Efficient synthesis and characterization of dibenzo[a,m]rubicenes and tetrabenzo-[a,f,r,m]rubicenes

Article abstract of DOI:10.1021/jo9015437

(Chemical Equation Presented) We report an efficient synthesis of dibenzo[a,m]rubicenes and tetrabenzo[a,f,r,m]rubicenes involving ICl-mediated benzannulation of 1,4-diphenyl-2,5-dialkynylbenzene 5 as the key step. Preliminary data on the photophysical pr

Full text of DOI:10.1021/jo9015437

pubs.acs.org/joc
luminescene,6 and application as a chemical sensor for metal
Efficient Synthesis and Characterization of
Dibenzo[a,m]rubicenes and Tetrabenzo-
[a,f,r,m]rubicenes
cations.6a Large PAHs such as hexabenzocorenene show
outstanding hole transport properties because of a strong
interaction to support columnar (liquid)-crystalline meso-
phases.7 To increase the ring sizes of rubicene frameworks,
herein we report an efficient synthesis of dibenzo
[a,m]rubicene (2) and tetrabenzo[a,f,r,m]rubicene (3) (Scheme 1);
the synthesis and characterization of these compounds remains
virtually unknown.8
Hung-Hung Hseuh, Ming-Yu Hsu, Tien-Lin Wu, and
Rai-Shung Liu*
Department of Chemistry, National Tsing-Hua Univeristy,
Hsinchu, Taiwan, ROC
SCHEME 1
rsliu@mx.nthu.edu.tw
Received July 30, 2009
Scheme 2 shows the synthetic protocol of dibenzo[a,m]-
rubicene 8 and 9 with ICl-mediated benzannulation9,10 of
1,4-diphenyl-2,5-dialkynylbenzene 5 as the key step. The
targeted molecules 8 and 9 bear four tert-butyl groups to
increase solubility in commonly used solvents. Preparation
of key species 5 begins with Sonogashira coupling11 of 1,4-
dibromo-2,5-diiodobenzene 3, followed by Suzuki cou-
pling12 of the resulting 1,4-dibromo-2,5-diakynylbenzene 4.
Treatment of compound 5 with ICl (2.2 equiv) in CH2Cl2 at
23 °C induced a 6-endo-dig benzannulation to give 6,13-
diiodobenzo[k]tetracene 6 in 74% yield; its subsequent Su-
zuki coupling with 4-substituted phenylboric acid afforded
5,6,12,13-tetraphenylbenzo[k]tetracene 7a and 7b, in 71%
and 70% yields, respectively. In the final step, we employed
FeBr312 in excess proportion (4 equiv) to transform species
7a and 7b into the desired dibenzo[a,m]rubicene 8 and 9 in
59% and 68% yields, respectively; this oxidative coupling
We report an efficient synthesis of dibenzo[a,m]rubicenes
and tetrabenzo[a,f,r,m]rubicenes involving ICl-mediated
benzannulation of 1,4-diphenyl-2,5-dialkynylbenzene 5
as the key step. Preliminary data on the photophysical
properties of these new indeno-PAHs are reported.
Significant attention has been directed to the synthesis of
indeno-PAHs, which are polycyclic aromatic hydrocarbons
(PAHs) containing one or more five-membered carbocyclic
units.1,2 Known for a century, rubicene (1) is an interesting
heptacyclic arene of which an efficient synthesis was
achieved a decade ago.3,4 The interesting properties and uses
of rubicene include a red color, yellow fluorescence, a large
rate of intersystem crossing (ISC),5 electrochemically generated
€
(7) (a) Iyer, V. S.; Wehmeire, M.; Brand, J. D.; Keegstra, M. A.; Mullen,
€
K. Angew. Chem., Int. Ed. 1997, 36, 1604. (b) Watson, M. D.; Fechtenkotter,
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no details for its synthesis and characterization. See: Farbenind, I. G. DE
655649, 1936; FTFVA6, Fortschr, Teerfarbenfahr. Verw. Industriezwige;
DE; German, 24, 965.
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8448 J. Org. Chem. 2009, 74, 8448–8451
Published on Web 10/02/2009
DOI: 10.1021/jo9015437
r
2009 American Chemical Society

Hseuh et al.
