J. Phys. Chem. 1996, 100, 13333-13337
13333
Polarized Electronic Energy Transfer in Langmuir-Blodgett Films John A. Pescatore, Jr., and Iwao Yamazaki* Department of Molecular Chemistry, Faculty of Engineering, Hokkaido UniVersity, Sapporo 060, Japan ReceiVed: March 7, 1996; In Final Form: May 8, 1996X
Aggregation of cyanine dyes within Langmuir-Blodgett (LB) films is exploited to form heterodimers within mixed dye monolayers. The geometrical constraints imposed on the amphiphilic dye molecules within an LB monolayer are utilized to facilitate polarized energy transfer. Time-correlated single-photon counting is used to measure the residual polarization of electronic energy transfer from donor oxacyanine molecules to acceptor oxacarbocyanine molecules. The residual polarization decay profiles are fit to a biexponential and are contrasted for two types of LB matrixes. In a neutral tripalmitin LB matrix, the residual polarization of this mixed cyanine dye system is as high as 0.13 and decays with decay times of 170 ps and 2.8 ns to 0.03. In an ionic fatty acid LB matrix the residual polarization reaches 0.11 and decays to 0.06 with decay times of 620 ps and 5.3 ns. The concentration depolarization of these residual polarizations suggests that heterodimers are randomly distributed throughout the LB monolayer.
Introduction The nanoseconds fluorescence lifetimes of cyanine dyes obtainable in solid-state matrixes and the large oscillator strength of their S1 r S0 transition have resulted in many spectroscopic studies of these dyes embedded in LB films.1-12 Concentration quenching of the fluorescence from cyanine dyes in LB films has been observed by many authors.2,4-12 Tamai et al.6 report that oxacyanine at ∼3 mol % in an LB matrix of a 1:1 (m/m) mixture of methyl arachidate and cadmium arachidate exists predominantly as dimers. In electronic energy-transfer studies of cyanine dyes in LB multilayer films at concentrations where aggregation in each monolayer is prominent, Yamazaki and coworkers11,12 discuss energy transfer as directional and, over distances greater than 25 Å, as occurring prior to internal conversion. In the present work LB samples of a mixture of oxacyanine and oxacarbocyanine in both tripalmitin and arachidate matrixes are prepared under identical LB deposition conditions to investigate the effect of the matrix on the distribution of dye molecules within an LB monolayer film and the directionality of electronic energy transfer. Time-resolved energy transfer is studied with fluorescence polarization spectroscopy using timecorrelated single-photon counting. Oxacarbocyanine as an acceptor molecule does not absorb the radiation used to excite oxacyanine donors. Furthermore, the fluorescence spectrum of oxacarbocyanine is clearly resolvable from that of oxacyanine (see Figure 1). As a result of these spectroscopic properties, any differential of the parallel and perpendicular components of oxacarbocyanine emission after photon absorption by oxacyanine is from electronic energy transfer which maintains the photoselected polarization. We observe this residual polarization in oxacarbocyanine emission and attribute it to emission from heterodimers formed during aggregation within the LB monolayers. To our knowledge these are the first observations of residual polarization in the acceptor dye fluorescence. Experimental Section N,N′-Dioctadecyloxacyanine perchlorate (NK-3050) and N,N′dioctadecyloxacarbocyanine perchlorate (NK-3444) purchased from Nippon Kankoh Shikiso are recrystallized from methanol X
Abstract published in AdVance ACS Abstracts, July 15, 1996.
