Controlling macromolecular superstructures of AIE-active porphyrin by manipulating pH in water

Abstract

Supramolecular superstructures are controlled by pH dependent self-assembly of octaphosphonatetetraphenyl-porphyrin- 1 (OPP-1) in aqueous solution. UV-Vis absorption spectroscopy shows decreasing in the intensity of OPP-1 in the pH range 3.0 to 6.0, which is correlated to H-type aggregates. However, absorption shifts to bathochromic (red-shift) under the influence of pH 8.0-11.0 range, which is typical for a J-type aggregate in absorbance spectrum as a result of supramolecular self-organisation constructed as compared to free OPP-1 (molecules transitioned to a monomeric positions). In these, the OPP-1 aggregated outside the surface pH 8.0-11.0 range is similar to growth of worm-like micelles. However, at pH range 3.0-6.0, OPP-1 is aggregated inside the surface. Self-assembled OPP-1 have been visualised on silicon wafer by Scanning Electron Microscopy (SEM), and it clearly shows that petite rod-like aggregates are formed at pH 5.0 and 6.0. Interestingly, OPP-1 assemble into directional growth to produce worm-like micelle aggregates at pH 8.0 and 9.0, respectively. The main driving force of assembled supramolecules are basically based on H-bonding and stacking between central core of porphyrin. Finally, Dynamic Light Scattering (DLS) study supports mode of aggregation of short rods and growth of wormlike micelles in basic pH and X-ray diffraction (XRD) study clearly demonstrates that self-assembled structures are crystalline in nature. This interesting pH-dependent self-assembly phenomenon is based on OPP-1 (synthetic analogue to natural occurring porphyrins) and can provide basis for development of novel conducting materials as well as biomaterials in the future.