J. Phys. Chem. B 2004, 108, 4669-4672
4669
Electron Delocalization in One-Electron Oxidized Aniline Oligomers, Paradigms for Polyaniline. A Study by Paramagnetic Resonance in Fluid Solution Birgit Grossmann,† Ju1 rgen Heinze,*,† Thomas Moll,† Cornelia Palivan,‡ Stanislav Ivan,‡ and Georg Gescheidt*,§ Institute of Physical Chemistry, UniVersity Freiburg, Albertstrasse 21, D-79014 Freiburg, Germany, Department of Chemistry, UniVersity of Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland, and Institute of Physical and Theoretical Chemistry, Graz UniVersity of Technology, Technikerstrasse 4/I, A-8010 Graz, Austria ReceiVed: December 17, 2003; In Final Form: January 28, 2004
How far is an electron delocalized in organic conducting polymers? Insights into this aspect are presented by analyzing the spin distribution in radical cations of seven differently substituted aniline oligomers. The paramagnetic stages of dimers, tetramers, and hexamers were studied by EPR/ENDOR spectroscopy in fluid solution and DFT calculations. The EPR isotropic hyperfine coupling constants are compared with calculated counterparts for two model systems. In all derivatives, the main portion of the spin and the charge are confined to the central phenylenediamine moiety; i.e., only a very limited electron delocalization into the adjacent phenyl substituents exists. These experimentally established results point to a substantial contribution of interchain hole transfer in conducting polyanilines.
Introduction
CHART 1
What are the intrinsic mechanisms for the conducting properties of organic materials? Are electrons delocalized within one polymer chain which is composed of conjugated π systems, and in how far does electron hopping between adjacent polymer chains contribute to conduction? In this contribution we wish to shed some light onto the amount of delocalization within a polyaniline chain. Polyaniline is a very popular example of organic conducting polymers. Anodic oxidation of aniline in acidic solution yields polymeric products with a vast palette of applications including conducting and electrochromic materials,1-5 thin films,6 nanotubes,7 ferromagnetic devices,8,9 and biosensors.10 The interest in these materials is due to their convenient accessibility, and the ease of structural variations in terms of different types of doping agents. The polymers obtained by coupling reactions can be regarded as a chain in which the backbone is formed by p-phenylenediamine moieties. The oxidation potential of these polymers (emeraldine base) is rather low (ca. 0.05 V vs Ag/ AgCl), and essentially, the radical cations (generally denoted as emeraldine salts) generated in this first oxidation step are responsible for the remarkable electronic properties. The electronic structure of the radical cations is determined by the energy and the shape of the singly occupied molecular orbital (SOMO), which can be readily characterized by the isotropic hyperfine coupling constants (hfc’s) determined by fluid-solution EPR and ENDOR spectroscopy. Unfortunately, the EPR spectra of polyaniline radical cations are not resolved, and thus their clear-cut characterization at the molecular level is hardly feasible. Therefore, radical cations derived from simple, small molecules representing structural subunits of a polymer like phenylenediamine have been chosen as models.11-19 †
University Freiburg. University of Basel. § Graz University of Technology. ‡
How is an unpaired electron delocalized within an aniline polymer backbone? To get more detailed insight into the electron delocalization within a polyaniline chain and to establish the differences between monomeric and polymeric anilines, we have chosen to investigate aniline oligomers. Molecules 1 and 2 (see Chart 1) represent the mono-N-phenyl-substituted dimer and tetramer of aniline, respectively. The acetylated derivative 3 allows us to distinguish if the substitution in the central phenylenediamine moiety leads to a redistribution of the spin and the charge. N-methylated derivatives 4-6 were prepared20 to check if conformational changes of the chain, induced by the CH3 group, influence the electron distribution. In this series, 4 represents the dimer, 5 the tetramer, and 6 the hexamer of aniline. In addition, the para position of the outermost phenyl substituents are protected by tert-butyl groups in tetramer 7. It is noteworthy that all derivatives formally possess a phenylenediamine constituent in the center which is symmetrically surrounded by N-aryl groups.
10.1021/jp0379042 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/23/2004
4670 J. Phys. Chem. B, Vol. 108, No. 15, 2004
Grossmann et al.
