α- and β-[Bmim][BiCl4 (2,2′-bpy)]: Two Polymorphic Bismuth-Containing Ionic Liquids with Crystallization-Induced Phosphorescence

Synthesis and structure-dependent optical properties of two new organic-inorganic hybrid antimony(III) chlorides

Two organic-inorganic hybrid antimony(III) chlorides, namely (C9H26N3)2Sb4Cl18 (1) and (C9H26N3)SbCl6 (2), were prepared using a facile solvent evaporation method. They have low-dimensional structures constructed from SbCl6 octahedra and triply protonated N,N,N',N'',N''-pentamethyldiethylenetriamine cations. The organic cations exhibit different conformations in the two compounds. Compound 1 crystallizes in the centrosymmetric space group P1̄, while compound 2 crystallizes in the noncentrosymmetric space group Pca21. Notably, compound 2 exhibits a moderate second harmonic generation (SHG) response of about 1.3 times that of KH2PO4 (KDP) under 1064 nm laser irradiation. Meanwhile, this compound displays green-yellow luminescence under 342 nm ultraviolet light irradiation, indicating its potential as a bifunctional optical material. Theoretical calculations based on density functional theory were also performed to gain insights into the correlation between their structures and optical properties.

Luminescence Enhancement and Temperature Sensing Properties of Hybrid Bismuth Halides Achieved via Tuning Organic Cations

Bismuth-halide-based inorganic-organic hybrid materials (Bi-IOHMs) are desirable in luminescence-related applications due to their advantages such as low toxicity and chemical stability. Herein, two Bi-IOHMs of [Bpy][BiCl₄(Phen)] (1, Bpy = N-butylpyridinium, Phen = 1,10-phenanthroline) and [PP14][BiCl₄(Phen)]·0.25H₂O (2, PP14 = N-butyl-N-methylpiperidinium), containing different ionic liquid cations and same anionic units, have been synthesized and characterized. Single-crystal X-ray diffraction reveals that compounds 1 and 2 crystallize in the monoclinic space group of P2₁/c and P2₁, respectively. They both possess zero-dimensional ionic structures and exhibit phosphorescence at room temperature upon excitation of UV light (375 nm for 1, 390 nm for 2), with microsecond lifetime (24.13 μs for 1 and 95.37 μs for 2). Hirshfeld surface analysis has been utilized to visually exhibit the different packing motifs and intermolecular interactions in 1 and 2. The variation in ionic liquids makes compound 2 have a more rigid supramolecular structure than 1, resulting in a significant enhancement in photoluminescence quantum yield (PLQY), that is, 0.68% for 1 and 33.24% for 2. In addition, the ratio of the emission intensities for compounds 1 and 2 shows a correlation with temperature. This work provides new insight into luminescence enhancement and temperature sensing applications involving Bi-IOHMs.

Luminescent Organic-Inorganic Hybrid Metal Halides: An Emerging Class of Stimuli-Responsive Materials

Luminescent organic-inorganic metal halides (OIMHs) are well known as a new materials family in recent years. Novel materials and applications of luminescent OIMHs have been explored by changing either the organic component or the metal halide species. Thereinto, the stimuli-responsive (SR) phenomena in OIMHs have drawn much attention recently, for not only their attractive application potential but also the helpfulness in understanding the stability of OIMHs to the external environment. Herein, the luminescent OIMHs that are sensitive to external stimuli including contact, pressure, mechanical grinding, light, heat, and gas molecules, are reviewed, with an emphasis on analyses of the structural change during the SR process. The applications of SR luminescent OIMHs in widespread fields, including gas sensing, information encryption, and rewritable luminescent paper are summarized. Finally, the challenges that deserve to be further explored in this research field are discussed, which provides certain guidance for the future study of SR luminescent OIMHs.

Tunable Photoluminescence Properties of Cotton Fiber With Gradually Changing Crystallinity

