SPECTROCHIMICA ACTA

PART A

E L S E V I E R Spectrochimica Acta Part A 53 (1997) 165-172

Spectrochemical investigations of fluorescence quenching agents Part 5. Effect of surfactants on the ability of nitromethane to

selectively quench fluorescence emission of alternant PAHs

S i d d h a r t h P a n d e y a, K r i s t i n A . F l e t c h e r a, J o y c e R . P o w e l l a, M a r y E . R . M c H a l e a,

A n n - S o f t M. K a u p p i l a a, W i l l i a m E. A c r e e , Jr . a,*, J o h n C. F e t z e r b, W e i D a i c,

R o n a l d G . H a r v e y c

aDepartment of Chemistry, University of North Texas, Denton, T.Y 76203-0068, USA bChevron Research and Technology Center, Richmond, CA 94802-0627, USA

CBen May Institute, University of Chicago, Chicago, IL 60637, USA

Received 29 June 1996; revised 27 July 1996; accepted 27 July 1996

Abstract

Applicability of the nitromethane selective quenching rule for discriminating between alternant vs. nonalternant polycyclic aromatic hydrocarbons (PAHs) is examined for 18 representative PAH solutes dissolved in micellar cetyltrimethylammonium chloride (CTAC1), micellar dodecyltrimethylammonium bromide (DTAB), micellar Brij-35 and micellar sodium octanoate (SO) solvent media. Experimental results show that nitromethane quenched fluores- cence emission of only the 10 alternant PAHs in the two cationic (CTAC1 and DTAB) and nonionic Brij-35 surfactant solvent media as expected. Emission intensities of nonalternant PAHs, except for the few exceptions noted previously, were unaffected by nitromethane addition. Unexpected quenching behavior was observed, however, in the case of nonalternant PAHs dissolved in micellar sodium octanoate solvent media. Nitromethane quenched fluorescence emission of all nonalternant PAHs studied in the SO solvent media, which is contrary to the selective quenching rule. © 1997 Elsevier Science B.V. All rights reserved.

Keywords: Fluorescence quenching; Polycyclic aromatic hydrocarbons; Surfactants

1. In troduct ion

Spectrofluorometric methods, alone or in con- junction with high performance liquid chromatog- raphy and/or supercritical fluid extraction, are becoming increasingly popular in the analysis of

* Corresponding author. E-mail: acree@casl.unt.edu

unknown mixtures of polycyclic aromatic hydro- carbons (PAHs), polycyclic aromatic nitrogen compounds and their substituted derivatives. Sev- eral aromatic compounds may absorb at a com- mon excitation wavelength; however, not all will emit at the wavelength(s) monitored by the photodetector. Utilization of selective fluorescence quenching agents further simplifies observed emis- sion spectra by eliminating signals from undesired chemical interferences having only slightly differ-

0584-8539/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0584-8539(96)01666-X

166 s. Pandey et aL / Spectrochimica Acta Part A 53 (1997) 165-172

ent molecular structures. On the basis of limited fluorescence measurements for perylene, diben- zo[b,k]chrysene, dibenzo[h,rst]pentaphene, naph- tho[1,2b]fluoranthene, indeno[1,2,3cd]pyrene, and 12,11-(perinaphthylene)-fluoranthene dissolved in a binary aqueous-acetonitrile mixture (20:80 by volume), Bliimer and Zander [1] noted that both nitromethane and nitrobenzene selectively quenched fluorescence emission of the so-called 'alternant' polycyclic aromatic hydrocarbons. Emission intensities of the three nonalternant PAHs (e.g. naphtho[1,2b]fluoranthene, in- deno[1,2,3cd]pyrene and 12,11-(perinaph- thylene)fluoranthene) were affected. (A recent study by Ogasawara et al. [2] suggests that ni- trobenzene is not as selective as originally be- lieved). Published studies [2-8] involving over 63 PAHs have identified dibenzo[hi,wx]heptacene, benzo[k]fluoranthene and naphtho[2,3b]-fluo- ranthene as among the few exceptions to the so-called nitromethane selective quenching rule in the benzenoid, fluorenoid, fluoranthenoid and 'methylene-bridged' cyclopenta-PAH subclasses. More recent measurements [9,10] revealed that nitromethane quenched fluorescence emission of all eighteen acenaphthylene- and acephenan- thrylene-derivatives studied thus far, which is completely contrary to what would be expected based upon the fact that the solutes are listed as examples of nonalternant PAH molecules. The unusual fluorescence quenching behavior of the acenaphthylene- and acephenanthrylene-deriva- tives results from the molecules' fixed double bond in the five-membered ring. The double bond is alkenic in nature, rather than aromatic as one might believe. This observation is confirmed by independent NMR coupling measurements [11- 14].

