J . Phys. Chem. 1986, 90, 1643-1647
1643
ODMR Investigation of the Donor-Acceptor Pair Orientation from Triplet-Triplet Energy Transfer within Frozen SDS Micelles Sanjib Ghosh, Michael Petrin, and August H, Maki* Department of Chemistry, University of California, Davis, California 95616 (Received: October 7 , 1985)
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Intermolecular nonradiative triplet-triplet energy transfer (T T ET) from benzophenone (B) to naphthalene-h, (N) was studied in frozen sodium dodecyl sulfate (SDS) micelles at 1.2 K by optically detected magnetic resonance (ODMR) spectroscopy in zero applied magnetic field. ODMR transitions of B and N observed in frozen SDS micelles suggest that the carbonyl group of B orients toward the surface and N penetrates into the hydrocarbon chains of the micelle. MicroT ET from B populates the T, sublevel wave-induced delayed phosphorescence (MIDP) measurements show that T of N 0,is the in-plane short axis) selectively, indicating a preferred configuration of the sensitizer-acceptor pair during T T ET within the micelles.
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Introduction
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Intermolecular nonradiative triplet-triplet energy transfer ( T T ET) has been studied from various aspects' since the first observation of sensitization phenomena by Terenin and Ermolaev.2 This type of energy transfer occurs through Dexter's exchange m e c h a n i ~ m ,where ~ T T E T is a spin-allowed process in contrast to the dipole-dipole mechanism. The rate constant for T T E T has been extensively studied as a function of the donor-acceptor separation within a rigid matrix by examining donor phosphorescence decay kinetics as a function of acceptor concentration following pulsed photoexcitation of the d ~ n o r . ~ - ~ The effective energy-transfer range (&) in rigid media is estimated to be on the order of 10-15 A.1$7 In the T T E T process, conservation of spin polarization is to be expected since the operator involved in energy transfer is independent of electron spin.3 El-Sayed et aI.* showed that spin alignment is conserved during T T E T in a system of crystalline naphthalene ( N ) doped with quinoxaline using a PMDR (phosphorescence-microwave double resonance) technique. In this experiment, naphthalene x traps sensitized by T T ET from quinoxaline dopant molecules were found to be populated predominantly in the T, sublevel (z is the long molecular axis) which is the same spin alignment as occurs in the donor. Brenner9 T E T from phenazine to anthracene in a diphenyl studied T single crystal host at 1.7 K by EPR methods and found that the zero-field triplet sublevel associated with the long molecular axis was populated preferentially in both donor and acceptor, indicating that spin polarization was conserved during the transfer. The conservation of spin orientation was also reportedi0 in the T T ET from free triplet excitons to triplet exciton traps in pure benzophenone (B) crystals a t 4.2 K. Recently, the transfer of T ET has been donor spin polarization to acceptor during T shown to occur in B / N and acetophenone/N systems in rigid glassy matrices at 77 K by using transient ESR techniques."
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( I ) J. B. Birks, "Photophysics of Aromatic Molecules'', Wiley-Interscience, London, 1970; N . J. Turro, "Modern Molecular Photochemistry", Benjamin, Menlo Park, CA, 1978. (2) A. N. Terenin and V. L. Ermolaev, Dokl. Akad. Nauk. SSSR, 85, 547 (1952); Trans. Faraday Soc., 52, 1042 (1956). (3) D. L. Dexter, J . Chem. Phys., 21, 836 (1953). (4) M. Inokuti and F. Hirayama, J. Chem. Phys., 43, 1978 (1965). (5) H. Kobashi, T. Morita, and N. Mataga, Chem. Phys. Lett., 20, 376 ( 1973). (6) G. B. Strambini and W. C. Galley, Chem. Phys. Lett., 39, 257 (1976). (7) K. Yamamoto, T. Takemura, and H. Baba, J . Lumin., 15,445 (1977). (8) M. A. El-Sayed, D. S.Tinti, and E. M. Yee, J . Chem. Phys., 51, 5721 (1969). (9) H. C. Brenner, J . Chem. Phys., 59, 6362 (1973). (10) M. Sharnoff and E. B. Iturbe, Izu. Akad. Nauk SSSR, Ser. Fiz., 37, 522 (1973). (11) T. Imamura, 0. Onitsuka, H. Murai, and K. Obi, J . Phys. Chem., 88, 4028 (1984).