S0022-3654(96)00707-1 CCC: $12.00
Figure 1. (a) Absorption spectrum and (b) uncorrected fluorescence spectrum of 3 mol % OXA mixed with 3 mol % OCC in an LB monolayer built from an arachidate matrix.
prior to use. The dyes are assayed for purity on silica TLC plates with chloroform:methanol (7:3, v/v). (For brevity the dyes are referred to as oxacyanine or OXA and oxacarbocyanine or OCC.) Tripalmitin (Kanko chemicals), arachidic acid, and methyl arachidate (Kodak) are each recrystallized from ethanol prior to use. Octadecyltrichlorosilane is used as received from Tokyo Kasei Chemicals. The solvents are all of spectragrade (Kanko Chemicals) and used as received. Water is purified with a Milli-Q water purification system (Millipore Co.) prior to use. © 1996 American Chemical Society
13334 J. Phys. Chem., Vol. 100, No. 32, 1996
Letters at a bandpass of 8 nm) and is detected with a Hamamatsu R2809U microchannel plate photomultiplier tube. The differential response of this system to vertical and horizontal polarizations is corrected with the isotropic emission of 9-cyanoanthracene.16 Photon counting rates are kept below 10 kHz and typical collection times for the fluorescence decay measurements are ∼15 min yielding maximum photon counts of ∼15 000. For each sample, the order of recording the orthogonal components of the polarized emission is reversed prior to repeating anisotropy measurements. Both the rates of emission and the emission spectra are observed before and after timeresolved measurements in order to verify the integrity of the samples. Results and Discussion
Figure 2. (a) LB deposition direction parallel to the Y axis. The incident laser radiation is Y-polarized and makes an angle of incidence R ∼86° with the surface normal, N. The emission is observed in the Z direction and is X-polarized (perpendicular) or Y-polarized (parallel to the excitation). (b) Transition dipoles of the OXA D and of the OCC A molecules in the LB monolayers have polar angles restricted to ∼90° but randomly oriented azimuthal angles with respect to the surface normal N. The plane common to the molecules is designated as σLB and the variable intermolecular distances as r.
Stock solutions (1 mM) of the dyes, tripalmitin, arachidic acid, and methyl arachidate are prepared in CH2Cl2. Hereafter, arachidate matrixes refer to a 1:1 (m/m) mixture of methyl arachidate and cadmium arachidate. Predetermined ratios of the dye solutions to the matrix solutions are mixed prior to pipetting onto a pH 6 10 mM aqueous solution of CdCl2. The LB film is compressed to and transferred under a constant 22 mN/m surface pressure onto both sides of silanized glass microscope slides13,14 in a Y-type fashion1 at a rate of 4 mm/ min (San-Yesu Instrument Co., FSD-20). In this fashion, each dye containing monolayer is covered with three layers of the matrix molecules. The deposited films are stored in a desiccator. Absorption, steady-state fluorescence, and fluorescence excitation spectra are taken under ambient conditions. The absorption spectrophotometer is a Jasco Ubest-50. The emission spectra are recorded on a Jasco FP-770F fluorimeter. Roomtemperature time-resolved fluorescence studies are done on samples under 1 mTorr vacuum. The electronics of this timeresolved system is described elsewhere.15 In this work, frequency-doubled Coherent Mira 900 laser excitation operating at a repetition rate of 2 MHz illuminates an area of ∼25 mm2. The geometrical layout for polarization measurements is shown in Figure 2a. An unfocused laser beam is attenuated with a variable neutral density filter to prevent photobleaching of the LB samples. The polarization of the laser beam is oriented with a half-wave plate parallel to the direction in which the microscope slide is dipped into the LB trough and purified with a Glan Taylor polarizer. The emission nearly parallel to the surface normal is collected with a 12 cm focal length lens and polarized with HNP′B polaroid film. The polarized emission is filtered of scattered laser light through three 2 mm thick Toshiba cutoff filters (one 380 and two 390 nm), is dispersed through a 0.25 m Czerny-Turner monochromator (Nikon, set
