Emission Color Change Research Articles

Multistimuli-responsive fluorescence behaviours of aggregation-induced emission small-molecule and electrospun nanofibre films for acid-base vapour sensing.

Multistimuli-responsive fluorescent materials have garnered great research interest benefited from their practical applications. Two twisted-structure compounds containing tetraphenylethylene (TPE) as the aggregation-induced emission (AIE) group and a pyridine unit as the acid reaction site to obtain new multistimuli-responsive fluorescent compounds (namely, TPECNPy: TPECNPy-2 and TPECNPy-3) were successfully synthesized through a one-step Knoevenagel condensation reaction. The multiple-stimuli response process of TPECNPy was investigated by means of photoluminescence (PL) spectra and emission colour. The results showed that both TPECNPy compounds with excellent AIE abilities displayed reversible emission wavelength and colour changes in response to multiple external stimuli, including grinding-fuming by CH2 Cl2 or annealing and HCl-NH3 vapour fuming. More importantly, fluorescent nanofibre films were prepared by electrospinning a solution of TPECNPy mixed with cellulose acetate (CA), and these exhibited reversible acid-induced discolouration, even with only 1wt% TPECNPy. The results of this study may inspire strategies for designing multistimuli-responsive materials and preparing fluorescent sensing nanofibre films.

Highly bright broadband cyan-emitting CaLu2HfGaAl3O12:Ce3+ garnet phosphors for high-color-quality white light-emitting diodes

In this work, novel highly efficient cyan-emitting Ce3+-activated CaLu2HfGaAl3O12 (abbreviated as CLHGAO) garnet-structured phosphors were synthesized using the high-temperature solid-state reaction route. Rietveld refinement results demonstrate the as-prepared samples belong to cubic space group Ia3‾d. These CLHGAO:xCe3+ (x = 0.5–8%) phosphors possessed a broad absorption band in the 320–450 nm wavelength range with a maximum at 408 nm. Upon 408 nm excitation, the CLHGAO:0.5%Ce3+ sample showed a broad cyan emission band within 425–750 nm wavelength range (emission peak: 493 nm; bandwidth: 95 nm), along with CIE color coordinates of (0.1939, 0.3533) and a high internal quantum efficiency of 81.9%. It is observed that, with increasing Ce3+ doping concentration from 0.5% to 8%, the emission peak position of CLHGAO:xCe3+ moves from 493 nm to 527 nm, and the corresponding emission color changes from cyan to green. Besides, the thermal stability and color stability of CLHGAO:0.5%Ce3+ phosphor at high temperature are also studied. A white LED device was fabricated by utilizing a 395 nm LED chip with as-prepared CLHGAO:0.5%Ce3+ cyan-emitting phosphor and commercial (Ba,Sr)2SiO4:Eu2+ green-emitting phosphor and commercial CaAlSiN3:Eu2+ red-emitting phosphor, and it demonstrates a high-color-quality white light with high color rendering index of 91, low color correlated temperature of 3547 K and good luminous efficacy of 58.71 lmW−1.

Pressure- and Rate-Dependent Mechanoluminescence with Maximized Efficiency and Tunable Wavelength in ZnS: Mn2+, Eu3.

Mechanoluminescence (ML) has received widespread attention because of potential application in stress sensors and imaging. However, pursuing highly efficient ML remains a challenge due to multifactorial limitations such as pressure and loading rate. Here, we systematically investigate pressure- and rate-dependent ML in Mn2+ and Eu3+ co-doped ZnS in a gigapascal pressure range by using a high-pressure dynamic diamond anvil cell and microsecond time-resolved fluorescent methods and demonstrate the giant tunability in both ML efficiency and wavelength. Compressed from ambient pressure to 11 GPa at different compression rates, ZnS: Mn2+, Eu3+ exhibits a volcano shape in ML emission efficiency with an optimum at ∼3.5 GPa and ∼211.1 GPa/s, at least 1000-fold higher than that measured in the MPa range. The pressure-dependent ML is accompanied with a tunable yellow-to-red emission color change. A combination of high-pressure X-ray diffraction and photoluminescence measurements reveals that the pressure- and rate-dependent ML behavior derives from pressure-induced strengthening of the crystal piezoelectric field and enhanced interaction between the host lattice and doped ions with a significant change of the energy level of the Mn ion. Significantly, the highly efficient ML of ZnS: Mn2+, Eu3+ at the GPa level is reproducible under a compression-decompression process and can be manipulated on a micron scale, implying great potential in mechanical-optical energy conversion and application in dynamic pressure imaging, stress sensors, and multicolor displays.

