Photon‐Triggered Complex Formation: Radical Complexes of o ‐Benzosemiquinone, Dopa, Dopamine and Adrenaline Formed by Electron‐Transfer Reaction from Excited Tris (2,2′‐bipyridyl)ruthenium (II)

The pyrocatechol derivatives, dopamine ( 2 ), adrenaline ( 3 ) and L ‐dopa (4) form in their semiquinoid oxidation states complexes with several closed‐shell metal ions like Zn 2+ , Cd 2+ and Y 3+ . This complex formation can be triggered by visible light via a one‐electron transfer reaction from the pyrocatechol derivative to ( 3 CT)Ru(bpy)   3 2+ , which is thereby reduced ( Schemes 1 and 2 ), The quenching of the triplet charge‐transfer state, ( 3 CT)Ru(bpy)   3 2+cannot be measured by conventional methods ( Stern ‐ Volmer plot), because the bimolecular electron‐transfer rate is too slow ( k q < 10 −7 M −1 s −1 ). ESR. spectroscopy shows, however, clearly the paramagnetic reaction products. The concentration of these paramagnetic species is strongly enhanced by complex formation. The hindered rotation of the side chain in L ‐dopa and in its Y 3+ ‐complex is described within a two‐jump model, which gives best agreement between calculated and measured spectra assuming an energy of 37 kJmol −1 for the rotational barrier.