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 . ...

Polymer networks with dynamic cross-links have generated widespread interest as tunable and responsive viscoelastic materials. However, narrow stoichiometric limits in cross-link compositions are typically imposed in the assembly of these materials to prevent excess free cross-linker from dissolving the resulting polymer networks. Here we demonstrate how the presence of molecular competition allows for vast expansion of the previously limited range of cross-linker concentrations that result in robust network assembly. Specifically, we use metal-coordinate cross-linked gels to verify that stoichiometric excessive metal ion cross-linker concentrations can still result in robust gelation when in the presence of free ion competing ligands, and we offer a theoretical framework to describe the coupled dynamic equilibria that result in this effect. We believe the insights presented here can be generally applied to advance engineering of the broadening class of polymer materials with dynamic cross-links.

No abstract