J. Phys. Chem. 1991,95, 8983-8989
8983
Effects of Environmental Factors on the Eiectrooptical Properties of Conjugated Polymers Containing Oligo(oxyethy1ene) Substituents Jean Roncali,* Li Hong Shi, and Francis Garnier Laboratoire des Matgriaux Molt?culaires, CNRS ER 241, 2 rue Henry Dunanr, 94320 Thiais. France (Received: April 1 1 , 1991)
The electrochemical and spectroelectrochemical properties of poly[3-(3,6-dioxaheptyl)thiophene](PDHT) films deposited on transparent IT0 electrodes have been analyzed in various electrolytic media (0.1 and 1 M Bu4NCI04and LiClO, in CH3CN and 0.1 and 3.5 M LiCIO, in H20) by cyclic voltammetry (CV), UV-visible absorption spectroscopy, and cyclic voltabsorptometry (CVA). The voltammograms recorded in these various media involve successive wave systems whme characteristicsare strongly dependent on the solvent and on the nature and concentration of the electrolyte cation. Bu,N+ leads to a positive shift of the peak's potential and to a drop of electroactivity, whereas, in contrast, Li+ improves the resolution of the voltammetric waves and produces a negative shift of the peak's potential and an increase of the doping level. PDHT remains highly electroactive in aqueous media in which the increase of the concentration of lithium cations produces also an enhancement of the voltammetric waves and an increase of the doping level. In situ optical spectra of films undoped in these various media show that, in CH3CN, the increase of [Li'] leads to a slight bathochromic shift of the bands' maxima and to an enhancement of the resolution of the vibronic fine structure, whereas the reverse effects are observed with Bu4N+or aqueous media. The CVAs recorded at the absorption maximum of the neutral polymer show successive slope inflexions tightly correlated to the CV waves and providing evidence for distinct oxidation stages and for structural relaxation during the redox processes. Li+ increases the magnitude of the first step and decreases the corresponding potential, whereas Bu4N+has the opposite effect and the CVA recorded in aqueous media involve only one step for both the oxidation and the reduction processes. These results are discussed in terms of feedback effects of the interactions between the oligo(oxyethy1ene) substituents and the chemical environment of the polymer on the processes of charge and mass transport in the polymer and on the geometry and rigidity of the conjugated poly(thiophene) backbone.
Introduction The combination of the electrical, electronic, and electrochemical properties of poly(thiophene) derivatives together with their structural versatility has given rise to intensive research efforts aimed at both the elucidation of their polymerization mechanism, structure, and electronic properties and the development of functional conjugated polymers specifically designed for selective applications.' Over the past few years, the derivatization of the monomer structure has contributed to substantiate the progress in these two research areas. Thus, the covalent grafting of alkyl,2 alkyl sulfonate, or ~arboxyalkyl,~ redox,4 o ~ y a l k y l or , ~ chira16 groups a t the fl position of the thiophene ring has led to the development of new conjugated polymers with unusited properties such as processability,2 self-doping, and water solubility3 and original electrochemical behavior such as ionoselective and enantioselective molecular recognition.Sbs68 In this context, poly(3-alkylthiophenes) (PATS) have attracted considerable interest. From a technological viewpoint, their improved solubility2 and fusibility7 allow their processing by con( I ) Proceedings of ICSM'88. Synth. Met. 1989, 28. (2) (a) Lemaire, M.; Roncali, J.; Gamier, F.; Garreau, R.; Hannecart, E. French Patent 86.04744, April 4, 1986. (b) Sato, M.; Tanaka, S.;Kaeriyama, K.J . Chem. Soc., Chem. Commun. 1986,873. (c) Jen, K. Y.; Miller, G. G.; Elsenbaumer, R. L. J . Chem. SOC.,Chem. Commun. 1986, 1346. (3) (a) Patil, A. 0.; Ikenoue, Y.; Wudl, F.; Heeger, A. J. J . Am. Chem. Soc. 1987, 109, 1858. (b) BAuerle, P.; Gaudl, K. U.; Wllrthner, F.;Sariciftci, N. S.;Neugebauer, H.; Mehring, M.; Chuanjian, Z.; Doblhofer, K. Adu. Mater. 1990, 2, 490. (4) (a) Mirrazaei, R.; Parker, D.; Munro, H. S . Synth. Met. 1989,30,265. (b) BAuerle, P.; Gaudl, K.W. Adu. M u m . 1990, 2, 185. (c) Bryce, M. R.; Chissel, A. D.; Gopal, J.; Kathirgamanathan, P.; Parker, D. Synth. Met. 1991, 39, 397. (5) (a) Bryce, M. R.;Chissel, A.; Kathirgamanathan, P.; Parker, D.; Smith, N. M. R. J . Chem. Soc., Chem. Commun. 1987,466. (b) Roncali, J.; Garreau, R.; Delabouglise, D.; Garnier, F.; Lemaire, M. J. Chem. Soc., Chem. Commun. 1989, 679. (c) Feldhues, M.; Kgmpf, G.; Litterer. T.; Mecklenburg, T.; Wegener, P. Synth. Met. 1989,28, C487. (d) Roncali, J.; Garreau, R.; Lemaire, M. J . Electroanal. Chem. 1990,278, 373. (e) Daoust, G.; Leclerc, M. Macromolecules 1991, 24, 455. ( 6 ) (a) Lemaire, M.; Delabouglise, D.;Garreau, R.; Guy, A.; Roncali, J. J. Chem. Soc., Chem. Commun. 1988,658. (b) Kotkar, K.; Joshi, V.;Gosh, K. J. Chem. SOC.,Chem. Commun. 1988, 917. (7) Yoshino, K.;Nakajima, S.; Oncda, M.; Sugimoto, R. Synth. Met. 1989, 28, C349.
