Anal. Chem. 1984, 56, 2811-2815
The intensity dependence of multiphoton absorption is P , n being the number of photons absorbed (2). These effects would not be expected to be important until the higher intensities since the cross sections for multiphoton transition are much lower than those of the single photon. The increase in signal with increasing pressures may be explained by the saturation phenomenon. With the high laser intensities that were used in this study, it is very likely that eq 14b describes the pressure-dependent focal length. The two pressure-dependent terms, dn/dT and p , are both directly proportional to pressure (IO). The pressure dependence of the TLS signal should then be due to the pressure dependence of I, only. The zero-time saturation signal does not give the maximum signal. But, the maximum signal trend should follow the zero-time signal. Since I , is proportional to the pressure, the pressure-dependent signal for intensities above I , should also increase with pressure.
CONCLUSION In conclusion, this study shows the viability of the use of high-energy COz lasers in TLS. In the future infrared TLS may be used to gain both structural and quantitative informations about analyte species and data. Infrared absorptions previously unaccessible to conventional IR spectrometers may be studied since TLS can measure absorbances < lo-’ (atm cm)-l. These things, along with the relative simplicity of TLS instrumentation, make this a very useful tool for studying future analytical problems. We are currently developing a more precise mathematical description of the saturation thermal lens and thermal deflection signals. Thermal deflection experiments under saturation conditions are also being performed. Preliminary results indicate that the detection limits for gas-phase infrared absorbers can be decreased by at least an order of magnitude
2811
over those of the saturation thermal lens experiments described here. Registry No. Dichlorodifluoromethane, 75-71-8.
LITERATURE CITED (1) Fang, H. L.; Swofford, R. L. In “Ultrasensitive Laser Spectroscopy”; Kiiger, D. S., Ed.; Academic Press: New York, 1983; Chapter 3. (2) Twarowski, A. J.; Kiiger, D. S. Chem. Phys. 1977, 20,253-258. (3) Barker, J. R.; Rotherm, T. Chem. Phys. 1982, 68, 331-339. (4) Carter, C. A.; Harris, J. M. Appl. Opt. 1984, 23,476-481. (5) Bialkowski, S. E. Chem. Phys. Lett. 1984, 104, 448-454. (6) Dovichi, N. J.; Harris, J. M. Anal. Chem. 1980, 52,2338-2342. (7) Carter, C. A.; Brady, J. M.; Harris, J. M. Appl. Spectrosc. 1982, 36, 309-314. (8) Carter, C. A.; Harris, J. M. Anal. Chem. 1983, 55, 1256-1261. (9) Long, M. E.; Swofford, R. L.; Albrecht, A. C. Science (Washington, D . C . ) 1976, 191, 183-185. (10) Bailey, R. T.; Cruikshank, F. R . ; Pugh, D.; Johnstone, W. J . Chem. SOC. Faraday Trans. 2,1980, 633-647. (11) Xing-Xiao, M.; Zhu-De, X. Chem. Phys. Lett. 1983, 9 6 , 563-565. (12) Siebert, D. R.; Grabiner, F. R.; Fiynn, G. W. J . Chem. Phys. 1974, 60, 1564-1574. (13) Mori, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1983, 55, 1075-1079. (14) Mayer, A.; Comera, J.; Charpentier, H.; Jausaud, C. Appl. Opt. 1978, 17, 391-393. (15) Sheldon, S. J.; Knight, L. V.; Thorne, J. M. Appl. Opt. 1982, 9 , 1663- 1669. (16) Madjar, C. V.; Parey, F.; Excoffier, J. L.; Bekassy, S. J . Chromatogr. 1981, 203,247-261. (17) Wood, R. R.; Gordon, P. L.; Schwarz, S. E. IEEEJ. Quantum. Electron. 1969, 10, 502-513. (18) Jackson, W. B.; Amer, N. M.; Boccara, A. C.; Fournier, D. Appl. Opt. 1981, 20,1333-1343. (19) Bacon, J. R.; Demas, J. N. Anal. Chem. 1983, 56,653-656. (20) Sveito, 0. “PrinciDles of Lasers”, 2nd ed.; Plenum Press: New York, 1982; pp 58-68.. (21) Fiygare, W. H. Acc. Chem. Res. 1968, 1 , 121-127,
RECEIVED for review June 22,1984. Accepted August 9,1984. This work was supported by a grant from Utah State University for faculty development. Much of the equipment was purchased through a grant from Research Corp.
