Langmuir 1987,3, 1173-1175
1173
Observation of Forster Energy Transfer in Monolayers at the Air-Water Interface F. Grieser, P. Thistlethwaite,* and P. Triandos Physical Chemistry Department, Melbourne University, Parkville, Victoria 3052, Australia Received August 19, 1987 Forster energy transfer between 5-(N-octadecanoylamino)fluorescein (donor) and 5-(N-hexadecanoylamino)eosin (acceptor), embedded in a lipid monolayer at the air-water interface, has been observed by using steady-state fluorescence spectral measurements. The dependence of energy transfer on monolayer compression has been studied.
Introduction Air-water monolayers provide an ideal means of assembling chromophores into two-dimensional arrays in which molecular interactions can be varied in a controlled way. One possible form of interaction is Forster energy transfer. Forster transfer is involved in the photosynthetic process in which sunlight absorbed by antennae chlorophylls and other pigments is transferred to particular chlorophyll a molecules to be used in the production of carbohydrate and oxygen. The thought that chromophores in a monolayer may form the basis of a light-harvesting system has stimulated many studies of energy transfer in monolayers. These studies have to date been almost totally restricted to cast films on solid substrates (Langmuir-Blodgett Improvements in fiber optic technology and fluorimeter performance have recently made it more readily possible to obtain fluorescence spectra of air-water monolayer^.^^ In a recent study of chlorophyll a in monolayers at the nitrogen-water interface, Forster energy transfer was suggested as the explanation of sel-f-quenching.4*7 For energy transfer studies there is an obvious advantage in the air-water monolayer in the ability to vary the donor-acceptor distance by continuous compression. Moreover, the natural orienting feature of monolayers and the possibility that molecular orientation will change with compression suggests that the study of Forster transfer in these systems would be of particular interest. In the pioneering work of Gaines and co-workers,* and in subsequent Forster transfer studies on air-water monolayers>' quenching of donor fluorescence was observed and Forster transfer to a nonfluorescent acceptor inferred. In this paper we report the observation of Forster transfer between two fluorescent chromophores, 54N-octadecanoylamin0)fluorescein (OAF) and 5-(N-hexadecanoylamino)eosin (HAE), embedded in a lipid monolayer at the airwater interface. Results and Discussion OAF and HAE obtained from Molecular Probes Inc., and dipalmitoyllecithin from Fluka, were used as received. All measurements were carried out on a 60 X 16.5 X 1.5 cm Teflon Langmuir trough with a motor-driven Teflon barrier. Surface pressure measurements were made by the (1)Drexhage, K. H.; Zwick, M. M.; Kuhn, H. Ber. Bunsen-Ges. Phys.
Chem. 1963,67,62.
(2) Inacker, 0.;Kuhn, H. Chem. Phys. Lett. 1974, 27, 317. (3) Mobius, D.; Dreizler, G. Photochem. Photobiod. 1973, 17,225. (4) Agrawal, M. L.; Chauvet, J.-P.; Patterson, L. K. J. Phys. Chem. 1985,89,2979. ( 5 ) Subramanian, R.; Patterson, L. K. J. Phys. Chem. 1985,89,1202. (6) Grieser, F.; Thistlethwaite, P.; Triandos, P. J. Am. Chem. SOC. 1986. 108. 3844.
(7) Bo&, L. G.; Patterson, L. K.; Chauvet, J. P.; Kozak, J. J. J. Chem. Phys. 1987,86, 503. (8) Tweet, A. G.; Bellamy, W. D.; Gaines, G. L. J. Chem. Phys. 1964, 41, 2068.
