Symmetry-Breaking Charge Transfer in Metal-Organic Frameworks.

High quantum-yield charge carrier generation from the initially prepared excitons defines a key step in the light-harvesting and conversion scheme. Photoinduced charge transfer in molecular electron donor-acceptor assemblies is driven by a sizable ΔG0, which compromises the potential of the generated carriers. Reminiscent of the special pair at the reaction center of the natural light-harvesting complex, symmetry-breaking charge transfer (SBCT) within a pair of identical struts of metal-organic framework (MOF) will facilitate the efficient generation of long-lived charge carriers with maximized potentials without incorporating any foreign redox species. We report SBCT in pyrene-based zirconium metal-organic framework (MOF) NU-1000 that leads to efficient generation of radical ions in a polar solvent and bound CT states in a low-polar solvent. The probe unveils the role of the low-lying non-Franck-Condon excitonic states as intermediates in the formation of the SBCT state from the initially prepared Franck-Condon S1 states. Ultrafast and transient spectroscopy─probed over 200 fs-30 µs time scale─evinces a kSBCT = (110 ps)-1 in polar media (εs = 37.5) forming solvated radical ions with recombination rate kCR = (∼45 ns)-1. A slower rate with kSBCT = (203 ps)-1 was recorded in low-polar (εs = 7.0) solvent manifesting a bound [TBAPy•+ TBAPy•-] state with kCR ≈ (17 µs)-1. This discovery, along with other unique photophysical features relevant to light harvesting, should define a MOF-based platform for developing heterogeneous artificial photon energy conversion systems.

Temporal (Dis)Assembly of Peptide Nanostructures Dictated by Native Multistep Catalytic Transformations.

Emergence of complex catalytic machinery via simple building blocks under non-equilibrium conditions can contribute toward the system level understanding of the extant biocatalytic reaction network that fuels metabolism. Herein, we report temporal (dis)assembly of peptide nanostructures in presence of a cofactor dictated by native multistep cascade transformations. The short peptide can form a dynamic covalent bond with the thermodynamically activated substrate and recruit cofactor hemin to access non-equilibrium catalytic nanostructures (positive feedback). The neighboring imidazole and hemin moieties in the assembled state rapidly converted the substrate to product(s) via a two-step cascade reaction (hydrolase-peroxidase like) that subsequently triggered the disassembly of the catalytic nanostructures (negative feedback). The feedback coupled reaction cycle involving intrinsic catalytic prowess of short peptides to realize the advanced trait of two-stage cascade degradation of a thermodynamically activated substrate foreshadows the complex non-equilibrium protometabolic networks that might have preceded the chemical emergence of life.

Induction and rationalization of supramolecular chirality in a highly flexible Zn(II)porphyrin dimer: structural, spectroscopic and theoretical investigations.

A highly flexible pyrrole-bridged Zn(II)porphyrin dimer has been successfully utilized as an efficient host which enables an accurate determination of the absolute configuration directly for a large number of chiral amino alcohols and 1,2-diols. The addition of substrates resulted in the formation of 1 : 1 sandwich complexes which, after the addition of excess substrates, produced 1 : 2 host-guest complexes. In principle, the 1 : 2 host-guest complexes can be stabilized in three possible conformations, viz. exo-exo, exo-endo, and endo-endo based on how a substrate binds to the metal. The endo-endo conformation is stabilized by two strong interligand H-bonds [O-H⋯O(H)] between encapsulated diols which thereby interlock the stereochemistry. In the absence of such interligand H-bonding interactions, exo-endo binding is preferred as it is indeed observed for amino alcohols which show weaker CD couplets due to the free movement of substrates. The sandwich complexes with amino alcohols show a more intense CD couplet compared to the diols due to the stronger binding of the amine functionality (-NH2) towards a Zn-atom over an alcoholic moiety (-OH). The CD amplitude showed linear dependence with a binding constant for the 1 : 1 sandwich complex upon varying the substrates. Spectroscopic investigations, single crystal X-ray structure determination of four such host-guest complexes and DFT studies have enabled us to rationalize systematically the origin of optical activity unambiguously in the 1 : 1 and 1 : 2 host-guest complexes, which lead to an absolute stereochemical determination of a large number of chiral substrates. The larger vertical and horizontal flexibility of a diethyl pyrrole spacer induces stronger binding of the substrates to form the 1 : 1 complex with a much larger torsional angle along with intense CD couplets. In contrast, a rigid dibenzothiophene-bridged tweezer, due to its limited horizontal and vertical flexibility, facilitates 1 : 2 complexation more as compared to the highly flexible pyrrole-bridged host which results in stronger binding of the substrate with the intense CD couplet for the former.

