The nitrile (Ctriple bondN) group is a powerful probe of structure and dynamics because its vibrational frequency is extraordinarily sensitive to the electrostatic and chemical characteristics of its local environment. For example, site-specific nitrile labels are useful indicators of protein structure because their infrared (IR) absorption spectra can clearly distinguish between solvent-exposed residues and residues buried in the hydrophobic core of a protein. In this work, three variants of the optimized quantum mechanics/molecular mechanics (OQM/MM) technique for computing Ctriple bondN vibrational frequencies were developed and assessed for acetonitrile in water. For the most robust variant, the transferability of the OQM/MM methodology to different solutes and solvents was evaluated by simulating the IR absorption spectra of para-tolunitrile in water and tetrahydrofuran and comparing to experiment and density functional theory (DFT) calculations. The OQM/MM frequencies compared favorably to DFT for para-tolunitrile/water, and the calculated IR absorption spectra are in qualitative agreement with experiment. This suggests that a single parametrization of the OQM/MM technique is reasonable for the calculation of nitrile line shapes when the probe is attached to different chemical moieties and when the label experiences local environments of different polarity.

Excitonic energy migration was studied using single molecule spectroscopy of individual conjugated polymer (CP) chains and aggregates. To probe the effect of interchain morphology on energy migration in CP, tailored interchain morphologies were achieved using solvent vapor annealing to construct polymer aggregates, which were then studied with single aggregate spectroscopy. We report that highly ordered interchain packing in regioregular poly(3-hexylthiophene) (rr-P3HT) enables long-range interchain energy migration, while disordered packing in regiorandom poly(3-hexylthiophene) (rra-P3HT), even in aggregates of just a few chains, can dramatically impede the interchain mechanism. In contrast to rr-P3HT, interchain energy migration in poly(3-(2'-methoxy-5'-octylphenyl)thiophene) (POMeOPT), a polythiophene derivative with bulky side chains, can be completely inhibited. We use simulated structures to show that the reduction in interchain coupling is not due simply to increased packing distance between backbones of different chains, but reflects inhibition of stacking due to side-chain-induced twisting of the contours of individual chains. A competition from intrachain coupling has also been demonstrated by comparing POMeOPT aggregates with different polymer chain sizes.

The spatial arrangement of the side chains of conjugated polymer backbones has critical effects on the morphology and electronic and photophysical properties of the corresponding bulk films. The effect of the side-chain-distribution density on the conformation at the isolated single-polymer-chain level was investigated with regiorandom (rra-) poly(3-hexylthiophene) (P3HT) and poly(3-hexyl-2,5-thienylene vinylene) (P3HTV). Although pure P3HTV films are known to have low fluorescence quantum efficiencies, we observed a considerable increase in fluorescence intensity by dispersing P3HTV in poly(methyl methacrylate) (PMMA), which enabled a single-molecule spectroscopy investigation. With single-molecule fluorescence excitation polarization spectroscopy, we found that rra-P3HTV single molecules form highly ordered conformations. In contrast, rra-P3HT single molecules, display a wide variety of different conformations from isotropic to highly ordered, were observed. The experimental results are supported by extensive molecular dynamics simulations, which reveal that the reduced side-chain-distribution density, that is, the spaced-out side-chain substitution pattern, in rra-P3HTV favors more ordered conformations compared to rra-P3HT. Our results demonstrate that the distribution of side chains strongly affects the polymer-chain conformation, even at the single-molecule level, an aspect that has important implications when interpreting the macroscopic interchain packing structure exhibited by bulk polymer films.

