Ditopic pillarene-based fluorescence enhancement material induced by [c2]daisy chain formation has been successfully fabricated. The highly fluorescent smart material exhibits great adaptivity and can act as a multi-responsive supramolecular sensor.

anticounterfeiting, as well as bioimaging, biodetection, and thermal dynamic therapy. [7][8][9][10][11][12][13][14][15][16] In order to satisfy various functional parameters, large amount of efforts have been invested aiming to produce upconversion luminescence with expected characteristics in terms of the position, intensity, and lifetime of the emission bands, via suitable manipulations assisted by external stimuli, including electric field, magnetic field, temperature, mechanical force, as well as hydrostatic pressure. [17] Indeed, Hao et al. achieved a 2.7-fold upconversion luminescence enhancement by applying electric field on Yb 3+ / Er 3+ -doped epitaxially grown ferroelectric BaTiO 3 thin film, [18] Qiu et al. reported obvious luminescence enhancement under magnetic field in Yb 3+ /Ho 3+ -doped NaYF 4 nanoparticles, [19] Carlos et al. fabricated nanothermometers for precise and remote temperature sensing by taking advantage of temperature-induced upconversion luminescence emission, [20,21] and Xu et al. utilized mechanical luminescence generated by the fracture of constructions such as bridge, concrete wall, and buildings to monitor the status of constructions. [22,23] Compared to the above endeavors, hydrostatic pressure could offer unique merits to regulate and explore the luminescent properties, especially of upconversion luminescent materials, which is growing as a prevalent topic. [24][25][26] The tunable hydrostatic pressure can be utilized not only to probe the crystal structure and coordination bonding information of luminescent materials, but also to influence the electronic state and/or crystal field symmetry of luminescent ions. [27,28] This helps unveil the relationship between upconversion luminescent properties and crystal lattice variation of hosts, which provides direct insights in search of suitable crystal structures with improved performance for various purposes. [29] In addition, pressure-induced upconversion emission changes have wide applications in health monitoring, bioimaging, intelligent signature, optical sensors, nanomanometry, nanothermometry, [24,[30][31][32][33][34] as well as real-time monitoring of the working status of constructions via special coatings. [35][36][37][38] However, majority of such studies focused on downshifting luminescence and very limited examples targeted at upconversion emission whereas unfortunately, the unwanted decreased luminescence intensities are usually obtained under pressure. [24,25,34] The reason might be due to the Promising applications of downshifting mechanoluminescence have raised wide research enthusiasms in the past decades. However, weakened upconversion luminescence is usually obtained in lanthanide-doped materials under hydrostatic pressures. In layer-structured KAlF 4 , the z-axis has smaller elastic constant than that of the x-or y-axis, which can effectively tolerate high pressure influence on the coordination environment of luminescent ions by reducing the inter-layer distance and lead to enhanced upconversion luminescence. Ind...

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Halide perovskite materials have emerged as a new type of optoelectronic materials serving as key active layer for next‐generation photovoltaics, light‐emitting diodes, lasers, and photodetectors. Manipulating the crystal size toward the so‐called perovskite nanocrystals (PNCs) will endow new properties due to quantum confinement and ligand effect. However, like their bulk crystalline film, the lead toxicity is still one of decisive concerns that holds‐back their public acceptance. Design of lead‐free (LF) PNCs requires efforts on replacing the B‐site element with other metal candidates from the periodic table. In the past half‐decade, hundreds of new LF PNCs have been developed with various sizes, subdimensionalities, ligands, and electron/quantum confinements, as well as wide applications. Although the lead‐based perovskites still dominate the ongoing research fields, the LF PNCs can have a large potential if novel nonlead B‐site element is introduced to render new type of LF PNCs material that is more stable, less toxic, and more efficient in device performance. In this review, recent progresses on the LF PNCs are revisited to seek these opportunities. The paper is organized in subtopics on material structures, synthesis, properties, and their state‐of‐the‐art applications of different LF PNCs, coupled with an in‐depth discussion on the perspectives and challenges.