Reversible Photoinduced Ligand Substitution in a Luminescent Chromium(0) Complex

Abstract

Light-triggered dissociation of ligands forms the basis for many compounds of interest for photoactivated chemotherapy (PACT), in which medicinally active substances are released or "uncaged" from metal complexes upon illumination. Photoinduced ligand dissociation is usually irreversible, and many recent studies performed in the context of PACT focused on ruthenium(II) polypyridines and related heavy metal complexes. Herein, we report a first-row transition metal complex, in which photoinduced dissociation and spontaneous recoordination of a ligand unit occurs. Two scorpionate-type tridentate chelates provide an overall six-coordinate arylisocyanide environment for chromium(0). Photoexcitation causes decoordination of one of these six ligating units and coordination of a solvent molecule, at least in tetrahydrofuran and 1,4-dioxane solvents, but far less in toluene, and below detection limit in cyclohexane. Transient UV−vis absorption spectroscopy and quantum chemical simulations point to photoinduced ligand dissociation directly from an excited metal-to-ligand charge-transfer state. Owing to the tridentate chelate design and the substitution lability of the first-row transition metal, recoordination of the photodissociated arylisocyanide ligand unit can occur spontaneously on a millisecond time scale. This work provides insight into possible self-healing mechanisms counteracting unwanted photodegradation processes and seems furthermore relevant in the contexts of photoswitching and (photo)chemical information storage.

Picosecond Metal-to-Ligand Charge-Transfer Deactivation in Co(ppy)3 via Jahn–Teller Distortion

“…This low barrier and large energy difference implies that the five-coordinate geometry is generated rapidly and irreversibly, resulting in the rapid deactivation of the excited state. Bidentate pyridylcarbene Fe II complexes exhibit similar elongation of one metal–nitrogen coordinate in their lowest-energy 3 T geometry, and as a result have similarly short excited-state lifetimes. − A Cr 0 scorpionate complex was also found to undergo dissociation of a single ligand in the 3 MLCT state, followed by solvent coordination to the empty site …”

“…Balancing the ligand field around the entire coordination sphere may also lead to the disfavoring of the single metal–ligand bond lengthening demonstrated here. Finally, if heteroleptic ligands mixing strong and weak field elements are desired, care should be taken to follow strategies that do not put strong-donor ligands at positions allowing easy e g * population. , Coordinating solvents could also be used to stabilize the five-coordinate species, extending the excited-state lifetime . Such strategies, combined with the inherently larger ligand field of Co III complexes, may lead to even longer lifetimes of potentially photoactive states.…”