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

The role of non-covalent interactions in stabilizing and organizing complex structures throughout nature is indisputable. Of the various classes of non-covalent 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. With the rise in mate-rials containing chalcogen atoms ever-expanding, it is of interest to probe how intramolecular interactions between nucleophilic groups and chalcogen atoms can be used to impart specific function or enhance desired properties. This report takes the simplistic approach of placing nucleophilic imines in close proximity to fused thiophene moieties within naphtho[2,1-b:3,4-b’]dithiophene (ɑ NDT) cores, wherein an intramolecular N···S interaction is poised to yield rigid chromophores. These types of N···S interactions have been observed in the solid-state for several decades, but their solution-state analysis remains unexplored, and is absent from the perspective of the photoactive bent naphthodithiophenes. Herein we detail how blending 1H/13C NMR spectroscopy, X-ray crystallography, and molecular modelling are used synergistically to describe the strength and impact of intramolecular N⋅⋅⋅S interactions on imine-functionalized ɑ NDT chromophores, ɑ(1)2. The remote substituents on the aryl amines employed as condensation partners have minimal structural impact on the N···S; 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, together with the methods employed to characterize and analyze the intramolecular N···S interactions, reveal how NDTs can be incorporated into complex architectures in a programmable manner.