Cross-Coupling Polymerization at Iodophenyl Thin Films Prepared by Spontaneous Grafting of a Diazonium Salt

Cross-Coupling Polymerization at Thin Films Prepared by Spontaneous Grafting of a Development of a method for the surface-initiated Kumada cross-coupling polymerization based on 4-iodophenyldiazonium salt thin films, and use of this method to make very thick polythiophene brushes. ABSTRACT 12 Cross-coupling at aryl halide thin ﬁlms has been well-established as a technique for the surface-conﬁnement of the Kumada catalyst transfer polymerization (KCTP) reaction. The spontaneous grafting of 4-iodobenzenediazonium tetraﬂuoroborate on gold substrates creates a durable thin ﬁlm which is effective as a substrate for cross-coupling reactions including the surface-initiated KCTP reaction. Using cyclic voltammetry of a surface-coupled ferrocene derivative, we have measured initiator surface coverage produced by oxidative addition of Pd(t-Bu 3 P) 2 and used the resulting initiator to prepare thick, well-deﬁned polythiophene thin ﬁlms.


INTRODUCTION
Since the development of the first polyacetylenes by Shirakawa and MacDiarmid in the 1970s, (Chiang 21 et al. (1977,1978)) applications of pi-conjugated polymers (CPs) have proliferated. A number of refined 22 synthetic approaches for forming these polymers have been developed. CPs based on arene repeat units 23 are the single largest category of CP with current practical applications, due to the inherent stability 24 of the aromatic system vs. oxidation. To prepare polyarenes, oxidative polymerization approaches are commonly used in the preparation of the aryl halide film. This approach is reasonably effective and 51 many interesting surface structures have been prepared using it. However, limitations of silane-based 52 initiators exist; purification of these materials is often problematic, the surface produced is highly variable 53 based on difficult-to-control factors such as moisture content, and the most effective coupling agents 54 for SAM formation contain a long central alkyl chain which limits electronic coupling between the 55 surface and the endgroup, an undesirable property for electronic applications. So, we sought to develop 56 a more convenient initiator system for a SI-CTP reaction, specifically the surface-initiated Kumada 57 polymerization (SI-KCTP).

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Reductive electrografting of aryl diazonium salts is well-established as a surface modification protocol. work and past studies that arenediazonium-based films formed spontaneously from acetonitrile generally 85 do not contain any nitrogen at all, in sharp contrast to aqueous-based films driven by diazohydroxide 86 deposition.

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As a component of this work, we needed to demonstrate the spontaneous grafting of the aryldiazonium 88 halide 4-iodobenzenediazonium tetrafluoroborate, and demonstrate that the resulting layer reacts to form 89 a surface-bound initiator for SI-KCTP. We found that 4-iodobenzenediazonium salt spontaneously forms 90 a thin film at a clean gold surface, and that the resulting aryl iodide layer is convenient and effective for 91 cross-coupling. (Scheme 2) In particular, this iodoarene layer yields a high density of surface-bound 92 Pd(II) sites active in the cross-coupling reaction as measured electrochemically using a well-known 93 ferrocenyl probe, ferrocenyl 2-(5-chloromagnesiothienyl)methane (FcCH 2 ThMgCl). This surface-bound 94 cross-coupling initiator also reacts effectively with a thiophene AB monomer to yield polythiophene 95 brushes.  In this work, we report a useful instance of spontaneous aryl diazonium salt grafting to a gold surface 97 to prepare a functionalized surface which can serve as an initiator platform for the Pd-catalyzed SI-KCTP 98 reaction. XPS survey scans of the functionalized surface revealed no nitrogen in the film, supporting 99 the hypothesis that gold can directly catalyze the dissociation of the diazonium salt to give dinitrogen.

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The resulting surface has a high density of reactive groups as measured under standard conditions for 101 evaluation of SI-KCTP initiator surfaces, and a remarkably thick, brushlike polythiophene film is formed 102 on the surface when used in the SI-KCTP reaction. The convenience of this preparation method, and the Figure 4. XPS of the spontaneously deposited 4-iodophenyl layer. a) Survey scan of the film reveals no nitrogen near 400 eV, and a strong Au 4f peak indicating a thin (ca. 2.5 nm) organic film. b) I 3d XPS of the film gives a strong signal. c) The breadth of the C 1s peak is consistent with two major species present.
representing the C-I species. The spectrum is consistent with that reported for cathodically electrografted 4-  The full width at half max (FWHM) of each peak is also consistent with a nearly ideal surface-confined 128 redox couple. For contrast, we performed the same coupling reaction using an indium tin oxide sub-  Figure 5. a) Cyclic voltammetry of PhI thin film after reaction with the ferrocene cross-coupling probe Fc-CH 2 -ThMgCl shows a densely packed surface (2.8x10 -10 mol/cm 2 ) and a near-ideal surface redox couple consistent with a thin arene layer. b) A commercial bromophenyl silane yields an order of magnitude lower surface coverage, larger FWHM of redox peaks, and larger peak-to-peak separation.
An important ancillary finding of this work is that the standard procedure for forming FcCH 2 ThMgCl),      source. The Au 4f 7/2 peak was used as a binding energy reference at 84.0 eV; no significant charging was 270 observed on these highly conductive substrates. 271 with Pd(0) complexes to give surface-bound Pd(II) initiator complexes, including a complex generated in 286 situ from the well-known air-stable Pd(II) cross-coupling catalyst i-Pr-PEPPSI. The surface-bound Pd(II) 287 complex produced from the "Fu catalyst," Pd(t-Bu 3 P) 2 , initiates polymerization with 2-chloromagnesio-5-288 bromothiophene solution to yield densely grafted and durable polythiophene brushes up to 1 micron in 289 thickness. This initiator system is synthetically convenient and is likely to find use in organic electronic 290 device construction.  Figure S1. EDS element mapping of the cracked substrate in Fig. 7b. The carbon and sulfur signals are strongly associated with the film.   Figure S4. TLC of water-quenched aliquots of the magnesiated ferrocene probe FcCH 2 ThMgCl as prepared under various conditions. Solutions with unconverted FcCH 2 ThBr, including the 1h/1eq. sample, underwent cross-coupling with the aryl iodide surface as measured by cyclic voltammetry. Figure S5. Cyclic voltammetry of aryl iodide-functionalized substrates coupled with ferrocene probe FcCH 2 ThMgCl prepared under various conditions. Despite lower conversion, use of an excess of FcCH 2 ThBr yields much higher surface coverage of ferrocenyl groups than an excess of iPrMgCl. Curves correspond to coupling solutions "1h/1eq" (red) and "1h/3eq" (black), respectively, in Figure  S4.