Sensitized Room Temperature Biacetyl Phosphorescence via Molecular Organization Frank J. DeLuccia and L. J. Cline Love* Seton Hall University, Department of Chemistry, South Orange, New Jersey 07079
Room temperature sensitized blacetyl phosphorescence enhanced via molecular organlratlon Is observed for many aromatlc compounds. Micelles composed of sodium dodecyl sulfate (SDS) and hosts such as p-cyclodextrln (p-CD) are used to enhance the energy transfer reactlon by organlzing the reactants In close proxlmlty to one another. The trlplettrlpiet energy transfer of several polynuclear aromatics, nltragen heterocyclics, and heavy-atom-substltuted species to blacetyl was evaluated. Both SDS and p-CD were found to provide a superior medlum for production of sensltired blacetyl phosphorescence than found with homogeneous soiutlon. Typically, limlts of detectlon of 1 X lo-* to lo-’ M In SDS and 1 X IO-’ M In P-CD were obtained. The sensltivltles of the methods are dependent on the solublllty of the donor in SDS, the ablllty of the donor to fit Into the 0-cyciodextrln cavlty, and the background slgnal produced by the blacetyl blank solution.
Recent developments in solution chemistry have made it possible to observe triplet state emission from a large number
of polynuclear aromatic (PNAs) compounds in fluid solution at room temperature. Cline Love and co-workers have demonstrated that excited phosphorescent compounds associated with micellar assemblies (1,2) or included into cyclodextrin cavities (3, 4) will deactivate via triplet state emission. In addition, phosphorescence was shown to occur in colloidal suspensions where the insoluble solute interacts with its crystalline neighbor (5). Each of these techniques requires a high degree of molecular organization to stabilize the triplet state and induce room temperature phosphorescence (RTP). An alternate method for studying the triplet state processes of molecules is by energy transfer interactions. Sensitized phosphorescence involves triplet-triplet energy transfer according to Donkerbroek’s eq 1 (6)
where D is the donor molecule, A the acceptor molecule, and So and TI are the ground state and excited triplet state, respectively. Almgren and co-workers studied the energy transfer from triplet state aromatic hydrocarbons to Tb3+and Eu3+ions in
0003-2700/84/0356-2811$01.50/00 1984 American Chemical Society
2812
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
Table I. Excitation and Emission Sensitized Phosphorescence Characteristics and Limits of Detection (LODs) for Selected Polynuclear Aromatic Compounds in Micellar Solution, P-Cyclodextrin, and Acetonitrile compound biphenyl 4-bromobiphenyl fluorene dibenzofuran dibenzothiophene naphthalene 2-bromonaphthalene chrysene phenanthrene triphenylene
XEXpa
nm
272
278 305 295 295 276
280 295 295 295
bH,b
nm
476 d
468 449 450 513 519 513 501 432
SDS micelles 4.0 x 10-9 1.0x
10-9 8.9 x 10-9 3.3 x 10-8 2.7 X 4.6 x 10-9 1.9 x 10-9 6.9 x 10-9
4.6 4.8
X X
LOD; M P-cyclodextrin
acetonitrile
5.4 x 10-7 7.1
2.1 x 10“ 1.0 x 10-6 d d d 1.6 X 10” 1.3 X 10” d d d
loF8 6.5 x 10-7 X
5.0 x 5.2 x 3.6 x 3.5 x
10-7 10-7 10-7 10-8
e
9.5 x 10-7 e
“Excitation wavelength of donors; f l nm. bPhosphorescencewavelength of maximum intensity in P-CD, ref 3 and 4; LODs were measured at biacetyl wavelength maximum of 522 nm. Precision of LODs typically f0.2. d Not determined. e Not detected. micellar solution (7). They found that naphthalene, bromonaphthalene, biphenyl, and phenanthrene in their first triplet state were capable of sensitizing Tb3+associated with the micellar surface. The authors concluded that the micelle organized the acceptor and donor molecules into a discrete volume region and facilitated the