Dual-State Emission of Pyrazolyl-Pyrrolo[3,4-b]pyridin-5-ones via Excited-State Intramolecular Proton Transfer (ESIPT): Multicomponent Synthesis and Optical Characterization

Abstract

The dual-state emission (DSE) phenomenon has become crucial for developing dual-state emission luminogens (DSEgens) that exhibit efficient luminescence in both solution and solid states, addressing the limitations of conventional, phase-restricted fluorophores. Compounds exhibiting excited-state intramolecular proton transfer (ESIPT) mechanisms are especially valuable for enhancing emission stability across these states, offering significant potential in optoelectronics, bioimaging, and sensing applications. In this study, we report the synthesis of six new pyrazolyl-pyrrolo[3,4-b]pyridin-5-ones through an Ugi-Zhu-3CR coupled to an aza-Diels-Alder/N-acylation/decarboxylation/dehydration cascade sequence. An x-ray ORTEP confirms unequivocally the structure of one of the synthesized compounds. These ones demonstrate intriguing photophysical properties such as large Stokes shifts (>11,900 cm⁻¹) in solution and robust solid-state emission via Excited-State Intramolecular Proton Transfer (ESIPT) mechanism. Complementary DFT and TD-DFT calculations confirm weak but allowed transitions involving both pyrazole and pyrrolo[3,4-b]pyridin-5-one moieties, in agreement with experimental observations. This work represents the first application of an isocyanide-based multicomponent reaction for DSEgen synthesis, paving the way for innovative advances in the design of organic luminescent materials.