Two cyclometalated complexes [Ir(dfppy)2(aip)](PF6) (1) and [Ir(ppy)2(aip)](PF6) (2) have been synthesized based on a photoactive anthracene-based ligand aip and cyclometalating ligands dfppyH and ppyH [dfppyH = (2-(2,4-difluorophenyl)-pyridine), ppyH = 2-phenyl-pyridine]. Their crystal structures indicate that an aip ligand uses its phenanthroline moiety to chelate an {Ir(dfppy)2}+ unit in 1, while an {Ir(ppy)2}+ unit in 2. In CH2Cl2, the anthracene units in aip, 1 and 2 underwent photo-oxidation upon irradiation with 365 nm light, forming species aip-O, 1-O and 2-O, respectively. This photo-oxidation resulted in luminescence switching, from a luminescent state (emission at 493 nm) to a non-luminescent state for aip, while from a non-luminescent state to a luminescent state with an emission at 519 nm for 1 and 578 nm for 2. Additionally, the luminescence of aip, 1-O and 2-O in CH2Cl2 can be modulated by using TFA to protonate the imidazole units and/or non-coordinated phenanthroline moiety in these compounds. Upon adding TFA, aip showed luminescence quenching, while species 1-O and 2-O revealed both luminescence-intensity decrease and emission-wavelength increase (Δλ = 9 nm for 1-O, and Δλ = 4 nm for 2-O). In this paper, we discuss the luminescence switching/modulation of aip, 1 and 2 by light-irradiation-induced photo-oxidation of their anthracene units and by TFA treatment.

Cyclometalated Ir(III) complexes [Ir(dfppy)2(qbiH)](PF6) (1), [Ir(dfppy)2(qbim)](PF6)·H2O (2), [Ir(dfppy)2(qbio)](PF6) (3) and [Ir(dfppy)2(qbi)] (4) have been designed and prepared, in which the N^N ligands qbiH, qbim and qbio incorporate different substituent groups R on their imidazole units (H atom, CH3 group and n-C8H17 group, respectively) in order to explore the influence of the substituent groups R and the protonation/deprotonation state of imidazole units in these Ir(III) complexes on their structures and luminescence behaviors. Crystal structures indicate that an {Ir(dfppy)2}+ unit is coordinated by neutral ligands qbiH in 1, qbim in 2 and qbio in 3, while a qbi− anion in 4. These Ir(III) complexes show clearly different molecular stacking modes. In compound 1, neighboring [Ir(dfppy)2(qbiH)]+ cations are linked into a supramolecular chain through π⋯π stacking interactions between adjacent dfppy−/qbiH ligands. In 2 and 4, two neighboring iridium complex units connect each other through π⋯π stacking interactions between dfppy− ligands in the former, while between qbi− ligands in the latter, forming supramolecular dimers. Compared to 1, 2 and 4, compound 3 only exhibits intermolecular van der Waals interactions. At room temperature, these Ir(III) complexes in CH2Cl2 reveal phosphorescence with a mixing of 3MLCT and 3LC characters, emissions at 558 and 585 nm for 1, 572 (or 573) and 600 nm for 2 and 3, and 546 nm for 4. Compared to 1–3, compound 4 displays relatively weak luminescence intensity. Interestingly, upon addition of NEt3/TFA, both 1 and 4 in CH2Cl2 can switch their luminescence between strong emission at 558 nm and weak emission at 546 nm, due to their acid-/base-induced structural interconversion between the protonation state and the deprotonation state of the qbiH ligand. The emissions of 1–4 in the solid state reveal different degrees of the red shift compared to their corresponding emissions in CH2Cl2, the broad emission bands at 542, 572 and 611 nm for 1, 553, 581 and 612 nm for 2, 544, 578 and 630 nm for 3, and 595 and 633 nm for 4. Based on the crystal structures of 1–4, this work discusses the luminescence modulation of these Ir(III) complexes by varying their substituent groups or the protonation/deprotonation state of the imidazole units.

Two isomeric Ir(III) complexes Ir–O and Ir–R arising from the different coordination mode of a naphthalene-containing ligand, show distinct luminescence, self-assembly ability and cellular imaging behaviors.

Based on ligands dfppyH and pidpyH, cyclometalated Ir(III) complexes [Ir(dfppy)2(pidpyH)](PF6) (1·PF6) and [Ir(dfppy)2(pidpy)] (2) have been synthesized. The crystal structures indicate that each {Ir(dfppy)2}+ unit is coordinated by a neutral ligand pidpyH in 1·PF6, while by a pidpy− anion in 2. The packing structure of 1·PF6 only exhibits electrostatic interactions and van der Waals interactions among [Ir(dfppy)2(pidpyH)]+ cations and PF6− ions. In contrast, the neighboring molecules in 2 are linked into a supramolecular chain structure through aromatic stacking interactions between two dfppy− ligands. In solution, 1·PF6 and 2 show acid/base-induced structural transformation due to the protonation/deprotonation of their pyridyl groups and/or imidazole units, which can be confirmed by their 1H NMR spectra. At room temperature, compounds 1·PF6, 2 and pidpyH in CH2Cl2 reveal TFA-induced luminescence switching behaviors, from a non-luminescence state to a luminescence state with an emission at 582 nm for both 1·PF6 and 2, and emission switching from 392 nm to 502 nm for pidpyH. These switching behaviors are associated with the protonation of pyridyl groups and/or imidazole units in 1·PF6, 2 and pidpyH. Moreover, compounds 1·PF6 and 2 were used as photosensitizers (PS) for reduction of water to hydrogen under the same experimental conditions. It was found that the amount of evolved hydrogen and the PS turnover number are 512 μmol and 102 for 1·PF6, and 131 μmol and 26 for 2, respectively. Thus, compound 1·PF6 has better photocatalytic activity than 2. In this paper, we discuss the modulation of luminescence and photocatalytic activities of 1·PF6 and 2 by varying the coordination mode and/or protonation extent of pidpyH/pidpy− ligands.

