Signaling Recognition Events with Fluorescent Sensors and Switches.

From molecules to crystal engineering: supramolecular isomerism and polymorphism in network solids.

Self-Assembly of Discrete Cyclic Nanostructures Mediated by Transition Metals

Anion recognition chemistry has grown from its beginnings in the late 1960s with positively charged ammonium cryptand receptors for halide binding to, at the end of the millennium, a plethora of charged and neutral, cyclic and acyclic, inorganic and organic supramolecular host systems for the selective complexation, detection, and separation of anionic guest species. Solvation effects and pH values have been shown to play crucial roles in the overall anion recognition process. More recent developments include exciting advances in anion-templated syntheses and directed self-assembly, ion-pair recognition, and the function of anions in supramolecular catalysis.

Anion Recognition and Sensing: The State of the Art and Future Perspectives

Fluorogenic and chromogenic chemosensors and reagents for anions.

Rigid macrocycles based on cis bridging ligation of transition metals represent an unusual,'-4 and largely new,5 class of compounds having tremendous promise in host-guest, inclusion, and molecular recognition chemistry. Most of the available systems are (1) derived from pt(I1) or Pd(I1) triflate species, (2) tetranuclear with respect to metal content, and therefore, (3) arranged in square or box-like geometries, with metal atoms defining the comers. We report a simple, but potentially very useful, variant and extension of this theme: incorporation of visible-light-addressable, luminescent metal-ligand components within a square assembly. Induction of photoluminescent characteristics is particularly attractive in the context of eventual molecular sensing applications because it suggests an altemative to 'H NMR spectroscopy for detection of guest inclusion. Induction of luminescence also opens up the possibility of electronic excited state reactivity and possible manipulation of reactivity by encapsulated guests. The light-emitting square complex, 1; was prepared by combining the chromophore fac-Re(CO)F1(4,4'-bpy)2 (2)6 (105 mg, 0.170 "01) (4,4'-bpy = 4,4'-bipyridine) with Pd(dppp)(triflate)zId (139 mg, 0.170 m o l ) (dppp = 1,3-(diphenylphosphino)propane) in 50 mL of dry CH2C12. After 12 h of stirring, the pale yellow product was collected as a powder on a glass frit, washed with methylene chloride, and dried under vacuum. The yield was 83%. Elemental analysis data are consistent with the formation of either 1 or a larger assembly with 1:l Re:Pd stoichiometry.' FAB+ mass spectral measurements (m-NBA) yielded intense peaks at m/z = 2871.7 (2871.2 expected for l*H+), 2721.0 (1 triflate) and 2564.9 (1 triflate 4,4'bpy), thereby establishing the tetranuclear metal content of the compound. No peaks for compounds of higher mass (e.g., hexanuclear or octanuclear complexes) were found. FT-IR and 'H NMR measurements (supporting information) yielded additional data supportive of molecular square formation.* Figure 1 shows that the lowest energy transition in the electronic absorption spectrum of 2 is shifted substantially red by formation of the square. The shift is similar to that observed upon ligand protonation in 26 and is consistent with assignment

Luminescent Rhenium/Palladium Square Complex Exhibiting Excited State Intramolecular Electron Transfer Reactivity and Molecular Anion Sensing Characteristics

As a prelude to studies of 'externally facilitated' superexchange coupling in energy- and electron-transfer processes, a series of crown-linked donor-acceptor (D-A) complexes have been prepared. Photoexcitation at the rhenium-pyridyl center leads to intramolecular charge transfer across the crown spacer to the remote acceptor (nitrobenzene) with a rate constant of 6 [times] 10[sup 6] s[sup [minus]1]. Studies with a neutral analog show that simple cations can be encapsulated by the crown spacer.

