Structural and Theoretical Study of Copper(II)-5-fluoro Uracil Acetate Coordination Compounds: Single-Crystal to Single-Crystal Transformation as Possible Humidity Sensor

This paper describes the synthesis and characterization of seven different copper(II) coordination compounds, as well as the formation of a protonated ligand involving all compounds from the same reaction. Their synthesis required hydrothermal conditions, causing the partial in situ transformation of 5-fluoro uracil-1-acetic acid (5-FUA) into an oxalate ion (ox), as well as the protonation of the 4,4'-bipyridine (bipy) ligand through a catalytic process resulting from the presence of Cu(II) within the reaction. These initial conditions allowed obtaining the new coordination compounds [Cu₂(5-FUA)₂(ox)(bipy)]_n·2n H₂O (CP2), [Cu(5-FUA)₂(H₂O)(bipy)]_n·2n H₂O (CP3), as well as the ionic pair [(H₂bipy)⁺² 2NO₃^-] (1). The mother liquor evolved rapidly at room temperature and atmospheric pressure, due to the change in concentration of the initial reagents and the presence of the new chemical species generated in the reaction process, yielding CPs [Cu(5-FUA)₂(bipy)]_n·3.5n H₂O, [Cu₃(ox)₃(bipy)₄]_n and [Cu(ox)(bipy)]_n. The molecular compound [Cu(5-FUA)₂(H₂O)₄]·4H₂O (more thermodynamically stable) ended up in the mother liquor after filtration at longer reaction times at 25 °C and 1 atm., cohabiting in the medium with the other crystalline solids in different proportions. In addition, the evaporation of H₂O caused the single-crystal to single-crystal transformation (SCSC) of [Cu(5-FUA)₂(H₂O)(bipy)]_n·2n H₂O (CP3) into [Cu(5-FUA)₂(bipy)]_n·2n H₂O (CP4). A theoretical study was performed to analyze the thermodynamic stability of the phases. The observed SCSC transformation also involved a perceptible color change, highlighting this compound as a possible water sensor.

Effect of non-covalent self-dimerization on the spectroscopic and electrochemical properties of mixed Cu(i) complexes

A series of six new Cu(i) complexes with ([Cu(N-{4-R}pyridine-2-yl-methanimine)(PPh₃)Br]) formulation, where R corresponds to a donor or acceptor p-substituent, have been synthesized and were used to study self-association effects on their structural and electrochemical properties. X-ray diffraction results showed that in all complexes the packing is organized from a dimer generated by supramolecular π stacking and hydrogen bonding. ¹H-NMR experiments at several concentrations showed that all complexes undergo a fast-self-association monomer-dimer equilibrium in solution, while changes in resonance frequency towards the high or low field in specific protons of the imine ligand allow establishing that dimers have similar structures to those found in the crystal. The thermodynamic parameters for this self-association process were calculated from dimerization constants determined by VT-¹H-NMR experiments for several concentrations at different temperatures. The values for K _D (4.0 to 70.0 M^-1 range), ΔH (-1.4 to -2.6 kcal mol^-1 range), ΔS (-0.2 to 2.1 cal mol^-1 K^-1 range), and ΔG ₂₉₈ (-0.8 to -2.0 kcal mol^-1 range) are of the same order and indicate that the self-dimerization process is enthalpically driven for all complexes. The electrochemical profile of the complexes shows two redox Cu(ii)/Cu(i) processes whose relative intensities are sensitive to concentration changes, indicating that both species are in chemical equilibrium, with the monomer and the dimer having different electrochemical characteristics. We associate this behaviour with the structural lability of the Cu(i) centre that allows the monomeric molecules to reorder conformationally to achieve a more adequate assembly in the non-covalent dimer. As expected, structural properties in the solid and in solution, as well as their electrochemical properties, are not correlated with the electronic parameters usually used to evaluate R substituent effects. This confirms that the properties of the Cu(i) complexes are usually more influenced by steric effects than by the inductive effects of substituents of the ligands. In fact, the results obtained showed the importance of non-covalent intermolecular interactions in the structuring of the coordination geometry around the Cu centre and in the coordinative stability to avoid dissociative equilibria.

