Cracking down on vapochromic salts: unveiling vapomechanical stress in gas-sorbing platinum complexes

This study explores the vapochromic and vapoluminescent behaviors of [Pt(tpy)Cl]PF6 host molecules (tpy = 2,2':6',2''-terpyridine) under acetonitrile (CH3CN) vapor guest, challenging the conventional view that these phenomena arise solely from direct host-guest interactions. Our findings reveal a cooperative mechanism where mechanochromic surface perturbations prime the Pt(II) host for guest incorporation, leading to initial color and luminescence changes prior to significant structural alterations. While the color transition between the yellow [Pt(tpy)Cl]PF6 form and the red/orange [Pt(tpy)Cl]PF6·CH3CN form is reversible, repeated vapor cycling induces a loss of crystallinity, as indicated by diffraction peak broadening and emission shifts. Scanning electron microscopy analyses show mechanical deformations such as bending and surface pitting, emphasizing the role of vapomechanical stress in altering optical properties. These insights highlight the need for integrated design strategies in developing robust vapochromic materials for gas sensing applications.

Recent Advances in Fluorescent Based Chemical Probes for the Detection of Perchlorate Ions

This review highlights recent advancements and challenges in fluorescence-based chemical sensors for selective and sensitive detection of perchlorate, a persistent environmental pollutant and global concern due to its health and safety implications. Perchlorate is a highly persistent inorganic pollutant found in drinking water, soil, and air, with known endocrine-disruptive properties due to its interference with iodide uptake by the thyroid gland. Human exposure mainly occurs through contaminated water and food. Additionally, perchlorates are prevalent in improvised explosives, causing numerous civilian casualties, making their detection important in a worldwide aspect. Fluorescence-based chemical sensors provide a valuable tool for the selective detection of perchlorate ions due to their simplicity and applicability across various fields, including biology, pharmacology, military, and environmental science. This review article overviews perchlorate chemistry, occurrence, and remediation strategies, compares regulatory limits, and examines fluorescence-based detection mechanisms. It systematically summarizes recent advancements in designing at least a dozen fluorescence-based chemical materials for detecting perchlorate in the environment over the past decade. Key focus areas include the design and molecular architecture of synthetic chemical chromophores for perchlorate sensing and the photochemistry mechanisms driving their effectiveness. The main findings indicate that there has been significant progress in the development of reliable and robust fluorescence-based sensors with higher selectivity and sensitivity for perchlorate detection. However, several challenges remain, such as improving detection limits and sensor stability. The review outlines potential future research directions, emphasizing the need for further innovation in sensor design and development. It aims to enhance understanding and spur advances that could create more efficient and robust chemical scaffolds for perchlorate sensing. By addressing current limitations and identifying opportunities for improvement, the review provides a comprehensive resource for researchers working to develop better detection methods for this significant environmental pollutant.

Selective Turn-On Luminescent Recognition of Perchlorate Ion Using Pyridyl-Benzimidazole-Based Probe

Anions play a crucial role in various environmental, chemical, and biological processes. Among various anions, the production of perchlorate (ClO4 -) ion is expected to rise in upcoming years, and thus, an efficient method for the detection of perchlorate ion is highly desirable. In this effort, a pyridyl-benzimidazole-based luminescent probe (RSB1) containing two N-H donor sites has been synthesized for selective detection of perchlorate ion. Different spectral techniques such as FT-IR, NMR, ESI-mass, UV-Vis, and fluorescence analyses have been used to characterize this probe. High selectivity of RSB1 for ClO4 - was realized even in presence of strongly interfering species in aqueous-acetonitrile (CH3CN-H2O; 4:1, v/v) solution. Notably, RSB1 served as a "turn-on" perchlorate-responsive probe and exhibited an emission enhancement at 363 nm when excited at 300 nm. The detection limit (LoD) and the binding constant (Kb) values were depicted to be 0.121 μM and 2.6 × 105 M-1, respectively, while the binding mechanism for RSB1-ClO4 - was validated via Job's plot, NMR, and DFT analyses. Furthermore, this probe was successfully employed to trace perchlorate in real samples such as tap water, distilled water, and soil samples with good to excellent recovery values.

