A zwitterionic probe for ratiometric fluorescent detection of aluminium(III) ion in aqueous medium and its application in bioimaging.

In the present study, we have synthesized an aminobenzoic acid containing Schiff base (compound 1) and its structure was confirmed through single crystal X-ray study. Importantly, the compound 1 crystallizes in the zwitterionic form, with an anionic carboxylate group (-COO-) and a cationic iminium group (-C = NH+-). The compound 1 is highly soluble in water due to its zwitterionic feature in the solid state. Interestingly, compound 1 acts as a ratiometric fluorescent probe for the selective detection of Al3+ ion in aqueous solution without organic cosolvent. It can also detect Al3+ ion by visual colour change to bluish-green fluorescence under 365 nm UV light. The association constant between compound 1 with Al3+ ion was estimated to be 1.67 × 104 M-1. The lowest detection limit for Al3+ ion was calculated to be 7.05 × 10-8 M in water. Compound 1 in combination with Al3+ ion demonstrated fluorescent imaging potential of the nucleus of in RAW 264.7 murine macrophage cell line. In addition, the sensing model is developed as paper based sensor ''Test Kit' 'for its practical applicability.

Crystallographic elucidation of an aluminium-bound amido Schiff base chemosensor: a selective turn-on fluorescent chemosensor for Al3+ ions.

We have synthesized a biphenyl-derived two arm-containing amido Schiff base, bis((2-hydroxynaphthalen-1-yl) methylene)-[1,1'-biphenyl]-2,2'-dicarbohydrazide (sensor 1), having hard donors to facilitate chelation with hard metal centers. The crystal structure of sensor 1 reveals that it crystallizes in the monoclinic system with the space group I2/a and shows several types of intra/inter-molecular H-bond interactions, which stabilized the crystal lattice. The sensing property of sensor 1 towards different metal ions has been demonstrated using different analytical techniques. In fact, sensor 1 shows high fluorescence selectivity and sensitivity towards Al3+ ions in aqueous DMF media. Importantly, we have reported the first structurally characterized six-coordinate dinuclear Al3+ complex ([Na(Al2L2)·2H2O·4DMF], complex 1 (where ligand, L = sensor 1). Complex 1 crystallizes in the space group P1̄. X-ray single crystal diffraction studies for complex 1 reveal that each Al3+ ion is hexa-coordinated by four oxygen and two nitrogen atoms from each arm of the two ligands. The sodium ion has penta-coordination in a highly distorted trigonal bipyramidal geometry, surrounded by two µ-bridging naphtholate oxygen atoms and three solvent DMF oxygen atoms. Upon addition of Na2EDTA to complex 1, no spectral change or naked-eye colour change was observed. Furthermore, "test kits" coated with sensor 1 showed successful selective detection of Al3+ ions under UV light.

Para-N-Methylpyridinium Pyrenes: Impact of Positive Charge on ds-DNA/RNA and Protein Recognition, Photo-Induced Bioactivity, and Intracellular Localisation.

The 2- and 2,7- substituted para-N-methylpyridinium pyrene cations show high-affinity intercalation into ds-DNAs, whereas their non-methylated analogues interacted with ds-DNA/RNA only in the protonated form (at pH 5), but not at physiological conditions (pH 7). The fluorescence from non-methylated analogues was strongly dependent on the protonation of the pyridines; consequently, they act as fluorescence ratiometric probes for simultaneous detection of both ds-DNA and BSA at pH 5, relying on the ratio between intensities at 420 nm (BSA specific) and 520 nm (DNA specific), whereby exclusively ds-DNA sensing could be switched-off by adjustment to pH 7. Only methylated, permanently charged pyrenes show photoinduced cleavage of circular DNA, attributed to pyrene-mediated irradiation-induced production of singlet oxygen. Consequently, the moderate toxicity of these cations against human cell lines is strongly increased upon irradiation. Detailed studies revealed increased total ROS production in cells treated by the compounds studied, accompanied by cell swelling and augmentation of cellular complexity. The most photo-active 2-para-N-methylpyridinium pyrene showed significant localization at mitochondria, its photo-bioactivity likely due to mitochondrial DNA damage. Other derivatives were mostly non-selectively distributed between various cytoplasmic organelles, thus being less photoactive.

Methyl Viologens of Bis-(4'-Pyridylethynyl)Arenes - Structures, Photophysical and Electrochemical Studies, and their Potential Application in Biology.

