Synthesis and properties of monodisperse oligofluorene-funcfionalized truxenes: Highly fluorescent star-shaped architectures

Article abstract of DOI:10.1021/ja039228n

This paper describes the strategy toward novel monodisperse, well-defined, star-shaped oligofluorenes with a central truxene core and from monofluorene to quaterfluorene arms. Introduction of solubilizing n-hexyl groups at both fluorene and truxene moieties results in highly soluble, intrinsically two-dimensional nanosized macromolecules T1-T4. The radius for the largest oligomer of ca. 3.9 nm represents one of the largest known star-shaped conjugated systems. Cyclic voltammetry experiments reveal reversible or quasi-reversible oxidation and reduction processes (E_ox = +0.74 to 0.80 V, E_red = -2.66 to 2.80 eV vs Fc/Fc⁺), demonstrating excellent electrochemical stability toward both p- and n-doping, while the band gaps of the oligomers are quite high (E_g^cv = 3.20-3.40 eV). Close band gaps of 3.05-3.29 eV have been estimated from the electron absorption spectra. These star-shaped macromolecules demonstrate good thermal stability (up to 400-420°C) and improved glass transition temperatures with an increase in length of the oligofluorene arms (from T_g = 63°C for T1 to 116°C for T4) and show very efficient blue photoluminescence (λ_PL = 398-422 nm) in both solution (Φ_PL = 70-86%) and solid state (Φ_PL = 43-60%). Spectroelectrochemical experiments reveal that compounds T1-T4 are stable electrochromic systems which change their color reversibly from colorless in the neutral state (～340-400 nm) to colored (from red to purple color; ～500-600 nm) in the oxidized state.

Full text of DOI:10.1021/ja039228n

Published on Web 10/02/2004
Synthesis and Properties of Monodisperse
Oligofluorene-Functionalized Truxenes: Highly Fluorescent
Star-Shaped Architectures
Alexander L. Kanibolotsky,†,‡ Rory Berridge,† Peter J. Skabara,*,†
Igor F. Perepichka,*,§ Donal D. C. Bradley, and Mattijs Koeberg
Contribution from the Department of Chemistry, UniVersity of Manchester,
Manchester M13 9PL, U.K., L. M. LitVinenko Institute of Physical Organic and
Coal Chemistry, National Academy of Sciences of Ukraine, Donetsk 83114, Ukraine, and
Blackett Laboratory, Imperial College London, London SW7 2BZ, U.K.
Received October 24, 2003; E-mail: peter.skabara@man.ac.uk; i_perepichka@yahoo.com
Abstract: This paper describes the strategy toward novel monodisperse, well-defined, star-shaped
oligofluorenes with a central truxene core and from monofluorene to quaterfluorene arms. Introduction of
solubilizing n-hexyl groups at both fluorene and truxene moieties results in highly soluble, intrinsically two-
dimensional nanosized macromolecules T1-T4. The radius for the largest oligomer of ca. 3.9 nm represents
one of the largest known star-shaped conjugated systems. Cyclic voltammetry experiments reveal reversible
or quasi-reversible oxidation and reduction processes (Eox ) +0.74 to 0.80 V, Ered ) -2.66 to 2.80 eV vs
Fc/Fc+), demonstrating excellent electrochemical stability toward both p- and n-doping, while the band
gaps of the oligomers are quite high (EgCV ) 3.20-3.40 eV). Close band gaps of 3.05-3.29 eV have been
estimated from the electron absorption spectra. These star-shaped macromolecules demonstrate good
thermal stability (up to 400-420 °C) and improved glass transition temperatures with an increase in length
of the oligofluorene arms (from Tg ) 63 °C for T1 to 116 °C for T4) and show very efficient blue
photoluminescence (λPL ) 398-422 nm) in both solution (ΦPL ) 70-86%) and solid state (ΦPL
)
43-60%). Spectroelectrochemical experiments reveal that compounds T1-T4 are stable electrochromic
systems which change their color reversibly from colorless in the neutral state (∼340-400 nm) to colored
(from red to purple color; ∼500-600 nm) in the oxidized state.
Introduction
Over the past decade, polyfluorenes have emerged as leading
electroluminescent materials with bright blue emission, high hole
mobility, and easily tunable properties through modifications
and copolymerizations.4 Recently, Lin et al. reported on starlike
polyfluorenes with a silsesquioxane core.5 Many linear oligo-
fluorenes have been synthesized and studied as model com-
pounds of polyfluorenes.6 In contrast to conjugated polymers,
monodisperse oligomers are characterized by a well-defined and
uniform molecular structure as well as superior chemical purity,
which can be easily reached, for example, by column chroma-
tography. These intrinsic features allow a deep insight into the
effects of chemical structure on electronic, photonic, and
Star polymers, which are usually defined as branched macro-
molecules that consist of linear polymer arms joined together
by a central core, have been of great interest in the past decade.1
A star-shaped architecture combined with conjugated character
within the arms would bring new electrical, optical, and mor-
phological properties to the system which, depending on the
structure of the central core, could have intrinsic two- or three-
dimensional character. Monodisperse, well-defined π-conjugated
oligomers have recently become one of the most promising
prospects in materials science, allowing a deep insight into the
photophysics of conjugated systems and the interpretation of
the supramolecular structure of conjugated polymers.2 In this
context, star-shaped conjugated architectures have attracted
increasing attention as possible alternatives to linear conjugated
oligomers in optoelectronic applications (light-emitting devices
(LED), photovoltaics, field-effect transistors, etc.).3
(3) (a) Li, B.; Li, J.; Fu, Y.; Bo, Z. J. Am. Chem. Soc. 2004, 126, 3430-3431.
