Two-dimensional excitation energy transfer between chromophoric

J . Phys. Chem. 1987, 91, 841-845

841

Two-Dimensional Excitation Energy Transfer between Chromophoric Carbazole and Anthracene in Langmuir-Blodgett Monolayer Films Naoto Tamai, Tomoko Yamazaki, and Iwao Yamazaki*

Downloaded by UNIV OF NEBRASKA-LINCOLN on August 28, 2015 | http://pubs.acs.org Publication Date: February 1, 1987 | doi: 10.1021/j100288a017

Institute for Molecular Science, Myodaiji, Okazaki 444, Japan (Received: July 1 , 1986; In Final Form: August 13, 1986)

Two-dimensional dipole-dipole energy transfer (Forster type) has been studied with Langmuir-Blodgett (LB) monolayer films containing chromophores of carbazole (donor) and anthracene (acceptor). Fluorescence decay curves of donor (1.5 mol %) in the presence of acceptors (1.5-8.5 mol %) were measured with a picosecond time-correlated, single-photon-counting apparatus..The decay curves were analyzed in terms of a time-dependentequation of excited-state survival in the two-dimensional energy-transfer system. At substantially higher concentrations of acceptor (>5 mol %), the fluorescence decay follows the equation of two-dimensional energy-transfer kinetics plus an exponential decay term. At lower concentrations, the energy transfer competes with the fluorescence quenching due to the electron transfer from a photoexcited carbazole chromophore to carboxyl groups of stearic acid of the LB film. A nonexponential decay curve of carbazole chromophore without acceptor, which is independent of the concentration, is found to be consistent with a theoretical decay function of the electron-transfer quenching.

Introduction

Transport and trapping of electronic excitation energy have been the subject of extensive theoretical and experimental work since the pioneering work of Forster.' Special attention has recently been paid to dynamics of the excitation energy transfer in restricted molecular arrangements or geometries of molecular assemblies such as micelles, vesicles, and molecular adsorbates on solid surfaces.2 A central problem lies in the influence of dimensionality of a system on energy-transfer kinetics. This problem is of current interest in relation to the development of an artificial molecular apparatus for light harvest and photosensitization of semiconductors. As regards the two-dimensional energy transfer and trapping, theoretical approaches have been made by several workers. The fluorescence decay laws of the donor have been proposed by Hauser et aL3 and Wolber and Hudson4 for randomly distributed systems and by Zumofen and BlumenSfor regular lattices. Experimentally, time-resolved fluorescence measurements have been made by use of submonolayers of dyes adsorbed on solid substrates; rhodamine B69' and cresyl violet.* Their experimental results revealed that the fluorescence decay profiles were strongly dependent on the surface number density of dyes. At surface coverages of dyes ranging over more than 2 orders of magnitude, the decay curves did not fit the two-dimensional Forster-type decay mode13q4but fit the superposition of the two-dimensional energy transfer and the intrinsic deactivation of the excited donor molecule. The fluorescence decay at higher surface coverage exhibited more rapid decay as a consequence of excitation trapping by dimer or higher aggregates of dyes. It appeared that these submonolayer systems necessarily incorporate multiple emitting species, resulting in complicated fluorescence decays. Correlation of the kinetic parameters to the submonolayer structure is not straightforward, and problems remain unsolved. The Langmuir-Blodgett (LB) film is typical of the two-dimensional molecular organization which is prepared by trans(1) FBrster, Th. 2.Naturforsch., A : Astrophys., Phys. Phys. Chem. 1949, 4, 321. (2) Thomas, J. K. The Chemistry of Excitation at Interfaces; American Chemical Society: Washington, D.C., 1984. ( 3 ) Hauser, M.; Klein, U. K. A.; Gosele, U.Z . Phys. Chem. (Munich) 1976, 101, S255. (4) Wolber, P. K.; Hudson, B. S. Biophys. J. 1979, 28, 197. ( 5 ) Zumofen, Z.; Blumen, A. J. Chem. Phys. 1982, 76, 3713. (6) Nakashima, N.; Yoshihara, K.; Willig, K. J . Chem. Phys. 1980, 73, 3553. (7) Kemnitz, K.; Murao, T.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. Chem. Phys. Lett. 1983, 101, 337. (8) Anfinrud, P.; Crackel, R. L.; Struve, W . S. J . Phys. Chem. 1984,88, 5873.

