The structure–property relationship study of electron-deficient dihydroindeno[2,1-b]fluorene derivatives for n-type organic field effect transistors

Abstract

A bridged syn triphenylene derivative, namely 5,7-dihydroindeno[2,1-b]fluorene, functionalized with dicyanovinylene units (2,1-b)-IF(C(CN)₂)₂ has been designed, synthesized and characterized. Its optical and electrochemical properties have been carefully studied through a combined experimental and theoretical approach and compared to those of three other structurally related dihydro[2,1-b]indenofluorene derivatives bearing methylenes, (2,1-b)-IF, carbonyls, (2,1-b)-IF(O)₂, or both carbonyl and dicyanovinylene, (2,1-b)-IF(O)(C(CN)₂) on the bridgeheads. (2,1-b)-IF(C(CN)₂)₂, which possesses a very low LUMO level, ca. −3.81 eV, has been successfully used as an active layer in n-channel OFETs using an epoxy based photoresist SU-8 as the gate insulator. (2,1-b)-IF(C(CN)₂)₂ based n-channel OFETs show promising properties such as a low threshold voltage functioning of 7.2 V (low gate–source and drain–source voltages), a high ratio between the on and the off currents (6.3 × 10⁵), interesting subthreshold swing (SS = 2.16) and electron mobility (>10⁻³ cm² V⁻¹ s⁻¹) and excellent stability under electrical stress. This electrical stability has allowed the incorporation of (2,1-b)-IF(C(CN)₂)₂ based n-channel OFETs in an integrated circuit. Thus, as a proof of concept, pseudo CMOS inverters made of n-type (2,1-b)-IF(C(CN)₂)₂-based OFETs have been fabricated and characterized highlighting the potential of this new family of materials.