Transition State Stabilizing Effects of Oxygen and Sulfur Chalcogen Bond Interactions.

Non-covalent chalcogen bond (ChB) interactions have found utility in many fields, including catalysis, organic semiconductors, and crystal engineering. In this study, the kinetic effects of ChB interactions of oxygen and sulfur were experimentally measured using a series of molecular rotors. The rotors were designed to form ChB interactions in the bond rotation transition states. This enabled their kinetic influences to be assessed by monitoring changes in rotational barriers. Despite forming weaker ChB interactions, the smaller chalcogens were able to stabilize transition states and had measurable kinetic effects. Sulfur stabilized the bond rotation transition state by as much as -7.2 kcal/mol without electron-withdrawing groups. The key was to design a system where the sulfur ðœŽ-hole was aligned with the lone pairs of the chalcogen bond acceptor. Oxygen rotors also could form transition state stabilizing ChB interactions but required electron-withdrawing groups. For both oxygen and sulfur ChB interactions, a strong correlation was observed between transition state stabilizing abilities and electrostatic potential (ESP) of the chalcogen, providing a useful predictive parameter for the rational design of future ChB systems.

Empirical Model of Solvophobic Interactions in Organic Solvents.

An empirical model was developed to predict organic solvophobic effects using N-phenylimide molecular balances functionalized with non-polar alkyl groups. Solution studies and X-ray crystallography confirmed intramolecular alkyl-alkyl interactions in their folded conformers. The structural modularity of the balances allowed systematic variation of alkyl group lengths. Control balances were instrumental in isolating weak organic solvophobic effects by eliminating framework solvent-solute effects. A 19 F NMR label enabled analysis across 46 deuterated and non-deuterated solvent systems. Linear correlations were observed between organic solvophobic effects and solvent cohesive energy density (ced) as well as changes in solvent-accessible surface areas (SASA). Using these empirical relationships, a model was constructed to predict organic solvophobic interaction energy per unit area for any organic solvent with known ced values. The predicted interaction energies aligned with recent organic solvophobic measurements and literature values for the hydrophobic effect on non-polar surfaces confirmed the model's accuracy and utility.

Pnictogen Interactions with Nitrogen Acceptors.

Stabilizing nitrogen pnictogen bond interactions were measured using molecular rotors. Intramolecular C=O⋅⋅⋅N interactions were formed in the bond rotation transition states which lowered the rotational barriers and increased the rates of rotation, as measured by EXSY NMR. The pnictogen interaction energies show a very strong correlation with the positive electrostatic potential on nitrogen, which was consistent with a strong electrostatic component. In contrast, the NBO perturbation and pyramidalization analyses show no correlation, suggesting that the orbital-orbital component is minor. The strongest C=O⋅⋅⋅N pnictogen interactions were comparable to C=O⋅⋅⋅C=O interactions and were stronger than C=O⋅⋅⋅Ph interactions, when measured using the same N-phenylimide rotor system. The ability of the nitrogen pnictogen interactions to stabilize transition states and enhance kinetic processes demonstrates their potential in catalysis and reaction design.

Cooperative and Orthogonal Switching in the Solid State Enabled by Metal-Organic Framework Confinement Leading to a Thermo-Photochromic Platform.

Cooperative behavior and orthogonal responses of two classes of coordinatively integrated photochromic molecules towards distinct external stimuli were demonstrated on the first example of a photo-thermo-responsive hierarchical platform. Synergetic and orthogonal responses to temperature and excitation wavelength are achieved by confining the stimuli-responsive moieties within a metal-organic framework (MOF), leading to the preparation of a novel photo-thermo-responsive spiropyran-diarylethene based material. Synergistic behavior of two photoswitches enables the study of stimuli-responsive resonance energy transfer as well as control of the photoinduced charge transfer processes, milestones required to advance optoelectronics development. Spectroscopic studies in combination with theoretical modeling revealed a nonlinear effect on the material electronic structure arising from the coordinative integration of photoresponsive molecules with distinct photoisomerization mechanisms. Thus, the reported work covers multivariable facets of not only fundamental aspects of photoswitch cooperativity, but also provides a pathway to modulate photophysics and electronics of multidimensional functional materials exhibiting thermo-photochromism.