Chemically Driven Rotatory Molecular Machines

Molecular machines are at the frontier of biology and chemistry. The ability to control molecular motion and emulating the movement of biological systems are major steps towards the development of responsive and adaptive materials. Amazing progress has been seen for the design of molecular machines including light-induced unidirectional rotation of overcrowded alkenes. However, the feasibility of inducing unidirectional rotation about a single bond as a result of chemical conversion has been a challenging task. In this Review, an overview of approaches towards the design, synthesis, and dynamic properties of different classes of atropisomers which can undergo controlled switching or rotation under the influence of a chemical stimulus is presented. They are categorized as molecular switches, rotors, motors, and autonomous motors according to their type of response. Furthermore, we provide a future perspective and challenges focusing on building sophisticated molecular machines.

Effect of Fluoroalkyl-Substituent in Bistolane-Based Photoluminescent Liquid Crystals on Their Physical Behavior

Photoluminescent liquid crystals (PLLCs) have attracted significant attention owing to their broad applicability in thermosensing and PL switching. Extensive efforts have been made to develop bistolane-based PLLCs containing flexible units at both molecular terminals, and it has been revealed that their PL behavior can switch with the phase transition between the crystalline and LC phases. Although slight modulation of the flexible unit structure dramatically alters the LC and PL behaviors, few studies into the modification of the flexible units have been conducted. With the aim of achieving dynamic changes in their physical behaviors, we developed a family of bistolane derivatives containing a simple alkyl or a fluoroalkyl flexible chain and carried out a detailed systematic evaluation of their physical behaviors. Bistolanes containing a simple alkyl chain showed a nematic LC phase, whereas switching the flexible chain in the bistolane to a fluoroalkyl moiety significantly altered the LC phase to generate a smectic phase. The fluoroalkyl-containing bistolanes displayed a stronger deep blue PL than their corresponding non-fluorinated counterparts, even in the crystalline phase, which was attributed to the construction of rigid molecular aggregates through intermolecular F···H and F···F interactions to suppress non-radiative deactivation.

Multitasking with Chemical Fuel: Dissipative Formation of a Pseudorotaxane Rotor from Five Distinct Components

A 3-fold completive self-sorted library of dynamic motifs was integrated into the design of the pseudorotaxane-based rotor [Zn(2·H⁺)(3)(4)]²⁺ operating at k₂₉₈ = 15.4 kHz. The rotational motion in the five-component device is based on association/dissociation of the pyridyl head of the pseudorotaxane rotator arm between two zinc(II) porphyrin stations. Addition of TFA or 2-cyano-2-phenylpropanoic acid as a chemical fuel to a zinc release system and the loose rotor components 2-4 enabled the liberated zinc(II) ions and protons to act in unison, setting up the rotor through the formation of a heteroleptic zinc complex and a pseudorotaxane linkage. With chemical fuel, the dissipative system was reproducibly pulsed three times without a problem. Due to the double role of the fuel acid, two kinetically distinct processes played a role in both the out-of-equilibrium assembly and disassembly of the rotor, highlighting the complex issues in multitasking of chemical fuels.

Toward Molecular Cybernetics - the Art of Communicating Chemical Systems

The emerging field of molecular cybernetics has the potential to widely broaden our perception of chemistry. Chemistry will develop beyond its current focus that is mainly concerned with single transformations, pure compounds, and/or defined mixtures. On this way, chemistry will become autonomous, networked and smart through communicating molecules each of which serves a control engineering purpose, like the set of wheels in the machinery of life. The present personal account describes our latest developments in this field.

Three-state switching in a double-pole change-over nanoswitch controlled by redox-dependent self-sorting

The four-arm nanomechanical switch 1 with four different terminals exhibits two switching arms (contacts A and D) and two distinct stations for binding (contacts B and C). In switching State I, the azaterpyridine arm is intramolecularly coordinated to a zinc(ii) porphyrin station (connection A ↔ B) while contact D (a ferrocenylbipyridine unit) and contact C (phenanthroline) remain disconnected. After addition of copper(i) ions (State II) both connections A ↔ B and C ↔ D are established. Upon one-electron oxidation, double-pole change-over switching cleaves both connections A ↔ B & C ↔ D and establishes the new connection A ↔ C (State III). Fully reversible three-state switching (State I → State II → State III → State II → State I) was achieved by adding appropriate chemical and redox stimuli.

