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A Comparative Account of the Kinetics of Light-Induced EZ Isomerization of an Anthracene-Based Organogelator in Sol, Gel, Xerogel, and Powder States: Fiber to Crystal Transformation

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journal contribution
posted on 09.05.2016, 00:00 by Sanjoy Mondal, Priyadarshi Chakraborty, Sujoy Das, Partha Bairi, Arun K. Nandi
The organogel of (E)-N′-(anthracene-10-ylmethylene)-3,4,5-tris­(dodecyloxy)­benzohydrazide (I) in methyl cyclohexane having a fibrillar network structure exhibits excellent fluorescence, which decreases sharply with time upon photoirradiation at λ = 365 nm. It has been attributed to the transformation of the E isomer of I to the Z isomer, and the kinetics of EZ isomerization are compared for the sol, gel, xerogel, and powder states. The rate constants at different temperatures are measured from Avrami plots and its increase with an increase in temperature, indicating temperature acts as a promoter for photoirradiated EZ isomeization along the imine (CN) bond. In the powder form, the rate constant values are the lowest compared to those of other states for all temperatures and the xerogels exhibit the highest rate of EZ isomerization. The rate constants of sol and gel states mostly lie between the two. The wide-angle X-ray scattering pattern changes after ultraviolet (UV) irradiation with the generation of new sharp peaks whose intensities increase with an increase in irradiation time. A polarized optical microscopic study indicates formation of small crystalline dots on the fibers in the gels, dendritic morphology on the xerogel fibers, and large needlelike morphology at the surface boundary of the solid. The dried I gel exhibits a melting peak at 96.7 °C, but upon irradiation, two peaks are observed at 98.5 and 152.7 °C; the latter has been attributed to the melting of crystals of Z isomers. Similar higher melting peaks are observed both for the xerogel and for powders after UV irradiation; the powders exhibit the highest meting peak at 159.4 °C. Possible reasons for the variation of rate constant values in the four different states and the difference in morphology and melting points of crystals of Z isomers of I are discussed.