Characterization of a Hg2+-Selective Fluorescent Probe Based on Rhodamine B and Its Imaging in Living Cells

A small organic molecule P was synthesized and characterized as a fluorometric and colorimetric dual-modal probe for Hg²⁺. The sensing characteristics of the proposed probe for Hg²⁺ were studied in detail. A fluorescent enhancing property at 583 nm (>30 fold) accompanied with a visible colorimetric change, from colorless to pink, was observed with the addition of Hg²⁺ to P in an ethanol-water solution (8:2, v/v, 20 mM HEPES, pH 7.0), which would be helpful to fabricate Hg²⁺-selective probes with “naked-eye” and fluorescent detection. Meanwhile, cellular experimental results demonstrated its low cytotoxicity and good biocompatibility, and the application of P for imaging of Hg²⁺ in living cells was satisfactory.     

A Quinoxaline−Naphthaldehyde Conjugate for Colorimetric Determination of Copper Ion

This work facilitates detection of bivalent copper ion by a simple Schiff base probe QNH based on a quinoxaline−naphthaldehyde framework. The detailed study in absorption spectroscopy and theoretical aspects and crystal study of the probe and probe−copper complex has been discussed. The detection limit of the probe in the presence of Cu²⁺ is 0.45 µM in HEPES−buffer/acetonitrile (3/7, v/v) medium for absorption study. The reversibility of the probe−copper complex has been investigated by EDTA. The selective visual detection of copper has been established also in gel form.     

Synthesis and Characterization of a Mg2+-Selective Probe Based on Benzoyl Hydrazine Derivative and Its Application in Cell Imaging

A simple benzoyl hydrazine derivative P was successfully synthesized and characterized as Mg²⁺-selective fluorescent probe. The binding of P with Mg²⁺ caused an obvious fluorescence enhancement at 482 nm. The fluorescent, UV-vis spectra, ¹H-NMR, and IR spectra confirmed the formation of P-Mg²⁺ complex, and the formation of a 1:1 stoichiometry complex was proved by Job’s plot and mass spectrometry. The recognition mechanism of P to Mg²⁺ was owing to the photoinduced electron transfer effect (PET). The fluorescent response was linear in the range of 0.9–4.0 µM with the detection limit of 0.3 µM Mg²⁺ in water–ethanol solution (1:9, v:v, pH10.0, 20 mM HEPES). In addition, the results of cell imaging of Mg²⁺ in Hl-7701 cells was satisfying.     

Cholesteric Molecular Tweezer Artificial Receptor for Rapid and Highly Selective Detection of Ag+ in Food Samples

Chiral cholesteric molecular tweezer 7a was synthesized, and its recognition properties for Ag⁺, Al³⁺, Ca²⁺ etc., were investigated by UV and fluorescence spectra. The results showed that in ethanol/Tris (1/1, v/v, pH 7.0) buffer solution, the host molecular tweezer 7a had a specific recognition ability for Ag⁺, the detection limit was up to 1 × 10⁻⁶ mol/L, and other metal ions had little effect on Ag⁺ recognition. At the same time, the naked-eye detection of Ag⁺ was realized by the light red color of the complex solution. Furthermore, the mechanism of recognition of Ag⁺ by molecular tweezer 7a was studied by a nuclear magnetic titration test and computer molecular simulation, and a rapid detection method of Ag⁺ using host molecular tweezer 7a was established. Through the determination of Ag⁺ in milk powder, quinoa and other food samples, it was proved that this novel method had a good application prospect for the detection of Ag⁺ in food.     

A Simple Turn-off Schiff Base Fluorescent Sensor for Copper (II) Ion and Its Application in Water Analysis

An aniline-functionalized naphthalene dialdehyde Schiff base fluorescent probe L with aggregation-induced enhanced emission (AIEE) characteristics was synthesized via a simple one-step condensation reaction and exhibited excellent sensitivity and selectivity towards copper(II) ions in aqueous media with a fluorescence “ turn-off ” phenomenon. The detection limit of the probe is 1.64 × 10⁻⁸ mol·L⁻¹. Furthermore, according to the results of the UV-vis/fluorescence titrations, Job’s plot method and ¹H-NMR titrations, a 1:2 stoichiometry was identified. The binding constant between L and Cu²⁺ was calculated to be K_a = 1.222 × 10³. In addition, the AIEE fluorescent probe L could be applied to detection in real water samples with satisfactory recoveries in the range 99.10–102.90% in lake water and 98.49–102.37% in tap water.     

