Giant Crystalline Molecular Rotors that Operate in the Solid State

Molecular motion in the solid state is typically precluded by the highly dense environment, and only molecules with a limited range of sizes show such dynamics. Here, we demonstrate the solid-state rotational motion of two giant molecules, i.e., triptycene and pentiptycene, by encapsulating a bulky N-heterocyclic carbene (NHC) Au(I) complex in the crystalline media. To date, triptycene is the largest molecule (surface area: 245 Ų; volume: 219 ų) for which rotation has been reported in the solid state, with the largest rotational diameter among reported solid-state molecular rotors (9.5 Å). However, the pentiptycene rotator that is the subject of this study (surface area: 392 Ų; volume: 361 ų; rotational diameter: 13.0 Å) surpasses this record. Single-crystal X-ray diffraction analyses of both the developed rotors revealed that these possess sufficient free volume around the rotator. The molecular motion in the solid state was confirmed using variable-temperature solid-state ²H spin-echo NMR studies. The triptycene rotor exhibited three-fold rotation, while temperature-dependent changes of the rotational angle were observed for the pentiptycene rotor.     

Structural and Theoretical Insights into the AIE Attributes of Phosphindole Oxide: The Balance Between Rigidity and Flexibility

Multiple intramolecular motions consume the excited‐state energy of luminogenic molecules upon photoexcitation and lower the emission efficiency. The low frequency rotational motion of aromatic rings can be facilely restricted by steric constraint in the condensed phase, but the high frequency bond stretching motion can hardly be suppressed by aggregation. In this work, three phosphorus‐containing heterocycles, 1,2,3,4,5‐pentaphenylphosphole‐1‐oxide (PPPO), 1,2,3‐triphenylphosphindole‐1‐oxide (TPPIO), and 1,2,3‐triphenylphosphindole (TPPI), were synthesized and characterized. Their optical properties, crystal‐packing manners, electronic features, and fluorescence dynamics were systematically investigated, and theoretical calculations were performed to decipher structure–property relationships. The results reveal that these luminogens are weak emitters in solutions but show strong emission in aggregates, exhibiting obvious aggregation‐induced emission (AIE) features. The aggregation‐insensitive stretching motion, which is dominant in PPPO, is lowered in TPPIO, enabling TPPIO to fluoresce much more efficiently than PPPO in aggregates. The stretching motion is even more lowered in TPPI, but its relatively planar conformation suffers emission quenching due to strong π–π stacking interactions in aggregates. Therefore, a twisted molecular conformation consisting of a rigid stator and a rotatable periphery is demonstrated to be a rational design for more efficient AIE luminogens.     

Synthesis,structural characterization,and photophysical studies of hexanuclear gold(I) chalcogenido complexes

A hexanuclear gold(I) selenido cluster and its sulfido counterpart, [Au₆{μ‐Ph₂PN(CH₂‐o‐Py)PPh₂}₃(μ₃‐E)₂](ClO₄)₂ (E = S, Se), with bridging bis(diphenylphosphino)amine ligands were synthesized and characterized. The X‐ray crystal structure of the selenido cluster was determined, with the gold core possessing a distorted heterocubane structure. Intramolecular aurophilic interactions with short Au(I)?Au(I) contacts of around 3.09–3.13 Å were observed. The complexes were found to emit strongly in the solid state with orange to red emission colors. Their electronic absorption and emission properties were also investigated.     

Thermally Induced Intra‐Carboxyl Proton Shuttle in a Molecular Rack‐and‐Pinion Cascade Achieving Macroscopic Crystal Deformation

Proton transport via dynamic molecules is ubiquitous in chemistry and biology. However, its use as a switching mechanism for properties in functional molecular assemblies is far less common. In this study, we demonstrate how an intra‐carboxyl proton shuttle can be generated in a molecular assembly akin to a rack‐and‐pinion cascade via a thermally induced single‐crystal‐to‐single‐crystal phase transition. In a triply interpenetrated supramolecular organic framework (SOF), a 4,4′‐azopyridine (azpy) molecule connects to two biphenyl‐3,3′,5,5′‐tetracarboxylic acid (H₄BPTC) molecules to form a functional molecular system with switchable mechanical properties. A temperature change reversibly triggers a molecular movement akin to a rack‐and‐pinion cascade, which mainly involves 1) an intra‐carboxyl proton shuttle coupled with tilting of the azo molecules and azo pedal motion and 2) H₄BPTC translation. Moreover, both the molecular motions are collective, and being propagated across the entire framework, leading to a macroscopic crystal expansion and contraction.     