ventional polymer techniques, while a t a fundamental level, the large modifications of the electrochemical and optical properties of the conjugated ?r system induced by the grafting of long alkyl chains on the poly(thiophene) backbone have been the subject of much theoretical and experimental work. As a matter of fact, although the electronic inductive effect of alkyl substituents is roughly independent of their size, the lengthening of the alkyl chain has been shown to produce simultaneously an improvement of the electrochemical reversibility, a bathochromic shift of the absorption maximum, and the appearance of a fine structure (FS) in the electronic absorption spectrum.8 Furthermore, PATS have been shown to undergo thermochromism?I0 solvatochromism:JO and piezochromism.' These phenomena have been the subject of several studies that have led to the conclusion that, besides their direct electronic and steric effects on the conjugated 7r system, the alkyl chains indirectly affect the electronic and electrochemical properties of the conjugated backbone through side chain interI action~.~-' Further evidence for such indirect substituent effects has been recently obtained on poly(thiophenes) derivatized by benzyl ethyl ethers on which it has been shown that the substitution of the phenyl ring controls the optical and electrochemical properties of the poly(thiophene) backbone without affecting its mean conjugation length.I2 Besides these "homogeneous internal effects" related to the interactions between substituents grafted on adjacent monomer units or on neighboring polymer chains, the structure and electronic and electrochemical properties of the conjugated backbone can be also indirectly affected by "heterogeneous external effects" related to the interactions between the side substituents and the physical and/or chemical environment of the polymer. In this (8) Roncali, J.; Yassar, A.; Marque, P.; Garreau, R.;Garnier, F.; Lemaire, M. J . Phvs. Chem. 1987. 91. 6706. (9) Rughooputh, S.D.'D. V.; Hotta, S.;Heeger, A. J.; Wudl, F. J . Polym. Sci. 1987, 25, 1071.
(IO) Inganas, 0.;Salaneck, W. R.; 6sterholm, J. E.; Laakso, J. Synth. Mer. 1988, 22, 395.
( I 1) Themans, B.; Salaneck, W. R.; Braas, J. L. Synth. Met. 1989, 28, c359. (12) Roncali, J.; Korri Youssoufi, H.; Garreau, R.; Garnier, F.; Lemaire, M. J. Chem. SOC.,Chem. Commun.1990. 414.
0022-3654191 r ,12095-8983%02.50/0 . - - ., . 0 1991 American Chemical Society
8984 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991
Roncali et al.
context, we have recently reported preliminary characterizations of a novel poly(thiophene) derivative poly[3-( 3,6-dioxaheptyl)thiophene] (PDHT).
PDHT Although the ether groups are electronically decoupled from the conjugated backbone by the -(CH2CH2)- spacer, the presence of the embryonic poly(ethy1ene oxide) chain at the 0 position induces large modifications of the electrochemical and optical properties of the poly(thiophene) backbone such as (i) the occurrence of two distinct redox systems in the CV, (ii) a specific electrochemical response in the presence of lithium cations,5b(iii) a significant increase of hydr~philicity,'~ and (iv) unusual spectral features involving a 30-nm bathochromic shift of the absorption maximum compared to the alkyl analogue poly(3-heptylthiophene) and the occurrence, already at room temperature, of a well-resolved FS.14,'5 These original electrochemical and optical properties make PDHT an attractive model compound for further analyses of the feedback effects of the interactions between the side substituents and the chemical environment, on the electronic and electrochemical properties of the conjugated backbone. In this paper, the electrochemical and spectroelectrochemical properties of PDHT films, electrodeposited on optically transparent electrodes, have been analyzed by cyclic voltammetry, in situ UV-visible absorption spectroscopy, and linear sweep cyclic voltabsorptometry in the presence of organic and inorganic cations in both organic and aqueous media. It is shown that the electrochemical properties of the poly(thiophene) backbone in PDHT are strongly dependent on the composition of the electrolytic medium, Le., the solvent, the nature, and the concentration of the electrolyte cation, and that these environmental factors also control, to a large extent, the optical and electrooptical properties of the polymer. These results are discussed in terms of feedback effects of the interactions between the oligo(oxyethy1ene) substituents and the chemical environment of the polymer on the processes of charge and mass transport in the polymer and on the conformation and rigidity of the conjugated backbone.
Experimental Section The monomer was synthesized by reaction of 3-thienylethanol with chlorodioxyheptane according to the already described method.I6 Solvents and electrolytes were purified as previously reported." Electropolymerizations were carried out in galvanostatic conditions (6 mA cm-2) at ambient temperature in a single-compartment cell containing 0.1 M monomer and 0.02 M Bu4NPF6in nitrobenzene. The solution was degassed by 15 nm of argon bubbling prior to electrodeposition, which was performed under an argon atmosphere. Films were grown on indium-tin oxide (ITO) coated glass electrodes by using a deposition charge of 60 mC cm-2. An I T 0 plate was used as the counter electrode. After deposition, the films were electrochemically undoped at -0.2 V/SCE until the residual cathodic current reached a constant value, rinsed with acetone, dried with an argon flow, and placed in a 1- X 1 -cm quartz cuvette. The cell, filled with the appropriate electrolytic medium, was equipped with a platinum ring counter electrode, a silver wire pseudoreference electrode the potential of which is very close to that of the Ag/AgCl electrode,I8and tubes for argon circulation. For UV-visible absorption spectroscopy, (13) Roncali, J.; Li, H.S.;Garrcau, R.; Garnier, F.;Lemaire, M.Synrh. Met. 1990, 36, 267. (14) Roncali, J.; Marque, P.;Garreau, R.; Garnier, F.; Lemaire, M. Macromolecules 1990, 23, 1347. (IS) Li, H.S.;Garnier, F.; Roncali, J. Solid Store Commun. 1991, 77,81 I. (16) Lemaire, M.; Garreau, R.; Roncali, J.; Delabouglise, D.;Korri Youssoufi.H.;Garnier, F. New J . Chem. 1W9,13, 863. (17) Roncali, J.; Garnier, F. New J . Chem. 1986, 10, 237. (18) Genies, E. M.;Lapkowski, M.J . Elecfroanal.Chem. 1987,236, 189.
e....
..-*. .
'.....'.'.
~
t
I
0.2 0.4 0.6 0.8
1 E V/Ag Figure 1. Cyclic voltammograms of PDHT films on ITO. Deposition charge 60 mC cm-*, scan rate IO mV/s. Dotted line: 0.1 M Bu4NCI04 in CH3CN. Full line: 1.0 M Bu4NC104in CH,CN. 0
r
l
0
0.2 0.4 0.6 0.8
l
l
l
l
l
l
l
l
~
1
E V/Ag Figure 2. Cyclic voltammograms of PDHT films on ITO. Deposition charge 60 mC cm-*, scan rate 10 mV/s. Dotted line: 0.1 M LiCI04 in CH3CN. Full line: 1.0 M LiCI04 in CH3CN.
r
l
l
l
l
l
l
l
W
0 0.2 0.4 0.6 0.8
l
l
.
