U-Shaped Helical Azaarenes: Synthesis, Structures, and Properties

Article abstract of DOI:10.1021/acs.joc.9b02895

The synthesis and properties of a series of U-shaped helical azaarenes are reported. Crystal structures of these helical azaarenes were obtained, and the solid-state structures unequivocally exhibited their helicity.

Full text of DOI:10.1021/acs.joc.9b02895

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U-shaped Helical Azaarenes: Synthesis, Structures and Properties
Kexiang Zhao, Guankui Long, Wenbo Liu, Dong-Sheng Li, Weibo Gao, and Qichun Zhang
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b02895 • Publication Date (Web): 04 Dec 2019
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U-shaped Helical Azaarenes: Synthesis, Structures
and Properties
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†
‡
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‡
Kexiang Zhao, Guankui Long, Wenbo Liu, Dong-Sheng Li, Weibo Gao, and Qichun
Zhang*,†
†School of Materials Science and Engineering, Nanyang Technological University, Singapore
6
39798, Singapore
‡Division of Physics & Applied Physics, School of Physical and Mathematics Science, Nanyang
Technological University Singapore, 639798 Singapore, Singapore
§College of Materials and Chemical Engineering, Key Laboratory of Inorganic Nonmetallic
Crystalline and Energy Conversion Materials, China Three Gorges University, Yichang, Hubei
443002, PR China
* Email: qczhang@ntu.edu.sg (Q.Z.).
ABSTRACT The synthesis and properties of a series of U-shaped helical azaarenes are
reported. Crystal structures of these helical azaarenes were obtained, and the solid-state
structures unequivocally exhibited their helicity.
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In the past few decades, we have witnessed the rapid progress of N-heteroarenes in
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organic electronics, especially in organic field effect transistors (OFETs). However, most
of the N-heteroarenes are synthesized in linear shape due to the lack of suitable building
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blocks to construct non-linear backbones. Consequently, the majority of non-linear N-
heteroarenes are HAT (1,4,5,8,9,12-hexaazatriphenylene)-based star-shaped azaarenes
employing the commercially available hexaketocyclohexane as the building block.4
Among the existing non-linear N-heteroarenes, helical azaarenes are seldomly reported
compared with star-shaped analogues.5
Currently, the majority of helical azaarenes are azahelicenes. Although azahelicenes
were synthesized prior to carbohelicenes, the latter has received more attention because
of their good functionality and the versatile synthetic approaches towards them. Until
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recently, more interesting properties have been observed in azahelicenes. The
incorporation of nitrogen atoms could alter the electronic structure and allow the fine
tuning of optoelectrical properties. Based on the nitrogen-containing heterocycles,
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azahelicenes could be classified into several types: pyridine-containing ones, pyrroles,
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pyridazine derivatives, etc. It is noteworthy that pyrazine-containing azahelicenes are
extremely rare.10 Such gap prompts us to synthesize novel pyrazine-containing helical
azaarenes (U7, U9 and U11).
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Starting from 2,7-dimethoxyanthracene 1 , demethylation using BBr in boiling
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dichloromethane furnishes 2 in quantitative yield; followed by oxidizing 2 with
benzeneseleninic acid anhydride (BSA) to tetraketone 3 in 71% yield. The two-fold
condensation reactions between tetraketone 3 and diamines 4, 5 and 6 gave the helical
azaarenes in moderate to good yields (Scheme 1).
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Scheme 1. Synthesis of Helical Azaarenes U7, U9 and U11.
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gap[ev] mes. /cal.b
d
Comp.
ABSmax
λ
onset
EMSmax
[nm]
LUMO [ev]
mes. /cal.
HOMO [ev]
mes. /cal.
a
b
c
b
[nm]
[nm]
482
594
701
U7
U9
471
561
664
478
602
692
-3.08/-2.82
-3.27/-3.07
-3.77/-3.53
-5.65/-5.68
-5.36/-5.25
-5.54/-5.31
2.57/2.86
2.09/2.18
1.77/1.78
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U11
Table 1. Optical and electronic properties of U7, U9 and U11
[
a] LUMOmes.= - (Ered(onset) + 4.8) (eV); [b] DFT calculations (B3LYP/6-31G*) using Gaussian 09;
TMS groups were used instead of TIPS; [c] EHOMO=ELUMO(CV) - Egap(opt) (eV); [d] gapmes was
calculated from 1240nm/λonset
.
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Figure 1. Normalized absorption spectra of U7, U9 and U11 in dichloromethane (10 M).
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Figure 1 shows the normalized absorption spectra of U7, U9 and U11. There is an obvious
increase in λmax (Table 1) from U7 to U11, which indicates the size-dependent
bathochromic shift in the absorption. Compared with the reported S-shape counterparts12
(
Figure S15), helical azaarenes show small hypsochromic shifts in λmax. It is worth
mentioning that the helical azaarenes have almost identical optical band gaps to their
constitutional isomers, which exhibits the similarity in their optical properties (Table S1).
