Induced internal conversion as a probe of enhanced energy transfer in a plasmonic field

Nonradiative energy transfer in plasmonic donor-acceptor films was studied experimentally and theoretically using both the classical Förster model and a quantum-chemical supermolecule approach, in which the transfer process was treated as internal conversion between excited states localized on the donor and acceptor. Langmuir-Blodgett films based on a trimethine cyanine donor and Nile red acceptor were investigated on glass and on silver island films, with donor-acceptor and molecule-nanoparticle distances controlled by PDAOM spacer layers. Quantum-chemical calculations were carried out at the CAM-B3LYP/6-31G(d,p)/COSMO level. The supermolecule model reproduced the distance dependence of the transfer rate more realistically than the classical Förster r^-6 law, especially at short separations, where the latter strongly overestimates the rate. The plasmon-induced enhancement was also described more consistently within the quantum-chemical framework, yielding values close to experiments, whereas the plasmon-modified Förster model tended to overestimate the enhancement at larger distances. The results show that the electric component of the plasmonic near field can significantly enhance nonradiative energy transfer and that this effect is described most adequately within the quantum-chemical supermolecule picture.