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

TL;DR

This paper explores how noncovalent N···S interactions stabilize naphtho[2,1-b:3,4-b′]dithiophene chromophores, enhancing their photophysical properties.

Contribution

The study introduces a programmable design strategy using intramolecular N···S interactions in NDT chromophores.

Findings

Intramolecular N···S interactions in α NDT chromophores are confirmed using crystallography and NMR.

Electron-deficient and polarizing end-caps enhance photophysical properties compared to neutral ones.

Remote substituents on aryl amines have minimal structural impact on the α(1)2 series.

Abstract

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.

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