Photoinduced E-Z isomerization across the imine bond in a multi-responsive organogel affecting morphology and structure

Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700 032, India

E-mail : psuakn@iacs.res.in

Manuscript received 02 November 2016, accepted 30 December 2016

A low molecular weight organogelator, (E)-N'-(anthracene-10-ylmethylene)-3,4,5-tris(dodecyloxy)benzohydrazide (OG) produces thermo- and mechano-responsive organogel in methyl cyclohexane. The organogel exhibits green fluorescent which is also retained in the xerogel state. The gel exhibits fibrillar three dimensional network morphology with average diameter 45±8 nm. Rheological experiments reveal shear thinning and thixotropic properties of the gel. The fluorescence intensity of the gel is ~6 times higher than that of the solution with a 14 nm red shift indicating the J-type aggregate formation. On UV-irradiation at 365 nm E-isomer of OG transforms into Z-isomer in both gel and xerogel state and 1H NMR spectra clearly points out the transformation. The kinetics of E-Z isomerization of OG in gel phase is carried out with duration of photo-irradiation at different temperatures by fluorescence spectroscopy. The rate constant values of photo-isomerization of the gels are obtained from the intercepts of Avrami plot and they are 0.16, 0.28, 0.31 and 0.49 min^–1 for 10, 20, 30 and 40 ºC, respectively. The increase of rate constants with temperature indicates the process is Arrhenius type. The WAXS spectrum changes with UV-irradiation characterizing the formation of Z-isomer. On UV-irradiation for 24 h the fibers of OG gel transforms into microscopic crystals and the microcrystallization occurs very fast in the xerogel state. The crystals are prominent in 4.5 h of UV-irradiation as evident from the bright birefringence pattern of the crystals with mostly dendritic morphology.