Ultrafast Excited-State Dynamics of Copper(I) Complexes

Abstract

Bis-diimine Cu(I) complexes exhibit strong absorption in the visible region owing to the metal-to-ligand charge transfer (MLCT) transitions, and the triplet MLCT (³MLCT) states have long lifetimes. Because these characteristics are highly suitable for photosensitizers and photocatalysts, bis-diimine Cu(I) complexes have been attracting much interest. An intriguing feature of the Cu(I) complexes is the photoinduced structural change called “flattening”. Bis-diimine Cu(I) complexes usually have tetrahedron-like D_2d structures in the ground (S₀) state, in which two ligands are perpendicularly attached to the Cu(I) ion. With MLCT excitation, the central Cu(I) ion is formally oxidized to Cu(II), which induces the structural change to the “flattened” square-planar-like structure that is seen for usual Cu(II) complexes. In this Account, we review our recent studies on ultrafast excited-state dynamics of bis-diimine Cu(I) complexes carried out using femtosecond time-resolved optical spectroscopy. Focusing on three prototypical bis-diimine Cu(I) complexes that have 1,10-phenanthroline ligands with different substituents at the 2,9-positions, i.e., [Cu(phen)₂]⁺ (phen = 1,10-phenanthroline), [Cu(dmphen)₂]⁺ (dmphen = 2,9-dimethyl-1,10-phenanthroline), and [Cu(dpphen)₂]⁺ (dpphen = 2,9-diphenyl-1,10-phenanthroline), we examined their excited-state dynamics by time-resolved emission and absorption spectroscopies with 200 fs time resolution, observed the excited-state coherent nuclear motion with 30 fs time resolution and performed complementary theoretical calculations. This combined approach vividly visualizes excited-state processes in the MLCT state of bis-diimine Cu(I) complexes. It was demonstrated that flattening distortion, internal conversion, and intersystem crossing occur on the femtosecond–early picosecond time scale, and their dynamics is clearly identified separately. The flattening distortion predominantly occurs in the S₁ state on the subpicosecond time-scale, and the precursor S₁ state retaining the initial undistorted structure appears as a metastable state before the structural change. This observation indicates that the traditional understanding based on the Jahn–Teller effect appears irrelevant for realistically discussing the photoinduced structural change of bis-diimine Cu(I) complexes. The lifetime of the precursor S₁ state significantly depends on the substituents in the three complexes, indicating that the flattering distortion requires a longer time as the substituents at 2,9-positions of the ligands become bulkier. It is suggested that the substituents are rotated to avoid steric repulsions to achieve the flattened structure at the global minimum of the S₁ state, implying the necessity of discussion based on a multidimensional potential energy surface to properly consider this excited-state structural change. After the flattening distortion, the S₁ states of [Cu(dmphen)₂]⁺ and [Cu(dpphen)₂]⁺, which have bulky substituents, relax to the T₁ state by intersystem crossing on the ∼10 ps time scale, while the flattened S₁ state of [Cu(phen)₂]⁺ relaxes directly to the S₀ state on the ∼2 ps time scale. This difference is rationalized in terms of the different magnitude of the flattening distortion and relevant changes in the potential energy surfaces. Clear understanding of the ultrafast excited-state process provides a solid basis for designing and using Cu(I) complexes, such as controlling the structural change to efficiently utilize the energy of the MLCT state in solar energy conversion.