Missing Links between the Structures and Optical Properties of Porphyrin Assemblies

The molecular assembly has evolved into a popular and effective method for bottom-up fabrication of functional nanomaterials with narrow size and shape distributions. While the optical properties of assembled molecules usually differ drastically from those of dissolved monomers, knowledge on the correlation between the structures and the optical properties of the molecular assemblies remains scant. Using the recent polarized resonance synchronous spectroscopic method in combination with the polarized anti-Stokes, on-resonance, and Stokes-shifted spectroscopic approach, we performed a quantitative investigation of the optical properties of the self-assembled porphyrins with different shapes, sizes, and chemical compositions. Light scattering extinction of aggregated porphyrins depends strongly on wavelength, contributing up to 90% of its total UV–vis extinction measured with the conventional UV–vis spectrophotometer. While the peak scattering extinction invariably appears in the wavelength region in which the porphyrin assemblies are absorbed, signifying the resonance light scattering, there is no apparent correlation among the sample total UV–vis extinction, absorption extinction, and scattering extinction spectra. Correlation between the light scattering depolarization spectrum and the shape of the porphyrin assemblies and that between the scattering extinction intensity and the size of the assemblies are also very weak. These data strongly suggest that the intermolecular interaction among assembled porphyrin molecules holds the key to the mechanistic understanding of the optical properties of the assembled porphyrins. While the methodology provided in this work should be valuable for enhancing the quantitative understanding of the optical properties of optically complex molecular assemblies, the presented findings shed light on the missing links between the structure and optical properties of porphyrin assemblies.