2-(Anthracenyl)-4,5-bis(2,5-dimethyl(3-thienyl))-1H-imidazole (anbdtiH) has been synthesized. Its three solid-state structures, anbdtiH·CH3Cl (1), anbdtiH·2CH3OH (2) and anbdtiH2·CF3COO·CH3OH·H2O (3), have been constructed by skillfully choosing CHCl3 or CH3OH as the solvent and using or not using CF3COOH, with the aim of modifying the intermolecular hydrogen bonds and/or π⋯π stacking interactions. The three distinct structures show significantly different solid-state luminescence behaviors, an orange-red emission at 603 nm for 1, a blue emission at 453 nm for 2, and a green emission at 533 nm for 3. Upon grinding, these emission wavelengths exhibit evident variations, a blue-shift of Δλ = 83 nm for 1, a red-shift of Δλ = 20 nm for 2, and a blue-shift of Δλ = 54 nm for 3. The emission color of 1 can be reversibly switched between orange-red and green upon regulation of the intermolecular (N–H)imidazole⋯Nimidazole hydrogen bonds by a grinding–heating process. Moreover, compound 3 can undergo a solid-state [4π + 4π] photodimerization reaction upon irradiation with sunlight, forming 3-dimer. Based on the crystal structures of 1–3, this work discusses the relationship between the molecular stacking mode and the luminescence behavior/photochemical reactivity.

Heteroleptic complexes [Ir(dfppy)2(BrL)]·3CH3OH (1) and [Pt(dfppy)(BrL)]·CH3OH (2) have been prepared based on the same ligands including bisthienylethene BrLH and dfppyH = (2-(2,4-difluorophenyl)-pyridine). Complexes 1 and 2 reveal distinct crystal structures. The BrL− anion uses its phenol-imidazole moiety to coordinate with an {Ir(dfppy)2}+ unit in the former, while with a {Pt(dfppy)}+ unit in the latter. Neighboring [Ir(dfppy)2(BrL)]/[Pt(dfppy)(BrL)] molecules are connected through extensive hydrogen bonds and aromatic stacking interactions, thus forming a supramolecular chain structure in 1, and a layer structure in 2. Upon irradiation with 380 nm light, compound 2 shows photochromic behavior in CH2Cl2, with a color change from nearly colorless to light green. However, no photochromism was observed in compound 1. At room temperature, compound 1 reveals phosphorescence with a predominant 3MLCT character both in CH2Cl2 solution (emissions at 495 and 513 nm) and in the solid state (emission at 524 nm). Compound 2 exhibits phosphorescence with a predominant 3LC character in CH2Cl2 solution (emission at 508 nm), but it is almost non-luminescent in the solid state. Our experimental results demonstrate that the metal centers in 1 and 2 could significantly influence their structures, photochromism, and luminescence behaviors.

Molecular assembly of bisthienylethene Th2im (1) and [ReCl6]2− anions leads to the complex (Th2imH)2[ReCl6] (2), in which a [ReCl6]2− anion connects two equivalent Th2imH+ cations through Cl⋯N/C hydrogen bonds. Crystal structures of 1 and 2 indicate that two thiophene groups of each Th2im/Th2imH+ molecule adopt a photoactive antiparallel conformation. Thus, two compounds show crystalline-phase photochromism (CPP), i.e. reversible structural transformation between the open form and the closed form upon alternately irradiating the sample with UV light (365 nm) and visible light (574 nm for 1, 624 nm for 2). It was found that the CPP behaviors of 1 and 2 could regulate their luminescence and/or magnetic properties. Their solid-state emissions (433, 448, 482, 531 and 570 nm for 1, and 460, 489, 535 and 593 nm for 2) exhibited weaker intensities after UV irradiation with 365 nm light. Besides CPP and luminescence, compound 2 shows field-induced slow magnetic relaxation. Before and after UV irradiation, this compound revealed different magnetic behaviors, including the differences in the shape of the χMT vs. T plot, D parameter, and the values of the relaxation barrier Ueff and the preexponential factor τ0.

Two new bisthienylethenes containing an imidazole bridge unit have been synthesized, namely tBuLH and tBuLMeH, which incorporate –C(CH3)3 and/or –CH3 substituent groups. Based on tBuLH and tBuLMeH, heteroleptic complexes [Ir(dfppy)2(tBuL)]·2CH3OH (1) and Ir(dfppy)2(tBuLMe)] (2) were prepared [dfppyH = 2-(2,4-difluorophenyl)-pyridine]. Crystal structures of tBuLH·HOAc and 1 were measured, indicating that two thiophene groups adopt antiparallel conformations in the former, while parallel conformations in the latter. In compound 1, a tBuL− ligand uses its phenol–imidazole moiety to coordinate with an {Ir(dfppy)2}+ unit, and neighboring [Ir(dfppy)2(tBuL)] molecules interact through van der Waals interactions. Upon irradiation with 326 nm/321 nm light, both tBuLH and tBuLMeH show photochromic behavior in CH2Cl2. However, no photochromism has been observed in 1 and 2. At room temperature, tBuLH and tBuLMeH in CH2Cl2 exhibit fluorescence emission at 464 nm and 479 nm, respectively. Compounds 1 and 2 in CH2Cl2 reveal phosphorescence emission at 514 nm and 507 nm, respectively, with a mixed 3MLCT/3LC character. This work discusses the influence of –C(CH3)3 and –CH3 substituent groups on the structures, photochromism and luminescence of tBuLH, tBuLMeH, 1 and 2.