The alkali mercerizing process of semicrystalline cotton fiber (CF) is widely used in the printing and dyeing industry. The crystallinity change in the mercerizing process has been studied and certain laws have been obtained, but there is still a certain distance between the theoretical research results and the practical applications. CF is almost composed of cellulose, combined with the photoluminescence (PL) phenomenon of cellulose; herein, the varying crystallinity is correlated with its PL behavior after being treated with different concentrations of NaOH. In line with the characteristics of nonconventional luminogens, CF enjoys excitation-dependent emission and persistent room temperature phosphorescence (p-RTP) behavior. The emission spectra of all samples under the same excitation wavelength indicate that the change of CF crystallinity has a significant impact on its fluorescence and p-RTP emission. As the concentration of NaOH increases, the varying trend of quantum efficiency (QY) is consistent with the changed crystallinity of CF. Interestingly, the lifetime of p-RTP is exactly the opposite of the crystallinity change law. Clustering-triggered emission (CTE), crystallization-Induced Phosphorescence (CIP) mechanism, and the swelling due to hydrated sodium ions can reasonably explain these interesting photophysical processes, which also can be supported by theoretical calculations. The above studies have basically clarified the inherent law between the crystalline change of CF and the PL emission behavior during the alkali treatment process, which can be used as a theoretical reference for real-time monitoring of CF crystallinity changes using the spectral method in the actual cotton mercerizing process.

Ionic indium(III) chloride hybrids incorporating a 2,2'-bipyrimidine ligand: studies on photoluminescence and structural transformation

Although luminescent indium(III) based halide perovskites have been widely investigated, the study of emissive indium(III) halide hybrids is limited. Three indium(III) chloride hybrids based on a bpym ligand were synthesized, namely [EPy]₂[InCl₄(bpym)InCl₄]·DMF (1), [EPy]₂[InCl₄(bpym)InCl₄] (2), and [BPy]₂[InCl₄(bpym)InCl₄] (3) (EPy = N-ethylpyridinium; BPy = N-butylpyridinium; bpym = 2,2'-bipyrimidine). They all exhibit a zero-dimensional structure, in which the ligand bpym interconnects two [InCl₄]^- to form a [InCl₄(bpym)InCl₄]^2- anion that is further charge-compensated by the corresponding pyridinium cations. This is the first time using bpym to coordinate with an In atom. At 298 K, 1 exhibits a weak emission at 600 nm while 2 and 3 exhibit emissions peaking at 500 nm and 540 nm, respectively. Interestingly, the DMF solvent molecule in 1 can be removed by heating, thus resulting in the structural transformation of 1 into 2 together with a photoluminescence (PL) change. Density functional theory (DFT) calculations confirm that halogen-to-ligand charge-transfer (HLCT) occurs in the emission process. To the best of our knowledge, this is the first report on PL of ionic indium(III) halide hybrids incorporating organic ligands.

Crystalline-Phase-Recognition-Induced Domino Phase Transition and Luminescence Switching for Advanced Information Encryption

Herein, a new mechanism, namely, crystalline phase recognition (CPR), is proposed for the single-crystal-to-single-crystal (SCSC) transition of metal halides. Chiral β-[Bmmim]₂ SbCl₅ (Bmmim=1-butyl-2,3-methylimidazolium) can recognize achiral α-[Bmmim]₂ SbCl₅ on the basis of a key-lock feature through intercontact of their single crystals, resulting in a domino phase transition (DPT). The concomitant photoluminescence (PL) switching enables observation of the DPT in situ. The liquid eutectic interface, stress-strain transfer, and feasible thermodynamics are key issues for the CPR. DFT calculations and PL measurements revealed that the optical absorption and emission of the isomers mainly originate from [SbCl₅ ]^2- anions. The structural effects (e.g., supramolecular interactions and [SbCl₅ ]^2- distortion) on the optical emission are clarified. As a novel type of stimuli response, the CPR-induced DPT and luminescence switching exhibit potential for application in advanced time-resolved information encryption.

Tunable Photoluminescence Properties of Microcrystalline Cellulose with Gradually Changing Crystallinity and Crystal Form

Nonconventional luminogens with persistent room temperature phosphoresce (p-RTP) are attracting increasing attention owing to their momentous significance and diverse technical applications in optoelectronic and biomedical. So far, the p-RTP emission of some amorphous powders or single crystals has been studied in depth. The p-RTP emission of amorphous and fully crystalline states and their emission properties are widely divergent, while the difference of their p-RTP emission mechanism is still controversial. The relevance between crystallinity change and p-RTP properties is rarely studied. Furthermore, there is almost no research on the photoluminescence (PL) property change and emission mechanism under the crystal form transformation of semi-crystalline polymer. Herein, microcrystalline cellulose (MCC) is chosen as a model compound to explore its crystallinity and the change in luminescence during the crystal form transformation to make up for this gap. By precisely adjusting the crystallinity and crystal cellulose conversion of MCC, the changing trend of quantum efficiency, and p-RTP lifetime is consistent with the change of crystallinity, and the cellulose I may be more beneficial to PL emission than cellulose II. Clustering-triggered emission mechanism can reasonably explain these interesting photophysical processes, which also can be supported by single-crystal analysis and theoretical calculations.

