Photophysical Behavior of Open-Shell First-Row Transition-Metal Octabutoxynaphthalocyanines: CoNc(OBu)8 and CuNc(OBu)8 as Case Studies

Ultrafast photodynamics and density functional theory/time-dependent density functional theory (DFT/TDDFT) results for complexes of Co and Cu with 5,9,14,18,23,27,32,36-octabutoxynaphthalocyanine [CoNc(OBu)8 and CuNc(OBu)8] are reported. As a basis for this work, details concerning the syntheses of these complexes and the corresponding Zn complex (used as a reference) are given. Transient absorption spectrometry with femtosecond time resolution combined with a detailed DFT/TDDFT analysis has been employed to construct a complete picture of the excited-state dynamics after Q-band excitation of the Co and Cu complexes and to gain an understanding of the relationship between the nature of the metal center and the excited-state lifetime. The Co complex was shown to return to its ground state via two competing channels: a 2T1(π, π*) state that decayed with a lifetime of 1 ps and a low-lying 2(d, d) state that repopulated the ground-state surface with a lifetime of 15 ps. CuNc(OBu)8 showed ground-state repopulation from the 2T1(π, π*) state via a lower-lying ligand-to-metal charge-transfer (LMCT) state that was completed within a few nanoseconds. The photophysical behavior of the cobalt and copper complexes was compared to that previously reported for the nickel analog in an effort to highlight the effect of the central metal on the nature and rates of the deactivation pathways. The results described in this work provide basic knowledge that is relevant to the use of these compounds as photothermal sensitizers in cancer therapy.