Trapping intermediate MLCT states in low-symmetry {Ru(bpy)} complexes

Abstract

The picosecond excited state dynamics of [Ru(tpm)(bpy)(NCS)]⁺ (RubNCS⁺) and [Ru(tpm)(bpy)(CN)]⁺ (RubCN⁺) (tpm = tris(1-pyrazolyl)methane, bpy = 2,2′-bipyridine) have been analyzed by means of transient absorption measurements and spectroelectrochemistry. Emissive ³MLCTs with (GS)HOMO(h⁺)–(GS)LUMO(e⁻) configurations are the lowest triplet excited states regardless of whether 387 or 505 nm photoexcitation is used. 387 nm photoexcitation yields, after a few picoseconds, the emissive ³MLCTs. In contrast, 505 nm photoexcitation populates an intermediate excited state that we assign as a ³MLCT state, in which the hole sits in a metal-centered orbital of different symmetry, prior to its conversion to the emissive ³MLCTs. The disparities in terms of electronic configuration between the intermediate and the emissive ³MLCTs have two important consequences. On one hand, both states feature very different fingerprint absorptions in transient absorption measurements. On the other hand, the reconfiguration is impeded by a kinetic barrier. As such, the conversion is followed spectroscopically and kinetically on the 300 ps timescale.