Chapter Introduction 1.1 Low band gap polycyclic hydrocarbons with either a closed-shell or an open-shell singlet biradical ground state Polycyclic hydrocarbons (PHs) with a low band gap (Eg ≤ 1.5 eV) represent a class of interesting molecules with intriguing electronic and optical properties, which are of fundamental importance in the field of structural organic chemistry and materials science.1 The band gap of a PH molecule is dependant on both the molecular size and molecular shape, when it is beyond a certain point, a low band gap can be achieved. Figure 1.1 lists a number of low band gap PH candidates, such as acenes, rylenes, periacenes, anthenes, zethrenes, bis(phenalenyl)s and indenofluorenes. Some of them, such as acenes and rylenes, are extensively studied and actively participated in material sciences,2 while the study of some candidates, such as higher order periacenes, still stay at the theoretical level. Nevertheless, those molecules are believed to be perfect models to link theoretical chemistry, synthetic chemistry and material sciences, and have become a rising hot topic nowadays. Figure 1.1 Examples of low band gap PHs. In general, polycylic hydrocarbons exhibit two types of electronic states, closed-shell state and open-shell state. Most π-conjugated molecules can be characterized by a closed-shell electron configuration, accommodating their π electrons only in bonding orbitals. In contrast, open-shell configuration refers to the existence of one or more unpaired electrons, also known as radicals, in the molecular structures. This ground state often exists for low band gap PH molecules. Particularly, the electronic states of open-shell systems with two unpaired electrons can be further divided into open-shell singlet, when the unpaired electrons adopt anti-parallel spin, or open-shell triplet, when the unpaired electrons adopt parallel spin. Among all of the electronic states, the one with the lowest energy defines the ground state of π-conjugated molecules. It is worth noting that the two-radical systems can either be termed as diradicals (i.e. m-xylene) in which two radicals barely interact with each other, or biradicals (i.e. p-xylene) in which two radicals weakly coupled with each other, leading to increased biradical character and diminished intramolecular convalency compared to closed-shell configuration. The intramolecular electron coupling of biradicals is always weaker than closed-shell systems, but stronger than localized diradicals, or pure diradicals.3 The origin of the biradical character is a small HOMO-LUMO energy gap, which facilitates the admixing of doubly excited configuration into the ground state configuration.4 The ground state of polycyclic hydrocarbons is dependant on the size (conjugation length) and shape (edge structure) of the molecule. For example, molecules with only zigzag edges, such as phenalenyl and triangulene, are intrinsically open-shell systems in that their structures cannot be depicted in a closed-shell manner.5 On the other hand, for systems possessing both zigzag and armchair edges, such as anthenes and periacenes, there is a critical point on the conjugation length (or the band gap) beyond which a singlet biradical ground state will emerge. The properties of open-shell polycyclic hydrocarbons can be studied by a combination of theoretical and experimental methods. Various computational calculation methods provide informative parameters, such as biradical character index, LUMO occupancy number, exchange interaction (KH,L), spin density and singlet-triplet energy gap (ΔES-T). On the other hand, experimental methods such as nuclear magnetic resonance (NMR), electron paramagnetic resonance (ESR), superconducting quantum interference device (SQUID), X-ray single crystal analysis, Raman spectroscopy and so on represent powerful tools to investigate the