Strength Enhancement of Nanostructured Organogels through Inclusion of Phthalocyanine-Containing Complementary Organogelator Structures and In Situ Cross-Linking by Click Chemistry

Stable photoactive organogels were successfully prepared by a two-step sequence involving: 1) formation of thermoreversible organogels by use of a combination of low-molecular-weight organogelators (LMOGs) and Zn-II-phthalocyanine (Zn-II-Pc) moieties containing complementary organogelator structures, and 2) strength enhancement of the gels by in situ cross-linking with the aid of Cu-1-catalysed azide-alkyne [3+2] cycloadditions (CuAACs). The optimum click reaction was carried out between a flexible C6 aliphatic diazide and a suitable dialkyne (molar ratio 1:1) added in a low proportion relative to the organogelator system [LMOG+(ZnPc)-Pc-II]. The dialkyne unit was incorporated into a molecule resembling the LMOGs structure in such a way that it could also participate in the self-assembly of [LMOG+(ZnPc)-Pc-II]. The significant compatibility of the multicomponent photoactive organogels towards this strengthening through CuAACs allowed their sol-to-gel transition temperatures (T-gel) to be enhanced by up to 15 degrees C. The T-gel values estimated by the inverse flow method were in good agreement with the values obtained by differential scanning calorimetry (DSC). Rheological measurements confirmed the viscoelastic, rigid, and brittle natures of all Pc-containing gels. Transmission and scanning electron microscopy (TEM, SEM) and atomic force microscopy (AFM) revealed the fibrilar nature of the gels and the morphological changes upon cross-linking by CuAAC. Emission of a red luminescence from the dry nanoscale fibrous structure-due to the self-assembly of the Pc-containing compounds in the organogel fibres-was directly observed by confocal laser scanning microscopy (CLSM). The optical properties were studied by UV/Vis and fluorescence spectroscopy. Fluorescence, Fourier-transform infrared (FTIR) and circular dichroism (CD) measurements were also carried out to complete the physicochemical characterization of selected gels. As a proof of concept, two different organogetators (cholesterol- and diamide-based LMOGs) were successfully used to validate the general strategy.