Malene
Plesner
,
Thomas
Hensel
,
Bjarne E.
Nielsen
,
Fadhil S.
Kamounah
,
Theis
Brock-Nannestad
,
Christian B.
Nielsen
,
Christian G.
Tortzen
,
Ole
Hammerich
and
Michael
Pittelkow
*
Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark. E-mail: pittel@kiku.dk
First published on 30th April 2015
Insights to the subtle reactivity patterns of hydroxy-substituted carbazoles allows the precise synthesis of unsymmetrical azatrioxa[8]circulenes by the reaction of N-benzyl-2,7-di-tert-butyl-3,6-dihydroxycarbazole with two different 1,4-benzoquinones in the presence of an oxidant (chloranil) and a Lewis acid (BF3OEt2). The unique synthetic control obtained originates from the selectivity obtained upon reacting N-benzyl-2,7-di-tert-butyl-3,6-dihydroxycarbazole with an electron-rich benzoquinone to give first the C–C bond formation and then subsequently the dibenzofuran formation with high regioselectivity. Herein the first synthesis of unsymmetrical antiaromatic azatrioxa[8]circulenes and the full characterization using NMR spectroscopy, optical spectroscopy, electrochemistry, computational techniques and single crystal X-ray crystallography is reported. The controlled stepwise condensation of N-benzyl-2,7-di-tert-butyl-3,6-dihydroxycarbazole with two different 1,4-benzoquinones gives selectively the unsymmetrical azatrioxa[8]circulenes.
Planarized conjugated cyclooctatetraenes (COTs) are among the classic antiaromatic moieties, and as part of heterocyclic [8]circulenes COTs have attracted the interest of Komatsu,8 Osuka,10 Nenajdenko,11 Wong,12 Christensen,13 Erdtman and Högberg,14 and us.15 We have previously used DTF based nuclear independent chemical shift (NICS) calculations on the diazadioxa[8]circulenes (1), azatrioxa[8]circulenes (2) and tetraoxa[8]circulenes (3) to address the question of the aromaticity/antiaromaticity of the central planarized COTs, and found that the COT indeed possess anti-aromatic character.16 Another feature that we have explored with heterocyclic [8]circulenes is their (blue) fluorescent properties. We have found that the azatrioxa[8]circulenes and the diazadioxa[8]circulenes fluoresce in the blue region making them attractive for application in light emitting devices.17 Aggregation of π-conjugated systems tends to shift the fluorescence from the blue to the green region, and we have found it useful to functionalize the [8]circulenes with tert-butyl groups to modulate aggregation behaviour.18
The previously described synthetic methodologies developed for heterocyclic [8]circulenes limit the range of compounds available to symmetrical structures. Preparing unsymmetrical π-extended heterocyclic[8]circulenes may open the possibility of systematically studying the interplay between aromatic and antiaromatic moieties in large π-conjugated frameworks. In this paper we showcase a new synthetic protocol towards this goal, and we report on the spectroscopic, computational and electrochemical properties. The work described in this paper focuses on utilizing the intrinsic reactivity of hydroxyl-substituted carbazoles for the construction of azatrioxa[8]circulenes. We have found that we can modulate the reactivity of the hydroxyl-substituted carbazoles by careful choice of benzoquinone reaction partners, and thus opening the pathway for construction of complex heterocyclic [8]circulenes.
We found that the 3,6-dihydroxycarbazole may be dimerized under oxidative conditions to yield diazadioxa[8]circulenes 1 by simply changing from a 1,4-benzoquinone substituted with electron donating substituents to 1,4-benzoquinones substituted with electron withdrawing substituents (1, Scheme 1, top).15b It is important that the 1,4-benzoquinones are substituted in the 2- and 3-positions and that the 3,6-dihydroxycarbazole is substituted in the 2- and 7-positions, as otherwise the reaction gives uncontrolled polymerisation.
When N-benzyl-2,7-di-tert-butyl-3,6-dihydroxycarbazole (4) is treated with only one equivalent of a 1,4-quinone that does not cause it to dimerize (the benzoquinone should not have an electron-withdrawing substituent) to the diazadioxa[8]circulene (1) the reaction mixture contains mainly the product where one C–C bond has been formed (5, Scheme 2, top).
