Polarizing energy transfer

More interesting is the use of nonpolar solvents (e. g. xylene, toluene, carbon tetrachloride, hydrocarbons), because these are transparent to MW and absorb them only weakly. They therefore enable specific absorption by the reactants. If these reactants are polar, energy transfer occurs from the reactants to the solvent and the results might be different under the action of MW and A. This effect seems to be clearly dependent on the reaction and is, therefore, the subject of controversy. In xylene under reflux, for example, no MW-specific effects were observed for the Diels-Alder reaction [5] whereas important specific effects were described for aryldiazepinone synthesis [33]. 

A Montali, C Bastiaansen, P Smith, and C Weder, Polarizing energy transfer in photoluminescent materials for display applications, Nature, 392 261-264, 1998. 

Typical application (as demonstrated in chapter) Characterization of quenching, spectral shifts, polarization, energy transfer, excited state reactions... 

In the case of polarized, but otherwise incoherent statistical radiation, one finds a rate constant for radiative energy transfer between initial molecular quantum states i and final states f... 

Jonas D M, Lang M J, Nagasawa Y, Joo T and Fleming G R 1996 Pump-probe polarization anisotropy study of femtoseoond energy transfer within the photosynthetio reaotion-oenter of Rhodobacter sphaeroides R26 J. Rhys. 

The environmental (i.e., solvent and/or protein) free energy curves for electron transfer reactions can be generated from histograms of the polarization energies, as in the works of Warshel and coworkers [79,80]. 

Any factor that affects the size or shape of a molecule, the hindered movement of a fluorophore within a molecule, or the energy transfer within the molecule will affect the measured depolarization of its fluorescence emission. Therefore, the conformation of humic fractions in solution can be studied as a function of pH, ionic strength, temperature, and other factors by depolarization measurements. The principle of the method is that excitation of fluorescent samples with polarized light stimulates... 

Steady-State Fluorescence Depolarization Spectroscopy. For steady state depolarization measurements, the sample is excited with linearly polarized lig t of constant intensity. Observed values of P depend on the angle between the absorption and emission dipole moment vectors. In equation 2 (9), Po is the limiting value of polarization for a dilute solution of fluorophores randomly oriented in a rigid medium that permits no rotation and no energy transfer to other fluorophores ... 

Brockhinke, A. et al.. Energy transfer in the OH A Z state The role of polarization and of multi-quantum energy transfer, Phys. Chem. Chem. Phys., 7, 874, 2005. 

Under some circumstances the rotationally anisotropy may be even further simplified for T-R energy transfer of polar molecules like HF (41). To explore this quantitatively we performed additional rigid-rotator calculations in which we retained only the spherically symmetric and dipole-dipole terms of the AD potential, which yields M = 3 (see Figures 1, 3, and 4). These calculations converge more rapidly with increasing N and usually yield even less rotationally inelastic scattering. For example Table 2 compares the converged inelastic transition probabilities... 

Polarization of the Emission. We have sought support for the weakly interacting chain segment model from measurements of room temperature fluorescence polarization (19) on dilute solutions of 1 in 3-methylpentane. An independent preliminary report of similar measurements on a dilute glassy solution at 77K and on a neat polymer has also appeared (21). In the latter case, the analysis is complicated by inter-chain energy transfer. 

In pure water, electron or energy transfer to carotenoid aggregates is obstructed by the membrane of outside-directed polar groups (Sliwka et al. 2007), Figure 3.14. 

---