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Langmuir 1996, 12, 2932-2938
Reaction of Zinc Phthalocyanine Excited States with Amines in Cationic Micelles Marta E. Daraio, Axel Vo¨lker, Pedro F. Aramendı´a,* and Enrique San Roma´n INQUIMAE, Departamento de Quı´mica Inorga´ nica, Analı´tica y Quı´mica Fı´sica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabello´ n 2, Ciudad Universitaria, 1428 Buenos Aires, Argentina Received November 28, 1995. In Final Form: March 11, 1996X Excited state deactivation of a zinc phthalocyanine (ZnPc) by amines was studied in micelles of hexadecyltrimethylammonium chloride. The singlet state deactivation was studied by fluorescence quenching. This technique allows us to determine equilibrium constants for the distribution of the amines between the aqueous and the micellar phases. It could be established that amines associate to the micelles in two different ways: with a greater affinity to a saturable number of sites, that we will term binding sites, and with a lower affinity by a partitioning mechanism. Equilibrium constants could be determined for aliphatic and aromatic amines. A kinetic scheme taking into account the simultaneous quenching of ZnPc fluorescence by the two types of micellized amines could be successfully applied to derive singlet quenching rate constants, under the assumption that micelles behave like closed compartments during the singlet deactivation. Aromatic amines are more efficient than aliphatic ones, and partitioned quenchers are more effective than bound quenchers. Aromatic amines also deactivate the triplet state of ZnPc. By flash photolysis, the absorption of the anion radical of ZnPc was detected. This species originates on singlet and triplet quenching, indicating that both proceed by electron transfer.
1. Introduction Extensive research has been carried out on reactions of excited states in micellar solutions.1-3 From quenching measurements of excited singlet or triplet states information can be obtained on reaction mechanisms and on distribution of molecules in microheterogeneous systems. The main mechanisms for association to the micelles are partitioning and binding. Partitioning resembles the distribution of a solute between two solvents, with practically no limit to the amount of solute in either of the phases.4 This mechanism is adequate to describe solute partitioning far from the saturation limit. On the other hand, binding5 results in the association of solute molecules to a limited number of sites in a micelle,6 resembling an adsorption mechanism. When the occupation is much lower than the saturation, the binding distribution coincides with the partitioning prediction. The only difference can be established on molecular interaction grounds. The determination of equilibrium parameters for these distribution processes, when they exist either separated or simultaneously, is well documented in the literature.7,8 The way of association of the solute to the micelle determines its distribution statistics,4,6,9,10 which deeply influences the kinetics in microheterogeneous systems. * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, May 1, 1996. (1) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: Orlando, FL, 1987. (2) Gra¨tzel, M. Heterogeneous Photochemical Electron Transfer; CRC Press: Boca Raton, FL, 1989. (3) Gehlen, M. H.; De Schryver, F. C. Chem. Rev. 1993, 93, 199. (4) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289. (5) The term binding might be associated with the existence of a stronger interaction of the solute with the micelle than in the case of partitioning. However, in this work, the two terms are used to distinguish between a solute which is at an occupation far away from the saturation limit, in the case of partitioning, and a solute that covers a concentration range attaining saturation, in the case of what we designate as binding. (6) Hunter, T. F. Chem. Phys. Lett. 1980, 75, 152. (7) Encinas, M. V.; Lissi, E. A. Chem. Phys. Lett. 1982, 91, 55. (8) Blatt, E.; Chatelier, R. C.; Sawyer, W. H. Chem. Phys. Lett. 1984, 108, 397.
