Binary organogelators which show light and temperature responsiveness

Abstract

The gelation ability of 10 alkylammonium (C_nH_{2n + 1}NH₃⁺ where n = 4–11, 12 and 16) anthracene-9-carboxylates (1_n) has been evaluated. In cyclohexane, 1₄, 1₅, 1₆ and 1₇ only provided precipitates whereas 1₁₁, 1₁₂ and 1₁₆ provided very viscous solutions. In contrast, 1₈, 1₉ and 1₁₀ resulted in gels. The critical gelation concentration of 1₁₀ was very low (5.0 × 10⁻⁴ mol dm⁻³). SEM observations showed that in the gel phase the morphology changes from straight fibrils to frizzy fibrils with the increase in n, whereas in the sol phase the formation of the sheet-like, two-dimensional aggregate is recognized. When the cyclohexane 1₁₀ gel was photoirradiated (λ > 300 nm), the UV-VIS absorption bands assignable to monomeric anthracene were decreased and the gel was changed into the sol. It was confirmed by dark-field optical microscopy that the fibrillar bundles supporting the gel formation gradually disappear with photoirradiation time. When this sol was warmed at 30 °C in the dark, the gel was not regenerated but the precipitation of 1₁₀ resulted. When this sol was heated once at the bp of cyclohexane and cooled to 15 °C, the solution was changed into the gel again. This finding indicates that the fibrillar structure required for the gel formation is not reconstructed at 30 °C but obtained only when the hot cyclohexane solution is cooled.