J. Phys. Chem. 1995, 99, 5431-5437
5431
Effects of Organized Media on the Excited-State Intramolecular Proton Transfer of 1O-Hydroxybenzo[h]quinoline E. L. Roberts, P. T. Chou? T. A. Alexander, R. A. Agbaria, and I. M. Warner* Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, and Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 Received: June 20, 1994; In Final Form: October 31, 1994@
Absorbance and fluorescence characteristics of 10-hydroxybenzo[h]quinoline(HBQ) are studed in the presence of a-,/3-, and y-cyclodextrins, and the surfactants, hexadecyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and polyoxyethylene(23) lauryl ether (Brij 35). Fluorescence measurements are used to investigate the effect of organized media on the excited-state intramolecular proton transfer (ESIPT) reaction of HBQ by monitoring the large Stokes-shifted tautomer emission. Absorbance measurements are used to further characterize the interactions of HBQ with the various media.
Introduction Substituted quinolines and isoquinolines are compounds known to have interesting photochemical and photophysical properties. These compounds are commonly used as fluorescent probes, standards, and fluorogenic substrates for enzymatic assays.' The research reported in this manuscript focuses on 1O-hydroxybenzo[h]quinoline,or HBQ (Figure la). HBQ has found application as a reagent in the preparation of optical filter agents in photographic emulsion processes and as a chelating agent for gold cation^.^^^ More recently, the compound has been reported to have potential application as an agent in radiationhard scintilla tor^.^ HBQ is a fused heterocyclic compound that contains both a pyridinic nitrogen and a phenol group. These functional groups are arranged in a position relative to each other so as to promote strong intramolecular hydrogen bonding. The arrangement of the nitrogen and phenol heteroatoms in HBQ promote the formation of a keto tautomer (Figure lb) through an excitedstate intramolecular proton transfer (ESIFT) reaction. A consequence of the E S I n reaction is an observed anomalously large Stokes-shifted emission for HBQ (Figure 2a'). Excited-state proton-transfer reactions have been the subject of numerous investigations in the The fundamentals and inter- and intramolecular reactions of this type have recently been reviewed.* The ESIFT process usually involves transfer of the hydroxyl or amino proton to an acceptor such as a carbonyl oxygen or a nitrogen atom. In the report here, we were interested in examining the influence of organized media, specifically cyclodextrins (CDs) and surfactants, upon the spectroscopic properties of HBQ. Cyclodextrins are cyclic oligosaccharides composed of six, seven, and eight glucopyranose units and are named a, b, and y-CD, respectively. These CD molecules are capable of incorporating guest molecules on the basis of size and hydrophobic interactions9 These host-guest interactions can be further influenced by factors such as hydrogen bonding, van der Waals forces, and molecular size.'0." The ability of cyclodextrin molecules to perturb certain photochemical and photophysical properties of compounds makes CDs widely useful in chemical processes such as modification of stationary ~
* Author to whom correspondence should be addressed.
' University of South Carolina. @
Abstract published in Advance ACS Abstracts, March 15, 1995.
0022-365419512099-5431$09.0010
Figure 1. Structures of (a) HBQ (the normal species), and (b) HBQ (the tautomer species).
phases in HPLC, in the design of models for enzymatic studies, and in the study of photochemical inclusion c ~ m p l e x e s . ' ~ - ' ~ Surfactants are another form of organized media which have been reported to influence photophysical properties of certain compound~.'~Surfactants are molecules characterized by hydrophobic regions and hydrophilic anterior regions that may be cationic, anionic, or nonionic. Surfactants can organize themselves into micelles at the critical micelle concentration
0 1995 American Chemical Society
Roberts et al.
5432 J. Phys. Chern., Vol. 99, No. 15, 1995
'
0.80 O0
1
V.""
