Tetraphenylethylene-Based Hydrogen-Bonded Organic Frameworks (HOFs) with Brilliant Fluorescence.

By synergistically employing four key strategies: (I) introducing tetraphenylethylene groups as the central core unit with aggregation-induced emission (AIE) properties, (II) optimizing the π-conjugated length by extending the building block branches, (III) incorporating flexible groups containing ethylenic bonds, and (IV) applying crystal engineering to attain dense stacking mode and highly twisty conformation, we successfully synthesized a series of hydrogen-bonded organic frameworks (HOFs) exhibiting exceptional one/two-photon excited fluorescence. Notably, when utilizing the fluorescently superior building block L2, HOF-LIFM-7 and HOF-LIFM-8 exhibiting high quantum yields (QY) of 82.1 % and 77.1 %, and ultrahigh two-photon absorption (TPA) cross-sections of 148959.5 GM and 123901.1 GM were achieved. These materials were successfully employed in one and two-photon excited lysosome-targeting cellular imaging. It is believed that this strategy, combining building block optimization and crystal engineering, holds significant potential for guiding the development of outstanding fluorescent HOF materials.

One/Two-Photon-Excited ESIPT-Attributed Coordination Polymers with Wide Temperature Range and Color-Tunable Long Persistent Luminescence.

The multiple metastable excited states provided by excited-state intramolecular proton transfer (ESIPT) molecules are beneficial to bring temperature-dependent and color-tunable long persistent luminescence (LPL). Meanwhile, ESIPT molecules are intrinsically suitable to be modulated as D-π-A structure to obtain both one/two-photon excitation and LPL emission simultaneously. Herein, we report the rational design of a dynamic CdII coordination polymer (LIFM-106) from ESIPT ligand to achieve the above goals. By comparing LIFM-106 with the counterparts, we established a temperature-regulated competitive relationship between singlet excimer and triplet LPL emission. The optimization of ligand aggregation mode effectively boost the competitiveness of the latter. In result, LIFM-106 shows outstanding one/two-photon excited LPL performance with wide temperature range (100-380 K) and tunable color (green to red). The multichannel radiation process was further elucidated by transient absorption and theoretical calculations, benefiting for the application in anti-counterfeiting systems.

One and Two-Photon Excited Fluorescence Optimization of Metal-Organic Frameworks with Symmetry-Reduced AIEgen-Ligand.

By designing a tetraphenylethylene (TPE)-based AIEgen-ligand with reduced symmetry, we obtained two alkaline-earth metal-based MOFs (LIFM-102 and LIFM-103) with dense packing structures and low porosity as proved by single-crystal X-ray diffraction and CO2 sorption data. Excitingly, the desolvated MOFs with rigid environment and reduced lattice free solvent exhibit high quantum yields (QY, 64.9 % and 79.4 %) and excellent two-photon excited photoluminescence (TPA cross-sections, 2946.6 GM and 2899.0 GM), while maintaining the external-stimuli-responsive properties suitable for anticounterfeit fields. The effect of ligand conformation was validated by comparing the structure and fluorescence properties of the samples before and after desolvation and further verified by theoretical calculations. This work expands the study on TPE-cored materials to symmetry-reduced ligand and might bring forward novel structures and excellent photoluminescent properties in the future.

High-Temperature and Dynamic RGB (Red-Green-Blue) Long-Persistent Luminescence in an Anti-Kasha Organic Compound.

Organic LPL (long-persistent luminescence) materials have sparked extensive research interest due to the ultralong-lived triplet states. Although numerous organic LPL materials have been reported, most of the triplet emission was static and monotonous. Therefore, LPL materials with dynamic triplet emission are urgently required. A triamino-s-triazine derivative 1 with dynamic LPL was fabricated. The single-crystal structure shows that the abundant intermolecular interactions and small free volume restrict the molecular motion and avoid the quenchers. Spectral and theoretical calculations upheld the existence of multiple excited states in 1, and the migration of electrons between multiple excited states is very sensitive to external stimuli. By modulating the stimulus, the residence of electrons in different triplet states can be manipulated to achieve RGB LPL. Importantly, blue LPL was achieved by manipulating the anti-Kasha emission. And the red LPL can still be observed at high temperature.

Thermally Activated Fluorescence vs Long Persistent Luminescence in ESIPT-Attributed Coordination Polymer.

Excited-state intramolecular proton transfer (ESIPT) molecules demonstrating specific enol-keto tautomerism and the related photoluminescence (PL) switch have wide applications in displaying, sensing, imaging, lasing, etc. However, an ESIPT-attributed coordination polymer showing alternative PL between thermally activated fluorescence (TAF) and long persistent luminescence (LPL) has never been explored. Herein, we report the assembly of a dynamic Cd(II) coordination polymer (LIFM-101) from the ESIPT-type ligand, HPI2C (5-(2-(2-hydroxyphenyl)-4,5-diphenyl-1H-imidazol-1-yl)isophthalic acid). For the first time, TAF and/or color-tuned LPL can be achieved by controlling the temperature under the guidance of ESIPT excited states. Noteworthily, the twisted structure of the HPI2C ligand in LIFM-101 achieves an effective mixture of the higher-energy excited states, leading to ISC (intersystem crossing)/RISC (reverse intersystem crossing) energy transfer between the high-lying keto-triplet state (Tn(K*)) and the first singlet state (S1(K*)). Meanwhile, experimental and theoretical results manifest the occurrence probability and relevance among RISC, ISC, and internal conversion (IC) in this unique ESIPT-attributed coordination polymer, leading to the unprecedented TAF/LPL switching mechanism, and paving the way for the future design and application of advanced optical materials.

Novel double layer lanthanide metal-organic networks for sensing applications.

A trifunctional aromatic building block (H2L) containing three different types of functional groups (carboxyl C([double bond, length as m-dash]O)OH, aldehyde C([double bond, length as m-dash]O)H, and O-ether) was applied for the hydrothermal synthesis of two novel lanthanide 2D coordination polymers [Ln(µ-HL)(µ3-L)(phen)]n {Ln = Tb (1) and Eu (2); H2L = 5-methoxy-(4-benzaldehyde)-1,3-benzene dicarboxylic acid; phen = 1,10-phenanthroline}. Both compounds 1 and 2 are isostructural and reveal very complicated 2D metal-organic double layers with the 3,4L27 topology. The presence of free aldehyde groups positioned outside of the double layers opens up a possibility of using them as functional groups toward sensing amines and small organic molecules. The fluorescence measurements for the Tb derivative 1 reveal that it acts as an efficient fluorescence sensor for p-phenylenediamine, benzidine and acetone molecules via a luminescence quenching effect. A similar sensing behavior was observed for the Eu compound 2. Moreover, thin-films of 1-PEG on glass (1-PEG-glass thin-film material) were fabricated and investigated for the detection of amine vapors.