Wilhelmy plate method, using a 4-cm-wide mica plate suspended from a calibrated Shinkoh 2-g weight capacity strain gauge feeding a chart recorder. The trough and barrier were cleaned with hexane, the mica plate was rinsed in nitric acid, and impurities were swept off the water surface before the monolayer was spread. Milli-Q-filtered water was used for the subphase. The efficacy of these procedures was checked by recording the surface pressure-area isotherm for stearic acid. Fluorescence measurements were made on a Perkin-Elmer LS-5 spectrofluorimeter linked to the Langmuir trough by fiber optic bundles. The two fiber optic bundles were positioned approximately 5 mm above the monolayer. The apparatus incorporated a provision for ensuring that this distance remained constant throughout the fluorescence measurements. The fluorescence (comparable in intensity to the subphase Raman scatter) is generally much weaker than the "background", due mainly to secondary emission in the emission fiber bundle. Accordingly, it is necessary to separately scan the "background" and "total" spectra and then subtract to obtain the fluorescence spectrum. Multiple scanning (usually 25 scans) and signal averaging are used to increase the signal-to-noiseratio. All experiments were carried out at room temperature (19 f 1 "C). The emission spectrum of the dianion form of OAF and the absorption spectrum of the dianion form of HAE show a large degree of overlap. Both molecules exhibit a highfluorescence quantum yield in either the mono- or dianionic form, while being much more weakly fluorescent when in the neutral f~rm.~JOThe large fluorescence quantum yield of the OAF donor, the high molar absorptivity of the HAE acceptor, and the large spectral overlap suggest, via the Forster equation,ll that energy transfer should be efficient. Fluorescence spectra were excited at 460 nm, a wavelength at which OAF absorbs strongly while HAE absorbs only very weakly. Figure 1shows emission spectra, taken with identical conditions of excitation and gain, for monolayers consisting of 10 mol % OAF in dipalmitoyllecithin (DPL) and 10 mol % HAE in DPL, over aqueous pH 11 subphases. The emission maximum for OAF is at 525 nm while that for HAE is at 548 nm. For both cases spectra were recorded at average areas per molecule of 302, 241, and 119 A2. As expected from the very low absorbance, the emission intensity of the HAE monolayer is much smaller and is only detectable at the lowest area per molecule. For the OAF/DPL monolayer the fluorescence intensity actually declines slightly as the average area per molecule falls from 302 to 241 A2. Thereafter it remains constant. (9) Martin, M. M.; Lindqvist, L. J.Lumin 1975, IO, 381. (10) Leonhardt, H.; Gordon, L.; Livingston, R. J. Phys. Chem. 1971, 75, 245. (11)Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970.
0743-7463/87/2403-1173$01.50/0 0 1987 American Chemical Society
1174 Langmuir, Vol. 3, No. 6, 1987
Letters
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o md O/O I-IAE
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I
r . xx, ' S b o ' S B o Ob (m) Figure 1. Fluorescence spectra of 10 mol % OAF in DPL and 10 mol % BAE in DPL at various average areas per molecule. Subphase pH 11, A, 460 nm.
A2
per mlec I,
202
2, 3,
241
119
wavelength (nm)
Figure 2. Fluorescence spectrum of 10 mol % OAF/10 mol % HAE in DPL (subphase pH 11, A, 460 nm) at various average
areas per molecule.
This effect could be due to self-quenching or possibly to the formation of solution aggregates of OAF, leading to loss of OAF from the monolayer into the subphase (see later). It has recently been suggested that self-quenching in chlorophyll a monolayers is due to energy transfer to nonfluorescent traps consisting of pairs of chlorophyll molecules spaced at less than a critical d i ~ t a n c e .This ~ would seem unlikely in the present case, as the OAF is present as an anion and the trend does not continue at higher surface densities. The fall in fluorescence cannot be attributed to a reprotonation of the OAF carboxyl group at higher compression. As the monolayer is compressed the interfacial pK of the OAF increases due to the increasing negative surface potential.12J3 However, given that the pK of the carboxyl group of fluorescein is ca. 4,1° and the subphase pH is 11, reprotonation would not be expected at the low compressions of Figure 1. Calculations based on the Gouy-Chapman equation14 support this conclusion. Figure 2 shows the fluorescence spectrum of the mixed monolayer of OAF and HAE in DPL in which the OAF and HAE concentrations and the experimental conditions are the same as in Figure 1. It can be seen that in the mixed monolayer the emission intensity of HAE is increased relative to Figure 1 while the OAF emission is completely quenched. The emission intensity of HAE increases as the average area per molecule declines, although not exactly in step with the increase in density of absorber molecules (see later). Below an average area per molecule of ca. 110 Azthe HAE emission intensity becomes approximately constant. Because energy transfer is complete for equal concentrations of OAF and HAE, as in Figure 2, the situation with lower acceptor concentration was studied. This experiment can also refute another possible mechanism for the energy (12)Fernandez, M. S.;Fromherz, P. J.Phys. Chem. 1977,81,1755. (13)Lovelock, B.;Grieser, F.; Healy, T. W. J. Phys. Chem. 1985,89, 501. (14)Davies, J. T.;Rideal, E. K. Interfacial Phenomena, 2nd ed.; Academic: New York, 1963.
