Dual emission from highly conjugated 2,2':6':2″-terpyridine complexes-a potential route to white emitters

Here, we present a new class of terpyridine complexes of the transition-metal ions, iron(II), ruthenium(II), and osmium(II), overcoming the poor emission properties typical for this class of polypyridyl complexes. These complexes show, besides an increased room-temperature emission quantum yield and a prolonged lifetime of the metal-to-ligand charge-transfer (MLCT) states, dual emission from two well-separated excited states of the same molecule. These experimental findings are attributed to a highly stabilized ligand chromophore, where photoinduced excited-state planarization causes an enhancement of electron delocalization. This planarization, in turn, reduces the potential energy of the S(1) state and minimizes electronic coupling to the MLCT state, which is prone to non-radiative deactivation via metal-centered excited states. Due to their dual emission the complexes presented here show emission covering the entire Vis spectral range upon excitation of the ππ* states in the near UV. Thus, by structurally tuning the electronic coupling of the ππ* and the MLCT states a new synthetic route toward white emitters, which can subsequently be incorporated into polymers, is opened.