Mixed organized media: Effect of micellar-cyclodextrin solutions on the phosphorescence of phenanthrene

Mixed Organized Media: Effect of Micellar-Cyclodextrin Solutions on the Phosphorescence of Phenanthrene ROBERT A. FEMIA AND L. J. CLINE LOVE ~ Seton Hall University, Department of ChemisttT, South Orange, New Jersey 07079 Received January 17, 1985; accepted March 29, 1985 Intense room-temperature phosphorescence (RTP) from phenanthrene occurs when it is included within the/3-cyclodextrin 03-CD) cavity in the presence of 1,2-dibromoethane (DBE). The phosphorescence spectral profile of phenanthrene in /3-CD exhibits considerably higher vibronic resolution than that observed in 70:30 Na/T1 dodecyl sulfate (Na/T1DS) micellar solutions, and the wavelength maximum is blue-shifted by 7 nm. Below the critical micelle concentration (CMC), in a mixed system of ~-CD: DBE and Na/T1DS, phosphorescence is observed characteristic of that induced by cyclodextrin. Upon addition of more surfactant, but still below the CMC, the phosphorescence intensity from the/3-CD microenvironment is enhanced. This is interpreted in terms of favorable effects produced by aggregation of the surfactant monomers at the open ends of the/3-CD torus and/or partial inclusion which serves to reduce phenanthrene-water contact. This increases the hydrophobicity of the phosphor's microenvironment and further shields it from quenching effects. At surfactant concentrations above the CMC, phenanthrene is transferred from the/3-CD to a micellar environment, as evidenced by sharp changes in its phosphorescence spectral profile, wavelength maximum, and sensitivity to quenching by oxygen. Results indicate that DBE also prefers to reside in the micellar aggregate over/3-CD, and this severely diminishes micelle-induced RPT by heavy-atom quenching. © 1985AcademicPress,Inc. INTRODUCTION

Room-temperature phosphorescence from polynuclear aromatic compounds (PNAs) has been observed in/3-cyclodextrin solution, both in the presence and absence of a heavy-atom species such as 1,2-dibromoethane (DBE) (14), and in surfactant systems consisting of 70: 30 Na/T1 dodecyl sulfate (Na/T1DS) mixtures (5, 6). More recently, room-temperature phosphorescence (RTP) in fluid solution from various types of phosphors has been documented as occurring in mixed anionic/nonionic surfactant systems (7). For systems consisting of cyclodextrin and surfactants at or above the critical micelle concentration (CMC), Edwards and Thomas proposed that solubilization of pyrene in a mixed media system offl-CD and sodium dodecyl sulfate (SDS) will preferentially occur in I TO whom correspondence should be addressed.

the micelle based on differences in the fluorescence band ratios and fluorescence lifetimes of pyrene in the two separate media (8). Also, it was found that addition of surfactant monomers below the CMC to a solution of the bimolecular complex of pyrene:/3-CD enhanced pyrene's fluorescence emission. It was postulated that the surfactant monomers aggregate at the open ends of the/3-CD torus, effectively "capping" the system and reducing the degree of fluorphore-water interaction. This provides a more hydrophobic solvation microenvironment for pyrene which produces enhanced singlet state emission. Addition of more surfactant, either at or above the CMC, caused migration of the fluorophore from the /3-CD to the miceUar aggregate, as evidenced by a 'change in the fluorescence band ratios and lifetimes of the incorporated species. This paper documents the effects of surfactant concentration both below and above the CMC on the triplet state properties of a tri271

Journalof Colloidand InterfaceScience,Vol. 108,No. 1, November1985

0021-9797/85 $3.00 Copyright© 1985by AcademicPress,Inc. All rightsof reproductionin any formrec.-.-.-.-.-.-.-.-~ved.

