J . Phys. Chem. 1992, 96. 2190-2195
2790
monomer-like, but a moderate number of dimers and trimers with the J-aggregate configuration have formed from statistical associations. In addition, a few defect states, possibly having a sandwich dimer configuration, have formed. Energy transfer from the monomer-like species to the dimers, trimers, and defect states causes the bulk of the fluorescence to come from these states. The defect-state emission dominates at 90 K, where it has a reasonably high quantum yield, while the room-temperature emission comes mainly from the small aggregates, since the defect state relaxes nonradiatively at this temperature. At 0.50 mole fraction, most the PIC molecules have formed small J-aggregates and extensive energy migration among these aggregates causes significant energy transfer to the defect states, resulting in a low yield of J-aggregate emission at room temperature. At 90 K, emission occurs from both the J-aggregates and the defect states, but an increased rate of nonradiative relaxation for the defect states formed at this concentration causes the overall yield of fluorescence to be lower than for the 0.125 mole fraction sample. For the undiluted PIC J-aggregates, the increased range of energy migration results in further quenching of the room-temperature J-aggregate fluorescence by the nonradiative defect states. At 90 K, the quenching persists and some emission from the defect states can be seen. The overall yield of fluorescence at this temperature remains quite low, presumably because the defect states associated with the undiluted J-aggregate have significant nonradiative relaxation at this temperature. One focus of our future work will be development of a better understanding of the nature of the defect states and a more quantitative model for their effect on
the excitation dynamics of PIC J-aggregates. While energy transfer from the J-aggregates to the defect state is the dominant factor controlling the size-dependent excitation dynamics, a weak superradiant enhancement of the J-aggregate radiative decay is also present for the samples containing the larger aggregates. The values obtained for this enhancement are much smaller than those observed for PIC J-aggregates in solution but can be rationalized by postulating strong coupling of the J-aggregate exciton to a low-frequency phonon. While the much larger disorder present in the surface-adsorbed J-aggregates may also be a factor in limiting the superradiance, disorder alone is not sufficient to explain the differences in enhancement observed between these aggregates and the solution aggregates. Both the energy transfer to the defect states and the enhanced radiative rate of the large J-aggregates compete with the desired process of electron transfer from the aggregate excited state to the AgBr conduction band. Consequently, the relative quantum efficiency of spectral sensitization by the dye decreases as the aggregate size increases. However, this decrease is not as large as would be expected because the electron-transfer rate constant from the aggregate excited state is found to increase with increasing aggregate size. Better understanding of the reasons underlying this size dependence of the J-aggregate electron-transfer rate constant is another area for future work. Acknowledgment. We are grateful to the National Science Foundation for a Science and Technology Center grant, CHE8810024.
Medium Effects on Photoinduced Electron Transfer in Langmuir-Blodgett Films Yong Hsu,+ Thomas L. Penner,l and David G. Whitten*,+ Center for Photoinduced Charge Transfer, University of Rochester, Hutchison Hall, Rochester, New York 14627, Department of Chemistry, University of Rochester, Rochester, New York 14627, and Corporate Research Laboratories, Eastman Kodak Company, Rochester, New York 14650 (Received: October 1, 1991; I n Final Form: January 24, 1992)
The Langmuir-Blodgett technique has been used to prepare films in which electron donor and acceptor layers are separated by a spacer layer one molecule thick in which both the length and the electronic nature of the molecules were changed without altering donor or acceptor layers. A thiacyanine dye located at the hydrophilic interface acted as an excited-state electron acceptor. The donor layer incorporated a long-chain carboxylic acid derivatized with a ferrocene chromophore at the hydrophobic interface as a ground-state electron donor. By monitoring the quenching of the thiacyanine fluorescence by the ferrocene as a function of the composition of the spacer layer, the influence of the medium on the electron-transfer process was determined. For both series of saturated fatty acids and trans-stilbene-derivatizedfatty acids the rate of electron transfer decreased with increasing separation distance of donor and acceptor, but the rate of attenuation was greater for the saturated fatty acids. Although the data do not conform to a simple exponential attenuation of electron transfer with separation distance, it is possible to fit it to this form by assuming that about half the excited thiacyanine dye is inactive in the electron-transfer process, or by assuming two populations which quench differently due to their different lifetimes. Both treatments lead to very low attenuation coefficients (p) for the fatty acid and trans-stilbene spacers; within the context of this analysis the attenuation coefficient is about 50% greater for the saturated fatty acids than the [runs-stilbene spacers.
