Excitation Transfer between Two Chromophores

However, one needs to take into account that all the calculations are done assuming a temperature of 0 K, and hence at room temperature these couplings will be minimal. Furthermore, in collaboration with Beljonne and co-workers, transition densities were calculated [25] for excitation transfer between two per-yleneimide chromophores coupled by a fluorene trimer (separation 3.4 nm) and found to be in line with the Forster approximations. 

In what follows we derive the Forster expression for the rate of electronic energy transfer between two chromophore molecules. We consider two such molecules, donor D and acceptor A, each represented by its ground and excited electronic states and the associated vibrational manifolds Id, 2D,/j ° ) forthe ground and excited state manifolds of molecule D and 1a, 2a,... 

Forster or fluorescence energy transfer (FRET) is a system for describing energy transfer between two chromophores. A donor chromophore in its excited state can transfer energy to an acceptor chromophore provided it is proximal (typically less than Inm) through dipole-dipole coupling. There are too many publications to describe all that use FRET, and a full discussion of them would be beyond the scope of this review, so a few publications are included here, but the list is not comprehensive. 

W(r,6, 62) is a general excitation transfer rate depending on the distance of separation between two chromophores r and the unit orientation vectors of their transition dipoles... 

In fact, the distinction between two-step and direct dynamics is rather fuzzy. The basic issue is what kind and amount of preliminary work is needed before starting a dynamical calculation. Direct ab initio dynamics [90,97-101] requires a minimum of preparation some tests to choose basis sets and other options may suffice. For large systems, however, fully ab initio calculations are impractical, and one has to resort to QM/MM or PCM approaches but then, a host of empirical parameters are introduced, which may need some readjustement to avoid artefacts and to improve the accuracy before starting the dynamical calculations. The same holds for the semiempirical methods in order to represent at best the excited states, one has to re-parameterize the hamiltonian. In particular, our FOMO-SCF-CI method [56-58] differs considerably from the normal SCF or SCF+CIS procedures, so that the standard parameters need to be modified. However, the parameter sets are fairly transferable, and their optimization can be limited to the atoms belonging to the chromophore. In the two-step strategies one fits the ab... 

Forster energy transfer or energy transfer at a distance occurs between two molecules, a donor (the excited fluorophore), and an acceptor (a chromophore or fluorophore). Energy is transferred by resonance, i.e., the electron of the excited molecule induces an oscillating electric field that excites the acceptor electrons. As a result of this energy transfer, the fluorescence intensity and quantum yield of the emitter will decrease. Energy transfer is described in Chapter 14. 

Optical excitation transfer can occur between molecules as much as 10 nm apart when the dipole-dipole coupling between molecules (one excited "photon donor" chromophore, the other an unexcited "photon acceptor" chromophore) by a mechanism known as Forster79 resonance transfer or fluorescence resonance energy transfer (FRET) its characteristic dependence on the distance r between the two chromophores is r 6. 

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