Coordination of Quantum Dots in a Polar Solvent by Small-Molecule Imidazole Ligands.

Colloidal quantum dots (QDs) are attractive fluorophores for bioimaging and biomedical applications because of their favorable and tunable optoelectronic properties. In this study, the native hydrophobic ligand environment of oleate-capped sphalerite CdSe/ZnS core/shell QDs was quantitatively exchanged with a set of imidazole-bearing small-molecule ligands. Inductively coupled plasma-optical emission spectroscopy and 1H NMR were used to identify and quantify three different ligand exchange processes: Z-type dissociation of the Zn(oleate)2, L-type association of the imidazole, and X-type anionic exchange of oleate with Cl-, all of which contributed to the overall ligand exchange.

A Dual Threat: Redox-Activity and Electronic Structures of Well-Defined Donor-Acceptor Fulleretic Covalent-Organic Materials.

The effect of donor (D)-acceptor (A) alignment on the materials electronic structure was probed for the first time using novel purely organic porous crystalline materials with covalently bound two- and three-dimensional acceptors. The first studies towards estimation of charge transfer rates as a function of acceptor stacking are in line with the experimentally observed drastic, eight-fold conductivity enhancement. The first evaluation of redox behavior of buckyball- or tetracyanoquinodimethane-integrated crystalline was conducted. In parallel with tailoring the D-A alignment responsible for "static" changes in materials properties, an external stimulus was applied for "dynamic" control of the electronic profiles. Overall, the presented D-A strategic design, with stimuli-controlled electronic behavior, redox activity, and modularity could be used as a blueprint for the development of electroactive and conductive multidimensional and multifunctional crystalline porous materials.

Transition-State Stabilization by n→π* Interactions Measured Using Molecular Rotors.

A series of 16 molecular rotors were synthesized to investigate the ability of n→π* interactions to stabilize transition states (TSs) of bond rotation. Steric contributions to the rotational barrier were isolated using control rotors, which could not form n→π* interactions. Rotors with strong acceptor π* orbitals, such as ketones and aldehydes, had greatly increased rates of rotation. The TS stabilization of up to ∼10 kcal/mol was consistent with the formation of a strong n→π* stabilization between the imide carbonyl oxygens and the ortho R group in the planar TS. Computational studies effectively modeled the TS stabilization and geometry, and NBO analysis confirmed the role of n→π* interactions in stabilizing the TS.

Electrostatically Driven CO-π Aromatic Interactions.

A series of N-arylimide molecular balances were developed to study and measure carbonyl-aromatic (CO-π) interactions. Carbonyl oxygens were observed to form repulsive interactions with unsubstituted arenes and attractive interactions with electron-deficient arenes with multiple electron-withdrawing groups. The repulsive and attractive CO-π aromatic interactions were well-correlated to electrostatic parameters, which allowed accurate predictions of the interaction energies based on the electrostatic potentials of the carbonyl and arene surfaces. Due to the pronounced electrostatic polarization of the C═O bond, the CO-π aromatic interaction was stronger than the previously studied oxygen-π and halogen-π aromatic interactions.

Flipping the Switch: Fast Photoisomerization in a Confined Environment.

Stimuli-responsive materials are vital for addressing emerging demands in the advanced technology sector as well as current industrial challenges. Here, we report for the first time that coordinative integration of photoresponsive building blocks possessing photochromic spiropyran and diarylethene moieties within a rigid scaffold of metal-organic frameworks (MOFs) could control photophysics, in particular, cycloreversion kinetics, with a level of control that is not accessible in the solid state or solution. On the series of photoactive materials, we demonstrated for the first time that photoisomerization rates of photochromic compounds could be tuned within almost 2 orders of magnitude. Moreover, cycloreversion rates of photoresponsive derivatives could be modulated as a function of the framework structure. Furthermore, through MOF engineering we were able to achieve complete isomerization for coordinatively immobilized spiropyran derivatives, typically exhibiting limited photoswitching behavior in the solid state. For instance, spectroscopic analysis revealed that the novel monosubstituted spiropyran derivative grafted to the backbone of the MOF pillar exhibits a remarkable photoisomerization rate of 0.16 s-1, typical for cycloreversion in solution. We also applied the acquired fundamental principles toward mapping of changes in material properties, which could provide a pathway for monitoring material aging or structural deterioration.