0022-3654/86/2090-1643$01.50/0
Since this type of energy transfer occurs via the Dexter exchange mechanism, it is expected that there may be some preferred configuration of the donoracceptor pair which provides maximum overlap of the electronic wave functions of the sensitizer and the sensitized molecules. Magnetophotoselection studies] did not reveal orientation effects a t large intermolecular separation. Optical photoselection methods employed by Roy et aI.,I3however, and the polarization of the triplet-triplet absorption of the acceptor observed by EisenthalI4 indicate a preferred relative orientation of molecules involved in T T ET. In these experiments B/phenanthrene and anthrone/phenanthrene systems in a rigid glassy matrix at 77 K were investigated, and preferred energy transfer was found to occur when the donor C=O axis is parallel to the molecular plane of the hydrocarbon acceptor. However, no preferred orientation of the molecules within the parallel configuration was observed in either of these systems. Recently, T T ET between B and N has been studied in frozen SDS micelles at 77 K by time-resolved ESR.15 Evidence was obtained that energy transfer is much more efficient in the micellar system compared with that observed in glassy matrices. This was ascribed to the preferential solubilization of the donor-acceptor pair in the micelles such that the in-plane short axis of N is parallel to the C=O axis of B. This conclusion is based on a comparison of the observed time-resolved ESR spectrum of the B/N system in SDS micelles with a computer simulation generated by using experimental values of the triplet-state zero-field splitting (zfs) parameters of N and by assuming that the in-plane short axis sublevel of the N triplet state is populated predominantly by B via T T ET. This fit was not in complete agreement with the observed spectrum, however, particularly in the high-field region.I5 These results and the assurance that micelles retain their structural integrity even at 4.2 K16 prompted us to reinvestigate the relative orientation of the B/N pair during T T E T in frozen SDS micelles using O D M R spectroscopy, in particular, utilizing the microwave-induced delayed phosphorescence (MIDP) technique in zero applied magnetic field a t 1.2 K to obtain the relative populating rates of the triplet-state spin sublevels of the acceptor. In this report, we present the results of these measurements which indicate a different relative populating pattern of the triplet sublevels of N when it is sensitized by B compared with that which is found when N is directly photoexcited through its singlet excited state manifold. The results have been examined to determine the T E T in SDS relative orientation of the B/N pair during T micelles. It is shown also that zero-field O D M R spectroscopy,
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(12) H. S. Judeikis and S . Siegel, J. Chem. Phys., 53, 3500 (1970). (13) J. K. Roy and M. A. El-Sayed, J . Chem. Phys., 30, 3442 (1964). (14) K. B. Eisenthal, J . Chem. Phys., 50, 3 120 (1969). (15) Y. Yamamoto, H. Murai, and Y. J. I'haya, Chem. Phys. Lett., 112, 559 (1984). (16) P. A. Narayana, A. S. W. Li, and L. Kevan, J . A m . Chem. SOC.,103, 3603 (1981), and references therein.
0 1986 American Chemical Society
1644
The Journal of Physical Chemistry, Vol. 90, No. 8, 1986
Ghosh et al.
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Figure 1. Phosphorescence spectra (uncorrected) at 7 7 K of various SDS micelle samples in water with emission monochromator resolution of I .5 M) with 365-nm nm and excitation slits of 8 nm: (a) B (2.0 X M) with 365-nm excitation: (b) B (2.0 X IO-3 M) and N (2.0 X M); (d) N (2.0 X excitation; (c) as in (b) but with N (6.0 X M) with 3 IO-nm excitation. The SDS concentration is 8.0 X 1 O-* M in
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each sample. which has been established as a sensitive tool in observing subtle molecular interactions, may be applied to investigate molecules incorporated into micelles. The zfs parameters and line widths of the O D M R transitions between triplet sublevels observed in the case of B / N in SDS micelles provide further evidence concerning the location and environment of the solubilized molecules within the micelles previously investigated by other
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Experimental Section Commercially available B was recrystallized several times from ethanol. N was purified by repeated sublimation, and S D S (Bethesda Research Lab, electrophoresis grade) was used as supplied. Solutions were prepared having fixed concentrations of B and SDS of 2.0 X and 8.0 X lo-* M, respectively, with varying amounts of N and triply distilled, deionized water as solvent. All solutions were ultrasonically agitated for 30 min and subsequently degassed by bubbling nitrogen for 30 min immediately prior to measurement. No impurity emission was observed in any sample. The emission and zero-field O D M R apparatus used nave been described elsewhere.I9 M I D P experiments using microwave fast passage were preformed employing a solid-state sweep oscillator (Hewlett-Packard Model 8350B) as described by Schmidt et These measurements were performed a t 1.2 K with the samples immersed in pumped liquid helium. In M I D P from the steady state, the sample excitation time was 10 s and