ABSTRACT
From the discovery of electrically conductive polyacetylenes in 1977 [1], the field of π-conjugated polymer science has continuously flourished and gained prominent attention in both fundamental chemistry and material science. In the development of new π-conjugated polymers, one of main subjects has been how to construct the appropriate π-conjugated system having desired chemical and physical properties such as efficient luminescence properties, high carrier-ability and good stability. In order to fulfill this purpose, numerous efforts have been directed to tune electronic properties by the incorporation of heteroatoms into their backbone [2–9]. For example, in order to obtain main chain-type conjugated polymers including heteroatoms, it is essential to obtain stable monomers which are applicable in commodity polymerization reactions such as metal-catalyzed couplings. However, bases are required in the commodity couplings such as Suzuki‒Miyaura, Heck‒Mizoroki, and Sonogashira‒Hagihara reactions. Especially, in the case of the complexes with group 13 elements, these bases often cause degradation 490during polymerization. One of valid strategies for suppressing undesired degradation is to introduce bulky substituents into the heteroatoms. In this case, polymerization was often disturbed due to steric hindrance. Thus, elaboration on the protection groups was of great significance to achieve polymeric products with large molecular weights. Three-coordinate group 13 elements intrinsically have electron deficiency and work as a Lewis acid, originating from the vacant p-orbital by the combination with π-conjugated system. In 1961, it was revealed by Good and Ritter that π-conjugated compounds incorporating boron exhibited the extension of π-conjugation through the vacant p-orbital [10, 11]. From this report, investigations into the electronic structures of π-conjugated organoboron compounds have been extensively carried out [12–15]. Tanaka, Yamabe, and coworkers also theoretically designed polyboracetylene, which could have no band gap between the valence and conduction bands [16]. Further, Salzner, Lagowski, and coworkers theoretically predicted in 1998 that polyborole might show an extremely small band gap, 0.12 eV or less [17]. Introduction of hole into a conjugated system by doping is essential for transforming conjugated polymers to electric conductive materials. In these theoretical studies, it was proposed that boron introduction into polymer main chains can have the same effect to doping. The vacant p-orbital of boron can play a similar role in hole even in the absence of cationic species. Initially, boron introduction seemed to be the potential effective strategy for enhancing electric conductivity of conjugated polymers without doping. After synthesis, luminescent properties were discovered. Thus, organoboron and boron-containing polymers are now regarded as a promising platform for obtaining optoelectronic materials. Moreover, the examples of π-conjugated polymers composed of the heavier group 13 elements have been burgeoning very recently. Due to synthetic difficulties caused especially by extremely high reactivity of group 13 elements to Lewis bases, it has been challenging to incorporate these elements into polymers. Owing to developed synthetic strategies and protection groups, it was found that various types of boron complexes are applicable for constructing polymers. Meanwhile, even now, the number of examples to demonstrate the introduction of aluminum complexes is still limited due to their extensively-low stability under conventional polymerization conditions. Compared to aluminum, gallium complexes have higher stability. Indeed, recent works proved that some of them were applicable for polymerization with metal-catalyzed coupling reactions. More recently, comparison studies of optical properties between boron and gallium have been accomplished with the same structures in conjugated polymers, and differences originating from the type of elements in group 13 have been gradually revealed. In this review, these findings are described.
