Langmuir 1997, 13, 3009-3015
3009
Intralayer and Interlayer Energy Transfer between Octadecyl Substituted Pyronine and Crystal Violet Incorporated in Langmuir-Blodgett Films: A Time-Resolved Study E. Vuorimaa* and H. Lemmetyinen Institute of Materials Chemistry, Tampere University of Technology, P.O. Box 541, 33101 Tampere, Finland
P. Ballet, M. Van der Auweraer, and F. C. De Schryver Laboratories for Molecular Dynamics and Spectroscopy, Chemistry Department, K. U. Leuven, Celestijnenlaan 200 F 3001, Leuven, Belgium Received November 6, 1996. In Final Form: March 10, 1997X The intralayer and interlayer excitation energy transfer between dioctadecylpyronine (PYR18) and dioctadecyl crystal violet (CV18) in alternating multilayer Langmuir-Blodgett (LB) films has been examined with picosecond time-resolved fluorescence decay measurements. The PYR18 fluorescence is efficiently quenched by energy transfer to CV18. The critical transfer distances of 65 and 67 Å for intralayer and interlayer energy transfer, respectively, were calculated from spectral overlap. For the intralayer energy transfer system the fluorescence decay can be fitted with a two-dimensional Fo¨rster energy transfer equation, whereas for the interlayer energy transfer system it was necessary to allow the dimension to float in the Fo¨rster equation. By use of global analysis, a “fractal dimension” equal to 2.6 was obtained. With singlecurve analysis the two-dimensional system is seen to approach the three-dimensional one with increasing CV18 concentration.
Introduction Recently a large interest has been shown in excitation energy transfer in artificial molecular assemblies with restricted molecular geometries such as LangmuirBlodgett (LB) films.1-7 The unique advantages of the LB technique are the precise control over the monolayer thickness, the possibility of selectively and easily orienting the molecules in the films, and the possibility of making multilayer structures with varying layer composition. The high degree of orientation of molecules in LB films can cause new configurational and dynamic behavior quite different from that of a homogeneous solution.8 Hence, understanding the relationship between the mechanism of excitation energy relaxation and the distribution and orientation of dye molecules in a two-dimensional plane is essential for the construction of functional molecular assemblies. The analysis of the fluorescence decays obtained for systems with restricted geometries, such as LangmuirBlodgett films, is a model dependent problem. Previous studies4,9,10 of single-curve and global analysis of both * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, May 15, 1997. (1) Yamazaki, I.; Tamai, N.; Yamazaki, T. J. Phys. Chem. 1990, 94, 516. (2) Tamai, N.; Matsuo, H.; Yamazaki, T.; Yamazaki, I. J. Phys. Chem. 1992, 96, 6550. (3) Ohta, N.; Tamai, N.; Kuroda, T.; Yamazaki, T.; Nishimura, Y.; Yamazaki, I. Chem. Phys. 1993, 177, 591. (4) Ballet, P.; Van der Auweraer, M.; De Schryver, F. C.; Lemmetyinen, H.; Vuorimaa, E. J. Phys. Chem. 1996, 100, 13701. (5) Vourimaa, E.; Belovolova, L. V.; Lemmetyinen, H. J. Lumin. 1997, 71, 57. (6) Tamai, N.; Yamazaki, T.; Yamazaki, I. Can. J. Phys. 1990, 68, 1013. (7) Caruso, F.; Thistlewaite, P. J.; Grieser, F.; Furlong, D. N. Langmuir 1994, 10, 3373. (8) Lee, W.; Esker, A. R.; Yu, H. Colloids Surf. 1995, 102, 191. (9) Van der Auweraer, M.; Ballet, P.; De Schryver, F. C.; Kowalczyk, A. Chem. Phys. 1994, 187, 399. (10) Medhage, B.; Almgren, M. J. Fluoresc. 1992, 2, 7.