Quenching behavioral differences between alter- nant and nonalternant PAHs can be rationalized in terms of processes originating from the vibra- tionally relaxed first electronic excited singlet state, ~PAH*. Deactivation from IPAH* state is governed by the competition between radiative and nonradiative processes. Rate constants of fluorescence decay, kn ...... for PAH fluorophores are generally insensitive to molecular environ- ment. Efficiencies of nonradiative processes, on

the other hand, depend to a large extent upon external perturbations resulting from interactions involving PAH solutes with solvent/quenching molecules. Three possible mechanisms include re- duction of fluorescence emission intensities through: (1) intersystem crossing processes involving exter- nal 'heavy atom' quenchers,

1pAH* + ~Quencher ~ 3pAH* + lQuencher;

(2) intermolecular transfer (or partial transfer, 8 + /8 - ) of a single electron to form free ions (or exciplex or charge transfer complex),

1PAH* + 1Quencher ~ (PAH -+ ---Quencher-V )

2pA H _+ + 2Quencher -v ;

and (3) intermolecular electronic energy transfer,

1PAH* + 1Quencher ~ 1pAH + IQuencher*,

with the quenching agent promoted from a ground electronic singlet state to an excited singlet state, requiring Energy(~PAH * > En- ergy(IQuencher*).

Of the aforementioned mechanisms, only the second could discriminate between alternant and nonalternant PAHs. Breymann et al. [7] attributed nitromethane's selectivity to an electron/charge transfer reaction. As argued by the authors, re- duction potentials of nonalternant PAHs are gen- erally 0.4 eV more positive than those of alternant PAHs. Quantum mechanical computations show the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies of nonalternant PAHs to be lowered against those of alternant PAHs of equal HOMO-LUMO separation. For the electron transfer reaction

IPAH* + 1Quencher ~ 2pAH + + 2Quencher-,

the change in free energy is expected to be more negative in the case of alternant PAHs (see Fig. 1). A more favorable electron transfer process should result in larger quenching rate constants which if comparable to (or larger than) kn .....

s. Pandey et al. / Spectrochimica Acta Part .4 53 (1997) 165-172 167

would cause a significant reduction in PAH emis- sion intensity. Slow electron transfer reactions (i.e. kq . . . . h <ka ..... ) are not expected to affect fluorescence intensities as the photon is emitted long before electron transfer from the PAH donor to the quencher acceptor can occur. A similar argument could be made for electron donor quenching agents; in that case, the quenching agent would be more likely to affect fluorescence emission of the nonalternant PAHs (see Fig. 2).

The electron transfer mechanism postulated above requires favorable reaction kinetics and thermodynamic conditions. From a strictly ther- modynamic point-of-view, it is conceivable that the extent of quenching could be altered simply by changing the electronic nature of the surrounding solvent media in order to either stabilize or desta- bilize the charge (or partial charge) that is tempo- rarily formed on the polycyclic aromatic hydrocarbon. Micellar solutions provide a very convenient means to introduce ionic character, and still have a solvent medium capable of solubi- lizing the larger, hydrophobic PAH solutes. An- ionic surfactants could perhaps stabilize a positive charge on the PAH ring system, whereas cationic surfactants would tend to inhibit fluorescence emission quenching by nitromethane. To date, no

~ . I . R A N E~ON S F E R J ( LUMO ) PN-I"

" , k . , -

( LUMO ) pall * 1 ~ QUENCHER °

/ E RI~.~r~N

{HOMO) pall

1L QUENCHER

NON-N.TERNANT PN'I QUENCHER N.TERNANT PAll ( ELECTRON DONOR ) { ELECTRON ACCEPTOR) { ELECTRON DONOR )

Fig. 1. Simplified molecular orbital diagram indicating favor- able conditions for electron transfer between an electron donor alternant PAH and an electron acceptor quenching agent, such as nitromethane. The quencher's LUMO and nonalternant PAH's LUMO were placed at energies so as to discourage electron transfer. The dotted line represents the potential of a reference electrode. More detailed molecular orbital diagrams are given elsewhere [7].

