Synthesis, optical, electrochemical, and thermal properties of conjugated α-fluorenyl oligothiophenes

Article abstract of DOI:10.1016/j.tetlet.2006.12.036

A series of new α-fluorenyl oligothiophenes up to the pentamer have been synthesized using Suzuki cross-coupling and bromination reactions. The optical, electrochemical, and thermal properties of these materials can be tuned by varying the number of thiophene rings. The longer oligomers (n ≥ 4) were stable, crystalline, and unreactive to electrochemical oxidative dimerization.

Full text of DOI:10.1016/j.tetlet.2006.12.036

Tetrahedron Letters 48 (2007) 919–923
Synthesis, optical, electrochemical, and thermal properties
of conjugated a-fluorenyl oligothiophenes
Vinich Promarak,* Auradee Punkvuang, Duangratchaneegorn Meunmat,
Taweesak Sudyoadsuk, Sayant Saengsuwan and Tinnagon Keawin
Advanced Organic Materials and Devices Laboratory, Department of Chemistry, Faculty of Science,
Ubon Ratchathani University, Warinchumrap, Ubon Ratchathani 34190, Thailand
Received 5 November 2006; revised 26 November 2006; accepted 8 December 2006
Abstract—A series of new a-fluorenyl oligothiophenes up to the pentamer have been synthesized using Suzuki cross-coupling and
bromination reactions. The optical, electrochemical, and thermal properties of these materials can be tuned by varying the number
of thiophene rings. The longer oligomers (n P 4) were stable, crystalline, and unreactive to electrochemical oxidative dimerization.
Ó 2006 Elsevier Ltd. All rights reserved.
The study of well-defined oligomers has recently become
useful to gain insight into structure–property relation-
ships in order to rationalize the behavior of the corre-
sponding polymers and also has provided information,
which may be used to improve strategies for the devel-
opment of novel organic materials.1 The advantages of
oligomers are that their physical properties can be easily
tuned to the desired properties by changing the struc-
ture, for example, solubilizing chains, terminal groups,
and different oligomer lengths. In particular, oligothio-
phenes had earlier been used as model compounds
for polymers2 and have been lately advanced as compo-
nents for molecular electronic and optoelectronic
devices.3,4 To vary the structure and the properties of
oligothiophenes, the introduction of different terminal
groups into their backbone is at of increasing synthetic
interest. For example, a series of oligothiophenes substi-
tuted at their terminal a,a0-positions with 2-n-hexyl-
with the aim of investigating the structure and physical
property relationships of the materials.
The synthetic approach to a series of a-fluorenyl oligo-
thiophenes is illustrated in Scheme 1. Due to the conve-
nience and high yields, palladium-catalyzed Suzuki
cross-coupling reactions11 were employed to achieve
growth of the oligothiophene segment. Bromine was
selectively introduced to the a-position of the terminal
thiophene ring by bromination with NBS to allow fur-
ther Suzuki cross-coupling reactions to be performed.
The fluorene compound 2 was synthesized in high yield
by bis-alkylation at C-9 of 2-bromofluorene 1 with n-
hexylbromide. Compound FT1 was readily obtained
via cross-coupling between 2 and 2-thiopheneboronic
acid in the presence of Pd(PPh3)4 as a catalyst and
2 M Na2CO3 as a base in THF at reflux in 79% yield.
Consequently, treatment of a FT1 with NBS in THF
afforded the intermediate bromo compound 3 in a good
yield. Repetitive cross-coupling and bromination reac-
tion sequences led to a series of a-substituted oligo-
thiophenes up to the pentamers, FT2, FT3, FT4, and
FT5, in good yields. Compounds FTn (n = 1–5) were
fluorene,5
9,9-diphenylfluorene,6
diphenylamine,7
cyclophane,8 pyrene,9 and bis(4-methylphenyl)amino-
phenyl4 have been synthesized and investigated recently.
To our best knowledge, however, there are only a few re-
ports on monosubtituted derivatives,10–12 whose synthe-
sis and physical properties still remain to be explored. In
this Letter, we report a successful synthesis of a series of
novel a-fluorenyl oligothiophenes up to the pentamer
1
fully characterized by H, 13C NMR, IR, and HRMS
analyses.13 They exhibited a good solubility in organic
solvents such as chlorinated solvents, THF, and acetone
at room temperature.
