J. Phys. Chem. 1981, 85, 1309-1371
1369
Photoinduced Charge Separation of the Duroquinone-N-Ethylcarbazole System in Micellar Solutions and Microemulsions Yoji Yamaguchi, Tokuji Miyashlta, and Mlnoru Matsuda" Chemlcal Research Institute of Nonaqueous Solutbns, Tohoku Unlverslty, Katahlra 24hom, Sendal, 980 Japan (Recelved: November 13, 1980; In Final Form: January 27, 1981)
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The photoinduced electron transfer reaction of the duroquinone (DQ)-N-ethylcarbazole (ECz) system, DQ + ECz hv e DQ-. ECz+. products, was investigated by means of flash photolysis in micellar solutions and microemulsions. In both anionic sodium lauryl sulfate (SLS) and cationic cetyltrimethylammonium nitrate (CTAN) micellar solutions,the back electron transfer reaction in the dark was retarded, and ionic species decayed separately; DQ-. decayed by the dismutation reaction with a rate constant of 2 X 108-3 X lo8 M-l s-l. (In acetonitrile, it decays by back electron transfer with a rate constant of -lO1o M-ls-l.) In both sodium cetyl sulfate (SCS) and cetyltrimethylammonium bromide (CTAB) microemulsion, the back electron transfer was also retarded because of fast protonation of DQ-*in the microemulsion droplet. Although no transient species appeared in CTAl3 micellar solutions, charge separation took place in CTAB microemulsions. This is explained by the difference in the solubilization site of DQ.
+
+
Introduction Photoinduced electron transfer reactions of the form
A+D& -A-+D+ (1) where A and D are the electron acceptor and donor, respectively, have recently attracted much attention. Generally, however, the back electron transfer in the dark is so fast that effective charge separation cannot be attained. One promising approach to overcome this problem is to employ microscopic heterogeneous fields such as micelles,l microemulsions,2~3vesicles,4 and polyelectrolyte^.^ The reaction kinetics in heterogeneous systems are known to deviate from the conventional rate law of homogeneous solutions. They are affected by the surface charge of aggregates, the solubilization sites of substrates, and pH or ionic strength in bulk solutions.6 We have studied the yields and the decay kinetics of transients in the duroquinone (DQ)-N-ethylcarbazole (ECz) redox couple by means of flash photolysis in anionic (SLS or SCS) or cationic (CTAN or CTAB) micellar solutions and microemulsions. We compare these results with observations for homogeneous solutions (benzene or acetonitrile) and discuss the mechanism for charge separation. Experimental Section Materiak. DQ and ECz were recrystallized 3 times from ethanol. Surfactants (SLS, CTAB, and SCS) and solvents were purified by usual methods. CTAN was prepared (1)D.Meisel, M. Matheson, and J. Rabini, J. Am. Chem. SOC.,100, 117 (1978);M. Grlitzel Micellization, Solubilization, Microemulsions, [Proc. Znt. Symp.], 2, 531 (1977);J. K. Thomas and M. Almgren in "Solution Chemistry of Surfactants", Vol. 2,K. L. Mittal, Ed., Plenum Press, New York, 1979,p 569; Y. Tsutaui, K. Takuma, T. Nishizima, and T. Matauo, Chem. Lett., 617 (1979);N. J. Turro, M. Griitzel, and A. M. Braun, Angew. Chem., Int. Ed. Engl., 19,675 (1980). (2)J. Kiwi and M. Griitzel, J. Am. Chem. SOC.,100, 6314 (1978). (3)M. Almgren, F. Grieser, and J. K. Thomas, J. Am. Chem. SOC.,102, 3188 (1980);C. A. Jones, L. E. Weaner, and R. A. Mackay, J. Phys. Chem., 84,1495 (1980). (4)P.P. Infelta, M. Grlitzel, and J. H. Fendler, J . Am. Chem. SOC., 102,1479 (1980). (5)D.Meisel and M. S. Matheson, J . Am. Chem. SOC.,99,6577(1977); C. D.Jonah, M. S. Matheson, and D. Meisel, J. Phys. Chem., 83,257 (1 ,--979) . ,. (6)J. H.Fendler and E. J. Fendler, "Catalysis in Micellar and Macromolecular Systems", Academic Press, New York, 1975;B. Katusin100,1679 (1978). h e m , M. Wong, and J. K. Thomas, J. Am. Chem. SOC.,
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0022-385418112085-1369$01.25/0
according to the procedure of Emerson et al.' and recrystallized from ethanol. Preparations of Micellar Solutions and Microemukions. DQ and ECz (1X 1 0 4 2 X M)were solubilized into freshly prepared aqueous SLS (0.1 M), CTAB (0.012 M), or CTAN (0.012 M) micellar solutions. Microemulsions were prepared by the method of Mackay et ale: and the components were SCS or CTAB as a surfactant (4.8 w t %), cyclohexanol as a cosurfactant (7.0 wt %), benzene (1.9 wt %), and water (86.2 wt % ) (concentrations of the substrates were 1 X lO-"2 X M). All of the sample solutions were treated with ultrasonic waves and degassed under vacuum (5 X torr) by four to five successive freeze-pump-thaw cycles. At least 24 h was allowed for equilibration. Flash Photolysis. The flash apparatus delivered a flash light with an energy of 72 J and a half-peak duration of 10 ps from xenon-filledlamps. The flash light shorter than 390 nm was cut off by filters to excite only DQ. Transient species were detected by fast kinetic spectroscopy.
