Singlet generation

In the two most important types of photosensitization, both of which are in accord with the Wigner rule, a triplet excited state generates another triplet and a singlet generates a singlet ... 

Singlet generated cyclopropyl carbenes, e.g. 253 (Z = CR), give cyclobutadienes by Wolff-like rearrangement whereas alkyne products most likely result from the triplet carbene 34 4.-347 nitrene analogue 253 (Z = N ) is thought to behave similarly but azacyclobutadiene is not isolable from the Sj reaction as it fragments to alkyne and nitrile. 

Rothe C, King S, A1 Attar HA, Monkman AP (2006) Direct measurement of the singlet generation yield in polymer light emitting diodes. Phys Rev Lett 97 076602... 

To generate the proper A, A2, and E wavefunetions of singlet and triplet spin symmetry (thus far, it is not elear whieh spin ean arise for eaeh of the three above spatial symmetries however, only singlet and triplet spin funetions ean arise for this two-eleetron example), one ean apply the following (un-normalized) symmetry projeetion operators (see Appendix E where these projeetors are introdueed) to all determinental wavefunetions arising from the e eonfiguration ... 

Remembering that the projection process does not lead to a normalized function, although it does generate a function of pure symmetry, one can finally write down the normalized symmetry-adapted singlet functions as ... 

Phenols with bulky ortho- and para-substituents, eg, phenoHc antioxidants, do not undergo this reaction however, they scavenge radicals generated by thermolysis of diacyl peroxides and other peroxides. Diacyl peroxides react with potassium superoxide, KO2, forming singlet oxygen (207). 

The excitation process may generate an excited molecule in any allowed vibrational state, but tbe excess vibrational energy is rapidly lost, and the excited state species may then emit a photon of frequency Vem, this singlet-singlet transition from the excited to ground state being fluorescence. 

Singlet oxygen, O2, can readily be generated by irradiating normal triplet oxygen, 2 in the presence of a sensitizer, S, which is usually a fluorescein-type dye, a polycyclic hydrocarbon or other strong absorber of light. A spin-allowed transition then occurs ... 

Generation of (excited state) singlet S in the presence of parafHns yields the corresponding mercaptan by a concerted single-step insertion ... 

The number of excited determinants thus grows factorially with the size of the basis set. Many of these excited determinants will of course have different spin multiplicity (triplet, quintet etc. states for a singlet HF determinant), and can therefore be left out in the calculation. Generating only the singlet CSFs, the number of configurations at each excitation level is shown in Table 4.1. 

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