Smartphone-assisted ratiometric sensing platform for on-site tetracycline determination based on europium functionalized luminescent Zr-MOF

Accurate on-site analysis of tetracycline (TC) is of great research value for ensuring food safety and estimating environmental pollution. Herein, a smartphone-based fluorescent platform for TC detectionhas been developed based on a europium functionalized metal–organic framework (Zr-MOF/Cit-Eu). Based on the inner filter and antenna effect between Zr-MOF/Cit-Eu and TC, the probe exhibited a ratiometric fluorescent response toward TC, resulting in an emission color change from blue to red. Excellent sensing performance was achieved with a detection limit of 3.9 nM, consistent with the linear operation spanning nearly four orders of magnitude. Subsequently, visual test strips based on Zr-MOF/Cit-Eu were prepared, possessing the potential for accurate testing of TC via RGB signals. Finally, the proposed platform was well applied in actual samples with satisfied recoveries (92.27 to 110.22%). This MOF-based on-site fluorescent platform holds great potential on constructing intelligent platform for visual and quantitative detection of organic contaminants.

Emission Color Change of Multi-Component Organic Crystals in Response to Mechanical Stimuli

Emission Color Change of Multi-Component Organic Crystals in Response to Mechanical Stimuli

Highly selective discriminating E, E/E, Z isomers of functionalized 1-phenyl-2,5-divinyl-1H-pyrrole for n-/sec-/tert-butyl amines and as temperature sensor

A series of novel E, E/E, Z isomers based on 5-(2-cyano-3-ethoxy-3-oxoprop-1-en-1-yl)-1H-pyrrol-2-yl acrylic acid have been successfully synthesized. The comparative studies between compounds presenting E, E/E, Z isomers on the properties of emission and effects of temperature had been discussed. Interestingly, the E, Z, and E, E isomers emitted different luminous colors and aggregation-induced emission (AIE) properties. Notably, the E, Z-OC2H5COOH and E, E-OC2H5COOH loaded on filter paper could identify alkaline compounds. Furthermore, E, E-OC2H5COOH strips could selectively discriminate for n-/sec-/tert-butylamine by changing the color of the fluorescence. In addition, E, E-OC2H5COOH, doped into polymer dimethyl silicone (PDMS) film could exhibit an evident change in emission color from orange to yellow at 90 °C and gradually vanished at 190 °C, while that of E, Z-OC2H5COOH didn't change significantly with heating. Therefore, the configuration of two double bonds can be used to regulate the change of fluorescence, improve the chemical and physical properties of materials, and break through the deficiency of single configuration regulation.

An elastic organic crystal with multilevel stimuli-responsive room temperature phosphorescence

