Luminescence Properties of the Mixed J-Aggregate of Two Kinds of

Sep 27, 2000 - The layer-by-layer alternate assemblies incorporating two kinds of cyanine dyes have been fabricated by alternately adsorbing a cationi...
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J. Phys. Chem. B 2000, 104, 9630-9637

Luminescence Properties of the Mixed J-Aggregate of Two Kinds of Cyanine Dyes in Layer-by-Layer Alternate Assemblies Noritsugu Kometani,*,† Hiroyoshi Nakajima,† Kenji Asami,† Yoshiro Yonezawa,† and Okitsugu Kajimoto‡ Department of Applied Chemistry, Faculty of Engineering, Osaka City UniVersity, Sugimoto 3-3-138, Sumiyoshi-ku, Osaka, 558-8585, Japan, and DiVision of Chemistry, Graduate School of Science, Kyoto UniVersity, Kitashirakawa, Sakyo-ku, Kyoto, 606-8502, Japan ReceiVed: April 27, 2000; In Final Form: June 29, 2000

The layer-by-layer alternate assemblies incorporating two kinds of cyanine dyes have been fabricated by alternately adsorbing a cationic polyelectrolyte and anionic cyanine dyes on the quartz plate. A thiacyanine dye (dye I) was employed as the donor and two kinds of thiacarbocyanine dye having a meso-alkyl groups m-ethyl (dye II), m-methyl (dye III)sas the acceptor. The mole fraction of the acceptor in the mixed J-aggregate, χ, was varied from 0 to 1. It is confirmed that these dye combinations form the mixed J-aggregate in the alternate assemblies. From steady-state fluorescence spectra of the molecular assemblies, excitation energy transfer from the donor J-aggregate to the acceptor J-aggregate is observed, whose kinetics obeys the SternVolmer relationship. The experimentally determined rate constant of energy transfer, kET, is fairly large, indicating efficient energy transfer due to exciton migration through the donor J-aggregate. The relative fluorescence quantum yield and the fluorescence lifetime of the acceptor aggregate decrease with increasing χ, implying the considerable self-quenching of acceptor fluorescence. The relative change of the coherent size of the dye II aggregate has been estimated from the J-band line width and the radiative decay rate constant. It is found that the coherent size of the dye II aggregate is increased by a factor of 4-5 with increasing χ from 0.008 to 1.

Introduction The J-aggregate of cyanine dyes exhibits several remarkable optical properties: a sharp absorption peak J-band which is redshifted from the monomer band, resonance fluorescence with ultra-short lifetime, and large optical nonlinearity.1-4 These properties arise from the delocalization of the Frenkel exciton over chromophores well-arranged in one- or two-dimensional structure.5,6 From a viewpoint of practical applications to optoelectronic devices in the future, the J-aggregate of cyanine dyes supported in a variety of matrixes has been fabricated: in liquids,1,2 in rigid solutions,7 in thin organic films such as Langmuir-Blodgett (LB) films8 and polymer films,9,10 in silver halide emulsions,11,12 etc. Recently, Decher et al. have devised a novel method to fabricate the ultrathin multilayers by means of the alternate adsorption of cationic and anionic polyelectrolytes.13 This method has the advantage of a relatively simple procedure in comparison with the conventional LB technique. In the previous studies,14,15 we successfully applied this method to the preparation of the layer-by-layer alternate assemblies (alternate assemblies) incorporating the J-aggregates of anionic cyanine dyes. It has been confirmed that the alternate assemblies have much higher stability and versatility than the LB films. Furthermore, we have observed the formation of the mixed J-aggregate of two kinds of cyanine dyes, thiacyanine dye (TC) and thiacarbocyanine dye (TCC). The fluorescence spectra of the mixed J-aggregate TC + TCC shows strong quenching of * Author to whom correspondence should be addressed. Fax: +81-66690-2743. E-mail: [email protected]. † Osaka City University. ‡ Kyoto University.

