequency band) toward smaller values has been considered as proof for the increase of the acid strength. and Wavenumbers of the high-frequency band taken from our35336 literature data1v3' are plotted in Figure 10. Dombrowski et al.38 showed that the high-frequency band is composed of several subbands and proposed an assignment to bridging hydroxyls with different numbers of aluminum atoms in second-neighbor framework positions. Accepting this idea, the shift of the band maximum should be parallel to the (1 - Q) curve. This is fulfilled satisfactorily (Figure 10). The fine structure of the high-frequency band contains in both HY and HX zeolites the same subbands with wavenumbers in the range 3606-3667 cm-' but of different intensities. The shift in the band maximum with decreasing aluminum content is due to the alteration in the intensities of the subbands, Le., to the gradual degradation of the high-frequency side of the band (representing the nonacidic hydroxyls) connected (35) Lohse, U.; Loffler, E.; Hunger, M.; Stkkner, J.; Patzelovl, V. Zeolites 1987, 7, 11. (36) Patzelovi, V.;Drahoridovi, E.; Tvaruzkovi, 2.; Lohse, U. Zeolites, in rDress. ---(37) Jacobs, P. A.; Uytterhoeven, J. B. J. Chem. Soc., Faraday Trans. 1 1973, 69, 359. (381 Dornbrowski, D.; Hoffmann. J.; Fruwert, J.; Stock, Th. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2257 ~~
3683
with the enhancement of the intensity of the low-frequency subbands. If all hydroxyls are acidic (at Si/AI> 9), then no further significant change in the intensity ratios of the subbands occurs, and therefore no further shift in the band maximum is observed.
Conclusions A stepwise decreasing acid strength was found for H Y and weakly dealuminated samples by the calorimetric measurement of NH3chemisorption. The zeolites with relatively high aluminum content behave in the neutralization process as a solid polyfunctional acid. At Si/A1 ratios smaller than 9 an increasing part of the bridging hydroxyl groups is nonacidic. The protons of these hydroxyls cannot be ion exchanged in the neutral medium; their pK value is higher than 7. The efficiency a of an acid site (number of acid sites per framework aluminum atom) was determined and correlates for a given hydroxyl group with the average occupancy of the second-neighbor framework positions with aluminum. It follows from the results obtained by calorimetry, TPD, and potentiometric titration that the entire acid strength spectrum is not shifted with progressive dealumination toward stronger values. The distribution of acid strengths becomes more diffusive at higher degrees of dealumination, which is explained by the variation of the local geometry of acid sites due to the disturbance of the structure. The effective number of acid centers ( N A I a )determines the catalytic activity, as is demonstrated on the basis of literature data. The number of acid centers located on the nonframework aluminum species is estimated to about 1% of the nonframework aluminum atoms. Al(H20)63+ions are formed via decomposition of the nonframework aluminum species in the hydrated samples. These ions neutralize a part of the acid sites. Acknowledgment. We thank R. Wendt for the calorimetric measurements.
Picosecond Spectroscopic Studies of Electronic Energy Relaxation in H-Aggregates of 1,l'-Diethyl-2,2'-dicarbocyanine on Colloidal Silica Sun-Yung Chen,+Miin-Liang Horng; and Edward L. Quitevis* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409 (Received: June 28, 1988; In Final Form: November 14, 1988)
Polarized transient bleaching measurementsof H-aggregates of 1,l'-diethyl-2,2'-dicarbocyanine on colloidal silica were performed using picosecond pump-probe spectroscopy. The aggregates were nonfluorescent but exhibited a broad absorption band, between 550 and 600 nm, which was blue-shifted from the monomer band at 71 1 nm. These spectral properties are consistent with the simple theory for one-dimensional excitons. A fit of the transient bleaching signals to a stretched exponential, exp[-(t/rp)"], was obtained for CY = 0.46 0.02 and T~ = 17 f 5 ps. The nonexponentialsignals are attributed to a distribution of aggregates with different lifetimes, with the most probable lifetime being equal to 120-130 ps. The absorption anisotropy was constant and equal to 0.30 f 0.04. The anisotropy is discussed in terms of the coherence properties of excitons.