The optical properties of FTn (n = 1–5) in CH2Cl2 are
depicted in Figures 1 and 2, and summarized in Table
1. All the UV–vis spectra consisted of two absorption
Keywords: Fluorene; Oligomer; Oligothiophene; Suzuki coupling.
*
Corresponding author. Tel.: +66 81 5930005; fax: +66 45 288379;
e-mail: pvinich@sci.ubu.ac.th
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.12.036

920
V. Promarak et al. / Tetrahedron Letters 48 (2007) 919–923
i
ii
Br
Br
H
S
S
98%
79%
FT1
R
R
R
iii
R
1
2
92%
iii
ii
80%
H
Br
Br
S
S
93%
2
2
R
R
4
R
ii
R
R
R
FT2
3
84%
iii
ii
Br
H
H
S
S
S
98%
87%
3
4
3
FT4
R
R
R
iii
R
R
R
FT3
5
95%
ii
R = n-hexyl
H
Br
S
S
76%
5
4
FT5
6
R
R
R
R
Scheme 1. Synthesis of FTn (n = 1–5). Reagents and conditions: (i) n-C6H13Br, n-Bu4NCl, 50% NaOH, DMSO; (ii) 2-thiopheneboronic acid,
Pd(PPh3)4, 2 M Na2CO3, THF, reflux; (iii) NBS, THF.
1.0
0.5
0.0
1.0
0.5
0.0
FT1
FT2
FT3
FT4
FT5
FT1
FT2
FT3
FT5
FT6
400
500
600
300
400
500
Wavelength (nm)
Wavelength (nm)
Figure 1. Absorption spectra of FTn (n = 1–5) in CH2Cl2.
Figure 2. Photoluminescence (PL) spectra of FTn (n = 1–5) in CH2Cl2.
bands, which were assigned in terms of the strong
absorption band at longer wavelength corresponding
to the p–p* electron transition of the entire conjugated
backbone and the less intense absorption bands origi-
nating from the p–p* local electron transition of the
individual aromatic units. As expected, the intensity
and absorption maxima of the former bands of FTn
(n = 1–5) progressively increased and were red-shifted
with increasing number of thiophene rings. Compounds
FTn (n = 1–5) exhibited maximum absorptions at 330,
362, 398, 423, and 439 nm, which are considerably red-
shifted relative to those of thiophene, bithiophene
(302),14 terthiophene (355),15 quaterthiophene (390),16
and quinquethiophene (416),15 respectively. The elec-
tronic absorption maxima for FT5 (439 nm) are quite
close to that of regioregular poly(3-octylthiophene)
(434 nm),17 while its regiorandom polymer (428 nm)18
has a similar electronic absorption band to that of the
FT4 (423 nm). These results suggested the formation
of a highly extended p-electron delocalization system
through the fluorene platform. The HOMO–LUMO
energy gaps (Eg) (Table 1), estimated from the onset
absorption edge, decrease with an increase in the p-con-
jugation length of the oligothiophene units. The photo-
luminescence (PL) spectra of FTn (n = 1–5) also showed
a gradual red-shift of the maximum emission peak with
increased number of thiophene rings. Moreover, an
excellent linear correlation of the absorption and emis-

V. Promarak et al. / Tetrahedron Letters 48 (2007) 919–923
Table 1. Summary of the physical data of FTn (n = 1–5)
921
Compound
k
abs/nm (loge/dm3/mol cm)a
k
em/nmb
Eg/eVc
E1/2/V (DE/mV)d
Tc/Tm/Td/°Ce
HOMO/eVg
LUMO/eVh
FT1
FT2
FT3
FT4
FT5
330 (4.59)
362 (4.83)
398 (5.11)
423 (5.34)
439 (5.77)
374
3.48
2.99
2.71
2.54
2.44
1.02 (200), 1.28 (170)
0.85 (50), 1.09 (150)
0.77 (60), 0.99 (150)
0.87 (70), 1.16 (100)
0.83 (70), 1.09 (90)
NA
NA
53/105/406
127f/172/417
190f/225/420
ꢀ5.65
ꢀ5.48
ꢀ5.36
ꢀ5.25
ꢀ5.22
ꢀ2.17
ꢀ2.49
ꢀ2.65
ꢀ2.71
ꢀ2.78
418, 434
456, 481
488, 514
516
The underlined values refer to an unresolved and irreversible process.