energy transfer reaction. Additional work by Tabushi and co-workers (8)demonstrated that selective inclusion of acceptor molecules into 0-cyclodextrin rigidly capped with benzophenone moieties produced specific and effective energy transfer between the derivatized cyclodextrin host and the included species. Recently, Donkerbroek and co-workers have shown that sensitized room temperature phosphorescence using conventional organic solvents has considerable potential as an analytical scheme (6,9-11). These workers studied the energy transfer of selected polynuclear aromatic compounds to biacetyl dissolved in hexane, methanol, acetonitrile, water, and dichloromethane. They concluded that acetonitrile was the solvent of choice to generate sensitized phosphorescence from biacetyl. This paper reports some photophysical and analytical characteristics of room temperature sensitized biacetyl phosphorescence in two fluid, microscopically organized media. Micellar assemblies and cyclodextrin molecules were found to facilitate the energy transfer reaction by organizing the analytes and necessary reactants on a molecular scale and arranging them in close proximity for more efficient interaction. The triplet-triplet energy transfer reactions for several polynuclear aromatics, aromatic nitrogen heterocyclics, and heavy-atom-substituted aromatic species were evaluated and found to be enhanced significantly in the micellar medium and less so using cyclodextrins. Analytical limits of detection for selected donor compounds in both microscopically organized media and acetonitrile are reported. EXPERIMENTAL SECTION Reagents. Fluorene, chrysene, and triphenylene were obtained from CHEM Services, Inc., and were used as received. Naphthalene (MCB), 2-bromonaphthalene (Aldrich),biphenyl (MCB), and 4-bromobiphenyl (Aldrich) were recrystallized once from ethanol. Dibenzofuran (Ultra Scientific), quinoline (Fisher), biacetyl (Merck), dibenzothiophene (Analab), phenanthrene, lepidine, quinazolene, 2-chloroquinoline, 2,4-dichloroquinoline, and sodium dodecyl sulfate (all six from Aldrich) were used without further purification. 0-Cyclodextrin (Aldrich) was recrystallized once from boiling water. Spectroscopicgrade methanol (Baker) and HPLC grade acetonitrile (Fisher) were used as received. Apparatus. Phosphorescence spectra were obtained by using a Fluorolog 2+2 spectrofluorometer (SPEX Industries, Inc., Metuchen, NJ) with double excitation and emission monochromators. The instrument was equipped with a 450-W xenon continuous light source and a water-cooled Hammamatsu R928
Table 11. Excitation and Emission Sensitized Phosphorescence Characteristics and Limits of Detection for Selected Nitrogen Heterocyclic Molecules in SDS Micellar and 0-Cyclodextrin Solutions LOD,’ M SDS P-cyclomicelles dextrin
XEX;
XEM,~
compound
nm
nm
quinoline lepidine 2-chloroquinoline 2,4-dichloroquinoline quinazoline
305 305 305 305
500 501 494
9.3 X 8.7 X
d
1.7
305
503
8.3 X
2.2 X X
5.2 X lo-’ 9.8 X 9.1 X 4.1 X 3.5 X loF7
” Excitation wavelength of donor; &1nm. bPhosphorescence wavelength of maximum intensity in P-cyclodextrin,ref 4; LODs determined at biacetyl emission maximum of 522 nm. Precision of LODs, typically f0.2. Not determined. photomultiplier tube. The SPEX Datamate computer was used to correct all spectra for lamp intensity and photomultiplier tube response. Printouts of the spectra were produced by a Houston Instruments x-y digital plotter. Procedure. All glassware was rinsed with spectrograde M) of the donors methanol before use. Stock solutions (1x and biacetyl were prepared in spectrograde methanol. The nitrogen heterocyclic compounds and biacetyl were stored in amber bottles away from direct light to prevent possible