Photophysics of Ru(II) complexes with hydroxylated diimine ligands: Photoinduced electron/proton transfer to anthraquinone

This manuscript reports the reaction of the 3MLCT excited states of two luminescent chromophores, [(bpy)2Ru(OHbpy)]2+ and [(bpy)2Ru(OMebpy)]2+ (bpy = 2,2′-bipyridine, OHbpy = 4-hydroxy-2,2′-bipyridine, OMebpy = 4-methoxy-2,2′-bipyridine), with anthraquinone (AQ). A series of luminescence, electrochemical, spectrophotometric and transient absorption studies were done in order to determine free energies for the potential reaction paths between the photoexcited complexes and AQ. For the OMebpy complex, only excited state electron transfer (ET*) from the 3MLCT state of the complex to AQ was possible. However, for the OHbpy complex, the excited state could react with AQ via a variety of pathways including excited state electron transfer, ET*, excited state proton transfer (PT*) and excited state proton coupled electron transfer (PCET*). The thermodynamic analysis revealed that, for the OHbpy complex PT* was very endergonic and not a viable reaction pathway, however both ET* and PCET* could occur. Luminescence quenching studies revealed that both the OHbpy and the OMebpy excited complexes reacted with AQ (kq ~ 109 M−1s−1 for both). Transient absorption analysis showed that, for the OMebpy complex, no photoproducts escaped the encounter complex associated with the quenching reaction. The result is consistent with strong electrostatic association of the 3+/1- encounter complex. For the OHbpy complex transient absorption results clearly show the formation of PCET* products from the encounter complex. The result represents one of a small number of examples of excited states of chromophores reacting via proton coupled electron transfer within an encounter complex.

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