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Influence of the Molecular Structure on the Lateral Distribution of Xanthene Dyes in Langmuir-Blodgett Films Dimitri Pevenage, Mark Van der Auweraer,* and Frans C. De Schryver Laboratory for Molecular Dynamics and Spectroscopy, KU Leuven, Department of Chemistry, Celestijnenlaan 200 F, B-3001 Heverlee, Belgium Received April 12, 1999. In Final Form: August 5, 1999 Steady-state and time-resolved fluorescence experiments suggest an important difference in the lateral distribution of two rhodamine dyes, differing by one nitrogen substituent, upon incorporation in LangmuirBlodgett (LB) films. While the fluorescence decay curves of both dyes could be analyzed as a stretched exponential, the decay parameters of the NH derivative suggest that the fluorescence is, over the complete emission band, due to a single species, probably the monomer. The concentration dependence of the decay parameters suggests furthermore that the molecules of the NH derivative are distributed homogeneously over the Langmuir-Blodgett film. The fluorescence decay parameters of the N-ethyl derivative suggest on the other hand a distribution of two phases with a different dye concentration. For the latter molecule the wavelength dependence of the decay parameters suggests the presence of several emitting species.
Introduction The preparation of supramolecular devices requires the knowledge and if possible the control over the spatial distribution of the constituting molecules. LangmuirBlodgett-Kuhn (LB) films have been used to obtain model systems for the light harvesting system in biological membranes and for the photophysical processes occurring in photographic and electrophotographic applications.1-3 Kuhn et al. proved that complex supramolecular systems could be built which were able to harvest the light and conduct it to a predetermined site by energy and electron transfer.4,5 The hole injection efficiency6 and fluorescence intensity7 of dioctadecyl oxacarbocyanine dyes incorporated in Langmuir-Blodgett films decreases at high concentration due to the formation of dimers and aggregates which compete for the excitation light and act as efficient energy traps of the monomer excitation. The extent of dye aggregation is dependent on the pH of the subphase. At higher pH values, a better mixing of the dye with the arachidate matrix leads to a decreased aggregate formation.7 The photophysical properties of a LB film are highly dependent on the chemical structure of the dyes, and even small structural changes e.g., in squarines dyes, will induce different distributions of the dye in the monolayer leading to different photophysical properties.8 Fluorescence decays have been used to investigate the distribution of chromophores in a LB film.9-11 Although * To whom correspondence should be addressed. (1) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandrosss, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (2) Roberts, G. G.; Vincett, P. S.; Barlow, W. A. Phys. Technol. 1981, 12, 69. (3) Roberts, G. In Langmuir-Blodgett-films; Roberts, G., Eds.; Plenum Press: New York and London, 1990. (4) Kuhn, H. Pure Appl. Chem. 1981, 53, 2105. (5) Kuhn, H. Thin Solid Films 1983, 99, 1. (6) Willig, F.; Van der Auweraer, M. Isr. J. Chem. 1995, 25, 274. (7) Biesmans, G.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 1990, 6, 277. (8) Li, Jin-ru; Li, Bao-fang; Li, Xing-chang; Tang, Ji’an; Jiang, Long Thin Solid Films 1996, 287, 247. (9) Ballet P.; Van der Auweraer, M.; De Schryver, F. C.; Lemmetyinen, H.; Vuorimaa, E. J. Phys. Chem. 1996, 100, 13701.
the fluorescence decay curves of LB films are sometimes analyzed as a sum of exponentials, assuming a compartimental distribution of the dyes in the LB film,12-16 most authors use a “stretched” exponential to analyze the decay curves.9,10,17 This stretched exponential decay is attributed to energy transfer from the excited monomers to dimers and aggregates. In the absence of donor-donor transfer, at acceptor concentrations that are sufficiently small to avoid multiple occupation of the same site in the layer and in case that transfer is operating in the dynamic or static limits, the fluorescence decay of a donor will, for a d-dimensional system, be given by a stretched exponential.18-20
I(t) ) I0 exp[-(t/τ) - γd(t)d/6]
(1)
which becomes for a two-dimensional system
I(t) ) I0 exp[-(t/τ) - γ2(t)1/3]
(2)
γ2 ) Γ(2/3)πσAR02(τ)-1/3
(3)
with
where I(0), τ, d, Γ, σA, and R02 correspond to respectively (10) Laguitton-Pasquier, L.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 1998, 14, 5172. (11) Vuorimaa, E.; Lemmetyinen, H.; Ballet, P.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 1997, 13, 3009. (12) Anfinrud, P.; Causgrove, T.; Struve, W. J. Phys. Chem. 1986, 90, 5887. (13) Anfinrud, P.; Crarkel, R. L.; Struve, W. J. Phys. Chem. 1984, 88, 5873. (14) Shigong, M.; Xingze, L.; Song, L.; Peng, W.; Zhou, J.; Yu, Z.; Wang, W.; Zhang, Z. Langmuir 1995, 11, 2751. (15) Sluch, M. I.; Vitukhnovsky, A. G.; Petty, M. C. Thin Solid Films 1996, 284, 622. (16) Yu, Q.; Vuorimaa, E.; Tkachenko, N. V.; Lemmetyinen, H. J. Lumin. 1997, 75, 245. (17) Tamai, N.; Yamazaki, T.; Yamazaki, I. Chem. Phys. Lett. 1988, 147, 25. (18) Klafter, J.; Blumen, A. J. Lumin. 1985, 34, 77. (19) Klafter, J.; Blumen, A. Chem. Phys. Lett. 1985, 119, 377. (20) Willig, F.; Blumen, A.; Zumofen, G. Chem. Phys. Lett. 1984, 108, 222.
