J. Phys. Chem. 1985, 89, 1897-1901
1897
Phosphorescence of Polynuclear Aromatic Hydrocarbons in Heptakis(6-bromo-6-deoxy-~-cyclodextrin) R. A. Femia and L. J. Cline Love* Department of Chemistry, Seton Hall University, South Orange, New Jersey 07079 (Received: September 1 1 , 1984)
Room-temperature phosphorescence spectra and excited-statelifetimes are reported for several polynuclear aromatic hydrocarbons (Br-P-CD) (synthesized by replacing @-CD'sprimary included within the cavity of heptakis(6-bromo-6-deoxy-~-cyclodextrin) hydroxyls with bromine). Appreciable phosphorescence only occurs when the lumiphor is shielded from quenchers by the cyclodextrin torus and are within the realm of influence of heavy atoms. By locking the heavy atom into a fixed position on the cyclodextrin molecule, the observed phosphorescence and dynamic interactions depend primarily on the entrance and exit rate constants of the lumiphor from the cyclodextrin, simplifying the kinetic scheme and data interpretation. A solvent mixture of N,N-dimethylformamide (DMF) and water was employed due to the extreme water insolubility of heptakis(6bromo-6-deoxy-@-cyclodextrin),and to assess the degree of bulk solvent hydrophobicity on the inclusion process. This latter effect was monitored by differences in phenanthrene's Br-@-CDspectral profile relative to that in aqueous unbrominated @-CD/1,2-dibromoethane and by fluorescence peak ratio variations as a function of the DMF/water ratio. Luminescence lifetime values provided indirect information on the relative exit-to-entrance rate constant ratios. The optimum DMF/water ratio for production of maximum phosphorescence intensity was 4/1, and the optimum concentrationof brominated @-cyclodextrin in the 4/1 DMF/water solvent was 0.01 M.
Introduction
Spectroscopy of the triplet state of polynuclear aromatic (PNA) molecules has received considerable attention in recent years. Documented techniques for achieving stable room-temperature phosphorescence (RTP) include cyclodextrins,' solid s ~ b s t r a t e , ~ , ~ micelle ~ t a b i l i z e d , sensitized,6 ~.~ and colloidal/microcrystalline.7 Recent work has centered on characterizing the cyclodextrin inclusion process and ascertaining requirements for phosphorescence emission of molecules included into a-,B-, and y-cyclodextrin cavities using 1,Zdibromoethane (DBE) as a source of heavy atom to enhance the rate of intersystem crossing to the triplet manifold and/or triplet emission. Cyclodextrin room-temperature phosphorescence (CD-RTP) was found to have some advantages over previously examined techniques for generating fluid solution RTP, such as higher resolution of vibronic structure compared to micelle stabilized (MS-RTP)' and partial insensitivity to quenching by oxygen.'*5 In CD-RTP, phosphorescence is thought to occur due to formation of a trimolecular complex consisting of C D (a,B, or y) with the phosphor and a molecule of DBE, both of which are included into the nonpolar C D torus.' For triplet emission to occur, the bromo compound and the P N A have to be within the cavity during approximately the same time frame in order to interact favorably, producing RTP,'ss but it was not determined which compound entered the C D cavity first, or if both the PNA and haloalkane entered at the same time by first forming a weakly associated dimer. An equilibrium model based on the different possible pathways for formation of the trimolecular complex was proposed. A much simpler kinetic model involves fixing the heavy atom directly on the C D carbon skeleton, thus minimizing alternate pathways for complex formation. Previous reports indicated that replacement of the primary hydroxyls, which point out into the bulk solution from the narrower end of the CD, by selective halogenation with bromine, was the simplest and most reasonable (1) Scypinski, S.; Cline Love, L. J . Anal. Chem. 1984, 56, 322. (2) Roth, M. J. Chromatogr. 1967, 30, 276. (3) Ford, C. D.; Hurtubise, R. J. Anal. Chem. 1980, 52, 656. (4) Cline Love, L. J.; Skrilec, M.; Habarta, J. G. Anal. Chem. 1980, 52, 754. ( 5 ) Skrilec, M.; Cline Love, L. J. Anal. Chem. 1980, 52, 1559. (6) Donkerbroek, J. J.; Gooijer, C.; Velthorst, N. H.; Frei, R. W. Anal. Chem. 1982, 54, 891. (7) Weinberger, R.; Cline Love, L. J. Spectrochim. Acta, Part A 1984, 40A, 49. (8) Zander, M. "Phosphorimetry"; Academic Press: New York, 1968; p 20.