In the absorption spectra of our LB samples a sublinear deviation from Beer’s law with a concentration increase of the chromophore in either LB matrix suggests the formation of aggregates with absorption extinction coefficients less than that of the monomers. The extent of OXA aggregation within the two matrixes may be gauged by the ratio of the absorbances at 360 nm to that at 380 nm.1 On the basis of the predominance of the 360 nm peak aggregation appears more facile in the tripalmitin matrix than in the fatty acid matrix. Site distribution within the LB films of either matrix is evident from fluorescence excitation spectra which depend on the wavelength of the monitored emission. This site distribution is attributed primarily to different degrees of aggregation. Energy transfer, studied by the time-correlated single-photoncounting technique, among aggregates within 2D and 3D isotropic systems is extensively discussed, e.g., by Struve et al.17-21 The extent of donor aggregation affects the efficiency of energy transfer to acceptor molecules. Electronic excitation which is dissipated among randomly aggregated donors and the lattice cannot be transmitted to acceptor dye molecules. OXA aggregation within the two different LB matrixes is further compared by time-resolved studies using time-correlated singlephoton counting of the dispersed emission. Concentration depolarization is more prominent for the low-energy sites in tripalmitin matrixes than in arachidate matrixes for similar absorbances of OXA. For example, long time residual polarizations of 3 mol % OXA in tripalmitin matrixes are P(∞) ) 0.25 at 400 nm and P(∞) ) 0.1 at 480 nm, while that of 1 mol % OXA in the arachidate matrix are P(∞) ) 0.25 at 400 nm and P(∞) ) 0.225 at 480 nm.22 On the basis of such concentration depolarization studies, direct energy transfer from OXA to OCC is expected to be most effective for OXA concentrations less than 3 mol % in tripalmitin and at ∼1 mol % in the arachidate matrixes. Examples of the time-resolved OCC 505 nm emission from OXA-OCC mixed monolayers in (a) arachidate and (b) tripalmitin LB matrixes are shown in Figure 3. The residual polarizations of energy transfer from OXA irradiated at 385 nm to OCC is seen in the left-hand panel as a contrast in the intensities of the polarized decay components of OCC emission. A quantitative comparison of the time-resolved residual polarizations, as calculated using eq 1, obtained in the two different LB matrixes is made in the right hand panel.
P(t) )
I|(t) - I⊥(t) I|(t) - I⊥(t)
(1)
Due to the finite rise time of the acceptor fluorescence, the fluorescence anisotropy of OCC requires time to reach a maximum value. The OCC 2D fluorescence anisotropy decay,
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J. Phys. Chem., Vol. 100, No. 32, 1996 13335
Figure 3. Time-resolved polarized 505 nm fluorescence decay profiles after 385 nm excitation from (a) 0.5 mol % OXA mixed with 0.5 mol % OCC in an arachidate LB matrix and from (b) 1.5 mol % OXA mixed with 1.5 mol % OCC in a tripalmitin LB matrix. The left-hand side panel shows OCC fluorescence components I|(t) (- - -) and I⊥(t) (‚ ‚ ‚). The intensities are normalized to max counts of I||(t). Polarization decay curves calculated using eq 1 are displayed in the right-hand side panel.
Figure 4. Time-resolved polarized 505 nm fluorescence decay profiles after 385 nm excitation of an arachidate OXA-OCC LB sample. The dye concentrations are 1.0 mol % OXA mixed with (a) 1.0 mol % OCC and (b) 3.0 mol % OCC in the top panels and 3.0 mol % OXA mixed with (a) 1.0 mol % OCC and (b) 3.0 mol % OCC in the bottom panels. The left-hand side panels show OCC fluorescence components I|(t) (- - -) and I⊥(t) (‚ ‚ ‚). The intensities are normalized to max counts of I|(t). Polarization decay curves calculated using eq 1 are displayed in the right-hand side panels.
that is, the residual polarization decay, is fit to eq 2 starting at this maximum value, P(0) and including ∼2 ns of decay. The P(0) and P(∞) values and the decay times determined by these fits together with the rise times for the data displayed in Figures 3-5 are presented in Tables 1 and 2.
