Synthesis and Characterization of Thieno[3,4-b]pyrazine-Based Terthienyls: Tunable Precursors for Low Band Gap Conjugated Materials

Abstract

Synthetic methods have been developed for the preparation of new 2,3-dihalo- and 2,3-ditriflato-5,7-bis(2-thienyl)thieno[3,4-b]pyrazines. From these reactive intermediates, a variety of new 2,3-difunctionalized 5,7-bis(2-thienyl)thieno[3,4-b]pyrazines have been produced as precursors to conjugated materials. Structural, electronic, and optical characterization of these new analogues illustrate the extent to which the electronic nature of the functional groups can be used to tune the electronic properties of these thieno[3,4-b]pyrazine-based terthienyl units.

Synthesis, Characterization, and Electropolymerization of Extended Fused-Ring Thieno[3,4-b]pyrazine-Based Terthienyls

“…The absorption properties of TP-based terthienyls are characterized by a broad, low energy transition, which is formally assigned as an ICT band [55]. This assignment is consistent with the known properties of monomeric TPs, for which previous photophysical studies have revealed that the lowest energy absorption is a broad ICT band resulting from a transition between a predominately thiophene-localized HOMO and a LUMO of greater pyrazine contribution [33,34].…”

“…These are the same methods previously reported for a variety of traditional TP-based terthienyls [23,55,56], as well as various extended fused-ring analogues [23,45,50,52], and involve the initial Stille cross-coupling of the common precursor 2,5-dibromo-3,4-dinitrothiophene ( 8 ) [57] with 2-(tributylstannyl)thiophene to generate 3′,4′-dinitro-2,2′:5′,2″-terthiophene ( 9 ) [55]. The critical step in this synthetic route is then reduction of 9 to the corresponding diamine 10 .…”

“…The reported reduction of 9 has been accomplished via a number of different reducing agents, including iron powder in AcOH [58,59], SnCl 2 in HCl [45,51,56], and tin metal in HCl [55]. Although all of these conditions will successfully reduce 9 , there are significant differences in the resulting product purity.…”

“…The extended fused-ring TP is generally incorporated into these materials as a TP-based terthienyl, which is then copolymerized with additional monomeric species (as illustrated in polymers P3 and P6 – P8 ). In addition to providing a structural motif that accommodates the increased size of the extended fused-ring TP, many favor TP-based terthienyls over the somewhat reactive nature of monomeric TPs [26,55]. Although the increased conjugation length of these TP-based terthienyls actually cause them to undergo oxidation at lower potentials than the analogous TP monomers, the combination of increased size and longer conjugation length results in slower reactivity and thus less production of unwanted byproducts via oxidative coupling or decomposition [26,55].…”

“…In addition to providing a structural motif that accommodates the increased size of the extended fused-ring TP, many favor TP-based terthienyls over the somewhat reactive nature of monomeric TPs [26,55]. Although the increased conjugation length of these TP-based terthienyls actually cause them to undergo oxidation at lower potentials than the analogous TP monomers, the combination of increased size and longer conjugation length results in slower reactivity and thus less production of unwanted byproducts via oxidative coupling or decomposition [26,55]. Considering the extensive use of the TP-based terthienyl motif in the materials shown in Figure 3, it is somewhat surprising that most have overly complicated polymeric structures.…”