Stimuli-responsive Luminescence Research Articles

Dynamic, multi-color, multi-level stimulus-responsive luminescence in a co-crystallized coordination polymer: Modulation of energy transfer by acid-induced ligand protonation and photo-induced radical generation

Stimulus-responsive luminescent materials (SLMs), which can respond to external stimuli by switching their luminescence properties, hold great promise for a variety of applications. Particularly, multi-level SLMs with a logical stimulus sequence and a dynamic, multi-color transition are highly desirable for anti-counterfeiting and information encryption, but an effective design strategy is lacking. Herein, a co-crystallized coordination polymer multi-level SLM [Cd(9-AC)4]·(HAD)2·(9-HAC)2 (9-HAC for anthracene-9-carboxylic acid and AD for acridine) with dual emission centers of coordination unit ([Cd(9-AC)4]2-) and exciplex ([9-HAC/HAD]+) has been successfully constructed by a stepwise solvent process. This SLM possesses a variety of stimulus-responsive properties, including mechanochromic (MC), mechanochromic luminescence (MCL), reversible acidochromic luminescence (ACL), and reversible photochromic luminescence (PCL). Notably, both ACL and PCL properties are achieved by modulating the energy transfer process between the coordination unit and the exciplex, while there is a logical sequence between these two stimulus actions. The acid-induced activation and radical-regulated PCL performance has been exploited in the development of anti-counterfeiting and information encryption applications. This work provides an effective strategy to realize dynamic, multi-color, multi-level stimuli-responsive luminescence by modulating the energy transfer through acid-induced ligand protonation and photo-induced radical generation.

Hydroxyl-driven construction of dual-emitting CsPbBr3/EuWO4(OH) composite for radiometric thermometer and information encryption applications

Self-assembled inorganic metal halide perovskite composites with high emitting efficiency, tunable multimode emission and stimuli-responsive luminescence demonstrated great potential in diversified luminescence applications. Here, a novel CsPbBr3/EuWO4(OH) composite was successfully prepared through the surface adsorption of Pb2+ ions on hydroxylated EuWO4(OH) microsheets and subsequent nucleation of CsPbBr3 peroviskite quantum dots (PQDs) by the anti-solvent crystallization in CsBr containing N,N-dimethylformamide (DMF) solution. Due to the distinct fluorescent thermochromic switching properties of temperature-insensitive Eu3+ and sensitive CsPbBr3 PQDs emitting centers, the resultant CsPbBr3/EuWO4(OH) composite exhibits excellent ratiometric temperature sensing performance. The CsPbBr3/EuWO4(OH) thermometer based on the fluorescence intensity ratio (FIR) technique originating from emissions of Eu3+ to CsPbBr3 PQDs was then explored with a high temperature sensitivity (Sr) of 5.8 % ℃−1 at 54 ℃. In addition, a high-security-level information encryption scheme was proposed by utilizing the excitation-wavelength-dependent switchable fluorescence of CsPbBr3/EuWO4(OH) composite. This study provides a simple and feasible strategy for assembling dual-emission CsPbBr3/EuWO4(OH) composite, showing potential applications in optical ratiometric thermometry and information encryption fields.

Trap engineering in bismuth activated NaLu(Gd)GeO4 persistent phosphors by doping Ln3+

Persistent phosphors, as a promising material for information storage and encryption, have garnered considerable attention owing to their rapid access and low-energy consumption. However, the lack of effective trap control in storage phosphors has resulted in limited storage capacity. In this study, we have prepared a series of NaLu(Gd)GeO4:Bi3+,Ln3+ (Ln = Eu, Tb, Dy, Pr, Sm, Er, Nd, Ho, Tm, and Yb) materials exhibiting tunable multimode and multicolor stimuli-responsive luminescence. The enhanced thermo-stimulated luminescence of five times is obtained in NaLuGeO4:Bi3+,Eu3+ relative to NaLuGeO4:Bi3+, where Bi3+ not only serves as the luminescent center but also as the hole trap center, and trace amounts of Eu3+ act as electron traps to form stable electron-hole pair traps together with Bi3+ hole traps. We propose a new method to improve trap density by constructing electron-hole pair traps via utilizing Bi3+ and Ln3+ as both trap makers and luminescence centers. Density functional theory calculations provide detailed information on the band structures of the matrix and electronic properties, which has confirmed the types of traps and then supports persistent luminescence mechanisms. Particularly, our phosphors are used as optical storage medium for multilevel optical data recording in a single physical layer through managing traps via setting the temperature. This study provides valuable insights for the design and fabrication of next-generation optical information storage and encryption materials.