P fit(t) ) A1exp(-t/τ1) + A2exp(-t/τ2) + P(∞)
(2)
Depolarization due to energy transfer among randomly oriented molecules is indicated by P(t) ) 0. The residual polarization of OCC fluorescence from the arachidate matrix sample shown in Figure 3a starts at 0.05 and decays to 0.02. At similar dye coverages in the tripalmitin matrix (see Figure 3b) the residual polarization is 0.09 and remains as high as 0.05
even after 2 ns. Apparently at these dye concentrations there is a greater degree of aggregation of OXA with OCC in the tripalmitin matrixes than in the arachidate matrixes. A difference in average OXA-OCC intermolecular distances is revealed by an OCC fluorescence rise time of 60 ps in the tripalmitin versus 100 ps in the arachidate matrixes. The contrast of the decay curves of the left-hand side panel of Figure 3 indicates that acceptor OCC molecules in the proximity of donor OXA molecules favor anisotropic orientations. Concentration depolarization of these residual polarizations in the two different matrixes is shown in Figures 4 and 5. In the top panels of Figure 4 the OXA donor concentration is kept at 1 mol % while the OCC acceptor concentration is (a) 1 mol
13336 J. Phys. Chem., Vol. 100, No. 32, 1996
Letters
Figure 5. Time-resolved polarized 505 nm fluorescence decay profiles after 385 nm excitation of a tripalmitin OXA-OCC LB sample. The dye concentrations are 3.0 mol % OXA mixed with (a) 3.0 mol % OCC and (b) 9.0 mol % OCC in the top panels and 9.0 mol % OXA mixed with (a) 3.0 mol % OCC and (b) 9.0 mol % OCC in the bottom panels. The left-hand side panels show OCC fluorescence components I|(t) (- - -) and I⊥(t) (‚ ‚ ‚). The intensities are normalized to max counts of I|(t). Polarization decay curves calculated using eq 1 are displayed in the right-hand side panels.
TABLE 1: Characteristics of 505 nm Emission from 385 nm Excitation of OXA-OCC LB Monolayers in an Arachidate Matrix XOXA
XOCC
trisea
P(0)b
P(∞)b
τ1 (A1)b
τ2 (A2)b
0.005 0.01 0.01 0.03 0.03
0.005 0.01 0.03 0.01 0.03
100 50 40 50 50
0.05 0.09 0.11 0.08 0.10
0.02 0.05 0.06 0.03 0.02
1.3 (0.75) 3.0 (0.50) 5.3 (0.46) 4.8 (0.54) 1.2 (0.15)
0.75 (0.25) 0.30 (0.50) 0.62 (0.54) 0.47 (0.46) 0.20 (0.85)
a Rise times are expressed as the picosecond time difference between 0.90 and 0.10 of max counts. b Parameters as determined by fitting to eq 2. The time unit is nanoseconds and P(0) ) A1 + A2 + P(∞). The tabulated amplitudes are normalized to the sum A1 + A2. Based on a range of values from three measurements, the uncertainties are 50%.
TABLE 2: Characteristics of 505 nm Emission from 385 nm Excitation of OXA-OCC LB Monolayers in a Tripalmitin Matrix XOXA
XOCC
trisea
P(0)b
P(∞)b
τ1 (A1)b
τ2 (A2)b
0.015 0.03 0.03 0.09 0.09
0.015 0.03 0.09 0.03 0.09
60 60 40 60 40
0.09 0.13 0.04 0.09 0.02
0.05 0.03 0.02 0.02 0.00
9.6 (0.48) 2.8 (0.26) 1.1 (0.10) 2.9 (0.21) 0.2 (1.00)
1.13 (0.52) 0.17 (0.74) 0.31 (0.90) 0.19 (0.79) 0.00 (0.00)
a
Rise times are expressed as the picosecond time difference between 0.90 and 0.10 of max counts. b Parameters as determined by fitting to eq 2. The time unit is nanoseconds and P(0) ) A1 + A2 + P(∞). The tabulated amplitudes are normalized to the sum A1 + A2. Based on a range of values from three measurements, the uncertainties are 50%.