Scheme 2 Pathway to unsymmetrical azatrioxa[8]circulene. R2 of the first 1,4-benzoquinone must be an electron donating group, otherwise the diazadioxa[8]circulene (1) is the main product. |
Addition of one equivalent of an oxidant such as chloranil to the tetrahydroxy compound 5 in the presence of BF3OEt2 yields the ring-closed furan 6. Subsequent addition of a second 1,4-quinone to this material gives the unsymmetrical azatrioxa[8]circulene 2 in a process where two new C–C bonds and two furan rings are formed (Scheme 2, bottom).
We have found that when using either 2-tert-butyl-1,4-benzoquinone or 2-methoxy-1,4-benzoquinone only one regioisomer of the tetrahydroxy compound 5 was formed, and as a consequence only one regioisomer of the dibenzofuran 6 is formed after oxidation. Thus, the R2 group is situated in the position indicated in Scheme 2 (red). When treating the tert-butyl derivative 5 generated in situ with either 2-methoxy-1,4-benzoquinone (43%), 2-thiododecyl-1,4-benzoquinone (35%, 2-tert-butyl-1,4-benzoquinone (13%) or 1,4-naphthoquinone (31%) we isolate the unsymmetrical azatrioxa[8]circulenes (2, Fig. 1) in good yields.
In all three cases where unsymmetrical azatrioxa[8]circulenes are made, one regioisomer is formed as the main product. This is a remarkable control of regiochemistry in a reaction protocol where three C–C bonds and three C–O bonds are formed while three molecules of water are eliminated. When starting by reacting the N-benzyl-2,7-di-tert-butyl-3,6-dihydroxycarbazole (4) with 2-methoxy-1,4-benzoquinone followed by 2-tert-butyl-1,4-benzoquinone it gave exactly the same azatrioxa[8]circulene product as when the reaction was performed with the reverse order, as seen by 1H-NMR analysis of the isolated pure product. We have also found that if the sequence is started by reacting the N-benzyl-2,7-di-tert-butyl-3,6-dihydroxycarbazole (4) with 1,4-naphthoquinone, and then with 2-tert-butyl-1,4-benzoquinone, then we obtained a mixture of the two azatrioxa[8]circulene regioisomers with respect to the tert-butyl group originating from the 2-tert-butyl-1,4-benzoquinone.
The unsymmetrical azatrioxa[8]circulenes were all crystalline compounds, however we were not able to grow crystals with large enough dimensions to be suitable for single crystal X-ray crystallography. We were able to grow crystals of the C2-symmetrical azatrioxa[8]circulene (2tButBu, Fig. 2). The single crystal X-ray structure confirms outcome of the azatrioxa[8]circulenes synthesis and also the regiochemical outcome of the reaction forming the azatrioxa[8]circulene with respect to the tert-butyl groups originating from the 2-tert-butyl-benzoquinones. The azatrioxa[8]circulene is planar with respect to the π-conjugated system and due to the four tert-butyl substituents no intermolecular π–π-stacking is observed.
Fig. 2 Single crystal X-ray structure of azatrioxa[8]circulene (2tButBu). Hydrogens are omitted for clarity. Left: top view. Right: side view. |
We have previously performed NICS calculations on planar heterocyclic [8]circulenes (1–3) in order to address the question of the antiaromaticity of the central planarized COTs.15,16 NICS(0) and NICS(1)zz values for 2tBuOMe were determined at the B3LYP/6-311+G(d,p) level of theory using geometries obtained from B3LYP/6-31G(d) confirming the antiaromatic nature of the COT (Fig. 3).
Fig. 3 NICS(0) and NICS(1)zz (lower numbers) values calculated for the methoxy substituted azatrioxa[8]circulene (2). |
The UV/Vis and fluorescence spectra of the series of new azatrioxa[8]circulene 2 were recorded to assess whether these derivatives would be suitable for applications as the fluorescent component in blue OLEDs (Fig. 4). Three common electronic transitions are centered at ∼260 nm (∼4.8 eV), ∼360 nm (∼3.4 eV) and at ∼425 nm (∼2.9 eV) with varying intensities.