0022-3654/85/2089-1897$01.50/0
replacement scheme for halogenating C D S . ~ " RTP experiments with this type of derivatized CD permit study of the fixed bromine groups' capability of exerting a heavy atom effect and provide an alternate kinetic model for interpreting spectroscopic results. This paper discusses room-temperature phosphorescence and fluorescence spectra and excited-state lifetimes of PNAs included into the heavy atom substituted P-cyclodextrin. Resolution of the vibronic fine structure is retained, but not the partial insensitivity to oxygen quenching characteristic of CD-RTP. More stringent solvent requirements are discussed, and luminescence lifetimes as well as fluorescence band ratios for phenanthrene are used to evaluate indirectly the degree of inclusion for solvents of varying DMF/H,O ratios. Experimental Section
Synthesis of Hepta kis(6-bromo-6-deoxy-~-cyclodextrin). The method used for the synthesis of bromo-substituted P-cyclodextrin was that of Takeo, Sumimoto, and Kuge.Io Other methods using a key intermediate formed from the reaction of p-toluenesulfonyl chloride with the primary hydroxyls on the cyclodextrin torus were found to have shortcomings such as production of a mixture of unequally substituted sugars and substitution of chlorines on the primary hydroxyls. This required use of extensive thin-layer chromatography to isolate the individual sugars, with low yield, on the basis of differences in the degree of tosylation and was rejected for the present study. Selective bromination based on an intermediate formed from tritylation of primary hydroxyls was also rejected due to the impossibility of tritylating all of the primary hydroxyls of 0-CD because of severe steric overlap of the bulky substituents between adjacent glucose units.I0 The method chosen uses methanesulfonyl bromide, formed by reaction of the chloride species with Br,I2 in N,N-dimethylformamide (DMF), to selectively replace the primary hydroxyls on the 0-CD ring.I0 The D M F solvent (Aldrich, gold label) was distilled at reduced pressure and stored over barium oxide (Fisher) to prevent adsorption of water which must be absolutely excluded to eliminate solvolysis reactions of the starting materiaLg The method is straightforward except during the recrystallization of the final product from cold water (IO% up to 20%) is required to enhance the degree of phosphor inclusion into the brominated 0-CD via hydrophobic repulsion. The reason for increasing phosphorescence intensity with increasing % D M F up to about 80% is not clear. A minimum amount of water is required, but larger amounts adversely affect the intensity. In contrast, the phosphorescence intensity in the 100%aqueous P-CDIDBE system was more intense than that of all the Br-PCD/solvent systems studied. Also, phenanthrene's phosphorescence was completely quenched by oxygen in the mixed solvents compared to the unsubstituted j3-CDIDBE aqueous system. Fluorescence Band Ratios. Lumiphors which exhibit phosphorescence in the Br-/3-CD have fluorescence band ratios which
1900 The Journal of Physical Chemistry, Vol. 89, No. 10, I985 L
5.24E 05 I
IV
i
Femia and Cline Love TABLE II: Phosphorescence Lifetimes of 2.6 X lo4 M Phenanthrene for Solutions of Varying DMF/Water Ratios with 0.01 M Br-P-CD
DMF/water
phosphorescence lifetime," ms
113
2.5 f 0.5 0.91 f 0.07 1.6 f 0.2 0.20 f 0.04
111 312 4/ 1 ..
