Rate constants for triplet energy transfer

Fig. 2. Flash spectroscopic rate constants for triplet energy transfer from donors of various excitation energies to biacetyl and to m-stilbene.

Fig. 1.35. Schematic diagram describing energy levels of singlet (So, Si), triplet (Ti, Tn), and charge-separated (CS) states of OPVn-C6o dyads. The energy transfer (A et) and indirect (fc s) and direct (k s) charge separation reactions are indicated with curved dotted arrows. The solid arrow describes the initial excitation of the OPVn moiety. Other symbols are kr and k[ for the radiative rate constants, knr and k m for the non-radiative decay constants, fcjcs and k ics for the intersystem crossing rate constants, and fcx and fcxX for the rate constants for triplet-energy transfer, in each case for OPVn and MP-Ceo, respectively...

Quenching rate constants for triplet energy transfer quenchers are often faster than for electron transfer quenchers. However the requirement for a triplet state... 

Cl" in both water and 50 v/v% aqueous CH3CN. A possible role of chloride is as a bridging species in quenching via electron transfer between MBH and Fe is examined. Further reports of the quenching of triplet Methylene Blue by complexes of cobalt(ii) have also been presented and the results discussed in terms of a reversible electron-transfer model. Rate constants for the energy-transfer quenching of the lowest excited triplet states of anthracene, acridine, and naphthalene by Cr have been presented.Low values were obtained and are attributed to low values of the pre-exponential factor of the energy transfer rate constants. 

Some rate constants for triplet excitation transfer are shown in Table III. All data available at this time indicate that an essential requirement for such a process to occur is the close proximity of the acceptor-donor pair. Triplet excitation transfer is diffusion-controlled provided that the triplet state energy of the donor is at least 3-5 kcal per mole higher than the triplet state energy of the acceptor (triplet state energies refer to the energy gap between So and To ). When the triplet excitation energies of acceptor and donor are within 3 kcal per mole of each other, excitation transfer is reversible. If the process is endothermic by more than 3 kcal per mole it becomes very inefficient. 

Table 6.5. Rate Constants for Triplet-Triplet Energy Transfer<58>...

If therefore the transfer process is diffusion-limited (kt kd), as found for a number of (energy) donor-acceptor systems by Dubois and coworkers,59 it must be concluded that kQC (kMC + 2i kf) with kCM kd. This is in contrast to the findings for triplet energy transfer where the measured rate constant 3kt falls below the diffusion-limited value (Eq. 7) as the energy separation 3A MQ of donor (3M ) and acceptor (3Q ) triplet states is reduced.60 With 3kQC k° + GT = 1/tp the appropriate form of Eq. (37)... 

The self-quenching mechanism which has been invoked for the gas phase system applies also to the liquid phase. Order of magnitude agreement between data from the various techniques for measuring tx seems to have been achieved. Cundall and Griffiths (233), obtained the first indications from studies on the benzene/ butene-2 system, that the triplet state lifetime was very short. The value for kgx tx was 21.2M at 25°C where kgx was the overall rate constant for triplet excitation energy transfer from benzene to butene-2, and Tx the lifetime of the benzene triplet. Assuming a value of about 5 x 10 for kgx> x appears of the... 

Current work in our laboratory (30) in two-component aromatic solute systems has indicated the feasibility of determining accurate rate constants for triplet-triplet energy transfer between aromatic donor-acceptor pairs by methods analogous to those used in our electron transfer investigations. Similarly, an attempt is being made to determine rate... 

The rate constant for triplet-triplet energy transfer (Art) in fluid solution depends on < and a ... 

A very early example is provided by the study of solvent viscosity effects on the rate constants for bimolecular quenching of triplet valerophenone by dienes. Several studies by the Wagner group on the competing type II reactions of a,co-diacylaIkanes have provided measures of the rates of internal triplet energy transfer between the two carbonyl groups. 

Porter and Wilkinson(56) measured the rates of quenching for a variety of triplet donors with triplet acceptors at room temperature in fluid solution by flash photolysis. The appearance of the triplet-triplet absorption spectrum of the acceptor and the simultaneous disappearance of the donor triplet-triplet absorption spectrum provided unequivocal evidence for the triplet-triplet energy transfer process. Table 6.5 provides some of the quenching rate constants reported in this classic paper. 

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