Aggregation Game: Changing Solid-State Emission Using Different Counterions in Monoalkynylphosphonium Pt(II) Complexes

Two series of heteroleptic monoalkynylphosphonium Pt(II) complexes decorated with 2,2':6',2''-terpyridine (terpy, N series) and 6-phenyl-2,2'-bipyridine (phbpy, C series) ligands, were prepared and characterized by spectroscopic methods. The complexes obtained exhibit triplet emission in solution, and the characteristics inside the series depend on the nature of the alkynylphosphonium ligand. The description of electronic transitions responsible for energy absorption and emission in discrete Pt(II) complexes was made on the basis of a detailed analysis of the results of DFT calculations, and has shown to involve MLCT, ILCT, and LLCT transitions. The complexes of both series exhibit triplet solid-state luminescence with parameters that also depend on the composition of the complexes, and the analysis of the experimental data indicates the realization of LC, MLCT, MMLCT, and MC transitions due to Pt⋯Pt metallophilic interactions and matrix rigidity. It was shown that the anion variation leads to a significant difference in the photophysical characteristics of the N complexes, which exhibit a smooth dependence of the luminescent properties on the anion size. Using quantum chemical modeling, it is demonstrated how the anion size influences the Pt⋯Pt distance in the solid state.

Reversible and irreversible stimuli-responsive chromism of a square-planar platinum(ii) salt

A new simple Pt(ii) terpyridyl salt that shows reversible response towards acetonitrile and irreversible response towards methanol has been reported, accompanied with the colorimetric/luminescent changing from red to yellow. Experimentally and theoretically, the spectroscopic change derives from the hydrogen bonds between crystal water in the Pt(ii) terpyridyl salt and external organic molecules, and the different strength of hydrogen bond leads either reversible or irreversible stimuli-response. Furthermore, this Pt(ii) terpyridyl salt has been on one hand applied as a probe for sensing acetonitrile in water solution, with high selectivity, good reversibility, proper sensitivity and fast response rate, and on the other hand as advanced anticounterfeiting materials. The current study provides a new approach to acquire and design either reversible or irreversible stimuli-responsive luminescent materials.

Structural modifications to platinum(II) pincer complexes resulting in changes in their vapochromic and solvatochromic properties

There is a need to develop rapidly responsive chemical sensors for the detection of low concentrations of volatile organic solvents (VOCs). Platinum pincer complexes have shown promise as sensors because of their colours and vapochromic and solvatochromic properties, that may be related to the non-covalent interactions between the pincer complexes and the guest VOCs. Here we report an investigation into a series of Pt(II) complexes based on the 1,3-di(pyridine)benzene tridentate (N⁁C⁁N) skeleton with the formula [Pt(N⁁C(R)⁁N)(CN)] (R = C(O)Me 2, C(O)OEt 3, C(O)OPh 4) with the fourth coordination site occupied by a cyanide ligand. Solid-state samples of the complexes have been tested with a range of volatiles including methanol, ethanol, acetone, dichloromethane and water, and while 2 displays thermochromism, 3 and 4 display rapidly reversible vapochromism and solvatochromism. These results are correlated with X-ray powder and single crystal X-ray structural data including an assessment of the crystal packing and the void space in the crystalline space. The cyanide ligand and the R substituents are involved in hydrogen bonding that creates the voids within the structures and interact with the solvent molecules that influence the Pt⋯Pt separation in the crystalline state.

Water's Role in Polymorphic Platinum(II) Complexes

Solvent plays a vital role in the recrystallization process and resulting crystallinity of materials. This role is of such importance that it can control the stability and utility of materials. In this work, the inclusion of a solvent in the crystalline lattice, specifically water, drastically affects the overall stability of two platinum polymorphs. [Pt(tpy)Cl]BF₄ (tpy = 2,2';6'2″-terpyridine) crystallizes in three forms, red (1R) and blue (1B) polymorphs and a yellow nonsolvated form (2). 1R is the more stable of the two polymorphs, whereas 1B loses crystallinity upon dehydration at ambient conditions resulting in the formation of 2. Close examination of the solid-state extended structures of the two polymorphs reveals that 1R has a lattice arrangement that is more conducive to stronger intermolecular interactions compared to 1B, thereby promoting greater stability. In addition, these two polymorphs exhibit unique vapochromic responses when exposed to various solvents.

pH-Mediated Colorimetric and Luminescent Sensing of Aqueous Nitrate Anions by a Platinum(II) Luminophore@Mesoporous Silica Composite

