Role of the Comonomeric Units in Reaching Linear Backbone, High Solid-State Order and Charge Mobilities in Heptacyclic Arene-Based Alternating Copolymers

Abstract

Unpredictable trends of charge mobility (mu) in multicyclic heteroarenes-contained polymers remain an obstacle in designing high performance polymers used in polymeric field effect transistors (PFETs). The roles of the comonomeric units in reaching high hole mobility (mu(h)) of copolymers containing a heptacyclic arene unit, dithienocyclopentacarbazole (DTCC) were investigated in this study. A series of four DTCC-based alternating copolymers, PDTCC-1T, PDTCC-3T, PDTCC-BDT, and PDTCC-TT, were synthesized from the Pd-catalyzed copolymerizations between DTCC and comonomeric units including thioene (1T), terthiophene (3T), benzodithiophene (BDT) and thienothiophene (TT) units. Among the four DTCC-based alternating copolymers, highest mobility of 1.36 X 10(-2) cm(2) s(-1) was reached in PDTCC-3T. Optoelectronic and 2D-WAXD studies revealed that strong electronic interaction and highly ordered solid-state structure were only observed in PDTCC-3T. It is attributed to the combination of two axisymmertric units, DTCC and 3T, linearized the polymer backbone, leading to a compact solid-state packing and high mu(h), while centrosymmertic comonomeric units, BDT and TT curved the polymer backbones of PDTCC-BDT and PDTCC-TT, which decreases solid-state order and mu(h). Furthermore, the short axisymmeritric 1T although results in a linear backbone of PDTCC-1T, comparing to 3T, it is too short to effectively reduce interchain steric hindrance caused by the solubilizing octyl chains on DTCC. Thus, effective pi-pi stacking is hindered in PDTCC-1T, resulting in low mu(h). The macroscopic performances of mu(h)s agreed well with the optoelectronic and 2D-WAXD studies. It is concluded that the linear backbone of DTCC copolymers is the prerequisite to reach ordered solid-state packing, which facilitate effective charge transport, and it depends on the prudent choose on the symmetry of the comonomeric unit. In addition, balance between the solubility and adequate crowdedness of lateral side chains is essential for a PFET material to be not only easily processed, but also effectively pi-pi stack.