Intersystem crossing pathway

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. 

S-To-type, and reports of S-T i-type CIDNP imder these conditions are very rare, even in low fields. All recent examples involve radical pairs with very large hyperfine coupling constants caused by an inorganic nucleus ( P or Sn). Even then, diffusion must favour longer dwell times around the level crossing, as is seen by the observation that low-field net CIDNP of P from the radical-pair trimethylbenzoyl/dimethoxyphosphonyl still arises to 60-70% through S-To-type CIDNP in the less viscous solvent acetonitrile, while in the more viscous solvent dioxane the contribution of this intersystem crossing pathway decreases to 20-50%. ... 

Although carotenoid triplet states are not formed in appreciable yield by the usual intersystem crossing pathway, they can be produced via triplet-triplet energy transfer from other species or by radical pair recombination processes in artificial reaction centers (Gust et al., 1 2 Liddell et al 1997 Carbonera et al., 1998). Thus, their transient absorption characteristics are well known (Mathis and Kleo, 1973 Bensasson et al., 1976). Typically, they have strong absorption maxima at about 540 nm (Cr - 1 = 10 ... 

The much higher rate of radiationless relaxation of Br-TMIT relative to the unhalogenated species is attributed either to a heavy-atom enhanced intersystem crossing pathway or to reversible photochemical C-Br dissociation in the Sj state of the former [72,81],... 

Energy level diagram for a molecule showing pathways for deactivation of an excited state vr Is vibrational relaxation Ic Is Internal conversion ec Is external conversion, and Isc Is Intersystem crossing. The lowest vibrational energy level for each electronic state Is Indicated by the thicker line. 

Section V.D described the competition of two pathways in the H2 + CO molecular channel. There are also multiple pathways to the radical channel producing H + HCO. In aU cases, highly vibrationally excited CH2O is prepared by laser excitation via the So transition. In the case of the radical channel discussed in this section, multiple pathways arise because of a competition between internal conversion (S o) and intersystem crossing ( i T ), followed by evolution on these electronic states to the ground-state H + HCO product channel. Both electronic states So and Ti correlate adiabatically with H + HCO products, as shown in Fig. 7. 

H. B. Gray Multiconfiguration SCF calculations by P. J. Hay indicate that the 166 -366 energy separation is over 1 eV, and there is no evidence for intervening states that could provide a facile intersystem pathway. Thus a relatively small singlet triplet intersystem crossing rate constant is not all that peculiar. 

Many aromatic aldehydes and ketones (e.g. benzophenone, anthrone, 1- and 2-naphthaldehyde) have a low-lying n-n excited state and thus exhibit low fluorescence quantum yields, as explained above. The dominant de-excitation pathway is intersystem crossing (whose efficiency has been found to be close to 1 for benzophenone). 

The photolysis of the diazobicyclo[2.2.2]heptene derivative (142) was studied at different temperatures and was found to give mixtures of syn (143) and anti (144) products. The experimental data support the homolytic (Xe2) pathway as the prevalent reaction channel at elevated temperatures for the generation of the sterically encumbered syn product, whereas at low temperatures the triplet pathway operates and loss of the syn selectivity is observed. The loss of syn selectivity at low temperatures is due to efficient intersystem crossing in the singlet-excited azoalkane to afford the planar, nitrogen-free triplet diradical which unselectively ring closes. 

Chlorine and bromine react under favourable conditions (activation by other substituent, appropriate nucleophile). Iodine may also be photosubstituted by nucleophiles but is easily induced to enter into homolytic reaction pathways. One has to bear in mind that the heavier substituents (iodine, bromine and even chlorine) increase the rate of intersystem crossing which, depending on the conditions, increases or decreases the quantum yield. 

If the intersystem crossing rate with diamagnetic iron porphyrins is slow then the observed correspondence in relaxation times between S=0 and S=2 iron(II) porphyrins may not necessarily indicate that the relaxation pathways are the same. 

A small value of AEst facilitates intersystem crossing. We expect singlet state to be fast depleted along this pathway if the lowest excited state is of (n, it ) type. This pathway is further promoted due to the fact that t y by a factor of ten, due to the forbidden character of... 

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