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
(Eg), 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.