Optical Properties and Kinetics: New Insights to the Porphyrin Assembly and Disassembly by Polarized Resonance Synchronous Spectroscopy.

With their unique photochemical properties, porphyrins have remained for decades the most interested chemicals as photonic materials for applications ranging from chemistry, biology, medicine, to photovoltaic. Porphyrins can self-assemble into higher order structures. However, information has been scant on the kinetics and structural evolution during porphyrin assembly and disassembly. Furthermore, quantitative understanding of the porphyrin optical activities is complicated by the complex interplay of photon absorption, scattering, and fluorescence emission that can concurrently occur in porphyrin samples. Using meso-tetrakis(4-sulfonatophenyl)porphyrin as the model molecule, reported herein is a combined UV-vis extinction, polarized Stokes-shifted fluorescence, and polarized resonance synchronous spectroscopic (PRS2) study of porphyrin assembly and disassembly in acidic solutions. Although porphyrin assembly and disassembly occur instantaneously upon the sample preparation, both processes last at least a few months before reaching their approximate equilibrium states. The two processes were monitored in situ by quantifying the porphyrin fluorescence and scattering depolarizations as well as its extinction, absorption, scattering, and fluorescence emission cross sections. In addition to a series of new insights to the porphyrin assembly and disassembly, the methodology described in this work opens the door for the in situ study of the structural and optical properties of photonic materials comprising molecular assembly.