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Photoinduced Electron Transfer between a Self-Assembled Resorcinarene-[60]Fullerene Complex and Poly(3-hexylthiophene) in Langmuir-Blodgett Films E. Vuorimaa,*,† T. Vuorinen,† N. Tkachenko,† O. Cramariuc,† T. Hukka,† S. Nummelin,‡ A. Shivanyuk,‡ K. Rissanen,‡ and H. Lemmetyinen† Institute of Materials Chemistry, Tampere University of Technology, P.O. Box 541, FIN-33101 Tampere, Finland; and Department of Chemistry, University of Jyva¨ skyla¨ , P.O. Box 35, FIN-40351 Jyva¨ skyla¨ , Finland Received May 29, 2001. In Final Form: August 16, 2001 In the present study, a etoxymethylresorcinarene (EMR) is used as a trap for C60 molecules at the air-water interface. The resulting self-assembled EMR:C60 complexes form stable well-ordered monolayers which can be easily transferred to a solid substrate. The interlayer charge transfer between poly(3hexylthiophene) (PHT) and C60 was studied using the time-resolved Maxwell displacement charge (TRMDC) method. The structural details of the EMR molecule were calculated by means of molecular mechanics, semiempirical and ab initio methods. The computational results predict that there is enough space for the C60 molecule to be placed between the tails of the resorcinarene molecule.
Introduction Molecular systems capable of performing light induced electron transfer (ET) reactions have attracted significant attention during the past few decades. Discovery and successful synthesis of fullerenes have introduced new molecular engineering routes, potentially interesting for applications in molecular electronics.1-8 [60]Fullerene (C60) is an electron acceptor which is capable of reversibly accepting up to six electrons.8 Because C60 consists solely of carbon atoms, it is essentially hydrophobic and tends to form three-dimensional aggregates at the air-water interface rather than a monomolecular layer.9-11 Calix[5]- and calix[8]arenes can strongly bind C60 to give crystalline complexes with 1:1 and 2:1 host:guest ratios in various organic solvents12-16 and at the air† ‡
Tampere University of Technology. University of Jyva¨skyla¨.
(1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162. (2) Kuciauskas, D.; Lin, S.; Seely, G. R.; Moore, A. L.; Moore, T. A.; Gust, D. J. Phys. Chem. 1996, 100, 15926. (3) Liddell, P. A.; Kuciauskas, D.; Sumida, J. P.; Nash, B.; Nguyen, D.; Moore, A. L.; Moore, T. A.; Gust, D. J. Am. Chem. Soc. 1997, 119, 1400. (4) Carbonera, D.; Valentin, M. D.; Corvaja, C.; Agostini, G.; Giacometti, G.; Liddell, P. A.; Kuciauskas, D.; Moore, A. L.; Moore, T. A.; Gust, D. J. Am. Chem. Soc. 1998, 120, 4398. (5) Imahori, H.; Hagiwara, M.; Aoki, M.; Akiyama, T.; Taniguchi, S.; Okada, T.; Shirakawa, M.; Sakata, Y.; J. Am. Chem. Soc. 1996, 118, 11771. (6) Guldi, D. M.; Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1997, 119, 974. (7) Sariciftci, N. S.; Wudl, F.; Heeger, A. J.; Maggini, M.; Scorrano, G.; Prato, M.; Bourassa, J.; Ford, P. C. Chem. Phys. Lett. 1995, 247, 510. (8) Imahori, H.; Sakata, Y. Adv. Mater. 1997, 9, 537. (9) Cardullo, F.; Diederich, F.; Echgoyen, L.; Habicher, T.; Jayaraman, N.; Leblanc, R. M.; Stoddart, J. F.; Wang, S. Langmuir 1998, 14, 1955. (10) Zhang, P.; Lu, J.; Qunji, X.; Liu, W. Langmuir 2001, 17, 2143. (11) Imae, T.; Ikeo, Y. Supramol. Sci. 1998, 5, 61. (12) Haino, T.; Yanase, M.; Fukazawa, Y. Tetrahedron Lett. 1997, 38, 3739. (13) Haino, T.; Yanase, M.; Fukazawa, Y. Angew. Chem., Int. Ed. Engl. 1998, 37, 997. (14) Yanase, M.; Haino, T.; Fukazawa, Y. Tetrahedron Lett. 1999, 40, 2781. (15) Yanase, M.; Matsuoka, M.; Tatsumi, Y.; Suzuki, M.; Iwamoto, H.; Haino, T.; Fukazawa, Y. Tetrahedron Lett. 2000, 41, 493.
