The Coumarin-Dimer Spring-The Struggle between Charge Transfer and Steric Interactions.

The synthesis of a weakly coupled, strongly polarized coumarin dimer has been achieved for the first time. The three-step strategy comprises the Skattebøl formylation followed by the Knoevenagel reaction and the formation of a tertiary amide by using a peptide-type procedure. The molecule consists of two different coumarin moieties: One is a classical donor-acceptor system and the second one possesses a weaker amide donor at the 7-position. The polarized coumarin dimer can form an electronically conjugated structure possessing an electric dipole larger than that of 7-(dimethylamino)coumarin-3-carboxylic acid. The limited flexibility of the inter-coumarin connection results in stable conformers of different electric dipole moments and complex photophysics. In the solid state, this compound has a strongly bent conformation with the two coumarin units forming an angle of around 74°. In solution, two conformers are in equilibrium. The existence of the molecule as two conformers in the ground state has been confirmed by optical studies, and further corroborated by molecular calculations. The fluorescence spectra possess a unique feature: A charge-transfer band (ca. 550 nm) is visible only in nonpolar or weakly polar solvents. Optical spectroscopy studies coupled with molecular calculations allowed us to rationalize this phenomenon: The large amplitude of intramolecular motions is responsible for the conformational isomerization as well as producing a conical intersection between the potential energy surfaces of the excited singlet state and the ground state, which opens an internal conversion channel that effectively competes with the fluorescence of the conformers.