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

Abstract

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.

Linear Extrapolation of the Analyte-Specific Light Scattering and Fluorescence Depolarization in Turbid Samples

“…The fact that TPE has such small fluorescence depolarization is likely due to its large aggregation size, which reduces its mobility in solution. This data is consistent with a recent report that dispersed porphyrin has fluorescence depolarization as large as 1 whereas the fluorescence depolarization of the aggregated porphyrin is as small as 0.5 . The fact that TPE emission depolarization is approximately independent of both its excitation and detection wavelengths is surprising, given the relatively broad size distribution of the TPE aggregates (particle diameter ranges from 316 to 416 nm, Figure S10).…”

“…14 Light-scattering depolarization has enabled in situ and nondestructive monitoring of the structural modification of plasmonic gold nanoparticle aggregation and porphyrin self-assembly. 13,18…”

“…In contrast, light scattering occurs instantaneously upon photon excitation (within femtosecond time scale): scatterers are essentially immobile in solutions during individual photon excitation and scattering cycles. The degree of the light-scattering depolarization depends solely on the scatterers’ geometries and their chemical compositions. ,,, As an example, the light-scattering depolarization spectrum of linear solvent molecule CS 2 is 0.5 crossing the entire UV–vis region whereas depolarization of the tetrahedral CCl 4 is essentially zero (below the limit of detection of 0.02) . Light-scattering depolarization has enabled in situ and nondestructive monitoring of the structural modification of plasmonic gold nanoparticle aggregation and porphyrin self-assembly. , …”

“…14 Light-scattering depolarization has enabled in situ and nondestructive monitoring of the structural modification of plasmonic gold nanoparticle aggregation and porphyrin self-assembly. 13,18 Whereas quantification of the sample light scattering and fluorescence depolarization is straightforward, reliable determination of the intrinsic analyte-specific fluorescence and scattering depolarization in turbid samples is challenging. This difficulty arises from the depolarization propagation occurring from the multiplicative light scattering and the interplay of light-scattering and fluorescence emission depolarizations.…”

Using resonance synchronous spectroscopy to characterize the reactivity and electrophilicity of biologically relevant sulfane sulfur

“…Individual fluorophores are often considered as simultaneous photon absorbers and emitters with no significant light scattering due to small sizes [37]. Consequently, RS 2 is often observed from the aggregated fluorophores, which usually are simultaneous photon absorbers, scatters, and fluorescence emitters [21].…”