10.1021/la990421y CCC: $18.00 © 1999 American Chemical Society Published on Web 09/23/1999
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Figure 1. Molecular structure of arachidic acid (AA), dioctadecyl rhodamine H (SRH), and dioctadecyl rhodamine B (SRB) and multilayer structure of the deposited films.
the fluorescence intensity at time zero, the decay time of the monomer in absence of acceptors, the dimension of the system, the Euler gamma function, the concentration of acceptors, and the critical distance for energy transfer. In several cases an additional monoexponential term has to be inserted in the decay function (eq 4).
t t I(t) ) I0 R exp - - γ τ τ
[
[
( ) ] + (1 - R)exp[- τt]] d/6
(4)
The additional monoexponential term is mostly attributed to the presence of isolated donors.21,22 The quenching of the emission of dioctadecyl rhodamine B (SRB) in mixed LB films, investigated by stationary absorption and emission measurements,23 was attributed to excitation energy transfer from monomers to groundstate dimers.24 The time-resolved fluorescence spectra and fluorescence decay curves obtained for SRB and other rhodamine B derivatives in arachidic acid were discussed by Tamai et al. in terms of energy migration among different sites with a fractal-like distribution due to an energetic or spatial disorder leading eventually to energy trapping by higher aggregates. Single curve analysis in the framework of eq 4, where d was left floating,17,25 yielded values of d between 2 and 3 which did furthermore depend on the dye concentration. For concentrations lower than 0.091% dye, a monoexponential decay could be obtained. Those results could be confirmed for SRB in LB films of dipalmitoylphosphatidic acid (DPPA) where a demixing leading to a two-phase system is suggested.9,26,27 However, when SRB is mixed with dioleylphosphatidic acid (DOPA) the recovered decay parameters suggest a homogeneous distribution of dimers and monomers.9 Simulations and the global analysis of fluorescence decay curves obtained for samples with a thiacyanine or rhodamine dye confirmed that using a fractal dimension is not necessary to analyze the fluorescence decay curves.10,28 In this contribution we report the analysis of fluorescence decays obtained for LB films of an amphiphilic (21) Hauser, M.; Klein, U. K. A.; Go¨sele, U. Z. Z. Phys. Chem. 1976, 101, 255. (22) Baumann, J.; Fayer, M. D. J. Phys. Chem. 1986, 85, 4087. (23) Verschuere, B.; Van der Auweraer, M.; De Schryver, F. C. Chem. Phys. 1991, 149, 385. (24) Van der Auweraer, M.; Verschuere, B.; De Schryver, F. C. Langmuir 1988, 4, 583. (25) Tamai, N.; Yamazaki, T.; Yamazaki, I. Can. J. Phys. 1990, 68, 1010. (26) Nakashima, K.; Duhamel, J. J. Phys. Chem. 1993, 97, 10702. (27) Gust, D.; Moore, T.; Moore, A.; Luttrull, D. K.; Degraziano, J.; Boldt, N. J.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 1991, 7, 1483. (28) Van der Auweraer, M.; Ballet, P.; De Schryver, F. C.; Kowalczyk, A. Chem. Phys. 1994, 187, 399.
rhodamine G (SRH) and an amphiphilic rhodamine B (SRB) distributed in an arachidic acid LB film. To investigate the concentration dependence of the stationary and time-resolved fluorescence, we investigated LB films containing 0.1-2% dye in the mixed monolayers. The fluorescence decay curves were recorded at different emission wavelengths and analyzed globally. Experimental Section Materials. N,N′-Dioctadecyl rhodamine B (SRB) and N,N′dioctadecyl rhodamine G (SRH) were prepared and purified according to the literature.29,30 Arachidic acid (AA) was obtained from Aldrich and used without purification. The molecular structures are given in Figure 1. Multilayer System. The multilayer systems were deposited on hydrophilic quartz substrata by the Langmuir-Blodgett method using a KSV 5000 ALT trough. The substrata were cleaned as described earlier.31 The subphase consisted of Milli-Q water with 5 × 10-4 M CdCl2 and pH 5.6 at room temperature. After spreading on the water surface, the monolayers were compressed at a rate of 10 mm min-1 until a surface pressure of 30 mN m-1 was reached. The monolayers were deposited at a constant surface pressure of 30 mN m-1 at a dipping speed of 10 mm min-1 for the AA monolayers and 7 mm min-1 for the mixed monolayers. Before the mixed monolayers were deposited, three monolayers of AA were deposited on the substratum. Then a mixed monolayer of AA and SRH or SRB was deposited followed by three layers of AA. The latter procedure was repeated several times until a sufficient fluorescence intensity of SRH or SRB was obtained. Deposition of three inert monolayers of AA between each pair of mixed monolayers will limit energy transfer to an intralayer process. In the mixed monolayers SRH/AA the molar ratio of the two components amounted to 1.7, 0.85, 0.42, and 0.17%. In the mixed monolayers SRB/AA the molar ratio of the two components amounted to 2.48, 1.24, 0.62, 0.25, and 0.12%. Depending on the concentration, 2-11 mixed monolayers were deposited to obtain fluorescence intensities sufficient for the “single photon timing” experiments. Instrumentation. The steady-state emission and excitation spectra were determined on a SPEX Fluorolog in a “front face” configuration.7,32 The spectra were corrected for the intensity fluctuations of the excitation source and the wavelength dependence of the excitation and detection system. Fluorescence decay curves of the different samples were obtained using timecorrelated single-photon counting. For excitation at 537 or 540 nm a cavity-dumped dye laser with a pyrromethene 556 dye (29) Hurd, C. D.; Schmerling, L. J. Am. Chem. Soc. 1937, 59, 112. (30) Ioffe, I. S.; Shapiro, A. L. J. Org. Chem. 1970, 6, 356. (31) Verschuere, B.; Van der Auweraer, M.; De Schryver, F. C. Thin Solid Films 1994, 244, 995. (32) Biesmans, G.; Van der Auweraer, M.; Catry, C.; Meerschaut, D.; De Schryver, F. C.; Storck, W.; Willig, F. J. Phys. Chem. 1991, 95, 3771.
