Special Organic Chemistry Seminar
Isoindigo (II) is an electron-accepting unit widely used as building block of donor-acceptor (D-A) type organic semiconductors.1 Thienoisoindigo (TII) is the analogue of II with higher planarity than II.2 We focus on planar TII unit, and synthesized TII- and II-based small molecules end-capping with benzothiophene or benzofuran units in order to study the influence of molecular planarity on device performance in field effect transistors (FETs) and photovoltaics (PVs). The results indicate that TII-based molecules TII(BTh)2 and TII(BFu)2 are highly one-dimensional organic semiconductors exhibiting well-balanced ambipolar FET performance, in which both hole and electron transport are possible, and moderate OPV performance.
In order to achieve two-dimensional conduction, we have developed conjugated polymers based on TII and thiophene-flanked diketopyrrolopyrrole (TDPP) units having either branched-alkyl or siloxane-terminated alkyl solubilizing groups, and investigated how the polymer backbones combined with the side chains influence the optical, electrochemical, and carrier transport properties as well as the solid state packing. Of particular interest is that the TII-based polymers PTII-SiC11 and PTII-BO absorb near infrared wavelengths and have extremely narrow optical bandgaps below 0.6 eV, originating from the unique property of the TII unit.
Next, we have designed and synthesized medium-sized TII molecules to clarify the origin of ultra-narrow bandgap of TII-based polymers. The optical and electrochemical results are basically consistent with the estimation of theoretical calculation. We consider that the present low bandgap polymers are beneficial for realizing ambipolar property, and then this low bandgap enables to make p- and n-type doped conductive polymers preferable for organic thermoelectric devices (TEs). Finally, we demonstrate the potential of diazaisoindigo derivatives as a new electron-accepting building block in organic electronics.
References
1) R. Stalder, J. Mei, K. R. Graham, L. A. Estrada, and J. R. Reynold, Chem. Mater., 26, 664-678 (2014).
2) R. S. Ashraf, A. J. Kronemeijer, D. I. James, H. Sirringhaus, and I. McCulloch, Chem. Commn., 48, 3939-3941 (2012).