Versatile Direct (Hetero)Arylation Polymerization of Electro-Deficient Unsubstituted Thiazolo[5,4-d]Thiazole: A Tool to Lower the LUMO Level.

A series of novel conjugated semiconducting polymers based on the unsubstituted thiazolo[5,4-d]thiazole (TzTz) unit is synthesized using atom-economic and environmentally friendly direct (hetero)arylation polymerization (DHAP). The versatility of the proposed polymerization conditions, employing a non-chlorinated and moderately toxic solvent and cooperative palladium/copper bimetallic catalytic system, is demonstrated through the use of seven comonomers with varying electron-withdrawing strength: 2,2'-bithiophene (BT), 6,7-difluoroquinoxaline (Qx), thieno[3,4-c]pyrrole-4,6(5H)-dione (TPD), 5,6-difluorobenzo[c][1,2,5]thiadiazole (BTD), isoindigo (IID), para-azaquinodimethane (AQM) and 2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP). The resulting TzTz-based copolymers exhibit optical bandgaps between 1.5 and 2.0 eV with HOMO/LUMO energy levels spanning from -5.2/-3.3 eV to -5.4/-3.9 eV. They all show satisfactory thermal stability for electronic applications (Td = 300-360 °C). Notably, TzTz-based copolymers are observed they generally exhibit improved backbone planarity and deeper LUMO levels than their thiophene derivatives. A synthesis tool to finely lower the LUMO levels of next-generation A-A' copolymers in view of increasing the performance and air-stability of doped organic electronics is believed to be provided by this work.