magnetic properties, the biradical characters and the intermolecular interactions. The stability is a crucial issue for low band gap polycylic hydrocarbons, from both synthetic and applications point of views. The low band gap polycylic hydrocarbons generally possess less benzenoid aromatic sextet rings and tend to be more reactive, so different substituents can be introduced to stabilize them, such as bulky groups for kinetic blocking of reactive sites and electron-withdrawing groups for lowering the HOMO. Especially, when it comes to those with singlet biradical character, delocalization of the radicals in the molecular skeleton and kinetic blocking of the reactive sites should be taken into consideration. Once the stability issue is properly addressed, many applications are opened up for low band gap PHs. They are attractive candidates as near-infrared (NIR) dyes and semiconductors, which open the door for diverse applications such as bio-imaging,6 optical recording,7 and fabrication of electronic devices such as organic field effect transistors (OFETs) and solar cells.8 Moreover, recent theoretical and experimental studies on open-shell polycyclic hydrocarbons have added new insights into their material applications for non-linear optics,9 for the future photovoltaic devices10 and for molecule-based spintronic devices.11 All of these studies create a rising hot topic for low band gap PHs in synthetic chemistry as well as in materials science. 1.1.1 PHs with a closed-shell ground state 1.1.1.1 Acenes Acenes refer to a series of laterally fused benzene rings which can serve as semiconducting materials. The band gap of acenes will decrease with more benzene rings fused, so a low band gap can be realized for the higher order candidates in this series. Interestingly, many theoretical works are dedicated to reveal the open-shell character of higher order acenes, and a singlet biradical ground state is commonly arrived using different calculation methods. This result was further extended to polyradical for even larger acenes. Despite the theoretical calculations, the experimental examination seems to be a tough task since the larger acenes are normally very reactive and synthetically difficult. Fortunately, the development of modern chemistry has allowed substitutions to be made on acenes and the stable derivatives can be isolated and characterized (Figure 1.2). The method to stabilize acenes developed in Anthony’s group is to introduce silyl ethynyl groups at the strategic positions, the carefully chosen substituents allow them to obtain single crystals for hexacene and heptacene for the first time.12 More stable heptacene derivatives were synthesized in Wudl,13 Miller14 and Chi15 group independently, by blocking more reactive sites at the zigzag edges using different substituents (Compounds 1-13-1-15). It’s worth noting that all the heptacene derivatives possess closed-shell configuration in the ground state as evidenced by sharp peaks in the NMR spectra. Recently, two nonacene derivatives were prepared in Miller’s and Anthony’s group, Miller’s nonacene 1-16a-b was protected by arylthio groups which by calculation would eliminate the total spin when located at terminal rings, and hence led to a closed-shell species. Indeed, the sharp NMR peaks seem to further support this conclusion.16 But later, Chen and Miller himself suggested an open-shell singlet biradical ground state for this nonacene by unrestricted broken spin-symmetry density functional theory (UBS-DFT) at B3LYP/6-31G* level, irrespective of the positions of the substituents.17 On the other hand, Anthony’s nonacene derivatives 1-17a-c were intensively protected by trialkylsilylethynyl and bis(trifluoromethyl)phenyl groups on the zigzag edges and fluorine atoms on the outer rings, and the structures of these nonacenes were unambiguously confirmed by single crystal analysis. The nonacene featured a prominent S0-S1 transitions at 1014 nm with an energy gap of 1.2 eV based on the absorption onset, while no fluorescence was observed in the visible region. Interestingly, the nonacene samples appear to be NMR silent and an ESR spectrum was found at ge = 2.0060. Although the origin of the signal is not clear, there is a possibility that it could be an intrinsic characteristic of open-shell nonacene.18 Figure 1.2 Functionalized high order acenes. 