Triplet state structure–property relationships in a series of platinum acetylides: effect of chromophore length and end cap electronic properties

Abstract

To develop quantitative structure–spectroscopic property relationships in platinum acetylides, we investigated the triplet state behavior of nominally centrosymmetric chromophores trans-Pt(PBu₃)₂(CC-Phenyl-X)₂, where X = diphenylamino, NH₂, OCH₃, t-Bu, CH₃, H, F, benzothiazole, CF₃, CN, and NO₂. We measured ground state absorption, phosphorescence, excitation and triplet state absorption spectra and triplet lifetimes. By DFT we calculated the phosphorescence emission energy (E_T), the spin density on the end cap (SD(X)), triplet state geometry and the distance between the triplet centroid and the central platinum atom (〈R_S–Pt(X)〉). Compounds with electron-donating X have smaller triplet state lifetime, blue-shifted phosphorescence and larger triplet potential energy surface displacement associated with the CC bond. Compounds with electron-withdrawing X have larger triplet lifetime, red-shifted phosphorescence and smaller triplet potential energy surface displacement associated with the CC bond. The range of spin–orbit-coupling between the platinum atom and the triplet centroid was determined to be 6 Å. The quantity 〈R_S–Pt〉 is shown to be a linear function of one-dimensional well length calculated from experimental E_T. The multiple examples demonstrate 〈R_S–Pt〉 is a useful descriptor for analyzing triplet state behavior.