利用電化學掃瞄穿隧顯微術之重複斷裂接合法研究口比啶胺三核金屬串的單分子電導性質

Abstract

本論文論述重點在於測量三核金屬串化合物的單分子電導值，並藉由改變工作電位的方式使分子發生氧化還原，以達成調控分子之導電能力。我們的目標分子為[Ru3(μ3-dpa)4(X)2] (dpa– = dipyridylamido anion，X = NCS–、CN–)以及[M3(μ3-dpa)4(NCS)2] (M = Cr、Co、Ni、Cu)，這兩類分子都是以四個呈螺旋狀結構的口比啶胺配位基將三核金屬架在分子中心，所形成的一維金屬串分子。研究方法為電化學掃瞄穿隧顯微術(ECSTM)所衍生的重複斷裂接合法(repeated break junction)，重複形成單分子尺度的電極間距，用於電化學條件下得到單分子電導值。根據本實驗室以往在多核金屬串分子的測量結果，所提出的金屬間鍵序影響金屬串導電能力的觀點，我們可藉由電化學方式調控分子氧化還原態，預期改變金屬串錯合物的電子組態及結構，進而達到調節單分子電導值的能力。結果顯示(1) [Ru3(μ3-dpa)4(NCS)2]比起[Ru3(μ3-dpa)4(CN)2]的導電能力較佳，可見分子的導電能力受軸向配基與電極的耦合(coupling)所影響。(2) [Ru3(μ3-dpa)4(CN)2]氧化態導電能力優於中性態，而[Ru3(μ3-dpa)4(NCS)2]導電性在氧化前後無差別。這是由於CN–與釕金屬有很好的π-back donation，降低三核釕金屬的π*軌域能量，使分子的HOMO成為π*軌域而非πnb軌域，氧化後的鍵序增加；相對地NCS–並無此效應。(3) [Cr3(μ3-dpa)4(NCS)2]中性態導電能力優於氧化態，我們可由氧化前後中心金屬電子從未定域化轉為定域化的觀點說明。(4)鈷、鎳、銅三核金屬串分子氧化態與中性態導電能力的異同，由金屬間鍵結的程度，也可得到合理的解釋。

In this thesis, we demonstrate that manipulation of the single molecular conductance by controlling the redox states of tri-metal string complexes. The target molecules are [Ru3(μ3-dpa)4(X)2] (dpa– = dipyridylamido anion, X = NCS–, CN–) and [M3(μ3-dpa)4(NCS)2] (M = Cr, Co, Ni, Cu). The metal atoms are collinear and wrapped helically by four ligands. The measurements were carried out by electrochemical STM break junction (ECSTM BJ), which forms the electrode gap of single molecular scale repeatedly under electrochemical conditions. We have proposed that the conductance is related to the bond order between adjacent metal ions. We expect the electronic configurations or structures for a molecular wire are different for the corresponding redox states. Therefore, the electrochemical control is an approach to tune the conductance of the molecules. The results show that (1) The conductance of [Ru3(μ3-dpa)4(NCS)2]0 is superior to that of [Ru3(μ3-dpa)4(CN)2]0. This indicates that the coupling between the axial ligand and electrode affects the conductivity of the metal string. (2) When [Ru3(μ3-dpa)4(CN)2]0 is oxidized, the conductance increases. This is due to the increase of the bond order. The formation of the π-backbonding from metal to cyanide reduces the π-antibonding energy, and thus the HOMO of this compound is π* orbital. In contrast, isothiocyanate doesn’t have this effect. The HOMO of [Ru3(μ3-dpa)4(NCS)2]0 is πnb orbital, so we can’t observe the change in conductance. (3) For [Cr3(μ3-dpa)4(NCS)2], the conductance in neutral state is superior to oxidized state. This can be ascribed to the degree of delocalization upon 1-e− oxidation. (4) With the viewpoint of bond order, plausible explanation regarding the conductance of the redox states of other metal strings will be provided.