Dong Sub Kim

Ion-Controlled On–Off Switch of Electron Transfer from Tetrathiafulvalene Calix[4]pyrroles to Li+@C60

Binding of chloride anion to a tetrathiafulvalene calix[4]pyrrole (TTF-C4P) donor results in ET to Li(+)@C(60) to produce the radical pair (TTF-C4P(•+)/Li(+)@C(60)(•-)), the structure of which was characterized by X-ray crystallographic analysis. The addition of tetraethylammonium cation, which binds more effectively than Li(+)@C(60)(•-) as a guest within the TTF-C4P cavity, leads to electron back-transfer, restoring the initial oxidation states of the donor and acceptor pair.

Chemoresponsive alternating supramolecular copolymers created from heterocomplementary calix[4]pyrroles

The importance of noncovalent interactions in the realm of biological materials continues to inspire efforts to create artificial supramolecular polymeric architectures. These types of self-assembled materials hold great promise as environmentally stimuli-responsive materials because they are capable of adjusting their various structural parameters, such as chain length, architecture, conformation, and dynamics, to new surrounding environments upon exposure to appropriate external stimuli. Nevertheless, in spite of considerable advances in the area of responsive materials, it has proved challenging to create synthetic self-assembled materials that respond to highly disparate analytes and whose environmentally induced changes in structure can be followed directly through both various spectroscopic and X-ray diffraction analyses. Herein, we report a new set of artificial self-assembled materials obtained by simply mixing two appropriately chosen, heterocomplementary macrocyclic receptors, namely a tetrathiafulvalene-functionalized calix[4]pyrrole and a bis(dinitrophenyl)-meso-substituted calix[4]pyrrole. The resulting polymeric materials, stabilized by combination of donor–acceptor and hydrogen bonding interactions, undergo dynamic, reversible dual guest-dependent structural transformations upon exposure to two very different types of external chemical inputs, namely chloride anion and trinitrobenzene. The structure and dynamics of the copolymers and their analyte-dependent responsive behavior was established via single crystal X-ray crystallography, SEM, heterocomplementary isodesmic analysis, 1- and 2D NMR, and dynamic light scattering spectroscopies. Our results demonstrate the benefit of using designed heterocomplementary interactions of two functional macrocyclic receptors to create synthetic, self-assembled materials for the development of “smart” sensory materials that mimic the key biological attributes of multianalyte recognition and substrate-dependent multisignaling.

Expanded Porphyrin-Anion Supramolecular Assemblies: Environmentally Responsive Sensors for Organic Solvents and Anions.

Porphyrins have been used frequently to construct supramolecular assemblies. In contrast, noncovalent ensembles derived from expanded porphyrins, larger congeners of naturally occurring tetrapyrrole macrocycles, are all but unknown. Here we report a series of expanded porphyrin-anion supramolecular assemblies. These systems display unique environmentally responsive behavior. Addition of polar organic solvents or common anions to the ensembles leads to either a visible color change, a change in the fluorescence emission features, or differences in solubility. The actual response, which could be followed easily by the naked eye, was found to depend on the specifics of the assembly, as well as the choice of analyte. Using the ensembles of this study, it proved possible to differentiate between common solvents, such as diethyl ether, THF, ethyl acetate, acetone, alcohol, acetonitrile, DMF, and DMSO, identify complex solvent systems, as well as distinguish between the fluoride, chloride, bromide, nitrate, and sulfate anions.

Redox- and pH-Responsive Orthogonal Supramolecular Self-Assembly: An Ensemble Displaying Molecular Switching Characteristics

Two heteroditopic monomers, namely a thiopropyl-functionalized tetrathiafulvalene-annulated calix[4]pyrrole (SPr-TTF-C[4]P 1) and phenyl C61 butyric acid (PCBA 2), have been used to assemble a chemically and electrochemically responsive supramolecular ensemble. Addition of an organic base initiates self-assembly of the monomers via a molecular switching event. This results in the formation of materials that may be disaggregated via the addition of an organic acid or electrolysis.

Ion-Regulated Allosteric Binding of Fullerenes (C60 and C70) by Tetrathiafulvalene-Calix[4]pyrroles

The effect of ionic species on the binding of fullerenes (C60 and C70) by tetrathiafulvalene-calix[4]pyrrole (TTF-C4P) receptors and the nature of the resulting supramolecular complexes (TTF-C4P + fullerene + halide anion + tetraalkylammonium cation) was studied in the solid state through single crystal X-ray diffraction methods and in dichloromethane solution by means of continuous variation plots and UV-vis spectroscopic titrations. These analyses revealed a 1:1 stoichiometry between the anion-bound TTF-C4Ps and the complexed fullerenes. The latter guests are bound within the bowl-like cup of the C4P in a ball-and-socket binding mode. The interactions between the TTF-C4P receptors and the fullerene guests are highly influenced by both the nature of halide anions and their counter tetraalkylammonium cations. Three halides (F(-), Cl(-), and Br(-)) were studied. All three potentiate the binding of the two test fullerenes by inducing a conformational change from the 1,3-alternate to the cone conformer of the TTF-C4Ps, thus acting as positive heterotropic allosteric effectors. For a particular halide anion, the choice of tetraalkylammonium salts serves to modulate the strength of the TTF-C4P-fullerene host-guest binding interactions and, in conjunction with variations in the halide anion, can be exploited to alter the inherent selectivity of the host for a given fullerene. Differences in binding are reflected in the excited state optical properties. Overall, the present four-component system provides an illustration of how host-guest binding events involving appropriately designed artificial receptors can be fine-tuned via the addition of simple ionic species as allosteric modulators.

