Bent naphthodithiophenes: synthesis and characterization of isomeric fluorophores

Abstract

Thiophene-containing heteroarenes are one of the most well-known classes of π-conjugated building blocks for photoactive molecules. Isomeric naphthodithiophenes (NDTs) are at the forefront of this research area due to their straightforward synthesis and derivatization. Notably, NDT geometries that are bent – such as naphtho[2,1-b:3,4-b′]dithiophene (α-NDT) and naphtho[1,2-b:4,3-b′]dithiophene (β-NDT) – are seldom employed as photoactive small molecules. This report investigates how remote substituents impact the photophysical properties of isomeric α- and β-NDTs. The orientation of the thiophene units plays a critical role in the emission: in the α(OHex)R₂ series conjugation from the end-caps to the NDT core is apparent, while in the β(Oi-Pent)R₂ series minimal change is observed unless strong electron acceptors, such as β(Oi-Pent)(PhCF₃)₂, are employed. This push–pull acceptor–donor–acceptor (A–D–A) fluorophore exhibits positive fluorosolvatochromism that correlates with increasing solvent polarity parameter, E_T(30). In total, these results highlight how remote substituents are able to modulate the emission of isomeric bent NDTs.