Synthesis, structural characterization, and luminescence properties of heteroleptic bismuth-organic compounds

The synthesis and photoluminescent properties of four bismuth-organic compounds, their lanthanide doped analogs, and an isostructural europium complex are reported.

2,2′-Bipyridyl-1,1′-dioxide based bismuth(iii) bromide hybrids: studies on crystal structure and luminescence

The choice of organic solvents could induce inorganic–organic bromobismuthate hybrids with diverse metal–ligand linking modes.

X-ray scintillation and photoluminescence of isomorphic ionic bismuth halides with [Amim]+ or [Ammim]+ cations

X-ray scintillation and cyan phosphorescence are observed for three isomorphic ionic bismuth halides.

Controlling information duration on rewritable luminescent paper based on hybrid antimony (III) chloride/small-molecule absorbates

Controlling the duration that information lasts on paper so that it disappears as desired is crucial for information security. However, this area is rarely studied. Here, we report [TEMA]₂SbCl₅ (1, TEMA⁺ = methyltriethylammonium), [TEA]₂SbCl₅ (2, TEA⁺ = tetraethylammonium), [TEBA]₂SbCl₅ (3, TEBA⁺ = benzyltriethylammonium), and [Ph₄P]₂SbCl₅ (4, Ph₄P⁺ = tetraphenylphosphonium) with structure-dependent reversible photoluminescent switching induced by the absorption and thermal release of small guest molecules including H₂O, methanol, and ethylene glycol. Comparing the structural disorder levels, bond lengths, and luminescent Stokes shifts of the compounds aided in understanding their selective absorption behavior. Our results indicated that the information duration on the rewritable paper coated with the title compounds is easily tuned by changing the cation of the compounds, the type of guest molecules, and laser heating power. Our study opens previously unidentified avenues for information security and extends the potential applications of rewritable paper.

Modulation of the Structure and Photoluminescence of Bismuth(III) Chloride Hybrids by Altering the Ionic-Liquid Cations

Two bismuth(III) halides hybrids with room-temperature phosphorescence (RTP), namely, [BPy]₂[Bi₂Cl₈(bpym)] (1, BPy = N-butylpyridinium) and [EPy]₂[Bi₂Cl₈(bpym)] (2, EPy = N-ethylpyridinium), were synthesized and characterized. Structural comparison reveals that 1 and 2 possess similar anionic zigzaglike chain of [Bi₂Cl₈(bpym)]_n^2n-; however, different packing modes of anion/cations and thus different weak interactions. Interestingly, the utilization of pyridinium cations with different length of alkyl chain could tune the RTP behaviors efficiently. The RTP quantum yield (QY) is increased more than 5-fold from 1 to 2 probably due to more rigid structure of 2 arising from the additional H-bond and anion-π interactions, as confirmed by Hirshfeld surfaces analyses and PLATON calculations. Moreover, additional π-π interactions in 1 could stabilize the triplet excitons, leading to an average lifetime of 1 (11.36 ms at 77 K and 1.407 ms at 298 K) being higher than 2 (0.3618 ms at 77 K and 0.07511 ms at 298 K). Density functional theory (DFT) calculations confirm that inorganic moiety to organic ligand charge-transfer (IOCT) is involved in the phosphorescence process. The present work provides a new sight into the design of RTP metal halides through studying the structure-RTP relationship.

Synthesis and photoluminescence of three bismuth(III)-organic compounds bearing heterocyclic N-donor ligands

Three bismuth(iii)-organic compounds, [Bi4Cl8(PDC)2(phen)4]·2MeCN (1), [BiCl3(phen)2] (2), and [Bi2Cl6(terpy)2] (3), were prepared from solvothermal reactions of bismuth chloride, 2,6-pyridinedicarboxylic acid (H2PDC), and 1,10-phenanthroline (phen) or 2,2';6',2''-terpyridine (terpy). The structures were determined through single crystal X-ray diffraction and the compounds were further characterized via powder X-ray diffraction, Raman and infrared spectroscopy, and thermogravimetric analysis. The photoluminescence properties of the solid-state materials were assessed using steady state and time-dependent techniques to obtain excitation and emission profiles as well as lifetimes. The compounds exhibit visible emission ranging from the yellow-green to orange region upon UV excitation. Theoretical quantum mechanical calculations aimed at elucidating the observed emissive behavior show that the transitions can be assigned as predominantly ligand-to-ligand and ligand-to-metal charge transfer transitions. The solid-state structural chemistry, spectroscopic properties, and luminescence behavior of the bismuth compounds are presented herein.

