Π-conjugated Ligands Research Articles

Inter-Ligand Energy Transfer Process in an Ir-Complex with Expanding π-Conjugated Ligand.

The inter-ligand energy transfer (ILET) process in heteroleptic iridium complex, [Ir(dfppy)2 (bpy-Im2 )]+ , where dfppy=2-(2,4-difluorophenyl)pyridine and bpy-Im2 =4,4'-bis(1,2-diphenyl-1H-benzo[d]imidazole)-2,2',-bipyridine, was investigated using a femtosecond transient absorption (fs-TA) spectroscopic technique. The photophysical properties of [Ir(dfppy)2 (bpy-Im2 )]+ with significantly expanding π-conjugated ligand are compared to those of [Ir(dfppy)2 (bpy)]+ (bpy=2,2'-bipyridine) and a free bpy-Im2 ligand. The emission spectrum of [Ir(dfppy)2 (bpy-Im2 )]+ shows no shift upon changing the solvent polarity, whereas the free ligand bpy-Im2 showed bathochromic fluorescence shifts with increasing solvent polarity, which is attributed to intramolecular charge transfer (ICT). The unique photophysical properties of [Ir(dfppy)2 (bpy-Im2 )]+ are due to the fast ILET process from 3 MLCTdfppy to 3 MLCT/3 LCbpy-Im2 , resulting in the phosphorescence emission originating from 3 MLCT/3 LCbpy-Im2 . On the other hand, the TA bands of bpy-Im2 are observed at 540 and 480 nm, corresponding to the singlet and triplet manifolds, respectively. In contrast, the TA spectrum of [Ir(dfppy)2 (bpy-Im2 )]+ showes broad bands centered at 420 and 600 nm, attributed to the transitions from 3 MLCTdfppy and 3 MLCT/3 LCbpy-Im2 , respectively. Time-resolved spectroscopic results confirm the efficient ILET dynamics from 3 MLCTdfppy to 3 MLCT/3 LCbpy-Im2 in [Ir(dfppy)2 (bpy-Im2 )]+ . From the relaxation times determined by singular value decomposition analysis and simple sequential kinetic model, we infer that the ILET process from 3 MLCTdfppy to 3 MLCT/3 LCbpy-Im2 occurs with a time constant of ca. 4 ps. The presented results in this study show that the introduction of an expanding π-conjugated ligand can lead to the efficient ILET dynamics for improving the OLED performance.

Formation of Three-Dimensional Electronic Networks Using Axially Ligated Metal Phthalocyanines as Stable Neutral Radicals

Organic π-radical crystals are potential single-component molecular conductors, as they involve charge carriers. We fabricated new organic π-radical crystals using axially ligated metal phthalocyanine anions ([MIII(Pc)L2]−) as starting materials. Electrochemical oxidation of [MIII(Pc)L2]− afforded single crystals of organic π-radicals of the type MIII(Pc)Cl2·THF (M = Co or Fe, THF = tetrahydrofuran), where the π-conjugated macrocyclic phthalocyanine ligand is one-electron oxidized. The X-ray crystal structure analysis revealed that MIII(Pc)Cl2 formed three-dimensional networks with π-π overlaps. The electrical resistivities of CoIII(Pc)Cl2·THF and FeIII(Pc)Cl2·THF at room temperature along the a-axis were 6 × 102 and 6 × 103 Ω cm, respectively, and were almost isotropic, meaning that MIII(Pc)Cl2·THF had three-dimensional electronic systems.

Study of photophysical behaviour of some Sm(III) complexes with 4-oxo-4H-1-benzopyran-3-carboxaldehyde and other N,N′-donor π-conjugated ligands

