Quantum Chemical Studies on Electron-Accepting Overcrowded Ethylene with a Polarizable Skeleton

We report the quantum chemical studies on the neutral and radical anion forms of an electron-accepting overcrowded ethylene (OCE1) featuring a highly polarizable skeleton based on the density functional theory (DFT) approach using the M06-2X hybrid functional. Calculated results indicate that OCE1 (bis{4H,8H-4-(dicyanomethylene)benzo[1,2-c:4,5-c']bis[1,2,5]thiadiazol-8-ylidene}) shows conformational behaviors and energetics similar to those of bianthrone (OCE2), a typical thermochromic overcrowded ethylene. Neutral OCE1 and its radical anion have antifolded (afOCE1) and twisted (tOCE1) isomers on their potential energy surfaces. The calculated isomerization barrier heights of OCE1 and its radical anion are considerably low, indicating that its conformation is susceptible to interactions with surrounding molecules. While two afOCE1 molecules can form a simple π-stacked dimer, tOCE1 tends to be converted to afOCE1 when the two tOCE1 molecules come close together, indicating the instability of tOCE1 in the homogeneous OCE1 solid state. The thermochromic behavior difference between OCE1 and OCE2 in solution is closely associated with the considerably small energy difference between the afOCE1 and the tOCE1 as compared with OCE2. The properties of OCE1 are also compared with other typical electron-accepting overcrowded ethylenes in terms of electronic structure, energetics, and conformational behaviors.