Multiple Triplet Metal‐Centered Jahn‐Teller Isomers Determine Temperature‐Dependent Luminescence Lifetimes in [Ru(bpy)₃]²⁺

Abstract

Understanding the factors that determine the luminescence lifetime of transition metal compounds is key for applications in photocatalysis and photodynamic therapy. Here we show that for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:msup><mml:mrow><mml:mo>[</mml:mo> <mml:mi>Ru</mml:mi> <mml:msub><mml:mrow><mml:mo>(</mml:mo> <mml:mi>bpy</mml:mi> <mml:mo>)</mml:mo></mml:mrow> <mml:mn>3</mml:mn></mml:msub> <mml:mo>]</mml:mo></mml:mrow> <mml:mrow><mml:mn>2</mml:mn> <mml:mo>+</mml:mo></mml:mrow> </mml:msup> <mml:annotation>${[{\rm{Ru}}({\rm{bpy}})_{\rm{3}} ]^{{\rm{2 + }}} }$</mml:annotation> </mml:semantics> </mml:math> (bpy = 2,2'-bipyridine), the generally accepted idea that emission lifetimes can be controlled optimizing the energy barrier from the emissive triplet metal-to-ligand charge-transfer (³ MLCT) state to the thermally-activated triplet metal-centered (³ MC) state or the energy gap between both states is a misconception. Further, we demonstrate that considering a single relaxation pathway determined from the minimum that is lowest in energy leads to wrong temperature-dependent emission lifetimes predictions. Instead, we obtain excellent agreement with experimental temperature-dependent lifetimes when an extended kinetic model that includes all the pathways related to multiple Jahn-Teller isomers and their effective reaction barriers is employed. These concepts are essential to correctly design other luminescent transition metal complexes with tailored emission lifetimes based on theoretical predictions.