Distribution of Xanthene Dyes (P-556, Exciton Inc.)33 was used. A sample compartment was constructed to obtain time-resolved fluorescence decay curves, unbiased by fluorescence depolarization, of dyes deposited on solid substrata.28 Horizontally polarized dye laser pulses were guided by a 60E prism to the quartz substratum, on which the multilayer assemblies were deposited, with an external incidence angle of 45E. The fluorescence was collected perpendicular to the substratum with a quartz lens. After the collecting lens, the emission passed a polarizer at 45E. In this way no fluorescence polarization or depolarization will be observed when the transition dipoles are oriented randomly in the plane of the LangmuirBlodgett film. A cutoff filter (Schott OG-570) blocked most of the scattered excitation light. For the SRH/AA LB films laser pulses scattered from a blank quartz plate were used for generating the instrument response functions (IRFs) necessary for the analysis of the fluorescence decays of the sample.9 To collect the latter signals, the cutoff filter was removed. For the SRB/AA LB films the fluorescence decay of a diluted solution of 2-(2-(3-(dimethylamino)phenyl)ethenyl)-N-methylpyridinium iodide (DASPI) in methanol (τ:26 ps)34 in a 1 mm cuvette was recorded at magic angle conditions to obtain a reference decay for the application of the reference convolution method.35 Using the reference convolution method allows correction for the (small) dependence of the transit time spread of the microchannel plate upon the wavelength of the light. As for the rhodamine dyes the emission and excitation wavelengths are both situated in the visible region and differ less than 100 nm; both methods are equivalent. Analysis of the Fluorescence Decays. The fluorescence decays were analyzed, either single curve or globally, by iterative reconvolution using a Marquardt algorithm.36,37 The fluorescence decays of the samples were analyzed in the framework of eqs 2 and 4.28 To reduce the number of adjustable parameters leading to a better model discrimination and a more accurate recovery of the parameters, decays obtained at different analysis wavelengths, different concentration, or time increments per channel were analyzed globally linking τ.38,39 When the IRF convolution method was used, Bδ(t), where B is the scatter parameter, is added to the different expressions for the decays to correct for scattered light and digitization errors.40,41 In the analyses the preexponential factor I(0) was always much larger than the scatter parameter B, suggesting that the function Bδ(t) does not hide an important fast decaying component. For the same reasons the reference decay time was not fixed to the “true” value of 26 ps when the data were analyzed using the reference convolution method. Furthermore, fixing the reference decay time to its “true” value is only possible when it is at least three to five times longer than the channel width. When the reference convolution method was used, no scatter parameter B was introduced. In this way both procedures (reference convolution and IRF convolution) have the same number of adjustable parameters. The goodness of fit was judged in terms of the statistical parameter χ2 (generally less than 1.2 for an acceptable fit), Zχ2, the runs test, the Durbin-Watson parameters, and visual inspection of the weighted residuals and their autocorrelation function. For the global analysis, the global value of χ2, χg2 had to be smaller than 1.3 for an acceptable fit.42 (33) Scheblykin, I. G.; Varnavsky, O. P.; Verbouwe, W.; De Backer, S.; Van der Auweraer, M.; Vitukhnovsky, A. G. Chem. Phys. Lett. 1998, 282, 269. (34) Hofkens, J. Ph.D. Thesis, KU Leuven, Leuven, 1994. (35) Boens, N.; Van den Zegel, M.; De Schryver, F. C. Chem. Phys. 1988, 121, 73. (36) Knutson, J. R.; Beechem, J. M.; Brand, L. Chem. Phys. Lett. 1983, 102, 501. (37) Eisenfeld, J. In Time-resolved fluorescence spectroscopy in Biochemistry and Biology; Cundall, R. B., Dale, R. E., Eds.; Plenum Press: New York; 1983; p 233. (38) Beechem, J. M.; Ameloot, M.; Brand, L. Chem. Phys. Lett. 1985, 120, 466. (39) Boens, N.; Janssens, L. D.; De Schryver, F. C. Biophys. Chem. 1989, 33, 77. (40) Periasamy, N. Biophys. J. 1988, 54, 341. (41) Holtom, G. R. In Artifacts and diagnostics in fast decaying fluorescence measurements, in Time-Resolved Laser Spectroscopy in Biochemistry II. SPIE 1991, 59, 341. (42) Szabo, A. G.; Bramall, L.; Krajcarski, D. T.; Selinger, B. Rev. Sci. Instrum. 1985, 56, 14.
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Figure 2. (a) Fluorescence excitation spectra of the SRH/AA LB films for emission at 570 nm: 0.17 mol % (1), 0.42 mol % ([), 1.7 mol % (b). (b) Fluorescence emission spectra of the SRH/AA LB films for excitation at 510 nm: 0.17 mol % (1), 0.42 mol % ([), 0.85 mol % (2), 1.7 mol % (b). Table 1. Stationary Photophysical Properties of the SRH/AA LB Multilayers concn (%)a
λemmax,b (nm)
λexcmax,c (nm)
fwhm(em)d (cm-1)
fwhm(ex)d (cm-1)
φfle
1.7 0.8 0.42 0.17
560 559 557 553
535 535 534 534
1630 1550 1530 1550
1780 1780 1710 1680
0.23 0.47 0.62 1
a The molar fraction (in %) of the dye in the mixed film. b The emission maximum. c The excitation maximum. d The full width at half maximum of the emission spectrum (em) and the excitation spectrum (ex). e Relative fluorescence quantum yield.
Results Stationary Spectra. Stationary Emission and Excitation Spectra of LB Films Containing SRH in an AA Matrix. The excitation (λem 570 nm) and emission (λex 500 nm) (Figure 2) maxima are presented in Table 1 together with the full width at halve maximum (fwhm). The emission spectra obtained at different concentrations allowed the determination of the relative change of the fluorescence quantum yield. In the calculation of the relative quantum yield of fluorescence, the relative quantum yield of the most diluted sample was put equal to one. The quantum yields of fluorescence were corrected for the number of layers and for the dilution as described for earlier experiments on SRB/AA or multilayers of dioctadecyl oxacarbocyanine and arachidic acid.6,32 A bathochromic shift of 7 nm (230 cm-1) can be observed in the emission upon increasing concentration from 0.2% to 1.7%. This shift is considerably smaller than the shift observed for SRB in AA multilayers.17,23 When the concentration is increased, the fwhm of the fluorescence spectra increases 80-100 cm-1 for the concentration range studied. The quantum yield of fluorescence decreases with the increasing concentration. The maxima of the excitation spectra are independent of the concentration of dye in the LB film.
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Figure 4. Decay cruves recorded of different concentrations of the SRH/AA LB films (excitation at 537 nm): 0.17 (9), 0.42% (1), 0.85% ([), 1.7% (2) and IRF (b). Time increment was 38 ps.
Figure 3. (a) Fluorescence excitation spectra of the SRB/AA LB films for emission at 600 nm: 0.12 mol % (9), 0.62 mol % (b), and 2.48 mol % (2). (b) Fluorescence emission spectra of the SRB/AA LB films for excitation at 520 nm: 0.12 mol % (9), 0.25 mol % (1), 0.62 mol % (b), 1.24 mol % ([), 2.48 mol % (2). Table 2. Stationary Photophysical Properties of the SRB/AA LB Multilayers concn (%)a
λemmax,b (nm)
λexcmax,c (nm)
FWHM(em)d (cm-1)
FWHM(ex)d (cm-1)
φfle
2.48 1.24 0.62 0.25 0.12
584 589 586 584 578
552 554 555 555 558
1420 1460 1380 1550 1580
2100 2060 2030 1930 1870
0.12 0.23 0.51 0.89 1
a The molar fraction (in %) of the dye in the mixed film. b The maximum of the emission spectrum. c The maximum of the excitation spectrum. d The full width at half maximum of the emission spectrum (em) and the excitation spectrum (ex). e Fluorescence quantum yield relative to the most diluted sample.