S0743-7463(95)01082-1 CCC: $12.00
The kinetics of various fluorescence quenching mechanisms has been analyzed and analytically solved. The models include the cases where partition,4 binding,9 or both processes11 describe the quencher association to the micelles. While the first two models4,9 quite generally include the competition between excited state deactivation and quencher exchange between phases or between micelles, the last one11 is restricted to the case of fast exchange of the quencher, i.e. the case when the equilibrium distribution of quencher between micelles is maintained during the quenching events. For molecules with short fluorescence lifetimes (of a few nanoseconds), this is not a realistic approach. Sensitizers in photodynamic therapy are believed to associate to cell membranes. Micelles are the simplest but easiest-to-realize models for biological membranes, as the competition between hydrophobic and hydrophilic interactions is responsible for the stability of both systems. Sensitizers act by two sensitization mechanisms named type I and type II,12 depending on the way the excited state of the sensitizer is deactivated. The type I mechanism involves the generation of radicals; electron transfer reactions are a special case of this type of photosensitization. Type II is the generation of singlet molecular oxygen (1∆g) by energy transfer from the triplet excited state of the sensitizer (generally) to dissolved ground state molecular oxygen. In an actual system both mechanisms should operate in parallel. On the other hand, electron transfer after light absorption is the main path for systems converting light into chemical energy.13 Microheterogeneous systems have been used for stabilization of species generated after charge transfer to increase the efficiency of charge (9) Tachiya, M. J. Chem. Phys. 1982, 76, 340. (10) Daraio, M. E.; Aramendı´a, P. F.; San Roma´n, E. Chem. Phys. Lett. 1996, 250, 203. (11) Blatt, E.; Chatelier, R. C.; Sawyer, W. H. Biophys. J. 1986, 50, 349. (12) Jori, G., Perria, C., Eds. Photodynamic Therapy of Tumors and Other Diseases; Libreria Progetto Editore: Padova, 1985. (13) Wilkinson, F. In Photoinduced Electron Transfer; Fox. M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part A, Chapter 1.5.
© 1996 American Chemical Society
Zinc Phthalocyanine Excited States plus Amines
separation.14 Therefore, there exists a great interest in the study of charge transfer reactions in microheterogeneous systems. In a series of previous papers15-17 we reported the synthesis and photochemical characterization of a phthalocyanine sensitizer, zinc dicarboxydiamide phthalocyanine (ZnPc), and we studied its energy transfer ability to ground state oxygen as well as its oxidative quenching by quinones. In the latter case, we could characterize the kinetics in a system where simultaneous quenching of singlet and triplet excited states takes place.18 Few investigations have been performed on the association parameters of amines to micelles.8,19-21 In order to characterize the reductive quenching of ZnPc, in the present paper we present results on the deactivation of the singlet and triplet states of this sensitizer by amines in micellar solutions of hexadecyltrimethylammonium chloride (CTAC). A model of fluorescence quenching derived previously,10 which takes into account simultaneous quenching by two types of micelle-associated quenchers, is applied to the study of singlet quenching of aromatic and aliphatic amines. The results allow us to know the type of association of these amines to micelles and to evaluate the intramicellar quenching constants. We further study excited state triplet quenching, and we identify the anion radical of ZnPc. 2. Experimental Section Chemicals. ZnPc was synthesized and purified as described previously.16 CTAC (Kodak), dimethyl sulfoxide (DMSO, Mallinckrodt), diethylamine (DEA, Merck), triethylamine (TEA, Merck), N,N-dimethylaniline (DMAn, Aldrich), and N,N-diethylaniline (DEAn, Aldrich) were used as supplied. N,N,N′,N′tetramethylbencidine (TMB, Aldrich) and pyrene (Merck) were recrystallized from ethanol. Water was purified by a Millipore (Milli-Q) system. Preparation of CTAC Solutions. Micellar solutions were allowed to stand for at least 48 h before use in order to avoid variations in the quenching rates due to changes in the micellar microviscosity.22 All samples were taken to pH ) 12 with NaOH to assure complete deprotonation of the amines. Spectra. Absorption spectra were recorded on a Shimadzu UV-160 A spectrophotometer. Fluorescence spectra and steady state fluorescence quenching measurements were carried out on a Perkin-Elmer LS-5 spectrophotometer at room temperature in air-equilibrated solutions. The excitation and emission wavelengths for the quenching experiments were 612 and 694 nm, respectively. Flash Photolysis. Laser flash photolysis experiments were carried out with a dye laser (Pyridine 1, 690 nm), excited by a frequency-doubled Nd-YAG laser (Spectron Lasers). Absorption detection used a 100 W tungsten-halogen lamp as monitoring light and a Hamamatsu R928 photomultiplier as detector. Transient absorption was digitalized and averaged with a HP54502 digital oscilloscope and stored in a PC-AT 286. All solutions were degassed by prolonged bubbling with nitrogen.