1.1.1.2 Rylenes Rylenes represent PHs with two or more naphthalene units peri-fused together. Only one aromatic sextet benzenoid ring can be drawn for each naphthalene unit and the zigzag edges exist at the terminal naphthalene units. On the basis of the number of fused naphthalenes, rylenes can be termed as perylene, terrylene, quaterrylene and so on. In pursuit of stable dyes with high extinction coefficients and long-wavelength absorption/emission, rylenes have received a great deal of attention.19 Among them, perylene and its imide derivatives have shown obvious advantages, including outstanding chemical, thermal and photochemical inertness, and have already been well investigated and documented.20 Extension of conjugation along the long molecular axis leads to higher order rylenes with low band gap and NIR absorption and emission, which are promising dyes in various of applications. For rylenes, the primary concern is the poor solubility arose from dye aggregation, so substituents are introduced to improve the solubility. Four tert-butyl groups were firstly introduced to solve the problem, but the scope was only limited to quaterrylene.21 Later, dicarboxylic imide group was proven to be a good solution and rylene diimide compounds up to hexarylene (1-20–1-23) were prepared showing NIR absorption and high extinction coefficient. 22 Additional solubility can be achieved by substitution at the bay regions. Moreover, core-expanded rylene diimides (1-24,1-25) were also synthesized as promising dyes for bio-labeling or laser-induced applications. Figure 1.3 High order rylenes and their diimide derivatives. Another interesting modification concept of rylene dyes is N-annulation. New opto-electronic properties are expected due to the electron-donating nature of amines. From the year 2009, a series of poly(peri-N-annulated perylene) up to hexarylene 1-26, 1-27 were achieved in Wang’s group23 (Figure 1.4), and oxidative ring fusion driven by DDQ/Sc(OTf)3 was found to be very effective for this system due to the electron-rich property of N-annulated perylene core. A large dipole moment along the short molecular axis favoring formation of H aggregates was reflected by decrease of intensity in absorption and a marked concentration dependence of the spectrum. The large dipole moments may favor the formation of highly ordered supramolecular structures, which may lead to enhanced charge carrier mobilities. In parallel to these work, the carboximide derivative 1-28 was developed in our group, and the presence of imide group not only enhanced the stability of the core by lowering the high-lying HOMO energy level, but also allowed the introduction of bulky diisopropylphenyl group which helped to increase the solubility together with the branched alkyl chain at the amine site. An alternative cyclodehydrogenation strategy by mild base was applied to synthesize 1-28, due to the existence of electron-withdrawing imide group. Compound 1-28 exhibited absorption at NIR region and emitted strong fluorescence with quantum yield up to 55% in dichloromethane. Such a high quantum yield is remarkable given that many NIR absorbing dyes usually exhibit low fluorescence quantum yield.24 Figure 1.4 Structures of N-annulated rylenes. 