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molecular complex consisting of phenanthrene, ~-CD, and DBE. Surfactant concentrations well below the CMC produce an enhancement of phenanthrene's phosphorescence intensity which is thought to be the result of sequestering of the open ends of the CD torus by the hydrophobic tails of the surfactant monomers. The binding of surfactant hydrophobic chains to cyclodextrins has been reported by Okubo et aL (9), who observed larger micelle CMCs in the presence of CD due to a decrease in available surfactant monomers. Above the CMC of the thallium/sodium counterion surfactant, the phosphorescence emission profile and wavelength maxima shift from that of the phosphor:C/-CD:DBE complex to that ofphenanthrene solubilized by the Na/ T1DS micelle, with a concomitant decrease in intensity due to quenching by the cosolubilized DBE in the micelle. The preference for a micellar environment over the CD is in agreement with a recent report comparing sensitized RTP in/3-CD and micelles which showed that the donor-acceptor pair interaction was significantly enhanced in micellar medium over that in/3-CD, presumably because of the micelle's greater solubilization power (10). The lumiphor can be dislodged from the micelle and forced back into the CD by physically agitating the solution. If fl-CD is added to a micellar solution containing DBE and phenanthrene, and the solution vigorously shaken, a fluorescence intensity enhancement is observed due to transfer of some of the lumiphor to the/3-CD cavity. Similar fluorescence intensity enhancements have been observed when 0.01 M ~- or 3'-CD is added to aqueous solutions of 1-anilino-8-naphthalenesulfonate (ANS), presumably due to transfer of the ANS from the aqueous medium to the apolar CD cavity (11). However, in the present experiments, the fluorescence enhancement is lost upon standing for 1 h, indicating a return of phenenthrene to a micellar environment from the/3-CD. The phosphorescence spectral profile and intensity, characteristic of a micellar environment quenched by DBE, remains unchanged in the shaken solution and Journal of Colloid and Interface Science, Vol. 108, No. 1, November 1985

the reequilibrated one. This suggests that the disturbance of equilibrium by shaking does not force the DBE back into the/3-CD cavity since no phosphorescence characteristic of the CD: haloalkane environment is observed. Thus, preference for a micellar environment by the DBE appears to be even greater than that of phenanthrene. MATERIALS

AND

METHODS

Reagents. Spectroscopic grade methanol and acetone, and thallous nitrate (all from Fisher), and gold label 1,2-dibromoethane (Aldrich) were used as received. Sodium dodecyl sulfate (Bio-Rad) was recrystallized from boiling water containing Darco decolorizing carbon, and the solution was hot-filtered through a Rainin 0.45-/~m-pore-diameter nylon filter. Synthesis of the 70:30 Na/T1 dodecyl sulfate is described elsewhere (5, 6)./3-Cyclodextrin (Aldrich) was recrystallized from boiling water. All water was deionized and triply distilled. Phenanthrene was of ultrahigh purity from a PNA kit from Chem Services, Inc., West Chester, Pennsylvania. Apparatus. A SPEX fluorolog 2 + 2 spectrofluorometer (SPEX Industries, Metuchen, N. J.) with double excitation and emission monochromators (spectral bandpass of 1.8 rim/ram) equipped with a 450-W Xe continuous light source and a Peltier-cooled Hammamatsu R928 photomultiplier, was used to acquire all luminescence information. The data was acquired and stored on a SPEX Datamate computer with dual floppy disks interfaced directly to the fluorometer. Hard copies were obtained using a Houston Instruments digital x-y plotter. All emission spectra reported were corrected for lamp intensity variation and photomultiplier tube response characteristics. Procedure. Experimental procedures for induction of phenanthrene RTP in/3-CD:DBE and 0. l M 70:30 Na/T1DS micelles have been documented previously (I, 2, 5, 6). All samples were prepared by pipetting the appropriate quantity of a 0.01 M phenanthrene stock so-