1 E VIAB Figure 3. Cyclic voltammograms of PDHT films on ITO. Deposition charge 60 mC cm-*, scan rate 10 mV/s. Dotted line: 0.1 M LiClO, in H20.Full line: 3.5 M LiCIOI in H20. the cell was inserted in a Cary 3032 spectrometer by means of micropositioners. Cyclic voltammetry (CV) and linear sweep cyclic voltabsorptometry (CVA) were performed in the same spectroelectrochemical cell placed in an optical system involving
I
The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8985
Electrooptical Properties of Conjugated Polymers
TABLE I: Eketroehemicrl Data for PDHT Films (60 mC cm-' on IT0 (S = 2 cm')) in Various Electrolytic Media' Epal, Epa2, Epa3, Epcl, Epc2, Epc3, Ipal, Ipa2, Ipa3, Ipcl, V V V V V V PA PA rA PA Li+/CH,CN 0.10 M 0.24' 0.49 0.84 0.13' 0.32 0.70 65 164 213 45 BuiN+/CHICN Li+/HIO
1.00 M 0.10 M 1.00 M 0.10 M 3.50 M
0.34' 0.48' 0.23b
0.30 0.56 0.71 0.43' 0.33
0.70 0.95
0.33' 0.42'
0.67' 0.72
0.14 0.52 0.60 0.16' 0.21'
0.60 0.78
0.53 0.54
80 13 120
223 189 187 150 275
214 256
243 219
70 60
Ipc2, PA 168 147 205 175 113 158
lpc3, PA 205 210 261 153 179 220
y,% 21.6 23.2 17.7 10.4 14.8 18.4
'All potentials refer to the Ag wire pseudoreference electrode. *Shoulder.
a 100-W tungsten halogen lamp, a Durrum grazing monochromator, and an 1P28 RCA photomultiplier tube. The potential was driven by a PAR 173 potentiostat galvanostat equipped with a PAR 175 universal waveform programmer and a PAR 179 plug-in digital coulometer. Electrical and optical responses to linear potential sweeps were recorded simultaneously by means of two Sefram TGM 164 recorders. Results and Mscussion Cyclic Voltammetry. Figures 1-3 show the CV traces recorded
a t 10 mV/s in CH3CN containing 0.1 and 1 M Bu4NC104and 0.1 and 1 M LiCIO, and in H20containing 0.1 and 3.5 M LiCIO,. A first comparison of these various CVs clearly shows that the electrochemical behavior of PDHT is strongly dependent on the composition of the electrolytic medium. The CV recorded in the presence of 0.1 M Bu,N+ in CH3CN (Figure 1) involves three successive anodic waves, the first shoulder (Ipal) at 0.34 V (Epal) followed by two waves (Ipa2 and Ipa3) at respectively 0.56 (Epa2) and 0.95 V (Epa3). The cathodic branch also exhibits three reduction waves, Ipc3, Ipc2, and Ipcl, at respectively 0.78 (Epc3), 0.52 (Epc2), and 0.33 V (Epcl). Increasing the electrolyte concentration to 1 M produces a shift of the first two pairs of voltammetric waves toward positive potentials, while the third pair of waves is no longer apparent in the analyzed potential range. Furthermore, the intensity of the current peaks decreases, whereas the overall electroactivity (expressed by the doping level (y) calculated from the amount of charge reversibly exchanged upon redox cycling (Q,) and the deposition charge (Qd) using y = 2Q,/Qd - Qr), decreases from 17.7 to 10.4% (Table I). The comparison of the CV recorded in the presence of 0.1 M Li+ in CH3CN (Figure 2) with Figure 1 shows that replacing Bu4N+ by Li+ leads to a negative shift of Epal, Epa2, and Epa3 to respectively 0.24,0.49, and 0.84 V, while Epcl, Epc2, and Epc3 shift to respectively 0.1 3, 0.32, and 0.70 V. Concurrently, the doping level increases from 17.7 to 21.6%. Contrary to the case of Bu4N+, the increase of the concentration of lithium cations (Figure 2) produces a marked improvement of the resolution of the voltammetric response with a sharpening of the current waves and a further negative shift of Epa2 and Epa3 to respectively 0.30 and 0.70 V, while the first pair of shoulders, Ipal and Ipcl, disappear and y increases from 21.6 to 23.2%. These results clearly show that the electrochemical behavior of PDHT is strongly dependent on the chemical nature of the electrolyte cation, Bu4N+, having deleterious effects on both the definition of the voltammetric response and the overall electroactivity, whereas Li+ leads to an improvement of these parameters. In a previous work, we have shown that, whereas the alkyl analogue of PDHT, poly(3-heptylthiophene) is quite electroinactive in aqueous media due to the presence of the hydrophobic alkyl substituent, the introduction of ether groups in the alkyl chain produces a considerable increase of hydrophilicity, allowing PDHT to retain a high level of electroactivity in aqueous media." However, whereas PDHT is stable up to N 1.2 V/SCE in CH3CN, in HzO, potentials positive of ca.0.9 V/SCE lead to decomposition. Similar differences of stability between organic and aqueous media have been previously observed on poly( 3-methylthi0phene).'~.~~ The CV recorded between -0.1 and 0.8 V in 0.1 M LiC1O4/Hz0 (19) Thackeray. J . W.; White, H. S.; Wrighton, M. S.J. Am. Chem. Soc. 1985, 89, 5133.
(20) Roncali, J.; Garnier, F. J . Chem. Soc., Chem. Commun. 1986,783.
