![]() Often, in organic molecules, only light atoms (C, N, O) contribute to the molecular structure thus the spin–orbit interaction is weak, and the ISC process is slow. The atomic spin–orbit coupling is proportional to the nuclear charge. If this happens, then the molecule undergoes intersystem crossing (ISC) and becomes a triplet state. The unpairing of the electron spins is possible only if there is some degree of spin–orbit coupling (in which the magnetic field arising from the orbital motion of the two electrons interacts with the spin magnetic moments). The ground state of a typical organic molecule is a singlet state (S = 0), and absorption of light leads to excited singlet states. The Jablonski diagram in Fig. 4.36 illustrates the relevant molecular electronic states and their population and decay routes. Pragst 74 used triplet sensitizers created in the recombination reaction of electrochemically produced radicals and reported 170 ± 10 kJ mol −1.Įlectronic triplet states in molecules are rare but during chemical and biological systems they are often formed thermally by photo-excitation or, in some cases, by chemical reactions, starting from singlet states. From their kinetic results, they inferred that the azobenzene acceptor should be twisted (phantom triplet) when accepting the energy and calculated the distortion energy to be 56 kJ mol −1 for the E-form and 77 kJ mol −1 for the Z-form. 73 located triplet states at 146 (E-azobenzene) and 121 kJ mol −1 (Z-azobenzene). 71,72 found two relevant triplet states at about 196 and 180 kJ mol −1 (E-azobenzene) and about 192 and 142 kJ mol −1 (Z-azobenzene). The energy of triplet states has been located only by “chemical spectroscopy,” i.e., the quenching of other molecules' triplet states by azobenzene. 47 Neither has phosphorescence been detected. Direct absorption of a 0.51 mol l −1 solution in C 7H 15J in 5 cm cells has not been detectable. Triplet state data for azobenzene-type azo compounds are very limited. ![]() Hermann Rau, in Photoreactive Organic Thin Films, 2002 1.3.1.1.7 The Triplet State
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