photodegradation. Inclusion complexes (3) and micellar solutions were prepared by adding aliquots of the donor of interest and biacetyl to a 10-mL volumetric flask followed by dilution with either 0.01 M aqueous P-cyclodextrin or 0.1 M aqueous sodium dodecyl sulfate solution. Blank samples were prepared in the same manner but without addition of the donor molecules. The solutions were placed, after mixing, into a standard fluorescence cuvette equipped with a Teflon stopper and deaerated for 20 min under high-purity nitrogen passed through an indicating oxygen trap. Solutions for initial screening usually contained donor and biacetyl concenM each. Blank samples contained 1 X trations of 1 X M biacetyl dissolved in the appropriate medium. Limits of Detection. The analytical limits of detection (LOD) were determined by extrapolating a plot of sensitized biacetyl phosphorescence intensity at 522 nm vs. donor concentration to the concentration where the signal equaled three times the noise. The noise in this study was taken to be the standard deviation of several measurements of the response from the biacetyl blank measured at 522 nm which had typical relative standard deviations of 2.5%.
RESULTS AND DISCUSSION Energy Requirements of Donor-Acceptor Pair. The energy diagram for a donor-acceptor pair was previously described (6). For sensitized phosphorescence to occur, the triplet state of the donor must be of higher energy than the
ANALYTICAL CHEMISTRY, VOL. 56,
NO. 14, DECEMBER 1984
2813
Table 111. Ratio of Phosphorescence to Fluorescence Intensities for Biacetyl in SDS, P-CD, and Acetonitrilea solvent
IPlIF
0.10 M SDS 0.01 M P-CD
2.7 3.0 4.5
acetonitrile
M; excitation wavelength, "iacetyl concentration = 1 X 415 nm: emission Wavelength, 522 nm.
I
I
'[
_ _ _ _ _---_-
I
O.OOE OB 2OB.00
Concentration Requirement of the Acceptor. The efficiency of triplet-triplet energy transfer etDAis defined by the following equation:
I 325.oa
450.00
Wavelength [nm) Excitation spectra of 1.0 X loe4M biacetyi in (A) 0.1 M SDS micelles and in (B) 0.01 M 0-cyclodextrin solution. Flgure 1.
I/
/-B
3.5
5
10
15
20
25
30
35
DEAERATION TIME C M i N l Flgure 2. Effect of deaeration time on the phosphorescence intensity M biacetyl sensitized by 2.0 X 10" M biphenyl excited of (A) 1 X at 272 nm in 0.10 SDS and (6)background signal produced by 1 X M biacetyi excited at 272 nm in 0.10 M SDS.
triplet state of the acceptor. The phosphorescence emission of biacetyl in both sodium dodecyl sulfate and 0-cyclodextrin occurs at 522 nm. Tables I and I1 list the triplet state energies of the donor molecules in p-CD. Donkerbroek has shown (12, 13) that reverse energy transfer or quenching of biacetyl phosphorescence occurs if biacetyl's triplet state energy equals or surpasses that of the donor. In this study the donors were selected so that energy transfer from the donor to biacetyl was the predominate phenomenon. The absorption spectra of biacetyl in SDS and /3-CD are shown in Figure 1. Biacetyl displays an absorbance maximum at 247 nm in SDS and a t 252 nm in P-CD but shows only low absorbance at wavelengths larger than 270 nm in either solvent. Donors were selected with excitation wavelengths greater than 270 nm so that biacetyl's phosphorescence signal occurred primarily through the energy transfer process. However, the small absorption of biacetyl up to 450 nm and the subsequent phosphorescence emission a t 522 nm did contribute to the background signal, thereby adversely affecting the analytical limits of detection. Furthermore, the biacetyl phosphorescence background emission intensity increased with increasing time of deaeration up to approximately 15 min after which it became essentially constant (Figure 2).