Increased levels of nitrate (NO₃^-) in the environment can be detrimental to human health. Herein, we report a robust, cost-effective, and scalable, hybrid material-based colorimetric/luminescent sensor technology for rapid, selective, sensitive, and interference-free in situ NO₃^- detection. These hybrid materials are based on a square-planar platinum(II) salt [Pt(tpy)Cl]PF₆ (tpy = 2,2';6',2″-terpyridine) supported on mesoporous silica. The platinum salt undergoes a vivid change in color and luminescence upon exposure to aqueous NO₃^- anions at pH ≤ 0 caused by substitution of the PF₆^- anions by aqueous NO₃^-. This change in photophysics of the platinum salt is induced by a rearrangement of its crystal lattice that leads to an extended Pt···Pt···Pt interaction, along with a concomitant change in its electronic structure. Furthermore, incorporating the material into mesoporous silica enhances the surface area and increases the detection sensitivity. A NO₃^- detection limit of 0.05 mM (3.1 ppm) is achieved, which is sufficiently lower than the ambient water quality limit of 0.16 mM (10 ppm) set by the United States Environmental Protection Agency. The colorimetric/luminescence of the hybrid material is highly selective to aqueous NO₃^- anions in the presence of other interfering anions, suggesting that this material is a promising candidate for the rapid NO₃^- detection and quantification in practical samples without separation, concentration, or other pretreatment steps.

Reversible stimuli-responsive chromism of a cyclometallated platinum(II) complex

We report the reversible chromism and luminescence of a cyclometalated platinum(ii) complex that forms dimers, with close PtPt interactions that can be modulated by solvent and temperature. The precise reversible control may be exploited in future stimuli-responsive chemosensing or optoelectronic devices.

Luminescent Pt(2,6-bis(N-methylbenzimidazol-2-yl)pyridine)X+: a comparison with the spectroscopic and electrochemical properties of Pt(tpy)X+ (X = Cl, CCPh, Ph, or CH3)

A series of platinum(ii) pincer complexes of the formula Pt(mbzimpy)X+, 1(a-d), (mbzimpy = 2,6-bis(N-methylbenzimidazol-2-yl)pyridine; X = Cl; (a), CCPh; (b), Ph; (c), or CH3; (d), CCPh = phenylacetylide, and Ph = Phenyl) have been synthesized and characterized. Electronic absorption and emission, as well as electrochemical properties of these compounds, have been investigated. Pt(tpy)X+ analogs (tpy = 2,2';6'2''-terpyridine), 2(a-d), have also been investigated and compared. Electrochemistry shows that 1 and 2 analogs undergo two chemically reversible one-electron reduction processes that are shifted cathodically along the a < b < c < d series. Notably, these reductions occur at slightly higher negative potentials in the case of 1. The absorption spectra of 1 and 2 in acetonitrile exhibit ligand-centered (1LC) transitions (ε ≈ 104 M-1 cm-1) in the UV region and metal-to-ligand-charge transfer (1MLCT) transitions (ε ≈ 103 M-1 cm-1) in the visible region. The corresponding visible bands of 1b and 2b have been assigned to 1(LLCT/MLCT) mixed state (LLCT: ligand-to-ligand-charge transfer). The preceding 1LC and 1MLCT transitions of 1 occur at lower energies than that of 2. These 1LC transitions have distinctly been blue-shifted along a < c < d in 2, but occur at nearly identical energies in 1. Conversely, 1MLCT transitions are red-shifted along a < c < d in both the analogs. The 77 K glassy solutions of 1 and 2 exhibit an intense vibronically-structured emission band at λmax(0-0) in the 470-560 nm range. This band is red-shifted along b < a ≤ c < d in 1 and along a ≤ d ≈ c ≪ b in 2. The main character of these emissions is assigned to 3LLCT emissive state in 1b and 2b, whereas to 3LC in the rest of the compounds. Relative stabilization of these spin-forbidden emissive states is discussed by invoking configuration mixing with the higher-lying 3MLCT state.

