Size and shape effects on the orientation of rigid molecules in nematic liquid crystals

Triplet electron transfer and spin polarization in a palladium porphyrin–fullerene conjugate

Transient electron paramagnetic resonance (TREPR) spectroscopy is used to investigate the pathway and dynamics of electron transfer in a palladium porphyrin–fullerene donor–acceptor conjugate.

Principal molecular axis and transition dipole moment orientations in liquid crystal systems: an assessment based on studies of guest anthraquinone dyes in a nematic host

Analyses of MD simulations assess different definitions of the axes along which molecules align in a nematic host, and the effects of molecular flexibility on transition dipole moment orientations.

Orientational mechanisms in liquid crystalline systems. 2. The contribution to solute ordering from the reaction field interaction between the solute electric quadrupole moment and the solvent electric field gradient

In the previous paper of this issue, [Celebre, G.; Ionescu, A. J. Phys. Chem. B doi: 10.1021/jp907310g], following a generalized reaction field approach in the linear response approximation, we were successful in obtaining an analytical compact expression for the mean-field anisotropic orientational potential U(Q-EFG) theoretically experienced by a highly idealized nonionic and apolar solute, considered as a point quadrupole immersed in a uniaxial polarizable continuum medium (model of a nematic solvent comprised of dipolar mesogenic molecules). The term U(Q-EFG) describes the electrostatic interaction between the electric quadrupole of the solute and the electric field gradient induced at the solute by the surrounding medium polarized by the distribution of electric charges representing the quadrupolar solute itself. In the present paper, the obtained potential has been considered as an additional orientational interaction contributing to the solute ordering, besides the well-recognized and very effective "short-range" (size-and-shape-dictated) mechanisms. Since in our theory the solvent is characterized by its dielectric tensor, the model has been widely tested by taking as references the experimental order parameters of several uniaxial and biaxial different small rigid probe molecules (H(2), N(2), acetylene, allene, propyne, benzene, hexafluorobenzene, 1,4-difluorobenzene, and norbornadiene) dissolved in the nematic solvents ZLI1132 (Deltaepsilon >> 0) and EBBA (Deltaepsilon < 0); moreover, the order parameters of the same solutes in the so-called nematic "magic mixture" (45 wt % EBBA + 55 wt % ZLI1132), where the short-range orientational effects are commonly believed to be very dominant, have been conventionally assumed as reference of the absence of electrostatic orientational effects. The experimental order parameters of the treated solutes, obtained in the past by liquid crystal NMR and available from literature, have been then compared with those theoretically predicted by our theoretical approach in order to obtain useful hints about two basic points, (a) the real physical nature of the interactions (other than the "size-and-shape") involved in the orientational mechanisms and (b) the conceptual effectiveness of the suggested mean-field approach in describing this kind of phenomena. Successes and failures of the approach in the predictions are discussed at length, along with their possible reasons and implications.

The smectic effect on solute order parameters rationalized by double Maier-Saupe Kobayashi-McMillan theory

From the dipolar couplings obtained by NMR spectroscopy we have calculated the order parameters of a wide variety of solutes in the nematic and smectic A phases of the liquid crystals 8CB and 8OCB. These measurements are then rationalized with the previously tested two Maier-Saupe Kobayashi-McMillan interaction potential from which smectic order parameters are calculated.

An Explicit Relationship between the Dielectric Anisotropy and the Average Electric Field Gradient in Nematic Solvents

Nowadays, the interaction between the electric quadrupole moment of a probe-solute and the so-called average electric field gradient (EFG) of the nematic medium is commonly suggested as an important long-range mechanism contributing to the ordering of small molecules dissolved in nematic solvents. Anyway, an explicit relationship between the solvent's EFG and some macroscopic property of the medium has never been established explicitly. In this work, a derivation is carried out leading to a simple formula that shows that the EFG of a nematic solvent is directly related to the dielectric permittivity of the medium (in particular, to its dielectric anisotropy, Deltaepsilon) and to the quadrupole moment of the solute. The obtained expression (a) allows for immediate considerations about the sign and the value of EFG (and could be very useful in designing proper nematic mixtures with null EFG) and (b) reconciles two existing conceptual ways of describing long-range orientational interactions, revealing they are only seemingly different.

Quantum-mechanical studies of NMR properties of solutes in liquid crystals: A new strategy to determine orientational order parameters

We present a quantum-mechanical method to calculate the tensorial NMR magnetic properties of molecules in liquid crystals. The method exploits a density functional description for the solute and the integral equation formalism version of the polarizable continuum model to include the effects of the dielectric anisotropy of the solvent on the wave function of the solute molecule and on its response to an applied magnetic field. Taking into account the dependence of the calculated molecular tensors on the solute orientation, we have derived the necessary expressions to relate such tensors to the spectral observables. These equations are then used to determine order parameters by calculating the nuclear shieldings of various solutes in the nematic ZLI-1167. Comparisons with experimental data of the same parameters are finally presented.

An analytical solution to the problem of the orientation of rigid particles by planar obstacles. Application to membrane systems and to the calculation of dipolar couplings in protein NMR spectroscopy

Nonspherical particles or molecules experience an ordering effect in the presence of obstacles due to the restrictions they place on the orientation of those molecules that are in their proximity. Obstacles may be the limits of a membrane in which the molecule is embedded, oriented mesoscopic systems such as bicelles, or membrane fragments used to induce weak protein alignment in a magnetic field. The overall shape of most proteins can be described to a good approximation by an ellipsoidal particle. Here we describe and solve analytically the problem of the orientation of ellipsoidal particles by planar obstacles. Simple expressions are derived for the orientational distribution function and the order parameter. These expressions allow the analytical calculation of the residual dipolar couplings for a protein of known three-dimensional structure oriented by steric effects. The results are in good agreement with experiment and with the results of previously described simulations. However, they are obtained analytically in a fraction of the time and therefore open the possibility to include the optimization of the overall shape in the determination of three-dimensional structures using residual dipolar coupling constraints. The equations derived are general and can also be applied to problems of a completely different nature. In particular, previous equations describing the orientation of particles embedded in membranes are verified and generalized here.