Stack the Bowls: Tailoring the Electronic Structure of Corannulene-Integrated Crystalline Materials.

We report the first examples of purely organic donor-acceptor materials with integrated π-bowls (πBs) that combine not only crystallinity and high surface areas but also exhibit tunable electronic properties, resulting in a four-orders-of-magnitude conductivity enhancement in comparison with the parent framework. In addition to the first report of alkyne-azide cycloaddition utilized for corannulene immobilization in the solid state, we also probed the charge transfer rate within the Marcus theory as a function of mutual πB orientation for the first time, as well as shed light on the density of states near the Fermi edge. These studies could foreshadow new avenues for πB utilization for the development of optoelectronic devices or a route for highly efficient porous electrodes.

Stabilizing Fluorine-π Interactions.

A series of N-arylimide molecular balances were designed to study and measure fluorine-aromatic (F-π) interactions. Fluorine substituents gave rise to increasingly more stabilizing interactions with more electron-deficient aromatic surfaces. The attractive F-π interaction is electrostatically driven and is stronger than other halogen-π interactions.

Redox-Active Corannulene Buckybowls in a Crystalline Hybrid Scaffold.

A porous crystalline corannulene-containing scaffold, which combines the periodicity, dimensionality, and structural modularity of hybrid frameworks with the intrinsic properties of redox-active π-bowls, has been prepared. Single-crystal and powder X-ray diffraction, ab initio density functional theory computations, gas sorption analysis, fluorescence spectroscopy, and cyclic voltammetry were employed to study the properties of the novel corannulene derivatives and the buckybowl-based hybrid materials. X-ray diffraction studies revealed the preservation of the corannulene bowl inside the prepared rigid matrix, which offers the unique opportunity to extend the scaffold dimensionality through the buckybowl curvature. Merging the inherent properties of hybrid frameworks with the intrinsic properties of π-bowls opens a new avenue for preparing redox-active materials and potentially improving charge transport in the scaffold.

Fulleretic Well-Defined Scaffolds: Donor-Fullerene Alignment Through Metal Coordination and Its Effect on Photophysics.

Herein, we report the first example of a crystalline metal-donor-fullerene framework, in which control of the donor-fullerene mutual orientation was achieved through chemical bond formation, in particular, by metal coordination. The (13) C cross-polarization magic-angle spinning NMR spectroscopy, X-ray diffraction, and time-resolved fluorescence spectroscopy were performed for comprehensive structural analysis and energy-transfer (ET) studies of the fulleretic donor-acceptor scaffold. Furthermore, in combination with photoluminescence measurements, the theoretical calculations of the spectral overlap function, Förster radius, excitation energies, and band structure were employed to elucidate the photophysical and ET processes in the prepared fulleretic material. We envision that the well-defined fulleretic donor-acceptor materials could contribute not only to the basic science of fullerene chemistry but would also be used towards effective development of organic photovoltaics and molecular electronics.

Pendant alkyl and aryl groups on tin control complex geometry and reactivity with H2/D2 in Pt(SnR3)2(CNBu(t))2 (R = Bu(t), Pr(i), Ph, mesityl).

The complex Pt(SnBu(t)3)2(CNBu(t))2(H)2, 1, was obtained from the reaction of Pt(COD)2 and Bu(t)3SnH, followed by addition of CNBu(t). The two hydride ligands in 1 can be eliminated, both in solution and in the solid state, to yield Pt(SnBu(t)3)2(CNBu(t))2, 2. Addition of hydrogen to 2 at room temperature in solution and in the solid state regenerates 1. Complex 2 catalyzes H2-D2 exchange in solution to give HD. The proposed mechanism of exchange involves reductive elimination of Bu(t)3SnH from 1 to afford vacant sites on the Pt center, thus facilitating the exchange process. This is supported by isolation and characterization of Pt(SnMes3)(SnBu(t)3)(CNBu(t))2, 3, when the addition of H2 to 2 was carried out in the presence of free ligand Mes3SnH (Mes = 2,4,6-Me3C6H2). Complex Pt(SnMes3)2(CNBu(t))2, 5, can be prepared from the reaction of Pt(COD)2 with Mes3SnH and CNBu(t). The exchange reaction of 2 with Ph3SnH gave Pt(SnPh3)3(CNBu(t))2(H), 6, wherein both SnBu(t)3 ligands are replaced by SnPh3. Complex 6 decomposes in air to form square planar Pt(SnPh3)2(CNBu(t))2, 7. The complex Pt(SnPr(i)3)2(CNBu(t))2, 8, was also prepared. Out of the four analogous complexes Pt(SnR3)2(CNBu(t))2 (R = Bu(t), Mes, Ph, or Pr(i)), only the Bu(t) analogue does both H2 activation and H2-D2 exchange. This is due to steric effects imparted by the bulky Bu(t) groups that distort the geometry of the complex considerably from planarity. The reaction of Pt(COD)2 with Bu(t)3SnH and CO gas afforded trans-Pt(SnBu(t)3)2(CO)2, 9. Compound 9 can be converted to 2 by replacement of the CO ligands with CNBu(t) via the intermediate Pt(SnBu(t)3)2(CNBu(t))2(CO), 10.