Photophysical Properties of Pt(II) Polypyridines with Five- versus Six-Membered Chelate Rings: Trade-Offs in Angle Strain

This report describes the synthesis and characterization of a series of eight [Pt(NNN)X]⁺ complexes where the tridentate NNN ligand is (2,2'-bipyrid-6-yl)(pyrid-2-yl)sulfide (btp) or methyl(2,2'-bipyrid-6-yl)(pyrid-2-yl)amine (bmap) and X is OMe, Cl, phenylethynyl (C₂Ph), or cyclohexylethynyl (C₂Cy). The expectation was that inserting a heteroatom into the backbone of 2,2':6',2″-terpyridine (trpy) would expand the overall intraligand bite angle, introduce ILCT character into the excited states, and improve the photophysical properties. Crystal structures of [Pt(bmap)C₂Ph]⁺ and [Pt(btp)Cl]⁺ reveal that atom insertion into the trpy backbone successfully expands the bite angle of the ligand by 8-10°. However, the impact on the photophysics is minimal. Indeed, of the eight systems investigated, only the [Pt(bmap)C₂Ph]⁺ and [Pt(btp)C₂Ph]⁺ complexes display appreciable emission in fluid solution, and they exhibit shorter emission lifetimes than [Pt(trpy)C₂Ph]⁺. One reason is that the bond angle preferences of platinum and the inserted heteroatom induce the six-membered rings to deviate from planarity and adopt a boat-like conformation, impairing charge delocalization within the ligand. In addition, angle strain induces the donor atoms about platinum to assume a pseudotetrahedral arrangement, which offsets any benefit due to the increase in overall bite angle by promoting deactivation via d-d excited states. The results reveal that, in order to improve the luminescence of a [Pt(NNN)X]⁺ system, one must take care to avoid trading one kind of angle strain for another.

Harnessing solid-state packing for selective detection of chloride in a macrocyclic anionophore

We report the synthesis of an inherently fluorescent macrocyclic receptor for chloride. The use of a disulphide tether provides for an excellent yield in the macrocyclization step. This compound binds chloride in the solution and solid state, and while unstable over time in aqueous solution, shows a selective response toward chloride over other anions in the solid state due to intermolecular interactions between fluorophore backbones. Surprisingly, the optoelectronic response to anions differs in solution and the films, with a distinct colorimetric response observed only in the film.

Analyte-responsive gated hollow mesoporous silica nanoparticles exhibiting inverse functionality and an AND logic response

A multifunctional nanoparticle with designed selectivity was made using hollow mesoporous silica, ship-in-a-bottle synthesis of a crystalline solid-state detector, and protection of the crystal by acid-responsive nanogates. The system demonstrates the inverse application of the usual trapping of contents by the gate followed by their release. Instead, the gate protects the contents followed by selective exposure. Crystallization of [Pt(tpy)Cl](PF₆) (tpy = 2,2':6',2''-terpyridine) inside the cavity of hollow mesoporous silica created the unique core/shell nanoparticle. The crystalline core becomes fluorescent in the presence of perchlorate. By condensing an acid-sensitive gate onto the particle, access to the pores is blocked and the crystal is protected. The new nanomaterial obeys Boolean AND logic; only the presence of both the analyte (ClO₄^-) and acid results in the optical response.

Proton-switchable vapochromic behaviour of a platinum(ii)–carboxy-terpyridine complex

We synthesized a carboxy-functionalized Pt(ii)–terpyridine complex that exhibits vapochromic behavior that is switchable via protonation/deprotonation of the carboxy group.

Pattern-Based Detection of Anion Pollutants in Water with DNA Polyfluorophores

Many existing irrigation, industrial and chemical storage sites are currently introducing hazardous anions into groundwater, making the monitoring of such sites a high priority. Detecting and quantifying anions in water samples typically requires complex instrumentation, adding cost and delaying analysis. Here we address these challenges by development of an optical molecular method to detect and discriminate a broad range of anionic contaminants with DNA-based fluorescent sensors. A library of 1296 tetrameric-length oligodeoxyfluorosides (ODFs) composed of metal ligand and fluorescence modulating monomers was constructed with a DNA synthesizer on PEG-polystyrene microbeads. These oligomers on beads were incubated with YIII or ZnII ions to provide affinity and responsiveness to anions. Seventeen anions were screened with the library under an epifluorescence microscope, ultimately yielding eight chemosensors that could discriminate 250 μM solutions of all 17 anions in buffered water using their patterns of response. This sensor set was able to identify two unknown anion samples from ten closely-responding anions and could also function quantitatively, determining unknown concentrations of anions such as cyanide (as low as 1 mM) and selenate (as low as 50 μM). Further studies with calibration curves established detection limits of selected anions including thiocyanate (detection limit ~300 μM) and arsenate (~800 μM). The results demonstrate DNA-like fluorescent chemosensors as versatile tools for optically analyzing environmentally hazardous anions in aqueous environments.

