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.
The spatial disposition of three benzene rings in triptycene permits through-space overlap of the orbitals of fused benzene sp2 carbons, leading to ‘homoconjugation’; the latter is a contentious issue in...
A set of three donor−acceptor azahelical coumarins (DA-AHCs), namely, H-AHC, Me-AHC, and Ph-AHC, were rationally designed and synthesized, and their excited-state properties were comprehensively investigated. All three DA-AHCs are shown to display very high fluorosolvatochromic shifts as a result of significant intramolecular charge transfer in their excited states. The para-quinoidal forms of the latter apparently contribute predominantly to large dipole moments in their excited states. By virtue of the fact that these helical systems structurally incorporate a highly fluorescent coumarin dye, they exhibit high quantum yields in both solution and solid states. Indeed, their emission behaviors in the crystalline media are shown to be remarkably correlated with their respective crystal packings. Incisive analyses demonstrate (i) strengthening of hydrogen bonding in the excited state promotes quenching (H-AHC), (ii) efficient crystal packing promotes high emission (Me-AHC) by precluding deactivations via vibrational motions, and (iii) loose crystal packing contributes to excited-state deactivation to account for low quantum yields of emission (Ph-AHC).
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