A Cages-on-Cluster Structure Constructed by Post-Clustering Covalent Modifications and Guest-Enabled Stimuli-Responsive Luminescence.

A gold(I)-cluster-based twin-cage has been constructed by post-clustering covalent modification of a hexa-aldehyde cluster precursor with triaminotriethylamines. The cages-on-cluster structure has double cavities and four binding sites, which show site-discriminative binding for silver(I) and copper(I) guests. The guests in the tripodal hats affect the luminescence of the cluster: the tetra-silver(I) host-guest complex is weakly red-emissive, while the bis-copper(I)-bis-silver(I) one is non-emissive but is a stimuli-responsive supramolecule. The copper(I) ion inside the tri-imine cavity is oxidation sensitive, which enables the release of the bright emissive precursor cluster triggered by H2O2 solution. The hybridization of a cluster with cavities to construct a cluster-based cage presents an innovative concept for functional cluster design, and the post-clustering covalent modification opens up new avenues for finely tuning the properties of clusters.

Mechano-responsive and acid stimuli-responsive luminescence of a hydrogen-bonded organic frameworks

Stimuli-responsive materials are capable of changing their chemical or physical properties when exposed to external stimuli, showing a wide range of potential applications such as drug delivery, sensing, smart coatings, and artificial muscles. Here, a micron-sized hydrogen-bonded organic framework PFC-1 was constructed and its mechanical and acid stimuli-responsive luminescence were investigated. It was found that the fluorescence of PFC-1 aqueous suspension is affected HCl concentrations. The pristine PFC-1 aqueous suspension exhibits an emission peak with high fluorescence intensity at 440 nm and two weak emission shoulders at 475 nm and 535 nm, respectively. Upon the addition of hydrochloric acid, there is a gradual reduction in the peak intensities at 440 nm and 475 nm, while the peak intensity at 535 nm remains relatively unchanged, resulting in a progressive shift in fluorescence color from blue to green. Mechanical grinding induces approximate 30 nm blue-shift in the fluorescence emission peak, a change that is reversible upon immersing the powder in ethanol for 60 min. This work contributes to the development of stimuli-responsive hydrogen-bonded organic framework materials with potential applications in acid and pressure fluorescence sensing.

Force/Acid-Activated and Radical-Regulated photochromic luminescence in a binuclear Zero-Dimensional complex for multi-level anti-counterfeiting

Traditional static optical anti-counterfeiting technology can only output a single emission signal, which is easy to replicate and difficult to meet the needs in current high-tech fields. Multi-level stimuli-responsive luminescent materials with internal logical relationship can improve the degree of anti-counterfeiting and resist against the flood of counterfeiting. In this work, we propose to use force and acid dual stimuli to restrict the dissipation path of photogenerated free radicals, which further lead to the radical-actuated photochromic luminescence (PCL) performance. Based on this strategy, a zero-dimensional (0D) dinuclear zinc complex Zn2(9-AC)4(IM)2 (1) with blue luminescent properties (450 nm) was designed and synthesized. The interaction between 9-AC and IM (as 9-AC/IM exciplex) can be cut off by force-induced rearrangement of structural units in the spatial dimensions or acid-induced protonation of 9-AC, achieving the luminescent transformation from 9-AC/IM exciplex (447 nm) to 9-AC/9-AC excimer (480 nm) or 9-HAC (485 nm). At the same time, the PCL performance was activated and the emission intensity quenching rate increased from 4.77 % to 93.86 % or 44.93 % under UV light irradiation, respectively. The solid-state ultraviolet–visible absorption spectra, electron paramagnetic resonance spectra and theoretical calculation confirmed that the activation of PCL performance is due to the fact that the 9-AC• radicals are more difficult to dissipate upon force- or acid-induced stimuli. Combining mechanochromic luminescence (MCL) and PCL, a dynamic anti-counterfeiting quick response (QR) code with multi-level and fast-responsive logic sequence have been designed, which are promising for the application in time-resolved information encryption. This work provides an effective strategy to construct multi-level stimuli-responsive luminescence as novel anti-counterfeiting materials by changing the photogenerated free radical dissipation pathway to activate PCL properties.

Stimuli-responsive luminescence of electronic capture phosphor for multimode anti-counterfeiting

Long persistent luminescence materials (i.e., electronic capture phosphors) with the tunability on luminescent performance are particularly suitable for information storage and anti-counterfeiting applications. However, traditional electronic capture phosphors face serious challenges in developing multilevel anti-counterfeiting applications using a single phosphor. Here, we developed a series of Eu3+, Tb3+, Pb2+ double or triple doped NaGdGeO4 electronic capture phosphors that exhibit changeable luminescent color in the temporal domain. In addition, the pre-irradiated phosphors also demonstrate the photo/thermo-stimulated luminescence. Based on these excellent optical features, the luminescence patterns are printed by using these phosphors as inks for advanced anti-counterfeiting, which cause anti-counterfeiting more difficult to imitate.