Introduction Excitation energy transport in molecular aggregates consisting of ordered arrays plays a central role in the light-harvesting processes of many biological systems.' To understand the dynamics of excitation energy transport in these complex systems, it is logical to choose model systems that are ordered but less complicated. Aggregates of cyanine dyes are ideally suited because the molecules are in well-defined structures.2 Cyanine aggregates 'Robert A. Welch Foundation Predoctoral Fellow.
0022-3654/89/2093-3683$0 1SO10
have been actively studied both experimentally and theoretically since they were first investigated by Jelley3 and Scheibe? These aggregates form in aqueous solution at high concentrations and on surfaces.s Aggregation occurs because of the strong dispersion (1) Bioenergetics of Photosynthesis; Govindjee, Ed.; Academic Press: New York, 1975. (2) (a) Emerson, E. S.; Conlin, M. A,; Rosenoff, A. E.; Norland, K. S.; Rodriguez, H.; Chin, D.; Bird, G.R. J. Phys. Chem. 1967, 71, 2396. (b) Graves, R. E.: Rose, P. 1. J. Phys. Chem. 1975, 79, 746. (3) Jelley, E. E. Nature (London) 1936, 138, 1009. (4) Scheibe, G . Angew. Chem. 1936, 49, 563.
0 1989 American Chemical Society
3684 The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 forces associated with the high polarizability of the chromophoric chain. In aqueous solution, the high dielectric constant of water facilitates dye aggregation by reducing the electrostatic repulsions between similarly charged dye molecules. Strong electronic coupling between the molecules in these aggregates gives rise to an absorption band which is either blueshifted or red-shifted from the monomer band. For example, the aggregates of 1,I’-diethyL2,2’-cyanine, or pseudoisocyanine (PIC), exhibit a narrow absorption band, called the J-band, at 4 7 0 nm, which is red-shifted from the monomer band at 523 nm1,39436 whereas the aggregates of carbocyanine dyes exhibit a band, called the H-band, which is blue-shifted from the monomer band.za,5a-c Several theoretical models7 have been developed that relate the structure of the aggregates to these spectral properties. In particular, by use of a one-dimensional exciton the orientation of molecules in the aggregate can be inferred from the spectral shifts. In this model the dye molecules are stacked as in a tilted deck of cards and dipolar coupling is assumed. The interactions of these dyes at small intermolecular distances may not be well-approximated by dipole-dipole interactions, because the transition dipole moment in these dyes is spread over a large *-electron system. Nonetheless, it is possible to obtain crude estimates of the molecular orientation in aggregates. In this model, spectral shifts are proportional to (1 - 3 cosz e), where 0 is the angle between the transition dipole moment of a molecule in the aggregate and the long axis of the aggregate. Thus, the aggregate bands are blue-shifted for 54’44’ < 8 < 90’ and red-shifted for 0’ < 0 < 54’44’. The one-dimensional exciton model7a also predicts that the N-degenerate singlet excited states of the N dye molecules in an aggregate are split into an N-fold band of levels. For 54’44’ < 8 < 90’, optical transitions are allowed between the ground state and the level at the top of the band but forbidden between the ground state and the level at the bottom of the band. The reverse situation holds