Hydrogen-Bonding Interactions Trigger Induction of Chirality via Formation of a Cyclic Dimer.

A rationalization for the chirality transfer mechanism in the supramolecular host-guest assemblies of an achiral Zn(II) porphyrin dimer (host) and a series of chiral diamines and diamino esters (substrates) via cyclic dimer formation has been reported for the first time. Stepwise formations of 2:2 host-guest cyclic dimers and 1:2 host-guest monomeric complexes have been observed via intermolecular assembling and disassembling processes. A large bisignate CD couplet was observed for the cyclic dimer, whereas the monomeric complexes exhibited negligible CD intensity. Crystallographic characterizations demonstrate that the strong intermolecular H bonding in cyclic dimers is responsible for their stability over the linear chain, which thereby display high-intensity bisignate CD couplets. In order to minimize the steric crowding within the host-guest assembly, the cyclic dimer switches its helicity toward the conformer having less steric hindrance. The cyclic scaffold is oriented according to the pre-existing chirality of the substrate in both the solid and solution phases: the substrates having R chirality display a negative CD couplet, whereas the substrates with S chirality display a positive couplet. Opposite signs for the CD couplets between R and S substrates suggest that the stereographic projection at the chiral centers solely dictates the overall helicity of the cyclic dimer. DFT studies further support the experimental observations.

Induction, control, and rationalization of supramolecular chirogenesis using metalloporphyrin tweezers: a structure-function correlation.

Supramolecular chirogenesis is one of the most rudimentary topics in the interdisciplinary sciences and essentially deals with various natural processes and innovative modern technologies. A comprehensive and rigorous understanding of such phenomenon is necessary to have a clear insight into the fundamental mechanisms and the various controlling factors, which would eventually lead to a range of practical applications of chiral supramolecular science. Metalloporphyrin tweezers have been extensively employed for such chirogenic processes due to their exciting physicochemical and tunable spectral properties, large stabilities, easily available synthetic protocols, and excellent abilities to form molecular assemblies. During the last few decades, various metalloporphyrin tweezers have been developed and considerably utilized by several research groups for assigning the absolute configuration to a variety of chiral diamines, conjugates of primary and secondary amines, amino alcohols, secondary alcohols, α-chiral carboxylic acids, etc. Our group has been at the forefront in trying to establish the structure-property correlation in this important area of interdisciplinary research. A brief account of our systematic investigation for understanding the underpinning mechanism of chirality induction and control at the molecular level over the last few years is presented in this Perspective article. The comprehensive understanding of such mechanistic details will be helpful in understanding various natural processes and designing modern technologies for various chirogenic functions in the fields of molecular sensors, nanotechnology, and supramolecular chemistry.

Complexation of Chiral Zinc(II)Porphyrin Tweezer with Chiral Guests: Control, Discrimination and Rationalization of Supramolecular Chirality.

A series of 1:1 sandwich complexes consisting of chiral zinc(II) bisporphyrin hosts and a series of chiral guests has been synthesized and has rationalized the underpinning mechanism of chirality transfer in the host-guest supramolecular assemblies. The number of stereogenic centers is also varied in both the host and guests, which provides insight into the overall helicity of the assembly. The interactions between the chiral host and chiral guests have been investigated by UV-visible, CD, and 1H NMR spectroscopic titrations along with extensive DFT studies. Interestingly, CD spectral changes are very different between chiral guests with one and two chiral centers. It has been observed that the sign of the CD couplet of the host-guest complexes is dictated by the chirality of the host only with guests having one chiral center. The match and mismatch of the chirality of the guest only affects the amplitude of the CD signal; the sign, however, remains intact. In sharp contrast, the helicity of the 1:1 sandwich complex is dictated by the chirality of the guests having two chiral centers. However, amplification of the CD couplet is observed upon matching of the chirality between the host and guest, while a mismatch leads to an inversion of the CD couplet. The enantiomeric host also displays similar trends with chiral guests but with opposite sign. The enantioselective host also displays excellent chiral discrimination ability toward enantiomeric guests with two chiral centers. The guest with the same chirality as the host binds much stronger as compared to its enantiomer. Remarkable enantiodiscrimination effects were also detected in the 1H NMR spectra of the diastereomeric complexes in which well-resolved separation of the signals is clearly visible. The theoretical calculations are consistent with those of the experiment.

Molecule to Supramolecule: Chirality Induction, Inversion, and Amplification in a Mg(II)porphyrin Dimer Templated by Chiral Diols.

