Excited state dynamics of symmetric and asymmetric Cr3(dpa)4Cl2 measured using femtosecond transient absorption spectroscopy

Abstract

Herein, the excited-state dynamics of an extended metal atom chain complex, Cr₃(dpa)₄Cl₂ (dpa = dipyridylamide), in tetrahydrofuran solution were investigated using femtosecond transient absorption spectroscopy. Upon excitation at a wavelength of 330 nm, two distinct excited-state absorption species with varied dynamics were identified and assigned to the symmetric (s-) and unsymmetric (u-) Cr₃(dpa)₄Cl₂. The major species is s-Cr₃(dpa)₄Cl₂ that undergoes rapid conversion at less than 100 fs from the ligand-centred π–π* state, which is the initially accessed state, to the metal-centred d–d state and then vibrational cooling accompanying the structural relaxation at a time constant ∼2.2 ps. Most of the s-form is recovered to the ground state at ∼200 ps. For u-Cr₃(dpa)₄Cl₂, a similar rapid conversion to d–d states is observed, and the geometric/vibrational relaxation is ∼0.8 ps. The second recovery of the ground state with approximately equal amplitude is observed at a time constant of ∼5 ns. This might be because many d–d states exist and about half of them inefficiently couple with the ground state surface.