Fluorination represents an important strategy in developing highperformance conjugated polymers for photovoltaic applications. Here, we use regioregular poly(3-ethylhexylthiophene) (P3EHT) and poly(3-ethylhexyl-4-fluorothiophene) (F-P3EHT) as simplified model materials, using single-molecule/aggregate spectroscopy and molecular dynamic simulations, to elucidate the impacts of backbone fluorination on morphology and excitonic coupling on the molecular scale. Despite its high regioregularity, regioregular P3EHT exhibits a rather broad distribution in polymer chain conformation due to the strong steric hindrance of bulky ethylhexyl side chains. This conformational variability results in disordered interchain morphology even between a few chains, prohibiting long-range effective interchain coupling. In stark contrast, the experimental and molecular dynamic calculations reveal that backbone fluorination of F-P3EHT leads to an extended rod-like single-chain conformation and hence highly ordered interchain packing in aggregates. Surprisingly, the ordered and close interchain packing in F-P3EHT does not lead to strong excitonic coupling between the chains but rather to dominant intrachain excitonic coupling that greatly reduces the molecular energetic heterogeneity.fluorinated polythiophenes | photophysics | single-molecule spectroscopy | morphology | organic electronics M orphology and excitonic coupling are among the most important factors dictating the functions and performance of conjugated polymers (CPs) in a variety of optoelectronic applications (1-4). To tune and optimize the morphological and optoelectronic properties of CPs, chemical structure modification of the backbone and side chains is one of the main and most effective approaches (5-7). It has been revealed that the position, size, and length of side chains dramatically affect the morphology and optoelectronic properties of CPs. In the efforts to develop high-performance photovoltaic CPs, the introduction of fluorine atoms onto aromatic comonomers, i.e., the fluorination of polymer backbone, offers a very appealing strategy to tune the electronic properties, morphology, and photochemical stability (6-8). It is proposed that the inherent electronwithdrawing nature of fluorine atoms lowers the highest energy occupied molecular orbital energy levels, thereby increasing the open-circuit voltage in photovoltaics. In addition, the strong electronegativity might induce F-S and F-H interactions and potentially modifies the molecular conformation and intermolecular organization (6). Furthermore, it has been shown that fluorination often leads to enhanced thermal and oxidative stability (9-11). Given a combination of these desirable properties, fluorine-containing CPs have led to most of the best-performing polymer-based solar cells to date (12-15).For typical CPs, the primarily created exciton is Coulombically bound upon photoexcitation mainly due to the low dielectric constant and strong electron-phonon interaction in polymers. The interchain and intrachain morp...

Recent experiments have reported that the self-assembly of conjugated polymers mimicking rod-coil-rod triblock copolymers (BCPs) in selective solvents exhibits unique aggregate morphologies. However, the nature of the arrangement of the polymers within the aggregates and the spatial organization of the aggregates remain an unresolved issue. We report the results of coarse-grained Langevin dynamics simulations, which investigated the self-assembly behavior of rod-coil-rod BCPs in a coil-selective solvent. We observe a rapid formation of cylindrically shaped multichain clusters. The initial stages of formation of the aggregates was seen to be independent of the strength of the solvent selectivity. However, for higher solvent selectivities, the clusters were seen to merge into larger units at later stages. A reduction in rod to coil block ratio was observed to decrease the size and number of clusters. In the limit of a highly concentrated solution, we observe the formation of a networked system of distinct clusters, which however retain the cylindrical arrangement observed at lower polymer concentrations.

It is well-known that the transport properties of monolayer graphene are degraded by charged impurities present between graphene and either a given substrate or air. Such impurities cause charge scattering of holes and electrons in graphene. In previous work, our group has used both fluoropolymer thin films and polar vapor molecules to dramatically improve graphene field-effect transistor (FET) device characteristics, including Dirac voltage and mobility. We attributed the graphene device improvements to mitigation of charged impurities and defects due to electrostatic interaction with the dipoles of the applied fluoropolymers and polar molecules. In this work, we present theoretical support to this hypothesis, in the form of computational chemical simulations involving the interaction of polar molecules and impurities on a graphene sheet. We examine two types of impurities which may occur at graphene interfaces: ionic impurities and molecular dipole impurities. Upon introduction of polar vapor molecules to an impurity/graphene system, we observed a dramatic reduction in the electrostatic potential in the plane of the graphene from the impurity. The magnitude of potential reduction scales with the average dipole moment of each polar molecule. We were able to determine two separate mechanisms which contribute to the total potential reduction, impurity displacement, and electrostatic screening of the impurity. The respective impacts of the mechanisms vary with distance from the impurity. Additionally, in the case of the molecular dipole impurity, the orientation of the impurity atop graphene is a key factor that determines the potential impact.