Halogen-Bonded Photoresponsive Materials (original) (raw)

2014, Topics in Current Chemistry

Halogen bonding (XB) is among the most recent and the least exploited non-covalent interactions in the toolbox of supramolecular sciences. It can be defined as an attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same molecular entity [1]. Most of the properties of the halogen bond, such as directionality, strength, and tunability, can be rationalized by considering the interaction as essentially electrostatic. Electrostatic forces give rise to an electropositive area on the surface of the halogen-bond donor atom, narrowly located on the extension of the R-X bond and called a σhole [2]. Due to this region, an electron donor (e.g., a Lewis base) will be attracted by the σ-hole and depleted by the rest of the atom surface. This explains the high directionality and linearity of halogen-bonded structures, both in the gas phase [3] and in the solid state [4]. The strength of the halogen bonding can be adjusted by tuning the polarizability of the halogen atom: the higher the polarizability, the greater the interaction strength. Also increasing the electron-withdrawing capability of the moiety to which the XB-donor is attached can increase the interaction strength. Within the past decade, halogen bonding has emerged as a first-class tool in fields as diverse as supramolecular chemistry [5], medicinal chemistry [6], crystal engineering [7], biochemistry [8], and materials science [9,10]. The present work is focused on a specific subset of the latter where halogen bonding seems particularly promising, namely photoresponsive azobenzene-containing materials. The first reports on halogen-bonded photoresponsive polymers and liquid crystals appeared as recently as in 2012 [11, 12]. The field is still in its infancy, but nonetheless highly potential and multifaceted, as highlighted by the examples given herein. The remainder of this review comprises a general introduction to azobenzene, the key photoactive unit of the studies presented. Subsequently, we will bring out the recent advances in halogen-bonded photoactive polymers, liquid crystals, and crystals, respectively, before concluding with some future perspectives.