El-Sayed s rules

Outline the basis of El-Sayed s rules and be able to use these to explain the differences in luminescence properties between aliphatic and aromatic carbonyl compounds. 

It is because of El-Sayed s selection rules that there is a great difference in luminescence behaviour between aliphatic and aromatic ketones. The principal reason for the difference is that 1kisc for aromatic ketones involves a change in orbital type whereas klsc for aliphatic ketones does not undergo a change in orbital type (Figures 5.5 and 5.6). Thus k is much greater for the aromatic ketones because of application of El-Sayed s rules. 

According to the selection rules for intersystem crossing known as El Sayed s rules (El Sayed, 1963) transitions... 

An unusual example of delayed fluorescence exemplifying El Sayed s rules (Section 2.1.6) was recently reported for the triplet sensitizer xanthone,127 which undergoes ultrafast ISC within 1 ps. Delayed fluorescence with a lifetime of 700 ps was observed in aqueous solution. Temperature-dependent steady-state and time-resolved fluorescence experiments indicate that the T2(n,it ) state, which is primarily accessed by ISC from Si(ji,ji ), is nearly isoenergetic with the Sj state. The delayed fluorescence is attributed to reverse ISC from T2(n,it ), in competition with internal conversion to Tl(7I,7l ). 

Figure 4.30 Left El Sayed s rule for ISC. Right the n,7i transition of formaldehyde...

An important example of El-Sayed s rules in action concerns the ISC efficiencies of simple ketones. Figure 16.7 B summarizes the effect. For a typical ketone. Si and Ti are ,Tr, while T2 is TT,TT. Therefore, S] and T1 interconversion is inefficient, but conversion to T2 may be efficient. Let s examine some cases. For a dialkyl ketone, T2 lies above S, and so ISC is inefficient (conversion to a higher energy state almost never occurs). However, aryl substitu-... 

ISC is more efficient for diaiyd ketones because T2, which is it,it, is comparable in energy to Si, allowing ISC, consistent with El-Sayed s rules. 

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