S0743-7463(96)01083-9 CCC: $14.00
simulated and measured fluorescence decay curves suggested that global analysis yielded the ability to discriminate between models, typical for microheterogeneity and self-similar distribution11,12 of acceptors characterized by dilatation symmetry. Examination of the possibility of accurate parameter recovery, using the iterative reconvolution method, has shown that global analysis allows a reliable analysis over a larger concentration range of acceptors than single-curve analysis. In the present study the intralayer and interlayer excitation energy transfer between PYR18 and CV18 in LB films has been examined using a picosecond time-resolved fluorescence decay method. The packing of these dyes in cadmium arachidate Langmuir-Blodgett films has already been studied.13-15 PYR18 forms miscible LB films with arachidic acid at concentrations under 1 mol %, whereas CV18 exists predominantly as dimers already at 1 mol % concentration. Efficient energy transfer can be expected between these two dyes in view of the large overlap between the absorption spectrum of CV18 and the fluorescence spectrum of PYR18. Experimental Section Dioctadecylpyronine (PYR18) and dioctadecyl crystal violet (CV18) were prepared as described previously.14,16 Arachidic acid (AA) (Aldrich) was used without further purification. The structures of PYR18 and CV18 are shown in Figure 1. The LB films were prepared with a KSV 5000 alternate LB system (KSV Instruments). A 0.5 mM CdCl2 solution in water purified by a Milli-Q system (Millipore) was used as a subphase. (11) Klafter, J.; Blumen, A. J. Chem. Phys. 1984, 80, 875. (12) Dewey, T. G. Chem. Phys. 1991, 150, 445. (13) Verschuere, B.; Van der Auweraer, M.; De Schryver, F. C. Chem. Phys. 1991, 149, 385. (14) Van der Auweraer, M. Habilitation; K. U. Leuven: Leuven, Belgium, 1990. (15) Ballet, P. PhD Thesis, K. U. Leuven, Leuven, Belgium, 1995. (16) Verschuere, B.; Van der Auweraer, M.; De Schryver, F. C. Chem. Phys. 1991, 149, 385.
© 1997 American Chemical Society
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Figure 3. Absorption spectra of “monolayer” LB films with different CV18 concentrations. The concentration of PYR18 is 0.5 mol % for all the films.
Figure 1. Molecular structures of PYR18 and CV18.
Figure 2. Schematic representation of a “monolayer” and “double layer” LB films.
concentration of CV18 varied from 0 to 10 mol %. Eighteen PYR18-containing layers and 19 CV18-containing layers were deposited on both sides of the substrate. The absorption spectra were recorded with a DW-2000 Aminco spectrophotometer. An uncoated quartz slide was used as a reference. The steady-state fluorescence and excitation spectra were measured with a Spex Fluorolog in a “front-face” configuration. The quartz plates were positioned perpendicular to the excitation, and the emission was detected at an angle of 26° with respect to the excitation. The spectra were obtained under reduced pressure (1 Torr) to avoid quenching or enhancement of the photodegradation by oxygen and were corrected for the intensity of the excitation source and the wavelength dependence of the excitation and detection systems. The fluorescence decay curves were measured under a reduced pressure (1 Torr) with a time-correlated single-photon counting system. Excitation occurred at 550 nm by a cavity-dumped dye laser with pyrromethene-556 dye (P-556 Exciton, Inc.) synchronously pumped by a mode-locked argon ion laser (514 nm, Spectra Physics Model 2020-5). To obtain fluorescence decays, unbiased by fluorescence depolarization, of the dyes in LB films, a selfdeveloped compartment described previously4 was used. To obtain the instrumental response function, a reference compound Daspi (2-(p-dimethylaminostyryl)pyridylmethyl iodide)19 with a very short fluorescence lifetime (12.4 ( 1.0 ps in methanol) was used.