TABLE 1: 1H and 14N hfc’s (mT) and g Factors of 1•+-3•+ Together with hfc’s Calculated for Model System 1•+ 1(calc) 1 2 3
2N
2 NH
H 2,3,5,6
H para
H ortho
H meta
g factor
0.502 0.550 0.543 0.560
-0.719 -0.613 -0.663 -0.655
-0.148 -0.154 -0.089 -0.270 (2H)
-0.188 -0.183
-0.152 -0.103 -0.122 -0.109
0.046 0.020 0.014
2.0030 2.0030 2.0029
-0.109
TABLE 2: 1H and 14N hfc’s (mT) and g Factors of 4•+-7•+ Together with the hfc’s Calculated for Model System 4•+ 4(calc) 4 5 6 7
2N
2 NCH3
H 2,5
H 3,6
H para
H ortho
H meta
0.619 0.592 0.575 0.585 0.545
0.671 0.652 0.636 0.652 0.673
-0.209 -0.205 -0.204 -0.198 -0.176
-0.133 -0.155 -0.160 -0.163 -0.123
-0.068 0.050
-0.070 0.050 0.053 0.054 0.080
0.065 0.014 0.005 0.040
g factor 2.0030 2.0030 2.0031 2.0030
It was shown that all model compounds are excellent electron donors.20 Cyclic voltammograms of 1-7 indicate stepwise reversible oxidation to the radical cations and dications. The tetramers 2 and 5 and hexamer 6 are able to form tri-, tetra, and hexacations, respectively. The potentials which have to be applied to generate the corresponding radical cations lie between 500 and 100 mV vs Ag/AgCl, with the lowest potential corresponding to hexamer 6. Here we present EPR studies of the radical cations generated from model compounds 1-7. The hyperfine data of the radical cations are established by ENDOR and TRIPLE spectroscopy. Calculations on the density-functional level of theory performed on dimer 1 and its N-methylated analogue 4 will assist in assigning the EPR data and reflecting the orbital character. Experimental Section The synthesis of compounds 1-7 has been described elsewhere.20,21 The radical cations were generated under high vacuum in CH2Cl2 (stored under high vacuum and molecular sieves) as the solvent, and triflouroacetic acid (TFAA, Fluka Buchs p.a.) served as the oxidant. EPR spectra were taken on a Varian E9 spectrometer, whereas the ENDOR and general TRIPLE measurements were performed on a Bruker ESP300 instrument. EPR simulations were done with the public domain program WinSim.22 Quantum mechanical calculations were performed on the UB3LYP/6-31G* level of theory using the Gaussian 94 program suite.23 Results and Discussion The parent compounds 1-7 were oxidized with trifluoroacetic acid (TFAA) (10% in CH2Cl2). Immediately after the contact of 1-7 with the oxidant, only very weak EPR signals were detected. However, after 10-120 min, intense colors developed (blue-green) and well-defined and well-resolved EPR spectra were observed. In all cases, 1H and 14N ENDOR signals could be obtained. Representative examples of the related dimers 1•+ and 4•+ are given in Figure 1a and Figure 1b, respectively. The EPR data of 1•+-3•+ are given in Table 1, and those of N-methylated derivatives 4•+-7•+ can be found in Table 2. The allotment of the specific positions is displayed in Figure 2. For all radical cations, the dominating splittings in the EPR spectra are caused by the interaction of the free electron with two equivalent 14N nuclei (14N hfc’s ca. 0.55-0.59 mT) and two (1-3) or six (4-7) protons (1H hfc’s ca. 0.56-0.65 mT). It is straightforward to assign these hfc’s to two apparently equivalent NH and NCH3 groups in 1•+-3•+ and 4•+-7•+, respectively. These values resemble those established for the very persistent radical cation of Wursters blue (8), in which the two equivalent
Figure 1. Experimental and simulated EPR and ENDOR spectra of (a) 1•+ and (b) 4•+.
nitrogens possess a 14N hfc of 0.701 mT and the protons of the adjacent methyl groups a 1H hfc of 0.683 mT (Figure 3) with virtually identical g factors of the EPR signals (Wursters blue, g ) 2.00305).24-26 The slightly lower 14N hfc’s in 1•+-7•+ compared to 8•+ can be traced back to the delocalization of the spin into the adjacent phenyl groups in 1•+-7•+ not present in 8•+. An analogous behavior also holds for the 1H hfc’s attributed to the NH (1-3) and NCH3 (4-7) protons. A spin distribution such as that in a diphenylamine27 can clearly be ruled out 12 (9, Figure 3) because it shows a considerably higher 14N hfc. In the N-methyl derivatives 4•+-7•+, the 14N and 1H hfc’s are slightly higher than in the N-H analogues 1•+-3•+. In contrast to 1•+-3•+, the steric hindrance by the N-methyl groups
Electron Delocalization in Aniline Oligomers
J. Phys. Chem. B, Vol. 108, No. 15, 2004 4671
Figure 2. Allotment of the protons and nitrogens in the central phenylenediamine moiety in the center of 1-7.
Figure 3. Structures of 8 and 9 and the 1H and 14N hfc’s (mT) of the corresponding radical cations (data of 8•+, see ref 24; data of 9•+, see ref 27).
