28 research outputs found
Enantioselective Component Selection in Multicomponent Supramolecular Gels
No full textWe investigate a
two-component acid–amine gelation system
in which chirality plays a vital role. A carboxylic acid based on
a second generation l-lysine dendron interacts with chiral
amines and subsequently assembles into supramolecular gel fibers.
The chirality of the amine controls the assembly of the resulting
diastereomeric complexes, even if this chirality is relatively “poor
quality”. Importantly, the selective incorporation of one enantiomer
of an amine over the other into the gel network has been demonstrated,
with the <i>R</i> amine that forms complexes which assemble
into the most stable gel being primarily selected for incorporation.
Thermodynamic control has been proven by forming a gel exclusively
with an <i>S</i> amine, allowing the <i>R</i> enantiomer
to diffuse through the gel network, and displacing it from the “solidlike”
fibers, demonstrating that these gels adapt and evolve in response
to chemical stimuli to which they are exposed. Excess amine, which
remains unincorporated within the solidlike gel fiber network, can
diffuse out and be reacted with an isocyanate, allowing us to quantify
the enantioselectivity of component selection but also demonstrating
how gels can act as selective reservoirs of potential reagents, releasing
them on demand to undergo further reactions; hence, component-selective
gel assembly can be coupled with controlled reactivity
Using EPR Spectroscopy as a Unique Probe of Molecular-Scale Reorganization and Solvation in Self-Assembled Gel-Phase Materials
No full textWe describe the synthesis of spin-labeled
bis-ureas which coassemble
with bis-urea gelators and report on self-assembly as detected using
electron paramagnetic resonance spectroscopy (EPR). Specifically,
EPR detects the gel–sol transition and allows us to quantify
how much spin-label is immobilized within the gel fibers and how much
is present in mobile solvent poolsî—¸as controlled by temperature,
gelator structure, and thermal history. EPR is also able to report
on the initial self-assembly processes below the gelation threshold
which are not macroscopically visible and appears to be more sensitive
than NMR to intermediate-sized nongelating oligomeric species. By
studying dilute solutions of gelator molecules and using either single
or double spin-labels, EPR allows quantification of the initial steps
of the hierarchical self-assembly process in terms of cooperativity
and association constant. Finally, EPR enables us to estimate the
degree of gel-fiber solvation by probing the distances between spin-labels.
Comparison of experimental data against the predicted distances assuming
the nanofibers are only composed of gelator molecules indicates a
significant difference, which can be assigned to the presence of a
quantifiable number of explicit solvent molecules. In summary, EPR
provides unique data and yields powerful insight into how molecular-scale
mobility and solvation impact on assembly of supramolecular gels
A General One-Step Synthesis of Alkanethiyl-Stabilized Gold Nanoparticles with Control over Core Size and Monolayer Functionality
No full textIn spite of widespread interest in the unique size-dependent
properties
and consequent applications of gold nanoparticles (AuNPs), synthetic
protocols that reliably allow for independent tuning of surface chemistry
and core size, the two critical determinants of AuNP properties, remain
limited. Often, core size is inherently affected by the ligand structure
in an unpredictable fashion. Functionalized ligands are commonly introduced
using postsynthesis exchange procedures, which can be inefficient
and operationally delicate. Here, we report a one-step protocol for
preparing monolayer-stabilized AuNPs that is compatible with a wide
range of ligand functional groups and also allows for the systematic
control of core size. In a single-phase reaction using the mild reducing
agent tert-butylamine borane, AuNPs that are compatible
with solvents spanning a wide range of polarities from toluene to
water can be produced without damaging reactive chemical functionalities
within the small-molecule surface-stabilizing ligands. We demonstrate
that the rate of reduction, which is easily controlled by adjusting
the period over which the reducing agent is added, is a simple parameter
that can be used irrespective of the ligand structure to adjust the
core size of AuNPs without broadening the size distribution. Core
sizes in the range of 2–10 nm can thus be generated. The upper
size limit appears to be determined by the nature of each specific
ligand/solvent pairing. This protocol produces high quality, functionally
sophisticated nanoparticles in a single step. By combining the ability
to vary size-related nanoparticle properties with the option to incorporate
reactive functional groups at the nanoparticle–solvent interface,
it is possible to generate chemically reactive colloidal building
blocks from which more complex nanoparticle-based devices and materials
may subsequently be constructed
Application of PAT Tools for the Safe and Reliable Production of a Dihydro-1<i>H</i>-imidazole
No full textThe application of two Process Analytical Technology (PAT) tools was studied and implemented for the safe and reliable synthesis of an advanced intermediate (<b>4<i>S</i>,5<i>R</i>-7</b>) of a member of the dihydro-1<i>H</i>-imidazole (<b>1</b>) class of compounds. Real time data were generated using ReactIR to track the complete breakdown of phosgene precursors (<b>2</b>) to phosgene (<b>3</b>) and confirm the absence of these hazardous materials prior to batch transfer operations. In addition, the chiral resolution by crystallization of <i><b>rac</b></i> <b>7</b> was monitored by a Lasentec FBRM probe-based system. Implementation of the latter helped to track the crystallization process to minimize the risk of cocrystallization of undesired isomer <b>4<i>R</i>,5<i>S</i>-7</b>
