Exciton Migration and Surface Trapping for a Photonic Crystal Displaying Charge-Recombination Fluorescence.

A compact donor-acceptor molecular dyad has been synthesized by attaching an N,N-dimethylamino fragment to a naphthalic anhydride residue. The dyad shows fluorescence from an intramolecular charge-transfer state (i.e., charge-recombination fluorescence) in solution, with the photo-physical properties being strongly dependent on the solvent polarity. Similar emission is seen for single crystals of the target compound, the molecules being aligned head-to-head, although time-resolved emission profiles display dual-exponential kinetics. A second polymorph with the head-to-tail alignment also gives rise to two lifetimes that differ somewhat from those of the first structure, which are assigned to bulk and surface-bound molecules. Growing the crystal in the presence of Rhodamine B localizes the dye around the surface. Excitation of the crystal is followed by sub-ps exciton migration along the aligned stacks, with occasional crossing to adjacent stacks and trapping at the surface. Rhodamine B present at very low levels acts as the acceptor for excitons entering the surface layer. Crystals embedded in a polyester resin form an artificial light-harvesting antenna able to sensitize an amorphous silicon solar cell.