Thierry Maris

Constructing monocrystalline covalent organic networks by polymerization

An emerging strategy for making ordered materials is modular construction, which connects preformed molecular subunits to neighbours through interactions of properly selected reactive sites. This strategy has yielded remarkable materials, including metal–organic frameworks joined by coordinative bonds, supramolecular networks linked by strong non-covalent interactions, and covalent organic frameworks in which atoms of carbon and other light elements are bonded covalently. However, the strategy has not yet produced covalently bonded organic materials in the form of large single crystals. Here we show that such materials can result from reversible self-addition polymerizations of suitably designed monomers. In particular, monomers with four tetrahedrally oriented nitroso groups polymerize to form diamondoid azodioxy networks that can be fully characterized by single-crystal X-ray diffraction. This work forges a strong new link between polymer science and supramolecular chemistry by showing how predictably ordered covalent or non-covalent structures can both be built using a single modular strategy. Modular construction using connectable molecular subunits is a powerful strategy for making new carbon-based materials. So far, large crystals have been produced only from subunits linked by weak interactions. Covalently bonded analogues have now been prepared by reversible self-addition polymerization of suitable monomers and structurally characterized by single-crystal X-ray diffraction.

Engineering New Metal-Organic Frameworks Built from Flexible Tetrapyridines Coordinated to Cu(II) and Cu(I)

A series of new metal-organic frameworks have been constructed by the coordination of Cu(II) and Cu(I) with pentaerythrityl tetrakis(4-pyridyl) ether (1 = PETPE), a flexible tetradentate ligand. Networks derived from Cu(OOCCH(3))(2), Cu(NO(3))(2), and CuBF(4) proved to have different topologies (diamondoid, PtS, and SrAl(2), respectively). This reflects (1) the ability of PETPE (1) to adopt diverse conformations and (2) the varied geometries of complexes of Cu(II) and Cu(I). Extended PETPE (2), a tetrapyridine with phenyl spacers inserted into the pentaerythrityl core of PETPE (1), yielded an expanded version of the PtS network derived from simple PETPE (1) and Cu(NO(3))(2). However, increases in the ability of the network to accommodate guests were largely offset by interpenetration of independent networks. Attempts to thwart interpenetration by converting ligand 2 into methyl-substituted derivative 3 led to the construction of networks with alternative topologies. In particular, the reactions of ligand 3 with both Cu(II) and Cu(I) yielded isostructural Pt(3)O(4) networks, despite the preference of the two oxidation states for coordination spheres with different geometries. Together, these observations demonstrate that PETPE (1) and related compounds are useful ligands for constructing metal-organic frameworks, with a distinctive ability to accommodate a single metal in different oxidation states, as well as to adapt to a metal in a single oxidation state but with different counterions or secondary ligands.

Two-dimensional hydrogen-bonded networks in crystals of diboronic acids

Crystallization of suitable diboronic acids favors the formation of hydrogen-bonded sheets, thereby underscoring the usefulness of the –B(OH)2 group in crystal engineering.

Using Pyridinyl-Substituted Diaminotriazines to Bind Pd(II) and Create Metallotectons for Engineering Hydrogen-Bonded Crystals

The pyridinyl groups of pyridinyl-substituted diaminotriazines 3a,b and 4a,b can bind metals, and the diaminotriazinyl (DAT) groups serve independently to ensure that the resulting complexes can participate in intercomplex hydrogen bonding according to characteristic motifs. As planned, ligands 3a,b and 4a,b form trans square-planar 2:1 complexes with PdCl(2), and further association of the complexes is directed in part by hydrogen bonding of the DAT groups. Similarly, ligands 3a,b and 4a,b form cationic square-planar 4:1 complexes with Pd(BF(4))(2), Pd(PF(6))(2), and Pd(NO(3))(2), and the complexes again typically associate by hydrogen bonding of the peripheral DAT groups. The observed complexes have predictable constitutions and shared structural features that result logically from their characteristic topologies and the ability of DAT groups to engage in hydrogen bonding. These results illustrate the potential of a hybrid inorganic/organic strategy for constructing materials in which coordinative bonds to metals are used in conjunction with other interactions, both to build the molecular components and to control their organization.

Engineering crystals built from molecules containing boron

The study of compounds containing boron continues to have an important impact on virtually every area of chemistry. One of the few areas in which compounds of boron have been neglected is crystal engineering, which seeks to develop and exploit an understanding of how the structure and properties of crystals are related to the individual atomic or molecular components. Although detailed predictions of crystal structures are not yet reliable, crystal engineers have developed a sound qualitative strategy for anticipating and controlling structural preferences. This strategy is based on the design of special molecules, called tectons, which feature carefully selected cores and multiple peripheral functional groups that can direct association and thereby place neighboring molecules in predetermined positions. Recent work has demonstrated that molecular crystals with unique properties can be constructed logically from tectons with boron in their cores or sticky sites of association. In particular, the -B(OH)2 group of boronic acids engages in reliable patterns of hydrogen bonding, and its use as a sticky site in tectons has emerged as an effective tool for controlling association predictably. In addition, replacement of tetraphenylsilyl or tetraphenylmethyl cores in tectons by tetraphenylborate leaves the overall molecular geometry little changed, but it has the profound effect of introducing charge. Tectons derived from tetraphenylborate can be used rationally to construct porous charged molecular networks that resemble zeolites and undergo selective ion exchange. In such ways, boron offers chemists exciting new ways to engineer molecular crystals with predetermined structures and properties.

