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Electron paramagnetic resonance studies of polymeric charge-transfer complexes
S~fnthetic Metals, 38 (1990) 53-62
53
E L E C T R O N P A R A M A G N E T I C R E S O N A N C E S T U D I E S OF POLYMERIC C H A R G E - T R A N S F E R C O M P L E X E S S. P ~ P A N
and D. N. SATHYANARAYANA*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 550 012 (India) (Received January 4, 1990; accepted February 23, 1990)
Abstract Electron paramagnetic resonance (EPR) spectroscopic studies are performed on a number of polymeric charge-transfer complexes with different polymer donors and low molecular weight acceptors. The g values, linewidths and spin concentrations at different temperatures have been determined. The results indicate that there is no change in the g values and linewidths with temperature. The signal intensity increases with temperature, which is typical of singlet-triplet systems separated by an activation energy. The spin concentration of the paramagnetic charge-transfer complexes generally increases with the strength of the d o n o r - a c c e p t o r tendencies. The charge-transfer complexes transform finally to a diamagnetic product and this was confirmed by electronic absorption spectroscopy.
Introduction The theoretical and practical interest in new highly conducting materials have recently stimulated a great deal of work on charge-transfer complexes based on polymers. The principle of introducing a polymeric molecule into the complex has been used, for example, to prepare a series of chelate compounds with polymeric phthalocyanine, polypyromellitimide and polymeric tetracyanoethylene as ligands. Interest in polymeric complexes is connected with the possibility of improving the conductance by increased intermolecular interaction on complex formation by chain molecules and with the use of the film-forming properties of polymers in semiconductor technology. Doped polymers, where the polymer acts as an electron donor or as an electron acceptor, find application in electrography [1] and as cell electrodes in implantable cardiac pacemakers [2]. There have been a number of investigations on the electrical and magnetic properties of polymeric charge-transfer complexes [3-6]. Part of this interest *Author to whom correspondence should be addressed.
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54 is derived from observations of electron paramagnetic resonance in polymeric charge-transfer complexes. The EPR spectra on doped polyacetylene [7-9], poly(paraphenylene) [10, 11 ] and polypyrrole [12, 13] have been investigated the most. Studies on other systems have been few [14-16]. The goal of the present investigation was to obtain information concerning the structure and the nature of the polymeric charge-transfer complexes from EPR and electronic spectral studies and to increase the radical concentration of the polymer with appropriate acceptors. For this purpose we have investigated the charge-transfer complexes of poly(4-vinylpyridine) (4PVP), poly(2-vinylpyridine) (2PVP) and poly(2-vinylpyridine-co-styrene) (2PVP-coSt) with eight different acceptors, namely, p-chloranil (pCHL), o-chloranfl (oCHL), 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ), 7,7',8,8'-tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE), 2,4,5,7-tetranitro9-fluorenone (TENF), 2,4,7-trinitro-9-fluorenone (TNF) and iodine (I2).
Experimental Poly(4-vinylpyridine) (Aldrich) was dissolved in AR-grade benzene and reprecipitated by excess of AR-grade n-hexane from the filtrate. The procedure was repeated thrice. The product was dissolved in chloroform and filtered (through a G-3 crucible) and the solvent was evaporated under reduced pressure. Poly(2-vinylpyridine) (Aldrich) and poly(2-vinylpyridine-co-styrene) (Aldrich) were also purified by the same method. p-Chloranil (B.D.H.) was purified by recrystallization from benzene, m.p. 292 °C. 2,3-Dichloro-5,6-dicyano-l,4-benzoquinone (Fluka) was twice crystaUized from dry benzene, m.p. 215 °C. 2,4,7-Trinitro-9-fluorenone (Fluka) was crystallized from dry benzene, m.p. 175 °C. 2,4,5,7-Tetranitro-9-fluorenone (Aldrich) was purified by recrystallization from hot acetic acid (20 ml) containing 1 ml acetic anhydride, m.p. 254 °C. Tetracyanoethylene (Aldrich) was recrystallized twice from dichloromethane and then sublimed, m.p. 198 °C. Resublimed reagent-grade iodine, o-chloranil (Sigma), m.p. 128 °C and 7,7'8,8'-tetracyanoquinodimethane (Aldrich), m.p. 296 °C were used. The EPR spectra of the complexes were obtained on a Varian E-109 spectrometer operating in the X-band. The sample temperature was varied from 100 °C down to liquid nitrogen temperatures. The g values, linewidths and spin concentration were estimated using charred dextrose as a standard. The charge-transfer complexes for the EPR study were prepared by refltLxing the solutions of the constituents (polymer and low molecular weight acceptor in the ratio 20:1) for 1 h in chloroform. The solution was cooled to room temperature and the solvent was evaporated under vacuum. Absorption spectra were obtained in chloroform at 25 °C using an Hitachi U-3400 spectrophotometer. The electronic spectra of the charge-transfer complexes were recorded in the wavelength range 2 0 0 - 1 2 0 0 nm using a pair of matched 3 ml-stoppered silica cells of 10-mm path length.
