6 research outputs found
Cooperativity Scale: A Structure–Mechanism Correlation in the Self-Assembly of Benzene-1,3,5-tricarboxamides
ConspectusThe self-assembly of small and well-defined
molecules using noncovalent interactions to generate various nano-
and microarchitectures has been extensively studied. Among various
architectures, one-dimensional (1-D) nano-objects have garnered significant
attention. It has become increasingly evident that a cooperative or
nucleation–elongation mechanism of polymerization leads to
highly ordered 1-D supramolecular polymers, analogous to shape-persistent
biopolymers such as actin. With this in mind, achieving cooperativity
in self-assembled structures has been actively pursued with significant
success. Only recently, researchers are focusing on the origin of
the mechanism at the molecular level in different synthetic systems.
Taking a step further, a thorough quantitative structure–mechanism
correlation is crucial to control the size, shape, and functions of
supramolecular polymers, and this is currently lacking in the literature.Among a plethora of molecules, benzene-1,3,5-tricarboxamides (BTAs)
provide a unique combination of important noncovalent interactions such as hydrogen bonding, π-stacking, and hydrophobic interactions,
for self-assembly and synthetic ease. Due to the latter, a diverse
range of BTA derivatives with all possible structural mutations have
been synthesized and studied during the past decade, mainly from our
group. With such a large body of experimental results on BTA self-assembly,
it is time to embark on a structure–mechanism correlation in
this family of molecules, and a first step toward this will form the
main focus of this Account. The origin of the cooperative mechanism
of self-assembly in BTAs has been ascribed to 3-fold intermolecular
hydrogen bonding (HB) between monomers based on density-functional
theory (DFT) calculations. The intermolecular hydrogen-bonding interaction
forms the central premise of this work, in which we evaluate the effect
of different moieties such as alkyl chains, and amino
acids, attached to the core amides on the strength of intermolecular
HB, which consequently governs the extent of cooperativity (quantified
by the cooperativity factor, σ). In addition to this, we evaluate
the effect of amide connectivity (C- vs N-centered), the role of solvents,
amides vs thioamides, and finally the influence of the benzene vs
cyclohexane core on the σ. Remarkably, every subtle structural
change in the BTA monomer seems to affect the cooperativity factor
in a systematic and rationalizable way.The take home message
will be that the cooperativity factor (σ) in the BTA family
forms a continuous spectrum from 1 (isodesmic) to <10<sup>–6</sup> (highly cooperative) and it can be tuned based on the appropriate
modification of the BTA monomer. We anticipate that these correlations
drawn from the BTA series will be applicable to other systems in which
HB is the main driving force for cooperativity. Thus, the understanding
gained from such correlations on a prototypical self-assembling motif
such as BTA will aid in designing more complex systems with distinct
functions
Amplifying Chiroptical Properties of Conjugated Polymer Thin-Film Using an Achiral Additive
Chiral conjugated polymers bearing
enantiopure side chains offer
the possibility to harness the effect of chirality in organic electronic
devices. However, its use is hampered by the low degree of circular
polarization in absorption (<i>g</i><sub>abs</sub>) in most
of the conjugated polymer thin-films studied. Here we demonstrate
a versatile method to significantly increase the <i>g</i><sub>abs</sub> by using a few weight percentages of a commercially
available achiral long-chain alcohol as an additive. This additive
enhances the chiroptical properties in both absorption and emission
by ca. 5–10 times in the thin-films. We envisage that the alcohol
additive acts as a plasticizer which enhances the long-range chiral
liquid crystalline ordering of the polymer chains, thereby amplifying
the chiroptical properties in the thin-film. The application of this
methodology to various conjugated polymers has been demonstrated
Perylene Based Porous Polyimides: Tunable, High Surface Area with Tetrahedral and Pyramidal Monomers
Perylene Based Porous
Polyimides: Tunable, High Surface Area with Tetrahedral and Pyramidal
Monomer
Dipole-Moment-Driven Cooperative Supramolecular Polymerization
While the mechanism of self-assembly
of π-conjugated molecules
has been well studied to gain control over the structure and functionality
of supramolecular polymers, the intermolecular interactions underpinning
it are poorly understood. Here, we study the mechanism of self-assembly
of perylene bisimide derivatives possessing dipolar carbonate groups
as linkers. It was observed that the combination of carbonate linkers
and cholesterol/dihydrocholesterol self-assembling moieties led to
a cooperative mechanism of self-assembly. Atomistic molecular dynamics
simulations of an assembly in explicit solvent strongly suggest that
the dipole–dipole interaction between the carbonate groups
imparts a macro-dipolar character to the assembly. This is confirmed
experimentally through the observation of a significant polarization
in the bulk phase for molecules following a cooperative mechanism.
The cooperativity is attributed to the presence of dipole–dipole
interaction in the assembly. Thus, anisotropic long-range intermolecular
interactions such as dipole–dipole interaction can serve as
a way to obtain cooperative self-assembly and aid in rationalizing
and predicting the mechanisms in various synthetic supramolecular
polymers
Solvent Clathrate Driven Dynamic Stereomutation of a Supramolecular Polymer with Molecular Pockets
Control over the
helical organization of synthetic supramolecular
systems is intensively pursued to manifest chirality in a wide range
of applications ranging from electron spin filters to artificial enzymes.
Typically, switching the helicity of supramolecular assemblies involves
external stimuli or kinetic traps. However, efforts to achieve helix
reversal under thermodynamic control and to understand the phenomena
at a molecular level are scarce. Here we present a unique example
of helix reversal (stereomutation) under thermodynamic control in
the self-assembly of a coronene bisimide that has a 3,5-dialkoxy substitution
on the imide phenyl groups (<b>CBI-35CH</b>), leading to “molecular
pockets” in the assembly. The stereomutation was observed only
if the CBI monomer possesses molecular pockets. Detailed chiroptical
studies performed in alkane solvents with different molecular structures
reveal that solvent molecules intercalate or form clathrates within
the molecular pockets of <b>CBI-35CH</b> at low temperature
(263 K), thereby triggering the stereomutation. The interplay among
the helical assembly, molecular pockets, and solvent molecules is
further unraveled by explicit solvent molecular dynamics simulations.
Our results demonstrate how the molecular design of self-assembling
building blocks can orchestrate the organization of surrounding solvent
molecules, which in turn dictates the helical organization of the
resulting supramolecular assembly
High Circular Polarization of Electroluminescence Achieved <i>via</i> Self-Assembly of a Light-Emitting Chiral Conjugated Polymer into Multidomain Cholesteric Films
We demonstrate a
facile route to obtain high and broad-band circular
polarization of electroluminescence in single-layer polymer OLEDs.
As a light-emitting material we use a donor–acceptor polyfluorene
with enantiomerically pure chiral side-chains. We show that upon thermal
annealing the polymer self-assembles into a multidomain cholesteric
film. By varying the thickness of the polymer emitting layer, we achieve
high levels of circular polarization of electroluminescence (up to
40% excess of right-handed polarization), which are the highest reported
for polymer OLEDs not using chiral dopants or alignment layers. Mueller
matrix ellipsometry shows strong optical anisotropies in the film,
indicating that the circular polarization of luminescence arises mainly
after the photon has been generated, through selective scattering
and birefringence correlated in the direction of the initial linear
polarization of the photon. Our work demonstrates that chirally substituted
conjugated polymers can combine photonic and semiconducting properties
in advanced optoelectronic devices