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Principles Of Fluorescence Spectroscopy

Fascination with supramolecular chemistry over the last few decades has led to the synthesis of an ever-increasing number of elegant and intricate functional structures with sizes that approach nanoscopic dimensions Today, it has grown into a mature field of modern science whose interfaces with many disciplines have provided invaluable opportunities for crossing boundaries both inside and between the fields of chemistry, physics, and biology This chemistry is of continuing interest for synthetic chemists; partly because of the fascinating physical and chemical properties and the complex and varied aesthetically pleasing structures that supramolecules possess For scientists seeking to design novel molecular materials exhibiting unusual sensing, magnetic, optical, and catalytic properties, and for researchers investigating the structure and function of biomolecules, supramolecular chemistry provides limitless possibilities Thus, it transcends the traditional divisional boundaries of science and represents a highly interdisciplinary field

In the early 1960s, the discovery of ‘crown ethers’, ‘cryptands’ and ‘spherands’ by Pedersen,1 Lehn,2 and Cram3 respectively, led to the realization that small, complementary molecules can be made to recognize each other through non-covalent interactions such as hydrogen-bonding, charge-charge, donor-acceptor, π-π, van der Waals, etc Such ‘programmed’ molecules can thus be self-assembled by utilizing these interactions in a definite algorithm to form large supramolecules that have different physicochemical properties than those of the precursor building blocks Typical systems are designed such that the self-assembly process is kinetically reversible; the individual building blocks gradually funnel towards an ensemble that represents the thermodynamic minimum of the system via numerous association and dissociation steps By tuning various reaction parameters, the reaction equilibrium can be shifted towards the desired product As such, self-assembly has a distinct advantage over traditional, stepwise synthetic approaches when accessing large molecules

It is well known that nature has the ability to assemble relatively simple molecular precursors into extremely complex biomolecules, which are vital for life processes Nature’s building blocks possess specific functionalities in configurations that allow them to interact with one another in a deliberate manner Protein folding, nucleic acid assembly and tertiary structure, phospholipid membranes, ribosomes, microtubules, etc are but a selective, representative example of self-assembly in nature that is of critical importance for living organisms Nature makes use of a variety of weak, non-covalent interactions such as hydrogen–bonding, charge–charge, donor–acceptor, π-π, van der Waals, hydrophilic and hydrophobic, etc interactions to achieve these highly complex and often symmetrical architectures In fact, the existence of life is heavily dependent on these phenomena The aforementioned structures provide inspiration for chemists seeking to exploit the ‘weak interactions’ described above to make scaffolds rivaling the complexity of natural systems

The breadth of supramolecular chemistry has progressively increased with the synthesis of numerous unique supramolecules each year Based on the interactions used in the assembly process, supramolecular chemistry can be broadly classified in to three main branches: i) those that utilize H-bonding motifs in the supramolecular architectures, ii) processes that primarily use other non-covalent interactions such as ion-ion, ion-dipole, π–π stacking, cation-π, van der Waals and hydrophobic interactions, and iii) those that employ strong and directional metal-ligand bonds for the assembly process However, as the scale and degree of complexity of desired molecules increases, the assembly of small molecular units into large, discrete supramolecules becomes an increasingly daunting task This has been due in large part to the inability to completely control the directionality of the weak forces employed in the first two classifications above Coordination-driven self-assembly, which defines the third approach, affords a greater control over the rational design of 2D and 3D architectures by capitalizing on the predictable nature of the metal-ligand coordination sphere and ligand lability to encode directionality Thus, this third strategy represents an alternative route to better execute the “bottom-up” synthetic strategy for designing molecules of desired dimensions, ranging from a few cubic angstroms to over a cubic nanometer For instance, a wide array of 2D systems: rhomboids, squares, rectangles, triangles, etc, and 3D systems: trigonal pyramids, trigonal prisms, cubes, cuboctahedra, double squares, adamantanoids, dodecahedra and a variety of other cages have been reported As in nature, inherent preferences for particular geometries and binding motifs are ‘encoded’ in certain molecules depending on the metals and functional groups present; these moieties help to control the way in which the building blocks assemble into well-defined, discrete supramolecules4

Since the early pioneering work by Lehn5 and Sauvage6 on the feasibility and usefulness of coordination-driven self-assembly in the formation of infinite helicates, grids, ladders, racks, knots, rings, catenanes, rotaxanes and related species,7 several groups - Stang,8 Raymond,9 Fujita,10 Mirkin,11 Cotton12 and others13,14 have independently developed and exploited novel coordination-based paradigms for the self-assembly of discrete metallacycles and metallacages with well-defined shapes and sizes In the last decade, the concepts and perspectives of coordination-driven self-assembly have been delineated and summarized in several insightful reviews covering various aspects of coordinationdriven self-assembly15 In the last decade, the use of this synthetic strategy has led to metallacages dubbed as “molecular flasks” by Fujita,16 and Raymond and Bergman,17 which due to their ability to encapsulate guest molecules, allowed for the observation of unique chemical phenomena and unusual reactions which cannot be achieved in the conventional gas, liquid or solid phases Furthermore, these assemblies found applications in supramolecular catalysis18,19 and as nanomaterials as developed by Hupp20 and others21,22

