Deconvoluting capping ligand influence on photophysical properties in tetrathiafulvalene-based diradicaloids

Tetrathiafulvalene-2,3,6,7-tetrathiolate (TTFtt) complexes are synthetically tunable and emit brightly in the near-infrared II region (NIR II, 1000-1700 nm). Their emission/absorption energies respond to the identity of the capping ligands on the metal center, but a detailed understanding of how ligand bonding interactions dictate photophysical properties is key to predictive design optimization. Here we assess the relative influence of ligand pi (π) backbonding <i>versus</i> sigma (σ) donation in these complexes across a new series of olefin- and phosphite-capped complexes. Increasing the backbonding character of peripheral ligands results in a hypsochromic shift in the absorption maxima, while stronger σ donation results in a bathochromic shift.