Trivalent lanthanide ions show interesting optical properties. Keeping this in view, five Sm(III) complexes were synthesized using 4-oxo-4H-1-benzopyran-3-carboxaldehyde (L) as primary ligand along with 1,10-Phenanthroline (Phen); Bipyridine (Bipy); Bathophenanthroline (Bathophen) and Neocuproine (Neo) as secondary ligands. Characterizations of prepared complexes were carried out by means of FTIR, elemental analysis, UV–vis, ESI-MS+, thermal studies, PXRD, FESEM, and luminescence studies. Thermal studies confirmed the good thermal stability of complexes. Sharp peaks in X-ray diffractograms suggest their crystalline nature. The photoluminescence process in complexes has been thoroughly studied and discussed in liquid and powder state. The complexes show 4G5/2→6Hj transitions where j = 5/2, 7/2, and 9/2, analogous to characteristic emission peaks of Sm(III) ion at ~ 566, 601, and 648 nm, respectively. The complexes demonstrate intense emission peaks at ~ 601 in liquid and ~ 648 nm in a powder state, which is responsible for bright orange and red emission, respectively. The synergistic effect of secondary ligands was responsible for longer luminescence lifetime and enhanced emission intensity in ternary complexes. Attractive photoluminescence properties of complexes could play a vital role in electroluminescent devices, bio-assays, liquid lasers, OLEDs, etc. Five new Sm(III) complexes were synthesized using 4-oxo-4H-1-benzopyran-3-carboxaldehyde (L) and other N,N′-donor π-conjugated ligands. Results were in favour of the formation of bright orange (in solution) and red (in powder state) luminescent complexes exhibiting sharp Sm(III)-centered emission peaks. Hence, the selected ligands act as efficient sensitizers in transferring their excitation energy towards Sm(III) ion.

RETRACTED: Two Cu(II)-based coordination polymers: Photocatalytic dye degradation and treatment activity combined with BDNF modified bone marrow mesenchymal stem cells on craniocerebral trauma via increasing complement C3 expression

Abstract Two novel mixed-ligand Cu(II) coordination polymers (CPs) {[Cu2(edpc)2(ga)2]·2H2O}n (1) and {[Cu(edpc)(bpdc)]·0.25(H2O)}n (2) (H2ga = glutaric acid, edpc = 9-ethyl-2,6-di-pyridin-4-yl-9H-carbazole, H2bpdc = biphenyl-4,4′-dicarboxylic acid) have been solvothermally formed through a π-conjugated carbazole-containing pyridine ligand under the presence of auxiliary ligands (H2ga for the complex 1 and H2bpdc for complex 2). Due to its good water stability of complex 2, it was applied in the photocatalytic rhodamine B (RhB), methylene blue (MB) and methyl orange (MO) degradation in unclean water, and the possible pathway of photocatalytic degradation was researched. Whether the compounds could enhance the treatment activity of the BDNF modified bone marrow mesenchymal stem cells (BMSC) against craniocerebral trauma was then evaluated. The recognition of the craniocerebral trauma mice was assessed by Morris water maze (MWM). The viability of the nerve cells in the brain was evaluated by Cell Counting Kit-8 (CCK-8). Then the enzyme-linked immuno sorbent assay (ELISA) detection was performed to detect the release content of the brain-originated neurotrophic factor and nerve promoting factor β in peripheral regions of cerebral hemorrhage. Finally, the expression levels of the complement C3 receptor on the nerve cells in the brain was also determined.

N-Heterocyclic Carbenes: A Door Open to Supramolecular Organometallic Chemistry.

