Intermolecular interaction of photoexcited Cu(TMpy-P4) with water studied by transient resonance Raman and picosecond absorption spectroscopies

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A photoinduced complex between Cu(TMpy-P4) and water molecules, reversibly axially coordinated to the central metal, was observed in picosecond transient absorption and nanosecond resonance Raman experiments. This complex is rapidly created ( τ 1 = 15 ± 5 ps) in the excited triplet (π, π ∗ ) state of Cu-porphyrin, and the subsequent relaxation is proposed to proceed via two parallel pathways. One is fast and efficient (⩾ 90% of molecules), and presumably involves a (π, d) charge-transfer state. The second pathway is slow ( τ 2 ⪢ 1 ns), has a low quantum yield (⩽ 10%) and involves the excited (d, d) state which is responsible for transient Raman features at ≈ 1553 cm −1 (ν 2 ∗) and ≈ 1347 cm −1 (ν 4 ∗) , and for low-intensity long-lived transient absorption features.