Introduction Langmuir-Blodgett (LB) assemblies, prepared by sequential transfer of spread, compressed monolayer films formed at the air-water interface, have proven to be useful media for examining a variety of intermolecular interactions.' The ability to construct a variety of functionalized amphiphiles and to incorporate them at fixed distances from other reagents has led to a number of studies of both long- and short-range phenomena. Particularly interesting have beem studies carried out in LB assemblies in which 'Department of Chemistry, University of Rochester 'Eastman Kodak Company.
the distance dependence of excited-state quenching via energy and electron transfer has been investigated. On the one hand, studies of singlet-singlet energy transfer via long-range resonance interactions in LB assemblies have shown generally good agreement between theory and experiment; indeed LB assemblies have proven to be an almost ideal medium for investigating this phenomenon which proves elusive for investigation in many other situations due to the presence or predominance of other competing deac( I ) Recent monographs reviewing the field are: Langmuir-Blodgetl Films; Roberts, G.G..Ed.; Plenum: New York, 1990. Ulman, A . An Introduction fo Ultrathin Organic Films; Academic: New York, 1991.
0022-3654/92/2096-2190$03.00/0 0 1992 American Chemical Society
Photoinduced ET in Langmuir-Blodgett Films tivation processes.24 In contrast, studies of excited-state electron transfer in LB assemblies have frequently resulted in quenching over distances greater than predicted by most theories or observed in studies in disordered organic mediass-’ The origin of these differences is not clear: several studies with a range of substrate-quencher combination and assembly architecture indicate an exponential or monotonic falloff of quenching with distance but with a slope 0 in the range of 0.3-0.4 A-’ compared to a value of 1-2 A-l for nonpolar organic While it has been suggested that the highly ordered medium provided by a compressed monolayer film may offer low-lying levels enhancing electron transfer, other explanations include the attribution of quenching to the presence of “defect sites” in which the actual substrate-quencher separation is much closer than the average separation measured by macroscopic studies. In the present paper we report an investigation of potential electron-transfer quenching as a function of substratequencher distance and the composition of the intervening medium. Most previous studies have relied on functionalized surfactants in which both substrate and quencher components are located in the hydrophilic portion of the amphiphile; in these cases the separation has normally been provided by saturated hydrocarbon chains of 14-22 carbon atoms, with a resulting monolayer separation of 20-30 A, by orienting this spacer layer with a hydrophobic to hydrophilic interface with one of the other layers. In the present study we have employed hydrophobically functionalized surfactants as both quencher and spacer media. The use of a hydrophobic ferrocene chromophore permits us to prepare films with excited substrate-quencher separations in the range 18-32 A with the intervening medium being either a saturated fatty acid or a “modified” fatty acid of similar length into which a trans-stilbene chromophore has been incorporated. All interfaces are either hydrophilic-hydrophilic or hydrophobic-hydrophobic contacts. We find that substantial reduction of substrate fluorescence by a hydrophobic ferrocene quencher occurs in this distance regime for both stilbene and fatty acid spacers. The results indicate that when the trans-stilbene derivatives are used as spacers the fluorescence quenching shows an even weaker dependence on distance than when saturated fatty acids are used.
