UV−Vis, NMR, and Time-Resolved Spectroscopy Analysis of Photoisomerization Behavior of Three- and Six-Azobenzene-Bound Tris(bipyridine)cobalt Complexes

The photoisomerization properties of tris(bipyridine)cobalt complexes containing six or three azobenzene moieties,
namely, [CoII(dmAB)3](BF4)2 {dmAB = 4,4‘-bis[3‘ ‘-(4‘ ‘‘-tolylazo)phenyl]-2,2‘-bipyridine}, [CoIII(dmAB)3](BF4)3, [CoII(mAB)3](BF4)2 {mAB = 4-[3‘ ‘-(4‘ ‘‘-tolylazo)phenyl]-2,2‘-bipyridine}, and [CoIII(dmAB)3](BF4)3, derived from the effect
of gathering azobenzenes in one molecule and the effect of the cobalt(II) or cobalt(III) ion were investigated using
UV−vis absorption spectroscopy, femtosecond transient spectroscopy, and 1H NMR spectroscopy. In the
photostationary state of these four complexes, nearly 50% of the trans-azobenzene moieties of the Co(II) complexes
were converted to the cis isomer, and nearly 10% of the trans-azobenzene moieties of the Co(III) complexes
isomerized to the cis isomer, implying that the cis isomer ratio in the photostationary state upon irradiation at 365
nm is controlled not by the number of azobenzene moieties in one molecule but rather by the oxidation state of
the cobalt ions. The femtosecond transient absorption spectra of the ligands and the complexes suggested that the
photoexcited states of the azobenzene moieties in the Co(III) complexes were strongly deactivated by electron
transfer from the azobenzene moiety to the cobalt center to form an azobenzene radical cation and a Co(II) center.
The cooperation among the photochemical structural changes of six azobenzene moieties in [CoII(dmAB)3](BF4)2
was investigated with 1H NMR spectroscopy. The time-course change in the 1H NMR signals of the methyl protons
indicated that each azobenzene moiety in [CoII(dmAB)3](BF4)2 isomerized to a cis isomer with a random probability
of 50% and without interactions among the azobenzene moieties.