A series of bis-(4'-pyridylethynyl)arenes (arene=benzene, tetrafluorobenzene, and anthracene) were synthesized and their bis-N-methylpyridinium compounds were investigated as a class of π-extended methyl viologens. Their structures were determined by single crystal X-ray diffraction, and their photophysical and electrochemical properties (cyclic voltammetry), as well as their interactions with DNA/RNA were investigated. The dications showed bathochromic shifts in emission compared to the neutral compounds. The neutral compounds showed very small Stokes shifts, which are a little larger for the dications. All of the compounds showed very short fluorescence lifetimes (<4 ns). The neutral compound with an anthracene core has a quantum yield of almost unity. With stronger acceptors, the analogous bis-N-methylpyridinium compound showed a larger two-photon absorption cross-section than its neutral precursor. All of the dicationic compounds interact with DNA/RNA; while the compounds with benzene and tetrafluorobenzene cores bind in the grooves, the one with an anthracene core intercalates as a consequence of its large, condensed aromatic linker moiety, and it aggregates within the polynucleotide when in excess over DNA/RNA. Moreover, all cationic compounds showed highly specific CD spectra upon binding to ds-DNA/RNA, attributed to the rare case of forcing the planar, achiral molecule into a chiral rotamer, and negligible toxicity toward human cell lines at ≤10 µM concentrations. The anthracene-analogue exhibited intracellular accumulation within lysosomes, preventing its interaction with cellular DNA/RNA. However, cytotoxicity was evident at 1 µM concentration upon exposure to light, due to singlet oxygen generation within cells. These multi-faceted features, in combination with its two-photon absorption properties, suggest it to be a promising lead compound for development of novel light-activated theranostic agents.

Transition Metal Catalyst-Free, Base-Promoted 1,2-Additions of Polyfluorophenylboronates to Aldehydes and Ketones.

A novel protocol for the transition metal-free 1,2-addition of polyfluoroaryl boronate esters to aldehydes and ketones is reported, which provides secondary alcohols, tertiary alcohols, and ketones. Control experiments and DFT calculations indicate that both the ortho-F substituents on the polyfluorophenyl boronates and the counterion K+ in the carbonate base are critical. The distinguishing features of this procedure include the employment of commercially available starting materials and the broad scope of the reaction with a wide variety of carbonyl compounds giving moderate to excellent yields. Intriguing structural features involving O-H⋅⋅⋅O and O-H⋅⋅⋅N hydrogen bonding, as well as arene-perfluoroarene interactions, in this series of racemic polyfluoroaryl carbinols have also been addressed.

2- and 2,7-Substituted para-N-Methylpyridinium Pyrenes: Syntheses, Molecular and Electronic Structures, Photophysical, Electrochemical, and Spectroelectrochemical Properties and Binding to Double-Stranded (ds) DNA.

Two N-methylpyridinium compounds and analogous N-protonated salts of 2- and 2,7-substituted 4-pyridyl-pyrene compounds were synthesised and their crystal structures, photophysical properties both in solution and in the solid state, electrochemical and spectroelectrochemical properties were studied. Upon methylation or protonation, the emission maxima are significantly bathochromically shifted compared to the neutral compounds, although the absorption maxima remain almost unchanged. As a result, the cationic compounds show very large apparent Stokes shifts of up to 7200 cm-1 . The N-methylpyridinium compounds have a single reduction at ca. -1.5 V vs. Fc/Fc+ in MeCN. While the reduction process was reversible for the 2,7-disubstituted compound, it was irreversible for the mono-substituted one. Experimental findings are complemented by DFT and TD-DFT calculations. Furthermore, the N-methylpyridinium compounds show strong interactions with calf thymus (ct)-DNA, presumably by intercalation, which paves the way for further applications of these multi-functional compounds as potential DNA-bioactive agents.

Rotation of a helical coordination polymer by mechanical grinding.

Anisotropic cell volume expansion by mechanical grinding of the solid facilitates the concerted rotation of the photo-inert helical coordination polymer, which causes the misaligned arms containing olefin functional groups in the neighbouring strands to align to undergo [2+2] cycloaddition reaction in 83% yield.

Comparison Study of the Site-Effect on Regioisomeric Pyridyl-Pyrene Conjugates: Synthesis, Structures, and Photophysical Properties.

