Synthesis and Intramolecular Charge-Transfer Interactions of a Donor

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Synthesis and Intramolecular Charge-Transfer Interactions of a Donor−Acceptor Type Polymer Containing Ferrocene and TCNAQ Moieties Wenyi Huang* and Chang Dae Han Department of Polymer Engineering, The University of Akron, Akron, Ohio 44325-0301, United States S Supporting Information *

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INTRODUCTION The basic concept of donor−acceptor interactions in molecules has been regarded as being an important subject to chemists. Hence, the subject has long been discussed in the literature, and the interested readers are referred to excellent review articles.1 This concept has successfully been used in the synthesis of organic conductors2 (e.g., charge-transfer salt of tetrathiafulvalene (TTF) and 7,7,8,8-tetracaynoquinodimethane (TCNQ)) and organic magnets3 (e.g., charge-transfer salt of decamethylferrocene and tetracyanoethylene (TCNE)). In recent years the research interests in the synthesis of donor−acceptor type polymers have been intensified due to their great potential applications in such emerging fields as light-emitting diodes,4 light-harvesting devices,5 nonlinear optical devices,6 and optoelectronic materials.7 However, the synthesis of polymers containing alternating strong donor (e.g., ferrocene or TTF) and acceptor (e.g., TCNQ, TCNE, or 11,11,12,12-tetracyanoanthraquinomethane (TCNAQ)) has remained a synthetic challenge until now, the reason behind which will be elaborated on later. Here we present a new approach for the synthesis of a unique polymer (named PFPA-Si) consisting of electrondonating ferrocene and electron-accepting TCNAQ through judicious molecular design. The synthesis scheme for PFPA-Si is given in Scheme 1. The covalent linkage of ferrocene unit with TCNAQ unit via phosphoranimine bond throughout the macromolecular chains has an obvious appeal because such

linkage would facilitate the electron transport between the two units leading to intramolecular charge-transfer interactions.

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RESULTS AND DISCUSSION Monomer Synthesis. Since ferrocene is a well-known electron donor with excellent chemical versatility,8 which leads to easy syntheses of a large number of ferrocene derivatives suitable for polymerizations, in this study ferrocene was selected as an electron donor. Referring to Scheme 1, the electron-donating monomer (named Fc-Si-PPh2) was obtained by first synthesizing 1,1′-bis(trimethylsilyl)ferrocene in accordance with the procedures described by Rausch and Ciapenelli,9 which was then dilithiated with n-butyllithium and N,N,N′,N′tetramethylethylenediamine in hexane and subsequently treated with chlorodiphenylphosphine. This reaction was borne out to be highly regio- and stereoselective determined by 1H, 13C, and 31 P NMR spectroscopy (as Figures S4−S6 in the Supporting Information), and thus Fc-Si-PPh2 has a well-defined structure, instead of a mixture of isomers. It is well established that among many candidates TCNE, TCNQ, and TCNAQ are known to be very effective electron acceptors. The effectiveness of electron-accepting capability of these three species is in the following order: TCNE > TCNQ > TCNAQ, which is attributed to an increase in bulkiness going from TCNE to TCNQ and to TCNAQ. In order to take advantage of the unique features of these three electronaccepting species to synthesize donor−acceptor type polymers, one must introduce a functional group(s) into them, such that each of them can react with an electron-donating monomer. However, the introduction of a functional group(s) into TCNE is a formidable task due to the absence of reactive hydrogen atom in the TCNE molecule. On the other hand, it is rather difficult to prepare TCNQ monomers having suitable functional groups for the synthesis of targeted polymers because many functional groups are not compatible with the quinone structure of TCNQ, resulting in the instability of the TCNQ derivative(s) prepared. Thus, in this study TCNAQ was chosen as the desired electron-accepting species due to its easy accessibility of functionalization and the stability of its derivative(s).10 Referring to Scheme 1, the electron-accepting monomer (named TCNAQ-N3) was synthesized using 2,6diamineanthraquinone as the starting compound. The reaction

Scheme 1. Reaction Scheme for PFPA-Si

Received: January 31, 2012 Revised: May 2, 2012 Published: May 10, 2012 © 2012 American Chemical Society

