Coumarin-Annelated Regioisomeric Heptahelicenes: Influence of Helicity on Excited-State Properties and Chiroptical Properties

Two
regioisomeric pairs of heptahelical mono- and biscoumarins
that are differentiated by “inward”
and “outward” disposition of the pyran-2-one
moiety have been synthesized and investigated to understand the influence
of helicity on excited-state and chiroptical properties. A slight
variation in the helicities is found to manifest in contrasting excited-state
properties of coumarin-annelated heptahelicenes; in addition to the
intramolecular charge transfer, structural relaxation in the excited
state is shown from theoretical calculations to cause decrease in
the fluorescence quantum yield for a system with higher helicity.
The optically pure enantiomers of heptahelical coumarins exhibit helicity-dependent
chiroptical properties, namely, specific rotations, molar ellipticities,
Cotton effects, and anisotropic dissymmetry factors. Theoretical calculations
point to factors that are not readily explicable.