Spectroscopic, Structural, and Kinetic Investigation of the Ultrafast Spin Crossover in an Unusual Cobalt(II) Semiquinonate Radical Complex.

A comprehensive spectroscopic and structural investigation of [Co (l-N  tBu )(dbsq)][B(p-C H Cl) ] (1, l-N  tBu =N,N'-di-tert-butyl-2,11-diaza3.3pyridinophane, dbsq =3,5-di-tert-butylsemiquinonate), the first known octahedral complex with a low-spin (ls) Co semiquinonate ground state, is reported. Above 200 K, solids as well as solutions of 1 exhibit thermally induced spin-crossover (SCO) from the ls to the high-spin (hs) Co semiquinonate state instead of the frequently observed valence tautomerism from ls Co catecholate to hs Co semiquinonate. DFT calculations demonstrate that the (closed shell) Co catecholate suffers from a triplet instability leading to the ls Co semiquinonate ground state. The thorough temperature-dependent spectroscopic study of the SCO enables a photophysical investigation. Thus, by selective photoexcitation of the ls fraction of 1 in solution at room temperature, ultrafast conversion to the hs state is observed using femtosecond electronic and IR-vibrational (infrared) transient absorption spectroscopy. The kinetics of the photocycle is described by a stretched exponential with τ=3.3-3.6 ps and β=0.52-0.54, representing an upper limit for the hs-ls relaxation time. This is, to our knowledge, the fastest interconversion ever determined for a SCO complex, and is attributed to the special situation that in 1 a Co complex is coordinated to a π-radical ligand allowing very efficient coupling between the ls and hs spin states.