320
396
472
548
624
700
Figure 2. Excitation region of absorbance spectrum (a) and emission M) in water. spectrum (a') of HBQ (1.0 x
(cmc). These micelles can enhance fluorescence by shielding compounds from external quenchers in aqueous solutions, e.g., dissolved oxygen.I6 In general, organized media have been used to examine many important photophysical processes. Specifically, cyclodextrins have been employed in the investigation of excimer formation, energy transfer, and analysis of the twisted intramolecular charge transfer state (TICT).I7-l9 The included host-guest complex of N-methyl-p-(p(dimethy1amino)phenyl)pyridinium(APP) with a- and p-CD has been reported. The spatial restrictions of APP within the CD cavity was believed to enhance the non-TICT fluorescence quantum yield and inhibit the formation of the TICT state. l 9 Pesavento studied the protonation of sulfonephthalein indicators and dyes in the presence of cationic surfactant, inferring that the distribution of charged species between the compound and surfactant is dependent upon the potential difference at the interface.20 Similarly, intermolecular excited-state proton transfer has been used to probe the microenvironments of micellar media,*' cyclodextrins,22 and proteins.23 In this study, however, we were interested in the interaction of HBQ, an excited-state intramolecular proton transfer (ESIPT) molecule, with micelles and cyclodextrin organized media. Such interactions could provide greater stability and enhancement of spectroscopic properties through inclusion and association phenomena. Thus, in this paper, we examine the effect of a-,p-, and y-CD, as well as the surfactants hexadecyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and polyoxyethylene(23) lauryl ether (Brij 35) on the spectroscopicproperties of 10-hydroxybenzo[h]quinoline. The focus of this study is to test the influence of these chemical environments upon the spectroscopic properties, and the ESIPT reaction of l0-hydroxybenzo[h]quinoline.
Experimental Section Materials. HBQ was synthesized by one of us (PTC) and was recrystallized once from heptane and EtOH. The a-, p-, and y-CDs were obtained from American Maize Products Co. (Hammond, IN). Hexadecyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and polyoxyethylene(23) lauryl ether (Brij 35) were obtained from Sigma (St. Louis, MO). All alcohol and other solvents were of HPLC and/or spectroquality grade (MallinckrodtEM Science) and were used without further purification. Apparatus. Absorbance measurements were performed on a Perkin-Elmer Lambda 3 spectrometer, and a Shimazdu UV3 lOlPC UV-vis-nearIR scanning spectrometer. Steady-state fluorescence measurements were acquired by use of a Spex Model P2T 21 1 spectrofluorimeter, and a Photon Technology International (PTI) LS- 100 luminescence spectrophotometer. Samples were measured in 1 cmz quartz cells using excitation
and emission bandwidths of 2.0-4.0 nm. All measurements were performed at ambient room temperature unless otherwise indicated. Method. Preparation of Samples. A. With Cyclodextrins. A 1.0 x M aqueous solution of HBQ was prepared by pipeting a 2.5 mL aliquot of a 0.010 M HBQEtOH stock solution into a 250 mL flask. The EtOH was evaporated under dry N2, and the flask containing the residue was filled to the mark with deionized water, sonicated for 20 min, and allowed to equilibrate for 12 h. A 5.0 mL aliquot of the aqueous HBQ solution was transferred to individual 10 mL flasks, and a volume of aqueous cyclodextrin (a,/?, y ) was added to yield the desired concentration. A 2.0 mL volume of phosphate buffer was added to the HBQKD samples to achieve pH = 7.0, and the flasks were filled to the mark with water, yielding a final HBQ concentration of 5.0 x lop6 M. The concentrations of the cyclodextrins ranged from 2.0 x lov4 to 0.010 M. The HBQ/cyclodextrin samples were allowed to equilibrate overnight and were purged with N2 gas prior to analysis. The reference solutions for absorbance studies contained the same concentrations of a-,p-, and y-cyclodextrins and were diluted to the mark with deionized water. B. With Surj-actants. An aqueous solution of HBQ (1.0 x M) was prepared as previously described. A 5 mL aliquot of the aqueous HBQ was transferred to 10 mL flasks, and a measured volume of aqueous surfactant solution was added to achieve the desired concentrations in solution. All of the flasks were filled to the 10.0 mL mark with deionized water to yield 5.0 x M HBQ in all cases. Surfactant concentrations ranged from 1.0 x to 5.0 x M for SDS, 1.0 x to 2.0 x M for CTAB, and 2.0 x to 1.0 x M for Brij 35. All blank solutions for absorbance studies contained the appropriate concentration of surfactant.