Experimental Section The structures of the compounds used in this study are shown in Chart I. The synthesis and isotherm studies of the amphiphilic ferrocene surfactant Fc16Awere previously de~cribed.~These studies showed that the ferrocene and fatty acids were well mixed and show no apparent phase separation. Arachidic or eicosanoic acid (C20)was used as received from Analabs, Inc. and kept under refrigeration. Stearic acid (cl8) was received from Sigma Co. and recrystallized three times from absolute methanol. All other fatty acids (myristic acid (C14),pentadecanoic acid (CIS),palmitic acid (cl6), heptadecanoic acid (C17),nonadecanoic acid (C,,), heneicosanoic acid (C21),behenic acid (C22),and lignoceric acid (C24))were used as received from Sigma Co. The aniphiphilic trans-stilbenes (,,,&A) were synthesized at the University of Rochester using methods described elsewhere.I0 N,N’-Dioctadecylcyanine p-toluenesulfonate was synthesized at the Kodak Research Laboratories according to the procedure of Sondermann.” Langmuir-Blodgett fabrications were carried (2) Kuhn, H.; Mobius, D.; Biicher, H. In Physical Methods of Chemistry: Weissberger, A,, Rossiter, B. W., Eds.; Wiley: New York, 1972; Vol. 1, pp 577-702. (3) Biicher, H.; Drexhage, K. H.; Fleck, M.; Kuhn, H.; Mobius, D.; Schafer, F. P.; Sondermann, J.: Sperling, W.; Tillman, P.; Wiegand, J. Mol. Cryst. 1967. 2, 199. (4) Mobius, D. Acc. Chem. Res. 1981, 14, 63. (5) Kuhn, H. J . Photochem. 1979, 10, 111. (6) Mobius, D. Ber. Bunsenges. Phys. Chem. 1978.82, 848. (7) Mooney, W. F.; Whitten, D . G . J . Am. Chem.Soc. 1986,108, 5712. ( 8 ) Wasielewski, M. R. In Photoinduced Electron Transfer; Fox, M. A,; Chanon, M., Eds.; Elsevier: New York, 1988, Part A, p 161. (9) Seiders, R. P.; Brookhart, M.; Whitten, D. G.Isr. J . Chem. 1979, 18, 272. (10) Mooney, W . F.; Brown, P. E.; Russell, J. C.; Costa, S. B.; Pedersen, L. G.; Whitten, D. G. J . Am. Chem. SOC.1984, 106, 5659.
The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 2791 CHART I: Structures of Compounds
cx>+o cl8H37
pts-
cl8ct37
Thiacyanine p-toluenesulfonate
I_.
“o- 0 4S4A
ram-stilbene surfactants
arachidic acid
”
out with water deionized and polished with a Millipore purification system and with spectroscopic grade solvents. The general methods used for preparing Langmuir-Blodgett film assemblies and for cleaning the various substrates have been described elsewhere.2 Film preparations were carried out on a KSV2200 or a KSV3000 automatic film balance. All films were deposited a t a pressure of 30.0 mN m-]. Subphase contained 3 X lo4 M-I CdC12 and 5 X M-’ N a H C 0 3 (pH 6.5). The substrates were quartz or glass slides for photoinduced-electrontransfer studies. Ellipsometry was performed using a Gaertner L1 1GB-Auto Gain ellipsometer equipped with a He-Ne laser (632.8 nm). LB samples for ellipsometry were deposited onto silicon wafer pieces cut from (1-0-0) face that were cleaned as previously described.12 Absorption spectra were measured on a Hewlett-Packard 8452A diode array spectrophotometer. A five-layer arachidate precoated slide was used as reference. All slides were supported on a home-made Teflon holder. Steady-state fluorescence spectra were obtained on a SPEX FLUOROLOG 2 spectrofluorometer. A home-made rotation stage was used to position the slide sample incident angle at 45’ and the detection angle at 90’. The excitation wavelength for quenching experiments was 430 nm. Time-resolved fluorescence measurements were performed using time-correlated single photon counting (SPC).I3 The excitation system consisted of a mode-locked frequency-doubled Nd:YAG laser (Quantronix Model 416 ) synchronously pumping a cavitydumped dye laser (Coherent, Model 703D) circulating dye 801. The Nd:YAG laser produced a series of pulses that were typically 5 ps in duration at a repetition rate of 38 MHz. The system had a time resolution of 10 ps limited by the detector. Excitation was
-
( I I ) Sondermann, J. Liebigs Ann. Chem. 1971, 749, 183. ( 1 2 ) Penner, T. L.; Schildkraut, J . S . ; Ringsdorf, H.; Schuster, A . Macromolecules 1991, 24, 1041. (13) Ci, X.;Whitten, D. G . J . Phys. Chem. 1991, 95, 1991.
Hsu et al.
2192 The Journal of Physical Chemistry, Vol. 96, No. 7, 1992
TABLE I: Multiexponential Components of the Fluorescence Decay Kinetics of the Thiacyanine Dye in a LB Monolayer Film Diluted 1:20 with Arachidic Acid
FehjA
Spacer
lifetime T , ns
relative quantum yield
0.2
1.o
0.1 0.5
2.1
0.4
Cyanine
TABLE I 1 Comparison of the Steady-State and Time-Resolved Quenching of Thiacyanine Fluorescence by Ferrocene for Fatty Acid and tmm-Stilhene Spacers
macer
length. A
AA *&A
26.7 26.3
I,lI
III, 0.83 f 0.02 0.75 f 0.01
-1
1,
ns
0.20
1
0.33
0.8-0.9"
'This is a weighted average of the lifetime components normalized to the values for the fatty acid spacer. Fcl6A
Stilbenes
Cyanine
0.6
trans-stilbene derivative mS&o-"-
I
E
Thiacyanine dye
d P
r
-.
