The aggregation of π-conjugated materials significantly impacts the photophysics and performance of optoelectronic devices. Nevertheless, little is known about the laws governing aggregate formation of π-conjugated materials from solution. In this Perspective, we compare, discuss, and summarize how aggregates form for three different types of compounds, that is, homopolymers, donor–acceptor type polymers, and low molecular weight compounds. To this end, we employ temperature-dependent optical spectroscopy, which is a simple yet powerful tool to investigate aggregate formation. We show how optical spectra can be analyzed to identify distinct conformational states. We find aggregate formation to proceed the same in all these compounds by a coil-to-globule-like first-order phase transition. Notably, the chain expands before it collapses into a highly ordered dense state. The role of side chains and the impact of changes in environmental polarization are addressed.

Temperature Induced Order–Disorder Transition in Solutions of Conjugated Polymers Probed by Optical Spectroscopy

The nonlinear properties and the photophysical behavior of two π-conjugated chromophores that incorporate an electron-deficient pyrimidine core (A) and γ-methylenepyrans as terminal donor (D) groups have been thoroughly investigated. Both dipolar and quadrupolar branching strategies are explored and rationalized on the basis of the Frenkel exciton model. Even though a cooperative effect is clearly observed if the dimensionality is increased, the nonlinear optical (NLO) response of this series is moderate if one considers the nature of the D/A couple and the size of the chromophores (as measured by the number of π electrons). This effect was attributed to a disruption in the electronic conjugation within the dyes' scaffold for which the geometry deviates from planarity owing to a noticeable twisting of the pyranylidene end-groups. This latter structural parameter also has a strong influence on the excited-state dynamics, which leads to a very efficient fluorescence quenching.

https://hal.archives-ouvertes.fr/hal-00867587/document

Synthesis, Photophysics and Nonlinear Optical Properties of Stilbenoid Pyrimidine-Based Dyes Bearing Methylenepyran Donor Groups

We have measured the dielectric relaxation spectra of eight glass-forming liquids in the presence of electric direct current (dc)-bias fields ranging from 100 to 500 kV/cm. For every sample, we observe two distinct field-induced effects: a reduction in the relaxation amplitude and an increase in the primary structural relaxation time that is associated with viscous flow. Whereas amplitude change is typical of the well-known dielectric saturation, the field-induced increase in viscosity is a source of nonlinear behavior that has been recognized only recently. We find that this electrorheological behavior occurs in all polar liquids of this study, and its magnitude is correlated with the field-induced change in thermodynamic entropy. It constitutes a significant source of nonlinear dielectric behavior, which occurs for both dc and alternating current fields.

Electrorheological Source of Nonlinear Dielectric Effects in Molecular Glass-Forming Liquids

Quantum mechanical/molecular mechanical simulations are used to explore the temperature dependence of intramolecular electron-transfer rates in systems that represent both the “normal” and the “inverted” regions of the Marcus curve. The treatment uses an approach that includes effects of vibrational relaxations and dephasing and is largely free of adjustable parameters. Effects of temperature on the distribution of the energy gap between the reactant and product (P(xo)), the electronic-interaction matrix element, and the rates of dephasing and vibrational relaxations are considered. The simulations reproduce the measured rate constant and temperature dependence well for photochemical charge separation in a porphyrin–benzoquinone cyclophane and for a ground-state charge-shift reaction in a biphenylyl-androstane–naphthylyl radical. They overestimate the rate of the charge-shift reaction in a biphenylyl-androstane–benzoquinone adduct but are in accordance with the observation that this reaction is almost ind...

Temperature Dependence of the Rate of Intramolecular Electron Transfer.

Variable-temperature fast field cycling NMR measurements are used to probe the surface dynamics of different liquids imbibed within a γ-alumina catalyst support material. The imbibed liquids were g...

Characterizing Solid–Liquid Interactions in a Mesoporous Catalyst Support Using Variable-Temperature Fast Field Cycling NMR

Both tetrahydrofuran (THF) and 2-methyltetrahydrofuran (MTHF) are studied systematically at desired temperatures using molecular dynamics simulations. The results show that the calculated densities are well consistent with experiment. Their glass transition temperatures are obtained: 115 K 130 K for THF and 131 K 142 K for MTHF. The calculated results from the dipolar orientational time correlation functions indicate that the “long time” behavior is often associated with a glass transition. From the radial and spatial distributions, we also find that the methyl has a direct impact on the structural symmetry of molecules, which leads to the differences of physical properties between THF and MTHF.

http://iopscience.iop.org/article/10.1088/1674-1056/21/8/086402/pdf

Liquid-to-glass transition of tetrahydrofuran and 2-methyltetrahydrofuran

Metal matrix for the impact protection often requires high strength and good ductility as well as large energy absorption capacity. However, the traditional metal matrix alloys show relatively low strength and ductility to limit the energy absorption capacity. Here, the effect of chemical composition on properties of CoxNi50-xFe25Cr25 high entropy alloys is studied using first-principal calculations. The integrated method considering the competitive relationship between the strength and ductility is proposed to evaluate the energy absorption capacity. The results show that CoNiFeCr HEA with high strength and good ductility exhibits the exceptional energy absorption capacity, where the specific strength and ductility are 889 kN m/kg and 0.99. This work gives a new light into the selection and evaluation of strong energy-absorbing alloys. 

Influence of chemical composition on mechanical properties and energy absorption of CoNiFeCr high entropy alloys: A first-principles study

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Liquid-to-glass transition of tetrahydrofuran and 2-methyltetrahydrofuran

Influence of chemical composition on mechanical properties and energy absorption of CoNiFeCr high entropy alloys: A first-principles study