Results The pH of the subphase was 5.7 and the temperature was 20 °C. The films were deposited on quartz plates cleaned as previously described.17 The monolayers were compressed at a rate of 5.7 × 10-3 nm2 molecule-1 min-1 until the deposition pressure of 30 mN m-1 was reached. The dipping rates were 2.1 × 10-3 nm2 molecule-1 min-1 downward and 1.0 × 10-3 nm2 molecule-1 min-1 upward. Two different multilayer systems were constructed (Figure 2). To study intralayer energy transfer, a double alternating deposition of monolayers of pure AA and mixed monolayers of the dyes in an AA matrix was made. Hence, the dyes were arranged in a tail-to-tail configuration, and interlayer dimer formation was not possible. This system is referred to as a “monolayer”. The concentration of PYR18 in the “monolayers” was 0.5 mol % and the concentration of CV18 varied from 0 to 3 mol %. Eighteen chromophore-containing layers and 21 pure AA layers were deposited on both sides of the substrate. To study interlayer energy transfer, a double alternating deposition of monolayers containing PYR18 and CV18 in an AA matrix was made. The PYR18-containing layers and CV18containing layers were separated by a distance of 50 Å.18 Interlayer dimer formation between two PYR18-containing layers or between two CV18-containing layers was possible. Hence, this system is referred to as a “double layer”. The concentration of PYR18 in the double layer films was 0.25 mol % and the (17) Verschuere, B.; Van der Auweraer, M.; De Schryver, F. C. Thin Solid Films 1994, 244, 995. (18) Roberts, G. Langmuir-Blodgett Films; Plenum: New York, 1990; p 139.
Intralayer Energy Transfer. Figure 3 shows the absorption spectra of the LB “monolayer” films at different CV18 concentrations. In the absence of CV18, the absorption of PYR18 monomers and dimers is observed at 554 and 520 nm, respectively.20,21 In the presence of CV18 the broad absorption of CV18 dimers, having maxima at 555 and 605 nm, dominates the spectra.22 Although the absorption spectra have maxima at 555 and 605 nm, the excitation spectra of mixed LB films of CV18 and dipalmitoylphosphatidylcholine, obtained at an analysis wavelength of 630 nm (monomer), have a maximum at 590 nm.15 Hence, the band at 605 nm is due to a transition to the lowest level of the singlet excited state of the dimer stabilized by exciton interaction rather than to a bathochromic shift of the monomer related to an increase of the polarizability at increased CV18 concentration. Figure 4 shows the normalized fluorescence spectra of “monolayer” films at different CV18 concentrations. Except at the highest CV18 concentrations, the spectra (19) Jonkman, A. M.; Van der Meulen, P.; Zhang, H.; Glasbeek, M. Chem. Phys. Lett. 1996, 256, 21. (20) Grauer, Z.; Grauer, G. L.; Avnir, D.; Yariv, S. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1685. (21) Gianneschi, L. P.; Kurucsev, T. J. Chem. Soc., Faraday Trans. 2 1974, 70, 1334. (22) Clark, F. T.; Drickamer, H. G. J. Chem. Phys. 1984, 81, 1024.
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Figure 4. Fluorescence spectra of “monolayer” LB films with different CV18 concentrations normalized at the maximum. The concentration of PYR18 is 0.5 mol % for all the films and the excitation wavelength is 540 nm.
are dominated by the fluorescence of PYR18 monomers,13,23 which show a maximum at 572 nm. The fluorescence intensity of PYR18 decreases drastically with increasing concentration of CV18, indicating that intralayer excitation energy transfer occurs from PYR18 to CV18. For 0.5 mol % CV18 concentration, the PYR18 fluorescence intensity is only 38% of that in the absence of CV18. As a result of the fluorescence quenching of PYR18, a weak fluorescence of CV18 dimers, which shows a maximum around 680 nm, is observed at high CV18 concentrations.14 No CV18 monomer fluorescence at 640 nm was observed.14 Thus, in the present films CV18 exists predominantly as dimers and those dimers, rather than CV18 monomers, will act as acceptors in the energy transfer. Even if, to some extent, the initial energy transfer would occur to CV18 monomers, it will always be followed by rapid energy transfer from CV18 monomers to CV18 dimers. The fluorescence decay curves of PYR18 obtained for the “monolayer” films at different CV18 concentrations are shown in Figure 5. The quenching of PYR18 fluorescence is observed as an increase of the fluorescence decay rate. If this quenching would be due to Fo¨rster energy transfer in a system with a self-similar distribution of acceptors,11,24 the fluorescence decay would be characterized by the equation
I(t) ) R exp[-t/τD - γdh (t/τD)dh /6]
(1)
where τD is the fluorescence decay time of the donor in the absence of acceptors, the parameter γdh is a function of the critical transfer distance R0, and the number of the h is the fractal dimension acceptors in a unit area σ0, and d of the distribution of the quenchers. In a two-dimensional system eq 1 becomes
I(t) ) R exp[-t/τD - γ2(t)1/3]
(2)
where γ2 ) Γ(2/3)πR02σ0(τD)-1/3 and Γ is the Euler gamma function. To study Fo¨rster energy transfer in heterogeneously distributed systems, an additional term has often been included because of the presence of donor molecules that are not affected by any acceptors1,4 (eq 3).
