Oxidation State-Dependent Intramolecular Electronic Interaction of

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Oxidation State-Dependent Intramolecular Electronic Interaction of Carbazole-Based Azacyclophanes with 9,10-Anthrylene Units Tetsuo Iwanaga,*,† Tomokazu Yamauchi,† Shinji Toyota,*,‡ Shuichi Suzuki,§,∥ and Keiji Okada§ †

Department of Chemistry, Faculty of Science, Okayama University of Science, 1-1 Ridaicho, Kita-ku, Okayama 700-0005, Japan Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152−8551, Japan § Department of Chemistry, Graduate School of Science, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan ‡

S Supporting Information *

ABSTRACT: We designed an azacyclophane comprising 3,6substituted carbazole and 9,10-anthrylene units to investigate the unique electronic properties of the oxidation states arising from the presence of multiple oxidizable azacyclophane. This compound and its analogue containing 1,4-phenylene in substitution of 9,10anthrylene units were synthesized by Buchwald−Hartwig coupling reactions. Cyclic voltammograms of both the 9,10-anthrylene and 1,4-phenylene derivatives exhibited four quasi-reversible oxidation processes. The absorption spectra of the oxidation states of the 9,10-anthrylene derivative exhibited broad absorption bands in the near-infrared region arising from charge-resonance and intramolecular charge-transfer interactions. The photophysical and electrochemical properties of the 9,10-anthrylene derivative were compared with those of the corresponding 1,4-phenylene derivative on the basis of theoretical calculations to evaluate the intramolecular electronic interactions. arbazole (Cz) units are useful building blocks in πconjugated compounds because of their rigid structures, specific optical properties, and chemical stability.1 Several Cz oligomers have found applications as functional materials in a variety of devices such as organic field-effect transistors,2 organic light-emitting diodes,3 and organic photovoltaics.4 3,6Substituted Cz derivatives and oligo(3,6-Cz) have been extensively studied because of the ease of electrophilic aromatic substitutions at the 3,6-positions.1,5 Recently, macrocycles have attracted intense interest due to their fascinating properties such as inclusion phenomena and supramolecular architectures.6 To design macrocyclic π-functional materials, the choice of spacers is important to realize the desired photochemical and electrochemical properties. For example, a 3,6-substituted Czbased macrocycle with methylene spacers, reported by Yan and co-workers,7a selectively binds ammonium ions via cation−π interactions. In addition, Jin and Nakamura reported that macrocycles composed of Cz and triazolium units exhibited self-association by π−π interactions and higher affinities for halide ions.7b On the contrary, azacyclophanes comprising phenylene units have been utilized as high-spin organic molecules in the molecular design of organic spin devices.8 The electrochemical and spectroscopic properties of the neutral and dicationic materials support these claims, including the generation of triplet spin states.9 Previously, we reported rigid cyclic oligomers consisting of 9,10-bis(phenylethynyl)anthracene units and diacetylene linkers.10 On the other

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© 2017 American Chemical Society

hand, the diphenylmethane11 and diphenylamino12 derivatives with 9,10-anthrylene units undergo reversible redox because of the introduction of the anthraquinone diimine group and the accompanying color change in the acidic solution. Azacyclophanes comprised m- or p-phenylenediamine units with 9,10anthrylene units that have formed unique spin density distributions due to the steric demand of the confronted anthrylene units.13 The 9,10-anthrylene moieties readily rotate along the macrocyclic frameworks, and this conformational change significantly influences both the structures and properties. Therefore, we are interested in the introduction of 9,10-anthrylene moieties into 3,6-substituted Cz-based macrocycles to create novel π-conjugated azacyclophanes. To this aim, we designed novel azacyclophane 1, in which two anthrylene units are connected with two 3,6-Cz units via phenylamino linkers. In each phenyl unit, a methoxy group is introduced to enhance the electron density and solubility. In this paper, we report the synthesis and properties of 1 and its 1,4-phenylene analogue 2, and their photochemical and electrochemical properties are compared (Figure 1). Compounds 1 and 2 were synthesized from 3,6-dibromo-N(p-methoxyphenyl)carbazole 314 by Buchwald−Hartwig coupling (Scheme S1). N,N′-Bis(4-methoxyphenyl)-p-phenylenediamine 515 was prepared by known methods. 9,10-N,N′-Bis(4Received: July 19, 2017 Published: September 7, 2017 10699

DOI: 10.1021/acs.joc.7b01688 J. Org. Chem. 2017, 82, 10699−10703

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The Journal of Organic Chemistry

Figure 1. Molecular structures of azacyclophanes 1 and 2 containing carbazole units.