MICELLE-CYCLODEXTRIN-ENHANCED PHOSPHORESCENCE lution into 10-ml volumetric flasks resulting in 2.5 × 10 -5 M phosphor. For studies of varying surfactant concentration in 0.01 M/3CD, the 70:30 Na/T1DS surfactant was added directly to the volumetric flask in the concentration range from 0 to 0.1 M. The effect of cyclodextrin on the micellar phosphorescence was studied by dissolving phenanthrene in 0.1 M Na/TIDS, followed by addition of DBE to give a 0.58 M concentration and equilibration for 1 h. The desired quantity of/3-CD was then added and the solution reequilibrated for another hour. This was repeated for/3-CD concentrations from 0 to 0.01 M. The effect of DBE on micellar phosphorescence was carried out by dissolving phenanthrene in 0.1 M Na/ TIDS and equilibrating for 1 h, followed by addition of varying amounts (0.0115 to 0.58 M) of DBE and equilibration for an additional hour. The solutions contained in cuvettes were deaerated for 15 min with high-purity nitrogen passed through a indicating oxygen trap (Alltech Associates, Inc.), then the cuvettes were sealed with Teflon stoppers. RESULTS AND DISCUSSION The differences in the phosphorescence spectral profiles of phenanthrene in 0.01 M :3CD:DBE and in 0.1 M 70:30 Na/T1DS micellar solution are evident in Fig. 1. The/3-CD room-temperature phosphorescence (CDRTP) has more highly resolved vibronic structure than that observed using the micellar medium, and the wavelength maximum is blueshifted at 501 nm compared to that of 508 nm for micelle-stabilized r o o m - t e m p e r a t u r e phosphorescence (MS-RTP) of phenanthrene. Differences in vibronic resolution and wavelength maxima enable one to determine which microenvironment the phenanthrene is preferentially solubilized at the time of triplet state emission. Oxygen sensitivity also provides information on the microenvironment, as CDRTP is partially insensitive to quenching by oxygen (1, 2), whereas MS-RTP is completely quenched by dissolved oxygen (5, 6). Figure 2 illustrates the enhancement in phenanthrene's phosphorescence intensity

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when 70:30 Na/T1DS surfactant is added to a fl-CD:DBE solution at concentrations below the CMC. The surfactant monomers appear to preferentially bind with the CD complex, producing a complex aggregate structure (8, 9). These data suggest that the aggregation phenomenon around the trimolecular CD complex produces a more favorable environment for phosphorescence emission from phenanthrene in the/3-CD cavity. Although the exact mechanism for this enhancement is unclear, the phosphor is probably better protected from the contact with the bulk water, reducing quenching pathways. Thus, as the aggregation of surfactant monomers about the CD increases with surfactant concentration below the CMC, the hydrophobicity of the phosphor's environment increases, resulting in a more favorable solvation sphere that stabilizes the triplet state. At concentrations of Na/TIDS near and above the CMC, there is a significant decrease in phenanthrene's/3-CD:DBE induced phosphorescence intensity compared to a pre-CMC concentration of 0.001 M surfactant (Fig. 3). At the higher concentration of 0.05 M, premicellar aggregates have formed which reduce the number of surfactant monomers available for association with the/3-CD torus and which Journal of Colloid and Interface Science, Vol. 108, No. 1, November 1985

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FIG. 4. Phosphorescence spectra of phenanthrene in a mixture of 0.1 M Na/T1DS, 0.01 M/3-CD, and 0.58 M DBE in an aerated solution (dashed line) and a deaerated solution (solid line). Emission from a micellar environment evident from wavelength maximum at 508 nm and loss of fine structure. In the presence of dissolved oxygen there is complete quenching of MS-RTP. Other conditions the same as in Fig. 1.

solubilize the phenanthrene and/or DBE. Because the CMC is raised by the fl-CD, these premicellar aggregates apparently are not organized enough to induce MS-RTP, but are capable of extracting the phenanthrene and/ or DBE from the 6-CD. At a surfactant concentration of 0.05 M, no phosphorescence characteristic of either medium is observed; at 0.1 M surfactant, micellization has occurred and weak MS-RTP emerges.