3. Results and Discussion 3.1. Deactivation of the Singlet Excited State. The interaction of the ZnPc singlet excited state with DEA, (14) Meisel, D.; Matheson, M. S. In Photocatalysis. Fundamentals and Applications; Serpone, N., Pelizzetti, E., Eds.; J. Wiley: New York, 1989; Chapter 12. (15) Negri, R. M.; Zalts, A.; San Roma´n, E. A.; Aramendı´a P. F.; Braslavsky, S. E. Photochem. Photobiol. 1991, 53, 317. (16) Daraio, M. E.; Aramendı´a, P. F.; San Roma´n, E. A.; Braslavsky, S. E. Photochem. Photobiol. 1991, 54, 367. (17) Daraio, M. E.; Aramendı´a, P. F.; San Roma´n, E. A. J. Photochem. Photobiol., A:Chem. 1994, 77, 41. (18) Daraio, M. E.; Aramendı´a, P. F.; San Roma´n, E. Chem. Phys. Lett. 1993, 204, 415. (19) Gehlen, M. H.; Berci Fo, P.; Neumann, M. G. J. Photochem. Photobiol., A: Chem. 1991, 59, 335. (20) Melo, E. C. C.; Costa, S. M. B. J. Phys. Chem. 1987, 91, 5635. (21) Atik, S. S.; Thomas, J. K. J. Am. Chem. Soc. 1981, 103, 3550. (22) Gra¨tzel, M.; Thomas, J. K. J. Am. Chem. Soc. 1973, 95, 6885.
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TEA, DEAn, DMAn, and TMB (quencher Q, in general) was studied by steady state fluorescence. The experiments were carried out in aqueous solutions of different CTAC concentrations, from 0.033 to 0.100 M, and in DMSO. No changes in the excitation and emission spectra of ZnPc were observed by adding amines, and the visible absorption spectra were unchanged after irradiation, indicating that ZnPc is not degraded by the quenching reaction. In the fluorescence quenching experiments no difference in the quenching capability could be detected during the measurement time, so the degradation of the amine is also negligible under these conditions. DEA does not quench ZnPc fluorescence in micelles. The inefficiency of quenching by TMB was attributed to the low solubility of this amine in CTAC solutions (ca. 1 mM). Results are summarized in Figure 1. Kinetic analysis of the results obtained in micellar solutions requires a previous evaluation of ZnPc and quencher distribution between the aqueous phase and the micellar pseudophase. Spectroscopic studies on the ZnPc dimerization equilibrium in cationic micelles16 showed that this molecule is fully incorporated into the micelles. Stern-Volmer type plots like those of Figure 1 contain all the equilibrium distribution information of the quencher and the kinetic information as well. In order to obtain quantitative information on the quencher distribution between the aqueous solution and the micellar pseudophase, Encinas and Lissi7 proposed a method that is independent of the quenching mechanism and the type of quencher distribution. This method considers that the relationship between the steady state fluorescence intensity in the absence of quencher, If0, and that obtained at a given quencher concentration, If, will be determined only by the mean occupation number of quencher molecules per micelle, 〈n〉. This is true under the assumptions that (i) the mean aggregation number of the micelles (the number of surfactant molecules that form a micelle) is constant in the surfactant concentration range of analysis (which is true for CTAC23) and (ii) the quenching mechanism and rate constants for elementary intramicellar events are independent of the quencher concentration.24 The mass balance for Q indicates that the total quencher concentration [Q]t is related to the micelle concentration [Mic] by