Selective detection of DABCO using a supramolecular interconversion as fluorescence reporter

The quantitative double self-sorting between the three-component rectangle [Cu₄(1)₂(2)₂]⁴⁺ and the four-component sandwich complex [Cu₂(1)(2)(4)]²⁺ is triggered by inclusion and release of DABCO (4). The fully reversible and clean switching between two multicomponent supramolecular architectures can be monitored by fluorescence changes at the zinc porphyrin sites. The structural changes are accompanied by a huge spatial contraction/expansion of the zinc porphyrin–zinc porphyrin distances that change from 31.2/38.8 Å to 6.6 Å and back. The supramolecular interconversion was used for the highly selective detection of DABCO in a mixture of other similar compounds.

Dinuclear ruthenium acetylide complexes with diethynylated anthrahydroquinone and anthraquinone frameworks: a multi-stimuli-responsive organometallic switch

The anthrahydroquinone (AHQ)/anthraquinone (AQ) system is of particular interest as a molecular switching system for molecule-based devices because AHQ and AQ can be interconverted by redox stimuli and their π-conjugated systems are distinct from each other. To date, however, a way to modify and/or functionalize the switching behavior based on the AHQ/AQ system is still limited. In the present contribution, the synthesis and properties of multi-responsive dinuclear molecular switches having Ru(dppe)₂ fragments bridged by the diethynylated diacetoxyanthracene (AcAHQ) and AQ units (μ-AcAHQ/AQ){C[triple bond, length as m-dash]C-Ru(R)(dppe)₂}₂ (R = Cl, C₄TMS) are presented. The terminal Ru(dppe)₂ fragments are redox-active and, therefore, the intermetallic interaction can be estimated by electrochemical as well as IVCT band analysis. As a result, the organometallic AcAHQ/AQ-Ru system turns out to be an effective bimodal molecular switch, which is triggered not only by redox stimuli but also by pH stimuli.

Machine Metathesis: Thermal and Catalyzed Exchange of Piston Rods in Multicomponent Nanorotor/Nanoslider Ensemble

Three-component nanorotor R1 ( k₂₉₈ = 80 kHz) and two-component slider-on-deck DS2 ( k₂₉₈ = 440 kHz) were prepared from rotator S1 and stator [Cu₃(1)]³⁺ and from S2 and deck D, respectively. Mixing of R1 with DS2 leads to clean metathesis, furnishing the slower nanodevices R2 ( k₂₉₈ = 29.6 kHz) and DS1 ( k₂₉₈ = 32.2 kHz). Exchange of the piston rods S1 and S2 is completed within 22 min (uncatalyzed) or 3 min (catalyzed) at 298 K.

Survey of short and long cuprophilic d10–d10 contacts for tetranuclear copper clusters. Understanding of bonding and ligand role from a planar superatom perspective

Polynuclear copper(i) complexes involving d¹⁰–d¹⁰ interactions have been studied to a lesser extent in comparison to their gold counterparts.

Blue and orange oxygen responsive emissions in the solid state based on copper(i) complexes bearing dodecafluorinated diphosphine and 1,10-phenanthroline derivative ligands

The copper(i) complexes bearing dodecafluoronated diphosphine and diimine ligands show reversible oxygen sensing abilities in the solid state.

Thermo-responsive fluorescence of AIE-active poly(N-isopropylacrylamides) labeled with highly twisted bis(N,N-dialkylamino)arenes

A thermo-responsive fluorescent polymer materials were synthesized fromN-isopropylacrylamides with AIE-active 9,10-bis(N,N-dialkylamino)arene monomers.

Smart Adsorbents with Photoregulated Molecular Gates for Both Selective Adsorption and Efficient Regeneration

Selective adsorption and efficient regeneration are two crucial issues for adsorption processes; unfortunately, only one of them instead of both is favored by traditional adsorbents with fixed pore orifices. Herein, we fabricated a new generation of smart adsorbents through grafting photoresponsive molecules, namely, 4-(3-triethoxysilylpropyl-ureido)azobenzene (AB-TPI), onto pore orifices of the support mesoporous silica. The azobenzene (AB) derivatives serve as the molecular gates of mesopores and are reversibly opened and closed upon light irradiation. Irradiation with visible light (450 nm) causes AB molecules to isomerize from cis to trans configuration, and the molecular gates are closed. It is easy for smaller adsorbates to enter while difficult for the larger ones, and the selective adsorption is consequently facilitated. Upon irradiation with UV light (365 nm), the AB molecules are transformed from trans to cis isomers, promoting the desorption of adsorbates due to the opened molecular gates. The present smart adsorbents can consequently benefit not only selective adsorption but also efficient desorption, which are exceedingly desirable for adsorptive separation but impossible for traditional adsorbents with fixed pore orifices.

