Intramolecular Stabilization of Naphtho[2,1-b:3,4-b']dithiophenes: Synthesis and Analysis of Imine-Based Chromophores

The role of noncovalent interactions in stabilizing and organizing complex structures throughout nature is indisputable. Of the various classes of noncovalent interactions, those that involve secondary bonding - attractive interactions between σ-hole and nucleophile - are of interest in the design of materials due to their strength and programmability. This report takes the approach of placing nucleophilic imines in close proximity to fused thiophene moieties within naphtho[2,1-b:3,4-b']dithiophene (α NDT) cores, where an intramolecular N···S interaction is poised to yield rigid chromophores. These types of intramolecular N···S interactions have been observed in the solid-state for several decades, but their solution-state analysis remains rare. Here we detail how crystallography, ¹H/¹³C NMR spectroscopy, and molecular modeling work synergistically to describe the strength and impact of intramolecular N···S interactions on α NDT chromophores α(1)₂. The remote substituents on the aryl amines (1) employed as condensation partners have minimal structural impact on the α(1)₂ series, but the photophysical properties of strongly electron-deficient (1d and 1dd) or polarizing (1c and 1cc) end-caps are enhanced in comparison to their neutral (1a) and weak (1b and 1bb) counterparts. This design strategy to incorporate intramolecular N···S interactions highlights how NDTs can be incorporated into complex architectures in a programmable manner.