Coherent nuclear dynamics in ultrafast photoinduced structural change of Bis(diimine)copper(I) complex

The photoinduced structural change of a prototype metal complex, [Cu(dmphen)₂]⁺ (dmphen = 2,9-dimethyl-1,10-phenanthroline) , was studied by ultrafast spectroscopy with time resolution as high as 30 fs. Time-resolved absorption measured with direct S₁ excitation clearly showed spectral changes attributable to the D_2d (perpendicular) → D₂ (flattened) structural change occurring in the metal-to-ligand charge transfer singlet excited state (¹MLCT) and the subsequent S₁ → T₁ intersystem crossing. It was confirmed that the two processes occur with time constants of ∼0.8 ps (structural change) and ∼10 ps (intersystem crossing), and their time scales are clearly well-separated. A distinct oscillation of the transient absorption signal was observed in the femtosecond region, which arises from the coherent nuclear motion of the perpendicular S₁ state that was directly generated by photoexcitation. This demonstrated that the perpendicular S ₁ state has a well-defined vibrational structure and can vibrate within its subpicosecond lifetime. In other words, the S₁ state stays undistorted in a short period, and the coherent nuclear motion is maintained in this state. Time-dependent density functional theory (TDDFT) calculations gave consistent results, indicating a very flat feature and even a local minimum at the perpendicular structure on the S₁ potential energy surface. The vibrational assignments of the S₁ nuclear wavepacket motion were made on the basis of the TDDFT calculation. It was concluded that photoexcitation induces a₁ vibrations containing the Cu-ligand bond length change and a b₁ vibration attributed to the ligand-twisting motion that has the same symmetry as the flattening distortion. Ultrafast spectroscopy and complementary quantum chemical calculation provided an overall picture and new understanding of the photoinduced structural change of the prototypical metal complex.