Energy transfer between Si nanocrystals and protoporphyrin molecules as a function of distance, orientation and size

The interaction between protoporphyrin molecules (donor) and Si nanocrystals (acceptor) up to 2.5 nm for varying distances and orientations is studied by DFT, semi-empirical and TDDFT methods. Simulations show an effect on electronic structure, indicative of electron charge transfer in parallel orientation and small distances and nonelectron energy transfer for different orientations and larger distances. An absorption-emission spectral overlap is observed. The calculations of coupling and energy transfer rates show a distance dependence typical of fluorescence resonance energy transfer (FRET) in the long range, while in the short and ultra-short range the distance dependence indicates electron transfer in addition to FRET. The Si NCs with the smallest size yield larger couplings than the larger nanocrystals. The PPIX/Si NC coupling was enhanced by adding a plasmon nanoparticle as a bridge in the donor-acceptor system. Results using Au nanoparticles show increased energy transfer rates up to 10⁴ and lower distance dependence.