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

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.