Langmuir 1989,5, 805-808 temperatures near 100 OC (Figure 2 and ref 14 and 15) and apparently by solution exposures/electrodeposition,as in Figure 9. The surface chemistries of phthalocyanine thin films are obviously complex and require additional studies beyond the scope of this paper. Nevertheless, there are several phenomena which have been observed for which we can formulate hypotheses as to the mechanism for the interaction of small molecules with these surfaces. We leave aside for the moment the question of the interaction of water vapor with the GapC-C1 surface, since the effects seen in Figure 3 have been observed both with atmosphere and pure O2 exposures. The chemisorption of O2 with most Pc surfaces, including the GaPc-C1 surface, is weak, and it is therefore not surprising that the effect of O2 on the photoconductivity response in Figure 3 is not seen until 10-3-Torr pressures are achieved.20 We assume that the first adsorbed O2occupies a true surface site (Figure 11,site A), which apparently assists in the photogeneration of charge, possibly through an exciton dissociation mechanism as has been previously p r o p o ~ e d . ~At J ~higher pressures we hypothesize that the chemisorbed O2undergoes diffusion to sites just below the surface (site B in Figure 11, also previously proposed),'lz1 where ita principle role is to act as a trap site, thereby lowering the overall photoconductivity. The activation energy for photoconduction in atmosphere for several Pc's is ca. 0.1-0.5 eV,14consistent with the idea that energetically shallow traps control the flow of charge in the near-surface region, after its generation. Prolonged purging with N2 probably removes some, but not all, of these trap sites and apparently lowers the O2level on the surface (A sites) to trace levels. The presence of such 02-filled, A sites is confirmed with the first introduction (20) (a) Dahlberg, S.C . Appl. Surf. Sci. 1982, 14, 47. (b) Dahlberg, S. C.; Mueser, M. E.Surf. Sci. 1979, 90,1.
(21) Bonham, J. Aust. J. Chem. 1978,31, 2117.
805
of NH3 as a small photoconduction drop. Further introduction of NH3 appears to affect photoconduction through the action of diffusion of the NH3 molecule into a chemisorption/trap site B and either displacement of the molecule responsible for the creation of this trap (O,?) or charge donation to the molecular entity which constitutes the trap. Prolonged exposure of the Pc thin film to O2in the atmosphere apparently creates more of the type A sites, which enhance photoconduction and are then acted upon by NH3 to decrease the photoconductivity (Figure 7). At elevated temperatures, the action of NH3 is apparently to act primarily upon those sites responsible for charge generation, thereby lowering the net photoconductivity. Because of the diversity of surface states on these and other Pc thin films, the development of chemical sensor applications of these materials probably requires a combination of (a) controllable introduction of "impurities", which provide a selective chemisorption site for analyte molecules of interest, and (b) better control over the ordering of Pc molecules or other cyclic conjugation molecular semiconductor materials, on both the MC and QCM, so as to provide a smaller range of chemisorption sites and therefore some hope of understanding the nature of their effect on the conduction and photoconduction processes. Research in progress using electron spin resonance spectroscopies and thermal desorption techniques seeks to identify the energetics of such chemisorption sites on better ordered molecular semiconductor t h i n - f h surfaces.
Acknowledgment. We wish to thank Fred Hickernell of Motorola for the generous donation of the microcircuits used in these and other recent studies. This work was assisted by support from the National Science Foundation (CHE 86-18181) and by the Materials Characterization Program-University of Arizona. Stimulating discussions with J.-P. Dodelet are also gratefully acknowledged. Registry No. GaPc-Cl, 19717-79-4;02,7782-44-7;NHS, 7664-41-7.
