Lifetime lengthening

M. Chergui Let me invoke a rather exotic behavior of Rydberg states that may or may not relate to the lifetime lengthenings discussed in the previous talk. 

Are the weak homogeneous fields present in most experiments sufficient to cause the observed lifetime lengthening, and if not, what is the relative importance of homogenous and inhomogeneous field effects ... 

Substituents on the methine chain can stabilize the dye radical cation if the substituent (like methyl) is located on the high electron density carbons. However, no significant stabilization occurs when alkyl groups are on the alternate positions (like 9, 11 for the dication in Fig. 9). Current results for several dyes including die arbo cyanines and carbocyanines indicate that electronic stabilization of the dication radical lengthens the radical lifetime and also enhances the reversibiUty of the dimerization process (37). 

We shall now examine the behavior of a fairly large sample of 10,000 cells using the same conditions as mentioned above. Again, use a single 5000 iteration run. (Check to see how many of the 10,000 starting A ingredients have ended up in the C states. Lengthen the run if too many have not completed then-decays after 5000 iterations.) Determine cpf, tf, and tp for this sample and compare these values with the deterministic values. For the lifetimes Xf and Tp, plot ln(A or B) versus time for the first 70% of each decay period and determine the decay rates k from the slopes. The lifetimes are the inverses of the slopes, r = k. 

Lozano and co-workers reported an interesting stabilization effect of IL for lipase-catalyzed reaction the authors discovered that the presence of an appropriate substrate was essential for stabilization of enzyme in an IL solvent. The half lifetime of native CAL was only 3.2 h in [emim][PFg] solvent, while it lengthened remarkably to 7500 h in the presence of the substrate. The authors succeeded in demonstrating an efficient lipase-recyclable use system based on SCCO2 solvent (Fig. 9). - ... 

If the energy is transferred by trivial emission/reabsorption, it will lengthen the measured lifetime of the donor emission, not shorten it as happens in resonance energy transfer. This comes about because intervening absorption and emission processes take place prior to the final fluorescence emission (the reabsorption cannot take place until the photon has been emitted) the two processes do not compete dynamically, but follow in a serial fashion. In FRET, such an emission/reabsorption process does not occur, and the fluorescence lifetime of the donor decreases. This is an experimental check for reabsorption/reemission. 

Table I lists the excited state lifetimes for 0s(phen)3 + at three temperatures. The striking result is the lack of any significant change in the LMCT statefs lifetime on going from 295 to 10 K. Table I also presents data on the effects of deuteration on the charge transfer state lifetimes of Os(bpy) 3 /3+ complexes. The lifetimes are lengthened, but only by factors of 2 and 2.5 respectively, for Os(bpy)33+ and 0s(bpy)3 +.

Flash photolysis studies with absorption or delayed fluorescence detection were performed to compare the binding of ground and excited state guests with DNA.113,136 The triplet lifetimes for 5 and 6 were shown to be lengthened in the presence of DNA.136 The decays were mono-exponential with the exception of the high excitation flux conditions where the triplet-triplet annihilation process, a bimo-lecular reaction, contributed to the decay. The residence time for the excited guest was estimated to be shorter than for the ground state, but no precise values for the rate constants were reported. However, the estimated equilibrium constants for the... 

The understanding of chemical reactions taking place in the normal laboratory time scale, seconds-to-days, requires insight into much faster processes, with ultimate consequences in the normal time scale. For example, free radicals frequently have lifetimes in the microsecond or millisecond time scale. There are normally two approaches to study short-lived intermediates. In one, the experimental conditions are adjusted, so as to lengthen the intermediate lifetime to the point... 

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