Crystallization-induced Emission Enhancement Research Articles

Cu34S6(PFBT)22(PPh3)6: A Cu12S6 Core‐Based Nanocluster with Long‐Lived Phosphorescent Crystallization‐Induced Emission Enhancement for Selectively Acetonitrile Sensing

AbstractAtomically precise copper nanoclusters with crystallization‐induced emission enhancement (CIEE) have garnered significant academic interests‐ but the rational design of high‐nuclearity Cu nanoaggregates with precise atomic structures and the CIEE effect remains a long‐standing challenge. Herein, a new atomically precise copper nanocluster co‐protected by thiolate, phosphine, and S2− ligands, formulated as Cu34S6(PFBT)22(PPh3)6 (Cu34, PFBT = pentafluorobenzenethiol, PPh3 = triphenylphosphine), is reported, which possesses a typical Cu12S6 core and a Cu22S22P6 shell. Under photoexcitation at 505 nm, Cu34 exhibits strong red‐light emission with a photoluminescence quantum yield of 4.1%. DFT calculations confirm that the emission stems from the ligand‐to‐metal charge transfer (LMCT) transition. This nanoaggregate shows a strong CIEE effect, with the crystalline form exhibiting an 800‐fold enhancement in emission intensity compared to its solution form. An in‐depth structural investigation of the ligand shell reveals that the extensive C−H···F, C−H···π, and π···π interactions significantly restrict the intra‐ and inter‐molecular rotations and vibrations, accounting for the CIEE phenomena. The red emission of Cu34 can be selectively quenched by CH3CN, with a lower detection limit of 9.36 µM (0.38 ppm). This study advances the development of high‐nuclearity copper nanocluster with the CIEE effect and holds significant potential for applications in selectively sensing volatile organic compounds (VOCs).

Aggregation/Crystallization-Induced Emission in Naphthyridine-Based Carbazolyl-Modified Donor-Acceptor Boron Dyes Tunable by Fluorine Atoms.

Four donor-acceptor boron difluoride complexes based on the carbazole electron donor and the [1,3,5,2]oxadiazaborinino[3,4-a][1,8]naphthyridine acceptor were designed, synthesized, and systematically spectroscopically investigated in solutions, in dye-doped polymer films, and in the solid states. The dyes exhibit an intense blue to red solid-state emission with photoluminescence quantum yields of up to 59 % in pure dye samples and 86 % in poly(methyl methacrylate) films. All boron complexes show aggregation-induced emission and reversible mechanofluorochromism. The optical properties of these dyes and their solid state luminescence can be tuned by substitution pattern, i. e., the substituents at the naphthyridine unit. Exchange of CH3 - for CF3 -groups does not only increase the intramolecular charge transfer character, but also provides a crystallization-induced emission enhancement.

Controlled nanocrystallization of gold nanoclusters within surfactant envelopes: enhancing aggregation-induced emission in solution.

The nanocrystallization of functional molecules has been a subject of recent interest in the current development of nanotechnology. Herein, we report the unprecedented synthesis of single nanocrystals of a molecular gold nanocluster in a homogeneous solution by using surfactant-based nano-envelopes. The co-assembling of a Au8 nanocluster carrying lipophilic phosphine ligands with sodium dodecyl sulfate (SDS) in an aqueous solution results in the formation of sphere-shaped amorphous nano-aggregates coated with the surfactant. Upon sonication, the spherical amorphous aggregates are smoothly shape-shifted into discrete rhombic nanocrystals, which can be tracked by TEM and solution XRD. The transformation into single nanocrystals occurs exclusively without further growth or agglomeration, implying that the crystal growth is restricted within the surfactant nano-envelopes. The robust but flexible nature of the wrapped surfactant is likely responsible for the controlled crystallization. We also demonstrate that the amorphous-to-nanocrystalline transition in solution remarkably enhances the photoluminescence emission from the nanocluster, providing a clear example of crystallization-induced emission enhancement. Notably, the obtained nanocrystals showed high stability in solution and retained their shape, size, and PL intensity even after several months, owing to the densely packed surfactant shell. The present surfactant-directed nanocrystallization method may be applicable to other molecular species to contribute to the development of nanocluster science as well as the designed synthesis of nanomaterials.

Au14(2-SAdm)9(Dppe)2]+: a gold nanocluster with a crystallization-induced emission enhancement phenomenon.

In this work, a gold nanocluster [Au14(2-SAdm)9(Dppe)2]+ was synthesized and structurally determined by X-ray crystallography. The crystals of this cluster exhibit a 50-fold enhancement in quantum yield (5.05% for crystals) compared with its solution. Crystallographic analysis reveals that the weak intermolecular interactions (C-H⋯π, π⋯π) can inhibit the molecular vibration and thus generate the crystallization-induced emission enhancement phenomenon.

