Real-time observation of the photoinduced structural change of bis(2,9-dimethyl-1,10-phenanthroline)copper(I) by femtosecond fluorescence spectroscopy: A realistic potential curve of the Jahn-Teller distortion

In copper(I) complex [Cu(dmphen)₂]⁺ (dmphen = 2,9-dimethyl-1,10-phenanthroline), a "flattening" structural change is induced with ¹MLCT excitation, which is a prototype of the structural change accompanied with Cu(I)/Cu(II) conversion in copper complexes. Femtosecond and picosecond emission dynamics of this complex were investigated in solution at room temperature with optically allowed S₂ - S ₀ photoexcitation. Time-resolved emission was measured in the whole visible region, and the lifetimes, intrinsic emission spectra, and radiative lifetimes of the transients were obtained by quantitative analysis. It was concluded that the initially populated S₂ state is relaxed with a time constant of 45 fs to generate the S₁ state retaining the perpendicular structure, and the D_2d- D₂ structural change (the change of the dihedral angle between the two ligand planes) occurs in the S₁ state with a time constant of 660 fs. The intersystem crossing from the S₁ state to the T₁ state takes place after this structural distortion with a time constant of 7.4 ps. Importantly, the temporal spectral evolution relevant to the structural change clearly exhibited an isoemissive point around 675 nm. This manifests that there exists a shallow potential minimum at the perpendicular geometry on the S₁ surface, and the S_i state stays undistorted for a finite period as long as 660 fs before the structural distortion. This situation is not expected for the structural change induced by the ordinary (pseudo-)Jahn-Teller effect, because the distortion should be induced by the spontaneous structural instability at the perpendicular structure. This result sheds new light on the present understanding on the structural change occurring in the metal complexes.