Excited-State Intramolecular Proton Transfer (ESIPT) Based Metal-Organic Supramolecular Optical Materials: Energy Transfer Mechanism and Luminescence Regulation Strategy

During the past few years, excited-state intramolecular proton transfer (ESIPT) has attracted great attention in the field of metal-organic optical materials due to their rich photophysical properties. Generally, the ESIPT process includes unique four-leveled photocycle and concomitant multiemissions, leading to complex luminescence mechanisms and variable luminescence phenomena. In contrast with the widely reported research on pure organic photoluminescent molecules, with the aid of modern techniques such as in situ X-ray diffraction and transient photophysical study, metal-organic supramolecular optical materials are emerging in recent years, which have adjustable frame structures, diverse emission types, and excellent optical performance. Through regulating the equilibrium and transformation relationship of various photophysical processes in the ESIPT excited-state (nonradiative transitions, radiative transitions, energy transfer, charge transfer, etc.), ESIPT-based metal-organic supramolecular optical materials demonstrate rich luminescence mechanisms and wide applications in displaying, sensing, imaging, lasing, etc. Given that the deep understanding of the properties of ESIPT metal-organic supramolecular materials is still in its infancy, numerous new strategies for regulating luminescence mechanisms need to be established urgently. In this short Account, we describe a construction and luminescence regulation system for ESIPT metal-organic supramolecular materials, including four types of energy levels conversion, corresponding photoluminescence (PL) regulation strategies, and potential application demonstrations.Based on our recent work and related reports from other groups, this Account proposes four strategies to synthesize new ESIPT metal-organic supramolecular materials and regulate their PL performance. Namely, the strategies can be described as ESIPT-process-directed enol/keto emission regulation strategy, ISC (intersystem crossing)/RISC (reverse intersystem crossing)-process-directed fluorescence/persistent luminescence regulation strategy, metal-centered (MC) emission regulation strategy, and monomer/excimer emission regulation strategy, respectively. It should be noted that the first two strategies can be implemented in ESIPT pure organic small molecule materials, and they are enhanced and optimized in metal-organic materials based on ligand-centered (LC) emission, while the last two are unique to supramolecular systems. We will shed light on the equilibrium and transformation relationship between various photophysical processes regulated by the ESIPT process.Collectively, the above approaches and strategies that we propose for the construction and luminescence regulation of ESIPT metal-organic supramolecular materials will be illustrated by the basic energy transfer mechanism understanding and specific examples (for elaboration on the related mechanism strategies, some organic examples will also be included) in this Account. We anticipate that the principles can be used for a better understanding and utilization of ESIPT processes to construct new ESIPT optical materials. And the potential photophysical and photoluminescent applications will be further envisaged, paving the way for the future design and application of more advanced optical materials.