emission was followed for 1 s. After a short time following the opening of the emission shutter, microwaves were swept through the ID + El and the 21EI transitions of N a t 3.53 and 0.96 GHz. respectively, in the shortest possible time of 10 ms. Leveled microwave power of ca. 200 mW obtained by amplification (Alfred Model 5-6868) was found to be sufficient to ensure saturation of sublevel populations. T o investigate the M I D P response following energy transfer between donor and acceptor, B was selectively excited a t 365 nm with 8-nm band-pass while the N phosphorescence origin was monitored at 473 nm through a Corning GG-435-2 glass cutoff filter. Non-steady-state M I D P utilizing short excitation times of 100 ms was performed to probe transient populations in N triplet sublevels as a result of direct excitation and via ET from (l?), (a) P. Mukherjee, J. R. Cardinal, and N. R. Desai in "Micellization, Solubilization and Microemulsions", K. L. Mittal, Ed., Vol. 1, Plenum Press, New York, 1977, p 241; (b) J . H. Fendler, E. J. Fendler, G . A. Infante, P. S . Shih, and L. K. Patterson, J . Am. Chem. SOC.,97, 89 (1975). (18) R. R Hautala, N. E. Schore, and N . J. Turro, J . Am. Chent. Sor.. 95, 5508 (1973). (19) S. Ghosh, J . Weers, M . Petrin, and A . H. Maki, Chem. 6'hj.s. L e i ? . . 108, 87 ( 1 984). (20) J . Schmidt, D. A . Antheunis. and J. 11. van der Waals, Mol. Phy.r., 22. I (1971)
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Figure 2. Zero-field slow passage ODMR transitions at 1.2 K of N and of B in SDS micelles monitoring the 0,Oband: (a, b) sample is N (8.0 X M) with 310-nm excitation; (c, d) sample is B (2.0 X m) and N (8.0 X M) with 365-nm excitation; (e) sample is B (2.0 X M ) in aqueous SDS with a total sweep time of ca. 10 ms; (f) as in (e) with a total sweep time of ca. 25 ms. The SDS concentration is 8.0 X M in water. The phosphorescence is monitored at 473 nm for N and at 421 nm for B.
the B donor. Repeated trials were signal averaged, and we chose the time interval sufficiently long in all cases to ensure complete decay of photoexcited states before repeating the cycle.
Results and Discussion Phosphorescence Spectra of B I N in SDS Micelles. The phosphorescence spectra of B I N in SDS micelles at 77 K obtained by excitation into the first excited singlet absorption band of B are shown in Figure la-c. The structured emission spanning 470 to 560 nm, whose intensity increases with an increase in N concentration as indicated in the figure, is attributed to the N acceptor phosphorescence. The phosphorescence spectrum of N alone in SDS micelles a t 77 K is presented in Figure Id for comparison. The phosphorescence emission observed here for N in SDS micelles is considerably narrower and better resolved compared to that observed by us in 20% aqueous glycerol medium under similar conditions. This indicates that N is completely incorporated into the micellar phase under the experimental conditions employed here. Lowering the temperature of the sample to 4.2 K has no discernible effect on these spectra. The SDS concentration was chosen to be roughly 10 times the critical micelle concentration (cmc) value of aqueous SDS at 43.8 OC." Figure 1 demonstrates that the efficiency of energy transfer is less than unity even for the highest concentration of acceptor used in our experiments. It is observed, however, that energy transfer is considerably more efficient in S D S micelles in relation to systems of B/N in rigid glassy matrices a t 77 K'-' when considering the relative concentrations of donor and acceptor pairs used in those studies. Slow Passage Zero-Field ODMR Transitions of B I N in SDS Micelles. In Figure 2 we present the slow passage zero-field (21) M. Abu-Hamdiyyah and I. A. Rahman, J . ?'hj,s. Chem., 89. 2377 (1985)
Energy Transfer within Frozen SDS Micelles
The Journal of Physical Chemistry, Vol. 90, No. 8, 1986 1645
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ODMR transitions of N in SDS micelles when it is directly excited (a, b) as well as that observed when sensitized by T T ET from B (c, d), while monitoring the 0,O phosphorescence band of N . In both instances the ID El (T, T,) and 21EI (T, T,) transitions occur at 3.52 and 0.95 GHz, respectively, with an increase in relative phosphorescence intensity and fairly narrow resonance line widths of ca. 25-30 MHz. In Figure 2e,f we show the O D M R transitions of B in SDS micelles at 1.2 K while monitoring B phosphorescence at 421 nm. The transitions exhibit very broad line widths suggesting a relatively inhomogeneous environment. The narrow zero-field ODMR line widths for transitions in the N triplet state produced by direct excitation or sensitized by T T E T from B indicate that N is in a relatively homogeneous environment. Although UV absorption studies'7a indicate that N resides in a relatively polar environment, N M R results'7bsuggest that N is solubilized into the inner hydrophobic core of the micelle. The efficient T T E T observed here combined with ODMR results suggests that N is neither buried deeply within the hydrophobic core nor adsorbed very near the polar surface region. W e are currently utilizing O D M R techniques to investigate in more detail the nature of N solubilization into anionic micelles of varying surfactant chain length. The transition line widths, however, are greater than those