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(N-methyl-4-pyridyl)porphyrin [TMPyP], 5,10,15,20-tetrakis(N-ethyl-4-pyridyl)porphyrin [TEPyP], and 5,10,15,20-tetrakis(N-n-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 via the electrostatic interaction between their peripheral cationic substituents and negatively surface-charged WO3 colloid particles, and most of the ones adsorbed eventually formed J-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 (H- and J-bands). Judging from the exciton coupling theory and adsorption measurements, we concluded that the J-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.