Resumen de evaluación

Polarised UV-visible absorption spectra were recorded from aligned samples of the dyes in E7 with the electric vector of the polarised light oriented parallel with the host director, as shown in Figure 4.3, along with those obtained from p-xylene solutions. It is evident from the spectra presented in Figure 4.3 that the visible absorption profiles of the dyes do not change significantly between the isotropic p-xylene and nematic E7 solvents. This match between the band shapes is consistent with the TD-DFT calculations, which indicated that the visible absorption band of each dye arises from a single electronic transition, or from transitions with very similar TDM orientations.

Additionally, the consistency in band shapes between the samples in p-xylene and E7 is indicative that aggregation of the dyes is unlikely to have occurred in the E7 samples, because molecular aggregation is typically associated with changes in spectral profile.²⁶⁹ The aggregation of an anthraquinone dye in 5CB has been reported in the literature,²⁷⁰ but no evidence of the hair-like structures described in this report was observed in the samples when viewed under a microscope. This observation is consistent with the structures of the five dyes studied here, in which the terminal alkyl chains typically confer greater dye solubility than that of unsubstituted structures²⁴ such as that for which aggregation has been reported.

Figure 4.3 Scaled UV-visible absorption spectra of the dyes at ca 1.5 wt % (except for 15NB3OH for which was at ca 0.75 wt % to ensure that the absorbance was within the instrument range) in E7 (black) and at 2 × 10⁻⁴ mol dm⁻³ in p-xylene (grey), shown for comparison.

A comparison of the λmax values of the dyes in E7 with those in p-xylene, listed in Table 4.1, shows a red shift in going to E7 for each of the dyes, and comparison with the dyes in other isotropic solvents (Table 3.4) shows that the same offset is observed with respect to all of the isotropic solvents used. This behaviour is consistent with red-shifts reported for many dyes on going from non-polar isotropic solvents to

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based liquid-crystalline solvents.37, 45, 271 The shift has been assigned to the polarisability of the cyanobiphenyl hosts being higher than those of the isotropic solvents in the case of an oxazine dye,²⁷² although no strong trends were observed between solvent polarisability and λmax values within the isotropic solvents used in the work reported here (see Appendix 1.2).

Table 4.1 Experimental λmax values of the aligned samples of dye-E7 mixtures obtained from the polarised UV-vis absorption spectra. Values in p-xylene are also shown for comparison.

Dye λmax / nm

E7 p-xylene

15SB3 460 439

15NB3 548 535

15NB3OH 693 680

26B3 367 358

26B3OH 477 471

Polarised UV-visible absorption spectra of aligned samples of the dyes in E7 with the director oriented parallel (A_Ⅱ) and perpendicular (A⊥) to the electric vector of the incident light are presented in Figure 4.4.

Figure 4.4 Polarized UV-visible absorption spectra of aligned samples of the dyes at ca 1.5 wt

% (except for 15NB3OH for which was at ca 0.75 wt % to ensure absorbance was within the instrument range) in E7, used to obtain their dichroic ratios. The spectra are offset for clarity.

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The spectra in Figure 4.4 show that the value of A_Ⅱ is greater than that of A⊥ for each of the dyes in E7, and this positive dichroism is consistent with the reported alignment of comparable structures studied previously.^{59, 60} Plots of the dichroic ratios, R, across the full range of the visible spectrum are presented for each of the dyes in Figure 4.5, and they show that the dichroic ratios are approximately constant over the range of each of the visible bands. This observation is consistent with the TD-DFT calculations and with the comparison of spectral profiles in Figure 4.3, indicating that the bands arise from single electronic transitions, discussed in Chapter 3.

The dichroic ratios and associated order parameters, Sexp, determined using Equation (4.4) are listed in Table 4.2, and a comparison of the order parameter values is shown graphically in Figure 4.6. The dichroic ratios were calculated at the absorption maxima for each of the dyes, except for 26B3 for which the dichroic ratio was determined at 390 nm to avoid overlap with E7 absorption below ca 370 nm (see Figure A2.1 in Appendix 2.1).

Figure 4.5 Plots of dichroic ratio against wavelength for each of the dyes in E7, obtained from the polarized UV-visible absorption spectra presented in Figure 4.4 which are shown at the top of this figure for clarity. Highlighted grey bands show the full-width-half-maxima of the visible bands, except in the case of 26B3 due to the absorption of E7 below ca 370 nm.

Table 4.2 Experimental dichroic ratios, R, and order parameters, Sexp, of aligned samples of the dyes in E7, obtained from the polarised UV-visible absorption spectra presented in Figure 4.4.

Calculated Sβ values presented and discussed in Chapter 3 (Table 3.10) are given here for

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Figure 4.6 Graphical comparison of the experimental order parameters of the dyes in E7, Sexp, and the values, Sβ, obtained from the calculated β angles of the dyes.

The experimental order parameters in E7 show significant variation between the dyes despite their relatively similar structures. A relatively high experimental order parameter is observed for 15SB3, consistent with measurements of other 1,5-disubstituted sulfide anthraquinones reported in the literature.^{64, 65, 68} Replacement of the sulfide linking group with an amine group causes a significant decrease in dichroic ratio, shown in going to 15NB3 and 15NB3OH, which is again consistent with values for 1,5-disubstituted aminoanthraquinones reported previously.59, 65, 66

The two 2,6-disubstituted dyes exhibit high order parameters, which is also consistent with reported order parameters of similar structures.⁶¹

A comparison of the experimental order parameters with the order parameters, Sβ, obtained from the calculated β angles, listed in Table 4.2 and shown in Figure 4.6, shows a good match in the trends between the dyes. This match indicates that the relative orientations of the TDMs within the dyes are significant in terms of the observed alignment of this set of dyes in E7, although the observation that the experimental values are lower than the calculated Sβ values indicate that there is also likely to be a significant contribution from the molecular alignment within the host for each of the dyes.

A comparison of the experimental order parameters of the dyes in E7 with the experimental order parameter of 0.64 for E7 alone shows that the order parameters of the amine dyes (15NB3 and 15NB3OH) are lower than that of the host, whereas the order parameters of the sulfide and 2,6-disubstituted dyes (15SB3, 26B3 and 26B3OH) are higher than that of the host. This type of behaviour has been linked with the relative sizes of the host and guest molecules, with dyes that are longer than the host molecules

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typically exhibiting higher order parameters than the host alone.²⁴ This observation is discussed further in Section 4.3.2.3.

4.3.2 Molecular dynamics simulations

A comparison of Sexp with Sβ, such as that discussed above, is only of limited use because it neglects the contribution of the molecular alignment to the experimental order parameter, as discussed in Chapter 1. Therefore, it is desirable to be able to calculate the molecular alignment contribution, Sθ, to the overall order parameter in order to be able to make a more meaningful comparison between experimental and calculated order parameters. The molecular alignment is of particular relevance to the dyes studied in this work because of the variation in shapes between the dyes, highlighted by the electronic structure calculations discussed in Chapter 3. In principle, the order parameter, Sθ, arising from the molecular alignment of the dyes within the host should be obtainable from guest-host molecular dynamics simulations. Prior to consideration of such guest-host systems, MD studies on the host alone were carried out, and are presented first in this section.