Deactivation of Naphthalene and Pyrene Derivatives Bound to Aerosol OT Reverse Micelles by Fumaronitrile and Acrylonitrile M. V. Encinas* and E. A. Lissi Departamento de Quimica, Facultad de Ciencias, Universidad de Santiago de Chile, Santiago, Chile
C. M. Previtali and J. Cosa Departamento de Qulmica y Flsica, Universidad Nacional de Rio Cuarto, 5800 Rio Cuarto, Argentina Received July 27, 1988. I n Final Form: January 13, 1989 Fluorescence quenching of cationic and anionic derivatives of naphthalene and pyrene by fumaronitrile and acrylonitrile in AOT reverse micelles was investigated as a function of the water content. For donors bound to the micellar interface, quenching rate constants ( k ~were ) ~evaluated ~ in terms of the interfacial quencher concentration. For both fumaronitrile (a diffusion-controlled quencher) and acrylonitrile (whose quenching rates are determined by the solvent polarity), the quenching rate constants obtained are considerably smaller than those obtained in homogeneous solvents, and they increase when the water/AOT ratio increases. For ppenetetrasulfonic acid (sodium salt) at high water/AOT ratios, the quenching rate constant obtained in terms of the quencher concentration in the water pool is a factor of 2 smaller than that obtained in bulk water and slightly increases when the water/AOT ratio increases.
Introduction The fluorescence quenching in reverse micelles and or water in oil microemulsions has been extensively studied
and reviewed.'$ The results obtained in these systems are generally discussed in terms of two limiting cases, based on whether or not the probe is associated with the mi-
0743-7463/89/2405-0S05$01.50/0@ 1989 American Chemical Society
806 Langmuir, Vol. 5,No. 3, 1989
celles.' For the quenching of probes that are totally associated with the micelles, there are extensive studies performed employing ions as quencher~.'-~On the other hand, there are very few studies employing as quencher neutral molecules that can be partitioned between the dispersion medium and the micellar aggregates. Most of the studies were devoted to determining the effect of water concentration (R = [H,O]/ [surfactant]) on the additive quenching efficiency. With regard to thiseffect, it has been reported that the quenching increases when R increases (quenching of pyrenesulfonic acid (PSA) in AOT by nitromethane4), decreases (quenching of PSA by diiodomethane4 in AOT or carbon tetrachloride in dodecylammonium propionate (DAPY and quenching of 241naphthy1)acetic acid and 6-(l-naphthyl)hexanoic acid by triethylamine in DAP6), or remains nearly constant (quenching of 2 4 1-naphthy1)acetic acid by m-dicyanobenzene in DAP6 and quenching of zinc tetramethylpyridylporphyrin by duroquinone in benzyldimethyl-nhexadecylammonium chloride (BHDC)'). Interpretation of these results is generally hindered by the lack of information regarding the distribution of the probe between the solvent, the interface, and the water pool. This precludes a meaning comparison of bulk quenching rate constant when R and/or the probe is changed. Furthermore, it has to be considered that the observed change in apparent quenching efficiency with R is going to be determined by different factors whether the observed process is diffusion controlled or not. Also another factor that complicates an analysis of the data is the fact that the average location of the probe can be changed when R changes.s We have reported previously the partition of a series of unsaturated compounds of widely different hydrophobicity between the oil pseudophase and the micellar pseudophase a t different R in the AOT/heptane/water ~ y s t e m . ~ Furthermore, applying a simplified three-pseudophase model, we evaluated the fraction of acrylonitrile and fumaronitrile incorporated into the water pool of the micelle. We report now data bearing on the quenching capacity of these compounds toward a series of micelle-incorporated probes and discuss the results in terms of the quencher partitioning and the average location of the probe.
