Solvent- and Proton-Responsive Optical Properties of Donor–Acceptor-Type Thieno[2,3- <i>h</i> ]isoquinoline-Based Aromatic Imides

Thieno[2,3-<i>h</i>]isoquinoline-based aromatic imide was designed as a fundamental scaffold to modify the electronic structure of aromatic imides. Utilizing the reactivity of the thiophene unit, methoxyphenyl and <i>N</i>,<i>N</i>-dimethylaminophenyl groups were introduced into the scaffold (<b>1</b> and <b>2</b>, respectively), and their optical and electrochemical properties were investigated. <b>1</b> and <b>2</b> exhibited solvent-dependent fluorescence, showing linear correlations between their emission maxima and the solvent polarity parameter <i>E</i>_T(30). <b>2</b> displayed greater sensitivity to solvent polarity than <b>1</b>, owing to the stronger electron-donating nature of the dimethylamino group relative to the methoxy group. Protonation of the pyridine units in <b>1</b> and <b>2</b> led to the formation of pyridinium ions, resulting in red-shifted absorption bands. Further protonation of the dimethylamino group in <b>2</b> disrupted the donor-acceptor electronic structure, leading to blue-shifted absorption and fluorescence bands. These findings highlight the potential of the thieno[2,3-<i>h</i>]isoquinoline-based aromatic imide scaffold as a versatile platform for modulating the electronic structure of the aromatic imides via diverse aryl substitutions and protonation.