Dyads for Photoinduced Charge Separation Based on Platinum Diimine Bis(acetylide) Chromophores: Synthesis, Luminescence and Transient Absorption Studies

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Abstract
The platinum diimine bis(acetylide) chromophore was utilized to explore photoinduced intramolecular reductive quenching with phenothiazine donors in chromophore−donor dyad complexes. Compounds of the general formula Pt(X2-bpy)(C⋮C-p-C6H4CH2(D))2 (where D = phenothiazine (PTZ) or trifluromethylphenothiazine (TPZ) and X = tBu or CO2Et) were synthesized from the corresponding Pt(X2-bpy)Cl2 and aryl acetylene by a CuI-catalyzed coupling reaction. Solvent dependence was explored for the system with X = tBu in MeCN, CH2Cl2, EtOAc, and toluene. Electron transfer quenching of the 3MLCT excited state of the platinum diimine bis(acetylide) takes place in MeCN leaving no intrinsic emission from the excited state, but in toluene both the PTZ and TPZ dyad complexes exhibit no emission quenching. Picosecond pump−probe transient absorption (TA) experiments were used to monitor decay of the 3MLCT excited state and electron transfer to form the charge-separated (CS) state. Electrochemical measurements were used to estimate the driving force for charge recombination (CR), with ΔECR based on the reduction potential corresponding to Pt(X2-bpy)(C⋮C−Ar)2 → Pt(X2-bpy•-)(C⋮C−Ar)2 and the oxidation corresponding to donor → donor•+. Kinetic information from the TA measurements was used to correlate rate and driving force with the electron transfer reactions. Concomitant with the decay of the 3MLCT excited state was the observation of a transient absorption at ca. 500 nm due to formation of the PTZ or TPZ radical cation in the CS state, with the rate of charge separation, kCS, being 1.8 × 109 to 2 × 1010 s-1 for the three dyads explored in MeCN and 1:9 CH2Cl2/MeCN. The fastest rate of CR occurs for X = CO2Et and D = PTZ, the compound with smallest ΔECR = 1.71 V. The rate of CR for dyads with X = tBu and D = PTZ or TPZ was estimated to be 1.7−2.0 × 108 s-1 in MeCN. The slower rate corresponds to a greater driving force for CR, ΔECR = 2.18 and 2.36 V for D = PTZ and TPZ, respectively, suggesting that the driving force for charge recombination places it in the Marcus inverted region.