The field of metallosupramolecular chemistry is clearly dominated by the use of O-, N-, and P-donor Werner-type polydentate ligands. These molecular architectures are of high interest because of their wide range of applications, which include molecular encapsulation, stabilization of reactive species, supramolecular catalysis, and drug delivery, among others. Only recently, organometallic ligands have allowed the preparation of a variety of supramolecular coordination complexes, and the term supramolecular organometallic complexes (SOCs) is gaining space within the field of metallosupramolecular chemistry. While the early examples of SOCs referred to supramolecular architectures mostly containing bisalkenyl, diphenyl, or bisalkynyl linkers, the development of SOCs during the past decade has been boosted by the parallel development of multidentate N-heterocyclic carbene (NHC) ligands. The first examples of NHC-based SOCs referred to supramolecular assemblies based on polydentate NHC ligands bound to group 11 metals. However, during the last 10 years, several planar poly-NHC ligands containing extended π-conjugated systems have facilitated the formation of a large variety of architectures in which the supramolecular assemblies can contain metals other than Cu, Ag, and Au. Such ligands are Janus di-NHCs and trigonal-planar tris-NHCs-most of them prepared by our research group-which have allowed the preparation of a vast range of NHC-based metallosupramolecular compounds with interesting host-guest chemistry properties. Although the number of SOCs has increased in the past few years, their use for host-guest chemistry purposes is still in its earliest infancy. In this Account, we describe the achievements that we have made during the last 4 years toward broadening the applications of planar extended π-conjugated NHC ligands for the preparation of organometallic-based supramolecular structures, including their use as hosts for some selected organic and inorganic guests, together with the catalytic properties displayed by some selected host-guest inclusion complexes. Our contribution describes the design of several Ni-, Pd-, and Au-based metallorectangles and metalloprisms, which we used for the encapsulation of several organic substrates, such as polycyclic aromatic hydrocarbons (PAHs) and fullerenes. The large binding affinities found are ascribed to the incorporation of two cofacial panels with large π-conjugated systems, which provide the optimum conditions for guest recognition by π-π-stacking interactions. We also describe a series of digold(I) metallotweezers for the recognition of organic and inorganic substrates. These metallotweezers were used for the recognition of "naked" metal cations and polycyclic aromatic hydrocarbons. The recognition properties of these metallotweezers are highly dependent on the nature of the rigid connector and of the ancillary ligands that constitute the arms of the tweezer. A peculiar balance between the self-aggregation properties of the tweezer and its ability to encapsulate organic guests is observed.

Self-crosslinkable polyaniline with coordinated stabilized CoOOH nanosheets as a high-efficiency electrocatalyst for oxygen evolution reaction

Polyaniline (PANI)-based composite materials have shown to be promising candidates for oxygen evolution reaction (OER) electrocatalysts because of their non-ignorable merits of conductivity, flexibility, durability and environmental friendly. Herein, we develop a facile strategy to realize in-situ assembly of CoOOH nanosheets into the PANI network, which is denoted as Co/PANI HNSs for OER performance. The nitrogen species derived from PANI building blocks can work as bridging sites to preferentially coordinate with Co metal ions, which impart coupling effects between CoOOH nanosheets and PANI as well as the structure stability. Besides the Co-N coordination, the occurred electron delocalization between Co d-orbitals and PANI π-conjugated ligands can also modulate the electronic structural states of Co/PANI HNSs, enabling the efficient interfacial electron transfer from CoOOH to PANI. In addition, the Co/PANI HNSs possesses a hierarchical porous with both structure of mesopores and macropores that allows electrolyte to be more efficiently transported to the highly oxidative active sites, resulting in fast reaction kinetics. In recognition of these advanced structural characteristics, the Co/PANI HNSs electrocatalyst can give a low overpotential of 291 mV at an anodic current density of 10 mA cm−2 and a small Tafel slope of 54 mV dec−1 in 1 M KOH electrolyte as well as a good durability.

Enhancing the Stability of Orthorhombic CsSnI3 Perovskite via Oriented π-Conjugated Ligand Passivation.

Lead-free orthorhombic CsSnI3 (Bγ-CsSnI3) perovskite has been emerging as one of the potential candidates of photovoltaic materials with superior performance. However, the instability induced by rapid reconstructive phase transition and the oxidation of Sn2+ greatly limits their future application. We thus reported a strategy, oriented π-conjugated ligand passivation, for enhancing the stability of Bγ-CsSnI3, simulated using a Bγ-CsSnI3 slab model based on the first-principles computation. The phase stability was found to be strongly dependent on the orientations of phenylethylammonium (PEA+) ligands. The passivated Bγ-CsSnI3 slab with the ligand molecule axis along [414] was demonstrated as the most stable with the lowest adsorption energy (Eads). Based on this configuration, the calculated formation energies (Eform) of half- and full-monolayer coverage were even more negative than that of yellow phase (Y-) CsSnI3 passivated by PEA+ ligands, verifying the enhanced phase stability. Furthermore, the surface states could be effectively suppressed and the downshifted conduction band minimum (CBM) resulted in a reduced band gap for the completely capped Bγ-CsSnI3. Moreover, the CBM and the valence band maximum (VBM) of the system with complete coverage were respectively donated by the surface and bulky components of the slab, which might benefit the separation and transfer of photogenerated carriers.