I(t) ) R exp[-t/τD - γ2(t)1/3] + (1 - R)exp[-t/τD] (3) (23) Pant, D. D.; Pant, K. C.; Joshi, N. B. Ind. J. Pure Appl. Phys. 1973, 11, 507. (24) Baumann, J.; Fayer, M. D. J. Chem. Phys. 1986, 85, 4087.
Figure 5. Fluorescence decay curves of “monolayer” LB films with different CV18 concentrations. The concentration of PYR18 is 0.5 mol % for all the films. The excitation wavelength is 540 nm, and the fluorescence is monitored at 590 nm.
Figure 6. Plot of γ2 as a function of the number density of CV18, σCV18, for “monolayer” films. Table 1. Parameters Obtained by Global Analysis of Fluorescence Decays of “Monolayer” Films with Different CV18 Concentrations in the Framework of Eq 3 Linking τD (χ2global ) 1.11 and τD ) 2.94 ( 0.12 ns) [CV18] (mol %)
γ2
0 0.5 1 2 3
0.803 1.239 2.605 3.857 4.528
(1 - R)
χ2
0.019 0.012
1.05 1.14 1.24 1.09 1.21
The results of the global analysis of the decay curves in the framework of eq 3 linking τD are listed on Table 1. At low CV18 concentrations satisfactory fits were obtained when (1 - R) was fixed at zero. For 2 and 3 mol % CV18 films the monoexponential term should be included in the analysis to obtain acceptable values of the statistical parameters. However, the relative contribution of the monoexponential decay, which is given by the factor 1 - R, remains small. Linking the decay time of the donor in the absence of quenchers over different concentrations yielded a value of 2.94 ( 0.12 ns at the emission wavelength of 590 nm. A value of 2.93 ( 0.07 ns was obtained by single-curve analysis of the decay curve in the absence of CV18. These results are in good agreement with the fluorescence lifetimes obtained for a dilute PYR18 solution: 2.0 ns in methanol, 2.3 ns in ethanol, and 2.7 ns in chloroform.15 The γ2 values increase linearly with increasing CV18 concentration but do not approach zero at low CV18 concentrations (Figure 6). The fluorescence decay curve in the absence of CV18 is nonexponential in spite of the low PYR18 concentration used. A similar phenomenon was also observed for dioctadecylrhodamine B and zinc(II) pyropheophytin-anthraquinone LB films.4,25 As revealed by analysis of simulated decays, the correlation between
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Table 2. Calculated Acceptor Concentrations σ0 and Total CV18 Dimer Concentrations, σCV18 [CV18] (mol %)
σCV18 (nm-2)
σ0 (nm-2)
σ0/σCV18 (%)
0.5 1 2 3
0.013 0.026 0.051 0.076
0.0035 0.0145 0.0247 0.0302
27 56 48 40
parameters γ2 and 1 - R can lead to an overestimation of parameter γ2 at low acceptor concentrations.9 Furthermore, a nonzero value of γ2 in the absence of acceptors can be attributed to intrinsic quencher sites in LB films. From the parameters γ2, τD, and R0 the number density of acceptors in a “monolayer” can be calculated. γ2 and τD were obtained from global analysis of the fluorescence decay curves. R0 was calculated from the spectral overlap of the donor fluorescence band FD(νj) (normalized to unity) with the absorption band A(νj) (molar extinction coefficient in M-1 cm-1) of the acceptor (i.e., CV18 dimer) according to eq 4
R06 )
9000 ln(10)κ2φD
FD(νj)A(νj)d(νj4)
128π5n4NA
νj4
∫0∞
Figure 7. Absorption spectra of “double layer” LB films with different CV18 concentrations. The concentration of PYR18 is 0.25 mol % for all the films.
