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

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 (d_z2,d_x2-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 → 4p_z 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.