Singlet energy transfer mechanism

Debreczeny, M. R, Wasielewski, M. R., Shinoda, D., Osuka, S. (1997). Singlet-singlet energy transfer mechanisms in covalently-linked fucoxanthin- and zeaxanthin-py-ropheophorbide molecules, J. Am. Chem. Soc., 119 6407. 

Faure, S., C. Stem, R. Guilard, and P.D. Harvey (2004). Role of the spacer in the singlet—singlet energy transfer mechanism (Forster vs Dexter) in cofacial bis-porphyrins. 7. Am. Chem. Soc. 126(4), 1253-1261. 

The most important mechanism for singlet energy transfer is the Forster resonance transfer ... 

All of the examples of singlet energy transfer we have considered take place via the long-range resonance mechanism. When the oscillator strength of the acceptor is very small (for example, n-> n transitions) so that the Fdrster critical distance R0 approaches or is less than the collision diameter of the donor-acceptor pair, then all evidence indicates that the transfer takes place at a diffusion-controlled rate. Consequently, the transfer mechanism should involve exchange as well as Coulomb interaction. Good examples of this type of transfer have been provided by Dubois and co-workers.(47-49)... 

Understand that both mechanisms for singlet-singlet energy transfer require an overlap between the florescence spectrum of the donor and the absorption spectrum of the acceptor. In addition, understand that a favourable orientation of donor and acceptor is necessary for the dipole mechanism. 

Two main models are usually discussed for the mechanism of the spectral sensitization. The excitation of the sensitizer by absorbed light and electron transfer from the excited sensitizer to the semiconductor is the first model. The alternative mechanism consists of the transfer of the excitation energy from the sensitizer to the semiconductor. This energy is used for photogeneration of the charge carriers in the sensitized photoconductor. In the first case the excited singlet level of the sensitizers has to be located above the conduction band of the semiconductor for realization of the electron transfer. For hole transfer the basic sensitizer level has to be located lower than the valence band of the sensitized photoconductor. The energy transfer mechanism does not need a special mutual location of the semiconductor and sensitizer levels. 

For quenching by energy transfer mechanism, the quencher must have suitable energy levels, singlet or triplet, near or below the energy level of the donor molecule. Such a transfer has the greatest probability if there is an approximate resonance between the donor and the acceptor energy levels. 

Both through-space and through-bond energy transfer mechanisms are known, by which singlet-singlet energy transfer occurs through both... 

The MP-CfioC i) state formed via the intramolecular singlet-energy transfer in the OPVn Cgo dyads is expected to decay predominantly via intersys-tem crossing to the MP-Cfjo(Ti) state, apart from some radiative decay. Consistent with this expectation, the PIA spectrum recorded for all four dyads in toluene solution shows the characteristic MP Cf,o T <— Tn absorption at 1.78 eV with a shoulder at 1.54 eV (Fig. 1.28a). The PIA bands increase in a near-linear fashion with the excitation intensity (—AT oc Ip, p = 0.80-1.00) consistent with a monomolecular decay mechanism. The lifetime of the triplet state lies in the range 140-280 ps. 

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