Two bisthienylethenes 2-(2-hydroxyphenyl)-4,5-bis[2,5-dimethyl(3-thienyl)]-1H-imidazole (L1H) and 2-(2-hydroxyphenyl)-4,5-bis(2,5-dimethyl(3-thienyl))-1-phenyl-imidazole (L2H), which have a chelating N,O-donor binding site attached to the photochromic core, have been synthesized using a one-pot condensation reaction, and used to prepare the heteroleptic complexes [Ir(dfppy)2(L1)]·2CH3OH (1) and [Ir(dfppy)2(L2)] (2) [dfppyH = 2-(2,4-difluorophenyl)-pyridine]. In the crystal structures of all four compounds, two thiophene groups of each bisthienylethene molecule adopt parallel conformation. Neighboring molecules in L1H and 1 are linked into supramolecular chains through hydrogen bonds. Particularly, the packing structure of 1 contains right- and left-handed 21 helical chains. In contrast, neighboring molecules in L2H and 2 interact only through van der Waals interactions. At room temperature, L1H and L2H in CH2Cl2 show fluorescence emission at 442 nm and 469 nm, respectively. Compounds 1 and 2 in CH2Cl2 reveal broad emission band characteristics of the Ir(III)/dfppy− chromophores at 508 nm and 494 nm, respectively, with a mixing of 3MLCT and 3LC characters. At room temperature, the photochromism ability of L2H in CH2Cl2 is clearly weaker than that of L1H. Moreover, no photochromism has been observed in 1 and 2. It has been demonstrated that both the substituent group and {Ir(dfppy)2}+ coordination could significantly influence the crystal structures, luminescence and photochromic properties of L1H, L2H, 1 and 2.

Based on two new bisthienylethenes containing N,O-donor binding sites, 2-(2-hydroxy-5-bromo-phenyl)-4,5-bis(2,5-dimethyl(3-thienyl))-1H-imidazole (BrLH) and 2-(2-hydroxy-5-diethylphosphono-phenyl)-4,5-bis(2,5-dimethyl(3-thienyl))-1H-imidazole (PLH), multifunctional mononuclear complexes Co(BrL)2·3CH3OH (1) and Co(PL)2·2CH3OH (2) have been synthesized and characterized by crystallographic analysis. In the molecular structures of 1 and 2, the Co(II) ion adopts a distorted tetrahedral coordination geometry, and is coordinated by two nonequivalent bisthienylethene molecules (BrL− in 1, PL− in 2), showing non-photoactive parallel and photoactive antiparallel conformations, respectively. Compounds 1 and 2 show a distinct distortion of Co(II) coordination geometry, with bond angles of N–Co–N = 112.71(12)° and O–Co–O = 99.87(11)° for 1 and N–Co–N = 119.93(12)° and O–Co–O = 107.31(13)° for 2. Thus, 1 and 2 revealed different magnetic behaviors, which are demonstrated by the χMT vs. T plots, and the frequency dependence of the χ′M and χ′′M signals at low temperature. Besides the field-induced slow magnetic relaxation, both 1 and 2 also showed photochromic behavior. Upon irradiation with 360 nm light for 1 and 343 nm light for 2, their CH2Cl2–CH3CN solutions could change color from being nearly colorless to blue purple. It was demonstrated that the substituent groups of Br atom and –PO(OEt)2 in 1 and 2, respectively, could significantly influence their crystal structures, magnetic relaxations and photochromic properties.

Complexes [Ir(dfppy)2(pbdtiH)](PF6)·2CHCl3 (1-H) and [Ir(dfppy)2(pbdti)] (1) were synthesized by the reaction of bisthienylethene pbdtiH and an [Ir(dfppy)2Cl]2 dimer under neutral and basic conditions, respectively. Thus, the {Ir(dfppy)2}+ unit is coordinated by pbdtiH in 1-H, and by pbdti− in 1, which are confirmed by their crystal structures. The structures of 1-H and 1 could be interconverted in solution, upon alternately adding NEt3 and TFA, thus resulting in reversible luminescence switching between the on-state of 1-H and the off-state of 1 at room temperature. In addition, both 1-H and 1 show solid-state luminescence, with a broad emission at 534 nm and 525 nm, respectively. The free pbdtiH ligand shows photochromic behavior in CH2Cl2 solution. However, no photochromism has been observed in 1-H and 1, indicating that the coordination of the pbdtiH/pbdti− ligand to the {Ir(dfppy)2}+ unit could suppress their photochromic behaviors.