where KT is the rate constant for triplet-triplet energy transfer, [A] is the molar concentration of acceptor, and T~~ is the lifetime of the donor's triplet state (6). In the studies performed by Donkerbroek, a biacetyl concentration of 1 X M was found to be optimum for both energy transfer requirements and low background signals. It can be seen from eq 2 that the efficiency of energy transfer is dependent on the lifetime of the donor molecule. Obviously, for sensitized phosphorescence to be significant, energy transfer must occur within the triplet state lifetime of the donor. Sensitized Biacetyl Phosphorescence in SDS. The 0.1 M SDS solutions used in these studies produced approximately 0.001 M micellar assemblies, which is at least an order of magnitude greater than the concentrations of the acceptor and several orders of magnitude greater than that of the donor (1,14). The energy transfer pair would tend to aggregate in or on the micelles due to favorable hydrophobic interactions with the hydrocarbon portion of the micelle compared to the bulk aqueous phase. Thus, the donor would be exposed to a greater "effective" concentration of acceptor molecules in SDS because of this condensing of the donor-acceptor pair into a smaller reaction volume (15). Figure 3 shows the phosphorescence spectra of biacetyl sensitized by biphenyl in (A) SDS, (B) P-CD and (C) acetonitrile. It is apparent that the sensitized phosphorescence intensities produced in the micellar solution are increased significantly over that observed in p-CD or acetonitrile. In order to determine if the increase in sensitized phosphorescence signal was due to a more efficient triplet-triplet energy transfer reaction (larger BtDA) or to enhanced triplet emission from biacetyl, the phosphorescence efficiency of biacetyl, BBBIAC, was measured in each of the three media. This was done according to the method of Donkerbroek (6) in which the ratio of the intensity at the phosphorescence wavelength maximum to the intensity a t the fluorescence maximum of a 1 x M biacetyl solution was determined. The results are presented in Table 111. The data show that the relative phosphorescence efficiencies, BpBIAC, are in the reverse order to that found for the sensitized phosphorescence signal in the three media, with lower biacetyl phosphorescence intensities in the more organized media. These data indicate that SDS and p-CD are actually decreasing BpBIAC, and that these media must be significantly enhancing the energy transfer efficiency, etDA, over that found in acetonitrile, most probable by organizing the donor-acceptor pair in close proximity. The extent of molecular organization must be considerable in order to counterbalance the decrease in biacetyl phosphorescence efficiency due to the organized media and produce a 50- to 1000-fold decrease in LODs. I t should be noted that the data given in Table I11 for acetonitrile are 6-7 times less than those observed by Donkerbroek and co-workers (6),and it is not possible to determine
2814
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
if the phosphorescence signal is less in the present study or the fluorescence signal is higher. The former could be explained if an impurity was present which quenched the phosphorescence or if our deareation step was less efficient. The presence of a fluorescing impurity could increase the latter term. Also, if incomplete correction factors were applied to the emission spectra for the two sets of data, one would expect some variation in the magnitudes of the ratios. Curiously, the Free University studies used a fluorescence wavelength maximum of 460 nm in contrast to 490 nm found in the present study. The LODs for ten selected polynuclear aromatic compounds using sensitized phosphorescence in the three different media are listed in Table I. The detection limits for the PNAs in SDS were at least an order of magnitude lower than those found in 0-CD or acetonitrile. In addition, a slight internal heavy atom effect was apparent for 2-bromonaphthalene and 