To investigate the "site effect" of pyridyl substituents on a pyrene core, four regioisomeric monopyridyl-pyrene (1-4) and five regioisomeric dipyridyl-pyrene (5-9) conjugates were synthesized and characterized and their structures confirmed by single-crystal X-ray diffraction. The photophysical properties and related frontier orbital features of these compounds have been studied both experimentally and theoretically and demonstrate the dependence of the properties of the compounds on the position of substitution of the pyridyl moieties connecting to the pyrene core. It was found that the absorption spectra of 2- and 4-substituted pyrene derivatives display similar and weak influence on the S2 ← S0 excitations, whereas they are quite different from those of 1-substituted isomers. The emission spectra of 1- and 4-substituted pyrenes are quite similar, whereas those of 2-substituted isomers display the largest bathochromic shift. The 1,6-disubstituted compound 5 exhibits a near-unity emission quantum yield in solution, which is nearly three times higher than those of other regioisomeric dipyridyl-pyrenes. In addition, the tetrasubstituted 1,6-dipyridyl-3,8-di-n-butyl-pyrene (10) exhibits the highest solid-state quantum yield of 0.24 among all of the 10 pyridyl-pyrenes prepared in this study.

Reversible single-crystal-to-single-crystal photochemical formation and thermal cleavage of a cyclobutane ring.

A [2+2] cycloaddition reaction has been observed in a number of solids. The cyclobutane ring in a photodimerized material can be cleaved into olefins by UV light and heat. The high thermal stability of the metal-organic salt K2SDC (H2SDC = 4,4'-stilbenedicarboxylic acid) has been successfully utilized to investigate the reversible cleavage of a cyclobutane ring. The two polymorphs of K2SDC undergo reversible cyclobutane formation by UV light and cleavage by heat in cycles. Of these, one polymorph retains its single-crystal nature during the reversible processes. Polymorphs are known to show different physical properties and chemical reactivities. This work reveals that the retention of single-crystal nature is strongly associated with the packing of molecules, which is controlled by kinetics and thermodynamics. The photoemissive nature of the products makes this as a promising material for photoswitches and optical data storage devices.

Influence of C-H···π interactions on the solid-state [2+2] cycloaddition reaction of a Ag(I) coordination complex in an inorganic co-crystal.

The C-H···π interaction plays a vital role in directing a single-crystal to single-crystal [2+2] cycloaddition reaction to form an intramolecular head-to-head photodimer compared to an intermolecular head-to-tail photoproduct of 4-styrylpyridine of a Ag(I) coordination complex in an inorganic co-crystal.

Solid-state structural transformations and photoreactivity of 1D-ladder coordination polymers of Pb(II).

An attempt has been made to design double-stranded ladder-like coordination polymers (CPs) of hemidirected Pb(II) . Four CPs, [Pb(µ-bpe)(O2 C-C6 H5 )2 ]⋅2H2 O (1), [Pb2 (µ-bpe)2 (µ-O2 C-C6 H5 )2 (O2 C-C6 H5 )2 ] (2), [Pb2 (µ-bpe)2 (µ-O2 C-p-Tol)2 (O2 C-p-Tol)2 ]⋅ 1.5 H2 O (3) and [Pb2 (µ-bpe)2 (µ-O2 C-m-Tol)2 (O2 C-m-Tol)2 ] (4) (bpe=1,2-bis(4'-pyridyl)ethylene), have been synthesised and investigated for their solid-state photoreactivity. CPs 2-4, having a parallel orientation of bpe molecules in their ladder structures and being bridged by carboxylates, were found to be photoreactive, whereas CP 1 is a linear one-dimensional (1D) CP with guest water molecules aggregating to form a hydrogen-bonded 1D structure. The linear strands of 1 were found to pair up upon eliminating lattice water molecules by heating, which led to the solid-state structural transformation of photostable linear 1D CP 1 into photoreactive ladder CP 2. In the construction of the double-stranded ladder-like structures, the parallel alignment of CC bonds in 2-4 is dictated by the chelating and µ2 -η(2) :η(1) bridging modes of the benzoate and toluate ligands. The role of solvents in the formation of such double-stranded ladder-like structures has also been investigated. A single-crystal-to-single-crystal transformation occurred when 4 was irradiated under UV light to form [Pb2 (rctt-tpcb)(µ-O2 C-m-Tol)2 (O2 C-m-Tol)2 ] (5).

Solid-state reactivity and structural transformations involving coordination polymers.

This tutorial review discusses recent literature on coordination polymers and metal-organic frameworks (MOFs) that exhibit solid-state reactivity and structural transformation under various experimental conditions. Removal or exchange of solvents and guest molecules, exposure to reactive vapours, and external stimuli such as heat, light or mechanochemical force cause such structural transformations and these are often manifested by various physical properties such as colour, magnetism, luminescence, chirality, porosity, etc. due to change in coordination number and geometry, dimensionality, interpenetration, etc. More drastic transformations related to the exchange of metal ions, pillar ligands and insertion of additional ligands between the layers have also been demonstrated. The vast area of dynamic behaviour of coordination polymers and their stimuli-responsive properties have also been addressed briefly.

