Phototunable Azobenzene Cholesteric Liquid Crystals with 2000 nm Range By Timothy J. White,* Rebecca L. Bricker, Lalgudi V. Natarajan, Nelson V. Tabiryan, Lisa Green, Quan Li, and Timothy J. Bunning* 1. Introduction Liquid crystals continue to be developed as dynamic optical materials. [1] To date, these materials have seen most extensive application in the visible spectra in displays, [2] filters, [3–6] and in a wide-range of specialty optical applications. [7–12] The emergence and growing utilization of infrared sources and their application in optical communication, the need for infrared optical materials and in particular, dynamic infrared optical materials are ever- growing. [13] Reconfigurable or tunable optical elements active across multiple spectral regions could present unique function- ality in nearly all these applications. Scientists have increasingly favored cho- lesteric liquid crystals (CLCs) as dynamic optical materials for liquid crystal displays, [1] lasers, [14–17] and optical filters. [18–20] The spectral position of these reflective materials is given in Equation 1: l b ¼ nP (1) where l b is the center wavelength of the reflection notch, n is the average of the ordinary (n o ) and extraordinary (n e ) refrac- tive indices for the LC ( n ¼ no þne ð Þ 2 ), and P is the pitch of the CLC. CLCs are formed from liquid crystal materials possessing inherent chirality (e.g., cholesterols) or by doping chiral molecules (chiral dopants) into nematic liquid crystals (NLCs). The chirality of the CLC encourages the liquid crystal molecules to align at a slight angle that yields a helical microstructure. The P of a CLC helix is defined as the unit length for a complete rotation. The sign and magnitude of P is controlled by the chiral nature of the LC or the helical twisting power (HTP) of the chiral dopant. The position of l b of CLCs formulated with chiral dopants is directly related to the concentration of the chiral compound. From Equation 1, the l b of the CLC can be spectrally shifted (e.g., tuned) by adjusting n or manipulating the length of P. Changes in the refractive index or the pitch can be induced with electric field, [21,22] heat, [20] or light [23–30] with advantages and disadvan- tages associated with each. Classically, the most studied approach has been electro-optic in systems made up of ‘‘static’’ chiral dopants, the HTP of which are unaffected by the application of a field. The interaction of CLCs that consist of the more common nematic LCs exhibiting a positive dielectric anisotropy (þDe) with an electric field has little effect on the pitch or the average refractive index in most CLC devices. Moderate electric field tuning has been observed in polymer-stabilized CLCs [31] and also recently, in CLCs based on liquid crystals with a negative dielectric anisotropy (–De). [20,21] An increasingly common means for fabricating dynamic, tunable CLC devices is to photosensitize the CLC material. [24,32] This approach can overcome some of the hurdles associated with electro-optic CLC devices while also extending application considerations. Phototunable CLC devices are formed through photosensitizing either the chiral dopant or the base NLC with chromophores that undergo photochemical transitions (isomer- ization, ring opening, etc.). Sackman first demonstrated photo- tuning in 1971, by demonstrating that the reflection of CLCs could FULL PAPER www.afm-journal.de [*] Dr. T. J. White, R. L. Bricker, Dr. L. V. Natarajan, Dr. T. J. Bunning Air Force Research Laboratory Materials and Manufacturing Directorate 3005 Hobson Way, Ste. 1 Wright Patterson Air Force Base, OH 45433 (USA) E-mail: timothy.white2@wpafb.af.mil, timothy.bunning@wpafb.af.mil R. L. Bricker SOCHE Inc. 3155 Research Blvd, Ste. 204, Dayton, OH 45420 (USA) Dr. L. V. Natarajan SAIC Inc. 4031 Colonel Glenn Hwy, Dayton, OH 45433 (USA) Dr. N. V. Tabiryan BEAM Engineering for Advanced Measurements 809 S. Orlando Ave, Winter Park, FL 32789 (USA) L. Green, Dr. Q. Li Department of Chemical Physics Liquid Crystal Institute Kent State University, P. O. Box 5190 Kent, OH 44242 (USA) DOI: 10.1002/adfm.200900396 Phototuning of more than 2000 nm is demonstrated in an azobenzene-based cholesteric liquid crystal (azo-CLC) consisting of a high-helical-twisting- power, axially chiral bis(azo) molecule (QL76). Phototuning range and rate are compared as a function of chiral dopant concentration, light intensity, and thickness. CLCs composed of QL76 maintain the CLC phase regardless of intensity or duration of exposure. The time necessary for the complete restoration of the original spectral properties (position, bandwidth, baseline transmission, and reflectivity) of QL76-based CLC is dramatically reduced from days to a few minutes by polymer stabilization of the CLC helix. 3484 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2009, 19, 3484–3488