Study on structure-property relationship of isoindigo and thienoisoindigo : synthesis, characterization and application for organic field-effect transistors

Abstract

Over the past decades, numerous lactam functionalized organic compounds have been synthesized for their application in optoelectronic devices to show promising advantages

stabilized LUMOs, broad absorption range, planar structure of lactam moiety, and so on. Among others, isoindigo (IID) and thienoisoindigo (TIID) derivatives showed remarkable performance as donor materials in organic solar cell devices and organic field-effect transistors. However, the structure-property relationship between IID and TIID-based molecules has rarely been explored in spite of their structural similarity. Characteristic twisted geometry comprising carbonyl oxygen and phenyl hydrogen in IID significantly hampers effective π-π interaction between IID molecules. Recently, with a view to overcome such an undesirable limit in extending π-conjugation, thienoisoindigo moiety (TIID) has newly been developed. Replacing phenyl ring with thiophene ring is believed to reduce the steric hindrance and thus enhance the planarity along the backbone. In this work, new series of IID and TIID-based small molecules and D-A type polymers have been synthesized to explore the structure-property correlation in them with special focus on their applications in OFET. To compare the fundamental properties of IID and TIID,core and brominated core of them were synthesized and characterized using UV/VIS, CV, IR and DFT calculation. It was clearly found that TIID was more planar in structure to have broader absorption range and lower bandgap energy. Furthermore, relatively larger electron donating property of thiophene ring in TIID resulted in its high-lying HOMO compared to that in IID. FT-IR spectra has also supported S∙∙∙O secondary bonding interaction for planarization of TIID backbone. Aiming at developing a new class of n-type OFET molecules, strong electron withdrawing dicyanovinyl units were introduced into the IID and TIID core structures. Structure-property relationship was investigated in terms of their optical and electrochemical properties measured by UV-VIS, CV, and DFT calculation. Top-contact bottom-gate type OFET devices were fabricated using vacuum deposition method. It was found that the IID based devices, EHIIDPDCV and EHIIDTDCV, showed decent n-type performance

maxima electron mobility were 1.70×10-3 cm2V-1s-1 and 1.83×10-2 cm2V-1s-1, respectively. On the other hand, novel class of D-A type polymers were designed and synthesized using IID and TIID as an effective acceptor(A) unit. From UV/VIS and CV data, D-A polymers were shown to have low bandgap energy and appropriate frontier MO levels for OFET application. DFT calculation suggested that TIID-based D-A polymer would have much more planar structure through polymer backbone compared to that in IID-based polymer. Solution-processed OFET devices were fabricated using 0.3wt% chloroform solution for each polymer. Both polymers were shown to have ambipolar charge transport ability. While IID-based D-A copolymer showed maxima mobilities of μe =3.09×10-4cm-1V-1s-1 and μh =3.04×10-2cm-1V-1s-1, TIID-based D-A copolymer showed more balanced ambipolar performances with maximum mobility of μe =4.44×10-3cm-1V-1s-1 and μh =1.14×10-2cm-1V-1s-1.