JOCNote
SCHEME 2
FIGURE 1. Absorption and emission spectra of compounds 8-11.
yields, respectively; the two compounds were purified chro-
matographically under argon to avoid oxidation with air. In
the presence of FeCl3, we envisage that species 7c and 7d
undergo one-electron oxidation to form a radical cation at
the C(7) carbon of the central benzo[k]tetracene framework
as represented by intermediate A; this reactive cationic center
reacts preferably with the C(2) carbon, rather than the C(5)
carbon, of the tethered naphthyl ring although the C(5)
carbon has a greater effect in the HOMO orbital.14 We
envisage that steric hindrance in the C(5)-addition prohibits
this benzannulation reaction.
SCHEME 3
Dibenzo[a,m]rubicene (8-9, 8-Cl, and 9-Cl) and tetrabe-
nzo[a,f,r,m]rubicene (10-11) are orange and red solids, but
the latter PAHs decomposed completely in CH2Cl2 within
12 h at 25 °C in the presence of air. Species 10 and 11 were
stored safely in sold forms at 0 °C for 24 h under argon. We
characterized these compounds with NMR, Maldi-mass,
and elemental analysis. 1H NMR signals of these new
compounds were assigned with the 1H NOE effect and
sequential proton decoupling; the data appear in the Sup-
porting Information.
Table 1 shows key data for the photopysical properties of
rubicene derivatives 8-10 and their chloro derivatives 8-Cl
and 9-Cl; the dichloromethane solutions were degassed care-
fully to avoid interference from oxygen. These indeno-PAHs
are highly fluorescent with quantum yields of 4.9-10.6%
despite their small energy gaps. We envisage that the at-
tached bulky tert-butyl groups minimize molecular aggrega-
tion to prevent intermolecular energy transfer. Figure 1
shows their absorption spectra measured at 1ꢀ10-6 M in
CH2Cl2. Parent compound 8 has four absorption bands at
3,11 357, 431, and 459 nm, and their substituented deriva-
tives 8-Cl, 9, and 9-Cl retain the same absorptions.
Tetrabenzo[a,f,r,m]rubicenes 10 and 11 have five absorp-
tions at 308, 351, 374, 448, and 476 nm, respectively, and the
latter three absorptions are shifted to large wavelengths by
(Scholl oxidation)13 was performed at a dilute concentration
-2
(1.0 ꢀ 10
M) in dry nitromethane at 23 °C for 10 min.
Under the same conditions, FeCl3 (4 equiv) gave poor yields
(5-12%) of the desired 8 and 9 accompanied by formation of
their chloro derivatives 8-Cl and 9-Cl in 31% and 5% yields,
respectively.
Scheme 3 shows the successful extension of this approach
to the synthesis of tetrabenzo[a,f,r,m]rubicenes 10 and 11.
The starting benzo[k]tetracenes 7c and 7d were prepared
efficiently from the Suzuki coupling11 of compound 6 with
4-R-1-naphthylboric acid (R = H, Me). We found that
oxidation of 7c and 7d with FeBr3 under various concentra-
tions (0.01-0.002 M) was unsuccessful with exclusive reco-
very of unreacted 7c and 7d, but use of FeCl3 (8 equiv) in
dilute solution ([substrate]=0.005 M) led to formation of
tetrabenzo[a,f,r,m]rubicenes 10 and 11 in 69% and 51%
(14) (a) Norman, R. O. C.; Coxon, J. M. In Principles of Organic Synthesis,
3rd ed.; Blackie Academic & Professional: Glasgow, UK, 1993; p 68. (b) Aihara,
J.-i. J. Phys. Chem. 1999, 103, 7487. (c) Ciesielski, A.; Krygowski, T. M.;
ꢀ
(13) King, B. T.; Kroulik, J.; Robertson, C. R.; Rempala, P.; Hilton,
C. L.; Korinek, J. D.; Gortari, L. M. J. Org. Chem. 2007, 72, 2279.
Cyranski, M. K.; Dobrowolski, M. A.; Balaban, A. T. J. Chem. Inf. Model. 2009,
49, 369.
