臺灣博碩士論文加值系統

此研究主要在超高真空下以4-methoxyphenyl azide（4-MPA）吸附在Cu（100）表面上，探討其光化學反應，同時藉由反射式紅外光譜（RAIRS）和熱程控脫附儀（TPD）來鑑定。4-MPA分子是根據參考文獻的步驟合成出的。實驗中，在室溫下將4-MPA直接從樣品管中昇華，並經過氣道漏氣閥將其蒸氣導引進入到超高真空腔體中。
經由實驗得知，吸附在銅單晶上的疊氮化物，可以藉由熱化學、光化學和金屬催化反應，使其脫去兩個氮原子而形成氮烯化合物。而透過紫外光照射可以讓N = N雙鍵形成，形成偶氮芳香烴化合物分子。在實驗中所使用光源的波長分別為365 nm和405 nm;然而不同波長的光源會使得N-N2斷鍵的速率不同，其原因來自於照射光會激發金屬基板的電子並轉移至吸附分子的最低未佔用軌域，促使鍵的斷裂。光解後生成穩定的中間體imide和活潑的triplet nitrene，triplet nitrene會迅速偶合形成偶氮芳香烴化合物。

Thermal, photochemical and metal-facilitated routes to aryl nitrenes via the extrusion of dinitrogen from 4-methoxyphenyl azide adsorbed on a single crystal Cu(100) surface have been investigated using reflection-absorption infrared spectroscopy and temperature-programmed desorption in ultrahigh high vacuum. Only by irradiation with UV light can an N=N double bond be made, rendering azoarene. The azido precursors are transparent to the 365 nm and 405 nm photons employed in our experiments; therefore, the observed wavelength-dependent N-N2 bond dissociation rates suggest a photo-induced substrate-mediated electron attachment mechanism leading concurrently to stable coordinated imide intermediates, as well as transitory reactive triplet nitrene species attributable to the on-surface azo-dimer synthesis.

Table of Content
Chapter 1. Introduction 1
1.1 Background. 1
1.2 Research Motivation 5
Chapter 2. Experimental Section 7
2.1 Ultrahigh vacuum (UHV) system 7
2.2 Surface 9
2.3 Reagents 10
2.4 Temperature-programmed desorption (TPD) 11
2.5 Reflection-absorption infrared spectroscopy (RAIRS) 12
2.6 Density functional theory (DFT) calculations 13
Chapter 3. Results 14
3.1 Synthesis and characterization of 4-methoxyphenyl azide (4-MPA) 14
3.2 Thermal and photochemical reactions of 4-MPA on Cu(100). 16
3.3 Coverage-dependent study of 4-MPA on Cu(100). 25
3.4 Identification of surface intermediates by density functional theory (DFT) calculations 31
3.5 Thermal and photochemical reactions of 2,6-difluorophenyl azide (2,6-DFPA) on Cu(100) 36
Chapter 4. Discussion and Conclusions 40
4.1 Possible reaction mechanisms for azoarene formation 40
4.2 Photodissociation mechanism: direct photolysis. 45
4.3 Photodissociation mechanism: dissociation by electron attachment 47
4.4 Conclusions 51
References 53