55 Results and discussion Generally the same EPR and electronic spectral features were observed between all the polymer donors for a c o m m o n acceptor. Initial studies on some of the samples indicated that the EPR absorption was unaffected by atmospheric oxygen. However, as a precautionary measure, all the solid samples were studied i n v a c u o . T h e g values and linewidths of the EPR spectra obtained for the various charge-transfer complexes of the pol ym er donors and low molecular weight acceptors are r e p o r t e d in Table 1. The EPR spectra of dope d 4PV P- T E NF and 4PVP-pCHL systems are shown in Figs. 1 and 2, respectively. Most of the systems reveal only one EPR signal as shown in Fig. 1 while some of the systems give two signals as in Fig. 2. For a representative 4PVP-pCHL system, the values are gl = 2.0054 and g 2 = 2 . 0 0 1 0 (with an uncertainty of ___0.00015 in the g values) with a total linewidth of 8.4 G. The shape of the EPR signal is the same for the complex of an a c c e p t o r with different polymer donors. The EPR spectra of 4PVP-TENF complex at different temperatures are shown in Fig. 1. The signal intensity (spin concentration) increases with t e m p e r a t u r e and this is presented in Table 2. This type of variation has been observed for most of the systems investigated. There is variation in the EPR signal intensity with t e m p e r a t u r e resulting from the changes in the population of the magnetic levels [ 1 7 - 1 9 ] . For a singlet-triplet system, the intensity increases with temperature, because of the population of the triplet level. For a system in the ground state consisting of an unpaired electron on the donor and an unpaired electron on the acceptor, and each acting independently of each other as doublet states, the intensity of the spin resonance signal would be inversely proportional to the temperature. For most of the systems presently investigated, EPR signal intensity was found to increase with temperature. This feature is typical of singlet-triplet systems separated by an activation energy and it is clearly reflected by Fig. 1 and data given in Table 2. The possibility that the system might be S = 1 (two unpaired electrons) was considered and a search for the forbidden ~Ms = _ 2 transitions was made. However no lines arising from AMs= _+2 transitions were found. Such a result is not conclusive because usually the signal intensity is very weak. EPR studies indicate that all the systems exhibit an exponential temperature d e p e n d e n c e of spin concentration characteristic of semiconductors, strongly suggesting the possibility that the resonance observed is due to a single unpaired electron either on the a c c e p t o r or on the donor. Apparently the two peaks observed are not due to two electron resonances associated with the donor and a c c e p t o r ions, but attributable rather to an anisotropic g tensor. Hence averaged g values are r e p o r t e d in Table 1 for the polymeric charge-transfer complexes where the acceptors are pCHL, oCHL and DDQ. The remaining systems exhibit a single EPR absorption. The charge-transfer complexes of h y d r o c a r b o n - h a l o g e n s [19] also show an exponential d e p e n d e n c e on t e m p e r a t u r e in agreem ent with the observed activation energy for electronic conduction. In the present
2.0001 2.0045 2.0057 2.0030 2.0067 2.0095 2.0032 2.0035 2.0086 2.0019 2.0044 2.0062 2.0036 2.0033 2.0081 2.0017 2.0037 2.0058
4PVP-TCNQ 2PVP-TCNQ (2PVP-co-St)-TCNQ 4PVP-DDQ 2PVP-DDQ (2PVP-co-St)-DDQ 4PVP-pCHL 2PVP-pCHL (2PVP-co-St)-oCHL 4PVP-TENF 2PVP-TENF (2PVP-co-St)-TENF 4PVP--oCHL 2PVP--oCHL (2PVP-co-St)-oCHL 4PVP-TNF 2PVP-TNF (2PVP-co-St)--TNF
"The activation energy could not be calculated.
g
Polymer complexes
6.2 8.8 4.6 5.2 4.8 5.4 8.4 8.2 8.8 3.8 3.8 4.0 7.4 7.3 7.6 4.4 4.3 3.7
AH.,. (G)
361.4 1472.0 223.3 86.0 457.1 26.7 66.4 137.7 33.8 46.1 51.9 28.6 5.4 8.3 2.1 1.2 1.9 0.5
× 10 m
Spins &r'
0.004 0.015 0.007 0.005 0.002 0.007 0.020 0.012 0.014 0.005 0.009 0.010 0.012 0.016 0.010 0.011 0.012 0.022
temperature
Low
E. (eV)
The g values, linewidths, spin concentration at 298 K and activation energies fl)r tile polymeric charge-transfer c o m p l e x e s
TABLE 1
0.031 0.029 0.048 0.078 0.055 0.106 " 0.061 " 0.039 0.106 0.062 0.072 0.155 • 0.254 " "
High temperature
¢91
57
143 K
173 K 203 K 233K
263 K
313K 323 K 333 K 343K 353K
I
I
I
I
32405 MAGNETIC FIELD (H)
4G
3240 G MAGNETIC FIELD (H)
)
)
Fig. 1. Variation of EPR spectra of the 4PVP-TENF system with temperature. (Receiver gain: 4 × 10 z and 5 × 102 for the range 143-263 K and 313-353 K, respectively.) Fig. 2. EPR spectrum of the 4PVP-pCHL complex at room temperature. TABLE 2 Spin concentration with temperature for the 4PVP-TENF system Spins g-1
Log (spins g-l)
T (K)
17.7671 17.7481 17.7301 17.7103 17.6972 17.1073 17.0802 17.0702 17.0496 17.0203
353 343 333 323 313 263 233 203 173 143
X 1017
5.85 5.60 5.37 5.13 4.98 1.28 1.20 1.17 1.12 1.05
study no hyperfine splitting was observed. It is therefore not possible from the present results to infer whether the electron spin is associated with the acceptor or the donor. The spin concentration of the polymeric charge-transfer complexes is highly dependent upon the nature of the donor and acceptor moieties. The spin concentration for a given donor is dependent on the nature of the acceptors (Table 1). The spin concentration of the polymeric charge-transfer complexes generally varies as TCNQ > DDQ > p C H L > TENF > oCHL > TNF.
