Molecular Origin of Isomerization Effects on Solid State Structures and Optoelectronic Properties: A Comparative Case Study of Isomerically Pure Dicyanomethylene Substituted Fused Dithiophenes

Introduction of a strong electron-withdrawing dicyanomethylene (-CH-(CN)(2)) group onto a fused bithiophene frame is a useful strategy to convert fused bithiophene derivatives from p-type organic semiconductor materials into n-type materials. Here, through systematic studies of isomerically pure 7-dicyanomethylene-7H-cyclopenta-[1,2-b:4,3-b']dithiophene (1), 4-dicyanomethylene-4H-cyclopenta[2,1-b:3,4-b']dithiophene (2), and 7-dicyanomethylene-7H-cyclopenta[1,2-b:3,4-b']dithiophene (3) as well as their oligomers and polymers, we report that isomerization has the potential to fine-tune the optoelectronic properties of these materials including band gap (E-g), electron affinities (EAs), ionization potentials (IPs), electrochemical polymerization behaviors, and the solid state molecular packing, all of which are important for the performance of semiconductor devices. The monomers of these isomers exhibit noticeable difference in maximum absorption energies; and the oligomers and polymers composed of these monomers exhibit increased band gap difference as predicted by DFT calculation. Furthermore, the isomer 2 exhibits better electrochemical polymerization behavior as well as profound electrochromic switching in the near to middle infrared region. X-ray diffraction and quantum mechanical calculations reveal that the difference of dipole and quadrupole moments in these isomers is likely responsible for the difference in the solid state packing and subsequent polymer assembly.