Molecular-Shape-Dependent Luminescent Behavior of Dye Aggregates: Bent versus Linear Benzocoumarins

Aggregation
patterns of dye molecules can govern their photophysical
properties in the solid state. The linear and bent shaped dipolar
benzocoumarins showed contrasting luminescence behavior in solution
and in the solid state. Single crystal structures of both compounds
showed π-stacking patterns with eclipsing but opposite dipole
moments when viewed orthogonal to the stacking plane. Although the
bent molecules are stacked in parallel in the solid state, they behave
as independent molecules owing to the unfavorable excited state resonance
interaction and hence emit strong fluorescence because each dipolar
molecule now is in a hydrophobic environment surrounded by other molecules.
This is an unusual example where the shape-dependent stacking governs
the solid-state luminescence of dyes, being suggested here as a nonresonant
π-stacking system.