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.

Mark T Sims,Stephen J Cowling,John N Moore,John W Goodby,Laurence C Abbott

doi:10.1039/c6cp05979a

Mark T Sims, Stephen J Cowling + Show 3 more

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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.

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Molecular alignment and its characterisation is a key aspect in the study of liquid-crystalline phases, to understand the nature of molecular ordering and because this alignment typically underlies the many applications of liquid-crystalline materials
We recently reported experimental UV-visible studies of a series of five anthraquinone dyes in the nematic host mixture E7, from which we obtained a set of experimental dichroic order parameters, Sexptl, that showed significant differences between the dyes, as given in Fig. 2.41,42 In parallel, we demonstrated that a combination of fully atomistic molecular dynamics (MD) simulations of the guest–host systems and time-dependent density functional theory (TD-DFT) calculations on optimised structures of the dye molecules could be used to obtain calculated dichroic order parameters, Sf, that are directly comparable with the experimental values, and which we used to rationalise the trend observed experimentally between the dyes
The equivalent angles were smaller for the other dyes, and the angles for all the dyes are given in Table 1, along with the molecular order parameters, Sy, determined using the aMOI axes against the host director, the values of Sy (MOI) using the minimum moment of inertia axes that we reported previously, and the differences between these values, D = Sy À Sy (MOI)

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Molecular alignment and its characterisation is a key aspect in the study of liquid-crystalline phases, to understand the nature of molecular ordering and because this alignment typically underlies the many applications of liquid-crystalline materials. An overall comparison of the data can be considered in terms of the average D values listed, from which it is evident that the minimum MOI and maximum aspect ratio axes are generally the worst and the surface tensor z-axis is generally the best of these descriptions from first principles of the molecular axes along which the dyes align within the simulations.

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