Effect of Donor-Acceptor Substitution on Optoelectronic Properties of Conducting Organic Polymers

The impact of donor-acceptor substitution on optical and electronic properties of conducting polymers was investigated with time-dependent density functional theory (TDDFT). A series of donor-acceptor systems with thiophene, 3,4-ethylenedioxythiophene, and pyrrole as donors and 3,4-difluorothiophene, diketopyrrolopyrrole, 2,1,3-benzothiadiazole, 4-dicyanomethylene-4H-cyclopenta[2,1-b;3,4-b']dithiophene, and indeno[1-2b]-fluorene-6,12-dimalonitrile as acceptors as examples of donor-acceptor systems with increasing donor-acceptor character was studied. Spectral properties were analyzed in terms of differences in ionization potentials and electron affinities of donors and acceptors, charge separations between donors and acceptors in ground and excited states, and electron distribution in the acceptor units. A shift in electron density away from the backbone caused by some of the acceptors correlates with localization of the conduction band on the acceptor. Localization does not correlate with energy level differences between donor and acceptor. Localization results in shift of oscillator strength from the HOMO-LUMO peak to a higher energy feature where the first delocalized orbital acts as the acceptor. The "camel back" absorption with two equally strong peaks that gives rise to green polymers is the intermediate case associated with partial localization of the conduction band. Stronger localization causes the HOMO-LUMO band to almost vanish.