The most well-known excitation energy transfer (EET) process is Förster resonant energy transfer (FRET). It is widely used as a biophysical imaging technique of protein dynamics as the EET probability and hence the signal strength of the acceptor emission depends on the distance between the donor and acceptor labels which are attached to the protein.
In our group, we have developed a monomer transition density (MTD) approach which is based on DFT/MRCI wave functions and allows to compute rate constants for Förster and Dexter EET beyond the ideal dipole approximation (IDA). In a collaboration with the Gohlke group, we evaluated trajectories of molecular dynamics simulations where an Alexa468 donor and a Cy5 acceptor were attached as FRET labels to an RNA backbone. These studies reveal that the IDA (which represents the basis for the interpretation of imaging experiments) fails for interchromophore distances below 10 Å. Fortunately, the orientation factor κ2 is small on the average in such situations, thus damping the failure of the IDA.
FRET can also be used in OLED technology to lower the residence time of the system in the excited state. In hyperfluorescent devices, the assistant TADF dopant transfers its excitation energy by FRET to a strongly fluorescent acceptor. In principle, a similar technique – hyperphosphorescence – could be used to increase the operational stability of phosphorescent OLEDs. In hyperphosphorescent devices, triplet-singlet FRET is assumed to be faster than phosphorescence emission. In both cases, Dexter triplet-triplet EET should be avoided as far as possible. Methods to compute electronic coupling matrix elements for triplet-triplet and triplet-singlet energy transfer are currently being developed.
A particular form of EET is singlet fission (SF). It has been observed, for example, in pentacene and diphenylhexatriene crystals, but also bichromophoric molecular systems have been shown to undergo SF. Herein, it is assumed that a local singlet excitation on one chromophore transfers its excitation energy in a spin-allowed process to a singlet-coupled triplet pair 1(T...T) which eventually splits into two independent triplet excitons. For this reason, SF was proposed to be used in photovoltaics for transforming highly energetic sunlight to lower energy excitons which better match the band widths of the semiconductor materials. We have only recently started working in this field. Interestingly, there are indications for a spin-forbidden interaction between singlet-coupled 1(T...T) and quintet-coupled triplet pairs 5(T...T) in SF and triplet-triplet annihilation upconversion (TTA-UC) which is considered the reverse of SF. To determine their coupling strengths, electronic spin-spin interaction (SSC) has to be invoked.