(b) de Bettignies, R.; Nicolas, Y.; Blanchard, P.; Levillain, E.; Nunzi, J.-
M.; Roncali, J. AdV. Mater. 2003, 15, 1939-1943. (c) Ponomarenko, S.
A.; Kirchmeyer, S.; Elschner, A.; Huisman, B.-H.; Karbach, A.; Drechsler
D. AdV. Funct. Mater. 2003, 13, 591-596. (d) Cherioux, F.; Guyard, L.
AdV. Funct. Mater. 2001, 11, 305-309.
(4) (a) Grice, A. W.; Bradley, D. D. C.; Bernius, M. T.; Inbasekaran, M.; Wu,
W. W.; Woo, E. P. Appl. Phys. Lett. 1998, 73, 629-631. (b) Redecker,
M.; Bradley, D. D. C.; Inbasekaran, M.; Wu, W. W.; Woo, E. P. AdV.
Mater. 1999, 11, 241-246. (c) Bernius, M. T.; Inbasekaran, M.; O’Brien,
J.; Wu, W. AdV. Mater. 2000, 12, 1737-1750. (d) Scherf, U.; List, E. J.
W. AdV. Mater. 2002, 7, 477-487. (e) Leclerc, M. J. Polym. Sci. A, Polym.
Chem. 2001, 39, 2867-2873. (f) Whitehead, K. S.; Grell, M.; Bradley, D.
D. C.; Jandke, M.; Strohriegl, P. Appl. Phys. Lett. 2000, 76, 2946-2948.
(5) Lin, W.-J.; Chen, W.-C.; Wu, W.-C.; Niu, Y.-H.; Jen, A. K.-Y. Chem.
Mater. 2004, 37, 2335-2341.
† University of Manchester.
§ National Academy of Sciences of Ukraine.
Imperial College London.
‡ On leave from the National Academy of Sciences of Ukraine.
(1) Hadjichristidis, N.; Pitsikalis, M.; Pispas, S.; Iatrou, H. Chem. ReV. 2001,
101, 3747-3792.
(2) Electronic Materials: The Oligomeric Approach; Mu¨llen, K., Wegner, G.,
Eds.; Wiley-VCH: Weinheim, New York, 1998.
9
10.1021/ja039228n CCC: $27.50 © 2004 American Chemical Society
J. AM. CHEM. SOC. 2004, 126, 13695-13702
13695

A R T I C L E S
Kanibolotsky et al.
Chart 1
morphological properties of materials. From a practical stand-
point, chemical purity and uniformity are often critical param-
eters of the performance of LEDs. Several publications dem-
onstrated that the performance and the stability of monodisperse
oligofluorenes in LEDs are higher than those for polyfluorenes,6d
in which fast degradation and an appearance of undesirable
green emission during the LED operation are observed due to
formation of fluorenone defects on the polymer chain.7 In the
absence of chain entanglements or defects (e.g., bends or kinks),
relatively short conjugated chains are believed to be conducive
to the formation of monodomain films. However, oligomers are
generally more inclined to form a crystalline state than polymers,
which can scatter the light and limit charge injection and
transport in LEDs. In this regard, oligomeric materials capable
of demonstrating a glass transition state while resisting crystal-
lization are better candidates for LED applications.