Stationary Emission and Excitation Spectra of LB Films Containing SRB in an AA Matrix. The maxima of the emission and excitation spectra (Figure 3) are represented in Table 2 together with their fwhm and the calculated quantum yield of fluorescence (Table 2). With an increase of the concentration of SRB from a molar fraction of 0.12% to a molar fraction of 1.24%, the emission maximum shifts by 11 nm (320 cm-1) to longer wavelengths (Figure 3). A further increase of the concentration leads again to a small blue shift of 150 cm-1. The red shift corresponds to the shift from 575 to 585 nm found by Tamai for a single monolayer upon increasing the concentration of rhodamine from 0.09% to 2.2%.17 The emission maxima are, for the same concentration range, at similar wavelengths as those observed by Verschuere43 who observed a shift from 585 to 600 nm upon increasing the dye concentration from 0.25% to 100%. On the other hand the maxima of the excitation spectra shift hypsochromically upon increasing the concentration from 558 (43) Verschuere, B. Ph.D. Thesis, KU Leuven, Leuven, 1989.
nm for the 0.12% to 552 nm for the 2.48% sample. This does not agree with the generally observed bathochromic shift at higher concentrations, attributed to an increase of the polarizability of the multilayers.23,24,44,45 The fwhm of the excitation spectra increases by 230 cm-1, upon increasing the concentration of SRB from 0.12% to 2.48%, while that of the emission spectra decreases by 160 cm-1 over the same concentration range. The fwhm of the fluorescence spectrum amounts at a mixing ratio of 0.25% to 1550 cm-1, which corresponds to the value of 1540 cm-1 found by Verschuere for the same concentration.23 The quantum yield of fluorescence decreases with increasing concentration of SRB in the LB film. The Fluorescence Decays. Fluorescence Decays of LB Films Containing SRH in an AA Matrix. Fluorescence decay curves were recorded for different concentrations and at different emission wavelengths (Figure 4) with a time increment of 31 or 38 ps per channel. Single curve analysis of the fluorescence decay curves using eqs 2 and 4 yielded acceptable statistical parameters for the goodness of fit. To obtain a better model discrimination and a more accurate parameter recovery, decays obtained at different concentrations or emission wavelengths were also analyzed globally. For the different concentrations the global analysis of the decays in the framework of eq 2 or 4, linking the decay time of the unquenched dye over decays obtained at different emission wavelengths yielded acceptable statistical parameters (Table 3). As local parameters γ, 1 - R, and a scatter parameter were obtained for the individual curves. Only γ and 1 - R have a physical meaning and are therefore represented in Table 3 for all curves. The scatter parameter did not change significantly for the different emission wavelengths and is only represented for the global analysis at 555 and 580 nm. The very small value of the scatter parameter observed in Table 3 excludes the presence of an important component with a short decay time below the time resolution of the setup. To confirm this observation, the quantum yields of fluorescence were calculated in a quantitative way using eq 5.10
φcal ) kf
∫0∞{R exp[- (τt) - γ(t)1/3] +
[ τt]} dt (5)
(1 - R) exp -
In eq 5 the value of kf obtained for a solution of SRH in methanol is used.46 Values for φcal of 0.09, 0.15, 0.24, and (44) Ray, K.; Dutta, A. K.; Misra, T. N. J. Lumin. 1997, 71, 123. (45) Dutta, A. K.; Misra, T. N.; Pal, A. J. Langmuir 1996, 12, 459.
Distribution of Xanthene Dyes
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Table 3. Global Analysis of the Fluorescence Decays of the SRH/AA LB Films Linked over Different Concentrationsa concn (%) 0.17 0.42 0.85 1.7
γ
λem 570 nm, τ 3.75 ns, χg2 1.13
λem 580 nm, τ 3.83 ns, χg2 1.13
(ns-1/3)
1-R
B
(ns-1/3)
1-R
B
γ (ns-1/3)
1-R
B
0.26 0.49 1.13 1.53
0 0 0.06 0.04
0.27 0.36 0.04 0.10
0.27 0.47 1 1.51
0 0 0.05 0.04
0.27 0.39 0.32 0.17
0.27 0.48 1.1 1.47
0 0 0.06 0.05
0.07 0.21 0.14 0.13
γ
λem 600 nm, τ 3.76 ns, χg2 1.15
a concn (%) gives the concentration of SRH in the AA film in mol %, λ em is the emission wavelength at which the decay is recorded, τ is the decay time in the absence of energy transfer, γ has a dimension of ns-0.33, a is the normalized pre-exponential factor, B is the normalized scatter parameter, and χg2 is the global statistical parameter; χ2 is the statistical parameter of the individual decays.
Figure 5. Dependence of γ and γr upon the concentration for SRH/AA in the LB film (excitation at 537 nm): γ [ (570 nm), 9 (580 nm), b (600 nm); γr ] (570 nm), 0 (580 nm), O (600 nm).
0.32 were obtained for the concentrations of 1.7, 0.85, 0.42, and 0.17% SRH, respectively, in the LB films. These values follow the same trend as the experimental values in Table 1 and reduce the possibility of a component with a decay time shorter than the time resolution of the experimental setup. Equation 2 can be used for the analysis of decay curves recorded at different emission wavelengths for a dye concentration ranging from 0.16 to 0.42%. Also for the higher concentrations the preexponential factor of the monoexponential term (1 - R) remains small. The presence of long living emission will be revealed most efficiently under conditions where its decay time differs most strongly from that of monomers susceptible to quenching by energy transfer to dimers or aggregates. The decay times obtained from the global analyses at different emission wavelengths and linking τ over different concentrations are situated between 3.5 and 3.6 ns. Since γ is proportional to the inverse cube root of the decay time (eq 3), a reduced value of γ, γr, which is corrected for changes of the decay time was calculated. γr depends only on the critical distance for energy transfer and on the number of quenchers per unit area in the mixed monolayer. The dependence of γ and γr on the concentration of SRH in the film is given in Figure 5. γr increases with increasing concentration of SRH in the LB film. The increase of γr is almost linear up to 0.85%. For the 1.7% film, the recovered value of γr is however smaller than expected on the basis of a linear relation between γr and the overall dye concentration. In Figure 5 one can observe that γ and γr decrease marginally upon increasing the emission wavelength where the decay curves were recorded. This phenomenon contradicts the results obtained by us (cf. infra) and others for mixed LB multilayers of SRB and AA.25 Fluorescence Decays of LB Films Containing SRB in an AA Matrix. Fluorescence decay curves have been recorded with a time increment per channel of 40 ps at different emission wavelengths (570, 580, 590, 600, and 620 nm) (46) Pevenage, D. Ph.D. Thesis, KU Leuven, Leuven, 1999.