0022-3654/87/2091-0841$01.50/0

ferring a compact monolayer spread on a water surface onto a s ~ b s t r a t e . ~One can prepare the LB film as a monomolecular layer containing chromophoric donor and acceptor molecules with their number densities being variable over a wide range. IJnlike the coating prepared by casting a dye solution onto a solid substrate, the LB film is expected to be an authentic two-dimensional monolayer or submonolayer. In the present study, the two-dimensional energy transfer between chromophoric donor and acceptors has been investigated with LB monolayer films consisting of stearic acid and small amounts of 1l-(carbazoI-9-yl)undecanoic acid and 16-(9-anthroyloxy)paImitic acid (hereafter referred to as CU and AP, respectively:

cu

AP

Fluorescence decay curves of donor CU in the presence of acceptor AP were measured with a picosecond time-resolved fluorescence spectrophotometer and analyzed in terms of the two-dimensional Forster-type energy-transfer kinetics. The results were compared with previous studies6-* of dye molecules adsorbed on solid substrates. It was found that, at lower concentration of AP, energy transfer competes with a fluorescence quenching due to the electron transfer from carbazole to the carboxyl group of stearic acid in the LB film. The decay curve of C U without acceptor was examined with a theoretical decay function of the electrontransfer quenching. Experimental Section

CU and AP were purchased from Molecular Probe Co. and further purified by repeated recrystallizations from ethanol. Stearic acid, obtained from Wako Chemical Co., Osaka, was purified five times by recrystallization from ethanol. A mixture of stearic acid and small amounts of C U and AP dissolved in benzene was spread onto the surface of the water subphase containing 3 X lo4 M CdC12. The subphase conditions were adjusted to a temperature of 17 OC and pH 6.5 by addingKHC0, buffer solution. Mixed monolayers were deposited on a quartz plate under a constant surface pressure of 2.5 X N m-l. The quartz plates used were precoated with three monolayers of stearic acid-cadmium salt to minimize the influence of the substrate (9) Kuhn, H.; Mobius, D.; Bucher, H . In Techniques of Chemistry; Weissberger, A,, Rossiter, B. W., Eds.; Wiley: New York, 1972; Vol. 1, Part 3B, pp 577-702.

0 1987 American Chemical Society

842

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The Journal of Physical Chemistry, Vol. 91, No. 4, 198'7

Tamai et al. stearic acid

cu AP

0 01

A

0.3

1 r I

I

1

04

AREA PER MOLECULE (nmz )


r L

02

Figure 1. Surface pressure-area isotherms of (1) single-component monolayer of 1 l-(carbazol-9-yl)undecanoic acid (CU), (2) mixed monolayer of CU (1.5 mol %) and 16-(9-anthroyloxy)palmiticacid (3 mol %) with stearic acid, and (3) single-component monolayer of stearic acid.

surface on energy-transfer kinetics. Corrected fluorescence spectra and fluorescence-excitation spectra were obtained with a Spex Fluorolog 2 spectrophotometer. The fluorescence decay curves and the time-resolved spectra were measured with a synchronously pumped, cavity-dumped dye laser and a time-correlated single-photon-counting apparatus. For a fluorescence detector, we used a proximity type of microchannel-plate photomultiplier (Hamamatsu R1564U) to obtain an instrumental response function with a 40-ps pulse width (fwhm) for the scattered laser light. Details of this system have been described elsewhere.I0 Fluorescence decay curves were analyzed by using a nonlinear, least-squares iterative convolution method based on the Marquardt Curve fitting was carried out with a Hitachi M-200H computer in the Computer Center of the Institute for Molecular Science.