Mimic of the green fluorescent protein ß-barrel: photophysics and dynamics of confined chromophores defined by a rigid porous scaffold.

Chromophores with a benzylidene imidazolidinone core define the emission profile of commonly used biomarkers such as the green fluorescent protein (GFP) and its analogues. In this communication, artificially engineered porous scaffolds have been shown to mimic the protein ß-barrel structure, maintaining green fluorescence response and conformational rigidity of GFP-like chromophores. In particular, we demonstrated that the emission maximum in our artificial scaffolds is similar to those observed in the spectra of the natural GFP-based systems. To correlate the fluorescence response with a structure and perform a comprehensive analysis of the prepared photoluminescent scaffolds, (13)C cross-polarization magic angle spinning solid-state (CP-MAS) NMR spectroscopy, powder and single-crystal X-ray diffraction, and time-resolved fluorescence spectroscopy were employed. Quadrupolar spin-echo solid-state (2)H NMR spectroscopy, in combination with theoretical calculations, was implemented to probe low-frequency vibrational dynamics of the confined chromophores, demonstrating conformational restrictions imposed on the coordinatively trapped chromophores. Because of possible tunability of the introduced scaffolds, these studies could foreshadow utilization of the presented approach toward directing a fluorescence response in artificial GFP mimics, modulating a protein microenvironment, and controlling nonradiative pathways through chromophore dynamics.

Additivity of substituent effects in aromatic stacking interactions.

The goal of this study was to experimentally test the additivity of the electrostatic substituent effects (SEs) for the aromatic stacking interaction. The additivity of the SEs was assessed using a small molecule model system that could adopt an offset face-to-face aromatic stacking geometry. The intramolecular interactions of these molecular torsional balances were quantitatively measured via the changes in a folded/unfolded conformational equilibrium. Five different types of substituents were examined (CH3, OCH3, Cl, CN, and NO2) that ranged from electron-donating to electron-withdrawing. The strength of the intramolecular stacking interactions was measured for 21 substituted aromatic stacking balances and 21 control balances in chloroform solution. The observed stability trends were consistent with additive SEs. Specifically, additive SE models could predict SEs with an accuracy from ±0.01 to ±0.02 kcal/mol. The additive SEs were consistent with Wheeler and Houk's direct SE model. However, the indirect or polarization SE model cannot be ruled out as it shows similar levels of additivity for two to three substituent systems, which were the number of substituents in our model system. SE additivity also has practical utility as the SEs can be accurately predicted. This should aid in the rational design and optimization of systems that utilize aromatic stacking interactions.

Self-assembly of pyridinium-functionalized anthracenes: molecular-skeleton-directed formation of microsheets and microtubes.

Two amphiphilic regioisomers, 9-AP (1-[11-(9-anthracenylmethoxy)-11-oxoundecyl]pyridinium bromide), and 2-AP (1-[11-(2-anthracenyl methoxy)-11-oxoundecyl]pyridinium bromide), were synthesized and their assembly behaviors were studied. Due to the anisotropic features of the anthracene structure, different substituted positions on the anthracene ring lead 9-AP and 2-AP to adapt "shaver" and "spatula"-like molecular shapes, respectively, which consequently dictate the structure of their final assemblies. While "shaver"-shaped 9-AP assembled into microsheets, driven by π-π interactions, "spatula"-shaped 2-AP assembled into microtubular structures, promoted primarily by charge-transfer interactions.