Stimuli-responsive luminescence from polar cyano/isocyano-derived luminophores via structural tailoring and self-assembly.

Polar cyano fragments and their isomeric isocyano counterparts have attracted great attention as stimuli-responsive luminescent materials in a wide range of fields including organic light-emitting diode devices, chemical fluorescent sensors, photoelectric semiconductors, anti-counterfeit products, etc., mainly because of their typical electron-deficient activity, noncovalent recognition ability, and variable coordination capacity. The electron-deficient and polar nature of these blocks have significant effects on the properties of the cyano/isocyano-based luminophore materials, especially concerning their condensed state-dependent electronic structures. Among them, donor-acceptor (D-A) derived unimolecular and co-assembled luminophores have attracted more attention because their large delocalized structures and noncovalent interaction recognition sites can rebuild the electronic transfer character in the aggregative state, thus endowing them with outstanding stimuli-responsive luminescent behavior via intermolecular and intramolecular charge transfer in polytropic morphologies. In this perspective paper, we give a brief introduction on stimuli-responsive organic and coordinated luminophores and the documented typical design concepts and applications in recent years. It is expected that this perspective article will not only summarize the recent developments of polar cyano/isocyano-derived luminophores and their coordination compounds via structural tailoring and self-assembly but also throw light on the future of the design of more sophisticated stimuli-responsive architectures and their versatile properties.

One-Dimensional Cu(I) coordination polymers with multi-stimuli-responsive luminescence

Luminescent coordination polymers (CPs) with one-dimensional (1D) closely-packed structures usually display poor performances in stimuli-responsive luminescence due to the low porosity and high rigidity. Here, we reported a multi-stimuli-responsive 1D luminescent [Cu2I2]-cluster-based CP (CuIP-OPY) with piezochromic, thermochromic and oxygen sensing properties. Attributing to the flexibility of the OPY ligand, which twists under external forces and heating, respectively, CuIP-OPY shows obvious piezochromic and thermochromic luminescence. Moreover, although its porosity is only 1.1%, oxygen can still quench its luminescence intensity by 57.53%. To further confirm the related mechanism, we also synthesized a [Cu2Cl2]-cluster-based CP with similar structure, namely CuClP-OPY, that displays higher rigidity, and compared the differences between these two CuXP-OPY (X = I, Cl) in the multi-stimuli-responsive behavior.

Self-Healing Luminescent Elastomers with Ultrahigh Mechanical Strength, Superior Toughness, and Stimuli-Responsive Luminescence

Self-Healing Luminescent Elastomers with Ultrahigh Mechanical Strength, Superior Toughness, and Stimuli-Responsive Luminescence

Tailoring Dye Emissions within Metal–Organic Frameworks for Tunable Luminescence and Ratiometric Temperature Sensing

Metal-organic frameworks (MOFs) have emerged as powerful platforms for tuning the luminescence characteristics of guests due to their various structures and functions. Tunable and stimuli-responsive luminescence of guests within MOFs can be achieved through a judicious choice of guests and hosts. Herein, we demonstrate a dramatic change in the luminescence of dye excimers encapsulated in MOFs. A polar dye presented largely red-shifted excimer emissions in MOFs with higher polarities, while a nonpolar dye showed very different excimer emissions. Interestingly, the excimer emissions tailored by the MOFs showed strong thermal quenching. Cz-Ant@ZIF-8, containing two luminescent dyes (carbazole (Cz) and anthracene (Ant)), was prepared, and it presented ratiometric temperature sensing properties (1.55% K-1) in the temperature range of 278-353 K. This work sheds light on the luminescence tuning of dyes confined in MOFs and the design of sensitive ratiometric thermometers.

Cascade Energy Transfer in Biopolymer-Templated Multi-Chromophoric Assemblies: Ratiometric Temperature Sensing.

Natural light-harvesting complexes collect energy from sunlight and transfer it to the reaction center through a cascade of energy and electron transfer steps. Artificial light-harvesting systems functioning in aqueous media mimic natural photosynthetic systems. However, their design remains a challenging task as closely packed antenna chromophores often undergo severe self-quenching. Herein, we report luminescent co-assemblies between cationic pyrene-appended imidazolium amphiphiles and two anionic biopolymeric scaffolds, heparin and DNA in aqueous media. These co-assemblies served as excellent platforms for constructing artificial light-harvesting systems as upon co-embedding of multiple external dyes, highly efficient single-step and cascade energy transfer was observed from the pyrene donors to the acceptor dyes. Most notably, the efficiency of the energy transfer process was possible to modulate by employing multiple stimuli such as pH and temperature, and this resulted in the generation of multi-color luminescent materials in solution and film states, and they were also exploited in ratiometric temperature sensing. Their stimuli-responsive luminescence in the solid state was found to be advantageous for encryption studies.

