El Sayed rules

El-Sayed rules The rate of intersystem crossing, e.g. from the lowest singlet state to the triplet manifold, is relatively large if the radiationless transition involves a change of orbital type. E.g. - n,7t is faster than - n,n and n,7i ... 

Ryzhikov et al. reported the quantum yield of fluorescence ( ) of DD and its analogue DHA is 0.003 and 0.26, respectively, which are quite different from each other. Their proposed mechanism is fast intersystem crossing to the low-lying triplet nn states for the low value of in DD in terms of the El-Sayed rule. ... 

Intersystem crossing (S —> T) HT12 1CT6 2.1.6 (El Sayed rule)... 

Rate Constants of Intersystem Crossing, El Sayed Rules... 

Intersystem crossing likewise may follow more complicated mechanisms. Generalizations such as that expressed by the El Sayed rules prohibiting S-T transitirm when both states were njt or nn have been reexamined, evidencing a new path for the Si(njr ) —> Ti(njt ) transition [25]. In nitroaromatics, intersystem crossing occurs within 100 fs following the path Si(jtJt ) T3(mt ) appar-... 

The natural phosphorescence rate constants suggest again that the Tj transitions are about 10 to 100 times more allowed for ketones than for cis-azoalkanes. It should be noted that the phosphorescence rate of azoalkanes is not accelerated by heavy atoms such as in 1,2-dibromoethane as solvent [50] or thallium in zeolites [147]. This is expected since phosphorescence from a (n,7i) state to the 71 ground state is an El-Sayed rule allowed process [143], which is inherently fast (in comparison to phosphorescence from (7r,7r) states) and is less likely to be much assisted by heavy atoms [153]. 

In the case of nitrogen heterocycUc compounds, the situation is more complicated but it is nonetheless possible to uniquely identify the promoting mode the El-Sayed rule determines the path of the intersystem crossing. 

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. 

Quite generally, El-Sayed 13,14) derived the following rules for spin-orbit coupling ... 

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...

The German chemist Erich Hiickel was the first to recognize that an aromatic compound must have an odd number of pairs of tt electrons. In 1931, he described this requirement by what has come to be known as Hiickel s rule, or the 4 -I- 2 rule. The rule states that for a planar, cyclic compound to be aromatic, its uninterrupted tt cloud must contain An + 1)tt electrons, where n is any whole number. According to Hiick-el s rule, then, an aromatic compound must have 2 n = 0), 6 (n = 1), 10 ( = 2), 14 (n = 3), 18 n = 4), etc., tt electrons. Because there are two electrons in a pair, Hiickel s mle requires that an aromatic compound have 1, 3, 5, 7, 9, etc., pairs of tt electrons. Thus, Hiickel s rule is just a mathematical way of saying that an aromatic compound must have an odd number of pairs of tt electrons. 

S)unmetry selection rules for radiationless transitions have also been derived by El-Sayed for nitrogen heterocycUcs (7). He showed that to the first order no spin-orbit coupling occurs between singlet and triplet states of the same configuration, i. e. 

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