Singlet-triplet switches

Since singlet-triplet transition is not allowed, consequently, the transition is slow. This makes phosphorescence emission long after the irradiating light is switched off. This does not happen in fluorescence. 

In addition, it was fotmd that during the lifetime of the HBC triplet state, additional selective excitations of the PMI chromophores were quenched by singlet-triplet annihilation as sketched in Fig. 20 [38]. This result establishes a direct link to Sect. 3.2 because the energy transfer pathway in 35 has been reversed. The energy now flows from PMI to HBC. In this respect, the HBC core could be viewed as a switch for the PMI fluorescence operating via intersystem crossing. 

In the limit of infinite atom separations, or if we switch off the Coulomb repui. sion between two electrons, all four wavefunctions have the same energy. But they correspond to different eigenvalues of the electron spin operator the first combination describes the singlet electronic ground state, and the other three combinations give an approximate description of the components of the first triplet excited state. 

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... 

Photoisomerization of 4-methoxy-4/-nitrostilbene 91 suffers from a strong solvent effect (Sch. 35). The (Z/E)pss ratio upon excitation at 366 nm is 91 9 in nonpolar petroleum ether, but the Z/E preference is dramatically switched to give the Z/E ratio of 17 83 in polar dimethylformamide [218]. Laser flash photolysis studies revealed the multiplicity of the excited state involved. The singlet mechanism is operative in the photoisomerization of ( )-4-cyano-4/-methoxystilbene and ( )-4-methoxy-4/-nitrostilbene [219,220], while the photoisomerization of nitrostilbenes involves the triplet state [221]. 

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