Dithienopyrrole-quinoxaline/pyridopyrazine donor-acceptor polymers: synthesis and electrochemical, optical, charge-transport, and photovoltaic properties

Soluble alternating copolymers of N-(3,4,5-tri-n-dodecyloxyphenyl)dithieno[3,2-b:2',3'-d]pyrrole donor groups and 2,3-di-n-decylquinoxaline, 2,3-di-n-decylpyrido[3,4-b]pyrazine, 2,3,6,7-tetrakis(n-decyloxy)benzo[a,c]phenazine, or 2,3,6,7-tetrakis(n-decyloxy) dibenzo[f,h]pyrido[4,3-b]quinoxaline acceptors were synthesised using Stille coupling reactions. Experimental absorption maxima in THF range from 645 to 770 nm. These optical data, along with the results of quantum-chemical calculations and electrochemical measurements, show that, as expected, the pyridopyrazine moiety acts as a stronger acceptor than quinoxaline and that the extended species benzophenazine and dibenzopyridoquinoxaline are stronger acceptors than quinoxaline and pyridopyrazine, respectively. Modest average hole mobilities of up to ca. 3.0 x 10(-4) cm(2) V-1 s(-1) were obtained in field-effect transistors. Bulk heterojunction photovoltaic devices made from blends of the benzo[a,c]phenazine-based polymer with 3'-phenyl-3'H-cyclopropa[1,9](C-60-I-h)[5,6]fullerene-3'-butanoic acid methyl ester (1 : 3 weight ratio) exhibited average power conversion efficiencies up to 1.4%.