c
a
H
O.04E
OS 325. DO
387.50
Wave 1 e n g t h [ nm I Figure 4. Comparison of fluorescence emission of 1.2 X anthrene in Br-@-CD/4:1DMF/water (-) and in DMF
'
45n.,&
M phen( 0 ) . Note
increase in intensity and spectral profile changes upon complexation with the Br-S-CD. Instrument conditions the same as in Figure 1.
differed substantially from systems that did not phosphoresce. Table I shows representative fluorescence band intensity ratios for phenanthrene in various solvent systems. The ratios are defined as [A] = III/I, [B] = IV/I, and [C] = A/B, and the exact wavelengths are given in the Figure 1 caption. Solvents in which phenanthrene phosphoresced appreciably have C values of approximately 1.3. All systems which did not phosphoresce had C values of approximately 1S,with the exception of the pure water solvent system. The similarity of the measured C values for the latter systems suggests a common polarity-type solvation sphere in each of the systems which prevents population of the triplet state or effectively depopulate it via nonradiative pathways. In pure water the phenanthrene fluorescence is weak, with a C value of 0.99. Upon addition of /3-CD, the intensity increased an order of magnitude, the spectral profile changed drastically (note changes in A and B band ratios), and the C value rose to 1.52. This results from the fluorophor being transferred to a solvation site in the @-CDcavity more favorable to singlet-state emission than the bulk aqueous solvent. In pure DMF, the phenanthrene fluorescence C value is 1.53, very similar to that of the aqueous /3-CD system, and the spectral intensity, but not the profile as noted above, is also comparable. These results show that both D M F and the @-CDcavity provide stabilization of the excited state, but by two different microenvironments. When Br-@-CDis added to the DMF/phenanthrene system, there is no discernible change in the fluorescence profile, showing no inclusion occurring. Only a slight increase in intensity is observed, and the Cvalue remains essentially constant, indicating that the fluorophor remains in the equally favorable D M F environment. Siege1 and Breslow have noted that nonaqueous solvents containing P-CD and a probe would form much weaker inclusion complexes than would aqueous 0-CD solutions, certainly too weak to support a stable, emitting triplet state species.22 Table I also gives C values for phenanthrene dissolved in dioxane, both with and without 8-CD present. This solvent was examined because it has been determined that its polarity most closely approximates the polarity of the p-CD interior." Both systems have similar C values of 1S , the same as the other solvent systems which do not support phosphorescence (with the exception of water). Addition of @-CD to a dioxane solution containing phenanthrene produced no changes in fluorescence intensity or relative spectral profile, giving no evidence for formation of an inclusion complex. Addition of water to the Br-P-CDIDMFlphenanthrene system drives the lumiphor into the C D cavity, most probably because the water introduces a hydrophobic repulsion between the phenanthrene and bulk water. This is supported by the appearance of phosphorescence and changes in the fluorescence spectral profile (Table I) relative to the same system without added water. However, the phosphorescence is weak, with fluorescence being
Average of three determinations f standard deviation.