Figure 6. Decay curves recorded of different concentrations of the SRB/AA LB films (excitation at 540 nm): 0.12 mol % (9), 0.26 mol % (1), 0.62 mol % ([), 1.24 mol % (2), and DASPI (b). Time increment was 40 ps.
for the SRB/SA samples. A diluted solution of DASPI was used to determine the reference decay (Figure 6). The decay curves could be analyzed individually using eq 1 or 2 and eq 4 while allowing the dimension to float or fixing it at 2. Contrary to what was expected on the basis of eq 1, 2, or 4, different values were obtained for τ at different dye concentrations. Furthermore for experiments at the same dye concentration but at different emission wavelengths different values of γ and 1 - R were observed. By use of eq 5, with the value of kf obtained for a solution of SRB in methanol, the fluorescence quantum yield φcal (eq 5) could be estimated.10,46 This estimation yielded values of 0.18, 0.15, 0.23, 0.45 and 0.45 for φcal for the concentrations of 2.48, 1.24, 0.62, 0.25, and 0.12% SRB, respectively, in the LB films. These values follow the same trend as the experimental values in Table 2. To investigate whether the changes in decay parameters were due to an inaccurate parameter recovery, the decays obtained at different emission wavelengths or concentrations were analyzed globally using eq 2 and linking τ. When no satisfactory statistical parameters could be obtained, eq 4 with the additional monoexponential term was used for the fluorescence decays. As at all dye concentrations the extra monoexponential term was necessary to obtain a satisfactory fit; only the results of the global analysis in the framework of eq 4 are shown in Table 4. The recovered decay time of the reference DASPI corresponds within the experimental error to the literature values.34 When the decay time τ is linked over different concentrations for decay curves recorded at a single emission wavelength, no linear increase of γ and γr with the concentration can be observed from 0.1% to 2.48% (Figure 7). The mean value of γr is 2.3, 2.5, 2.1, 1.1, and 1.7 for the 2.48, 1.24, 0.62, 0.25, and 0.12 mol % LB films, respectively. Between 0.5% and 2% γ and γr are independent upon the concentration within the experimental error. Although the concentration dependence of γr at lower concentrations is less clear, there is no indication for a systematic decrease exceeding the experimental error. At concentrations of
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Table 4. Global Analysis of the Fluorescence Decays of the SRB/AA Samples Linked over Different Concentrationsa λem 570 nm, τ 3.23 ns, χg2 1.13 concn (%) 0.12 0.25 0.62 1.24 2.48
γ
(ns-1/3)
2.10 2.20 2.10
γr
3.09 3.24 3.11
1-R
0.09 0.04 0.06
λem 580 nm, τ 3.60 ns, χg2 1.11
τref (ps)
γ
(ns-1/3) 1.90 1.30 1.40 1.70 1.70
12 12 18
γr
1-R
τref (ps)
γ (ns-1/3)
γr
1-R
τref (ps)
2.90 1.98 2.14 2.59 2.59
0.37 0.27 0.09 0.05 0.06
19 26 20 15 23
1.35 0.81 1.17 1.46 1.33
2.08 1.25 1.80 2.25 2.05
0.44 0.29 0.09 0.04 0.06
27 30 20 18 27
λem 600 nm, τ 3.93 ns, χg2 1.12 concn (%) 0.12 0.25 0.62 1.24 2.48
γ
(ns-1/3) 0.55 0.33 0.89 1.28 1.16
λem 590 nm, τ 3.71 ns, χg2 1.07
λem 620 nm, τ 3.94 ns, χg2 1.12
γr
1-R
τref (ps)
0.86 0.52 1.39 2.01 1.82
0.31 -0.11 0.04 0.04 0.05
32 33 26 18 28
γ
(ns-1/3)
γr
1-R
τref (ps)
0.57 0.38
0.90 0.60
0.26 -0.03
33 20
1.14
1.79
0.03
29
a
concn (%) gives the concentration of SRB in the AA film in mol %, λem is the emission wavelength at which the decay is recorded, τ is the decay time of the unquenched dye, γ has a dimension of ns-0.33, λr is g corrected for the decay time and has no dimension, R is the normalized preexponential factor, τref is the decay time of the reference in picoseconds, and χg2 is the global statistical parameter. χ2 is the statistical parameter of the individual decay.
Figure 7. Dependence of γr upon the concentration for SRB/ AA in the LB film (excitation at 540 nm); γr [ (570 nm), 9 (580 nm), 2 (590 nm), b (600 nm), and O (620 nm).
0.5% and higher γr decreases by about 30% (Table 4) when the emission wavelength is increased from 570 to 590 nm. Although this increase is only two or three times larger than the precision with which γr is determined, it is observed at three different concentrations. Furthermore, this effect was not observed for SRH in arachidic acid multilayers. While the preexponential factor of the monoexponential term in eq 4 does not change between 0.62 and 2.48%, it increases sharply from 0.62% over the 0.25% to the 0.12% samples (Figure 7). The increase of 1 - R at low concentrations (Figure 8) could suggest the presence of isolated monomers decaying monoexponentially. It can however also be an artifact of the algorithm used to recover the parameters. Simulations showed that at very low quencher concentrations the recovered values of 1 - R and γ become both too large.28 The error bars in Figure 7 on these values are also higher at low concentration. To study the dependence of the decay parameters recovered by global analysis on the emission wavelength, decay curves recorded at different emission wavelengths were analyzed globally using eq 4 (Table 5). The values of γ, 1 - R, and the reference decay time are almost identical to those obtained when the global analysis is performed with decays obtained from samples with different concentrations. This suggests a reliable parameter recovery of the parameters by the global analysis method. The recovered decay time of the reference DASPI is furthermore within the experimental error identical to the literature values.34 The decay time of the unquenched
Figure 8. Dependence of 1 - R upon the concentration of SRB for different emission wavelengths: [ (570 nm), 9 (580 nm), 2 (590 nm), b (600 nm), O (620 nm). The negative values are not presented because they have no physical meaning.
