Triplet decay, kinetic measurement

The absorption bands measured by the flash spectrographic method are often assigned by (a) comparison with known singlet-singlet absorption spectra, (b) comparison of the lifetime of the species responsible for the absorption with the phosphorescence lifetime, (c) comparison with calculated energies and intensities of the various possible absorptions by semi-empirical molecular orbital methods, and (d) comparison with published triplet absorption spectra and decay kinetics of model compounds. 

The effect of oxygen on cyclic 1,3-diradicals shows that conformation can affect the triplet state lifetime ST Time resolved resonance Raman spectroscopy has been used to examine triplet states produced from different isomers of p-carotene. A theoretical study has also been reported on the a-cleavage of the triplet states of symmetric and non-symmetric ketones S mechanism for triplet state relaxation of aromatic molecules has been used to explain experimental data for substituted benzenes. The decay kinetics of triplet-triplet fluorescence in the mesitylene biradical (two sub-levels) have been measured between 10 and 77K in Shpolski matrices triplet state of dimesityl... 

Thus, the transient experiments described above should be carried out at optical pumping rates that are sufficiently low that the apparent rate constants are no longer influenced by further reductions in K. This problem is avoided completely, on the other hand, by carrying out the kinetic measurements in the absence of optical pumping, that is, during decay of the triplet state. 

Besides fluorescence spectroscopy, time-resolved spectroscopy can rely on the measurement of excited (singlet or triplet) state absorption. Similarly to ground-state absorption, the spectral and absorbance properties may be altered by CyD complexation and yield information about the behavior of the complex in the excited state in addition, the time dependence (formation and decay) of the excited state absorption yields information about the kinetics and dynamics of the system. This is illustrated by the behavior of the lowest triplet state of naphthalene as measured by nanosecond spectroscopy using a Q-switched Nd YAG laser at 266 nm for excitation [21]. The triplet-triplet absorption spectra were measured in neat solvents (water and ethanol) and in the presence of a- and -CyD (Fig. 10.3.3). The spectra in ethanol and H2O had the same absorption maximum, but the transition was considerably weaker and broadened in H2O. Both CyDs induced a red shift, and a-CyD additionally narrowed the main band considerably. Fig. 10.3.4 shows the effect of a-CD concentration on the time evolution of the triplet-triplet absorption at 416 nm in the microsecond range. Triplet decay was caused by O2 quenching a detailed kinetic analysis of the time dependence yielded two main components which could be assigned to the free guest and the 1 2 complex, in full... 

The intermediate diphenylhydroxymethyl radical has been detected after generation by flash photolysis. Photolysis of benzophenone in benzene solution containing potential hydrogen donors results in the formation of two intermediates that are detectable, and their rates of decay have been measured. One intermediate is the PhjCOH radical. It disappears by combination with another radical in a second-order process. A much shorter-lived species disappears with first-order kinetics in the presence of excess amounts of various hydrogen donors. The pseudo-first-order rate constants vary with the structure of the donor with 2,2-diphenylethanol, for example, k = 2 x 10 s . The rate is much less with poorer hydrogen-atom donors. The rapidly reacting intermediate is the triplet excited state of benzophenone. 

The kinetic decay rate constant of the y-triplet level determined by ESE measurements and reported in Table I can be converted to the equivalent lifetime. This lifetime equals 1 05 ms for TPP in a 1 1 mixture of toluene and ethanol as the solvent. This is consistent with ODMR measurements (13) of the same level for TPP in n-octane where the lifetime equaled 1.45 ms. 

This demonstrates that unless the sensitizer has a route of decay to the ground state other than energy transfer to the acceptor, all the energy which goes into the sensitizer will eventually be measured as a reaction of the acceptor, and the multiple energy transfers will be kinetically undetectable. However, it is unlikely that a sensitizer with kd = 0 will be found, for it would have an infinite triplet lifetime. 

Finally, transient absorption measurements were deemed necessary to confirm the photoproducts in 21a,b and 22a,b. Due to overlapping absorptions of Cso, oFL and ZnP, which would impede a clear analysis, we have focused first on the selective excitation of ZnP. To this end, transient absorption spectra of the reference compounds (19 and 20a,b) reveal the instantaneous formation of the ZnP singlet excited state with maxima at 460 and 800 nm and minima at 565 and 605 nm. Furthermore, an isosbestic point at 500 nm as it develops on a time scale of 3000 ps reflects the intersystem crossing process at which end the triplet excited state of ZnP stands. The latter includes maxima at 530, 580 and 640 nm (Fig. 9.57a). Equally important is the fact that the decay of the singlet excited state matches the formation of the triplet excited state kinetics (Fig. 9.57b). 

The core of the PS-II reaction center has been prepared. Flash absorption showed that this core is able of efficient charge separation to form the primary radical pair which decays in about 30ns. The recombination populates the P-680 triplet state, which does not transfer to beta-carotene and can be detected by spin-polarized ESR. The yield of formation and kinetics of decay of the radical-pair have been measured in various PS-II preparations. The data are in favor of an equilibrium between the radical-pair and chlorophyll excited state in the antenna. 

The triplet-photochrome labeling method has been used to study very rare encounters in a system containing the Erythrosin B sensitiser and SITC photochrome probe (Mekler and Likhtenshtein, 1986). Both types of the molecules were covalently bound to a-chymotrypsin. The photoisomerisation kinetics was monitored by fluorescence decay of the frans-SITS. The rate constants of the triplet-triplet energy transfer between Erythrosin B and SITS (at room temperature and pH 7) were found k,r = 2 xlO7 NT s-1 and ktT = 107 M V. It should be emphasized that the concentration of the triplet sensitiser attached to the protein did not exceed 10 7 M in those experiments, and the collision frequencies were close to 10 s 1 which are 8-9 orders of magnitude less than those measured with the regular luminescence or ESR techniques. 

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