1.1.1.3 Bisanthenes Bisanthene refers to a class of PH with two anthracene units peri-fused together, it is an unstable compound but can be stabilized by proper substitution. One example of stable bisanthene derivative 1-29 was reported in Kubo’s group by introducing tert-butyl groups to the periphery of the bisanthene, this method provides sufficient stability and solubility but leave the most reactive meso-positions exposed.25 One strategy developed in our group is to block the meso-positions by different subsituents, such as imide,26 aryl groups or triisopropylsilylethynyl groups.27 Bisanthene bisimide 1-30 was prepared using base-promoted cyclization reaction as a key step. Compared to the parent bisanthene, 1-30 exhibited a red shift of 170 nm at the absorption maximum together with enhanced stability and solubility, indicating 1-30 as a promising candidate for NIR absorbing materials. An alternative approach was synchronously developed by means of meso-substitution with aryl or alkyne substituents to block the most reactive sites. Based on this consideration, three meso-substituted bisanthenes 1-32a-c were prepared by nucleophilic addition of aryl Grignard reagent or alkyne organolithium reagent to the bisanthenequinone followed by reduction/aromatization of the as-formed diol. The obtained compounds not only showed enhanced stability and solubility, but also exhibited absorption and emission in the NIR region as well as amphoteric redox behaviors, which qualified them as NIR dyes and hole/electron transporting materials. The same synthetic strategy was also applied to prepare quinoidal bisanthene 1-31, which can be regarded as a rare case of stable and soluble PAH with a quinoidal character.28 Figure 1.5 Stable bisanthene derivatives. 1.1.1.4 Indenofluorenes Indenofluorene molecules are a class of π-conjugated molecules with five-membered rings. These systems can be viewed as antiaromatic analogues of acenes, and are very interesting in terms of their bonding pictures. Indenofluorene derivatives 1-33 and 1-34 were reported in Haley’s group. Compounds 1-33 were prepared as stable indenofluorene derivatives from the corresponding diketone precursors, and X-ray crystallographic analysis of the single crystals allowed a rare glimpse of the p-quinodimethane (p-QDM) core. The bond length showed alternation in the central p-QDM core but homogeneous for the peripheral benzenes, thus those molecules should be described as fully conjugated 20-π-electron hydrocarbon with fused s-trans 1,3-diene linkages across the top and bottom portions of the carbon skeleton. 29 In order to further explore how the substituents can influence the indenofluorene core, a number of 6,12-diethylnylindenofluorenes 1-34 were prepared in the same group. The crystal packing for 1-34b and 1-34h was observed in 1D stacks with contacts around 3.40 Ǻ, being different from the herringbone packing mode of 1-33. Together with the optical and electrochemical properties, these results suggest that these molecules can be promising semiconductors.30 Notably, due to the relatively large HOMO-LUMO energy gap, no open-shell biradical ground state was observed for this system. Recently, a series of diaryl-substituted indenofluorene derivatives 1-35 were prepared in Haley and Yamashita’s group independently. The aryl substituents were found to have a profound impact on the physical properties such as stability, HOMO-LUMO energies and redox properties. FET device was fabricated on vapor-deposited thin films in Yamashita’s group, an interesting ambipolar transporting behavior was observed for 1-35k with electron mobility of 8.2 × 10-6 cm2 V-1 s-1 and hole mobility of 1.9 × 10-5 cm2 V-1 s-1. The relative low mobilities were due to the less-ordered molecular arrangements in the thin films.31 In parallel, a single crystal OFET with 1-35j as active component was reported in Haley’s group, the device exhibited ambipolar behavior with hole and electron mobilities as × 10-4 and × 10-3 cm2 V-1 s-1, which represented one of very few single crystal OFETs from organic semiconductors. 