Liquid crystals are traditionally classified as thermotropic, lyotropic or polymeric, based on the stimulus that governs the organization and order of the molecular system. The most widely known and applied class of liquid crystals are a subset of thermotropic liquid crystals known as calamitic, in which adding heat can result in phase transitions from or into the nematic, cholesteric and smectic mesophases. Photoresponsive liquid-crystal materials and mixtures can undergo isothermal phase transitions if light affects the order parameter of the system within a mesophase sufficiently. In nearly all previous examinations, light exposure of photoresponsive liquid-crystal materials and mixtures resulted in order-decreasing photo-induced isothermal phase transitions. Under specialized conditions, an increase in order with light exposure has been reported, despite the tendency of the photoresponsive liquid-crystal system to reduce order in the exposed state. A direct, photo-induced transition from the isotropic to the nematic phase has been observed in a mixture of spiropyran molecules and a nematic liquid crystal. Here we report a class of naphthopyran-based materials that exhibit photo-induced conformational changes in molecular structure capable of yielding order-increasing phase transitions. Appropriate functionalization of the naphthopyran molecules leads to an exceedingly large order parameter in the open form, which results in a clear to strongly absorbing dichroic state. The increase in order with light exposure has profound implications in optics, photonics, lasing and displays and will merit further consideration for applications in solar energy harvesting. The large, photo-induced dichroism exhibited by the material system has been long sought in ophthalmic applications such as photochromic and polarized variable transmission sunglasses.

We study the phase diagram of director structures in cholesteric liquid crystals of negative dielectric anisotropy in homeotropic cells of thickness d which is smaller than the cholesteric pitch p. The basic control parameters are the frustration ratio d/p and the applied voltage U. Upon increasing U, the direct transition from completely unwound homeotropic structure to the translationally invariant configuration (TIC) with uniform in-plane twist is observed at small d/p < or = 0.5. Cholesteric fingers that can be either isolated or arranged periodically occur at 0.5 < or = d/p<1 and at the intermediate U between the homeotropic unwound and TIC structures. The phase boundaries are also shifted by (1) rubbing of homeotropic substrates that produces small deviations from the vertical alignment; (2) particles that become nucleation centers for cholesteric fingers; (3) voltage driving schemes. A novel reentrant behavior of TIC is observed in the rubbed cells with frustration ratios 0.6 < or = d/p < or = 0.75, which disappears with adding nucleation sites or using modulated voltages. In addition, fluorescence confocal polarizing microscopy (FCPM) allows us to directly and unambiguously determine the three-dimensional director structures. For the cells with strictly vertical alignment, FCPM confirms the director models of the vertical cross sections of four types of fingers previously either obtained by computer simulations or proposed using symmetry considerations. For rubbed homeotropic substrates, only two types of fingers are observed, which tend to align along the rubbing direction. Finally, the new means of control are of importance for potential applications of the cholesteric structures, such as switchable gratings based on periodically arranged fingers and eyewear with tunable transparency based on TIC.

The helicoidal structure of cholesteric liquid crystals (CLCs) selectively refl ects light according to the Bragg condition, which at normal incidence simplifi es to:where n is the average index of the mixture and P is the pitch, defi ned as the length of one complete rotation of the helix. [ 1 ] Researchers have examined the utility of this mesophase in a range of topical areas including optics (switchable or tunable fi lters), photonics (selective shutters), and lasing (mirrorless feedback cavity). [ 1 , 2 ] Like other liquid-crystal phases, CLCs are responsive to a range of stimuli including temperature, electric fi elds, and light. The color switching or tuning response to each of these stimuli has been recently reviewed. [ 3 ] In particular, photoresponsive CLCs offer the capability of dynamic selective refl ection cued by light itself. Photoresponsive CLCs have been examined for more than forty years. Early examinations looked at photodegradation of halide-containing cholestryl mixtures [ 4 , 5 ] as well as guest-host mixtures with photochromics such as azobenzene. [ 6 ] Vinogradov et al. were the fi rst to demonstrate comparably larger optical responses by employing photoresponsive chiral materials to formulate the CLC mixture. [ 7 ] In this case, a menthone chiral dopant yielded a approximately 400-nm tuning of the refl ection bandgap with exposure to a 442-nm helium-cadmium laser. Convergent to this effort, Feringa, [ 8 , 9 ] Schuster, [10][11][12] and others pursued the synthesis of novel photochiral materials, which at the time were focused towards the development of an optical switch. In these examinations, as well as others that have followed, a variety of photochromic chiral materials have been examined including menthone, [ 7 , 13 , 14 ] chiral olefi ns, fulgides, [ 15 , 16 ] azobenzene, [17][18][19][20][21][22][23][24] and overcrowded alkenes. [25][26][27] Phototuning of the CLC refl ection notch has been presented in a number of recent reports. These works have primarily utilized azobenzene nematic liquid-crystal hosts [ 20 , 28-32 ] or chiral dopants. [17][18][19][20][21][22][23][24] The limited tuning range of guest-host azobenzene mixtures ( ≈ 50 nm) is expanded through the utilization of azobenzene nematic liquid-crystal hosts ( ≈ 250 nm) or further expanded by the use of azobenzene chiral dopants ( > 2000 nm). Unfortunately, a limitation of azo-based CLCs is the slow dark relaxation. While the at times days-long dark relaxation has been dramatically reduced through polymer stabilization, [ 32 ] materials with inherently faster dark relaxation are desirable.Towards this end, this work uses an overcrowded alkene (OA1; 9-(2-phenyl-2,3-dihydro-cyclopenta[a]naphthalen-1-ylidene)-9 H -fl uorene) as a chiral dopant. Overcrowded alkenes are known to have comparably rapid relaxation. Feringa and co-workers have championed these materials and showed their utility as optical switches, molecular motors, and as phototunable CLCs. [8,9] The structure and photoinversion of OA1 is shown in Figure 1 . ...