Electronic structures and band gaps of chains and sheets based on phenylacetylene units

We investigate the electronic structures of polymers composed of π-conjugated phenylacetylene (PA) units, m-PA-based and p-PA-based wires, at the extended Hückel level of theory. It is demonstrated that these conjugated systems should have a variety of electric conductance. All of the one-dimensional (1D) chains and the two-dimensional (2D) sheet based on the m-PA unit are insulators with large band gaps of 2.56 eV because there is no effective orbital interaction with neighboring chains. On the other hand, p-PA-based 1D chains have relatively small band gaps that decrease with an increase in chain width (1.17-1.74 eV) and are semiconductive. The p-PA-based sheet called “graphyne”, a 2D-limit of the p-PA-based 1D chains, shows a small band gap of 0.89 eV. The variety of band electronic structures is discussed in terms of frontier crystal orbitals.