Pendant Group Functionalization of Cyclic Olefin for High Temperature and High-Density Energy Storage.

High-temperature flexible polymer dielectrics are critical for high density energy storage and conversion. The need to simultaneously possess a high bandgap, dielectric constant and glass transition temperature forms a substantial design challenge for novel dielectric polymers. Here, by varying halogen substituents of an aromatic pendant hanging off a bicyclic mainchain polymer, a class of high-temperature olefins with adjustable thermal stability are obtained, all with uncompromised large bandgaps. Halogens substitution of the pendant groups at para or ortho position of polyoxanorborneneimides (PONB) imparts it with tunable high glass transition from 220 to 245 °C, while with high breakdown strength of 625-800 MV/m. A high energy density of 7.1 J/cc at 200 °C is achieved with p-POClNB, representing the highest energy density reported among homo-polymers. Molecular dynamic simulations and ultrafast infrared spectroscopy are used to probe the free volume element distribution and chain relaxations pertinent to dielectric thermal properties. An increase in free volume element is observed with the change in the pendant group from fluorine to bromine at the para position; however, smaller free volume element is observed for the same pendant when at the ortho position due to steric hindrance. With the dielectric constant and bandgap remaining stable, properly designing the pendant groups of PONB boosts its thermal stability for high density electrification.

Influence of Internal Bond Rotation on Ultrafast IR Anisotropy Measurements and the Internal Rotational Potential.

Measurement of molecular orientation relaxation using ultrafast infrared (IR) pump-probe experiments is widely used to understand the properties of liquids and other systems. In the simplest situation, the anisotropy decay is a single exponential reflecting diffusive orientational relaxation. However, the anisotropy decay is frequently biexponential. The faster component is caused by solvent caging restricting angular sampling until constraint release permits all angles to be sampled. Here, we describe another mechanism that limits the range of sampling, i.e., sampling of a restricted range of angles via internal bond reorientation on a rotational potential surface with barriers. If the internal angular sampling occurs faster than the entire molecule's diffusive orientational relaxation, it will produce a fast component of anisotropy decay with a cone angle determined by the shape of the internal rotation potential. We studied four molecules to illustrate the effects of internal bond rotations on anisotropy decay. The molecules are p-chlorobenzonitrile, phenylselenocyanate, phenylthiocyanate, and 2-nitrophenylselenocyante in the solvent N,N-dimethylformamide. The CN stretch is used as the IR chromophore. p-Chlorobenzonitrile does not have internal rotation; its anisotropy decays as a single exponential. The other three have bent geometries and internal rotation of the moieties containing the CN occurs; the anisotropies decay as biexponentials. The faster of the two decays can be understood in terms of motions on the rotational potential surface. A method is developed for extracting the intramolecular rotational potential surface by employing a modification of the harmonic cone model, and the results are compared to density functional theory calculations.

Rethinking Vibrational Stark Spectroscopy: Peak Shifts, Line Widths, and the Role of Non-Stark Solvent Coupling.

A vibration's transition frequency is partly determined by the first-order Stark effect, which accounts for the electric field experienced by the mode. Using ultrafast infrared pump-probe and FT-IR spectroscopies, we characterized both the 0 → 1 and 1 → 2 vibrational transitions' field-dependent peak positions and line widths of the CN stretching mode of benzonitrile (BZN) and phenyl selenocyanate (PhSeCN) in ten solvents. We present a theoretical model that decomposes the observed line width into a field-dependent Stark contribution and a field-independent non-Stark solvent coupling contribution (NSC). The model demonstrates that the field-dependent peak position is independent of the line width, even when the NSC dominates the latter. Experiments show that when the Stark tuning rate is large compared to the NSC (PhSeCN), the line width has a field dependence, albeit with major NSC-induced excursions from linearity. When the Stark tuning rate is small relative to the NSC (BZN), the line width is field-independent. BZN's line widths are substantially larger for the 1 → 2 transition, indicating a 1 → 2 transition enhancement of the NSC. Additionally, we examine, theoretically and experimentally, the difference in the 0 → 1 and 1 → 2 transitions' Stark tuning rates. Second-order perturbation theory combined with density functional theory explain the difference and show that the 1 → 2 transition's Stark tuning rate is ∼10% larger. The Stark tuning rate of PhSeCN is larger than BZN's for both transitions, consistent with the theoretical calculations. This study provides new insights into vibrational line shape components and a more general understanding of the vibrational response to external electric fields.

Ultrafast excited state dynamics of silver ion-mediated cytosine-cytosine base pairs in metallo-DNA.

To better understand the nexus between structure and photophysics in metallo-DNA assemblies, the parallel-stranded duplex formed by the all-cytosine oligonucleotide, dC20, and silver nitrate was studied by circular dichroism (CD), femtosecond transient absorption spectroscopy, and time-dependent-density functional theory calculations. Silver(I) ions mediate Cytosine-Cytosine (CC) base pairs by coordinating to the N3 atoms of two cytosines. Although these silver(I) mediated CC base pairs resemble the proton-mediated CC base pairs found in i-motif DNA at first glance, a comparison of experimental and calculated CD spectra reveals that silver ion-mediated i-motif structures do not form. Instead, the parallel-stranded duplex formed between dC20 and silver ions is proposed to contain consecutive silver-mediated base pairs with high propeller twist-like ones seen in a recent crystal structure of an emissive, DNA-templated silver cluster. Femtosecond transient absorption measurements with broadband probing from the near UV to the near IR reveal an unusually long-lived (>10 ns) excited state in the dC20 silver ion complex that is not seen in dC20 in single-stranded or i-motif forms. This state is also absent in a concentrated solution of cytosine-silver ion complexes that are thought to assemble into planar ribbons or sheets that lack stacked silver(I) mediated CC base pairs. The large propeller twist angle present in metal-mediated base pairs may promote the formation of long-lived charged separated or triplet states in this metallo-DNA.

DNA-like Photophysics in Self-Assembled Silver(I)-Nucleobase Nanofibers.

Supramolecular assemblies form when silver nitrate is added to an aqueous solution of adenine (Ade) or 2-aminopurine (2AP) in a 2:1 mole ratio. Atomic force microscopy images reveal nanofibers that are ∼30 nm in diameter and micrometers in length in the dried film formed from a room-temperature solution. Femtosecond broadband transient absorption spectroscopy was used to investigate the dynamics of excited states formed by UV excitation of the nanofibers in room-temperature aqueous solutions in an effort to learn how nonradiative decay pathways of the uncomplexed nucleobases are altered in the silver-ion-mediated assemblies. The changes in the spectroscopy and dynamics of Ade and 2AP upon forming nanofibers with silver ions closely parallel the ones seen when these bases are organized into DNA strands. The similarities strongly suggest that these structures feature extensive π-π stacking interactions between nucleobases. The results show that time-resolved spectroscopy combined with growing understanding of the photophysics of DNA strands can deliver new insights into the properties of metal-nucleobase nanoassemblies.