a Measured in dilute CH2Cl2 solution.
b Excited at the absorption maxima.
c Estimated from the onset of absorption spectra (Eg = 1240/konset).
d Measured using a platinum rod counter electrode, a glassy carbon working electrode, and a SCE reference electrode in CH2Cl2 containing
n-Bu4NPF6 as a supporting electrolyte with a scan rate of 50 mV/s under an argon atmosphere.
e Obtained from DSC measurements during the second heating cycle with a heating rate of 10 °C/min under N2.
f Observed upon cooling.
g Calculated by the empirical equation: HOMO = ꢀ(4.44 + Eonset).
h Calculated from LUMO = HOMO + Eg.
3.6
12
8
Absorption
Emission
Eg
3.2
2.8
2.4
2.0
4
λ = 508 nm
FT1
FT2
FT3
FT4
FT5
0
2.09 eV
λ = 599 nm
-4
0.6
0.8
1.0
1.2
1.4
1.6
0.0
0.2
0.4
Voltage vs Ag/Ag+ (V)
Inverse number of thiophenes (1/n)
Figure 4. CV curves of FTn (n = 1–5) in CH2Cl2 at a scan rate of
50 mV/s.
Figure 3. Plots of absorption and emission maxima, and energy gap
(Eg) for FTn (n = 2–5) in CH2Cl2 with the inverse number of thiophene
rings (1/n).
curves were gradually shifted to a lower oxidation po-
tential. This is in accord with the longer conjugation
length achieved.21 The CV curves of compounds FT1,
FT2, and FT3 demonstrated one oxidation process at
1.28, 1.09, and 0.99 V, respectively, corresponding to
the removal of electrons from the terminal thiophene
ring to form the radical cation and a weak oxidation
process at lower potential (1.02, 0.85, and 0.77 V,
respectively) corresponding to the removal of electrons
from the dimer originating from an oxidative coupling
of the radical cation. However, compounds FT4 and
FT5 behaved totally differently and exhibited two
reversible oxidation processes at 0.87 and 1.16 V, and
0.83 and 1.09 V, respectively. The first oxidation process
corresponds to the removal of an electron from the ter-
minal thiophene ring and the second oxidation process
can be assigned to the removal of electrons from the
interior moieties. Upon repeated scanning, there was
no weak oxidation process due to an oxidative coupling
at lower potential being observed. This result indicates
that radical cations for the tetramer and pentamer are
stable to an oxidative coupling and the radical cations
of shorter oligomers (n 6 3) tend to dimerize inherently.
In addition, no obvious reduction process was detected
sion maxima, and energy gaps (Eg) with the inverse
number of thiophene rings for the fluorene platform ser-
ies in CH2Cl2 was observed (Fig. 3). Extrapolation to a
hypothetical infinite number of thiophene rings models
the properties of an ideal polymer (in solution) and gives
a maximum absorption at 508 nm, a maximum emission
at 599 nm and an energy gap (Eg) of 2.09 eV. These val-
ues are lower than those found experimentally for poly-
thiophene film (solid) due to defects and interruptions of
the conjugated backbone in the polymer. However, on
the basis of the spectroscopic investigation of oligothio-
phenes, the effective conjugation length is not much
longer than 15–20 thiophene units.19,20 Therefore, this
correlation allows an estimation of the mean conjuga-
tion length in the real polymer, which is in this case
for a polythiophene of about 20 thiophene units and
thus differs significantly from the mean conjugation
length of the polymer.
