Investigating the Optical Properties of Thiophene Additions to s-Indacene Donors with Diketopyrrolopyrrole, Isoindigo, and Thienothiophene Acceptors

New donor-acceptor light harvesting polymers with either a 4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene or a 4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithienothiophene donor subunit and either a 2-(nonadecan-9-yl)-5-(2-octyldodecyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione, (E)-1,1′-bis(2-octyldodecyl)-[3,3′-biindolinylidene]-2,2′-dione, or 3-ethyl-1-(thieno[3,4-b]thiophen-2-yl)heptan-1-one acceptor were synthesized. In this study, the effects of donor conjugation length and donor-acceptor combination on the optical properties of the synthesized polymers were investigated using steady state, up-conversion, quantum chemical simulations and ultrafast and transient spectroscopic techniques. At high energies, it was observed that polymers with extended donor conjugation had significantly enhanced absorption, whereas those with stronger acceptors showed higher extinction coefficients at lower energies. Fluorescence up-conversion experiments revealed that the donor conjugation length increased the fluorescence lifetimes for the extended conjugated polymers. Two-photon absorption experiments and quantum chemical simulations showed that polymers with stronger acceptors exhibited enhanced charge-transfer characteristics. Transient absorption investigations revealed new optical species for the extended donor polymers. This study has shown the considerable effect that donor conjugation has on the optical properties of donor-acceptor light harvesting conjugated polymers. The results from this work can be used in the design and synthesis of new donor-acceptor polymers for organic photovoltaics.