Pressure/Thermo-Induced Hypsochromic-Shifted and Enhanced Luminescence Based on Carbazole Emitter

Most organic mechanochromic luminogens show bathochromic-shifted and quenched emission with increasing pressure. Design of turn-on mechanochromic material with hypsochromic-shifted luminescence is still a challenge. Herein, a novel emitter (namely DCzPy) was prepared through laboratory-synthesized carbazole to study its stimuli-responsive luminescence. The crystalline powders of DCzPy emitted blue-green fluorescence at 490 nm. Upon gently grinding, the fluorescence intensity of DCzPy was dramatically enhanced with a hypsochromic shift by 33 nm. Further heavily grinding, its emission wavelength returned to 490 nm. Powder X-ray diffraction patterns indicated that such behaviors were attributed to the solid-state morphology transition from crystal to crystal. Interestingly, a large bathochromic-shifted and weakened fluorescence was achieved under isotropic compression. Moreover, thermoresponsive behavior of DCzPy was studied in the film to find that its emission turned from green to blue as the temperature increased. Given the thermo-induced fluorescence switching of DCzPy, a two-dimensional code was fabricated for information anticounterfeiting.

Investigating stimuli-responsive luminescence and aggregation-induced emission properties of o-carborane-based luminophores modified with phenanthrene or anthracene

Incorporation of a diphenylphosphine group and an o-carborane unit into luminophores leads to polymorphs and stimuli-responsive luminescence behavior.

Two Cholesterol-Containing Pyrene Derivatives: Subtle Spacer Difference, Diverse Stimuli-Responsive Luminescence, Chirality, and Self-Assembly Behaviors.

Two chiral molecules 1 and 2 were designed and synthesized with a pyrene moiety directly linked to a chiral cholesterol moiety and connected through a methylene spacer, respectively. Influence of the spacer on their stimuli-responsive luminescence, chirality, and self-assembly behaviors was systematically investigated. Molecules 1 and 2 had similar aggregation-induced emission enhancement (AIEE) in solution, because of carrying the same fluorescence moiety. Both molecules displayed mechanochromism (MC) property but with different color contrast, whereas only 2 showed mechanoluminescence (ML) activity. When doping in liquid crystal molecule 5CB, both molecules induced the formation of chiral nematic liquid crystals (N*-LCs) with strong circularly polarized luminescence (CPL). Molecule 2 induced single handedness signal, irrespective of doping ratios, while 1-doped N*-LCs showed an inversion of CPL signal from negative to positive upon the increase of doping ratios. Molecules 1 and 2 also self-assembled into different coassemblies with 5CB. Their distinct behaviors were attributed to the influence of the methylene spacer, which caused different molecular conformation and steric bulkiness; accordingly, it changed intermolecular interactions and molecular packing of the two molecules and led to diverse chirality and luminescence. This work provided important model molecules to better understand the molecular structure-property relationship and guide the design of novel functional molecules.

Stimuli-responsive luminescent nanomaterial based on lanthanide modified carbon dots for multilevel information encryption

Multistimuli responsive luminescence material have tremendous potential in various advanced photonic applications, especially in high-level information encryption. Here, a facile and effective way was presented to construct multistimuli intelligent luminescent nanomaterial based on europium modified carbon dots (CDs-Eu) for advanced information encryption. This nanocomposite exhibiting controllable response behaviors upon exposure to acid-base, 2,6-dipicolinic acid (DPA) and Cu2+ hold an enormous potential in multilevel encryption application. The fluorescence (FL) color of CDs-Eu can be reversibly switched between colorless and green upon acid-base stimulation. Furthermore, in the presence of the deciphering agent (DPA), the fluorescence color of CDs-Eu exhibited a green to red color change, which can be restored upon another external stimulus (Cu2+). Combined with Morse code and anti-counterfeiting pattern, the encryption, decryption and anti-counterfeiting of important information can be realized. This stimuli-responsive luminescence material with high confidentiality shows great potential in advanced information encryption.

Coordination-Guided Conformational Locking of 1D Metal-Organic Frameworks for a Tunable Stimuli-Responsive Luminescence Region.

