5,15-二氧雜卟啉之頭基接點影響單分子電性的量子干涉模擬

Abstract

量子干涉(quantum interference, QI)在單分子的電子傳輸領域中有建設性(constructive, CQI)與破壞性(distructive, DQI)兩種現象。5,15-二氧雜卟啉(5,15-dioxaporphyrin, DOP)是個反芳香性分子(antiaromatics)，其meso位置被氧原子取代，在氧化還原前後皆具鉗合金屬的能力，並能維持芳香性分子的平面結構，比一般卟啉分子擁有更高的電化學操作耐受性。本研究以軌域對稱性法則(orbital symmetry rule)和延伸彎箭頭法則(extended curly arrow rules)預測三種DOP分子在反芳香性的20個π電子(簡稱20π)氧化成為芳香性的18個π電子(簡稱18π)時，發生的CQI和DQI現象。通過第一原理計算分析頭基接點對3,13-雙對胺基苯基DOP(3,13-bis(4-aminophenyl)DOP, 3,13-DOP)、3,17-雙對胺基苯基DOP (3,17-DOP)與10,20-雙對胺基苯基DOP (10,20-DOP)的量子干涉特性。我們發現DOP分子由20π氧化為18π時，因為分子軌域相位的加乘或抵消，使得3,17-DOP的電荷傳輸能譜由DQI轉變成CQI，而10,20-DOP的轉換則相反。3,13-DOP在兩種氧化態均呈現DQI特徵。模擬電化學電位控制下的改變工作電極電位(Ewk)獲得的I-Ewk圖顯示在DQI特徵附近，呈現四個數量級以上的電流開關比值。

Constructive and destructive quantum interference (CQI and DQI) are especially featured in the charge transport of single-molecule junctions. 5,15-dioxaporphyrin (DOP) is a meso-oxygen-substituted counterpart of highly π-conjugated porphyrin which can maintain planar structure and ability to chelate nickel ion during redox reactions. In this study, we utilized orbital symmetry rule to predict the CQI and DQI features of the DOP when it was oxidized from 20π-electron neutral specie to 18π-electron dication. Through first-principles calculation, the switches between DQI and CQI caused by altering contact-positions of anchoring groups on 3,13-bis(4-aminophenyl)DOP (3,13-DOP), 3,17-bis(4-aminophenyl)DOP (3,17-DOP) and 10,20-bis(4-aminophenyl)DOP (10,20-DOP) were analyzed. When the 20π-electron 3,17-DOP was oxidized to the 18π-electron state, the QI patterns in the transmission spectra would turn from DQI to CQI by the addition and the offset of the phases of certain molecular orbitals, and vice versa for 10,20-DOP. 3,13-DOP feature DQI at both oxidation state. Calculated electrochemical gating I-Ewk curves result in up to 4 orders of current on/off ratio near the DQI pattern.