[Q]t ) [Q]w + 〈n〉[Mic]
(1)
where 〈n〉 ) [Q]m/[Mic] and [Q]m and [Q]w denote the Q concentration (referred to the total volume) in the micellar and aqueous phase, respectively. [Mic] was calculated considering the surfactant concentration and taking 105 as the aggregation number for CTAC.1,23 According to the hypotheses of the method, horizontal intercepts of plots of If0/If versus [Q]t (Figure 1) correspond to systems with equal 〈n〉. Values of 〈n〉 as a function of [Q]w were obtained from plots of [Q]t versus [Mic] (Figure 2), as indicated by eq 1. In this set of data is the information of equilibrium parameters of Q. As 〈n〉/[Q]w depends on [Q]t, a simple distribution mechanism cannot be applied (see Figure 3), and the situation is more complex than a pure partition. Data have been analyzed taking into account the possibility of two different association mechanisms: partitioning and (23) Lianos, P.; Viriot, M. L.; Zana, R. J. Phys. Chem. 1984, 88, 1098. (24) Daraio, M. E.; Aramendı´a, P. F.; San Roma´n, E. J. Braz. Chem. Soc. 1995, 6, 161.
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Figure 2. Encinas-Lissi plots for TEA and DEAn in CTAC, at different If0/If ratios.
Figure 1. Fluorescence quenching of ZnPc by (a) TEA, (b) DEAn, and (c) DMAn in CTAC micelles at different surfactant concentrations. The lines are the best fit curves according to eq 3. The corresponding fitting parameters are listed in Table 1.
binding.25 In this case, the following equation can be derived:8
(〈n〉/[Q]w) ) VmKp + (NKb/(1 + Kb[Q]w))
(2)
where Vm is the molar volume of the micelles (17 M-1 for CTAC26), Kp is the partition constant, and Kb (M-1) is the (25) Haigh, E. A.; Thulborn, K. R.; Nichol, L. W.; Sawyer, W. H. Aust. J. Biol. 1978, 31, 447.
binding constant. N is the maximum number of molecules allowed in binding sites. No interaction among quenchers is taken into account besides the exclusion effect of occupation of a binding site. [Q]w depends on N, Kb, Kp, [Q]t, and [Mic],11 and it is experimentally obtained as the intercept of the [Q]t versus [Mic] plot (eq 1 and Figure 2). From slope-intercept ratios of the plots of Figure 2, the equilibrium parameters N, Kb, and Kp are obtained by a nonlinear fit to eq 2, as shown in Figure 3. The distribution parameters obtained are summarized in Table 1. As stated above, DEA does not quench ZnPc fluorescence, but it quenches pyrene fluorescence. From these later experiences, the equilibrium parameters for the distribution of DEA in CTAC micelles were obtained. As Table 1 shows, the partition mechanism is negligible but DEA associates to the micelles by a binding process. We can conclude that the absence of fluorescence quenching of ZnPc by DEA is a consequence of low reactivity. Were partitioning the only mechanism responsible for the quencher distribution, according to eq 2, the plots of Figure 3 should be a horizontal line. On the other hand, following the same equation, if only binding would exist, (〈n〉/[Q]w)-1 should be linear with [Q]w. In fact, these kinds of plots are straight lines with a good degree of approximation but, for kinetic reasons (see below) and previous evidence on two types of association of DMAn to (26) Encinas, M. V.; Lissi, E. A. Photochem. Photobiol. 1985, 42, 491.
