Engineering photoresponsive liquid crystals through halogen bonding

Photochromic organic molecules are playing an important role in present-day research on materials science and photonics, since they offer the unique possibility to control the properties of a system with light [1]. Due to their efficient and reversible photoisomerization, azobenzene derivatives are widely used as photo-switches [2]. Such photo-switches have been implemented in a variety of materials ranging from optical devices to smart polymers and liquid crystals (LCs) [1]. The simplest way to obtain a photoresponsive LC is to incorporate a small amount of azobenzene units into the liquid crystal. Such systems allow, e.g., for isothermal LC-to-isotropic phase transition that can be reversibly controlled with light [3]. However, it is also possible to incorporate the photoactive moiety as a part of a supramolecular LC system [4]. We pursued both these strategies, introducing halogen bonding [5] as a new tool to design photoresponsive liquid-crystalline complexes (Figure 1a). We are now able to combine highly efficient photoalignment and surface-relief grating formation (Figure 1b,c) in azobenzene-containing, low-molecular-weight supramolecular LCs [4]. On the other hand, adding a small amount of azobenzene dopant containing a halogen-bond donor moiety into the widely used LC 4-cyano-4'-pentylbiphenyl, promotes the photomodulation of the LC alignment due to halogen bonding to the cyano group [6]. These materials are the first example of photoresponsive halogen-bonded liquid crystals, having potential applications in photonics and material chemistry