-
P
0.4-
P
0
Fatty acid Spacer CI4-cW
P
Ferrocene derivative Fci6A
P P
Figure 1. Structure of Langmuir-Blodgett films containing monolayers of thiacyanine dye and ferrocene derivative separated by a single spacer
monolayer of either fatty acid or stilbene-derivatizedcarboxylic acid mixed with fatty acid (labeled as shown).
P
P
o,z! 0.0 19
1.0 r
21
23
25
27
29
31
3
d(A)
Figure 3. Dependence of the extent of electron transfer between thiacyanine and ferrocene layers on the thickness of the fatty acid spacer
layer.
>'..& ,,...-J I\
300
\ I
I
400
500
'NT---
600
I 700
Wavelength (nm)
Figure 2. Absorption and fluorescence spectra of thiacyanine dye, fer-
rocene, and stilbene incorporated into LB monolayers diluted with arachidic acid at the levels of 1:20, 1:4, and 1:1, respectively; thiacyanine absorption (-), thiacyanine fluorescence (--), stilbene absorption (-- -), stilbene fluorescence ferrocene absorption X 10 (dash-dotted line). (..a),
at 420-430 nm. Kinetic analysis was done using a nonlinear least-squares fitting program which could fit up to three components. Results Deposition of the monolayers comprising the film structures proceeds quantitatively, with deposition ratios of 1.OO f 0.05 for all layers with the exception of the film containing 4S4Aat room temperature, as discussed below. Figure 1 shows a schematic representation of the layer structure of the films. Multilayer films of cadmium arachidate alone, deposited onto silicon, gave thicknesses of 28.3 and 27.3 A per monolayer measured, respectively, by ellipsometry and X-ray scattering, consistent with the literature report.I4 As seen in the absorption spectra in Figure 2, both the transstilbene derivatives mixed 1:l and the thiacyanine dye mixed 1:20 (14) Fromherz, P.; Reinbold, G. Thin Solid Films 1988, 160, 347.
with arachidic acid show a tendency toward aggregation in the LB films, as has been reported in the literature for these materials.2*10The absorption spectrum of ferrocene derivative F c , ~ A mixed 1:4 with arachidic acid also shown in Figure 2 has only very weak absorption in this spectral regione9 In the case of the thiacyanine dye this microheterogeneity is reflected in the multiexponential nature of its fluorescence decay. The lifetimes and relative quantum yields of the different components are given in Table I. Two components dominate, each with about half the fluorescence quantum yield, probably reflecting kinetic processes originating from monomer and dimer of the dye. The presence of stilbene derivative overlayers does not alter the fluorescence lifetime or steady-state intensity, but addition of a layer of the ferrocene derivative Fc16A,diluted 1:4 with arachidic acid and deposited subsequent to a layer of either fatty acid or stilbene derivative as a spacer layer causes a quenching of the thiacyanine fluorescence intensity and a reduction in the overall lifetime of the thiacyanine excited state. Significantly, a comparison of the extent of quenching for arachidic acid and a 1:l mixture of arachidic acid and stilbene derivative sS4A, which has essentially the same layer thickness, indicates a substantially greater quenching for the stilbene derivative spacer film. As shown in Table 11, this is true for both steady-state intensity and the excited-state lifetime. Systematic variation of the effect of the separation of the excited thiacyanine dye layer from the F q 6 A on the thiacyanine fluorescence intensity I was examined by fabrication of a series of LB films in which the spacer layer was varied from myristic acid (C,J to lignoceric acid (C24)with linear saturated fatty acids. The film slides were fabricated with the spacer layer overcoated with the ferrocene derivative Fc,,A-arachidate mixture for half its length and with cadmium arachidate for the remainder. In this way a reference fluorescence intensity in the absence of the ferrocene derivative (Io)was obtained for each sample. The results, plotted in Figure 3, indicate that quenching of the excited thia-
The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 2793
Photoinduced ET in Langmuir-Blodgett Films
SCHEME I cy+
-
z
04
I+
004
21
1
CY+'
hu
cy+'
kd
CY+
___L
f Cy"
?
