Pyridine‐Linked Diarylmaleimide Dimers Bearing Thiophene Chains: Optical and Proton‐Responsive Properties

Diarylmaleimides are promising organic dyes owing to their structural simplicity and the tunability of their electronic structures through aryl substitution. Herein, we designed and synthesized pyridine‐linked diarylmaleimide dimers 1 – 3 bearing thiophene chains. The pyridine unit imparts proton responsiveness, whereas the length of the thiophene chains controls the electron‐donating ability. To evaluate the effect of the number of maleimide frameworks, h1 , a half‐structure of 1 , was also synthesized. Comparison of the absorption and fluorescence spectra of 1 – 3 with those of h1 revealed that the maleimide dimer framework enhances light‐harvesting efficiency and induces larger Stokes shifts. Extending the π‐conjugation through thiophene rings led to redshifted absorption and fluorescence bands, accompanied by increased molar absorption coefficients. As part of their proton‐responsive behavior, acid titration induced larger absorption spectral changes in 1 – 3 than those observed for h1 . Notably, the absorption band of protonated 3 extended into the near‐infrared region. According to 1 H NMR analysis and density functional theory (DFT) calculations, 1 exhibited structure‐specific proton responsiveness, involving a conformation in which both the pyridine nitrogen atom and an imide carbonyl oxygen atom participate in proton capture. Additionally, 1 – 3 displayed viscosity‐dependent fluorescence arising from the restriction of molecular motions.