Crossing intersystem

This refers to efficient energy transfer between the singlet and triplet manifolds. Subsequent internal conversion usually leads to the lowest triplet state (T i), which does not fluoresce. The rate of intersystem crossing is governed by the spin-orbit coupling term, which is enhanced by heavy atoms, such as those of the sulphur atoms in oligothiophenes. 

Accordingly, the open shell states of rectangular CBD (D2/1) have the irreps  

In square-planar CBD (D4/1), the irreps of the overall state functions of the two lowest open shell states are  

The symmetry requirements for crossing from a singlet to a triplet potential energy surface have been dealt with in a variety of ways, with particular emphasis on intersystem crossing (1C), most commonly observed when an excited open 

The condition for a symmetry-allowed pathway for a reaction in which a singlet (S) and triplet (T) are interconverted is the retention of overall symmetry along the pathway [2, p. 206]  

Consequently, the process is allowed if the irrep of the space function of the singlet equals the overall irrep of any one of the triplet components, i.e. that of its space function multiplied by the irrep of Ry or R  

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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. ... 

By examining the expression for Q ( equation (B1.16.4)). it should now be clear that the nuclear spin state influences the difference in precessional frequencies and, ultimately, the likelihood of intersystem crossing, tlnough the hyperfme tenn. It is this influence of nuclear spin states on electronic intersystem crossing which will eventually lead to non-equilibrium distributions of nuclear spin states, i.e. spin polarization, in the products of radical reactions, as we shall see below. 

Since Ag is positive and is negative, Q is larger for the p state than for the a state. Radical pairs in the p nuclear spin state will experience a faster intersystem crossing rate than those in the a state with the result that more RPs in the p nuclear spin state will become triplets. The end result is that the scavenging product, which is fonned primarily from triplet RPs, will have an excess of spins in the p state while the recombination product, which is fonned from singlet RPs, will have an excess of a nuclear spin states. 

Because the spin-orbit interaction is anisotropic (there is a directional dependence of the view each electron has of the relevant orbitals), the intersystem crossing rates from. S to each triplet level are different. 

Figure C 1.5.10. Nonnalized fluorescence intensity correlation function for a single terrylene molecule in p-terjDhenyl at 2 K. The solid line is tire tlieoretical curve. Regions of deviation from tire long-time value of unity due to photon antibunching (the finite lifetime of tire excited singlet state), Rabi oscillations (absorjDtion-stimulated emission cycles driven by tire laser field) and photon bunching (dark periods caused by intersystem crossing to tire triplet state) are indicated. Reproduced witli pennission from Plakhotnik et al [66], adapted from [118].

Bernard J, Fleury L, Talon FI and Orrit M 1993 Photon bunching in the fluorescence from single molecules a probe for intersystem crossing J. Phys. Chem 98 850-9... 

The natural processes of intersystem crossing and internal conversion will quickly (e.g. 50 ns) carry the molecule from this excited electronic surface to the ground electronic surface without a collision,... 

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. 

Interstitial cystitis Interstitial-free steels Intersystem crossing Intestinal flukes... 

VD = vibrational deactivation IC = internal conversion F = fluorescence IX = intersystem crossing and P = phosphorescence. 

The second type of photoinitiators, ie, those that undergo electron transfer followed by proton transfer to give free-radical species, proceed as follows, where is the rate constant for intersystem crossing. 

The common case, and the one that will be emphasized here, is triplet sensitization. In this case, the intersystem crossing of the sensitizer must be faster than energy transfer to the reactant or solvent from the singlet excited state. 

Sensitizer singlet formed Intersystem crossing of sensitizer Energy transfer to reactant molecule... 

These reactions usually occur via the triplet excited state T,. The intersystem crossing of the initially formed singlet excited state is so fast (fc 10 s ) that reactions of the S state are usually not observed. The reaction of benzophenone has been particularly closeh studied. Some of the facts that have been established in support of the general mechanisir. outlined above are as follows ... 

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