I
22
,
I
23
,
8
,
I
25
24
,
3
26
,
I 27
d(A)
Figure 4. Dependence of the extent of electron transfer between thiacyanine and ferrocene layers on the thickness of the trans-stilbene-con-
taining spacer layer. cyanine dye falls off monotonically with the fatty acid spacer layer thickness as derived from literature X-ray measurement^.'^ The plotted points represent the average of several independent experiments. As seen later in the Discussion section, the value of Zo/Z- 1 is proportional to the rate of quenching under steady-state assumptions. Similar experiments were carried out using the stilbene-derivatized carboxylic acids ,,,S4A, where m = 4,6, and 8, all mixed in equimolar amounts with the fatty acid of nearest molecular length (palmitic acid c16, stearic acid c18, and arachidic acid CZO, respectively). In this case the lengths of the molecules were calculated using the molecular modeling program MACROMODEL, version 2.5.1. For 6S4Athis calculated length was compared with ellipsometric measurements and shown to be very similar-23.9 and 24.5 A, respectively. At room temperature the film containing 4S4Awas not entirely stable at the air-water interface. Therefore, the entire stilbene derivative spacer series was repeated with the spacer layer deposited from the subphase at 10 O C . Under these conditions all layers were stable and the deposition ratio was unity. For 6S4Aand sS4A the extent of quenching for films deposited at room temperature and 10 O C were identical within our experimental reproducibility (-5%). In the case of 4S4Athe quenching by ferrocene at room temperature was substantially higher than that at 10 OC, reflecting the 4S4Afilm's instability and consequent incomplete deposition. This point was excluded from the data set. The results of the fluorescence quenching experiments with these stilbene derivative spacers are shown in Figure 4. As with the saturated fatty acid spacers the quenching shows a monotonic decrease with the separation distance but with a slower fall off than the fatty acid series.
Discussion The quenching of the thiacyanine fluorescence in the LB films is due to an interaction with the ferrocene chromophore since in the absence of FcI6Ano quenching is observed. We believe that the net process is the transfer of an electron from the ferrocene in its ground state to the excited thiacyanine dye. This is an energetically favorable process (by about 1.2 eV) based on solution redox potentials of the thiacyanine dye15 and ferrocene.I6 Attempts to detect such redox intermediates in the LB films have not yet been made. Energy transfer via a Forster type resonance mechanism is unlikely in this system. The ferrocene derivative has only very weak absorption transitions (measured to be t I 100 M-l cm-l in solution at wavelengths longer than the thiacyanine fluorescence -400 From these spectroscopic data and the fluorescence quantum yield of the thiacyanine dye in the LB film, the critical transfer distance for energy transfer was estimated to be 3.3 .&.* As a consequence, energy-transfer (15) Lenhard, J. J . Imag. Sci. 1986, 30, 27. (16) Gordon, K. R.; Warren, K. D. Inorg. Chem. 1978, 17, 987. (17) Kuwana, T.; Bublitz, D. E.; Hoh, G. J . Am. Chem. SOC.1960, 82, 5811.
+
Fclg
k,, ___)
Cy,
+
Fc,,A'
quenching at spacings on the order of 20 A should be negligible. The observation of a monotonic decrease in the extent of electron-transfer quenching with the separation distance between donor and acceptor is consistent with current theories and experiments on electron-transfer processes of weakly coupled donor-acceptor systems.18-22The electronic coupling between donor and acceptor through the overlap of the extended tails of the molecular wave functions at large distances provides the mechanism for long-range electron t r a n ~ f e r . ~Since ~ . ~ ~the degree of coupling is quite sensitive to the quantitative particulars of the choice of wave functions and to the medium through which they interact, it is essentially impossible to calculate the overlap quantitatively in an a b initio manner. But both through-space and through-bond mechanisms of charge transfer would predict an exponential decrease in the rate constant with donor-acceptor separation of the formZS k,, = kOe-bd where k,, and ko are the rate constants of the electron-transfer process at separation distance d and at contact, respectively. j3 is the attenuation coefficient of the distance dependence of the electron-transfer process. As