the donor fluorescence and sensitization of the acceptor fluorescence. For a past decade, excited-state dynamics of the J-aggregates has attracted much interest.3,4,9-12,16-21 The coherent size N* of Frenkel excitons is one of the most important factors to determine the optical properties and the radiative decay rate of the J-aggregate, and it is generally smaller than the physical size of the J-aggregate.3,16 However, the works directly related to the size effect on excited-state dynamics of the J-aggregate in thin organic films have been rather rare. In the course of the former work,15 we have noticed the shift and the broadening of the J-bands of the donor and the acceptor aggregates as the mole fraction of the acceptor χ in the mixed J-aggregates is changed. This indicates a possibility that the coherent size N* of the J-aggregate may be controlled by χ, which gives us the opportunity to investigate size-dependent excited-state dynamics of the J-aggregate. In this study, we have fabricated the layer-by-layer alternate assemblies incorporating the mixed J-aggregate by using two different combinations of thiacyanine dye and thiacarbocyanine dye. The optical properties of the mixed J-aggregate have been examined by means of steady-state absorption spectroscopy, fluorescence spectroscopy, and picosecond fluorescence lifetime measurements. On the basis of experimental observations are discussed excited-state dynamics of the mixed J-aggregate, excitation energy transfer from the donor J-aggregate to the acceptor J-aggregate, self-quenching of the acceptor J-aggregate, and the coherent size of the acceptor J-aggregate. Hereafter, we shall call one J-aggregate whose J-band is located at shorter wavelength a donor (J-)aggregate and another one an acceptor (J-)aggregate.

10.1021/jp001614t CCC: $19.00 © 2000 American Chemical Society Published on Web 09/27/2000

Luminescence of J-Aggregate of Cyanine Dyes

Figure 1. Molecular structures of PDDA and cyanine dyes (dye I-III) used in this study. The schematic structure of the alternate assembly is illustrated at the bottom.

J. Phys. Chem. B, Vol. 104, No. 41, 2000 9631 The amount of cyanine dyes incorporated in the alternate assembly was determined by the following procedure. The sample plate was immersed in a 5 mL hot 0.1% (w/v) PSS aqueous solution for 15 min to dissolve dye molecules into the solution. After dilution up to 10 mL, the solution was subjected to spectrophotometric analysis. In the absorption spectra, monomer bands of the donor and the acceptor were observed, from whose absorbance dye concentrations were determined. UV-vis absorption spectra were recorded on a UV-2200 spectrophotometer (Shimadzu Seisakusho). Steady-state fluorescence spectra were measured with a PMA-10 detection system (Hamamatsu Photonics) with the excitation source of a 500 W xenon arc lamp whose wavelength was adjusted through a monochromator. Picosecond lifetime measurements were carried out by the method of time-correlated single-photon counting. The second harmonic radiation (420 nm, 8.2 MHz) of the mode-locked Ti: sapphire laser (Spectra Physics, Tsunami Model 3950) was used as an excitation source. The pulse width of the excitation light was about 1.0 ps. The wavelength of monitoring fluorescence was selected through the monochromator. The arrangement of the optical system was almost the same as that previously reported.22 In this measurement, fwhm of the instrumental response function was about 40 ps. To examine the excitedstate dynamics of the donor and the acceptor aggregates separately, the fluorescence decay curves were measured at 475 nm (donor) and 640 nm (acceptor), respectively. Fluorescence decay curve was fitted to the multiple exponential function deconvoluted with the instrumental response function.23 Typically, two exponential functions were necessary to reproduce the observed decay curve. The weighted mean fluorescence lifetime was calculated from the fitted parameters. It was noticed that the amplitude factor for the fast component of the doubleexponential function is usually more than 9 times larger than that for the slow component. The experimental error of the finally obtained lifetime was estimated to be about (10 ps. All the measurements were performed at room temperature in air.

Experimental Section The molecular structures of cyanine dyes and PDDA used in this study are illustrated in Figure 1. 3,3′-Disulfopropyl-5,5′dichloro-thiacyanine sodium salt (dye I), 3,3′-disulfopropyl-5,5′dichloro-9-ethyl-thiacarbocyanine potassium salt (dye II), and 3,3′-disulfopropyl-5,5′-dichloro-9-methyl-thiacarbocyanine triethylammonium salt (dye III) were purchased from the Japanese Research Institute for Photosensitizing Dyes (Okayama). PDDA (MW ) 400 000-500 000) and poly(sodium styrenesulfonate) (PSS, MW ) 70 000) were obtained from Aldrich. These materials were used without further purification. The alternate adsorption method has been carried out by the conventional manner.14,15 Briefly, the surface of the quartz plate, 10 × 10 × 1 mm3 in size, was first made hydrophilic by immersion in a hot H2SO4/H2O2 (7:3) bath for 1 h and then in a H2O/H2O2/ NH3 (5:1:1) bath at 60 °C for 30 min. Then, the plate was carefully rinsed in distilled water. The alternate assembly was built up by immersing the plate alternately for 30 min in a 1.0% (w/v) PDDA aqueous solution and a mixed solution of cyanine dyes in water. Here we have selected two combinations of cyanine dyes, donor + acceptor; dye I + dye II, dye I + dye III. The molar mixing ratio of [donor]:[acceptor] in the aqueous solution was varied from 1:0 to 0:1, while the total concentration was kept constant (5 × 10-5 M). All the samples were prepared by repeating the above cycle three times. A schematic structure of the alternate assembly is illustrated at the bottom of Figure 1.