Results and Discussion HBQ in Cyclodextrins. Excited-state intramolecular proton transfer for HBQ has recently been reported! The ESIPT process usually involves transfer of a hydroxy proton to an acceptor such as a nitrogen or carbonyl oxygen atom in the excited state. The hydroxyl proton and acceptor group are more acidic and basic, respectively, in the excited state than in the ground state. Thus, the basic nitrogen or carbonyl atom can accept a proton from the acidic group and through delocalization of the positive charge about the heterocyclic ring, form a species that has a Stokes-shiftedtautomer emission. Compounds which undergo intramolecular reactions of this type have found application in lasing systems for chemicals,24molecular energy storage,25 and as high-energy radiation detectors.26 The most studied ESIPT reaction has been that involving 3 - h y d r o ~ y f l a v o n e . ~There ~ - ~ ~ have been only a few reported cases of excited-state proton transfer between the nitrogen atom and hydroxyl group in two separated aromatic rings of the same compound.4 One case involves 7-hydroxyquinoline (7HQ) which, in the presence of protic solvents, can undergo intermolecular proton transfer. In the case of HBQ, there is an additional fused benzene ring relative to 7HQ, which leaves the hydroxy group in the 10th position (Figure la). With this particular orientation, strong intramolecular hydrogen bonding between the OH proton and nitrogen atom of HBQ is more favorable. Moreover, this configuration for HBQ leads to a large Stokes-shifted tautomer emission (600 nm, & x 0.02), which is believed to be one of the largest among the ESIPT systems ~ t u d i e d . Furthermore, ~ HBQ exhibits three distinct qualities which make it more advantageous in application over other ESIPT molecules: (1) good stability, which minimizes
Absorbance Characteristics of HBQ
J. Phys. Chem., Vol. 99, No. 15, 1995 1
270 1
242
214
1 i
I
i
0.44
L
0.38
U
.
a 0.28
'
0
I 2
4
6
6
0.20
10
580
Wavelength
i
a-CD
(3-CD
i
A Y-CD
'
I 2.40
4.80
7.20
0.60
12.00
[CD]m Y
Figure 3. Influence of cyclodextrins on the tautomer emission of HBQ (iex = 370.0 nm, iem = 608.0 nm). a-, p-. and y-cyclodextrin to 0.01 M. concentrations ranged from 2.0 x
540
f
0
-
0.00
[CD] mM
0' 500
i : T
i
T
.->u
130
5433
620
660
I 700
(nm)
Figure 4. Influence of y-CD upon the tautomer emission of HBQ ( 5 . x M): (a) absence of y-CD; (b) 2.0 x M y-CD; (c) 6.0 x M y-CD (5,= 370.0 nm).
photodecomposition in high-intensity beam radiation research, (2) long-wavelength emission, which is useful for avoiding possible fluorescence interference from radiation sources, and (3) large separation of absorption and emission energies (Figure 2a, a and a'), minimizing reabsorption effect^.^ Thus we were interested in studying this new ESIPT system in the presence of organized media, and using fluorescence and absorbance measurements to depict changes in the microenvironment experienced by HBQ along with its effect upon the ESIPT reaction of the compound. Figure 3 depicts the influence of the various cyclodextrins (a,P, and y) upon the Stokes-shifted emission of HBQ. A marked enhancement of the emission of HBQ at 590 nm is observed for solutions in p- and y-cyclodextrin, most dramatically in P-cyclodextrin. A spectral shift of the emission spectra of HBQ to longer wavelengths (588-610 nm) occurs upon increasing the concentrations of both P- and y-cyclodextrin to 0.010 M. The shift relative to HBQ alone in y-cyclodextrin is shown in Figure 4. These spectral shifts suggest a preferential binding of HBQ within the hydrophobic (nonpolar) cyclodextrin cavity. Further, these "red" shifts are in agreement with solution studies of HBQ vs solvent polarity. As solvent polarity is decreased, the fluorescence of HBQ is observed to experience a shift to longer wavelength. The fluorescence intensity of HBQ increases linearly with increasing concentrations of p- and y-cyclodextrin, and begins to level off at concentrations above 6 mM. These trends could suggest concentrations at which complete complexation is
Figure 5. Influence of cyclodextrin HBQ (5.0 x M).