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stable at ambient conditions, enabling us to characterize its chemical structure despite its high molecular weight. Gel permeation chromatography (Figure S22) shows that PFPA-Si has Mn = 38 720 and Mw = 53 670 against polystyrene standards in THF. Thermogravometric analysis indicates that PFPA-Si is thermally stable up to ∼370 °C under a nitrogen atmosphere (Figure S23). Because of its rigid chemical structure and the intramolecular charge-transfer (ICT) interactions between the donor and acceptor units, PFPA-Si has a high glass transition temperature (Tg = 227 °C), as evidenced by differential scanning calorimetry (Figure S24). Owing to the very mild reaction conditions employed for the polymerization, we learned that oxidation of the iron (Fe) element in the ferrocene unit of PFPA-Si did not occur as evidenced by X-ray diffraction (Figure S25) and wide-angle powder diffraction (Figures S26) patterns because there were no reflections peaks that could be assigned to the crystalline structure of iron oxide. The above observations have led us to conclude that the chemical structure of PFPA-Si is very stable, and the ICT interactions illustrated below must have originated intrinsically from macromolecular chains, not from extrinsic sources or the change of molecular structure during experiments. It is worth mentioning at this juncture that some previous research groups22 attempted to synthesize, via the Staudinger reaction, polymers having phosphoranimine bonds by reacting 1,4-diazidobenzene with bis(diphenylphosphino)alkyl or -aryl compounds, but they only obtained either insoluble solids or oligomers. Intramolecular Charge-Transfer Interactions. An ICT between the ferrocene unit and the TCNAQ moiety of PFPASi is manifested by the appearance of a strong, broad absorption band in the 450−700 mm region of UV−vis spectrum shown in Figure 1. The maximum value of this ICT band is about 510

schemes and experimental procedures for the synthesis of TCNAQ-N3 including the intermediate compounds are described in the Supporting Information, and their 1H, 13C NMR, and FTIR spectra are also given (Figures S8−S17). In this study we learned that TCNAQ-N3 was very stable and could easily be purified and stored at an ambient condition. The stability of TCNAQ-N3 is in conformity with the empirical rule that the number of nitrogen atoms in a compound containing an azide group must not exceed that of carbon atoms, and the total number of carbon and oxygen atoms should be 3 times greater than or equal to that of nitrogen atoms.11 Polymer Synthesis. Initially, we encountered great difficulty in developing suitable polymerization methods to synthesize the targeted polymer. Among the reasons for the difficulty, we learned that the tetracyano group in TCNAQ tended to form complexes with metal ions in catalysts; for example, the palladium catalysts used for the Sonogashira reaction12 of triple bond and the Suzuki reaction13 or the Stille coupling reaction14 of C−C bond coupling. As a result, the palladium catalysts lost their effectiveness during polymerization. Thus, we concluded that the use of such catalysts was inappropriate for synthesizing target polymers having TCNAQ moiety. Also, we learned that the tetracyano group in TCNAQ was unstable under the base conditions employed. Thus, we concluded that the Knoevenagel reaction,15 Wittig reaction,16 or Horner−Wadsworth−Emmons reaction17 could not be used to synthesize conjugated polymers having CC conjugated bond when the polymers had TCNAQ moiety. Further, we observed that the reaction rates for forming CN conjugated bond via the Schiff base reaction18 between a TCNAQ derivative having diamine group and a ferrocene derivative having dialdehyde group were so slow that only oligomers could be obtained. Other polymerization methods such as the imine metathesis reaction19 and the aza-Wittig reaction20 were found to be not effective to synthesize polymers having CN conjugated bond. After encountering the difficulties described above, we realized that we had to develop a new synthesis scheme which would enable us to successfully synthesize a donor− acceptor type polymer containing ferrocene and TCNAQ moieties, without using a catalyst, under neutral conditions. Finally, we succeeded, via the Staudinger reaction,21 in synthesizing our targeted polymer. The Staudinger reaction enables one to obtain phosphoranimine bond from the reaction between an azide and a phosphine or phosphate under moderate conditions without requiring a metallic catalyst. Specifically, in this study our targeted polymer, PFPA-Si, was polymerized by reacting Fc-Si-PPh2 with TCNAQ-N3 in tetrahydrofuran (THF) at 25−35 °C (Scheme 1). The experimental procedures employed for the synthesis of PFPASi and the characterization of the chemical structures of various compounds are presented including 1H, 13C, and 31P NMR spectra (Figures S18−S20) and FTIR spectra (Figure S21). The attractive feature of the Staudinger reaction lies in that the reaction rate is very fast and the side product, nitrogen gas, can easily be removed by bubbling out during the reaction. It should be mentioned that without introducing the bulky trimethylsilyl side group into 1,1′-bis(diphenylphosphino)ferrocene we only obtained oligomers due to its limited solubility. PFPA-Si has been found to be very soluble in common organic solvents (e.g., THF and chloroform) and chemically