32 The 24-π-electron antiaromatic system 1-36 possessing a bond-localized 2,6-naphthoquinone dimethane unit was recently presented by the same group.33 Although the structure of 1-36 could be drawn in a biradical form, the absence of line broadening in NMR, the silence in ESR for both solid and solution samples together with a large bond alternation all lead to a conclusion of a closed-shell ground state. Other isomers of indenofluorenes, such as 1-37 and 1-38 were also reported, but they are all proven to be closed-shell molecules from different experimental measurements.34,35 Figure 1.6 Indenofluorene derivatives. 1.1.2 PHs with an open-shell ground state 1.1.2.1 Higher order anthenes Higher order anthenes refer to those with three or more anthracene units fused together, the biradical character of anthenes will increase with more anthryl units fused. According to the calculation at the CASSCF(2,2)6-31G level, the singlet biradical character (y) values are estimated to be 0.07 for bisanthene, 0.54 for teranthene and 0.91 for quarteranthene. Because the unpaired electrons are fixed to the meso-positions of anthenes, the effect of delocalization is minimized and the discussion of biradical character can be focused on the aromatic stabilization effect. Therefore, anthenes represent excellent models to study how formation of aromatic sextet rings affects biradical/polyradical characters in PAHs with Kekulé type structures, and to investigate the spin-polarized state in zigzag edged GNRs. Inspiringly, derivatives from bisanthene to quarteranthene are prepared and isolated in the crystalline form in Kubo’s group, allowing a detailed examination of their molecular structure, chemical behavior and physical properties (Figure 1.7).36,37 Due to the solubility and stability problems, tert-butyl substituents are introduced to the periphery of the anthenes and aryl groups are introduced to the meso-positions to block the reactive sites. For teranthene and quarteranthene derivatives 1-39 and 1-40, moderate to large biradical character and an edge localization of unpaired electrons are confirmed by a combination of physical measurements and DFT calculations. Both teranthene and quarteranthene derivatives 1-39 and 1-40 are NMR silent at room temperature, and the peaks become sharp upon cooling for 1-39. However, the NMR baseline of 1-40 was flat even when the temperature is lowered down to 183 K. The absence of NMR signals is due to the large population of a thermally accessible triplet diradical species for 1-40. The NMR results can also be explained by SQUID measurements, which showed a small singlet-triplet gap for both compounds (1920 K for 1-39 and 347 K for 1-40a). Single crystals suitable for X-ray analysis for both 1-30 and 1-40a were successfully obtained, revealing a high planarity and symmetry for the anthene core. Moreover, as shown in the resonance structures, the contribution from the biradical resonance will shorten the bond a due to the enhanced double bond character. From the bond lengths information provided by the X-ray analysis, the bond length of bond a for quarteranthene (1.412 Å) is much shorter than that in teranthene (1.424 Å) and bisanthene (1.447 Å). Furthermore, the harmonic oscillator model of aromaticity (HOMA) values of ring A is highest for quarteranthene and lowest for bisanthene, indicating that quarteranthene has more benzenoid character for the peripheral rings, hence, a larger biradical character. Another interesting property for quarteranthene is the absorption behavior. The room temperature absorption spectrum located at 920 nm derives from a mixture of triplet and singlet species, while at lower temperature (183 K), a bathochromic shift to 970 nm was observed corresponding to the singlet biradical ground state. The shape of the two spectra is quite similar due to their similar distribution of the unpaired electrons at the zigzag edges. The investigations of the anthene series have paved the way to understand the intrinsic properties of zigzag edged GNRs and the fabrications of nanographene-based optical and magnetic devices. Figure 1.7 Teranthene/quarteranthene derivatives with singlet biradical ground states. 