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against 〈n〉. This is seen in Figure 4. When a similar plot is done for the other amines, different curves are obtained. Therefore, it is impossible to obtain equilibrium parameters for DMAn by this method. From data on fluorescence quenching of anthracene by DMAn in CTAB, association constants for this quencher are reported:11 N ) 2, Kb ) 1.2 × 105 M-1, and Kp ) 190. The method is successful in this case because of the longer singlet lifetime of anthracene compared to ZnPc. The high occupation number attained in some experiments does not seem to alter the surfactant aggregation number of the micelles, one of the hypotheses of the method to derive equilibrium constants. This is reflected in the fact that the plots of Figure 2 present good linear correlation. The mechanism that describes the fluorescence quenching process in micelles by partitioned and bound quenchers can be represented by the following equations: k0
M*b,p 98 Mb,p bk-b
M*b,p 98 M*b-1,p + Qw pk-p
M*b,p 98 M*b,p-1 + Qw (1 - b/N)k+b[Q]w
M*b,p + Qw 98 M*b+1,p k+p[Q]w
M*b-1,p + Qw 98 M*b,p+1 pkqp
M*b,p 98 Mb,p bkqb
M*b,p 98 Mb,p where b and p indicate the occupation of Q in binding and partitioning, respectively. The asterisk represents a micelle containing a sensitizer in an excited state. All rate constants refer to processes for a single quencher molecule. The assumption that total quenching rates are proportional to b and p is coherent with the hypothesis that quencher molecules behave as independent in the micelles.4,9 Under continuous irradiation the fluorescence intensity is described by10
If ) If0 e(-VmKp[Q]w) Figure 3. Dependence of (〈n〉/[Q]w) on [Q]w for DEA, TEA, and DEAn. Lines are the best fit curves according to eq 2. The corresponding parameters are listed in Table 1. 27
micelles, we interpret the distribution as being due to partitioning and binding. In the case of DMAn, (〈n〉/[Q]w) cannot be evaluated by this method because the intercept of the plot of [Q]t versus [Mic] is zero within experimental error. We can conclude that this quencher is completely incorporated into the micelles. Therefore 〈n〉 ) [Q]t/[Mic], and the data of Figure 1 can be reduced to a single curve when they are plotted (27) Katusin-Razem, B.; Wong, M.; Thomas, J. K. J. Am. Chem. Soc. 1978, 100, 1679.
N
∞
∑∑
N!
(Kb[Q]w)b(VmKp[Q]w)p
(1 + Kb[Q]w)Nb)0p)0b!(N - b)! p![1 + (bkqb + pkqp)/k0] (3) where the equilibrium constants Kb and Kp are related to the corresponding entrance and exit rate constants by
VmKp ) k+p/k-p
(4)
Kb ) k+b/Nk-b
(5)
In the solution of the rate equations derived from the mechanism, we took into account neither the exchange of the quencher among micelles and water nor that of quenchers between binding and partitioning; i.e., during the singlet quenching event, micelles are considered as
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Table 1. Deactivation of ZnPc Singlet Excited State by Amines (Q) and Equilibrium Parameters for Q Association to CTAC Micelles
a
Q
N
Kb/M-1
Kp
kqb/s-1
kqp/s-1
E°(Q‚+/Q)a/V versus SCE
DEAb TEA DEAn DMAn
80 249 34 2c
22 10.5 320 1.2 × 105 c
,1 8 23 190c
1 × 105 4 × 106 ,107
9 × 106 6 × 107 1.3 × 107
0.78 0.76 0.76 0.81
From ref 30. b Pyrene was used as fluorescent probe. c From reference 11.
Figure 4. Fluorescence quenching of ZnPc by DMAn at different CTAC concentrations: (O) 0.100 M; (3) 0.075 M; (]) 0.067 M; (4) 0.033 M. The line is the best fit curve to a quadratic polynomial function for experimental data at 0.033 M CTAC. Table 2. Excited State Energies (in singlet state triplet state a
eV)30
ZnPc
TEA
DEAn
DMAn
TMB
1.83a 1.13a
>3.90 >3.90
3.90 2.95
3.85 2.99
3.60 2.73
From ref 29.