Figure 4 illustrates the effect of quenching of the phosphorescence spectrum by dissolved oxygen observed for phenanthrene dissolved in a mixture of 0.01 M fl-CD:DBE and 0.1 M 70:30 Na/T1DS. The upper spectrum with no fine structure in its lowest energy bands is that from the micellar medium, and it is completely quenched if the solution is aerated. Although this portion of the spectrum is clearly that characteristic of MS-RTP, it is diminished

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MICELLE-CYCLODEXTRIN-ENHANCED

in intensity compared to that in purely micellar media. No CD-RTP is observed in the aerated solution because the phosphor is preferentially solubilized in the micellar aggregates. At concentrations of surfactant below the CMC, phenanthrene CD-RTP is observed in aerated solution indicating it is solubilized in the fl-CD. The quenching effect of increasing concentrations of DBE on the phosphorescence intensity of phenanthrene dissolved in the micellar medium without/~-CD present is illustrated in Fig. 5. At 0.58 M 1,2-dibromoethane, a drastic reduction in phosphorescence intensity is found, indicating strong solubilization of the nonpolar DBE by the hydrophobic portions of the micellar assembly. Thus, the low triplet emission intensity also observed in Fig. 4 is most probably the result of increased radiationless deactivation ofphenanthrene's excited states by the high concentration of haloalkane heavy atoms solubilized by the micelle in the proximity of the phosphor. Variation of the concentration of/3-CD from 0 to 0.1 M in solutions containing 0.1 M 70:30 Na/T1DS, 0.58 DBE, and 2.5 × 10-5 M phenanthrene produced no changes in the phosphorescence spectral profile; the spectra remained identical to that in micellar medium with DBE. Unusual changes in the fluorescence intensity of phenanthrene dissolved in the mixed media system with surfactant present at concentrations above the CMC were observed when the solutions were shaken. Phenanthrene's fluorescence intensity slightly increased immediately after agitation, while the phosphorescence spectral intensity characteristic ofa micellar microenvironment remained unchanged. Figure 6 shows this fluorescence enhancement effect for a sample that was shaken and the spectrum immediately recorded. When the same sample was allowed to sit for 1 h after shaking, the fluorescence intensity obtained was identical to that recorded prior to agitation. Thus, although the migration of phenanthrene between the/3-CD to the micelle is reversible if the micellar assembly is momentarily disrupted, releasing

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some of the lumiphor into the bulk phase where it can encounter and include into a CD species, the equilibrium favors association of the phosphor with the micellar aggregate. The possibility of incomplete solubilization of the probe present as microcrystals on the walls of the cuvette can be discounted based on two reasons. First, the concentrations of surfactant (0.1 M) and CD (0.01 M) are sufficiently large compared to the concentrations of lumiphor (down to 1 × 10-6 M where the effect is still observed), that complete solubilization can be assumed. Second, in other studies in our laboratory, we have studied fluorescence and phosphorescence phenomena of microcrystals and found for phenanthrene that its phosphorescence was considerably blue-shifted and had much higher resolution compared to its micelle-induced phosphorescence (12). Also, its microcrystalline fluorescence exhibits severe self-absorption of its O-O band, and a redshifted excitation spectrum compared to those in a methanol environment (13). None of these features characteristic of phenanthrene's microcrystalline luminescence were observed in the present study; only features characteristic of micelle- or CD-induced luminescence were found, discounting incomplete solubilization. Similar studies with 1,2-dibromoethane revealed different behavior. If agitation of the Journal of Colloid and Interface Science, Vol. 108, No. 1, November 1985