A new bisthienylethene 2-(2-hydroxynaphthyl)-4,5-bis(2,5-dimethyl(3-thienyl))-1H-imidazole (hnbdtiH) has been synthesized and applied for the coordination-assembly generation of two heteroleptic complexes [Ir(dfppy)2(hnbdti)]·2CH3OH (1) and [Ir(ppy)2(hnbdti)]·CH3OH (2) [dfppyH = 2-(2,4-difluorophenyl)-pyridine, ppyH = 2-phenyl-pyridine]. In the crystal structure of hnbdtiH, neighboring molecules are linked into a helical chain through N–H⋯π interactions between imidazole and naphthol rings. In 1 and 2, the {Ir(dfppy)2}+ or {Ir(ppy)2}+ unit is chelated by a hnbdti− ligand using both N and O binding sites, and the intramolecular aromatic stacking interactions are formed between the thiophene ring and the 2,4-difluorophenyl/phenyl group. The crystal structures of 1 and 2 indicate that neighboring molecules are connected by CH3OH molecules through hydrogen bond interactions, forming the [Ir⋯(CH3OH)4⋯Ir] dimer in the former, and the [Ir⋯(CH3OH)2⋯Ir] dimer in the latter. These dimers in 1 pack together through the intermolecular aromatic stacking interactions, while there are only van der Waals interactions in 2, resulting in their distinct luminescence behavior. At room temperature, 1 exhibits aggregation-induced phosphorescence emission. In contrast, 2 is almost non-luminescent both in solution and in the solid state. Compared with 1 and 2, free ligand hnbdtiH shows fluorescence both in CH2Cl2 solution and in the solid state. Moreover, no photochromism has been observed in hnbdtiH, 1 and 2.

A new dithienylethene compound 2-(4-diethylphosphonophenyl)-4,5-bis(2,5-dimethyl(3-thienyl))-1H-imidazole (dbi) has been synthesized and applied for the coordination-assembly generation of a mononuclear complex [Dy(hfac)3(dbi)2]·CH3OH (1) (hfac = hexafluoroacetylacetonate), in which dbi ligands coordinate to Dy(III) ion through phosphoryl oxygen atoms. In the crystal structure of dbi, each molecule shows a parallel conformation, and neighboring molecules are linked into an interesting helical chain through hydrogen bonds. In contrast, each dbi ligand in 1 exhibits a photoactive antiparallel conformation, thus resulting in crystalline-phase photochromism (CPP) behavior of 1. Before and after CPP reaction, 1 shows different magnetic and luminescence properties. After irradiation with 365 nm light, the shape of the χMT vs. T plot revealed drastic changes, and field-induced χM′′ peaks were observed. Compound 1 exhibits ligand- and Dy(III)-centered fluorescence emissions. The irradiation with 365 nm light resulted in the red shift of dbi-centered emissions, and the decrease in intensity of Dy(III)-centered emission.

Mononuclear complex Co(hpbdti)2·3CH3OH (1) was synthesized [hpbdtiH = 2-(2-hydroxyphenyl)-4,5-bis(2,5-dimethyl(3-thienyl))-1H-imidazole], showing multiple-step field-induced slow magnetic relaxation behaviors, and photochromic properties in CH2Cl2–CH3CN solution.

Two kinds of solid-state structures of 5-phosphonomethyl-8-hydroxyquinoline (5pm8hqH3) have been obtained, namely 1·HCl·H2O and 1·H2O, involving different hydrogen bonds and/or aromatic stacking interactions. As a derivative of 5pm8hqH3, 5-phosphonomethyl-8-(carboxymethoxy)quinoline (5pm8cmoqH3) was synthesized. Based on 5pm8hqH3 and 5pm8cmoqH3, three new metal phosphonates have been hydrothermally prepared, including Zn(5pm8hqH)(H2O)·H2O (2), Cu(5pm8cmoqH)·2H2O (3) and Fe(5pm8cmoqH) (4), exhibiting layered structures for 2 and 4, and a three-dimensional open framework for 3. The 8-hydroxyquinoline moieties in 1·H2O and 2–4 exhibit three kinds of interesting aromatic stacking modes, including pyridine ring–pyridine ring stacking between a pair of moieties, double benzene ring–pyridine ring stacking between a pair of moieties and alternating benzene ring–benzene ring and pyridine ring–pyridine ring stacking among a number of moieties in the layered structure. The solid-state fluorescence measurements indicate the emissions of 1·HCl·H2O and 1·H2O are significantly different due to their distinct packing structures. Compound 2 exhibits both ligand-centered (LC) and ligand-to-metal charge transition (LMCT) emissions. Magnetic studies reveal dominant antiferromagnetic interactions in 3 and 4.

Four mononuclear lanthanide complexes, [Ln(hfac)3(depma)(H2O)] [Ln(III) = Dy (1), Gd (2)], [Dy(hfac)3(depma)2]2·H2O (3) and [Gd(hfac)3(depma)2]·2H2O (4), have been obtained (hfac = hexafluoroacetylacetonate, depma = 9-diethylphosphonomethyl anthracene) by using one (for 1 and 2) or two (for 3 and 4) depma molecules to substitute coordination water molecules of Ln(hfac)3(H2O)2. It was found that the number of introduced depma ligands can modify the coordination geometry of Ln(III) ions, showing a distorted biscapped triangular prism geometry in isostructural 1 and 2 and a distorted square-antiprismatic geometry in 3 and 4. Magnetic studies reveal that both 1 and 3 show field-induced slow magnetic relaxation under the applied dc field of 1000 Oe. The solid-state fluorescence measurements indicate the presence of multicomponent emissions in 1 and 3, including ligand-centered (LC) emissions from hfac and depma, and yellow emission from Dy(III) ions and only ligand-centered (LC) emissions in 2 and 4.

