Distant- and Shape-Dependent Excitation Energy Transfer in Nanohybrid Systems: Computations on a Pheophorbide-α CdSe Nanocrystal Complex.

The combination of semiconductor nanocrystals (NCs) and molecules for efficient electronic excitation energy transfer is expected to be a promising ingredient of novel hybrid photovoltaic devices. Here energy transfer from a CdSe NC to the tetrapyrrole-type Pheophorbide-a molecule (Pheo) is studied theoretically. The rate expression accounts for the correct NC-Pheo transfer coupling, for the multitude of NC single exciton levels as well as their thermal distribution, and for the electron-vibrational Pheo states. A spherical Cd1159Se1450 NC is compared with a similar large NC of pyramidal and hemisphere shape. Because of the different exciton energies and wave functions, the transfer rates differ somewhat. For all three types of NC, however, the Coulomb correlation essentially determines the magnitude of the transfer coupling and the exciton energy. In any case, the energy-transfer coupling is below 1 meV, excluding hybrid-state formation.