Results Figure 1 shows the surface pressure-area isotherms for pure CU, pure stearic acid, and a mixture of CU, AP, and stearic acid. The isotherm of pure C U (curve 1) can be explained as follows. At surface areas larger than 0.40 nm2, molecules lie parallel to the water surface; the nitrogen atoms of carbazole and the carboxyl group are both in contact with the water subphase. In the region of 0.40-0.34 nm2 (from point A to B in the figure), molecules are oriented perpendicular to the water surface. The CU monolayer collapses at 0.34 nm2 (point B). A second collapse occurs at 0.13 nm2 (point C). Such a behavior in the isotherm of C U indicates that pure CU monolayer forms no compressed-phase m ~ n o l a y e r . ~ On the other hand, an isotherm of a mixture of C U and A P in stearic acid (curve 2 ) exhibits a sharp increase in the surface pressure at a surface area of 0.23 nm2, similar to that of pure stearic acid (curve 3). These isotherms show that a mixed monolayer diluted with stearic acid forms a compressed-phase monolayer, although the pure C U (and the pure AP) form no stable monomolecular film. The limiting area of 0.22 nm2 is consistent with a space-filling molecular model for approximately vertical packing of the hydrocarbon chain in which carboxyl groups are in contact with the water surface. The structures of the LB films used here are illustrated schematically in Figure 2 . Five or six layers were deposited on a quartz plate in the following order: (1) three layers of stearic acid-cadmium salt contacted with the quartz substrate, (2) monolayer or bilayer consisting of stearic acid and small amounts of C U and AP, and (3) monolayer of stearic acid. An outer layer of stearic acid prevented the mul(10) Yamazaki, I.; Tamai, N.; Kume, H.; Tshuchiya, H.; Oba, K. Reu. Sci. Instrum. 1985, 56, 1187. (11) O'Connor, D. V.; Ware, W. R.; Andre, J. C. J . Phys. Chem. 1979,

83, 1333. (12) Marquardt, D. W. J. SOC.Ind. Appl. Math. 1963, 11, 431.

(a 1 (b) Figure 2. Schematic illustration of the structure of LB films used in the present study. Chromophoric monolayer (a) and bilayer (b) are deposited on the quartz substrate precoated by three stearic acid monolayers. The open circles stand for the carboxyl group. See text for details. i

I

r

,

I r i

c

,

I

I

250

1

1 .

300 350 WAVELENGTH ( n m )

4 00

Figure 3. Excitation spectra of LB monolayer films containing (1) CU (1.5 mol %) and AP (3 mol %), (2) AP (3 mol W ) , and (3) CU (1.5 mol W ) . The monitoring wavelengths are 460 nm in (1) and (2) and 370 nm in (3). A difference spectrum between (1) and (2) is identical with (3). The excitation wavelength (295 nm) in the time-resolved measurement is shown by an arrow.

tilayered structure of the LB film from being destroyed. The chromophoric bilayer (Figure 2b) was used in order that fluorescence emissions from the LB film of low C U concentration were detected with sufficient intensity. As regards time-resolved fluorescence characteristics, no difference was observed between the chromophoric monolayer and bilayer. The fluorescence excitation spectra are shown in Figure 3 for the t h e e LB films of C U and AP (curve l ) , AP (curve 2), and CU (curve 3). The difference spectrum between spectra of the mixed film and the AP film is identical with that of the CU film. This indicates that the excitation energy transfer occurs between chromophoric carbazole and anthracene in the LB monolayer film. In the figure, the excitation wavelength (295 nm) employed in the time-resolved measurements is shown by an arrow. Figure 4 shows time-resolved fluorescence spectra for the LB films of donor CU (a) and of C U and AP (b). In the mixed film (Figure 4b), each of the time-resolved spectra consists of the two fluorescence bands due to C U and AP, Le., the former with vibrational peaks at 351 and 368 nm and the latter with no vibrational structure having a peak at 450 nm. It is seen that decay of the CU fluorescence band is accompanied by a rise of the AP fluorescence band. On the other hand, no change was observed in the time-resolved spectra of the CU film without acceptor (Figure 4a). This means that the energy transfer occurs between carbazole and anthracene chromophores and that the fluorescence which we observe here is emitted almost exclusively from the carbazole chromophore and not from aggregates or impurities.

The Journal of Physical Chemistry, Vol. 91, No. 4, I987

Energy Transfer in Langmuir-Blodgett Films 1

I

843

TABLE I: Analysis of Fluorescence Decay Curves of Donor CU in the LB Films with Two-Dimensional FBrster-Tvoe Eauation (Ea 5) ~ _ _ accept0r concn: nA,b mol % lo4 7 ~ns, 27, A2/A, R O ,8,~ x2! 1.46 7.18 17.5 3.02 0.033 34.4 1.98 2.87 14.1 16.7 3.85 0.028 27.8 1.29 5.58 30.0 15.2 4.89 0.027 22.6 1.17 8.53 40.8 15.5 4.92 0.026 23.2 1.18 ‘Molar fraction of acceptor AP in the LB monolayer film consisting of AP, CU,and stearic acid. bSurface number density calculated from the concentration and the limiting surface area spread on a water surface. cCritical transfer distance calculated from eq 2 by using nA and yA values. dReduced x2 in the curve fitting.


not change on decreasing the concentration to 0.01 mol %. At C U concentrations greater than 10 mol %, however, the fluorescence decay becomes more rapid than the decay at lower concentrations, probably due to the concentration quenching.

