Excited-State Electronic Coupling and Photoinduced Multiple Electron Transfer in Two Related Ligand-Bridged Hexanuclear Mixed-Valence Compounds

The synthesis, characterization, electrochemical, photophysical, and photochemical properties of two hexanuclear mixed-valence compounds are reported. Each supramolecular species consists of two cyano-bridged [(NC)5FeII-CN-PtIV(NH3)3L-NC-FeII(CN)5] triads that are linked to each other through a PtIV-L-PtIV bridge, where L = 4,4‘-dipyridyl (bpy) or 3,3‘-dimethyl-4,4‘-dipyridyl (dmb). The major difference between the two compounds is the electronic nature of the bridging ligand between the two Pt atoms. Both species exhibit a broad Fe(II) → Pt(IV) intervalent (IT) absorption band at 421 nm with an oscillator strength that is approximately four times that for [(NC)5FeII-CN-PtIV(NH3)5] and twice that for [(NC)5FeII-CN-PtIV(NH3)4-NC-FeII(CN)5]. - When L = bpy, the resonance Raman spectrum obtained by irradiating the IT band at 488 nm exhibits several dipyridyl ring modes at 1604, 1291, and 1234 cm-1 which are not present in the spectrum when L = dmb. In addition, femtosecond pump−probe spectroscopy performed at 400 nm yields a transient bleach of the IT absorption band with a single exponential decay of 3.5 ps for L = bpy, compared with only 1.8 ps for L = dmb and 2.1 ps for [(NC)5FeII-CN-PtIV(NH3)4-NC-FeII(CN)5]. - Last, prolonged irradiation of the complexes at 488 nm leads to the formation of 4 equiv of ferricyanide with a quantum efficiency of 0.0014 for L = bpy and 0.0011 for L = dmb. The transient absorption, resonance Raman, and photochemical data suggest that the degree of excited electronic coupling in these compounds is tunable by changing the electronic nature of the Pt-L-Pt bridging ligand.