J . Phys. Chem. 1989, 93, 6198-6201
6198
Picosecond Polarized Spectroscopy of J-Aggregates of Pseudoisocyanine on Colloidal Silica Miin-Liang Horngt and Edward L. Quitevis* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409 (Received: February 9, 1989)
Polarized transient bleaching signals of J-aggregates of I,l’-diethyL2,2’-cyanineor pseudoisocyanine on colloidal silica were obtained by using picosecond pump-probe spectroscopy. The laser was tuned to the peak of the J-band at 5 7 0 nm. This absorption band arises from the exciton states of the aggregate. The nonexponential signals were proportional to the laser intensity. However, the decay kinetics appeared to be independent of the laser intensity. For 0 < t < 160 ps, the absorption anisotropy was constant and equal to 0.42 & 0.04. These results suggest that singletsinglet exciton annihilation is not occurring and that the excitons are delocalized.
Introduction
In aqueous solution at high concentrations, in concentrated salt solutions, or on surfaces, 1,l’-diethyl-2,2’-cyanine or pseudoisocyanine (PIC) forms molecular aggregates.’ These aggregates are ideal models for light-harvesting arrays in biological systems because the molecules are in well-defined structures.* Sat0 and co-workers3recently showed that J-aggregates can be incorporated into small unilamellar vesicles, which are simple analogues of biomembrane~.~The strong electronic coupling between the molecules in a J-aggregate of PIC gives rise to a narrow absorption band at =570 nm which is red-shifted from the monomer band at 523 nm. Several theoretical models5 have been developed that relate aggregate structure and size to the spectral properties of these aggregates. In simple one-dimensional exciton models, which assume dipolar coupling, the dye molecules are stacked as in a tilted deck of card^.^,^^,^ In these models, the red shift is attributed to a tilt angle of 0’ < 0 < 54’ 44’ between the transition moments of the molecules and the axis of the aggregate. The J-aggregates of PIC in solution have also been extensively investigated in picosecond fluorescence,6transient absorption and bleaching,’ and accumulated photon-echo experiments.* We recently showed that J-aggregates of PIC and H-aggregates of I , I’-diethyl-2,2’-dicarbocyanine(DDC) can be formed on colloidal silica and that the transient bleaching signals of these aggregates decay none~ponentially.~*’~ The H-aggregates of DDC on colloidal silica are characterized by a broad absorption band between 550 and 600 nm which is blue-shifted from the monomer band at 71 1 nm. The spectral data and nonexponential signals for both dye aggregates can be explained in terms of a distribution of aggregates with different exciton lifetimes. In this paper we report that the nonexponential bleaching signals for J-aggregates of PIC on colloidal silica depended linearly on the laser intensity. However, the decay kinetics was independent of the laser intensity. For 0 < t < 160 ps, the absorption anisotropy was constant and equal to 0.42 f 0.04. These results suggest that singlet-singlet exciton annihilation is not occurring and that the excitons are delocalized. Experimental S e c t i o n
1 ,I’-DiethyL2,2’-cyanineiodide (Kcdak Chemical Co.) was used without further purification. The stock solution of colloidal silica (Nalco 1 1 15) was a 15% (wt/vol) aqueous suspension at pH 10.4. The average particle size in this polydispersed suspension was 40 f 10 A. Ten-milliliter samples were made by diluting 50-pL aliquots of a I O mM methanol solution of PIC with 5 mL of colloidal silica and 5 mL of deionized water at constant pH. The final concentrations were =50 pM PIC and 7 . 5 % colloidal silica. Detailed descriptions of the picosecond apparatus and the ~ ~ ’ excitation ~ source method of analysis were given p r e v i o ~ s l y . The was a synchronously pumped rhodamine 6G dye laser, tuned to
’Robert 4
Welch Foundation Predoctoral Fellow
0022-3654/89/2093-6 I98$01.50/0
TABLE I: Biexponential Fitting Parameters for the Polarized Transient Bleaching Signals in Figure 1.
signal parallel magic angle
perpendicular
AI 0.66 0.66 0.66
TI.
DS
23.94 22.4 20.75
A, 0.34 0.34 0.34
7,.
DS
335.91 336.43 334.31
Y2
5.5 5.0
X io-’ X lo-’
5.6 X IO-’
570 nm and operating at a repetition rate of =76 MHz. Noncollinear, copropagating pump and probe beams were used. The polarization of the probe beam with respect to the pump beam was changed with a polarization rotator. The beams were focused with a 10-cm focal length achromat to a spot size of =200 pm in a sample contained in a rotating 3.1-mm path length cell. 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 Megaherz lock-in amplifier. 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
( 1 ) (a) Jelley, E. E. Nature (London) 1936, 138, 1009. (b) Scheibe, G . Angew. Chem. 1936,49,563. (c) Mattoon, R. W. J . Chem. Phys. 1944,12, 268. (d) Zimmermann, H.; Scheibe, G. Z. Electrochem. 1956,60,566. (e) Cooper, W. Chem. Phys. Lett. 1970, 7, 73. (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. I. J. Phys. Chem. 1975, 79, 746. (3) Sato, T.; Yonezawa, Y.; Hada, H . J. Phys. Chem. 1989, 93, 14. (4) Fendler, J. H. Acc. Chem. Res. 1980, 13, 7. (5) (a) McRae, E. G.; Kasha, M. In Physical Processes in Radiation Biology; Augenstein, L., Mason, R., Rosenberg, B., Eds.; Academic Press: New York, 1964; pp 23-42. (b) McRae, E. G.; Kasha, M. J . Chem. Phys. 1958, 28, 721. (c) McRae, E. G. Aust. J. Chem. 1961, 14, 354. (d) Czikkely, V. Forsterling, H . D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 207. (e) Scherer, P. 0. J.; Fischer, S. F. Chem. Phys. 1984, 86, 269. (f) Knapp, E. W. Chem. Phys. 1984, 85, 73. (g) Juzeliunas, G . Z. Phys. D 1988, 8, 379. (6) (a) Fink, F.; Klose, E.; Teuchner, K.; Dahne, S. Chem. Phys. Lett. 1977, 45, 548. (b) Yu, Z. 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. (7) (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. (c) Sundstrom, V.; Gillbro, T.; Gadonas, R. A,; Piskarskas, A. J . Chem. Phys. 1988, 89, 2754. ( 8 ) DeBoer, S.;Vink, K. J.; Wiersma, D. A. Chem. Phys. Lett. 1987, 137, 99. (9) (a) Quitevis, E. L.; Horng, M.-L.; Chen, S.-Y. J . Phys. Chem. 1988, 92. 256. (b) Quitevis, E. L.; Horng, M.-L.;Chen, S.-Y. In Advances in Laser Science I I I , Optical Science and Engineering Series 9; Tam, A. C . , Gale, J. L., Stwalley, W. C . , Eds.; American Institute of Physics: New York, 1988; pp 697-699. (10) Chen, S . - Y . ;Horng, M.-L.;Quitevis, E. L. J . Phys. Chem. 1989, 93, 3683.
0 1989 American Chemical Society
J-Aggregates of Pseudoisocyanine 1650
The Journal of Physical Chemistry, Vol. 93, No. 16, 1989 6199
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