Michal Juríček

Induced-fit catalysis of corannulene bowl-to-bowl inversion

Stereoelectronic complementarity between the active site of an enzyme and the transition state of a reaction is one of the tenets of enzyme catalysis. This report illustrates the principles of enzyme catalysis (first proposed by Pauling and Jencks) through a well-defined model system that has been fully characterized crystallographically, computationally and kinetically. Catalysis of the bowl-to-bowl inversion processes that pertain to corannulene is achieved by combining ground-state destabilization and transition-state stabilization within the cavity of an extended tetracationic cyclophane. This synthetic receptor fulfils a role reminiscent of a catalytic antibody by stabilizing the planar transition state for the bowl-to-bowl inversion of (ethyl)corannulene (which accelerates this process by a factor of ten at room temperature) by an induced-fit mechanism first formulated by Koshland.

Ultrafast Conformational Dynamics of Electron Transfer in ExBox4+⊂Perylene

Multielectron acceptors are essential components for artificial photosynthetic systems that must deliver multiple electrons to catalysts for solar fuels applications. The recently developed boxlike cyclophane incorporating two extended viologen units joined end-to-end by two p-phenylene linkers-namely, ExBox(4+)-has a potential to be integrated into light-driven systems on account of its ability to complex with π-electron-rich guests such as perylene, which has been utilized to great extent in many light-harvesting applications. Photodriven electron transfer to ExBox(4+) has not previously been investigated, however, and so its properties, following photoreduction, are largely unknown. Here, we investigate the structure and energetics of the various accessible oxidation states of ExBox(4+) using a combination of spectroscopy and computation. In particular, we examine photoinitiated electron transfer from perylene bound within ExBox(4+) (ExBox(4+)⊂perylene) using visible and near-infrared femtosecond transient absorption (fsTA) spectroscopy. The structure and conformational relaxation dynamics of ExBox(3+)⊂perylene(+) are observed with femtosecond stimulated Raman spectroscopy (FSRS). From the fsTA and FSRS spectra, we observe that the central p-phenylene spacer in one of the extended viologen units on one side of the cyclophane becomes more coplanar with its neighboring pyridinium units over the first ∼5 ps after photoreduction. When the steady-state structure of chemically generated ExBox(2+) is investigated using Raman spectroscopy, it is found to have the central p-phenylene rings in both of its extended viologen units rotated to be more coplanar with their neighboring pyridinium units, further underscoring the importance of this subunit in the stabilization of the reduced states of ExBox(4+).

Non-Interpenetrated Metal-Organic Frameworks Based on Copper(II) Paddlewheel and Oligoparaxylene-Isophthalate Linkers: Synthesis, Structure and Gas Adsorption

Two metal-organic framework materials, MFM-130 and MFM-131 (MFM = Manchester Framework Material), have been synthesized using two oligoparaxylene (OPX) tetracarboxylate linkers containing four and five aromatic rings, respectively. Both fof-type non-interpenetrated networks contain Kagomé lattice layers comprising [Cu2(COO)4] paddlewheel units and isophthalates, which are pillared by the OPX linkers. Desolvated MFM-130, MFM-130a, shows permanent porosity (BET surface area of 2173 m(2)/g, pore volume of 1.0 cm(3)/g), high H2 storage capacity at 77 K (5.3 wt% at 20 bar and 2.2 wt% at 1 bar), and a higher CH4 adsorption uptake (163 cm(3)(STP)/cm(3) (35 bar and 298 K)) compared with its structural analogue, NOTT-103. MFM-130a also shows impressive selective adsorption of C2H2, C2H4, and C2H6 over CH4 at room temperature, indicating its potential for separation of C2 hydrocarbons from CH4. The single-crystal structure of MFM-131 confirms that the methyl substituents of the paraxylene units block the windows in the Kagomé lattice layer of the framework, effectively inhibiting network interpenetration in MFM-131. This situation is to be contrasted with that of the doubly interpenetrated oligophenylene analogue, NOTT-104. Calculation of the mechanical properties of these two MOFs confirms and explains the instability of MFM-131 upon desolvation in contrast to the behavior of MFM-130. The incorporation of paraxylene units, therefore, provides an efficient method for preventing network interpenetration as well as accessing new functional materials with modified and selective sorption properties for gas substrates.

Triazole–pyridine ligands: a novel approach to chromophoric iridium arrays

We describe a novel modular approach to a series of luminescent iridium complexes bearing triazole–pyridine-derived ligands that were conveniently prepared by using “click” chemistry. One, two or three triazole–pyridine units were effectively built into the heteroaromatic macromolecule using versatile acetylene- and azide-functionalised precursors. Using this approach, a series of iridium-derived molecules, that differ in the number of iridium centres, the structural characteristics of the cyclometalating ligand and the backbone, were synthesised. The preliminary photophysical properties of the prepared complexes indicate that there is only limited interaction (through space or through the backbone) between the iridium centres within one molecule and that each iridium centre retains its individual properties. The results show that our approach can be generally applied towards covalently linked multichromophoric systems with potential application, for instance, in the design and preparation of tunable light emitters. As a demonstration of this concept, a single molecule white-light emitter, constructed from two iridium centres (yellow emission) and a fluorene unit (blue emission), is presented.

