Determining the Magnitude and Direction of Photoinduced Ligand Field Switching in Photochromic Metal–Organic Complexes: Molybdenum–Tetracarbonyl Spirooxazine Complexes

The ability to optically switch or tune the intrinsic properties of transition metals (e.g., redox potentials, emission/absorption energies, and spin states) with photochromic metal–ligand complexes is an important strategy for developing “smart” materials. We have described a methodology for using metal–carbonyl complexes as spectroscopic probes of ligand field changes associated with light-induced isomerization of photochromic ligands. Changes in ligand field between the ring-closed spirooxazine (SO) and ring-opened photomerocyanine (PMC) forms of photochromic azahomoadamantyl and indolyl phenanthroline-spirooxazine ligands are demonstrated through FT-IR, 13C NMR, and computational studies of their molybdenum–tetracarbonyl complexes. The frontier molecular orbitals (MOs) of the SO and PMC forms differ considerably in both electron density distributions and energies. Of the multiple π* MOs in the SO and PMC forms of the ligands, the LUMO+1, a pseudo-b1-symmetry phenanthroline-based MO, mixes primarily with the Mo(CO)4 fragment and provides the major pathway for Mo(d)→phen(π*) backbonding. The LUMO+1 is found to be 0.2–0.3 eV lower in energy in the SO form relative to the PMC form, suggesting that the SO form is a better π-acceptor. Light-induced isomerization of the photochromic ligands was therefore found to lead to changes in the energies of their frontier MOs, which in turn leads to changes in π-acceptor ability and ligand field strength. Ligand field changes associated with photoisomerizable ligands allow tuning of excited-state and ground-state energies that dictate energy/electron transfer, optical/electrical properties, and spin states of a metal center upon photoisomerization, positioning photochromic ligand–metal complexes as promising targets for smart materials.