Figure 2. Cyclic voltammograms of 1 (red line) and 2 (blue line).

Table 1. Electrochemical Data and HOMO−LUMO Energy Gaps of Compounds 1 and 2

methoxyphenyl)diaminoanthracene 4 was prepared from 9,10dibromoanthracene and p-anisidine by Buckwald−Hartwig coupling under conventional conditions.16 The Buckwald− Hartwig coupling of 3 with 4 under conventional conditions gave 1 in 14% yield as a red solid, and the same coupling reaction of 3 with 5 gave 2 in 29% yield as a greenish-yellow solid. These compounds were reasonably well characterized by NMR and FAB mass spectra (Figures S1−S3). In the 1H NMR spectra, 1 gave two doublets of doublets due to the anthracene protons appearing at δ = 7.22 and 8.21, and 2 afforded one singlet due to the phenylene protons at δ = 6.95, which is consistent with the molecular pseudosymmetry. Compounds 1 and 2 gave molecular ion peaks at m/z 1378.54 and 1178.47, respectively, in their mass spectra. Because we could not obtain single crystals suitable for X-ray analysis, the molecular structures were examined using the DFT calculations. The structures of the methoxy-free model compounds 1′ and 2′ were calculated at the B3LYP/631G(d) level of theory to save computational time (Figures S4 and S5).17 Compound 1′ has a bowl shape, in which the two planes of the Cz units rotate by 132° in the opposite direction to the macrocyclic cage. The dihedral angle between the Cz and the 9,10-anthrylene units was almost perpendicular (97.0°). Regarding the cavity size of 1′, the distance between the center of the five-membered ring of the two Cz units is 9.8 Å, and that between the centers of the 9,10-anthrylene units is 7.9 Å. Compound 2′ has almost the same structure as 1′, including a similar dihedral angle between the Cz and phenylene units. To investigate the electronic effect of the 9,10-anthrylene units, the oxidation potentials of 1 and 2 were measured by cyclic voltammetry in CH2Cl2 using n-Bu4NPF6 as an electrolyte (Figure 2 and Table 1). The cyclic voltammograms of 1 in CH2Cl2 exhibited two one-electron oxidations at E1/2ox = −0.02 and +0.15 V (vs Fc/Fc+) and one two-electron oxidation at +0.36 V, corresponding to the formation of the radical cation, di(radical cation), and tetracation, respectively. Although the distance between the two diaminoCz units of 1 is too large to expect intramolecular interactions, this result suggests the presence of weak interactions between the radical cation of the diaminoCz moiety and the neutral diaminoCz moiety. The formation of the first radical cation of one diaminoCz moiety should decrease the electron density of the other. As a result, one-electron oxidation of the radical cation moiety may become harder than the first one-electron oxidation. The third two-electron oxidation of the di(radical cation) of 1 is indicative of the similar potential value due to the weak interaction between the radical cations of the diaminoCz

compound

E1/2ox [V]a

1

−0.02, +0.15 +0.36 (2e) −0.23, −0.13 +0.26, +0.41

2

EHOMO [eV]b

ELUMO [eV]b

Eg [eV]

Eopt [eV]c

−4.52

−1.77

2.75

2.11

−4.26

−0.90

3.36

2.72

Measured in CH2Cl2 (0.5 mmol L−1) with n-Bu4NPF6 (0.10 mol L−1) as the supporting electrolyte and Ag/Ag+ as the reference electrode. b Calculated at the B3LYP/6-31G(d) theory level for methoxy-free derivatives. cThe optical band gap determined from the band onset with a molar absorptivity of ε = 500 L mol−1 cm−1. a

moieties (Figure 3). The cyclic voltammogram of phenylene derivative 2 exhibited four reversible one-electron oxidation

Figure 3. Proposed structures for the oxidized species 12+ and 22+.