10,15-Dihydro-5H-diindeno[1,2-a;1′,2′-c]fluorene (truxene,
1), a polycyclic aromatic system with C3 symmetry,8 has been
recognized as a potential starting material for the construction
of bowl-shaped fragments of the fullerenes,9 C3 tripodal
materials in chiral recognition,10 and liquid crystalline com-
pounds.11,12 Some syn-trialkylated truxenes (monoalkylated at
each CH2 group) have been shown to self-associate in solution
through arene-arene interactions.13 Recently, Echavarren and
co-workers synthesized a series of sterically crowded 5,10,15-
triarylated truxenes (aryl ) 1- or 2-naphthyl, 9-phenanthryl,
9-anthracenyl). They also prepared overcrowded 5,10,15-tri(9-
fluorenylidene)truxene, which showed reversible reduction peaks
in cyclic voltammetry at -0.94, -1.18, and -1.45 V (vs SCE).14
Truxene can be considered as three “overlapping” fluorene
fragments, and in this sense it represents an excellent choice as
a core for construction of two-dimensional star-shaped oligo-
fluorenes. Its CH2 groups can be easily functionalized (e.g., by
alkyl subsituents to increase the solubility and processability
of the materials) in the same manner as that exploited in oligo/
polyfluorenes; subsequent oligomers constituting oligofluorene
arms and the central truxene core can be virtually considered
as “no-core” star-shaped oligofluorenes. Recently, Pei and co-
workers reported on employing the truxene core for star-shaped
oligothiophene architectures and an elegant synthesis of truxene-
based dendrimers.15
In this paper, we describe the strategy toward novel mono-
disperse, star-shaped oligofluorenes T1-T4 (Chart 1) with a
central truxene core and up to quaterfluorene arms, which results
in nanosized macromolecules. In the case of T4, the radius of
the macromolecule is ca. 4 nm, which represents the largest
known star-shaped conjugated system. We also report on their
highly efficient blue light emission, together with high thermal
and electrochemical stability.
(6) (a) Geng, Y.; Trajkovska, A.; Culligan, S. W.; Ou, J. J.; Chen, H. M. P.;
Katsis, D.; Chen, S. H. J. Am. Chem. Soc. 2003, 125, 14032-14038. (b)
Geng, Y.; Trajkovska, A.; Katsis, D.; Ou, J. J.; Culligan, S. W.; Chen, S.
H. J. Am. Chem. Soc. 2002, 124, 8337-8347. (c) Wong, K.-T.; Chien,
Y.-Y.; Chen, R.-T.; Wang, C.-F.; Lin, Y.-T.; Chiang, H.-H.; Hsieh, P.-Y.;
Wu, C.-C.; Chou, C. H.; Su, Y. O.; Lee, G.-H.; Peng, S.-M. J. Am. Chem.
Soc. 2002, 124, 11576-11577. (d) Geng, Y.; Culligan, S. W.; Trajkowska,
A.; Wallace, J. U.; Chen, S. H. Chem. Mater. 2003, 15, 542-549. (e) Geng,
Y.; Chen, A. C. A.; Ou, J. J.; Chen, S. H. Chem. Mater. 2003, 15, 4352-
4360. (f) Culligan, S. W.; Geng, Y.; Chen, S. H.; Klubek, K.; Vaeth, K.
M.; Tang, C. W. AdV. Mater. 2003, 15, 1176-1180. (g) Lee, S. H.
Nakamura, T.; Tsutsui, T. Org. Lett. 2001, 3, 2005-2007. (h) Klaerner,
G.; Miller, R. D. Macromolecules 1998, 31, 2007-2009.
(7) (a) List, E. J. W.; Guentner, R.; de Freitas, P. S.; Scherf, U. AdV. Mater.
2002, 14, 374-378. (b) Gaal, M.; List, E. J. W.; Scherf, U. Macromolecules
2003, 36, 4236-4237. (c) Gong, X.; Iyer, P. K.; Moses, D.; Bazan, G. C.;
Heeger, A. J.; Xiao, S. S. AdV. Funct. Mater. 2003, 13, 325-329.
(8) Go´mez-Lor, B.; de Frutos, OÄ .; Ceballos, P. A.; Granier, T.; Echavarren,
A. M. Eur. J. Org. Chem. 2001, 2107-2114.
(9) (a) Abdourazak, A. H.; Marcinow, Z.; Sygula, A.; Sygula, R.; Rabideau,
P. W. J. Am. Chem. Soc. 1995, 117, 6410-6411. (b) Demlow, E. V.; Kelle,
T. Synth. Commun. 1997, 27, 2021-2031. (c) Plater, M. J.; Praveen, M.;
Howie, A. R. J. Chem. Res. (S) 1997, 46-47; J. Chem. Res. (M) 1997,
0430-0436. (d) Go´mez-Lor, B.; de Frutos, OÄ .; Echavarren, A. M. Chem.
Commun. 1999, 2431-2432. (e) Go´mez-Lor, B.; Gonza´lez-Cantalapiedra,
E.; Ruiz, M.; de Frutos, OÄ .; Ca´rdenas, D. J.; Santos, A.; Echavarren, A.
M. Chem. Eur. J. 2004, 10, 2601-2608.
Results and Discussion
Synthesis. The strategy used in the synthesis of truxene-
oligofluorenes T1-T4 was different from the repetitive diver-
gent approach exploited by Pei and co-workers for the design
of star-shaped oligomers (an increase of the arm size by
repetitive addition of arm units to the star-shaped oligomer
through Suzuki coupling).15a,16 In our case, the divergent method
gave very low yields, and the separation of the desired oligomers
from mono- and disubstituted byproducts and homocoupling
(10) For the review on C3-symmetrical receptors, see: Moberg, C. Angew.
Chem., Int Ed. 1998, 37, 248-268.
