Experimental and molecular dynamics studies of anthraquinone dyes in a nematic liquid-crystal host: a rationale for observed alignment trends

Abstract

Five anthraquinone dyes with bis(4-propylphenyl) substituents, connected via sulfide or amine linking groups at the 1,5-positions or directly at the 2,6-positions, have been studied as guests in the nematic liquid crystal host, E7. Polarized UV-visible absorption spectra of aligned samples were used to obtain experimental dichroic order parameters, which exhibit values in the range 0.51-0.74. Fully atomistic MD simulations of these guest-host systems were carried out, generally using default parameters but using new force constants derived here for the dyes containing flexible phenyl-sulfide and phenyl-amine linking groups. An analysis of the alignment of the dye molecules in these simulations provides calculated molecular order parameters, which are combined with calculated order parameters for the alignment of the transition dipole moments within the dyes, reported previously, to give calculated dichroic order parameters. The trend in the calculated dichroic order parameters between the dyes shows a good match with the trend in the experimental values, enabling the observed variation to be rationalised primarily by changes in the alignment of the calculated transition dipole moments within the dyes; the calculated molecular order parameters show a relatively small variation between the dyes. The results indicate that this computational approach may be used generally to rationalise trends in the alignment of guest molecules in liquid crystal hosts, suggesting that it may also be able to provide a predictive aid in the design of guest dyes.

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

“…The average values over the last 500 ns of the simulations, given in Table 4, suggest that all three dye molecules exhibit lower molecular order parameters than the host molecules, which is consistent with the less rod-like shapes of the dye molecules. In our previous work, we observed that the trend in the calculated order parameters of the host molecules in guest-host mixtures matched that of the guest molecules, and that is the case again for these simulations [31], demonstrating that the addition of guest molecules has an influence on the calculated alignment of the overall mixture.…”

“…In particular, the relationship between the molecular structure and the dichroic order parameter is not always intuitive. 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].…”

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

“…2. 41,42 In parallel, we demonstrated that a combination of fully atomistic 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, S f , that are directly comparable with the experimental values, and which we used to rationalise the trend observed experimentally between the dyes. In these studies, the principal molecular axes were defined as the minimum MOI axes, providing consistency with other reported MD studies, and the molecular order parameters, S y (MOI), were calculated from the MD simulations according to eqn (1) for the host and according to the first term in eqn (2) for the guest dyes.…”

“…2 Structures and abbreviations of the five anthraquinone dyes studied in this work, along with the respective S exptl values we reported previously. 41,42 axes may not be defined readily by symmetry. The work reported here extends our studies of the set of five anthraquinone dyes in the nematic host E7, which provide a useful data set to assess some general assumptions that are often made about molecular axis systems, and for which the results may be expected to have wider relevance than these particular systems.…”

“…Hence, we present a further and more detailed analysis of the fully atomistic guest-host MD simulations of the dyes in E7 that we reported previously, we present additional MD simulations and analyses of E7 alone, and we include direct comparisons with our reported experimental data. 41,42 The results are structured into two main sections. First, the guest-host simulations are analysed using several different definitions of the principal molecular axes of the dyes, including those defined by the minimum circumference and surface tensor approaches outlined above, as well as other definitions based on minimum area and maximum aspect ratio approaches.…”