Ladder-Type Polyenazine Based on Intramolecular S ... N Interactions: A Theoretical Study of a Small-Bandgap Polymer

Ladder-type polyenazine (LPEA, 2), a nitrogen-containing polymer analogous to polyacetylene (PA), is proposed, based on molecule I synthesized and characterized earlier.(8) By using density functional theory calculations, we show that the ladder-type structure of the proposed polymer arises from the intramolecular S center dot center dot center dot N interactions, which reduce the bond length alternation of the polymer and hence lead to a significantly narrower bandgap compared to the parent PA. A sigma-conjugated pathway is found along the center dot center dot center dot S-S center dot center dot center dot N-N center dot center dot center dot S-S center dot center dot center dot linkage due to the noncovalent S center dot center dot center dot N interactions and NN through-space interactions. The theoretically predicted electron affinity of the proposed LPEA lies in the range for n-type semiconductors. We examine the energy barrier from the equilibrium geometry to the geometry of uniform bond length pattern and find that the estimated energy barrier is very similar to that of the parent PA.