Methylthiolation Modified 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, a methylthio-substituted BAI derivative, compound 1, is successfully synthesized and the impact of methylthio substitution on its optoelectronic properties is investigated. UV-vis absorption and fluorescence spectra reveal that compound 1 displays a significant redshift compared to the nonmethylthio-substituted compound 2. Cyclic voltammetry measurements and density functional theory calculations indicate that compound 1 has a narrower highest occupied molecular orbital and lowest unoccupied molecular orbital gap, demonstrating the prominent influence of methylthio side chains in modulating molecular electronic properties. Importantly, the organic field-effect transistor device based on compound 1 exhibits a hole mobility 3.5 times higher than that of the nonmethylthio-substituted compound 2. Furthermore, atomic force microscopy 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.