J. Org. Chem. Vol. 74, No. 21, 2009 8449

Hseuh et al.
JOCNote
TABLE 1. Photophysical Properties of Compounds 8-11
compd
absmaxa (nm)
PLmaxa (nm)
Q.Y.a,b (%)
9.1
E1/2ox (V)
HOMO-LUMOc (eV)
band gapd (eV)
2.26
8
311, 354, 428, 453
522
0.86
-5.66
-3.40
-5.68
-3.35
-5.58
-3.28
-5.59
-3.35
-5.46
-3.46
-5.38
-3.40
8-Cl
9
306, 356, 425, 453
303, 312, 358, 431, 459
315, 358, 428, 456
311, 372, 446, 475
372, 441, 475
531
538
536
611
616
4.9
10.6
5.8
0.88
0.78
0.79
0.66
0.58
2.33
2.30
2.24
2.00
1.98
9-Cl
10
8.5
11
4.9
aIn CH2Cl2, 1 ꢀ 10-5 M. bCoumarin I as a standard with a standard deviation of 3%. cThe energy levels of HOMO are determined from CV. dBand
gaps were calculated from uv absorption.
16-18 nm, relative to those of dibenzo[a,m]rubicene 8.
Figure 1b showed normalized photoluminescence (PL) spec-
tra with emission maxima at 524-536 nm for the family of
dibenzo[a,m]rubicenes, and at longer wavelengths of
ca. 611-616 nm for tetrabenzo[a,f,r,m]rubicene species 10
and 11.
1-diyl)bis(tert-butylbenzene) (4) (2.78 g, 5.07 mmol) in 83%
yield. Spectral data for 4: IR (neat, cm-1) 3054 (m), 2918 (m),
2112 (w), 1630 (w), 1539 (w), 1365 (s), 1130 (m); 1H NMR
(400 MHz, CDCl3) δ (ppm) 7.77 (s, 2 H), 7.52 (d, J=8.4 Hz,
4 H), 7.40 (d, J = 8.4 Hz, 4 H), 1.34 (s, 18 H); 13C NMR
(100 MHz, CDCl3) δ (ppm) 152.5, 135.9, 131.5 (2ꢀCH), 126.4,
125.4 (2ꢀCH), 123.7, 119.3, 96.9, 86.4, 34.9, 31.1; HRMS calcd
for C30H28Br2 546.0558, found 546.036.
Dibenzo[a,m]rubicenes (8-9, 8-Cl, and 9-Cl) and
tetrabenzo[a,f,r,m]rubicenes 10 and 11 show one-electron
reversible oxidation in CH2Cl2 according to cyclic voltam-
metry measurements (see the Supporting Information). We
estimated their HOMO-LUMO gaps from the intersection
of the absorption and fluorescence spectrum, giving
2.23-2.33 and 1.98-2.00 V for dibenzo[a,m]rubicenes
(8, 9, 8-Cl, and 9-Cl) and tetrabenzo[a,f,r,m]rubicenes 10
and 11, respectively. We deduced the HOMO energy levels
from their oxidation potentials (E1/2ox), and calculated the
LUMO levels from the measured energy gaps. As shown in
Table 1, the family of dibenzo[a,m]rubicene species have the
HOMO and LUMO orbitals at 5.59-5.67 V and 3.25-3.40
eV, and the two tetrabenzo[a,f,r,m]rubicenes have corre-
sponding values at 5.38-5.46 V and 3.40-3.45 eV, respec-
tively. The observed low-lying LUMO and high-lying
HOMO indicate that these new indeno-PAHs may have
ambipolar properties to conduct both electrons and holes
transport efficiently.
In summary, we have developed a new and efficient
synthesis of dibenzo[a,m]rubicenes and tetrabenzo[a,f,r,m]-
rubicenes involving ICl-mediated benzannulation of 1,4-
diphenyl-2,5-dialkynylbenzene 5 as the key step. These in-
deno-PAHs absorb light in the visible light regions, and have
small energy gaps (2.33-1.99 eV) and fair quantum yields.
Their HOMO-LUMO energy levels indicate ambipolar
properties. Further application of these indeno-PAHs in
materials application is under current investigation.
