Impact of the Crystallite Orientation Distribution on Exciton Transport in Donor-Acceptor Conjugated Polymers.

Conjugated polymers are widely used materials in organic photovoltaic devices. Owing to their extended electronic wave functions, they often form semicrystalline thin films. In this work, we aim to understand whether distribution of crystallographic orientations affects exciton diffusion using a low-band-gap polymer backbone motif that is representative of the donor/acceptor copolymer class. Using the fact that the polymer side chain can tune the dominant crystallographic orientation in the thin film, we have measured the quenching of polymer photoluminescence, and thus the extent of exciton dissociation, as a function of crystal orientation with respect to a quenching substrate. We find that the crystallite orientation distribution has little effect on the average exciton diffusion length. We suggest several possibilities for the lack of correlation between crystallographic texture and exciton transport in semicrystalline conjugated polymer films.

Dielectric and phase behavior of dipolar spheroids.

The Stockmayer fluid, composed of dipolar spheres, has a well-known isotropic-ferroelectric phase transition at high dipole densities. However, there has been little investigation of the ferroelectric transition in nearly spherical fluids at dipole densities corresponding to those found in many polar solvents and in guest-host organic electro-optic materials. In this work, we examine the transition to ordered phases of low-aspect-ratio spheroids under both unperturbed and poled conditions, characterizing both the static dielectric response and thermodynamic properties of spheroidal systems. Spontaneous ferroelectric ordering was confined to a small region of aspect ratios about unity, indicating that subtle changes in sterics can have substantial influence on the behavior of coarse-grained liquid models. Our results demonstrate the importance of molecular shape in obtaining even qualitatively correct dielectric responses and provide an explanation for the success of the Onsager model as a phenomenological representation for the dielectric behavior of polar organic liquids.

Nanoscale phase analysis of molecular cooperativity and thermal transitions in dendritic nonlinear optical glasses.

A broad nanoscopic study of a wide-range of dendritic organic nonlinear optical (NLO) self-assembly molecular glasses reveals an intermediate thermal phase regime responsible for both enhanced electric field poling properties and strong phase stabilization after poling. In this paper, the focus is on dendritic NLO molecular glasses involving quadrupolar, liquid crystal, and hydrogen bonding self-assembly mechanisms that, along with chromophore dipole-dipole interactions, dictate phase stability. Specifically, dendritic face-to-face interactions involving arene-perfluoroarene are contrasted to coumarin-containing liquid crystal mesogen and cinnamic ester hydrogen interactions. Both the strength of dendritic interactions and the impact of dipole fields on the relaxation behavior have been analyzed by nanoscale energetic probing and local thermal transition analysis. The presence of dendritic groups was found to fundamentally alter transition temperatures and the molecular relaxation behavior. Thermal transition analysis revealed that molecules with dendritic groups possess an incipient transition (T(1)) preceding the glass transition temperature (T(2)) that provides increased stability and a well-defined electric field poling regime (T(1) < T < T(2)), in contrast to molecular groups lacking dendrons that exhibit only single transitions. On the basis of enthalpic and entropic energetic analyses, thermally active modes below T(1) were found to be intimately connected to the dendron structure. Their corresponding activation energies, which are related to thermal stability, increased moving from cinnamic ester groups to coumarin moieties to arene-perfluoroarene interacting groups. While dendritic NLO materials were found to possess only enthalpic stabilization energies at temperatures relevant for device operation (T < T(1)), the apparent molecular binding energies above T(1) contain a substantial amount (up to ~80%) of cooperative entropic energy. The multiple interactions (from dipole-dipole interactions to local noncovalent dendritic interactions) are discussed and summarized in a model that describes the thermal transitions and phases.

Nano-engineering lattice dimensionality for a soft matter organic functional material.

A high performing electro-optic (EO) chromophore with covalently attached coumarin-based pendant groups exhibits intermolecular correlation of coumarin units through molecular dynamics (MD) simulations. Unique, orthogonal molecular orientations of the chromophore and coumarin units are also evident when investigated optically. Such molecular orientation translates to reduced lattice dimensionality of the bulk C1 soft matter material system, leading to increased acentric order and EO activity. Results are corroborated by nanorheological experimental methods.

Measuring order in contact-poled organic electrooptic materials with variable-angle polarization-referenced absorption spectroscopy (VAPRAS).

Organic nonlinear electrooptical (ONLO) chromophores must be acentrically ordered for the ONLO material to have electrooptic (EO) activity. The magnitude of the order is characterized by the acentric order parameter, , where ß is the major Euler angle between the main axis of the chromophore and the poling field which imposes the acentric order. The acentric order parameter, which is difficult to measure directly, is related to the centrosymmetric order parameter, defined as = ½(3-1), through the underlying statistical distribution. We have developed a method to determine centrosymmetric order of the ONLO chromophores when the order is low (i.e., < 0.1). We have extended the method (begun by Graf et al. J. Appl. Phys. 1994, 75, 3335.) based on the absorption of light to determine the centrosymmetric order parameter induced by a poling field on a thin film sample of ONLO material. We find that the order parameters, analyzed by two different methods, are similar and also consistent with theoretical estimates from modeling of the system using coarse-grained Monte Carlo statistical mechanical methods.

Reduced dimensionality in organic electro-optic materials: theory and defined order.

Identification of electronic intermolecular electrostatic interactions that can significantly enhance poling-induced order is important to the advancement of the field of organic electro-optics. Here, we demonstrate an example of such improvement achieved through exploitation of the interaction of coumarin pendant groups in chromophore-containing macromolecules. Acentric order enhancement is explained in terms of lattice-symmetry effects, where constraint of orientational degrees of freedom alters the relationship between centrosymmetric and acentric order. We demonstrate both experimentally and theoretically that lattice dimensionality can be defined using the relationship between centrosymmetric order and acentric order. Experimentally: Acentric order is determined by attenuated total reflection measurement of electro-optic activity coupled with hyper-Rayleigh scattering measurement of molecular first hyperpolarizability, and centrosymmetric order is determined by the variable angle polarization referenced absorption spectroscopy method. Theoretically: Order is determined from statistical mechanical models that predict the properties of soft condensed matter.