Rate constant intersystem crossing

Figure 6.3. Experimentally determined ratio for the intersystem crossing rate constants, (a) Pyrazine,(19) KJKz > 20. (b) Quinoxaline, 25 KJK2 > 10. (c) 2,3-Dichloroquinoxaline,<25) KX/K2 >15.

Since (/iv//av)/(/i//2) can be measured and kx and k2 are just the observed reciprocal decay constants of levels tx and r2, it is possible to determine the ratio of the intersystem crossing rate constants K KX. This information is difficult if not impossible to obtain in other ways. The results for several molecules are shown in Figure 6.3. 

H. B. Gray Multiconfiguration SCF calculations by P. J. Hay indicate that the 166 -366 energy separation is over 1 eV, and there is no evidence for intervening states that could provide a facile intersystem pathway. Thus a relatively small singlet triplet intersystem crossing rate constant is not all that peculiar. 

Intersystem crossing rate constants of ortho- and meta-substituted singlet phenylni-trenes are presented in Table 1Mono- and di-o-fluorine substituents have no influence on ISC rate constants.No effect with meta, meta-difluoro substitution is observed either. Pentafluoro substitution has no effect on fcisc in pentane although a modest acceleration is observed in the more polar solvent methylene chloride.i° - i... 

TABLE 11.3. Intersystem Crossing Rate Constants of Ortho and Meta Substituted Phenylnitrenes"... 

Table 3 Intersystem crossing rate constants of ortho- and meM-substituted phenyl-nitrenes.

Quantum yields for isomerization of t-1 to c-1 via intersystem crossing are similar to those obtained in triplet sensitization, indicating that t is formed with near unit efficiency. Similar results were reported for the interaction of t with inorganic anion electron donors (127b). Rate constants for intersystem crossing of the t-1-ethyldiisopropylamine and triethylamine exciplexes obtained from the measured exciplex lifetimes are 5 x 107 s"1 (116), similar to the values for the exciplexes of pyrene (128) and naphthalene (121) with triethylamine. Larger intersystem crossing rate constants are observed for the exciplexes of t-1 with the bicyclic amines l-azabicyclo[2.2.2]-... 

The high value for the quenching of 3,4-dimethoxyacetophenone by phenol suggests that it is probable that within the lignin structure hydroxyl groups are able to quench carbonyls by a static mechanism to yield phenoxy-ketyl radical pairs which decay on a timescales faster than the time resolution of our laser flash photolysis apparatus. Intersystem crossing rate constants for triplet radical pairs in the restricted environments of micelles have been demonstrated to be of the order of 2 -5 x 106 s-1 (25, 24). However, in the lignin matrix where diffusional processes are likely to be... 

Fig. 1.35. Schematic diagram describing energy levels of singlet (So, Si), triplet (Ti, Tn), and charge-separated (CS) states of OPVn-C6o dyads. The energy transfer (A et) and indirect (fc s) and direct (k s) charge separation reactions are indicated with curved dotted arrows. The solid arrow describes the initial excitation of the OPVn moiety. Other symbols are kr and k[ for the radiative rate constants, knr and k m for the non-radiative decay constants, fcjcs and k ics for the intersystem crossing rate constants, and fcx and fcxX for the rate constants for triplet-energy transfer, in each case for OPVn and MP-Ceo, respectively...

Table 5 Equilibrium Constants (K, ), Relaxation Times (t) and Intersystem Crossing Rate Constants (kx, k t in Equation 21) For Some [Fe(X-salmeen)] f Complexes in Acetone/Methanol Solution1...

The lack of the triplet-triplet absorption of carbocyanine dyes is due to low values of the intersystem crossing rate constants as compared with the rate constants of competing processes [5, 9]. The dye-DNA interactions lead to an increase in the quantum yield of the triplet state of the dye molecules, since the complexation impedes the processes of photoisomerization and vibrational relaxation (nonradiative deactivation), thus permitting the detection of T-T absorption spectra of the bound dyes upon direct photoexcitation. In the presence of DNA in the solutions, the triplet lifetimes of the dyes comprise himdreds of microseconds [10]. 

Fluorination and chlorination cause a large decrease in the nonradiative decay rates. The intersystem crossing rate constant drops from 4 x 10 s for acetone to 1.5 x 10 s- - for hexafluoroacetone. For the cyclobutanone/perfluorocyclobutanone pair, the rates are 4.2 x 10 s- - and 1 x 10 s l, respectively. A similar decrease is likely for the glyoxal/oxalyl fluoride pair, where zero-pressure tp values of 2.4 ps and 24 ps, respectively, have been measured (19,25,26). The fluorescence yields of these compounds must be measured before the true extent of this effect is known. Mixed chlorofluoroacetones exhibit nonradiative rates of intermediate value, with k jR increasing at low energy excitation with increasing chlorination from 2.7 x 10 s l for chloropentafluoroacetone to 3.9 x 10 s l and 8.0 x 10 s l for dichlorotetrafluoroacetone and trichloro-trifluoroacetone, respectively (94). 

In this case, the K s are the intersystem crossing rate constants. A detailed description of the different experimental techniques used at the present time, together with equations derived for molecules whose emission results from two zf levels, is given in a recent review of the subject (87). In this section we will give only a brief description of these PMDR methods. In all but the pulsed-excitation method, these assumptions are made (a) the spin-lattice relaxation is absent and (b) the microwave radiation either saturates or inverts the population of... 

The differential equations for these processes can be solved analytically and the results have been presented by Mangir and co-workers (6). They studied this system at room temperature and showed that the intersystem crossing rate constant, k = 2.7 x... 

TABLE 6.3 Arrhenius Parameters of the Elementary Steps on the Singlet and Triplet PES Including the S T and T S Intersystem Crossing Rate Constants... 

Rate constant for internal conversion Rate constant for intersystem crossing Rate constant for non-radiative decay Observed decay rate constant... 

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