Figure 1. Structure of EMR.
water interface.17 Furthermore, LB films of amphiphilic calixresorcinarenes have been shown to have a nanoporous and flexible structures and are found to provide a suitable matrix for incorporation of aromatic molecules.18-20 This stimulated us to study the complex formation between tetrakis(C-undecyl)-p-ethoxymethylresorcinarene (EMR)21 (Figure 1) and C60 at the air-water interface. The crown isomer of EMR is a macrocyclic amphiphile and is very attractive for the fabrication of LB films. The molecule consists of two rims of different cross-sectional areas. The hydrophilic rim consists of a cyclic carbon skeleton with eight OH groups and four ethoxymethyl groups attached to the aromatic rings of the skeleton. The four 11-carbonlong hydrocarbon tails form the hydrophobic rim. Molecular modeling and pressure-area isotherm measurements of related substances have revealed that the hydrophilic rim has a cross section18-20 of ca. 1.3 nm2 while the cross (16) Bourdelande, J. L.; Font, J.; Gonzalez-Moreno, R.; Nonell, S. J. Photochem. Photobiol. A: Chem. 1998, 115, 69. (17) Castillo, R.; Ramos, S.; Cruz, R.; Martinez, M.; Lara, F.; RuizGarcia, J. J. Phys. Chem. 1996, 100, 709. (18) Hassan, A. K.; Nabok, A. V.; Ray, A. K.; Lucke, A.; Smith, K.; Stirling, C. J. M.; Davis, F. Mater. Sci. Eng. C 1999, 8, 251. (19) Dutton, P. J.; Conte, L. Langmuir 1999, 15, 613. (20) Ichimura, K.; Fujimaki, M.; Matsuzawa, Y.; Hayashi, Y.; Nakagawa, M. Mater. Sci. Eng. C 1999, 8, 353-359. (21) Rissanen, K.; Nummelin, S.; Shivaniuk, A. Manuscript in preparation.
10.1021/la010786u CCC: $20.00 © 2001 American Chemical Society Published on Web 11/06/2001
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section of the sum of the four alkyl chains22 of the hydrophobic rim is estimated to be ca. 0.75 nm2. Consequently, a two-dimensional free volume should be secured in a molecular film even though the molecules cover the surface very densely. To study the interlayer charge transfer between poly(3-hexylthiophene) (PHT) and C60 samples with two EMR: C60 layers and one layer of PHT mixed with octadecylamine (ODA) (PHT:ODA) were prepared. The vectorial charge transfer was measured using the time-resolved Maxwell displacement charge (TRMDC) method.23-26 The method gives direct information about the charge shift in the direction perpendicular to the plane of the film. A high sensitivity of the method allows us to study the charge transition between single layers of the donor and acceptor molecules. The structural details of the EMR molecule were calculated by means of molecular mechanics and semiempirical and ab initio methods. The aim of the computational study was to determine whether the C60 molecule could be incorporated between the long hydrocarbon tails of the EMR molecule. The computational details are presented in the last section. Experimental Section Chloroform and toluene of analytical-reagent grade (Merck) were used for solution preparation. Octadecylamine (ODA) was of 99% grade (Sigma) and Buckminsterfullerene, C60, was of >98% grade (Fluka). The synthesis of tetrakis(C-undecyl)-p-ethoxymethylresorcinarene (EMR) will be reported elsewhere. The LB films were prepared with a KSV 5000 LB system (KSV Instruments). A 0.6 mM phosphate buffer solution (pH ) 7.0) in water purified by a Milli-Q system (Millipore) was