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 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 π‐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 ( λ max =389 nm) and in the solution ( λ 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.