Figure 5. Comparison of calculated (1•+, 4•+) and experimental 1H and 14N hfc’s of NH (1•+, 2•+) and NCH3 (4•+-6•+ together with reference compound 8•+).
Figure 4. Calculated (UB3LYP/6-31G*) geometries of 1•+ and 4•+.
in 4•+-7•+ prevents the adjacent aryl substituents from adopting a coplanar arrangement with the central phenylenediamine moiety. This is mirrored in the calculated (UB3LYP/6-31G*) structures of 1•+ and 4•+ (Figure 4), which were used to accomplish the assignment of the experimentally determined hfc’s. Whereas the angle between the plane of the central π system and the N-phenyl substituents is 42° in 1•+, it increases to 63° in 4•+. Consequently, the 1H hfc’s attributed to the ortho, meta, and para protons in the phenyl groups of 4•+-7•+ are smaller than those in 1•+-3•+; moreover, because the rotation of the phenyl groups is hindered, the 1H hfc’s of the four inner protons (positions 2, 3, 5, 6, see Figure 2) being virtually equivalent in 1•+ are clearly distinct in 4•+ (Tables 1 and 2). The major coupling constants show only slight variations when the chain length increases. This is shown in Figure 5. Starting from the unsubstituted building block, the radical cation of 8 (Wursters blue), a decrease of the 14N hfc becomes apparent. Within the series dimer (4•+), tetramer (5•+), hexamer (6•+), it is only about 10%. The same also holds for the 1H hfc’s of the adjacent NH and NCH3 protons. No additional 14N hfc’s can be established for the tetramers 2•+ and 5•+ and hexamer 6•+, corroborating that no considerable amount of spin and charge are injected into the adjacent aniline moieties. This electron distribution is hardly altered by the introduction of substituents to the central moiety as in derivative
3 or by introducing tert-butyl groups to the outermost p-phenyl positions (7). On the hyperfine time scale, the multiplicities of the hfc’s indicate pairwise equivalent nuclei in the range from 193 to 293 K. This is in line with a 2-fold symmetry of the radical cations, and thus the spin and the positive charge have to be confined to the central phenylenediamine unit with a very limited spin-transfer distance into adjacent π systems of the oligomer chain. Conclusions The experimental data presented in this study show that, in the radical cations of oligoanilines 1-7, electron delocalization occurs only to a small extent. The unpaired electron is confined to a central phenylenediamine unit and the adjacent phenyl groups. The EPR line widths are not temperature dependent, revealing that no dynamic phenomena such as changes of conformation or inter/intramolecular electron transfer occur on the hyperfine time scale. The limited amount of charge and spin delocalization is illustrated by the decreasing differences between the first and second oxidation potential, ∆E1-2 of 4, 5, and 6, being 500, 290, and 190 mV, respectively (see Figure 2 in ref 20), and is in line with data reported previously (although the authors interpret the occurrence of hfc’s attributed to the phenyl substituents in 1•+ in terms of a “considerable delocalization”).11 Even under conditions promoting intramolecular electron transfer, a small extent of electron delocalization exists along the oligoaniline backbone. The observation that it is possible to produce radical trications at potentials as low as 1.42 V (tetramer 5) and 1.030 V vs Ag/AgCl (hexamer 6) underpins that the conjugation length is indeed rather short; i.e., Coulomb repulsion does not impede the third oxidation step which takes
4672 J. Phys. Chem. B, Vol. 108, No. 15, 2004
Figure 6. Charge and electron delocalization in polyanilines.
part at a more distant phenylenediamine unit. In other words, aniline or, better, phenylenediamine units which are not directly adjacent to the already oxidized moiety can be regarded as almost isolated electroactive domains (cf. Figure 2). Such a structure has already been postulated for emeraldine salts.28 Besides conformational effects, ion pairing phenomena29 are likely to be responsible for this low tendency of electron delocalization in the radical cations of 1-7. The above structural features are in very good agreement with recent investigations carried out in the fluid and in the solid state. UV-vis, IR, and Raman spectra of FeCl3-doped polyaniline dimers (in acetonitrile) reveal that only the central ring possesses a quinoid structure whereas the outer rings keep their aromatic character.30 According to measurements utilizing lowand high-field EPR, the conducting properties of polyanilines (emeraldine salts) are predominantly caused by three-dimensional charge hopping (Figure 6) between different polymeric chains rather than by one-dimensional solitary conducting chains.31,32 Similar conclusions can be drawn from the observation that the presence of triflate anions does not affect the conductivity of emeraldine salts.33 This is, moreover, in line with near-field scanning electrochemical measurements on ultrathin layers34 and theoretical predictions.35 References and Notes (1) Mortimer, R. J. Chem. Soc. ReV. 1997, 26, 147. (2) Novak, P.; Mueller, K.; Santhanam, K. S. V.; Haas, O. Chem. ReV. 1997, 97, 207. (3) MacDiarmid, A. G. Angew. Chem., Int. Ed. 2001, 40, 2581. (4) MacDiarmid, A. G.; Yang, L. S.; Huang, W. S.; Humphrey, B. D. Synth. Met. 1987, 18, 393. (5) Pouget, J. P.; Oblakowski, Z.; Nogami, Y.; Albouy, P. A.; Laridjani, M.; Oh, E. J.; Min, Y.; MacDiarmid, A. G.; Tsukamoto, J. Synth. Met. 1994, 65, 131.