closed and frozen compartments regarding quencher distribution. For this reason it is also irrelevant whether the quencher exchange between micelles takes place via migration through the aqueous phase or via direct exchange by micellar collision.24 Because N, Kb, and Kp were previously obtained by the above procedure, the only remaining parameters to be fitted by eq 3 are the two quenching rate constants kqb and kqp, as k0 is known to be 3.3 × 108 s-1.17,18,28 For a given amine, the same values of kqb and kqp are obtained by a nonlinear fit to eq 3 irrespective of the CTAC concentration. The fits are shown by the solid lines in Figure 1, and the best values for kqb and kqp are given in Table 1. Our data for DMAn in CTAC can be fitted to eq 3 using the equilibrium constants of ref 4. The best fit value for kqp is 1.3 × 107 s-1 (see Table 1) lower than kqp for DEAn. Due to the low occupation for b, no value of kqb can be obtained. The deactivation of 1ZnPc by an energy transfer mechanism can be ruled out on energetic grounds (see Table 2). Taking into account the values of Table 2 and the approximate value E°(ZnPc/ZnPc‚-) ) -0.89 V versus SCE,29 electron transfer from Q to 1ZnPc requires E°(Q‚+/Q) < 0.94 V versus SCE. This is the case for all the amines studied, and therefore any observable quenching of 1ZnPc should be ascribed to electron transfer. Further evidence of this (28) Xu, H.; Shen, T.; Zhou, Q.; Shen, S.; Liu, J.; Li, L.; Zhou, S.; Zhang, X.; Yu, Q.; Bi, Z.; Xiao, X. J. Photochem. Photobiol., A: Chem. 1992, 65, 267. (29) Darwent, J. R.; Douglas, P.; Harriman, A.; Porter, G.; Richoux, M. C. Coord. Chem. Rev. 1982, 44, 83.
Figure 5. Fluorescence quenching of ZnPc in DMSO. For TMB and DMAn, lines are the best fit curves according to eq 6. The fitting parameters are shown in Table 3. Table 3. Dynamic (kq) and Static (W) Quenching Constants of 1ZnPc by Amines in DMSO Q
E°(Q‚+/Q)/V versus SCE
kq/M-1 s-1
W/M-1
TMB DMAn TEA
0.32 0.81 0.76
4 × 109 2 × 109 3 × 108
30 3
fact will be given by fluorescence quenching in DMSO (see below) and by flash photolysis experiments (see Deactivation of the Triplet Excited State). In spite of the similar redox potentials of DEA, TEA, and DEAn, the aromatic amine is a more efficient quencher. Efficiency differences in the quenching of excited states by alkyl- and arylamines have been reported previously.30 The difference arises in the geometry around the nitrogen atom: planar amines are favored in comparison to alkylamines, that have to undergo a geometry change during the interaction with the excited state. We performed fluorescence-quenching measurements in DMSO in order to know the relative efficiency of amines with increasing favorable redox potentials for singlet quenching. Figure 5 shows the Stern-Volmer plots obtained and the lines of best fit to eq 6.31
(If0/If) exp(-W[Q]) ) 1 + kq[Q]/k0
(6)
where W is the static quenching constant (M-1) and kq is the second-order dynamic rate constant. In the case of TMB and DMAn the results in DMSO can only be explained assuming both dynamic and static quenching. For TEA, the quenching efficiency is so low, even at 0.12 M, that a rough estimate of kq assuming only dynamic quenching could be obtained. The fitting parameters kq and W are shown in Table 3. The order of the dynamic quenching rate constants is TMB > DMAn > TEA, which agrees with the increasing order of the redox potential E°(Q‚+/Q) for TMB and DMAn. These results also point to (30) Kavarnos, G. J.; Turro, N. J. Chem. Rev. 1986, 86, 401. (31) Frank, J. M.; Wawilow, S. J. Z. Phys. 1931, 69, 100.
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Figure 6. Data of Figure 1 for DEAn. The solid lines are the best fits to eq 25 in ref 9. [CTAC]: (O) 0.100 M; (3) 0.067 M; (4) 0.033 M.