(a,p, and y) upon absorbance of
achieved. The spectral red shifts and the enhancement of HBQ tautomer emission in P- and y-cyclodextrins further suggest that the molecule is located within the apolar cavity of the cyclodextrin. The lack of enhancement of the Stokes-shifted emission of HBQ in solutions containing a-cyclodextrin is believed to be due to spatial restrictions of the a-CD cavity. Hansen et aL30 studied the intermolecular excited state proton transfer of protonated 1-aminopyrene complexed with p-cyclodextrin and found that the rate of proton transfer was increased by a factor of 2-3 orders of magnitude relative to that in water. It was inferred that the hydrogen-bonding interactions around the cavity of the P-cyclodextrin molecule influenced the protontransfer rate, based on a spectral blue shift of 1-aminopyrene, along with increased quantum yield of fluorescence. Similarly, Chatt~padhyay~' studied the rate of excited-state intermolecular proton transfer for carbazole and 2-naphthylamine (2NA) using steady-state and time-resolved emission spectroscopy. The results indicated an increase in the deprotonation rate of the carbazole-CD complex and a decrease in that of the 2NACD complex. These data suggest that excited-state proton transfer is dependent not only on the microenvironment of the molecule imposed by cyclodextrin but also upon the nature (Le., structure) of the compound itself. Recently, Baraka et al.32 studied the inclusion complexes of the three electronic states of 4-(N,N-dimethylamino)benzonitrilewith P-cyclodextrin. A 1:1 stoichiometry was found for all three reactions, and the data further suggest that the different excited states occupy different microenvironments within the p-CD cavity. With respect to HBQ, the rate of ESIPT is ultrafast (>>lo" s-I) in both aprotic and protic solvents,33resulting in approximately unit efficiency production of the tautomer in the excited state. Furthermore, purging the samples of HBQ:CD with NZ gas prior to analysis did not result in any change in the tautomer emission intensity of HBQ, suggesting that deactivation by quenching (e&, by dissolved 0 2 ) is minimal. Similarly, solution studies of HBQ reveal an emission lifetime (z) less than 1.5 ns, with no tautomer phosphorescence ~ b s e r v e d .Thus, ~ the increase of the quantum yield of the tautomer emission for HBQ is not due to the enhancement of the rate of ESIPT but rather the decrease of the rate of the radiationless transition due to the spatial and hydrophobic interaction with p- and y-cyclodextrin. This proposed mechanism of HBQ included in both p- and y-cyclodextrins can be definitively proven by the steady-state absorption spectra. Figure 5 depicts the general trend of HBQ absorbance versus concentration of cyclodextrin. The absorbance maximum, at 238 nm, was observed to undergo a slight decrease with inceasing concentrations of cyclodextrin (Le., in
Roberts et al.
5434 J. Phys. Chem., Vol. 99, No. IS, 1995
p- and y-CD).
Analysis of the So-S, (nn*) excitation region of the spectra (Le., 370 nm) reveals similar trends with increasing concentrations of cyclodextrin, indicating that the electronic configuration of HBQ is mediated by the spatial and hydrophobic interaction with p- and y-cyclodextrins. In addition, a slight red shift is observed relative to HBQ alone as [p-CD] is increased, both at the maximum wavelength (238 nm) and the &-SI excitation region of the spectrum. Such shifts are usually observed upon interaction of a probe molecule with cyclodextrin. Stoichiometric Ratio and Binding Strength between CDs and HBQ. Benesi-Hildebrand plots34 can be used to better understand the stoichiometric relationships between host (CD) and guest molecules, as well as the strength of an arrangement of association. Assuming a 1:l complex between HBQ and CD, the equation would be given by HBQ
+ C D = [HBQCD]
I
0' 100.0
416.0
732.0
1048.0
1364.0
1680.0
[l/p-CD] x 1 0 - * M
(1)
and the equilibrium constant for the complex is given by
K , = [HBQCD]/[HBQ][CD]
(2)
where KI is the equilibrium constant, and [HBQI, [CD], and [HBQCD] are the concentrations of the HBQ, CD, and the complex, respectively. The concentration of cyclodextrin is large with respect to the concentration of the complex, Le., [CD] >> [HBQCD]. We can assume that [CDIo = [CD], where [CDIo is the initial analytical concentration of cyclodextrin. Thus, the following equation is derived 1
