1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

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1 Introduction complexes are usually prepared from metal salts (most often acetates or acetylacetonates) in organic solvents at room or elevated temperatures. These procedures are generally well suitable especially for most of the lighter transition metals (e.g. Mn, Fe, Co, Ni, Cu, Zn) and were also applied in the course of this thesis. Sometimes, a metal carbonyl approach (e.g. for Ru, Os) or the utilization of metal amides as base is necessary but those are less common. 40 After having discussed the build-up of systems, it shall now be focused on the systems properties. <strong>1.1</strong>.3 Aromaticity, Spectroscopy & Electronic Properties Most porphyrins have a planar, conjugated π-system consistent of C-C double bonds and nitrogen lone pairs. In analogy to [18]annulenes, porphyrins are considered as bis-etheno bridged aza-analogues meaning that only nine double bonds and none of the nitrogen lone pairs are directly involved in the aromaticity. 41 Another approach based on computational molecular dynamics assumes the presence of an aromatic “inner cross”. Both views are depicted in Scheme 10. Independent of the points of view, the systems always fulfill HÜCKEL’s rule (4n+2 π-electrons) and hence are considered aromatic. 42,43 This feature strongly influences NMR and UV/Vis spectroscopy as well as reactivities. 10 NH N N N HN HN NH N N NH N Scheme 10. Mesomeric structures of porphin 2 in respect to the abovementioned assumptions for the aromatic character. Analogy to diaza-[18]annulene (left) and the inner cross approach (right). The magnetic field used with NMR spectroscopy causes a ring current within the macrocycle. So the β-pyrrolic protons get deshielded and their resonances shift to lower field. The effect is stronger than for benzene analogues and thus the signals are usually observed around 9 ppm. The same ring current effect is responsible for the shielding of the inner ring amine protons and gives rise to a strong shift to higher field making the resonances detectable at negative δ values (-2 to -4 ppm). 44 HN
Introduction 1 Also the absorption of light differs from that of benzoidic aromatic systems since the porphyrins are intensely colored. This can be explained by the enlargement of the aromatic system causing shifts of the absorption bands to higher wavelengths and thus into the visible region of the spectrum (see Figure 4 and Table 1). 0.01 nm 1 nm 100 nm 1 mm 1 cm 1 m 1 km 400 nm 700 nm Figure 4. Illustration of the electromagnetic spectrum. 45 Table 1. Absorptions of exemplary aromatic compounds with rising size of the aromatic system. 46 Compound λmax [nm] λ [nm] remainder bands benzene 189 208, 262 [10]annulene 257 265 [18]annulene 379 456, 764 TPP 13 420 518, 553, 592, 648 A simple model to explain the observed UV/Vis spectra for porphyrins was established by GOUTERMAN in the 1960s (Four-Orbital Model). 47 Accordingly, the absorption spectra of regular porphyrins – like most of the porphyrins within this thesis – can generally be divided into two major regions. The first region below 500 nm comprises the most intense absorption – the so-called SORET band (B-band) around 400 nm. It arises from an allowed π → π * transition (usually S0 → Sm, m>2) and the molar extinction coefficient ε can reach several hundred thousand M -1 ·cm -1 . In the region above 500 nm, the so-called Q-bands are observed which also represent π → π * transitions (usually S0 → S1 and S0 → S2). Their ε values are much lower as those transition are only quasi-allowed (typically around 10.000 M -1 ·cm -1 ). In free base systems in general, four bands are detectable due to non-equivalent transition dipole moments resulting in two non-degenerate perpendicularly polarized π → π * 11
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3 Discussion and Results other TPP
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3 Discussion and Results cyanomethy
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3 Discussion and Results phenyl sub
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4 Summary As the characterization d
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5 Zusammenfassung 5 Zusammenfassung
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5 Zusammenfassung Da in guten Ausbe
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6 Experimental Section UV/Vis spect
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6 Experimental Section 6.2 Studied
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6 Experimental Section 13 C NMR (10
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6 Experimental Section (cyanomethyl
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6 Experimental Section 6.2.5.5 αβ
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6 Experimental Section UV/Vis (CH2C
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6 Experimental Section broad signal
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7 References 16 H. Fischer, K. Zeil
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7 References 57 S. Schwartz, K. Abs
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7 References 89 (a) R. C. Fuson, J.
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7 References 131 (a) J. Perdew, Phy
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S. Jasinski, N. Jux, “Investigati
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Ein großer Dank gilt auch den Ange
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Cirruculum vitae Stefan Jasinski Ge
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1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

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