Unlike the constitutional isomers, cyclic voltammetry of helical azaarenes was performed
in DCM at room temperature due to their much-improved solubility. The measured
LUMOs of helical azaarenes are apparently shallower than that of S-shaped isomers.
This could be ascribed to their twisted helical structures. The two pyrazine units near the
centre are highly distorted because of the torsional strain of the helix. Hence, the
conjugation in these two pyrazine units is rather weak resulting in smaller π-systems
compared to S-shaped azaarenes, which further leads to the shallower LUMOs. Taking
U11 as an example, unlike its S-shaped isomer whose LUMO is deeper than the linear
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TIPS-TAP, U11 has a shallower LUMO than TIPS-TAP. The calculated energy levels
are in good agreement with the experimental values and also show the same trend (Table
1).
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The crystals suitable for X-ray analysis were obtained via slow diffusion of methanol vapor
into the chlorobenzene solutions of helical azaarenes. The solid-state structures and
stacking patterns are shown in Figure 2. From the crystal structures, one can conclude that
the helicity of these azaarenes is caused by the steric hindrance between the two inner TIPS
groups. Moreover, all three racemates are racemic compounds, which means that the P and
M-isomers are both present in the crystal with the ratio of 1:1. The packing motifs of U7,
U9 and U11 are similar. P and M-isomers pack in pairs such that the arms of the helix are
on top of each other to different extents in an inverted manner. U9 has the largest
overlapping area and longest stack distance, whereas U11 possesses the smallest
overlapping area but the shortest stack distance. U7, on the other hand, bears intermediate
overlapping area and stack distance. The TIPS groups strongly hinder the π–π stacking due
to their bulkiness, and the helical backbone further obstructs the effective π–π stacking.
Consequently, helical azaarenes possess much improved solubility compared with their S-
shaped counterparts, and they could be easily dissolved in chlorinated solvents and other
common solvents such as dichloromethane, hexane, THF, DMF and toluene.
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Figure 2 Crystal structures and packing motifs of U7 (a), U9 (b) and U11 (c). ORTEP plot of U9
(d). Hydrogen atoms and solvent molecules are omitted for clarity. Thermal ellipsoids drawn at the
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0% probability level.
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The interplane angles of U7, U9 and U11 are 38.9⁰, 48.2⁰ and 39.7⁰, respectively. The sum
of torsional angels increases from 52.7⁰ to 82.0⁰ as the helical skeleton becomes larger.
Unlike the common helicenes, where all the aromatic rings are fused inward encircling the
axis, there are only two phenanthrene segments folding inward in the as-obtained helical
azaarene molecules. Thus, although the interplane angle of U9 is significantly larger than
that of U7, there is little difference in the sum of their torsional angles. Upon close
inspection on their torsional angles (Table S2), it is found that the largest torsional angles
of U7, U9 and U11 are almost identical. More interestingly, all of those largest torsional
angles belong to the phenanthrene segments, which substantiates the similarity in helical
skeletons among the as-obtained helical azaarenes. Although the helicity of U-shaped
azaarenes is clearly shown here, one should not mistreat the U-shaped helical azaarenes as
azahelicenes, since they do not comply with the definition of helicenes. Instead, the U-
shaped azaarenes might unveil another bulky-substituent-induced helical configuration
other than the traditional helicenes.
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In conclusion, a series of helical azaarenes have been prepared by the condensation
reactions between tetraone 3 and diamines 4 – 6. The optical and electronic properties of
as-obtained compounds were investigated and compared with their S-shaped constitutional
isomers. Helical skeletons of those helical azaarenes were unequivocally revealed by the
single crystal X-ray analysis.
EXPERIMENTAL SECTION
Materials. Reagents and solvents for synthesis were, if not otherwise specified, purchased from
Sigma Aldrich and used without further purification. THF and DCM were dried over sodium and
CaH respectively, then freshly distilled prior to use. Technical grade dichloromethane (DCM) and
2
hexane for column chromatography were purchased from Aik Moh and used as received.
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Benzeneseleninic anhydride was purchased from ACROS Organics. Compounds 1 , 4 , 5 and
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were synthesized according to the reported literature.
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Methods. H NMR and 13C NMR spectra were recorded on a Bruker Advance 300 NMR
1
spectrometer at ambient temperature with tetramethylsilane (TMS) as an internal standard. High
resolution mass spectrum (HRMS) was performed on a Waters Q-Tof premier mass spectrometer.
UV-Vis absorbance was recorded on UV-Vis-NIR spectrometer Cary 5000. Cyclic voltammetry
measurements were conducted on a CHI 604E electrochemical workstation, where platinum wires
were used as bath counter electrode and reference electrode and glassy carbon (diameter: 1.6 mm;
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area: 0.02 cm ) was employed as working electrode. Ferrocene was used as an external standard.
