The effect of orientation and distance between donor and acceptor molecules on the efficiency of singlet–singlet energy transfer in Langmuir–Blodgett films

Singlet–singlet energy transfer between molecules of fluorescein and oxazine dyes in Langmuir–Blodgett films is studied experimentally. The dependence of the energy-transfer efficiency on the distance shows that the quenching of the donor fluorescence is the most efficient when the layers of the donor and acceptor molecules are in a direct contact. An increase in the distance between the donor and acceptor layers leads to a decrease in the energy-transfer efficiency. To establish the mutual orientation of the donor and acceptor molecules, quantum-chemical calculations of the energy transfer process in the donor–acceptor pair are carried out. The calculations show that the best correlation of the experimental and calculated values of the energy-transfer efficiency is observed when the interacting particles are shifted relative to each other by about ~0.12 nm in parallel planes. The presented approach can be used to estimate the relative orientation of interacting particles in multimolecular ensembles.