•Elastic organic RTP crystal with multilevel stimuli responsiveness was first reported •Highest RTP quantum yield of 25.1% was recorded in elastic organic RTP crystal •Modulation of RTP was realized via the transition of excited state conformation •RTP single to double emission under external stimuli in single-component system In this work, we report a series of single-component molecules crystals named dialkyl 4,4′-oxalyldibenzoate (n-BZ) with both efficient room temperature phosphorescence (RTP) emission, multilevel stimuli responsiveness, and elasticity or flexibility. The crystalline n-BZ will change from original RTP single emission to RTP dual emission after grinding or thermal annealing (TA). Interestingly, the emission wavelength difference between the RTP dual-emission peaks after grinding or TA reaches 82 nm with emission color changes from cyan blue to orange. In addition, to our knowledge, the largest RTP quantum yield of 25.1% (6-BZ) has been achieved as the highest recorded for organic RTP elastic crystals. Through a series of experiments and theoretical calculations, we demonstrate that the multilevel stimuli responsiveness of n-BZ originates from the transition of excited state conformation and its elasticity from many weak intermolecular interactions, which will provide a molecular design and construction strategy for organic stimuli-responsive RTP materials. In this work, we report a series of single-component molecules crystals named dialkyl 4,4′-oxalyldibenzoate (n-BZ) with both efficient room temperature phosphorescence (RTP) emission, multilevel stimuli responsiveness, and elasticity or flexibility. The crystalline n-BZ will change from original RTP single emission to RTP dual emission after grinding or thermal annealing (TA). Interestingly, the emission wavelength difference between the RTP dual-emission peaks after grinding or TA reaches 82 nm with emission color changes from cyan blue to orange. In addition, to our knowledge, the largest RTP quantum yield of 25.1% (6-BZ) has been achieved as the highest recorded for organic RTP elastic crystals. Through a series of experiments and theoretical calculations, we demonstrate that the multilevel stimuli responsiveness of n-BZ originates from the transition of excited state conformation and its elasticity from many weak intermolecular interactions, which will provide a molecular design and construction strategy for organic stimuli-responsive RTP materials.

Bulk assemblies of organic antimony chloride with multiple reversible photoluminescence switching for anti-counterfeiting and information encryption

Forgery and leakage of confidential information will bring great social harm and economic loss, so it is necessary to take simple and effective strategies to ensure information security. Here, two different zero-dimensional (0D) organic antimony chlorides of [Ph3S]2SbCl5·2C2H3N (1) and [Ph3S]2SbCl5 (2) were obtained by inserting the organic ligand of Ph3SCl (triphenylsulfonium chloride) into SbCl3 lattice via different chemical synthesis methods. Under photoexcitation, compound 1 shines bright yellow emission with a photoluminescence quantum efficiency (PLQE) of 98.2%, while compound 2 shows efficient red emission with a PLQE of 99.6%. Both of them exhibit dual-emission bands, which stem from the singlet self-trapped excitons (STEs) and triplet STEs, respectively. Particularly, the dynamic transformation processes between compounds 1 and 2 are witnessed via the acetonitrile (C2H3N) stripping/insertion under different external stimuli, which was demonstrated by the results of powder X-ray diffraction and photoluminescence (PL) spectra. In addition, the continuous heating of compound 2 can make the emission color change from red to yellow again, accompanied by the lattice distortion of [SbCl5]2- pyramid. Finally, the multiple anti-counterfeiting and information encryption models were fabricated based on the reversible PL switching of the title compounds under different external stimuli.

Diphenyl imidazole-based fluorescent chemosensor for Al3+ and its Al3+ complex toward water detection in food products

A sophisticated fluorescent chemosensor, 2-(4-nitrophenyl)-4,5-diphenyl-1H-imidazole (NPDI), was designed and synthesized through a one-step condensation reaction. NPDI exhibited a fluorescence enhancement response toward Al3+, accompanied by significant emission color change without interference from other tested metal ions. The binding stoichiometry and mechanism was corroborated using various techniques. The limit of detection (LOD) for Al3+ could reach 7.25 × 10−8 mol/L and the binding constant was found to be 1.47 × 105 L/mol. Furthermore, the in-situ formed NPDI·Al complex functioned a secondary chemosensor for water by quenching effect. The fluorescence quenching mechanism could be attributed to hydrogen bonding interaction of nitro substituent with water. The LOD was calculated to be 0.012 %, indicating NPDI·Al heightened sensitivity to water. Additionally, NPDI·Al complex was employed for the moisture detection in the surroundings. Finally, the practical application of NPDI·Al complex had been successfully used in the determination of water content in food products.