Synthesis and preliminary photophysical studies of intramolecular electron transfer in crown-linked donor- (Chromophore-) acceptor complexes

: Dioxorhenium(V) tetrapyridyl species are currently under active investigation as model systems for interfacial two-electron, two-proton transfer reaction sequences We now find that the corresponding oxo, methoxo complexes can be prepared from dioxo species and methyl trifluoromethanesulfonate. The new complexes behave nearly identically to the analogous oxo, hydroxo complexes-with one important exception: CH3(+), unlike H(+), does not dissociate from the oxo ligand. As a direct consequence, the usually elusive rhenium oxidation state, IV, is stabilized with respect to redox disproportionation and is observable for several complexes at high pH. The ability to detect this state, in turn leads to: (1) direct access to the formal reduction potentials for the isolated 1e(-) redox couples comprising the overall two electron transfer (key information for understanding multi-ET kinetics) (2) elucidation of the profound structural and energetic consequences of the initial protonation (methylation) step in the dioxorhenium(V) reduction kinetics.

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High-Valent Oxo, Methoxorhenium Complexes: Models for Intermediates and Transition States in Proton-Coupled Multi-Electron Transfer Reactions

Ligand-bridged mixed-valence ions can exist in either valence-localized double-minimum ground potential energy surface or valence-delocalized (single-minimum ground PE surface) form The determining factor is the balance between twice the initial-state/final-state electroniccoupling energy (H[sub if]) and the sum of the zeroth-order reorganization or trapping energy ([chi]) and any redox asymmetry [delta]E For the Creutz-Taube ion (NH[sub 3])[sub 5[minus]] Ru[sup 11][sup 1/2](NH[sub 3])[sub 5][sup 5+], (1), redox asymmetry is absent, the valencies are completely delocalized, and 2H[sub if] evidently exceeds [epsilon] We reasoned, however, that [open quotes]electronic isomerization[close quotes] to a valence-localized form might be possible if significant redox asymmetry could be introduced We report here the introduction of the desired asymmetry in an unusual way: via binding of a macrocyclic ether to just one of the two available ruthenium ammine sites We also report optical evidence for the onset of valence localization in both 1 and a trans pyridine (py) substituted analog (2), in the presence of either dibenzo-36-crown-12(DB-36-C-12) or dibenzo-30-crown-10 (DB-30-C-10) species 17 refs, 3 figs

Perturbation of the electronic structure of the Creutz-Taube ion via asymmetric encapsulation with macrocyclic ether species

A luminescent complex, [Re(CO)3(Cl)(LL)] where LL is 1,11-bis(4-pyridylcarboxy)-3,6,9-trioxaundecane, was prepared, and the crystal structures of two solvates were determined. The flexible polyether chain of the chelating ligand adopts an all gauche conformation about the aliphatic C–C bonds, and does not appear to introduce any appreciable constraints on the geometry at the octahedral metal center. The cavity formed by the “crown ether” has a diameter of ca. 7 A. Binding of cations causes a decrease of the luminescence lifetime of the ReIILL– excited state, consistent with an “energy gap” effect and with a rapid excited-state equilibrium between bound and unbound moieties. Binding constants were determined for several alkali metal ions, and for the ammonium cation. The chemical affinity of the crown-functionalized ligand is not specific for any of the cations studied, but, instead, is similar for a range of cations. Comparative studies with a complex containing ester-functionalized pyridine ligands, but lacking the polyether linkage necessary to form the crown, also showed excited state binding of small cations, indicating that the anionic ester pyridyl fragment, rather than the polyether fragment, is responsible for the binding.

A luminescent tricarbonylchlororhenium(I) complex featuring a flexible “crown ether” ligand. Manipulation of photoexcited state properties via binding of small cations†

Dong I. Yoon

Papers

Luminescent Rhenium/Palladium Square Complex Exhibiting Excited State Intramolecular Electron Transfer Reactivity and Molecular Anion Sensing Characteristics

Synthesis and preliminary photophysical studies of intramolecular electron transfer in crown-linked donor- (Chromophore-) acceptor complexes

High-Valent Oxo, Methoxorhenium Complexes: Models for Intermediates and Transition States in Proton-Coupled Multi-Electron Transfer Reactions

Perturbation of the electronic structure of the Creutz-Taube ion via asymmetric encapsulation with macrocyclic ether species

A luminescent tricarbonylchlororhenium(I) complex featuring a flexible “crown ether” ligand. Manipulation of photoexcited state properties via binding of small cations†