(4)
where κ2 is an orientation factor, which depends on the relative orientation of the fluorescence and absorption dipoles, φD is the fluorescence quantum yield of the donor (in the absence of energy transfer), NA is the Avogadro number, and n is the refractive index of the medium. For φD the fluorescence quantum yield of PYR18 in a diluted solution of ethanol at a temperature of 25 °C (φD ) 0.5) is used. The refractive index of the medium n equals 1.5. The A(νj) value used in eq 4 is that of an isotropic solution. To relate it to the molar extinction coefficient determined from the absorption spectrum of a LB film, film j), the orientation of CV18 molecules in the film A (ν should be known. The orientation of CV18 molecules in LB films must be between an edge-on and an in-plane orientation. For the in-plane orientation the orthogonal transition dipoles of the two degenerate transitions of j) is 3/2 A(νj). CV18 are in the plane of the LB film and film A (ν film 2 Therefore, the A (νj) must be multiplied by /3 to get the isotropic value used to calculate R0. For the orientation factor κ2, a value of 5/4, characteristic for randomly oriented transition dipoles in a two-dimensional plane, is used. For the edge-on orientation only 50% of the oscillator strength of CV18 is observed, since the transition dipole perpendicular to the LB film will not interact with the incident light. For the transition polarized in the plane of the LB film the factor 3/2 remains. To find A(νj ), film j) must be multiplied by 2 × 2/3 ) 4/3. Since the only A (ν transition dipole moment of the donor PYR18 is in the plane of the film, only the in-plane transition dipole of CV18 will contribute to the energy transfer and a κ2 of 5/4 × 1/2 ) 5/8 should be used. Thus, independent of the orientation of CV18, a κ2 value of 5/4 and a correction factor of 2/3 ) 4/3 × 1/2 should be introduced in eq 4. A value for the critical transfer distance of R0 ) 65 Å was calculated in this way. In Table 2 the acceptor concentrations, σ0, obtained from γ2 after subtracting γCV18)0, are compared with the CV18 dimer concentrations, σCV18, determined from the pressure-area isotherms of the films under the assumption that all CV18 molecules are present as dimers. Global analysis in the framework of eq 1, linking h of decays obtained at different CV18 parameters τD and d concentration, resulted in acceptable statistical fits in the (25) Tkachenko, N. V.; Tauber, A. Y.; Lemmetyinen, H.; Hynninen, P. H. Thin Solid Films 1996, 280, 244.
Figure 8. Fluorescence spectra of “double layer” LB films with different CV18 concentrations normalized at the maximum. The concentration of PYR18 is 0.25 mol % for all the films and the excitation wavelength is 540 nm.
CV18 concentration range 0-1 mol %. For 2 and 3 mol % CV18 films the monoexponential term should be included in the analysis to obtain acceptable values of the statistical parameters. The decay time of 3.00 ( 0.20 ns corresponds well with the values obtained in the framework of eq 3. A fractal dimensionality of 2.09 ( 0.24 can be calculated. This suggests again that the distribution of acceptors, i.e., CV18 dimers, in the films is, within the experimental error, nearly uniform over the concentration range used. Interlayer Energy Transfer. The absorption spectra of the LB “double layer” films at different CV18 concentrations are shown in Figure 7. In the absence of CV the relative intensity of the PYR18 dimer band at 520 nm is higher for “double layers” than for “monolayers”, suggesting the presence of interlayer dimers in the “double layer” films. In the presence of CV the relative intensity of the CV18 dimer bands is lower and the spectra are broader for “double layer” films than for “monolayer” films. For “double layer” films the formation of interlayer dimers is possible besides the formation of intralayer dimers. Thus, part of the CV18 dimers in “double layers” could have different packing than the dimers in “monolayer”. The fluorescence spectra of “double layer” films at different CV18 concentrations, normalized at the maximum, are shown in Figure 8. Again, the fluorescence intensity of PYR18 decreases drastically with increasing concentration of CV18; the fluorescence intensity of 1 mol % CV18 film is 15 % of that in the absence of CV18. Thus, the interlayer excitation energy transfer occurs from PYR18 to CV18. The weak fluorescence of CV18 dimers is now observed at 715 nm. This large red shift of the dimer fluorescence
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Figure 9. Fluorescence decay curves of “double layer” LB films with different CV18 concentrations. The concentration of PYR18 is 0.25 mol % for all the films. The excitation wavelength is 540 nm, and the fluorescence is monitored at 590 nm. Table 3. Parameters Obtained by Global and Single-Curve Analyses of Fluorescence Decays of “Double Layer” Films with Different CV18 Concentrationsa [CV18] (mol %)
d h
γdh
0 1 2 3 10
Global Analysis 2.60 0.576 2.60 2.363 2.60 2.733 2.60 3.028 2.60 6.820
0 1 2 3 10
Single-Curve Analysis 2.00 0.622 2.65 2.273 2.51 2.732 2.35 3.179 3.00 6.777
1-R
χ2
0.072
1.07 1.19 1.22 1.27 1.02
0.009
1.05 1.12 1.23 1.19 0.92
Figure 10. Plot of γdh as a function of the number density of CV18, σCV18, for “double layer” films: (]) global analysis; (×) single-curve analysis.