A New Coumarin-Acridone Compound as a Fluorescence Probe for Fe3+ and Its Application in Living Cells and Zebrafish

A new coumarin-acridone fluorescent probe S was designed and synthesized, and the structure was confirmed with ¹H/¹³C NMR spectrometry, single-crystal X-ray diffraction, and high-resolution mass spectrometry. This probe has high sensitivity and selectivity for Fe³⁺ over other testing metal ions at 420 or 436 nm in acetonitrile–MOPS (3-Morpholinopropanesulfonic Acid) buffer solution (20.0 μM, pH = 6.9, 8:2 (v/v)). Under physiological conditions, the probe displayed satisfying time stability with a detection limit of 1.77 µM. In addition, probe S was successfully used to detect intracellular iron changes through a fluorescence-off mode, and the imaging results of cells and zebrafish confirmed their low cytotoxicity and satisfactory cell membrane permeability, as well as their potential biological applications.     

A Cd(II) Luminescent Coordination Grid as a Multiresponsive Fluorescence Sensor for Cr(VI) Oxyanions and Cr(III), Fe(III), and Al(III) in Aqueous Medium

Hydro(solvo)thermal reactions of Cd(NO₃)₂, N-(pyridin-3-ylmethyl)-4-(pyridin-4-yl)-1,8-naphthalimide (NI-mbpy-34), and 5-bromobenzene-1,3-dicarboxylic acid (Br-1,3-H₂bdc) afforded a luminescent coordination polymer, {[Cd(Br-1,3-bdc)(NI-mbpy-34)(H₂O)]∙2H₂O}_n (1). Single-crystal X-ray diffraction analysis showed that 1 features a two-dimensional (2-D) gridlike sql layer with the point symbol of (4⁴·6²), where the Cd(II) center adopts a {CdO₅N₂} pentagonal bipyramidal geometry. Thermogravimetric (TG) analysis confirmed the thermal stability of 1 up to about 340 °C, whereas XRPD patterns proved the maintenance of crystallinity and framework integrity of 1 in CH₂Cl₂, H₂O, CH₃OH, and toluene. Photoluminescence studies indicated that 1 displayed intense blue fluorescence emissions in both solid-state and H₂O suspension-phase. Owing to the good fluorescent properties, 1 could serve as an excellent turn-off fluorescence sensor for selective and sensitive Cr(VI) detection in water, with LOD = 15.15 μM for CrO₄²⁻ and 14.91 μM for Cr₂O₇²⁻, through energy competition absorption mechanism. In addition, 1 could also sensitively detect Cr³⁺, Fe³⁺, and Al³⁺ ions in aqueous medium via fluorescence-enhancement responses, with LOD = 2.81 μM for Cr³⁺, 3.82 μM for Fe³⁺, and 3.37 μM for Al³⁺, mainly through an absorbance-caused enhancement (ACE) mechanism.     

A Fluorescent Probe for Al3+ Based on Calix[4]arene Containing Salicylaldehyde-Derived Schiff Base

Two novel fluorescent probes L1 and L2 for recognizing Al³⁺ have been prepared by cooperating salicylaldehyde-derived Schiff base groups onto the upper rim of the calix[4]arene. Of these probes, L2 is the most effective at recognizing Al³⁺, displaying a highly selective fluorescence “on” response with an emission wavelength at 478 nm and a Stokes shift of 88 nm. Additionally, probe L2 can form a 1 : 1 coordination complex with Al³⁺ with a binding constant of 2.6×10¹⁰ M⁻¹. Furthermore, its fluorescence intensity exhibits a good linear relationship with Al³⁺ concentration within the range of 2.0×10⁻⁵ M to 1.4×10⁻⁴ M, and the probe has a low detection limit of 4.36×10⁻⁷ M.     

