Dongwhan Lee

Conjugated Polymer Sensors Built on π-Extended Borasiloxane Cages

An efficient 2 + 2 cyclocondensation with dihydroxysilane converted simple arylboronic acids to bifunctional borasiloxane cage molecules, which were subsequently electropolymerized to furnish air-stable thin films. The extended [p,pi]-conjugation that defines the rigid backbone of this new conjugated polymer (CP) motif gives rise to longer-wavelength UV-vis transitions upon oxidative doping, the spectral window and intensity of which can be modified by interaction with Lewis basic reagents. Notably, this boron-containing CP undergoes a rapid and reversible color change from green to orange upon exposure to volatile amine samples under ambient conditions. This direct naked-eye detection scheme can best be explained by invoking the reversible B-N dative bond formation that profoundly influences the p-pi* orbital overlap.

Torsionally Responsive C3-Symmetric Azo Dyes: Azo−Hydrazone Tautomerism, Conformational Switching, and Application for Chemical Sensing

An efficient triple azo coupling reaction between anilines and phloroglucinol furnished a series of C(3)-symmetric molecules 7-9 supporting multiple conjugation pathways that converge at the molecular core. A combination of (1)H/(13)C NMR spectroscopy, X-ray crystallography, and density functional theory computational studies provided a coherent picture of the [n,pi]-conjugated molecular core, which is best described as the tris(hydrazone) [rather than tris(azo)] tautomer stabilized by resonance-assisted hydrogen bonding. For a homologous series of compounds, an increase in the torsional angles between the planar molecular core and the peripheral aryl groups results in a systematic blue shift in the low-energy electronic transitions (7, 523 nm; 8, 505 nm; 9, 445 nm in CHCl(3)) that qualitatively correlates with the shrinkage of effective conjugation through structural distortion. Similar spectral shifts could also be induced by amine substrates that interact with the intramolecular hydrogen-bonding network to trigger bond-twisting motions. Specifically, a brief exposure of a thin film of 7 to vapor samples of butyl-, hexyl-, diethyl-, and diisopropylamine resulted in a rapid and reversible color change from pink to dark-orange. Under similar conditions, however, triethylamine did not elicit any detectable color change, despite the fact that it has a significantly higher vapor pressure than n-hexylamine. These findings implicate that the hydrogen-bonding donor ability is a key requirement for the binding-induced conformational switching, which allows for direct naked-eye detection of volatile amines under ambient conditions.

Layer-by-layer synthesis of metal-containing conducting polymers: caged metal centers for interlayer charge transport.

Metal-templated [2 + 3]-type cocondensation of a pi-extended boronic acid and nioxime furnished a series of cage molecules, which were electropolymerized to prepare metal-containing conducting polymers (MCPs). Despite sharing essentially isostructural organic scaffolds, these materials display metal-dependent electrochemical properties as evidenced by different redox windows observed for M = Co, Fe, Ru. Consecutive electropolymerization using two different monomers furnished bilayer MCPs having different metals in each layer. In addition to functioning as heavy atom markers in cross-sectional analysis by FIB and EDX, redox-active metal centers participate in voltage-dependent interlayer electron transport to give rise to cyclic voltammograms that are distinctively different from those of each layer alone or random copolymers. A simple electrochemical technique can thus be used as a straightforward diagnostic tool to investigate the structural ordering of electrically conductive layered materials.

Iron Complexes of Dendrimer-Appended Carboxylates for Activating Dioxygen and Oxidizing Hydrocarbons

The active sites of metalloenzymes are often deeply buried inside a hydrophobic protein sheath, which protects them from undesirable hydrolysis and polymerization reactions, allowing them to achieve their normal functions. In order to mimic the hydrophobic environment of the active sites in bacterial monooxygenases, diiron(II) compounds of the general formula [Fe2([G-3]COO)4(4-RPy)2] were prepared, where [G-3]COO- is a third-generation dendrimer-appended terphenyl carboxylate ligand and 4-RPy is a pyridine derivative. The dendrimer environment provides excellent protection for the diiron center, reducing its reactivity toward dioxygen by about 300-fold compared with analogous complexes of terphenyl carboxylate ([G-1]COO-) ligands. An FeIIFeIII intermediate was characterized by electronic, electron paramagnetic resonance, Mössbauer, and X-ray absorption spectroscopic analyses following the oxygenation of [Fe2([G-3]COO)4(4-PPy)2], where 4-PPy is 4-pyrrolidinopyridine. The results are consistent with the formation of a superoxo species. This diiron compound, in the presence of dioxygen, can oxidize external substrates.

Dendritic molecular switch: chiral folding and helicity inversion.