(Figure 3) shows that replacing CH3CN by H 2 0 produces a decrease of the resolution of the CV. Thus, Ipal, Ipa2, and Ipa3 appear only as weak shoulders hardly perceptible, whereas Ipcl and Ipc2 occur as a broad cathodic wave extending from 0.4 to -0.1 V. Taking into account the lower anodic limit of the applied potential, the value of y (14.8%) confirms the high electroactivity of PDHT in aqueous media. The CV recorded in the presence of 3.5 M LiClO, (Figure 3) shows that, as in CH3CN, the increase of the concentration of lithium cations leads also to an improvement of the definition of the CV with the disappearance of Ipal, a strong enhancement of the intensity of 4x2, which becomes predominant, and a significant increase of y from 14.8 to 18.4%. Despite the lesser definition of the CV recorded in aqueous media, these results confirm the positive effects of lithium cations on the electrochemical behavior of PDHT. The interpretation of these results is not straightforward, and various aspects of the interactions between the polymer and the cation must be considered. It is well acknowledged that the electrochemical p doping of conducting polymers proceeds by the extraction of electrons from the conjugated r system together with the ingress of electrolyte anions in the polymer to ensure electroneutrality. The undoping can proceed then via two different pathways involving either anion expulsion and/or cation incorporation, depending on the relative mobility of both species in the polymer matrix.21g22 Thus, whereas in the case of small anions such as ClO,, PFs-, or BF, associated with hydrophobic tetraalkylammonium cations the redox process involves essentially anion transport, the doping/undoping of polymers in which the anionic dopant is physically23or chemically3 immobilized occurs through cation transport. Furthermore, lithium cation transport has been shown to contribute to the redox process of poly(pyrr01e)~~ and poly(3-methylthi0phene).~lJ~ In this latter case, it was shown that Li+ leads to faster and more efficient doping than Bu4N+, this effect being attributed to the smaller ionic radius of Li+. However, the specificity and the considerably larger magnitude of the effects of Li+ on the electrochemical behavior of PDHT suggests that, in addition to the size of the cation, other factors must be also taken into account. Since it is evident that the unusual electrochemical properties of PDHT are related to the presence of the oligo(oxyethy1ene) substituent at the 0 position, this specific behavior must originate from the interactions between the ether groups and the chemical environment of the polymer. On the one hand, it seems likely that hydrophilic lithium cations are more mobile in the hydrophilic environment constituted by the PDHT matrix than hydrophobic tetrabutylammonium cations. On the other hand, the spacing of two ether functions in the side chain by two -(CH2)- groups represents the optimal geometry for efficient complexation and transport of alkali cations,2s as in the case of glymes or poly(ethy1ene o ~ i d e ) . ~ ~The . ~ ' cation-de(21) Chao, F.; Baudoin, J. L.; Costa, M.; Lang, P. Makromol. Chem. Makromol. Symp. 1987, 8, 173. (22) Marque, P.; Roncali, J.; Garnier, F. J . Elecrroanal. Chem. 1987,218, 107. (23) Shimitzu, T.; Ohtani, A,; Iyoda, T.; Honda, K. J . Chem. Soc., Chem. Commun. 1987, 327. (24) (a) Genies, E. M.; Pernaut, J. M. Synrh. Mer. 1984/85,10, 117. (b) Kaufman. J. H.;Kanazawa. K. K.:Street. G. B. Phvs. Rev. Leu. 1984.53, 2461. (25) Shatenstein, A. I.; Petrov, E. S.;Yakovleva, E. A. J . Polym. Sci. 1%7, 16, 1729. (26) Fenton, F. D. E.; Parker, J. M.; Wright, P. W. Polymer 1974, 14, 589.
8986
The Journal of Physical Chemistry, Vol. 95, No. 22, 199
pendent electrochemical properties of linear or cyclic oligo(oxyethylene) compounds coupled to redox active moieties such as nitrobenzene, quinones, or ferrocene have been widely described.28 However, in these cases, the complexation of metal cations by the polypodant results generally in an increase of the oxidation potential of the redox system. Thus, the fact that, in the case of PDHT, the complexation of Li' leads to the opposite effect must be related to the specificity of the redox properties of conjugated polymers. As a matter of fact, the complexation of Li+ by the oligo(oxyethy1ene) side chains can affect the electrochemical behavior of PDHT by means of various mechanisms concerned with both the transport of charges and ionic species and the structure of the polymer. The complexation of Li+ is expected to be maximal in the neutral state; upon doping, the creation of positive charges along the polymer chains leads to the expulsion of Li' by Coulombic repulsion while electrolyte anions are incorporated in the polymer to ensure electroneutrality. However, the expulsion of Li' is only a consequence of the polymer oxidation, which remains the only charge-consuming process detected by CV. Thus, the fact that increasing [Li'] leads to an increase of Ipal and y and to a cathodic shift of Epal must be an indirect consequence of the complexation of Li+. Two kinds of explanations can be proposed to account for these effects. First, the complexation of Li' by the oligo(oxyethy1ene) side chains contributes to localize the counter anion in the vicinity of the conjugated backbone by electrostatic interactions. Since, among other factors, the doping rate is limited by the diffusion rate of the counteranion in the polymer bulk, this localization effect could increase both the rate and the efficiency of the doping process. Second, LiC10, has been shown to undergo significant contact ion pairing in a ~ e t o n i t r i l eand , ~ ~ such ion pairs have been already detected in poly(3-methylthiophene) during the redox process.30 In this regard, the complexation of Li' by the polyether side chains contributes to dissociate these ion pairs,3' thus increasing the mobility of the anion and the doping rate. In addition to these effects, the complexation of Li+ is likely to increase the ionic conductivity of the polymer matrix. Since in the beginning of the electrochemical doping the rate of charge transport in conducting polymers is limited by the insulating character of the neutral polymer, this increased conductivity can contribute to the overall charge propagation process, which has been shown to involve simultaneously both ohmic and redox condu~tivities.~~ Besides these effects on the rate and efficiency of the processes of charge and mass transport in the polymer, the complexation of Li+ can indirectly affect the electrochemical doping/undoping process by modifying the structure and the geometry of the polymer chains. Thus, in the neutral state, the adoption by the polyether side chains of the optimal geometry for Li+ complexation can contribute to enhance the coplanarity of the conjugated backbone, leading thus to an increase of the effective mean conjugation length and hence to a decrease of the oxidation potential. UV-Visible Absorption Spectroscopy. In order to gain more detailed information on the effects of the complexation of Li+ on the structure and properties of the conjugated backbone, the electronic absorption spectra of PDHT films have been recorded in situ after electrochemical undoping in the above-described electrolytic media. Figure 4 shows the spectra of PDHT films after reduction in 1 M Bu4NC104 and LiC104 in CH,CN and 3.5 M LiCIO, in HzO. Although both spectra recorded in CH3CN (27) Armand. G . M.; Chabagno, J.; Duclot, M. 2nd Int. Conf. on Solid Electrolytes, St. Andrew, U.K., 1978. (28) (a) Kaifer, A.; Echegoyen, L.; Gustowski, D.; Goli, D. M.; Gokel, G . W. J . Am. Chem. Soc. 1983, 105, 7168. (b) Wolf, R. E.; Cooper, S. R. J . Am. Chem. SOC.1984, 106,4646. (c) Saji, T. Chem. Lett. 1986, 275. (d) Maruyama, K.; Sohmiya, H.; Tsukube, H. J . Chem. SOC.,Perkin Trans. 1 1986, 2069. (29) Berman, H.A.; Stengle, T. J . Phys. Chem. 1979, 79, 1001. (30) Baudoin, J. L.; Chao, F.; Costa, M. J . Chim. Phys. 1989, 86, 181. (31) (a) Lehn: J. M.; Sauvage, J. P.; Dietrich, B. J . Am. Chem. Soc. 1970, 92,2916. (b) Roitman, J. N.; Cram, D. J. J. Am. Chem. Soc. 1971.93.2231. (c) Zavada, J.; Svoboda, M.; Pankova, M. Tetrahedron Letr. 1972, 711. (32) Huanyu, M.; Pickup, P. G . J . Am. Chem. SOC.1990, 112, 1776.