Experimental Section AOT was purified by the procedure previously described.1° 1-(1-Naphthy1)ethylamine(NEA) (Aldrich) and 24-naphthyl)-
acetic acid (NAA) (Fluka) were employed as received. Their absorption and emission spectra were in agreement with those reported in the literature. These compounds show a monoexponential fluorescence decay. 1-Methylpyrene(MP), pyrenesulfonic acid (PSA),11-(1-pyreny1)undecyltrimethylammonium iodide (PUTMA),1-pyrenyl(methy1)trimethylammoniumiodide (1)Kalyasundaran, K. Photochemistry in MicroheterogeneousSystems; Academic Press: New York, 1987;p 143. (2)Geladd, E.; De Schryver, F. C. Reverse Micelles: Fluorescence,a method to obtain information about reverse micellar systems; Luisi, P. L., Straub, B. E., Ne.; Plenum: New York, 19&1;p 143. (3)Verbeeck, A.; De Schryver, F. C. Langmuir 1987,3,494. (4)Thomas, J. K.Chem. Rev. 1980,80,283. (5)Correll, G.D.; Cheaer, R. N.; Nome, F.; Fendler, J. H. J.Am. C h m . SOC.1978,100,1254. (6)(a) GeladB, E.;Boens, N.; De Schryver, F. C. J. Am. Chem. SOC. 1978,82,423.(b) Geladd, E.;De Schryver, F. C. J. Am. Chem. SOC.1982, 104,6288. (7)Costa, S. M. B.; Lopes, J. M. F. M.; Martins, M. J . T.J. Chem. SOC.,Faraday Trans. 2 1986,82,2371. (8)Geladd, E.;De Schryver, F. C. J. Photochem. 1982,18,223. (9)Encinas, M. V.; Lissi, E. A. Chem. Phys. Lett. 1986, 132,545. (10)Maitra, A. N.;Eicke, H. F. J.Phys. Chem. 1981,85,2687.
Encinas et al. Table I. Quenching of Pyrene Derivatives by
FumaronitrileO
donor
MP
solvent heDtane
R
heptane:EtOH (1:l) EtOH
PMTMA W
Et0H:W (1:l) EtOH AOT
PUTMA
PSA
5 10 20 30
Et0H:W (1:l)
EtOH AOT
5 10 20 30
W
Et0H:W (1:l) EtOH
AOT
PTS
5 10 20 30
W
EtOH AOT
6 15 25
(kdbdk
21 12 10 11.6 6.4 7.3 6.2 6.0 6.3 7.7 6.7 8.8 1.2 1.0 1.1 1.1 7.6 4.3 7.8 8.2 8.3 8.8 9.1 4 2.8 4.3 9.67 10
(ka)ht
k J - 1
0.16 0.19 0.23 0.27 0.027 0.029
0.036 0.045
0.21 0.25 0.31 0.36
0.14 2 2.6
"All rate constants are given in log M-' s-l. Table 11. Quenchingof l-(l-NaDhthul)ethulamineSinglet" quencher solvent R ( k p ) m (k&t acrylonitrile EtOH 1.2 EtOHW (1:l) 2.0 W AOT
fumaronitrile
13 5 10 15 20 25 30
EtOH Et0H:W (1:l) W AOT
5 10 15 20 25 30
0.72 0.72 0.96 0.84 0.97 1.2 65 53 85 145 145 157 158 145 156
0.056 0.057
0.080 0.074 0.092 0.127
3.6 4.4 5.2 5.5 5.2 5.8
All rate constants are given in lo* M-' s-l.