Tuning the photoluminescence properties of SLE- and MRL-active tricarbonylrhenium(I) complexes through minor structural changes of the organic ligand

Solid-state luminescence properties depend on numerous parameters, including the molecular geometry and the intermolecular interactions that take place in the solid. To clarify the role played by these parameters on the photoluminescence (PL) properties of tricarbonylrhenium(I) complexes, four molecules incorporating a 3-(2-pyridyl)-1,2,4-triazole (pyta) ligand with appended phenylbenzoxazole (PBO) unit were compared. One or two methyl groups were inserted at different places of the organic ligand of the parent compound RePBO, resulting in three new complexes RePBO-Me1, RePBO-Me2 and RePBO-Me3. As shown by NMR, the presence of the methyl group(s) induced some changes in molecular flexibility, but the electronic effects were relatively weak. As a result, the electrochemical and optical properties were little impacted, and the four complexes behaved almost similarly in organic solution, in agreement with theoretical calculations. In contrast, marked differences appeared between the complexes when considering the aggregation-induced emission (AIE) effect, mainly due to the formation of tiny microcrystals in aqueous medium. In the same way, the three methylated complexes in the form of microcrystalline powders showed clear crystallization-induced emission enhancement (CIEE) with respect to solutions, but with distinct characteristics. They emitted less intensely and at longer wavelengths than the unsubstituted complex. Most likely, the methyl groups strongly affect the geometry and the packing mode of the molecules in the crystals, which influence the PL properties in the solid state. After grinding the powders, the emission spectra of the three methylated complexes were shifted to the red, although this shift was weaker than that previously observed for RePBO. This effect was almost reversible after THF fuming. It was attributed to transitions between the crystalline and amorphous phases. Remarkably, in the amorphous phase where molecules regain their mobility, the emission differences between the four complexes almost disappeared. It was then concluded that the amplitude of the mechanoresponsive luminescence (MRL) effect strongly depends on the geometry of the molecules in the pristine powder. This study is one more step toward the rational design of photoluminescent tricarbonylrhenium(I) complexes. More generally, it is also a good example of how very small structural modifications can drastically govern the PL and MRL properties.

Front Cover: Effects of Central Elements on the Properties of Group 13 Dialdiminate Complexes (Chem. Eur. J. 38/2023)

Group 13 elements diversify the luminescent behavior of dialdiminate complexes. Aluminium and gallium complexes exhibit Lewis base-responsive emission, and aluminium enhances the responsiveness. The indium system encompasses ligand-exchange reactions in solution states, thus enabling us to alter the luminescence behavior dynamically. We also demonstrate their solid-state emissive properties involving aggregation- and crystallization-induced emission enhancement. More information can be found in the Research Article by K. Tanaka and co-workers (DOI: 10.1002/chem.202300654).

Effects of Central Elements on the Properties of Group 13 Dialdiminate Complexes.

Novel luminescent dialdiminate complexes of the Group 13 elements were prepared to evaluate the effects of the central element on their properties. We demonstrate that their absorption wavelength and the response to Lewis bases apparently depend on the central atom. The aluminum complex exhibited the absorption band in the higher-energy region than the gallium and indium congeners. Theoretical calculations suggest that the aluminum complex has a lower-lying highest-occupied molecular orbital than the other complexes. Additionally, the emission intensity of the aluminum complex clearly changed in response to a Lewis base. Quantum chemical calculations suggest that these element-dependent optical properties could originate from the difference in the electric charges on the central elements. Interestingly, the ligand exchange reactions were observed in the indium complexes together with the changes in the optical properties and controlled by the addition of InCl3 and InMe3 . Furthermore, all the complexes showed aggregation-induced emission enhancement (AIEE) and crystallization-induced emission enhancement (CIEE) properties. These results lead to proposing a practical strategy for manipulating the optoelectronic properties coupled with the reactivities of complexes by choosing the central elements in the same group.

Multi-layer 3D Chirality and Double-Helical Assembly in a Copper Nanocluster with a Triple-Helical Cu15 Core.