Thermo- and mechanical-grinding-triggered color and luminescence switches of the diimine-platinum(II) complex with 4-bromo-2,2'-bipyridine

The square-planar diimine-platinum(II) complex, Pt(4-Brbpy)(C≡CC6H5)2 (1) (4-Brbpy = 4-bromo-2,2'-bipyridine), was prepared and characterized. Solid-state 1 exhibits reversible thermo- and mechanical-grinding-triggered color and luminescence changes. When crystalline 1·2(CH2Cl2) or 1·2(CHCl3) are heated or ground, the original bright yellow-green emission centered at 525 (549, sh) nm changed to 637 and 690 nm, corresponding to thermo- and mechanochromic response shifts of approximately 88-112 nm and 141-165 nm, respectively. Meanwhile the crystalline state changes into an amorphous phase in both processes. Once the amorphous sample absorbs organic vapors, it can be reverted to the original crystalline state, along with red luminescence turning back to yellow-green emission. The reversibility of thermo- and mechanical-grinding-triggered chromic luminescence properties has been dynamically monitored by emission spectra and X-ray diffraction patterns. The dramatic thermo- and mechanical-grinding-triggered emission red shifts are most likely due to the conversion of the (3)MLCT/(3)LLCT emission state into the (3)MMLCT triplet state.

In-house and synchrotron X-ray diffraction studies of 2-phenyl-1,10-phenanthroline, protonated salts, complexes with gold(III) and copper(II), and an orthometallation product with palladium(II)

Different salts of the 2-phenyl-1,10-phenanthrolin-1-ium cation, (pnpH)(+), are obtained by reacting 2-phenyl-1,10-phenanthroline (pnp), C18H12N2, (I), with a variety of anions, such as hexafluoridophosphate, C18H13N2(+)·PF6(-), (II), trifluoromethanesulfonate, C18H13N2(+)·CF3SO3(-), (III), tetrachloridoaurate, (C18H13N2)[AuCl4], (IV), and bromide (as the dihydrate), C18H13N2(+)·Br(-)·2H2O, (V). Compound (I) crystallizes with Z' = 2, with both independent molecules adopting a coplanar conformation. In (II)-(IV), a hydrogen bond exists between the cation and anion, while one of the lattice water molecules serves as a hydrogen-bonded bridge between the cation and anion in (V). Reaction of (I) with HAuCl4 gives the salt complex (IV); however, reaction with KAuCl4 produces the monodentate complex trichlorido(2-phenyl-1,10-phenanthroline-κN(10))gold(III), [AuCl3(C18H12N2)], (VI). Dichlorido(2-phenyl-1,10-phenanthroline-κ(2)N,N')copper(II), [CuCl2(C18H12N2)], (VII), results from the reaction of CuCl2·2H2O and (I), in which the Cu(II) center adopts a tetrahedrally distorted square-planar geometry. The pendent phenyl ring twists to a bisecting position relative to the phenanthroline plane. The square-planar Pd(II) complex, bromido[2-(phenanthrolin-2-yl)phenyl-κ(3)C(1),N,N']palladium(II), [PdBr(C18H11N2)], (VIII), is obtained from the reaction of (I) with [PdCl2(cycloocta-1,5-diene)], followed by addition of bromine. A coplanar geometry for the pendent ring is adopted as a result of the tridentate bonding motif.

Structural, spectroscopic and theoretical studies of a vapochromic platinum(ii) terpyridyl complex

A combination of spectroscopic methods, crystallographic analyses, and theoretical studies provides a rationale to understand the nature of the structural and electronic response of the simplest platinum(ii) terpyridyl complex to acetonitrile.

A highly selective and sensitive luminescent chemosensor for Zn2+ ions based on cyclometalated platinum(II) complexes

A novel luminescent Zn(2+) ions chemosensor, a cyclometalated platinum(II) bipyridyl acetylide complex, was designed. Of particular significance is that it shows a high sensitivity towards Zn(2+) ions without interference from other biologically important cations in acetonitrile. The tautomerization of amide favors detecting Zn(2+) ions among other HTM (heavy and transition metal) ions in aqueous systems.

Chromogenic and fluorogenic chemosensors and reagents for anions. A comprehensive review of the years 2010-2011

This review focuses on examples reported in the years 2010-2011 dealing with the design of chromogenic and fluorogenic chemosensors or reagents for anions.