+
Potentials were recorded versus Fc /Fc in anhydrous DCM with tetrabutylammonium
-1
hexafluorophosphate (TBAPF , 0.1 M) as supporting electrolyte at a scan rate of 100 m Vs . After
6
the acquirement of E (0.25 eV in our setup), the value of LUMO can be calculated applying
Fc
empirical formulas ELUMO = -[4.8 - E + E ro en set] eV. Optical bandgaps were calculated from 1240
Fc
nm/λonset. The value of HOMO level is then obtained using EHOMO = ELUMO – Egap(opt) (eV). Crystal
structure analysis was accomplished using diffractometers with Mo Kα radiation. The heat source
used in this work was an oil bath over the Heidolph Hei-Tec magnetic stirrer, equipped with Pt
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000 temperature probe.
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,7-Dihydroxyanthracene (2). In a heatgun-dried Schlenk flask purged with argon, 1 (477 mg, 2
mmol, 1 eq) was dispersed in dry DCM (30 ml). The suspension was cooled to 0 °C using an
ice/water bath, followed by the dropwise addition of BBr (1 ml, 5 eq). Then the cooling bath was
3
removed, and the mixture was heated to reflux in an oil bath for 4h. After being cooled to 0 °C
again, ice-cold water was carefully added to the mixture to quench the unreacted BBr . The
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resulting slurry was extracted with diethyl ether (3 x 50 ml), and the organic phase was combined
and dried with sodium sulfate. The removal of solvent under reduced pressure gave 2 as light
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brown solid (420 mg, quant.). mp 159 °C; H NMR (300 MHz, Acetone-d ) δ = 8.49 (s, 2H), 8.18
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(
s, 1H), 7.84 (s, 1H), 7.76 (d, J = 9.1 Hz, 2H), 7.07 (d, J = 2.3 Hz, 2H), 6.96 (dd, J = 9.1, 2.4 Hz,
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H); C{ H} NMR (75 MHz, DMSO-d ) δ = 154.7, 133.2, 129.8, 125.9, 125.3, 119.8, 118.9,
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105.9;IR νmax 3392, 3218, 1632, 1592, 1547, 1455, 1410, 1378, 1330, 1275, 1227, 1198, 1169,
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+
1137, 972, 893, 843, 798, 745, 648, 591, 549, 476, 422 cm ; HRMS (ESI) m/z [M + H] calcd for
C H O 211.0759, found 211.0758.
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Anthracene-1,2,7,8-tetraone (3). In a heatgun-dried round bottom flask, benzeneseleninic acid
anhydride (1.8 g, 5 mmol) was suspended in dry THF (80 ml) under argon. To the stirring
suspension in an oil bath, a solution of 2 (0.5 g, 2.38 mmol) in dry THF (40 ml) was added dropwise
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at 50 °C. After the addition, heat was turned off, and the resulting deep red solution was allowed
to stand overnight. Upon the completion of reaction, the solvent was removed in vacuo. The
residual was washed with hot diethyl ether (15 ml), and DCM (10 ml) then dried in vacuo. 3 was
obtained as deep reddish brown solid (403 mg, 71%). This product was used directly in next step
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without further purification. H NMR (300 MHz, THF-d ) δ = 6.81 (s, 1H), 5.86 (s, 2H), 5.83 (s,
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1H), 4.77 (d, J = 10.2 Hz, 2H); C{ H} NMR was not taken due to the poor solubility of 3 in
common solvents; IR νmax 3052, 1706, 1665, 1602, 1586, 1429, 1322, 1261, 1230, 1177,
-1
+
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166, 1124, 1088, 945, 914, 838, 796, 675, 524 cm ; HRMS (ESI) m/z [M + H] calcd for
C H O 211.0759, found 211.0758.
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General procedure (GP). To a round bottom flask equipped with a condenser, 3 (20 mg, 0.084
mmol) and diamine (0.2 mmol) were added to a solvent mixture of chloroform (5 ml) and acetic
acid (15 ml). The stirring solution was heated to 100 °C in an oil bath for 1 d. The mixture was
cooled to RT, and the solvent was removed in vacuo. This crude product was further purified by
column chromatography on silica gel using a Hexane/DCM solvent mixture.
