Synthesis and Characterization of a Low-Bandgap Poly(arylene ethynylene) Having Donor-Acceptor Type Chromophores in the Side Chain

■ INTRODUCTION As one of the most important π-conjugated polymers, poly(arylene ethynylene)s (PAEs) have unique features of aromatic rings and ethynylene units alternating in the macromolecular chains, giving rise to distinct electronic and optical properties as well as outstanding mechanical properties with enhanced thermal stability and photostability. Such features originate, at least in part, from the cylindrical symmetry of ethynyl bonds, which stabilizes the conjugation of adjacent aromatic rings irrespective of the orientation of aromatic planes. However, PAEs usually exhibit large bandgaps greater than 2.0 eV due to their inherent chemical structures. This restricts their potential applications in many important areas such as solar cells, light-emitting diodes, field-effect transistors, and supercapacitors, where low bandgaps of the conjugated polymers are highly preferred. For this reason, a plethora of strategies were developed to narrow down the bandgap of PAEs, for example, the introduction of the silole or heterocylic units into the aromatic systems. Among these, PAEs with electron donor and acceptor moieties are of particular interest because the intramolecular charge-transfer interactions between donor and acceptor would lead to a lower bandgap. The main idea behind this lies in that the highest occupied molecular orbital (HOMO) energy (EHOMO) level of the donor and the lowest unoccupied molecular orbital (LUMO) energy (ELUMO) level of the acceptor are closer than those in pristine PAE systems, since the bandgap energy (Eg) may be defined by Eg = ELUMO − EHOMO. 8,9 To date, the attainable Eg of PAEs having donor−acceptor interactions was reported to be as low as 1.8 eV. To achieve a lower bandgap of PAEs, the strength of both electron donating and accepting units must be further increased, which can be accomplished by employing stronger electron donating groups like dialkylamine on the donor to raise the HOMO energy level as well as stronger electron withdrawing groups such as −CN and −NO2 on the acceptor to reduce the LUMO energy level. Tetracyanoethylene (TCNE) is one of the strongest electron acceptors but has sparsely been used for the synthesis of donor−acceptor type polymers. This is attributed to the difficulty in synthesizing TCNE derivatives suitable for polymerization. Furthermore, it should be pointed out that many conventional approaches for synthesizing conjugated polymers, especially those involved with the use of metal-ion catalysts (e.g., palladium or nickel), are not appropriate for the applications in the synthesis of polymers having TCNE derivatives because these TCNE derivatives would always form strong complexes with these metal-ion catalyst and reduce their catalytic performance. This synthetic obstacle can be circumvented by postfunctionalization with TCNE after polymerization, which requires that the reaction must be extremely reactive and highly selective. This can be realized by the “click” type reaction of tetracyanoethylene (TCNE) with electron-rich alkyne to afford 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) derivatives in a quantitative yield. The underlying mechanism lies in the fact that the electron accepting TCNE would undergo thermal [2 + 2] cycloaddition with electron-rich alkynes followed by spontaneous electrocyclic ring-opening of the initially formed cyclobutenes under mild conditions without the necessity of any catalyst. This cycloaddition/retroelectrocyclization reaction has been borne out to be very fruitful in the synthesis of donor−acceptor type organic molecules or dendrimers having interesting electrical, electrochemical, or nonlinear optical properties. Very recently, this synthetic protocol has been generalized into macromolecular systems using aromatic polyamines bearing electron-rich alkyne side chains and PAEs containing ferrocene in the main chain as precursors. In the latter case, only partial adduction of TCNE occurred due to the low electron donating property of ferrocene as well as the high steric hindrance in the main chain. Herein, we report an effective approach, for the first time, to synthesizing a low-bandgap PAE having donor−acceptor type chromophores in the side chain. Specifically, a PAE precursor with electron-rich alkynes in the side chain was accomplished through judicious molecular design to endow the polymer with reasonably high molecular weight. The extremely selective reaction of TCNE with dialkylanilino-activated alkynes in the side chains of the polymer afforded donor−acceptor type chromophores while maintaining the important features of alternating aromatic ring and ethynylene bonds in the main chain of the polymer, which have never been accomplished before. Such a unique polymer exhibited interesting intramolecular charge-transfer interactions, leading to a lower bandgap, potent redox activity, and improved thermal stability.