58 Amongst the three donors the spin concentration changes as 2PVP> 4 P VP > (2PVP-co-St). The positive charge on the nitrogen of the polymer can be better stabilized when the - - C H - - C H 2 - - group is in the para position than when it is in the ortho position due to stereoelectronic effects. Hence in 4PVP the lone pair of electrons on the nitrogen is more easily available than in 2PVP. Consequently it is inferred that an ionic c o m p o u n d is formed in the case of charge-transfer complexes of 4PVP, whereas in the case of charge-transfer complexes of 2PVP and the copolymer it takes a longer time to form the ionic compound. For 4PVP complexes, spin pairing possibly takes place which leads to the observed decrease in spin concentration. The spin concentration of the 2PVP complexes are hence greater than 4PVP complexes. The presence of styrene with 2-vinylpyridine in the copolymer decreases the spin concentration relative to that of the pure poly(2-vinylpyridine), since the acting donor group is r e d u c e d in the copolymer. Most of the charge-transfer complexes exhibit an exponential t e m p e r a t u r e d ep en d en ce of spin concentration characteristic of semiconductors [18, 19]. This t e m p e r a t u r e dependence may be e x pressed in terms of EPR spin concentration as
N=No expC-Ea/RT)
(1)
where N and No are the num ber of spins per gram of the sample and reference, respectively, and Ea is the observed activation energy for spin concentration evaluated from a p l o t of log N versus lIT (Fig. 3). The activation energy for the polymeric charge-transfer complexes is re po r ted in Table 1. For some systems it was not possible to calculate the activation energy because of the small variation of spin concentration with temperature. However there is a definite increase in spin concentration with temperature. The activation energy for the 4PV P- T E N F system was found to be 0.039 eV at higher temperatures and 0.005 eV at lower temperatures. The activation
17.8
c
•~.
~ab~a = O.039 eV
17.4
~.~__o~.
Eo =
0.005 eV
17.0 i
I
i
0.004
I
0.006
i
0.008
(TIK~ "I
Fig. 3. Variation of EPR spin concentration with temperature for the 4PVP-TENF system.
59 energy varies from 0.02 to 0.26 eV at high t em perat ure and from 0.001 to 0.02 eV at low temperatures. Two factors contribute to the spin concentration in the polymeric charge-transfer complexes: (i) a thermally activated species at high temperatures; and (ii) a constant num ber of spins at low temperatures. The activation energies will therefore be different at lower and higher temperatures. Consequently eqn. (1) is not applicable in the whole t em perat ure range and a break in the intermediate t em p erat ure region is found. A similar behaviour has be e n found for aromatic hydrocarbons with iodine by Singer and Kommandeur [20]. Solutions containing the interacting donor (D) and a c c e p t o r (A) species show not only the absorption bands of D and A, but also most often exhibit a new band or bands which are assignable to charge-transfer transition(s) of the complex as a whole. Furthermore, the electronic spectra also show another general feature of charge-transfer interaction -- the possibility of complete electron transfer from the donor to the a c c e p t o r with the formation of radicals (intramolecular excitation bands of ionic acceptors). The chargetransfer complex may further undergo an irreversible chemical reaction to give a final product. The absorption maxima of the donors, the acceptors and their charge-transfer complexes are given in Table 3. In most of the systems colour develops immediately when the polymer donor and low molecular weight acceptors (such as TCNQ, TCNE, DDQ and I2) are mixed in chloroform and the optical density of this solution increases with time and attains a m a xi m um ( e xc e pt when TCNE is the acceptor). This colour is ascribed to the formation of the charge-transfer complexes between the reactants. In the case of pCHL and oCHL as accept ors no new bands were observed. However, intensity of the a c c e ptor band gradually increases with time indicating the formation of the charge-transfer complexes. Finally these bands disappear and the absorption m axi m um shifts to the UV region or, in some cases, a new absorption maximum arises. From this, it is inferred that the initially f or m e d charge-transfer complexes transform into a final product. New bands have not be e n observed in the polymeric charge-transfer complexes where the acceptors are TNF and TENF. Equimolecular quantities of 4PVP and pCHL dissolved in chloroform show a narrow single EPR line. The same p h e n o m e n o n was observed for the complexes of all three polymers with acceptors, oCHL, pCHL, TCNQ and DDQ. The g values and linewidths of EPR absorptions are given in Table 4. The EPR spectra were measured immediately after the chloroform solutions of the reactants were mixed. The signal intensity increases rapidly, attains a maximum and then gradually decreases. The paramagnetism of the solution finally vanishes. A very weak EPR signal was observed in the polymeric charge-transfer complexes where TENF is an a c c e p t o r and no EPR signal was observed in the case of polymeric charge-transfer complexes where TNF, TCNE and Iz are the acceptors. The same p h e n o m e n o n was observed in the solid state. However, the reaction in the solid state is extremely slow relative to that in solution.