Conceptually mimicking biomolecules' ability to construct multiple-helical aggregates with emergent properties and functions remains a long-standing challenge. Here we report an atom-precise 18-copper nanocluster (NC), Cu18H(PET)14(TPP)6(NCS)3 ( Cu18H ) which contains a pseudo D3-symmetrical triple-helical Cu15 core. Structurally, Cu18H may be also viewed as sandwich type of sulfur-bridged chiral copper cluster units [Cu6-Cu6-Cu6], endowing three-layered 3D chirality. More importantly, the chiral NCs are aggregated into an infinite double-stranded helix supported by intra-strand homonuclear C-H···H-Cdihydrogen contacts and inter-strand C-H/π and C-H/S interactions. The unique multi-layered 3D chirality and the double-helical assembly of Cu18H are evocative of DNA. Moreover, the collective behaviours of the aggregated NCs not only exhibit crystallization-induced emission enhancement (CIEE) and aggregation-induced emission enhancement (AIEE) effects in the deep-red region, but also efficiently catalyzes electron transfer (ET) reaction. This study thus presents that hierarchical assemblies of atomically defined copper NCs could be intricate as observed for important biomolecules like DNA with emergent properties arising from aggregated behaviours.

Multi‐layer 3D Chirality and Double‐Helical Assembly in a Copper Nanocluster with a Triple‐Helical Cu15 Core

AbstractConceptually mimicking biomolecules’ ability to construct multiple‐helical aggregates with emergent properties and functions remains a long‐standing challenge. Here we report an atom‐precise 18‐copper nanocluster (NC), Cu18H(PET)14(TPP)6(NCS)3 (Cu18H) which contains a pseudo D3‐symmetrical triple‐helical Cu15 core. Structurally, Cu18H may be also viewed as sandwich type of sulfur‐bridged chiral copper cluster units [Cu6−Cu6−Cu6], endowing three‐layered 3D chirality. More importantly, the chiral NCs are aggregated into an infinite double‐stranded helix supported by intra‐strand homonuclear C−H⋅⋅⋅H−C dihydrogen contacts and inter‐strand C−H/π and C−H/S interactions. The unique multi‐layered 3D chirality and the double‐helical assembly of Cu18H are evocative of DNA. Moreover, the collective behaviours of the aggregated NCs not only exhibit crystallization‐induced emission enhancement (CIEE) and aggregation‐induced emission enhancement (AIEE) effects in the deep‐red region, but also efficiently catalyze electron transfer (ET) reaction. This study thus presents that hierarchical assemblies of atomically defined copper NCs could be intricate as observed for important biomolecules like DNA with emergent properties arising from aggregated behaviours.

Crystal packing and mechanofluorochrmism of distyrylanthracene derivatives modulated by hydrogen bonds

Two distyrylanthracene derivatives with ester and carboxyl moieties (AnDMB and AnDBA) were synthesized to investigate the influence of terminal group on their photophysical, crystal stacking and mechanofluorochromic (MFC) properties. Spectral and theory calculation results indicate that two compounds have similar absorption because their similar energy levels although they have different terminal groups. Both compounds exhibited crystallization-induced emission enhancement (CIEE). AnDMB crystals are rod-like and emitted strong blue-green fluorescence because of weak π-stacking. The blocky AnDBA crystals with DMAC (N, N-dimethylacetamide) molecules have a red-shifted emission. Moreover, instead of forming a hydrogen bond dimer between the carboxyl groups, a strong hydrogen bond is formed between the carbonyl group of DMAC and the carboxyl group. Crystal structure indicates that enhanced π-stacking should be responsible for its longer wavelength emission. Moreover, the crystals of two compounds could change their emission colors under force stimuli, but AnDMB exhibited higher color contrast than that of AnDBA. A shift of 40 nm might be observed when the AnDMB crystals were ground, while an emission spectral shift of 18 nm emerged after AnDBA crystals were ground. These results indicated that hydrogen bond did not only regulate molecular stacking and fluorescence properties in crystals, but also adjusted force-stimuli responsiveness.

Alkynyl-Protected Ag12Cu4 Cluster with Aggregation-Induced Emission Enhancement

Photoluminescence (PL) of atomically precise metal clusters is attractive for various applications, including light emission, chemical sensing, and bioimaging. Herein, we report the synthesis and total structure of an alkynyl-protected bimetallic AgCu cluster, Ag12Cu4(C≡CR)14(PPh3)4 (Ag12Cu4: HC≡CR = 3,5-bis(trifluoromethyl)phenylacetylene; PPh3 = triphenylphosphine), which exhibits crystallization-induced emission enhancement and aggregation-induced emission enhancement phenomena. As revealed by the single crystal X-ray diffraction data, the Ag12Cu4 cluster comprises an octahedron Ag6 core, which is stabilized by two large Ag3Cu2(C≡CR)7(PPh3)2 metal–ligand motifs. The free valence electron count suggests that Ag12Cu4 is a two-electron superatom. Interestingly, the weak PL of Ag12Cu4 solution at ≈730 nm (near-infrared region) is found to be significantly enhanced in the solid state (15 and 8 fold respectively, for the crystalline and aggregated forms). A detailed structural analysis of the Ag12Cu4 cluster reveals the presence of abundant intercluster C–H···F interactions, which significantly restrict the intramolecular rotations and vibrations, leading to strong PL enhancement in the solid state. This work will trigger the studies of other alloy metal nanoclusters with promising photophysical properties for optoelectronic and other applications.

