A PULSE RADIOLYSIS and PULSED LASER STUDY OF THE VITAMIN D3 TRIPLET STATE: LIFETIME, RELAXATION and ‘NONVERTICAL’EXCITATION

The triplet state of vitamin D3 in benzene has been characterised in terms of its absorption spectrum, Λmax 315 nm, its lifetime, 300 ns, its rate constant for reaction with oxygen, 4.2 times 109mol−1 s−1 and the efficiency with which it sensitizes the formation of singlet oxygen, 25%. There is a large difference in the electronic excitation energies of the spectroscopic and relaxed triplets, ˜ 237 kJ mol−1 and147–168 kJ mol−1, respectively. It has been shown that, in the endothermic situation, the vitamin D3 molecule is a 'nonvertical'acceptor of triplet energy. This is in accord with the non‐planar character of its acyclic conjugated Il‐system.