60 TABLE 3 Absorption maxima of the donors, acceptors and their charge-transfer complexes in chloroform Compound
~
(nm)
4PVP 2PVP (2PVP-co-St) I~ 4PVP-I.)a 2PVP-I.,a (2PVP-co-St)-Ie a TCNQa 4PVP-TCNQa 2PVP-TCNQa (2PVP-co-St)-TCNQa DDQ~ 4PVP-DDQ ~ 2PVP-DDQ a (2PVP-co-St)-DDQ ~ TCNE 4PVP-TCNE a 2PVP-TCNE a (2PVP-co-St)-TCNE a pCHL oCHL TENF TNF
242.0 257.5 239.5 503.0 366.0 365.5 367.0 402.0 684.0 684.0 685.0 387.5 730.0 b 483.5 484.0 267.0 399.0 399.5 399.5 287.0 250.0 285.5 281.5
256.0 263.0 258.0
263.0 269.5 263.0
269.0
750.5 751.0 750.5
768.0 768.5 768.0
851.5 851.5 852.5
725.0 b 727.0 b 277.0 420.0 419.0 420.0 362.0 288.0 323.5 320.0
445.0 345.0 340.0
~I'he spectra were recorded in the range 350-1200 nm. bBroad band. The i n i t i a l l y f o r m e d c h a r g e - t r a n s f e r c o m p l e x e s t r a n s f o r m finally t o a d i a m a g n e t i c p r o d u c t w h e r e t h e a c c e p t o r s a r e TCNQ, TCNE, DDQ, p C H L , oCHL a n d I2. T h e c h a r g e - t r a n s f e r c o m p l e x f o r m a t i o n o c c u r s w i t h t h e f o r m a t i o n o f f r e e r a d i c a l s w h e r e TCNQ, DDQ, p C H L a n d oCHL a r e t h e a c c e p t o r s . W i t h TCNE a n d I2 t h e c h a r g e - t r a n s f e r c o m p l e x f o r m a t i o n o c c u r s w i t h o u t t h e f o r m a t i o n o f f r e e r a d i c a l s . C h a r g e - t r a n s f e r c o m p l e x f o r m a t i o n is w e a k w h e r e t h e a c c e p t o r s a r e T E N F a n d TNF. The g values and linewidth are t e m p e r a t u r e independent. The a b s e n c e o f h y p e r f i n e s p l i t t i n g in t h e c o m p l e x e s c o u l d b e c a u s e d b y s e v e r a l p o s s i b l e e x c h a n g e p r o c e s s e s , e.g., i n t r a m o l e c u l a r e x c h a n g e b e t w e e n t h e u n p a i r e d electrons, exchange between independently wandering holes and electrons a n d i n t e r m o l e c u l a r e x c h a n g e b e t w e e n t h e u n p a i r e d e l e c t r o n s in o n e c o m p l e x molecule.
Conclusions T h e s h a p e o f t h e E P R s i g n a l is s a m e f o r t h e c o m p l e x o f a n a c c e p t o r with different p o l y m e r donors. The g values and linewidths are i n d e p e n d e n t
61 TABLE 4 The g values and linewidths for the polymeric charge-transfer complexes in chloroform Polymer complexes
g
z~r-/tpp(G)
4PVP-TCNQ 2PVP-TCNQ (2PVP-co-St)-TCNQ 4PVP-DDQ 2PVP-DDQ (2PVP-co-St)-DDQ 4PVP--~pCHL 2PVP-pCHL
2.0024 2.0080 2.0042 2.0053 2.0029 2.0068 2.0045 2.0039 2.0066 2.0054 2.0028 2.0049 2.0063 2.0034 2.0057
5.7 4.6 5.5 9.0 2.1 3.0 7.5 1.2 3.0 2.9 1.6 2.2 3.0 2.0 2.7
(2PVP-co-St)-pCHL
4PVP-TENF 2PVP-TENF (2PVP-co-St)-TENF 4PVP--oCHL 2PVP-oCHL (2PVP-co-St)--oCHL
of t e m p e r a t u r e . The spin c o n c e n t r a t i o n of the c h a r g e - t r a n s f e r c o m p l e x e s varies as TCNQ > DDQ > p C H L > T E N F > oCHL > TNF a n d a m o n g the d o n o r s it varies as 2PVP > 4PVP > (2PVP-co-St). The p a r a m a g n e t i c c h a r g e - t r a n s f e r c o m p l e x e s exhibit an e x p o n e n t i a l t e m p e r a t u r e d e p e n d e n c e o f spin c o n c e n tration characteristic o f s e m i c o n d u c t o r s . The initially f o r m e d c h a r g e - t r a n s f e r c o m p l e x e s t r a n s f o r m finally to a d i a m a g n e t i c p r o d u c t w h e n the a c c e p t o r s are TCNQ, TCNE, DDQ, p C H L , oCHL a n d I2. The c h a r g e - t r a n s f e r c o m p l e x f o r m a t i o n o c c u r s with the f o r m a t i o n of free radicals w h e r e TCNQ, DDQ, p C H L a n d oCHL are the a c c e p t o r s and for the TCNE and I2 s y s t e m s the c h a r g e - t r a n s f e r c o m p l e x f o r m a t i o n o c c u r s w i t h o u t the f o r m a t i o n of free radicals. C h a r g e - t r a n s f e r c o m p l e x f o r m a t i o n is w e a k w h e r e the a c c e p t o r s are T E N F and TNF. The n a t u r e o f the c o m p l e x s e e m s to be d e t e r m i n e d mainly by the a c c e p t o r . However, the r e a c t i o n in the solid state is e x t r e m e l y slow relative to that in solution.
References 1 R. M. Shaffert, IBM J. Res. Dev., 15 (1979) 75. 2 A. A. Schneider, W. Greatbatch and R. Mead, in D. H. Collins (ed.), Power Sources 5 (Proc. 9th Int. Power Sources Symp., Brighton, U.K., 19743, Academic Press, London, 1975, Paper No. 3. 3 H. Hogel, J. Phys. Chem., 69 (1965) 755. 4 A. Rembaum, A. M. Hermann, F. E. Stewart and F. Gutmann, J. Phys. Chem., 73 (1969) 513. 5 A. M. Hermann and A. Rembaum, J. Polym. Sci., Part C, 17 (1967) 107. 6 C. U. Pittman and P. L. Grube, J. Appl. Polym. Sci., 18 (1974) 2269. 7 A. J. Epstein, H. Rommelmann, R. Bigelow, H. W. Gibson, D. M. Hoffmann and D. B. Tanner, J. Phys. (Paris), 44 (1983) C3-61.