A dual-response fluorescent probe for Zn and Al detection in aqueous media: pH-dependent selectivity and practical application

A Schiff base-type fluorescent probe (1) consist of 2-hydroxynaphthaldehyde and glutamide moieties has been designed and synthesized for detection Zn²⁺ and Al³⁺. The probe shows pH dependent dual-selectivity for Zn²⁺ and Al³⁺ in Tris-HCl buffer, viz. that can selectively recognized Zn²⁺ at pH 7.4 and Al³⁺ at pH 6.0, respectively. From Job's plots and MS data, the stoichiometric ratios of the probe with Zn²⁺ and Al³⁺ appeared to be 1:1 and 2:1, respectively. The probe can detect as low as 5.5 × 10⁻⁸ M⁻¹ Zn²⁺ and 1.27 × 10⁻⁷ M⁻¹ Al³⁺, whereas respective association constants are 4.27 × 10⁴ M⁻¹ and 3.50 × 10⁹ M⁻¹. Furthermore, it is also confirmed that the probe has good cell-permeability and could thus be used to selectively sense intracellular Zn²⁺ and Al³⁺ by bioimaging in different pH environment. Finally the probe has been used successfully for determination of the analytes in real drug samples.     

A Highly Selective and Sensitive Turn-On Fluorescent Chemosensor Based on Rhodamine 6G for Iron(III)

Recently, more and more rhodamine derivatives have been used as fluorophores to construct sensors due to their excellent spectroscopic properties. A rhodamine-based fluorescent and colorimetric Fe³⁺ chemosensor 3’,6’-bis(ethylamino)-2-acetoxyl-2’,7’-dimethyl-spiro[1H-isoindole-1,9’-[9H]xanthen]-3(2H)-one (RAE) was designed and synthesized. Upon the addition of Fe³⁺, the dramatic enhancement of both fluorescence and absorbance intensity, as well as the color change of the solution, could be observed. The detection limit of RAE for Fe³⁺ was around 7.98 ppb. Common coexistent metal ions showed little or no interference in the detection of Fe³⁺. Moreover, the addition of CN⁻ could quench the fluorescence of the acetonitrile solution of RAE and Fe³⁺, indicating the regeneration of the chemosensor RAE. The robust nature of the sensor was shown by the detection of Fe³⁺ even after repeated rounds of quenching. As iron is a ubiquitous metal in cells and plays vital roles in many biological processes, this chemosensor could be developed to have applications in biological studies.     

Complexation and redox chemistry of neptunium,plutonium and americium with a hydroxylaminato ligand

There is significant interest in ligands that can stabilize actinide ions in oxidation states that can be exploited to chemically differentiate 5f and 4f elements. Applications range from developing large-scale actinide separation strategies for nuclear industry processing to carrying out analytical studies that support environmental monitoring and remediation efforts. Here, we report syntheses and characterization of Np(iv), Pu(iv) and Am(iii) complexes with N-tert-butyl-N-(pyridin-2-yl)hydroxylaminato, [2-(^tBuNO)py]⁻(interchangeable hereafter with [(^tBuNO)py]⁻), a ligand which was previously found to impart remarkable stability to cerium in the +4 oxidation state. An[(^tBuNO)py]₄ (An = Pu, 1; Np, 2) have been synthesized, characterized by X-ray diffraction, X-ray absorption, ¹H NMR and UV-vis-NIR spectroscopies, and cyclic voltammetry, along with computational modeling and analysis. In the case of Pu, oxidation of Pu(iii) to Pu(iv) was observed upon complexation with the [(^tBuNO)py]⁻ ligand. The Pu complex 1 and Np complex 2 were also isolated directly from Pu(iv) and Np(iv) precursors. Electrochemical measurements indicate that a Pu(iii) species can be accessed upon one-electron reduction of 1 with a large negative reduction potential (E_1/2 = −2.26 V vs. Fc^+/0). Applying oxidation potentials to 1 and 2 resulted in ligand-centered electron transfer reactions, which is different from the previously reported redox chemistry of U^IV[(^tBuNO)py]₄ that revealed a stable U(v) product. Treatment of an anhydrous Am(iii) precursor with the [(^tBuNO)py]⁻ ligand did not result in oxidation to Am(iv). Instead, the dimeric complex [Am^III(μ₂-(^tBuNO)py)((^tBuNO)py)₂]₂ (3) was isolated. Complex 3 is a rare example of a structurally characterized non-aqueous Am-containing molecular complex prepared using inert atmosphere techniques. Predicted redox potentials from density functional theory calculations show a trivalent accessibility trend of U(iii) < Np(iii) < Pu(iii) and that the higher oxidation states of actinides (i.e., +5 for Np and Pu and +4 for Am) are not stabilized by [2-(^tBuNO)py]⁻, in good agreement with experimental observations.

The coordination modes and electronic properties of a strongly coordinating hydroxylaminato ligand with Np, Pu and Am were investigated.Complexes were characterized by a range of experimental and computational techniques.     

