Photochemical Modulation of Color and Transmittance in Chiral Nematic Liquid Crystal Containing an Azobenzene as a Photosensitive Chromophore

Abstract

All-optical switching of selectively reflected colors based on isomerization of an azobenzene (Azo) and subsequent change in transmittance was investigated in cholesteric liquid crystals (LCs) which reflect light in the visible region. Photoirradiation at 366 nm, which causes an efficient trans-cis isomerization of Azo, led to change in color of cholesteric LCs toward shorter wavelengths with a concomitant lowering of phase transition temperature from a cholesteric to an isotropic phase (T Ch-I ). Reversible change in color was induced all-optically by irradiating alternatively at effective wavelengths for reversible isomerization of Azo. A remarkable change in transmittance was also observed when the photoinduced change in colors was measured by a probe light with the same handedness as the helical sense of the cholesteric LCs. In the wavelength regions of reflected colors before photoirradiation, the spectral position played an important role in producing a normal-mode and a reverse-mode switching in photoinduced modulation of transmittance of the cholesteric samples with respect to the probe light.

Photocontrollable Liquid‐Crystalline Actuators

“…Then the optical switching of selectively refl ective colors was investigated based on photoinduced change in the helical pitch of CLCs doped with a chiral AZ. [ 45 ] Doping of cyanobiphenyl nematic LCs with chiral dopants possessing strong HTP produced CLC phases with refl ection bands in a visible region. Photoirradiation of the CLCs containing the chiral AZ resulted in a change in the helical pitch due to trans-cis isomerization of the chiral AZ, followed by the change in the refl ective wavelength.…”

“…Photoirradiation of the CLCs containing the chiral AZ resulted in a change in the helical pitch due to trans-cis isomerization of the chiral AZ, followed by the change in the refl ective wavelength. [ 45 ] Such a change in the helical pitch can be interpreted in terms of the change in the phase transition temperature caused by changing the molecular shape of the guest AZs (Figure 2 C). Recently, a series of dicholecteric ester oligomers was developed, which can fi x the cholesteric color by immobilizing the molecular ordering in a glassy state and shows applicable to optics materials with a static function.…”

Photoresponsive Cholesteric Liquid Crystals

“…Taking into account that the molecular shape for cis isomers of 3,3 -disubstituted azobenzenes (rod-like) and 4,4 -disubstituted azobenzenes (bent) is different, these results strongly support the theory that the pitch changes are related to the order parameter affected by the photoisomerization of azobenzene molecules. The multiple dopants system may also contain photoresponsive chiral dopants and non-photoresponsive codopants [32,82,96,97,[105][106][107][108][109][110]. In these systems, the pitch lengths are determined by the HTPs of both dopants, and the photoresponsive dopant has the additional function of photosensitization.…”

Synthesis, Liquid‐Crystalline Properties, and Photo‐optical Studies of Photoresponsive Oligomeric Mesogens as Dopants in a Chiral Glassy Liquid Crystal

“…[3,4] The sensitivity of ChLCs to external parameters, such as temperature, the presence of impurities, and light, has led to many practical applications and has been the subject of much theoretical research. [2,[5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] Several mechanisms have been proposed to explain the role that the dopant molecules play in the modulation of the macroscopic chirality of ChLCs. [11,[24][25][26][27][28] Recently, we reported that the mechanism of the helical-pitch shift that is induced by achiral dopants (as well as in dimesogenic compounds) is due to enhancement of smectic A (SmA)-like clusters in the cholesteric phases.…”