used as a subphase. The temperature of the subphase was 18.0 ( 0.5 °C. The films were deposited on quartz plates cleaned with sulfochromic acid, plasma etched with nitrogen and precoated with nine ODA bottom layers. The plasma cleaner PDC-23G (Harrick) was used, the pressure of nitrogen was about 0.15 mbar and the plates were etched for 15 min. EMR was spread on the subphase from chloroform solution. Because of the poor solubility of C60 in chloroform, it was dissolved in toluene and the mixture of EMR and C60 was spread from toluene: chloroform ()3:8) solution. The monolayers were compressed at a rate of 1.55 Å2 molecule-1 min-1, to the deposition pressure of 30 mN m-1. The deposition rates were 30 mm min-1. For charge-transfer measurements the films were deposited on quartz plates coated by an ITO semitransparent thin electrode. Before deposition the substrates were precoated with nine layers of ODA. This was needed to isolate the donor and acceptor layers from the semiconductor ITO film. In addition, the donor and acceptor layers prepared for TRMDC measurements were coated with 10 ODA layers to prevent any interaction between the donor or acceptor molecules and the top electrode. The ratio of PHT monomer unit:ODA was 3:2. The deposition pressure was 20 mN m-1 and the deposition rates were 4 and 7 mm min-1 for upward and downward direction, respectively. The absorption spectra were measured with a Shimadzu UV2501PC spectrophotometer. An uncoated clean quartz plate was used as a reference. The apparatus used for the time-resolved Maxwell displacement charge measurements is schematically shown in Figure 2. The excitation wavelength was 532 nm (second harmonic of the (22) Roberts, G. Langmuir-Blodgett Films; Plenum: New York, 1990; pp 21-22. (23) Ikonen, M.; Sharonov, A.; Tkachenko, N.; Lemmetyinen, H. Adv. Mater. Opt. Electron. 1993, 2, 115. (24) Tkachenko, N. V.; Hynninen, P. H.; Lemmetytinen, H. Chem. Phys. Lett. 1996, 261, 234. (25) Tkachenko, N. V.; Vuorimaa, E.; Kesti, T.; Alekseev, A. S.; Tauber, A. Y.; Hynninen, P. H.; Lemmetyinen, H. J. Phys. Chem. B 2000, 104, 6371. (26) Tkachenko, N. V.; Tauber, A. Y.; Hynninen, P. H.; Sharonov, A. Y.; Lemmetyinen, H. J. Phys. Chem. A 1999, 103, 3657.
Figure 2. Arrangement of the samples for the time -resolved Maxwell displacement charge (TRMDC) measurements. Nd:YAG Q-switched laser) and the time resolution of the instrument was 15 ns (determined by the excitation pulse width). The sample structure for the photoelectric measurements was as follows: ITO/ODA insulating layers/EMR:C60/PHT:ODA/ ODA insulating layers/InGa liquid-metal drop electrode. Therefore, the TRMDC signals were produced by charge motions between the EMR:C60/PHT:ODA layers and could not be affected by a semiconductor-chromophore interaction. The active EMR: C60/PHT:ODA layer pair was deposited also in a reverse order, i.e. PHT:ODA/EMR:C60, thus producing samples with opposite donor-acceptor orientations. The samples had extremely low conductivity (Rs > 1012 Ω) and could be treated as pure capacitors (typical Cs was 100-200 pF). The preamplifier input resistance was Rin g 100 MΩ, and therefore, in a time scale shorter than 10 ms, the TRMDC measurements were done in a photovoltage mode (RinCs > 10 ms), which means that the measured signals are directly proportional to the charge displacement.