Fig. 4 UV/Vis (top) and normalized fluorescence spectra (bottom) of the series of azatrioxa[8]circulenes (CH2Cl2). |
The oscillator strength and Franck–Condon factors for each transition are not identical when comparing the transitions at the low energy transition. The transition at ∼425 nm is weak in intensity in the spectrum of the three azatrioxa[8]circulenes without a naphthalene units (extinction coefficient of 15000–20000 M−1 cm−1) compared to the spectra of the two azatrioxa[8]circulenes with either one or two naphthalene moieties (extinction coefficients of ∼35000–45000 M−1 cm−1). The two C2 symmetrical azatrioxa[8]circulenes (2PhePhe and 2tButBu) have a slightly blue-shifted fluorescence as compared to the three unsymmetrical azatrioxa[8]circulenes. Again the two azatrioxa[8]circulenes containing naphthalene units (2PhePhe and 2tBuPhe) give fluorescence spectra with comparable features, but the spectrum of the compound containing two naphthalenes is blue-shifted by ca. 20 nm. The fluorescence spectra of the three unsymmetrical azatrioxa[8]circulenes with identical π-systems (2tBuOMe, 2tBuSDo and 2tButBu) have similar features, with the high-energy bands of the 2tBuOMe (10 nm) and 2tBuSDo (15 nm) slightly redshifted as compared to 2tButBu. We also determined the fluorescence quantum yields for the series, and the quantum yields increase significantly with the increasing area of the π-conjugated system (Table 1).
Compound | Quantum yielda | Oxidation potentialb |
---|---|---|
a Quantum yields determined in CH2Cl2. b Formal potentials determined by cyclic voltammetry as the average of Eoxp and Eredp for the first oxidation wave. Solvent: dichloromethane, supporting electrolyte: (nBu)4PF6 (0.1 M), voltage sweep rate: 0.1 V s−1, working electrode: glassy carbon (d = 3 mm). | ||
2PhePhe | 0.89 | 0.67 V |
2tButBu | 0.31 | 0.67 V |
2tBuOMe | 0.17 | 0.62 V |
2tBuSDo | 0.28 | 0.66 V |
2tBuPhe | 0.67 | 0.65 V |
The optical data suggests that the π-extended azatrioxa[8]circulenes may be suitable candidates as the fluorescent component in light emitting devices, as this derivative fluoresce in the blue region and it does not aggregate in solution. Analysis of the crystal structure of 2PhePhe15a suggests that this derivative aggregates in the solid state, so despite having the highest quantum yield of fluorescence it may be more desirable to utilize the 2tBuPhe derivative in light emitting devices.
The electrochemical oxidation of the five compounds was studied by cyclic voltammetry in dichloromethane with (nBu)4PF6 (0.1 M) as the supporting electrolyte. Reversible one-electron oxidations corresponding to the formation of persistent radical cations were observed for 2tBuOMe, 2tBuSDo, 2tBuPhe and 2tButBu (Fig. S14†). In the case of 2PhePhe, the oxidation process appeared as a double peak composed of two closely spaced electron transfer processes (Fig. 5). The voltammogram gradually developed into that for a simple one-electron transfer upon dilution. In contrast, the double peak behaviour became increasingly pronounced with decreasing temperature (Fig. S15†). Similar observations were recently reported for indenofluorene-extended tetrathiafulvalenes and were shown to originate from the formation of radical cation-neutral and radical cation-radical cation associates.20 It is likely that similar associates result from the oxidation of 2PhePhe that in contrast to the four other members of the series has one dibenzofuran subunit not carrying a space filling tert-butyl group, but rather an extended π-system resulting from the presence of two additional benzene rings. The values of the formal potentials for the one-electron oxidations are summarized in Table 1 and it is seen that the minor structural changes that distinguish the five azatrioxa[8]circulenes are reflected by oxidation potentials that are very similar. A second oxidation wave corresponding to the subsequent formation of the dication was observed close to the solvent wall for all compounds (Fig. S16†). For 2tBuSDo the dication was observed to react with residual water; deprotonation resulted in the formation of the corresponding S-oxide that was further oxidized to a persistent radical cation at the potential of the second wave (Fig. S17†).