1 + K , [HBQI,
-
[HBQCDI - ~ l [ H B Q I o [ ~ D l o
+
0 100.0
(3)
+
where [HBQlo = [HBQI [HBQCD] and [CDIo = [CD] [HBQCD] = [CD]. The fluorescence intensity of HBQ in the presence (I)and absence (10)of CD is proportional to [HBQCD] and [HBQ], respectively. The values can be substituted into eq 2 to yield 1 l + 1 1-10 I ,-Io Kl [CD],(Z-Z,-,)
(4)
Thus, in the case of a 1:l complex, a plot of U(1-IO) vs 1/[CD] should yield a straight line. Similarly, for a 2:l complex, the equilibrium constant and equation are given by
+ [2CDl*
[HBQCD,]
(5)
K2 = [HBQCDI/[HBQI[CDl*
(6)
[HBQI
Assuming that [CD] >> [HBQCD2][HBQCD], one can derive the equation 1 -
l zi-z~
+
1 K2[CD],2(ZI-Z0)
(7)
A straight line should be obtained when U(1-10) vs 1/[CDI2is plotted. A Benesi-Hildebrand plot assuming 1:1 association between p-CD and HBQ reveals a linear regression with a correlation of r = 0.994 (Figure 6a). A plot assuming a 2: 1 association of HBQ:p-CD reveals a regression showing an upward curvature (Figure 6b). This deviation suggests that the stoichiometry of the complex is not 2:l. Similarly, a Benesi-Hildebrand plot assuming a 1:1 host:guest interaction between HBQ and y-CD reveals a linear regression with a correlation of 0.988. Con-
5680.0 11260.0 16840.0 22420.0 28000.0
[l/P-CD]Z X 1 0 2 M
Figure 6. Benesi-Hildebrand plots for the P-CDIHBQ complex: (a, top) plot of 1/(F - Fa) vs l/@CD]; (b) plot of 1/(F- Fo) vs l/@CDI2.
versely, a plot assuming a 2: 1 association reveals a regression showing upward curvature, again suggesting that the complex formation does not have a 2: 1 stoichiometry (HBQ:y-CD). Both p-CD and y-CD have identical cavity lengths, but different cavity diameters (Le., 7.8 and 9.5 A,respectively). A 2:l HBQ: y-CD complex is plausible considering cavity diameter. Both p- and y-CD, however, have similar cavity lengths (0.78 A), which may restrict the accommodation of two HBQ molecules into one p- or y-CD cavity. The highest degree of linearity was observed for HBQ in p-CD, suggesting that the stoichiometry of p-CD with HBQ is mostly 1:l. The formation constants ( K ) of the complexes for HBQ in p-CD and y-CD can easily be calculated from the linearized Benesi-Hildebrand plots by dividing the intercept by the slope. Benesi-Hildebrand plots, however, place more emphasis on lower concentrations values than on higher concentration values. Thus, the slope of the line is more sensitive to ordinate values of the point with the smallest concentration. An alternative approach to this method is to use the K values calculated from BenesiHildebrand plots (Le., a linear regression) as estimates for parameters in the nonlinear regression (NLR) graphical method.35 The NLR program provides estimates for K by fitting the data through iteration, using the following equation (assuming a 1:l interaction)
where 1 is the measured intensity of HBQ at a given CD
Absorbance Characteristics of HBQ
J. Phys. Chem., Vol. 99, No. 15, 1995 5435
TABLE 1: Estimated Formation Constants for HBQ in Organized Media organized media formation const ( K ) log K p-CD y-CD CTAB
SDS Brij-35
513 263 25 1 115 70.8
r-----l
0.1 O.’O
2.71 2.42 2.40 2.06 1.85
concentration. The estimated formation constants for HBQ in /3 and y-CD appear in Table 1. HBQ in Micellar Media. Spectroscopic properties of HBQ were also analyzed in the presence of charged (SDS and CTAB) and nonionized (Brij 35) micellar media. Surfactants, through formation of micelles, can provide a region of hydrophobicity upon interaction of probe molecules with the hydrophilic head of the specific surfactant. The organized micellar media can then serve to protect the compound from external quenchers in solution. Thus, absorbance and fluorescence measurements of HBQ were performed in SDS, CTAB, and Brij-35 surfactant. Upon addition of all three surfactants individually, HBQ was observed to undergo a slight increase in absorbance. With the nonionized Brij-35 surfactant, HBQ experiences an enhancement of absorbance with a loss in vibronic fine structure in solutions containing higher concentrations of Brij-35 (-1.0 x M). No apparent shifts in the absorbance spectrum of HBQ were observed in the presence of Brij-35. In the presence of SDS (cationic) and CTAB (anionic) surfactants, however, there was an observed enhancement of HBQ absorbance