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 versus 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.