Accessing lanthanide-based,in situ illuminated optical turn-on probes by modulation of the antenna triplet state energy

Luminescent lanthanides possess ideal properties for biological imaging, including long luminescent lifetimes and emission within the optical window. Here, we report a novel approach to responsive luminescent Tb(iii) probes that involves direct modulation of the antenna excited triplet state energy. If the triplet energy lies too close to the ⁵D₄ Tb(iii) excited state (20 500 cm⁻¹), energy transfer to ⁵D₄ competes with back energy transfer processes and limits lanthanide-based emission. To validate this approach, a series of pyridyl-functionalized, macrocyclic lanthanide complexes were designed, and the corresponding lowest energy triplet states were calculated using density functional theory (DFT). Subsequently, three novel constructs L3 (nitro-pyridyl), L4 (amino-pyridyl) and L5 (fluoro-pyridyl) were synthesized. Photophysical characterization of the corresponding Gd(iii) complexes revealed antenna triplet energies between 25 800 and 30 400 cm⁻¹ and a 500-fold increase in quantum yield upon conversion of Tb(L3) to Tb(L4) using the biologically relevant analyte H₂S. The corresponding turn-on reaction can be monitored using conventional, small-animal optical imaging equipment in presence of a Cherenkov radiation emitting isotope as an in situ excitation source, demonstrating that antenna triplet state energy modulation represents a viable approach to biocompatible, Tb-based optical turn-on probes.

The rational, analyte-mediated modulation of the relative energy of the lanthanide-sensitizing triplet state produces Tb-based luminescence, observable by a conventional optical imager in presence of the Cherenkov radiation emitting radioisotope ¹⁸F.     

A Zn(II)-Based Sql Type 2D Coordination Polymer as a Highly Sensitive and Selective Turn-On Fluorescent Probe for Al3+

A luminescent coordination polymer with the overall formula {[Zn(tr₂btd)(bpdc)]∙DMF}_n (where tr₂btd = 4,7-di(1H-1,2,4-triazol-1-yl)-2,1,3-benzothiadiazole; bpdc = 4,4′-biphenyldicarboxylate) was synthesized and characterized by single-crystal and powder X-ray diffraction, thermogravimetric, infrared spectroscopy, and elemental analyses. Luminescent properties of the obtained compound were studied in detail both in the solid state and as a suspension in N,N-dimethylacetamide (DMA). It was found that {[Zn(tr₂btd)(bpdc)]∙DMF}_n exhibits bright turquoise luminescence with excellent quantum efficiency and demonstrates turn-on fluorescence enhancement effect upon soaking in DMA Al³⁺ solution. Fluorescence titration experiments were carried out and the detection limit for Al³⁺ ions was calculated to be 120 nM, which is among the lowest reported values for similar materials. Moreover, compound demonstrated excellent selectivity and reusability, and the mechanism of the response is discussed. These results indicate that {[Zn(tr₂btd)(bpdc)]∙DMF}_n is a promising probe for sensitive fluorescent Al³⁺ detection.     

Protein encapsulation: a new approach for improving the capability of small-molecule fluorogenic probes

Herein, we report a protein-based hybridization strategy that exploits the host-guest chemistry of HSA (human serum albumin) to solubilize the otherwise cell impermeable ONOO⁻ fluorescent probe Pinkment-OAc. Formation of a HSA/Pinkment-OAc supramolecular hybrid was confirmed by SAXS and solution-state analyses. This HSA/Pinkment-OAc hybrid provided an enhanced fluorescence response towards ONOO⁻versusPinkment-OAc alone, as determined by in vitro experiments. The HSA/Pinkment-OAc hybrid was also evaluated in RAW 264.7 macrophages and HeLa cancer cell lines, which displayed an enhanced cell permeability enabling the detection of SIN-1 and LPS generated ONOO⁻ and the in vivo imaging of acute inflammation in LPS-treated mice. A remarkable 5.6 fold (RAW 264.7), 8.7-fold (HeLa) and 2.7-fold increased response was seen relative to Pinkment-OAc alone at the cellular level and in vivo, respectively. We anticipate that HSA/fluorescent probe hybrids will soon become ubiquitous and routinely applied to overcome solubility issues associated with hydrophobic fluorescent imaging agents designed to detect disease related biomarkers.

Herein, we report a protein-based hybridization strategy that exploits the host–guest chemistry of HSA (human serum albumin) to solubilize the otherwise cell impermeable ONOO⁻ fluorescent probe Pinkment-OAc.     