accompanied by a shift to longer wavelength. These shifts were observed for both the region of maximum absorbance and for the SI-& (nn*) region of the absorbance spectrum of HBQ. The spectral enhancement and spectral shifts for HBQ in the presence of SDS are shown in Figure 7 (a and b). The shift to longer wavelength is indicative of a less polar microenvironment for HBQ. With respect to the observed enhancement in absorbance, HBQ undergoes a more pronounced fluorescence enhancement in the presence of these micelles. The most pronounced enhancement was observed for HBQ in CTAB and SDS. The influence of SDS and CTAB upon HBQ fluorescenceis depicted in Figure 8, a and b, respectively. Both plots show an initial linear transition at lower surfactant concentrations followed by a plateau region at moderate concentrations. At higher surfactant concentrations, the fluorescence shows another region of increased emission, followed by another region where the spectrum reaches a plateau. The transitions in the graphy may correspond to the formation of different comicellar structures with HBQ. Earlier studies of HBQ showed no evidence of dimerization or solvent-solute complex f ~ r m a t i o n .Further~ more, upon addition of higher concentrations of SDS and CTAB (>20 and 2 mM, respectively), the Stokes-shifted emission undergoes a red shift, from 588 to 610 nm. This spectroscopic shift is, again, indicative of HBQ in less polar media and suggests the formation of a micellar complex. Surfactants have been found to aggregate or deaggregate dyes such as PABT (1-(9’-phenathronyl)-2(2‘-(N-ethyl)benzothiazo1ium)ethylene bromide) and Auromine 036 based on the type of charge possessed by the specific surfactant.I6 With HBQ, however, enhanced absorbance and fluorescence are observed for both nonionized and ionized surfactants. The ionized surfactants, however, induce a red shift in the ESIPT emission at higher concentrations of surfactant. These data suggest the formation of a stronger binding interaction of HBQ in the vicinity of the charged surfactants as opposed to the nonionized Brij-35 surfactant. Transitions in the emission intensity of HBQ
0.1
0.10 ....
I
I
230
234
238
242
246
250
WaWlEngth (nm)
O.OO 0.60
t
1 I
0.00 340
350
360
370
380
390
Wavelength (nm)
M) in the Figure 7. Change in absorbance of HBQ (5.0 x presence of sodium dodecyl sulfate (SDS) between (a, top) 230-250 nm and (b, bottom) 350-370 nm: (1) absence of SDS; (2) 5 mM SDS; (3) 10 mM SDS.
with increasing concentrations of SDS and CTAB surfactants are observed at concentrations near the cmcs of the surfactants and at concentrations above the cmcs (Figure 8a,b). These regions of maximum emission intensity with increasing surfactant concentration were recently observed for interactions of Auromine 0 with Brij-78 and SDS.36 The leveling off of the dye intensity observed with increasing surfactant concentration is believed to be the result of a complete dye-surfactant interaction, along with subsequent disaggregation of the Auromine 0 dye by the surfactant^.^^ It was further assumed that, in the case of a complete probe:surfactant interaction, the molecules of Auromine 0 are bound individually to the micelles. If we assume a 1:l interaction of HBQ with the surfactants SDS, CTAB, and Brij-35, double reciprocal plots can be used to calculate estimates of the apparent HBQ: micelle binding constants (eq 4). Typical double reciprocal plots of HBQ in Brij-35 and SDS surfactants are shown in Figure 9, a and b, respectively. The K values obtained from these linear plots were obtained and used as estimates in the nonlinear regression program (eq 8). The estimated K values appear in Table 1 and further suggest stronger interactions of HBQ with the charged (SDS, CTAB) surfactants than with the nonionized Brij-35 surfactant. The transitions observed for HBQ fluorescence with increasing concentration of surfactant could further indicate concentrations at which the cmc of the various micelles occur. Chattopadhy et aL3’ studied the excited-state proton-transfer reaction for carbazole in nonionic (Triton X-100) and ionic (CTAB and SDS) micellar media using spectroscopic methods. In aqueous solutions of carbazole at specific pH’s, changes in the slopes of the curves of intensity ratio of carbazole versus concentration
5436 .I. Phys. Chem., Vol. 99, No. 15, 1995 310
Roberts et al.
