Structural change dynamics of heteroleptic Cu(i) complexes observed by ultrafast emission spectroscopy

[Cu(i)(dmp)(P)₂]⁺ (dmp = 2,9-dimethyl-1,10-phenanthroline derivatives; P = phosphine ligand) is one of the most promising photosensitizers used in a photo-catalytic system for reducing CO₂, for which the quantum yield is as high as 57%. In this work, time-resolved emission spectra of Cu(i) complexes in solutions were investigated using femtosecond fluorescence up-conversion and nanosecond time-resolved emission spectroscopic systems. The temporal profiles of emission intensities less than 10 ps in acetonitrile solution were reproduced using a tri-exponential function with three-time constants of 0.040 ps, 0.78 ps and 8.0 ps. We found that only the second time constant is dependent on the solvent (acetonitrile: 0.78 ps, butyronitrile: 1.4 ps), indicating that the 0.78 ps spectral change is attributed to the structural change of the Cu(i) complex. The oscillator strengths of transition species are derived from the intensities in the time-resolved emission spectra (species-associated spectra). Based on the oscillator strengths, we concluded that the 0.040 ps process is the S_n → S₁ internal conversion and the 0.78 ps process is a structural change in the S₁ state. The final time constant of 8.0 ps is assigned to the S₁ → T₁ intersystem crossing because the ³MLCT state (τ_T1 = 97 ns) is generated after the decay. The DFT calculation showed that the 0.78 ps spectral change (∼600 cm⁻¹ redshift) is attributed to Jahn-Teller distortion around the metal center, and there is a large structural change in the ligand, which results in a large Stokes shift in the S_n state (7.3 × 10³ cm⁻¹).