Roles of Octabutoxy Substitution and J‑Aggregation in Stabilization of the Excited State in Nickel Phthalocyanine

Nickel phthalocyanine (NiPc) complexes are known to show a rapid nonradiative deactivation of the photoexcited state through the internal conversion. This could be exploited in practical applications, such as photoprotection and photodynamic therapy. The butoxy substitution of NiPc plays an important role for drug delivery but also greatly influences its photophysics. We prepared novel peripherally substituted 2,3,9,10,16,17,23,24-octabutoxy nickel(II) phthalocyanine and characterized the deactivation pathway of its photoexcited state in solution by femtosecond transient absorption spectroscopy and quantum chemical calculations. We bring experimental evidence for the kinetic model, in which the photoexcitation evolves in two independent branches. In the first branch, assigned to the monomer, it undergoes ultrafast intersystem crossing to a triplet state, which subsequently decays to the ground state through a pathway involving lower-lying triplet states, with a ground-state recovery lifetime of 814 ps. It is about three-times longer than the lifetime published for unsubstituted NiPc. In the second branch, the photoexcitation decayed to a triplet state with an orders of magnitude longer lifetime, with the quantum yield of about 4%. This state showed spectral features of J-aggregates. These findings are important for the applications that rely on singlet oxygen formation or fast nonradiative deactivation of the excited state.