T
L
t A 262
1 1 . 1 1
,TI
I
,
I
f
'
T
A
214
.
0.60 4.00
11.20
18.40
23.60
32.80
40.00
l/[Brij 351 .lo-'
9.0
0.0
18.0
27.0
36.0
40.00
45.0
/'
Concentration (mM)
32.03
r
24.03 /
0.15 0.40
-+
-+ , 0.88
1.36
1.84
2.32
2.80
l/[SDS] x 10''
70 "i 00
'
Figure 9. Double reciprocal plots for the (a, top) HBQBrij-35 complex (UI-IO vs l/[Brij-35]) and (b, bottom) the HBQSDS complex (W-I" vs l/[SDS]) assuming 1: 1 interactions. ' 4 0
I 80
120
160
200
Concentration (mu) Figure 8. Influence of (a, top) SDS (0.0-45.0 mM) and (b, bottom) CTAB (0-20.0 mM) surfactants on the Stokes-shifted emission of HBQ (5.0 x M). The CMCs of SDS and CTAB are 8.2 x and 9.2 x M, respectively.
of surfactant was observed. These breaks were believed to correspond to the cmcs of the surfactants in the specific media (acidic or basic). Similar slope changes in the curves of HBQ fluorescence intensity vs surfactant concentration are observed (Figure 8a,b). Slight transitions in the plots are observed in the vicinity of the cmcs of the three micelles SDS (8.2 x M), CTAB (9.2 x M), and Brij-35 (7.0 x M)38 in aqueous media at neutral pH. Adjustment of pH in these systems could provide a more distinct transition in each curve, with the ESIPT of HBQ used as a probe for monitoring the phase transformation of micelles in water. Conclusion
In view of our findings, the enhancement of HBQ's fluorescence in cyclodextrins, as well as the molecules absorbance and fluorescence enhancement in micellar media, may be attributed to the provision of a more favorable environment by the organized media. This hydrophobic environment seems to reduce the rate of radiationless transition of the excited tautomer state. The observed enhanced and spectral-shifted ESIPT emission could have future application in improving the characteristics of HBQ as a potential agent in radiation-hard scintillators. Furthermore, the ability of HBQ to depict changes in the microenvironment of cyclodextrins and micellar media suggests its potential application as a probe of these media.
Acknowledgment. This work was supported by a grant from the Department of Energy (DE-FG05-91ER14219). The authors thank Jian Wang and A. Yvette Will for their technical assistance. The authors are also grateful to G. A. Reed of American Maize Products for providing the CDs used in this study. References and Notes (1) Katzenellenbogen,J. A.; Haroutounian, S. A. Phoiochem. Photobiol. 1988, 47 (4). 503.