for 0’ < 8 < 54O44’. If fluorescence occurs from the bottom of the band, J-aggregates will fluoresce strongly, whereas H-aggregates will not fluoresce or will fluoresce very weakly. These predictions are borne out by the fact that the excitation of J-aggregates produces an intense narrow fluorescence band at approximately the same wavelength as the absorption band. In contrast, the fluorescence from H-aggregates is very weak. The J-aggregates of PIC in solution have been investigated in picosecond fluorescence,8 transient absorption? and accumulated photon-echo experiments.I0 We recently reported the picosecond transient bleaching of J-aggregates of PIC adsorbed on 40-Adiameter silica particles.” We explained the line width of the J-band and the nonexponential decay of the transient bleaching signal in terms of a distribution of J-aggregates with different lifetimes. Despite the numerous time-resolved studies of J-aggregates, there have been no measurements of the electronic energy relaxation in H-aggregates of cyanine dyes. Although H-ag(5) (a) West, W.; Carroll, B. H.; Whitcomb, D. L. J . Phys. Chem. 1952, 56, 1054. (b) West, W.; Carroll, B. H.; Whitcomb, D. L. Ann. N.Y. Acad. Sci. 1954, 58, 893. (c) West, W.; Pearce, S . J . Phys. Chem. 1965, 69, 1894. (d) Herz, A. H. In The Theory ofthe Photographic Process, 4th ed.; James, T. H., Ed.; MacMillan: New York, 1977; pp 235-250. (6) (a) Mattoon, R. W. J . Chem. Phys. 1944.12.268. (b) Zimmermann, H.; Scheibe, G. 2.Electrochem. 1956, 60, 566. (c) Cooper, W. Chem. Phys. Lett. 1970, 7 , 73. (7) (a) McRae, E. G.; Kasha, M. J . Chem. Phys. 1958, 28, 721. (b) McRae, E. G. Ausr. J . Chem. 1961, 14, 354. (c) Czikkely, V.; Forsterling, H . D.; Kuhn, H . Chem. Phys. Lett. 1970, 6, 207. (d) Scherer, P. 0. J.; Fischer, S. F. Chem. Phys. 1984, 86, 269. (8) (a) Fink, F.; Klose, E.; Teuchner, K.; Dahne, S . Chem. Phys. Lett. 1977, 45, 548. (b) Yu, 2. X.; Lu, P. Y . ;Alfano, R. R. Chem. Phys. Lett. 1983, 79, 289. (c) Brumbaugh, D. V.;Muenter, A. A,; Knox, W.; Mourou, G.; Wittmershaus, B. J . Lumin. 1984, 31, 783. (9) (a) Rentsch, S. K.; Danielius, R. V.; Gadonas, R. A,; Piskarskas, A. Chem. Phys. Lett. 1981,84,446. (b) Kopainsky, B.; Kaiser, W. Chem. Phys. Lett. 1982, 88, 357. ( I O ) DeBoer, S . ; Vink, K. J.; Wiersma, D. A. Chem. Phys. Lett. 1987, 137, 99. ( 1 1 ) Quitevis, E. L.; Horng, M.-L.; Chen, S.-Y. J . Phys. Chem. 1988, 92, 256.
Chen et al.
!Xa
(AVSTRO~Y’
WAVEL~TH
Figure 1. Absorption spectra of 20 fiM l,l’-diethyl-2,2’-dicarbmyanine iodide in (E) ethanol, (W) water, and ( S ) 3.75% colloidal silica. The extinction coefficient is in units of M-’ cm-’.
gregates fluoresce very weakly, their strong absorption makes them amenable to study by picosecond transient bleaching. This article describes picosecond transient bleaching measurements of electronic energy relaxation in H-aggregates of 1,l ’-diethyl-2,2’-dicarbocyanine (DDC) adsorbed on colloidal silica. This dye was chosen because its H-band lies within the tuning range (570-610 nm) of our picosecond laser. The transient bleaching signals for H-aggregates of DDC on colloidal silica were nonexponential. The spectral data and nonexponential signals can be rationalized in terms of a distribution of aggregates with different exciton lifetimes. Finally, the absorption anisotropy is discussed in terms of the coherence of the excitons in these aggregates.