Synthesis of 1,4-Di(tert-butylphenyl)-2,5-di(tert-butylacetyle-
nyl)benzene (5). To a solution of dppf (282.5 mg, 0.51 mmol) in
dioxane (20 mL) was added 4,40-(2,5-dibromo-1,4-phenylene)-
bis(ethyne-2,1-diyl)bis(tert- butylbenzene) (4) (2.78 g, 5.1 mmol)
at room temperature, and the solution was degassed with
nitrogen. To this mixture was added 4-tert-butylphenylboronic
acid (2.17 g, 12.2 mmol), Pd(OAc)2 (282.5 mg, 0.51 mmol,
10 mol %), and 5 M NaOH(aq) (3 mL, 15.3 mmol), and the
resulting mixture was heated to 105 °C for 6 h. After completion
of the reaction, the solution was cooled to room temperature;
the aqueous layer was extracted with dichloromethane, washed
with brine solution, dried over MgSO4, and concentrated. The
residues were purified by washing several times with acetone to
give compound 5 as a white solid (2.42 g, 3.72 mmol, 73% yield).
Spectral data for 5: IR (neat, cm-1) 3051 (m), 2921 (m), 2110 (w),
1
1620 (w), 1541 (w), 1362 (s), 1130 (m); H NMR (400 MHz,
CDCl3) δ 7.68 (s, 2H), 7.70 (d, J=8.0 Hz, 4H), 7.48 (d, J=8.0
Hz, 4H), 7.29-7.24 (m, 8H), 1.39 (s, 18H), 1.29 (s, 18H); 13C
NMR (100 MHz, CDCl3) δ 151.5, 150.5, 142.0, 136.6, 130.2,
133.7(4ꢀCH), 131.2(4ꢀCH), 125.3 (2ꢀCH), 124.9 (4ꢀCH),
121.7, 120.4, 93.8, 89.0, 34.8, 34.6, 31.4, 31.2; HRMS calcd for
C50H54 654.4226, found 654.421. Anal. Calcd for C50H54: C,
91.69; H, 8.31. Found: C, 91.56; H, 8.46.
Synthesis of 3,10-Di(tert-butyl)-5,12-di[4-(tert-butyl)phenyl]-
6,13-diiododibenzo[a,h]anthracene (6). To a CH2Cl2 solution
(15 mL) of compound 5 (12.43 g, 3.72 mmol) was added ICl
(1.0 M in CH2Cl2, 3.7 mL, 9.3 mmol) at -78 °C, and the mixture
was stirred at -40 °C for 0.5 h before being quenched with
sodium thiosulfate solution. The solution was extracted with
CH2Cl2, dried over MgSO4, and purified by washing several
times with acetone to give compound 6 as a light yellow solid
(2.49 g, 2.75 mmol) in 74% yield. Spectral data for 6: IR (neat,
cm-1) 3062 (m), 2932 (m), 1628 (w), 1541 (w), 1362 (s), 1130 (m);
1H NMR (400 MHz, CDCl3) δ 9.73 (s, 2H), 8.89 (d, J=8.4 Hz,
2H), 7.79 (dd, J=8.4, 2.0 Hz, 2H), 7.58 (d, J=8.4 Hz, 4H), 7.36
Experimental Section
Synthesis of 4,40-(2,5-Dibromo-1,4-phenylene)bis(ethyne-2,1-
diyl)bis(tert-butylbenzene) (4). To a Et3N solution (20 mL) of
CuI (45.8 mg, 0.24 mmol, 4 mol %) was added compound 3
(3.0 g, 6.12 mmol); the mixture was degassed with nitrogen for
15 min at 23 °C. To the resulting solution was added Pd(PPh3)4
(0.141 mg, 0.12 mmol, 2 mol %); the mixture was stirred for
15 min before addition of 1-tert-butyl-4-ethynylbenzene (2.43 g,
14.7 mmol). The solution was stirred at 25 °C for 3 h, filtered
through a Celite bed, concentrated, and eluted through a silica
column to give 4,40-(2,5-dibromo-1,4-phenylene)bis(ethyne-2,
(s, 2H), 7.28 (d, J=6.4 Hz, 4H), 1.45 (s, 18H), 1.24 (s, 18H); 13
C
NMR (100 MHz, CDCl3) δ 151.1, 150.5, 141.4, 134.8, 133.9,
133.1, 129.5, 129.2, 128.0, 126.8, 125.4, 125.2 (4ꢀCH), 124.2,
123.3, 105.0, 34.9, 34.8, 31.5, 30.8; MALDI-MS calcd for
C50H52I2 906.2158, found 906.262. Anal. Calcd for C50H52I2:
C, 66.23; H, 5.78. Found: C, 66.76; H, 5.56.