(11) (a) Destrade, C.; Malthete, J.; Tinh, N. H.; Gasparoux, H. Phys. Lett. A
1980, 78, 82-84. (b) Tinh, N. H.; Malthete, J.; Destrade, C. Mol. Cryst.
Liq. Cryst. 1981, 64, 291-298. (c) Destrade, C.; Gasparoux, H.; Babeau,
A.; Tinh, N. H.; Malthete, J. Mol. Cryst. Liq. Cryst. 1981, 67, 693-703.
(d) Foucher, P.; Destrade, C.; Tinh, N. H.; Malthete, J.; Levelut, A. M.
Mol. Cryst. Liq. Cryst. 1984, 108, 219-230. (e) Tinh, N. H.; Foucher, P.;
Destrade, C.; Levelut, A. M.; Malthete, J. Mol. Cryst. Liq. Cryst. 1984,
111, 277-292. (f) Raghunathan, V. A.; Madhusudana, N. V.; Chan-
drasekhar, S.; Destrade, C. Mol. Cryst. Liq. Cryst. 1987, 148, 77-84. (g)
Buisine, J. M.; Cayuela, R.; Destrade, C.; Tinh, N. H. Mol. Cryst. Liq.
Cryst. 1987, 144, 137-160.
(13) (a) de Frutos, OÄ .; Go´mez-Lor, B.; Granier, T.; Monge, M. AÄ .; Gutie´rrez-
Puebla, E.; Echavarren, A. M. Angew. Chem., Int. Ed. 1999, 38, 204-207.
(b) (a) de Frutos, OÄ .; Granier, T.; Go´mez-Lor, B.; Jime´nez-Berbero, J.;
Monge, AÄ .; Gutie´rrez-Puebla, E.; Echavarren, A. M. Chem. Eur. J. 2002,
8, 2879-2890.
(12) (a) Perova, T. S.; Vij, J. K. AdV. Mater. 1995, 7, 919-922. (b) Sandstroem,
D.; Nygren, M.; Zimmermann, H.; Maliniak, A. J. Phys. Chem. 1995, 99,
6661-6669. (c) Fontes, E.; Heiney, P. A.; Ohba, M.; Haseltine, J. N.; Smith,
A. B. Phys. ReV. A 1988, 37, 1329-1334. (d) Lee, W. K.; Wintner, B. A.;
Fontes, E.; Heiney, P. A.; Ohba, M.; Haseltine, J. N.; Smith, A. B. Liq.
Cryst. 1989, 4, 87-102. (e) Lee, W. K.; Heiney, P. A.; Ohba, M.; Haseltine,
J. N.; Smith, A. B. Liq. Cryst. 1990, 8, 839-850. (f) Maliszewskyj, N. C.;
Heiney, P. A.; Blasie, J. K.; McCauley, J. P.; Smith, A. B. J. Phys. II
1992, 2, 75-85. (g) Goldfarb, D.; Belsky, I.; Luz, Z.; Zimmermann, H. J.
Chem. Phys. 1983, 79, 6203-6210.
(14) Ruiz, M.; Go´mez-Lor, B.; Santos, A.; Echavarren, A. M. Eur. J. Org. Chem.
2004, 858-866.
(15) (a) Pei, J.; Wang, J.-L.; Cao, X.-Y.; Zhou, X.-H.; Zhang, W.-B. J. Am.
Chem. Soc. 2003, 125, 9944-9945. (b) Cao, X.-Y.; Zhang, W.-B.; Wang,
J.-L.; Zhou, X.-H.; Lu, H.; Pei, J. J. Am. Chem. Soc. 2003, 125, 12430-
12431.
(16) Zhou, X.-H.; Yan, J.-C.; Pei, J. Org. Lett. 2003, 5, 3543-3546.
9
13696 J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004

Monodisperse Oligofluorene-Functionalized Truxenes
A R T I C L E S
Scheme 1. Synthesis of Hexahexyltruxene (T0) and Tribromo-hexahexyltruxene 2
Scheme 2. Synthesis of Oligofluorene-2-boronic Acids F1, F2, F3, and F4, and 2-Bromo-oligofluorenes 5, 6, and 7
byproducts proved difficult. Instead, we attached oligofluorene
arms of corresponding length directly to the central truxene core
of T0. For this purpose we first synthesized, starting from
truxene 1, the hexahexylated truxene core with bromine terminal
substituents 2 (Scheme 1), exploiting procedures similar to those
described for alkylation17 and bromination18 of fluorene in the
synthesis of 2,7-dibromo-9,9-dihexylfluorene 3.
The synthesis of 2-bromo-oligofluorenes 5-7 and oligofluo-
rene-2-boronic acids F1-F4 is depicted in Scheme 2. In general,
we used a repetitive procedure for the conversion of bromo
derivatives 4-7 into the corresponding boronic acids F1-F4
via lithiathion with n-BuLi, followed by quenching with
triisopropyl borate and hydrolysis under acidic conditions. The
boronic acids were then coupled by Suzuki methodology with
2,7-dibromo-9,9-dihexylfluorene 3, affording the next generation
of 2-bromo-oligofluorene (a 3-fold excess of 3 over boronic
acids F1-F3 was used to diminish bis-coupling at both sides
of 2,7-dibromo 9,9-dihexylfluorene 3).
