Quenching by energy transfer

Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

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. 

Figure 6.14 Three donor-acceptor energy level schemes for quenching by energy transfer. 1. Benzophenone (D)+naphthalene (A) ...

Upon laser flash photolysis of a cyclohexane solution containing DTT and DNB, no transient absorption band ascribable to the products of electron-transfer reactions was observed. The transient absorption due to t(326) disappeared within 500 ns after laser excitation, and a new absorption band appeared at wavelengths <350 nm. The new band can be attributed to the triplet excited state of DNB. The result indicates that t(326) is quenched by energy transfer to DNB. The decay rate constant of t(326) for the energy transfer was estimated to be 8.8 x 109 M 1 s. As for cyclohexane solution of 327 and DNB, quenching of T(327) by energy transfer was also observed and the rate constant was 1.1 x 1010 M-1 s... 

The quenching of 0(XZ)) by N2, reaction [4], can be a very efficient source of vibrationally excited nitrogen [260] even though the curve-crossing mechanism may not permit more than one or two quanta of N2 vibrational energy per Of1/)) quench [258, 259]. Hunten and McElroy [56] have pointed out that whereas 0(XZ>) is readily quenched by energy transfer to N2, O S) is not, even though it is nearly resonant with v = 16. 

Other groups " have investigated various [Co(cage)] complexes as electron relays with varying degrees of success. Many of these species seem to quench by energy transfer or do not charge separate in the encounter complex but a notable exception is [Co(sepulchrate)] 13+3 159-161 complex oxidatively quenches [Ru(bipy)3] " with a rate =... 

We therefore developed a much simplified, but mathematically exact, version of a shell model which is particularly suitable for the analysis of cross relaxation in high symmetry systems such as the lanthanide elpasolites. Since the vibronic structure in the emission and absorption spectra of these compounds is both intense and broad (relative to the electronic origins), there are many cases where the vibronic structure of one electronic transition in emission overlaps with the vibronic structure of another electronic transition of a chemically identical ion in absorption. The emission of the excited ion may then be partially quenched by energy transfer. Implicit in this formulation is the assumption that the interionic coupling is weak compared with the vibronic coupling this is certainly true for the lanthanide elpasolites where the lanthanide ions are separated by distances of more than 0.7 nm. We refer to this process as cross relaxation by the electric dipole vibronic-electric dipole vibronic (EDVEDV) mechanism. 

Singlet Quenching by Energy Transfer and Exciplex Formation.—In a series of papers Shetlar has examined the kinetic consequences of there being more than one quenching process in a system. The non-linear Stern-Volmer type expressions derived will be useful in both luminescence studies and studies of photochemical reactions.108... 

Ce -REE energy transfer is strong for any REE ion that has transitions in the UV, such as Eu, Pr, Tm and Tb. Therefore in CL or photoluminescence spectra of apatites (and other minerals), Ce emission may be quenched by energy transfer to these and other REE species, even with their concentration levels substantially lower than that of the Ce. An example is given by the spectra of two fluorescent apatites from the Col dwell alkaline complex in Ontario (Mitchell et al. 1997). Both of these apatites have dominant Ce but only very weak emission in the general area of the Ce bands. 

When a metal complex like osmium(II) fris(bipyridyl) or a metallocene, as in compound 3, is appended to the ligand, the MLCT excited state is quenched by energy transfer to the adjacent metal center. The rate of this quenching decreases as an anionic substrate binds between the two metal eomplexes. A similar switching mechanism is seen in the calix[4]diquinone-appended complex, 4. In this case, the fluorescence emission is quenched via an intramolecular electron transfer to the quinone. Anion binding between the Ru complex and the quinone blocks the quenching process, and the emission intensity is significantly recovered. 

---