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solution drives micelle-associated species into aphthene, fluorene, dibenzothiophene, dibenthe t3-CD, then emergence of CD-RTP should zofuran, and biphenyl. Above the CMC, the occur if shaking also drives the DBE, along mieeUe attraction for the hydrophobic phoswith the phenanthrene, into the/3-CD to re- phor is much greater than that of the CD, and form the trimolecular complex. However, no the 13-CD:DBE:phosphor trimolecular comfl-CD-induced triplet state emission was ever plex is broken up by solubilization of DBE observed, despite vigorous shaking of the and phosphor into the miceUar aggregates. sample, suggesting that DBE much prefers to However, the MS-RTP intensity which reside, microscopically, with the micelle. emerges is weak due to the associated heavy A rationale for why phenanthrene and DBE atom-containing alkane. Interestingly, inprefer to associate with the micellar microen- creases in/3-CD concentration did not meavironment can be given based on the different surably affect the association of phosphor with mechanisms of micelle association dynamics the micelle, indicating that the miceUe-phosand 13-CD inclusion equilibrium. The cyclod- phor equilibrium constant must be considerextrin torus possesses considerably rigidity, ably larger than that of the CD complex. The and offers a similar solvation sphere for a wide dynamic nature of the micellar assembly is variety of compounds forming inclusion emphasized by its sensitivity to physical discomplexes (1, 2). In contrast, the micellar as- turbances such as agitation. Curiously, agitasembly, being composed of flexible hydrocar- tion temporarily increases the fluorescence inbon chains with varying degrees of disorder tensity suggestive of inclusion of some of the and possible water penetration, offers many phenanthrene into the CD, but no decrease in different types of solvation sites for different MS-RTP is seen. Some phenanthrene molesolubilizates based on individual solvation cules appear to be dislodged from the micelle, preferences (14, 15). An associated species can producing CD-enhanced singlet state emissheath itself with its own custom-made mi- sion; the lack of observable change in triplet croenvironment within the flexible hydrocar- state emission may be due to differences in the bon chains using Van der Waals attractions, quantum yields of the two states. For hydroa possibility not offered by the cyclodextfin. phobic species such as phenanthrene, the micelle provides a favored environment over that of the cyclodextrin. CONCLUSIONS The cyclodextrin and micelle organized media act in a synergistic fashion to enhance the phosphorescence intensity of phenanthrene at concentrations of surfactant below the CMC. This effect was also observed for several other PNAs. The monomers would be expected to associate preferentially with the cyclodextrin torus based on hydrophobie attraction, most probably at the wider end of the torus with the secondary hydroxyls, thus increasing the hydrophobicity experienced by the phenanthrene. Compounds observed to undergo redistribution into Na/T1DS micelles from ~-CD, based on wavelength shifts, oxygen sensitivity, resolution changes and quenching by DBE, were naphthalene, acenJournal of Colloid and Interface Science, Vol. 108, No. 1, November 1985

ACKNOWLEDGMENT This work was supported in part by a grant to L.J.C.L from the National Science Foundation, Grant CHE8216878. REFERENCES 1. Scypinski, S., and Cline Love, L. J., Anal. Chem. 56, 322 (1984). 2. Scypinski, S., and Cline Love, L. J., Anal. Chem. 56, 332 (1984). 3. Turro, N. J., Cox, G. S., and Li, X., Photochem. Photobiol. 37, 149 (1983). 4. Turro, N. J., Bolt, J., Kuroda, Y., and Tabushi, I., Photochem. Photobiol. 35, 69 (1982). 5. Cline Love, L. J., Skrilec, M., and Habarta, J. G., Anal. Chem. 52, 754 (1980).

MICELLE-CYCLODEXTRIN-ENHANCED PHOSPHORESCENCE 6. Skrilec, M., and Cline Love, L. J., Anal. Chem. 52, 1559 (1980). 7. Noroski, J., and Cline Love, L. J., in "Abstract of Papers," 35th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, N. J., March, 1984; Pittsburgh Conference: Pittsburgh, Pa., 1984; Abstr. 779. 8. Edwards, H. E., and Thomas, J. K., Carbohydr. Res. 65, 173 (1978). 9. Okubo, T., Kitano, H., and Ise, N. J., Ji Phys. Chem. 80, 2661 (1976).

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10. DeLuccia, F., and Cline Love, L. J., Anal. Chem. 56, 2811 (1984). 11. Bender, M. L., and Komiyama, M., "Cyclodextrin Chemistry," pp. 15-16. Springer-Verlag, New York, 1978. 12. Weinberger, R., and Cline Love, L. J., Appl. Spectrosc. 39, 516 (1984). 13. Weinberger, R., and Cline Love, L. J., Spectrochim. Acta Part A 40, 49 (1984). 14. Menger, F.,Acc. Chem. Res. 12, 111 (1979). 15. Cline Love, L. J., Habarta, J. G., and Dorsey, J. G., Anal. Chem. 56, 1132A (1984).

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