OI'It=I~I'IF"R" 1

Is ( LUMO ) pa l l "

(wMo) PAH" T EREDUCnON

. o . o ¢ o o o . ~ . • • • • • • •

° ° " c H "

(HOMO) pAH NON-AL'I~RNANT pall QUENCHER ALTERNANT P/U-I ( ELECTRON ACCEPTOR ) ( ELEGTRON DONOR ) ( ELECTRON AGCEPTOR )

Fig. 2. Simplified molecular orbital diagram indicating favor- able conditions for electron transfer between an electron ac- ceptor nonalternant PAH and an electron donor quenching agent, such as 1,2,4-trimethoxybenzene. The quencher's HOMO and alternant PAH's LUMO were placed at energies so as to discourage electron transfer. The dotted line repre- sents the potential of a reference electrode. More detailed molecular orbital diagrams are given elsewhere [7].

one has systematically examined how micellar so- lutions affect nitromethane's selectivity for dis- criminating between alternant vs. nonalternant PAHs. In this paper, we report our initial obser- vations concerning PAH fluorescence emission quenching in micellar cetyltrimethylammonium chloride (CTACI), micellar dodecyltrimethylam- monium bromide (DTAB), micellar Brij-35 and micellar sodium octanoate (SO) slutions. Also included is a discussion of the fluorescence behav- ior of 4-H-benzo[g]cyclopenta[mno]chrysene and 4-H-benzo[f]cyclopenta[pqr]picene dissolved in organic nonelectrolyte solvents of varying polarity in order to screen both polycyclic aromatic hydro- carbon solutes for possible solvent polarity probe character. (Molecular structures of 4-H-ben- zo[g]cyclopenta[mno]chrysene and 4-H-ben- zo[f]cyclopenta[pqr]picene are depicted in Fig. 3).

A B

Fig. 3. Molecular structures of zo[g]cyclopenta[mno]chrysene (A) and zo[f]cyclopenta[pqr]picene (B).

4-H-ben- 4-H-ben-

168 S. Pandey et al. / Spectrochimica Acta Part A 53 (1997) 165-172

Table 1 Summary of nitromethane quenching results for alternant and nonalternant polycyclic aromatic hydrocarbons dissolved in micellar CTAC1, micellar DTAB, micellar Brij-35 and micellar SO solvent media

Chemical name 2ex CTAC1 DTAB Brij-35 SO (nm) Red a'b Red a'b Red a'b Red a'b

Alternant polycyclic aromatic hydrocarbons (benzenoids) Pyrene 338 91% Perylene 403 47%

Coronene 334

Benzo[ghi]perylene 380 Benzo[e]pyrene 335 Chrysene 320

42% 54% ~ 86%

78%

Anthracene 340 46%

Alkylated alternant polycyclic aromatic hydrocarbons (benzenoids) 2-Methylpyrene 333 96% 4-H-Benzo[g]cyclopenta[mno]chrysene 300 84% 4-H-Benzo[qcyclopenta[pqr]picene 298 76% Nonalternant fluoranthenoids and fluorenoids Benz[def]indeno[1,2,3hi]chrysene 406 3%

Benzo[k]fluoranthene 306

Dibenzo[a,e]fluoranthene Naphtho[1,2b]fluoranthene

390 0% 350 0%

Benzo[a]fluoranthene Naphtho[2,1 a]fluoranthene Naphtho[2,3b]fluoranthene

406 0% 400 0% 316

Benzo[b]ftuoranthene 346 < 1%

84% 90% 94% 63% 35% 70%

59% c 33% 41%

61% c 87% c 79% 85% 93% 73% 74% 97% 61% 79% 80% 77% ¢ 890/0 c 20% 33% 46% 39% ~ 64% ~

95% 94% 91% 66% 70% 88% 56% 57% 81%

< 1% 3% 27% 8°/o e 51%c

20% 15% 77% 29% c 100% c

0% 0% 38% 0% 6% 66%

130/0 ~ 91%c <2%

0% 0% 38% 12% 16% 68% 33% d 350/0 c < 1% 7% 49%

Unless otherwise indicated, 25 ~tL pipette droplet of neat niromethane was added to 11 mL of solution. b% Reduction = 100 x(Finitial-Ffinal)/Finitial, where Fini t ia I refers to the corrected PAH emission intensity prior to nitromethane addition and Fnna~ is the corrected emission intensity after nitromethane addition. c 50 laL of nitromethane added to 11 mL of solution. d 75 ~tL of nitromethane added to 11 mL of solution. e 100 ~tL of nitromethane added to 11 mL of solution.