Sb- and Bi-based coordination polymers with N-donor ligands with and without lone-pair effects and their photoluminescence properties

Fifteen new sublimable Sb- and Bi-based chlorido, bromido and iodido coordination polymers (CPs) with linear bispyridyl ligands are presented in this work and compared in terms of their crystal structures and photoluminescence properties. The Sb-CPs occur in two structural motifs: 1∞[Sb2X6(L)2] (X: Cl (a), Br (b), I (c); L: 1,2-bis(4-pyridyl)ethylene (bpe) (1), 1,2-bis(4-pyridyl)ethane (bpa) (2), 4,4'-bipyridine (bipy) (X: Br, I; 3)) with two polymorphs showing negligible stereochemical demand of the lone-pair and 1∞[SbCl3(bipy)] (3a) featuring a stereochemically active lone pair with significant 5p-contribution at SbIII. This is accompanied by differences in the coordination polyhedra being octahedral for high s-character, whereas a high p-character of the lone pair results in a square pyramid as the coordination sphere. The Bi-CPs are represented by the general formula 1∞[Bi2X6(L)2] (X: Cl (a), Br (b), I (c); L: 1,2-bis(4-pyridyl)ethylene (bpe) (4), 1,2-bis(4-pyridyl)ethane (bpa) (5)) and thus show no significant 6p-character of the lone pairs. For examining the parallels and differences between the SbIII- and BiIII-CPs, both are compared in terms of structures and luminescence properties, as well as with related literature known CPs. Altogether, this comparison of structures and properties allows for gaining new insights into the photoluminescence mechanisms of the Sb and Bi-containing CPs. For the first time, distinct hints on the participation of inter-valence charge transfer transitions in E3+-pairs (E: Sb, Bi) were observed for the Sb- and Bi-containing coordination polymers constructed from N-donor ligands.

Phase transitions and photoluminescence switching in hybrid antimony(iii) and bismuth(iii) halides

In this review, the stimuli-responsive phase transitions and photoluminescence switching in inorganic–organic hybrid antimony(iii)/bismuth(iii) halides were summarized.

Highly Luminescent Ionic Liquids Based on Complex Lanthanide Saccharinates

Four strongly luminescent ionic liquids with complex lanthanide saccharinate anions, [C₄mim]₃[Eu(Sac)₆(H₂O)₂] (1), {C₄mpy}₅{[Ln(Sac)₆(H₂O)₂][Ln(Sac)₅(H₂O)₃]}{(H₂O)₂(CH₃CN)₂} (Ln = Sm for 2a; Ln = Eu for 2b), and [C₄mpy]₃[Eu(Sac)₆][CH₃CN] (3) (C₄mim = 1-butyl-3-methylimidazolium; C₄mpy = N-butyl-4-methylpyridinium; Sac = saccharinate), have been obtained by reacting the ionic liquids 1-butyl-3-methylimidazolium saccharinate, [C₄mim][Sac], and N-butyl-4-methylpyridinium saccharinate, [C₄mpy][Sac], with the respective lanthanide saccharinates. Single-crystal X-ray diffraction analyses reveal the respective lanthanide center to be six- or eight-coordinated by five or six saccharinate anions and two or three aqua ligands in the cases of 1 and 2 when lanthanide saccharinated hydrate was employed as the starting material. Coordination of water to the lanthanide can be avoided when using the anhydrous lanthanide saccharinate as shown by the structure of 3. Using a co-solvent, acetonitrile, to facilitate the reaction led to incorporation of solvent molecules into the crystal structure of the final materials (2 and 3). Differential scanning calorimetry analyses reveal that 1 is an ionic liquid whereas 2 and 3 are low-temperature molten salts. The three europium(III)-containing compounds (1, 2b, and 3) all show characteristic intense red emissions of Eu(III) upon excitation into levels of the saccharinate ligands (321 nm) or Eu(III) (393 nm). At room temperature, the decay times of 1 and 2b are all ∼0.5 ms, whereas the decay time of 3 amounts to 3.85 ms due to removal of aqua ligands in the first coordination sphere; accordingly, the quantum efficiencies of 1, 2b, and 3 were determined to be 15.9%, 22.95%, and 57.75%, respectively. The CIE chromaticity coordinates for all Eu compounds are in the red region and approach the NTSC standard CIE values when the temperature is increased. Sm(III)-containing compound 2a shows characteristic Sm(III) emission peaks. As expected, the CIE coordinates of samarium compound 2a fall in the orange-red region.