It is known that arylboronic acids form dimers through
intermolecular hydrogen bonding19 or infinite networks in the
case of polyfunctional boronic acids.20 As a result, OH protons
are often “invisible” in their 1H NMR spectra. Moreover, they
easily lose water, yielding the corresponding tricyclic anhy-
drides, boroxines.21 In our case, we observed this when com-
1
paring the H NMR spectra of “as obtained” fluorene boronic
acids (from the column after purification) with the spectra of
(17) (a) Ranger, M.; Leclerc, M. Chem. Commun. 1997, 1597-1598. (b) Ranger,
M.; Rondeau, D.; Leclerc, M. Macromolecules 1997, 30, 7686-7691.
(18) Cho, H. N.; Kim, J. K.; Kim, D. Y.; Kim, C. Y.; Song, N. W.; Kim, D.
Macromolecules 1999, 32, 1476-1481.
9
J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004 13697

A R T I C L E S
Kanibolotsky et al.
Scheme 3. Dehydration of Fluorene Boronic Acids F1-F4 into Boroxines F1b-F4b
Table 1. Fluoreneboronic Acids F1, F2, F3, and F4
calculated (acid), %
C
calculated (anhydride), %
found, %
1
1
compd
MW, g mol-
H
B
MW, g mol-
C
H
B
C
H
B
F1
F2
F3
F4
378.36
710.88
1043.40
1375.92
79.36
84.48
86.33
87.29
9.32
9.50
9.56
9.60
2.86
1.52
1.04
0.79
1081.02
2078.58
3076.15
4073.71
83.33
86.67
87.85
88.45
9.23
9.46
9.53
9.58
3.00
1.56
1.05
0.80
82.79
86.32
87.30
87.49
9.39
9.82
9.63
9.84
2.81
1.49
1.16
0.87
Scheme 4. Suzuki-Coupling Route to T1 and T2 Truxene-Fluorene Oligomers
the same samples when they were dried in vacuo at 40-70 °C.
Initially complicated spectra became simpler as a result of the
transformation of the acids into boroxines (Scheme 3). Accord-
ing to elemental analyses (the analysis on carbon is the most
sensitive for this transformation), drying the samples converted
fluorene-2-boronic acids F1 and F2 almost completely into the
corresponding boroxines (Table 1), whereas the longer fluorene
chain products probably existed as a mixture of the acid and
the boroxine.
(19) (a) Rettig, S. J.; Trotter, J. Can. J. Chem. 1977, 55, 3071-3075. (b)
Zhdankin, V. V.; Persichini, P. J., III; Zhang, L.; Fix, S.; Kiprof, P.
Tetrahedron Lett. 1999, 40, 6705-6708. (c) Schilling, B.; Kaiser, V.;
Kaufmann, D. E. Chem. Ber. 1997, 130, 923-932. (d) Akita, T.; Kobayashi,
K. AdV. Mater. 1997, 9, 346-348. (e) Bradley, D. C.; Harding, I. S.; Keefe,
A. D.; Motevalli, M.; Zheng, D. H. J. Chem. Soc., Dalton Trans. 1996,
3931-3936. (f) Pilkington, M.; Wallis, J. D.; Larsen, S. J. Chem. Soc.,
Chem. Commun. 1995, 1499-1500. (g) Gainsford, G. J.; Meinhold, R. H.;
Woolhouse, A. D. Acta Crystallogr., Sect. C 1995, C51, 2694-2696. (h)
Scouten, W. H.; Liu, X.-C.; Khangin, N.; Mullica, D. F.; Sappenfield, E.
L. J. Chem. Cryst. 1994, 24, 621-626. (i) Soundararajan, S.; Duesler, E.
N.; Hageman, J. H. Acta Crystallogr., Sect. C 1993, C49, 690-693. (j)
Feulner, H.; Linti, G.; Noth, H. Chem. Ber. 1990, 123, 1841-1843.
Because the reactivities of both boronic acids and boroxines
are similar and the presence of water in the Suzuki coupling
reaction results in the hydrolysis of boroxines back to the acids,
we use the term “fluorene boronic acids” throughout the paper,
(20) Fournier, J.-H.; Maris, T.; Wuest, J. D.; Guo, W.; Galoppini, E. J. Am.
Chem. Soc. 2003, 125, 1002-1006.
(21) (a) Li, W.; Nelson, D. P.; Jensen, M. S.; Hoerrner, R. S.; Cai, D.; Larsen,
R. D.; Reider, P. J. J. Org. Chem. 2002, 67, 5394-5397. (b) Wu, Q. G.;
Wu, G.; Brancaleon, L.; Wang, S. Organometallics 1999, 18, 2553-2556.