Crystallization-induced emission enhancement of alkyl chain-dependent pyrene-based luminogens: Visual detection of nitro-explosives

In view of the widespread applications and urgent need for efficient blue emitting materials, the synthesis of such materials has seen renewed interest. In the present work, three alkyl chain-dependent blue emitting pyrene-based luminogens were synthesized and characterized. The experimental results show that these luminogens exhibited high photoluminescence efficiency both in solution (>86%) and in the solid state (>35%). Interestingly, 1,3,6,8-tetrakis (pentylphenyl)pyrene (TPPy), as a photoelectric functional material, was endowed with higher quantum yield (72%) and competitiveness due to a remarkable crystallization-induced emission enhancement (CIEE) property. More importantly, TPPy exhibits high sensitivity and selectivity for ortho-nitroaniline (o-NA) with low limit of detection (9.99 × 10−8 M), which is mainly due to the distinctive odd-even effects of the terminal alkyl chain. The results demonstrate that this type of luminogen can potentially be utilized for practical applications in the field of contaminant or explosive detection as a sensitive and portable fluorimeter.

Mechanochromism and crystallization-induced emission enhancement of carbazole derivatives with different terminal groups

Two D-A carbazole derivatives (TBCA and NCA) were synthesized to investigate the influence of terminal group on the photophysical and mechanofluorochromic (MFC) properties in solution and crystal state. Results suggest that TBCA with t-butylphenyl as terminal group has a slightly smaller optical bandgap than that of NCA with naphthyl as terminal group because NCA adopts a more distorted conformation. Both compounds exhibited crystallization-induced emission enhancement (CIEE). TBCA crystals are rod-like and emitted strong blue fluorescence and the lamellar NCA crystals have a red-shifted emission in comparison with that of TBCA. The single-crystal structure analysis indicates that the two molecules adopt a more distorted conformation in their crystal. CN⋯H–CC hydrogen bonds help TBCA molecules to form a 1D chain stacking, but no strong π-stacking between TBCA molecules exists. Obvious π-π interaction is present between the carbazole units in NCA crystal, which results in a longer emission wavelength that of TBCA. Furthermore, two compounds exhibited reversible MFC behaviors. Force stimuli promoted a larger bathochromic shift for TBCA than that of NCA because a more pronounced planarization for TBCA existed after force damaged ordered stacking in crystal.

Structural Isomerization in Cu(I) Clusters: Tracing the Cu Thermal Migration Paths and Unveiling the Structure-Dependent Photoluminescence

Structural Isomerization in Cu(I) Clusters: Tracing the Cu Thermal Migration Paths and Unveiling the Structure-Dependent Photoluminescence

Multistimuli-Responsive Squaraine Dyad Exhibiting Concentration-Controlled Vapochromic Luminescence.

Stimuli-responsive organic materials with controllable luminescence are of enormous importance because of their potential applications in sensing, data security, and display devices. In this study, a multistimuli-responsive squaraine dyad (SQ-d) composed of two rigid squaraine moieties and a flexible ethylene linker was rationally designed and synthesized. SQ-d exhibits polymorphic luminescence, which can be reversibly switched by various external stimuli, including solvent vapor exposure, heat, and shear force. Unexpectedly, the weakly luminescent phase (O1) of SQ-d exhibits concentration-controlled vapochromic behavior. Film O1 can convert to a highly green-emissive phase (G1) under a low concentration of CHCl3 vapor and convert to a highly yellow-emissive phase (Y) under a high concentration of CHCl3 vapor; these originate from two distinct crystallization-induced emission enhancement processes. To the best of our knowledge, this is the first investigation of the effect of vapor concentration on the phase transitions of organic vapochromic luminophores. By analyzing the single-crystal structures and photophysical properties of SQ-d, we concluded that the green and yellow emissions probably originated from a zigzag stacking mode and an H-type π-π stacking mode, respectively. Finally, two prototypes based on SQ-d for applications in information encryption and vapor sensing were successfully demonstrated.

