| 摘要: | 有機物吸附在金電極上可以理解為有機/無機電化學界面的模型，這與分子電子學和有機薄膜半導體的研究有關。本實驗室曾研究 terthiophene (TT)在 Au(111) 電極上的吸附行為 (Chen et al, J. Electroanal. Chem. 2022 (921) 116651)，結果顯示將 Au(111) 電極浸泡在由乙醇配製成的 TT 溶液中，使 TT 分子自動形成一排列整齊的單層薄膜。
目前的研究聚焦在Au(111) 上吸附 3′,4′-bis(hexylthio)-2,2′:5′,2′′-terthiophene (DTDST)，它具有 TT 骨架，與兩個 C6 鏈通過硫醚與 TT 連接。我獲得了高質量的 STM 圖像，揭示了 DTDST 分子的內部和二維空間結構。電位極大地影響了 DTDST 在 Au(111) 上的排列，將原本無序的 DTDST 分子吸附層轉變為有序的結構。在 0.1 M 的 H2SO4 和 HClO4 中，當電位小於 0 V (vs. Ag/AgCl) 時，產生的結構分別為 Au(111) - (33 9) 和 (53 26)。在正電位下引起硫酸氫根的共吸附，導致在 0.1 M H2SO4 中從 Au(111) - (33 9) 轉變成 (13 15)。這種有序的HSO4- + DTDST 共吸附層與在過氯酸中看到的混亂形成對比。STM 以清楚解析 DTDST 的內部分子結構，可以闡明分子基團與 Au(111) 電極作用，而烷基鏈的排列方式使分子間產生凡得瓦力。原位 STM 也顯示了吸附的 DTDST 分子的氧化產生了低聚噻吩。
另外還研究了 3′-(hexylthio)-2,2′:5′,2′′-terthiophene (DTST) 在 Au(111) 上的吸附。這個分子有一個 C6 鏈通過硫醚連接到 TT 核心上。它與DTDST一樣，電位和陰離子都影響 DTST 在 0.1 M 硫酸和過氯酸中的空間結構。在電位小於0 V (vs. Ag/AgCl)，產生的結構分別為 Au(111) - (33 5) 和 (13 27)。在 0.1 M 硫酸中，硫酸氫根陰離子與DTST共吸附，使Au(111) - (33 5) 轉變成 Au(111) - (13 16)。值得注意的是，STM 顯示 TT 的順反形式的異構化，這可能和分子內 S – S 相互作用有關，它可能抑制 C – C 單鍵的旋轉。
研究在電位控制下 C4H4N2 異構物（噠嗪PD、嘧啶PM、吡嗪PZ）在 Pt(111) 電極的吸附。在過氯酸鹽介質中進行的循環伏安實驗，這些分子導致了一個不可逆的還原峰。當 pH 值從 1 增到 3 時，向負電位移動 120 mV，表明這是一個 1 H+/1e- 的過程，推測是分子吸附結構的不可逆變化，同時也是 Pt(111) 上的氫原子吸附。這些分子的兩個N端在還原反應之前皆鍵結於 Pt(111) 上，但在負電位時失去了其中一個 Pt – N 表面的結合，這導致這些分子重新定向到一個更直立的構型，並可能在稍後階段在Pt(111)上質子化，這些質子化的 C4H4N2H+ 無法恢復它們原來的吸附構型。
我們從不可逆還原特徵的電荷中估計每個分子的覆蓋率，PD、PM 和 PZ 的電荷分別為 80.5、78.2 和 82.3 µC/cm2，表明這些分子在 Pt(111) 電極上具有相同的覆蓋度，推測它們有類似的吸附構型，但有不同的傾斜角度。在 pH1 過氯酸中，PZ 的還原峰最小，這歸因於它在 Pt 電極上的水平方向。除了 N 端，芳香環 (PZ) 的 π - 電子也有助於與Pt表面結合，導致它和 Pt(111) 有較強的鍵結，因此較難改變它的吸附構型。
;The adsorption of organic species at gold electrode serves as a model to understand the organic/inorganic electrified interface, which has been relevant to the study of molecular electronics and organic thin film semiconductors. Our previous study on terthiophene (TT) adsorption on Au(111) electrode (Chen et al, J. Electroanal. Chem. 2022 (921) 116651.) shows that highly ordered TT molecular thin film can be installed on an Au(111) crystal by immersing in a TT dosing solution made of ethanol.
The current study focused on the adsorption of 3′,4′-bis(hexylthio)-2,2′:5′,2′′-terthiophene (DTDST), a molecule with a TT backbone attached with two C6 chains via thiolethers, on an ordered Au(111) electrode. High – quality STM images were obtained to reveal the internal and 2D spatial structures of DTDST admolecules. Potential greatly influenced the organization of DTDST, which transformed the pristine DTDST disarray to ordered structures, characterized as Au(111) - (33 9) and (53 26) in 0.1 M H2SO4 and HClO4, respectively, when the potential was more negative than 0 V (vs. Ag/AgCl). Bisulfate coadsorption at positive potential resulted in conversion from (33 9) to (13 15) in 0.1 M H2SO4. This ordered HSO4- + DTDST adlayer contrasts with a disarray seen in perchloric acid. The –SC6 modifiers of DTDST admolecules were clearly imaged by the STM, suggesting that these groups interacted with the Au electrode. The alkyl chains arranged in a way that allowed intermolecular van der Waals force interactions. Oxidation of adsorbed DTDST molecules to yield oligomers was also revealed by in situ STM.
The adsorption of 3′-(hexylthio)-2,2′:5′,2′′-terthiophene (DTST) on Au(111) was also examined. This molecule has one C6 chains attached to the TT core via thiolether. As with DTDST, both the potential and the anion affect the spatial structures of DTST fpmd in 0.1 M sulfuric acid and perchloric acids. At potential negative of 0 V (vs. Ag/AgCl), the structures fond in and H2SO4 and HClO4 are Au(111) - (33 × 5) and (13 × 27). In 0.1 M sulfuric acid, bisulfate anion is coadsorbed with DTST, transforming Au(111) - (33 × 5) to (13 × 16). It is noteworthy that the STM imaging reveals the isomerization of cis-trans forms of TT, suggesting that the S - S interaction might be important enough to refrain the rotation C – C single bond.
The second part - the adsorption of C4H4N2 isomers (Pyridazine PD, Pyrimidine PM, Pyrazine PZ) from solution onto Pt(111) electrode under potential control is also examined. These admolecules result in an irreversible reduction peak in the cyclic voltammetry experiments performed in perchlorate media. This feature shifts 120 mV negatively as the pH incrases from 1 to 3, suggestging it is a 1H+/1e- process, presumed to be an irreversible change of molecular adsorption configuration, coupled with hydrogen adsorption at the Pt electrode. It is proposed that two N-ends of these molecules are tethered to the Pt electrode prior to the reduction reaction, but loses one of these Pt – N surface bonding at negative potential. This leads to reorientation to a more upright configuration of these molecules and possibly protonation at a later stage on the Pt electrode. These protonated C4H4N2H+ do not restore their original adsorption configurations.
We estimate the coverage of each molecule from the charge involved in the irreversible reduction feature. The charges are 80.5, 78.2 and 82.3 µC/cm2 for PD, PM, and PZ, suggesting that these molecules have similar adsorption configurations with different tilted angle on the Pt(111) electrode. PZ is the smallest reduction peak at pH1 perchloric acid, which is attributed to its horizontal orientation on the Pt electrode. In addition to N – ends, the π- electron of the aromatic ring (PZ) also contributes to the surface binding with the Pt surface. |