9
13698 J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004

Monodisperse Oligofluorene-Functionalized Truxenes
A R T I C L E S
Scheme 5. Negishi-Coupling Route to T2, T3, and T4 Truxene-Fluorene Oligomers
9
J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004 13699

A R T I C L E S
Kanibolotsky et al.
Figure 1. Cyclic voltammogram of truxene-fluorene derivatives T1 (top)
and T3 (bottom); electrolyte 0.1 M Bu4NPF6, scan rate 100 mV s-1
.
Figure 2. Absorption (dash lines) and photoluminescence (solid lines)
spectra of truxene-oligofluorenes T1-T4 in toluene (A) and in films (B).
Table 2. CV Data for Compounds T1-T4
1/
2
1/
2
CV
Eg ,
c eV
compd
Eox
,
a V vs Fc/Fc+
Ered
,
b V vs Fc/Fc+
V (vs Fc/Fc+), the oxidation processes were more complex
(although a negative shift in the first oxidation peaks from +0.80
to +0.74 V in the range of T1-T4 was observed). For longer
oligomers T3 and T4, current values for the first and second
oxidation waves are very close, indicating that the same number
of electrons are involved in both oxidation processes. Due to
partial overlapping of the first and second oxidation waves, the
difference between the anodic and cathodic peaks cannot be
precisely determined. The difference in oxidation/reduction
peaks is somewhat higher than the theoretical value of ∆Epa-pc
) 59 mV, but both oxidation processes are essentially single-
electron steps (resulting in radical cation and dication species,
respectively). Recently, Bard et al. showed that 9,9-disubstituted
terfluorenes show two consecutive reversible single-electron
oxidation peaks with a difference E2ox - E1ox ≈ 0.16-0.17 eV,
which is close to our data (Table 2).23 Thus, for longer oligomers
due to delocalization of the charges along the arms, no
substantial electronic interactions between the arms are observed
and CV peaks belong to tris(radical cation) and tris(dication)
species. For the shorter oligomer T1, the current value for the
second oxidation is twice the height of the first one, and
oligomer T2 showed several overlapped oxidation peaks (Figure
S1, Supporting Information). This could be a result of electronic
interactions between the oligofluorene arms upon oxidation,
which are not electronically isolated. Moreover, the central
truxene core can also be involved in the oxidation process for
short oligomers. All the oligomers are high-band-gap (Eg)
materials; EgCV estimated from the difference between the onsets
of the reduction wave and the first oxidation peak was found
to vary in the range 3.20-3.40 eV (Table 2).
T1
T2
T3
T4
0.80, 1.05
0.76, 0.84, 1.03
0.76, 0.94
-2.80
3.40
3.30
3.24
3.20
-2.74
-2.70, -2.83
-2.66, -2.74
0.74, 0.87
a DCM, 0.1 M Bu4NPF6, 100 mV s-1
s-1 c Band gap estimated from the onsets for the reduction and oxidation
peaks.
.
b THF, 0.1 M Bu4NPF6, 100 mV
.
understanding, however, that the isolated compounds were (after
drying) the boroxines in some cases.
For shorter oligomers T1 (yield 50%) and T2 (yield 23%),
we used Suzuki cross-coupling of 2-fluorene(bifluorene) boronic
acids with tribromotruxene derivative 2 (Scheme 4). For the
synthesis of longer oligomers, we prepared 9,9-dihexylated
2-bromo-oligofluorenes by an iterative procedure of Suzuki
coupling of (n)-fluorene-2-boronic acids with an excess of 9,9-
dihexyl-2,7-dibromofluorene (3), resulting in (n+1)-fluoren-2-
yl bromides (in these syntheses the yields achieved were good
to excellent, 83-94%); see also ref 22. The monobromo-
oligomers were then converted into zinc derivatives via reaction
with BuLi and ZnCl2 for Negishi type cross-coupling with 2,
affording derivatives T2-T4 in 29-37% yields (Scheme 5).
1
Oligomers T1-T4 were adequately characterized by H and
13C NMR, MALDI-TOF MS, and elemental analyses data.
Electrochemistry. Cyclic voltammetry (CV) experiments
were conducted to probe the electrochemical properties of
the oligomers. The results revealed two sequential single-
electron reversible oxidation peaks [in dichloromethane (DCM)]
and reversible or partly reversible reductions [in tetrahydro-
furan (THF)], demonstrating good electrochemical stability
of the oligomers toward both p- and n-doping (Figure 1,
Table 2).
(22) Geng, Y.; Trajkovska, A.; Katsis, D.; Ou, J. J.; Culligan, S. W.; Chen, S.
H. J. Am. Chem. Soc. 2002, 124, 8337-8347.
(23) Choi, J.-P.; Wong, K.-T.; Chen, Y.-M.; Yu, J.-K.; Chou, P.-T.; Bard, A. J.
