Mechanism of Peripheral Substituent Effects on Adsorption–Aggregation Behaviors of Cationic Porphyrin Dyes on Tungsten(VI) Oxide Nanocolloid Particles

The adsorption and aggregation behaviors of the cationic porphyrin derivatives such as 5,10,15,20-tetrakis­(4-pyridyl)­porphyrin [TPyP], 5,10,15,20-tetrakis­(<i>N</i>-methyl-4-pyridyl)­porphyrin [TMPyP], 5,10,15,20-tetrakis­(<i>N</i>-ethyl-4-pyridyl)­porphyrin [TEPyP], and 5,10,15,20-tetrakis­(<i>N</i>-<i>n</i>-propyl-4-pyridyl)­porphyrin [TPPyP] (hereafter called “TPyP derivatives”) in the tungsten­(VI) oxide (WO₃) colloid aqueous solution at weak acidic pH were studied by UV–vis spectroscopy. The TPyP derivatives were strongly adsorbed as monolayer onto the WO₃ surface <i>via</i> the electrostatic interaction between their peripheral cationic substituents and negatively surface-charged WO₃ colloid particles, and most of the ones adsorbed eventually formed <i>J</i>-type dimers aligned in the head-to-tail fashion. These different dimerization states were effectively analyzed by the change of ratios among the intensities of exciton split Soret bands (<i>H</i>- and <i>J</i>-bands). Judging from the exciton coupling theory and adsorption measurements, we concluded that the <i>J</i>-dimer geometry of the TPyP derivatives adsorbed on the WO₃ colloid particle surface is strongly dependent on the presence and difference of peripheral substituents. The results described here indicate a new and promising way of designing surface supramolecular structures combination of two principles, the self-association of organic dyes, and the steric repulsive interaction between the peripheral substituents and the inorganic semiconductor surfaces