monomer could be linked over the different emission wavelengths. It is independent of the concentration and equals the decay time of SRB in solution (τCHCl3: 3.6 ns). The values of γ and γr evolve in the same way as in the global analysis where decays obtained at different concentration were linked. γ and γr are within the experimental error independent upon the dye concentration for concentrations exceeding 0.5%. As the large value of γ and γr observed for the SRB/AA 0.1% LB film could be an artifact, it is difficult to draw a definitive conclusion on the concentration dependence of γ and γr at low concentrations. Although the concentration dependence of γr at lower concentrations is less clear, there is no indication for a systematic decrease exceeding the experimental error. However, it is clear that when the dye concentration is decreased, γ and γr approach in no way asymptotically to zero or even to the values found for SRH in arachidic acid. The preexponential factor of the monoexponential term varies slightly with emission wavelength at the lowest concentrations. Discussion Stationary Fluorescence and Fluorescence Excitation Spectra. Rhodamine dyes have a slightly broader emission and excitation spectrum than carbocyanines. The shoulder observed in both the emission and absorption spectra is due to vibrational progression. Changing the hydrogen of a rhodamine G (as in SRH) to
Distribution of Xanthene Dyes
Langmuir, Vol. 15, No. 24, 1999 8471
Table 5. Global Analysis of the Fluorescence Decays of the SRB/AA LB Films Linked over Different Emission Wavelengthsa concn (%) 2%, τ 3.60 ns, χg2 1.11 λem (nm) 570 580 590 600 620
γ
(ns-1/3) 1.88 1.74 1.39 1.34 1.27
concn (%) 1%, τ 3.39 ns, χg2 1.14
γr
1-R
τref (ps)
2.87 2.65 2.12 2.04 1.94
0.04 0.06 0.07 0.11 0.08
23 25 26 26 27
γ
(ns-1/3) 2.14 1.85 1.61
γr
1-R
τref (ps)
γ (ns-1/3)
γr
1-R
τref (ps)
3.20 2.77 2.41
0.04 0.06 0.07
14 14 16
1.73 1.40 1.20 1.03
2.65 2.15 1.84 1.58
0.06 0.08 0.10 0.12
19 21 20 22
concn (%) 0.2%, τ 3.72 ns, χg2 1.14 λem (nm) 580 590 600 620
γ
(ns-1/3) 1.04 0.79 0.62 0.70
concn (%) 0.5%, τ 3.66 ns, χg2 1.18
concn (%) 0.1%, τ 3.75 ns, χg2 1.15
γr
1-R
τref (ps)
1.60 1.22 0.96 1.08
0.22 0.29 0.31 0.32
19 30 32 18
γ
(ns-1/3) 1.27 1.19 1.14 1.08
γr
1-R
τref (ps)
1.96 1.84 1.76 1.67
0.35 0.43 0.51 0.46
26 28 29 29
a concn (%) gives the concentration of SRB in the AA film in mol %, λ em is the emission wavelength at which they decay is recorded, τ is the decay time of the unquenched dye, γ has a dimension of ns-0.33, γr is g corrected for the decay time and has no dimension, R is the normalized preexponential factor, τref is the decay time of the reference in picosecond, and χg2 is the global statistical parameter. χ2 is the statistical parameter of the individual decay.
an ethyl group of a rhodamine B (as in SRB) induces a bathochromic shift of about 20 nm in the fluorescence spectra. This can be explained by a better delocalization of the positive charge over the xanthene moiety of the rhodamine dye, due to the better electron donating properties of the N-ethyl group.47 The fwhm increases slightly more at higher concentrations of dye when SRB (160 cm-1) is incorporated in the LB film instead of SRH (80 cm-1). The changes in fluorescence spectra are in agreement with earlier data.24,25 When SRB is mixed in DOPA or DPPA in an alternating layer deposition architecture, a bathochromic shift is observed in the emission spectra, while no change in the absorption or excitation maxima is observed upon increasing the dye concentration in a similar concentration range as used here. A hypsochromic shift of the absorption maxima is also observed for a head to head deposition of mixed monolayers of DPPA and SRB at a mixing ratio of 5 mol %. The concentration dependence of the emission maxima can be explained to a large extent by an increase of the polarizability (refractive index) of the samples upon increasing the concentration. However one must always consider that a bathochromic shift observed upon increasing the concentration of the dye could indicate the conversion of the emission of the monomers into that of dimers and aggregates, which always emit at longer wavelength.48-50 As the effect of the increased polarizability would be the same for SRB and SRH, the slightly larger influence of the concentration on the emission maxima observed when SRB is incorporated in the film suggests probably a more extensive aggregation or exciton interaction of SRB compared to SRH. Furthermore the independence of the excitation maxima of SRH upon the concentration and the blue shift of 6 nm observed for those of SRB make it difficult to attribute the red shift of the fluorescence maxima exclusively to an increased polarizability. The dependence of the fwhm on the concentration is more pronounced for SRB than for SRH. Both differences can be attributed to a more extensive aggregation or exciton interaction for SRB compared to SRH in the AA matrix. The data obtained from the mixing of SRH in AA suggest a more homogeneous (47) Rettig, W.; Braun, D.; Suppan, P.; Vouthey, E.; Rotkiewicz, K.; Lubardski, R.; Suwinska, K. J. Phys. Chem. 1993, 174, 425. (48) Kasha, M. Radiat. Res. 1963, 20, 55. (49) McRae, E. G.; Kasha, M. Physical Processes in Radiation Chemistry; Academic Press: New York, 1964; p 23. (50) Mo¨bius, D. Adv. Mater. 1995, 437.
distribution of the SRH dyes compared with the SRB dyes. When a homogeneous distribution over the matrix is assumed for the rhodamine molecules, the average distance between two neighboring rhodamine molecules will amount to 36 Å for the mixed monolayers with 2% rhodamine. When some aggregation of the rhodamine molecules is assumed, the concentration of monomers will be smaller than the analytical concentration of rhodamine leading to a larger average intermolecular distance. Applying the point dipole model to describe the exciton interaction,45,46,51-53 a hysochromic shift of 1-2 nm is expected for a parallel sandwich “packing” of two SRB or SRH molecules with a transition dipole moment of 2 Å at a distance of 36 Å. However as the transition dipoles of “neighboring” dye molecules will have a random orientation one can expect that the average shift of the absorption spectrum due to exciton interaction of randomly oriented molecules will be even smaller than 1-2 nm. The larger hypsochromic shift observed for SRB suggests that most molecules experience a larger average concentration of SRB or the formation of nonrandom dimers. For the emission spectra we have a slightly different situation because we can expect that emission will preferentially occur from the lowest state formed by a splitting of the energy levels of two interacting dye molecules. As the interaction in the random dimers is never much larger than kT, thermal equilibrium and vibronic coupling between both exciton levels will prevent a drastic change of the features of the fluorescence spectrum and decay time. Hence, while the exciton interaction of randomly oriented dye molecules will probably lead to no shift at all or a very small blue shift of the absorption spectra, it will always lead to a red shift of the emission spectra. The small red shift observed upon increasing the dye concentration should be attributed to a combination of very small exciton interactions between randomly oriented molecules and to an increased polarizability. Furthermore energy transfer to more stabilized molecules or strongly interacting dimers could also play a role.54,55 While the results obtained for SRH are still compatible with exciton interaction between homogeneously distrib(51) Fo¨rster, T. Pure Appl. Chem. 1962, 4, 121. (52) Fo¨rster, T. Pure Appl. Chem. 1963, 7, 73. (53) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371. (54) Kemnitz, K.; Tamai, N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. J. Chem. Phys. 1986, 90, 5094. (55) Kemnitz, K.; Murao, T.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. Chem. Phys. Lett. 1983, 159, 337.