F l u o r e s c e n c e p rope r t i e s o f the r e m a i n i n g 16 P A H

solu tes h a v e been r e p o r t e d in o u r ear l ie r inves t iga-

t ions.

2. E x p e r i m e n t a l

M i c e l l a r so lu t ions o f c e t y l t r i m e t h y l a m m o n i u m ch lo r ide (Aldr ich , ~ 1.0 × 10 - 2 M), d o d e -

c y l t r i m e t h y l a m m o n i u m b r o m i d e (A ld r i ch 99%, 5 . 0 × 10 - 2 M) , Bri j-35 (Aldr ich , ~ 5 . 0 × 10 - 3

M ) a n d s o d i u m o c t a n o a t e ( F l u k a 99 + % a n d

S i g m a 99 + %, 5.0 x 10 1 M ) were p r e p a r e d by

d i s so lv ing the su r f ac t an t in d o u b l y de - i on i zed wa-

ter. E x p e r i m e n t a l I / I I I b a n d emis s ion in tens i ty

ra t ios fo r p y r e n e a n d benzo[gh i ]pe ry lene c o n f i r m

tha t the so lu t ions are i n d e e d a b o v e the cr i t ical

mice l le c o n c e n t r a t i o n . 4 - H - B e n -

zo [g ] cyc lopen t a [mno]ch rysene a n d 4 - H - b e n -

zo [ f ]cyc lopen ta [pqr ]p icene were p r e p a r e d a n d

pur i f ied by p r o c e d u r e s desc r ibed e l sewhere [15].

Syn the t i c re fe rences a n d / o r c o m m e r c i a l suppl ie rs

S. Pandey et al. / Spectrochimica Acta Part A 53 (1997) 165-172 169

for the remaining 16 PAH solutes contained in Table 1 are listed in our earlier papers (for a single source listing see Tucker [16]). Stock solu- tions were prepared by dissolving the solutes in dichloromethane, and were stored in closed amber glass bottles in the dark to retard any photochem- ical reactions between the PAH solutes and dichloromethane solvent. Carbon tetrachloride and chloroform (to a much lesser extent) are reported to react with polycyclic aromatic hydro- carbons via a hypothesized concerted transannu- lar addition with free radical formation [17-22]. Small aliquots of each stock solution were trans- ferred into test tubes, allowed to evaporate, and diluted with the micellar solvent medium of inter- est. All solutions were ultra-sonicated, vortexed and allowed to equilibrate for a minimum of 24 h before any spectrofluorometric measurements were made. Experimental results were unaffected by longer equilibration times.

Absorption spectra were recorded on a Milton Roy Spectronic 1001 Plus and a Hewlett-Packard 8450A photodiode-array spectrophotometer in the usual manner. The fluorescence spectra were mea- sured on a Shimadzu RF-5000U spectrofluorime- ter with the detector set at high sensitivity. Solutions were excited at the wavelengths listed in Table 1. Fluorescence data were accumulated in a 1 cm 2 quartz cuvette at 21°C (ambient room temperature) with excitation and emission slit width settings of 15 and 3 nm, respectively. The fluorescence spectra represent a single scan which was then solvent blank corrected and verified by repetitive measurements.

Emission intensities associated with the quench- ing measurements were corrected for primary in- ner-filtering artifacts arising from the absorption of excitation radiation according to the following expression [23-25]:

fpr im = Fc°~r/F°bs

= 2.303 A(y - x)/[10 -Ax - 10- AY], (i)

which differs slightly from the approximate form [26]

fprim ~ 100"5 A (2)

where F c°rr and F °bs refer to the corrected and observed fluorescence emission signal, respec- tively, A is the absorbance per centimeter of path- length at the excitation wavelength, and x and y denote distances from the boundaries of the inter- rogation zone to the excitation plane. Several of the PAHs have excitation wavelengths in the 300- 320 nm spectral region and a few drops of ni- tromethane gave solutions having appreciable absorbances. Computational procedures and in- terrogation zone dimensions are discussed in greater detail elsewhere [3-5,27,28]. Every effort was made to work at solution absorbances of A c m - ~ ~< 0.95 (fpnm ~< 3.0), where the inner-filter- ing equation is valid. Secondary inner-filtering corrections were not necessary as nitromethane is 'optically transparent' in most of these PAH emis- sion ranges.