waves at E1/2ox = −0.23, −0.13, +0.26, and +0.41 V due to the formation of the radical cation, di(radical cation), dicationradical cation, and tetracation species, respectively.18 The first two-electron oxidation potentials of 2 were cathodically shifted by −0.21 and −0.28 V, respectively, compared with those of 1. This is attributed to the delocalized radical cations over the pphenylenediamine units (Figure 3). After the second oxidation, the two carbazole units of compound 1 become electronically insulated from each other. These phenomena could occur when the each 9,10-anthrylene linker gets locked into a perpendicular (relative to carbazole plane) conformation, however, the sterically smaller phenylene linkers could retain coplanarity through all four oxidations. The absorption spectra of 1 and 2 were measured in CH2Cl2 (Figure S6 and Table S1). Compound 1 exhibited a broad band in the range 450−560 nm, whereas 2 gave an absorption band 10700


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Figure 4. Absorption spectra on oxidation states by TBPA·SbF6 of 1 (a) and 2 (b) in CH2Cl2 (black: neutral; red: 1 equiv of oxidant; blue: 2 equiv of oxidant).

(LUMO) were mainly located on the Cz units, respectively (Figure S9), whereas the 313β (HOMO) appears to be located on the 9,10-bis(phenylamino)anthracene units. On the basis of these results, the broad absorption peaks of 1•+ in the longest wavelength region are assigned to the charge-resonance (CR) band (312β → 314β) and the intramolecular charge-transfer (CT) absorption (313β → 314β).19 On the other hand, the di(radical cation) of 1 exhibited a 2-fold increased absorption coefficient compared with that of the radical cation 1•+ due to the accumulated two radical cations of the Cz units, and a slightly blue-shifted absorption peak relative to 1•+ as a result of the weak electrostatic repulsion between two radical cations of the Cz unit.20 The calculated excitaition at the longest wavelength and the contributing transitions are as follows: di(radical cation) of 1, 0.8160 eV, 1519 nm (f = 0.038, transition: 311β → 313β and 312β → 314β). As the 312β (HOMO) and 314β (LUMO+1) were mainly located on the Cz units and 9,10-bis(phenylamino)anthrylene units, respectively (Figure S10), the absorption in the longest wavelength region is assigned to the inverse intramolecular CT absorption (312β → 314β) compared to 1•+. The electronic spectra of the radical cation and di(radical cation) prepared from 2 exhibited absorption bands at 1265 and 976 nm, whereas it showed no NIR absortion band, which was in contrast to the result obtained for the same oxidation states of 1 (Figure 4b). This observation suggests that the π-conjugation of 2 is slightly larger than that of 1 due to the nonbulky groups of the phenylenediamine units. In addition, the stability of the radical cation and di(radical cation) of 2 is attributed to the phenylenediamine units being easy-to-make semiquinone structures (Figure S11 and S12).13 In conclusion, we synthesized the novel azacyclophanes 1 comprising Cz and 9,10-anthrylene units and its phenylene analogue 2 via Buchwald−Hartwig coupling reactions. These compounds were highly stable at oxidation states. The radical cation of the anthrylene derivative 1 showed the intramolecular CT interaction from the HOMO of the 9,10-diaminoanthrylene units to the LUMO of the Cz units at one-electron oxidation, while the di(radical cation) of 1 showed the inverse CT interaction compared to the radical cation of 1. We succeessfully elucidated the electronic properties at the oxidation state of Cz-based azacyclophane due to the sterically hindered 9,10-antrhylene units. Further studies involving the synthesis of azacyclophanes with substituted anthrylene units and their application in the design of organic spin electronic devices are in progress.

oxidant were determined using UV−vis titration (Figure S8). Upon oxidation, the solution color of 1 changed dramatically from orange (neutral) to brown (radical cation) and then to green [di(radical cation)]. The absorption data of the radical cation and di(radical cation) of 1 and 2 are summarized in Table S2. The electronic spectrum of the radical cation prepared from 1 with 1 equiv of TBPA·SbF6 revealed a very broad absorption in the near-infrared (NIR) region (λmax > 2000 nm) in addition to the absorption at 770 nm (Figure 3a). TD-DFT calculations using the UB3LYP/6-31G(d) level were performed to clarify the electronic features of these compounds. The calculated excitation at the long wavelength and their contributing transition are as follows: 1•+, 0.4717 eV, 2629 nm ( f = 0.1616, transition: 313β → 314β) and 0.5518 eV, 2247 nm ( f = 0.1870, transition: 312β → 314β). The calculated MOs of 1•+ indicated that the 312β (HOMO−1) and 314β