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uted molecules with a random mutual orientation, the blue shift of the excitation spectra and the larger red shift of the emission spectra observed for SRB probably suggest a higher local dye concentration or the formation of more strongly interacting dimers or aggregates. This leads to the assumption that for SRB the monolayer consists of at least two phases.9,10 Besides a diluted phase containing a very small concentration of isolated monomers, there is a more concentrated phase containing a higher concentration of randomly interacting monomers and strongly interacting dimers and/or aggregates. This inhomogeneous distribution of xanthene dyes in a fatty acid matrix has been suggested earlier by Ballet9 and Vuorimaa.56 The decrease of the quantum yield of fluorescence with increasing concentration has been observed in other studies and is attributed to energy transfer to strongly interacting dimers and aggregates when the concentration of SRH and SRB in the LB film is increased.25,56,57 The dimers and aggregates are expected to emit at longer wavelength than the monomers. To the extent they are characterized by a “sandwich” packing they will also have a smaller fluorescence rate constant and probably a lower fluorescence quantum yield.23,24,58 To a minor extent one should also consider the competition for the excitation light by those dimers and aggregates. Both exciton coupling of randomly oriented molecules and energy transfer can explain the concentration-dependent changes of the stationary fluorescence spectra. Hence, fluorescence decay experiments are necessary to assess the relative importance of both effects. Fluorescence Decays. Fluorescence Decays of LB Films Containing SRH in an AA Matrix. The independence of τ, on the emission wavelength, as obtained by global analysis linking decays obtained at different emission wavelength and at two dye concentrations, suggests that the emission of the different SRH/AA LB films is over the complete emission band due to a single species. The independence of γr of the emission wavelength indicates that the number of quenchers, which are capable of accepting energy from the emitting species, does not decrease for the emission observed at longer wavelengths. This confirms, in agreement with the spectral data, that all fluorescence is due to a single species, probably the SRH monomer, over the whole emission band studied. There is furthermore no indication for a broad distribution of the excitation energies of this species. In the latter case dye molecules in a more “favorable” or “stabilizing” environment and emitting at longer wavelengths would be characterized by a smaller overlap between their emission spectrum and the absorption spectrum of the dimers or aggregates and hence by smaller values of R0 and γ or γr. Although γ and γr, who monitor the extent of energy transfer to quenching sites (dimers), decrease upon decreasing the dye concentration down to 0.16%, no further decrease is observed at lower concentrations. This phenomenon has already been observed for SRB in DOPA and is attributed to an amount of quenchers, which are inherent for the films studied, or to imperfections of the LB film.9,10,59 When an identical critical distance for energy transfer is assumed for all concentrations studied, γr should increase at least linearly with the analytical dye concen(56) Vuorimaa, E.; Ikonen, M.; Lemmetyine, H. Chem. Phys. 1994, 188, 289. (57) Vuorimaa, E.; Ikonen, M.; Lemmetyinen, H. Thin Solid Films 1992, 214, 243. (58) Kasha, M. Rev. Mod. Phys. 1959, 31, 162.
Pevenage et al.
tration. Actually, as the equilibrium for dimer or aggregate formation shifts to the latter species at higher analytical dye concentrations, one could even expect a slightly superlinear behavior. At the highest dye concentrations used, this contradicts the observed decrease of the slope of a plot of γ or γr versus the dye concentration. It should be noted that a linear or superlinear increase of γ or γr with the overall dye concentration has been observed very rarely up to now for fluorescence decays of LangmuirBlodgett films.9,10,11,28 The concentration dependence of γ or γr observed for SRH in arachidic acid parallels that observed for SRB in DOPA where it was considered compatible with a homogeneous distribution of the dye in the LB film. In the DOPA samples the deviations observed at higher concentrations were attributed to a decrease of the equilibrium constant for aggregation. This is no surprise, as a concentration of 2% would already correspond to 0.01 mol/L. Another possibility could be an increase of the aggregation number of the aggregates at higher dye concentration. A naı¨ve exploration of this hypothesis10 suggests that the decrease of the number density or concentration of acceptors would not be compensated by the increase of R02. Fluorescence Decays of LB Films Containing SRB in an AA Matrix. The use of eq 4, with an extra monoexponential term, instead of eq 2 in the global analysis of decay curves recorded for SRB/AA LB films indicates the presence of isolated donors in the LB film in addition to a phase where energy transfer60-62 occurs from monomers to dimers or aggregates. While γr depends marginally upon the dye concentration, a small but reproducible dependence upon the emission wavelength at which the decay curves were recorded is observed. While γr depends on the critical distance for energy transfer and on the number of quenchers per unit area present in the film, 1 - R gives information on the presence of a phase containing isolated chromophores. Hence, the concentration dependence of these local parameters suggests an inhomogeneous distribution behavior of SRB in the AA matrix. Whether this inhomogeneous distribution relates to the presence of two phases in the true thermodynamic sense or rather of “pseudophases” or regions is difficult to asses. In the diluted phase were monomers decay monoexponentially, the distance between monomers must at all analytical dye concentration exceed the critical distance for energy transfer. Extrapolating the change of 1 - R to lower concentrations indicates that a monoexponential decay is approached asymptotically for the SRB/AA LB films upon decreasing the dye concentration. This is in agreement with the results of Tamai et al. but different from results of Ballet et al. where SRB is mixed in a DPPA or DOPA matrix.8,28 In the DOPA systems the supplementary monoexponential term must only be included for the higher concentration range studied. Contrary to the fractal-like dimension invoked by Tamai et al.,17,25 the dimension was fixed at 2 when fitting the fluorescence decays to eq 4 using global analysis. This means that when the heterogeneity of the layers is taken into account by the factor 1 - R, the phase with a high dye concentration can be regarded as a two-dimensional (59) Yamazaki, I.; Ohta, N.; Yoshinari, S.; Yamazaki, T. Microchemistry, Spectroscopy and Chemistry in Small Domains; Masuhara, H., De Schryver, F. C., Kitamura, N., Tamai, N., Eds.; North-Holland Elsevier Science B.V.: Amsterdam, 1994; p 431. (60) Tamai, N.; Yamazaki, T.; Yamazaki, I. J. Phys. Chem. 1987, 91, 841. (61) Urquhart, R.; Grieser, F.; Thistlethwaite, P. J. Phys. Chem. 1992, 96, 7808. (62) Nishiyama, Y.; Azuma, T.; Obato, N.; Kasatani, K.; Sato, H. J. Photochem. Photobiol., A 1991, 59, 341.
