Synthesis and characterization of polybrominated fluorenes and their conversion to polyphenylated fluorenes and cyclopenta[def]triphenylene

Article abstract of DOI:10.1016/j.tetlet.2014.02.002

We report the synthesis and characterization of polyphenylated fluorene derivatives and a ring cyclized product containing cyclopenta[def]triphenylene core. Polybromination on fluorene was achieved either by solid state reaction with bromine or utilizing

Full text of DOI:10.1016/j.tetlet.2014.02.002

Tetrahedron Letters 55 (2014) 1931–1935
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and characterization of polybrominated fluorenes
and their conversion to polyphenylated fluorenes and
cyclopenta[def]triphenylene
b
Sushil Kumar a, Durai Karthik a, K. R. Justin Thomas a, , Maninder Singh Hundal
⇑
a Organic Materials Laboratory, Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India
b Department of Chemistry, Guru Nanak Dev University, Amristar 143 005, India
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the synthesis and characterization of polyphenylated fluorene derivatives and a ring cyclized
product containing cyclopenta[def]triphenylene core. Polybromination on fluorene was achieved either
by solid state reaction with bromine or utilizing Br2/KBrO3 in AcOH/H2SO4 mixture. The bromofluorenes
were converted to the corresponding polyphenylated fluorenes by Suzuki coupling protocol. A hexab-
romofluorene underwent a multifold Suzuki coupling followed by C–H activation to produce a cyclo-
penta[def]triphenylene derivative. Fluorene ring showed a severe distortion from planarity beyond
tetra-substitution which manifested in the optical properties.
Received 17 December 2013
Revised 30 January 2014
Accepted 2 February 2014
Available online 10 February 2014
Keywords:
Fluorene
Polysubstitution
Suzuki coupling
Crystal structure
Optical properties
Ó 2014 Elsevier Ltd. All rights reserved.
Organic semiconducting materials derived from polyaromatic
hydrocarbons such as fluorene,1,2 anthracene,3 triphenylene,4 fluo-
ranthene,5 perylene,6 pyrene,7 and acenes8 have attracted im-
mense attraction in recent years due to their display of novel
properties such as charge transport, tunable luminescence, and
non-linear optical properties such as two-photon absorption and
hyperpolarizability. They are widely used as emitters in organic
light-emitting diodes (OLED), semiconducting layers in thin-film
modifications. Generally, functionalization of fluorene at 2-, 4-, 7-,
and 9-positions has been easily accomplished by the established
electrophilic substitution procedures followed by metal-catalyzed
coupling reactions.9 But, introduction of chromophores on several
other positions on fluorene required synthetic methodologies gen-
erating fluorene framework from acyclic building blocks.10 Notable
methods are the intramolecular dehydrogenative cyclization of 1-
amino-1,1-diarylalkanes11 or triarylmethanols12 catalyzed by
Rh(III) or Ir(III) catalysts. Access to 3,6-disubstituted fluorene
derivatives from phenanthrene-9,10-dione involving conventional
synthetic strategies have also been reported.13 Though, these
methods allow the precise positioning of the substituents on the
fluorene nucleus, they suffer from limited options available for
substituents. Alternatively, variety of synthetic explorations can
be used to generate polysubstituted fluorenes, if polyhalogenated
fluorenes can be obtained from the commercial fluorene. To the
best of our knowledge, polyhalogenations have not been demon-
strated for fluorene.14 This is partially due to the deactivating influ-
ence of the bromo substituents. In this Letter, we report tri-, tetra-,
penta- and hexabromination of fluorene and the use of the result-
ing polybromides in the generation of polyphenylated fluorenes by
palladium-catalyzed cross-coupling reactions. Polyphenylated are-
nes,15 commonly termed as ‘Müllen dendrons’ are attractive due to
their wide use in electro-optical devices16 and as precursors for
nanosized graphene molecules possessing one atom thick planar
sheet structures.17
transistors (TFT), reporters in fluorescent sensors, and p-conjugat-
ing linkers in nonlinear optical materials and organic sensitizers
suitable for dye-sensitized solar cells (DSSC). As a result of their ex-
tended conjugation and rigidity, they display strong emission with
relatively low Stokes shifts. Also, due to their planar structure they
exhibit long-range solid state electronic communications facili-
tated by intermolecular
p-interactions which enhance the charge
transporting properties usually beneficial for their application in
electronic devices such as TFT, OLED, and bulk heterojunction solar
cells.