Results and Discussion Flash Photolysis of DQ-ECz in Homogeneous Solutions (Benzene or Acetonitrile). The flash photolysis of the acetonitrile or benzene solution containing only DQ produced the transient absorption spectrum of the excited triplet state (DQT) of DQ; the decay of the absorption maximum of DQT (480 nm)O obeyed first-order kinetics with a specific rate constant of e 4 . 0 X lo4 s-l. In the presence of ECz, transient absorptions with maxima at 420 and 780 nm (ECz+.)1° instead of 480 nm (DQT) were observed in acetonitrile (Figure l),whereas in benzene only DQT absorption is observed. Duroquinone radical anion (DQ-e) has an absorption maximum a t 440 nm and a shoulder at 420 nm, while durosemiquinone radical (DQH-) has an absorption maximum at 420 nm." Although no absorption maximum appeared around 440 nm in Figure 1, DQ-+is believed to occur because the absorption at 440 nm decays more rapidly than that at 420 nm. The charge (7)M. F. Emerson and A. Holtzer, J. Phys. Chem., 71,1898 (1967). (8)K. Letts and R. A. Mackay, Inorg. Chem., 14,2990 (1975). (9)E.J. Land, Trans. Faraday SOC.,65, 2815 (1969). (10)Y.Taniguchi, T.Nishina, and N. Mataga, Bull. Chem. SOC.Jpn., 46,1646 (1973). (11)K. B. Pate1 and R. L. Willson, Trans. Faraday SOC.,69, 814 (1973).
0 1981 American Chemical Society
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The Journal of Physical Chemistry, Vol. 85, No. 10, 1981
Yamaguchi et ai. i n
""I
500
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Flgure 1. Transient absorption spectra obtained from the flash photolysis of acetonitrile solution containing DQ (1.5X M) and ECz (2.0 X lo3 M) at 50 (-) and 100 ps (---) after flash.
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o h
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400
/--J
_____ -----
700
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/f*'
750
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Figure 2. Transient absorption spectra obtained from the flash photolysis of SLS iO.1 M) micellar solution containing DQ (2 X lo4 M) and ECz (2 X 10- M) at 0.5 (-) and 4.0 ms (---) after flash.
separation in this system appears to result from the reaction of DQT with ECz by the well-known quenching mechanism.12 The decay of both DQ-• and ECz+. deviated from the simple second-order kinetics for the back electron transfer reaction (eq 2), showing that the first-order reactions,
+
DQ-• ECZ+DQ + ECZ (2) D Q - - Z DQH. and ECz+. product, concurrently occurred. The rate constant for the back electron transfer was calculated to be 1O1O M-I s-l by computer simulation of the decay curves. DQH. decayed by reaction 3 via 2DQH. DQ + DQHz (3) second-order kinetics after the completion of the decay of DQ-., and the rate constant was calculated to be 2.1 X lo9 M-' s-l assuming 6 = 4700 M-I ~ m - l . ~ DQ-ECz in Aqueous Micellar Solutions. Flash photolysis of anionic SLS micellar solutions containing DQ and ECz produced the absorption spectra of DQ-- and ECz+. (Figure 2). On the other hand, no transient absorption spectra could be observed in cationic CTAB micellar solutions. It has been reported that halogen ions inhibit the photoreaction of quinones; l3the DQT is quenched by Brin this case, too. This effective quenching by Br- suggests that the DQ is located in the vicinity of Br-, Le., near the Stern layer. When cationic CTAN having the counteranion of NO3- was used as a surfactant, the charge separation occurred and both DQ-. and ECz+*were produced as well as in SLS micellar solutions. The lifetimes of transient species in micellar solutions were longer than those in acetonitrile. In both SLS and CTAN micellar solutions, the decay rate of ECz+. was much faster than that of DQ-., suggesting that ECz+-did not react with DQ;; the back electron transfer reaction did
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(12)D. Ream and A. Weller, Ber. Bunsenges. Phys. Chem., 73, 834 (1969);H.Masuhara, T.Hino, and N. Mataga, J.Phys. Chem., 79,894 (1976);R. Scheerer and M. Gratzel, J.Am. Chem. SOC.,99,866(1977). (13)K, C. Kurien and P. A. Robins, J. Chem. Soc. B , 866 (1970);K. Kano and T. Mataga, Chem. Lett., 1127 (1973).
