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

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 (τ1 = 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 (τ2 >> 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 (ν2*) and ≈ 1347 cm−1 (ν4*), and for low-intensity long-lived transient absorption features.