Melting points are uncorrected. NMR spectra were measured on a 400 spectrometer (1H: 400 MHz, 13C: 100 MHz). High-resolution mass spectra were measured by the FAB method (double-focusing magnetic sector analyzer). The FAB-MS spectra were recorded with mnitrobenzyl alcohol as a matrix. UV spectra were measured with a 10 mm cell in CH2Cl2 solution. Cyclic voltammograms were performed using a cell equipped with a glassy carbon as working electrode (diameter: 3 mm), a platinum wire as the counter electrode, Ag wire/0.01 M AgNO3 (acetonitrile) as the reference electrode, and 0.1 M n-Bu4NPF6 as the supporting electrolyte in CH2Cl2 solution. Column chromatography was carried out with Silica gel 60N (70−100 mesh) and Silica gel 60 (NH). DFT Calculation. The calculations were carried out using the Gaussian 09W program17 on a Windows computer. The structures of 1′ and 2′ (in which methoxy groups are replaced by H atoms) were optimized by the hybrid DFT method at the B3LYP/6-31G(d) level.

at 323 nm and a shoulder band extending to ca. 450 nm. This significant red-shift for 1 relative to 2 is attributed to the introduction of the 9,10-anthrylene units; however, the absorption coefficient was small. This weak absorption of 1 is most likely assignable to a forbidden transition comparable to the π−π* transition from the Cz to the anthrylene units obtained by time-dependent DFT (TD-DFT) calculations (Figure S8, 2.29 eV or 541 nm). In contrast, the broad absorption band of 2 could be assigned to the transition from HOMO to LUMO+1 using TD-DFT calculations (Figure S7, 2.96 eV or 419 nm). Compounds 1 and 2 exhibited unique electrospectroscopic properties depending on their oxidation state. The chemical oxidation of 1 or 2 with n equiv of tris(4-bromophenyl)aminium hexafluoroantimonate (TBPA·SbF6) as an oxidant in CH2Cl2 afforded the corresponding cationic species 1n+ or 2n+ (n = 1 or 2), respectively (Figure 4). The amounts of the added