Three-coordinated mononuclear Cu(I) complexes with crystallization-enhanced thermally activated delayed fluorescence characteristics

Triangular three-coordinated copper(I) halide complexes (1 (I), 2 (Br)) were developed based on a rigid bidentate P-donor ligand functionalized with electron-deficient pyridoimidazole moiety. The results of the temperature-dependent spectroscopic properties reveal that both Cu(I) complexes in crystalline powder at 298 K achieve orange thermally activated delayed fluorescence (TADF) with high photoluminescence quantum yields of 77% for 1 and 56% for 2, and microsecond lifetimes of 55.1 μs (1) and 73.5 μs (2), respectively. Additionally, aromatic CH···π interactions in molecular packing structures constitute an appealing class of crystallization-induced emission enhancement (CIEE) complexes. The results suggest that such highly efficient copper-based stimuli-responsive materials are beneficial for a broad range of applications.

Self‐assembly into Polymorphic 2D Nanosheets with Crystallization‐Induced Emission Enhancement

Comprehensive SummaryOrganic micro/nanocrystals with polymorphic tailorability and perfect platform to modulate decode the complex relationship among molecular structures, supramolecular/noncovalent interactions, aggregates and exciton behaviors. Herein, we precisely tune the polymorphic two‐dimensional organic nanosheets (2DONs) of cyano‐spirocyclic aromatic hydrocarbon (2'‐CN‐SFX), where the crystal growth processes are dominated by precursor concentration. The low concentration (ca. 2 mmol/L) affords the rhombus‐like nanosheets with monoclinic P21/c space group, in which molecules adopt antiparallel and slipped stacking mode. In contrast, the high concentration (ca. 4 mmol/L) favors herringbone‐like molecular packing pattern and then generates shuttle‐like nanosheets with monoclinic C2/c space group. Both above polymorphic nanosheets exhibit unprecedented crystallization‐induced emission enhancement (CIEE) feature that increases the photoluminescence quantum yields (PLQYs) from 16% in solution, 21% in amorphous film to 37%—39% in crystalline states. This result will offer promising opportunities for nanophotonics and organic intelligent semiconductors.

Tunable full-color solid-state fluorescent carbon dots for light emitting diodes

Carbon dots (CDs) as a class of new fluorescent materials have attracted numerous attention. However, the solid-state photoluminescence quenching of CDs is still a big challenge. In this work, self-quenching-resistant solid-state fluorescent CDs are achieved, which exhibit tunable full-color solid-state emission without any other additional solid matrices. These CDs were prepared via one-step microwave method using phloroglucinol and urea as sources by regulating reactant ratio and microwave power. The maximum emission of CDs gradually shifts from 445 to 643 nm, and solid-state quantum yield of five typical CDs with blue (B), green (G), yellow (Y), orange (O) and red (R) emission reaches up 48.2%, 26.0%, 18.5%, 13.7% and 5.7%, respectively. Moreover, multicolor solid-solid fluorescence mechanism of CDs is investigated. The G-, Y-, O-, R-CDs show aggregation-induced red-shift behavior, while B-CDs exhibit unique crystallization-induced emission enhancement property. Finally, CDs are utilized as phosphors combing with ultraviolet chip to fabricate multicolor and white light emitting diodes (LEDs). The obtained white LEDs exhibit an adjustable correlated color temperature (3937–8588 K) and high color rendering index (80–88). This study provides a novel guidance for developing self-quenching resistant CDs with multicolor emission.

Development of NIR emissive fully-fused bisboron complexes with π-conjugated systems including multiple azo groups.

Development of novel near-infrared (NIR) emitters is essential for satisfying the growing demands of advancing optical telecommunication and medical technology. We synthesized elemental skeletons composed of robust π-conjugated systems including two boron-fused azo groups, which showed an intense emission in the red or near-infrared (NIR) region both in solution and solid states. Two types of bisboron complexes with different aromatic linkers showed emission properties with larger bathochromic shifts and emission efficiencies in solution than the corresponding monoboron complex. Transient absorption spectroscopy disclosed that the inferior optical properties of the monoboron complex can be attributed to fast nonradiative deactivation accompanied by a large structural relaxation after photoexcitation. The expanded π-conjugated system through multiple boron-fused azo groups can contribute to rigid molecular skeletons followed by improved emission properties. Moreover, the anti-form of the bisboron complex with fluorine groups in the opposite directions to the π-plane exhibited crystallization-induced emission enhancement in the NIR region. The molecular design by using multiple boron-fused azo groups is expected to be a critical strategy for creating novel NIR emitters.

Mechanochromism and Crystallization-Induced Emission Enhancement of Carbazole Derivatives with Different Terminal Groups

Mechanochromism and Crystallization-Induced Emission Enhancement of Carbazole Derivatives with Different Terminal Groups