noted in the Introduction, many systems, particularly in glassy matrix solutions26or donoracceptor systems covalently bound by a rigid saturated hydrocarbon bridge,27-29but also including a self-assembled monomolecular film system,30 have attenuation constants 9 of about 1 A-1. The photoinduced-electron-transfer process is shown in Scheme I. Under the steady-state assumption, one obtains That is, the rate constant of electron transfer is proportional to the quantity Zo/Z - 1. Z and Io are the fluorescence intensity of thiacyanine dye with and without the ferrocene acceptor present, respectively. kd is the rate constant for other radiative and nonradiative processes. If an exponential dependence of k,, on distance is assumed, substitution of (2) in eq 1 results in the relationship In (Zo/Z - 1) = -pd + constant (3) A linear dependence of In (Zo/Z - 1) on distance d has been reported previously for LB film s y ~ t e m s . ~ However, , ~ - ~ ~ when the data in Figure 3 were plotted according to eq 3 a curved line was obtained; the plot shows curvature with a negative deviation from linearity at short distances, d. The curvature indicates either that the photoinduced electron-transfer process is not a purely quantum tunneling or that it occurs with more than one rate constant. If (18) Kuhn, H. Chem. Phys. Lipids 1972, 8, 401. (19) Beratan, D. N.; Hopfield, J. J. J . Am. Chem. SOC.1984, 106, 1584. (20) Closs, G. L.; Calcaterra, L. T.; Green, N. J.; Penfield, K. W.; Miller, J. R. J . Phys. Chem. 1986, 90, 3673. (21) Oevering, H.; Paddon-Row, M . N.; Heppener, M.; Oliver, A. M.; Cotsaris, E.; Verhoeven, J. W.; Hush, N. S. J . Am. Chem. SOC.1987, 109, 3258. (22) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265. (23) Onuchic, J . N.; Beratan, D. N. J . Am. Chem. Soc. 1987,109,6771. (24) McLendon, G. Acc. Chem. Res. 1988, 21, 160. (25) Onuchic. J. N.; Beratan, D. N. J . Chem. Phys. 1990, 92, 722. (26) Miller, J. R.; Beitz, J. V. J . Chem. Phys. 1981, 74, 6746. (27) Miller, J. R.; Calcaterra, 1.T.; Closs, G. L. J . Am. Chem. Soc. 1984, 106, 3047. (28) Verhoeven, J. W. Pure Appl. Chem. 1986, 58, 1285. (29) Closs, G. L.; Miller, J . R. Science 1988, 240, 440. (30) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J . Am. Chem. SOC.1990, 112, 4301. (31) Miyashita, T.; Hasegawa, Y.; Matsuda, M. J . Phys. Chem. 1991, 95, 9403.
2194 The Journal of Physical Chemistry, Vol. 96, No. 7, 1992
Hsu et al.
of the electron transfer with distance in LB films is modified by * the nature of the medium.
the separation distance d, a straight line should be obtained. Indeed that is the case as shown in Figure 5. The values of x i = 0.50 and 0.47 were obtained by determining the best fit of eq 2d to the data for fatty acid and trans-stilbene spacers, respectively, in Figure 3 and Figure 4. These values of x l are consistent within experimental error with the fractions of the two major lifetime components of the excited thia~yanine.~~ The slopes of these plots, corresponding to -@ values for the electron transfer, were determined to be -0.3 and -0.2 A-’ for the fatty acid series and trans-stilbene spacer series, respectively. The intercepts for the two cases are 7.3 and 5.9, respectively. In view of the limited trans-stilbene data and the extent of extrapolation to the y-axis, this is a good agreement. An alternative treatment)) is to assume that two dye populations are active, but that they quench differently due to their different lifetimes. From such a treatment, with x for each component set a t 0.5 (reasonable in view of the lifetime measurements), we obtain reasonable linear fits for both yspacers”but with even lower values for the attenuation coefficient, fl (0.2 and 0.12 for fatty acids and trans-stilbenes) and somewhat different intercepts (kdapprent)for fatty acids (2.4 X 1O’O s-l) and trans-stilbenes (4.1 X lo9 sd). Kuhn and =workers have reported the use of azobenzene derivatized fatty acids as ‘molecular wire” to increase the electron-transfer rate across LB spacer layers.34 But the present results report the first evidence that the attenuation
