Spin–orbit coupling effects hidden behind the photophysics of phosphorescent chiral cyclometalated Pt(ii) complexes

Abstract

The electronic and (chiro-) optical properties of [Pt(pCpy)(acac)] 1 and [Pt(pCpz)(acac)] 2 (pCpy = 2-[2,2]-paracyclophane-4-yl)pyridyl; pCpz = 1-[2,2]-paracyclophane-4-yl)pyrazolyl; acac = dimethyl-substituted acetylacetonato), representative of phosphorescent chiral cyclometalated Pt(II) complexes, are investigated by means of density functional theory (DFT) and its time-dependent extension so-called TD-DFT, including spin–orbit coupling (SOC) effects. The computed absorption, phosphorescence and circularly polarized luminescence (CPL) spectra are compared to the available experimental spectra, and analysed on the basis of spin–orbit interactions and electronic excited state sub-levels. The major role of the SOC is established and deciphered for both complexes. Spin–orbit sub-levels of the low-lying triplet manifold not only perturb the absorption spectra by a 60–70 nm shift to the red, but entirely control the phosphorescence and CPL activities, in terms of intensity and composition. It is shown that the substitution of a pyridyl ligand in 1 with a pyrazolyl in 2 has dramatic consequences on the photophysics of these “case-study” molecules. Indeed, the character and the energetics of the lowest triplet states participating in the emission properties are drastically affected by this change of ligands. Whereas [Pt(pCpy)(acac)] 1 can be considered as an “easy case”, both experimentally and theoretically, [Pt(pCpz)(acac)] 2 represents a challenge computationally due to the presence of two nearly degenerate emissive triplet states. The correlation between the structural/electronic properties of the excited states contributing to the spectra is discussed as well as the early time (<1 ps) photophysics simulated by non-adiabatic quantum dynamics for the two complexes.