High-Throughput Single-Molecule Spectroscopy Resolves the Conformational Isomers of BODIPY Chromophores

A borondipyrromethene
(BODIPY) chromophore is connected to a benzoxazole,
benzothiazole, or nitrobenzothiazole heterocycle through an olefinic
bridge with trans configuration. Rotation about the
two [C–C] bonds flanking the olefinic bridge occurs with fast
kinetics in solution, leading to the equilibration of four conformational
isomers for each compound. Ensemble spectroscopic measurements in
solutions fail to distinguish the coexisting isomers. They reveal
instead averaged absorption and emission bands with dependence of
the latter on the excitation wavelength. Using high-throughput single-molecule
spectroscopy, two main populations of single molecules with distinct
spectral centroids are observed for each compound on glass substrates.
Computational analyses suggest the two populations of molecules to
be conformational isomers with antiperiplanar and periplanar arrangements
of the BODIPY chromophores about its [C–C] bond to the olefinic
bridge. Thus, statistical analysis of multiple single-molecule emission
spectra can discriminate stereoisomers that would otherwise be impossible
to distinguish by ensemble measurements alone.