Appropriately designed chemical architectures can fold to adopt well-defined secondary structures without the need for structural motifs of biological origin. We have designed tris(N-salicylideneaniline)-based hyperbranched molecules that spontaneously collapse to compact three-blade propeller geometry of either (P)- or (M)-handedness. For a homologous series of compounds, a direct correlation was established between the absolute screw sense, either (P)- or (M)-, of this helical folding and the absolute configuration, either (R)- or (S)-, of the chiral alcohol groups introducing local asymmetric bias to the conformationally restricted molecular backbone. 1H NMR and CD spectroscopic studies provided significant insights into structural folding and unfolding of these chiral molecules in solution, which proceed via reversible assembly and disassembly of the C3-symmetric hydrogen-bonding network. Notably, solvents profoundly influenced this dynamic process. A strong correlation between the solvent donor number (DN) or solvent basicity (SB) parameters and the change in the Cotton effects pointed toward specific O-H...solvent interactions that drive structural unfolding and eventual refolding to apparently opposite helicity. This unusual chirality inversion process could also be induced by installation of chemical protecting groups that simulate specific solvent-solute interactions. Removal of this covalent mimic of the solvent shell restored the original screw sense of the parent molecule, thus establishing the feasibility of covalently triggered helicity inversion as a new mode of operation for chiroptical molecular switches.

Fluorescence Switching of Imidazo[1,5-a]pyridinium Ions: pH-Sensors with Dual Emission Pathways

Imidazo[1,5-a]pyridinium ions are identified as highly emissive and water-soluble fluorophores accessed by an efficient three-component coupling reaction. Synthetic modifications of groups conjugated to the polyheterocyclic core are shown to profoundly impact the emission properties of these molecules. Notably, two structural isomers of functionalized imidazo[1,5-a]pyridinium ions were found to exhibit distinct de-excitation pathways, which are responsible for either a fluorescence turn-on or ratiometric response to pH change.

Estimation of breeding values for mean and dispersion, their variance and correlation using double hierarchical generalized linear models

The possibility of breeding for uniform individuals by selecting animals expressing a small response to environment has been studied extensively in animal breeding. Bayesian methods for fitting models with genetic components in the residual variance have been developed for this purpose, but have limitations due to the computational demands. We use the hierarchical (h)-likelihood from the theory of double hierarchical generalized linear models (DHGLM) to derive an estimation algorithm that is computationally feasible for large datasets. Random effects for both the mean and residual variance parts of the model are estimated together with their variance/covariance components. An important feature of the algorithm is that it can fit a correlation between the random effects for mean and variance. An h-likelihood estimator is implemented in the R software and an iterative reweighted least square (IRWLS) approximation of the h-likelihood is implemented using ASReml. The difference in variance component estimates between the two implementations is investigated, as well as the potential bias of the methods, using simulations. IRWLS gives the same results as h-likelihood in simple cases with no severe indication of bias. For more complex cases, only IRWLS could be used, and bias did appear. The IRWLS is applied on the pig litter size data previously analysed by Sorensen & Waagepetersen (2003) using Bayesian methodology. The estimates we obtained by using IRWLS are similar to theirs, with the estimated correlation between the random genetic effects being -0·52 for IRWLS and -0·62 in Sorensen & Waagepetersen (2003).

Copper clusters built on bulky amidinate ligands: spin delocalization via superexchange rather than direct metal-metal bonding.

Entry into a new class of tetra- and dicopper clusters was assisted by a fine steric tuning of bulky amidinate ligands that provide spin-delocalizing superexchange pathways in class III mixed-valence clusters, the properties of which are best understood without invoking metal-metal bonding.

Epoxidation of an alkene promoted by various nickel(II) multiaza macrocyclic complexes

Abstract The epoxidation of trans-β-methylstyrene promoted by various Ni(II) complexes of macrocyclic ligands (cyclam and 1–5) using PhIO as a terminal oxidant has been investigated. In terms of the rate of epoxide formation, the complexes of monocyclic ligands (cyclam, 1 and 2) are better catalysts than those of polycyclic ligands (3–5) and the cyclam complex without pendant arms is better catalyst than those (1 and 2) with pendant arms. However, a series of the complexes show remarkably similar reactivity in the transfer of oxygen from active high-valent intermediate to the alkene and they provide nearly the same final yield in certain reaction conditions. Therefore, the yield of epoxide produced in a given period depends mainly on the rate of reaction of the complex with PhIO, which is greatly affected by the ligand structure. In order to become a better catalyst, the complex should have low Ni(II)/Ni(III) oxidation potential and the macrocyclic ligand should exert less steric hindrance around the Ni(II) center to allow easy axial approach of the oxidant.

Molecular engineering of two-dimensional π-conjugation: expected and unexpected photophysical consequences of a simple particle-in-a-box approach

A highly modular and convergent synthetic route was devised to construct a series of planar π-conjugated molecules with systematically varied structural dimensions and electronic characteristics. High-yielding triple Schiff base condensation reactions between π-extended bulky anilines and 1,3,5-triformylphloroglucinol furnished a series of pseudo C3-symmetric tris(N-salicylideneamine)s displaying intense absorptions at λmax = 445–475 nm and emissions at λmax = 470–504 nm. X-Ray crystallographic studies revealed that intricate hydrogen-bonding networks sustain the planar conjugation of these discotic molecules, the HOMO–LUMO gaps of which decrease with increasing conjugation area. This reduction in excitation energy is accompanied by a nearly 4-fold enhancement in emission quantum yield (ΦF). Past a structural threshold, however, increasing conjugation area leads to either (i) decrease in ΦF or (ii) development of localized electronic transitions. These findings provide a well-defined structural window for future elaboration of this emerging family of dynamic 2-D conjugation, the luminescence properties of which have already been shown to reversibly change in response to external stimuli.