The effect of the donor unit on the optical properties of polymers

Abstract

A series of donor–acceptor–donor (DAD) type π-conjugated monomers containing 10,11,12,13-tetrahydrodibenzo[ a, c]phenazine moiety in the backbone as the acceptor unit and thiophene derivatives as the donor unit were prepared by Stille coupling. Thiophene, 3-hexyl thiophene and 3,4-ethylenedioxythiophene were used as the donor units to understand the effects of donor unit on electrochemical and optoelectronic properties of the resulting polymers. Electroactivity of monomers and redox behavior of their polymers were investigated by cyclic voltammetry. The presence of the strong electron-donating ethylenedioxy groups on the aromatic structure increased electron density on 2,7-bis(2,3-dihydrothieno[3,4- b][1,4]dioxin-5-yl)-10,11,12,13-tetrahydrodibenzo[ a, c]phenazine (DTBP), thus the oxidation potential of DTBP shifts to a lower value than that of 2,7-bis(4-hexylthiophen-2-yl)-10,11,12,13-tetrahydrodibenzo[ a, c] phenazine (HTBP) and 2,7-di(thiophen-2-yl)-10,11,12,13-tetrahydrodibenzo[ a, c]phenazine (TTBP). The electronic band gaps, defined as the onset of the π–π ∗ transition, were found to be 2.5 eV for HTBP, 2.0 eV for DTBP and 2.2 eV for TTBP. Both of PDTBP and PTTBP films showed multi-color electrochromism and also fast switching times (<1 s) in the visible and NIR regions.