Effect of Molecular Structure on Chiro-Optical and Photo-Optical Properties of Smart Liquid Crystalline Polyacrylates Alexey Bobrovsky, †, * Valery Shibaev, † Alexej Bubnov, ‡ Vĕ ra Hamplova ́ , ‡ Miroslav Kas ̌ par, ‡ and Milada Glogarova ́ ‡ † Faculty of Chemistry, Moscow State University, Leninskie gory, Moscow, 119992 Russia ‡ Institute of Physics, Academy of Sciences of the Czech Republic, 182 21 Prague, Czech Republic * S Supporting Information ABSTRACT: Comparative study of chiro- and photo-optical properties of three novel chiral photochromic liquid crystalline azobenzene-containing comb-shaped polyacry- lates was performed. The synthesized polymers contain the same polyacrylate backbones and uniform spacer size (10 methylene units), but have different structure of azobenzene-containing mesogenic groups with different chiral tail. The polymers are characterized by crystalline, SmA*, SmC*, and N* phases formation depending on chemical structure of the variable fragments. Presence of the lateral chlorine substituent in the mesogen fragment leads to an extreme decrease of isotropization temperature and causes a disappearance of cholesteric mesophase. Circular dichroism (CD) spectra demonstrate formation of helical order elements even in the amorphousized spin- coated polymer films. Possibility of manipulation of CD values and degree of the helical order by light and annealing was established and studied. UV-irradiation of all polymers induces E-Z isomerization of azobenzene groups and thus disrupting the LC-order. Besides, irradiation of films by polarized light leads to photoorientation process due to cycles of E-Z-E isomerization. As a result, appearance of significant linear dichroism dependent on the relative position of azobenzene moiety in aromatic core was evidenced. ■ INTRODUCTION Azobenzene-containing polymer systems are very promising materials from both academical and practical points. Their application in photonics and optoelectronics is very promising owing to their high photosensitivity combined with strong fatigue resistance. 1 They are very interesting because position of absorbance band and photooptical features are predetermined not only by the molecular chromophore structure (presence of different substituents) 1-3 but also by the supramolecular structure, for example, by self-assembled liquid crystalline (LC) media forming nematic, smectic and cholesteric phases. 4 Moreover, E-Z isomerization of azobenzene groups results in changes of their molecular shape and polarity, which usually causes significant structural transformations having impact on phase transitions, 5-7 wettability and surface tension modifica- tion, 8-10 and even photoactuation phenomena. 11-15 In most cases the action of polarized light on azobenzene- containing systems induces photoorientation of chromophores perpendicular to the polarization plane of excitation light that can be used for of LC-director reorientation, 16 induction of birefringence and dichroism, 1 preparation of “command surfaces”, 17 etc. In recent years remarkable attention is focused on the study of photoswitchable chiral self-organized systems (i.e., possess- ing the liquid crystalline behavior see, e.g., refs 18 and 19) or photoinduced chirality induction. 20-31 For example, chiro- optical properties and photoinduced changes in supramolecular organization under light action were studied for a number of chiral-photochromic azobenzene-containing LC polymers having different structure of chiral terminal group. 23 Let us point out that similar investigations of photo-chiro- optical properties of LC polymers were performed by us earlier. 29-33 We have also demonstrated a complicated photo- and thermoinduced behavior of the azobenzene-containing cholesteric copolymers and found elements of chiral organ- ization even in amorphousized films of cholesteric polymers prepared by spin-coating technique. The E-Z isomerization induced by the UV-irradiation reversibly disrupts the helical superstructure, whereas an annealing of the films leads to more pronounced helical structure formation. 30,33 In the case of smectogenic copolymers, homeotropic alignment of chromo- phores and mesogens (with the molecular axis normal to the film plane) is observed. 31,33 However, despite a large number of papers devoted to the azobenzene-containing LC systems, there are still a number of questions related to the structure-properties relationship regarding the chiro-optical behavior, mechanism of photo- orientation phenomena, etc. Moreover, up to now it is almost impossible to predict the polymorphism type for LC polymer just considering only their chemical structure. Taking into account the above-mentioned problems in the present paper we continued the study of chiral-photochromic Received: May 16, 2013 Revised: May 20, 2013 Published: May 29, 2013 Article pubs.acs.org/Macromolecules © 2013 American Chemical Society 4276 dx.doi.org/10.1021/ma401010t | Macromolecules 2013, 46, 4276-4284