400

500

WAVELENGTH ( n m )

500

400

WAVELENGTH (nm )

Figure 4. Time-resolved fluorescence spectra of the LB films of (a) CU (1.5 mol %) and (b) CU (1.5 mol 5%) and AP (1.5 mol 5%). The excitation wavelength is 295 nm. Each spectrum is normalized to the maximum intensity. The time zero corresponds to the time at which the excitation laser pulse reaches maximum intensity. io4

io3

Discussion The LB films used in the present study are monolayer or bilayer films as far as the layers including chromophores are concerned (Figure 2). The chromophoric monolayer consists of donor C U (1.5 mol %), acceptor AP, and stearic acid. The concentration of AP is varied, ranging from 1.5 to 8.5 mol %. The carbazole and anthracene chromophores are thought to be randomly distributed in the films. The mean distance between carbazole chromophores is estimated to be 37 A for 1.5 mol % of CU. This value is larger than the critical transfer distance of energy miThus, the energy migration gration among donors, Ro = 21 between donor molecules can be neglected under the present conditions. The change of the fluorescence spectrum with time (Figure 4b) manifests directly the excitation energy transfer between chromophoric carbazole and anthracene in the monolayer. Moreover, in the change of the decay curve profile of donor (Figure 5), the fluorescence decays more rapidly as the acceptor concentration is raised; this is attributable to the energy transfer from CU to AP. Two-Dimensional Energy Transfer in the LB Film. For the two-dimensional energy transfer, the fluorescence decay function of the donor p(t) is expressed in the following

v) I-

p(t) = exp[-t/7D - 2YA(t/7D)1’3]

z

= 102

(1)

0

u

10‘ exc.

pulse

1

20

40

60

80

T I M E (ns)

Figure 5. Fluorescence decay curves of donor CU in the LB film in the presence of acceptor AP with concentrations of (1) 0, (2) 1.46, and (3) 5.58 mol %. A broken line is the exponential decay curve obtained for CU in ethanol solution; the lifetime is 20 ns. The solid lines depicted on the experimental curves are the best-!it curve obtained with a simulating calculation.

Figure 5 shows the fluorescence decay cudes of donor with and without acceptor, together with that of C U in ethanol solution. It can be seen that the fluorescence decay becomes faster on increasing the concentration of acceptor AP and that the slope of the slow decay at long-time region is close to that of C U in solution (15.5 ns). It should be noted that the fluorescence decay curves of donor without acceptor deviate significantly from single-exponential form. The decay curve profile of the CU film did

where 7 D is the mean lifetime of the donor without acceptor, nA is the density of acceptor molecules, and Ro is the critical transfer distance where the energy transfer and the intrinsic energy deactivation of the excited donor compete with equal probability. Curve fitting on the basis of eq 1 was carried out by varying the parameter values of rDand Y*, but the theoretical decay curves did not fit the experimental curves. It appears in Figure 5 that the experimental curves include a long-lifetime component whose lifetime values are close to the intrinsic decay of carbazole. Taking the contribution of this slow-decay component into consideration, we modified eq 1 such that an exponential decay term is involved: p(t)

=

exp[-t/TD - 2yA(t/TD)1’31 +

exp(-t/7D)

(3)

Examples of best-fit curves are shown in Figure 5. The ratio of the preexponential factor ( A l / A 2 )TD, , and were determined for various concentrations of the acceptor. These parameter values thus obtained are listed in Table I. As is seen from the reduced x2 values, analyses for the decay curves at lower AP concentrations give no good curve fitting. The fluorescence decay for low acceptor concentrations must be determined by a quenching process other than the energy transfer which will be discussed later. Hence, we are concerned here with the values obtained for higher acceptor concentrations. ( 1 3) Berlman, I. B. Energy Transfer Parameters of Aromatic Compounds; Academic: New York, 1973.