62 8 B. R. Weinberger, J. Kaufer, A. J. Heeger, A. Profi and A. G. MacDiarmid, Phys. Rev. B, 20 (1979) 223. 9 S. Ikehata, J. Kaufer, T. Woerner, A. Profi, M. A. Druy, A. Sivak, A. J. Heeger and ~ G. MacDiarmid, Phys. Rev. Lett., 45 (1980) 1123. 10 L. D. Kispert, J. Joseph, G. G. Miller and R. H. Baughman, J. Chem. Phys., 81 (1984) 2119. 11 F. Maurice, C. Fontaine, A. Morisson, J. Y. Goblot and G. Froyer, MoL Cryst. Liq. Cryst., 118 (1985) B319. 12 F. Devreux, F. Genoud, M. Nechtschein, J. P. Travers and G. Bidan, J. Phys. (Paris), 44 (1983) C3-621. 13 J. C. Scott, P. Pfluger, M. T. Krounbi and G. B. Street, Phys. Rev. B, 28 (1983) 2140. 14 E. T. Kang, A. P. Bhatt, E. Villaroel, W. A. Anderson and P. Ehrlich, Mol. Gryst. Liq. Cryst., 83 (1982) 1339. 15 G. E. Wnek, J. C. W. Chien, F. E. Karasz and C. P, LiUya, Polymer, 20 (1979) 1441. 16 L. D. Kispert, L. A. Files, J. E. Frommer, L. W. Shacklette and R. R. Chance, J. Chem. Phys., 78 (1983) 4858. 17 S. N. Bhat and C.N.R. Rao, Can. J. Chem., 47 (1969) 3899. 18 D. B. Chesnut and W. D. Phillips, J. Chem. Phys., 35 (1961) 1002. 19 J. Kommandeur and R. Hall, J. Chem. Phys., 34 (1961) 129. 20 L. S. Singer and J. Kommandeur, J. Chem. Phys., 34 (1961) 133.
53
E L E C T R O N P A R A M A G N E T I C R E S O N A N C E S T U D I E S OF POLYMERIC C H A R G E - T R A N S F E R C O M P L E X E S S. P ~ P A N
and D. N. SATHYANARAYANA*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 550 012 (India) (Received January 4, 1990; accepted February 23, 1990)
Abstract Electron paramagnetic resonance (EPR) spectroscopic studies are performed on a number of polymeric charge-transfer complexes with different polymer donors and low molecular weight acceptors. The g values, linewidths and spin concentrations at different temperatures have been determined. The results indicate that there is no change in the g values and linewidths with temperature. The signal intensity increases with temperature, which is typical of singlet-triplet systems separated by an activation energy. The spin concentration of the paramagnetic charge-transfer complexes generally increases with the strength of the d o n o r - a c c e p t o r tendencies. The charge-transfer complexes transform finally to a diamagnetic product and this was confirmed by electronic absorption spectroscopy.
Introduction The theoretical and practical interest in new highly conducting materials have recently stimulated a great deal of work on charge-transfer complexes based on polymers. The principle of introducing a polymeric molecule into the complex has been used, for example, to prepare a series of chelate compounds with polymeric phthalocyanine, polypyromellitimide and polymeric tetracyanoethylene as ligands. Interest in polymeric complexes is connected with the possibility of improving the conductance by increased intermolecular interaction on complex formation by chain molecules and with the use of the film-forming properties of polymers in semiconductor technology. Doped polymers, where the polymer acts as an electron donor or as an electron acceptor, find application in electrography [1] and as cell electrodes in implantable cardiac pacemakers [2]. There have been a number of investigations on the electrical and magnetic properties of polymeric charge-transfer complexes [3-6]. Part of this interest *Author to whom correspondence should be addressed.
0379-6779/90/$3.50
© Elsevier Sequoia/Printed in The Netherlands
54 is derived from observations of electron paramagnetic resonance in polymeric charge-transfer complexes. The EPR spectra on doped polyacetylene [7-9], poly(paraphenylene) [10, 11 ] and polypyrrole [12, 13] have been investigated the most. Studies on other systems have been few [14-16]. The goal of the present investigation was to obtain information concerning the structure and the nature of the polymeric charge-transfer complexes from EPR and electronic spectral studies and to increase the radical concentration of the polymer with appropriate acceptors. For this purpose we have investigated the charge-transfer complexes of poly(4-vinylpyridine) (4PVP), poly(2-vinylpyridine) (2PVP) and poly(2-vinylpyridine-co-styrene) (2PVP-coSt) with eight different acceptors, namely, p-chloranil (pCHL), o-chloranfl (oCHL), 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ), 7,7',8,8'-tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE), 2,4,5,7-tetranitro9-fluorenone (TENF), 2,4,7-trinitro-9-fluorenone (TNF) and iodine (I2).
Experimental Poly(4-vinylpyridine) (Aldrich) was dissolved in AR-grade benzene and reprecipitated by excess of AR-grade n-hexane from the filtrate. The procedure was repeated thrice. The product was dissolved in chloroform and filtered (through a G-3 crucible) and the solvent was evaporated under reduced pressure. Poly(2-vinylpyridine) (Aldrich) and poly(2-vinylpyridine-co-styrene) (Aldrich) were also purified by the same method. p-Chloranil (B.D.H.) was purified by recrystallization from benzene, m.p. 292 °C. 2,3-Dichloro-5,6-dicyano-l,4-benzoquinone (Fluka) was twice crystaUized from dry benzene, m.p. 215 °C. 2,4,7-Trinitro-9-fluorenone (Fluka) was crystallized from dry benzene, m.p. 175 °C. 2,4,5,7-Tetranitro-9-fluorenone (Aldrich) was purified by recrystallization from hot acetic acid (20 ml) containing 1 ml acetic anhydride, m.p. 254 °C. Tetracyanoethylene (Aldrich) was recrystallized twice from dichloromethane and then sublimed, m.p. 198 °C. Resublimed reagent-grade iodine, o-chloranil (Sigma), m.p. 128 °C and 7,7'8,8'-tetracyanoquinodimethane (Aldrich), m.p. 296 °C were used. The EPR spectra of the complexes were obtained on a Varian E-109 spectrometer operating in the X-band. The sample temperature was varied from 100 °C down to liquid nitrogen temperatures. The g values, linewidths and spin concentration were estimated using charred dextrose as a standard. The charge-transfer complexes for the EPR study were prepared by refltLxing the solutions of the constituents (polymer and low molecular weight acceptor in the ratio 20:1) for 1 h in chloroform. The solution was cooled to room temperature and the solvent was evaporated under vacuum. Absorption spectra were obtained in chloroform at 25 °C using an Hitachi U-3400 spectrophotometer. The electronic spectra of the charge-transfer complexes were recorded in the wavelength range 2 0 0 - 1 2 0 0 nm using a pair of matched 3 ml-stoppered silica cells of 10-mm path length.