(PMTMA),and pyrenetetrasulfonic acid (sodiumsalt) (PTS) were Molecular Probe products. Their fluorescence decays were monoexponential. Fluorescence-quenchingexperiments were carried out in a LS-5 Perkin-Elmer spectrofluorimeter. Pyrene derivative quenching studies were also performed by following the change in fluorescence lifetimes. These experiments were performed by recording the fluorescence decay in a Tektronik 7633 oscilloscope after excitation with the pulse of a Nitronite nitrogen laser. Naphthalene derivative lifetimes were measured by employing fluorescence lifetime equipment that consists of an arc lamp (Xenon nanolamp; 10-ns fwhm) as the excitation source. The sample was located in the cavity of a TRW 75-A filter fluorometer. The signal of the photomultiplier was displayed, averaged, and digitized by a Hewlett-Packard 54200A oscilloscope. It was then transferred via an IEEEinterface to an IBM-XTcomputer, where it was processed. All measurements were carried out at room temperature in heptane/AOT (0.2 M)/water solutions. NAA was incorporated as sodium salt to water pools prepared with water at pH 10. NEA
Langmuir, Vol. 5, No. 3, 1989 807
Deactivation of Naphthalene and Pyrene Derivatives Table 111. Quenching of 2 4 1-Naphthy1)aceticAcid Singlet" a uencher solvent (kahulL (ko)et acrylonitrile EtOH 0.54 Et0H:W (1:l) 2.03 W 3.8 AOT 5 1.5 0.10
Table IV. Values of (kp)ht at R = 10 Compared with Those for Homogeneous Ethanol
~
8 10 20
fumaronitrile
EtOH Et0H:W (19) W AOT
10 16.7 25
" All rate constants are given in lo8 M-'
1.6 2.0 2.1 8.4 5.8 8.6 14 10 6.7
0.13 0.17 0.20
fumaronitrile
acrylonitrile
0.05" 0.026 0.003 0.032 0.068 0.033
0.048 0.31
"Value obtained at R = 6. 0.28 0.34 0.34
s-'.
was introduced as an ammonium salt to water pooh prepared with water at pH 3.
Results and Discussion All fluorescence decays of the probes in the absence of quenchers could be fitted, within the limits of our experiments, by a single exponential. Stationary quenching experiments of all probes studied with varying concentrations of quenchers (expressed as analytical concentrations) gave lineal Stern-Volmer plots. From these plots, values of (kg)bulk70were obtained. The results obtained are shown in Tables 1-111. For the pyrene derivatives, for which quenching efficiencies were evaluated from fluorescence intensity changes and lifetimes, similar ( k Q ) m values were obtained from P / I or T O / T plots. This normal behavior is that expected under our experimental conditions, since in all the systems studied the average number of quencher molecules per micelle is considerably larger than 1. Quenching of PMTMA, PUTMA, PSA, NEA, and NAA by Fumaronitrile. The data of Tables 1-111 show that the quenching by fumaronitrile in homogeneous solvents is diffusion controlled. For 1-methylpyrene, a model compound soluble in nonpolar solvents, the quenching is diffusion controlled even in n-heptane. Furthermore, all the probes employed can be considered to remain associated to the interface in the AOT solutions at all the R values Nevertheless, different locations of the chromophore can be expected when the alkyl chain increases (i.e., between PMTMA and PUTMA) or for compounds of different charge (Le., between PMTMA and PSA or NEA and NAA). If it is assumed that the probe remains associated to the interface, its decay can be expressed as
where the bimolecular quenching rate constant (kQ)i,tis given in terms of the quencher concentration at the interface [&Ihp This concentration was evaluated from the amount of quencher associated with the interface and the volume of the interfacial region. The amount of quencher in the interface was obtained from the partition constants reported previo~sly.~The volume of the interface was estimated from the area per surfactant head and the thickness of the interface." The values of ( K ) obtained with this procedure are given in Tables I-IfiYIt can be seen that for all compounds considered (kQ)intincreases when R increases. Since the quenching process is diffusionally controlled, this type of effect can be associated to (11)Maitra, A. J. Phys. Chem. 1984,88, 5122.
