The substituent effect on the MLCT excited state dynamics of Cu(i) complexes studied by femtosecond time-resolved absorption and observation of coherent nuclear wavepacket motion

Abstract

The substituent effect on the excited-state dynamics of bis-diimine Cu(I) complexes was investigated by femtosecond time-resolved absorption spectroscopy with the S₁ ← S₀ metal-to-ligand charge transfer (MLCT) photoexcitation. The time-resolved absorption of [Cu(phen)₂]⁺ (phen = 1,10-phenanthroline) showed a slight intensity increase of the S₁ absorption with a time-constant of 0.1–0.2 ps, reflecting the flattening distortion occurring in the S₁ state. The transient absorption of the ‘flattened’ S₁ state was clearly observed, although its fluorescence was not observed in the previous fluorescence up-conversion study in the visible region. The flattened S₁ state decayed with a time constant of ∼2 ps, and the S₀ bleaching recovered accordingly. This clarifies that the S₁ state of [Cu(phen)₂]⁺ is predominantly relaxed to the S₀ state by internal conversion. The time-resolved absorption of [Cu(dpphen)₂]⁺ (dpphen = 2,9-diphenyl-1,10-phenanthroline) showed a 0.9 ps intensity increase of the S₁ absorption due to the flattening distortion, and then exhibited a 11 ps spectral change due to the intersystem crossing. This excited-state dynamics of [Cu(dpphen)₂]⁺ is very similar to that of [Cu(dmphen)₂]⁺ (dmphen = 2,9-dimethyl-1,10-phenanthroline). In the ultrafast pump–probe measurements with 35 fs time resolution, [Cu(phen)₂]⁺ and [Cu(dpphen)₂]⁺ exhibited oscillation due to the nuclear wavepacket motions of the initial S₁ state, and the oscillation was damped as the structural change took place. This indicates that the initial S₁ states have well-defined vibrational structures and that the vibrational coherence is retained in their short lifetimes. The present time-resolved absorption study, together with the previous time-resolved fluorescence study, provides a unified view for the ultrafast dynamics of the MLCT excited state of the Cu(I) complexes.