3. Results and discussion

Fluorescence emission spectra were recorded for 4-H-benzo[g]cyclopenta[mno]chrysene and 4- H-benzo[qcyclopenta[pqr]picene dissolved in cy- clohexane, 2,2,4-trimethylpentane, n-hexadecane, dibutyl ether, methyl t-butyl ether, methylben- zene, butyl acetate, 2-propanol, 1-propanol, dichloromethane, chloroform, carbon tetrachlo- ride, methanol, acetonitrile, and dimethyl sulfox- ide. The nonelectrolyte solvents were judiciously selected so as to encompass a broad range of solvent polarity, from the nonpolar n-hexadecane solvent to the moderately polar butyl acetate and dichloromethane solvents to the very polar dimethyl sulfoxide, which is the most polar sol- vent considered in this present study. Care- ful examination of the entire spectral file for the different organic solvents used reveals that the emission bands of 4-H-benzo[g]cyclopenta[mno]- chrysene and 4-H-benzo[f]cyclopenta[pqr]picene are poorly resolved. Calculated emission intensity ratios are essentially constant, irrespective of sol- vent polarity. Both PAH solutes are thus classified as nonprobe molecules.

Table 1 summarizes our fluorescence quenching measurements for 10 alternant and 8 nonalternant

170 s. Pandey et al. / Spectrochimica Acta Part A 53 (1997) 165-172

polycyclic aromatic hydrocarbons dissolved in mi- cellar CTAC1, micellar DTAB, micellar Brij-35 and micellar SO solvent media. Experimental re- sults are reported in the third-sixth columns as the percent reduction in the original fluorescence emission intensity observed after the addition of nitromethane. Unless otherwise noted, numerical entries correspond to one 25 ~tL Eppendorf pipette droplet of neat quenching agent added to 11 mL of solution. All emission intensities used in the computations were corrected for primary in- ner-filtering as discussed in the last paragraph of the Experimental section. Estimated nitromethane molar concentrations are as follows: 0.04 M (25 gL), 0.08 M (50 laL), 0.12 M (75 gL) and 0.16 M (100 gL). Molar concentrations are calculated using an average droplet mass of 0.0175 g ob- tained by weighing 25 individual 25 gL Eppendorf pipette droplets of nitromethane, which ranged in size from 0.0267-0.0275 g.

Careful examination of Table 1 reveals that nitromethane quenches the fluorescence emission of all 10 alternant benzenoids and alkylated derivatives, in accordance with the nitromethane selective quenching rule. Similar behavior is ob- served for all four surfactant solutions. No special significance is given to the slightly larger percent reductions observed in the SO micelle solution as such differences may result simply from the size of the added nitromethane droplet. More interesting quenching behavior is observed in the case of the 8 nonalternant fluoranthenoid and fluorenoid derivatives. Two of the nonalternant PAHs, ben- zo[k]fluoranthene and naphtho[2,3b]fluoranthene, are quenched by nitromethane in the micellar DTAB solvent media. Both compounds have been previously noted to be exceptions to the ni- tromethane selective quenching rule [3,6,29], and their behavior here is not totally unexpected. (Note: naphthol[1,2b]fluoranthene was considered to be a 'borderline case' in our initial quenching study [3], and its behavior in DTAB is. also in agreement with earlier findings.) What is unex- pected, however, is the large number of nonalter- nant PAHs that are quenched in the SO solution.

Rationalization of why nitromethane quenches the fluorescence emission of nonalternant PAHs dissolved in the anionic SO surfactant solution

must take into account the fact that most of the fluorophore molecules reside somewhere within the micellar interior. Polycyclic aromatic hydro- carbons, especially the larger multi-ring ones, are extremely hydrophobic and they have very limited aqueous solubilities. Moreover, the high sodium (SO), chloride (CTAC1) and bromide (DTAB) counter-ion concentrations in the aqueous pseu- dophase should further reduce the PAH solubility through the so-called 'salting out' effect. In sev- eral earlier solvent polarity probe studies, we noted that it was impossible with our Shimadzu spectrofluorimeter to determine meaningful emis- sion intensity ratios for coronene, methylcoro- nene, 1,2-dimethylcoronene and other larger PAHs because of extremely weak fluorescence signals [30,31]. We even substituted neat acetoni- trile for the recommended aqueous-acetonitrile solvent media (20:80 by volume) in our quenching studies [32] in order to solubilize the larger PAHs. The 100-fold increase in emission intensity that is observed in the micellar CTAC1, micellar DTAB and micellar SO solvents results from the en- hanced PAH solubility, combined with the inabil- ity of oxygen to effectively quench PAH molecules residing inside the micellar pseu- dophase.

