Synthesis, structures, and properties of two isomeric naphthodithiophenes and their methyl, methylthio, and 2-thienyl derivatives; application to conductive charge-transfer complexes and low-bandgap polymers

Two isomeric naphthodithiophenes (syn-NDT and anti-NDT), isoelectronic with pyrene, as well as their dimethyl, bis(methylthio), and di(2-thienyl) derivatives have been successfully synthesized as novel perifused heteroaromatics. Among them, the parent syn-NDT and the di(2-thienyl) derivative of anti-NDT were examined by X-ray crystallographic analyses, revealing that the molecular structures comprise fairly planar, but considerably strained skeletal rings, and their crystal structures are characterized by a herringbone arrangement of uniform stacking columns. The UV/vis spectra of these heteroaromatics show considerable red-shifts of the π-π* transitions as compared to that of pyrene, making them chromogens ranging from yellow to purple. Cyclic voltammetry indicates that they have much stronger electron donating abilities than pyrene. Consequently, all of them, except for the methyl derivatives, can form highly conductive molecular complexes with iodine and DDQ. In addition, the electrochemical oxidation of the parent naphthodithiophenes and the di(2-thienyl) derivatives leads to the formation of conductive polymers, which show low HOMO-LUMO bandgaps upon undoping. In particular, both polymers of syn-NDT and anti-NDT have very low bandgaps of ca. 0.8 eV (77 kJ mol^-1).