Molecular compasses and gyroscopes with polar rotors: synthesis and characterization of crystalline forms

We report the highly convergent synthesis and solid-state characterization of six crystalline "molecular compasses" consisting of a central phenylene rotor with polar substituents, or compass needle, and two trityl groups axially connected by acetylene linkages to the 1,4-positions. Compounds with fluoro-, cyano-, nitro-, amino-, diamino-, and nitroamino substituents are expected to emulate the parent compound which was shown to form crystals where the central phenylene can rotate about its 1,4-axis with rate constants in the 10(3) -10(6) s(-)(1) dynamic ranges near ambient temperature, depending on crystal morphology. With data from single-crystal X-ray diffraction analysis, solid-state CPMAS (13)C NMR, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA), it is shown that a relatively small structural perturbation by a single polar group (F, CN, NO(2), NH(2)) results in isomorphous structures with analogous properties. In analogy to the parent compound, crystals grown from benzene formed clathrate structures in the space group Ponemacr; with one molecular compass and two benzene molecules per unit cell. Solvent-free crystals with the same space group obtained by a first-order phase transition between 60 and 130 degrees C were shown to be spectroscopically identical to those obtained by slow solvent evaporation from a mixture of CH(2)Cl(2) and hexanes. A qualitative analysis of the positionally disordered phenylene groups in terms of the expected solid-state rotational dynamics suggests a nonsymmetric, 2-fold rotational potential, or a process involving full 360 degrees turns.     

Tunable Trimers: Using Temperature and Pressure to Control Luminescent Emission in Gold(I) Pyrazolate‐Based Trimers

A systematic investigation into the relationship between the solid‐state luminescence and the intermolecular Au???Au interactions in a series of pyrazolate‐based gold(I) trimers; tris(μ₂‐pyrazolato‐N,N′)‐tri‐gold(I) ( 1 ), tris(μ₂‐3,4,5‐ trimethylpyrazolato‐N,N′)‐tri‐gold(I) ( 2 ), tris(μ₂‐3‐methyl‐5‐phenylpyrazolato‐N,N′)‐tri‐gold(I) ( 3 ) and tris(μ₂‐3,5‐diphenylpyrazolato‐N,N′)‐tri‐gold(I) ( 4 ) has been carried out using variable temperature and high pressure X‐ray crystallography, solid‐state emission spectroscopy, Raman spectroscopy and computational techniques. Single‐crystal X‐ray studies show that there is a significant reduction in the intertrimer Au???Au distances both with decreasing temperature and increasing pressure. In the four complexes, the reduction in temperature from 293 to 100 K is accompanied by a reduction in the shortest intermolecular Au???Au contacts of between 0.04 and 0.08 Å. The solid‐state luminescent emission spectra of 1 and 2 display a red shift with decreasing temperature or increasing pressure. Compound 3 does not emit under ambient conditions but displays increasingly red‐shifted luminescence upon cooling or compression. Compound 4 remains emissionless, consistent with the absence of intermolecular Au???Au interactions. The largest pressure induced shift in emission is observed in 2 with a red shift of approximately 630 cm^?1 per GPa between ambient and 3.80 GPa. The shifts in all the complexes can be correlated with changes in Au???Au distance observed by diffraction.     

Crystal structure and dynamics of Mg(ND3)6Cl2

The crystal structure and dynamics of Mg(ND(3))(6)Cl(2) have been investigated by powder neutron diffraction and molecular dynamics. The powder diffraction data can be well described by 4 partly occupied deuterium sites in a square arrangement around the N atoms, which is seemingly inconsistent with the 3-fold symmetry of the ND(3) molecule. Molecular dynamics show highly correlated rotational and translational motion of the ND(3) molecules which explains the apparent 4-fold symmetry of the deuterium arrangement. A more disordered structure model based on the molecular dynamics results gives a better fit to the experimental data and is in agreement with the 3-fold symmetry of ND(3).     