Figure 1. UV−vis spectrum of PFPA-Si in THF.

nm. Additionally, an absorption band at 416 nm is assigned to ferrocene unit. In contrast, TCNAQ-N3 exhibits two broad absorption bands at 387 and 319 nm (Figure S27a), which correspond to the lowest electronic transition of its tetracyano center in the neutral state, while Fc-Si-PPh2 only shows two absorption bands at 296 and 254 nm, which correspond to ferrocene and phenyl units, respectively (Figure S27b). These results confirm that ICT interactions have really occurred in PFPA-Si. In order to further confirm the existence of the intramolecular charge-transfer interactions in PFPA-Si, we ran UV−vis spectroscopy measurement for the equimolar mixture of ferrocene and 2,6-diamine-11,11,12,12-tetracyanoanthraquinodimethane trifluorodiacetate (TCNAQ−DACF3) in THF 4426

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better understand our interpretation of the cyclic voltammogram presented in Figure 2, we have prepared enlarged voltammograms (Figures S29 and S30) and then identified the locations of oxidation and reduction potentials as well as the onset values of oxidation and reduction potentials. The values of redox potentials of PFPF-Si, Fc-Si-PPh2, and TCNAQ-N3 summarized (Table S1) will be very helpful to have a better understanding of our interpretation. It can be seen from Figure S31 that intermolecular chargetransfer interactions do not occur in the equimolar mixture of ferrocene and TCNAQ-N3. In other words, intramolecular charge-transfer interactions must be dominant in PFPA-Si. Because of the electron-donating property of phosphoranimine bond,24 it would facilitate the intramolecular charge-transfer interactions between ferrocene and TCNAQ units in PFPA-Si. Because of the ICT interactions between ferrocene (and/or phosphoranimine) and TCNAQ units, upon scanning anodically, PFPA-Si exhibits three irreversible oxidation waves at the potentials of 0.62, 0.78, and 1.07 V (Figure S29). These three values can easily be identified in Figure 2, and they are also included (Table S1), which are assigned to ferrocene unit and phosphoranimine unit, respectively. The large positive shift of 0.60 V, which comes from (Epa(2)ox(PFPA-Si) − Epaox(Fc-SiPPh2) = 0.78 − 0.18 = 0.60 V, in the oxidation potential of ferrocene unit in PFPA-Si, relative to that of Fc-Si-PPh2, is attributable to the electron transfer of ferrocene unit, via the phosphoranimine bond, to the TCNAQ unit. If there had been no ICT interactions along the macromolecular chain, at least we should have observed a reversible oxidation of ferrocene in the cyclic voltammogram, similar to that of Fc-Si-PPh2. The above statement is due to the fact25 that the electrochemical behaviors of ferrocene are reversible, and thus one expects to observe cathodic and anodic peaks during CV experiments. Because ferrocene was used as the internal standard during the cyclic voltammetry measurements,25 the energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were determined (see Supporting Information for the justification) by the onset oxidation potential (Φox = 0.49 V) and onset reduction potential (Φred = −1.13 V) with an assumption that the redox potential of Fc+/Fc has an absolute energy level of 4.80 eV below the vacuum level.26 Accordingly, EHOMO = −(Φox + 4.80) (eV) = −5.29 eV and ELUMO = −(Φred + 4.80) (eV) = −3.67 eV. The bandgap energy (EgCV = −(EHOMO − ELUMO)) for PFPA-Si based on cyclic voltammetric results is 1.59 eV, which is close to Egopt = 1.77 eV determined by UV−vis spectroscopy (refer to the discussion of Figure 1 given above). Therefore, PFPA-Si may be regarded as a low-bandgap polymer, which is consistent with the observations made in other donor−acceptor type polymers.27