J. Phys. Chem. B 2003, 107, 14407-14413.
Whereas the reduction of the oligomers T1-T4 to radical
anions demonstrated regular positive shifts from -2.80 to -2.66
9
13700 J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004

Monodisperse Oligofluorene-Functionalized Truxenes
A R T I C L E S
Table 3. Optical Properties of Star-Shaped Oligofluorene-Truxene Derivatives T1-T4
a
a
opt
b
λ
abs, nm [log
(toluene)
ꢀ]
λ
abs, nm
λ
PL, nm
λ
(film)
PL, nm
ΦPL
ΦPL
Eg
,
compd
(film)
(toluene)
(toluene)
(film)
nm
T1
T2
T3
T4
343 [4.97]
360 [5.50]
370 [5.61]
374 [5.67]
343
359
369
372
375sh, 396, 416sh
399, 422, 443sh
408, 431, 460sh
411, 436, 460sh
380sh, 398, 419.5
404, 425.5, 449
417sh, 436, 462sh
422, 442, 467sh
0.70
0.83
0.83
0.86
0.43
0.51
0.60
0.59
3.29
3.14
3.08
3.05
a Estimated error in PL quantum efficiencies ∼(10%. Quinine sulfate was used as a standard in estimation of PL efficiencies in solutions (ΦPL
)
0.546%). b Band gap estimated from the red edge of the longest wavelength absorption (in toluene).
Absorption and Emission Spectra. The absorption spectra
for compounds T1-T4 both in solution and in the solid state
show strong π-π* electron absorption bands, which progres-
sively red-shift with increasing chain length from 343 to 374
nm (Figure 2, Table 3). The maxima and shapes of the
absorption peaks are the same for the solutions (cyclohexane
or toluene) and films (Figure 2A, Figure S3, and Table 3), and
the energies are in linear correlation (r ) 0.998-0.999) with
the inverse number of benzene units (n) in each arm (Figure
S4). The absorption maxima corresponding to the energy values
extrapolated to n ) ∞ (λn)∞ ) 399 nm in cyclohexane or
toluene, 396 nm in solid) are between the extrapolated values
for linear oligofluorenes (402 nm in tetrahydrofuran) and the
experimental value of λmax ) 388 nm for poly(9,9-di-
hexylfluorene).6h Oligomers T1-T4 are highly fluorescent
materials both in solution and in the solid state, with photolu-
minescence (PL) quantum efficiencies (ΦPL) for higher oligo-
mers close to those for linear polyfluorenes. PL spectra are red-
shifted from T1 to T4 and show vibronic structure typical for
polyfluorenes. An interesting feature of the solution-phase
spectra is that the spectral shape is independent of conjugation
length, as opposed to those for the solid state, where the
vibrational progression bands change relative intensity. The band
gaps for oligomers T4-T1 of 3.05-3.29 eV, estimated from
the red edge of the longest wavelength absorption in the
electronic spectra, was found to be somewhat lower (by
0.11-0.16 eV) but still close to those determined from the
electrochemical experiments (Tables 2 and 3).
Figure 3. Absorption spectra of chemically generated radical cations from
truxene-oligofluorenes T1-T4 (λmax ) 497/525, 543, 567, and 572 nm,
respectively).
Spectroscopy of Radical Cations. Considering the revers-
ibility in the oxidation of oligomers T1-T4 under CV condi-
tions, we performed oxidative titration with 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ)/CF3CO2H in DCM.24 The
radical cations generated show intense absorption in the visible
region of the spectrum (497-572 nm) with red shifts from T1
to T4 (Figure 3).
Radical cations from truxene-oligofluorenes T1-T4 have also
been generated and studied by spectroelectrochemistry (SEC)
(Figure 4; Figures S5-S7, Supporting Information). On elec-
trochemical oxidation of compounds T1-T4, the intensity of
absorption at ∼340-400 nm decreased and new intense bands
at ∼460-600 nm appeared, the maxima and the shape of which
corresponded well to the chemically generated radical cations
(Figure 3). It should be noted that, on full conversion of the
neutral molecules to radical cations, the short-wavelength bands
(∼340-400 nm) did not completely disappear (Figures 4A, S5,
S6, and S7A). Upon further oxidation of the longer oligomers
T3 and T4, we also observed a decrease in the absorption
maximum for the radical cation species and a hypsochromic
shift of short-wavelength bands at more positive potentials
Figure 4. Spectroelectrochemistry of truxene-fluorene derivative T3, DCM,
0.1 M Bu4NPF6, Pt wire reference electrode. (A) Generation of radical cation
from T3; (B) its further oxidation.
(Figures 4B and S7B). When the oxidation is limited to a
potential range corresponding to the generation of radical cat-
ions, the process is completely reversible, and the initial spectra
of the neutral species are completely restored. Several cycles
of oxidation-reduction did not show any degradation of the
(24) Sep, W. J.; Verhoeven, J. W.; De Boer, Th. J. Tetrahedron 1979, 35, 2161-
2168.
9
J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004 13701

A R T I C L E S
Kanibolotsky et al.