Anion directed structural diversity in zinc complexes with conformationally flexible quinazoline ligand: structural, spectral and theoretical studies

In this paper, we report the synthesis, and structure of four new complexes of conformationally flexible 6-chloro-4-phenyl-2-(pyridin-2-yl)quinazoline ligand (L) with Zn(ii). Significance of ring twisting on supramolecular assembly and photophysical properties have also been highlighted.

Selective adsorption and efficient regeneration via smart adsorbents possessing thermo-controlled molecular switches

A new generation of adsorbents possessing thermo-controlled molecular switches was fabricated and consequently realized selective adsorption and efficient desorption simultaneously.

Redox-dependent self-sorting toggles a rotary nanoswitch

The pyridine–pyrimidine (py–pym) arm as the moving part of the two-state nanomechanical rotary switch [Cu(1)]⁺ is toggled reversibly between two stations using one-electron oxidation/reduction.

Highly emissive copper(i) complexes bearing diimine and bis(diphenylphosphinomethyl)-2,2-dimethyl-1,3-dioxolane

Highly emissive copper(i) complexes bearing 4,5-bis(diphenylphosphinomethyl)-2,2-dimethyl-1,3-dioxolane and 1,10-phenanthroline derivatives were newly synthesized. The quantum yields of the photoluminescence are up to 0.38.

From self-sorted coordination libraries to networking nanoswitches for catalysis

This feature article sketches our long way from the development of dynamic heteroleptic coordination motifs to the self-sorting of multi-component libraries and finally the design of a new family of triangular nanomechanical switches, which are useful for ON–OFF control of catalysis and in bidirectional communication.

Heating/cooling stimulus induces three-state molecular switching of pseudoenantiomeric aminomethylenehelicene oligomers: reversible nonequilibrium thermodynamic processes

A 1:1 mixture of pseudoenantiomeric aminomethylenehelicene (P)-tetramer and (M)-pentamer formed three states, namely, the heterodouble helices B and C and the random coil A. At high temperatures, A is the most stable. At low temperatures, C is the most stable, and the structural changes from A to the metastable state B to the product C occur, where B and C have pseudoenantiomeric helical structures. Heating then converts C to A. Essentially, all the molecules change their structure from A to B to C to A. Various nonequilibrium reversible thermodynamic responses appeared depending on thermal conditions: The metastable states A and B can be interconverted with thermal hysteresis without forming C in a far-from-equilibrium manner; three-state hysteresis occurs; states A and B can be frozen at low temperatures and defrosted by warming. An energy and population model for the three-state switching is given, involving inversion of thermodynamic stability and thermal hysteresis.

Structural modification on copper(I)-pyridylpyrimidine complexes for modulation of rotational dynamics, redox properties, and phototriggered isomerization

The redox properties of copper pyridylpyrimidine complexes, which undergo linkage isomerism based on pyrimidine ring rotation, were compared under different coordination environments. A newly synthesized compound, [Cu(Mepypm)(L(Mes))]BF4 (1·BF4, Mepypm = 4-methyl-2-(2'-pyridyl)pyrimidine, L(Mes) = 2,9-dimesityl-1,10-phenanthroline) was compared with previously reported complexes of [Cu(MepmMepy)(L(Mes))]BF4 (2·BF4, MepmMepy = 4-methyl-2-(6'-methyl-2'-pyridyl)pyrimidine), Cu(Mepypm)(DPEphos)]BF4 (3·BF4, DPEphos = bis[2-(diphenylphosphino)phenyl]ether), [Cu(Mepypm)(L(Anth))]BF4 (4·BF4, L(Anth) = 2,9-bis(9-anthryl)-1,10-phenanthroline), and [Cu(Mepypm)(L(Macro))]BF4 (5·BF4). Isomer ratios, isomerization dynamics, redox properties, and photoelectron conversion functions varied with the coordination structure. Methyl substituents on the 6-position of the pyridine moiety increased steric repulsion and contributed to quicker rotation, enhanced photoluminescence, and increased photodriven rotational isomerization.