Right- and Left-Handed Helices, What is in between? Interconversion of Helical Structures of Alternating Pyridinedicarboxamide/<i>m</i>‑(phenylazo)azobenzene Oligomers

Some unnatural polymers/oligomers have been designed to adopt a well-defined, compact, three-dimensional folding capability. Azobenzene units are common linkages in these oligomer designs. Two alternating pyridinedicarboxamide/<i>m</i>-(phenylazo)­azobenzene oligomers that can fold into both right- and left-handed helices were studied computationally in order to understand their dynamical properties. Helical structures were shown to be the global minima among the many different conformations generated from the Monte Carlo simulations, and extended conformations have higher potential energies than compact ones. To understand the interconversion process between right- and left-handed helices, replica-exchange molecular dynamic (REMD) simulations were performed on both oligomers, and with this method, both right- and left-handed helices were successfully sampled during the simulations. REMD trajectories revealed twisted conformations as intermediate structures in the interconversion pathway between the two helical forms of these azobenzene oligomers. This mechanism was observed in both oligomers in current study and occurred locally in the larger oligomer. This discovery indicates that the interconversion between helical structures with different handedness goes through a compact and partially folded structure instead of globally unfold and extended structure. This is also verified by the nudged elastic band (NEB) calculations. The temperature weighted histogram analysis method (T-WHAM) was applied on the REMD results to generate contour maps of the potential of mean force (PMF). Analysis showed that right- and left-handed helices are equally sampled in these REMD simulations. In large oligomers, both right- and left-handed helices can be adopted by different parts of the molecule simultaneously. The interconversion between two helical forms can occur in the middle of the helical structure and not necessarily at the termini of the oligomer

Self-Assembly of a Donor–Acceptor Nanotube. A Strategy To Create Bicontinuous Arrays

The self-assembly of bolaamphiphile <b>1</b> into nanotubes containing a nanostructured electron donor/acceptor heterojunction is reported. In 10% MeOH/H₂O, the tetraphenylporphyrin (TPP) and 1,4,5,8-naphthalenetetracarboxylic acid diimide chromophores engage in strong <i>J</i>-type π–π interactions within monolayer rings that further stack into the nanotube assemblies. In 10% MeOH/H₂O at pH 1 or 11 or in pure MeOH, assembly is driven exclusively by the TPP ring, leading to the formation of nonspecific, unstructured aggregates. Steady-state, time-resolved fluorescence and femtosecond transient absorption spectroscopy revealed a strong dependence of the fluorescence decay and electron-transfer/charge-recombination time constants on the nature of the assemblies. These studies highlight the importance of local nanostructure in determining the photophysical properties of optoelectronic materials

Strategy for the Co-Assembly of Co-Axial Nanotube–Polymer Hybrids

Nanostructured materials having multiple, discrete domains of sorted components are particularly important to create efficient optoelectronics. The construction of multicomponent nanostructures from self-assembled components is exceptionally challenging due to the propensity of noncovalent materials to undergo structural reorganization in the presence of excipient polymers. This work demonstrates that polymer–nanotube composites comprised of a self-assembled nanotube wrapped with two conjugated polymers could be assembled using a layer-by-layer approach. The polymer–nanotube nanostructures arrange polymer layers coaxially on the nanotube surface. Femtosecond transient absorption (TA) studies indicated that the polymer–nanotube composites undergo photoinduced charge separation upon excitation of the NDI chromophore within the nanotube