Excited state apparent temperature

A major discrepancy that remains unresolved in the excited-state properties of the [Fe384]° cluster in D. gigas Fdll concerns the existence of a low-lying, fully valence-delocalized state that becomes populated at temperatures above 25 K. 8uch a state is clearly apparent in the temperature-dependent Mossbauer studies of reduced D. gigas Fdll (29) and P. furiosus 3Fe Fd (198) and is represented by one quad-rupole doublet with AEq 0.9 mm/s and S = 0.45 mm/s. 8uch a... 

Figure B8.2.1 shows the fluorescence spectra of DIPHANT in a polybutadiene matrix. The h/lu ratios turned out to be significantly lower than in solution, which means that the internal rotation of the probe is restricted in such a relatively rigid polymer matrix. The fluorescence intensity of the monomer is approximately constant at temperatures ranging from —100 to —20 °C, which indicates that the probe motions are hindered, and then decreases with a concomitant increase in the excimer fluorescence. The onset of probe mobility, detected by the start of the decrease in the monomer intensity and lifetime occurs at about —20 °C, i.e. well above the low-frequency static reference temperature Tg (glass transition temperature) of the polybutadiene sample, which is —91 °C (measured at 1 Hz). This temperature shift shows the strong dependence of the apparent polymer flexibility on the characteristic frequency of the experimental technique. This frequency is the reciprocal of the monomer excited-state...

Lastly, photochemically unstable ligands should be avoided. Re(bpy)(CO)3Cl shows a moderately efficient MLCT emission at room temperature (R. M. Ballew, unpublished results from our laboratory). However, the apparently closely related Re(dpk)(CO)3Cl (dpk = 2,2 -dipyridyl ketone) shows a benzophenone like phosphorescence at 77K indicating that the n-n excited state of the ketone in complex is the lowest state of the complex. No luminescence is seen at room temperature, and even at 77K the dpk triplet state is such a powerful hydrogen atom extractor that it removes protons from alcohol glasses as seen by the formation of the intense blue color of the keto free radical. The absence of an MLCT emission is caused by the greater difficulty of reducing dpk relative to bpy, which pushes the MLCT states above the dpk ligand states. 

These data led to the model already described several times above. The enzyme executes a search for a tunneling sub-state, apparently 13 kcaFmol in energy above the principal state from this state the hydrogen atom tunnels with no further vibrational excitation. Probably motion of the secondary center is coupled into the tunneling coordinate. The result is large, temperature-independent primary and secondary isotope effects in the context of an isotope-independent activation energy. 

Since the values of i/ depend on several factors noted above, in the absence of additional data such as the temperature dependence of the electron transfer rate constants for i-2 it is difficult to analyze the apparent difference between i/ for the charge separation reaction and that of the radical ion pair recombination reaction. However, the difference between these two values of u is not unreasonable given that the charge separation involves oxidation of an excited state of the donor, while radical ion pair recombination involves two ground state radicals. Small changes in the nuclear coordinates of the donor and acceptor for these two reactions should be sufficient to produce the observed difference in i/. The electronic coupling factor between ZnTPP and AQ should be different than that between ZnTPP " and AQ". 

As a consequence of the extensive efforts devoted to this attractive and intriguing area of asymmetrical photochemistry, the chirality transfer mechanisms operating in both uni- and bimolecular enantiodifferentiating photosensitizations have been understood in considerable detail, which in turn enabled us not only to obtain optical yields much higher than those achieved in earlier studies but also to utilize a variety of internal and external, or electronic, structural, and environmental, factors in the critical control of enantioselectivity in the excited state. From a wider chemical viewpoint, it should be emphasized that the entropy-related environmental factors, such as temperature, pressure, and solvent, play much more important roles than previously expected, and in typical cases even the product chirality may be switched by these apparently supplementary factors. 

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