4-bromobiphenyl. Increased sensitized phosphorescence intensities were found for these compounds resulting in lower detection limits compared to the unsubstituted parent compounds. Most likely, the addition of a heavy atom substituent increased the donor's degree of intersystem crossing producing an enhanced sensitized phosphorescence emission signal by increasing the number of triplet state donor molecules available for sensitizing biacetyl (5). Limit of detection studies in acetonitrile were confined to naphthalene, 2-bromonaphthalene, biphenyl and 4-bromobiphenyl because of solvent stability problems. In most cases freshly opened containers of acetonitrile yielded reproducible results, but several hours after initial use the sensitized phosphorescence signal went to zero. Degradation of acetonitrile to methyl- and ethylamine appears the likely cause of this effect (15). Experiments in this laboratory have shown amines to be strong quenchers of phosphorescence. This stability problem made LOD determinations in acetonitrile difficult and the results unreliable; therefore, these studies were not pursued in this media. The solvent stability problem found with acetonitrile was not observed in either SDS or p-CD, and this appears to be a distinct advantage to working with these media. Table I1 shows the LODs found for five nitrogen heterocyclic species. The detection limits are substantially higher for these compounds in SDS than those observed for the polyaromatic hydrocarbons. The heavy atom effect previously observed for halosubstituted PNAs was also apparent with the chloroquinoline species where lower limits of detection were observed compared to the other heterocyclic species. The generally higher LODs can be attributed, in part, to the increased water solubility of the nitrogen heterocyclics compared to that of the PNAs. If the residence time of the donor molecules in the micelle is short, minimal micellar interactions would be expected. Studies by Weinberger (15) have shown that the anticonvulsant drug, phenobarbital, did not sensitize biacetyl in SDS, although the molecule was quite responsive to sensitization of biacetyl in acetonitrile. It was postulated that the absence of sensitized phosphorescence in SDS was due to the low solubility (short residence time) of phenobarbital and the high solubility of biacetyl in the micelles. Another study of micelle-enhanced RTP of nitrogen heterocyclics such as quinoline has shown that weak emission is observed only when a fraction of the sodium counterions are replaced by silver heavy-atom ions (16). It was postulated that silver plays a dual role in enhancing the RTP emission from nitrogen heterocyclic species, one involving the conventional heavy atom effect and a second involving a direct complexation of silver ion with either the nitrogen nonbonding electrons or with the r-electron system itself. This complexation would serve to affix the complex near the micellar
7.79E 04
t
A '
I\" I \
-! t
I
t
\
I
I
i 00 .
550.00
7W.00
Wavelength Inml Flgure 3. Corrected phosphorescence emission spectra of 1 X M biacetyl sensitized by 1.3 X M biphenyl in (A) 0.10 M SDS, (B) 0.01 M P-CD, and (C) acetonitrile: excitation wavelength, 272 nm; slits, 14.4 nm EX and EM; scan rate, 1 nm/s; filter, 470 cutoff.
aggregate,decreasing the exit rate of water soluble species from the micelle. The relationship between the relative solubility of the donor and acceptor in the micelle, the donor's triplet state lifetime, and the efficiency of energy transfer producing sensitized phosphorescence profoundly affects the magnitude of the LOD observed experimentally. For simple arenes the triplet state lifetime in micellar solution is inversely proportional to the species solubility in water, a result that means that highly water soluble species will have short phosphorescence lifetimes (1,14). This can be easily related to the efficiency of energy transfer (eq 2), where short lifetimes, T ~ will ~ have , a large reciprocal value and will decrease the efficiency of biacetyl excitation.