Results Dye I + Dye II Assembly. The absorption spectra of dye I + dye II assembly with varying dye II mole fractions χdye II ) [dye II]/([dye I] + [dye II]) are shown in Figure 2a. The sharp peaks around 465 and 625 nm are assigned to the J-bands of dye I and dye II, respectively. The absorbance at the J-band of dye I decreases and that of dye II increases approximately linearly with increasing χ. This means that the dye concentration in the alternate assembly is well controlled by changing the ratio of [dye I]:[dye II] in the solution. The peak position of the J-band is plotted as a function of χdye II in Figure 2b. It is found that the J-bands of both dye I and dye II tend to be blue-shifted and become broader on decreasing the concentration of the corresponding dye. The increase in χdye II would bring about the decrease in the physical size of the dye I aggregate, causing the reduction of coherent size of the dye I aggregate as evidenced by the blue shift and broadening of the dye I J-band.5,24 At the same time, coherent size of the dye II aggregate increases, resulting in the red shift and narrowing of the dye II J-band. The fluorescence spectra of the dye I + dye II assembly are shown in Figure 3a. The excitation wavelength is 410 nm where only dye I aggregate is excited. In Figure 3a, we can see strong resonance fluorescence of the dye I aggregate at 470 nm in the absence of dye II (χdye II ) 0). However, resonance fluorescence of the dye I aggregate is almost completely quenched and that

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Figure 2. (a) Absorption spectra of the dye I + dye II alternate assemblies with various mole fractions of dye II. (b) Dependence of the peak position of the J-band on χdye II.

Figure 3. (a) Fluorescence spectra of the dye I + dye II alternate assemblies for χdye II ) 0.000-0.700 under the excitation of 410 nm light. (b) Plots of the normalized fluorescence intensity of the donor (dye I) aggregate and the acceptor (dye II) aggregate as functions of χdye II.

of the dye II aggregate (640 nm) develops in return when a small amount of dye II (e.g., χdye II ) 0.0133 in Figure 3a) is mixed. These observations would demonstrate efficient energy transfer from the donor (dye I) J-aggregate to the acceptor (dye II) J-aggregate. To treat the fluorescence quenching quantitatively, the normalized fluorescence intensity was evaluated by the spectral area of resonance fluorescence divided by the absorbance at the excitation wavelength. Figure 3b shows the plots of the normalized fluorescence intensity of the donor aggregate and the acceptor aggregate as functions of χdye II. It is clear in this figure that the donor fluorescence is rapidly quenched and the acceptor fluorescence is sensitized by excitation energy transfer. The fluorescence lifetime of the alternate assembly was measured by using the 420 nm laser pulse as an excitation source. Figure 4 shows the instrumental response function and the fluorescence decay curves of the donor aggregate in the dye I + dye II assembly monitored at 475 nm. As a result of the deconvolution analysis, the fluorescence lifetime 120 ( 10 ps was obtained for the pure dye I aggregate (χdye II ) 0), while it decreases to less than the resolution limit for χdye II ) 0.013, in agreement with fast excitation energy transfer to the dye II aggregate. Dye I + Dye III Assembly. Figure 5a shows the absorption spectra of the dye I + dye III alternate assembly. The J-band

Figure 4. The instrumental response function and fluorescence decay curves of the dye I + dye II assemblies for χdye II ) 0.000, 0.013 monitored at 475 nm. The wavelength of the excitation laser light is 420 nm.

of dye III appears around 630 nm in addition to the J-band of dye I at about 465 nm. A peak corresponding to the H-aggregate (H-band) of dye III is also seen around 475 nm for χdye III higher

Luminescence of J-Aggregate of Cyanine Dyes

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Figure 5. (a) Absorption spectra of the dye I + dye III alternate assemblies with various mole fractions of dye III. (b) Dependence of the peak position of the J- or H-band on χdye III.

