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.