Proton-induced reversible modulation of the luminescent output of Re(I), Ir(III) and Ru(II) tetrazolate complexes

One of the distinct features of metal-tetrazolate complexes is the possibility of performing electrophilic additions onto the imine-type nitrogens of the coordinated five-membered ring. These reactions, in particular, provide a useful tool for varying the main structural and electronic properties of the starting tetrazolate complexes. In this paper, we demonstrate how the use of a simple protonation-deprotonation protocol enables us to reversibly change, to a significant extent, the light-emission output and performance of a series of Re(I)-tetrazolate-based phosphors of the general formulation fac-[Re(CO)3(N^N)L], where N^N denotes diimine-type ligands such as 2,2’ bipyridine (bpy) or 1,10 phenantroline (phen) and L represents a series of different 5-aryl tetrazolates. Indeed, upon addition of triflic acid to these neutral Re(I) complexes, a consistent blue shift (max ca. 50 nm) of the emission maximum is observed and the protonated species also display increased quantum yield values (4-13 times greater than the starting compounds) and longer decay lifetime. This alteration can be reversed to the initial condition by further treating the protonated Re(I) complex with a base such as triethylamine. Interestingly, the reversible modulation of luminescent features by the same protonation-deprotonation mechanism appears as a quite general characteristic of photoactive metal tetrazolate complexes, even for compounds in which the 2-pyridyl tetrazolate ligands coordinate the metal centre with a bidentate mode, such as the corresponding Ir(III) cyclometalates [Ir(C^N)2L] and the Ru(II) polypyridyl derivatives [Ru(bpy)2L] + . In these cases, the protonation of the starting materials leads to red shifted and more intense emissions for the Ir(III) complexes, while almost complete quenching is observed in the case of the Ru(II) analogues.