Intersystem crossing efficiency

A similar method for determining intersystem crossing efficiencies has been developed by Parker and Joyce 7> using acceptor delayed fluorescence (P-type, see Section 5.2a). The processes involved in this method are... 

Thus a plot of t //p vs. 1/[A] should yield a straight line with intercept kf + kic + klsc)IKkisc or 1 imlsc. Thus phosphorescence intensity and lifetime as a function of [A] (within the limits of our assumptions), the intersystem crossing efficiency of the donor can be obtained. 

The subject of delayed fluorescence was discussed in Section 5.2a. It was seen that there are two common types of delayed fluorescence, that arising from thermally activated return from the triplet state to the lowest excited singlet (E-type delayed fluorescence) and that arising from collision of two excited triplet molecules resulting in a singlet excited molecule and a ground state molecule (P-type delayed fluorescence). The P-type delayed fluorescence can be used as a convenient tool for the determination of intersystem crossing efficiencies[Pg.125]

Midinger and Wilkinson<54> have used flash photolysis and fluorescence quenching by heavy atoms to determine the intersystem crossing efficiencies of anthracene and a number of its derivatives. As discussed in Section 5.2b, heavy atoms present as molecular substituents or in the solvent serve to promote multiplicity forbidden transitions. When anthracene is excited the following processes can occur ... 

Figure 5.8. Processes involved in the determination of intersystem crossing efficiencies by energy transfer.

A. R. Harrochs, A. Kearvell, K. Tickle, and F. Wilkinson, Mechanism of fluorescence quenching in solution II. Quenching by xenon and intersystem crossing efficiencies, Trans. Faraday Soc. 62, 3393-3399 (1966). 

As has already been mentioned (vide supra, p. 118) the population of the triplet state of a carbene depends on the effectiveness of the intersystem crossing steps kisc and k lse- These rates can be altered by dilution. Another technique which exclusively populates the triplet state of a carbene is sensitization, or energy transfer. A triplet sensitizer is required for this purpose — usually an aromatic ketone. In these ketones the intersystem crossing efficiency is almost 100%. Energy is then transferred from the sensitizer triplet to the diazoalkane, thus populating the triplet state of the latter. 

It was recognized at an early stage that the fact that m-nitrophenyl phosphate and m-nitroanisole give a much cleaner and more efficient reaction than their ortho- and para-isomers, in itself, is not sufficient to conclude that the rate of reactions of the excited meta-compound with the nucleophile is higher. Such a more efficient photoreaction might also be explained by a longer lifetime of the excited metacompound, by a different intersystem crossing efficiency, etc. 

Quantitative data on rates of reaction have been obtained for some of the triplet reactions. Assuming triplet quenching to be approximately diffusion controlled, the rate constants for the reactions between excited species and nucleophile are 10 -10 1 mole s . The data show that in comparing and interpreting quantum yields—even in the case of related systems—one should proceed to determine separately rate constants as well as intersystem crossing efficiencies and lifetimes of the reacting excited species. 

Figure 2. Schematic representation of some relevant ground and excited-state properties of Ru(bpy)j. MLCT and MLCT are the spin-allowed and spin-forbidden metal-to-ligand charge transfer excited states, responsible for the high intensity absorption band with = 450 nm and the luminescence band with = 615 nm, respectively. The other quantities shown are intersystem crossing efficiency energy (E°°) and lifetime (x) of the MLCT state luminescence quantum yield ( ) quantum yield for ligand detachment (O,). The reduction potentials of couples involving the ground and the MLCT excited states are also indicated.

Several examples of heavy atom quenching of aromatic hydrocarbon states are known for example, carbon tetrabromide is an efficient quencher of the fluorescence of anthracene167 and carbon tetrachloride behaves similarly with p-terphenyl.188 Since quenching results in formation of the triplet state, it has been possible to use the heavy atom effect to measure intersystem crossing efficiencies ( ). Because of the elegance of this technique 169 and the importance of the results in photochemistry, we shall cover it in some detail. 

In Table 1 we have collected data on measured lifetimes and quantum yields in various solvents for [Ru(bpy)3]2+. Also included is a calculated quantum yield from the observed lifetime, assuming a constant value for the radiative lifetime (14 ps174,192)). Demas and Crosby188 have argued that the intersystem crossing efficiency to populate the emitting MLCT triplet is unity and should be independent of solvent Bolletta et al.193)... 

Ethoxyisoindolenone (50) sensitized cis-trans isomerization of c/s-piperylene has been used in a triplet counting experiment to measure the intersystem crossing efficiency. A linear triplet counting plot is obtained which gives an intersystem crossing efficiency of 0.96 0.01 and a triplet lifetime of 1.5 x 10 7 sec. for 50 in methylene chloride solvent at ambient temperature in the absence of olefin. 

Sensitized photolysis is a much more efficient method to produce triplet nitrenes than the collisional deactivation method. This technique, which populates the triplet state of a nitrene exclusively by energy transfer, employs triplet sensitizers such as aromatic ketones with a high intersystem-crossing efficiency. Energy is... 

When more conjugated diimine or pyridine ligands are used, the excited states of rhenium(I) carbonyl complexes can have substantial IL character. While the MLCT emission is often broad, with a lifetime in the submicrosecond to microsecond timescale, the IL emission usually has noticeable structural features, even in fluid solutions at ambient temperature. The emission lifetime is usually very long. A simple and widely applicable approach is to evaluate the ratio of the emission quantum yield and the emission lifetime (the product of the intersystem crossing efficiency and radiative decay-rate constant). Experimental values of... 

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