The role of Fe3+ ions in fluorescence detection of H2S by a bimetallic metal-organic framework

A novel bimetallic MOF (i.e., FexAl1-x-MIL) with fluorescence quenching in the probe, was constructed to detect H2S based on a “turn-on” effect in an aqueous system. Interestingly, a trace amount of Al3+ replaced by Fe3+ in the parent MOF Al-MIL-53-NH2 causes significant fluorescence quenching in the bimetallic MOF, which is attributed to the strong ligand to metal charge transfer between unpaired electrons in Fe3+ and π-conjugated BDC-NH2 ligands. After H2S treatment, a fluorescence augmentation was observed, with a good linear relationship between H2S concentration (0–38.46 ​μM) and fluorescence intensity, indicating that Fe0.05Al0.95-MIL could be used for quantitative H2S detection. Particularly, Fe3+ in the bimetallic MOF seized by S2− facilitated the partial degradation and subsequent release of BDC-NH2 ligands, which were determined to be real fluorophores that contributed to the fluorescence enhancement. This study offers new insights into the luminescent bimetallic MOF design and would expand its application in chemical sensing.

Two-dimensional halide perovskite lateral epitaxial heterostructures.

Epitaxial heterostructures based on oxide perovskites and III-V, II-VI and transition metal dichalcogenide semiconductors form the foundation of modern electronics and optoelectronics1-7. Halide perovskites-an emerging family of tunable semiconductors with desirable properties-are attractive for applications such as solution-processed solar cells, light-emitting diodes, detectors and lasers8-15. Their inherently soft crystal lattice allows greater tolerance to lattice mismatch, making them promising for heterostructure formation and semiconductor integration16,17. Atomically sharp epitaxial interfaces are necessary to improve performance and fordevice miniaturization.However, epitaxial growth of atomically sharp heterostructures of halide perovskites has not yet been achieved, owing to their high intrinsic ion mobility, which leads to interdiffusion and large junction widths18-21, and owing to their poor chemical stability, which leads to decomposition of prior layers during the fabrication of subsequent layers. Therefore, understanding the origins of this instability and identifying effective approaches to suppress ion diffusion are of great importance22-26. Here we report an effective strategy to substantially inhibit in-plane ion diffusion in two-dimensional halide perovskites by incorporating rigid π-conjugated organic ligands. We demonstrate highly stable and tunable lateral epitaxial heterostructures, multiheterostructures and superlattices. Near-atomically sharp interfaces and epitaxial growth are revealed by low-dose aberration-corrected high-resolution transmission electron microscopy. Molecular dynamics simulations confirm the reduced heterostructure disorder and larger vacancy formation energies of the two-dimensional perovskites in the presence of conjugated ligands. These findings provide insights into the immobilization and stabilization of halide perovskite semiconductors and demonstrate a materials platform for complex and molecularly thin superlattices, devices and integrated circuits.

Interfacial charge transfer processes in 2D and 3D semiconducting hybrid perovskites: azobenzene as photoswitchable ligand.

In the vast majority of studies on semiconductor particles ligands or capping agents are used that bind to the surface of the particles covering them with an electrically insulating shell. Since the transport of charge carriers and/or energy across interfaces is desirable for a variety of applications, the use of π-conjugated ligands becomes increasingly interesting. Among them are compounds that react to external stimuli. Molecular switches in particular are fascinating because the properties of the interfaces can be potentially adjusted as required. However, there is debate about how the properties of such special ligands are influenced by the presence of a semiconductor and vice versa. Here ammonium-modified azobenzene compounds were selected as prototypes for molecular switches and organic–inorganic hybrid perovskites as semiconductor materials. The class of ammonium–lead–halide phases as prototypes is peculiar because, in addition to the surface functionalization of 3D crystals, organic compounds can actually be incorporated into the crystal as 2D phases. Thus, for example, layered Ruddlesden–Popper phases are obtained. We present photoswitchable azobenzene ligands with different head-group lengths for the synthesis of 2D and 3D hybrid perovskite phases. The energy transfer mechanisms are influenced by the length of the molecular spacer moiety, which determines the distance between the π system and the semiconductor surfaces. We find huge differences in the photoswitching behaviour between the free, surface-coordinated and integrated ligands between the perovskite layers. Photoswitching of azobenzene ligands incorporated in 2D phases is nearly quenched, while the same mechanism for surface-coordinating ligands is greatly improved, compared to the free ligands. The improvement originates from an energy transfer from perovskite to azobenzene, which is strongly distance-dependent. This study provides evidence for the photoswitching of azobenzenes as ligands of hybrid perovskites, which depends on the spacing between the chromophore and the perovskite phase.