Synthesis of 3,10-Di(tert-butyl)-5,12-di[4-(tert-butyl)phenyl]-
6,13-diphenyldibenzo[a,h]anthracene (7a). To a solution of dppf
8450 J. Org. Chem. Vol. 74, No. 21, 2009

Hseuh et al.
JOCNote
(33.18 mg, 0.061 mmol, 10 mol %) in dioxane (20 mL) was added
3,10-di(tert-butyl)-5,12-di[4-(tert- butyl)phenyl]-6,13-diiododibenzo-
[a,h]anthracene (6) (0.56 g, 0.61 mmol) at room temperature,
and the solution was degassed with nitrogen. To this mixture
was added phenylboronic acid (0.17 g, 1.46 mmol) and Pd(OAc)2
(13.6 mg, 0.061 mmol, 10 mol %) and 5 M NaOH(aq) (0.36 mL,
1.83 mmol), and the resulting mixture was heated to 105 °C for
6 h. After completion of the reaction, the solution was cooled to
room temperature and washed with water. The aqueous layer
was extracted with dichloromethane, washed with brine solu-
tion, dried over MgSO4, and concentrated. The residues were
purified by washing several times with acetone to give com-
pound 7a as a light yellow solid (2.42 g, 3.72 mmol) in 71% yield.
Spectral data for 7a: IR (neat, cm-1) 3053 (m), 2916 (m), 1622
(w), 1539 (w), 1362 (s), 1130 (m); 1H NMR (400 MHz, CDCl3)
δ 8.90 (s, 2H), 8.31 (d, J=8.4 Hz, 2H), 7.59 (d, J=8.4 Hz, 2H),
7.51 (s, 2H), 7.33-7.25 (m, 12H), 7.10 (d, J=8.4 Hz, 4H), 1.29
(s, 18H), 1.22 (s, 18H); 13C NMR (100 MHz, CDCl3) δ 148.9,
148.7, 139.8, 137.4, 136.8, 136.5, 131.9, 131.2, 131.1, 130.6,
130.2, 128.2, 127.8, 126.3, 124.3, 124.2, 123.8, 122.3, 121.1,
34.5, 34.1, 31.2, 31.1; MALDI-mass calcd for C62H62
806.4852, found 806.470. Anal. Calcd for C62H62: C, 92.26; H,
7.74. Found: C, 92.00; H, 7.66.
Synthesis of 3,12-Di(tert-butyl)-5,14-di[4-(tert-butyl)phenyl]-
dibenzo[a,m]rubicene (8). Compound 7a (100 mg, 0.124 mmol)
was dissolved in dry CH2Cl2 (2 mL), and the solution was
bubbled with argon for 20 min. To this solution was added
dry CH3NO2 (0.248 mL) and FeBr3 (270 mg, 1.24 mmol)
dropwise at 23 °C; the reaction was quenched on adding
methanol (10 mL) after 30 min. The solvent was removed under
reduced pressure and purified by a silica column to give com-
pound 8 as an orange solid (58.7 mg, 59%). IR (neat, cm-1) 3023
(m), 2915 (m), 2110 (w), 1628 (w), 1539 (w), 1365 (s), 1130 (m);
1H NMR (600 MHz, CD2Cl2) δ ppm 9.15 (d, J=8.4 Hz, 2H),
8.60 (d, J=7.8 Hz, 2H), 7.67 (dd, J=8.4, 1.8 Hz, 2H), 7.64 (d, J=
7.8 Hz, 4H), 7.62 (s, 2H), 7.42 (d, J=7.8 Hz, 4H), 7.22 (t, J=
7.2 Hz, 2H), 6.97 (t, J=7.2 Hz, 2H), 6.69 (d, J=7.2 Hz, 2H), 1.44
(s, 18H), 1.26 (s, 18H); 13C NMR (150 MHz, CD2Cl2) 152.1,
151.2, 141.9, 140.1, 138.3, 136.1 (2ꢀCH), 135.8, 132.7, 132.5,
130.2, 130.0, 129.3, 128.9, 128.5, 128.0, 127.4, 126.6, 125.3,
125.0, 124.6, 124.5, 35.5, 35.4, 32.0, 31.7; MALDI-mass calcd
for C62H58 802.4539, found 802.528. Anal. Calcd for C62H58: C,
92.72; H, 7.28. Found: C, 92.43; H, 7.40.