A highly sensitive and selective fluorescent probe for trivalent aluminum ion based on rhodamine derivative in living cells

A rhodamine spirolactam derivative (1) is developed as a colormetric and fluorescent probe for trivalent aluminum ions (Al³⁺). It exhibits a highly sensitive “turn-on” fluorescent response toward Al³⁺ with a 70-fold fluorescence intensity enhancement under 2 equiv. of Al³⁺ added. The probe can be applied to the quantification of Al³⁺ with a linear range covering from 5.0 × 10⁻⁷ to 2.0 × 10⁻⁵ M and a detection limit of 4.0 × 10⁻⁸ M. Most importantly, the fluorescence changes of the probe are remarkably specific for Al³⁺ in the presence of other metal ions, which meet the selective requirements for practical application. Moreover, the experiment results show that the response behavior of 1 towards Al³⁺ is pH independent in neutral condition (pH 6.0–8.0) and the response of the probe is fast (response time less than 3 min). In addition, the proposed probe has been used to detect Al³⁺ in water samples and image Al³⁺ in living cells with satisfying results.     

A 1-Hydroxy-2,4-Diformylnaphthalene-Based Fluorescent Probe and Its Detection of Sulfites/Bisulfite

A novel 1-hydroxy-2,4-diformylnaphthalene-based fluorescent probe L was synthesized by a Knoevenagel reaction and exhibited excellent sensitivity and selectivity towards sulfite ions (SO₃²⁻) and bisulfite ions (HSO₃⁻). The detection limits of the probe L were 0.24 μM using UV-Vis spectroscopy and 9.93 nM using fluorescence spectroscopy, respectively. Furthermore, the fluorescent probe L could be utilized for detection in real water samples with satisfactory recoveries in the range 99.20%~104.30% in lake water and 100.00%~104.80% in tap water by UV-Vis absorption spectrometry, and in the range 100.50%~108.60% in lake water and 102.70%~103.80% in tap water by fluorescence spectrophotometry.     

Anion-Responsive Fluorescent Supramolecular Gels

Three novel bis-urea fluorescent low-molecular-weight gelators (LMWGs) based on the tetraethyl diphenylmethane spacer—namely, L1, L2, and L3, bearing indole, dansyl, and quinoline units as fluorogenic fragments, respectively, are able to form gel in different solvents. L2 and L3 gel in apolar solvents such as chlorobenzene and nitrobenzene. Gelator L1 is able to gel in the polar solvent mixture DMSO/H₂O (H₂O 15% v/v). This allowed the study of gel formation in the presence of anions as a third component. An interesting anion-dependent gel formation was observed with fluoride and benzoate inhibiting the gelation process and H₂PO₄⁻, thus causing a delay of 24 h in the gel formation. The interaction of L1 with the anions in solution was clarified by ¹H-NMR titrations and the differences in the cooperativity of the two types of NH H-bond donor groups (one indole NH and two urea NHs) on L1 when binding BzO⁻ or H₂PO₄⁻ were taken into account to explain the inhibition of the gelation in the presence of BzO⁻. DFT calculations corroborate this hypothesis and, more importantly, demonstrate considering a trimeric model of the L1 gel that BzO⁻ favours its disruption into monomers inhibiting the gel formation.     

Linear Schiff-base fluorescence probe with aggregation-induced emission characteristics for Al detection and its application in live cell imaging

A simple Schiff-base derivative with salicylaldehyde moieties as fluorescent probe 1 was reported by aggregation-induced emission (AIE) characterization for the detection of metal ions. Spectral analysis revealed that probe 1 was highly selective and sensitive to Al³⁺. The probe 1 was also subject to minimal interference from other common competitive metal ions. The detection limit of Al³⁺ was 0.4 μM, which is considerably lower than the World Health Organization standard (7.41 μM), and the acceptable level of Al³⁺ (1.85 μM) in drinking water. The Job's plot and the results of ¹H-NMR and FT-IR analyses indicated that the binding stoichiometry ratio of probe 1 to Al³⁺ was 1:2. Probe 1 demonstrated a fluorescence-enhanced response upon binding with Al³⁺ based on AIE characterization. This response was due to the restricted molecular rotation and increased rigidity of the molecular assembly. Probe 1 exhibited good biocompatibility, and Al³⁺ was detected in live cells. Therefore, probe 1 is a promising fluorescence probe for Al³⁺ detection in the environment.     

