Surprising Properties of a Furo‐Furanone

The electronic absorption, fluorescence, and excitation spectra of furo[3,4‐c]furanone ( 1 ) have been measured in different solvents at different concentrations. We observed a complex dependence of absorption and excitation spectra as a function of the concentration in CH₂Cl₂ and THF due to aggregate formation. Interestingly, the fluorescence spectra were not affected. Resolving the puzzle was made possible by the fact that 1 fits perfectly into the channels of zeolite L (ZL) microcrystals to form 1 –ZL guest–host composites. The geometry of the ZL channel system ensures a well‐defined orientation of the embedded dye molecules, thereby leading to a preferred orientation of their electronic transition dipole moment (ETDM) and thus to objects with pronounced optical anisotropy properties. This enabled us to understand that in solution the monomers that are present at low concentration form an aggregate in which the molecules sit on top of each other and arrange into a J‐type aggregate configuration at higher concentrations. The signature of the latter is observed in the 1 –ZL composites. This seems to be the first example in which the insertion of molecules into a nanochannel microcrystal has helped in understanding the weak intermolecular interactions that take place in solution.