the monoexponential term should be included in the analysis to obtain acceptable values of the statistical parameters. Single-curve analysis in the framework of h values for eq 5 fixing τD ) 3.30 ns resulted in different d different CV18 concentrations. The results are listed in Table 3. The recovered values of d h suggest that the system approaches a three-dimensional one with increasing CV18 concentration. No changes in the parameters were observed if τD was fixed at 3.77 ns. Plots of γdh against the σCV18 for both the global and singlecurve analysis are shown in Figure 10. The γdh values are nearly equal for both analysis methods. Although γdh increases with increasing CV18 concentration, the increase is not linear. A similar phenomenon was also observed for dioctadecylrhodamine B and octadecyl crystal violet LB films.3,4 Discussion
I(t) ) R exp[-t/τD - γdh (t)dh /6] + (1 - R)exp[-t/τD] (5)
“Monolayer” Films. In the present films CV18 could exist both as monomers and as dimers. For the “monolayer” films complex formation between PRY18 and CV18 is possible. However, no direct spectral evidence of complex formation was found, and thus, its existence is not taken into account in the present study. When the concentration of CV18 changes, the equilibrium between monomers and dimers changes. Hence, the absorption spectrum and the overlap with the PYR18 emission spectrum could change. In the framework of the Kasha model26 the oscillator strength of the dimer is twice that of the monomer. Thus, assuming the same normarlized overlap for both monomers and dimers, R02 will be 21/3 times larger for dimers compared with monomers. On the other hand, converting two monomers into one dimer decreases the acceptor concentration by 50%. Hence, the predicted values of the product R02σ0 will be 22/3 times too small if it is assumed that the dye molecules are only present as monomers while in reality they are present as dimers. When both monomers and dimers of CV18 are present, the energy transfer cannot be expected to take place exclusively to monomers or to dimers. Both species can act as a primary acceptor, and in a second step the monomers will transfer the energy further to dimers. Thus, R02σ0 should become σ0MR0M2 + σ0DR0D2. This effect does not change the mathematical expression describing the decay of the donors. It will, however, change the mathematical expression27 for the decay law of the CV18 monomers.
h yielded acceptable Global analysis linking τD and d statistical parameters with τD ) 3.77 ( 0.36 ns (Table 3). A fractal dimensionality equal to 2.60 ( 0.14 was obtained. At low CV18 concentrations satisfactory fits were obtained when (1 - R) was fixed at zero. For 10 mol % CV18 film
(26) (a) Kasha, M. Rev. Mod. Phys. 1959, 31, 1. (b) Kasha, M. Radiat. Res. 1963, 20, 55. (c) Mac Rae, E. G.; Kasha, M. The Molecular Exciton Model. Physical Processes in Radiation Biology; Academic Press: New York, 1964; p 23-42. (27) Agranovich, V. M.; Galanin, M. D. Electronic Excitation Energy Transfer in Condensed Matter; North-Holland: New York, 1982.
a τ for global analysis was 3.77 ns, and for single-curve analysis D τD was fixed at 3.30 ns.