Cyclic voltammetry (CV) was performed to investigate
the redox properties of FTn (n = 1–5). The results are
shown in Figure 4 and are summarized in Table 1. On
increasing the number of thiophene rings, the oxidation

922
V. Promarak et al. / Tetrahedron Letters 48 (2007) 919–923
6. Wong, K.-T.; Wang, C.-F.; Chou, C. H.; Su, Y. O.; Lee,
G.-H.; Peng, S.-M. Org. Lett. 2002, 4, 4439–4442.
7. Tabet, A.; Schro¨der, A.; Hartmann, H.; Rohde, D.;
Dunsch, L. Org. Lett. 2003, 5, 1817–1820.
in all cases. The HOMO and LUMO energy levels of the
oligomers were determined using the onset positions of
the oxidation and energy gap (Eg) and are summarized
in Table 1.
8. Guyard, L.; Dumas, C.; Miomandre, F.; Pansu, R.;
´
Renault-Meallet, R.; Audebert, P. New J. Chem. 2003,
The thermal properties of oligomers FTn (n = 3–5) were
investigated by differential scanning calorimetry (DCS).
All were found to be thermally stable, with the onset of
decomposition temperatures above 406 °C under nitro-
gen. The results are summarized in Table 1. As expected,
the increasing number of thiophene rings resulted in an
increase of the phase transition temperatures. The DSC
curves of samples recrystallized from CH2Cl2/methanol
exhibited clear endothermic melting peaks (Tm) during
the first heating scan at 105, 172, and 225 °C, respec-
tively. On subsequent cooling, only compounds FT4
and FT5 were found to recrystallize with the recrystalli-
zation temperatures (Tc) being observed at 127 and
190 °C, respectively. When the non-recrystallized sample
FT3 was reheated, the second DSC curve showed an
exothermic crystallization peak (Tc) at 53 °C followed
by an endothermic peak due to melting of the same crys-
talline form at 105 °C. In the cases of FT4 and FT5, only
sharp endothermic melting peaks (Tm) were detected
during the second heating scan at 172 and 225 °C,
respectively.
27, 1000–1006.
9. Aso, Y.; Okai, T.; Kawaguchi, Y.; Otsubo, T. Chem. Lett.
2001, 420–421.
10. Pei, J.; Wang, J.-L.; Cao, X.-Y.; Zhou, X.-H.; Zhang,
W.-B. J. Am. Chem. Soc. 2003, 125, 9944–9945.
11. Liu, X.-M.; Xu, J.; He, C. Tetrahedron Lett. 2004, 45,
1507–1510.
12. Zotti, G.; Schiavon, G.; Berlin, A.; Pagani, G. Chem.
Mater. 1993, 5, 430–436.
13. Characterization data. Compound FT1: light green
viscous oil; IR (KBr) 2927, 1467, 1214, 1052, 808, and
1
693 cmꢀ1; H NMR (300 MHz, CDCl3) d 0.73–0.78 (4H,
m), 0.81–0.86 (6H, m), 1.12–1.22 (12H, m), 2.05–2.10 (4H,
m), 7.15 (1H, dd, J = 5.1 Hz, J = 3.6 Hz), 7.33–7.45 (5H,
m), 7.66–7.69 (2H, m), and 7.74–7.77 (2H, m); 13C NMR
(75 MHz, CDCl3) d 14.04, 22.62, 23.79, 31.52, 40.47,
55.21, 119.75, 120.08, 120.24, 122.87, 122.91, 124.47,
124.94, 126.87, 127.15, 128.05, 133.25, 140.70, 140.78,
150.94, and 151.57; HRMS-ESI m/z: [MH+] calcd for
C29H37S1 417.2610; found, 417.2618.
