Reversible energy transfer

Reversible Energy Transfer. It is almost always assumed that the transfer of triplet energy from the sensitizer to substrate is irreversible. Back transfer, transfer of triplet energy from the acceptor back to the sensitizer, can, however, take place under the right set of circumstances. The following scheme illustrates the mechanism for back transfer. 

In a classic study of back transfer, Sandros determined the forward and reverse energy transfer rate constants for biacetyl with a number of compounds of varying triplet energies.102,103 These values are given in Table III.102... 

The observed reversible energy transfer (Equation 21) places the energy of MPT at 255 kJ.mol- (19). 

However, the adequate kinetic description of such a process is richer. Let us derive it from the rate consideration of the pair, subjected to reversible energy transfer accompanied by light excitation and radiationless decay. According to the simplest reaction scheme... 

The extension of DET to reversible energy transfer was intuitively made only once [7,64] and for the particular case of the resonance intramolecular transfer (AG = 0), when Wa = Wg = W. The set of equations that substituted for Eq. (3.91) assumed the following form ... 

At s = 0 the concentration corrections in Eq. (3.667) become the rates of excitation quenching by any partner that does not belong to a given couple (reactant pair). These bachelors compete for an excitation with the reactants in a couple when they move apart for a while between successive recontacts. Similar results were obtained with the many-particle theory of diffusion-influenced reactions based on the revised superposition approximation and became known as MPK1 [51]. The authors were the first who managed to obtain concentration corrections to the IET result for the kinetics of reversible energy transfer. In a subsequent modification of their theory, named MPK3 [126], the same authors reached the full correspondence with MET. 

Reversible Energy Transfer in Systems with Aromatic Hydrocarbon Units... 

An analogous mechanism of reversible energy transfer is operative in a dyad where an Os(bpy)3 + unit is connected to an anthracene system by an ethynyl bridge the lifetime of the Os(II)(bpy) fragment (t = 420 ns) is almost ten times longer than that of the parent complex (z = 60 ns) [193]. 

Sandros, K., Transfer of Triplet State Energy in Fluid Solutions. III. Reversible Energy Transfer, Acta Chem. Scand. 1964, 18, 2355 2374. 

A variation of the quenching method leading to Eq. (25) is to follow the decay of the triplet state from the emission of a luminescent quencher, which is much more intense than the phosphorescence of the probe [192]. In this case, the observed rate constant corresponds to the growth and subsequent decay in the emission profile of the quencher. However, an additional rate constant, corresponding to the emission lifetime of the quencher, has to be included in Eq. (25). The exit rate constant was determined to be 2.5 x 10 s for 1-bromonaphthalene when quenched by either Eu or Tb. This value is the same (Table 16) as that determined using the anionic quencher, NOf [62], showing that this method is useful. However, care should be taken to eliminate the possibility of reverse energy transfer. 

Reversible energy transfer between monomeric and dimeric forms of rhoda-mine 6G in ethylene glycol has been observed" and the concentration dependence of the overall fluorescence quantum yield has been modelled by Monte-Carlo simulations. Triplet energy transfer in disordered polymers has been analyzed on the basis of Bassler s model in which the trap energies have a Gaussian distribution." Energy transfer has also been observed in mono-layers and for photoswitchable molecular triads." The structural requirements for efficient energy transfer from a carotenoid to chlorophyll have been... 

Park, P.J., Lee, S. Diffusion-influenced reversible energy transfer reactions between polymers. J. Chem. Phys. 2001,115, 9594. 

The detailed balancing requirement puts a constraint on the reverse energy transfer rates (fiE) is again the equilibrium distribution function) ... 

The triplet state has received rather less attention than the singlet in micellar photochemistry. Phosphorescent decay of solubilized polynuclear aromatic hydrocarbons from their Tj state may be observed in heavy-metal ion lauryl sulphate micelles. This involves a conventional intersystem crossing from Si- Ti promoted by spin-orbit coupling with the heavy atom. 1-Bromonaphthalene readily forms a triplet excited-state in micelles, which may be quenched by added sodium nitrite in water, the lifetime then being reduced from 2.8 x 10 s to 5 x 10 s. There is efficient triplet energy transfer from N-methylphenothiazine (Ti) to trans-stilbene (So), which is irreversible but reversible energy-transfer to naphthalene (So) occurs. ... 

---