Molecular design strategies for spin-crossover (SCO) metal complexes (Fe(II) and Co(II)) for thermoelectricity

TEC are developing technology to generate electricity from thermal energy. It causes current flow when redox solution disturbs the electrochemical potential equilibrium between the electrolyte and two inert electrodes upon application of a temperature gradient. We explore the spin-crossover materials which have been shown as promising TEC candidates upon its spin-state transition by change of temperature. Particularly, we analyse the correlation of the spin-state transition, redox properties, molecular structures and consequently on TEC performance. Three complexes, [M2(CH3COO)4(L)2] and [M(L)3](BF4)2, where M = Fe(II) and Co(II), and L = extended π-conjugated bipyridyl ligand and N3-Schiff bases were successfully synthesised and characterised. The best performance recorded by Complex 2 owing to the molecular octahedral structure with high-spin which allows higher diffusion of ions. It also noted that the largest change of εmax value (Complex 2) arises from spin-states transition brings an additional contribution to the entropy change that accounted for TEC performance.

Heteroleptic Ir(iii) complexes with varied π-conjugated diimine ligands: synthesis, tunable triplet states and nonlinear absorption properties.

Four heteroleptic Ir(iii) complexes (Ir-1-Ir-4) with varied π-conjugated diimine ligands were synthesized. Their optical properties were investigated systematically via spectroscopic methods, in order to elucidate the influence of the extended π-conjugation on the singlet and triplet states of the molecules. All these Ir(iii) complexes exhibit ligand localized 1π,π* transitions below 370 nm, and broad metal-to-ligand and ligand-to-ligand charge transfer (MLCT/LLCT) absorption bands in the visible region. Extending the π-conjugation in diimine ligands influences the electron distribution of the triplet state. The 3π,π* dominated Ir(iii) complexes (Ir-3 and Ir-4) exhibit long-lived triplet states and low phosphorescence quantum yields, compared with the charge transfer dominated Ir(iii) complexes (Ir-1 and Ir-2). Complexes Ir-3 and Ir-4 exhibit broadband transient absorption spectra from 420 to 750 nm, between which the nonlinear absorption could be observed by virtue of reverse saturable absorption. The results of nonlinear transmission measurements using 532 nm laser pulses further elucidate that selected complexes Ir-3 and Ir-4 are promising candidates for optical limiting applications.

Visual self-assembly and stimuli-responsive materials based on recent phosphorescent platinum(ii) complexes

Organoplatinum(ii) complexes with various π-conjugated ligands exhibit intriguing luminescent properties and they are a promising candidate to construct smart luminescent materials in optical devices, chemical sensors, and bio-imaging.

Isomeric Tetradentate Ligand-Containing Cyclometalated Gold(III) Complexes.

A simple one-pot two bond-forming reaction for the rapid construction of cyclometalated gold(III) complexes with fully π-conjugated tetradentate ligand is reported. The coupling of the bifunctional gold(III) precursor with the bifunctional aromatic compound has led to the formation of two regioisomers with either C- or N-coordination. Through monitoring by high-throughput high performance liquid chromatography, the regioselectivity of the reaction has been effectively tuned toward the formation of a single isomer, allowing easy separation of the metal complexes. The structures of the complexes have been determined by X-ray crystallography, and the photophysical, electrochemical, and electroluminescence (EL) studies have been carried out. Computational study has been performed to provide insights into the nature of the excited states. Isomeric effect has been shown to have a significant influence on the EL behavior of the organic light-emitting devices.