Excited-state lifetimes of PAHs are on the order of nanoseconds [33,34], and the micelle-to- aqueous pseudophase transfer rate is on the order of microseconds (micelle residence times being several microseconds [35,36]). Any hypothesized quenching mechanism must have the PAH molecule residing somewhere within the interior of the micelle during the electron transfer. There is insufficient experimental data at the present time for us to completely explain the observed differences in PAH quenching behavior in micel- lar cationic (CTAC1 and DTAB), nonionic (Brij- 35) and anionic (SO) surfactant solvent media. A possible explanation involves the surfactants' neg- ative vs. positive charges. As noted in the Intro- duction, nitromethane selectively quenches alternant PAHs through a presumed electron transfer process, with the polycyclic aromatic hy- drocarbon being the electron donor. If the solubi- lized PAH molecule resided in the interior region in close proximity to the negatively-charged exte-

S. Pandey et al. / Spectrochimica Acta Part A 53 (1997) 165-172 171

rior surface of the SO micelle, then ionic interac- tions could stabilize the positive charge (or partial positive charge) on the PAH as it develops. Natu- rally, this would facilitate electron transfer for both alternant and nonalternant polycyclic aro- matic hydrocarbons, perhaps to the point where all quenching selectivity is lost. Conversely, the positively charged surface of the cationic CTAC1 (and DTAB) micelle would discourage electron transfer. This would explain why nitromethane retains its selectivity for discriminating between alternant vs. nonalternant PAHs in the case of the micellar CTAC1 and DTAB solvent media.

We cannot rule out the possibility that differ- ences in the alkyl chain length may also play an important role in the quenching mechanism. Sodium octanoate does have a shorter alkyl chain (7 vs. 12 and 16 carbon atoms), and one would expect that a PAH solubilized in a SO micelle would in all likelihood reside closer to the exterior surface region. Closer proximity of the PAH fluorophore to the negatively-charged surface would strengthen any stabilizing ionic interac- tion(s) that might exist. In the case of CTAC1 and DTAB, the alkyl chain length may be of sufficient length to solubilize the PAH molecule much deeper within the micellar hydrocarbon-like inte- rior.

'Electrostatic contact' with the positively- charged CTAC1 (or DTAB) exterior surface would be reduced (perhaps even eliminated). Ad- ditional measurements are planned with other an- ionic and cationic surfactants having different alkyl-chain lengths in hopes of developing a better understanding of why the nitromethane selective quenching rule fails in the micellar sodium oc- tanoate solvent media.

Acknowledgements

This work was supported in part by the Univer- sity of North Texas Research Council. The au- thors also acknowledge support from the National Institutes of Health, Institute for Envi- ronmental Health Sciences (ES 04732) and the National Cancer Institute (CA 67937).

References

[1] G.-P. Bliimer and M. Zander, Fresenius Z. Anal. Chem., 296 (1979) 409.

[2] F.K. Ogasawara, Y. Wang and V.L. McGuffin, Appl. Spectrosc., 49 (1995) 1.

[3l S.A. Tucker, W.E. Acree Jr., B.P. Cho, R.G. Harvey and J.C. Fetzer, Appl. Spectrosc., 45 (1991) 1699.

[4] V.L. Amszi, Y. Cordero, B. Smith, S.A. Tucker, W.E. Acree Jr., C. Yang, E. Abu-Shaqara and R.G. Harvey, Appl. Spectrosc., 46 (1992) 1156.

[5] S.A. Tucker, H. Darmodjo, W.E. Acree Jr., J.C. Fetzer and M. Zander, Appl. Spectrosc., 46 (1992) 1260.

[6] H. Dreeskamp, E. Koch and M. Zander, Z. Naturforsch., 30A (1975) 1311.