Grossmann et al. (6) Baba, A.; Advincula, R. C.; Knoll, W. J. Phys. Chem. B 2002, 106, 1581. (7) Zhang, L.; Wan, M. J. Phys. Chem. B 2003, 107, 6748. (8) Tanaka, K.; Yoshizawa, K.; Takata, A.; Yamabe, T.; Yamauchi, J. Synth. Met. 1991, 43, 3297. (9) Goto, H.; Iino, K.; Akagi, K.; Shirakawa, H. Synth. Met. 1997, 85, 1683. (10) Krishnamoorthy, K.; Contractor, A. Q.; Kumar, A. Chem. Commun. 2002, 240. (11) Wolf, J. F.; Forbes, C. E.; Gould, S.; Shacklette, L. W. J. Electrochem. Soc. 1989, 136, 2887. (12) Male, R.; Allendoerfer, R. D. J. Phys. Chem. 1988, 92, 6237. (13) Moyer, M.; Quillard, S.; Louarn, S.; Lefrant, S.; Rebour, E.; Monkman, A. P. Synth. Met. 1997, 84, 787. (14) Quillard, S.; Louarn, S.; Buisson, J. P.; Boyer, M.; Lapkowski, M.; Pron, A.; Lefrant, S. Synth. Met. 1997, 84. (15) Yang, S. M.; Chiang, J. H.; Shiah, W. M. Synth. Met. 1991, 41, 753. (16) Dunsch, L.; Petr, A. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 436. (17) Niu, J. J.; Lee, J. Y.; Neudeck, A.; Dunsch, L. Synth. Met. 1999, 99, 133. (18) MacDiarmid, A. G.; Zhou, Y.; Feng, J. Synth. Met. 1999, 100, 131. (19) Rodrigue, D.; Domingue, M.; Riga, J.; Verbist, J. J. Synth. Met. 1993, 57, 4802. (20) Moll, T.; Heinze, J. Synth. Met. 1993, 55-57, 1521. (21) Heinze, J.; Tschuncky, P. In Electronic Materials: the Oligomer Approach; Mu¨llen, K., Wegner, G., Eds.; Wiley-VCH: Weinheim, 1998; p 479. (22) Duling, D. R. PEST Winsim. J. Magn. Reson., B 1994, 104, 105. (23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; HeadGordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, Revision B.3; Gaussian, Inc.: Pittsburgh, PA, 1995. (24) Latta, B. M.; Taft, R. W. J. Am. Chem. Soc. 1967, 89, 5172. (25) Bolton, J. R.; Carrington, A.; Dossantosveiga, J. Mol. Phys. 1962, 5, 615. (26) Weissman, S. I. J. Chem. Phys. 1954, 22, 1135. (27) Neugebauer, F. A.; Bamberger, S. Angew. Chem., Int. Ed. Engl. 1971, 10, 71. (28) Jansen, S. A.; Duong, T.; Major, A.; Wei, Y.; Sein Jr., L. T. Synth. Met. 1999, 105, 107. (29) Khan, M. N.; Palivan, C.; Barbosa, F.; Amaudrut, J.; Gescheidt, G. J. Chem. Soc., Perkin Trans. 2 2001, 1522. (30) Boyer, M. I.; Quillard, S.; Louarn, G.; Froyer, G.; Lefrant, S. J. Phys. Chem. B 2000, 104, 8952. (31) Krinichnyi, V. I.; Roth, H. K.; Hinrichsen, G.; Lux, F.; Luders, K. Phys. ReV. B 2002, 65, 155205/1. (32) Mizoguchi, K. Synth. Met. 2001, 119, 35. (33) Polk, B. J.; Potje-Kamloth, K.; Josowicz, M.; Janata, J. J. Phys. Chem. B 2002, 106, 11457. (34) Mandler, D.; Unwin, P. R. J. Phys. Chem. B 2003, 107, 407. (35) Kwon, O.; McKee, M. L. J. Phys. Chem. B 2000, 104, 1686.
