Methylthiolation Modifed Bay‐Annulated Indigo Derivatives for Enhanced Performance in Organic Field‐Effect Transistors

Indigo is one of the most well‐known natural dyes and has attracted significant research interest due to its low cost and exceptional stability. Notably, bay‐annulated indigo (BAI) has been reported as an effective electron acceptor and is widely used in various applications. Herein, we successfully synthesize a methylthio‐substituted BAI derivative, compound 1 and investigate the impact of methylthio substitution on its optoelectronic properties. UV‐vis absorption and fluorescence spectra reveal that compound 1 displays a significant red shift compared to the non‐methylthio‐substituted compound 2. Cyclic voltammetry measurements and DFT calculations indicate that compound 1 has a narrower HOMO‐LUMO gap, demonstrating the prominent influence of methylthio side chains in modulating molecular electronic properties. Importantly, the organic field‐effect transistor (OFET) device based on compound 1 exhibits a hole mobility 3.5 times higher than that of the non‐methylthio‐substituted compound 2. Furthermore, atomic force microscopy (AFM) characterization reveals the formation of needle‐like crystallites in the compound 1 film after annealing, whereas compound 2 forms an amorphous thin film. These results suggest that methylthiolation is an effective strategy for tuning intermolecular interactions in novel BAI derivatives, and compound 1 is a promising hole‐transporting material.