Synthesis and Properties of Poly ( ferrocenylsilane ) s Containing Si-Vinylene Units in the Main Chain

Poly(ferrocenylsilane)s containing Si-vinylene unit in the main chain have been synthesized by Mizoroki-Heck reaction of 1,1’-bis(dimethylvinylsilyl)ferrocene (MVFS) and dibromo aryl compounds using palladium diacetate as a catalyst. The alternating copolymers with relatively low molecular weight were obtained in good yields. Cyclic voltammetry of the copolymers in CH2Cl2 solutions showed redox potential derived from the ferrocene units in the copolymers. The voltammograms the copolymers indicated that almost no difference in numbers of electron transfer compared with MVFS. By contrast, diffusion coefficients of the ferrocene units in the copolymers were smaller than that of MVSF. The copolymers showed reversible electrochromism from yellow to blue-green due to the redox of ferrocene units in the main chain by 3V of application.

We developed fluorene-and carbazole-based alternating copolymers containing Si-vinylene units in the main chain (Naga, Tagaya, Noda, Imai, & Tomoda, 2008;Naga, Ohkura, Tagaya, & Tomoda, 2011).These copolymers were synthesized by alternating copolymerization of dibromofluorene or dibromocarbazole with Si containing divinyl or diallyl compounds using Mizoroki-Heck reaction with a Pd catalyst.Incorporation of Si and unsaturated vinylene units in the main chain of the conjugated polymers was effective to improve solubility of the polymers in organic solvents.Furthermore, the copolymerization is usable to control the photophysical properties of the conjugated polymers.We came to an idea to synthesize poly(ferrocenylsilane)s containing Si-vinylene units in the main chain using the alternating copolymerization.This paper reports the alternating copolymerization of 1,1-bis(dimethylvinylsilyl)ferrocene (MVSF) with aromatic dibromo compounds (ArBr 2 ) using Mizoroki-Heck reaction with a Pd catalyst, as shown in Scheme 2, and the photophysical and electro chemical properties of the resulting copolymers.
Indium tin oxide (ITO) coated glass slide (surface resistivity: 8-12 /sq) was purchased from Sigma-Aldrich Co. LLC.Chloroform and CH 2 Cl 2 (spectroscopy grade) were commercially obtained from Dojindo Molecular Technologies, Inc. Tetrabutylammonium hexafluorophosphate was commercially obtained from Tokyo Chemical Industry Co., Ltd., and used as received.

Synthesis of 1,1-bis(dimethylvinylsilyl)Ferrocene (MVSF)
MVSF was synthesized according to the previous report (Majchrzak, Marciniec, Kubicki, & Pawelczyk, 2005).A n-hexane solution of n-butyllithium (40.0 mmol, 1.63 M, 24.5 mL) was added dropwise to a hexane (80 mL) solution of TMEDA (40.8 mmol) and ferrocene (19.9 mmoL) at room temperature under nitrogen atmosphere, and the reaction system was stirred for 20 h.The reaction system was cooled to -78ºC, and chlorodimethylvinylsilane (41.8 mmol) was added dropwise to the reaction solution, and stirred for 25 min.The reaction system was gradually warmed to room temperature, and stirred for 8 h.The solvent was evaporated, and the products was purified by silica gel column using n-hexane as an eluent.

Synthesis of Poly(ferrocenylsilane)s Containing Si-vinylene Unit in the Main Chain
The copolymerization was carried out in a 100 mL glass reactor equipped with a magnetic stirrer.Equivalent molar of MVSF (1.25 mmol) and a ArBr 2 (1.25 mmol) were added to the reactor under nitrogen atmosphere.P(o-Tol) 3 (0.25 mmol), NEt 3 (3.13mmol), and a DMF solution (4.0 mL) of Pd(OAc) 2 (0.05 mmol) were introduced to the reactor.The copolymerization was conducted at 100ºC for 24 h.The reaction was terminated by adding a small amount of methanol.
The polymer was precipitated in a large excess of methanol and recovered by decantation.The copolymer obtained was dissolved in chloroform and re-precipitated in methanol.The precipitate was collected by decantation and dried in vacuo at 60°C for 6 h.

