Efficiency of Intersystem Crossing

The efficiency or quantum yield of intersystem crossing is expressed by the following equation  

---

It has been possible to employ the heavy-atom solvent effect in determining the rate constants for the various intercombinational nonradiative transitions in acenaphthylene and 5,6-dichIoroacenaphthylene.<436,c,rate constants, which are not accessible in light-atom solvents due to the complexity of the mechanism and the low efficiency of intersystem crossing from the first excited singlet to the first excited triplet, can be readily evaluated under the influence of heavy-atom perturbation. 

However, the heavy atom effect can be small for some aromatic hydrocarbons if (i) the fluorescence quantum yield is large so that de-excitation by fluorescence emission dominates all other de-excitation processes (ii) the fluorescence quantum yield is very low so that the increase in efficiency of intersystem crossing is relatively small (iii) there is no triplet state energetically close to the fluorescing state (e.g. perylene)10 . 

Fluorescein and related quinonelike dyes such as eosin have found extensive use as low-energy sensitizers, especially in photooxidation studies. The efficiency of intersystem crossing in fluorescein varies markedly with pH. For the neutral molecule, the quantum yield of fluorescence is only 0.31, while it is 0.91 for the dianion.449 In strong acid solutions, however, protonated fluorescein is strongly phosphorescent. In this state (dissolved in boric acid) fluorescein was the first compound for which T-T absorption in an excited triplet state was observed.7... 

Polar solvents also may affect the geometrical isomerization of ethylenes by affecting the efficiency of intersystem crossing [96,97]. Thus, even in the absence of polar substituents, solvent polarity can be of importance in the photochemical isomerization of aryl-substituted ethylenes. For example, cis-l-(9-anthryl)-2-(l-naphthyl)ethylene 74 in cyclohexane isomerizes to the trans-isomer 75 with a quantum yield of 0.05, while the quantum yield in acetonitrile solution is 0.31. Any trans->cis isomerization 75—74 in cyclohexane has not been observed, but the reaction does proceed with a preparatively useful quantum yield of 0.05 in acetonitrile [98],... 

First, the exchange interaction often does not fall off to zero. On the one hand, this reduces the efficiency of intersystem crossing between IS) and ITq). On the other hand, it opens up another intersystem crossing pathway, namely, between IS) and IT j) (or, rarely, IT + i)) because the potential energy curves of these states intersect at some point, and the system spends more time in that region if diffusion is not free. 

Undoubtedly the most important distinction between (n, Jt ) and (n, n ) excited states of aromatic carbonyl compounds is the much greater efficiency of intersystem crossing (i.e. Si Ti conversion) in the former cases. This follows because, as already noted, triplet states are the longest lived and most useful intermediates in organic photochemistry. Two factors contribute to the high probability of intersystem crossing between (n, n ) states as compared to that between (jr, n ) states ... 

Let us first consider the situation where initial excitation is followed by relaxation to a bound LEES, which is then responsible for the ligand substitution chemistry. In accord with the above discussion, the quantum yield <1>S for ligand substitution from that state would be fl>lscfcst, where intersystem crossing from the state(s) initially formed, ks is the rate constant for ligand substitution from the LEES, and r = kd1 (kd being the sum of the rate constants for the decay of the LEES). The apparent activation volume for the photoreaction quantum yields is therefore defined as... 

Because orbital momentum, and thus orbital shape and orientation, are involved in intersystem crossing driven by spin-orbit coupling, molecular symmetry and electronic configuration come into play. Consequently, the efficiency of intersystem crossing is usually different for the sublevels of a given multiplicity. This effect is the basis of the so-called triplet mechanism of electron spin polarizations as far as CIDNP is concerned, Hso only plays a role for systems with restricted diffusion (biradicals, radical pairs in micelles). 

However, the quantum yield is defined slighdy differently since the efficiency of intersystem crossing has to be taken into account. 

As with other reactions involving triplet excited states, the efficiency of intersystem crossing appears to be a major factor in the outcome of the reaction. The... 

Radiative and Non-radiative Processes.—The variation of the ratio of phosphorescence to fluorescence for CeH5, C De, CeH6CH8, and - 4( )2 in methylcyclohexane at 77 can be accounted for by an increase in the importance of triplet-triplet annihilation at high solute concentration. The detection of delayed fluorescence in these systems supports the triplet-triplet annihilation mechanism.184 This conclusion is different from that reached in earlier reports,186 when it was suggested that an increase in concentration of the aromatic affected the efficiency of intersystem crossing (Sj, -> T ). 

Because the lower molar absorptivity of the n Ti transitions translates to lower values of fef, the relative importance (efficiency) of intersystem crossing is in turn enhanced. The smaller energy differences between Si and Ti for n,7r states also increases the rate of intersystem crossing. Therefore, if Si is an... 

As in fluorescence, phosphorescence signals depend upon both structural and environmental factors. Under appropriate experimental conditions, the intensity of the phosphorescent emission measured is linearly related to the concentration of the phosphor. This relationship, which constitutes the theoretical basis of quantitative phosphorescence analysis, is very similar to that obtained for fluorescence, except that the efficiency of intersystem crossing , which populates the triplet, should also be taken into account. Therefore, at very low concentrations of the analyte (trace analysis), the basic equation is... 

Our preliminary results on the role of torsion on the efficiency of intersystem crossing in a series of substituted dibiphenylene ethylenes, II, indicate that increasing the... 

---