Aggregation induced luminescence chromism molecule for preparation of physically unclonable function labels

Organic stimuli-responsive luminescent materials have attracted much attention due to their great potential in security fields such as fluorescent inks and information security. However, the widespread aggregation quenching effect limits the organics’ practical application in anti-counterfeiting technology. To overcome this challenge, this study developed a novel molecule 4-(4-phenyl-6-(pyren-1-yl)-1,3,5-triazin-2-yl)benzyl acrylate (PPTBA) which exhibits high luminescence efficiency in both monodisperse and aggregated states, with an obvious aggregation-induced emission color change. The copolymer P(NIPAM-PPTBA) was obtained by copolymerizing PPTBA and NIPAM (N-Isopropyl acrylamide). The polymer exhibits a blue-to-yellow luminescence chromism upon adding an appropriate amount of water. The polymer film can perfectly hide letters when combined with stamp technology and can be developed with water for general-purpose anti-counterfeiting verification. The polymer is utilized to prepare fluorescence physically unclonable functions (PUF) labels with a high-security level by using a convenient spraying technique. The random localization of polymeric domains yields unclonable surfaces with moderate uniformity, uniqueness, and good coding capacity. In particular, this PUF exhibits good temperature stability and photostability, and the luminescent hydrochromism of the polymer further enhances the encryption level. This work demonstrates the practicality and reliability of an organic light-emitting system for anti-counterfeiting applications and provides a realistic solution for preparing low-cost PUF.

Dual-Color Photoluminescent Functionalized Nanoparticles for Static-Dynamic Anticounterfeiting and Encryption: First Collaboration of Spiropyran and Coumarin.

Increasing the security of anticounterfeiting materials has been the most important challenge in recent years, and the development of dual-color photoluminescent inks with multi-level security, static/dynamic emission, and dynamic color change is an important solution to overcome this problem. In this study, the multi-functionalized copolymer nanoparticles containing different functional groups (with a concentration of 20 wt %), including ester, carboxylic acid, hydroxyl, epoxide, amide, and amine groups were synthesized successfully by the emulsion polymerization method. The results showed that the particle size and morphology of nanoparticles are affected by the polarity of functional groups. The prepared multi-functionalized copolymer nanoparticles were modified physically with spiropyran (photochromic and red fluorescence emission) and coumarin (cyan emission) derivatives to develop dual-color photoluminescent polymer nanoparticles with application in static-dynamic photoluminescent anticounterfeiting inks, which have multi-level security. The investigation of optical properties indicates that the kinetics of photochromism and photoluminescence properties of samples containing spiropyran is dependent on the local polarity on the surface of polymer nanoparticles. Hence, an increase in the polarity (functionalization with amide, carboxylic acid, and hydroxyl groups) has resulted in fast photochromism, high-intensity photoluminescence emission and increased the efficiency of the photoswitchable color change of emission from cyan to pink. Dual-color photoluminescent anticounterfeiting inks were prepared by mixing polymer nanoparticles containing spiropyran with polymer nanoparticles containing coumarin, in different ratios (1:1, 1:3, 1:5, 1:8, and 1:10). Obtained results showed that prepared samples have cyan emission under UV light of 254 nm (static mode), and a dynamic photoswitching of fluorescence emission from cyan to pink (as a function of irradiation time) was also observed under UV-light irradiation of 365 nm, which is well known as a dynamic mode of emission. The responsivity and intensity of dynamic photoluminescence emission are dependent on the local polarity of the surface functional groups, in which the samples based on amide functionalized copolymer nanoparticles displayed high-intensity emission in the static mode and high-intensity photoswitchable dual-color emission in the dynamic mode, in the case of all ratios of colloid solution mixtures. Printing security tags on cellulose paper by dual-color photoluminescent inks indicates advantages such as maximum printability, resolution, brightness, and static-dynamic photoluminescence emission with high intensity for inks based on amide functionalized nanoparticles. The static-dynamic dual-color photoluminescent anticounterfeiting ink with unique properties and multi-level security was reported for the first time by the collaboration of spiropyran and coumarin. This study can open a new approach and window to the future of advanced and high-security anticounterfeiting technologies.