Fluorene-based small molecules1 and polymers2 have been
demonstrated as promising emitters for electroluminescent de-
vices and light-harvesting chromophores in photovoltaic devices.
Their emission properties can be effectively tuned by chemical
⇑
Corresponding author. Tel.: +91 1332 285376.
E-mail address: krjt8fcy@iitr.ac.in (K.R.J. Thomas).
http://dx.doi.org/10.1016/j.tetlet.2014.02.002
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

1932
S. Kumar et al. / Tetrahedron Letters 55 (2014) 1931–1935
Mono- and dibromination of fluorene is conveniently achieved
after a pentafold Suzuki coupling reaction. Suzuki-Heck-type cou-
pling cascade reactions producing fused polycycles have been pre-
viously demonstrated for other substrates, particularly peri-
dihalo-polyaromatic hydrocarbons.24 It has been found that for
sterically less demanding substrates, addition of bulky phosphines
as additive was required to force the annulation. In the present
case, the exclusive formation of cyclized product without additives
is probably driven by a steric release on annulation.
The structures of two bromo-derivatives (4 and 6) and the
cyclopenta[def]-triphenylene derivative (10) (Fig. 1) have been
established by single crystal X-ray diffraction measurements. The
fluorene unit is planar in the tetra-substituted derivative (4). But
the hexa-bromo derivative (6) and the cyclopenta[def]-triphenyl-
ene derivative (10) exhibited significant deviation from the planar-
ity for the central cores. The benzene rings were tilted (8.54°,
8.26°) from the central cyclopentadiene ring in 6 owing to the sub-
stitution at the bridgehead positions. Though the compound 10,
showed good planarity for the fluorene moiety the appended ben-
zene ring was titled (12.15°) from the fluorene segment
considerably.
by treating with bromine in dichloromethane.18 By controlling
the stoichiometry, either 2-bromo- or 2,7-dibromofluorene can
be obtained with reasonable purity. Our initial attempts to obtain
tribromo- or tetrabromo-derivatives under similar conditions, but
using excess bromine, failed. However, in the initial experiments,
we observed the formation of 2,4,7-tribromo-9,9-diethyl-9H-fluo-
rene (3) and other higher substitution products when 9,9-
diethyl-9H-fluorene (1) was reacted with bromine in neat. Optimi-
zation experiments revealed the formation of 3 in major amount
when 1.5 equiv of bromine were used.19 Attempts to obtain highly
brominated fluorenes in pure form under these conditions did not
yield the desired results. Use of excess bromine under neat condi-
tions produced inseparable mixture of polybrominated products.
So, we turned our attention to other available brominating meth-
20
ods. Bromination using Br2/KBrO3
reagent system in acetic
acid/sulfuric acid mixture was found to be tenable. The tri-substi-
tuted derivative, 3 on reaction with 1.0 equiv of Br2/KBrO3 reagents
produced 2,3,5,7-tetrabromo-9,9-diethyl-9H-fluorene (4) exclu-
sively.21 Use of the same reagent system, albeit under extreme
harsh conditions, produced 2,3,4,6,7-pentabromo-9,9-diethyl-9H-
fluorene (5) and 2,3,4,5,6,7-hexabromo-9,9-diethyl-9H-fluorene
(6) in reasonable yields (Scheme 1).