Coordination Chemistry of a Bis(Tetrazine) Tweezer: A Case of Host-Guest Behavior with Silver Salts

The carbon-carbon cross-coupling of phenyl s-tetrazine (Tz) units at their ortho-phenyl positions allows the formation of constrained bis(tetrazines) with original tweezer structures. In these compounds, the face-to-face positioning of the central tetrazine cores is reinforced by π-stacking of the electron-poor nitrogen-containing heteroaromatic moieties. The resulting tetra-aromatic structure can be used as a weak coordinating ligand with cationic silver. This coordination generates a set of bis(tetrazine)-silver(I) coordination complexes tolerating a large variety of counter anions of various geometries, namely, PF₆⁻, BF₄⁻, SbF₆⁻, ClO₄⁻, NTf₂⁻, and OTf⁻. These compounds were characterized in the solid state by single-crystal X-ray diffraction (XRD) and diffuse reflectance spectroscopy, and in solution by ¹H-NMR, mass spectrometry, electroanalysis, and UV-visible absorption spectrophotometry. The X-ray crystal structure of complexes {[Ag(3)][PF₆]}_∞ (4) and {[Ag(3)][SbF₆]}_∞ (6), where 3 is 3,3′-[(1,1′-biphenyl)-2,2′-diyl]-6,6′-bis(phenyl)-1,2,4,5-tetrazine, revealed the formation of 1D polymeric chains, characterized by an evolution to a large opening of the original tweezer and a coordination of silver(I) via two chelating nitrogen atom and some C=C π-interactions. Electrochemical and UV spectroscopic properties of the original tweezer and of the corresponding silver complexes are reported and compared. ¹H-NMR titrations with AgNTf₂ allowed the determination of the stoichiometry and apparent stability of two solution species, namely [Ag(3)]⁺ and [Ag(3)₂]²⁺, that formed in CDCl₃/CD₃OD 2:1 v/v mixtures.     

Correlations between ligand field Δo,spin crossover T1/2 and redox potential Epa in a family of five dinuclear helicates

A family of five new bis-bidentate azole–triazole Rat ligands (1,3-bis(5-(azole)-4-isobutyl-4H-1,2,4-triazol-3-yl)benzene), varying in choice of azole (2-imidazole, 4-imidazole, 1-methyl-4-imidazole, 4-oxazole and 4-thiazole), and the corresponding family of spin-crossover (SCO) and redox active triply bridged dinuclear helicates, [Fe^II₂L₃]⁴⁺, has been prepared and characterised. X-ray crystal structures show all five Fe(ii) helicates are low spin at 100 K. Importantly, DOSY NMR confirms the intactness of these SCO-active dinuclear helicates in D₃-MeCN solution, regardless of HS fraction: γ_HS(298 K) = 0–0.81. Variable temperature ¹H NMR Evans and UV-vis studies reveal that the helicates are SCO-active in MeCN solution. Indeed, the choice of azole in the Rat ligand used in [Fe₂L₃]⁴⁺ tunes: (a) solution SCO T_1/2 from 247 to 471 K, and (b) reversible redox potential, E_m(Fe^II/III), from 0.25 to 0.67 V for four helicates, whilst one has an irreversible redox process, E_pa = 0.78 V, vs. 0.01 M AgNO₃/Ag. For the four reversible redox systems, a strong correlation (R² = 0.99) is observed between T_1/2 and E_pa. Finally, the analogous Ni(ii) helicates have been prepared to obtain Δ_o, establishing: (a) the ligand field strength order of the ligands: 4-imidazole (11 420) ∼ 1-methyl-4-imidazole (11 430) < 2-imidazole (11 505) ∼ 4-oxazole (11 516) < 4-thiazole (11 804 cm⁻¹), (b) that Δ_o ([Ni^II₂L₃]⁴⁺) strongly correlates (R² = 0.87) with T_1/2 ([Fe^II₂L₃]⁴⁺), and (c) interestingly that Δ_o strongly correlates (R² = 0.98) with E_pa for the four helicates with reversible redox, so the stronger the ligand field strength, the harder it is to oxidise the Fe(ii) to Fe(iii).

Choice of non-coordinated diazole heteroatom in five robust triply bridged dinuclear helicates tunes Δ_o, spin crossover and redox potential. Regardless of fraction high spin (0–0.81), DOSY NMR confirms the helicates are intact in solution.