Single‐Crystal‐to‐Single‐Crystal Transformation and Solvochromic Luminescence of a Dinuclear Gold(I)–(Aza‐[18]crown‐6)dithiocarbamate Compound

The treatment of [AuCl(SMe₂)] with an equimolar amount of NaO₅NCS₂ (O₅NCS₂=(aza‐[18]crown‐6)dithiocarbamate) in CH₃CN gave [Au₂(O₅NCS₂)₂] ? 2 CH₃CN ( 2? 2 CH₃CN), and its crystal structure displays a dinuclear gold(I)‐azacrown ether ring and an intermolecular gold(I) ??? gold(I) contact of 2.8355(3) Å in crystal lattices. It is noted that two other single crystals of 2 ?tert‐butylbenzene?H₂O and 2? 0.5 m‐xylene can be successfully obtained from a single‐crystal‐to‐single‐crystal (SCSC) transformation process by immersing single crystals of 2? 2 CH₃CN in the respective solvents, and both also show intermolecular gold(I) ??? gold(I) contacts of 2.9420(5) and 2.890(2)–2.902(2) Å, respectively. Significantly, the emissions of all three 2 ?solvates are well correlated with their respective intermolecular gold(I) ??? gold(I) contacts, where such contacts increase with 2? 2 CH₃CN (2.8355(3) Å)< 2? 0.5 m‐xylene (2.890(2)–2.902(2) Å)< 2? tert‐butylbenzene?H₂O (2.9420(5) Å), and their emission energies increase with 2? 2 CH₃CN (602 nm)< 2? 0.5 m‐xylene (583 nm)< 2? tert‐butylbenzene?H₂O (546 nm) as well. In this regard, we further examine the solvochromic luminescence for some other aromatics, and finally their emissions are within 546–602 nm. Obviously, the above results are mostly ascribed to the occurrence of intermolecular gold(I) ??? gold(I) contacts in 2 ?solvates, which are induced by the presence of various solvates in the solid state, as a key role to be responsible for their solvochromic luminescence.     

Design and evaluation of a crystalline hybrid of molecular conductors and molecular rotors

Combining recent concepts from the fields of molecular conductivity and molecular machinery we set out to design a crystalline molecular conductor that also possesses a molecular rotor. We report on the structures, electronic and physical properties, and dynamics of two solids with a common 1,4-bis(carboxyethynyl)bicyclo[2.2.2]octane (BABCO) functional rotor. One, [nBu(4)N(+)](2)[BABCO][BABCO(-)](2), is a colorless insulator where the dicarboxylic acid cocrystallizes with two of its monoanionic conjugated bases. The other is self-assembled by electrocrystallization in the form of black, shiny needles, with highly conducting molecular slabs of (EDT-TTF-CONH(2))(2)(+) (EDT-TTF = ethylenedithiotetrathiafulvalene) and anionic [BABCO(-)] rotors. Using variable-temperature (5-300 K) proton spin-lattice relaxation, (1)H T(1)(-1), we were able to assign two types of Brownian rotators in [nBu(4)N(+)](2)[BABCO][BABCO(-)](2). We showed that neutral BABCO groups have a rotational frequency of 120 GHz at 300 K with a rotational barrier of 2.03 kcal mol(-1). Rotors on the BABCO(-) sites experience stochastic 32 GHz jumps at the same temperature over a rotational barrier of 2.72 kcal mol(-1). In contrast, the BABCO(-) rotors within the highly conducting crystals of (EDT-TTF-CONH(2))(2)(+)[BABCO(-)] are essentially "braked" at room temperature. Notably, these crystals possess a conductivity of 5 S cm(-1) at 1 bar, which increases rapidly with pressure up to 50 S cm(-1) at 11.5 kbar. Two regimes with different activation energies E(a) for the resistivity (180 K above 50 and 400 K below) are observed at ambient pressure; a metallic state is stabilized at ca. 8 kbar, and an insulating ground state remains below 50 K at all pressures. We discuss two likely channels by which the motion of the rotors might become slowed down in the highly conducting solid. One is defined as a low-velocity viscous regime inherent to a noncovalent, physical coupling induced by the cooperativity between five C(sp3)-H···O hydrogen bonds engaging any rotor and five BABCO units in its environment. The rotational barrier calculated with the effect of this set of hydrogen bonds amounts to 7.3 kcal mol(-1). Another is quantum dissipation, a phenomenon addressing the difference of dynamics of the rotors in the two solids with different electrical properties, by which the large number of degrees of freedom of the low dimensional electron gas may serve as a bath for the dissipation of the energy of the rotor motion, the two systems being coupled by the Coulomb interaction between the charges of the rotors (local moments and induced dipoles) and the charges of the carriers.     