((kQ)int)R-lO/ (kQ)EM)H
compound PTS PMTMA PUTMA PSA NEA NAA
a decrease in the viscosity of the interface.12 Nevertheleas, the interpretation of these results is not straightforward since the quenching rate constant will also be strongly dependent upon the average location of the probe and the quencher in the interfacial region. In the present case, and due to the fact that different probes show similar effects, we considered that changes in the properties of the media offer a more general explanation than changes on average locations for explaining the results obtained. Furthermore, the diffusionally controlled rate between a nonionic quencher an excited probe totally incorporated to the micelles could be dependent on the quencher entrance rate to the micelles, a process that also can be considered to be dependent on the value of R.l8 However, under conditions such as those prevailing in the present work (large occupation number), the process can be considered as "intramicellar", and the quencher incorporation rates must have only a negligible effect on the observed quenching rate constants. Another relevant point is that, for all the compounds considered, (kQ)btvalues are considerably smaller than k~ values in homogeneous solvents, and noticeable differences are observed with regard to the (kQ)bt values of different compounds. Values of (( k Q ) ~ J ~ l l o / ( K q ) ~ ~are H collected in Table IV. This table shows that the charge of the ion has little significance, indicating that for monocharged hydrophobic ions the location of the aromatic ring barely depends on the ion charge. On the other hand, the large difference between PMTMA and PUTMA clearly reflects the different average location of the pyrenyl moiety in both compounds. A deeper penetration of the chromophore in the n-heptane pseudophase will sense a lower fumaronitrile concentration which will be reflected in a lower value of (KQ)hv Similar conclusions regarding the relative location of the pyrene derivatives have been derived from oxygen-quenching e~periments.'~ In a study of the quenching of pyrene derivatives by carbon tetrachloride in DAP aggregates? it has been re( k Q ) d o h m is considerably larger for ported that pyrenebutyric acid than for PSA. Similar results were obtained in the quenching of 6 4 1-naphthy1)hexanoicacid and 2-(l-naphthyl)acetic acid by triethylamine in DAP reverse micelles? In the last work, it was found that an increase in R decreases the apparent bimolecular rate constant, a result explained in terms of a decrease of the ability of the quencher to reach the micelle-bound probe. The difference between these results and the results reported in the present work can be explained in terms of the larger hydrophilicity of fumaronitrile compared to that of carbon tetrachloride or triethylamine. Quenching by Acrylonitrile. Quenching of I W A and NAA by acrylonitrile in homogeneous solvents is not a diffusion-controlled process, and the rate of the process (12)Zinsli, P.E.J. Phys. Chem. 1979,83, 3223. (13)Kikuchi, K.;Thomas, J. K. Chem. Phys. Lett. 1988, 148, 245. (14) Abuin, E. B.; Lissi, E. A,; Saez, M. Lungmuir, submitted.
808
Langmuir 1989,5, 808-816
is determined by the solvent polarity.16 In spite of this, the data of Tables I1 and I11 show that the quenching rate constanta in reverse micelles follow a similar pattern to that obtained employing fumaronitrile as quencher: (kQ)ht values are smaller than those obtained in water or ethanol and increase when R increases. For acrylonitrile, this increase must then reflect a more favorable spatial distribution and/or a more polar environment when R increases. Quenching of PTS by Fumaronitrile. Quenching of PTS by fumaronitrile in homogeneous solvents is slower than that obtained for the other pyrene derivatives. Furthermore, and due to ita high negative charge, PTS will be incorporated to the micellar water p00l.9.~ The values of (kQ)min AOT solutions show a different pattern than those observed for the other pyrene derivatives, increasing by a factor of nearly 2 when R increases. However, at R = 6, when it can be considered that a significant "pool" is not present,16the value of (kQ)ht/ (kQ)EaHis similar to that obtained for other pyrene derivatives whose chromophores can be considered to remain in the interfacial region (see Table IV). At larger R values, PTS must be displaced toward the water pools? Under these conditions, (15) Encinae, M.V.;LiSei, E. A. J. Photochem. 1986,29, 386. (16) (a) Wong,M.; Thomas, J. K.; Now&, T. J. Am. Chem. SOC.1977, 99,4730. (b) Baedez, E.;Monnier, E.;Valeur, B. J. Phys. Chem. 1986, 89, 5031.
the fluorescence decay of PTS must be represented by T-l
=
(To)-'
+ (k~)~i[Ql,i
(2)
where [Q] is the average quencher concentration in the water pooYThese values can be evaluated from the reported values of the quencher partition constant between the n-heptane pseudophase and the poolg and the size of the pool," which can be approximately evaluated from the amount of water associated to it. The values show a small increase with R, and absolute values are less than a factor of 2 smaller than those in bulk water. It is important to note that, since PTS must be concentrated toward the center of the pool3and since fumaronitrile, due to its lack of charge, can be considered as nearly equally distributed over all the pool, the obtained rate constant can be considered as a representative measure of the characteristics of the center of the core of the water pool of the reverse micelle as a reaction media.
