Identification of H-aggregate in a monolayer amphiphilic porphyrin-TiO₂ nanoparticle heterostructure assembly and its influence on the photoinduced charge transfer

The spectral properties of the monolayer 5,10,15-tri(4-hydroxyphenyl)-20-(4-hexadecyloxyphenyl) porphyrin molecule (TriHHP)-TiO₂ nanoparticle heterostructure assembly were investigated with absorption, fluorescence emission and excitation spectra, in comparison with those of CHCl₃ solution and monolayer films deposited from pH 3 and pH 10 water subphases. A new fluorescence emission band, arising from a blue-shifted Q-absorption band, was observed in the assembly or the monolayer film deposited from pH 3 water subphase. An H-aggregate type species from the protonated TriHHP molecules was assigned to cause this new emission band. According to the Frontier orbital theory, this H-aggregate is suggested in the configuration of face-to-face π-π stacking interaction among the protonated TriHHP molecules. In the heterostructure assembly, the efficient fluorescence emission quenching of TriHHP by the photoinduced charge transfer occurs under excitation in Soret band region of the unprotonated TriHHP molecules. No contribution to the photoinduced charge transfer.