Efficient modulation of photoluminescence by hydrogen bonding interactions between inorganic [MnBr4]2- anions and organic cations

The different hydrogen bond interactions in two organic-inorganic hybrid manganese halide compounds, namely [A]2[MnBr4] (A = N-butyl-N-methylpyrrolidinium ([P14]+) for (1) and N-butyl-N-methylpiperidinium ([PP14]+) for (2)), lead to distinct photoluminescence quantum yields (81% for 1; 55% for 2). Further applications of luminescent 1 are also developed.

Bismuth(iii)-thiophenedicarboxylates as host frameworks for lanthanide ions: synthesis, structural characterization, and photoluminescent behavior

Three bismuth-2,5-thiophenedicarboxylates (Bi-TDC) and two europium-2,5-thiophenedicarboxylates (Eu-TDC) were synthesized under ambient conditions. The structures were determined through single crystal X-ray diffraction, and three of the phases were further characterized by powder X-ray diffraction, Raman spectroscopy, and thermogravimetric analysis. Reactions of bismuth nitrate, 2,5-thiophenedicarboxylate, and pyridine in an acidic solution of acetic acid and ethanol yield Hpy[Bi(TDC)2(H2O)]·1.5H2O (1), whereas reactions in a water/ethanol mixture produce a minor phase, [Hpy]3[Bi2(TDC)4(HTDC)(H2O)]·xH2O (2) along with a major product, (Hpy)2[Bi(TDC)2(HTDC)]·0.36H2O (3). The structures of 1-3 are all built from anionic Bi-TDC chains that are further bridged through additional TDC linkages into interpenetrated 2D sheets. Addition of an aqueous lanthanide solution to the reaction mixtures that yielded 1 and 2-3 resulted in the formation of doped phases, Hpy[Bi1-xLnx(TDC)2(H2O)]·1.5H2O (Bi1-xLnx-1), where Ln = Nd, Sm, Eu, Tb, Dy, and Yb, and (Hpy)2[Bi0.99Eu0.01 (TDC)2(HTDC)]·0.36H2O (Bi0.99Eu0.01-3). Using europium nitrate rather than the bismuth precursor resulted in the formation of two homometallic europium based phases, [Eu(TDC)(NO3)(H2O)]n (4) and [Eu2(TDC)3(H2O)9]·5H2O (5), which adopt an extended 3D network and an interpenetrated 2D structure, respectively. Photophysical measurements were carried out for 1 and the lanthanide containing phases and quantum yield and lifetime values were determined for the visible light emitters. Herein, the structural chemistry, spectroscopic properties, and luminescence of the bismuth phases, their lanthanide doped analogs, and the europium compounds are presented.

Bismuth as a versatile cation for luminescence in coordination polymers from BiX3/4,4'-bipy: understanding of photophysics by quantum chemical calculations and structural parallels to lanthanides

Coordination polymers (CPs) with bismuth(iii) as a connectivity centre have been prepared from BiX3 (X = Cl-I) and 4,4'-bipyridine (bipy) in order to implement Bi-based luminescence. The products were obtained via different synthetic routes such as solution chemistry, melt syntheses or mechanochemical reactions. Five neutral and anionic 1D-CPs are presented that show a chemical parallel to trivalent lanthanides forming isostructural or closely related 1D-CPs, of which five additional compounds are described. Bi3+ proves to be a versatile cation for luminescence resulting from energy transfer processes between a metal and a ligand in the presented CPs. Quantum chemical calculations were carried out to investigate Bi3+-participation in the luminescence processes. The calculated results allow an assignment of the bright transitions composed of mainly metal-to-ligand-charge transfer (MLCT) character. These results show that Bi3+ can form strongly luminescent coordination compounds with N-donor ligands.

A novel N-methylimidazolium-based poly(ionic liquid) to recover trace tetrachloroaurate from aqueous solution based on multiple supramolecular interactions

Two novel ionic liquids, [Bzmim]Cl and [Bzmim][AuCl₄], were synthesized to recover Au(iii) in aqueous solution.

The structures, water stabilities and photoluminescence properties of two types of iodocuprate(i)-based hybrids

The effects of aromatic and aliphatic cations on the structure, water stability and photoluminescence of iodocuprate(i)-based materials were disclosed.