Thermal cleavage of cyclobutane rings in photodimerized coordination-polymeric sheets.

Three coordination polymers, [Cd(2)(pvba)(2)(tbdc)(dmf)(2)] (1), [Co(2)(pvba)(2)(tbdc)(dmf)(2)(H(2)O)(2)] (2), and [Ni(2)(pvba)(2)(tbdc)(dmf)(2)(H(2)O)(2)] (3) (H(2)tbdc = 2,3,5,6-tetrabromobenzenedicarboxylic acid, Hpvba = trans-2-(4'-pyridyl)vinylbenzoic acid), were synthesized by solvothermal methods. The solid-state structures of compounds 1 and 2 were determined by X-ray crystallography. In compounds 1 and 2, the bimetallic cores acted as secondary building units that connected the tbdc ligands in one direction and a pair of pvba ligands, which were aligned in a head-to-tail parallel manner, in the orthogonal direction to form sheet structures. The C=C bonds in these pvba ligand pairs in all three compounds were well-aligned to undergo quantitative [2+2] cycloaddition reactions in the solid state under UV irradiation, thereby yielding their cyclobutane derivatives. This photochemical reaction appeared to facilitate structural transformations from one 2D structure into another in the solid state. The photoreactive Co(II)- and Ni(II) coordination polymers exhibited a reversible dehydration-rehydration reaction that was accompanied by color changes from pink to purple and green to yellow, respectively, owing to a change in coordination number from six to five. Magnetic studies showed that compound 2 was an antiferromagnet, which displayed a field-dependent transition with a critical field (H(c)) of 40 kOe at 2 K; the antiferromagnetic interaction between the Co(2) units was strengthened and weakened by dehydration and UV irradiation, respectively. The cyclobutane ligand in the photodimerized products was cleaved on heating to yield a mixture of trans- and cis-isomers of pvba, as monitored by (1)H NMR spectroscopy. The Cd(II) coordination polymer underwent quantitative cleavage of the cyclobutane ring whilst the other two underwent partial cleavage.

Photoreactive gold(I) macrocycles with diphosphine and trans,trans-muconate ligands.

A pair of trans,trans-muconate ligands have been successfully aligned in two novel Au(I) macrocycles by design from phosphino metal precursors that undergo photochemical cycloaddition reactions quantitatively, resulting in the formation of cyclooctadiene derivatives.

Role of anions in the synthesis of cyclobutane derivatives via [2 + 2] cycloaddition reaction in the solid state and their isomerization in solution.

trans-3-(4'-Pyridyl)acrylic acid (4-PA) is inert to photodimerization reaction both in solution and solid state. It is made photoreactive by forming salts with various acids. The anions of these salts play a key role in directing the packing of 4-PAH(+) in the solid state. The anions CF(3)CO(2)(-), Cl(-), ClO(4)(-), and BF(4)(-) direct the parallel alignments of 4-PAH(+) in head-to-tail (HT) fashion and lead to the formation of HT-photodimer. On the other hand, bivalent anion SO(4)(2-) directs parallel alignment of 4-PAH(+) in head-to-head (HH) fashion and lead to the formation of HH-photodimer. The details of the anion-controlled stereoselective syntheses of these two cyclobutane derivatives are presented. Interestingly, both cyclobutane compounds undergo isomerization from rctt-form to rctc-form in solution catalyzed by acid.

A new ligand for metal-organic framework and co-crystal synthesis: mechanochemical route to rctt-1,2,3,4-tetrakis-(4'-carboxyphenyl)-cyclobutane.

A novel method to access rctt-1,2,3,4-tetrakis-(4'-carboxyphenyl)-cyclobutane in quantitative yield from an environmentally benign route is presented. The cyclobutane derivative is demonstrated as a potential candidate to serve as an organic building block for making co-crystal and MOF.

Anion-controlled stereoselective synthesis of cyclobutane derivatives by solid-state [2 + 2] cycloaddition reaction of the salts of trans-3-(4-pyridyl) acrylic acid.

Two stereoisomers of cyclobutane derivatives with pyridyl and carboxylic acid functionalities have been stereoselectively synthesized by a solid-state photochemical [2 + 2] cycloaddition reaction in quantitative yields. The head-to-head and head-to-tail parallel orientations of the monomers, required to obtain these two isomers, have been controlled by the anions present in the salts. The photoinert behavior of these salts in solution signifies the importance of the solid-state synthesis of these cyclobutane derivatives.