Solvent- and Proton-Responsive Optical Properties of Donor-Acceptor-Type Thieno[2,3-h]isoquinoline-Based Aromatic Imides.

Abstract

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