Multireference perturbation theory reveals the intricate excited-state electronic structure of the 2-phenylpyridine ligand

Abstract

2-Phenylpyridine (2-ppy) is a widely used cyclometalating ligand in transition-metal complexes (TMCs), yet its intrinsic excited-state electronic structure remains poorly characterized. Here, we investigate its singlet excited states of ππ* nature accessible in the ultraviolet spectral window, both as an isolated molecule and when coordinated in the TMC Ir(2-ppy)3, using extended multi-state restricted active-space second-order perturbation theory (XMS-RASPT2). Our study reveals multiple low-lying excited states, whose relative energy ordering and oscillator strengths are strongly dependent on the choice of the orbital active space. The XMS-RASPT2 calculations accurately reproduce the experimental absorption spectrum of the isolated 2-ppy and show that the characteristic two-band profile arises from mixing between the same bright configuration and dark transitions. Coordination to the iridium in Ir(2-ppy)3 induces an overall redshift of the ligand-centered transitions accompanied by a change in their energy ordering. In contrast, inter-ligand couplings, evaluated using a Frenkel exciton model, are found to have an almost negligible impact on the electronic structure of the 2-ppy manifold.