Orientation change of porphyrin in aggregate caused by counterion

Aggregates of the diacid form of meso-tetraphenylporphyrin (TPP) were formed by evaporating organic solutions on quartz substrate. The effect of counterions on TPP aggregates, which were obtained through adding various kinds of acids HX(X = HSO₄, NO₃, Br and Cl) to TPP dichloromethane solution, has been investigated through the combined use of UV–vis absorption and Raman scattering spectroscopy. The protonated TPP and the resulting aggregates exhibited spectroscopic features that were remarkably influenced by the nature of counterions. For all the aggregates, split of the B state absorptions with components red- and blue-shifted relative to their diacid solution values indicated the J-type arrangement of TPP molecules was formed in the aggregates, and the magnitude of this shift was markedly influenced by the nature of the counterions; the (C_b ? H) in-plane bending vibrational bands in the Raman spectra of the aggregates also showed dependence on counterions. A model for TPP J-aggregates has been proposed on the basis of the angle between transition moment along the chain and the projected line connecting the centers of porphyrins onto the porphyrin plane, and the extended evidence was also obtained by investigation of the Raman spectra of the aggregates from the sulfuric diacid TPP organic (toluene, dichloromethane, acetone and chloroform) solutions, and T(4-NO₂P)P, T(4-MeOP)P dichloromethane solutions.