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 Zn2+, Cd2+ and Y3+. This complex formation can be triggered by visible light via a one-electron transfer reaction from the pyrocate...
Ausführliche Beschreibung

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 Zn2+, Cd2+ and Y3+. This complex formation can be triggered by visible light via a one-electron transfer reaction from the pyrocatechol derivative to (3CT)Ru(bpy)32+, which is thereby reduced (Schemes 1 and 2), The quenching of the triplet charge-transfer state, (3CT)Ru(bpy)32+ cannot be measured by conventional methods (Stern-Volmer plot), because the bimolecular electron-transfer rate is too slow (kq < 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 Y3+-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. Ausführliche Beschreibung