Aggregation-Enhanced Room-Temperature Phosphorescence from Au(I) Complexes Bearing Mesogenic Biphenylethynyl Ligands

Gold(I) complexes, enabling to form linear coordination geometry, are promising materials for manifesting both aggregation-induced emission (AIE) behavior due to strong intermolecular Au–Au (aurophilic) interactions and liquid crystalline (LC) nature depending on molecular geometry. In this study, we synthesized several gold(I) complexes with rod-like molecular skeletons where we employed a mesogenic biphenylethynyl ligand and an isocyanide ligand with flexible alkoxyl or alkyl chains. The AIE behavior and LC nature were investigated experimentally and computationally. All synthesized gold(I) complexes exhibited AIE properties and, in crystal, room-temperature phosphorescence (RTP) with a relatively high quantum yields of greater than 23% even in air. We have demonstrated that such strong RTP are drastically changed depending on the crystal-size and/or crystal growth process that changes quality of crystals as well as the aggregate structure, of e.g., Au–Au distance. Moreover, the complex with longer flexible chains showed LC nature where RTP can be observed. We expect these rod-like gold(I) complexes to have great potential in AIE-active LC phosphorescent applications such as linearly/circularly polarizing phosphorescence materials.     

Motion Capture and Manipulation of a Single Synthetic Molecular Rotor by Optical Microscopy

Single‐molecule imaging and manipulation with optical microscopy have become essential methods for studying biomolecular machines; however, only few efforts have been directed towards synthetic molecular machines. Single‐molecule optical microscopy was now applied to a synthetic molecular rotor, a double‐decker porphyrin (DD). By attaching a magnetic bead (ca. 200 nm) to the DD, its rotational dynamics were captured with a time resolution of 0.5 ms. DD showed rotational diffusion with 90° steps, which is consistent with its four‐fold structural symmetry. Kinetic analysis revealed the first‐order kinetics of the 90° step with a rate constant of 2.8 s^?1. The barrier height of the rotational potential was estimated to be greater than 7.4 kJ mol^?1 at 298 K. The DD was also forcibly rotated with magnetic tweezers, and again, four stable pausing angles that are separated by 90° were observed. These results demonstrate the potency of single‐molecule optical microscopy for the elucidation of elementary properties of synthetic molecular machines.     

Argentophilic Interactions

The decade 1990–2000 saw a growing interest in aurophilic interactions in gold chemistry. These interactions were found to influence significantly a variety of structural and other physical characteristics of gold(I) compounds. The attention paid to this unusual and counterintuitive type of intra‐ and intermolecular bonding between seemingly closed‐shell metal centers has rapidly been extended to also include silver chemistry. Hundreds of experimental and computational studies have since been dedicated to the argentophilicity phenomenon. The results of this development are reviewed herein focusing on molecular systems where two or more silver(I) centers are in close contact leading to specific structural characteristics and a variety of novel physical properties. These include strongly modified ligand‐to‐metal charge‐transfer processes observed in absorption and emission spectroscopy, but also colossal positive and negative thermal expansion on the one hand and unprecedented negative linear compressibility of crystal parameters on the other.     

Molecular dynamics simulation of an electric field driven dipolar molecular rotor attached to a quartz glass surface

Molecular dynamics simulations of the response of a dipolar azimuthal 3-chloroprop-1-ynyl rotor mounted on the surface of quartz glass to a rotating electric field were performed. The rotor motion was classified as synchronous, asynchronous, random, or hindered, based on the value of the average lag of the rotor behind the field and a comparison of the intrinsic rotational barrier V(b) with kT. A phase diagram of rotor behavior was deduced at 10, 300, and 500 K as a function of field strength and frequency. A simple model for the rotor motion was developed, containing the driving force, the temperature, the height of the torsional barrier, and the friction constant of the rotor. Defining E(bo) to be the electric field strength necessary to get rotational response from the rotor ("breakoff field") and mu to be the rotor dipole moment component in the plane of rotation, we find that E(bo) is frequency independent when 2 microE(bo) is less than either V(b) or kT (the driving force needs to overcome the more important of the two, the intrinsic barrier or random thermal motion). At higher frequencies, E(bo) is a quadratic function of the frequency and the driving force fights friction, which is dictated by intramolecular vibrational redistribution (IVR) from the pumped rotational mode to all others. Fitting the simple model to simulation data, we derived a friction constant of 0.26 ps eV x (nu - 0.5)/THz between 500 and 1000 GHz.     

