Nickel-Catalyzed Suzuki Polycondensation for Controlled Synthesis of Ester-Functionalized Conjugated Polymers

Controlled synthesis of conjugated polymers with functional side chains is of great importance, affording well-defined optoelectronic materials possessing enhanced stability and tunability as compared to their alkyl-substituted counterparts. Herein, a chain-growth Suzuki polycondensation of an ester-functionalized thiophene is described using commercially available nickel precatalysts. Model compound studies were used to identify suitable catalysts, and these experiments provided guidance for the polymerization of the ester-substituted monomer. This is the first report of nickel-catalyzed Suzuki cross-coupling for catalyst-transfer polycondensation, and to further illustrate the versatility of this method, block and alternating copolymers with 3-hexyl­thiophene were synthesized. The presented protocol should serve as an entry point into the synthesis of other electron-deficient polymers and donor–acceptor copolymers with controlled molecular weights and low dispersity

Conjugated Polymers with Repeated Sequences of Group 16 Heterocycles Synthesized through Catalyst-Transfer Polycondensation

Periodic π-conjugated polymers of the group 16 heterocycles (furan, thiophene, and selenophene) were synthesized with controlled chain lengths and relatively low dispersities using catalyst-transfer polycondensation. The optical gap and redox potentials of these copolymers were fine-tuned by altering the heterocycle sequence, and atomic force microscopy revealed nanofibrillar morphologies for all the materials. Grazing incidence wide-angle X-ray scattering of the thiophene-selenophene copolymers indicated that the π-stacking distance increased with incorporation of the larger heteroatom (from ∼3.7–4.0 Å), while the lamellar spacing decreased (from ∼15.8–15.2 Å). The study also revealed that periodic sequences allow electronic properties to be tuned while retaining nanofibrillar morphologies similar to those observed for poly­(3-hexylthiophene)

Synthesis of Polyfuran and Thiophene-Furan Alternating Copolymers Using Catalyst-Transfer Polycondensation

There is intense interest in the rational design of semiconducting materials to improve organic electronics. Furan is a particularly attractive monomer for building biorenewable and biodegradable π-conjugated frameworks. In this report, regioregular head-to-tail and head-to-head poly­(3-hexylfuran) were synthesized using chain-growth polycondensation. The resultant polyfurans have relatively low molecular weights but also low dispersities. The head-to-head polyfuran adopted a nearly identical coplanar backbone conformation as its head-to-tail analog in the solid state, as determined by UV–visible spectroscopy and atomic force microscopy. Extensive aggregation of the furan homopolymer during polymerization led to the investigation of an alternating furan-thiophene copolymer, confirming that furyl-based monomers can polymerize in a chain-growth manner. All of the synthesized polymers are sensitive when exposed to both oxygen and light

Systematic Investigation of Benzodithiophene-Benzothiadiazole Isomers for Organic Photovoltaics

Two new donor–acceptor small molecules based on benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene (BDT) and benzo­[<i>c</i>]­[1,2,5]­thiadiazole (BT) were designed and synthesized. Small molecules 4,4′-[(4,8-bis­(5-(2-ethylhexyl)­thiophen-2-yl)­benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene-2,6-diyl)­bis­(2,2′-bithiophene)-5,5′-diyl]­bis­(benzo­[<i>c</i>]­[1,2,5]­thiadiazole) (BDT-TT-BT) and 4,4′-(4,8-bis­(5-(2-ethylhexyl)­thiophen-2-yl)­benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene-2,6-diyl)­bis­[7-(2,2′-bithiophene-5-yl)­benzo­[<i>c</i>]­[1,2,5]­thiadiazole] (BDT-BT-TT) are structural isomers with the 2,2-bithiophene unit placed either between the BDT and BT units or at the end of the BT units. This work is targeted toward finding the effect of structural variation on optoelectronic properties, morphology, and photovoltaic performance. On the basis of theoretical calculations, the molecular geometry and energy levels are different for these two molecules when the position of the 2,2-bithiophene unit is changed. Optical and electrochemical properties of these two small molecules were characterized using UV–vis and cyclic voltammetry. The results showed that BDT-BT-TT has broader absorption and an elevated HOMO energy level when compared with those of BDT-TT-BT. The performance of these two isomers in solar cell devices was tested by blending with [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM). Power conversion efficiencies as high as 3.22 and 3.71% were obtained in conventional solar cell structures for BDT-TT-BT and BDT-BT-TT, respectively. The morphology was studied using grazing incident wide-angle X-ray scattering and transmission electron microscopy, which revealed different phase separations of these two molecules when blended with PC₇₁BM