Figure 6. (a) Fluorescence spectra of the dye I + dye III alternate assemblies for χdye III ) 0.000-0.807 under the excitation of 410 nm light. (b) Plots of the normalized fluorescence intensity of the donor (dye I) aggregate and the acceptor (dye III) aggregate as functions of χdye III.

than 0.8 in Figure 5a. The peak position of the J- or H-band is plotted as a function of χdye III in Figure 5b, which displays almost analogous behavior as observed in the dye I + dye II assembly (Figure 2b). Broadening of the J-band at the lower concentration of the corresponding dye is evident in Figure 5a, too. Fluorescence spectra of the dye I + dye III assembly are shown in Figure 6a. The normalized fluorescence intensity of the donor and the acceptor aggregates is plotted against χdye III in Figure 6b, which demonstrates efficient energy transfer from the donor (dye I) aggregate to the acceptor (dye III) aggregate. It is noted that in the case of the dye I + dye III assemblies, the 410 nm light excites not only the J-band of the donor aggregate but also the H-band of the acceptor in the large χdye III region. Discussion Excitation Energy Transfer between the Donor and the Acceptor J-Aggregates. To describe excitation energy transfer between the donor (dye I) aggregate to the acceptor (dye II, dye III) aggregate, we have introduced the Stern-Volmer kinetics as an approximation:25,26

I0 ) 1 + K[A] I

(1)

where I0 and I are the normalized fluorescence intensity of the donor aggregate in the absence and the presence of the acceptor aggregate, [A] is molar concentration of the acceptor (molecules‚ nm-2). K denotes a Stern-Volmer constant (nm2‚molecules-1), from which one can estimate the rate constant of energy transfer kET (nm2‚molecules-1‚s-1):

K ) kETτD

(2)

where τD (s) is the fluorescence lifetime of the donor aggregate in the absence of the acceptor. Since dye I aggregate is a common donor in our study, the fluorescence lifetime of pure dye I aggregate τD of about 120 ( 10 ps (Figure 4) has been used. Figure 7 represents the Stern-Volmer plots for (a) dye I + dye II and (b) dye I + dye III, respectively. Considering the weak fluorescence intensity of the donor aggregate, we carried out the measurements repeatedly. It should be noted that these plots cover only a small χ region where the concentration of the acceptor is rather low (χdye II ) 0-0.14, χdye III ) 0-0.29). Such limitations come from rather weak donor fluorescence at high acceptor concentrations. In Figure 7, both plots approximately fit to straight lines within the experimental errors. The slopes obtained from least-squares fitting are summarized in Table 1, together with the rate constant kET. It is noticed in

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Figure 8. The instrumental response function and fluorescence decay curves of (a) dye I + dye II assemblies for χdye II ) 0.013, 0.050, 1.000, and (b) dye I + dye III assemblies for χdye III ) 0.008, 0.052, 1.000 monitored at 640 nm. Figure 7. Stern-Volmer plots of the normalized fluorescence intensity of the donor aggregate in (a) the dye I + dye II, (b) the dye I + dye III, respectively.

TABLE 1: Summary of K and kET for the Dye I + Dye II and Dye I + Dye III Assemblies cyanine dye dye I + dye II dye I + dye III

K kET (103 nm2‚molecules-1) (1013 nm2‚molecules-1‚s-1) 2.5 ( 0.3 1.7 ( 0.2

2.1 ( 0.3 1.4 ( 0.2

Table 1 that the Stern-Volmer constants are in the order of 103 nm2‚molecules-1 and thus kET are in the order of 1013 nm2‚molecules-1‚s-1. The decreased fluorescence lifetime of the donor aggregate less than the resolution limit for χdye II ) 0.013 (Figure 4) is consistent with fairly large kET. The Stern-Volmer constants in Table 1 are as large as that reported for excitation energy transfer from the oxacyanine aggregate (donor) to thiacyanine molecules (acceptor) in the Langmuir-Blodgett films.26 In the small χ region (Figure 7), where the physical size of the donor aggregate is fairly large, acceptor molecules would take the form of small aggregates and be distributed in the mixed J-aggregate. As the coherent size of the molecular exciton of the dye I aggregate is usually much smaller than the physical size, the molecular exciton generated inside the donor aggregate should migrate many times until reaching the vicinity of the acceptor aggregate, at which site excitation energy transfer takes place.26,27 Fluorescence Self-Quenching of the Acceptor Aggregate. The fluorescence decay curves of the acceptor aggregate in the

