Abstract
Photochromism is the reversible structural change of the molecules upon light irradiation having different spectral pattern before and after irradiation. Liquid crystal (LC) is a long-range orientational order of the molecules in between solid and isotropic liquid phases which share the properties of the crystal and liquid phases. 1-Alkyl-2-(ary-lazo)imidazoles, (Raai-CnH2n+1, R = H, Me and n = 10, 12, 14, 16, 18, 20, 22) exhibits photochromism and two of them (Haai-C18H37 and Haai-C22H45) show Liquid crystal (LC) properties. One of these compounds, Meaai-C16H33 is structurally characterized by single crystal X-ray diffraction study. The compounds upon irradiation with UV light show E-to-Z isomerisation. Quantum yields (φE→Z) of E-to-Z isomerisation vary with the molar mass of the compounds. The reverse transformation, Z-to-E, is recorded in thermal condition. The activation energy (Ea) of isomerisation is determined by the controlled temperature reaction. Liquid crystal property of the molecules is examined by polarizing optical microscopy (POM) and DSC. The carbon chain length n = 18 shows nematic (N)-isotropic (I) phase transition and an unknown Smectic (Sm) phase is observed with n = 22 on heating cycle.
Highlights
Smart molecules can respond to a specific external stimulus in a specific manner and are of increasing interest in areas as diverse as microbiology [1], microfluidics [2], organic electronics [3,4], high density optical data storage devices [5,6] etc
We describe a preliminary account on photochromic liquid crystal (LC) (PLC) activity of some long chain alkyl group substituted 1-alkyl-2(arylazo)imidazoles, Raai-CnH2n+1 (n = 10, 12, 14, 16, 18, 20, 22)
The liquid crystalline properties were established by thermal microscopy (Nikon polarizing microscope LV100POL attached with Linkam Temperature controlled stage model Examina-THMS600, and the phase transitions were confirmed by differential scanning calorimetry (Perkin-Elmer Diamond DSC Pyris1 system)
Summary
Smart molecules can respond to a specific external stimulus in a specific manner and are of increasing interest in areas as diverse as microbiology [1], microfluidics [2], organic electronics [3,4], high density optical data storage devices [5,6] etc. The research field has received a renewable thrust in the 1980s when the photo-induced reversible transformation was observed for organic molecules like spirooxazines, diarylethenes etc. The molecules have been used as photoresponsive functional devices utilized as smart polymers [16], liquid crystals [17], molecular switches [18] and machines [19]. Liquid crystals are widely used in information display technology and the anisotropic fluid states of rigid polymers are used for processing of high strength fibers. Arylazoimidazoles have several advantages of possessing both the photoactivity related to the azo unit, the capability of self-assembly through the imidazolyl group and coordinating ability to metal ions. Attachment of long chain alkyl groups to aromatic molecules may develop liquid crystal (LC) property.
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