Intramolecular triplet energy transfer

The photophysical properties of the luminescent complexes formed between 2,2 -bipyridine (bipy) and certain transition metals, most notably Re, Ru or Os but 

As mentioned earlier, triplet energy transfer can occur by way of Forster-type through-space interactions [100] or via Dexter-type electron exchange [101], In the former case, the rate constant for Forster-type triplet energy transfer (kp) can be expressed as follows  

In the context of designing molecular-scale wires, the Dexter mechanism is more relevant since it provides direct information on the propensity of the connector to conduct electrons. This mechanism involves the simultaneous transfer of an electron, by way of bridging LUMOs, and a positive hole, via the bridging HOMOs, from donor to acceptor [102]. It is a relatively short-range effect (Fig. 2.11) and, to be eflRcient, requires orbital overlap between donor and acceptor. The rate constant 

fi is an attenuation coefficient that describes the resistivity of the bridge and the term V refers to the coupling element at orbital contact. The coupling element can also be related to the energy gap (Hdb) between orbitals on the donor and on the bridge (Fig. 2.12) [107-110]. 

In this latter expression, db and ba refer, respectively, to atomic orbital coefficients describing coupling between the donor and the first member of the bridge 

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The intramolecular triplet energy transfer studies described so far generally have used phosphorescence emission measurements to follow the... 

Because of our obvious prejudices we will describe this study in some detail. The energy levels of the acetophenone and -methylstyrene moieties are ideally situated for intramolecular triplet energy transfer (see Figure 6.12). If these two chromophores are not interacting, then the absorption spectra for these compounds should be the composite of the acetophenone-/ -methylstyrene spectra. Figure 6.13 indicates that this is true for n = 4 (and also 2 and 3) however, it is not correct for n = 1 (Figure 6.14). In the case of n = 1 the increased intensity of the vibronic structure of the n - n ... 

Table 6.7. Rates and Rate Ratios for Intramolecular Triplet Energy Transfer in Compounds (7), n = 2,3,4...

Excitation of [33] in the dimethoxybenzene part leads, after very efficient Intramolecular triplet energy transfer, to reaction of the norbornene part of the molecule. 

Wagner, P. J., Klan, P., Intramolecular Triplet Energy Transfer in Flexible Molecules Electronic, Dynamic, and Structural Aspects, J. Am. Chem. Soc. 1999, 212, 9626 9635. 

Engel, P. S., Horsey, D. W., Scholz, J. N., Karatsu, T., Kitamura, A., Intramolecular Triplet Energy transfer in Ester linked Bichromophoric Azoalkanes and Naphthalenes, J. Phys. Chem. 1992, 96, 7524 7535. 

Finally, diphenylsulfonium-bis-(methoxycarbonyl)methylide, IX [87], and salts of p,p -bis-[(triphenylphosphonio)methyl]-benzophenone, X [88], have been claimed as photoinitiators of free radical polymerizations [89]. They presumably photolyze homolytically in the case of X, intramolecular triplet energy transfer from the benzophenone chromophore to the phosphonium moieties may likely be involved. 

The donor acceptor norbornadiene derivative (116) exhibits high quantum yields for valence isomerization using blue light/ Intramolecular electron transfer is involved in ring opening of the quadricyclane derivative (117) to the corresponding norbornadiene. The study has indicated that electron transfer occurs from the quadricyclane moiety to the BF2 chromophore. Further work on such systems has shown that intramolecular triplet energy transfer from the carbazole to the norbornadiene moiety also occurs in the molecule (118). ... 

Closs GL, Piotrowiak P, Maccinis JM, and Fleming GR. Determination of long-distance intramolecular triplet energy transfer rates - a quantitative comparison with electron transfer. J. Am. Chem. Soc. 1988 110 2652-... 

Figure 2.8. Examples of molecular dyads used to study intramolecular triplet energy transfer.

Figure 2.11. Effect of separation distance on the rate constant for intramolecular triplet energy transfer as calculated for (a) Dexter electron exchange with /S = 0.1, (b) Dexter electron...

A set of bipy-based Ru"/Os" binudear complexes, comprising compounds 21-23 (Fig. 2.13), was prepared such that the energy gap between triplet states localized on donor and acceptor units remained at ca. 0.34 eV [112]. These complexes differed in the site of attachment of the acetylene bridge and used both ethynylene and butadiynylene connectors. Of course, with such systems it is difficult to be definitive about the exact nature of the bridge. It was found, however, that intramolecular triplet energy transfer was extremely fast in each case. There was htde... 

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