One-dimensional (1D) metal-organic frameworks (MOFs) have shown great potential for designing more sensitive and smart stimuli-responsive photoluminescence metal-organic frameworks (PL-MOFs). Herein, we propose a strategy for constructing the 1D MOFs with tunable stimuli-responsive luminescence regions based on coordination-guided conformational locking. Two flexible 1D MOF microcrystals with trans- and cis-coordination modes, respectively, were synthesized by controlling the spatial constraint of solvents. The two 1D frameworks possess different conformation lockings of gain ligands, which have a great influence on the rotating restrictions and corresponding excited-state behaviors, generating the remarkably distinct color-tunable ranges (cyan-blue to green and cyan-blue to yellow, respectively). On this basis, the two 1D MOF materials, benefiting from the varied stimuli-responsive ranges, have displayed great potential in fulfilling the anticounterfeiting and information encryption applications. These results provide valuable guidance for the development of smart MOF-based stimuli-responsive materials in information identification and data encryption.

A Series of D–A–D Structured Disilane-Bridged Triads: Structure and Stimuli-Responsive Luminescence Studies

A series of σ-π extended octamethyltetrasilanes, which have phenothiazine, 9,9-dimethyl-9,10-dihydroacridine, or phenoxazine (1, 2, and 3) groups as donor moieties and thienopyrazine or benzothiadiazole (a and b) groups as acceptor fragments, has been prepared, and their optical properties have been studied as an extension of our work. All six compounds exhibited fluorescence in the solid state with maximum wavelengths centered in the range of 400 and 650 nm upon excitation by a UV lamp. Compound 2b showed apparent dual emission behavior in solution, which depends on solvent polarity, and a reversible photoluminescent change under mechanical and thermal stimuli in the solid state. Quantum chemical calculations suggest the contribution of a quasi-axial conformer of the 9,9-dimethyl-9,10-dihydroacridine moiety in 2b to the dual emission in solution and the mechanofluoroluminescence in the solid state, similarly to 1a. These studies provide new insight into the preparation of disilane-bridged triads capable of responding to multiple stimuli.

Tuning and reviving the luminescence of a new class of pyridyl β-diketonate Eu(III) metallomesogens: From molecules to entrapment in polymer particles

A new class of polynuclear Eu(III) compounds bearing pyridyl β-diketonate ligands have been synthesised, and their thermal and photophysical behaviour studied. Since they behave as highly emissive materials, polymer thin films were prepared by using the Eu(III) compounds as a dopant and poly(methyl methacrylate) as the matrix. Interestingly, the luminescence properties of the new materials can be tuned by protonation of the nitrogen atom at the pyridine moiety of the ligands, which allows controlling the energy transfer between the antenna ligands and the Eu(III) centre. This stimuli-responsive luminescence behaviour causes a drastic colour change from red to green that can be useful to detect highly acidic media. The antenna effect has been also explored at the nanoscale. Several colloidal dispersions of poly(methyl methacrylate) microparticles encapsulating the luminescent Eu(III) compounds have been obtained from oil-in-water droplets. The polymer matrix acts as a barrier and protects the coordination environment of the Eu(III) compounds, avoiding the quenching effect by water coordination. This method of entrapment can constitute a powerful tool to revive the luminescence of Eu(III) species in aqueous media.

Zn(II) Complexes Based on a Schiff Base: Mechanochromism- and Solvent Molecule-Dependent Acidochromism

A new Schiff base molecule (HL) with a triphenylamine group and its three zinc(II) complexes (1, 1-THF, and 1-DMF) were synthesized and characterized. As expected, three kinds of loosely packed structures that are susceptible to mechanical stimulation were adjusted through twisted conformations and weak intermolecular interactions (C–H···π, π···π, and hydrogen bonds). They all exhibit remarkable mechanochromic luminescence (MCL) behavior with high contrast upon mechanical grinding due to the phase transition between a crystalline and an amorphous state, as confirmed by powder X-ray diffraction (PXRD) studies. Especially, grinding of 1-THF was accompanied by the departure of lattice tetrahydrofuran (THF) molecules. Further investigation revealed that 1-THF and 1-DMF showed outstanding acidochromic behavior. The PXRD and thermogravimetric analysis (TGA) results proved that fumigation with HCl could destroy the interaction between the solvent and ligand molecules, resulting in the phase transition from the crystalline state to the amorphous state, which is essential for their acidochromic properties. The acidochromic mechanism is different from the reported protonation and deprotonation process. Both HL and complex 1 exhibit no acidochromism. This work will shed light on the mechanism of acidochromic properties of Schiff base zinc(II) complexes and expand the potential solid-state stimuli-responsive luminescence materials.