55 Results and discussion Generally the same EPR and electronic spectral features were observed between all the polymer donors for a c o m m o n acceptor. Initial studies on some of the samples indicated that the EPR absorption was unaffected by atmospheric oxygen. However, as a precautionary measure, all the solid samples were studied i n v a c u o . T h e g values and linewidths of the EPR spectra obtained for the various charge-transfer complexes of the pol ym er donors and low molecular weight acceptors are r e p o r t e d in Table 1. The EPR spectra of dope d 4PV P- T E NF and 4PVP-pCHL systems are shown in Figs. 1 and 2, respectively. Most of the systems reveal only one EPR signal as shown in Fig. 1 while some of the systems give two signals as in Fig. 2. For a representative 4PVP-pCHL system, the values are gl = 2.0054 and g 2 = 2 . 0 0 1 0 (with an uncertainty of ___0.00015 in the g values) with a total linewidth of 8.4 G. The shape of the EPR signal is the same for the complex of an a c c e p t o r with different polymer donors. The EPR spectra of 4PVP-TENF complex at different temperatures are shown in Fig. 1. The signal intensity (spin concentration) increases with t e m p e r a t u r e and this is presented in Table 2. This type of variation has been observed for most of the systems investigated. There is variation in the EPR signal intensity with t e m p e r a t u r e resulting from the changes in the population of the magnetic levels [ 1 7 - 1 9 ] . For a singlet-triplet system, the intensity increases with temperature, because of the population of the triplet level. For a system in the ground state consisting of an unpaired electron on the donor and an unpaired electron on the acceptor, and each acting independently of each other as doublet states, the intensity of the spin resonance signal would be inversely proportional to the temperature. For most of the systems presently investigated, EPR signal intensity was found to increase with temperature. This feature is typical of singlet-triplet systems separated by an activation energy and it is clearly reflected by Fig. 1 and data given in Table 2. The possibility that the system might be S = 1 (two unpaired electrons) was considered and a search for the forbidden ~Ms = _ 2 transitions was made. However no lines arising from AMs= _+2 transitions were found. Such a result is not conclusive because usually the signal intensity is very weak. EPR studies indicate that all the systems exhibit an exponential temperature d e p e n d e n c e of spin concentration characteristic of semiconductors, strongly suggesting the possibility that the resonance observed is due to a single unpaired electron either on the a c c e p t o r or on the donor. Apparently the two peaks observed are not due to two electron resonances associated with the donor and a c c e p t o r ions, but attributable rather to an anisotropic g tensor. Hence averaged g values are r e p o r t e d in Table 1 for the polymeric charge-transfer complexes where the acceptors are pCHL, oCHL and DDQ. The remaining systems exhibit a single EPR absorption. The charge-transfer complexes of h y d r o c a r b o n - h a l o g e n s [19] also show an exponential d e p e n d e n c e on t e m p e r a t u r e in agreem ent with the observed activation energy for electronic conduction. In the present
2.0001 2.0045 2.0057 2.0030 2.0067 2.0095 2.0032 2.0035 2.0086 2.0019 2.0044 2.0062 2.0036 2.0033 2.0081 2.0017 2.0037 2.0058
4PVP-TCNQ 2PVP-TCNQ (2PVP-co-St)-TCNQ 4PVP-DDQ 2PVP-DDQ (2PVP-co-St)-DDQ 4PVP-pCHL 2PVP-pCHL (2PVP-co-St)-oCHL 4PVP-TENF 2PVP-TENF (2PVP-co-St)-TENF 4PVP--oCHL 2PVP--oCHL (2PVP-co-St)-oCHL 4PVP-TNF 2PVP-TNF (2PVP-co-St)--TNF
"The activation energy could not be calculated.
g
Polymer complexes
6.2 8.8 4.6 5.2 4.8 5.4 8.4 8.2 8.8 3.8 3.8 4.0 7.4 7.3 7.6 4.4 4.3 3.7
AH.,. (G)
361.4 1472.0 223.3 86.0 457.1 26.7 66.4 137.7 33.8 46.1 51.9 28.6 5.4 8.3 2.1 1.2 1.9 0.5
× 10 m
Spins &r'
0.004 0.015 0.007 0.005 0.002 0.007 0.020 0.012 0.014 0.005 0.009 0.010 0.012 0.016 0.010 0.011 0.012 0.022
temperature
Low
E. (eV)
The g values, linewidths, spin concentration at 298 K and activation energies fl)r tile polymeric charge-transfer c o m p l e x e s
TABLE 1
0.031 0.029 0.048 0.078 0.055 0.106 " 0.061 " 0.039 0.106 0.062 0.072 0.155 • 0.254 " "
High temperature
¢91
57
143 K
173 K 203 K 233K
263 K
313K 323 K 333 K 343K 353K
I
I
I
I
32405 MAGNETIC FIELD (H)
4G
3240 G MAGNETIC FIELD (H)
)
)
Fig. 1. Variation of EPR spectra of the 4PVP-TENF system with temperature. (Receiver gain: 4 × 10 z and 5 × 102 for the range 143-263 K and 313-353 K, respectively.) Fig. 2. EPR spectrum of the 4PVP-pCHL complex at room temperature. TABLE 2 Spin concentration with temperature for the 4PVP-TENF system Spins g-1
Log (spins g-l)
T (K)
17.7671 17.7481 17.7301 17.7103 17.6972 17.1073 17.0802 17.0702 17.0496 17.0203
353 343 333 323 313 263 233 203 173 143
X 1017
5.85 5.60 5.37 5.13 4.98 1.28 1.20 1.17 1.12 1.05
study no hyperfine splitting was observed. It is therefore not possible from the present results to infer whether the electron spin is associated with the acceptor or the donor. The spin concentration of the polymeric charge-transfer complexes is highly dependent upon the nature of the donor and acceptor moieties. The spin concentration for a given donor is dependent on the nature of the acceptors (Table 1). The spin concentration of the polymeric charge-transfer complexes generally varies as TCNQ > DDQ > p C H L > TENF > oCHL > TNF.