Roncali et al. 0.d A
933
400
600
700
800 5nm
Figure 4. Electronic absorption spectra of undoped P D H T films (deposition charge 60 m C cm-* on ITO)recorded in situ after a 10 mV/s voltammetric cycle in 1 M Bu4NClO4 in CH3CN, (-) 1 M LiClO, in CH3CN, and (- -) 3.5 M LiCIOl in H20.
-
(e..)
involve a FS with an absorption maximum around 550 nm and two well-resolved shoulders at ca. 520 and 605 nm, the comparison of these spectra shows that replacing Bu4N+by Li' leads to a slight but significant bathochromic shift of the absorption maximum and to an enhancement of the FS resolution. In contrast, cycling PDHT in aqueous medium produces a ca. 40-nm hypsochromic shift of the absorption maximum together with almost complete loss of FS. In a recent short communication, we have shown that these solid-state ionochromic and solvatochromic effects are reversible and that the spectral changes resulting from the complexation of Li+ are still apparent even after the films have been dried under vacuum.15 It has been shown already that, in contrast to unsubstituted poly(thiophene), the electronic absorption spectrum of several substituted polymers exhibits a FS involving spectral features around 510, 550, and 605 nm.5b,cJ'-12J4,15In the case of electrogenerated PATs, the position of the absorption maximum and the resolution of the FS have been shown to depend on the length of the alkyl chain,* on the electrosynthesis conditions, and on the film thickne~s.~"~ Thermochromism and solvatochromism have been observed on PATs solution^,^ and more recently, thermochromism was also reported for solution-cast PAT films.Io The cooling of PAT films or solutions or the addition of a poor solvent such as methanol to these solutions results generally in a strong intensification of the FS with changes in the relative intensities of the three main absorption features. Recently, we have shown that similar effects can be structurally induced by the modification of the interactions among the side substituents.12 The fact that similar absorption features differing only by their relative intensities are observed on poly(thi0phenes) derivatized by substituents of different chemical structure has led to the conclusion that these spectral characteristics are inherent to the conjugated backbone and rendered more or less apparent by side chain interactions. The origin of the FS in the electronic spectra of PATs has been the subject of much debate, and it has been attributed to vibronic couplings involving the C=C stretching mode in the thiophene ring (I450cm-I, 0.18 eV)9 or to rotational defects around single bonds leading to discrete lengths of conjugated segments.I0 In fact, these two interpretations are not mutually exclusive since the resolution of the vibronic FS in the spectra of polyconjugated systems depends generally on the conformation, the extent, and the rigidity of the conjugated system. These effects are well documented in the case of free or rigidified trans-stilbene and biphenyl and also in the case of polyenes for which (33) (a) Roncali, J.; Yassar, A.; Garnier, F. J . Chem. SOC.,Chem. Commun. 1988, 581. (b) Yassar, A.; Roncali, J.; Garnier, F. Macromolecules 1989, 22, 804. (34) Suzuki, H. Bull. Chem. SOC.Jpn. 1960,33, 396. (35) Ogawa, K.; Suzuki, H.; Futakami, M. J . Chem. SOC.,Perkin Trans. 2 1988, 39.
The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8987
Electrooptical Properties of Conjugated Polymers
I,,
, , , , ,\, 0.2 0.4 0.6 0.8
I,,
, , ,
*
, ,
0.2 0.4 0.6 0.8
1
E V/Ag
1
E VIAg
Figure 5. Cyclic voltabsorptometric responses of PDHT films on IT0 (deposition charge 60 mC cm-2, scan rate 10 mV/s) recorded in (a) 0.1 M Bu4NCI04 in CH3CN (A = 543 nm) and (b) 1.0 M Bu4NC104 in CH,CN (A = 543 nm).
