Ab Initio Study of Excitation Energy Transfer between Quantum Dots and Dye Molecules

We investigate excitation energy transfer between a semiconductor nanocrystal (NC) and an organic dye molecule, using ab initio calculations. The electronic coupling is evaluated based on the full Coulombic interaction between the NC and dye transition densities. We explore the effect of the NC-dye relative configurations on the electronic coupling, using a (CdSe)(6) cluster and a rhodamine cation as a simplified model system. Our analysis demonstrates the limitations of the commonly used Forster theory for the NC-dye system and highlights the importance of considering the full Coulombic interactions of donor and acceptor. We find that energy transfer involves several electronic states and can occur even from optically dark states of the NC. The consequences for larger NC-dye systems are discussed.