58 Amongst the three donors the spin concentration changes as 2PVP> 4 P VP > (2PVP-co-St). The positive charge on the nitrogen of the polymer can be better stabilized when the - - C H - - C H 2 - - group is in the para position than when it is in the ortho position due to stereoelectronic effects. Hence in 4PVP the lone pair of electrons on the nitrogen is more easily available than in 2PVP. Consequently it is inferred that an ionic c o m p o u n d is formed in the case of charge-transfer complexes of 4PVP, whereas in the case of charge-transfer complexes of 2PVP and the copolymer it takes a longer time to form the ionic compound. For 4PVP complexes, spin pairing possibly takes place which leads to the observed decrease in spin concentration. The spin concentration of the 2PVP complexes are hence greater than 4PVP complexes. The presence of styrene with 2-vinylpyridine in the copolymer decreases the spin concentration relative to that of the pure poly(2-vinylpyridine), since the acting donor group is r e d u c e d in the copolymer. Most of the charge-transfer complexes exhibit an exponential t e m p e r a t u r e d ep en d en ce of spin concentration characteristic of semiconductors [18, 19]. This t e m p e r a t u r e dependence may be e x pressed in terms of EPR spin concentration as
N=No expC-Ea/RT)
(1)
where N and No are the num ber of spins per gram of the sample and reference, respectively, and Ea is the observed activation energy for spin concentration evaluated from a p l o t of log N versus lIT (Fig. 3). The activation energy for the polymeric charge-transfer complexes is re po r ted in Table 1. For some systems it was not possible to calculate the activation energy because of the small variation of spin concentration with temperature. However there is a definite increase in spin concentration with temperature. The activation energy for the 4PV P- T E N F system was found to be 0.039 eV at higher temperatures and 0.005 eV at lower temperatures. The activation
17.8
c
•~.
~ab~a = O.039 eV
17.4
~.~__o~.
Eo =
0.005 eV
17.0 i
I
i
0.004
I
0.006
i
0.008
(TIK~ "I
Fig. 3. Variation of EPR spin concentration with temperature for the 4PVP-TENF system.
59 energy varies from 0.02 to 0.26 eV at high t em perat ure and from 0.001 to 0.02 eV at low temperatures. Two factors contribute to the spin concentration in the polymeric charge-transfer complexes: (i) a thermally activated species at high temperatures; and (ii) a constant num ber of spins at low temperatures. The activation energies will therefore be different at lower and higher temperatures. Consequently eqn. (1) is not applicable in the whole t em perat ure range and a break in the intermediate t em p erat ure region is found. A similar behaviour has be e n found for aromatic hydrocarbons with iodine by Singer and Kommandeur [20]. Solutions containing the interacting donor (D) and a c c e p t o r (A) species show not only the absorption bands of D and A, but also most often exhibit a new band or bands which are assignable to charge-transfer transition(s) of the complex as a whole. Furthermore, the electronic spectra also show another general feature of charge-transfer interaction -- the possibility of complete electron transfer from the donor to the a c c e p t o r with the formation of radicals (intramolecular excitation bands of ionic acceptors). The chargetransfer complex may further undergo an irreversible chemical reaction to give a final product. The absorption maxima of the donors, the acceptors and their charge-transfer complexes are given in Table 3. In most of the systems colour develops immediately when the polymer donor and low molecular weight acceptors (such as TCNQ, TCNE, DDQ and I2) are mixed in chloroform and the optical density of this solution increases with time and attains a m a xi m um ( e xc e pt when TCNE is the acceptor). This colour is ascribed to the formation of the charge-transfer complexes between the reactants. In the case of pCHL and oCHL as accept ors no new bands were observed. However, intensity of the a c c e ptor band gradually increases with time indicating the formation of the charge-transfer complexes. Finally these bands disappear and the absorption m axi m um shifts to the UV region or, in some cases, a new absorption maximum arises. From this, it is inferred that the initially f or m e d charge-transfer complexes transform into a final product. New bands have not be e n observed in the polymeric charge-transfer complexes where the acceptors are TNF and TENF. Equimolecular quantities of 4PVP and pCHL dissolved in chloroform show a narrow single EPR line. The same p h e n o m e n o n was observed for the complexes of all three polymers with acceptors, oCHL, pCHL, TCNQ and DDQ. The g values and linewidths of EPR absorptions are given in Table 4. The EPR spectra were measured immediately after the chloroform solutions of the reactants were mixed. The signal intensity increases rapidly, attains a maximum and then gradually decreases. The paramagnetism of the solution finally vanishes. A very weak EPR signal was observed in the polymeric charge-transfer complexes where TENF is an a c c e p t o r and no EPR signal was observed in the case of polymeric charge-transfer complexes where TNF, TCNE and Iz are the acceptors. The same p h e n o m e n o n was observed in the solid state. However, the reaction in the solid state is extremely slow relative to that in solution.
