Investigation of the photoinduced axial ligation process in the excited state of nickel(II) phthalocyanine

Abstract

Photoinduced axial ligation process in the excited state of nickel(II) octabutoxyphthalocyanine (NiPcOBu8) in pyridine solvent has been investigated by ultrafast optical transient absorption and X-ray transient absorption spectroscopies. Although Ni phthalocyanine complexes have been reported to have no tendency to form six-coordinate structure, the (dz2,dx2-y2) state of NiPcOBu8 generated by photoexcitation of Q-band was discovered to axially ligate with the pyridine molecule very rapidly, on a timescale of ˜100 ps. Although the presence of an excited unligated (d,d) state cannot be confirmed due to the time resolution limit of the synchrotron X-ray probe pulse with ˜80 ps FWHM, the biexponential kinetics of the ground state recovery detected by the 1s → 4pz feature suggests the decay of the unligated (d,d) species with an 82 ps time constant, while that of the ligated NiPcOBu8 species is 3–5 ns. The photoinduced ligation dynamics of NiPcOBu8 is vastly different compared to that of Ni porphyrin complexes. This study provides a structural basis on the ligation chemistry in both the ground and excited states of nickel complexes based on their electronic and nuclear structures.