Results and Discussion Film Properties. The pressure-area isotherms obtained for pure EMR monolayer and mixed monolayers of EMR and C60 are shown in Figure 3a. The pure EMR monolayer shows a smooth rise in the surface pressure and a clear collapse point at 50 mN m-1. For the mixed films the collapse pressure depends on the composition increasing from 32 mN m-1 for EMR:C60 ) 1:2 to 55 mN m-1 for EMR:C60 ) 2:1. The limiting area of the pure EMR film, determined by extrapolating the linear part of the isotherms to zero pressure, 130 Å2, is in good agreement with the values between 130 and 160 Å2 observed in previous studies of related substances.18,27,28 For the mixed films the mean molecular area decreases with increasing C60 proportion. Since C60 does not form a monolayer at the air-water interface, the limiting molecular areas taking only EMR molecules into account were calculated and plotted as a function of C60 proportion. The results are compared with the measured limiting molecular areas in Figure 3b. In short, the added C60 does not increase the total area of the EMR monolayer, (27) Nabok, A. V.; Lavrik, N. V.; Kanzantseva, Z. I.; Nesterenko, B. A.; Markovskiy, L. N.; Kalchenko, V. I.; Shivaniuk, A. N. Thin Solid Films 1995, 259, 244. (28) Nabok, A. V.; Ray, A. K.; Hassan, A. K.; Omar, O.; Taylor, R.; Richardson, T.; Pavier, M. Thin Solid Films 1998, 327-329, 104.
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Figure 3. (a) Pressure-area isotherms for mixed monolayers with (1) pure EMR, (2) EMR:C60 ) 2:1, (3) 1:1, and (4) 1:2. (b) Measured and calculated limiting molecular areas of EMR as a function of molar fraction of C60.
although the shapes of the pressure area isotherms in the presence of C60 are clearly different from that of pure EMR. This could indicate that the C60 molecules are totally squeezed out of the monolayer. Another explanation is that the EMR and C60 molecules are organized as molecular pairs and the pairs are oriented at the airwater interface with the EMR at the water surface and the C60 molecule on top of the EMR. This orientation is very likely in view of the strongly hydrophobic nature of the C60 molecules. The pure EMR and EMR:C60 ) 1:1 films at deposition pressure of 30 mN m-1 were very stable and easily transferred to the solid substrates with transfer ratios of 1.00 ( 0.05. The absorption spectra of the pure EMR and EMR:C60 ) 1:1 films measured after deposition of every two layers are shown in Figure 4. In the absence of C60, the absorption of EMR is observed at 205 and 290 nm. In the presence of C60, in addition to the EMR absorption at 205 nm, the broad absorption bands of C60, having maxima at 340 and 270 nm, are clearly visible.14,15 As observed previously for amphiphilic C60 derivatives in LB films,14,15 the characteristic sharp C60 absorption at 428 nm has almost disappeared. For both films the absorbance increases linearly as a function of the number of layers deposited (Figure 4), indicating that the Langmuir films are effectively transferred onto the substrate at least up to 18 layers. If the C60 molecules were just squeezed out of the monolayer during the compression, linear increase in the C60 absorption as a function of the number of layers could not be expected. Thus, it seems that the EMR:C60 ) 1:1 mixture does form an organized monolayer containing both EMR and C60 molecules. The EMR:C60 ) 2:1 and 1:2 films were also deposited at 30 mN m-1. The 2:1 film behaves as the 1:1 film: the transfer ratios were 1.00 ( 0.05 and the absorbance increases linearly as a function of the number of layers deposited. As expected, the intensity of the C60 absorption bands are half of those observed for EMR:C60 ) 1:1 film. The EMR:C60 ) 1:2 film did not behave as nicely: the
Figure 4. Absorption spectra of (a) 100% EMR and (b) EMR: C60 ) 1:1 films measured after every two layers deposited. Absorbance at the absorption band maxima as a function of the number of layers is shown in the inserts.