Compound FT2: green viscous oil; IR (KBr) 2927, 1467,
1212, 1054, 809, and 691 cmꢀ1 1H NMR (300 MHz,
;
CDCl3) d 0.68–0.70 (4H, m), 0.79 (6H, t, J = 7.0 Hz),
1.09–1.17 (12H, m), 2.02 (4H, t, J = 8.0 Hz), 7.07 (1H, t,
J = 4.3 Hz), 7.20 (1H, d, J = 3.9 Hz), 7.25 (2H, d,
J = 3.9 Hz), 7.31 (1H, d, J = 3.9 Hz), 7.35–7.38 (3H, m),
7.57 (1H, d, J = 1.0 Hz), 7.60 (1H, dd, J = 7.9 Hz,
J = 1.6 Hz), and 7.70 (2H, dd, J = 8.0 Hz, J = 2.2 Hz);
13C NMR (75 MHz, CDCl3) d 13.90, 22.56, 23.74, 29.69,
31.47, 40.41, 55.18, 119.72, 119.85, 120.08, 122.88, 123.46,
123.54, 124.29, 124.55, 124.63, 126.84, 127.16, 127.87,
132.83, 136.35, 137.58, 140.59, 140.89, 144.02, 150.60, and
151.91; HRMS-ESI m/z: [MH+] calcd for C33H39S2
499.2488; found, 499.2496.
Compound FT3: yellow solid; mp 105 °C; IR (KBr) 2930,
1467, 1214, 1050, 810, and 692 cmꢀ1; 1H NMR (300 MHz,
CDCl3) d 0.66–0.70 (4H, m), 0.78 (6H, t, J = 6.8 Hz),
1.08–1.16 (12H, m), 2.01 (4H, t, J = 8.5 Hz), 7.04 (1H, dd,
J = 5.1 Hz, J = 3.6 Hz), 7.13 and 7.14 (2H, AA0BB0,
J = 3.7 Hz), 7.19 (1H, d, J = 3.9 Hz), 7.20 (1H, dd,
J = 3.7 Hz, J = 1.2 Hz), 7.24 (1H, dd, J = 5.1 Hz,
J = 1.2 Hz), 7.31 (1H, d, J = 3.9 Hz), 7.33–7.37 (3H, m),
7.56 (1H, d, J = 1.2 Hz), 7.59 (1H, dd, J = 7.9 Hz,
J = 1.5 Hz), and 7.70 (2H, d, J = 8.1 Hz); 13C NMR
(75 MHz, CDCl3) d 13.96, 22.54, 23.72, 29.67, 31.45,
40.39, 55.17, 119.71, 119.84, 120.08, 122.88,123.55, 123.70,
124.07, 124.42, 124.50, 124.53, 124.57, 126.83, 127.17,
127.89, 132.72, 135.98, 136.25, 137.58, 140.59, 140.99,
144.18, 150.91, and 151.92; HRMS-ESI m/z: [MH+] calcd
for C37H41S3 581.236; found, 581.2372.
In conclusion, we have presented a convenient and
efficient synthetic approach to a series of a-fluorenyl
oligothiophenes up to the pentamer using Suzuki
cross-coupling and bromination reactions. The presence
of the fluorene offers good solubility and extends the
p-electron delocalization system of the oligomers.
Substantially red-shifted absorption and emission spec-
tra, improved transition temperatures and decreased
oxidation potentials of the oligomers were observed as
more thiophene rings were introduced. The tetramer
and pentamer were crystalline and stable to electro-
chemically oxidative dimerization.
Acknowledgments
This research was supported by the National Metal and
Materials Technology Centre (MTEC) of Thailand
(Grant MT-S-46-POL-24-249-G). We also thank Chu-
labhorn Research Institute (CRI) of Thailand for
HRMS measurements.
Compound FT4: orange yellow solid; mp 172 °C; IR
References and notes
(KBr) 2928, 1469, 1214, 1050, 807, and 691 cmꢀ1 1H
;
NMR (300 MHz, CDCl3) d 0.65–0.71 (4H, m), 0.78 (6H, t,
J = 6.8 Hz), 1.08–1.17 (12H, m), 2.02 (4H, t, J = 8.0), 7.04
(1H, dd, J = 5.1 Hz, J = 3.6 Hz), 7.12–7.16 (4H, m), 7.21
(2H, t, J = 3.0 Hz), 7.24 (1H, d, J = 4.8 Hz), 7.31 (1H, d,
J = 3.9 Hz), 7.33–7.37 (3H, m), 7.57 (1H, s), 7.59 (1H, dd,
J = 7.8 Hz, J = 1.2 Hz), and 7.70 (2H, d, J = 7.8 Hz); 13C
NMR (75 MHz, CDCl3) d 13.96, 22.55, 23.73, 29.68,
31.46, 40.40, 55.18, 119.72, 119.84, 120.09, 122.88, 123.59,
123.78, 124.15, 124.25, 124.36, 124.42, 124.58, 124.65,
126.84, 127.19, 127.91, 132.70, 135.80, 135.92, 136.38,
136.48, 137.07, 140.54, 140.98, 144.26, 150.92, and 151.92;
1. Martin, R. E.; Diederich, F. Angew. Chem., Int. Ed. 1999,
38, 1350–1377.
