Ultrastrong Electron‐Phonon Coupling in Uranium‐Organic Frameworks Leading to Inverse Luminescence Temperature Dependence

Abstract

Electron‐phonon interactions, crucial in condensed matter, are rarely seen in Metal‐Organic Frameworks (MOFs). Detecting these interactions typically involves analyzing luminescence in lanthanide‐ or actinide‐based compounds. Prior studies on Ln‐ and Ac‐based MOFs at high temperatures revealed additional peaks, but these were too faint for thorough analysis. In our research, we fabricated a high‐quality, crystalline uranium‐based MOF (KIT‐U‐1) thin film using a layer‐by‐layer method. Under UV light, this film showed two distinct "hot bands," indicating a strong electron‐phonon interaction. At 77 K, these bands were absent, but at 300 K, a new emission band appeared with half the intensity of the main luminescence. Surprisingly, a second hot band emerged above 320 K, deviating from previous findings in rare‐earth compounds. We conducted a detailed ab‐initio analysis employing time‐dependent density function theory to understand this unusual behaviour and to identify the lattice vibration responsible for the strong electron‐phonon coupling. The KIT‐U‐1 film's hot‐band emission was then utilized to create a highly sensitive, single‐compound optical thermometer. This underscores the potential of high‐quality MOF thin films in exploiting the unique luminescence of lanthanides and actinides for advanced applications.