Quater-, quinque-, and sexithiophene organogelators: unique thermochromism and heating-free sol-gel phase transition.

A series of quater-, quinque-, and sexithiophene derivatives bearing two cholesteryl groups at the alpha-position, which are abbreviated as 4 T-(chol)(2), 5 T-(chol)(2), and 6 T-(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 pi-block moieties of the oligothiophenes are stacked in an H-aggregation mode. Surprisingly, an AFM image shows that 4 T-(chol)(2) forms unimolecular fibers in a left-handed helical sense, whereby one pitch of the helical fiber is constructed by 400-540 4 T-(chol)(2) molecules. Very interestingly, the conformational change in the oligothiophene moieties can be visually detected: for example, 6 T-(chol)(2) shows a specific absorption maximum in the gel (lambda(max) = 389 nm) and in the solution (lambda(max) = 439 nm). In addition, a sol-gel phase transition of the 6 T-(chol)(2) gel was implemented by addition of oxidizing and reducing reagents such as FeCl(3) and ascorbic acid, respectively. The stimuli-responsive functionality of the oligothiophene-based organogels makes them promising candidates for switchable opto- and electronic soft materials.