TABLE 2: Fluorescence Lifetime, τ, and Relative Fluorescence Quantum Yield, O, of the Acceptor Aggregate for Dye I + Dye II and Dye I + Dye III Assemblies at Various Mole Fractions of the Acceptor dye I + dye II assembly χdye II 0.000 0.013 0.020 0.027 0.050 0.099 0.253 0.534 0.700 1.000

τ/ps 759 510 436 270 161 60 45 107 51

dye I + dye III assembly

φ

χdye III

τ/ps

φ

1.000 0.733 0.612 0.342 0.236 0.167 0.181 0.193 0.214

0.000 0.008 0.025 0.040 0.052 0.108 0.349 0.692 0.807 1.000

725 565 261 225 104 72 75 19 92

1.000 0.842 0.428 0.380 0.192 0.074 0.067 0.053 0.029

dye I + dye II and dye I + dye III assemblies monitored at 640 nm are given in Figure 8a,b, respectively. Since the excitation wavelength is 420 nm, the acceptor aggregate is excited through energy transfer from the donor aggregate. In the case of χdye III higher than 0.8, certain acceptor (dye III) aggregate is excited via the H-band of dye III as well. In the analysis of the decay curves, no rise time was detected, indicating that the excitation of the acceptor aggregate is almost instantaneous. This is consistent with the fast decay of the donor fluorescence in Figure 4. The fluorescence lifetime of the acceptor aggregate in the dye I + dye II and dye I + dye III assemblies is summarized in Table 2. It is found that the fluorescence lifetime of the acceptor aggregate remarkably

Luminescence of J-Aggregate of Cyanine Dyes

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Figure 9. The fluorescence spectra of the dye I + dye II assemblies for χdye II ) 0.013, 0.253, 1.000 under the excitation with 565 nm light.

decreases with increasing χ, in particular for χ ) 0-0.1. These observations mean the enhancement of radiative and/or nonradiative decay rate processes of the acceptor aggregate. We have attempted to estimate the relative fluorescence intensity of the acceptor aggregate by direct excitation with 565 nm light. The typical fluorescence spectra of the dye I + dye II assemblies are shown in Figure 9. The relative fluorescence quantum yield φ of the acceptor aggregate was calculated from the spectral area divided by the absorbance at the excitation wavelength and is included in Table 2. φ rapidly decreases with increasing χ from 0 to 0.1 for both assemblies, in agreement with serious quenching of acceptor fluorescence (self-quenching). This means that the decrease of the fluorescence lifetime in Table 2 is at least in part ruled by the increase of nonradiative decay rate of the acceptor aggregate. To get further insight into the self-quenching of the molecular exciton inside the acceptor aggregate, we have tested the empirical relationships to represent the concentration dependence of the fluorescence intensity and lifetime of the acceptor aggregate (quasi-Stern-Volmer relation):28

log log

( ) ( )

I0 - 1 ) n log [A] + K I

(3)

τ0 - 1 ) n log [A] + K τ

(4)

where I0 and I are the normalized fluorescence intensity of the acceptor aggregate, τ and τ0 the corresponding fluorescence lifetimes, [A] the concentration of the acceptor, and n, K the constants. For convenience, I and τ at the smallest concentration of the acceptor (χdye II ) 0.013, χdye III ) 0.008) are chosen as I0 and τ0, respectively. Figure 10a,b shows the logarithm plots of (I0/I - 1) and (τ0/τ - 1) against [A] for the dye I + dye II and dye I + dye III assemblies, respectively. For both assemblies, the plots are almost linear and overlapped with each other for the concentration less than 0.4 nm-2. This corresponds to χ ) 0-0.1. Parameters in eqs 3 and 4 are n ) 0.6, K ) 2.9 for the dye I + dye II assembly, and n ) 0.8, K ) 3.2 for the dye I + dye III assembly. To search for the nature of the “quencher” controlling the fluorescence self-quenching, the fluorescence spectrum of the alternate assembly was measured at 77 K. Figure 11 shows fluorescence spectra of dye I + dye III assemblies at room temperature and 77 K, respectively. For χdye III ) 0.008 where the fluorescence self-quenching is not appreciable, the spectra at room temperature and 77 K are almost the same except that the peak position is slightly red-shifted at 77 K. On the other hand, for χdye III ) 1.0 where the fluorescence self-quenching

Figure 10. Logarithm plots of (I0/I - 1) (circle) and (τ0/τ - 1) (square) against the concentration of the acceptor for (a) dye I + dye II and (b) dye I + dye III assemblies, respectively.