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EXPERIMENTAL SECTION



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The frequency analysis was carried out for the global minimum structures of all compounds and resulted in no imaginary frequencies. The calculations of the excited states of 1′ and 2′ were performed by TD-DFT method at the B3LYP/6-31G(d) level of theory to afford the excitation energies, oscillator strengths, transition velocity dipole moments, and transition magnetic dipole moments for the lowest 20 excited states. Each excitation was treated as a Gaussian-type function with a half bandwidth of 1500 cm−1. 9,10-Bis[(4-methoxyphenyl)amino]anthracene (4). A solution of 9,10-dibromoanthracene (1.80 g, 5.36 mmol), p-anisidine (1.45 g, 18.7 mmol, 2.2 equiv), Pd(t-Bu3P)2 (137 mg, 268 μmol, 5 mol %), and NaOt-Bu (1.80 g, 18.7 mmol, 3.5 equiv) in degassed toluene (60 mL) was refluxed for 4 h under Ar. The solvent was then removed by evaporation, and the residue was dissolved in CH2Cl2 (150 mL × 2). The organic solution was washed with water (100 mL), dried over Na2SO4, and evaporated. The crude product was purified by chromatography on silica gel (60N) with CH2Cl2 eluent to give the desired product 4 as an orange solid. Yield 1.85 g (82%); mp 248−251 °C; 1H NMR (CDCl3, δ, 400 MHz) 3.73 (s, 6H), 5.90 (s, 2H), 6.58 (d, J = 8.4 Hz, 4H), 6.74 (d, J = 8.8 Hz, 4H), 7.43 (dd, J = 3.2, 6.8 Hz, 4H), 8.24 (dd, J = 3.2, 6.8 Hz, 4H); 13C NMR (CDCl3, 100 MHz) δ 55.7, 114.8, 115.2, 124.3, 125.8, 129.4, 131.8, 142.2, 152.8; HRMS−FAB m/z [M]+ calcd for C28H24N2O2: 420.1838, found 420.1851. CzAnt2mer (1). A solution of 3,6-dibromocarbazole 3 (620 mg, 1.44 mmol), anthracene 4 (605 mg, 1.44 mmol), Pd(t-Bu3P)2 (89 mg, 174 mmol, 12 mol %), and NaOt-Bu (415 mg, 4.32 mmol) in degassed toluene (100 mL) was refluxed for 48 h under Ar. The solvent was then removed by evaporation, and the residue was dissolved in CHCl3 (300 mL). The organic solution was washed with water (100 mL), dried over Na2SO4, and evaporated. The crude product was purified by chromatography on silica gel (60N) with hexane/CH2Cl2 (2:1−0:1) eluent to give the desired product 1 as a red solid. This solid was further purified by recrystallization from CHCl3/hexane. Yield 138 mg (14%); mp 415−419 °C (decomp.); 1H NMR (CDCl3, 400 MHz) δ 3.69 (s, 12H), 3.89 (s, 6H), 6.64 (d, J = 9.2 Hz, 8H), 6.73 (d, J = 8.8 Hz, 8H), 7.13 (d, J = 8.8 Hz, 4H), 7.22 (dd, J = 2.8, 6.4 Hz, 8H), 7.29 (d, J = 8.8 Hz, 4H), 7.34 (s, 4H), 7.51 (d, J = 8.8 Hz, 4H), 7.71 (d, J = 6.8 Hz, 4H), 8.21 (dd, J = 3.2, 6.8 Hz, 8H); 13C NMR (CDCl3, 100 MHz) δ 55.5, 55.6, 110.3, 114.4, 114.6, 115.0, 120.75, 120.77, 123.6, 125.5, 126.0, 128.6, 132.0, 137.6, 138.0, 141.9, 144.0, 153.4, 158.7 (one aromatic peak is missing); HRMS−FAB m/z [M]+ calcd for C94H70N6O6: 1378.5357, found 1378.5365. CzPh2mer (2). This compound was prepared from 3,6dirbromocarbazole 3 (208 mg, 0.48 mmol), diphenylamine 5 (154 mg, 0.48 mmol), Pd(t-Bu3P)2 (30 mg, 59 μmol, 12 mol %) and NaOtBu (139 mg, 1.45 mmol) according to a similar procedure to the synthesis of 1. The reaction mixture was refluxed for 24 h. The crude product was purified by chromatography on silica gel (NH) with hexane/CH2Cl2 (1:1−0:1) eluent to give the desired compounds as a greenish yellow solid. This material further purified by preparative size exclusion chromatography with toluene as eluent. Yield 82.0 mg (29%); mp 265−268 °C; 1H NMR(benzene-d6, 400 MHz) δ 3.20 (s, 6H), 3.24 (s, 12H), 6.67 (d, J = 8.0 Hz, 4H), 6.69 (d, J = 9.2 Hz, 8H), 7.11 (d, J = 8.0 Hz, 4H), 7.12 (s, 8H), 7.16 (d, J = 8.8 Hz, 4H), 7.21 (d, J = 9.2 Hz, 8H), 7.24 (dd, J = 2.0, 8.8 Hz, 4H), 8.23 (d, J = 2.4 Hz, 4H); 13C NMR (benzene-d6, 100 MHz) δ 54.0, 111.1, 115.1, 115.3, 117.2, 123.0, 124.7, 125.3, 126.7, 128.8, 130.9, 138.9, 142.3, 142.6, 144.0, 156.0, 159.1 (one methoxy peak is missing); HRMS−FAB m/z [M]+ calcd for C78H62N6O6: 1178.4731, found 1178.4712.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel/Fax: +81 86 259 9779. *E-mail: [email protected]. ORCID

Tetsuo Iwanaga: 0000-0002-2224-3746 Keiji Okada: 0000-0002-2817-9335 Present Address ∥

Department of Chemistry, Graduate School of Engineering Science, Osaka University, Machikaneyama, Toyonaka, Osaka 560−8531, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by JSPS KAKENHI Grant-inAid for Young Scientists (B) (Grant Number 16K17874) and Grant for Promotion of OUS Research Projects. The authors thank Prof. Haruo Yamada and Prof. Kan Wakamatsu of Okayama University of Science for their useful suggestions. The authors thank Prof. Takashi Kubo and Dr. Tomohiko Nishiuchi of Osaka University for the measurements of UV−vis−NIR absorption spectra. The authors thank Prof. Kazunari Yoshizawa, Prof. Yoshihito Shiota and Dr. Takashi Kamachi of Kyushu University for the useful suggestion of DFT calculations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b01688. 1 H and 13C NMR Spectra of all new compounds, electronic spectra, UV−vis−NIR titration and computational results (PDF) 10702


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Oxidation State-Dependent Intramolecular Electronic Interaction of

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