To check the utility of the polybrominated fluorenes, we have
first chosen the Suzuki coupling reactions with phenylboronic acid
as the coupling partner. Initial attempts employing the conven-
tional catalyst system, Pd(OAc)2/K2CO3/toluene:H2O were not suc-
cessful in replacing all the bromines in 5 and 6. However, the use of
reaction conditions involving Pd(PPh3)2Cl2 and PPh3, K2CO3 and
DMF:H2O (45/5) gave desired results.22 Interestingly, with 6, in-
stead of the expected hexaphenyl derivative, a cyclized product,
1,2,6,7-tetrasubstituted cyclopenta[def]-triphenylene (10), was
formed.23 The formation of 10 probably involves C–H activation
Absorption and emission spectra of the compounds 7–10 re-
corded in dichloromethane are presented in Figure 2 and relevant
data listed in Table 1. All the fluorene derivatives (7–9) displayed a
moderate p–
p⁄ absorption band in the 300–350 nm range. Though
the absorption pattern of the present compounds is not signifi-
cantly different from 9,9-dioctyl-2,7-diphenyl-9H-fluorene
(kabs = 327 nm; e
max = 76,100 MÀ1 cmÀ1 in CHCl3)25 reported ear-
lier, their molar extinction coefficients are drastically reduced. This
points that, introduction of phenyl groups on additional fluorene
nuclear sites do not extend the chromophore conjugation but
diminishes the transition probability mainly due to the twisting
in the fluorene segment. The absorption spectrum of 10 is signifi-
cantly blue shifted (>45 nm) with a concomitant increase in the
Scheme 1. Synthesis of polyphenylated fluorenes and cyclopenta[def]triphenylene.

S. Kumar et al. / Tetrahedron Letters 55 (2014) 1931–1935
1933
Figure 1. ORTEP plots of the compounds 4 (top-left), 6 (top-right), and 10 (bottom).
larger than that observed for 10. It suggests that compound 10 re-
sists structural reorganization in the excited state.
All the derivatives underwent an irreversible oxidation at mod-
erate redox potentials (1.26–1.33 V) with reference to internal fer-
rocene, attesting the reasonable electron-richness of the core
structure. Additionally, they have also exhibited decent decompo-
sition temperatures in the range 383–431 °C. Interestingly, the
annulated derivative, 10 showed the highest thermal decomposi-
tion temperature in the series which probably originates from its
rigid structure.
In summary, we have devised high yield methods for the syn-
thesis of polybrominated fluorene derivatives by the use of simple
reagents. These bromides can be used to construct polyarylated
fluorene derivatives as demonstrated in this work by the multifold
Suzuki coupling reactions with phenylboronic acid. A reaction cas-
cade involving pentafold Suzuki coupling reaction and a final Heck
type C–H activation with the hexabromide leading to a unique
pentacyclic product is demonstrated. The synthetic pathway pre-
sented in this Letter may be extended further to obtain fluorene
containing polymers and oligomers with additional functional
chromophores on the peripheral sites of the fluorene nucleus.
Work to extend this strategy to obtain functional materials dis-
playing pure blue emission and suitable for solution processed or-
ganic light-emitting diodes are under progress in our lab.
Figure 2. Absorption (unfilled symbols) and emission (solid symbols) spectra of the
compounds recorded in CH2Cl2.
molar extinction coefficient due to the change in the nature of the
absorbing chromophore. All the compounds showed violet-blue
fluorescence (Fig. 2) with high quantum efficiency. The Stokes
shifts observed for the fluorene derivatives (7–9) were significantly
Table 1
Optical data for the compounds
b
Compound
kabs.a, nm (
e
 103, MÀ1cmÀ1
)
kem/nma
UF
Stokes shift, cmÀ1
kem/nmc
7
8
9
10
331 (41.9), 264 (16.0)
330 (30.3), 261 (41.9)
325 (31.4), 258 (45.3)
279 (88.5)
382
379
371
367, 383
0.78
0.74
0.82
0.68
4033
3918
3815
607
392
432
404
409, 386
a
b
c
Measured for CH2Cl2 solution.