Triarylamines designed to form molecular glasses. Derivatives of tris(p-terphenyl-4-yl)amine with multiple contiguous phenyl substituents.

The principles of crystal engineering can be used in a contrary way to help devise molecules that resist crystallization and form long-lived glasses. This can be achieved by making structural changes that thwart established patterns of crystallization. In using this strategy to block the crystallization of triarylamines, we have found that the introduction of methylpentaphenyl groups is particularly effective, presumably because they inhibit efficient molecular packing and normal intermolecular interactions.

Photophysical, Electrochemical and Crystallographic Investigations of the Fluorophore 2,5-Bis(5-tert-butyl-benzoxazol-2-yl)thiophene

The photophysics of 2,5-bis(5-tert-butyl-benzoxazol-2-yl)thiophene (BBT) were investigated for assessing its limitations for use as a universal fluorophore and as a viable sensor for both polymeric and solution studies. This is of importance given the limitations of currently used materials. BBTs steady-state and time-resolved fluorescence were additionally investigated to correlate its solid-state features, observed by fluorescence spectroscopy when mixed in poly(1,4-butylene succinate) (PBS) films, with its single crystal characteristics. The conjugated fluorophore was found to be highly fluorescent, with absolute quantum yields of (Φ(fl)) ≥ 0.60. The Φ(fl) values were high, regardless of solvent polarity and proticity and whether alone or in polymeric films. The major competitive fluorescence quenching pathway was found to occur by intersystem crossing to the triplet state. This was confirmed by laser flash photolysis in which the BBT triplet absorbed at 500 nm. The triplet transient was confirmed by quenching studies with 1,3-cyclohexadiene. Meanwhile, nonradiative deactivation of BBTs singlet excited state by internal conversion was found to be negligible. In solution and especially when distributed in semicrystalline PBS, BBT exhibits spectral changes and a bathochromic shift as a function of concentration due to aggregation of ground state molecules, which is present even at low BBT concentrations. Consistent monoexponential lifetimes on the order of ∼2 ns were observed regardless of solvent and independent of both the excitation wavelength and concentration. The constant excited state kinetics confirm the absence of a singlet excited state deactivation by excimer formation. The electrochemistry of BBT demonstrated that it is irreversibly oxidized and the resulting radical cation is unstable. Conversely, the cathodic process, resulting in the radical anion, is reversible, confirming its n-doping character. Crystallographic studies revealed that the planes described by the benzoxazolyl moieties are twisted from the plane described by the central thiophene. Several weak C-H···π and π-π intermolecular interactions were also observed. BBTs high solubility in common solvents combined with its measured enhanced optoelectronic properties make it a candidate as a universal fluorophore reference and smart material for both polymeric and solution studies.

Engineering Hydrogen-Bonded Hexagonal Networks Built from Flexible 1,3,5-Trisubstituted Derivatives of Benzene.

2,4-Diamino-1,3,5-triazinyl (DAT) groups are known to form N-H···N hydrogen bonds according to reliable patterns of self-association. In compounds 3a-c, three DAT groups are attached to trigonally substituted phenyl cores via identical flexible arms. Crystallization of compounds 3a-c produces robust networks in which each molecule is linked to its immediate neighbors by a total of 10-12 hydrogen bonds. In compound 3a, the DAT groups are designed to lie close to the plane of the phenyl core, thereby giving hydrogen-bonded sheets built from hexameric rosettes. In contrast, the more highly substituted phenyl cores of analogues 3b and 3c favor conformations in which the DAT groups are no longer coplanar, leading predictably to the formation of three-dimensional networks. In general, the nominally trigonal topologies of compounds 3a-c favor structures in which hexagonal networks are prominent, so they behave like trimesic acid despite their greater complexity and flexibility. The structures of all crystals incorporate open networks with significant fractions of volume accessible to guests (32-60%). Despite their flexibility, compounds 3a-c appear to be unable to assume conformations that pack efficiently and simultaneously allow the DAT groups to engage in normal hydrogen bonding.

Building Giant Carbocycles by Reversible C-C Bond Formation.

We describe a simple way to build giant macrocyclic hydrocarbons by the reversible formation of carbon-carbon bonds. Specifically, extended spirobifluorene-substituted derivatives of Wittigs hydrocarbon were synthesized and found to undergo oligomerization, giving the largest hydrocarbon that has been crystallized and characterized by X-ray diffraction to date.

Engineering homologous molecular organization in 2D and 3D. Cocrystallization of aminoazines and alkanecarboxylic acids

Amino-substituted azines such as aminotriazines have simple planar structures with an affinity for adsorption on graphite. In addition, aminoazines associate predictably with carboxylic acids according to a reliable hydrogen-bonded motif. Together, these properties help predispose aminoazines and alkanecarboxylic acids to cocrystallize and to coadsorb on graphite to give related 3D and 2D structures built from essentially planar tapes. These results illustrate how molecular materials that reliably adopt similar ordered structures in both adlayers and bulk samples can be devised by choosing components that combine an affinity for surfaces with a tendency to form sheets held together by strong coplanar intermolecular interactions.