Tuning of photo-redox behaviours, and thermodynamic and kinetic aspects of intercomponent energy transfer in trimetallic complexes of Ru(II) and Os(II) by exploiting their second coordination sphere

This paper deals with a thorough investigation of pH-induced tuning of the ground and excited state photophysical as well as electrochemical behaviours of two series of our recently reported homo- and heterotrimetallic complexes of the type [(bpy)2Ru(d-HIm-t)M(t-HIm-d)Ru(bpy)2]6+ and [(bpy)2Os(d-HIm-t)M(t-HIm-d)Os(bpy)2]6+ (M = RuII and OsII) derived from a heteroditopic bpy-tpy (d-HIm-t) type bridging ligand through the exploitation of their second coordination sphere. A small bathochromic shift of the absorption and emission spectral band along with substantial alteration of emission intensity and lifetime of the triads is noted upon deprotonation of the NH motifs at elevated pH values. The lowering of the half wave potential of a M3+/M2+ couple is also observed upon removal of the NH protons. Both ground and excited state pKa values of the triads are estimated from their absorption/emission versus pH spectral profiles. In addition, the variation of the free energy change (ΔG) and the rate of intercomponent energy transfer (ken) in the triads upon stepwise deprotonation of the NH motifs are also addressed in the present study.