Highly Stable Lead-Free Perovskite Field-Effect Transistors Incorporating Linear π-Conjugated Organic Ligands

Sn(II)-based halide perovskite semiconductor materials are promising for a variety of electronics and optoelectronics applications but suffer from poor intrinsic materials stability. Here, we report the synthesis and characterization of a stable Sn (II)-based two-dimensional perovskite featuring a π-conjugated oligothiophene ligand, namely (4Tm)2SnI4, where 4Tm is 2-(3″',4'-dimethyl-[2,2':5',2″:5″,2″'-quaterthiophen]-5-yl)ethan-1-ammonium. The conjugated ligands facilitate formation of micrometer-size large grains, improve charge injections, and stabilize the inorganic perovskite layers. Thin film field-effect transistors based on (4Tm)2SnI4 exhibit enhanced hole mobility up to 2.32 cm2 V-1 s-1 and dramatically improved stability over the previous benchmark material (PEA)2SnI4. Stabilization mechanisms were investigated via single-crystal structure analysis as well as density functional theory calculations. It was found that the large conjugated organic layers not only serve as thick and dense barriers for moisture and oxygen but also increase the crystal formation energy via strong intermolecular interactions. This work demonstrates the great potential of molecular engineering for organic-inorganic hybrid perovskite materials toward applications in high-performance electronics and optoelectronics.

Direct Observation of Primary C-H Bond Oxidation by an Oxido-Iron(IV) Porphyrin π-Radical Cation Complex in a Fluorinated Carbon Solvent.

Oxido-iron(IV) porphyrin π-radical cation species are involved in a variety of heme-containing enzymes and have characteristic oxidation states consisting of a high-valent iron center and a π-conjugated macrocyclic ligand. However, the short lifetime of the complex has hampered detailed reactivity studies. Reported herein is a remarkable increase in the lifetime (80 s at 10 °C) of FeIV (TMP+. )(O)(Cl) (2; TMP=5,10,15,20-tetramesitylporphyrin dianion), produced by the oxidation of FeIII (TMP)(Cl) (1) by ozone in α,α,α-trifluorotoluene (TFT). The lifetime is 720 times longer compared to that of the currently most stable species reported to date. The increase in the lifetime improves the reaction efficiency of 2 toward inert alkane substrates, and allowed observation of the reaction of 2 with a primary C-H bond (BDEC-H =ca. 100 kcal mol-1 ) directly. Activation parameters for cyclohexane hydroxylation were also obtained.

Light-harvesting metal-organic framework nanoprobes for ratiometric fluorescence energy transfer-based determination of pH values and temperature.

Light-harvesting nanoprobes were developed by self-assembly of nanoscale metal-organic frameworks (NMOFs) and stimuli-responsive polymers for fluorometric sensing of pH values and temperature. Two kinds of fluorescent NMOFs (acting as the energy donor) and stimuli-responsive polymers conjugated to fluorophores (acting as energy acceptors) were prepared and characterized. The NMOFs include zirconium(IV) and π-conjugated dicarboxylate ligands. The fluorophores inclued cyaine dyes and aBodipy dye. The energy donor and energy acceptor form a Förster resonance energy transfer (FRET) nanosystem. In the light-harvesting system, the chain lengths of the stimuli-responsive polymers vary when the local pH value or temperature change. Ratiometric sensing of pH and temperature was accomplished by monitoring fluorescence. pH values were can be sensed between 3.0 and 8.0 under 420nm excitation and by ratioing the emission peaks at 645 and 530nm. Temperature can be sensed in the range from 25 to 50°C under 550nm excitation and by ratioing the emission peaks at 810 and 695nm. The nanoprobes display excellent water dispersibility and cell membrane permeability. They were applied to image pH values and temperature in HeLa cells. Graphical abstract Schematic presentation of an effective strategy to fabricate light-harvesting nanoprobes by self-assembly of MOFs and stimuli-responsive polymers for ratiometric pH and temperature sensing. The distance as the polymer length between energy donor and acceptor is crucial for energy transfer efficiency.

