Measuring molecular aggregates in sub-attomolar solutions

This contribution presents fluorescence spectra of monomer and assembled tetratolylporphyrin molecules in ultralow concentrated solutions. The molecules were dissolved in acetone. Fluorescence signals could be detected from solutions whose concentration ranged from 10−14 to 10−19 M. The detection limit of molecular fluorescence emissions was found at 10−20 M. The molecular assemblies could be determined as J-aggregates in which the monomer units are shared along the y-axis (N–H⋯H–N) of the porphyrin coordinate system. Two different types of J-aggregates are present. One type appeared in freshly prepared solutions only and turned into the second stable form after few hours