Acknowledgment. This work was carried out as part of a project supported by CONICET (Argentina) and CONICYT (Chile), with financial support from FONDECYT (Grant No. 1433). Redstry NO.AOT, 577-11-7; NEA,3309-13-5;NAA (sodium Salt), 61-31-4; MP, 2381-21-7; PSA, 38886-82-7; PUTMA, 103692-03-1;PMTMA, 72185-47-8;PTS (sodium salt), 10829262-2; fumaronitrile, 764-42-1; acrylonitrile, 107-13-1;heptane, 142-82-5.
General Anesthetic Agents and the Conformation of Proteins. 2. Strengthening of Hydrophobic Interaction in @-Lactoglobulinby 1-Alkanols and the "Cutoff"Effect in Anesthetic Potency Mohammad Abu-Hamdimah* and Kamlesh Kumari Chemistry Department, University of Kuwait, Safat, 13060,Kuwait Received June 16,1988. I n Final Form: January 12,1989 By use of the ability of a nonionic amphiphile to depress the levorotation of a protein at 546 nm as a measure of the ability to strengthen hydrophobic interaction (HI) in the protein solution, the effect of homologous 1-alkanols (ethanol, propanol, butanol, hexanol, octanol, decanol, dodecanol, tridecanol, tetradecanol, pentadecanol, and hexadecanol) on the optical rotation of &lactoglobulin (j3LG) has been determined at 25 "C. For the anesthetically active alcohols, we have investigated the effect at concentrations that do not exceed the concentrationnecessary to anesthetize 50% of subjecta (EDSO)and verified the previous conclusion that anesthetic potency is linearly related to the ability to strengthen HI in the protein solution. The ability to strengthen HI depends on the protein concentration and the alcohol chain length. HI increases with decreasing protein concentration. The effect of chain length shows two distinctive regions, the first region from C2to C14showing an initial linear increase up to Cswith the ability thereafter tending to level off at dodecanol. In the second region, the ability to strengthen HI increases linearly from C13 to Cia. The onset of curvature in the ability to strengthen HI after C8 suggests the size of hydrophobic region to be about Cs. HI in the second region is interpreted as probably due to intramolecular bridging of two hydrophobic regions on the surface of the protein. The corresponding strengthening of HI by 1-alkanols in solutions of sodium dodecyl sulfate (SDS)was also determined. The ability to strengthen HI as a function of chain length shows no diecontinuity or significant curvature. It is argued that the hydrophobicinteraction of anesthetic agents with membrane proteins, forming ionic or hydrophilic channels, is represented by the interaction observed in the first region (C2-C12)in BLG, thus rationalizing the "cutoff" effect in anesthetic potency in homologous 1-alkanols.
Introduction Studies on the effect of amphiphiles on proteins in aqueous solutions indicate that there are two main effects observed.'-" The first, which has been extensively in(1) Duggan, E.L.; Luck, J. M. J. B i d . Chem. 1948,172, 205. (2) Tanford, C.; De,P. K.; Taggart J.Am. Chem. SOC.1960,82,6028.
vestigated, occurs at relatively high concentration of the additive resulting usually in denaturation of the protein (3) Tanford, C.; De, P. K. J. Biol. Chem. 1961,236, 1711. (4) Roaenberg, R. N.; Rogers, D. W.; Haebig, J. E.; Steck, T.L. Arch. Biochem. Biophys. 1962,97,433. ( 5 ) Herskovitz, T. T.; Mescanti, L. J. Biol. Chem. 1966, 240, 639.
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