Moisture-Induced reversible structure conversion of Zero-Dimensional organic cuprous bromide hybrids for multiple photoluminescent anti-Counterfeiting, information encryption and rewritable luminescent paper

With social development and the technology advancement, the demand for high-security photoluminescence (PL) anti-counterfeiting and information encryption materials is increasing constantly. Herein, two organic cuprous bromide hybrids, namely (TPA)CuBr2 and (TPA)2Cu4Br6, with variable coordination geometries were synthesized via a facile solution method. (TPA)CuBr2 shines bright cyan emission with a photoluminescence quantum efficiency (PLQE) of 81 % under 254 nm excitation, while (TPA)2Cu4Br6 exhibits orange emission with a PLQE of 95 % upon 365 nm irradiation. Interestingly, a reversible structural conversion could be triggered by water or heat between (TPA)CuBr2 and (TPA)2Cu4Br6, accompanied by a back-and-forth emission color change between cyan and orange under its respective excitation. The emission mechanism of above two compounds was investigated, and the results show that they stem from self-trapped excitons emission. Such PL color switching under the external stimuli of water, heat and different excitation wavelengths enables us to construct a series of anti-counterfeiting and information encryption scenarios. In addition, the rewritable PL paper based on (TPA)CuBr2 with water as ink is realized. These two compounds not only provide a good way to modulate zero-dimensional Cu(I)-based metal halide with highly efficient emission, but also a new example to develop multifunctional applications by organic–inorganic hybrid metal halides.

Spirocyclic rhodamine B benzoisothiazole derivative: a multi-stimuli fluorescent switch manifesting ethanol-responsiveness, photo responsiveness, and acidochromism.

Multi-stimuli fluorescent switching materials have been extensively employed in chemistry, biochemistry, physics, and materials science. Although rhodamine-based spirolactams have been specifically considered for metal ion sensing by photoluminescence, only some of them manifest photochromic behavior, and further development of rhodamine B (RHB)-based photochromic materials is required. RHB and its cyclic amides are advantageous in various sensing applications owing to their colorimetric responses to external stimulation. Hence, the current work reports a novel multifunctional active molecular material (3',6'-bis(diethylamino))-2-(5-nitrobenzo[c]isothiazol-3-yl)spiro[isoindoline-1,9'-xanthen]-3-one (RHBIT) by linking rhodamine B with 3-amino,5-nitro[2,1]benzoisothiazole (ANB) in a facile synthetic pathway; that perceives both emission color change and switching between off-on states. RHBIT shows acidochromism, photochromism, and pH sensitivity accompanied by unique ethanol responsiveness, with potential applications in anti-counterfeiting and drug delivery. Notably, RHBIT is highly acid sensitive and reverts to the ring-closed form on treatment with triethylamine (base), visible with the naked eye amidst colorless-pink-colorless transformations. On short UV irradiation, RHBIT provides a two-fold rise in the lifetime for the ring-open form in CHCl3 and DCM compared to the spirolactam (closed form). DFT and TDDFT studies provide electronic characterization for the absorption spectra of the open and closed forms. Using the photoresponsive feature of RHBIT, an information protection application has been enacted via a rewritable platform.

Acid-responsive Emission Color Changes of Diarylaminobenzylideneanilines

Emission color changes of 4-[bis(4-methylphenyl)amino]benzylideneaniline (BMBZA) and 4-[bis(9,9-dimethylfluoren-2-yl)amino]benzylideneaniline (BFBZA) in response to hydrochloric acid (HCl) vapor and solid organic acids, pentafluorobenzoic acid (PFBA) and benzoic acid (BA), have been investigated in their different solid states. Emission color of BMBZA crystals changed from greenish blue to reddish orange by exposure to HCl vapor, whereas the emission color of BFBZA crystals did not. However, powder sample obtained by grinding the BFBZA crystal exhibited emission color change by exposure to HCl vapor. Similar results of emission color changes of BMBZA and BFBZA crystals in response to PFBA were obtained. In the case of BMBZA–BA and BFBZA–BA systems, grinding of the crystalline mixtures allowed their emission color changes without change in their object colors.

