Excited-State Dynamics of the Metal String Complex Co3(dpa)4(NCS)2from Femtosecond Transient Absorption Spectra

Abstract

Transient absorption spectroscopy is used to study the excited-state dynamics of Co(3)(dpa)(4)(NCS)(2), where dpa is the ligand di(2-pyridyl)amido. The pi pi*, charge-transfer, and d-d transition states are excited upon irradiation at wavelengths of 330, 400 and 600 nm, respectively. Similar transient spectra are observed under the experimental temporal resolution and the transient species show weak absorption. We thus propose that a low-lying metal-centered d-d state is accessed immediately after excitation. Analyses of the experimental kinetic traces reveal rapid conversion from the ligand-centered pi pi* and the charge-transfer states to this metal-centered d-d state within 100 fs. The excited molecule then crosses to a second d-d state within the ligand-field manifold, with a time coefficient of 0.6-1.4 ps. Because the ground-state bleaching band recovers with a time coefficient of 10-23 ps, we propose that an excited molecule crosses from the low-lying d-d state either directly within the same spin system or with spin crossing via the state (2)B to the ground state (2)A(2) (symmetry group C(4)). In this trimetal string complex, relaxation to the ground electronic surface after excitation is thus rapid.