5390
Macromolecules 1997, 30, 5390-5396
Poly(aryleneethynylene) Type Polymers Containing a Ferrocene Unit in the π-Conjugated Main Chain. Preparation, Optical Properties, Redox Behavior, and Mo¨ssbauer Spectroscopic Analysis Takakazu Yamamoto,* Takashi Morikita, and Tsukasa Maruyama Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan
Kenji Kubota Faculty of Engineering, Gunma University, Tenjincho, Kiryu 376, Japan
Motomi Katada Radioisotope Research Center, Tokyo Metropolitan University, Minami-osawa, Hachioji, Tokyo 192-03, Japan Received March 31, 1997; Revised Manuscript Received June 24, 1997X
ABSTRACT: Palladium-catalyzed polycondensation between diiodoferrocenes (1,1′-diiodoferrocene and 1,6-diiodo-1′,6′-biferrocene) and diethynyl aromatic compounds HCtCsArsCtCH (e.g., 2,5-diethynylpyridine and 2,5-diethynyl-3-hexylthiophene) gives poly(aryleneethynylene) (PAE) type polymers containing the ferrocene unit in the π-conjugated main chain. The prepared polymers include (sFcsCtCsPhsCtCs)n, PAE-Fc-1, (sFcsCtCsPysCtCs)n, PAE-Fc-2, and (sFcsCtCshexThsC tCs)n, PAE-Fc-3 (Fc ) 1,1′-ferrocenylene; Ph ) p-phenylene; Py ) pyridine-2,5-diyl; hexTh ) 3-hexylthiophene-2,5-diyl). 1H-NMR and IR spectra of the polymers are reasonable for their structures. The PAE type polymer containing the pyridine unit (PAE-Fc-2) is soluble in formic acid, and the polymer containing the hexylthiophene unit (PAE-Fc-3) is soluble in common organic solvents such as CHCl3, THF, and benzene. UV-visible spectra of the polymers exhibit a main π-π* absorption peak at about 330 nm and a d-d absorption peak at about 450 nm. The cyclic voltammogram of the polymers in a CH3CN/CH2Cl2 solution shows a reversible Fe(II) a Fe(III) redox cycle at about 0.25 V vs Ag/Ag+, and the redox peaks are broadened compared with those of low-molecular-weight ferrocenes. Exchange of electrons along the main chain is considered to be the origin of the broadening. The polymers themselves are insulating, however, they are converted into semiconducting materials with conductivity of 10-7 to 10-4 S cm-1 by formation of adducts with iodine. Mo¨ssbauer spectra of the polymers reveal oxidation of Fe(II) in the ferrocene unit of PAE-Fc-1 to Fe(III), and the ease of the oxidation reflects the electronic properties of the polymer.
Introduction Extensive studies have been made on ferrocene and its derivatives. They have various interesting electronic and optical properties. For example, they form charge transfer complexes,1 generate mixed valent states by oxidation,2 and show nonlinear optical properties.3 Polymers constituted of the ferrocene units have also attracted attention of chemists, and preparation of poly(ferrocene) (PFc) and its related polymers as well as their conversion into electrically conducting materials by partial oxidation have been reported.4-6
physical properties have been restricted by low solubility of PFc containing the ferrocene (Fc) unit. On the other hand, Pd-catalyzed polycondensation (eq 1) between dihalo aromatic compounds and diethynyl aromatic compounds provides a useful synthetic tool for π-conjugated poly(aryleneethynylene) (PAE) type polyPdsCu
nXsArsX + nHCtCsAr′sCtCH 9 8 NR 3
(sArsCtCsAr′sCtCs)n (1) PAE mers.7 Application of this polycondensation to appropriate monomers containing the Fc unit is expected to give soluble PAE type polymers containing the Fc unit in the π-conjugated main chain, and revealing their optical and electrical properties is considered to be intriguing. Here, we report preparation and properties of such PAE type polymers containing the Fc unit7f,g in the π-conjugated main chain.
Delocalization of electrons along the PFc main chain and intermolecular exchange of electrons are considered to be the origin of the electrical conductivity. However, characterization of PFc and revealing its chemical and X Abstract published in Advance ACS Abstracts, August 15, 1997.
S0024-9297(97)00436-1 CCC: $14.00
Results and Discussion Preparation. The Pd-catalyzed polycondensation expressed by eq 2 (in cases of PAE-Fc-1, -2, -3, and -4) and by eq 3 (PAE-Fc-4) gives the corresponding PAE type polymers containing the ferrocenylene unit Fc. Table 1 summarizes results of the polymerization. © 1997 American Chemical Society
Macromolecules, Vol. 30, No. 18, 1997
Poly(aryleneethynylene) Type Polymers 5391
Pd(0) complex + CuI
IsFcsI + HCtCsArsCtCH 9 8 i-Pr NH 2
(sFcsCtCsArsCtCs)n (2) Pd(II) complex
8 IsFcsI + BrMgCtCsArsCtCMgBr 9 THF (sFcsCtCsArsCtCs)n (3) Use of 1,6-diiodo-1′,6′-biferrocenylene IsFcsFcsI, instead of IsFcsI, also gives the corresponding polymer, PAE-Fc-5, according to eq 3.