(Figure S28) and found out that there were no intermolecular charge-transfer interactions existing in the mixture. This observation is similar to that reported for the mixture of tetrathiafulvalene (TTF) and TCNAQ derivatives.23 In the UV region of Figure 1, strong absorption bands at around 363 and 288 nm are characteristic of π−π* transitions located on the phenyl and TCNAQ subunits. The Fourier transform infrared spectrum shows that the −CN stretching frequency of TCNAQ-N3 occurs at 2222 cm−1 (Figure S17), whereas the −CN stretching frequency of PFPA-Si is shifted to 2215 cm−1 (Figure S21). This further verifies the presence of ICT interactions taking place in PFPA-Si. If there had been no ICT interactions in PFPA-Si, there would not have been such a large difference in −CN stretching between TCNAQ-N3 and PFPA-Si. It is worth noting that the onset wavelength of PFPASi, λonset = 700 nm, shown in Figure 1, corresponds to an optical bandgap energy (Egopt = 1242/λonset) of 1.77 eV, in which Egopt was calculated from the definition, Egopt = hC/λonset, with h being Planck’s constant (6.626 × 10−34 J s), C being the speed of light (3.0 × 108 m/s), and λonset being the onset wavelength (nm × 10−9 m). The redox properties of PFPA-Si were studied by cyclic voltammetry (CV) using tetrabutylammonium hexafluorophosphate (nBu4NPF6) as a supporting electrolyte in dichloromethane. As shown in Figure 2, there is a large potential

Figure 2. Cyclic voltammogram (third scan) of PFPA-Si vs Fc+/Fc in dichloromethane solution containing 0.1 M nBu4NPF6. Arrows describe the directions of scan.

difference (ΔE = 1.17 V) (Table S1) between the cathodic peak (at −1.61 V) and corresponding anodic peak (at −0.44 V), which might have arisen from the conformational changes between the distorted neutral TCNAQ and the planar dianion of TCNAQ during the charge-transfer process. Accordingly, PFPA-Si exhibits a half-wave potential (E1/2red) of −1.03 V vs Fc+/Fc, which is ascribed to the formation of the TCNAQ2− dianion. In contrast, Fc-Si-PPh2 has an E1/2ox of 0.12 V, while TCNAQ-N3 has an E1/2red of −0.79 V vs Fc+/Fc with a potential difference (ΔE) of 0.26 V with an enlarged cyclic voltammogram (Figure S29). The cathodic shift (E1/2red(PFPASi) − E1/2red(TCNAQ-N3) = −1.03 − (−0.79) = −0.24 V) under the same conditions of the reduction values for PFPA-Si with respect to that of TCNAQ-N3 can be attributable to the fact that the presence of the ferrocene and phosphoranimine electron donors directly linked to the TCNAQ unit increases the electron density of TCNAQ, thus slightly lowering its electron accepting capability. Note that because E1/2red(PFPASi) is higher than E1/2red(TCNAQ-N3) in terms of absolute value, the electron-accepting strength of TCNAQ in PFPA-Si is lower than that in TCNAQ-N3. In order to help the readers

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CONCLUSION In summary, a donor−acceptor type polymer, PFPA-Si, containing ferrocene and TCNAQ moieties has been successfully synthesized via the Staudinger reaction. This unique polymer has reasonably high molecular weight as well as good solubility in common solvents. PFPA-Si possesses remarkable optical and electrochemical properties, which originate from ICT interactions along the macromolecular chains. The electron-donating property of phosphoranimine bonds facilitates the intramolecular charge-transfer interactions between ferrocene and TCNAQ units in PFPA-Si. 4427

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(23) de Miguel, P.; Bryce, M. R.; Goldenberg, L. M.; Beeby, A.; Khodorkovsky, V.; Shapiro, L.; Niemz, A.; Cuello, A. O.; Rotello, V. J. Mater. Chem. 1998, 8, 71−76. (24) Escobar, M.; Jin, Z.; Lucht, B. L. Org. Lett. 2002, 4, 2213−2216. (25) Gagné, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980, 19, 2854−2855. (26) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R. F.; Bässler, H.; Porsch, M.; Daub., J. Adv. Mater. 1995, 7, 551−554. (27) Zhang, Q. T.; Tour, J. M. J. Am. Chem. Soc. 1998, 120, 5355− 5362.

ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental procedures and characterizations of monomers and polymers. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph 1-330-622-3807; Fax 1-330-972-5290. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge with gratitude that this study was supported in part by the National Science Foundation under Grant CBET-0755763.

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REFERENCES

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