U7. GP was applied to 3 (20 mg) and 4 (94 mg). Column chromatography (Hexane/DCM = 5:1)
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yielded U7 as a yellow crystalline solid (54 mg, 58 %). mp 337 °C; H NMR (300 MHz, CDCl )
3
δ = 10.99 (s, 1H), 8.37 (s, 1H), 8.17 (d, J = 9.3 Hz, 2H), 8.02 (d, J = 9.3 Hz, 2H), 7.99 (s, 4H),
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.29 (s, 42H), 1.23 – 1.14 (m, 6H), 1.02 (dd, J = 10.8, 6.9 Hz, 36H); C{ H} NMR (75 MHz,
CDCl ) δ = 144.2, 143.4, 143.1, 141.4, 134.3, 133.5, 133.4, 133.3, 130.2, 129.2, 127.1, 125.7,
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125.0, 123.6, 104.0, 103.8, 101.7, 100.3, 18.93, 18.87, 11.7, 11.6; IR νmax 2943, 2891, 2865,
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152, 1491, 1462, 1350, 1319, 1243, 1213, 1127, 1060, 1033, 1017, 996, 922, 884, 846,
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790, 700, 677, 587, 544, 500, 462, 419 cm ; HRMS(ESI) m/z [M + H] , calcd for C H N Si ,
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0
95
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104.6630, found 1104.6643.
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U9. GP was applied to 3 (20 mg) and 5 (104 mg). Column chromatography (Hexane/DCM = 4:1)
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yielded U9 as a red crystalline solid (81 mg, 80 %). mp > 350 °C; H NMR (300 MHz, CDCl ) δ
3
=
11.01 (s, 1H), 8.91 – 8.74 (m, 4H), 8.28 (s, 1H), 8.10 (d, J = 9.4 Hz, 2H), 7.98 (d, J = 9.4 Hz,
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1
2
H), 7.80 – 7.66 (m, 4H), 1.36 (s, 42H), 1.23 – 1.14 (m, 6H), 1.03 (t, J = 7.3 Hz, 36H); C{ H}
NMR (75 MHz, CDCl ) δ = 145.3, 144.3, 141.0, 139.3, 135.0, 134.8, 134.7, 133.9, 130.5, 129.7,
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127.9, 127.8, 127.61, 127.56, 127.50, 126.3, 121.5, 120.0, 109.3, 107.3, 103.1, 102.8, 18.99, 18.95,
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1.71, 11.67; IR νmax 3060, 2942, 2890, 2864, 2135, 1614, 1517, 1463, 1386, 1342, 1315,
246, 1217, 1115, 1043, 1014, 995, 920, 883, 827, 762, 747, 678, 597, 571, 513, 484,
1
4
-1
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14 cm ; HRMS(ESI) m/z [M + H] calcd for C H N Si , 1204.6943, found 1204.6902.
7
8
99
4
4
U11. GP was applied to 3 (20 mg) and 6 (114 mg). Column chromatography (Hexane/DCM = 2:1)
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yielded U11 as a dark purple crystalline solid (49 mg, 45 %). mp > 350 °C; H NMR (300 MHz,
CDCl ) δ = 11.00 (s, 1H), 8.34 – 8.25 (m, 4H), 8.23 (s, 1H), 8.10 (d, J = 9.4 Hz, 2H), 7.98 (d, J =
3
9
3
.4 Hz, 2H), 7.91 – 7.83 (m, 4H), 1.41 (s, 42H), 1.26 – 1.17 (m, 6H), 1.09 (dd, J = 7.0, 2.9 Hz,
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1
6H); C{ H} NMR (75 MHz, CDCl ) δ = 146.4, 145.3, 145.13, 145.07, 143.2, 142.91, 142.88,
3
1
1
41.2, 135.4, 135.0, 131.8, 131.6, 130.73, 130.68, 130.5, 130.0, 128.0, 127.1, 123.7, 121.6, 113.6,
11.0, 103.1, 102.8, 19.02, 18.93, 11.83, 11.80; IR νmax3063, 2942, 2890, 2864, 2348, 1613, 1530,
1
501, 1461, 1382, 1356, 1314, 1246, 1228, 1188, 1128, 1071, 1042, 1020, 996, 921, 883, 754,
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20, 677, 588, 512, 460, 415; HRMS(ESI) m/z [M + H] , calcd for C H N Si , 1309.7061, found
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2
99
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1309.7117.
ASSOCIATED CONTENT
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Supporting Information.
The Supporting Information is available free of charge on the ACS Publications website
at DOI:
HRMS, NMR spectra, PL spectra, cyclic voltammetry, details of X-ray crystal structures
and computational details (PDF)
Crystallographic data of U7, U9 and U11 (CIF)
AUTHOR INFORMATION
Corresponding Author
* Email: qczhang@ntu.edu.sg (Q.Z.).
Notes
The authors declare no competing financial interest
ACKNOWLEDGMENT
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K.Z. thanks Prof. Michinobu for his kind assistance in attempted chiral resolution of helical
azaarenes. Q.Z. acknowledges AcRF Tier 1 (Grants RG 111/17, RG 2/17, RG 114/16, RG
1
13/18) and Tier 2 (Grants MOE 2017-T2-1-021 and MOE 2018-T2-1-070), Singapore.
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