The slope of 0.3 A-I for the fatty acid is similar to other electron-transfer experiments measured in LB film^.^-'*^' The systems studied have included both Y-type (Le., hydrophilic-tohydrophilic and hydrophobic-to-hydrophobic) and Z-type (hydrophilic-to-hydrophobic) structures with either ground-state or excited-state species at the hydrophilic or hydrophobic interfaces. All appear to have values of j3 in the range of 0.3-0.4 A-1.5-7As stated in the Introduction this differs substantially from the values that have been obtained from the other systems like glassy matrix solution and covalent-bridged donoracceptor. K ~ h n and, ) ~ more recently, Mobius36have written about the possible reasons why the attenuation of electron transfer with distance may be different in LB films from other systems. Among these are (i) the greater degree of order in the spacer chain with an extended all-trans conformation and highly anisotropic quasi-crystalline structure, (ii) the existence of interfacial sites acting as transitory traps for electrons at smaller distances than the full donor-acceptor separation, (iii) under the conditions of fast electron transfer in LB films, absence of nuclear motion leading to a tunneling probability as much as twice that calculated for the same barrier height when nuclear motion accompanies the electron transfer. There remains the possibility that the electron-transfer process is dominated by defects where the donor-acceptor separation is much smaller than the bulk film thickness. It is difficult to rule this process out completely. If such defects are the cause of the small attenuation constant seen in LB films, they must be similar in density and nature for the different systems reported to date. As stated, these systems include a variety of film structures and active components. That defect-dominated kinetics can show an apparent exponential distance dependence is shown by Mallouk and co-workers who have reported a layer system prepared by sequential adsorption of zirconium phosphonates in which the distance dependence of electron transfer appears to be exponential but for which they have electrochemical evidence that the process is defect dominated.)’ In order for defects to be the dominant sites for electron transfer either they must be sufficiently numerous or large on the one hand or on the other hand energy or charge migration must occur with sufficient efficiency to allow the charge-transfer process to funnel through relatively few such sites. In the system of Mallouk, electron exchange in the ferrocene-containing layer provides charge mobility. In our system, energy transfer in the thiacyanine layer could be the operative method. From the Forster overlap integral we estimated that the critical transfer radius for energy migration in the thiacyanine dye layer is 6 A. The average spacing of thiacyanine molecules is about 12 A so that energy migration is a very inefficient process, about 5% per transfer. (The presence of dimers would lower the energy-transfer prccess even more since the dimer absorption overlaps more poorly with the fluorescence and the distance between energy-transfer sites would be greater.) In our system the maximum quenching of thiacyanine dye by ferrocene observed was 40%. If it is assumed that this was entirely due to a fast, defect-controlled electron transfer, one can calculate that the rate of energy transfer to a defect site would be 0.65 the rate of excited-state decay. From the distance-dependence equation for quenching in two dimensions,2this requires a spacing between donor and defect of 7 A. This is much smaller than the spacing between donor molecules which means that the effective spacing between defect sites would also need to be about 7 A. If the minimum defect size is assumed to be one molecular diameter, or about 20 AZ,a conservative estimate of the area density of defects would be about 15% of the total layer area. We believe that this is highly unlikely both because our film depositions have been of very good quality and because of the excellent agreement
(32) Using values for x, in the range of 0.45-0.50 changes neither the slopes nor the intercepts appreciably. (33) We thank a referee for suggestions concerning this approach. (34) (a) Kuhn, H. Proc. Robert A. Welch Foundation ConJ Chem. Res. 1986,30, 339. (b) Mobius, D. In Phorochemical Conversion and Storage of Solar Energy; Rabani, J., Ed.; Weizman Science Press: Jerusalem, 1982.
( 3 5 ) Kuhn, H. Thin Solid Films 1989,178, 1. (36) Mobius, D.; Ahuja, R. C.; Caminati, G.; Chi, L. F.; Cordroch, W.; Li, 2.-M.; Matsumoto, M., submitted for publication. We thank Dr. Mobius for providing us a copy of this manuscript prior to publication. (37) Hong, H.-G.; Mallouk, T. E. Langmuir 1991, 7,2362. We thank Dr. Mallouk for providing us a copy of this manuscript prior to publication.