Figure 11. The fluorescence spectra of the dye I + dye III assemblies for (a) χdye III ) 0.008, and (b) χdye III ) 1.000 measured at room temperature and 77 K.

is most remarkable, a broad peak develops around 750 nm at 77 K. In addition, it is observed that the intensity of this band gradually increases with increasing χdye III for χdye III ) 0.0081.000. From these observations, we propose that the broad peak around 750 nm assigned to certain defect species inside the J-aggregate29 may play the role of the “quencher” for the fluorescence self-quenching. Muenter and co-workers have

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Figure 13. The schematic illustration of the structure of mixed J-aggregates for χ , 1, ∼0.5, and ∼1.

Figure 12. Plots of the relative coherent size N* of the dye II aggregate against the concentration of dye II. The circle and triangle represent N* calculated by eqs 5 and 6, respectively.

reported a similar broad fluorescence band of the J-aggregate of pseudoisocyanine dye at low temperature.12 Considering the linear relation in Figure 10a,b, the concentration of defect species would increase with increasing [A] in the power of 0.60.8. This would reflect the rapid decrease of τ with increasing χ in the small χ region and the gradual decrease in the large χ region as seen in Table 2. Coherent Size and Structure of the J-Aggregate. Several theories have been used to estimate the coherent size N* of the J-aggregate from the spectroscopic properties: the J-band shift, the J-bandwidth, the radiative decay rate constant.3,16 A simple relation between N* and the J-bandwidth ∆ν1/2(N*) with respect to the monomer bandwidth ∆ν1/2(M) is given by5

∆ν1/2(M) ∆ν1/2(N*)

) xN*

(5)

Further, the equation connecting the radiative decay rate constant of the J-aggregate kr(N*) with that of the monomer kr(M) has been frequently used:27,30

kr(N*) ) N*kr(M)

(6)

By using eqs 5 and 6, we can estimate the relative change of N* as a function of χ. The bandwidth was evaluated from absorption spectra of the acceptor aggregate. The ratio kr(N*)/ kr(M) was calculated by the fluorescence lifetime and the relative fluorescence quantum yield φ of the acceptor aggregate. Figure 12 shows the relative change of N* of the acceptor aggregate in the dye I + dye II assembly. N* increases with increasing χdye II by a factor of 4-5. N* obtained from the J-bandwidth is not inconsistent with one obtained from the radiative decay rate constant. These results unequivocally suggest that the coherent size of the dye II aggregate actually increases with increasing χdye II. A saturating behavior of N* in the large χdye II region indicates that N* has a finite value even if the physical size of the dye II J-aggregate is very large as a result of the disturb of the coherence between chromophores due to, e.g., the excitonphonon coupling, disorder of the J-aggregate, and so on.30,31 N* of the dye III aggregate estimated by the J-bandwidth (Figure 5a) increases with increasing χdye III. However, at the same time, N* from the radiative decay rate constant does not change so much. Such a disagreement may come from the presence of the H-band of dye III in the large χdye III region, which makes the estimation of N* by eqs 5 and 6 rather obscure.

