Intersystem crossing rate, spin conversion

The intersystem crossing process has opposite effects on the yields of fluorescence and phosphorescence since it depletes the singlet state and populates the triplet state. It is commonly known that heavy ions, such as iodide and bromide, increase intersystem crossing by spin-orbit coupling.(1617) For proteins, fluorescence can be quenched as phosphorescence yield is enhanced. 8,19) However, although the phosphorescence yield is increased, the lifetime is decreased. This effect arises because spin-orbit coupling, which increases the intersystem crossing rate from 5, to Tt, also increases the conversion rate from T, to S0. 

The only difference from the single-channel EM outlined above (Section V.A) is the substitution of k et by the sum of the spin-allowed and spin-forbidden transfer rates k et + k c, to the ground and triplet states, respectively. Like k-eh the intersystem crossing rate kKC does not depend on viscosity. Moreover, EM does not separate the two different steps of the forbidden transition spin conversion to the triplet RIP and subsequent allowed electron transfer into the triplet product [212-216]. However, as has been shown in Section XI.A, even in the case of a single channel but spin-forbidden reaction (I), one should discriminate between the spin conversion and subsequent recombination through electron transfer. The qualitative difference between the spin-allowed and... 

Once the excited molecule reaches the S state it can decay by emitting fluorescence or it can undergo a fiirtlier radiationless transition to a triplet state. A radiationless transition between states of different multiplicity is called intersystem crossing. This is a spin-forbidden process. It is not as fast as internal conversion and often has a rate comparable to the radiative rate, so some S molecules fluoresce and otliers produce triplet states. There may also be fiirther internal conversion from to the ground state, though it is not easy to detemiine the extent to which that occurs. Photochemical reactions or energy transfer may also occur from S. ... 

The heavy-atom effect on intersystem crossing, due to spin-orbit coupling, is very well known.118-120 On the other hand, very little is known about internal conversion. This process is mostly considered to be of negligible importance, but as was pointed out by El-Sayed121 there exists little rate data to support this assumption. As far as xanthene dyes and related dyes are concerned, results indicate the occurrence of internal conversion.7,93,103,110 122... 

We conclude that zero-order crossing of potential curves can enhance spin-forbidden processes. Spin-forbidden processes, such as intersystem crossing, may also occur in the absence of zero-order crossings, though at a slower rate in general. The formulation here is time dependent. Some experimental phenomena which have been interpreted as time-dependent phenomena (for example, S2 > S1 internal conversion) may also be interpreted in a time-independent188,199 formulation. 

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. 

Various primary processes induced upon photon absorption by this molecule are also shown in Fig. 2-1. The photon absorption processes associated with the vibrational-electronic transitions from So to Si and S2 are represented by So ->Si Abs. and So —>82 Abs., respectively. By internal conversion (IC) we mean a radiationless process between two different electronic states of the same spin multiplicity. In Fig. 2-1, IC from S2 to S and IC from Si to So are shown. Usually, the rate constants of S2 Si IC and Si —>So IC are more than lO s and 10 -10 s , respectively. By intersystem crossing (ISC) we mean a radiationless process between two different electronic states of two different spin multiplicities. In Fig. 2-1, Si Ti ISC and Ti—>So ISC are shown. The rate constants of the... 

The transition between two excited states with similar energy but of different multiplicity is called intersystem crossing. It is a spin-forbidden process thus the rate is slower than that of internal conversion. The rate varies from 10" to 107 s-1. 

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