% and (b) 3 mol % in the arachidate matrix. At 3 mol % OCC it is inevitable that homodimers and possibly OCC aggregates are formed. As seen in the left-hand side top panel of Figure 4
aggregate excitation leads to nonradiative decay, thus the decrease in the decay times of the (b) 1:3 (OXA:OCC) over that of the (a) 1:1 samples. The increase in OCC concentration reduces the risetime of the OCC fluorescence because of a decrease in the average intermolecular distance to OXA donors; see Table 1. The smallest intermolecular distances between OXA and OCC are represented by heterodimers. The larger residual polarizations and the longer residual polarization decay times of the (b) 1:3 samples than those of the (a) 1:1 samples are attributed to a higher concentration of such heterodimers. The fluorescence decay curves shown in the bottom panels of Figure 4 are from LB arachidate monolayers with OXA concentrations increased to 3 mol % and mixed with OCC concentrations at (a) 1 mol % and (b) 3 mol %. The increase in the OCC decay times of the bottom left-hand side panel in comparison to the counterparts of the top left-hand side panel of Figure 4 shows that a larger range of transfer rates from OXA is sampled. Since the decay times for identical LB arachidate matrixes containing solely OXA as a chromophore decrease on increasing the concentration from 1 to 3 mol %,6 the increase in the OCC decay times after increasing the OXA concentration from 1 to 3 mol % in these mixed-dye monolayers is attributed to indirect energy transfer. In other words, in the single-dye system, donor-donor energy transfer leads to nonradiative decay. However, in the presence of a suitable acceptor dye, direct energy transfer (i.e., donor-acceptor) as well as indirect energy transfer (e.g., donor-donor-acceptor) become alternative channels of relaxation. The earliest OCC emission arises predominantly from OCC excited directly from photon-excited OXA. As the contribution from indirect excitation increases with time, the residual polarization necessarily decreases. This
Letters is evident by comparing the magnitudes and rates of the residual polarization decay profiles shown in Figure 4 and Table 1. Analyses of similar mixtures of OXA and OCC in tripalmitin LB samples are presented in Figure 5 and Table 2. In contrast to Figure 4 and Table 1 there is a larger degree of aggregation and thus concentration depolarization in the tripalmitin matrix than in the arachidate matrix. This increase in the degree of aggregation and thus the concentration depolarization is consistent with both stationary-state and time-resolved spectroscopic comparisons of these matrixes for single dye LB films. The theoretical maximum residual polarization of electronic energy transfer within 2D isotropic systems and measured under the present experimental geometry is 0.025.21 Consequently, the larger magnitudes of residual polarizations measured for our compressed LB monolayer samples (e.g., P(0) ) 0.11 for 1:3 OXA:OCC in arachidate) are attributed to anisotropic molecular orientations such as heterodimers, in which an oxacyanine molecule and an oxacarbocyanine molecule are aligned parallel to each other, (similar to the depiction of an OXA homodimer shown in Figure 10 of ref 6). We note that an amphiphilic molecule of a compressed LB monolayer may assume any azimuthal angle about the normal to the LB plane but is restricted in its range of polar angles. The restricted range of polar angles essentially keeps the transition dipoles of the donor and acceptor molecules within the LB plane; see Figure 2b. The concentration depolarization measurements show that the heterodimers are both randomly oriented and randomly located within an LB plane. In summary, directional energy transfer from photon-excited donors to acceptors of heterodimers occurs in mixed-dye LB monolayer films. The extent of dimerization and therefore of directional energy transfer is somewhat controllable by the choice of dye concentrations and of the LB matrix. Given the 1/r6 dependence for dipolar interactions,23 donor-donor energy transfer impedes this donor-to-acceptor directionality in LB multilayers with separated donor and acceptor dye layers containing dye concentrations suitable for aggregation. Residual polarization measurements promise to be useful to further energy-transfer studies under the restricted geometries and low dimensionalities characteristic of LB films.