Headgroup motion in a lyotropic lamellar phase by deuterium and nitrogen-14 relaxation studies

Deuterium (2H) and nitrogen-14 (14N) NMR spectroscopy were used to investigate the molecular dynamics of a lyotropic liquid crystal. Deuterium spectral densities of motion for the C1 deuterated site on the chain of the molecule decylammonium chloride (DACl) at the Larmor frequency 61.4 MHz and those for the (14)N at the headgroup (NH(3)(+)) at 28.9 MHz are analyzed quantitatively in the lamellar phase of the DACl-d(11)/water binary system to shed light on the headgroup dynamics. The motional model used is the small step rotational diffusion for reorientations plus internal rotations of the methylene group in the strong collision limit. The tumbling motion of the long axis of the DACl molecule in the aggregates seems to be as rigorous as the molecular spinning motion, likely due to the proposed motional model. The similarity of deuterium spectral densities from the C1 and C2/C3 sites may indicate a relatively rigid unit of C1-C2-C3 in the backbone.     

On the Non‐Metallicity of 2.2 nm Au246(SR)80 Nanoclusters

The transition from molecular to plasmonic behaviour in metal nanoparticles with increasing size remains a central question in nanoscience. We report that the giant 246‐gold‐atom nanocluster (2.2 nm in gold core diameter) protected by 80 thiolate ligands is surprisingly non‐metallic based on UV/Vis and femtosecond transient absorption spectroscopy as well as electrochemical measurements. Specifically, the Au₂₄₆ nanocluster exhibits multiple excitonic peaks in transient absorption spectra and electron dynamics independent of the pump power, which are in contrast to the behaviour of metallic gold nanoparticles. Moreover, a prominent oscillatory feature with frequency of 0.5 THz can be observed in almost all the probe wavelengths. The phase and amplitude analysis of the oscillation suggests that it arises from the wavepacket motion on the ground state potential energy surface, which also indicates the presence of a small band‐gap and thus non‐metallic or molecular‐like behaviour.     

Ultra-fast rotors for molecular machines and functional materials via halogen bonding: crystals of 1,4-bis(iodoethynyl)bicyclo[2.2.2]octane with distinct gigahertz rotation at two sites

As a point of entry to investigate the potential of halogen-bonding interactions in the construction of functional materials and crystalline molecular machines, samples of 1,4-bis(iodoethynyl)bicyclo[2.2.2]octane (BIBCO) were synthesized and crystallized. Knowing that halogen-bonding interactions are common between electron-rich acetylenic carbons and electron-deficient iodines, it was expected that the BIBCO rotors would be an ideal platform to investigate the formation of a crystalline array of molecular rotors. Variable temperature single crystal X-ray crystallography established the presence of a halogen-bonded network, characterized by lamellarly ordered layers of crystallographically unique BIBCO rotors, which undergo a reversible monoclinic-to-triclinic phase transition at 110 K. In order to elucidate the rotational frequencies and the activation parameters of the BIBCO molecular rotors, variable-temperature (1)H wide-line and (13)C cross-polarization/magic-angle spinning solid-state NMR experiments were performed at temperatures between 27 and 290 K. Analysis of the (1)H spin-lattice relaxation and second moment as a function of temperature revealed two dynamic processes simultaneously present over the entire temperature range studied, with temperature-dependent rotational rates of k(rot) = 5.21 × 10(10) s(-1)·exp(-1.48 kcal·mol(-1)/RT) and k(rot) = 8.00 × 10(10) s(-1)·exp(-2.75 kcal·mol(-1)/RT). Impressively, these correspond to room temperature rotational rates of 4.3 and 0.8 GHz, respectively. Notably, the high-temperature plastic crystalline phase I of bicyclo[2.2.2]octane has a reported activation energy of 1.84 kcal·mol(-1) for rotation about the 1,4 axis, which is 24% larger than E(a) = 1.48 kcal·mol(-1) for the same rotational motion of the fastest BIBCO rotor; yet, the BIBCO rotor has three fewer degrees of translational freedom and two fewer degrees of rotational freedom! Even more so, these rates represent some of the fastest engineered molecular machines, to date. The results of this study highlight the potential of halogen bonding as a valuable construction tool for the design and the synthesis of amphidynamic artificial molecular machines and suggest the potential of modulating properties that depend on the dielectric behavior of crystalline media.     