Two-in-one platform based on conjugated polymer for ultrasensitive ratiometric detection and efficient removal of perfluoroalkyl substances from environmental water

Continuous emergence of persistent organic pollutants perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) in various water bodies around the world poses a serious threat to the global ecosystem. The exploration of advanced detection/removal techniques to monitor/treat such type of toxicants is urgently required. Herein, we unveiled a donor-acceptor type conjugated polymer PF-DBT-Im as a first-of-its-kind ratiometric fluorescent probe for visual, amplified, and specific monitoring of PFOA and PFOS with ultra-low detection limits of 6.12 nM (PFOA) and 14.3 nM (PFOS), respectively. PF-DBT-Im undergoes strong aggregation after binding with PFOA/PFOS as evident by transmission electron microscopy, zeta potential measurements, and dynamic light scattering studies. This promotes interchain Förster resonance energy transfer process to endorse an obvious emission color change from blue-to-magenta under ultraviolet lamp excitation. Consequently, a smartphone-integrated portable device is fabricated for realizing rapid and on-site detection of PFOA/PFOS. Besides, a new class of magnetic adsorbent Fe3O4@NH2&F13 is also prepared and used in combination with PF-DBT-Im to remove PFOA/PFOS from the environmental water effectively and rapidly as confirmed by liquid chromatography-mass spectrometry analysis. Thus, utilizing the excellent signal amplification property of PF-DBT-Im and the remarkable magnetic separation capability of Fe3O4@NH2&F13, a multifunctional system is developed for step-wise recognition and separation of PFOA/PFOS from the environmental water proficiently and rapidly.

Ultrafast and Multicolor Luminescence Switching in a Lanthanide-Based Hydrochromic Perovskite.

Hydrochromic materials characterized by noticeable color change upon water exposure have attracted pervasive attention for their frontier applications in sensing and information technologies. However, existing hydrochromic materials typically suffer from a slow hydrochromic response as well as limited stability and color tunability. This work describes a novel hydrochromic perovskite crystal composed of zero-dimensional Cs3TbF6:Eu3+, which displays switchable luminescence due to the constituent Tb3+ and Eu3+ ions. Mechanistic investigation reveals that the hydrochromic property stems from a water-induced phase transformation into a one-dimensional structure through a CsF-stripping process. The phase transformation triggers energy coupling between Tb3+ and Eu3+ ions in adjacent lanthanide halide polyhedra, resulting in an emission color change from green to orange. Notably, the phase transformation is ultrafast (20 ms) and reversible, and the emission color in each phase can be fine-tuned by controlling the Eu3+ doping concentration along with Y3+ co-doping. The advances in these hydrochromic luminescent materials offer exciting opportunities for information security and data storage.

Naked-Eye Optical Recognition of Ammonia Vapor and Melamine in Water Using a Fluorophore Appended Polymer Matrix.

The generation of solid-state emitters is a challenge due to the intrinsic aggregation-caused quenching feature of the fluorophores. A conformationally twisted pyridyl π-conjugate as a solid-state emitter is appended with well-known and inexpensive poly(methylmethacrylate) [PMMA] to afford a handy, portable, and reusable solid-state emitting polymer matrix. Entrapment of the probe is noticed through non-covalent interactions, resulting in a green-emitting platform. It quickly accepts a proton upon acid vapor exposure and switches emission from green to red with a significant 107nm redshift. This shift is reversible with red to green emissions while exposed to base vapor. Thus, polymer-blended, homogeneous red-emitting pyridyl salt is employed as potential material to detect various basic vapors optically. Among different bases, naked-eye detection of essential analytes such as ammonia vapor and melamine shows potential demands. Hence, we have established an easy detection of ammonia vapor and aqueous melamine as low as 2.5 and 0.126ppm, respectively, using this solid-state emitter that displays an emission color change with an enhancement of emission intensity even in an aqueous solution.