the dominant process as was seen in Figure 1. Figure 4 compares the fluorescence profiles of phenanthrene in pure D M F and in 4/1 D M F / H 2 0 systems containing Br-@-CD. The concentration of Br-@-CD producing maximum phosphorescence intensity from phenanthrene was determined over the range 0.001-0.015 M in the 4/1 D M F / H 2 0 solvent system. The 0.01 M Br-8-CD concentration produced the most favorable results, with larger concentrations giving considerably turbid solutions, diminishing the observable phosphorescence intensity. Luminescence Lifetime Studies. Solute-micelle dynamics can serve as a model for Drobable solute-CD dvnamics that can alter the observed excited triplet-state lifetime: T . An equation describing the numerous deactivation processes occurring in solution (based on ref 23) is given below
and shows that the observed phosphorescence lifetime depends on (a) rate constants for exit from the CD, (k-), radiative deactivation within the CD, (kmp),and outside of the CD, (k,); (b) products of the concentration of quencher within the CD and its quenching rate constant, (ksin[Qlin),and the concentration of external bulk solvent quencher and its quenching rate constant, (k,[Q],), and the product of the entrance rate constant of solute into the C D and the concentration of CD, (k+[CD]). Radiationless deactivation pathways of the phoshor are contained within the k,, and k, terms. The D M F bulk solvent effectively serves as the quencher, [Q],,, and as the quencher, [Q],,, if it is also included within the C D cavity. In the absence of included quencher and at a constant C D concentration, the observed phosphorescence lifetime should depend primarily on the exit rate, k-, of phenanthrene from the C D and on the intrinsic phosphorescence lifetime of phenanthrene within the CD. To observe appreciable phosphorescence intensity, the entrance rate constant should be significantly larger than the exit rate constant, and the rate constant for phosphorescence, k,,, should be large compared to the exit rate. One would expect the entrance rate to increase and the exit rate to decrease as the water content of the bulk solvent increases due to the greater repulsion of phenanthrene from the bulk solvent. Also, reducing the [DMF] would reduce the contribution of the k,,,[DMF],, term. Thus, the D M F / H 2 0 solvent mixtures generally should have longer observed phosphorescence lifetimes as the water content is increased, in the absence of secondary effects. This was confirmed experimentally. Table I1 lists the triplet-state lifetimes obtained from various DMF/water solvent mixtures containing phenanthrene and Brp-CD. The mixtures with longer phosphorescence lifetimes had weaker emission intensities. The most intensely phosphorescent mixture had the least water (4/1) and shortest phosphorescence lifetime (0.20 ms), and the least intensity was observed from the mixture with the most water (1/3) and longest lifetime (2.5 ms). The relative lifetime values could be rationalized by exit rates which are larger than emissive rates in the 4/1 system causing appreciable quenching of phenanthrene by the bulk D M F before it can emit; however, the relative intensities are opposite to what is expected on the basis of relative solubilities and observed lifetimes of phenanthrene in the 4/ 1 and 1/3 bulk solvent systems. Obviously secondary effects are present which prevent a simple explanation. The CD may offer different protection of the phosphor from quencher contaminant influx in the differnt solvent
The Journal of Physical Chemistry, Vol. 89, No. 10, 1985 1901
Polynuclear Aromatic Hydrocarbons TABLE III: Fluorescence Lifetimes of 2.6 X lo-” M Phenanthrene in Various Solvents
solvent
fluorescence lifetime,”ns
0.01 M Br-@-CDIDMF 0.01 M Br-@-CD/1:3DMF/water 0.01 M Br-@-CD/l:l DMF/water 0.01 M Br-@-CD/3:2DMF/water 0.01 M Br-@-CD/4:1DMF/water 0.01 M @-CDIDMF 0.01 M @-CDlwater
1.23 f 0.06 2.4 f 0.1 9.1 0.4 9.2 f 0.4 11.1 f 0.5 10.3 f 0.3 8.5 f 0.4 24.9 f 1
DMF
a
*
Average of three determinations f standard deviation.
