Synthesis and properties of an efficient and switchable photosensitizing unit, [Ru(4,4′-diphenyl-2,2′-bipyridine)2-(7-amino-dipyrido[3,2-a:2′,3′-c] phenazine)]2+, for a photo-induced energy transfer system

A Ru(II) complex containing the 4,4'-diphenyl-2,2'-bipyridine (Ph(2)bpy) and 7-amino-dipyrido[3,2-a:2',3'-c]phenazine (dppz-NH2) ligands was synthesized and found to be an excellent switchable photosensitizing unit suitable for photo-induced energy transfer, which has large light absorptivity, superior excited state properties and an amino functional unit for connecting acceptor units. [Os(tpy)2](2+) (tpy = 2,2':6',2"-terpyridine) unit was connected to the amino group of this photosensitizing unit by an amide bond to construct a heterodinuclear Ru(II)/Os(II) photo-induced energy transfer system. This system showed stronger Os(II) centered emission compared to the similar heterodinuclear Ru(II)/Os(II) complex containing 2,2'-bipy (bpy) instead of Ph(2)bpy. Analysis of the photophysical processes of these complexes indicated that the stronger Os(II) emission was ascribed not only to the higher absorptivity but also to the increased energy transfer efficiency from the Ru(II) to the Os(II) center. It also demonstrated that photosensitization by energy transfer to the Os(II) center was switched on and off reversibly by protonation/deprotonation of the dppz moiety.