Yield: 96% (266 mg: 1.92 mmol) melting point: 47–49 °C TLC: Rf-value = 0.26 (CH2Cl2) 1H-NMR (500 MHz, CDCl3): δ = 6.71 (m, 2H), 5.95 (s, 1H), 3.83 (s, 3H) ppm. 13C-NMR (126 MHz, CDCl3): δ = 187.6 (Cq), 181.9 (Cq), 158.8 (Cq), 137.4 (CH), 134.6 (CH), 107.9 (CH), 56.4 (–OCH3) ppm. Elemental analysis: C: 60.90%, H: 4.10%, N: 0% (theoretical – C: 60.87%, H: 4.38% N: 0%). GC-MS: calcd for C7H6O3 [M]+m/z 138.0, found 138.1.
Yield: 82.5% (773 mg: 1.92 mmol) melting point: 205–209 °C TLC: Rf-value = 0.35 (MeCN/toluene, 1:8) 1H-NMR (500 MHz, DMSO): δ = 8.87 (s, 2H, 2 × ROH), 7.28 (t, J = 7.4 Hz, 2H), 7.24 (s, 2H), 7.23 (s, 2H), 7.22–7.17 (m, 3H), 5.46 (s, 2H, R2NCH2Ph), 1.40 (s, 18H, 2 × RC(CH3)3) ppm. 13C-NMR (126 MHz, DMSO): δ = 149.2 (Cq), 138.7 (Cq), 134.8 (Cq), 134.6 (CH), 128.5 (Cq), 127.1 (CH), 126.9 (CH), 119.7 (CH), 106.7 (CH), 105.6 (CH), 45.7 (CH2), 35.0 (Cq), 29.7 (CH3) ppm. Elemental analysis: C: 81.00%, H: 7.76%, N: 3.41% (theoretical – C: 80.76%, H: 7.78% N: 3.49%), MALDI HRMS: calcd for C27H31NO2 [M]+m/z 401.23548, found 401.23464.
Yield: 49% (309 mg: 0.48 mmol) melting point: 304–308 °C TLC: Rf-value = 0.57 (toluene/n-heptane, 1:4) 1H-NMR (500 MHz, CDCl3): δ = 8.71–8.63 (m, 4H), 7.77–7.70 (m, 4H), 7.49 (s, 2H), 7.36–7.29 (m, 5H), 5.81 (s, 2H), 1.78 (s, 18H) ppm. 13C-NMR (126 MHz, CDCl3): δ = 150.5 (Cq), 148.9 (Cq), 148.8 (Cq), 137.9 (Cq), 137.6 (Cq), 134.1 (Cq), 129.0 (CH), 127.7 (CH), 126.9 (CH), 126.2 (CH), 126.0 (CH), 122.2 (CH), 121.9 (CH), 121.0 (Cq), 120.8 (Cq), 117.4 (Cq), 114.1 (Cq), 113.1 (Cq), 112.6 (Cq), 104.9 (CH), 47.8 (CH2), 35.5 (C(CH3)3), 30.7 (R-CH3) ppm. MALDI HRMS: calcd for C47H35NO3 [M]+m/z 661.26169, found 661.26095.