Reaction of the cyclic phosphonate ester of anthracen-9-yl(5,5-dimethyl-2-oxo-2λ5-1,3,2-dioxaphosphinan-2-yl)-methanol (1) with TMSBr unexpectedly generated phosphonates 2-(10-bromoanthracen-9-ylmethyl)-5,5-dimethyl-2λ5-1,3,2-dioxa-phosphinan-2-one (2) and (10-bromoanthrancen-9-yl)methyl-phosphonate mono(3-bromo-2,2′-dimethylpropyl) ester (3) through a series of bond cleavages and formations. Compound 3 could transform into 2 in CH3OH–H2O solution, but it is stable in dry CH3CN. The structures of 1–3 have been characterized by IR spectra, 1H NMR spectra, and crystal structures. In contrast to 1, the dioxaphosphorinane ring is open in 3, but not changed in 2. In addition, both 2 and 3 contain a bromine-substituted anthracene group, and have no hydroxyl group as that in 1. For compounds 1–3, their crystal structures and fluorescent properties in CH2Cl2 solution and in the solid state have been studied.

Reactions of N-(phosphinomethyl)iminodiacetic acid (H3L) and cobalt or copper salts in aqueous solutions result in two new metal phosphinates, [Co(H2O)6][Co2L2]·2H2O (1) and [Cu(H2O)4][Cu2L2] (2). Both compounds exhibit different supramolecular layered structures, which result from the assembly of cationic and anionic moieties through hydrogen bond interactions. In 1, {Co2O2} dimers are linked through O–P–O units into a {[Co2L2]}n2n− double chain and [Co(H2O)6]2+ serve as a counterion. Compound 2 contains two kinds of chain-like moieties, i.e. {[Cu2L2]}n2n− and {[Cu(H2O)4]}n2n+. The former is made up of corner-sharing {CuO4N} square pyramids and {PHO2C} tetrahedra and the latter is formed through edge-sharing connections of {CuO6} octahedra. Magnetic studies reveal a dominant ferromagnetic interaction mediated between the magnetic centers in 1, while antiferromagnetic interactions are dominant in 2.

Isostructural lanthanide oxalatophosphonates Ln(5pm8hqH3)(C2O4)1.5(H2O)·2H2O [Ln(III) = Eu(1), Gd(2), Tb(3), Dy(4)] have been obtained through hydrothermal reactions of 5-phosphonomethyl-8-hydroxyquinoline (5pm8hqH3), oxalate, and Ln(NO3)3·6H2O. Four compounds reveal layer structures, in which {LnO8} polyhedra are linked through oxalate units into a netlike {Ln(C2O4)1.5}n layer containing 24-member ring composed of six Ln(III) ions and six oxalate anions. The noncoordinated 8-hydroxyquinoline groups protrude from two sides of the layer. Neighboring layers are further connected to each other through hydrogen bonds and aromatic stacking of the 8-hydroxyquinoline rings, forming a supramolecular 3D structure. In magnetic measurements, compound 2 shows weak antiferromagnetic interactions. The magnetic behaviors of 1, 3 and 4 are ascribed to the depopulation of the Stark levels and weak antiferromagnetic interactions. In the solid-state fluorescent measurements, strong ligand-centered emissions and weak Ln(III)-centered emissions can be observed for 1 and 4, while only ligand-centered emissions for 2 and 3.

Reactions of 2-(1-Imidazole)-1-hydroxyl-1,1′-ethylidenediphosphonic acid (ImhedpH4) and cobalt or manganese salts under hydrothermal conditions result in three new metal diphosphonates: β-Co3(ImhedpH)2(H2O)4·2H2O (1), Co3(ImhedpH)2(H2O)4 (2), and Mn(ImhedpH2)·H2O (3). In compound 1, the columns made up of {Co12O2} dimers and {PO3C} tetrahedra through corner-sharing are cross-linked through {Co2O6} octahedra, forming an inorganic layer. Neighboring layers are pillared by coordinated imidazole groups of ImhedpH− ligands, leading to a three-dimensional open framework containing two kinds of channels with sizes of 8.256 × 9.851 Å and 8.030 × 4.745 Å (van der Waals radii not accounted for). Compound 2 shows a layer structure, in which Co3(ImhedpH)2(H2O)4 trimer units are connected through the corner-sharing of {Co1O5} trigonal bipyramids and {PO3C} tetrahedra, forming an inorganic layer containing 20-member rings composed of six Co atoms, two μ3-O1 units, and four O−P−O units. The noncoordinated imidazole groups protrude from two sides of the layer. Compound 3 shows a ladder structure, where the Mn(II) ions are bridged by ImhedpH22− ligands through double O−P−O units to form a single chain, and two such chains are further fused together by sharing edges of {MnO5} trigonal bipyramids. The magnetic properties of 1−3 have been studied. Ferrimagnetism and field-induced magnetic transition from ferrimagnetism to a fully polarized state are observed in 1. Compounds 2 and 3 reveal dominant antiferromagnetic interactions between metal centers, and two-step field-induced magnetic phase transitions are found in 2.