A clear and unambiguous rationalization of chirality induction, amplification, and subsequent inversion processes has been demonstrated using an achiral Mg(II)porphyrin dimer (host) and a series of chiral diols (guests) upon stepwise formation of a 1:1 host-guest polymer and 1:2 host-guest monomer via intermolecular assembling and disassembling processes. Crystallographic characterizations are reported here for both the polymer and the monomeric complexes, which enable us to completely scrutinize the structural and geometrical changes systematically in rationalizing their optical properties. The sign of the CD couplets for both the polymer and monomer are just opposite between R and S guests, which suggests that the chirality is dictated solely by the stereogenic projection of the chiral centers. Stronger intra- and intermolecular coupling in the polymeric complexes is responsible for the highly enhanced CD couplets as compared to the monomer and have only intramolecular coupling as also observed in their X-ray structures. DFT studies clearly support the experimental observations.

Complexation of Chiral Zinc(II) Porphyrin Tweezer with Achiral Aliphatic Diamines Revisited: Molecular Dynamics, Electronic CD, and 1H NMR Analysis.

We have reported here the complexation and chiroptical behavior of the host-guest complexes using a new chiral Zn(II) bisporphyrin tweezer host and a series of achiral aliphatic diamine guests varying the chain length. (1R,2R)-Cyclohexanediamine covalently links two Zn(II) porphyrin moieties, which thereby produces a strong chiral field around the bisporphyrin tweezer framework. The chiral tweezer upon complexation with achiral guest exhibited large changes in the UV-vis spectra and CD exciton couplets due to a sudden change in the porphyrin disposition, which is controlled by the host-guest stoichiometry as well as the chain lengths of the diamine guest. Addition of smaller diamines (n: 2-5) to the host resulted in the formation of 1:1 sandwich and 1:2 open complexes, respectively, at the low and high guest concentration, which eventually display two-step inversions of the CD couplet. With longer diamines (n: 6-8), however, only 1:1 sandwich complexes are formed with retention of the CD sign. Similar observations were also reported by us recently using another chiral bisporphyrin tweezer having (1R,2R)-diphenylethylenediamine as the spacer. In an effort to obtain deeper insights into the sudden changes of interporphyrin disposition just by changing the length of the achiral diamines, we have extended a series of computational studies and correlated closely with the results obtained from the experiment. While the previously published study has relied on commonly applied Monte Carlo (MC) sampling of the potential energy surface in addition to being guided by porphyrin effective transition moment approximation, the present study uses a considerably more robust molecular modeling protocol, namely Molecular Dynamics (MD) simulations followed by full ab initio geometry optimization and TD-DFT CD prediction. The experimental data corroborate with the results obtained from the theoretical conformational analysis. The latter are also supported by experimental 1H NMR data empowered by the porphyrin ring-current effect. The NMR spectral patterns of pyrrolic protons of the free host and the 1:1 sandwich complexes appear very diagnostic and reflect the changes in the mutual porphyrin disposition on moving from the free host to the complexed ones with short and long diamines. Overall, the experimental NMR data underscore the sensitivity of pyrrolic protons chemical shifts to subtle alterations of the geometrical features, and as such, they come in agreement with the theoretically derived models.

Formation of Polymeric Housene Molecules of Group 15 Elements (N, P, As, and Sb): A DFT Study.

Recently, the formation of the dimeric stibahousene molecule, bis(stibahousene), has been reported. In line with the report, the formation of dimeric housene molecules with N, P, and As is examined in light of density functional theory. Moreover, the extension of the study from dimeric to tetrameric and hexameric molecules (N, P, As, and Sb) is also performed. The study supports the formation of such polymeric housene analogues.

Density Functional Study on the Adsorption of 5-Membered N-Heterocycles on B/N/BN-Doped Graphene: Coronene as a Model System.

Adsorption of seven 5-membered N-heterocycles on B/N/BN-doped graphene (with coronene as a model system) has been studied using density functional theory (DFT). The geometry of the complexes validated the involvement of both π···π stacking and N-H···π interaction in the adsorption process. The stability of the complexes is measured in terms of stabilization energy, and the results suggested that the complexes are stable enough (stabilization energies are in the range of 7.61-14.77 kcal mol-1). Studies confirmed the stability of complexes in the solvent phase too irrespective of the dielectric of the solvent. Dispersive force is the major mode of interaction in stabilizing the complexes. Natural bond orbital analysis indicated a small contribution from electrostatic and covalent interactions. Thermochemical analysis revealed that the complexation is exothermic in nature and favorable at a lower temperature. Adsorption of N-heterocycles exerts a nominal impact on the electronic properties of the undoped/doped graphene. The study presents a simple approach to introduce an arbitrary functionality to undoped/doped graphene by preserving its electronic properties.