.
500
wavelength (nm)
Figure 3. Emission spectra of monolayers containing 10 mol % OAF in DPL and varying concentrations of HAE (Aex 465 nm,
subphase pH 11).
transfer. It is possible that the transfer occurs only from OAF donors that exist prior to excitation in close proximity to HAE acceptors (static quenching of the OAF excited state by energy transfer). If this was the case, the efficiency of energy transfer would depend on the probability of formation of these donor-acceptor pairs, which would in turn depend on the product of the surface densities of the two species. This possibility is eliminated by the results shown in Figure 3. The fluorescence spectrum of monolayers containing 10 mol % OAF' in DPL and progressively larger concentrations of HAE was monitored. Figure 3 shows that even 1mol % of HAE causes the almost complete removal of OAF emission. As the excitation will be randomly distributed, and not confined to the small fraction of OAF molecules that could now be paired with HAE molecules, this observation eliminates the above possibility. I t is noteworthy that in Figure 2 the intensity change between curves 1and 2 is greater than would be predicted on the basis of the number of absorber molecules per unit area and constant energy-transfer probability. Between curves 2 and 3 the intensity change is less than expected on the same basis. This suggests that the energy-transfer probability is rising as the average area per molecule drops from 302 to 241 A2. The quantum yield of OAF emission is sufficiently low at all three compressions to prevent detection of OAF emission. It appears that as the area per molecule falls from 302 to 241 A2 energy transfer competes more effectively with other nonradiative processes, and then, between an average area per molecule of 241 and 119 A2,other effects come into play, which reduce the HAE intensity. Energy transfer involving diffusion of the donor and acceptor into close proximity following excitation can be ruled out. For 10 mol % of both OAF and HAE an average area per molecule of 302 A2 corresponds to an average "nearest-neighbor" OAF-HAE distance of ca. 38 A.15 Even given this proximity, the high value of the diffusion coefficient applicable at low surface pressures,16 and the appreciable size of the chromophores, the calculated time for the chromophores to diffuse into contact is several times the fluorescence lifetime of fluorescein (estimated to be 3-4 ns17). Furthermore, the energy-transfer efficiency does not fall dramatically at higher surface pressures whereas the diffusion coefficient drops by a factor of 1000 when the DPL passes through its phase transition (at approximately 90 A2 molecule-1.16 A decline in energy-transfer efficiency can account for a slowing in the rate of rise of HAE emission as seen in Figure 2. However, this process cannot continue for long without the reappearance of OAF emission. In the absence of any (15) Barraclough, C. G., private communication. (16)Peters, R.; Beck, K. Roc. Natl. Acad. Sci. U.S.A. 1983,80,7183. (17)Fleming, G. R.; Knight, A. E. W.; Morris, J. M.; Morrison, R. J. S.; Robinson, G. W. J.Am. Chem. SOC.1977,99, 4306.
Langmuir 1987, 3, 1175-1178 reappearance of OAF emission, the ultimate constancy of HAE emission intensity from the 10 mol 90OAF/lO mol 9O HAE monolayer requires another explanation. Above pH 7 OAF is reported to form solution aggregates with associated fluorescence quenching.18 Formation of aggregates and associated loss of OAF from the monolayer would lead to OAF becoming a less effective donor and a reduction in HAE intensity. In view of the constant emission intensity seen for the OAF/DPL monolayer at higher compression (Figure l),this seems the most plausible explanation for the present results. The loss of a small fraction of the OAF at higher compression would be difficult to detect from pressure-area isotherms, given the small concentration of OAF in the monolayer. Isotherms for each of OAF' and HAE at a concentration of 10 mol % in DPL and the isotherm for the mixed monolayer in the molar ratio 1:1:8were very similar to that for DPL alone. The large degree of overlap of the OAF and HAE emissions precludes the obtaining of quantitative data on the variation of transfer efficiency with surface density for comparison with the Forster treatment in two dimen(18) Haugland, R. P. Handbook of Fluorescent Probes and Research Chemicals; Molecular Probes Inc.: Junction City, OR, 1985.