Sensitized Biacetyl Phosphorescence in /3-Cyclodextrin. 0-Cyclodextrin is made up of seven glucose moieties which are coupled to produce a conical structure with a hydrophobic interior containing highly energetic water molecules ( I 7). Species satisfying the size criteria of fitting into the 0-CD cavity can easily displace the thermodynamically unfavored included water (3). Favorable enthalpy changes give cyclodextrins their ability to form stable inclusion complexes with a wide variety of organic and inorganic compounds (18). Scypinski and Cline Love have shown that the optimum p-CD concentration for induction of room temperature phosphorescence in PNAs and nitrogen heterocyclic species is 0.01 M ( 3 , 4 ) . Higher concentrations did not significantly increase the phosphorescence signal and caused some solubility problems. It has also been shown by Yorozu (19) and Turro (20) that multiple guest molecules could reside in a single p-CD cavity. It was postulated in the present study that donor and acceptor molecules would be brought into close proximity to one another by inclusion into the same CD cavity, producing a trimolecular complex of CD:donor:acceptor. Figure 3 shows that the sensitized phosphorescence intensity obtained in p-CD solution is significantly lower than that found in SDS solution and the LODs in p-CD found for the ten PNAs listed in Table I were substantially higher. However, @-CDwas found to increase the sensitized phosphorescence signal by approximately an order of magnitude over that found using acetonitrile. An internal heavy atom effect producing enhanced sensitized phosphorescence intensities and lower LODs was also found in p-CD for Z-bromonaphthalene, 4-bromobiphenyl, and 2,4-dichloroquinoline. The decrease in sensitivity using p-CD compared to SDS solutions is most likely due to the size limitations imposed by the cyclodextrin cavity. The p-CD interior has specific dimensions which places constraints on the number of donor
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
t
9.93E 04
\ t 4U8.8U
\
558. eo
t
t 780.UU
Wave) e n g t h [ nml M biacetyi senFlgure 4. Corrected emission spectra of 1.0 X M phenanthrene in 0.01 p C D (A) deaerated for sitized by 5.0 X 20 rnin under high-purity nitrogen and (B) aerated: excitation wavelength, 295 nm; slits, 14.4 nm EX and EM; scan rate, 1 nm/s.
and acceptor molecules which can occupy the cavity. If only one of the energy transfer pair is fully included into the cavity, no significant enhancement in sensitized RTP would be expected. If both the donor and acceptor molecules are partially included, some enhancement might be possible depending on the geometry of inclusion. What is clear from the results is that the p-CD did organize the energy transfer reactants and did enhance the energy transfer process over that observed for homogeneous acetonitrile solutions of the reactants. Triphenylene and chrysene did not produce any measurable sensitized phosphorescence signal in 0-CD, although they produced appreciable sensitized biacetyl emission in SDS solution. Most likely these two PNAs and/or biacetyl are excluded from the P-CD cavity due to the size requirements. These two PNAs may show enhanced sensitized phosphorescence in the larger y-cyclodextrin which could more effectively include and position the analyte and biacetyl in close proximity for enhanced interaction. The LODs found for the nitrogen heterocyclic molecules in p-CD were equal in magnitude to those found for the PNAs. The similar LODs found for the PNAs and nitrogen heterocycles in 0-CD, unlike those obtained in SDS, were most likely due to the inclusion process. Any species satisfying the size criteria of the 0-CD cavity can easily displace the included water, therefore rendering solubility characteristics less critical for 0-CD induced phosphorescence compared with SDS induced RTP. Figure 4 shows the phenanthrene-sensitized phosphorescence spectra of biacetyl in aerated and deaerated P-CD solutions. The appearance of an additional peak at 490 nm was found which was not quenched by oxygen. The emission wavelength of this peak corresponds to that found for biacetyl fluorescence in p-CD. The peak, although present in the blank sample, showed enhancement upon the addition of not only phenanthrene, but with several other PNAs and nitrogen heterocyclic species as well. The exact nature of this peak found only in the P-CD experiments is unknown at this time and studies are under way to determine its origin,
CONCLUSIONS Both SDS and P-CD provide more favorable media for induction of sensitized room temperature phosphorescence
2815
than homogeneous solutions of reactants in acetonitrile. These media most probably organize the donor-acceptor pair in a smaller reaction volume producing a higher effective concentration of the acceptor seen by the donor, thus facilitating the triplet-triplet energy transfer reaction. This demonstrates another application of the enormous ability of organized media, particularly micelles in this case, to enhance analytical sensitivity. The observed LODs are significantly lower in the organized media compared to LODs obtained from homogeneous solutions. The LODs were limited by the high background signal obtained for the blank biacetyl sample, a common problem encountered in luminescence measurements and particularly in sensitized RTP. Although assay development is not the purpose of this paper, it should be noted that the biacetyl concentration should be optimized for each donor analyte molecule for optimum sensitivity. Further developments of microscopically organized energy transfer reactions such as biacetyl may find use as a detection scheme in highperformance liquid chromatography using micellar and cyclodextrin mobile phases where a compromise constant concentration of biacetyl would be necessary.