Figure 3. Comparison of the decay of absorption maxima (A) of ECz+. in the following solutions: (a) acetonitrile containing DQ (1.5 X M) and ECz (2.0 X lo3 M) (A = 780 nm); (b) SCS microemuision containing DQ (1.0X M) and ECz (1.1 X lo3 M) (A = 790 nm); (c) CTAB microemuision containing DQ (1.2 X M) and ECz (1.4 X lo-' M) (A = 780 nm).
not occur. The decay of the absorption at 440 nm obeyed second-order kinetics with similar rates in both SLS and CTAN micellar solutions, and the rate constants are 2.1 X 10s and 3.0 X 10s M-l s-l, re~pective1y.l~In anionic SLS micellar solution, DQ is solubilized near the Stern layer as in CTAB micellar solution. ECz is located in the interior of the micelle because of a hydrophobic interaction. DQ--, produced by the electron transfer from ECz to DQT, escapes from the micelle into the aqueous phase because of the electrostatic repulsion between DQ-. and the negatively charged micellar surface. DQ-. subsequently decays by reaction 4 mainly in the aqueous phase in SLS micellar 2DQ-- DQ + DQ2(4) solution, whereas in the micellar surface in CTAN micellar s~lution.'~ On the other hand, ECz+. is stabilized by Coulombic interaction with the anionic SLS micelle and may be located at the micellar surface region and decays via firstorder kinetics with a specific rate of (5.0 f 0.6) X lo2 s-l. In CTAN micellar solution ECz+. would be expelled from the micelle because of the electrostatic repulsion, and it decays with simultaneous first- and second-order kinetics with rate constanta calculated by the computer simulation analysis to be 2 X lo2 s-l and k / t N 9 X lo4 cm s-l, respectively. DQ-ECz in Microemukions. The main transient species observed in the flash photolysis of the SCS microemulsion were ECz+. and DQH.. In the SCS microemulsion, both DQ and ECz may be present in the benzene phase which is a core of the droplet and is surrounded by surfactant SCS and cosurfactant cyclohexanol phase. DQ-. formed from the charge separation is protonated by cyclohexano12 before escaping from the droplet. If DQ-. escaped from the droplet and reacted in the aqueous phase as well as in micellar solutions, the absorption spectrum of DQ-* could be observed. A large yield of DQH. instead of DQ--, however, was formed in the SCS microemulsion. This indicates that DQ-• was protonated in the microemulsion droplet before diffusing into the aqueous phase. When only DQ was present in the system, where DQH. is formed from the reaction of DQT with cyclohexanol in low yield,
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(14)In CTAN micellar solution, DQ-. exists near the Stern layer of the micelle (M) in an equilibrium with DQ-- in the aqueous phase, DQ-., + M * DQ;, (K). Thus, the decay of DQ-. by the disproportionation can ] k,q[DQ--,,]2 = be expressed as -d[DQ-.] dt = k,[DQ-.,][DQ-&&[MI + k,)/(l + K[M]{z[DQ--]2= k,~[DQ--]2:where k, and k are the rate constants for the reaction in the micellar surface and that:i the aqueous hase, respectively. Under our experimental conditions, [MI = 1.8 X 10qM, k = 2.1 X l@ M-' s-' (the rate constant in SLS),and k o w = 3.0 X 10s M-%l, when K is lO'-l@ M-l, k, is estimated to be (6-28)&,; k, > k, is reasonable because the positive charge of the micellar surface favors the disproportionationof DQ-. due to the decrease in the repulsion between DQ-., and DQ-.,.
+
J. Phys. Chem. 1981, 85, 1371-1380
DQH. decayed via second-order kinetics due to the disproportionation reaction. In the presence of ECz, however, DQH. decayed via simultaneous first- and second-order kinetics. Their rate constants were calculated to be (1.9 f 0.7) X lo2s-l and (1.7 f 0.3) X lo8 M-l s-l, respectively. The first-order reaction of DQH. can be attributed to an intradroplet r e a ~ t i 0 n . l ~ Both DQH. and ECz+. were formed, though the yields were low, in the CTAB microemulsion in contrast to the CTAB micellar solution where no transient absorption spectrum was obtained. This is ascribed to the difference in the solubilization site of DQ in the micelle and the microemulsion droplet. In the CTAB micellar solution, (15) A. J. Frank, M. Gratzel, and J. Kozak, J.Am. Chem. SOC.,98,3317 (1976).
