Stimuli-Triggered Reversible Switching Mechanism between H- and J-Type Supramolecular Assemblies of Cationic Porphyrins Adsorbed on Tungsten(VI) Oxide Surface
K. Adachi, Y. Ura, N. Kanetada
J. Porphyrins Phthalocyanines, 2018, 22, 658-669.
DOI : 10.1142/S1088424618500372

Abstract: Supramolecular organic dye–inorganic semiconductor nanocrystal assemblies are potentially useful in a broad range of technologies and applications, including photovoltaic systems, but the molecular basis of the adsorption of dye molecules onto the semiconductor surfaces remains poorly understood. Herein, we investigated the pH-dependent adsorption and conformational change of two cationic porphyrin stereoisomers [5,10-diphenyl-15,20-di(N-methyl-4-pyridyl)porphyrin (cis-DMPyP) and 5,15-diphenyl-10,20-di(N-methyl-4- pyridyl)porphyrin (trans-DMPyP)] on the tungsten(VI) oxide (WO3) colloid nanoparticle in aqueous media by means of UV-vis absorption spectroscopy. In accordance with the combination of a modified Langmuir adsorption model and Kasha’s exciton coupling model, the molecular orientation and stacking arrangement of DMPyP derivatives on the WO colloid surface are discussed in detail. In the trans-DMPyP/WO3 aqueous system, trans-DMPyP molecules adopted flat-on orientation with respect to the WO colloid surface and eventually formed head-to-tail J-dimers regardless of pH conditions. cis-DMPyP molecules in the acidic system also lay flat-on and mainly formed H-dimers on the WO3 colloid surface, whereas ones in the neutral system exhibited a dominant edge-on orientation and had a higher tendency to form face-to-face -dimers. Additionally, we have also convincingly demonstrated the pH-triggered switchable π-stacking geometry of cis-DMPyP molecules from H- to J-dimer and vice versa on the WO3 colloid surface. Such findings will undoubtedly provide a pertinent guideline for the rational design of stimuli-responsive organic-inorganic materials.