Comparative Electrochemical and Photophysical Studies of Tetrathiafulvalene‐Annulated Porphyrins and Their ZnII Complexes: The Effect of Metalation and Structural Variation

A series of tetrathiafulvalene (TTF)-annulated porphyrins, and their corresponding Zn(II) complexes, have been synthesized. Detailed electrochemical, photophysical, and theoretical studies reveal the effects of intramolecular charge-transfer transitions that originate from the TTF fragments to the macrocyclic core. The incremental synthetic addition of TTF moieties to the porphyrin core makes the species more susceptible to these charge-transfer (CT) effects as evidenced by spectroscopic studies. On the other hand, regular positive shifts in the reduction signals are seen in the square-wave voltammograms as the number of TTF subunits increases. Structural studies that involve the tetrakis-substituted TTF-porphyrin (both free-base and Zn(II) complex) reveal only modest deviations from planarity. The effect of TTF substitution is thus ascribed to electronic overlap between annulated TTF subunits rather than steric effects. The directly linked thiafulvalene subunits function as both π acceptors as well as σ donors. Whereas σ donation accounts for the substituent-dependent charge-transfer transitions, it is the π-acceptor nature of the appended tetrathiafulvalene groups that dominates the redox chemistry. Interactions between the subunits are also reflected in the square-wave voltammograms. In the case of the free-base derivatives that bear multiple TTF subunits, the neighboring TTF units, as well as the TTF(⋅+) generated through one-electron oxidation, can interact with each other; this gives rise to multiple signals in the square-wave voltammograms. On the other hand, after metalation, the electronic communication between the separate TTF moieties becomes restricted and they act as separate redox centers under conditions of oxidation. Thus only two signals, which correspond to TTF(⋅+) and TTF(2+), are observed. The reduction potentials are also seen to shift towards more negative values after metalation, a finding that is considered to reflect an increased HOMO-LUMO gap. To probe the excited-state dynamics and internal CT character, transient absorption spectral studies were performed. These analyses revealed that all the TTF-porphyrins of this study display relatively short excited-state lifetimes, which range from 1 to 20 ps. This reflects a very fast decay to the ground state and is consistent with the proposed intramolecular charge-transfer effects inferred from the ground-state studies. Complementary DFT calculations provide a mechanistic rationale for the electron flow within the TTF-porphyrins and support the proposed intramolecular charge-transfer interactions and π-acceptor effects.

Use of solvent to regulate the degree of polymerisation in weakly associated supramolecular oligomers

Using a tetrathiafulvalene functionalised calix[4]pyrrole (TTF-C[4]P; ) and alkyl diester-linked bis-dinitrophenols (), it was found that the solvent polarity and linker length have an effect on the molecular aggregation behaviour. 2D (1)H NOESY, DOSY NMR and UV-vis-NIR spectroscopic studies, as well as single crystal X-ray diffraction analyses support these conclusions.

Control over multiple molecular states with directional changes driven by molecular recognition

Recently, ligand–metal coordination, stimuli-responsive covalent bonds, and mechanically interlinked molecular constructs have been used to create systems with a large number of accessible structural states. However, accessing a multiplicity of states in sequence from more than one direction and doing so without the need for external energetic inputs remain as unmet challenges, as does the use of relatively weak noncovalent interactions to stabilize the underlying forms. Here we report a system based on a bispyridine-substituted calix[4]pyrrole that allows access to six different discrete states with directional control via the combined use of metal-based self-assembly and molecular recognition. Switching can be induced by the selective addition or removal of appropriately chosen ionic guests. No light or redox changes are required. The tunable nature of the system has been established through a combination of spectroscopic techniques and single crystal X-ray diffraction analyses. The findings illustrate a new approach to creating information-rich functional materials.Multistate molecular systems usually rely on external energy inputs to switch between states. Here, the authors show that a bispyridyl calixpyrrole system directed by only weak noncovalent interactions and metal coordination can access six discrete structural states, with directional and sequential control.

CCDC 988772: Experimental Crystal Structure Determination

Related Article: Steffen Bahring, Dong Sub Kim, Troels Duedal, Vincent M. Lynch, Kent A. Nielsen, Jan O. Jeppesen, Jonathan L. Sessler|2014|Chem.Commun.|50|5497|doi:10.1039/C4CC01514B