the intensity of the vibronic FS has been shown to depend on the number of conjugated double bonds" and on their conformati~n.~~ These examples strongly suggest that, in the case of poly(thiophenes), the intensity of the vibronic FS depends on the mean conjugation length, the conformation, and the rigidity of the conjugated backbone. In this context, the changes induced in the optical spectrum of PDHT by the modification of the composition of the electrolytic medium in which the polymer has been undoped could reflect changes in the conformation and the rigidity of the conjugated backbone due to the interactions between the polymer and its chemical environment. Thus, the red shift of ,A, and the enhancement of the FS observed after reduction in the presence of a high concentration of Li+ suggest that the complexation of Li+ by the oligo(oxyethy1ene) side chains has two consequences: (i) the adoption by these side chains of the optimal geometry for Li+ complexation enhances the coplanarity of the conjugated backbone, which results in a bathochromic shift of the absorption maximum and an enhancement of the vibronic FS,and (ii) the "ionic cr~ss-linking"~~ resulting from the complexation of Li+ increases the rigidity of the polymer framework, which contributes also to enhance the resolution of the vibronic FS. In contrast, the hypsochromic shift of the absorption maximum and the loss of the FS observed after reduction in aqueous media can be attributed to two causes. On the one hand, the complexation of Li+ is expected to be less effective in aqueous media owing to the larger solubility of LiC104 in H 2 0 , which modifies the partage between the polymer and the aqueous phase to the detriment of the former and also to the larger solvation number of Li+ in H20.@ On the other hand, it has been shown that oligo(oxyethy1ene) compounds form specific hydration complexes of approximately 2H20:10CH2CH241 Thus, in the case of PDHT, the formation of a solvent cage involving four water molecules around the diether side chain leads to a large increase of the effective volume of the substituent and hence of the steric crowding that produces distortions in the conjugated backbone and hence an hypsochromic shift of the absorption maximum and a loss of vibronic FS. In short, the tight correlation observed between the effects of the composition of the electrolytic medium on the electrochemical and spectroscopic properties of PDHT films provides strong evidence for a control of the conformation and rigidity of the conjugated backbone by the interactions between the oligo(oxyethylene) substituents and the chemical environment of the polymer. Cyclic Voltabsorptometry. In order to confirm this hypothesis and to determine more precisely what stages of the redox process are more specifically affected by environmental factors, the optical ~
~~~~
_____
(36) Jones, N . J . Am. Chem. SOC.1945,67, 2127. (37) Feichtmayr, F.; Heilbronner, E.; NBrrenbach, A.; Pommer, H.; Schlag, J. Tetrahedron 1969, 25, 5383. (38) Tanaka, M.; Watanabe, A.; Tanaka, J . Bull. Chem. Soc. Jpn. 1980, 53. 3430. (39) Bowden, E. F.; Dautartas, M. F.; Evans, J. F. J. Elecrroanal. Chem. 1987, 219. 91. (40) Reichart, C. Soluenr Effects in Organic Chemistry; Verlag Chemie: Weinheim, New York, 1979. (41) (a) Kjellander, R.; Florin, E. J. Chem. Soc., Faraday Trans. 1981, 77,2053. (b) Matsuura, H.; Fukuhara, K. Bull. Chem. SOC.Jpn. 1986,59, 763.
0.2 0.4 0.6 0.8
1
E V/Ag
0.2 0.4 0.6 0.8
1
E VlAg
Figure 6. Cyclic voltabsorptometric responses of PDHT films on IT0 (deposition charge 60 mC cm-2, scan rate 10 mV/s) recorded in (a) 0.1 M LiCI04 in CH$N (A = 550 nm) and (b) 1.0 M LiCIO, in CH$N (A = 556 nm).
0.2 0.4 0.6 0.8
0.2 0.4 0.6 0.8
E V/Ag E VlAg Figure 7. Cyclic voltabsorptometric responses of PDHT films on I T 0 (deposition charge 60 mC cm-2, scan rate IO mV/s) recorded in (a) 0.1 M LiC104 in H 2 0 (A = 520 nm) and (b) 3.5 M LiCIO, in H20(A = 520 nm).
changes associated with the electrochemical doping/undoping process have been recorded simultaneously to the CVs of Figures 1-3. Furthermore, in an attempt to make precise the origin of the low-energy absorption band, the cyclic voltabsorptometric responses (CVAs) have been recorded also at 605 nm. Figures 5-7 show the CVAs recorded at the absorption maxima of the undoped films. The absorbances have been normalized to the highest initial values for ease of comparison. The responses obtained in CH,CN show several slope inflexions, indicating that the doping process occurs through distinct oxidation stages. Previous works have shown that this is also the case for poly( t h i ~ p h e n e )and ~ ~ .that, ~ ~ in the case of PATS, the differentiation between the successive oxidation stages increases with the length of the alkyl ~ h a i n . 4The ~ examination of these CVAs against the corresponding CV (Figures 1 and 2) shows that, in each case, the maxima of the slope of the tangent to the CVAs are correlated to Epal and Epa2. This correlation suggests a maximum optical efficiency of the injected positive charges at the potentials corresponding to the current peaks in the CV. However, this correlation is no longer observed for Epa3. As a matter of fact, although this third anodic wave appears as the most intense in the CV, the CVA shows that it occurs when ca. 80% of the active sites are already oxidied. This absence of correlation suggests that the last anodic wave in the CV is not of purely Faradaic origin but involves a large part of capacitive charging current. As a matter of fact, both theoretical and experimental works have shown that the capacitive charge represents a large part of the overall amount of charge stored in polyheterocycles during their electrochemical doping.44 Furthermore, this conclusion is consistent with previous derivative cyclic voltabsorptometry experiments performed on PDHT.I4 The comparison of the CVAs recorded in 0.1 M Bu4N+ and Li+ (Figures 5a and 6a) shows that the doping occurs at less positive potential in the presence of Li+, in agreement with the CVs. Furthermore, although moderate hysteresis is observed in both cases, for Bu4N+,the absorbance is larger during the undoping process, suggesting that in this case (42) Zotti, G.; Schiavon, G. Synrh. Mer. 1989, 31, 347. (43) Marque, P.; Roncali, J. J . Phys. Chem. 1990, 94, 8614. (44) (a) Bull, A. R.; Fan, F. R.; Bard, A. J. J. Elecrrochem. Soc. 1982, 129, 1013. (b) Feldberg, S. W. J. Am. Chem. SOC.1984, 106, 4671. (c) Marque, P. Thesis, Paris, 1988. (d) Servagent,S.; Vieil, E. Synrh. Mer. 1989, 31, 127. (e) Tanguy, J.; Slama, M.; Hoclet, M.; Baudoin, J. L. Synrh. Mer. 1989, 28, C145.
8988 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991
Roncali et al. L o o t
LOOF--\
0.2 0.4 0.6 0.8
1
0.2 0.4 0.6 0.8
I
Figure 8. Cyclic voltabsorptometric responses of PDHT films on IT0 Scan rate 10 mV/s) recorded at 605 nm (deposition charge 60 mC in (a) 0.1 M Bu4NCI0, in CHJCN and (b) 1.0 M Bu,NCI04 in CH3CN.
