Chiroptical Switching between Liquid Crystalline Phases

The modulation of the mesophases and physical properties of liquid crystalline (LC) materials by reversible photochemical reactions, i.e., photoswitching of LC phases, is a major challenge for information technology. The photochromic behavior of organic molecules has also attracted renewed interest with a view to the development of materials for optical data storage and molecular optical devices. It is well-known that small amounts of optically active guest molecules added to a nematic host induce a cholesteric phase and that the helical pitch in the twisted nematic phase is very sensitive to chemical modifications in the chiral guest molecule. Reversible switching of a liquid crystal between a twisted and a nontwisted phase by photochemical modification of the chirality of the doping molecule, although predicted, has thus far not been realized. In their pioneering efforts to develop a chiral liquid crystalline phototrigger, Schuster and co-workers 6 showed that photoracemization of optically active binaphthyl derivatives induces a cholesteric to nematic transition in a doped phase. We describe herein the first successful demonstration of the reversible photochemical modulation of the helical pitch of a twisted nematic LC phase and the reversible transition between cholesteric and nematic phases using as dopant an optically active photoresponsive molecule. The chiroptical molecular switch employed is based on the donor-acceptor substituted inherently dissymmetric alkenes cisand trans-2-nitro-7-(dimethylamino)-9-(2 ,3 -dihydro-1 H-naphtho[2,1-b]thiopyran1 -ylidene)-9H-thioxanthene (1 and 2) (Scheme 1). Addition of 1 wt % of enantiomerically pure P-trans-2 to the nematic phase, obtained from 4 -(pentyloxy)-4-biphenylcarbonitrile 3,