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 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 (λ_max = 389 nm) and in the solution (λ_max = 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.