Analytical Procedures
1 H NMR spectra of the copolymers were recorded at room temperature on a JEOL-JNM-LA300 spectrometer in pulse Fourier transform mode.The sample solution was made in CDCl 3 as a solvent and the resonance of CDCl 3 (7.24ppm) was used as an internal reference.Molecular weight and molecular weight distribution of the copolymers were measured at 40ºC by means of gel-permeation chromatography, SHIMADZU Prominence GPC System, using chloroform as a solvent, and calibrated with standard polystyrene samples.UV-vis absorption spectroscopy of the copolymers was conducted with SHIMADZU UV-1200 in a chloroform or CH 2 Cl 2 solution.Photoluminescence (PL) spectroscopy was investigated with a SHIMADZU RF-1500 in a chloroform solution of the copolymer.Cyclic voltammetry was conducted with Electrochemical Measurement System HZ-5000 (Hokuto Denko) using Ag|AgCl reference electrode, glassy carbon working electrode, and Pt counter electrode at a scan rate of 0.1 V/s.Voltage for electrochlomism observation was applied by GP 025-5 (Takasago Ltd.) ranged from 0 to 3.0 V at a scan rate of 10 mV/s.

Synthesis and Structure of Poly(ferrocenylsilane)s
Copolymerizations of MVSF with ArBr 2 compounds have been investigated with Pd(OAc) 2 in DMF at 100ºC.The results are summarized in Table 1.Although the molecular weights of the copolymers were relatively low, the corresponding copolymers were obtained in good yield.a Evaluated in CHCl 3 (10 -8 mol/L of ferrocene unit).b Emission was exited at λ max of absorption.

Optical Properties
UV-vis and PL spectra of the copolymers were acquired in chloroform.Figure 2 shows absorption and emission spectra of P1.The wavelengths UV-vis and PL spectra of the copolymers are summarized in Table 1.All the copolymers showed an absorption peak at 275 nm, and emission peaks at around 380 and 420 nm.These wavelengths were almost same those of MVSF or ferrocene.The absorption and emission of the copolymers are derived from ferrocene units, and their wavelengths are independent of structure of the aryl units.

Electrochemical Properties
Wang et al. reported effect of molecular structure of poly(ferrocenlysilane)s on electrochemical behavior (Wang, Wang, Wang, Chen, & Chen, 2006;Wang, Wang, Wang, & Chen, 2007).Electrochemical properties of the present copolymers in CH 2 Cl 2 solution containing 0.1 M of tetrabutylammonium hexafluorophosphate were investigated with cyclic voltammetry.The results are summarized in Figure 3, and Table 2.The copolymers showed redox potential, derived from ferrocene units in the main chain.All the copolymers and MVSF, which have dimethylsilylene-vinylene units, showed higher E 1/2 values than that of ferrocene, indicating shift of the voltammograms to anode due to the electron withdrawing of the ferrocene units by the vinylene units.E values of the copolymers (0.08 -0.11 V) were almost same with those of the MVFS.The results indicate almost no difference in numbers of electron transfer between the copolymers and MVFS.By contrast, I pa and I pc of the copolymers were smaller than those of MVFS.Small diffusion coefficient of the ferrocene units in the copolymers, which was caused by large molecular weight, should decrease the I pa and I pc values of the copolymers.

Conclusions
Poly(ferrocenylsilane)s containing Si-vinylene unit in the main chain were obtained with Mizoroki-Heck reaction of bis(dimethylvinylsilyl)ferrocene and dibromo aryl compounds using Pd(OAc) 2 catalyst.The copolymers showed good solubility in CHCl 3 and CH 2 Cl 2 .The CHCl 3 solutions of the copolymers showed absorption and photo luminescence derived from the ferrocene units in the copolymers.The absorption and luminescence wavelengths were almost independent of the molecular structure of the aryl compounds.The cyclic voltammograms of the alternating copolymers indicated the electron withdrawing of the ferroecene units by the vinylene units, and small diffusion coefficient of the ferrocene units in the copolymers caused by low mobility derived from large molecular weight.The copolymers also showed reversible electrochromism due to the redox of the ferrocene units in the main chain.
Although, the present copolymerization should be one of the useful methods to design and synthesize the poly(ferrocenylsilane)s containing conjugation, the molecular weights of the copolymers were too low to form the films.Furthermore, the optical properties of the copolymers were independent with the structure of Ar units.The ferrocene units are dominant of the optical properties of the copolymers.As the next step, we are trying to synthesize the high molecular weight poly(ferrocenylsilane)s containing long conjugation, which can control the optical properties of the resulting copolymers, and the results will be reported elsewhere.

Figure 2 .
Figure 2. UV-vis (a) and PL (b) spectra of P1 in CHCl 3 solution, 10 -8 mol/L of ferrocene unit, emission was exited at λ max of absorption of 275 nm.

Table 1 .
Synthesis of alternating copolymers of MVFS with ArBr 2 , and their structure and optical properties