[7] U. Breymann, H. Dreeskamp, E. Koch and M. Zander, Chem. Phys. Lett., 59 (1978) 68.

[8] S.H. Chen, C.E. Evans and V.L. McGuffin, Anal. Chim. Acta, 246 (1991) 65.

[9] S.A. Tucker, H.C. Bates, V.L. Amszi, W.E. Acree Jr., H. Lee, P.D. Raddo, R.G. Harvey, J.C. Fetzer and G. Dyker, Anal. Chim. Acta, 278 (1993) 269.

[10] S.A. Tucker, J.M. Griffin, W.E. Acree Jr., P.P.J. Mulder, J. Lugtenburg and J. Cornelisse, Analyst, 119 (1994) 2129.

[11] P.P.J. Mulder, J. Olde Boerrigter, B.B. Boere, A. Baart, J. Cornelisse and J. Lugtenburg, Reel. Trav. Chim. Pays° Bas, 112 (1993) 22.

[12] R. Sangaiah, A. Gold and G.E. Toney, J. Org. Chem., 48 (1983) 1632.

[13] P.P.J. Mulder, J. Olde Boerrigter, B.B. Boere, H. Zuihof, C. Erkelens, J. Cornelisse and J. Lugtenburg, Reel. Trav. Chim. Pays-Bas, 112 (1993) 287.

[14] A.W.H. Jans, C. Tintel, J. Cornelisse and J. Lugtenburg, Magn. Reson. Chem., 24 (1986) 101.

[15] W. Dai and R.G. Harvey, manuscript in preparation. [16] S.A. Tucker, Ph.D. Dissertation, University of North

Texas, Denton, Texas, 1994. [17] N. Selvarajan, N.M. Panicker, S. Vaidyanathan and V.

Ramakrishnan, Indian J. Chem., 18A (1979) 23. [18] M.V. Encinas, M.A. Rubio and E.A. Lissi, Photochem.

Photobiol., 37 (1983) 125. [19] M.V. Encinas, M.A. Rubio and E.A. Lissi, J. Pho-

tochem., 18 (1982) 137. [20] W.M. Wiczk and T. Latowski, Z. Naturforsch., 42A

(1987) 1290. [21] E.J. Bowen and K.K. Rohatgi, Disc. Faraday Soc., 14

(1953) 146. [22] S.A. Tucker, L.E. Cretella, R. Waris, K.W. Street Jr.,

W.E. Acree Jr. and J.C. Fetzer, Appl. Spectrosc., 44 (1990) 269.

[23] C.A. Parker and W.J. Barnes, Analyst, 82 (1957) 606. [24] J.F. Holland, R.E. Teets, P.M. Kelly and A. Timnick,

Anal. Chem., 49 (1977) 706. [25] M.C. Yappert and J.D. Ingle, Appl. Spectrosc., 43 (1989)

759.

172 S. Pandey et al . / Spectrochimica Acta Part A 53 (1997) 165-172

[26] J.R. Lakowixz, Principles of Fluorescence Spectroscopy (Plenum, New York, 1983).

[27] S.A. Tucker, W.E. Acree Jr., J.C. Fetzer and J. Jacob, Polycyclic Aromat. Compds., 3 (1992) 1.

[28] S.A. Tucker, V.L. Amszi and W.E. Acree Jr., J. Chem. Educ., 69 (1992) A8.

[29] S.A. Tucker, H.C. Bates, W.E. Acree Jr. and J.C. Fetzer, Appl. Spectrosc., 47 (1993) 1775).

[30] S.A. Tucker, W.E. Acree Jr., J.C. Fetzer and R.H. Mitchell, Appl. Spectrosc., 47 (1993) 1040.

[31] S.A. Tucker, W.E. Acree Jr. and J.C. Fetzer, Appl. Spectrosc., 49 (1995) 8.

[32] S.A. Tucker and W.E. Acree Jr., Appl. Spectrosc., 46 (1992) 1388.

[33] I.B. Berlman, Handbook of Fluorescence Spectra of Aro- matic Molecules (Academic, New York, 1965).

[34] K. Nakashima and I. Tanaka, Langmuir, 9 (1993) 90. [35] M. Almgren, F. Greiser and J.K. Thomas, J. Chem. Soc.,

Faraday Trans. 1, 75 (1979) 1674. [36] R.J. Hunter, Foundations of Colloid Science, Vol. I (Ox-

ford Science, Oxford, 1989).