Metal-Center-Driven Supramolecular Chirogenesis in Tweezer Amino Alcohol Complexes: Structural, Spectroscopic, and Theoretical Investigations.

An apparently rigid dibenzothiophene-bridged zinc(II)/magnesium(II) bisporphyrin host (1M) has been explored for an accurate determination of the absolute configuration of a large series of amino alcohols. At lower substrate concentration, a 1:1 sandwich complex is formed which, upon addition of excess of substrate, converts to the 1:2 host-guest complex with complete inversion of the CD exciton couplet. The intensities of the couplet vary widely just by changing the metal ion (Zn vs Mg) and also vary between 1:1 and 1:2 host-guest complexes. Crystallographic characterizations are reported here for both 1:1 sandwich and 1:2 host-guest complexes using the same pair of host and guest, for the first time, which enable us to scrutinize the structural and geometrical changes systematically in rationalizing their optical properties. The intensity of the CD couplet is largely dependent on how strongly the substrate binds with the host and also their mode of binding. No CD couplet is observed in the spectral region of porphyrin absorption when substrate binds in either exo-endo or exo-exo fashion in the 1:2 host-guest complex. However, intermolecular H bonding between two encapsulated substrates in the 1:2 host-guest complex stabilizes the endo-endo conformer in which two porphyrin macrocycles are forced to be oriented in a clockwise/anticlockwise direction to produce an intense CD couplet. Such an endo-endo binding of (S)-2-aminobutan-1-ol (S-AB) has resulted in a highly intense CD couplet with 1Mg, while no chiroptic response was observed upon changing the metal to zinc, since S-AB would then bind in an exo-endo form. With an increase in the bulk of the substrate, the endo-endo form first transforms into an exo-endo form which, upon further increase in the bulk of the substrate, converts into an exo-exo complex.

Complexation of Chiral Zinc-Porphyrin Tweezer with Achiral Diamines: Induction and Two-Step Inversion of Interporphyrin Helicity Monitored by ECD.

We report here the synthesis of a new chiral Zn(II) bisporphyrin tweezer in which two achiral Zn(II) porphyrin moieties are covalently linked by (1R,2R)-diphenylethylenediamine, which produces a strong chiral field around the porphyrin moieties. The chiral tweezer exhibits not only intensity modulation in UV-vis and CD exciton couplets but also a dramatic change, namely, the inversion in the sign of the interporphyrin helicity upon binding of achiral diamines of varying lengths. The stoichiometry-controlled formation of a 1:1 sandwich complex followed by a 1:2 open complex was realized with smaller achiral diamines (n: 2-5) at their low and high concentration regions, respectively, leading to two-step inversion of chirality. With longer achiral diamines (n: 6-8), however, only 1:1 sandwich complexes are formed with no change of sign in the CD couplet. As compared to a 1:2 open complex, a 1:1 sandwich complex shows an enhanced CD response as two porphyrin units come closer in space. Structural insights of the host-guest complexes have been obtained spectroscopically along with molecular mechanics minimizations with the newly implemented OPLS-3 force field followed by geometry optimization using density functional theory of the most stable conformer. The amide bridge in the Zn(II) bisporphyrin has a low rotational barrier, which provides conformational flexibility to change interporphyrin helicity between 1:1 and 1:2 binding depending on the size of the achiral guests in order to minimize host-guest steric interactions.

Catalytic properties of cobalt(III)-oxo cubanes in the TBHP oxidation of benzylic alcohols.

Two series of oxo-bridged Co(III) complexes of the type Co(4)O(4)(O(2)CC(6)H(4)-X)(4)(py)(4) (1) and Co(4)O(4)(O(2)CC(6)H(4)-X)(4)(4-Mepy)(4) (2), where X = H (a), Me (b), MeO (c), Cl (d), NO(2) (e), have been synthesized and characterized in detail. The molecular structures of the complexes consist of a cubelike Co(4)(mu(3)-O)(4) core having Co and O atoms at alternate vertices with carboxylato ligands bridging the Co(3+) ions along four face diagonals of the approximate cube. Nitrogen atoms from pyridyl ligands complete the distorted-octahedral coordination around each Co(III). These neutral Co(III) complexes undergo a nearly reversible one-electron oxidation involving the redox couple [Co(4)(III)(mu(3)-O)(4)](4+) <--> [Co(3)(III)Co(IV)(mu(3)-O)(4)](5+) at potentials (approximately 0.7-1.0 V) that linearly depend on the electronic influence of X. The cobalt(III) clusters of types 1 and 2 have been found to effectively promote the TBHP oxidation of benzylic alcohols under homogeneous conditions to produce the corresponding carbonyl compounds.