Framework complexes of group 2 metals organized by homochiral rods and π···π stacking forces: a breathing supramolecular MOF.

The reactions of the potassium salts of the ligands (S)-2-(1,8-naphthalimido)propanoate (KL(ala)), (S)-2-(1,8-naphthalimido)-3-hydroxypropanoate (KL(ser)), and (R)-2-(1,8-naphthalimido)propanoate (KL(ala)*), enantiopure carboxylate ligands containing a 1,8-naphthalimide π···π stacking supramolecular tecton, and, in the case of L(ser)(-), an alcohol functional group with calcium or strontium nitrate under solvothermal conditions produce crystalline [Ca(L(ala))2(H2O)]·(H2O) (1); [Ca(L(ser))2]·(H2O)2 (2); [Sr(L(ala))2(H2O)]·(H2O)3 (3); [Sr(L(ala)*)2(H2O)]·(H2O)3 (3*); and [Sr(L(ser))2(H2O)] (5). Placing 3 under vacuum removes the interstitial waters to produce [Sr(L(ala))2(H2O)] (4) in a single-crystal to single-crystal transformation; introduction of water vapor to 4 leads to the reformation of crystalline 3. Each of these new complexes has a solid-state structure based on homochiral rod secondary building unit (SBUs) central cores. Supramolecular π···π stacking interactions between 1,8-naphthalimide rings link adjacent rod SBUs into three-dimensional structures for 1, 3, 4, and 5 and two-dimensional structure for 2. Compounds 1 and 3 have open one-dimensional channels along the crystallographic c axis that are occupied by disordered solvent. For 3, these channels close and open in the reversible single-crystal conversion to 4; the π···π stacking interactions of the naphthalimide rings facilitate this process by rotating and slipping. Infrared spectroscopy demonstrated that the rehydration of 4 with D2O leads to 3d8, and the process of dehydration and rehydration of 3d8 with H2O leads to 3, thus showing exchange of the coordinated water in this process. These forms of 3 and 4 were characterized by (1)H, (2)H, and (13)C solid-state NMR spectroscopy, and thermal and luminescence data are reported on all of the complexes.

Hydroxide-bridged cubane complexes of nickel(II) and cadmium(II): magnetic, EPR, and unusual dynamic properties.

The reactions of M(ClO4)2·xH2O (M = Ni(II) or Cd(II)) and m-bis[bis(1-pyrazolyl)methyl]benzene (Lm) in the presence of triethylamine lead to the formation of hydroxide-bridged cubane compounds of the formula [M4(µ3-OH)4(µ-Lm)2(solvent)4](ClO4)4, where solvent = dimethylformamide, water, acetone. In the solid state the metal centers are in an octahedral coordination environment, two sites are occupied by pyrazolyl nitrogens from Lm, three sites are occupied by bridging hydroxides, and one site contains a weakly coordinated solvent molecule. A series of multinuclear, two-dimensional and variable-temperature NMR experiments showed that the cadmium(II) compound in acetonitrile-d3 has C2 symmetry and undergoes an unusual dynamic process at higher temperatures (ΔGLm‡ = 15.8 ± 0.8 kcal/mol at 25 °C) that equilibrates the pyrazolyl rings, the hydroxide hydrogens, and cadmium(II) centers. The proposed mechanism for this process combines two motions in the semirigid Lm ligand termed the "Columbia Twist and Flip:" twisting of the pyrazolyl rings along the Cpz­Cmethine bond and 180° ring flip of the phenylene spacer along the CPh­Cmethine bond. This dynamic process was also followed using the spin saturation method, as was the exchange of the hydroxide hydrogens with the trace water present in acetonitrile-d3. The nickel(II) analogue, as shown by magnetic susceptibility and electron paramagnetic resonance measurements, has an S = 4 ground state, and the nickel(II) centers are ferromagnetically coupled with strongly nonaxial zero-field splitting parameters. Depending on the Ni­O­Ni angles two types of interactions are observed: J1 = 9.1 cm(­1) (97.9 to 99.5°) and J2 = 2.1 cm(­1) (from 100.3 to 101.5°). "Broken symmetry" density functional theory calculations performed on a model of the nickel(II) compound support these observations.