The Pd-catalyzed polycondensation expressed by eq 1 is based on the Pd-catalyzed coupling reaction between the acetylenic compound and organic halide8,9 Pd(0) complex + CuI
RX + R′CtCH 9 8 RCtCR′ NR′′ 3
(4)
and we have confirmed that this type of coupling reaction also proceeds well with 1,1′-diiodoferrocene (eq 5).
For example, 1,1′-bis(phenylethynyl)ferrocene,10a-c diEtnyl-Fc, can be obtained in 90% isolation yield by the reaction. However, 1,1′-dibromoferrocene has low reactivity against the coupling reaction, and gives di-EtnylFc only in low yield. Based on these results, IsFcsI is used as the monomer in the present polycondensation. We have also tried synthesis of 1,1′-bis(arylethynyl)ferrocenes like di-Etnyl-Fc from reactions between 1,1′-diethynylferrocene (HCtCsFcsCtCH) and aryl halide (ArsX) in order to seek the possibility for carrying out the polycondensation (eq 2) with a reverse type of combination of monomer, 1,1′-diethynylferrocene
(HCtCsFcsCtCH) and dihalo aromatic compound (XsArsX). However, attempts to synthesize 1,1′-bis(arylethynyl)ferrocenes by this route have not been successful; 1,1′-diethynylferrocene, which is prepared in situ by hydrolysis of Me3SiCtCsFcsCtCSiMe3, is unstable and makes cyclized compounds like ferrocenophane.10d PAE-Fc-1 is obtained in 96% yield, and about 25% of this polymer is soluble in organic solvents such as CHCl3, toluene, and THF. Since IR spectra of the soluble and insoluble polymers are identical, the fraction with relatively low molecular weights is considered to be dissolved in the solvents. PAE-Fc-2 is only partly soluble in usual organic solvents (e.g., about 20 wt % is soluble in CHCl3); however, it is fully soluble in formic acid, similar to other PAE type polymers with the pyridine unit.7b,c Removal of formic acid from the formic acid solutions of PAE-Fc-2 by evaporation gives original PAE-Fc-2 as proved by IR spectroscopy. PAE-Fc-3 is soluble in organic solvents; however, its yield is not high (run 3 in Table 1). 2,5-Diethynyl-3-hexylthiophene (HCtCshexThsCtCH) has high reactivity7c and may give some byproducts under the polymerization conditions. In the preparation of PAE-Fc-4 and -5, THF is well suited to be the polymerization solvent and the polymers are obtained in 100% yield. These polymers have high solubility in organic solvents due to the long alkyl substituent. IR spectra of these polymers show the ν(CtC) absorption peak of disubstituted acetylene7c at about 2200 cm-1 as well as several absorption peaks characteristic of the ferrocenylene unit. The ν(CsH) stretching absorption peak of terminal acetylene (at about 3250 cm-1) is not observable. 1H-NMR data of the polymers are also reasonable for their structures. For example, Figure 1 exhibits 1HNMR spectrum of PAE-Fc-4, and peaks at δ ) 4.2-4.6 ppm are assigned to the Fc protons. The two kinds of Fc protons (protons at 2- and 5-position and those at the 3- and 4-position) give rise to two peaks at δ 4.35 and 4.56 ppm, respectively. Appearance of additional two weak peaks near the two peaks may be attributed to the terminal groups, although the presence of rotamer(s) in view of the angle between the two ethynyl groups attached at the Fc groups may also result in such splitting of the Fc protons. The relative peak area of the Fc protons against those of CH2 and CH3 groups in the dodecyl substituent as well as against that of phenylene H’s agrees with the structure of PAE-Fc-4. The 1H-NMR absorption pattern of the Fc-H’s of PAEFc-5 becomes more complicated because of the presence of the sFcsFcs unit, which will make the presence of several kinds of rotamers possible. 13C{1H}-NMR data of the polymers are also consistent with the structures shown above. PAE-Fc-4 gives rise to two peaks of ethynylene sCtCs carbons at δ 85.6 and 91.8 ppm and it has phenylene signals at δ 121.8, 131.3, and 140.7 ppm, respectively (the inset in Figure 1). Its 1H-13C COSY NMR spectrum also agrees with the molecular structure of the polymer. Light scattering analysis11 of the CHCl3 solution of PAE-Fc-2 (CHCl3-soluble part) and PAE-Fc-3 indicates that they have weight average molecular weights Mw of 2.1 × 104 and 3.4 × 104, respectively. He-Ne laser with λ ) 632.8 nm where the polymer does not show absorption is used for the light scattering analysis. GPC analysis shows PAE-Fc-2 (CHCl3-soluble part) and PAEFc-3 have Mw of 5.7 × 103 and 5.8 × 103, respectively.
5392 Yamamoto et al.
Macromolecules, Vol. 30, No. 18, 1997 Table 1. Preparation of PAE Type Polymers Containing Ferrocene Unit λmax (nm)
run
polymer
methoda
temp (°C)
time (h)
yieldb (%)
10-4 Mwc
Fvd