Table 4. Glass Transition Temperatures and Thermal Stability of
Derivatives T1-T4
and ΦPL of the pristine spin-coated (from toluene) and thermally
treated (150-200 °C, 0.5 h) films are essentially the same
(within the error of the measurement), thereby confirming their
good thermal and morphological stability.
1
compd
Mw, g mol-
Tg,a
°
C
Td,b
°C [TGA, 5% mass loss]
T1
T2
T3
T4
1844.95
2842.52
3840.08
4837.65
63
86
101
116
401
408
410
413
Conclusion
In summary, we have presented a facile approach to highly
luminescent soluble monodisperse star-shaped oligofluorenes
with a truxene central core. The materials possess high ther-
mal (>400 °C according to TGA) and electrochemical sta-
bility (to both p- and n-doping) and emit bright blue light (λPL
≈ 400-420 nm) with quantum efficiencies of ∼50-60% in
the solid state. The two-dimensional architecture of these
oligomers facilitates their increased stability toward self-asso-
ciation through arene-arene stacking, which improves the amor-
phous properties of the materials in the solid state. Excellent
film-forming properties of the high-generation oligomers and
nanoscale size of the macromolecules (∼1.7 × 7 nm for T4)
make them potentially fascinating nano-objects for several
optoelectronic applications, which are now under investigation.
We also demonstrated electrochromic behavior of truxene-
oligofluorenes T1-T4, which reversibly change their color from
colorless in the neutral state (∼340-400 nm) to red or purple
color in the oxidized state (∼500-600 nm).
a Measured at 10 °C/min. b Measured at 5 °C/min. When heating rate
was increased to 25 °C/min, the decomposition temperatures were ca. 10-
15 °C higher.
materials, providing further evidence for a highly reversible
process. Thus, oligomers T1-T4 reperesent reversible electro-
chromic systems with intense purple color in the oxidized state.
Thermogravimetric Analysis and Differential Scanning
Calorimetry. For optoelectronic applications, the thermal and
morphological stabilities are critical factors for device stability
and lifetime. The compounds T1-T4 demonstrated good
thermal stability with no decomposition below 400 °C in an
inert atmosphere (Table 4, Figure S8). The thermal stability of
the oligomers was slightly increased from T1 to T4 by 10-15
°C. TGA experiments showed loss of ca. 45-50% of mass in
the range of 420-480 °C for all the oligomers and further mass
stability until 600 °C. This corresponds well to the abstraction
of all hexyl substituents from the oligomers and probably results
in stable polyarene materials. All of the oligomers are amor-
phous materials (some crystallinity is seen for T1) at room
temperature, and their glass transition temperatures are improved
from 63 °C for T1 to 116 °C for T4. These values are 20-28
°C higher than those for the corresponding star-shaped oligo-
fluorenes with a benzene central core,15 while for the largest
oligomer T4 the glass transition temperature is even higher than
that for poly(9,9-dihexylfluorene) (Tg ) 103 °C; Td ) 390 °C).25
Both linear and spiro-configured oligo(9,9-dialkylfluorenes)
have been shown to possess comparable glass transition
temperatures (∼60-150 °C), which vary substantially with the
alkyl substituents at position 9 of the fluorene moiety.6d,f,26
Heating films of the materials under a microscope using a cross-
polarized configuration did not show any liquid crystalline
behavior, contrary to what is generally observed in polyfluor-
enes.27,4d Furthermore, upon cooling, the materials stay in an
amorphous glassy phase. The shapes of the fluorescence spectra
Acknowledgment. We thank The Royal Society and NATO
for a Postdoctoral Fellowship (A.L.K.) and NATO for an Expert
Visit to Manchester (I.F.P.).
Supporting Information Available: Experimental Section
(instrumentation, synthetic procedures, and characterization for
all novel compounds, CV and spectroelectrochemistry experi-
ments); CV, UV-vis, SEC, TGA, and molecular modeling
details (Figures S1-S9; Table S1) (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
JA039228N
(25) Liu, B.; Yu, W.-L.; Lai, Y.-H.; Huang, W. Chem. Mater. 2001, 13, 1984-
1991.
(26) Katsis, D.; Geng, Y. H.; Ou, J. J.; Culligan, S. W.; Trajkovska, A.; Chen,
S. H.; Rothberg, L. J. Chem. Mater. 2002, 14, 1332-1339.
(27) (a) Grell, M.; Bradley, D. D. C.; Inbasekaran, M.; Woo, E. P. AdV. Mater.
1997, 9, 798-802. (b) Grell, M.; Redecker, M.; Whitehead, K.; Bradley,
D. D. C.; Inbasekaran, M.; Woo, E. P. Liq. Cryst. 1999, 26, 1403-1407.
(c) Sirringhaus, H.; Wilson, R. J.; Friend, R. H.; Inbasekaran, M.; Woo, E.
P.; Grell M.; Bradley, D. D. C. Appl. Phys. Lett. 2000, 77 406-408.
9
13702 J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004