J. Phys. Chem., Vol. 100, No. 32, 1996 13337 Acknowledgment. J.A.P. gratefully acknowledges the Japan Society for the Promotion of Science for a foreign researcher postdoctoral fellowship for his support. References and Notes (1) Kuhn, H.; Mo¨bius, D.; Bu¨cher, H. Techniques of Chemistry; Weissberger, A., Rossiter, B. W., Eds.; John Wiley and Sons: New York, 1972; Vol 1, Part 3B, pp 577-702. (2) Leitner, A.; Lippitsch, M. E.; Draxler, S.; Riegler, M.; Aussenegg, F. R. Thin Solid Films 1985, 132, 55. (3) Mo¨bius, D.; Kuhn, H. Isr. J. Chem. 1979, 18, 375. (4) Fromherz, P.; Reinhold, G. Thin Solid Films 1988, 160, 347. (5) Yamazaki, I.; Tamai, N.; Yamazaki, T.; Murakami, A.; Mimuro, M.; Fujita, Y. J. Phys. Chem. 1988, 92, 5035. (6) Tamai, N.; Matsuo, H., Yamazaki, T.; Yamazaki, I. J. Phys. Chem. 1992, 96, 6550. (7) Yamazaki, I.; Tamai, N.; Yamazaki, T. J. Lumin. 1988, 40, 47. (8) Yamazaki, I.; Tamai, N.; Yamazaki, T. J. Phys. Chem. 1990, 94, 516. (9) Tamai, N.; Matsuo, H.; Yamazaki, T.; Ohta, N.; Yamazaki, I. Chem. Func. Dyes 1993, 2, 810. (10) Tamai, N.; Yamazaki, T.; Yamazaki, I. Thin Solid Films 1989, 179, 451. (11) Yamazaki, I.; Ohta, N.; Yoshinari, S.; Yamazaki, T. Microchemistry: Spectroscopy and Chemistry in Small Domains; Masuhara, H., DeSchryver, F. C., Kitamura, N., Tamai, N., Eds.; North-Holland: Amsterdam, 1994; p 431. (12) Yamazaki, I.; Ohta, N. Pure Appl Chem 1995, 67, 209. (13) Brzoska, J. B.; Azouz, I. B.; Rondelez, F. Langmuir 1994, 10, 4367. (14) Brzoska, J. B.; Shahidzadeh, N.; Rondelez, F. Nature 1992, 360, 719. (15) Yamazaki, I.; Tamai, N.; Kume, H.; Tsuchiya, H.; Oba, K. ReV. Sci. Instrum. 1985, 56, 1187. (16) O’Connor. D. V.; Phillips, D. Time-Correlated Single Photon Counting; Academic Press: London, 1984. (17) Anfinrud, P. A.; Crackel, R. L.; Struve, W. S. J. Phys. Chem. 1984, 88, 5873. (18) Anfinrud, P. A.; Hart, D. E.; Hedstrom, J. F.; Struve, W. S. J. Phys. Chem. 1986, 90, 2374. (19) Anfinrud, P. A.; Hart, D. E.; Hedstrom, J. F.; Struve, W. S. J. Phys. Chem. 1986, 90, 3116. (20) Anfinrud, P. A.; Struve, W. S. J. Phys. Chem. 1987, 91, 5058. (21) Anfinrud, P. A.; Hart, D. E.; Struve, W. S. J. Phys. Chem. 1988, 92, 4067. (22) Pescatore, Jr., J. A.; Yamazaki, I., unpublished results. (23) Fo¨rster, T. Discuss. Faraday Soc. 1959, 27, 7.
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