A Novel Strategy for the Solid Phase Synthesis of Ultra Pure Organic Semiconductors

A new solid phase synthetic strategy for the production of organic semiconductors has been developed. The strategy uses a germanium-based linker and Suzuki-type cross-coupling protocols and has been demonstrated for the iterative synthesis of both a regio-regular oligo-3- alkyl-thiophene and an oligoarylamine. The process also incorporates a novel "double- coupling" after each iteration which minimizes deletion sequences. The key steps exploits the susceptibility of α-silyl- or α-silyloxy- but not germyl-substituted derivatives toward nucleophilic ipso-protodemetalation. INTRODUCTION π-Conjugated heterocyclic oligomers are promising candidates for organic semiconductors but for these, high levels of purity are critical. Although solution phase chemistry may be used to target polyheterocycles and oligoheterocycles using repetitive transition metal catalyzed cross-coupling of the monomers in solution, the purification strategies required to meet the requisite levels of purity are inefficient requiring careful purification of chromatographically similar intermediates after each successive iteration; a process which is time-consuming, and inefficient (1). Such methods are therefore of questionable commercial applicability. Moreover, conventional methods for preparing oligo-heterocycles (such as oligo-thiophenes) using solution phase cross-coupling (e.g. Suzuki, Kharasch, Stille or Negishi type processes) are plagued by undesirable side reactions such as homo-coupling and loss of functional groups again making purification arduous and inefficient. Recent publications applying solid phase organic synthesis (SPOS) to the preparation of oligo-3-alkyl-thiophenes demonstrate that SPOS offers an attractive solution to some of the purification issues (2). However, the ultimate purity of the cleaved oligomer is critically dependent on the yields attained for each individual cross-coupling step. Incomplete cross- coupling results in deletions and leads to a distribution in the final oligomer length. We describe in this work the development of a novel and efficient strategy for the iterative solid phase preparation of oligo-β-3-alkyl-thiophenes) and also for an oligoarylamine organic semiconducting material of well-defined lengths and high purity involving a 'double-coupling' strategy. The method employs a germanium-based linker (3) and exploits the orthogonal susceptibility of α-silyl and α-germyl substituted thiophene-derivatives and α-silyloxy and α- germyl substituted arylamine-derivatives towards nucleophilic ipso-protodemetalation. The germanium-based linker also allows for final cleavage by electrophilic ipso-degermylation (3). Cleavage with acid will yield α-H terminated oligomers, whereas cleavage with halonium ions will yield α-halo terminated oligomers (4). Such α,ω- differentiated telechelic oligomers are valuable substrates for block co-oligomer preparation and for oligomer end-capping. The advantages of solid phase chemistry over solution phase chemistry include ease of purification, amenability to automation, the ability to use excess reagents to drive reactions to completion without the penalty of making purification tedious and dilution effects (site isolation) which prevent homo-coupling. For these reasons, solid phase synthesis is seen as an attractive alternative for preparing poly-and oligo- conjugated organic semiconductors on a large scale which as yet has undergone little investigation.