Synthesis and characterization of pendant phenyl ester-substituted thiophene based copolymers

In an attempt to synthesize low band gap copolymers, three new monomers 3-ethyl-2-(2,5-dibromothiophene)-3-ylacetate (ETA), 3-phenylethyl-2-(2,5-dibromothiophene)-3-ylacetate (PTA), and 3,3-diphenylpropyl-2-(2,5-dibromothiophene)-3-ylacetate (DPTA) were synthesized. ETA, PTA, and DPTA were copolymerized with 2,5-bis(trimethylstannyl)thiophene to yield poly [3-ethyl 2-(thiophene-3-yl)acetate-2,2′-thiophene] (PETAT), poly[3-phenylethyl 2-(thiophene-3-yl)acetate-2,2′-thiophene] (PPTAT) and poly[3,3-diphenylpropyl 2-(thiophene-3-yl)acetate-2,2′-thiophene] (PDPTAT) respectively. Additionally, these mildly electron-donating monomers as well as 2,5-bis(trimethylstannyl)thiophene were incorporated with the electron withdrawing monomer, 4,7, dibromobenzo[c]-1,2,5-thiadiazole (BT), to develop three push-pull polymeric systems; poly[3-ethyl 2-(thiophene-3-yl)acetate-2,2′-thiophene-2,6-diyl-alt-2,1,3-benzothiadiazole-4,7-diyl] (PETATBT), poly[3-phenylethyl 2-(thiophene-3-yl)acetate-2,2′-thiophene-2,6-diyl-alt-2,1,3-benzothiadiazole-4,7-diyl] (PPTATBT), and poly[3,3-diphenylpropyl 2-(thiophene-3-yl)acetate-2,2′-thiophene-2,6-diyl-alt-2,1,3-benzothiadiazole-4,7-diyl] (PDPTATBT). Copolymers PETAT, PPTAT, and PDPTAT exhibit band gaps from 1.94–1.92 eV respectively, where as PETATBT, PPTATBT, and PDPTATBT show a reduction in the band gaps to 1.66–1.48 eV. The impact of the phenyl moiety was investigated using atomic force microscopy; increased order within films of PPTAT and PPTATBT can be attributed to the increased π-π stacking in the solid state from the pendant phenyl moiety.