1371
DQ is located near the Stern layer (vicinity of Br-), but in the benzene phase in the CTAB microemulsion. Therefore, DQT would be quenched in the CTAB micellar solution more effectively than in the CTAB microemulsion. The yields of the formation and the decay curves of ECz+. in microemulsions are compared with those in acetonitrile in Figure 3, which demonstrates the high effectiveness of the charge separation in microemulsions. ECz+- decayed via simultaneous first- and second-order kinetics in both SCS and CTAB microemulsions. In conclusion, it is clear that the microscopic heterogeneous systems such as micelles and microemulsions are effective in the photoinduced charge separation of DQ and ECz. The mechanism promoting the charge separation, however, is different in micellar solutions and microemulsions.
Theoretical Determination of the Crystalline Packing of Chain Molecules S. K. Tripathy,+ A. J. Hopfinger,”+ and P. L. Taylor$ Department of Macromolecular Science and Department of Physlcs, Case Instnute of Technohyy, Case Western Reserve University, Cleveland, Ohio 44106 (Received: November 17, 1880; In Flnal Form: February 3, 1981)
A general method is presented to minimize the packing energy of a polymer crystal. The polymer chain conformation is fixed so that the corresponding lattice can be decomposed into rows of atoms possessing the periodicity of the conformational repeat unit. A set of expressions is derived to compute the interaction energy of each unique row of atoms, treated as periodic force centers, with an atom not belonging to the lattice generated by the set of row atoms. The total energy is assumed to be the pairwise additive sum of atomic interactions expressed in terms of inverse powers of interaction distances by using classical Lennard-Jones-type dispersive-repulsive and Coulomb-type electrostatic potential functions. Mathematical tractability is obtained by expanding the series sums with the periodicity of the row lattice in a Fourier series in terms of the reciprocal lattice vectors. Rapid convergence of these series facilitates truncation to desired accuracy. The packing energy, defined to be the interaction energy of a central conformational repeat unit interacting with a set of user-prescribed polymer chains, is minimized with respect to the unit cell parameters for an assumed symmetry. The chain packing of polyethylene and phase I of poly(viny1idene fluoride) (PVF,)have been studied in detail to establish general criteria for investigating crystalline phases of polymers under arbitrary packing symmetry. Different proposed sets of potentials have been employed in the calculationsand some comments are presented regarding the choice of the set of potentials.
Introduction Multidimensional minimization techniques have been used to predict the stable intramolecular conformations of molecules, including polymer chains.’Y2 Attempts have also been made to predict macromolecular organization in polymer ~ r y s t a l s . l ~All ~ - ~of these calculations involve the minimization of the free energy of the system as a function of the structural variables. The internal energy contribution to the free energy is calculated via interatomic potential functions. For crystal calculations, the intermolecular potentials have been limited to a dispersive repulsive contribution. Thus the crystalline structures of polar polymers have not been considered by use of general lattice models. In most crystal structure calculations the valence geometry is fixed so that the bonded interaction energy contribution to the total energy is constant. If the intramolecular conformation is also fixed, the Debye temperature 0 and the potential energy are functions of the mot Department of Macromolecular Science.
Department of Physics. 0022-3654/81/2085-1371$01.25/0
lecular arrangement in a crystal only. The condition of crystal stability is then given by 6F = 64 + 6RT 60/0 = 0 where F is the free energy and 4 the packing energy of the system. At absolute zero the condition of stability changes to 64 = 0. Even at slightly higher temperatures the error introducted in the crystal structure evaluation by neglecting the second term is usually less than l%.e Thus a crystal structure can be reasonably predicted by minimizing the packing energy. However, this application requires summing pair potentials over a very large number of intermolecular atom pairs if reasonable accuracy is (1) Hopfinger, A. J. “Conformational Properties of Macromolecules”; Academic Press: New York, 1973. (2) Hopfinger, A. J. “Intermolecular Interactions and Biomolecular Organization”; Wiley: New York, 1977. (3) Farmer, B. L.; Hopfinger, A. J.; Lando, J. B. J. Appl. Phys. 1972, 43. (4) Yokouchi, M.; Tadokoro, H.; Chatani, Y. Macromolecules 1974, 7, 769. ( 5 ) Yokouchi, M.; Chatani, Y.; Tadokoro, H.; Tani, H1. Polym. J. 1974, 6, 248. ( 6 ) Kitaigorodski, A. I. Acta Crystallogr. 1965,18, 585.
0 1981 American Chemical Society
