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

Abstract

An assessment of five different definitions of the principal molecular axis along which molecules align in a nematic liquid crystal system has been made by analysing fully atomistic molecular dynamics (MD) simulations of a set of anthraquinone dyes in the cyanobiphenyl-based nematic host mixture E7. Principal molecular axes of the dyes defined by minimum moment of inertia, minimum circumference, minimum area, maximum aspect ratio, and surface tensor models were tested, and the surface tensor model was found to give the best description. Analyses of MD simulations of E7 alone showed that the surface tensor model also gave a good description of the principal molecular axes of the host molecules, suggesting that this model may be applicable more generally. Calculated dichroic order parameters of the guest-host systems were obtained by combining the surface tensor analysis with fixed transition dipole moment (TDM) orientations from time-dependent density functional theory (TD-DFT) calculations on optimised structures of the dyes, and the trend between the dyes generally matched the trend in the experimental values. Additional analyses of the guest-host simulations identified the range of conformers explored by the flexible chromophores within the dyes, and TD-DFT calculations on corresponding model structures showed that this flexibility has a significant effect on the TDM orientations within the molecular frames. Calculated dichroic order parameters that included the effects of this flexibility gave a significantly improved match with the experimental values for the more flexible dyes. Overall, the surface tensor model has been shown to provide a rationale for the experimental alignment trends that is based on molecular shape, and molecular flexibility within the chromophores has been shown to be significant for the guest-host systems: the computational approaches reported here may be used as a general aid in the predictive design of dyes with appropriate molecular shapes and flexibilities for guest-host applications.

Free volume, molecular grains, self-organisation, and anisotropic entropy: machining materials

“…By the early 1990s, there became a need at research centres such as DERA (Malvern, Worcestershire, UK) to estimate 'material properties' of targeted materials before costly synthesis was considered, and so began bridging the gap between the nano-molecular structures and those of the complex macroworld. Indeed, it is only recently that chemists have started to use modelling as a tool in the rational design of lead compounds (with optimum properties) in their synthetic programmes [5,6]. For want of a better expression, electrostatics appeared to lead the way not only in material design but also in determining mesophase formation and stability.…”

Guest–host systems containing anthraquinone dyes with multiple visible transitions giving positive and negative dichroic order parameters: an assessment of principal molecular axes and computational methods

“…In the case of the 1,5-disubstituted anthraquinone dyes we have studied previously, the minimum MOI axes and surface tensor z-axes were found to exhibit only slight differences in orientation [32], but for the structures studied here, dye 1 exhibits surface tensor z-axes and minimum MOI axes that are orthogonal, whereas 2 has these axes offset by 19°, and 3 has these axes coincident. The angles, β, between both definitions of the principal molecular axes and the calculated TDM vectors are listed in Table 3, along with their contributions, S β , to the dichroic order parameters.…”

“…The surface tensors and principal axes were defined using the method described previously, in which we define the surface tensor z-axis as the eigenvector associated with the eigenvalue with the smallest magnitude in the diagonalised Cartesian surface tensor [32]. The surface tensor y-axis was defined as the eigenvector associated with the eigenvalue with the largest magnitude, and the surface tensor x-axis was defined as orthogonal to the y and z axes.…”

“…The minimum MOI axes were used here to define the principal molecular axes because we have shown previously that they provide a very close match with the surface tensor z-axes of the E7 component molecules whilst avoiding the significant computational expense of the surface tensor analysis for the full simulations [32]. The average values of S θ over the last 500 Table 3.…”

“…A general approach to understanding this complex relationship has been provided by our recent development of computational methods to aid in dye design [29][30][31][32]. We have shown that a novel combination of molecular dynamics (MD) simulations and time-dependent density functional theory (TD-DFT) calculations, based on the application of Equation (4), can provide a rationale for observed trends in order parameters from calculations of S θ and S β that are combined to give values of S ϕ that are directly comparable with experimental values of S. These methods have resulted in the successful prediction of order parameter trends for 1,5-and 2,6-disubstituted anthraquinones dyes and have demonstrated that the choice of principal molecular axis, which affects calculations of both S θ and S β , may have a significant influence on the calculated dichroic order parameter [32]. For the structures we have studied to date, we have found that an improved match with experimental trends was obtained by using the surface tensor z-axes, which are defined by the shape of the molecular surface [33], as the principal molecular axes rather than the more conventionally used minimum moment of inertia (MOI) axes.…”