Figure 5. Aborption spectra of one layer of PHT:ODA, two layers of EMR:C60 ) 1:1, and the alternate film.
transfer ratios were lower, about 0.9, and the absorption of C60 does not increase as much as it should. This indicates a less ordered structure of the film, probably due to aggregation of C60 molecules.9 Interlayer Charge Transfer between C60 and Poly(3-hexylthiophene). The absorption spectra of one layer of PHT:ODA, two layers of EMR:C60 ) 1:1, and the alternate film are shown in Figure 5. The absorption spectrum of the alternate films is a superposition of the two other spectra. The excitation wavelength in the photovoltage measurements is 532 nm; thus, only PHT is excited. The photovoltage responses of the films are shown in Figure 6. The photovoltage response of a sample containing only two layers of EMR:C60 ) 1:1 between the insulating layers is very weak as compared to PHT-C60 samples. In the latter case two structures with opposite orientation of the donor and acceptor layers, PHT-C60 and C60-PHT, were prepared. Ideally, the photovoltage responses of these two samples should have opposite signs, but otherwise
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Figure 7. Structure of the EMR molecule optimized at the HF/3-21G* level of theory.
Figure 6. Photovoltage responses of the (1) PHT:ODA/EMR: C60, (2) EMR:C60 and (3) EMR:C60/PHT:ODA samples at (a) 150 µs and (b) 8 ms time scales. Table 1. Calculated Total Energies for the EMR Molecule method
energy (kJ/mol)
energy (au)
PM3 3-21G*
-2882.25 -10855958.03
-1.098 -4135.603
the signals should be identical. In the microsecond time domain, the signs of the signals are opposite and the shapes of the responses are similar (Figure 6a). This confirms the photoinduced electron transfer from PHT to C60. The amplitudes of the photovoltage responses are somewhat different, indicating that EMR:C60 layers deposited in upward and downward directions are not equivalent. At a longer delay time (millisecond time domain, Figure 6b), the signals of both alternate structures reach a small negative level. Most probably this part of the photovoltage response is determined by the conductivity in the polymer (PHT) layer. Then the polarity and value of the photoresponse is controlled by electric field existing between the electrodes made of different materials.24,26 The electron transfer between PHT and C60 layers takes place in less than 10 ns, since it was not resolved in TRMSC measurements. For covalently linked fullereneoligothiophene systems the electron transfer has been observed to take place in less than 2 ns.29,30 Thus, the signals observed in this study present the charge recombination, which is a rather complex multiexponential process. There are several reasons for this. (1) Since the donor, PHT, and acceptor, C60, are in different layers, a distribution of distances can be expected for the donoracceptor pairs, and, consequently, distribution of the charge transfer and recombination rates is natural. (2) (29) van Hal, P. A.; Knol, J.; Langeveld-Voss, B. M. W.; Meskers, S. C. J.; HUmmelen, J. C.; Janssen, R. A. J. Phys. Chem. A 2000, 104, 5974. (30) Fujitsuka, M.; Matsumoto, K.; Ito, O.; Yamashiro, T.; Aso, Y.; Otsubo, T. Res. Chem. Intermed. 2001, 27, 73.