Obtained with reference to 2-aminopyridine (UF = 60%) in 0.1 N H2SO4.
Measured for spin-cast film.

1934
S. Kumar et al. / Tetrahedron Letters 55 (2014) 1931–1935
14. (a) Chakraborty, C.; Dana, K.; Malik, S. J. Colloid Interface Sci. 2012, 368, 172–
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K.R.J.T. is thankful to CSIR and DST for financial support. Finan-
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Supplementary data
Supplementary data (synthetic procedures, characterization
details and spectra for new compounds) associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/
j.tetlet.2014.02.002. Crystallographic data for the compounds 4,
6, and 10 (CCDC 936751, 936752 and 937481) can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or
e-mail:deposit@ccdc.cam.ac.uk.)
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19. Preparation of 3: Bromine (1.0 mL, 20.0 mmol) was added drop wise to 9,9-
diethyl-9H-fluorene (1) (1.10 g, 5.0 mmol) with constant mixing using a glass
rod. After 30 min, the brown reaction mass was poured into aqueous solution
of sodium metabisulfite and extracted with dichloromethane. On removal of
volatiles a colorless solid was formed. It was crystallized from methanol to
obtain the analytically pure title compound. Yield: 1.88 g (82%). mp = 75 °C. 1
H
NMR (CDCl3, 500.13 MHz): d 0.29 (t, J = 7.5 Hz, 6H), 1.94–2.04 (m, 4H), 7.38 (d,
J = 2.0 Hz, 1H), 7.44 (d, J = 2.0 Hz, 1H), 7.50 (dd, J = 8.0 Hz, 2.0 Hz, 1H), 7.66 (d,
J = 2.0 Hz, 1H), 8.36 (d, J = 8.0 Hz, 1H). 13C NMR (CDCl3, 125.77 MHz): d 8.3,
33.0, 56.8, 117.2, 121.2, 122.5, 124.5, 125.1, 125.9, 130.1, 134.3, 138.0, 139.0,
152.1, 154.1. MALDI-TOF MS: calcd for C17H15Br3 455.87 (M+), found 455.86.
Anal. Calcd for C17H15Br3: C, 44.48; H, 3.29. Found: C, 44.54; H, 3.23.
20. Perepichka, I. I.; Perepichka, I. F.; Bryce, M. R.; Palsson, L.-O. Chem. Commun.
2005, 3397–3399.
21. Preparation of 4 and data for 5 and 6: A suspension of 2,4,7-tribromo-9,9-
diethyl-9H-fluorene (3) (2.30 g, 5.0 mmol) in acetic acid (25 mL) and 20%
H2SO4 (2.4 mL) was treated with potassium bromate (0.50 g, 3.0 mmol) and
bromine (0.30 mL, 6.0 mmol) and heated at 100 °C for 24 h. On completion of
the reaction, excess bromine was quenched by the addition of sodium
metabisulfite and reaction mixture extracted with dichloromethane. After
evaporation of the volatiles, the residue was purified by crystallization from
hexane to obtain the compound as colorless solid. Yield: 1.80 g (67%).
mp = 80 °C. 1H NMR (CDCl3, 500.13 MHz): d 0.30 (t, J = 7.5 Hz, 6H), 1.98 (q,
J = 7.5 Hz, 4H), 7.38 (d, J = 1.5 Hz, 1H), 7.55 (s, 1H), 7.67 (d, J = 1.5 Hz, 1H), 8.74
(s, 1H). 13C NMR (CDCl3, 125.77 MHz): d 8.4, 32.9, 56.7, 117.5, 122.0, 123.1,
124.4, 125.2, 127.6, 127.8, 134.5, 136.9, 140.8, 150.6, 154.3. MALDI-TOF MS:
calcd for C17H14Br4 537.78 (M++4), found 537.76. Anal. Calcd for C17H14Br4: C,
37.96; H, 2.62. Found: C, 38.07; H, 2.56. 5: Yield: 0.93 g (30%). mp 150–152 °C;
1H NMR (CDCl3, 500.13 MHz) d 0.30 (t, J = 7.5 Hz, 6H), 1.96–2.01 (m, 4H), 7.54
(s, 1H), 7.56 (s, 1H), 8.84 (s, 1H); 13C NMR (CDCl3, 125.77 MHz) d 8.3, 32.8, 56.3,
120.7, 123.1, 125.0, 125.1, 126.8, 127.7, 128.3, 139.3, 140.8, 150.8, 152.2.
