Crossover singlet-triplet

Figure 8 The singlet-triplet gap, Est, near the neutral-ionic transition of (36) in the restricted basis as a function of T = A — U/2. Open and closed symbols refer to boundary conditions with and without crossovers, respectively, and the stars as joint N —> oo extrapolations based on both[97].

A laser flash photolysis study of the sensitized and heavy atom induced production of the acenaphthylene triplet in solution emphasizes the importance of heavy atom effects on intersystem crossing. The role of the T2 state in 9,10-dibromoanthracene excited singlet-triplet crossover has been assessed by the use of 1,3-octadiene which for this system is a selective triplet quencher for triplet states higher than the lowest, T-. 0 The T2 state mediates intersystem crossing but does not participate in any T-T annihilation process. Triplet state properties of 9-acetoxy-10-phenylanthracene derivatives have also been reported in detail. 

The rise in iso with pressure (and the resulting decrease in Or) can now be interpreted as arising from faster vibrational relaxation of the triplet from the point of crossover from the singlet with increasing numbers of molecular collisions. At lower pressures, where vibrationally deactivating collisions are few, the triplet has the possibility of crossing back over to the singlet level before deactivation has occurred. 

Such a process is called an intersystem crossover, since the molecule changes from triplet to singlet. Letting M represent a molecule M with two unpaired electrons, i.e., a triplet [T] state, and M the d same molecule... 

Here the symbol M M o represents the intermediate resonance hybrid II) at Q s Qo- An equivalent symbol for M <-> could be M -. Another notation for this intersystem crossover process is obtained if one denotes singlet [5] by f j and triplet T] by f, ... 

Intersystem crossover processes between singlet and triplet states are well known in photochemistry. Intersystem crossing processes have been of importance in interpreting benzene photochemistry. Anticipating that... 

Thus various lines of evidence lead to the conclusion that there is a process which is of varying importance dependent on wavelength and probably on other conditions which destroy the singlet or the triplet state, or both. Since the extent of this process depends on wavelength one must conclude that the rate of crossover depends on the vibrational level of the upper electronic state. 

Equation (VI-67) involves a crossover from the initially formed singlet to a repulsive triplet state of NOCI. The photolysis of NOCI in the vacuum ultraviolet produces NO(/l2I, r = 0,1,2) [Welgc (1030)] and possibly other excited states of NO [Len/.i and Okabe (625)]. The fluorescence yield near 1500 A is about 4" , (625). 

Nitroxides are the most common of the oxygen-centred biradicals to be reported. As a model for spin-crossover molecules, the nitronyl nitroxide (105) was prepared and by oxidation with PbC>2 afforded the triplet biradical (106) which was characterized by ESR (Scheme 15).242 The one-electron oxidation of (105) afforded the singlet cation (107) which was seen to exist in equilibrium with (106) in solution. The authors claim that pH-controlled interconversion between two species of different spin multiplicities in this way may provide die basis for novel magnetic switches or pH sensors. The N.N-dialkylamino nitronyl nitroxides (108) were prepared and afforded die diplet-state biradical cation species by one-electron oxidation with iodine.243 The authors propose that, by the similarity of die electronic structures, diese structures can be regarded as hetero-analogues of trimethylenemethane. A paper confirms die conversion of 3,3-dimethyldioxetane into die corresponding ring-opened 1,2-diol but refutes the... 

Another example is found with benzene vapor at room temperature. The emission yield (fluorescence only) is about 0.20 0.04 and essentially independent of pressure100,101. There are products formed, although there is some disagreement on this subject and the yields in the gas phase have not been determined. Also there is a crossover from the singlet to the triplet state so that all or nearly all of the primarily excited singlet state molecules either fluoresce or cross over to the triplet state. Nevertheless, in the vapor phase at room temperature, about 80 per cent of the molecules which absorb neither react chemically nor emit radiation100, 101. 

If contributions from all states, including those of different multiplicity, must be considered in developing the complete wave function, it is not possible in one sense to say that collisions or foreign fields induce crossovers from singlet to triplet states. If, however, a molecule is to be stabilized in a triplet state enough energy must somehow be removed so that the wave function can be mainly triplet. Thus, to say that collisions play no role in the crossover process would be fallacious. 

Figure 6 Band-to-correlated crossover based on E B-u) = 2E(13BU) of IV-site Hubbard chains with alternating transfer integrals t = 0(1 ) The U = 0 and 21+ points are exact for the infinite chain and dimers, respectively. The dashed Uc(6) line is based on perturbation theory at large 6 and (inset) separate estimates of the singlet and triplet thresholds at small <5[80].

---