A series of quater-, quinque-, and sexithiophene derivatives bearing two cholesteryl groups at the α-position, which are abbreviated as 4T-(chol)2, 5T-(chol)2, and 6T-(chol)2, respectively, have been synthesized. It has been found that these oligothiophene derivatives act as excellent organogelators for various organic fluids and show the unique thermochromic behaviors through the sol-gel phase transition. It was shown on the basis of extensive investigations, performed with UV-visible spectroscopy, circular dichroism (CD), transmission electron" microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM), that these gelators self-assemble into the one-dimensional structures in the organogels, in which the π-block moieties of the oligothiophenes are stacked in an H-aggregation mode. Surprisingly, an AFM image shows that 4T-(chol) 2 forms unimolecular fibers in a left-handed helical sense, whereby one pitch of the helical fiber is constructed by 400-540 4T-(chol)2, molecules. Very interestingly, the conformational change in the oligothiophene moieties can be visually detected: for example, 6T-(chol)2 shows a specific absorption maximum in the gel (λmax = 389 nm) and in the solution (λmax = 439 nm). In addition, a sol-gel phase transition of the 6T-(chol)2 gel was implemented by addition of oxidizing and reducing reagents such as FeCl3 and ascorbic acid, respectively. The stimuli-responsive functionality of the oligothiophene-based organogels makes them promising candidates for switchable opto- and electronic soft materials.
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