1.1.2.2 Bis(phenalenyl)s Connection of two phenalenyl radicals with π-conjugated systems will produce a series of closed-shell quinoidal molecules 1-41–1-43 with biradical characters (Figure 1.8) and these 10 11. 2D COSY 1H NMR spectrum of 3-13 (500 MHz, CDCl3, rt) 123 Intens. [a.u.] 12. Mass spectrum (MALDI-TOF) of 3-1 873.4594 874.4720 5000 4000 3000 2000 1000 868 870 872 874 876 878 880 882 m/z 13. Mass spectrum (EI) of 3-2 A:\11APR12_EIHR\0411wu-zd10-c3-av4 EIHR 11/08/2012 03:57:05 PM 0411wu-zd10-c3-av4 #1 RT: 7.44 AV: NL: 3.58E5 T: + c Full ms [ 860.95-892.94] 866.9420 100 zd10 884.3303 878.9390 90 890.9432 70 885.3293 60 50 880.9401 882.2843 40 864.9448 30 20 867.9409 883.2962 872.9453 868.9570 861.9461 879.9545 886.3425 876.9383 10 891.9495 862 864 866 868 870 872 874 876 878 880 882 884 886 888 890 892 m/z 0411wu-zd10-c3-av4#1 RT: 7.44 T: + c Full ms [ 860.95-892.94] m/z= 884.16-884.52 m/z Intensity Relative Theo. Mass 884.3303 358327.0 100.00 884.3309 Delta RDB Composition (ppm) equiv. -0.68 32.0 C 54 H 48 O 10 N 14. Mass spectrum (MALDI-TOF) of 3-3 Intens. [a.u.] Relative Abundance 80 914.4092 2000 1500 913.3976 915.4010 1000 911.6255 500 912.6398 916.4276 908 910 912 914 916 918 920 m/z 15. Mass spectrum (MALDI-TOF) of 3-4 124 Intens. [a.u.] 1002.3864 5000 4000 3000 1001.3778 2000 1000 992.5 995.0 997.5 1000.0 1002.5 1005.0 1007.5 1010.0 1007.5 1010.0 1012.5 1015.0 m/z Intens. [a.u.] 16. Mass spectrum (MALDI-TOF) of 3-5 1000.3681 1250 1000 1001.3713 750 1002.3782 500 999.3606 250 1003.3807 992.5 995.0 997.5 1000.0 1002.5 1005.0 1012.5 1015.0 m/z 125 Appendix 1. 1H NMR spectrum of 4-4 (500MHz, CDCl3, rt) 126 2. HR-ESI mass spectrum for 4-3 Simulated: Intens.[a.u.] m/z= 805.34-815.34 m/z Theo. Mass 810.3426 810.3452 x10 Delta RDB Composition (ppm) equiv. -3.22 35.0 C 56 H 46 O N 810.3422 2.5 811.3480 2.0 1.5 1.0 812.3500 0.5 813.3580 0.0 806 808 810 812 814 816 818 m /z 3. MALDI-TOF Mass spectrum of 4-4 127 Appendix 1. 1H NMR spectrum of 5-1 (500MHz, CDCl3, rt) 128 2. 13C NMR spectrum of 5-1 (500MHz, CDCl3, rt) 129 3. 1H NMR spectrum of 5-1 (500MHz, toluene-d8, rt) 130 4. 1H NMR spectrum of 5-1 (500MHz, C6D6, rt) 131 5. 2D-COSY NMR spectrum of 5-1 (500MHz, C6D6, rt) 132 6. VT 1H NMR spectra of 5-2 in CD2Cl2 7. VT 1H NMR spectra of 5-2 in toluene-d8 133 8. 1H NMR spectrum of 5-2 (500MHz, toluene-d8, -70oC) 134 9. 2D-COSY NMR spectrum of 5-2 (500MHz, toluene-d8, -70oC) 135 10. 2D-COSY NMR spectrum of 5-2 (500MHz, THF-d8, -70oC) 136 11. HR-MASS spectrum (MALDI-TOF) of 5-1 137 12. HR-MASS spectrum (MALDI-TOF) of 5-2 138 [...]... following part of this chapter, the synthesis, reactivity, photophysical and electrochemical properties and theoretical calculations will be discussed Figure 2. 1 Structures of zethrene 2- 1, 7,14-disubstituted zethrene 2- 2 and zethrene bis(dicarboximide) 2- 3 2. 2 Results and Disccusion 2. 2.1 Synthesis and mechanism study Different from previous work, we chose 4,6-dibromo-1,8-naphthalimide (2- 6) as precursor... 1693 4000 3000 20 00 1000 wavenumber (c) 1715 1645 1675 C=O at 7,14-position imide C=O 4000 3000 20 00 1000 wavenumber Figure 2. 2 (a) MALDI-TOF Mass spectrum of 2- 8, (b) FT-IR spectrum of 2- 3, (c) FT-IR spectrum of 2- 8 30 Scheme 2. 4 Proposed mechanism for the formation of 2- 8 2. 2 .2 Theoretical calculations Time-dependent density function theory calculations were conducted for 2- 3 and 2- 8 and their optimized... arrows (Å) The blue and red color denote negative and positive charges, respectively 2. 