Spectral data for tetrabenzo[a,f,r,m]rubicenes (10):. IR (neat,
cm-1) 3022 (m), 2914 (m), 2113 (w), 1623 (w), 1532(w), 1353 (s),
1134 (m); 1H NMR (600 MHz, CD2Cl2) δ 9.09 (d, J=8.4 Hz,
2H), 8.72 (d, J=8.4 Hz, 2H), 8.14 (s, 2H), 7.80 (d, J=8.4 Hz,
2H), 7.75-7.73 (m, 4H), 7.64 (d, J=8.4 Hz, 4H), 7.58 (d, J=
8.4 Hz, 4H), 7.20-7.17 (m, 4H), 6.76 (t, J=7.8 Hz, 2H), 1.50 (s,
18H), 1.40 (s, 18H); 13C NMR (150 MHz, CD2Cl2) δ 152.8,
151.5, 142.1, 140.7, 138.8, 136.4 (6ꢀC), 135.9, 135.0, 132.8, 132.6
(4ꢀCH), 130.7, 130.3, 129.9, 129.3, 129.1, 127.9, 127.0 (4ꢀCH),
126.5, 126.2, 125.5, 125.1, 125.0, 123.5, 36.2, 35.9, 32.4, 32.2;
MALDI-MS calcd for C70H62 902.4852, found 902.470.
Anal. Calcd for C70H62: C, 93.08; H, 6.92. Found: C, 92.99;
H, 7.05.
Spectral data for compound 11:. IR (neat, cm-1) 3022 (m),
1
2914 (m), 2103 (w), 1623 (w), 1532 (w), 1364 (s), 1134 (m); H
NMR (400 MHz, CD2Cl2) δ 9.01 (d, J = 8.4 Hz, 2H), 8.61
(s, 2H), 8.14 (s, 2H), 7.90 (d, J=8.0 Hz, 2H), 7.72 (dd, J=8.0,
1.6 Hz, 4H), 7.62(d, J=8.0 Hz, 4H), 7.56 (d, J=8.0 Hz, 4H), 7.29
(d, J=8.4 Hz, 2H), 7.23 (t, J=8.4 Hz, 2H), 6.76 (t, J=8.4 Hz,
2H), 2.74 (s, 6H), 1.50 (s, 18H), 1.40 (s, 18H); 13C NMR (150
MHz, CD2Cl2/CS2) δ 152.1, 151.0, 141.6, 139.5, 138.5, 136.2,
136.1, 135.8, 135.6, 134.6, 133.5, 132.6, 132.3 (2ꢀCH), 130.3,
130.1, 128.7, 128.0, 126.6 (4ꢀCH), 126.0, 125.4, 125.2, 125.0
(4ꢀCH), 124.6, 124.3, 124.2, 35.8, 35.5, 32.0, 31.9, 31.8, 21.1;
MALDI-MS calcd for C72H66 930.5165, found 930.476. Anal.
Calcd for C72H66: C, 92.67; H, 7.33. Found: C, 92.39; H, 7.59.
Acknowledgment. We thank the National Science Coun-
cil, Taiwan, for financial support of this work.
Supporting Information Available: Spectral data, NMR
spectra, and Maldi-Mass and cyclic voltammetry curves of new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
J. Org. Chem. Vol. 74, No. 21, 2009 8451