Synthesis of ExnBox Cyclophanes

A rapid and efficient synthesis of the extended bipyridinium-based class of cyclophanes--that is, Ex(n)Box(4+) (n = 0-3), where n is the number of p-phenylene rings inserted between the pyridinium rings--is demonstrated, resulting in much higher yields of products along with a reduced output of oligomeric byproducts. Although each cyclophane can be synthesized readily without the use of a precise stoichiometric amount of template, ExBox(4+) can be prepared in 66% yield (following crystallization) using six equivalents of pyrene in a template-directed protocol. This new methodology has been employed to synthesize, in modest yield, a nearly 2.5 nm long cyclophane consisting of 12 aromatic rings.

Fusing Triazoles: Toward Extending Aromaticity

A novel method to extend aromaticity by one benzene and two triazole rings was developed and optimized. This two-step route employs the copper-catalyzed azide-haloalkyne cycloaddition reaction of an ortho-bis(iodoacetylene) system and the subsequent intramolecular homocoupling fusion of the neighboring iodotriazoles, a process in which an additional benzene ring is formed. This versatile methodology allows one to extend the core size of chromophores and, consequently, to tune the materials properties.

Three-Dimensional Architectures Incorporating Stereoregular Donor-Acceptor Stacks

We report the synthesis of two [2]catenane-containing struts that are composed of a tetracationic cyclophane (TC(4+)) encircling a 1,5-dioxynaphthalene (DNP)-based crown ether, which bears two terphenylene arms. The TC(4+) rings comprise either 1) two bipyridinium (BIPY(2+)) units or 2) a BIPY(2+) and a diazapyrenium (DAP(2+)) unit. These degenerate and nondegenerate catenanes were reacted in the presence of Cu(NO3)2⋅2.5 H2O to yield Cu-paddlewheel-based MOF-1050 and MOF-1051. The solid-state structures of these MOFs reveal that the metal clusters serve to join the heptaphenylene struts into grid-like 2D networks. These 2D sheets are then held together by infinite donor-acceptor stacks involving the [2]catenanes to produce interpenetrated 3D architectures. As a consequence of the planar chirality associated with both the DNP and hydroquinone (HQ) units present in the crown ether, each catenane can exist as four stereoisomers. In the case of the nondegenerate (bistable) catenane, the situation is further complicated by the presence of translational isomers. Upon crystallization, however, only two of the four possible stereoisomers--namely, the enantiomeric RR and SS forms--are observed in the crystals. An additional element of co-conformational selectivity is present in MOF-1051 as a consequence of the substitution of one of the BIPY(2+) units by a DAP(2+) unit: only the translational isomer in which the DAP(2+) unit is encircled by the crown ether is observed. The overall topologies of MOF-1050 and MOF-1051, and the selective formation of stereoisomers and translational isomers during the kinetically driven crystallization, provide evidence that weak noncovalent bonding interactions play a significant role in the assembly of these extended (super)structures.

Semiconducting single crystals comprising segregated arrays of complexes of C60

Although pristine C60 prefers to adopt a face-centered cubic packing arrangement in the solid state, it has been demonstrated that noncovalent-bonding interactions with a variety of molecular receptors lead to the complexation of C60 molecules, albeit usually with little or no control over their long-range order. Herein, an extended viologen-based cyclophane—ExBox2(4+)—has been employed as a molecular receptor which, not only binds C60 one-on-one, but also results in the columnar self-assembly of the 1:1 inclusion complexes under ambient conditions. These one-dimensional arrays of fullerenes stack along the long axis of needle-like single crystals as a consequence of multiple noncovalent-bonding interactions between each of the inclusion complexes. The electrical conductivity of these crystals is on the order of 10(-7) S cm(-1), even without any evacuation of oxygen, and matches the conductivity of high-quality, unfunctionalized C60-based materials that typically require stringent high-temperature vaporization techniques, along with the careful removal of oxygen and moisture, prior to measuring their conductance.

Electron Transfer and Multi‐Electron Accumulation in ExBox4+

Molecules capable of accepting and storing multiple electrons are crucial components of artificial photosynthetic systems designed to drive catalysts, such as those used to reduce protons to hydrogen. ExBox(4+), a boxlike cyclophane comprising two π-electron-poor extended viologen units tethered at both ends by two p-xylylene linkers, has been shown previously to accept an electron through space from a photoexcited guest. Herein is an investigation of an alternate, through-bond intramolecular electron-transfer pathway involving ExBox(4+) using a combination of transient absorption and femtosecond stimulated Raman spectroscopy (FSRS). Upon photoexcitation of ExBox(4+), an electron is transferred from one of the p-xylylene linkers to one of the extended viologen units in ca. 240 ps and recombines in ca. 4 ns. A crystal structure of the doubly reduced species ExBox(2+) was obtained.

Configurational Stability of [5]Helicenes

A series of [5]helicenes difunctionalized in the fjord region with either fluoro, methoxy, or methyl groups was synthesized via photochemical and benzylic coupling route. Resolution of each compound into enantiomers and determination of the Gibbs activation energies of enantiomerization (ΔG⧧(T)) revealed high configurational stability in all three cases. The ΔG⧧(T) values of difunctionalized [5]helicenes were compared with those of their monofunctionalized analogues and the parent [5]helicene. Within this series, an exponential correlation between the torsional twist and ΔG⧧(T) was found. The dimethyl derivative exhibits one of the highest configurational stabilities among [n]helicenes reported to date, comparable to that of [9]helicene.