Nonconventional Luminophores: Emission Mechanism, Regulation, and Applications.

Abstract

ConspectusNonconventional luminophores, characterized by the absence of extended (hetero)aromatic building blocks and alternating single-double/triple bonds, are composed primarily of electron-rich moieties, such as heteroatoms, double bonds, aliphatic amines, carbonyls, hydroxyls, cyano groups, amides, and their grouped functionalities. These unique structural features, coupled with their intriguing luminescent properties, have garnered significant interest for both fundamental research and promising applications, thus enabling widespread exploration. They generally benefit from abundant resources, simple synthesis, outstanding biocompatibility, and excellent photostability, empowering their potential applications in bioimaging, data storage and encryption, anticounterfeiting, bio- and chemosensing, etc. However, their research is preliminary, and the luminescence mechanisms remain elusive. For diverse systems, proposed conjectures, including tertiary amine oxidation, proton transfer, impurities, hydrogen bonding, and peptide bond electron delocalization, lack consistent correlation and universality, with some being subsequently invalidated. This lack of a unifying framework has hampered the development of effective guidelines for molecular design and photoluminescence (PL) regulation. To address these issues, a clustering-triggered emission (CTE) mechanism, focusing on the electron-molecule-aggregate multilevel structure-activity relationships, has been proposed. Specifically, it identifies the "clustered chromophores" of electron-rich moieties as emissive species. The CTE mechanism not only elucidates the emission behaviors of diverse nonconventional luminophores but also guides the PL regulation and further development of novel multifunctional luminescent materials.This Account begins with a concise introduction to the proposed CTE mechanism, highlighting the significance of electron delocalization (through-space conjugation) within the "clustered chromophores" of electron-rich groups. It then delves into insights gained from various nonconventional luminescent systems, identifying three core components of the CTE mechanism: electron-rich moieties, their clustering, and the conformational rigidity of the resulting clusters. The CTE mechanism proves to be rational and universally applicable, encompassing natural products, (macro)biomolecules, and synthetic compounds and extending from singlet fluorescence to triplet phosphorescence. By strategically coordinating these elements, it is feasible to modulate intra/intermolecular interactions, through-space conjugation, and spin-orbit coupling within the clusters, thus enabling effective PL regulation and achieving red/near-infrared (NIR) room-temperature phosphorescence (RTP) in these systems through both internal/chemical (e.g., incorporating additional bridging units and heavy atoms) and external/physical (e.g., pressurization, conformation adjustments) methods. Furthermore, we investigate the integration of these emitters with other conventional functional groups or substrates to realize intriguing tunable photophysical properties by controlling their clustering states. This approach leads to new multifunctional luminescent materials exhibiting synergistic merits such as high efficiency, film-forming ability, excitation- and time-dependent afterglows, and photochromic luminescence arising from subtle molecular rearrangement in crystals. The potential applications of these materials in information storage, anticounterfeiting, luminescent fibers, and bioimaging are also explored. Finally, the Account concludes with a forward-looking perspective on the challenges and future development of nonconventional luminophores.