Platinum Chromophore-Based Systems for Photoinduced Charge Separation: A Molecular Design Approach for Artificial Photosynthesis

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• 作者：Soma Chakraborty ; Thaddeus J. Wadas ; Heidi Hester ; Russell Schmehl ; and Richard Eisenberg
• 刊名：Inorganic Chemistry
• 出版年：2005
• 出版时间：October 3, 2005
• 年：2005
• 卷：44
• 期：20
• 页码：6865 - 6878
• 全文大小：329K
• 年卷期：v.44,no.20(October 3, 2005)
• ISSN：1520-510X

文摘

Photoinduced charge separation is a fundamental step in photochemical energy conversion. In the design ofmolecularly based systems for light-to-chemical energy conversion, this step is studied through the construction oftwo- and three-component systems (dyads and triads) having suitable electron donor and acceptor moieties placedat specific positions on a charge-transfer chromophore. The most extensively studied chromophores in this regardare ruthenium(II) tris(diimine) systems with a common ³MLCT excited state, as well as related ruthenium(II) bis(terpyridyl) systems. This Forum contribution focuses on dyads and triads of an alternative chromophore, namely,platinum(II) di- and triimine systems having acetylide ligands. These d⁸ chromophores all possess a ³MLCT excitedstate in which the lowest unoccupied molecular orbital is a mages/gifchars/pi.gif" BORDER=0 >* orbital on the heterocyclic aromatic ligand. Theexcited-state energies of these Pt(II) chromophores are generally higher than those found for the ruthenium(II)tris(diimine) systems, and the directionality of the charge transfer is more certain. The first platinum diimine bis(arylacetylide) triad, constructed by attaching phenothiazene donors to the arylacetylide ligands and a nitrophenylacceptor to 5-ethynylphenanthroline of the chromophore, exhibited a charge-separated state of 75-ns duration. Thefirst Pt(tpy)(arylacetylide)⁺-based triad contains a trimethoxybenzamide donor and a pyridinium acceptor and hasbeen structurally characterized. The triad has an edge-to-edge separation between donor and acceptor fragmentsof 27.95 Å. However, while quenching of the emission is complete for this system, transient absorption (TA) studiesreveal that charge transfer does not move onto the pyridinium acceptor. A new set of triads described in detailhere and having the formula [Pt(NO₂phtpy)(p-Cmages/entities/tbd1.gif">C-C₆H₄CH₂(PTZ-R)](PF₆), where NO₂phtpy = 4'-{4-[2-(4-nitrophenyl)vinyl]phenyl}-2,2';6',2' '-terpyridine and PTZ = phenothiazine with R = H, OMe, possess an unsaturatedlinkage between the chromophore and a nitrophenyl acceptor. While the parent chromophore [Pt(ttpy)(Cmages/entities/tbd1.gif">CC₆H₅)]PF₆ is brightly luminescent in a fluid solution at 298 K, the triads exhibit complete quenching of the emission, asdo the related donor-chromophore (D-C) dyads. Electrochemically, the triads and D-C dyads exhibit a quasi-reversible oxidation wave corresponding to the PTZ ligand, while the R=H triad and related C-A dyad display afacile quasi-reversible reduction assignable to the acceptor. TA spectroscopy shows that one of the triads possessesa long-lived charge-separated state of ~230 ns.