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The Journal of Physical Chemistry, Vol. 91, No. 4, 1987

From Y~ and nA values in Table I, the R, value can be deduced through eq 2. At higher acceptor concentrations, the Rovalues are about 23 A. This value is in good agreement with the literature The A 2 / A I value for the carbazole and anthracene pair (27.7 value falls in between 0.02 and 0.03. Thus, apart from small contributions of the single-exponential decay components, the two-dimensional energy transfer can be analyzed in terms of the kinetic equation of Hauser et Furthermore, this result indicates that the LB film is two-dimensional in nature. The small but finite values for A 2 / A 1 might suggest an irregular distribution of chromophores in the LB films and indicate that a small fraction of carbazole chromophores exist in the LB film as isolated and noninteracting species. Recently, we have shown a similar irregularity in the distribution of chromophores also in the LB films containing pyrene chromophores with the same amount of contrib~ti0n.I~ Kinetic equations similar to eq 3 were used in previous studiesbs on fluorescence quenching for dye molecules adsorbed on semiconductors or insulators Downloaded by UNIV OF NEBRASKA-LINCOLN on August 28, 2015 | http://pubs.acs.org Publication Date: February 1, 1987 | doi: 10.1021/j100288a017

~

t= )

ex~[-t/T, -

+ A , exp(-t/T2)

(4)

Tamai et al.

1 1 1 7 1 1 1 ' 1 104

lo3

u) I-

z

:

102

V

10'

where T~ and T 2 can take different values, contrary to the present study where these quantities are identical (eq 3). In the submonolayer of cresyl violet on amorphous quartz,* both T , and T 2 are strongly dependent on the coverage of dye; T~ varies between 0.5 and 3.4 ns, and T 2 between 9 and 90 ps. In the case of the submonolayer of rhodamine B adsorbed on glass and naphthalene single the fluorescence decay profiles at lower coverage were analyzed with a sum of two expressions of the first term of eq 4 with T ] = 3.8 ns and T ] ' = 0.9 ns. In the highest coverage, the decay profile can be simulated with an additional term of a single-exponential decay term with 7-2 = 0.2-0.3 ns. These behaviors are associated with the existence of several kinds of emitting species decaying with different time constants. It appears that different types of emitting species are probably due to different conformations of dye aggregates on the solid surface and that their structures are dependent on the dye coverage. On the other hand, the present study for the LB monolayer films appears to be fairly simple apart from the fluorescence quenching due to electron transfer to stearic acid as is shown in the following. Electron-Transfer Quenching in the LB Film. Let us proceed with the discussion on fluorescence quenching of excited carbazole due to a process other than the energy transfer and trapping. In the LB film of CU without the acceptor AP, the experimental results are summarized as follows. (1) The fluorescence decay is nonexponential, and its profile is independent of CU concentration when the concentration is lower than 5 mol %. At higher CU concentrations, the decay becomes faster and deviates much more from an exponential form probably due to the concentration quenching by dimer or higher aggregates of carbazole chromophores. (2) In the LB films at lower CU concentrations (1 mol %) or pure stearic acid, a very weak emission different from that of carbazole could be observed. This is attributable to impurities involved in the LB film, because the intensity of this emission is clearly dependent on the number of recrystallizations of stearic acid. On the contrary, the decay curve profile of carbazole is independent of the purification procedures for stearic acid. This means that the deviation from exponential decay in the CU film is not due to fluorescence quenching by impurities involved in the LB film. (3) Recently, we have examined fluorescence decay kinetics with LB films of chromophoric pyrene14*1s and rhodamine B.16 Both LB films exhibit a single-exponential decay curve at low concentrations of individual species. At higher concentrations, however, the decay profile deviates from a single-exponential form, owing to an excimer formation in the case of pyrene and to a concentration quenching in the case of rhodamine B. All these

Figure 7. Fluorescence decay curve of CU (1.1 mol %) in the LB film with no acceptor AP and theoretical curves calculated based on the electron-transfer quenching of three-dimensionalsystem (eq 12) under the Bohr radius L = 1.42, Reff= 7.04 8, ( l ) , 7.53 8, (2), 8.18 8, (3), 8.60 8, (4), 9.17 8, ( 5 ) , 9.46 8, (6). 9.82 8, (7), and 10.31 8, (8).