observed for N in an n-pentane polycrystalline Shpol'skii matrix, where N exhibits phosphorescence from a single site with O D M R transitions of ca. 5 M H z in widthI9 a t 1.2 K, which occur with a decrease in phosphorescence intensity. Comparison suggests that, for N in SDS micelles, inhomogeneous broadening may originate from various distributions and orientations of N from micelle to micelle. However, for N in n-decane, which does not form a Shpol'skii matrix, we observed that the ID El and 2(EI transitions of N exhibit an increase of phosphorescence intensity and line widths of ca. 30 MHz. Furthermore, for the n-decane matrix, the phosphorescence emission spectrum is nearly identical with that observed for N in the S D S micelles a t 4.2 K under similar conditions and concentration of N. Only the origin of the phosphorescence is redshifted by ca. 2 nm in n-decane relative to S D S micelles. Although the phosphorescence spectra of B in SDS micelles22 did not provide information regarding the type of environment experienced by B triplets, Turro et al.23 suggested from phosphorescence studies that B molecules are located within an aqueous environment in sodium perfluorooctanoate micelles, which form "minimicelles" having low aggregation numbers of ca. 7. However, it has been proposed'* as a result of absorption and proton magnetic resonance studies that B is positioned near the exterior of the SDS micelle with the C=O group pointing toward the surface and with the aromatic groups oriented toward the hydrophobic center. The degree of solubilization of B is believed to be governed by C=O hydrogen bonding in the outer micelle region in the vicinity of the charged head groups and by hydrophobic interactions experienced by the remainder of the substrate molecule. The very broad zero-field O D M R transitions observed in our measurements for B triplets are consistent with this model since large bandwidths are indicative of a very inhomogeneous environment. A large degree of broadening (ca. 1000 MHz) of B ODMR transitions has been observed in the case of B adsorbed on an alumina surface, where both phenyl groups as well as the C=O moiety are believed to interact with hydroxyl groups on the support surface.24 MIDP Measurements Yielding Relative Sublevel Populating Rates. MIDP measurements were made on the B / N system in SDS micelles following optical pumping of the lowest singlet state and of B at 365 nm. Concentrations of B and N were 2.0 X 1.6 X M, respectively. Although this concentration of N was not enough to effect maximum quenching of B phosphorescence, it was used for all MIDP measurements to avoid possible com-
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(22) W. J. Leigh and J. C. Scaiano, Chem. Phys. Lett., 96, 429 (1983). (23) N. J. Turro and P. C. C. Lee, J . Phys. Chem. 86, 3367 (1982). (24) D. G . Frank, K. A . Martin, and A. M . Nishimura, J . Phys. Chem., 88, 2961 (1984).
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Figure 3. M I D P response of N (ca. 1.6 X M ) in S D S micelles a t 1.2 K monitored a t 473 nm followed by microwave fast passage through the zero-field transition: (a) D E ; (b) 2E, with steady-state excitation of 10 s; (c) D E ; (d) 2E, with pulsed excitation of 0.1 s. The SDS concentration is 8.0 X lo-* M in water.
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plications due to acceptor aggregation and T T ET between acceptor molecules occupying different sites. Measurements were made under identical conditions for N in SDS in the absence of B to ensure a good control. The experiments were performed at the lowest pumped liquid helium temperature obtainable of ca. 1.2 K to minimize the effects of spin-lattice relaxation (SLR) on individual sublevel decay kinetics, while monitoring the naphthalene phosphorescence origin at 473 nm. MIDP responses were observed for the T, T, and T, Tytransitions of N after a long (10 s) and short (0.1 s) excitation time in order to obtain equilibrium steady-state population ratios within the sublevels as well as the "instantaneous" relative sublevel populating rates, respectively. In the latter instance, nonequilibrium MIDP20 was applied with minimum delay after the emission shutter opened to minimize the effects of S L R which relaxes the spin alignment of the sublevels. Figure 3a-d shows the MIDP response of N in S D S micelles for the ID + E( and 21EI transitions for steady-state and pulsed excitation conditions. Measurements by Six1 and S ~ h w o e r e r have *~ shown that k,' > kyl > k i , where k,' is the radiative rate constant of sublevel u, while the lifetimes are in the order of T, < T, < T,. The positive (increase of relative phosphorescence) response observed for both transitions under steady-state conditions (Figure 3a,b) indicates that N," > N," and Nyo > N,", where Nua is the steady-state population of sublevel u . This population pattern is consistent with the positive slow passage O D M R responses observed for N in SDS micelles (Figure 2), since the change in phosphorescence intensity accompanying a microwave slow passage through any two triplet sublevels u and v is given by AI,,u 0: (Q, - Qu)(N," - N u " ) , where Q, = k,'/k, represents the radiative quantum yield of sublevel u; k , is the total sublevel decay rate constant of sublevel u . For pulsed excitation of sufficiently short duration (t,,,