On the basis of (4-carboxypiperidyl)-N-methylenephosphonic acid (4-cpmpH3), four isostructural lanthanide phosphonates Ln(O3PCH2−NC5H9−COO)(H2O)2·xH2O [Ln = Eu(1), Gd(2), Tb(3), Dy(4)] have been synthesized. All exhibit three-dimensional open framework structures, in which inorganic double chains made up of {Ln2O14} dimers and {PO3C} linkages are connected through organic moieties of 4-cpmp3−. Uniform parallelogram-like channels are observed in the [100] direction. Luminescent properties of 1 and 3, and the magnetic properties of 2 are investigated.

Four mononuclear lanthanide complexes, [Ln(hfac)3(depma)(H2O)] [Ln(III) = Dy (1), Gd (2)], [Dy(hfac)3(depma)2]2·H2O (3) and [Gd(hfac)3(depma)2]·2H2O (4), have been obtained (hfac = hexafluoroacetylacetonate, depma = 9-diethylphosphonomethyl anthracene) by using one (for 1 and 2) or two (for 3 and 4) depma molecules to substitute coordination water molecules of Ln(hfac)3(H2O)2. It was found that the number of introduced depma ligands can modify the coordination geometry of Ln(III) ions, showing a distorted biscapped triangular prism geometry in isostructural 1 and 2 and a distorted square-antiprismatic geometry in 3 and 4. Magnetic studies reveal that both 1 and 3 show field-induced slow magnetic relaxation under the applied dc field of 1000 Oe. The solid-state fluorescence measurements indicate the presence of multicomponent emissions in 1 and 3, including ligand-centered (LC) emissions from hfac and depma, and yellow emission from Dy(III) ions and only ligand-centered (LC) emissions in 2 and 4.

Based on two new bisthienylethenes containing N,O-donor binding sites, 2-(2-hydroxy-5-bromo-phenyl)-4,5-bis(2,5-dimethyl(3-thienyl))-1H-imidazole (BrLH) and 2-(2-hydroxy-5-diethylphosphono-phenyl)-4,5-bis(2,5-dimethyl(3-thienyl))-1H-imidazole (PLH), multifunctional mononuclear complexes Co(BrL)2·3CH3OH (1) and Co(PL)2·2CH3OH (2) have been synthesized and characterized by crystallographic analysis. In the molecular structures of 1 and 2, the Co(II) ion adopts a distorted tetrahedral coordination geometry, and is coordinated by two nonequivalent bisthienylethene molecules (BrL− in 1, PL− in 2), showing non-photoactive parallel and photoactive antiparallel conformations, respectively. Compounds 1 and 2 show a distinct distortion of Co(II) coordination geometry, with bond angles of N–Co–N = 112.71(12)° and O–Co–O = 99.87(11)° for 1 and N–Co–N = 119.93(12)° and O–Co–O = 107.31(13)° for 2. Thus, 1 and 2 revealed different magnetic behaviors, which are demonstrated by the χMT vs. T plots, and the frequency dependence of the χ′M and χ′′M signals at low temperature. Besides the field-induced slow magnetic relaxation, both 1 and 2 also showed photochromic behavior. Upon irradiation with 360 nm light for 1 and 343 nm light for 2, their CH2Cl2–CH3CN solutions could change color from being nearly colorless to blue purple. It was demonstrated that the substituent groups of Br atom and –PO(OEt)2 in 1 and 2, respectively, could significantly influence their crystal structures, magnetic relaxations and photochromic properties.

Two bisthienylethenes 2-(2-hydroxyphenyl)-4,5-bis[2,5-dimethyl(3-thienyl)]-1H-imidazole (L1H) and 2-(2-hydroxyphenyl)-4,5-bis(2,5-dimethyl(3-thienyl))-1-phenyl-imidazole (L2H), which have a chelating N,O-donor binding site attached to the photochromic core, have been synthesized using a one-pot condensation reaction, and used to prepare the heteroleptic complexes [Ir(dfppy)2(L1)]·2CH3OH (1) and [Ir(dfppy)2(L2)] (2) [dfppyH = 2-(2,4-difluorophenyl)-pyridine]. In the crystal structures of all four compounds, two thiophene groups of each bisthienylethene molecule adopt parallel conformation. Neighboring molecules in L1H and 1 are linked into supramolecular chains through hydrogen bonds. Particularly, the packing structure of 1 contains right- and left-handed 21 helical chains. In contrast, neighboring molecules in L2H and 2 interact only through van der Waals interactions. At room temperature, L1H and L2H in CH2Cl2 show fluorescence emission at 442 nm and 469 nm, respectively. Compounds 1 and 2 in CH2Cl2 reveal broad emission band characteristics of the Ir(III)/dfppy− chromophores at 508 nm and 494 nm, respectively, with a mixing of 3MLCT and 3LC characters. At room temperature, the photochromism ability of L2H in CH2Cl2 is clearly weaker than that of L1H. Moreover, no photochromism has been observed in 1 and 2. It has been demonstrated that both the substituent group and {Ir(dfppy)2}+ coordination could significantly influence the crystal structures, luminescence and photochromic properties of L1H, L2H, 1 and 2.

