Abstract
<jats:p>Transition metal complexes containing abundant transition metals are receiving increasing attention in a broad range of photophysical and photochemical applications as a replacement for complexes with rare metals like ruthenium, osmium, iridium or platinum. Particularly, near-infrared (NIR)-absorptive and emissive materials are needed for various technological and biological applications, yet luminescent materials emitting in the NIR-II spectral region (> 1000 nm) are extremely rare. Formal CH-to-N isolobal substitution in a pyridine acceptor ligand gives an electron-deficient pyrazine ligand lowering the energy of the π* acceptor orbital and hence the metal-to-ligand charge transfer (MLCT) states in a molybdenum(0) complex. The resulting 3MLCT emission band of the pyrazine complex spans from the visible to the NIR-II spectral region. Solvatochromic effects shift the MLCT absorption and photoluminescence bands to lower energies. Coordination of medium-strong to strong boron-based Lewis acids to the accessible basic pyrazine nitrogen atom lowers the MLCT energies even further. The experimental photophysical effects are rationalized within the frameworks of the energy-gap law, thermally activated decay via metal-centered excited states, thermally activated ground state recovery and resonant energy transfer to vibrational overtones.</jats:p>