List of Figures
Figure 1.1 Examples of organic bond-forming reactions which were pursued in the on-surface synthesis. 2
Figure 1.2 Colors of azoarenes, such as Allura red (1), Chrysoin resorcinol (2), Janus green (3), and Direct blue (4). 3
Figure 1.3 Butterfly-like motion of the azoarene-crown ether. A large alkali-metal cation is selectively captured by the cis-isomer. 3
Figure 1.4 Photo-regulation of the formation and dissociation of DNA duplexes with azobenzene labeled oligonucleotides. 4
Figure 1.5 Various reactions pathways for aryl azides. 6
Figure 2.1 Layout of the UHV system. 8
Figure 2.2 (Left) Cu (100) mounted on a heating/cooling element. (Right) LEED pattern of Cu (100). Electron beam energy = 150 eV. 9
Figure 3.1 1HNMR spectrum of 4-MPA (400 MHz, CDCl3) Δ6.94 – 6.97 (d, H (H1, H3)), 6.87 – 6.90 (m, ΔH (H2, H4)), 3.79 (s, ΔH (H5)). 15
Figure 3.2 (a) Standard mass pattern of 4-MPA from NIST Chemistry WebBook. (b) Mass spectrum of 4-MPA characterized by our QMS. 15
Figure 3.3 Temperature-dependent RAIR spectra: (a) after adsorption of 1.0 L 4-MPA on Cu(100) at 100 K, (b) followed by 250 K anneal, and (c) followed by 650 K anneal. 17
Figure 3.4 Multiplex TPD spectra after adsorption of 1.0 L 4-MPA on Cu(100) at 100 K. 18
Figure 3.5 RAIR spectra: (a) after adsorption of 1.0 L 4-MPA on Cu(100) at 100 K, (b) (a) subjected to 10 minutes UV irradiation and (c)-(e) followed by annealing to 300 K, 500 K and 650 K. 19
Figure 3.6 Post-irradiation TPD spectra after adsorption of 1.0 L 4-MPA at 100 K on Cu(100) followed by UV illumination. 20
Figure 3.7 RAIR spectra of 4-MPA subjected to photolysis by 365 nm and 405 nm photoirradiation. 22
Figure 3.8 QMS response as a function of irradiation time. The synchronous rise-and-fall pattern of m/z 28 and 14 indictes the release of N2 due to photolysis of the adsorbed 4-MPA. The lack m/z 149 signals indicates there is no molecular photodesoprtion. 23
Figure 3.9 Photo-ejected N2 decay profiles fitted by exponential functions to extract the kinetic information. 24
Figure 3.10 Side-by-side comparison of PITPD for 1 L 4-MPA after by 365 nm and 405 nm UV light exposure for 10 minutes. 24
Figure 3.11 The coverage-dependent TPD spectra of m/z 149 after 0.5 L, 1.0 L, 2.0 L and 3.0 L 4-MPA exposures. 26
Figure 3.12 The coverage-dependent TPD spectra of m/z 123 for final aniline product after 0.5 L, 1.0 L, 2.0 L, 3.0 L and 4.0 L 4-MPA exposures. 27
Figure 3.13 The coverage-dependent RAIR spectra at various 4-MPA exposures. 28
Figure 3.14 The coverage-dependent post-irradiation TPD experiments of azoarene product (m/z 242) after 0.5 L, 1.0 L, 2.0 L, 3.0 L and 4.0 L 4-MPA exposures. 29
Figure 3.15 The coverage-dependent post-irradiation RAIR spectra as a function of 4-MPA exposures. 30
Figure 3.16 (a) RAIR spectra of surface species isolated by dissociatively adsorbing 4-MPA at 250 K on Cu(100), (b) computed IR spectrum of NAr moiety coordinated to single-layer Cu25 cluster, and (c) optimized Cu25NAr structure. 32
Figure 3.17 (a) RAIR spectrum taken right after 365 nm UV irradiation for 1.0 L 4-MPA on Cu(100) at 100 K, (b) (a) subjected to 500 K anneal, (c) difference spectrum between Figure 3.17 (a) and (b), and (d) DFT calculated spectrum of trans-azoarene on Cu25, and (e) optimized Cu25-(trans)ArN=NAr structure. 34
Figure 3.18 Mass spectrum of 2,6-DFPA characterized by QMS. 36
Figure 3.19 (a) Experimental RAIR spectrum after 1.0 L 2,6-DFPA adsorption on Cu (100), (b) and (c) Post-irradiation RAIR spectra after UV 365 nm and 405 nm illumination, respectively, and (d) RAIR spectrum after 1.0 L 2,6-DFPA adsorption at 250 K on Cu (100). 38
Figure 3.20 TPD spectrum after 1.0 L 2,6-DFPA adsorption on Cu(100) at 100 K. 39
Figure 3.21 Comparison of 2,6-DFPA PITPD by utilizing 365 nm and 405 nm UV irradiation, respectively. 39
Figure 4.1 (a) Post-irradiation TPD spectrum at 250 K adsorption of 1.0 L 2,6-DFPA. (b) 1.0 L 2,6-DFPA adsorbed at 100K. 42
Figure 4.2 (a) PITPD spectra of three possible azoarenes taken after co-adsorption of 0.5 L 4-MPA and 0.5 L 2,6-DFPA/Cu(100). (b) PITPD spectra of self-coupled and cross-coupled azoarenes taken after sequential adsorption of 1.0 L 4-MPA at 250 K and 0.5 L 2,6-DFPA at 100 K/Cu(100). 43
Figure 4.3 Calculated frontier orbitals of 4-MPA. 46
Figure 4.4 The UV-vis spectrum of 4-MPA diluted in ethyl acetate (EA) with a volume ratio of 0.001. 46
Scheme 4.1 Pathway involving bimolecular coupling of metal imide species. 40
Scheme 4.2 Pathway involving cycloaddition of aryl azide to metal imide to form a reactive metal-tetrazene species. 41
Scheme 4.3 Pathway involving rapid dimerization of triplet nitrene species. 44
Scheme 4.4 photo-induced substrate-mediate electron attachment processes and potential energy curves along the reaction coordinates. 50

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