Geminally Diaurated Aryls Bridged by Semirigid Phosphine Pillars: Syntheses and Electronic Structure

Geminally diaurated μ₂‐aryl complexes have been prepared where gold(I) centers were bridged by the semirigid diphosphine ligands bis(2‐diphenylphosphinophenyl)ether (DPEphos) and 4,6‐bis(diphenylphosphanyl)dibenzo[b,d]furan (DBFphos). Diaurated complexes were synthesized in ligand redistribution reactions of the corresponding di‐gold dichlorides with di‐gold diaryls (six of them new) and silver(I) salts. Diaurated complexes were isolated as salts of the minimally coordinating anions SbF₆^? and ReO₄^?. Efforts to prepare salts of the tetraarylborate [B(3,5‐(CF₃)₂C₆H₃)₄]^? led to transmetalation from boron, with crystallization of the fluorinated aryl complex. The new complexes were characterized by multinuclear NMR, absorption and emission spectroscopies, 77 K emission lifetimes, and by combustion analysis; three are crystallographically characterized. Structures of geminally diaurated aryl ligands are compared to those of mono‐aurated analogues. Both crystal structures and density‐functional theory calculations indicate slight but observable disruptions of aryl ligand aromaticity by geminal di‐gold binding. An intermolecular aurophilic interaction in one structurally authenticated complex was examined computationally.     

Molecular dynamics simulation of the nematic liquid crystal phase in the presence of an intense magnetic field

The influence of an intense external field on the dynamics of the nematic liquid crystal phase is investigated using a molecular dynamics simulation for the Gay-Berne nematogen under isobaric-isothermal conditions. The molecular dynamics as a function of the second-rank orientational order parameter P<2> for a system consisting of a nematic liquid crystal in the presence of an intense magnetic field is compared with that of a similar system without the field. The translational motion of molecules is determined as a function of the translational diffusion coefficient tensor and the anisotropy and compared with the values predicted theoretically. The rotational dynamics of molecules is analyzed using the first- and the second-rank orientational time correlation functions. The translational diffusion coefficient parallel with respect to the director is constrained by the intense field, although the perpendicular one is decreased as the P<2> is increased, just as it is in the system without the field. However, no essential effect of the strong magnetic field is observed in the rotational molecular dynamics. Further, the rotational diffusion coefficient parallel with respect to the director obtained from the first-rank orientational time correlation function in the simulation is qualitatively in agreement with that in the real nematic liquid crystalline molecules. The P<2> dependence of the rotational diffusion coefficient for the system with the intense magnetic field shows a tendency similar to that for the system without the field.     

Superposition of quantum and classical rotational motions in Sc2C2@C84 fullerite

The superposition of the quantum rotational motion (tunneling) of the encapsulated Sc(2)C(2) complex with the classical rotational motion of the surrounding C(84) molecule in a powder crystal of Sc(2)C(2)@C(84) fullerite is investigated by theory. Since the quantum rotor is dragged along by the C(84) molecule, any detection method which couples to the quantum rotor (in casu the C(2) bond of the Sc(2)C(2) complex) also probes the thermally excited classical motion (uniaxial rotational diffusion and stochastic meroaxial jumps) of the surrounding fullerene. The dynamic rotation-rotation response functions in frequency space are obtained as convolutions of quantum and classical dynamic correlation functions. The corresponding Raman scattering laws are derived, and the overall shape of the spectra and the width of the resonance lines are studied as functions of temperature. The results of the theory are confronted with experimental low-frequency Raman spectra on powder crystals of Sc(2)C(2)@C(84) [M. Krause et al., Phys. Rev. Lett. 93, 137403 (2004)]. The agreement of theory with experiment is very satisfactory in a broad temperature range.