2 18
20
22
24
26
28
30
32
d(A1 Figure 5. Dependence of the relative rate of electron transfer between thiacyanine dye and ferrocene layers on the thickness of the spacer layers. Open circle represents fatty acid spacer case, and solid circle represents trans-stilbene-containingspacer case.
there exists more than one electron-transfer process leading to fluorescence quenching in this LB film system, eq 2 should be rewritten as (2b), where kd is the rate constant for electron transfer
and kd is the rate constant for other nonradiative processes for each electron-transfer component and xiis the molar fraction of the ith electron-transfer component. In the most simple case of eq 2b, only two primary deactivation processes affect the fluorescence intensity of the excited thiacyanine dye, and only one of these involves electron transfer. Then, eq 2c should be followed:
Assuming electron transfer is an exponential function of distance (eq l), eq 2d is derived By plotting the left side of eq 2d versus
J . Phys. Chem. 1992, 96, 2795-2800 seen between calculated and measured film thicknesses. Although our results, analyzed in accordance with eq 2d, are consistent with an exponential distance dependence of the electron-transfer rate with a very low attenuation coefficient, 0, for various arbitrary models containing differentially active components of the excited thiacyanine dye, such fits do not prove the mechanism. Thus, in the absence of more extensive fluorescence lifetime measurements we cannot determine the actual kinetics in this system. A more complete investigation of the timeresolved fluorescence kinetics is currently underway. This should establish whether there is more than one electron-transfer component or even inactive excited thiacyanine dye. In addition, a search for electron-transfer intermediates is also planned using transient absorption spectroscopy,both to codm the chargetransfer nature of the process and to complement the fluorescence kinetics. Even in the absence of a specific kinetic mechanism, we can say that the electron transfer has a lower attenuation with distance for stilbene-containing spacer molecules than for the saturated fatty acids. When treated by the exponential attenuation eq 2d, the fatty acid spacer case leads to a coefficient (3 consistent with previous LB studies if it is assumed that about half the fluorescing excited thiacyanine dye does not participate in the electron-transfer process. Using the same assumptions for the tram-stilbene series gives an attenuation coefficent about 50% smaller. The alternative approach, assuming two active components, gives even lower 0 values for both “spacers” but here again 0 for the tram-stilbenes is lower by a similar amount. Thus, the inclusion of a substantial aromatic functionality in the spacer decreases the attenuation of electron-transfer rate with distance significantly. Within the context of a tunneling model in which the spacer is viewed as a continuum barrier this would imply a lowering of the barrier potential with 0 decreasing as the
2795
square root of the barrier height. On the other hand, the smaller attenuation for the stilbenes is also consistent with a superexchange process, whereby the overlap of donor and acceptor wave functions is enhanced by mixing in energy states of the medium, although the electron does not directly reside in these medium state^.^^^^^ In its simplest form, the superexchange mechanism predicts that /3 will decrease logarithmically with a decrease in the energy difference between the nearest states of the medium and the donor or acceptor energy levels.38 Thus both mechanisms predict a decrease in /3 as a result of decreasing the difference in energy levels between the spacer and the electron donor or acceptor. Qualitatively it is expected that a stilbene-containing spacer will have its energy levels closer to those of the thiacyanine dye than a saturated hydrocarbon spacer will have.39 The application of these different electron-transfer mechanisms in LB films has been discussed in the literature for fatty acid spacers.40 But in the absence of more quantitative data on the energy levels of the present materials in the present system, particularly for the stilbenes, a more extensive treatment is not merited. Acknowledgment. The stilbenecarboxylic acids were synthesized by Cristina Geiger. Financial support was provided by the National Science Foundation in the form of a Science and Technology Center grant (CHE-8810024). We acknowledge the suggestions of two referees in the kinetic treatment of the quenching data in terms of multicomponent electron-transfer kinetics. (38) McConnell, H. J . Chem. Phys. 1961, 35, 508. (39) Lewis, F. D. Ado. Phorochem. 1986, 13, 165. (40) De Schryver, F. C.; Van der Auweraer, M.; Verschuere, B.; Willig, F. In Supramolecular Photochemistry; Balzani, V., Ed.; Reidel: Baston, 1987; p 385.
Probing Femtosecond Solvation Dynamics at the Condensed Phase-Vacuum Interface Mlungisi Kwini, Martin J. Iedema, James P. Cowin,* Pacific Northwest Laboratory,‘ Box 999, Richland, Washington 99352
and Terry L.Gilton Micron Technology, Inc., Boise, Idaho 83706 (Received: November 4, 1991; In Final Form: January 22, 1992)
The photoelectron-induceddissociation of CH3C1adsorbed on top of a multilayer deposit ejects methyl radicals into the gas phase. The kinetic energies of these methyls vary with the identity of the underlying multilayer (H20, hexane, CH3Cl!, from 0.44 to 0.7 eV at the peak, and are much higher than seen for the gas-phase dissociative electron attachment to this molecule. The additional energy is understood in terms of the effects of the “prompt”solvation (
