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Nonconventional luminophores devoid of remarkable conjugates have attracted considerable attention due to their unique luminescence behaviors, updated luminescence mechanism of organics and promising applications in optoelectronic, biological and medical fields. Unlike classic luminogens consisting of molecular segments with greatly extended electron delocalization, these unorthodox luminophores generally possess nonconjugated structures based on subgroups such as ether (-O-), hydroxyl (-OH), halogens, carbonyl (CO), carboxyl (-COOH), cyano (CN), thioether (-S-), sulfoxide (SO), sulfone (OSO), phosphate, and aliphatic amine, as well as their grouped functionalities like amide, imide, anhydride and ureido. They can exhibit intriguing intrinsic luminescence, generally featuring concentration-enhanced emission, aggregation-induced emission, excitation-dependent luminescence and prevailing phosphorescence. Herein, we review the recent progress in exploring these nonconventional luminophores and discuss the current challenges and future perspectives. Notably, different mechanisms are reviewed and the clustering-triggered emission (CTE) mechanism is highlighted, which emphasizes the clustering of the above mentioned electron rich moieties and consequent electron delocalization along with conformation rigidification. The CTE mechanism seems widely applicable for diversified natural, synthetic and supramolecular systems.

Nonconventional luminophores without significant conjugations generally possess excitation-dependent photoluminescence (PL) owing to the coexistence of diverse clustered chromophores, which strongly implies the possibility of achieving color-tunable PL and/or persistent room temperature phosphorescence (p-RTP) from their crystals assisted by effective intermolecular interactions. Compared with traditional luminogens, however, nonconventional luminophores generally suffer from poor PL efficiencies. Here, inspired by the highly stable double-helix structure and multiple hydrogen bonds in DNA, we report a series of planar or twisted nonconventional luminophores based on hydantoin (HA), demonstrating unprecedentedly high PL/p-RTP efficiencies and p-RTP lifetimes of up to 87.5%/21.8% and 1.74 s, respectively, accompanying color-tunable p-RTP from sky-blue to yellowish-green. These findings will advance the exploitation of efficient nonconventional luminophores and provide deeper mechanistic insights into the origin of color-tunable p-RTP.

Compounds bearing aliphatic amines can be emissive under appropriate conditions. However, their ionized counterparts, namely, quaternary ammonium salts (QASs), which are widely used as phase-transfer catalysts, ionic liquids, disinfectants, and surfactants, are known as luminescence quenchers and considered nonemissive. Herein, unprecedented intrinsic fluorescence/phosphorescence dual emissions from various QASs are reported, which can be finely regulated by changing the excitation wavelength, alkyl chain length, counterion, and mechanical stimuli. The bright photoluminescence along with distinct afterglow and tunable multicolor emissions enables the application of QAS solids in advanced multimode anticounterfeiting. This finding refreshes the understanding of QASs and may inspire emerging applications based on the utilization of the intrinsic luminescences of QASs. Furthermore, it opens opportunities for the investigation of QAS-related processes and functions via a photophysical approach and affords strong implications for the fabrication of novel nonconventional luminophores.

Compounds bearing aliphatic amines could be emissive under proper conditions. Their ionized counterparts, quaternary ammonium salts (QASs), which are widely used as phase-transfer catalysts, ionic liquids, disinfectants and surfactants, however, are known as luminescence quenchers and thought to be nonemissive. Here we report the unprecedented intrinsic fluorescence-phosphorescence dual emission from a diversity of QASs, which can also be finely regulated by excitation wavelength, alkyl chain length, counter ions, mechanical stimuli, etc. The bright photoluminescence along with distinct afterglow and tunable multicolor emissions enables advanced multimode anticounterfeiting for QAS solids. This finding refreshes the cognition of QASs and would inspire emerging applications by utilizing their intrinsic luminescence. Furthermore, it opens opportunities for the investigation of QAS related processes and functions in a photophysical approach and affords strong implications for the fabrication of novel nonconventional luminophores.

Pure organic luminogens with long-persistent luminescence have been extensively studied, on account of their fundamental research significance and diverse utilizations in anticounterfeiting, bioimaging, encryption, organic light-emitting diodes, chemo-sensing, etc. However, time-dependent color-tunable afterglow is rarely reported, especially for single-component materials. In this work, we reported an organic luminogen with time-dependent afterglow, namely, benzoyleneurea (BEU), with multiple persistent room-temperature phosphorescence (p-RTP) and thermally activated delayed fluorescence (TADF) in single crystals. While the lifetime of TADF is relatively short (~1.2 ms), those for p-RTP are as long as around 369~754 ms. The comparable but different decay rates of diversified p-RTP emissions endow BEU crystals with obvious time-dependent afterglow. The existence of multiple emissions can be reasonably illustrated by the clustering-triggered emission (CTE) mechanism. Single-crystal structure illustrates that the combination of benzene ring and nonconventional chromophores of ureide helps facilitate divergent intermolecular interactions, which contribute to the formation of varying emissive species. Moreover, its methyl- and chloro-substituted derivatives show similar multiple p-RTP emissions. However, no time-dependent afterglows are observed in their crystals, due to the highly approaching lifetimes. The afterglow color variation of BEU crystals grants its applications in advanced anticounterfeiting field and information encryption.

Halogen-halogen short contacts, especially halogen bonds (XBs) have been widely utilized in multifarious fields, owing to its bridging function among luminophores as well as well-known heavy atom effect. However, little attention has been paid to the luminescent ability of halogen clusters. It remains unknown whether they are emissive. Herein, inspirited by the clustering-triggered emission of nonconventional luminophores, we report the first examples of emissive halogen clusters with fluorescence-phosphorescence dual emission in aggregated state and even under ambient conditions. Additionally, multi-tunable PL in response to excitation wavelength, temperature, and pressure are noticed. These results shed new lights on the underlying emission mechanism and would inspirit further exploration of nonconventional luminophores involving halogen moieties.

Persistent room-temperature phosphorescence (p-RTP) from pure organics is attractive due to its fundamental importance and potential applications in molecular imaging, sensing, encryption, anticounterfeiting, etc.1-4 Recently, efforts have been also made in obtaining color-tunable p-RTP in aromatic phosphors5 and nonconjugated polymers6,7. The origin of color-tunable p-RTP and the rational design of such luminogens, particularly those with explicit structure and molecular packing, remain challenging. Noteworthily, nonconventional luminophores without significant conjugations generally possess excitation-dependent photoluminescence (PL) because of the coexistence of diverse clustered chromophores6,8, which strongly implicates the possibility to achieve color-tunable p-RTP from their molecular crystals assisted by effective intermolecular interactions. Here, inspirited by the highly stable double-helix structure and multiple hydrogen bonds in DNA, we reported a series of nonconventional luminophores based on hydantoin (HA), which demonstrate excitation-dependent PL and color-tunable p-RTP from sky-blue to yellowish-green, accompanying unprecedentedly high PL and p-RTP efficiencies of up to 87.5% and 21.8%, respectively. Meanwhile, the p-RTP emissions are resistant to vigorous mechanical grinding, with lifetimes of up to 1.74 s. Such robust, color-tunable and highly efficient p-RTP render the luminophores promising for varying applications. These findings provide mechanism insights into the origin of color-tunable p-RTP, and surely advance the exploitation of efficient nonconventional luminophores.