Comparison of photoinduced reorientation of ortho-, meta-, and para-methyl red-doped nematic liquid crystals on rubbed polyimide

We compare the photoinduced reorientation of the easy axis on rubbed polyimide surfaces for the nematic E7 doped with three isomers of methyl red; ortho, meta, and para. Using optical techniques, the angle and the pitch of the director at the polymer surface were measured before, during, and after photoexcitation of the dye. Optical absorbances were also measured before and after photoexcitation. Extrapolation lengths, hence anchoring energies, were determined with the on/off application of a magnetic field for meta- and para-methyl red-doped nematics. Because of an elastic reorientation of the easy axis in the presence of the magnetic field, we could not determine the extrapolation length of the ortho-methyl red-doped nematic. Our results confirm that photoinduced reorientation is facilitated by desorption of all dyes from the polymer surface. While there is little evidence of weak photoinduced adsorption of meta- and para-methyl red to the surface during photoexcitation, there is strong evidence of photoinduced adsorption of ortho-methyl red, which is long lasting.

Figure 1

Structure of ortho-methyl red. $o, m$, and $p$ refer to the ortho, meta, and para sites on the phenyl ring.

Figure 2

Experimental design. P: polarizer; BS: beam splitter; D1: pump input detector; D2: signal detector; L: lens; EM: electromagnet: PEM: photoelastic modulator; M: mirror; S: shutter; A: analyzer. The applied magnetic field is parallel to the plane of the sample and perpendicular to the probe beam.

Figure 3

Ortho-methyl red-doped sample: Director angle and twist (1/pitch) in degrees and degrees/μm. The twist becomes more negative (or decreases when the twist is positive) when the magnetic field is applied, and it becomes more positive when the magnetic field is removed.

Figure 4

Meta-methyl red-doped sample: Director angle and twist (1/pitch) in degrees and degrees/μm. The twist becomes more positive when the magnetic field is applied, and it becomes more negative when the magnetic field is removed.

Figure 5

Para-methyl red-doped sample: Director angle and twist (1/pitch) in degrees and degrees/μm. The twist becomes more positive when the magnetic field is applied, and it becomes more negative when the magnetic field is removed.

Figure 6

Ortho-methyl red-doped sample immediately after irradiation. Director angle versus twist. Data are fit to $\varphi =a+b{\varphi }^{\prime }$; $a=-5.{560}^{\circ }$, and $b=18.54\mu \mathrm{m}$. $R^2=0.91945$.

Figure 7

Ortho-methyl red-doped sample before irradiation. Director angle and twist (1/pitch) in degrees and degrees/μm. The twist and the angle become more negative when the magnetic field is applied, and they become more positive when the magnetic field is removed.

Figure 8

Meta-methyl red-doped sample before irradiation. Director angle and twist (1/pitch) in degrees and degrees/μm. The twist increases and the angle decreases when the magnetic field is applied. The twist decreases and the angle increases when the magnetic field is removed.

Figure 9

Para-methyl red-doped sample before irradiation. Director angle and twist (1/pitch) in degrees and degrees/μm. The twist increases and the angle decreases when the magnetic field is applied. The twist decreases and the angle increases when the magnetic field is removed.

Figure 10

Absorbance spectra of ortho-methyl red before (dash) and after (solid) photoexcitation.

Figure 11

Absorbance spectra of meta-methyl red before (dash) and after (solid) photoexcitation.

Figure 12

Absorbance spectra of para-methyl red before (dash) and after (solid) photoexcitation.