UV-vis induced emission-color switching in Eu-doped BaNa2MgP2O8

This paper demonstrates emission-color change of BaNa2MgP2O8: Eu through photoredox reactions of dopant europium ions. Europium doped BaNa2MgP2O8 was prepared by high-temperature solid state reaction in air. Under UV excitation, Eu-doped BaNa2MgP2O8 exhibited weak mixed-color emission involving orange and blue components. They were derived from 4 f6-4 f6 intraconfigurational electron transition in Eu3+ and 4 f65d1-4 f7 interconfigurational electron transition in Eu2+, respectively. Since Eu3+ ⇄ Eu2+ photoredox reactions were both facilitated by UV–vis photostimulation, Eu3+/Eu2+ ratio approached to an equilibrium value depending on the stimulation wavelength. The resultant emission color was continuously varied by stimulation light. Optical absorption character and ESR spectroscopy revealed that the photoredox reactions in Eu were caused by electron transfer between Eu and PO43- within BaNa2MgP2O8.

Humidity visualization through a simple thermally activated delayed fluorescent emitter: The role of hydrogen bonding

Humidity monitoring is of great importance in our daily life and industrial process. However, humidity sensors based on fluorescent materials are usually hybrid system, which inevitably causes poor stability and bad repeatability. Herein, a conceptually new class of humidity-responsive thermally activated delayed fluorescent (TADF) material, AcHPM, is presented. AcHPM displays significant emission color changes from deep blue (446 nm) to green (510 nm) under different relative humidity (RH) from 1 % to 95 %. In addition to the high contrast of fluorescent emission (>60 nm), the AcHPM-sensor also shows ultrafast response time (about 20 ms) and good reversibility (>10 cycles). In view of the excellent humidity-responsive performance of AcHPM, the potential application of real-time RH monitoring, anti-counterfeiting, and fingerprint identification were presented by using an AcHPM-equipped test paper. To disclose the mechanism behind the humidity-dependent fluorescence, we further carry out the Fourier transform infrared spectroscopy, powder X-ray diffraction, and theoretical simulation, which jointly verify the critical role of hydrogen bond between water and AcHPM molecule. This work not only extends an avenue to develop humidity sensor with single component for various applications in our daily life, but also provides a new sight to explore the mechanism for the fluorescent-responsive processes.

Moisture-induced phase transition and emission color change for lead-free manganese (II)-Based halide perovskite crystals with greatly improved photoluminescence quantum yield

Halide perovskites with phase transition-sensitive fluorescence color have attracted great attentions in the field of sensing and detection. Though some achievements have been made, the photoluminescence quantum yield (PLQY) for such materials is usually less than 10% before and after phase transition, which is an intrinsic restriction for their application. Herein, we reported a novel lead-free DMAxMnBrx+2 perovskite crystals with moisture-sensitive emission color as well as high PLQY. The result shows that the octahedral coordination of Mn2+ in DMAMnBr3 gives rise to orange emission around 610 nm with PLQY of 41%. While the tetrahedral coordination of Mn2+ in DMA2MnBr4 causes bright green emission nearby 517 nm with a high PLQY of 70%, which is nearly 10 times higher than that of previously reported results. The in situ formation of DMA+ in solution is the key for the dramatically enhanced PLQY. Moreover, as-prepared orange emitted DMAMnBr3 crystals are sensitive to moisture. Just within several tens of second after exposing in the moisture environment, the original orange emission from DMAMnBr3 will turn to green light emission from DMA2MnBr4. The high PLQY of as-prepared DMAxMnBrx+2 perovskite crystals make them possible to be dispersed into non-luminous matrix at a low concentration for moisture detection.