After the primary charge transfer, the hole will delocalize over a few polymer units in the PHT layer. (3) The charge can migrate along the polymer chain and between the fullerene units. This leads to a rather long separation of the charges and, thus, to the formation of a relatively long-living charge transfer state, up to few milliseconds in the present study. Computer Modeling of the EMR Molecule. A. Computational Details. The geometry optimization of the EMR molecule was carried out using several, separate steps, as described below. In each step, the initial optimization of the structure was performed using the semiempirical PM3 method within the PC SpartanPro (version 1.0.5) software.31 The final equilibrium geometries were obtained from the ab initio self-consistent field (SCF) molecular orbital calculations, using the spin-restricted Hartree-Fock (HF) method. The geometry optimizations were carried out using the 3-21G*32 basis set with the Gaussian 98 (version A.9) program.33 It has been shown that the hydroxyl substituted crown conformer of the calix[4]arene has the structure with the minimum energy.34 Therefore, the crown conformer of calix[4]arene was chosen for the basic building unit of the EMR molecule. The crown conformer was optimized at the HF/3-21G* level of theory. The two classes of substituents, the eight hydroxyl groups and the four ether groups, were added to the crown in two separate steps. After the addition of each class, a grid conformational search was performed with the Cerius2 (version 4.0) software.35 During the conformational search the optimized crown structure was kept constrained, while the (31) SPARTAN 5.1; wavefunction, Inc. 18401 Von Karman Ave., Ste. 370 Irvine, CA 92612. (32) Pietro, W. J.; Francl, M. M.; Hehre, W. J.; DeFrees, D. J.; Pople, J. A.; Binkley, J. S. J. Am. Chem. Soc. 1982, 104, 5039. (33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.9; Gaussian, Inc.: Pittsburgh, PA, 1998. (34) van Hoorn, W. P.; Morshuis, M. G. H.; van Vegel, F. C. J. M.; Reinhoudt, D. N. J. Phys. Chem. A 1998, 102, 1130. (35) Molecular Simulation Inc., 9685 Scranton Road, San Diego, CA 92121-3752.
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Table 2. Calculated Distances (nm) between the Carbon Atoms on Opposite Tails of the EMR Molecule C (1)
C (2)
dist (nm)
C(3)
C(4)
dist (nm)
224 220 209 195 181 168 154 134 116 97 70
191 176 164 142 126 105 87 66 53 38 30
1.931 1.936 1.669 1.671 1.410 1.411 1.159 0.998 0.924 0.923 0.723
222 218 213 199 185 172 158 137 120 101 74
189 174 162 140 122 103 83 65 49 37 26
1.931 1.936 1.669 1.671 1.410 1.411 1.159 0.998 0.924 0.923 0.723
rotation of the C-C, C-O, and O-H bonds of the added groups was allowed. The energies of the conformers were calculated using the cvff_300_1.01 force field.36 The conformer with the lowest energy was selected for the geometry optimization. Eventually, the optimized structures of the hydrocarbon tails were added and the equilibrium geometry of resulting EMR molecule was computed. The calculated energies are presented in Table 1. B. Results and Discussions. The structure of the EMR molecule optimized at the HF/3-21G* level of theory is presented in Figure 7. To be able to estimate whether the C60 molecule can be placed between the hydrophobic tails, the interatomic distances between carbon atoms on opposite tails were calculated taking into account the van der Waals radius of the methylene groups. A comparison of the distances between carbon atoms, given in Table 2, shows that the arrangement of the tails is symmetric. Because C60 has a radius of approximately 0.5 nm, the calculations predict that it can be placed between the tails down to the atoms C(154, 87, 158 and 83). (36) (a) Hagler, A. T.; Huler, E.; Lifson, S. J. Am. Chem. Soc. 1974, 96, 5318. (b) Hagler, A. T.; Lifson, S. J. Am. Chem. Soc. 1974, 96, 5327. (c) Dauber-Osguthorpe, P.; Roberts, V. A.; Osguthorpe, D. J.; Wolff, J.; Genest, M.; Hagler, A. T. Proteins: Struct., Funct. Genetics 1988, 4, 31.
Figure 8. Representation of the contact surface between the hydrophobic tails of the EMR molecule and C60.
A second evaluation has been done using a probe sphere of 0.5 nm (the approximate radius of C60), which was rolled on the hydrophobic carbon tails. The surface of contact between the C60 molecule and the van der Waals surface of the tails was computed with the Cerius2 program. The graphical representation of this surface is presented in Figure 8. About the topology of the calculated surface, it can be seen that C60 can reach the same depth level. Summary The present substituted resorcinarene molecule can be used to incorporate unsubstituted C60 molecules to LBfilms. The computational results also predict that there is enough space for the C60 molecule to be placed between the tails of the resorcinarene molecule. The resulting films are stable and can be easily transferred to solid supports. Interlayer photoinduced charge transfer from PHT to C60 takes place in the present films. LA010786U