Figure 7. Difference absorption spectrum for ZnPc in 0.1 M CTAC immediately after laser pulse excitation at 690 nm of degassed solutions.
an electron transfer quenching. The quenching rate constant for TMB attains the diffusion value expected in DMSO (viscosity 2 cp). The difference observed between the quenching constants in solution for DMAn and TEA, whose redox potentials are essentially the same, can be explained by the arguments on alkyl- and arylamines previously cited. Application of our kinetic model leads to a good fit of the Stern-Volmer plots and a kinetically consistent picture of the quenching dynamics. None of the models available in the literature fulfilled simultaneously these two requisites. The simple partitioning model4 is incompatible with the equilibrium data, as is evident from Figure 3. If we assume that binding is the only association mechanism for the amines to the micelles, the plots of Figure 3 can be fitted by eq 2 setting Kp ) 0. This procedure gives fits which are almost indistinguishable from the ones shown, but the best fit values of N and Kb are slightly different: for example the values for DEAn change to N ) 42 and Kb ) 243 M-1. The kinetic equations for the model considering binding alone have been exactly solved.9 When the data of Figure 1 for DEAn are fitted to eq 25 in ref 9, Figure 6 results. The best fit values are kqb ) 6.0 × 106 s-1 and k-b , kqb. As Figure 6 shows, the fit provided by this model is very poor. Therefore, on kinetic grounds we are forced to discard a simple binding model. Finally, the model of partitioning and binding in the fast exchange limit11 provides good fits to the kinetic data, which are coincident with the ones shown in Figure 1 as fits to eq 3. But the model is not consistent with the short lifetime of the excited state (from 3 to 1 ns), which does not allow us to maintain a distribution equilibrium for the quencher all the time. The best fit values for the Stern-Volmer plots of DEAn to eq 9 of ref 11 are kqb ) 6.2 × 107 s-1 and kqp ) 1.0 × 109 s-1. This latter value is too high for a quenching rate constant in micelles. We discard this model because it is kinetically inconsistent. A few comments on the validity of the assumptions made are worthwhile. We assumed that quenchers can be distributed among two essentially different site classes in the micelle, characterized as partitioning and binding sites. It follows from our calculations that quenchers associated to binding sites are less reactive, in accordance with what was previously observed.8,27 This result is consistent with a lower free energy of quenchers in binding sites. Depending on the amine, kqb fitting values are one or two orders of magnitude lower than kqp (see Table 1). At 298 K this implies a free energy difference of 0.06-
0.12 eV. These values are consistent with micropolarity effects affecting quenchers in different regions of the micelle.1 A more general model could be developed, in which quenchers are distributed among a continuous set of site classes, each one characterized by its own weight factor, free energy of interaction, and reactivity (the two latter are related). As quencher concentration increases, the system tends to behave as an “ideal” solution of quenchers in the micellar pseudophase, leading to partition statistics. As low free energy sites are saturated, occupation of the more reactive partition sites becomes more and more important. This model, describing a more realistic situation, is very difficult to work out without simplifying assumptions. The model used in this work is an approximation which provides an insight to the extreme behavior of quenchers in the micelle. 3.2. Deactivation of the Triplet Excited State. The interaction of 3ZnPc with amines was studied by laser flash photolysis. After laser pulse excitation at 690 nm, a solution of ZnPc in CTAC gives a transient species with a monoexponential decay with a lifetime of 250 µs. Its difference absorption spectrum, shown in Figure 7, corresponds to the triplet excited state of zinc phthalocyanine in homogeneous solvents.32,33 Favorable conditions for ZnPc triplet quenching by electron transfer require E°(Q‚+/Q) < 0.24 C versus SCE, which is not the case for these amines. Nevertheless, aromatic amines quench the triplet state, due to its longer lifetime (200 µs in deaerated micelles) compared to the singlet lifetime (3 ns). In 0.1 M CTAC, with 36 mM DMAn, the decay of the triplet absorption takes place with a lifetime of ca. 50 µs. If we assume a rapid exchange of the amine during the triplet lifetime (τt), the triplet decay should be monoexponential with a first-order rate constant: (τt)-1 ) (τ0t)-1 + 〈n〉kqt, where τ0t and kqt are the unquenched triplet lifetime and the quenching rate constant of the triplet by the amine, respectively. Taking into account the values of 50 and 200 µs for τt and τ0t and 〈n〉 ≈ 30 in the above-mentioned conditions, we estimate kqt ≈ 5 × 102 s-1. Figure 8 shows the difference absorption spectra obtained for ZnPc in 0.1 M CTAC with 36 mM DMAn by laser pulse excitation at 690 nm. In addition to the triplet (32) Tuirko, M. P.; Sapunov, V. V.; Salovev, K. N. Opt. Spectrosk. 1973, 34, 1094. (33) Ohno, T.; Kato, S.; Lichtin, N. N. Bull. Chem. Soc. Jpn. 1982, 55, 2753.
