Paramagnetic Species in Photochromism and Photodegradation

Because the moieties present in the spiro compound are orthogonal, as are their n-systems, the possibility that the biradical 44 is intermediate in the conversion has been considered.45 

Laser flash-photolysis (LFP) studies in the nanosecond region carried out on 47 in hexane solution also suggested the presence of a triplet biradical, which, however, would not lie on the main reaction coordinate of the photochromic process, being instead involved in the dimerization of 47.49 As already mentioned, it is not possible to directly detect triplet biradicals in solutions by means of EPR spectroscopy, yet indirect indications of their presence in fluid benzene solutions of some spiro compounds50 could be obtained using the so-called double spin trapping technique.51 

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