Tani and co-workers have reported the aggregate-size (N)dependent fluorescence lifetime of the J-aggregate of dye II in the AgBr emulsions.11 Lanzafame and co-workers have controlled the size of the J-aggregate of the same dye by diluting with thiacyanine dye (statistical dilution method).32 It has been known that dye II molecules can form many types of Jaggregate, each of which is characterized by the different J-band.33 Kemnitz and co-workers have reported that the J-band of dye II adsorbed on SiO2 is located at λ ) 620 nm, while that on AgBr is located at λ ) 643-651 nm.34 Although dye II aggregate exhibits certain complicated behaviors according to the preparation conditions, we have assumed that dye II aggregate in the alternate assemblies in this study may be compared with those reported by Tani and co-workers and Lanzafame and co-workers, as we are much interested in the luminescence properties of the dye II aggregate. Tani and coworkers have attributed the decrease of the fluorescence lifetime with increasing N to the superradiane.11 In contrast to Tani et al., Lanzafame et al. have concluded that the decrease of the fluorescence lifetime with the increase of N is caused by the increase of the nonradiative decay rate.32 If we assume the absolute fluorescence quantum yield of the dye II aggregate to be 0.01-0.1 at χ ) 1.0,35 about 15-20 times increase of the nonradiative decay rate constant, together with 4-5 times increase of the radiative decay rate constant with χ are estimated. Such considerations mean that the increase of the nonradiative decay rate constant, which ultimately causes the self-quenching of the dye II fluorescence, is more operative in the decrease of the fluorescence lifetime than that of the radiative decay rate constant. Reconsidering the works of Tani et al. and Lanzafame et al. from such a viewpoint, it is concluded that both radiative and nonradiative decay rate constants are dependent on N. Finally, we would mention the structure of the mixed J-aggregates in this study. In the former paper,36 we have classified the structure of the mixed J-aggregate into four cases according to their spectral properties. According to the apparent structure, one extreme situation is the separate type (S-aggregate) in which two kinds of cyanine dyes are completely separated and two independent J-bands are observed. Another extreme is the homogeneous mixing of two components in which either a single amalgamated J-band appears (HA type) or two J-bands persist (HP-type). In addition to those extremes, there is an intermediate, mosaic type (M-aggregate) in which two kinds of cyanine dyes are mixed like a mosaic pattern and the positions of two J-bands somewhat depend on the molar mixing ratio of dyes. In this study, we have observed that the mixed Jaggregates, dye I + dye II, dye I + dye III, are characterized by the two J-bands in the absorption spectra whose peak positions are blue-shifted with decreasing mole fraction of the corresponding dye (Figures 2, 5). Therefore, we may call them a partitioning type (P-aggregate) which belongs in general to the M-aggregate but partitioning of the donor J-aggregate by the acceptor J-aggregate and vice-versa are evident (Figure 13).

Luminescence of J-Aggregate of Cyanine Dyes This is somewhat similar to the concept; “statistical dilution” of the J-aggregate proposed by Lanzafame et al.32 It is obvious that excitation energy transfer depends on the structure of the mixed J-aggregate. In the case of the Saggregate, the donor aggregate and the acceptor aggregate are regarded as a kind of supermolecules, respectively. As an example, the combination of 5,6,5′,6′-dibenzo-1,1′-diethyl-2,2′cyanine chloride (Sch: donor) and 5,5′-dichloro-3,3′-diethyl9-phenyl-thiacarbocyanine chloride (Thia(φ): acceptor) forms the S-aggregate in the LB films.37 Excitation energy transfer from the Sch aggregate to the Thia(φ) aggregate is characterized by rather small Perrin area of 7 nm2‚molecules-1. On the other hand, in the case of the P-aggregate, the donor J-aggregate and the acceptor J-aggregate are not isolated but somewhat mixed each other. Therefore, the Stern-Volmer constant as determined in the present work; 1.7-2.5 × 103 nm2‚molecules-1 is considerably larger than 7 nm2‚molecules-1 (Sch + Thia(φ) assembly). Summary and Conclusions In this article, we have examined excited-state dynamics of the mixed J-aggregate of two kinds of cyanine dyes incorporated in layer-by-layer alternate assemblies. For the dye I + dye II and dye I + dye III combinations, the mixed J-aggregates are formed, whose structure is classified into partitioning type. These mixed J-aggregates are characterized by efficient quenching and sensitization of resonance fluorescence and fairly large rate constant of energy transfer kET in the order of 1 × 1013 nm2‚molecules-1‚s-1. The marked self-quenching of acceptor fluorescence has been observed when χ is large. The fluorescence spectrum of the dye I + dye III assembly at 77 K shows a broad peak around 750 nm, which is assigned to the defect species responsible to the “quencher” for the self-quenching. In both dye I + dye II and dye I + dye III assemblies, the red-shift and the sharpening of the J-band of the acceptor aggregate with increasing χ are observed. The estimation of the relative coherent size N* of the acceptor aggregate for the dye I + dye II assembly indicates that N* is increased by a factor of 4∼5 in the region χdye II ) 0.008-1.0. In conclusion, we have suggested the existence of two kinds of size dependence of the dye II aggregate in the mixed assemblies; enhancement of the radiative decay by superradiance and increased nonradiative decay with increasing χ. Acknowledgment. The authors thank Prof. Y. Miura, Osaka City University, for permission to use a UV-2200 spectrophotometer.

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