in “double layer” films compared with the “monolayer” films is probably due to the formation of interlayer dimers. For the “monolayer” films CV18 dimer fluorecence was observed already at a CV18 concentration of 3 mol %. For the “double layer”, CV18 dimer fluorescence was only observed for 10 mol % CV18 film. Hence, either the energy transfer is less efficient in “double layer” than in “monolayer” films or the fluorescence efficiency of the interlayer dimer is much weaker than that of the intralayer dimer. When the critical transfer distance R0 was calculated from the spectral overlap, a value of 67 Å was obtained, which was in good agreement with the 65 Å obtained for the “monolayer” films. Figure 9 shows the fluorescence decay curves of PYR18 obtained for the “double layer” films at different CV18 concentrations. Global analysis, linking τD, in the framework of eq 3 of decays obtained at different CV18 concentrations yielded unacceptable statistical fits in the CV18 concentration range from 1 to 10 mol %. In the absence of CV18 acceptable statistical fits were obtained with τD ) 3.30 ( 0.06 ns. This is slightly larger than the value obtained for “monolayers”. The decay curves both in the precence and in the absence of CV18 were analyzed using eq 5:
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Upon increasing the CV18 concentration there are two possibilities: either the equilibrium shifts from monomers to dimers or the monomer-dimer ratio does not change significantly over the concentration range used. In the first case a slightly sublinear increase of γ2 upon increasing the total CV18 density, σ0, is expected. Hence, when R02 is calculated from the ratio γ2/σ0 and σ0 is estimated with the assumption that all dye molecules are present as dimers, a small decrease of R02 would be observed upon increasing CV18 concentration. In this case the recovered values of R02 will be too large at low dye concentrations and will approach the correct values upon increasing the dye concentration. In the latter case γ2 will depend in a linear way upon the total dye density, σ0. Even if all dye molecules were present as monomers, the recovered values of R02 will be only a factor of 22/3 too small. For the present “monolayer” films γ2 increases linearly with increasing CV18 concentration over the concentration range used. Thus, the equilibrium between monomers is not assumed to shift significantly over the concentration range used. This is supported by the independence of the absorption spectrum of the CV18 concentration. Since at 1 mol % concentration CV18 molecules are already mostly present as dimers, no great error is made when assuming σ0M ) 0. The values calculated for the quencher concentration, σ0, are about 40% of the total CV18 dimer concentration σCV18. The small value obtained at the lowest CV18 concentration indicates that the γCV18)0 value of 0.878 is, to some extent, due to incorrect parameter recovery. It is not due to a shift of the monomer-dimer equilibrium, since the decrease of the oscillator strength of the acceptor species upon dissociation of the dimers should, in a twodimensional system, be overcompensated by the increase of the number of quenching species. The effect would be enhanced by a better spectral overlap between the absorption spectrum of CV18 monomers and the PYR fluorescence. “Double Layer” Films. The relative intensity of the CV18 dimer bands is lower and the spectra are broader for “double layer” films than for “monolayer” films. Besides the different dimer geometry, these changes could be due to the presence of monomers in the “double layer” films. For octadecylrhodamine 6G LB films it was found that the proportion of dimers is greater in a “monolayer” than in a “double layer” film.28 This was attributed to the saltinduced dimerization caused by cadmium stearate in the layer adjacent to the “monolayer”. The concentration of octadecylrhodamine 6G in the films was very high, 40 mol %, compared with the CV18 concentrations used in the present study. Nevertheless, a similar phenomenon could take place in the present films. Thus, two acceptors would be present in the “double layer” films: CV18 dimers and CV18 monomers. Since CV18 monomer absorption is at 590 nm, exactly at the same wavelength as PYR18 fluorescence, it is a more efficient acceptor than CV18 dimer. However, no CV18 monomer fluorescence was observed. This could be due to efficient energy transfer from CV18 monomers to CV18 dimers. For the “double layer” films the “fractal dimension” d h increases with increasing CV18 concentration, becoming equal to 3 for a CV18 concentration of 10 mol %. In h previous studies1,3,5,6 of dye-containing LB films the d values have been found to vary with dye concentration. For monolayer films values smaller than 2 for d h have been found, whereas for multilayer films d h increases upon increasing layer number. The small values of d h found by (28) Ikonen, M.; Vuorimaa, E.; Moritz, V.; Lemmetyinen, H. Thin Solid Films 1993, 226, 275.
