Temperature- and matrix-dependent excited state equilibria in pyrene-linked ruthenium and iridium complexes

The triplet reservoir effect is an established concept to extend triplet state lifetimes in molecular transition metal complexes. However, the delicate balance between the excited states is impacted by various parameters such as ligand design, the central metal ion, but also the solvent environment. In this study, we present an in-depth investigation of the excited-state nature and relaxation pathways of structurally related Ru(II)- and Ir(III)-based dyads bearing a pyrene moiety in the ligand backbone to serve as triplet reservoir in two different matrices, (frozen) methanol and potassium bromide. By temperature-dependent luminescence spectroscopy and excited-state lifetime measurements, pronounced sensitivity of the energetic position of the 3 MLCT state relative to the pyrene 3 ππ* state to the environment is revealed, resulting in an inversion of the excited-state ordering in dependence of the surrounding medium in the Ru(II) dyad. Nevertheless, the triplet reservoir effect can be exploited in both investigated surroundings. In contrast, the lowest-lying excited state in the Ir(III) dyad is found to be of 3 ππ* pyrene character independently of the environment, as confirmed by transient absorption spectroscopy and temperature-dependent emission measurements, showing a characteristic spectral position and long lifetime of phosphorescence. Moreover, three simultaneously emissive states were revealed in (frozen) solution, whereas pyrene-based phosphorescence is the dominating emission pathway if embedded in KBr. This implies limited interaction of the different excited state manifolds and limited back energy transfer to the 3 MLCT/ 3 LLCT state due to an increased energy gap to the pyrene triplet state. The solvent environment tunes the energy gap, though never reaching efficient back electron transfer necessary to exploit the triplet reservoir. Our work demonstrates the importance of the intricate design of such dyads as well as the complex interplay of steric, electronic, and environmental effects.