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 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 pi pi* states in the near UV. Thus, by structurally tuning the electronic coupling of the pi pi* and the MLCT states a new synthetic route toward white emitters, which can subsequently be incorporated into polymers, is opened.