Distribution of Xanthene Dyes
mixture of monomers and dimers or higher aggregates. The wavelength and concentration dependence of γ and γr shown in Tables 4 and 5 suggests that at concentrations of 0.5 mol % and higher the number density of quenchers, σ, does not depend on the analytical dye concentration. This means that the “concentrated” phase is already saturated at a dye concentration of 0.5%. Increasing the total dye concentration will increase the area covered by the “concentrated” phase and hence decrease 1 - R. Another possibility to explain the independence of γ or γr on the concentration is a decrease of the critical distance of energy transfer due to a smaller overlap between the emission spectrum of the monomers or weakly coupled aggregates and the absorption spectrum of the strongly interacting dimers or aggregates, at higher concentration. This decreased overlap corresponds with the red shift of the emission spectra at higher concentration. A distribution of weakly coupled aggregates, or of monomers in different sites, implicating that the fluorescence at longer emission wavelengths is due to monomers or weakly coupled aggregates emitting at longer wavelengths can explain the decrease of R0(γr) with increasing emission wavelength.17 The decrease of R0 upon increasing the emission wavelength and the overall dye concentration is probably rather due to the formation of a manifold of random aggregates than due to conformational effects. In the latter case it is difficult to see why this phenomenon does not occur for SRH. However when the formation of aggregates is predominant, the higher local concentration in the concentrated phase of the SRB/AA multilayer will make this effect more important than that in the homogeneously distributed SRH/AA multilayer. These redshifted monomers or random aggregates can be excited directly or by energy transfer from other more energetic monomers (the latter only at the high end of the dye concentrations used). While monomer monomer transfer (or transfer from monomers to weakly coupled aggregates) can be important to explain the apparent decrease of γ and γr at longer wavelengths, it cannot explain the apparent decrease of R0 at higher overall dye concentration. There is no reason the concentration of quenching aggregates should increase slower than that of monomers of weakly coupled aggregates upon increasing the overall dye concentration. Hence the rate of energy transfer from a monomer to a quenching aggregate should increase faster than that of transfer to another (more stabilized) monomer and to the same extent as that of transfer to a weakly coupled aggregate. Also the increase of 1 - R when the decay curves are recorded at longer wavelengths can be understood in the models suggested above. Due to a lower number of chromophores available as acceptor for the energy transfer, the relative contribution of the first term in eq 4 will decrease at longer emission wavelengths. Conclusions The fluorescence decay curves of both dye-arachidic acid systems could be analyzed in the framework of eq 4 assuming a bidimensional distribution of the dye dimers or aggregates in the mixed LB film. A global analysis, linking the fluorescence decay time of the unquenched dye, performed as a function of the emission wavelength and the concentration of both dyes incorporated in LB films of AA yields values for the fluorescence decay times of unquenched SRH and SRB that are within the experimental error equal to the fluorescence decay time of SRH and SRB in a dilute chloroform solution. Together
Langmuir, Vol. 15, No. 24, 1999 8473
with the spectral data this suggests that the fluorescence decay should be attributed to monomers or very weakly coupled random dimers in the LB film.63,64 The different amplitude of the monoexponential term, which was attributed to a phase with isolated monomers which decay monoexponentially, indicates a very different distribution of SRH and SRB. While in the SRH films only a low number of chromophores are isolated, the large contribution of the monoexponential term found for the SRB films suggests the presence of a large fraction of SRB molecules which are isolated in the sense that the intermolecular distance is at all overall dye concentrations much larger than the critical distance for energy transfer. Besides a concentrated phase containing monomers and strongly coupled dimers or aggregates and characterized by stretched exponential fluorescence decay, a diluted phase with a monoexponentially fluorescence decay exists in the films of AA and SRB. The different distribution of SRH and SRB is also reflected in the different concentration and emission wavelength dependence of γ and γr. For SRH the linear increases of γ and γr with the overall dye concentration suggest a homogeneous distribution of SRH in the LB film. On the other hand, for SRB, γ and γr are above a concentration of 0.5% independent of the overall dye concentration. At lower dye concentration their concentration dependence is less clear but γ and γr remain larger than the values found for SRH. This behavior is compatible with the presence of a “concentrated” phase which is at an overall dye concentration of 0.5% already saturated. Besides this “concentrated” phase there is a dilute phase where the fluorescence decays exponentially up to an overall dye concentration of 2%. The dependence of γ on the emission wavelength is also very different for the SRH LB films and SRB LB films. The independence of γr or the emission wavelength suggests that in films with SRH only one species is contributing to the fluorescence over the whole emission band. On the other hand, for LB films containing SRB the decrease of γr upon increasing the analysis wavelength suggests a distribution of weakly coupled dimers or aggregates due to higher local concentration of SRB in the “concentrated” phase. The strongly different miscibility of SRH and SRB with arachidic acid or arachidate corroborates with the more extensive dimerization observed by Vuorimaa et al.56 for a long chain substituted rhodamine B compared to a similar rhodamine 6G in dioleoyl phosphatidyl choline. This difference is related to the presence of the (acid) proton in SRH which could for hydrogen bonds56 with carboxylate ions or carboxylic acid groups of the matrix, leading to a higher affinity of SRH for the matrix compared to SRB. Acknowledgment. M. Van der Auweraer is a “Onderzoeksdirecteur” of the Fonds voor Wetenschappelijk Onderzoek (F.W.O.). D. Pevenage thanks the “Vlaams instituut voor de bevordering van het wetenschappelijk en technologisch onderzoek in de industrie (I.W.T.)” for financial support. The continuing support of the “Fonds voor Wetenschappelijk Onderzoek”, the “Nationale Loterij” and the DWTC through IUAP IV-11 is gratefully acknowledged. We also thank Dr. W. Stork (Fritz Haber Institute) and Professor. F. Willig (H. Meitner Institute) for a gift of SRH. LA990421Y (63) Chambers, R. W.; Kajiwara, T.; Kearns, D. R. J. Phys. Chem. 1974, 78, 380. (64) Vogel, M.; Rettig, W.; Sens, R.; Drexhage, K. H. Chem. Phys. Lett. 1988, 147, 452.
