Organic soluble indigoids derived from 3-hydroxybenzaldehyde for N-type organic field-effect transistor (OFET) applications

Two new series of organic soluble indigoids 7-7′-dialkoxyindigoids (2a, 2b) and 4,4′-dibromo-7,7′-dialkoxyindigoids (3a, 3b) (alkoxy = n-butoxy and n-octyloxy) were synthesized starting from the inexpensive 3-hydroxybenzaldehyde. The indigoids were soluble in common organic solvents including chloroform, dichloromethane, toluene, ethyl acetate and ethers. The enhanced solubility was suggested to be a lack of intermolecular hydrogen-bonds as confirmed by single crystal X-ray diffraction analyses. It was found that intramolecular hydrogen-bonds in indigoids are crucial to the exhibition of field-effect in OFETs, while intermolecular hydrogen-bonds only caused insolubility of the indigoids. Compared to the pristine insoluble indigo (LUMO = -3.55 eV and E_g = 1.91 eV), the soluble indigoids containing electron donating alkoxy side chains at the indigoid 7 and 7′ positions were shown to have their LUMO decreased by -0.13 to -0.26 eV. Among the indigoid studied, the soluble indigoid 3a containing electron donating alkoxy side chains at the indigoid 7 and 7′ positions and bromine groups at the indigoid 4 and 4′ positions exihibited a narrowest bandgap energy with E_g = 1.66 eV. Employing the same fabrication technique and a bottom-gate-top-contact OFET configuration, the soluble indigoids were found to have electron mobility similar to and within an order of magnitude of the pristine indigo.