Experimental Section 1,lf-Diethy1-2,2’-dicarbocyanine iodide (Kodak Chemical Co.) was used without further purification. The stock solution of colloidal silica (Nalco 1115) was a 15% (w/v) aqueous suspension at pH 10.4. The average particle size in this polydispersed suspension was 40 i 10 A. Ten-milliliter samples were made by diluting 1-mL aliquots of a 200 pM ethanol solution of DDC with appropriate volumes of colloidal silica and deionized water at constant pH. Centrifugation of the samples at ~ 6 4 0 0 g0 for 15 h and spectrophotometry of the supernatant revealed that the dye was quantitatively removed from the solution by colloidal silica. A description of the picosecond apparatus used to measure the transient bleaching signals with collinear, copropagating, orthogonally polarized pump and probe beams was given previously.” The excitation source was a synchronously pumped rhodamine 6G dye laser, operating at a repetition rate of ==76 MHz. In the experiments described here, the optical configuration of the apparatus was modified in order to use noncollinear, copropagating pump and probe beams. The beams were focused with a IO-cm focal length achromat to a spot size of ~ 2 0 pm 0 in a sample contained in a rotating 3.1-mm path length cell. (The focal diameter was determined by transmission measurements through calibrated pinholes.) The average incident pump power was 5 2 0 mW. The pump beam was modulated at 10.24 MHz with an acoustooptic modulator before being focused into the sample. The modulation induced on the probe beam was detected with a photomultiplier detector, whose output was fed into a megahertz lock-in amplifier. Transient bleaching measurements were taken with the probe light polarized parallel (11) and perto the polarization of the pump beam. Transient pendicular (I) bleaching at the “magic angle” of 54.7’ was also measured in order to determine isotropic decay due to ground-state recovery in these aggregates. Background-free pulse intensity autocorrelation traces were obtained by replacing the sample by a KDP crystal and measuring the second harmonic signal as a function of the probe delay. The fwhm of the autocorrelation trace was typically 10-12 ps. The maximum of the autocorrelation was used to establish the position of zero delay. All the experiments were carried out at room temperature (22 f 1 ‘C). Results Spectral Data. The absorption spectra of DDC at a concentration of 20 pM in water, ethanol, and 3.75% colloidal silica are
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3685
H-Aggregates of DDC on Colloidal Silica
TABLE I: Comparison of the Fitting Parameters for the Polarized Transient Bleaching Signals in Figure 3 biexponential signal
AI
parallel magic angle perpendicular
0.767 0.767 0.767
719
PS
19.55 20.48 20.1
A2 0.233 0.233 0.233
stretched exponential 72,
PS
189.03 186.01 204.07
X2
a
6.9 x 10-3 5.6 x 10-3 6.9 x 10-3
0.458 0.460 0.455
7pr
PS
X2
6.1 x 10-3 5.0 x 10-3 5.6 x 10-3
19.6 18.9 20.3
E55
I
53
-EC
~
L .
- --
Figure 2. Absorption spectra of 20 WMD D C in (0)0.94%, (A)3.7556, and ( X ) 13.5% colloidal silica.
presented in Figure 1. The peak at 71 1 nm and the secondary peak at 653 nm in the spectrum of DDC in ethanol are respectively the 0 0 and 1 0 transitions of a vibronic progression of the monomer. The band at 621 nm in the spectrum of DDC in water can be assigned to the dimer. The spectra of DDC in water at neutral pH and pH 10.4 were identical. However, both the monomer and dimer bands were absent in the spectrum of the dye in colloidal silica. None of the samples exhibited a measurable fluorescence spectrum. The spectrum of DDC in colloidal silica was dominated by the H-band. The H-band shifted from 593 to 558 nm and became narrower as the colloid concentration was decreased from 13.5% to 0.94% (Figure 2). A peak whose maximum was nearly coincident with that of the monomer band in ethanol was clearly discernible in the spectrum of the sample containing the highest amount of silica. These spectral features were observed even when the pH was not kept at 10.4. Transient Bleaching Data. The curves SI,, S,, and SS4,,in Figure 3 were obtained respectively with probe light polarized parallel, perpendicular, and 54.7O to the polarization of the pump beam. Because of the coherent coupling artifact, the signals were peaked at zero delay.]* A semilogarithmic plot of SS4,(Figure 4) shows that the isotropic component was nonexponential. The transient bleaching curves were fitted to empirical decay functions. Straightforward deconvolution over the entire decay range can lead to an inaccurate description of the signal due to the coherent coupling artifact at zero delay. This inaccuracy can be very serious if the width of the coherent coupling artifact is on the order of the decay times. In order to extract the true decay behavior, we used the method of antisymmetrization.I3 This method makes use of the symmetry of the coherent coupling artifact about zero delay and removes the coherent coupling artifact while retaining the contributions to the signal from molecular decay processes. The antisymmetrized signal S,(t), which is given by sa(t) = (1/2)[S(t) - s(-t)l (1)
