Rationalizing the fluorescence emission enhancement of imidazo[1,5-a]pyridines in the solid state upon hydrogenation

The development of organic materials with high quantum efficiency in the solid state remains a significant challenge, particularly for deep-blue emitters. In this work, we investigate the solid-state photophysical properties of a series of hydrogenated imidazo[1,5-a]pyridine derivatives, which exhibit exceptional deep-blue fluorescence with absolute photoluminescence quantum yields (PLQYs) up to 0.96. This intense emission is highly counterintuitive when compared to their fully conjugated counterparts, which are nearly non-emissive in the crystalline phase. Through a detailed comparative analysis and Hirshfeld surface investigations, we demonstrate that the high efficiency of the hydrogenated species is rooted in their specific crystalline arrangement. While the aromatic precursors are characterized by strong intermolecular hydrogen bonds and H-π interactions that activate non-radiative decay pathways, the hydrogenated derivatives adopt a packing motif dominated by non-directional H-H interactions and intramolecular hydrogen bonds. This structural alteration effectively suppresses detrimental quenching mechanisms, allowing the radiative transition to dominate. We define this phenomenon as packing-induced fluorescence enhancement (PIFE). These results provide critical insights into the rational design of high-efficiency solid-state emitters for optoelectronic applications, such as luminescent down shifting (LDS) layers for photovoltaics and active layers in blue OLEDs.