Relaxation of exciton confinement in CdSe quantum dots by modification with a conjugated dithiocarbamate ligand

Coordination of phenyldithiocarbamate (PTC) ligands to solution-phase colloidal CdSe quantum dots (QDs) decreases the optical band gap, E(g), of the QDs by up to 220 meV. These values of DeltaE(g) are the largest shifts achieved by chemical modification of the surfaces of solution-phase CdSe QDs and are-by more than an order of magnitude in energy-the largest bathochromic shifts achieved for QDs in either the solution or solid phases. Measured values of DeltaE(g) upon coordination to PTC correspond to an apparent increase in the excitonic radius of 0.26 +/- 0.03 nm; this excitonic delocalization is independent of the size of the QD for radii, R = 1.1-1.9 nm. Density functional theory calculations indicate that the highest occupied molecular orbital of PTC is near resonant with that of the QD, and that the two have correct symmetry to exchange electron density (PTC is a pi-donor, and the photoexcited QD is a pi-acceptor). We therefore propose that the relaxation of exciton confinement occurs through delocalization of the photoexcited hole of the QD into the ligand shell.