Two kinds of solid-state structures of 5-phosphonomethyl-8-hydroxyquinoline (5pm8hqH3) have been obtained, namely 1·HCl·H2O and 1·H2O, involving different hydrogen bonds and/or aromatic stacking interactions. As a derivative of 5pm8hqH3, 5-phosphonomethyl-8-(carboxymethoxy)quinoline (5pm8cmoqH3) was synthesized. Based on 5pm8hqH3 and 5pm8cmoqH3, three new metal phosphonates have been hydrothermally prepared, including Zn(5pm8hqH)(H2O)·H2O (2), Cu(5pm8cmoqH)·2H2O (3) and Fe(5pm8cmoqH) (4), exhibiting layered structures for 2 and 4, and a three-dimensional open framework for 3. The 8-hydroxyquinoline moieties in 1·H2O and 2–4 exhibit three kinds of interesting aromatic stacking modes, including pyridine ring–pyridine ring stacking between a pair of moieties, double benzene ring–pyridine ring stacking between a pair of moieties and alternating benzene ring–benzene ring and pyridine ring–pyridine ring stacking among a number of moieties in the layered structure. The solid-state fluorescence measurements indicate the emissions of 1·HCl·H2O and 1·H2O are significantly different due to their distinct packing structures. Compound 2 exhibits both ligand-centered (LC) and ligand-to-metal charge transition (LMCT) emissions. Magnetic studies reveal dominant antiferromagnetic interactions in 3 and 4.

Complexes [Ir(dfppy)2(pbdtiH)](PF6)·2CHCl3 (1-H) and [Ir(dfppy)2(pbdti)] (1) were synthesized by the reaction of bisthienylethene pbdtiH and an [Ir(dfppy)2Cl]2 dimer under neutral and basic conditions, respectively. Thus, the {Ir(dfppy)2}+ unit is coordinated by pbdtiH in 1-H, and by pbdti− in 1, which are confirmed by their crystal structures. The structures of 1-H and 1 could be interconverted in solution, upon alternately adding NEt3 and TFA, thus resulting in reversible luminescence switching between the on-state of 1-H and the off-state of 1 at room temperature. In addition, both 1-H and 1 show solid-state luminescence, with a broad emission at 534 nm and 525 nm, respectively. The free pbdtiH ligand shows photochromic behavior in CH2Cl2 solution. However, no photochromism has been observed in 1-H and 1, indicating that the coordination of the pbdtiH/pbdti− ligand to the {Ir(dfppy)2}+ unit could suppress their photochromic behaviors.

Based on ligands dfppyH and pidpyH, cyclometalated Ir(III) complexes [Ir(dfppy)2(pidpyH)](PF6) (1·PF6) and [Ir(dfppy)2(pidpy)] (2) have been synthesized. The crystal structures indicate that each {Ir(dfppy)2}+ unit is coordinated by a neutral ligand pidpyH in 1·PF6, while by a pidpy− anion in 2. The packing structure of 1·PF6 only exhibits electrostatic interactions and van der Waals interactions among [Ir(dfppy)2(pidpyH)]+ cations and PF6− ions. In contrast, the neighboring molecules in 2 are linked into a supramolecular chain structure through aromatic stacking interactions between two dfppy− ligands. In solution, 1·PF6 and 2 show acid/base-induced structural transformation due to the protonation/deprotonation of their pyridyl groups and/or imidazole units, which can be confirmed by their 1H NMR spectra. At room temperature, compounds 1·PF6, 2 and pidpyH in CH2Cl2 reveal TFA-induced luminescence switching behaviors, from a non-luminescence state to a luminescence state with an emission at 582 nm for both 1·PF6 and 2, and emission switching from 392 nm to 502 nm for pidpyH. These switching behaviors are associated with the protonation of pyridyl groups and/or imidazole units in 1·PF6, 2 and pidpyH. Moreover, compounds 1·PF6 and 2 were used as photosensitizers (PS) for reduction of water to hydrogen under the same experimental conditions. It was found that the amount of evolved hydrogen and the PS turnover number are 512 μmol and 102 for 1·PF6, and 131 μmol and 26 for 2, respectively. Thus, compound 1·PF6 has better photocatalytic activity than 2. In this paper, we discuss the modulation of luminescence and photocatalytic activities of 1·PF6 and 2 by varying the coordination mode and/or protonation extent of pidpyH/pidpy− ligands.

Cyclometalated Ir(III) complexes [Ir(dfppy)2(qbiH)](PF6) (1), [Ir(dfppy)2(qbim)](PF6)·H2O (2), [Ir(dfppy)2(qbio)](PF6) (3) and [Ir(dfppy)2(qbi)] (4) have been designed and prepared, in which the N^N ligands qbiH, qbim and qbio incorporate different substituent groups R on their imidazole units (H atom, CH3 group and n-C8H17 group, respectively) in order to explore the influence of the substituent groups R and the protonation/deprotonation state of imidazole units in these Ir(III) complexes on their structures and luminescence behaviors. Crystal structures indicate that an {Ir(dfppy)2}+ unit is coordinated by neutral ligands qbiH in 1, qbim in 2 and qbio in 3, while a qbi− anion in 4. These Ir(III) complexes show clearly different molecular stacking modes. In compound 1, neighboring [Ir(dfppy)2(qbiH)]+ cations are linked into a supramolecular chain through π⋯π stacking interactions between adjacent dfppy−/qbiH ligands. In 2 and 4, two neighboring iridium complex units connect each other through π⋯π stacking interactions between dfppy− ligands in the former, while between qbi− ligands in the latter, forming supramolecular dimers. Compared to 1, 2 and 4, compound 3 only exhibits intermolecular van der Waals interactions. At room temperature, these Ir(III) complexes in CH2Cl2 reveal phosphorescence with a mixing of 3MLCT and 3LC characters, emissions at 558 and 585 nm for 1, 572 (or 573) and 600 nm for 2 and 3, and 546 nm for 4. Compared to 1–3, compound 4 displays relatively weak luminescence intensity. Interestingly, upon addition of NEt3/TFA, both 1 and 4 in CH2Cl2 can switch their luminescence between strong emission at 558 nm and weak emission at 546 nm, due to their acid-/base-induced structural interconversion between the protonation state and the deprotonation state of the qbiH ligand. The emissions of 1–4 in the solid state reveal different degrees of the red shift compared to their corresponding emissions in CH2Cl2, the broad emission bands at 542, 572 and 611 nm for 1, 553, 581 and 612 nm for 2, 544, 578 and 630 nm for 3, and 595 and 633 nm for 4. Based on the crystal structures of 1–4, this work discusses the luminescence modulation of these Ir(III) complexes by varying their substituent groups or the protonation/deprotonation state of the imidazole units.