Vuorimaa et al.
other authors1,3,5,6 for monolayer films have been explained by an irregular distribution of dye molecules on the film. The increase of d h upon increasing the number of layers for a multilayer system3 was explained by energy transfer occurring between dye molecules that are embedded in nonadjacent layers. Thus, the decay law for energy transfer in a two-dimensional system cannot be applied to stacked multilayer systems when the interlayer distance is not much larger than R0. The present “double layer” films are double alternating multilayer films in which the distance between PYR18-containing and CV18-containing layers is always 50 Å (Figure 2). Thus, there is a CV18 layer on both sides of the PYR18 and energy transfer to both these layers is equally probable. On the other hand, energy transfer to the second closest pair of CV18containing layers is not likely to take place, since the distance is already quite large, 150 Å. Yamazaki et al.1,29 used a superposition of Fo¨rster-type kinetics in two- and three-dimensional energy transfer to analyze their results of dyes in vesicles. The amplitude of the three-dimensional term increased with increasing acceptor concentration. They attributed this behavior to the nonuniform and nonrandom distribution of the dye molecules on the planar vesicle surface. Taking into account the results shown in Table 3, their analysis could also indicate a gradual transition from a two- to a three-dimensional system. The large difference between the τD values obtained by the global and single-curve analyses is probably due to this tendency of the system to approach a three-dimensional one with increasing CV18 concentration. The global h gives a longer lifetime, since all analysis linking τD and d the curves are forced to have an equal d h value. No literature data of R0 values exist for the present donor-acceptor pair. The R0 values observed for similar dyes, i.e., for rhodamine 6G as donor and malachite green as acceptor, vary from 52 to 64 Å.30-35 Thus, the obtained R0 values of 65 and 67 Å for monolayer and double layer films, respectively, seem to be reasonable. Conclusions In the present study the intralayer and interlayer excitation energy transfer between PYR18 and CV18 in LB films were examined with picosecond time-resolved fluorescence decay measurements. The PYR18 fluorescence is efficiently quenched by energy transfer to CV18 dimers. For “double layer” films the formation of interlayer dimers is observed as broader absorption spectra and a large red shift of fluorescence maximum compared with “monolayer” films. The critical transfer distances of 65 and 67 Å for intralayer and interlayer energy transfer, respectively, were calculated from spectral overlap. For “monolayer” films the fluorescence decays can be fitted to the two-dimensional Fo¨rster energy transfer equation, indicating uniform distribution of acceptors. For “double layer” films it was necessary to allow the dimension to float in the Fo¨rster equation. With global analysis a “fractal dimension” equal to 2.6 was obtained for the “double layers”. With single-curve analysis the twodimensional system is seen to approach the threedimensional one with increasing CV18 concentration. (29) Tamai, N.; Yamazaki, T.; Yamazaki, I.; Mizuma, A.; Mataga, N. J. Phys. Chem. 1987, 91, 3503. (30) Choi, K.-J.; Turkevich, L. A.; Loza, R. J. Phys. Chem. 1988, 92, 2248. (31) Miller, R. J. D.; Pierre, M.; Fayer, M. D. J. Chem. Phys. 1983, 78, 5138. (32) Pines, D.; Huppert, D.; Avnir, D. J. Chem. Phys. 1988, 89, 1177. (33) Lin, C.; Dienes, A. J. Appl. Phys. 1973, 44, 5050. (34) Pines, D.; Huppert, D. Isr. J. Chem. 1989, 29, 473. (35) Porter, G.; Tredwell, C. J. Chem. Phys. Lett. 1978, 56, 278.
Energy Transfer
Acknowledgment. The reference compound Daspi was a gift from Dr. Shamir Farid, Eastman Kodak. E.V. and H.L. gratefully acknowledge the financial support of the Academy of Finland and the Technology Development Center of Finland for support of our program of photochemistry of organic films. P.B. acknowledges the K. U. Leuven for financial support. M.V.d.A is an “Onder-
Langmuir, Vol. 13, No. 11, 1997 3015
zoeksdirecteur” of the F.K.F.O. The authors gratefully acknowledge the support of the COST D4 project, in collaboration with P.B. and M.V.d.A., and the continuing support from DWTC (Belgium). LA961083T