Yield: 13% (45 mg: 0.06 mmol) melting point: 245–248 °C TLC: Rf-value = 0.69 (toluene/n-heptane, 1:4) 1H-NMR (500 MHz, CDCl3): δ = 7.73 (s, 2H), 7.39 (s, 2H), 7.36–7.27 (m, 5H), 5.75 (s, 2H), 1.77 (s, 18H), 1.69 (s, 18H) ppm. 13C-NMR (126 MHz, CDCl3): δ = 152.9 (Cq), 151.3 (Cq), 151.1 (Cq), 138.0 (Cq), 137.3 (Cq), 134.8 (Cq), 134.3 (Cq), 129.0 (CH), 127.7 (CH), 126.9 (CH), 117.1 (Cq), 116.8 (Cq), 115.1 (Cq), 112.7 (Cq), 107.4 (CH), 105.5 (CH), 47.8 (CH2), 35.4 (Cq), 35.3 (Cq), 30.5 (CH3) 30.4 (CH3) ppm. MALDI HRMS: calcd for C47H47NO3 [M]+m/z 673.35559, found 673.35398.
Yield: 31% (60 mg: 0.09 mmol) melting point: 300–303 °C TLC: Rf -value = 0.66 (toluene/n-heptane, 1:4) 1H-NMR (500 MHz, CDCl3): δ = 8.71–8.66 (m, 2H, ArH), 7.79 (s, 1H), 7.76 (dt, J = 6.3, 3.5 Hz, 2H), 7.47 (s, 1H), 7.46 (s, 1H), 7.36–7.28 (m, 5H), 5.79 (s, 2H), 1.83 (s, 9H), 1.78 (s, 9H), 1.71 (s, 9H) ppm. 13C-NMR (126 MHz, CDCl3): δ = 153.2 (Cq), 151.3 (Cq), 151.0 (Cq), 150.5 (Cq), 149.2 (Cq), 148.8 (Cq), 138.0 (Cq), 137.6 (Cq), 137.3 (Cq), 135.0 (Cq), 134.3 (Cq), 134.1 (Cq), 129.0 (CH), 127.7 (CH), 126.9 (CH), 126.4 (CH), 126.3 (CH), 122.2 (CH), 122.2 (CH), 121.4 (Cq), 121.1 (Cq), 117.7 (Cq), 117.4 (Cq), 116.8 (Cq), 115.0 (Cq), 113.3 (Cq), 113.2 (Cq), 112.7 (Cq), 112.7 (Cq), 106.8 (CH), 105.1 (2 × CH), 47.8 (CH2), 35.6 (Cq), 35.5 (Cq), 35.4 (Cq), 30.7 (CH3), 30.6 (CH3), 30.4 (CH3) ppm. MALDI HRMS: calcd for C47H41NO3 [M]+m/z 667.30864, found 667.30724.
Yield: 43% (139 mg: 0.21 mmol) melting point: 326–329 °C TLC: Rf -value = 0.66 (toluene/n-heptane, 1:4) 1H-NMR (500 MHz, CDCl3): δ = 7.75 (s, 1H), 7.39 (s, 1H), 7.36 (s, 1H), 7.34 (s, 1H), 7.33–7.27 (m, 5H), 5.74 (s, 2H), 4.25 (s, 3H, O-CH3), 1.76 (s, 9H), 1.68 (s, 9H) ppm. 13C-NMR (126 MHz, CDCl3): δ = 153.3 (Cq), 153.0 (Cq), 151.8 (Cq), 151.1 (Cq), 150.8 (Cq), 145.8 (Cq), 142.7 (Cq), 138.0 (Cq), 137.5 (Cq), 137.3 (Cq), 135.3 (Cq), 134.2 (Cq), 134.2 (Cq), 129.0 (CH), 127.6 (CH), 126.8 (CH), 117.6 (Cq), 117.2 (Cq), 117.0 (Cq), 116.6 (Cq), 115.3 (Cq), 112.7 (Cq), 112.3 (Cq), 110.0 (Cq), 107.8 (CH), 105.5 (CH), 104.4 (CH), 94.9 (CH), 57.2 (O–CH3), 47.7 (CH2), 35.5 (Cq), 35.4 (2 × Cq), 30.5 (CH3), 30.5 (CH3), 30.4 (CH3) ppm. MALDI HRMS: calcd for C44H41NO4 [M]+m/z 647.30356, found 647.30208.
Footnote |
† Electronic supplementary information (ESI) available: NMR spectra for new compounds, general experimental procedures, a detailed NMR assignment and further electrochemical assignments. CCDC 1032471. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ob00676g |
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