Porphyrin J-aggregates as a probe for chiral impurities as demonstrated by their symmetry breaking by confinement in montmorillonite clay

In this paper, we demonstrate that chiral J-aggregates of porphyrins are able to detect minute chiral impurities, in this case, the presence of right-handed quartz in acid-activated K10 montmorillonite clay. Aggregation and symmetry breaking of 5,10,15,20-(tetra-4-carboxyphenyl) porphyrin (TCPP) and 5,10,15,20-(tetra-4-sulfonatophenyl) porphyrin (TPPS) were observed upon interaction with acid-activated montmorillonite clay (MMT-K10). A panel of characterization techniques, including UV-visible, electronic circular dichroism, IR, and vibrational circular dichroism spectroscopies, as well as X-ray scattering, were employed to investigate the aggregation of the confined TPPS and TCPP. An intriguing and persistent negative exciton effect was detected in the electronic circular dichroism spectra at the early stages of J-aggregation. The enrichment of a right-handed quartz impurity in the clay detected by vibrational circular dichroism may explain the symmetry breaking in the porphyrin assemblies. As a result, we propose that chiral porphyrin aggregates could serve as innovative probes for detecting subtle chirality in inorganic nanomaterials.