(2) Idelson, E. M. US.Patent 3, 920 667, 1975; Chem. Abstr. 1976, 84, 84. (3) Inazu, T. J. Am. Chem. Soc. 1966, 39, 1065. (4) Martinez, M. L.; Cooper, W. C.; Chou, P. T. Chem. Phys. Lett. 1992, 193 (1-3), 151. (5) Barbara, P. F.; Walsh, P. K.; Brus, L. E. J . Phys. Chem. 1989, 93, and references therein. (6) Special Issue (Spectroscopy and Dynamics of Elementary Proton Transfer in Polyatomic Systems), edited by P. F. Barbara and H. D. Trommsdorff Chem. Phvs. 1989, 136, 153-360. (7) Special Issue (M: Kasha Festschirift): J. Phys. Chem. 1991, 95, 10220- 10524. (8) Amautt, L. G.; Formosinho, S. J. J . Photochem. Photobiol. A: Chem. 1993, 75, 1. (9) Szejtli, J. Cyclodexti-ins and the inclusion complexes; Akademiai Kiado: Budapest, 1982. (10) Schutte, J. M.; Ndou, T.; Muiioz de la Peiia, A,; Greene, K. L.; Williamson, C. K.; Warner, I. M. J . Phys. Chem. 1991, 95, 4897. (11) Muiioz de la Peiia, A.; Ndou, T.; Zung, J. B.; Warner, I. M. J . Phys. Chem. 1991, 95, 3330. (12) Husain, N.; Anigbou, V. C.; Cohen, M. R.; Warner, I. M. J . Chromatogr. 1993, 635, 21 1. (13) Skvoinski. S.: Cline Love. L. J. Anal. Chem. 1984. 56. 331. (14) Y&ozu, T.; Hoshimo, M.; Imamura, M.; Shizuka, H. J . Phys. Chem. 1982, 86, 4422. (15) Rosen. M. J. Surfacianis and Interfacial Phenomena; Wiley: New York. 1989. (16) Green, M. D.; Patonay, G.; Ndou, T.; Warner, I. M. Appl. Spectrosc. 1992, 46 ( l l ) , 1724.
J. Phys. Chem., Vol. 99, No. 15, 1995 5431
Absorbance Characteristics of HBQ (17) Agbaria, R. A.; Gill, D. J. Phys. Chem. 1988, 92, 1052. (18) Murai, H.; Nizunuma, Y.; Ashikawa, K.; Yamamoto, Y. Chem. Phys. Lett. 1988, 144, 417. (19) Lyapustina, S. A,; Metelitsa, A. V.; Bulgarevich, D. S.; Alexeev, Y. E.; Knyazhansky, M. I. J. Phofochem. Photobiol. A,: Chem., 1993, 75, 119. (20) Pesavento, M. J. Chem. SOC., Faraday Trans. 1992,88 (14), 2035. (21) Bardez, E.; Monnier, E.; Valeur, B. J. Phys. Chem. 198.5, 89,50315036. (22) Eaton, D. F. Tetrahedron 1987, 43, 1551. (23) Loken, M. R.; Hayes, J. W.; Gohlke, J. R.; Brad, L. Biochemistry 1972, I I, 4779. (24) Chou, P. T.; Mcmorrow, T. J. A,; Kasha, M. J. Phys. Chem. 1984, 88, 3596. (25) Nishiya, T.; Yamauchi, S.; Hirota, N.; Baba, M.; Hanazaki, I. J. Phys. Chem. 1986, 90, 5730. (26) Harrah, L. A,; Renschler, C. L. Nucl. Inst. Methods Phys. Rev. 1985, A235, 41. (27) Chou, P. T.; Martinez, M. L.; Clements, J. H. Chem. Phys. Left. 1993, 204 (5,6), 395-399.
(28) Ormson, S. M.; Brown, R. G.; Vollmer, F.; Rettig, W. J . Phofochem. Photobiol. A: Chem. 1994, 81, 65-72. (29) Chou, P. T.; Martinez, M. L. Radiat. Phys. Chem. 1993,42(1-2), 373-378. (30) Hansen, J. E.; Dines, E.; Fleming, G. R. J. Phys. Chem. 1992, 96, 6904. (31) Chattopadhyay, N. J. Photochem. Photobiol. A: Chem. 1991, 58, 31. (32) Baraka, M. E.; Garcia, R.; Quiiiones, E. J. Photochem. Photobiol. A: Chem. 1994, 79, 181-187. (33) Chou, P. T., unpublished results. (34) Benesi, H. A.; Hilderbrand, J. H. J.Am. Chem. SOC.1949, 71, 2703. (35) SASISTAT, Release 6.03, Cary, NC: SAS Institute Inc., 1988. (36) Mwalaupindi, A. G.; Rideau, A,; Agbaria, R. A,; Warner, I. M. Tulanta 1994, 41, (4), 599-609. (37) Chattopadhyay, N.; Dutta, R.; Chowdhury, M. J. Photochem. Photobiol. A: Chem. 1989, 47, 259. (38) McIntire, G. L. Crit. Rev. Anal. Chem. 1990, 21 (4), 257. JP9415424