(14) Yamazaki, I.; Tamai, N.; Yamazaki, T. Utrafast Phenomena V; Fleming, G. R., Siegman, A. E., Eds.; Springer-Verlag: West Berlin, in press. ( I 5 ) Yamazaki, T.; Tamai, N.: Yamazaki, I. Chem. Phys. Lett. 1986, 124, 326. (16) Tamai, N.; Yamazaki, T.; Yamazaki, I., to be published.

experimental results (1-3) suggest that the nonexponential decay of carbazole is essential in the LB film. Note that carbazole chromophores are surrounded by a large number of stearic acid molecules and that, in a pair of carbazole and acids, most of the acids can function as electron

1 0

20

40

60

T I M E (ns) Figure 6. Fluorescence decay curve of CU (1.1 mol %) in the LB film with no acceptor AP and theoretical curves calculated based on the electron-transfer quenching of two-dimensional system (eq 11) under the Bohr radius L = 1.13, Reff= 4.26 8, ( l ) , 4.65 A (2), 5.36 A (3), 5.95 8, (4), 6.46 8, (5), and 6.86 8, (6).

L

1

I

'

, 0

20

40

60

TIME (ns)

The Journal of Physical Chemistry, Vol. 91, No. 4, 1987


Energy Transfer in Langmuir-Blodgett Films It is known that photoexcited carbazole in solution can easily undergo fluorescence quenching owing to the electron t r a n ~ f e r . ' ~ - ' ~ In the present study, by substituting stearic acid of the LB film by methyl stearate up to 20 mol %, we found the fluorescence decay curve to be fairly close to the single-exponential form. Thus, the deviation from a single-exponential form in the C U fluorescence decay might be due to the fluorescence quenching through the carboxyl group of stearic acid of the LB film. Recently, kinetics of the electron transfer in restricted molecular arrangements such as micelles and cellulose have been studied.20q21 The long-distance fluorescence quenching due to electron transfer has been examined with a variety of electron-deficient or -rich compounds in rigid g l a ~ s e s . ~ * With : ~ ~ the time-resolved experiment, several g r o ~ p s ~have ~ - *demonstrated ~ that the fluorescence decay function in the electron-transfer quenching follows the equation of Inokuti and Hirayama for the exchange interaction.26 Blumen et al.27,28further examined the decay function to get a theoretical formulation for the respective spatial dimensionalities of the system. Recently, Dominique and Fayer26 proposed equations in which the orientation of molecules is taken into consideration. Now we consider the nonexponential decay of CU without acceptor following the manner of Blumen et a1.28 It will be shown that the nonexponential decay results from the electron-transfer quenching by stearic acid in the LB film. Considering a system in which electron donors and acceptors are randomly distributed, the electron-transfer rate at distance r is given by w(r) = a exp(-yr) (5) where y = 2/L, L is the Bohr radius, and a is the frequency factor. Introducing the effective radius Reffat which the electron-transfer rate equals the inverse of the lifetime of electron donor without , can rewrite eq 5 in another form: acceptor T ~ we w(r) = (1/7D) exp[y(Reff - r)l (6) After taking an ensemble average of w(r) over all possible configurations of acceptors, the fluorescence decay function observable is expressed as follows: p(t)

= eXp[-t/TD - nAvA7-*gA(%t)1

(7)

where gA(z) = AJm[l - exp(-ze-Y)]yA-' dy nA is the number density of acceptor molecules, VAis the volume of a unit sphere in a A-dimensional space, and b is a cutoff parameter of the order of the nearest-neighbor distance. gA(z) can be reduced to an approximate form for two- and three-dimensional systems.

+ 1.154 In z + 1.978 g3(z) = In3 z + 1.732 In2 z + 5.934 In z + 5.445 g2(z) = In2 z

(9) (10)

Equation 7 can be written for the two- and three-dimensional systems respectively as eq 11 and eq 12.

(17) Pfister, G.; Williams, D. J. J . Chem. Phys. 1973, 59, 2683. (18) Pfister, G.;Williams, D. J.; Johnson, G. E. J. Phys. Chem. 1974, 75, 2009. (19) Johnson, G. E. J . Phys. Chem. 1980, 84, 2940. (20) Costa, S. M. B.; Aires, M. R.; Conde, J. P. J . Photochem. 1985, 28, 153. (21) Milosavljevic, B . H.; Thomas, J. K. J . Physi Chem. 1985.59, 1830. (22) Miller, J. R.; Peeples, J. A,; Schmitt, M. J.; Closs, G. L. J . A m . Chem. SOC.1982, 104, 6488. (23) Namiki, A.; Nakashima, N.; Yoshihara, K. J. Chem. Phys. 1979, 71, 925. (24) Strauch, S.: McLendon, G.;McGuire, M.; Guarr, T. J. Phvs. Chem. 1983,87, 3579. (25) Dominique, R. P.; Fayer, M. D. J . Chem. Phys. 1985, 83, 2242. (26) Inokuti, M.; Hiravama, F. J . Chem. Phvs. 1965. 43. 1978. (27) Blumen, A. J. Chem. Phys. 1980, 7 2 , 2632. (28) Blumen, A.; Klafter, 3 ; Silbey, R. J . Chem. Phys 1980, 72, 5320