2.3 Photophysical and electrochemical properties Both compounds 2- 3 and 2- 8 are soluble in common organic solvents and 2- 3 exhibits a blue color while 2- 8 appears to be red The UV-Vis absorption and fluorescence spectra recorded in chloroform are shown in Figure 2. 4 Compound 2- 3 has a major absorption band in far-red... photo-stability and the absorption spectra of its solution did not change after standing under ambient conditions for weeks 1 .2 Normalized Absorption 1.0 0.8 0.6 2- 8 2- 3 0.4 Normalized emission 2- 3 UV-Vis 2- 3 PL 2- 8 UV-Vis 2- 8 PL 0 .2 0.0 400 500 600 700 800 wavelength(nm) Figure 2. 4 Normalized UV-vis absorption spectra (5 x 10-5 M) and fluorescence spectra (5 x 10-5 M) of 2- 3 and 2- 8 in chloroform 0.5 0.5... intrinsic properties of zethrene- based molecules and to seek the possibilities of using them as functional materials, the study in this thesis aim to develop novel and facile synthetic methodologies to prepare soluble and stable zethrene derivatives and homologues, and therefore study the physical properties of this interesting class of PH 20 1.4 Reference 1 (a) Lambert, C Angew Chem Int Ed 20 11, 50,... F.; Wang, Z J Am Chem Soc 20 10, 1 32, 420 8– 421 3 24 Jiao, C.; Huang, K.; Luo, J.; Zhang, K.; Chi, C.; Wu, J Org Lett 20 09, 11, 4508–4511 25 Hirao, Y.; Konishi, A.; Matsumoto, K.; Kurata, H.; Kubo, T AIP Conf Proc 20 12, 1504, 863−866 26 Yao, J H.; Chi, C.; Wu, J.; Loh, K P Chem.–Eur J 20 09, 15, 929 9–93 02 27 Li, J.; Zhang, K.; Zhang, X.; Chi, C.; Wu, J J Org Chem 20 10, 75, 856–863 28 Zhang, K.; Huang, K.;... PCT Int Appl WO 03/0 026 87, A1, 20 03; (b) Moon, J M KR 20 07101430, A, 20 07 25 Chapter 2: Zethrene bis(dicarboximide) and its unexpected oxidation 2. 1 Introduction Zethrene (2- 1, Figure 2. 1), a hydrocarbon whose synthesis was established a long time ago, has been forgotten for a long time The first synthesis of zethrene was achieved by Clar in 1955,1 and a more convenient access to zethrene was found accidentally... in the bay region of zethrene 17 Scheme 1.5 New synthetic route to zethrene and its Diels-Alder addition reaction In addition to the synthesis of zethrenes, the synthesis of higher order zethrene, namely heptazethrene and octazethrene, is recently achieved in our group.56 The synthesis of triisopropylsilylethynyl substituted heptazethrene 1-63 and octazethrene 1-65 took advantage of the corresponding... 648, 2- 3 0.53 0.93 -0.84 -0.99 -1.30 - -5.50 -3.96 1.54 1.81 0.005 - -0.55 -0.87 -1 .21 -1.67 - -4.31 - 2. 14 596 477, 580, 5 820 2- 8 528 616 Table 2. 1 Photophysical and electrochemical properties of compounds 2- 3, 2- 8 Eox and Ered are half-wave potentials for respective redox waves with Fc/Fc+ as reference HOMO and LUMO energy levels were calculated according to equations: HOMO = - (4.8 + Eoxonset) and. .. K Chem Ber 1991, 124 , 20 91 21 00 22 (a) Quante, H.; Mü llen, K Angew Chem., Int Ed 1995, 34, 1 323 –1 325 ; (b) Avlasevich, Y.; Mü ller, S.; Erk, P.; Mü llen, K Chem.–Eur J 20 07, 13, 6555–6561; (c) Pschirer, N G.; Kohl, C.; Nolde, F.; Qu, J.; Mü llen, K Angew Chem., Int Ed 20 06, 45, 1401–1404; (d) Avlasevich, Y.; Mü llen, K Chem Commun 20 06, 4440–44 42 22 23 (a) Li, Y.; Wang, Z Org Lett 20 09, 11, 1385–1387; . zethrenes are more prone to exhibit biradical characters. 15 Figure 1.11 Resonance structures for zethrene and higher order zethrenes. 1 .2. 1 Synthesis and reactivity for zethrene- based. teranthene and quarteranthene derivatives 1-39 and 1-40, moderate to large biradical character and an edge localization of unpaired electrons are confirmed by a combination of physical measurements and. region of zethrene. 18 Scheme 1.5 New synthetic route to zethrene and its Diels-Alder addition reaction. In addition to the synthesis of zethrenes, the synthesis of higher order zethrene,