Core-Substituted Pyromellitic Diimides: A Versatile Molecular Scaffold for Tunable Triplet Emission

Arylene diimides represent a versatile class of n-type organic semiconductors, widely recognized for tunable photophysical properties, making them highly relevant across various optoelectronic applications. While their fluorescence can be finely modulated through core substitution, triplet-state emission has received comparatively little attention. This is particularly surprising given the growing field of ambient-organic triplet harvesting materials, such as thermally activated delayed fluorescence and phosphorescent systems, which would greatly benefit from structural modifications to the π-conjugated backbone and core substitution of arylene diimides to achieve the desired properties. Realizing tunable triplet states within a family of molecules is crucial for advancing organic triplet-based materials for applications in lighting, photocatalysis, and beyond. In this context, we present an unprecedented study demonstrating tunable triplet emission in pyromellitic diimides, the smallest member of the arylene diimide family, with an accessible emissive triplet state due to a narrow singlet–triplet energy gap. Herein, we report the synthesis of a series of core-substituted pyromellitic diimides (cPmDIs) using diverse synthetic strategies. Core substitution not only induces a wide spectrum of fluorescence colors but, notably, enables a wide-range phosphorescence spanning across the visible spectrum, depending on the core substituent. This article details the synthesis and photophysical and electrochemical characterization of a library of cPmDIs, supported by theory. Furthermore, we demonstrate the potential of this molecular design in achieving ambient-orange phosphorescence, as exemplified by the thiophenyl-cPmDI derivative, which exhibits triplet emission in the crystalline and film states by minimizing vibrational dissipation. In this regard, we envision that the present study represents a significant step toward the predictive structure–property design of ambient-organic phosphors and triplet harvesting materials.