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Figure 8. Difference absorption spectra for ZnPc in degassed 0.1 M CTAC with 36 mM DMAn. Spectra are shown immediately and 0.1 ms after laser pulse excitation at 690 nm.
Figure 9. Transient absorption for the sample of Figure 8 observed at 605 nm after excitation with a laser pulse at 690 nm.
absorption band centered at 480 nm, another band appears around 580 nm, which resembles the spectrum of ZnPc‚-.34,35 When DMAn is present, the decays are no longer fitted by a single exponential. The traces at a fixed wavelength show an initial fast decay with a lifetime of ca. 50 µs, as mentioned above, and a slower decay. The absorbance previous to the pulse recovers some milliseconds after excitation. If we take the spectrum at t ) 0 from Figure 8 and we subtract from it the triplet spectrum of Figure 7, multiplied by an adequate factor taking into account the singlet quenching (i.e. If/If0), we obtain a spectrum which coincides with the ZnPc‚- spectrum published and with the spectrum taken 0.1 ms after excitation as shown in Figure 8. The band at 580 nm present at t ) 0 indicates that ZnPc‚- originates on the singlet quenching. The signal increase in the range 600-610 nm (Figure 9), which takes place with the same rate as the quenched triplet decay, points out that ZnPc‚- also originates on triplet quenching. We can conclude that the triplet quenching also occurs by electron transfer. In CTAC, following the transfer of one electron to ZnPc excited states, the resulting DMAn+‚ should be rapidly displaced from the cationic micelles owing to strong Coulomb repulsive forces, making its reentry unfavorable.
This fact results in the slow decay for ZnPc‚-, which can be observed. In these systems we did not observe permanent chemical change. If the main channel for recovery of ZnPc is the recombination of ZnPc‚- and Q‚+, its kinetics must be of second order. The recovery of ZnPc was monitored at its absorption maximum (690 nm) and at the maximum of the absorption of the radical anion (580 nm). The decays were not monoexponential. The attempts to verify secondorder behavior failed because the signals were too low after the triplet decay (∆A < 2 × 10-3, see Figure 8). Moreover, the rate of this long decay is very much influenced by oxygen traces remaining after prolonged nitrogen bubbling of micellar solutions. The singlet quenching shows that DEAn is a more efficient quencher of 1ZnPc* than DMAn. In spite of this fact, the flash photolysis experiments with DEAn showed a much lesser ZnPc- formation than in the case of DMAn. This result suggests that the more hydrophobic DEAn is located deeper in the micelle and that, after transfer of an electron to ZnPc*, DEAn‚+ diffuses away from the micellar surface more slowly than DMAn‚+. TEA does not quench 3ZnPc at all. The difference spectrum at zero time of a solution 0.35 M in 0.05 M CTAC showed only the triplet state, and the triplet lifetime remained unchanged.
(34) Mack, J.; Stillman, M. J. J. Am. Chem. Soc. 1994, 116, 1292. (35) Clack, D. W.; Yandle, J. R. Inorg. Chem. 1972, 11, 1738.
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