0.2 0.4 0.6 0.8
1
E VIAg
E VIA8
E VIA8
0.2 0.4 0.6 0.8
1
E VIA8
E VIAg Figure 9. Cyclic voltabsorptometric responses of PDHT films on IT0 (deposition charge 60 mC Scan rate IO mV/s) recorded at 605 nm in (a) 0.1 M LiCIO, in CH$N and (b) 1.0 M LiCIO, in CH3CN.
the reduction is faster than the oxidation. The CVAs obtained in 1 M electrolyte (Figures 5b and 6b) confirm, in agreement with the CVs, that the doping requires the application of more positive potentials when [Bu4N+] is increased. In contrast, the increase of [Li+] leads to the opposite effect. Furthermore, in this case, the first inflexion point correlated to Ipal at 0.2 V disappears together with Ipal in the CV, while the second one and Ipa2 shift cathodically. Interestingly, during the undoping process, the curve exhibits a net absorbance minimum at 4 . 1 5 V correlated to Ipcl. This phenomenon shows unambiguously that high concentrations of lithium cations lead to larger conformational relaxations of the conjugated backbone. Since the complexation of Li+ is likely to occur after the neutralization of the positive charges on the conjugated system during the reduction process, this result suggests that the complexation of Li+ by the oligo(oxyethy1ene) side chains increases the amplitude of the structural relaxation of the conjugated backbone. As appears in Figure 7, the CVAs obtained in aqueous media are much simpler than the previous ones and involve only a single step for both the doping and the undoping processes. This result could reflect a restriction of the amplitude of structural relaxations due to the solvation of the oligo(oxyethylene) substituents by water molecules. The CVAs recorded at 605 nm (Figures 8-10) show a striking contrast with those obtained at the absorption maximum of the films. As a matter of fact, although the responses obtained in the various electrolytic media differ on several points, all of them involve two distinct potential regions: first a decrease of absorbance followed by an inversion of the slope of the tangent to the curve and by a further decrease of absorbance. Interestingly, in each case, the potential of the slope inversion is correlated to Epal and to the maximum slope of the tangent to the CVA recorded at the absorption maximum. These contrasting behaviors clearly show that the origin of the 605-nm absorption band is different from that of the main absorption band. The comparison of the CVAs obtained with 0.1 and 1 M Bu4N+ (Figure 8) shows that the first step, which corresponds to ca. 75% of the overall absorbance variation in 0.1 M Bu4N+, represents 100%of the absorbance variation in 1 M Bu4N+, with a slight reincrease of absorbance beyond 0.75 V. In contrast, in the case of Li+ (Figure 9), the relative magnitude of the first step is not drastically affected when increasing the concentration of Li+ and corresponds in both cases to ca. 50% of the overall absorbance variation. However, the increase of the concentration of lithium cations strongly enhances the contrast between the two successive steps and the dissymmetry between the doping and the undoping processes. The
E VIA$
Figure 10. Cyclic voltabsorptometricresponses of PDHT films on IT0 (deposition charge 60 mC cm-*, scan rate 10 mV/s) recorded at 605 nm in (a) 0.1 M LiCIO, in H20 and (b) 3.5 M LiC104 in H20.
amplitude of the absorbance variation at 605 nm relative to that a t the absorption maximum decreases from 0.41 for Li+ to 0.28 for Bu4N+, which is consistent with the lower intensity of the 605-nm band in the absorption spectra recorded after undoping in Bu4NC104. As could be expected from the absorption spectra obtained in aqueous media, this relative amplitude of absorbance variation at 605 nm shows a further decrease to 0.11 and 0.14 for the CVAs recorded in respectively 0.1 and 3.5 M LiCIO, in H 2 0 (Figure 10). The comparison of Figure 10 shows that the increase of [Li+] from 0.1 to 3.5 M in H 2 0 produces a large decrease of the relative magnitude of the first step and a positive shift of the potential of slope inversion. This behavior, which contrasts with the effects of the increase of [LP] on the CV, confirms that in contrast to the bleaching of the main absorption band, that of the 605 nm one is not directly related to the injection of positive charges in the conjugated backbone. These two sets of experiments clearly show that the successive slope inflexions in the CVAs recorded at the absorption maximum and at 605 nm are tightly correlated to the voltammetric waves. However, the absorbance at these two wavelengths evolves according to quite different behaviors since the current peaks correspond to maximal s l o p in the CVAs recorded at the absorption maximum and to minimal slopes in those recorded a t 605 nm. These results rule out the attribution of the 605-nm band to discrete lengths of more extensively conjugated segments, which has been proposed for PATs'O and which were adopted also in the preliminary characterizations of the optical properties of PDHT.I4 As a matter of fact, in this case, these longer conjugated segments should oxidize a t less anodic potentials than the main distribution of conjugation lengths corresponding to the absorption maximum, and hence, the bleaching of the 605-nm band should begin at a less positive potential than that of the main absorption band. In a previous work on PDHT, we have shown that Epal corresponds also to the maximum intensity of the 840-nm (1.5-eV) band and thus to the maximum concentration of bipolarons in the conjugated ba~kbone.'~ On the other hand, it has been shown that the transition from the neutral to the bipolaron state is accompanied by an evolution of the polymer structure from an aromatic structure, in which adjacents monomer units can rotate around single bonds, to a rigidified quinoid structure, in which such rotations are h i n d e ~ d . 4 Furthermore, ~ ab initio calculations have shown that this transition is accompanied by modifications of the various bond lengths in the poly(thiophene) backbone.4s On the basis of the hypothesis of a vibronic origin of the 605-nm band, such structural changes can be expected to strongly affect the magnitude of the coupling of the C = C stretching mode with the electronic structure. In this context, the minimal slope in the 605-nm CVA could reflect a minimum in the intensity of the vibronic coupling correlated to the geometrical relaxations that accompany the transition from the neutral state to the bipolaronic doping regime. In the frame of this hypothesis, the enhanced contrast between the two potential regions observed in the 605-nm CVA recorded in the presence of high concentration of lithium cations would result from the enhanced planarity and rigidity of the conjugated backbone in the neutral polymer due to the com~~
(45) Bredas, J. L.;Thdmans, Synth. Met. 1984, 9, 265.
E.: Andrd, J. M.;Chance, R. R.;Silky, R.
J . Phys. Chem. 1991,95, 8989-8995 plexation of Li+ by the oligo(oxyethy1ene) side chains.