Optimisation of octahedral iron(II) and cobalt(II) spin-crossover metal complex for thermoelectric application

Four spin-crossover (SCO) complexes with general formulae, [M2(CH3COO)4(L)2] and [M(L)3](BF4)2, where M = Fe(II) and Co(II), containing extended π-conjugated bipyridyl ligand and N3-Schiff bases appended with linear C16 carbon chains at the N atoms were successfully synthesized and characterized. Correlation of its structural properties to thermoelectric behavior is investigated: (1)structure of complexes, (2)choice of metal centre and (3)choice of counter ions. It was found that the structure of the molecule, i.e. molecular versus ionic has the largest impact on the SCO behavior. We found that the molecular complexes with higher percentage of high-spin (73.4% HS for Fe-dinuc and 78% HS for Co-dinuc) have produced the highest Seebeck values in mV K−1 (−0.57 ± 0.01 for Fe-dinuc and −0.58 ± 0.01 for Co-dinuc) due to the weaker metal-to-ligand bonds resulting in the increase mobility of the I− during agglomeration formed, thus increased the entropy in the solution. Additionally, choice of metal centre also was a factor to determine the magnitude of Seebeck performance due to the spin state transition during electron transfer. For counter ion effect, it has the effect of determining the sign of the Seebeck value where I− is easier to oxidize/reduce process compared to CH3COO− and BF4− due to lower redox potential. These findings will assist in a systematic molecule design pathway for high potential SCO complexes for thermoelectric applications.

Unprecedented Ultralow Detection Limit of Amines using a Thiadiazole-Functionalized Zr(IV)-Based Metal-Organic Framework.

A luminescent Zr(IV)-based metal-organic framework (MOF), with the underlying fcu topology, encompassing a π-conjugated organic ligand with a thiadiazole functionality, exhibits an unprecedented low detection limit of 66 nM for amines in aqueous solution. Markedly, this ultralow detection is driven by hydrogen-bonding interactions between the linker and the hosted amines. This observation is supported by density functional theory (DFT) calculations, which clearly corroborate the suppression of the twisting motion of thiadiazole core in the presence of amine, reducing significantly the nonradiative recombination pathways and subsequently enhancing the emission intensity. Credibly, nicotine regarded as a harmful chemical and bearing an amine pending group is also detected with high sensitivity, positioning this MOF as a potential sensor for practical environmental applications. This finding serves also as a benchmark to understand the sensing mechanism in MOFs.

Creating Well-Defined Hexabenzocoronene in Zirconium Metal-Organic Framework by Postsynthetic Annulation.

The incorporation of large π-conjugated ligands into metal-organic frameworks (MOFs) can introduce intriguing photophysical and electrochemical properties into the framework. However, these effects are often hindered by the strong π-π interaction and the low solubility of the arylated ligands. Herein, we report the synthesis of a porous zirconium-based MOF, Zr6(μ3-O)4(μ3-OH)4(OH)6(H2O)6(HCHC) (PCN-136, HCHC = hexakis(4-carboxyphenyl)hexabenzocoronene), which is composed of a hexacarboxylate linker with a π-conjugated hexabenzocoronene moiety. Direct assembly of the Zr4+ metal centers and the HCHC ligands was unsuccessful due to the low solubility and the unfavorable conformation of the arylated HCHC ligand. Therefore, PCN-136 was obtained from aromatization-driven postsynthetic annulation of the hexaphenylbenzene fragment in a preformed framework (pbz-MOF-1) to avoid π-π stacking. This postsynthetic modification was done through a single-crystal-to-single-crystal transformation and was clearly observable utilizing single -crystal X-ray crystallography. The formation of large π-conjugated systems on the organic linker dramatically enhanced the photoresponsive properties of PCN-136. With isolated hexabenzocoronene moieties as photosensitizers and Zr-oxo clusters as catalytic sites, PCN-136 was employed as an inherent photocatalytic system for CO2 reduction under visible-light irradiation, which showed increased activity compared with pbz-MOF-1.