60 TABLE 3 Absorption maxima of the donors, acceptors and their charge-transfer complexes in chloroform Compound
~
(nm)
4PVP 2PVP (2PVP-co-St) I~ 4PVP-I.)a 2PVP-I.,a (2PVP-co-St)-Ie a TCNQa 4PVP-TCNQa 2PVP-TCNQa (2PVP-co-St)-TCNQa DDQ~ 4PVP-DDQ ~ 2PVP-DDQ a (2PVP-co-St)-DDQ ~ TCNE 4PVP-TCNE a 2PVP-TCNE a (2PVP-co-St)-TCNE a pCHL oCHL TENF TNF
242.0 257.5 239.5 503.0 366.0 365.5 367.0 402.0 684.0 684.0 685.0 387.5 730.0 b 483.5 484.0 267.0 399.0 399.5 399.5 287.0 250.0 285.5 281.5
256.0 263.0 258.0
263.0 269.5 263.0
269.0
750.5 751.0 750.5
768.0 768.5 768.0
851.5 851.5 852.5
725.0 b 727.0 b 277.0 420.0 419.0 420.0 362.0 288.0 323.5 320.0
445.0 345.0 340.0
~I'he spectra were recorded in the range 350-1200 nm. bBroad band. The i n i t i a l l y f o r m e d c h a r g e - t r a n s f e r c o m p l e x e s t r a n s f o r m finally t o a d i a m a g n e t i c p r o d u c t w h e r e t h e a c c e p t o r s a r e TCNQ, TCNE, DDQ, p C H L , oCHL a n d I2. T h e c h a r g e - t r a n s f e r c o m p l e x f o r m a t i o n o c c u r s w i t h t h e f o r m a t i o n o f f r e e r a d i c a l s w h e r e TCNQ, DDQ, p C H L a n d oCHL a r e t h e a c c e p t o r s . W i t h TCNE a n d I2 t h e c h a r g e - t r a n s f e r c o m p l e x f o r m a t i o n o c c u r s w i t h o u t t h e f o r m a t i o n o f f r e e r a d i c a l s . C h a r g e - t r a n s f e r c o m p l e x f o r m a t i o n is w e a k w h e r e t h e a c c e p t o r s a r e T E N F a n d TNF. The g values and linewidth are t e m p e r a t u r e independent. The a b s e n c e o f h y p e r f i n e s p l i t t i n g in t h e c o m p l e x e s c o u l d b e c a u s e d b y s e v e r a l p o s s i b l e e x c h a n g e p r o c e s s e s , e.g., i n t r a m o l e c u l a r e x c h a n g e b e t w e e n t h e u n p a i r e d electrons, exchange between independently wandering holes and electrons a n d i n t e r m o l e c u l a r e x c h a n g e b e t w e e n t h e u n p a i r e d e l e c t r o n s in o n e c o m p l e x molecule.
Conclusions T h e s h a p e o f t h e E P R s i g n a l is s a m e f o r t h e c o m p l e x o f a n a c c e p t o r with different p o l y m e r donors. The g values and linewidths are i n d e p e n d e n t
61 TABLE 4 The g values and linewidths for the polymeric charge-transfer complexes in chloroform Polymer complexes
g
z~r-/tpp(G)
4PVP-TCNQ 2PVP-TCNQ (2PVP-co-St)-TCNQ 4PVP-DDQ 2PVP-DDQ (2PVP-co-St)-DDQ 4PVP--~pCHL 2PVP-pCHL
2.0024 2.0080 2.0042 2.0053 2.0029 2.0068 2.0045 2.0039 2.0066 2.0054 2.0028 2.0049 2.0063 2.0034 2.0057
5.7 4.6 5.5 9.0 2.1 3.0 7.5 1.2 3.0 2.9 1.6 2.2 3.0 2.0 2.7
(2PVP-co-St)-pCHL
4PVP-TENF 2PVP-TENF (2PVP-co-St)-TENF 4PVP--oCHL 2PVP-oCHL (2PVP-co-St)--oCHL
of t e m p e r a t u r e . The spin c o n c e n t r a t i o n of the c h a r g e - t r a n s f e r c o m p l e x e s varies as TCNQ > DDQ > p C H L > T E N F > oCHL > TNF a n d a m o n g the d o n o r s it varies as 2PVP > 4PVP > (2PVP-co-St). The p a r a m a g n e t i c c h a r g e - t r a n s f e r c o m p l e x e s exhibit an e x p o n e n t i a l t e m p e r a t u r e d e p e n d e n c e o f spin c o n c e n tration characteristic o f s e m i c o n d u c t o r s . The initially f o r m e d c h a r g e - t r a n s f e r c o m p l e x e s t r a n s f o r m finally to a d i a m a g n e t i c p r o d u c t w h e n the a c c e p t o r s are TCNQ, TCNE, DDQ, p C H L , oCHL a n d I2. The c h a r g e - t r a n s f e r c o m p l e x f o r m a t i o n o c c u r s with the f o r m a t i o n of free radicals w h e r e TCNQ, DDQ, p C H L a n d oCHL are the a c c e p t o r s and for the TCNE and I2 s y s t e m s the c h a r g e - t r a n s f e r c o m p l e x f o r m a t i o n o c c u r s w i t h o u t the f o r m a t i o n of free radicals. C h a r g e - t r a n s f e r c o m p l e x f o r m a t i o n is w e a k w h e r e the a c c e p t o r s are T E N F and TNF. The n a t u r e o f the c o m p l e x s e e m s to be d e t e r m i n e d mainly by the a c c e p t o r . However, the r e a c t i o n in the solid state is e x t r e m e l y slow relative to that in solution.
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