Influence of structure-property relationships of two structural isomers of thiophene-flanked diazaisoindigo on carrier-transport properties

Based on the electron-accepting building block of 7,7'-diazaisosindigo (DAM, two structural isomers of thiophene-flanked diazaisoindigo, 6,6'-substituted 6,6'-T-DAII for full conjugation and 5,5'-substituted 5,5'-T-DAII for cross-conjugation, have been designed and synthesized to study the influence of the connecting positions of the flanking thiophene on the optoelectronic properties, crystal structures, and solid-state microstructures, being well associated with the carrier-transport properties. The results indicate that the 5,5'-substitution on the DAII core of 5,5-T-DAII stabilizes both the HOMO and LUMO levels and reduces the oscillation strength of the low-energy absorption band as compared to the 6,6'-substitution. In the crystals, 6,6'-T-DAII, 5,5'-T-DAII, and 6,6'-dibromo-DAII adopt a flat platform with co-facial slipped pi-pi stacking. The organic field-effect transistors based on 6,6'-T-DAII and 5,5'-T-DAII on a tetratetracontane (TTC) modified substrate exhibit an n-dominant ambipolar performance with the hole and electron mobilities of 10(-3) cm(2) V-1 s(-1). Of particular note is that 5,5'-T-DAII show obviously higher carrier-mobilities compared to 6,6'-T-DAII, suggesting that the cross-conjugation of the 5,5'-connection on the DAII core could be designed to improve the carrier-transport properties for FET applications.