MALDI-TOF MS: calcd for C17H13Br5 615.69 (M++4), found 615.65. Anal. Calcd
for C17H13Br5: C, 33.10; H, 2.12. Found: C, 33.02; H, 2.04. Compound 6: Yield:
1.60 g (46%). mp = 160 °C. 1H NMR (CDCl3, 500.13 MHz): d 0.32 (t, J = 7.5 Hz,
6H), 2.00 (q, J = 7.5 Hz, 4H), 7.54 (s, 2H). 13C NMR (CDCl3, 125.77 MHz): d 8.4,
33.1, 57.2, 119.5, 125.6, 126.2, 128.8, 141.9, 152.4. MALDI-TOF MS: calcd for
C
17H12Br6 695.60 (M++6), found 695.58. Anal. Calcd for C17H12Br6: C, 29.35; H,
1.74. Found: C, 29.21; H, 1.68.
22. Compound 7: A mixture of 2,4,7-tribromo-9,9-diethyl-9H-fluorene (3) (2.30 g,
5.0 mmol), phenyl boronic acid (16.0 mmol), potassium carbonate (6.9 g,
50.0 mmol), Pd(PPh3)2Cl2 (120 mg), PPh3 (80 mg), DMF, (45 mL) and water
(5 mL) were heated at 110 °C under inert atmosphere for 2 h. The residue
obtained on evaporation of the volatiles was purified by column
chromatography using dichloromethane/hexanes (1:5) to obtain the title
compound as colorless powder. Yield: 1.80 g (80%). mp = 135–137 °C. 1H NMR
(CDCl3, 500.13 MHz): d 0.46 (t, J = 7.5 Hz, 6H), 2.14 (q, J = 7.5 Hz, 4H), 6.98 (d,
J = 8.0 Hz, 1H), 6.94 (dd, J = 8.0, 1.5 Hz, 1H), 7.33–7.36 (m, 2H), 7.42–7.48 (m,
6H), 7.51–7.54 (m, 3H), 7.57–7.58 (m, 3H), 7.61–7.63 (m, 2H), 7.71–7.72 (m,
2H). 13C NMR (CDCl3, 125.77 MHz): d 8.7, 33.2, 55.7, 120.3, 121.3, 122.8, 125.6,
127.1, 127.2, 127.3, 127.6, 128.0, 128.5, 128.8, 129.3, 137.5, 137.8, 139.5, 139.7,
140.4, 141.2, 141.3, 141.5, 151.3. ESI HRMS: calcd for C35H30+Na 473.2240
(M++Na), found 473.2257. Anal. Calcd for C35H30: C, 93.29; H, 6.71. Found: C,
93.37; H, 6.64. Compound 8: Yield: 2.16 g (82%). mp = 105–107 °C. 1H NMR
(CDCl3, 500.13 MHz): d 0.56–0.59 (m, 6H), 2.17–2.21 (m, 4H), 7.02 (s, 2H), 7.07
(s, 1H), 7.15 (s, 3H), 7.20–7.25 (m, 4H), 7.37–7.41 (m, 3H), 7.49–7.54 (m, 6H),
11. Morimoto, K.; Itoh, M.; Hirano, K.; Satoh, T.; Shibata, Y.; Tanaka, K.; Miura, M.
Angew. Chem., Int. Ed. 2012, 51, 5359–5362.
12. Itoh, M.; Hirano, K.; Satoh, T.; Shibata, Y.; Tanaka, K.; Miura, M. J. Org. Chem.
2013, 78, 1365–1370.