Molecular assembly of bisthienylethene Th2im (1) and [ReCl6]2− anions leads to the complex (Th2imH)2[ReCl6] (2), in which a [ReCl6]2− anion connects two equivalent Th2imH+ cations through Cl⋯N/C hydrogen bonds. Crystal structures of 1 and 2 indicate that two thiophene groups of each Th2im/Th2imH+ molecule adopt a photoactive antiparallel conformation. Thus, two compounds show crystalline-phase photochromism (CPP), i.e. reversible structural transformation between the open form and the closed form upon alternately irradiating the sample with UV light (365 nm) and visible light (574 nm for 1, 624 nm for 2). It was found that the CPP behaviors of 1 and 2 could regulate their luminescence and/or magnetic properties. Their solid-state emissions (433, 448, 482, 531 and 570 nm for 1, and 460, 489, 535 and 593 nm for 2) exhibited weaker intensities after UV irradiation with 365 nm light. Besides CPP and luminescence, compound 2 shows field-induced slow magnetic relaxation. Before and after UV irradiation, this compound revealed different magnetic behaviors, including the differences in the shape of the χMT vs. T plot, D parameter, and the values of the relaxation barrier Ueff and the preexponential factor τ0.

Heteroleptic complexes [Ir(dfppy)2(BrL)]·3CH3OH (1) and [Pt(dfppy)(BrL)]·CH3OH (2) have been prepared based on the same ligands including bisthienylethene BrLH and dfppyH = (2-(2,4-difluorophenyl)-pyridine). Complexes 1 and 2 reveal distinct crystal structures. The BrL− anion uses its phenol-imidazole moiety to coordinate with an {Ir(dfppy)2}+ unit in the former, while with a {Pt(dfppy)}+ unit in the latter. Neighboring [Ir(dfppy)2(BrL)]/[Pt(dfppy)(BrL)] molecules are connected through extensive hydrogen bonds and aromatic stacking interactions, thus forming a supramolecular chain structure in 1, and a layer structure in 2. Upon irradiation with 380 nm light, compound 2 shows photochromic behavior in CH2Cl2, with a color change from nearly colorless to light green. However, no photochromism was observed in compound 1. At room temperature, compound 1 reveals phosphorescence with a predominant 3MLCT character both in CH2Cl2 solution (emissions at 495 and 513 nm) and in the solid state (emission at 524 nm). Compound 2 exhibits phosphorescence with a predominant 3LC character in CH2Cl2 solution (emission at 508 nm), but it is almost non-luminescent in the solid state. Our experimental results demonstrate that the metal centers in 1 and 2 could significantly influence their structures, photochromism, and luminescence behaviors.

2-(Anthracenyl)-4,5-bis(2,5-dimethyl(3-thienyl))-1H-imidazole (anbdtiH) has been synthesized. Its three solid-state structures, anbdtiH·CH3Cl (1), anbdtiH·2CH3OH (2) and anbdtiH2·CF3COO·CH3OH·H2O (3), have been constructed by skillfully choosing CHCl3 or CH3OH as the solvent and using or not using CF3COOH, with the aim of modifying the intermolecular hydrogen bonds and/or π⋯π stacking interactions. The three distinct structures show significantly different solid-state luminescence behaviors, an orange-red emission at 603 nm for 1, a blue emission at 453 nm for 2, and a green emission at 533 nm for 3. Upon grinding, these emission wavelengths exhibit evident variations, a blue-shift of Δλ = 83 nm for 1, a red-shift of Δλ = 20 nm for 2, and a blue-shift of Δλ = 54 nm for 3. The emission color of 1 can be reversibly switched between orange-red and green upon regulation of the intermolecular (N–H)imidazole⋯Nimidazole hydrogen bonds by a grinding–heating process. Moreover, compound 3 can undergo a solid-state [4π + 4π] photodimerization reaction upon irradiation with sunlight, forming 3-dimer. Based on the crystal structures of 1–3, this work discusses the relationship between the molecular stacking mode and the luminescence behavior/photochemical reactivity.

Mononuclear complex Co(hpbdti)2·3CH3OH (1) was synthesized [hpbdtiH = 2-(2-hydroxyphenyl)-4,5-bis(2,5-dimethyl(3-thienyl))-1H-imidazole], showing multiple-step field-induced slow magnetic relaxation behaviors, and photochromic properties in CH2Cl2–CH3CN solution.