Reverse intersystem crossing

Equation (49) applies to both the forward and reverse rate constant, /clh and Ichl- Consequently, the thermodynamic parameters for the intersystem crossing process are related according to ... 

High stereospecificity is observed when the rotation of the diradical intermediate is slow in comparison with cyclization to cycloadduct or reversion to reactants. With the presence of external heavy atoms, it could facilitate the intersystem crossing (ISC) of the first-formed singlet diradical to the longer-lived triplet counterpart. The triplet diradical will have a chance to undergo rotation before it reverts back to singlet and cyclizes or cleaves to reactants. This then accounts for the reduced stereospecificity. The alternative possibility of a zwitterionic intermediate is considered unlikely because there is no interception of zwitterions by water. 

Thermally activated delayed fluorescence Reverse intersystem crossing Ti — Si can... 

The effects of nitro substituents on the cis-trans isomerization of stilbenes has been reviewed70 (equation 63). The trans-to-cis isomerization occurs from a triplet excited state, whereas the reverse cis-to-trans isomerization occurs through a main route which bypasses the triplet state. A nitro substituent usually causes a significant enhancement of the quantum yield of the intersystem crossing. Nitro substituent effects on the photoisomerization of trans-styrylnaphthalene71 (equation 64), trans-azobenzenes72 and 4-nitrodiphenylazomethines73 (equation 65) have been studied for their mechanisms. 

The rates for the reverse intersystem crossing (from upper singlet to triplet) can also be calculated if we make the following assumptions. We first assume that the singlet-triplet conversion takes place only after the molecule in the excited singlet state has lost its excess of vibrational energy. The singlet-triplet conversion (rate = kt) is then in direct com-... 

The rate determining step in intersystem crossing is the transfer from the thermally relaxed singlet state to the vibronically excited triplet state S/ >7 (j > k). This is followed by vibrational relaxation. The spin-orbital interaction modifies the transition rates. A prohibition factor of 10 — 10 is introduced and the values of kiSc lie between 101 and 107 s-1. The reverse transfer from the relaxed triplet to vibronically excited singlet is also possible. 

The wavy arrows in the Jablonski diagram of Figure 3.23, p. 50, correspond to the non-radiative transitions of internal conversion (ic) and the short arrows to intersystem crossing (isc) the former are spin allowed, as they take place between energy states of the same multiplicity the latter are spin forbidden and are therefore much slower. The rate constants of ic and isc span extremely large ranges because they depend not only on the spin reversal (for isc) but also on the energy gap between the initial and final states. 

The high pA"a for HNO would normally not be expected to entirely preclude reactivity of NO- at neutral pH. However, the HNO/NO pair is unique in that proton transfer requires a spin change and that both species are consumed by rapid self-dimerization [(168) 8 x 106M 1 s 1 for Eq. 3 (106)]. The intersystem crossing barrier slows proton transfer by as much as seven orders of magnitude (169) thus allowing dimerization (and other reactions) to not only become competitive with, but to exceed, the rate of proton transfer. Thus for the HNO/ NO pair, an acid-base equilibrium has little relevance the chemistry is instead dependent on the forward and reverse rate constants for proton transfer relative to consumption pathways. 

Both photoinduced LS —> HS and HS > LS transitions involve transition through a 3Ti state, from which the system can relax into the LS and HS ground states via intersystem crossing processes. This reversible state switching has been summarized as light-induced excited spin state trapping (LIESST) effect,29 and especially for Fe-based compounds it can be conveniently traced by Mossbauer spectroscopy. 

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