13. (a) Fomina, N.; Bradforth, S. E.; Hogen-Esch, T. E. Macromolecules 2009, 42,
6440–6447; (b) Kobin, B.; Grubert, L.; Blumstengel, S.; Henneberger, F.; Hecht,
S. J. Mater. Chem. 2012, 22, 4383–4390.

S. Kumar et al. / Tetrahedron Letters 55 (2014) 1931–1935
1935
7.64–7.65 (m, 3H), 7.76–7.77 (m, 2H). 13C NMR (CDCl3, 125.77 MHz): d 8.8,
33.0, 55.6, 120.3, 124.8, 124.9, 126.0, 126.3, 127.2, 127.6, 127.8, 128.4, 128.8,
129.2, 129.9, 130.0, 132.0, 133.6, 133.8, 137.4, 137.8, 138.5, 139.1, 139.6, 140.4,
23. Compound 10: Yield: 2.16 g (72%). mp = 235–238 °C. 1H NMR (CDCl3,
500.13 MHz): 0.67 (t, J = 7.5 Hz, 6H), 2.25 (q, J = 7.5 Hz, 4H), 6.94–6.96
(m, 2H), 7.16–7.19 (m, 10H), 7.25 (s, 2H), 7.27–7.29 (m, 8H), 7.56–7.58
(m, 4H). 13C NMR (CDCl3, 125.77 MHz):
9.3, 31.9, 58.0, 123.3, 125.0,
125.8, 126.7, 127.2, 128.4, 128.5, 130.4, 131.1, 133.2, 135.4, 136.7, 141.8,
142.3, 143.5, 147.8. ESI HRMS calcd for
47H36+Na+ 600.2817 (M++Na),
found 600.2816. Anal. Calcd for C47H36: C, 93.96; H, 6.04. Found: C, 94.08;
H, 6.02.
24. (a) Wegner, H. A.; Scott, L. T.; de Meijere, A. J. Org. Chem. 2003, 68, 883–887;
(b) Quimby, J. M.; Scott, L. T. Adv. Synth. Catal. 2009, 351, 1009–1013; (c)
Navarro, O.; Kelly, R. A., III; Nolan, S. P. J. Am. Chem. Soc. 2003, 125, 16194–
16195.
d
141.0, 141.2, 141.7, 142.3, 149.8, 151.9. ESI HRMS calcd for
C
41H34+Na
d
549.2553 (M++Na), found 549.2567. Anal. Calcd for C41H34: C, 93.49; H, 6.51.
Found: C, 93.21; H, 6.59. Compound 9: Yield: 2.35 g (78%). mp = 210–213 °C.
1H NMR (CDCl3, 500.13 MHz): d 0.59–0.62 (m, 6H), 2.11–2.15 (m, 4H), 6.31 (s,
1H), 6.86–6.87 (m, 2H), 6.93–6.94 (m, 5H), 7.06–7.07 (m, 3H), 7.15–7.25 (m,
15H), 7.34 (s, 1H), 7.43 (s, 1H). 13C NMR (CDCl3, 125.77 MHz): d 9.0, 32.8, 55.4,
123.8, 123.9, 124.9, 125.1, 125.5, 125.9, 126.0, 126.2, 126.6, 126.8, 127.4, 127.5,
127.6, 127.8, 128.0, 130.0, 130.1, 130.3, 131.7, 136.8, 138.3, 138.4, 138.6, 138.8,
139.6, 139.9, 140.2, 140.9, 141.6, 142.4, 142.6, 149.8. ESI HRMS calcd for
C
C
47H38+Na 625.2866 (M++Na), found 625.2881. Anal. Calcd for C47H38: C, 93.65;
H, 6.35. Found: C, 93.72; H, 6.23.
25. Kotaka, H.; Konishi, G.-I.; Mizuno, K. Tetrahedron Lett. 2010, 51, 181–184.