Organogels from unsymmetrical π-conjugated 1,3,4-oxadiazole derivatives

In this study, the unsymmetrical pi-conjugated gelators, 2-(3,4-bis(alkoxy)phenyl)-5-(pyridine-4-yl)-1,3,4-oxadiazole [4-POXD-Bn (n = 4, 8, 12)], were synthesized and screened for gelation in various solvents. The tendencies of the critical gelation concentration (CGC) and the gel-sol phase transition temperatures (T-gel) of the 4-POXD-B8 gels, as well as the relationship between the end-chain length of gelator and gelation ability, indicated that the solvophobic effect has a critical role in gelation. For the specific gelator that we studied, we observed that the strength of the solvophobic force was related to the solvent properties. The solvent effect on gelation was studied quantitatively using the solvent polarity parameters: polar solubility parameter (delta(a)), dielectric constant (epsilon), and polarity parameter (E-T(30)). The results revealed that the favorable delta(a) domain and E-T(30) domain for the gelation of 4-POXD-B8 was in the range of 8.1-12.6 (cal cm(-3))(1/2) and 43.3-55.7 kcal mol(-1), respectively. Meanwhile, a decrease of delta(a) and E-T(30) in the solvents is unfavorable for gelation. Furthermore, the solvent effect on the microstructure of the gels was studied via scanning electron microscopy (SEM). The SEM images revealed that the gels from 4-POXD-B8 in DMSO and DMF were composed of helical ribbons, whereas those formed in other solvents consisted of flat ribbons.