845

Equation 12 is the equation identical with the one derived by Inokuti and Hirayama for the three-dimensional system.26 A simulating calculation was carried out by using eq 11 and 12 with L, rD, and a being varied. Figures 6 and 7 show respectively the results of curve fitting for two- and three-dimensional equations. In this calculation, we used a value of T~ = 15.5 ns which was obtained from a lifetime measurement of CU in alcohol solution. For two-dimensional analysis (eq 1I ) , we used a value of nA = 5X estimated from the limiting molecular area (20 A2) of stearic acid, in the surface pressure-area curve. As is seen in Figure 6, the experimental curve can be simulated by the theoretical curve of L = 1.13 8, and Reff= 5.36 A ( a = 7 X 10" SKI). For the three-dimensional analysis (eq 12), we used nA = 1.99 X k3, estimated from the specific weight of stearic acid, d = 0.941 (20 "C), and a molecular weight of 284. As is seen in Figure 7, a theoretical curve with L = 1.42 8, and Reff= 8.60 A ( a = 9 X 10l2 s-I) gives the best-fit curve. It appears from Figures 6 and 7 that the theoretical decay curve by either two- or three-dimensional equations can fit the experimental curve with different sets of parameter values. However, by reference to other cases of electron-transfer donor-acceptor pairs, it is found that the three-dimensional curve fitting would give more favorable parameter values with regard to magnitudes of the Reffvalue. In the case of indole (donor) and chloromethane (acceptor) in rigid glass,23L = 1.04 A and Re,y = 9.8 A. In the case of pentacene (donor) and duroquinone (acceptor) in rigid glass,25L = 0.70 8,and Reff= 14.3 8,. In the case of a ruthenium complex (donor) and methylviologen ( a c ~ e p t o r )L, ~=~ 1.42 A and R,ff = 17.3 A. All these cases show that the Reffvalues fall between 9 and 18 A, in accord with the electron-transfer occurring in a fairly short range owing to the exchange interaction. Consequently, the electron-transfer quenching of carbazole fluorescence by stearic acid would be regarded as responsible for the nonexponential decays in the CU films and also in the CU and AP film with low AP concentration. In the present case of the LB film, distribution of the carboxyl group around the carbazole chromophore would not be two-dimensional but nearly threedimensional, since the LB films studied are multilayers insofar as stearic acid is concerned. In order to get further kinetic aspects of the electron transfer in the LB film, it will be necessary to examine systematically a variety of LB films consisting of different types of chromophores having electron-donating and -accepting abilities.

Concluding Remarks In the present study, kinetics of the two-dimensional excitation energy transfer has been investigated with LB monolayer films containing chromophoric carbazole (donor) and anthracene (acceptor). It was found that, at higher concentrations of acceptor, the fluorescence decay curve of donor can be simulated by the modified two-dimensional Forster-type equation involving a contribution of a single-exponential decay which means the existence of noninteracting donor chromophores. At lower acceptor concentrations, the excitation energy competes with the fluorescence quenching due to the electron transfer to the carboxyl group of stearic acid in the LB film. The electron-transfer quenching is responsible for nonexponential decay in the LB film containing only carbazole chromophore even at concentrations as low as 0.01 mol %. This makes a striking contrast with our recent studies of the LB monolayer films of chromophoric pyrenel4.l5and rhodamine BL6where the excited singlet chromophore exhibits a single-exponential fluorescence decay at low chromophore concentrations. In these latter cases, however, excited species undergoes concentration quenching due to energy migration and trapping by dimer and/or higher aggregated species. Acknowledgment. The authors acknowledge Dr. H. Nakahara of Saitama University and Dr. M. Sugi of Electrochemical Laboratory (Ibaragi, Japan) for their advice and help in preparation of the LB films. The authors are deeply indebted to Prof. K. Fukuda of Saitama University for many helpful suggestions and discussions. Registry No. Cu, 73025-00-0; AP, 64821-29-0; stearic acid, 57-1 1-4

Two-dimensional excitation energy transfer between chromophoric

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