Exploiting exciton coupling of ligand radical intervalence charge transfer transitions to tune NIR absorption

We detail the rational design of a series of bimetallic bis-ligand radical Ni salen complexes in which the relative orientation of the ligand radical chromophores provides a mechanism to tune the energy of intense intervalence charge transfer (IVCT) bands in the near infrared (NIR) region. Through a suite of experimental (electrochemistry, electron paramagnetic resonance spectroscopy, UV-vis-NIR spectroscopy) and theoretical (density functional theory) techniques, we demonstrate that bimetallic Ni salen complexes form bis-ligand radicals upon two-electron oxidation, whose NIR absorption energies depend on the geometry imposed in the bis-ligand radical complex. Relative to the oxidized monomer [1˙]⁺ (E = 4500 cm^-1, ε = 27 700 M^-1 cm^-1), oxidation of the cofacially constrained analogue 2 to [2˙˙]²⁺ results in a blue-shifted NIR band (E = 4830 cm^-1, ε = 42 900 M^-1 cm^-1), while oxidation of 5 to [5˙˙]²⁺, with parallel arrangement of chromophores, results in a red-shifted NIR band (E = 4150 cm^-1, ε = 46 600 M^-1 cm^-1); the NIR bands exhibit double the intensity in comparison to the monomer. Oxidation of the intermediate orientations results in band splitting for [3˙˙]²⁺ (E = 4890 and 4200 cm^-1; ε = 26 500 and 21 100 M^-1 cm^-1), and a red-shift for [4˙˙]²⁺ using ortho- and meta-phenylene linkers, respectively. This study demonstrates for the first time, the applicability of exciton coupling to ligand radical systems absorbing in the NIR region and shows that by simple geometry changes, it is possible to tune the energy of intense low energy absorption by nearly 400 nm.