Energy back-transfer model

Fig. 20. Energy back-transfer model describing the energy transfer between the semiconductor host and the lanthanide ion R3+ (Culp et al., 1997 Takarabe et al., 1995).

Measuring the variations of the intensity ratio /inp/Zyb under pressure, Takarabe (1996) was able to determine the energy difference bt and found a pressure-induced shift of 70 meV/GPa which is close to the shift of the band-edge related luminescence due to the bound e-h pairs. Furthermore, under pressure it was possible to completely recover the thermally quenched luminescence of the Yb3+ ion at temperatures of 220 K and 260 K (Takarabe et al., 1994) as well as at room temperature (Takarabe, 1996). The minimum pressure at which the luminescence could be observed again was shown to increase with increasing temperature. All these facts fitted well to the proposed back-transfer model, which was thus strongly supported by the pressure experiments. 

The irreversibility and efficiency of the ESIPT cannot be understood by a model consisting of only one low frequency mode and one high frequency proton mode. During every period of the low frequency vibration the ON-separation adopts a suitable distance where the energy barrier is suppressed and a back transfer of the proton should be possible. As discussed in Section 11.4.3 this inconsistency can be resolved by assuming that several low frequency modes are directly involved in the ESIPT. 

The decreased emission intensity and constant hfetime with pressure indicate that pressure is influencing the population of the state rather than its non-radiative decay rate. Webster and Drickamer developed a Dj excitation model based on feeding rates from the charge transfer state involved in the excitation process. According to the model [256,257], the Dj states are populated through transfer of excitation energy from the charge transfer state and depopulated by back transfer. Upon excitation, transfer occurs sequentially to the... 

The Butler model for the distribution of excitation energy within the photochemical apparatus of photosynthesis assumes that the rate constant for energy transfer to PSII reaction centres exceeds the rate constant for the back-transfer of energy from the reaction centre to the antenna (Butler, 1978). This led to the concept of the PSII unit as an energy funnel. However, more recent fluorescence lifetime measurements indicate that the equilibration of excitation energy between antennae and reaction centres is one order of magnitude faster than charge separation. Thus, PSII appears to be trap limited and the reaction centre appears to act not as a funnel but as a shallow trap (Schatz et al., 1988 Blankenship, 2002). 

Photoinduced ET at liquid-liquid interfaces has been widely recognized as a model system for natural photosynthesis and heterogeneous photocatalysis [114-119]. One of the key aspects of photochemical reactions in these systems is that the efficiency of product separation can be enhanced by differences in solvation energy, diminishing the probability of a back electron-transfer process (see Fig. 11). For instance, Brugger and Gratzel reported that the efficiency of the photoreduction of the amphiphilic methyl viologen by Ru(bpy)3+ is effectively enhanced in the presence of cationic micelles formed by cetyltrimethylammonium chloride [120]. Flash photolysis studies indicated that while the kinetics of the photoinduced reaction,... 

Conclusions from the reference 28 review article are as follows (1) Several models backed by experiment place S4 near the groove between adjacent subunits, while the MacKinnon group model places S4 near the periphery of the protein (2) lanthanide-based resonance energy transfer (LRET) places two S4 residues in segments across the tetramer from each other at a distance of 45 A (3) a method based on tethered quaternary ammonium pore blockers places the extracellular ends of the SI and S3 helices further away from the ion conduction pore than the S3-S4 linker, arguing that the S4 helix resides... 

In the photochemical conversion model (Fig. 3), the most serious problem is the undesired and energy-consuming back electron transfer (shown as dotted arrows) as well as side electron transfer, e.g., the electron transfer from (Q) to (T2)ox. It is almost impossible to prevent these undesired electron transfers, if the reactions are carried out in a homogeneous solution where all the components encounter with each other freely. In order to overcome this problem, the use of heterogeneous conversion systems such as molecular assemblies or polymers has attracted many researchers. The arrangement of the components on a carrier, or the separation of the Tj—Q sites from the T2—C2 ones in a heterogeneous phase must prevent the side reactions of electron transfer. 

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