TABLE IV: Excitation and Phosphorescence Emission Wavelengths (nm) of Polynuclear Aromatics in Br-b-CD compd
excitation wavelength
emission wavelength
naphthalene acenaphthene fluorene dibenzofuran dibenzothiophene phenanthrene pyrene
290 309 306 299 300 298 335
514 522 470 422 439 505 595
systems. Alternatively, the weak intensity may be the result of low population of the triplet by the Br-fl-CD 1/3 solvent system. Table 111 lists the fluorescence lifetimes of phenanthrene in various solvent mixtures. Addition of Br-8-CD to a DMF/ phenanthrene solution resulted in a small increase in the fluorescence lifetime. However, addition of water to this system substantially increased the lifetime, indicating effective inclusion due to the hydrophobic repulsion of phenanthrene from water. The longest lifetime value of 24.9 ns was observed from phenanthrene dissolved in 0.01 M aqueous unsubstituted P-CD. This value decreased to 8.5 ns for unsubstituted j3-CD in pure DMF, and further decreased to 2.4 ns for the Br-j3-CD in pure DMF. Probably the presence of a pure water bulk phase causes the fluorophor to reside, on the average, in the C D cavity for a longer time, and the addition of seven bromo moities decreases the strength of the inclusion process along with depopulating the excited singlet state via the heavy atom effect. The lifetimes were not significantly different going from 25% to 80% DMF. Thus, it is difficult to separate the influence of the bulk solvent (DMF or water) from the influence of unsubstituted and substituted CDs on the formation of an inclusion complex. However, it is clear that the presence of water allows for a more stable inclusion complex. Table IV lists the polynuclear aromatic molecules studied which gave observable phosphorescence. The lack of phosphorescence from molecules like chrysene can be explained simply because the Br-0-CD cavity is too small for good inclusion of bulky molecules of this sort. Previous studies found that chrysene phosphoresced only in the larger 7-CD.l Two molecules, biphenyl and tri-
phenylene, which were phosphorescent in aqueous 8-CD, were found to be nonphosphorescent in Br-B-CDIDMF. Both of these molecules have discontinuous *-electron structures, not delocalized over the entire structure. For example, biphenyl exists in a twisted, nonplanar configuration in the ground state, with less interactions between the two rings prior to e~citation.2~3~~ The electrical center of gravity of these smaller aromatic systems should be more easily approachable by the D M F solvent and have stronger interactions to retain them in the bulk solvent over the C D interior. Naphthalene, with a fully aromatic two-ring system less susceptible to D M F solvent interactions compared to biphenyl, did exhibit phosphorescence, probably because of formation of a more stable CD-inclusion complex. Triphenylene belongs to the class of condensed PNAs which Clar et al. postulated in 1958 to have a a-electrons not uniformly distributed over the entire molecule, with charge densities located in groups of six on the outer rings.26 The central ring is actually not aromtic, so little, if any, electronic communication occurs between the three aromatic rings. It would be expected that triphenylene would interact more strongly with the D M F solvent than would fully aromatic, similarly sized molecules, causing a lesser extent of inclusion with the CD/DMF system compared to the aqueous j3-CD. These studies indicate that the heavy atom effect is capable of exerting its influence either through attachment of bromine to the fmed carbon skeleton of the P-CD complexed to a lumiphor or from the close approach of the lumiphor to the bromo-substituted, narrow end of the CD. The observation of phosphorescence in a bulk solvent containing DMF, which normally quenches the triplet state, by means of inclusion into a 0-CD molecule, is a marked example of the power of cyclodextrin complexes to provide unique and interesting spectroscopic data. The effect of the composition of the bulk solvent on luminescence lifetimes indicates that relative solubilities of the lumiphor in the bulk solvent and the solvent’s quenching ability are important in determining the probability of triplet emission.
Acknowledgment. We thank Rodney J. Woods for his help in acquiring the luminescence lifetime values, Stephen Scypinski for helpful discussions, Matt Petersheim for the use of his laboratory facilities for the synthesis of Br-j3-cyclodextrin and for assistance in interpreting the N M R spectrum, and Chou Tann of Schering Corp. (Union,NJ) for obtaining the N M R data. This work was supported in part by a grant to L.J.C.L. from the National Science Foundation, Grant No. CHE-8216878. This work was presented in part at the 1985 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, New Orleans, LA, Feb 25-Mar 1, Abstract No. 954. Registry No. Br-@-CD,53784-83-1; naphthalene, 91-20-3; acenaphthene, 83-32-9; Fluorene, 86-73-7; dibenzofuran, 132-64-9; dibenzothiophene, 132-65-0 phenanthrene, 85-01-8; pyrene, 129-00-0. (24) Tinland, B. J . Mol. Srrucr. 1969, 3, 161. (25) Gropen, 0.; Seip, H. Chem. Phys. Letr. 1971, 11, 445. (26) Zander, M. “Phosphorimetry”; Academic Press: New York, 1968; p 71-5.