Conclusion In summary, these results show that the electrochemical optical and electmoptical properties of PDHT films are strongly dependent on the nature and composition of the chemical environment of the polymer. PDHT exhibits a complex electrochemical behavior involving successive redox systems whose potential and magnitude depend on the composition of the electrolytic medium. Hydrophobic tetraalkylammonium cations have deleterious effects on the electrochemical response of PDHT and lead to an increase of the oxidation potential and a decrease of the doping level. In contrast, lithium cations produce an improvement of the definition of the voltammetric response, a decrease of the oxidation potential, and an increase of the overall electroactivity. These effects of lithium cations are observed also to a lesser extent in aqueous media in which PDHT remains highly electroactive. The electronic absorption spectrum of neutral PDHT shows a marked dependence on the composition of the electrolytic medium in which the film has been undoped, indicating that the effects observed in electrochemistry are at least partly due to a medium-induced control of the conformation of the conjugated backbone. Cyclic
8989
voltabsorptometry provides definitive evidence for conformational changes asswiated with distinct oxidation stages. These effects, which are particularly important in the presence of high concentrations of lithium cations, suggest that the complexation of lithium cations by the embryonic polyether side chains indirectly affects both the conformation and the rigidity of the conjugated poly(thiophene) backbone. Finally, the cyclic voltabsorptometric responses recorded at the maximum of the low-energy absorption band a t 605 nm bring further support to the occurrence of electrochemically induced conformational relaxation and to an indirect control of the geometry and rigidity of the conjugated backbone through the complexation of lithium cations. Although further work is still required to fully elucidate the complex problem of the feed back effects of the interactions between the substituents and the chemical environment on the structure and properties of the conjugated backbone, these results, which confirm that PDHT represents the first step toward the design of ion-sensitive conjugated polymers, already have interesting implications for the development of sensors based on both the electrochemical and the optical properties of functional conjugated polymers. Registry No. PDHT, 120245-40-1 ; CH,CN, 75-05-8; Bu,NCIO,, 1923-70-2; LiC104,7791-03-9.
Effect of Urea on Micellar Properties of Aqueous Solutions of Nonionic Surfactants G. Briganti: S. Puvvada, and D. Blankscbtein* Department of Chemical Engineering and Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received: April 15, 1991)
We present results of a systematic experimental and theoretical investigation of the effect of urea on micellization, micellar growth, and phase separation of aqueous micellar solutions of the nonionic surfactant n-dodecyl hexaethylene oxide (CI2E6). The experimental studies, which cover a wide range of urea concentrations from 0 to 6 M, involve determinations of the critical micellar concentration (cmc), micellar shape and size as a function of temperature, and liquid-liquid phase separation coexistence curves. We find that the addition of urea significantly affects micellar solution properties. In particular, (i) the cmc increases, (ii) micellar size decreases, (iii) the “sphere-to-rod” shape transition temperature shifts to higher values, and (iv) the critical point, associated with the coexistence curve, shifts to a higher temperature and concentration. We also find that the measured coexistence curves, at 0, 2, 4, and 6 M urea concentrations, collapse onto a single universal curve when plotted in reduced coordinates. In addition, we find that plots of the mean micellar hydrodynamic radius, Rh,vs T, - T,where T,is the critical temperature and Tis the actual temperature, overlap for 0, 2,4, and 6 M urea concentrations, with the resulting universal curve exhibiting a pronounced break at a value of T, - T = 37 “C.
I. Introduction Nonionic surfactants belonging to the alkyl polyethylene oxide family, typically abbreviated as C,E,, are widely used as detergents, solubilizers, and emulsifiers. Their practical importance has triggered a significant effort to gain a fundamental understanding of their micellization characteristics, as well as their phase behavior in both aqueous and nonaqueous media.’ The unique chemical structure of these surfactants offers a convenient model system to study how systematic variations in the hydrophobic/hydrophilic character of the surfactant affects micellar solution propertiesq2 In particular, in aqueous solutions, the hydrophobic/hydrophilic character can be altered in a number of ways, which include (a) varying the number of methylene groups, i, or the number of ethylene oxide groups, j , of the surfactant, (b) varying the temperature of the solution, and (c) modifying the properties of the aqueous solvent. A common method to modify the solvent consists of adding electrolytes, such as simple salts, or nonelectrolytes, such as urea. In particular, urea and its derivatives are well-known denaturants To whom correspondence should be addressed. ‘Permanent address: Dipartimento di Fisica, Universita di Roma “La Sapienza”, Piazzale Aldo Moro, 00185 Rome, Italy.
0022-3654/91/2095-8989%02.50/0
of proteins, and this has been attributed to the isothermal unfolding of the protein molecule (a change in conformation) due to weaker hydrophobic interactions in the presence of urea.3 Several studies have been conducted to probe the effect of urea on the properties of aqueous micellar solutions.4d Two different mechanisms for urea action have been proposed: (i) an indirect mechanism, whereby urea decreases the “structure” of water to facilitate the hydration of the nonpolar solute,’,* and (ii) a direct mechanism, whereby urea replaces some of the water molecules in the hy(1) For a comprehensive survey on nonionic surfactants, including the alkyl polyethylene oxide family (C,E,), see: Nonionic Surjucrunrs; Shick, M. J., Ed.; Arnold: London, 1967. (2) Mitchell, D. J.; Tiddy, G. J. T.; Warring, L.; Bostock, T.; McDonald, M.P. J. Chem. Soc., Furaduy Trans. I 1983, 79,975, and references cited therein. (3) Tanford, C. J . Am. Chem. Soc. 1964, 86, 2050. (4) Shick, M. J. J . Phys. Chem. 1964, 68, 3585. ( 5 ) Emerson, M. F.; Holtzer, A. J. Phys. Chem. 1967, 71, 3320. (6) Franks, F., Ed. Wuter: A Comprehemive Treutise; Plenum: New York, 1978; Vol. 4 and references cited therein. (71 Wetlaufer, D. B.; Malik, S.K.; Stoller, L.; Coffin, R. L.J . Am. Chem. Soc. 1964, 86, 508. (8) Franks, H. S.; Franks, F. J . Chem. Phys. 1968,18,4746.
0 1991 American Chemical Society