2. Hananishi, H.; Aso, Y.; Otsubo, T. Synth. Met. 1999, 101,
604–605.
3. Hajlaoui, R.; Horowitz, G.; Garnier, F.; Arce-Brouchet,
A.; Laigre, L.; Kassmi, F. A. E.; Demanze, F.; Kouki, F.
Adv. Mater. 1997, 9, 389–391.
4. Noda, T.; Ogawa, H.; Noma, N.; Shirota, Y. J. Chem.
Mater. 1999, 9, 2177–2181.
5. Meng, H.; Zheng, J.; Lovinger, A. J.; Wang, B.-C.; Van
Patten, P. G.; Bao, Z. Chem. Mater. 2003, 15, 1778–1787.

V. Promarak et al. / Tetrahedron Letters 48 (2007) 919–923
923
HRMS-ESI m/z: [MH+] calcd for C41H43S4 663.2242;
found, 663.2230.
14. Ba¨uerle, P.; Wurthner, F.; Go¨tz, G.; Effenberger, F.
¨
Synthesis 1993, 1099–1103.
Compound FT5: red yellow solid; mp 225 °C; IR (KBr)
15. Martinez, F.; Voelkel, R.; Naegele, D.; Naarmann, H.
Mol. Cryst. Liq. Cryst. 1989, 167, 227–235.
16. Nakayama, J.; Konishi, T.; Marubayashi, S.; Hoshino, M.
Heterocycles 1987, 26, 1793–1796.
17. McCullough, R. D.; Lowe, R. D.; Jayaraman, M.;
Anderson, D. L. J. Org. Chem. 1993, 58, 904–912.
18. Chen, T. A.; Wu, X. M.; Rieke, R. D. J. Am. Chem. Soc.
1995, 117, 233–244.
19. Hoeve, W.; Wynberg, H.; Havinga, E. E.; Meijer, E. W. J.
Am. Chem. Soc. 1991, 113, 5887–5889.
20. Hananishi, H.; Aso, Y.; Otsubo, T. Synth. Met. 1999, 101,
604–605.
2927, 1466, 1214, 1053, 805, and 692 cmꢀ1 1H NMR
;
(300 MHz, CDCl3) d 0.66–0.68 (4H, m), 0.78 (6H, t,
J = 7.0 Hz), 1.08–1.16 (12H, m), 2.01 (4H, t, J = 8.2 Hz),
7.05 (1H, t, J = 3.9 Hz), 7.09–7.14 (6H, m), 7.20 (2H, br s),
7.24 (1H, d, J = 5.1 Hz), 7.31–7.37 (4H, m), 7.56 (1H, s),
7.59 (1H, d, J = 8.1 Hz), and 7.70 (2H, d, J = 7.8 Hz); 13
C
NMR (75 MHz, CDCl3) d 14.00, 22.57, 23.73, 29.69,
31.47, 40.41, 55.17, 119.74, 119.82, 120.11, 122.88, 123.60,
123.80, 124.18, 124.28, 124.34, 124.43, 124.54, 124.67,
126.84, 127.20, 127.93, 132.75, 135.82, 135.92, 136.00,
136.38, 136.49, 137.05, 140.53, 141.10, 144.24, 150.90, and
151.61; HRMS-ESI m/z: [MH+] calcd for C44H45S5
741.3140; found, 741.1334.
21. Jestin, I.; Frere, P.; Mercier, N.; Levillain, E.; Stievenard,
D.; Roncali, J. J. Am. Chem. Soc. 1998, 120, 8150–8158.