ACKNOWLEDGMENT The authors are grateful to Stephen Scypinski, Rod Woods, and Robert Weinberger for their helpful discussions and suggestions. Registry No. SDS, 151-21-3;p-CD, 7585-39-9; biacetyl, 43103-8; biphenyl, 92-52-4;4-bromobiphenyl, 92-66-0;fluorene, 8673-7; dibenzofuran, 132-64-9;dibenzothiophene, 132-65-0;naphthalene, 91-20-3; 2-bromonaphthalene, 580-13-2;chrysene, 21801-9; phenanthrene, 85-01-8; triphenylene, 217-59-4; quinoline, 91-22-5; lepidine, 491-35-0; 2-chloroquinoline, 612-62-4; 2,4-dichloroquinoline, 703-61-7; quinazoline, 253-82-7.
LITERATURE CITED Cline Love, L. J.; Skrilec, M.; Habarta, J. G. Anal. Chem. 1980, 52, 754. Skrllec, M.; Cline Love, L. J. Anal. Chem. 1980, 52, 1559. Scypinski, S.;Cline Love, L. J. Anal. Chem. 1984, 56,322-327. Scypinski, S.;Cline Love, L. J. Anal. Chem. 1984, 56,331-336. Weinberger, R.; Cline Love, L. J. Spectrochim. Acta, Part A 1984, 40A 49-55. Donkerbroek, J. J.; Gooijer, C.; Velthorst, N. H.; Frei, R. W. Anal. Chem. 1982, 54,891. Almgren, M.; Grieser, F.; Thomas, J. K. J . Am. Chem. SOC. 1979, 101, 2021. Tabushi, I.; Fujlta, K.; Yuan, L. C. Tetrahedron Lett. 1977, 29, 2503. Donkerbroek, J. J.; Elzas, J. J.; Gooijer, C.; Frei, R. W.; Velthorst, N. H. Talanta 1981, 28, 717. Donkerbroek, J. J.; van Eikema Hommes, N. J. R.; Gooijer, C.; Verlhorst, N. H.; Frei, R. W. Chromatogrphia 1982, 15,218. Donkerbroek, J. J.; van Eikema Hommes, N. J. R.; Gooijer, C.; Velthorst, N. H.; Frei, R. W. J. Chromatogr. 1983, 255,581. Donkerbroek, J. J. Appl. Spectros. 1983, 37, 188. Donkerbroek, J. J.; Veitkamp, A. C.; Gooijer, C.; Velthorst, N. H.; Frei, R. W. Anal. Chem. 1983, 55, 1886. Cline Love L. J.; Habarta, J. G.; Skrilec, M. Anal. Chem. 1981, 53, 437. Weinberger, R. Ph.D. Dissertation, Seton Hall University, South Orange, NJ, 1983. Woods, R.; Cline Love, L. J. Spectrochlm. Acta, fart A , In press. Szejtli, J. "Cyclodextrin and Their Inclusion Complexes"; Akademial Kiado: Budapest, Hungary, 1982; Chapter 2. Saenger, W. Angew. Chem., Int. Ed. Engl. 1980, 19,344. Yorozu, T.; Hoshino, M.; Imamura, M. J. J . fhys. Chsm. 1982, 86, 4426. Turro. N. J.; Boit, J. D.; Kuroda, Y.; Tabushi, I. fhotochem. fhotoblol. 1982, 35,69. I
RECEIVED for review April 4,1984. Resubmitted and accepted June 29,1984. This work was supported in part by the National Science Foundation Grant No. CHE-8216878.