1175
s i ~ n s . l * ~The ~ possibility of self-quenching and/or loss from the monolayer is a further complication. In future work it will be of interest to study the same two chromophores in a different lipid matrix, e.g., dioleoyllecithin. We are also currently building up photon-counting equipment for the measurement of polarized emission with a view to obtaining information on head group orientation. The measurement of polarization ratio and the correlation of this quantity with spectral and isotherm data will aid in a more thorough analysis of the energy transfer in two dimensions. Acknowledgment. Financial support from the Australian Research Grants Scheme is gratefully acknowledged. Registry No. OAF, 110698-53-8; HAE, 85735-45-1; dipalmitoyllecithin, 2644-64-6. (19) Wolber, P. K.;Hudson, B. S. Biophys. J. 1979,28, 197. (20) Doody, M. C.; Sklar, L. A.; Pownall, H. J.; Sparrow, J. T.; Gotto, A. M.; Smith, L. C. Biophys. Chem. 1983,17, 139. (21) Dewey, T. G.;Hammes, G. G. Biophys. J. 1980, 32, 1023. (22) Leitner, A.; Lippitach, M. E.; Draxler, S.;Riegler, M.; Aussenegg, F. R. Thin Solid F i l m 1985,132, 55. (23) Nakashima, N.; Yoshihara, K.; Willig, F. J.Chem. Phys. 1980, 73, 3553.
Effect of Si02 on CO Hydrogenation over Cobalt Catalysts As Observed by Isotopic Transient Techniques Xuezhi Zhang* Department of Chemical Engineering, University of Akron, Akron, Ohio 44325
Paul Biloent Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261 Received July 14, 1987. In Final Form: September 1 , 1987 Generally regarded as "inert" to cobalt-catalyzedCO hydrogenation, SiOzactually affects both coverage and reactivity of the surface-reactive intermediates in a compensating manner, as observed by isotopic transient methods. Introduction Si02 has been used as a support in heterogeneous catalysis for a long time. Yet, its effect on metal catalysts is still a controversial subject. Many researchers believe that Si02 is "inert" to the reaction, while others tend to disagree with this proposa1.l"' Very recent research, for example, indicates the important role of Si02in accommodating spiltover species and chemical reactions between those specie^.^ In general, however, the effect of Si02on the reaction rate, turnover frequency (TOF), rate constant (k),and the coverage of the surface intermediates (8) is seldom reported. To modify a catalyst toward a given reaction. the role of SiO,, one of the most often used supports, is obviously woith investigating. Two extreme support effects have been observed: the strong support interaction (SMS1) effect On' say' Pt/Ti025and the weak metal support interaction (WMSI) 'Deceased.
effect in systems such as metal supported on Si02 or A1203.2 A SMSI effect is easily reflected in TOF and chemisorptionproperty changes, as reported by Sinfelt and Lucchesi.6 A WMSI effect, on the other hand, will result in far less change in TOF and chemisorption properties. However, since the Si02 support itself is capable of accepting spiltover hydrogen, as disclosed by Lenz and Conner; we suspected that there should be a support effect on the real reaction rate constant (k)and surface coverage (8) even with this WMSI system. The effect of support on k and 8 might oppose each other in such a compensating manner that the gross output (k0 = TOF) remains constant. It is apparent that investigating this WSMI effect (1) Serman, P. A.; Bond, G. C. Catal. Reu. 1973, 8, 211. (2) Bond, G. C. In Metal-Support and Metal-Additive Effects in Catalysis; Imelik, B., et. al., Eds.; Elsevier: Amsterdam, 1982; p 1. (3) Conner, W. C., Jr., In Aduances in Catalysis; Academic: New York, 1986; Vol. 34, p 1. (4) Lenz, D. H.; Conner, W. C., Jr. J. Catal. 1987, 104, 288. (5) Sinfelt, J. M.;Lucchesi, S. J. J. Am. Chem. SOC.1963, 85, 3365.
0743-7463/87/2403-ll75$01.50/0 0 1987 American Chemical Society
