Toward Exceeding the Shockley−Queisser Limit: Photoinduced Interfacial Charge Transfer Processes that Store Energy in Excess of the Equilibrated Excited State

Nanocrystalline (anatase), mesoporous TiO2 thin films were functionalized with [Ru(bpy)2(deebq)](PF6)2, [Ru(bq)2(deeb)](PF6)2, [Ru(deebq)2(bpy)](PF6)2, [Ru(bpy)(deebq)(NCS)2], or [Os(bpy)2(deebq)](PF6)2,
where bpy is 2,2‘-bipyridine, bq is 2,2‘-biquinoline, and deeb and deebq are 4,4‘-diethylester derivatives.
These compounds bind to the nanocrystalline TiO2 films in their carboxylate forms with limiting surface
coverages of 8 (± 2) × 10-8 mol/cm2. Electrochemical measurements show that the first reduction of these
compounds (−0.70 V vs SCE) occurs prior to TiO2 reduction. Steady state illumination in the presence of
the sacrificial electron donor triethylamine leads to the appearance of the reduced sensitizer. The thermally
equilibrated metal-to-ligand charge-transfer excited state and the reduced form of these compounds do
not inject electrons into TiO2. Nanosecond transient absorption measurements demonstrate the formation
of an extremely long-lived charge separated state based on equal concentrations of the reduced and oxidized
compounds. The results are consistent with a mechanism of ultrafast excited-state injection into TiO2 followed
by interfacial electron transfer to a ground-state compound. The quantum yield for this process was found
to increase with excitation energy, a behavior attributed to stronger overlap between the excited sensitizer
and the semiconductor acceptor states. For example, the quantum yields for [Os(bpy)2(dcbq)]/TiO2 were
φ(417 nm) = 0.18 ± 0.02, φ(532.5 nm) = 0.08 ± 0.02, and φ(683 nm) = 0.05 ± 0.01. Electron transfer to
yield ground-state products occurs by lateral intermolecular charge transfer. The driving force for charge
recombination was in excess of that stored in the photoluminescent excited state. Chronoabsorption
measurements indicate that ligand-based intermolecular electron transfer was an order of magnitude faster
than metal-centered intermolecular hole transfer. Charge recombination was quantified with the Kohlrausch−Williams−Watts model.

采用[Ru(bpy)₂(deebq)](PF₆)₂、[Ru(bq)₂(deeb)](PF₆)₂、[Ru(deebq)₂(bpy)](PF₆)₂、[Ru(bpy)(deebq)(NCS)₂]或[Os(bpy)₂(deebq)](PF₆)₂对纳米晶锐钛矿相介孔二氧化钛（TiO₂）薄膜进行功能化修饰。其中2,2'-联吡啶（2,2‘-bipyridine，后文简称bpy）、2,2'-联喹啉（2,2‘-biquinoline，后文简称bq）、deeb与deebq均为4,4'-二乙酯基衍生物（4,4‘-diethylester derivatives）。上述化合物以羧酸根形式结合于纳米晶TiO₂薄膜表面，极限表面覆盖率为8(±2)×10⁻⁸ mol/cm²。电化学测量显示，这些化合物的第一还原过程（相对于饱和甘汞电极（saturated calomel electrode, SCE）为-0.70 V）发生于TiO₂还原之前。在牺牲电子给体三乙胺（triethylamine）存在的条件下进行稳态光照，可生成还原态敏化剂。热平衡状态下的金属到配体电荷转移（metal-to-ligand charge-transfer, MLCT）激发态，以及这些化合物的还原形式，均不会向TiO₂注入电子。纳秒瞬态吸收（nanosecond transient absorption）测量结果证实，基于还原态与氧化态化合物等浓度的电荷分离态具有极长的寿命。上述结果与如下机理相符：激发态经超快电子注入进入TiO₂，随后发生界面电子转移至基态化合物。该过程的量子产率随激发能量升高而增大，这一现象归因于激发态敏化剂与半导体受体态之间的重叠程度更强。例如，[Os(bpy)₂(dcbq)]/TiO₂体系的量子产率分别为：φ(417 nm) = 0.18 ± 0.02，φ(532.5 nm) = 0.08 ± 0.02，φ(683 nm) = 0.05 ± 0.01。生成基态产物的电子转移通过侧向分子间电荷转移完成。电荷复合的驱动力超过了光致发光激发态所储存的能量。计时吸收（chronoabsorption）测量表明，配体基元的分子间电子转移速率比金属中心的分子空穴转移速率快一个数量级。采用科尔劳施-威廉姆斯-沃茨（Kohlrausch−Williams−Watts）模型对电荷复合过程进行了定量分析。