This review focuses on the journey of early coordination-driven self-assembly paradigms to more complex and discrete 2D and 3D supramolecular ensembles over the last decade We begin with a discussion of various approaches that have been developed by different groups to assemble finite supramolecular architectures The subsequent sections contain detailed discussions on the synthesis of discrete 2D and 3D systems, their functionalizations and applications

Supramolecular coordination: self-assembly of finite two- and three-dimensional ensembles.

2.5. Stabilization of IL Emulsions by Nanoparticles 2623 3. Hydrogenations in ILs 2623 3.1. Hydrogenation on IL-Stabilized Nanoparticles 2623 3.1.1. Hydrogenation of 1,3-Butadiene 2623 3.1.2. Hydrogenation of Alkenes and Arenes 2624 3.1.3. Hydrogenation of Ketones 2624 3.2. Homogeneous Catalytic Hydrogenation in ILs 2624 3.3. Hydrogenation of Functionalized ILs 2625 3.3.1. Selective Hydrogenation of Polymers 2625 3.4. Asymmetric Hydrogenations 2626 3.4.1. Enantioselective Hydrogenation 2626 3.5. Role of the ILs Purity in Hydrogenation Reactions 2628

Catalysis in ionic liquids

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

This review gives a survey on the latest most representative developments and progress concerning ionic liquids, from their fundamental properties to their applications in catalytic processes. It also highlights their emerging use for biomass treatment and transformation.

Ionic liquids and catalysis: Recent progress from knowledge to applications

On the basis of 7Li NMR studies the coordination mode of phenantroline (phen) and bipyridine (bipy) to Li+ ions was found to be [Li(phen)2] and [Li(bipy)2] in the weakly coordinating solvent nitromethane. A large chemical shift of the 7Li signal indicated a strong interaction between the ligand and the metal center. The related sp2-hybridized N-donor ligand 2,2′-bis[(4S)-4-benzyl-2oxazoline] (biox) showed a negligible effect on the 7Li shift, suggesting that almost no interaction occurs between the Li+ center and biox as compared to Li+ and the solvent γ-butyrolactone. Corresponding DFT (RB3LYP/LANL2DZp) calculations have clearly indicated that the poor coordination of biox is not caused by steric effects but rather by the electronic nature of the heterocyclic biox system.

/pdf/ligand-exchange-processes-on-solvated-lithium-cations-vi-4rur27u8wg.pdf

Ligand Exchange Processes on Solvated Lithium Cations. VI. Determination of Coordination Numbers by Ligand Substitution and 7Li NMR

Host‐Guest Complexes of Dodeka(ethylene)octamine: Prediction of Ion Selectivity by Quantum Chemical Calculations IX

DFT-calculations (RB3LYP/LANL2DZp) show that the migration of a proton inside [1.1.1]- and [2.2.2]-cryptand from one nitrogen atom to the other follows different paths. While the proton in [H⊂.1.1.1]-cryptand moves via an ether oxygen atom (activation energy: 19.2 kcal/mol), the proton in [Hc2.2.2]-cryptand moves directly from one nitrogenatom to the other (activation energy: 16.1 kcal/mol). Our calculations rule out the application of doubly protonated [2.2.2]-cryptands as anion hosts.

Protons on Tour -DFT-study of H+-Migration in [1.1.1]-and [2.2.2]-Cryptands

https://chemistry-europe.onlinelibrary.wiley.com/doi/pdf/10.1002/chem.201290149

Cover Picture: Gutmann Donor and Acceptor Numbers for Ionic Liquids (Chem. Eur. J. 35/2012)

Based on DFT calculations (B3LYP/LANL2DZp) and the application of model equations, as well as a comparison of M-N-donor length with an unstrained reference ([M(solvent)6]n+), the selectivit...

Ralph Puchta

Papers

Ligand Exchange Processes on Solvated Lithium Cations. VI. Determination of Coordination Numbers by Ligand Substitution and 7Li NMR

Host‐Guest Complexes of Dodeka(ethylene)octamine: Prediction of Ion Selectivity by Quantum Chemical Calculations IX

Protons on Tour -DFT-study of H+-Migration in [1.1.1]-and [2.2.2]-Cryptands

Cover Picture: Gutmann Donor and Acceptor Numbers for Ionic Liquids (Chem. Eur. J. 35/2012)

Host-guest complexes of the Beer-Can-cryptand: prediction of ion selectivity by quantum chemical calculations XI