Tuning Hofmeister ion sensitivity in porphyrin J-aggregation through molecular charge density

This study investigates the self-assembly kinetics of three structurally related sulfonato-phenylporphyrins, 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS4), 5,10,15-tris (4-sulfonatophenyl)-20-phenyl-porphyrin (TPPS3) and 5,15-bis(4-sulfonatophenyl)-10,20-phenyl-porphyrin (t-TPPS2) into supramolecular J-aggregates in acidic aqueous solutions, using a series of inorganic acids. The aggregation process, followed by UV/Vis spectroscopy, monitoring the characteristic B-band disappearance (434 nm) and the sharp, red-shifted J-band appearance (490 nm), exhibits typical sigmoidal, autocatalytic kinetic profiles. Kinetic parameters derived from Pasternack’s model reveal that TPPS4 and t-TPPS2 aggregation rates adhere to the Hofmeister series for anions (SO42− > Cl− > Br− > NO3− > ClO4−), correlating linearly with the anions’ structure-making/breaking abilities. Notably, t-TPPS2 kinetics are an order of magnitude faster than TPPS4. In contrast, TPPS3 exhibits deviations from Hofmeister behavior together with smaller critical nucleus sizes, likely reflecting its distinct nano-ribbon morphology compared with the nano-tubes/stacked platelets structures formed by the other porphyrins. This work highlights how subtle molecular changes and the ionic environment tune macroscopic material properties via non-covalent interactions.