Ab initio study of the electronic structures and conduction properties of some donor-acceptor polymers and their copolymers

Abstract

The electronic structures and conduction properties of four donor-acceptor polymers containing alternating electron-donating group SiF2 and electron-accepting group Y [Y=>C=CH2 in PSIFCH, >C=O in PSIFCO, >C=CF2 in PSIFCF, and >C=C(CN)2 in PSIFCN] along the conjugated trans-cisoid polyacetylene backbone, obtained on the basis of the ab initio Hartree-Fock crystal orbital method are compared. All the polymers are predicted to have band gap values ranging between 1 and 2 eV. The calculated band gap values are found to decrease in the order PSIFCO>PSIFCH>PSIFCF>PSIFCN. PSIFCO is predicted to be the strongest candidate for oxidative (p) doping while PSIFCN is found to have the greatest capacity for reductive (n) doping. The π-bond order data as well as the geometric structures of these polymers show them to have aromaticlike electronic structures. Using the ab initio band structure results of PSIFCO (A)x and PSIFCN (B)x, the electronic density of states of various quasi-one-dimensional compositional superlattices (copolymers) (AmBn)x, belonging to the type-II-staggered class have been determined using the negative-factor counting method, taking multineighbor interactions (up to six neighbors) into account. The trends in the electronic structures and conduction properties of these copolymers as a function of (i) block sizes m and n, (ii) composition (m/n), and (iii) arrangement of blocks (periodic or aperiodic) in the copolymer chain are discussed. The results are important guidelines for molecular designing of copolymers with tailor-made conduction properties.