Synthesis and optical behaviour of monodispersed oligo(fluorenylidene)s

Article abstract of DOI:10.1016/j.tetlet.2008.01.126

The synthesis of the first series of monodispersed oligo(fluorenylidene)s is reported. The synthetic sequence envisages as key steps the obtainment (a) of a bis-pinacolboronate fluorenylidene dimer by a Ni(cod)₂ promoted homocoupling of the suitable 7-bromo-fluorenylidene boronic ester and (b) of the bis-pinacolboronate fluorenylidene trimer and tetramer by a Ni(cod)₂ promoted coupling of a 7-bromo-fluorenylidene boronic ester with 0.5 equiv of the corresponding 2,7-dibromofluorenylidene derivative. The optical properties of the obtained oligomers were studied. In the case of OF3 the optical behaviour was compared to that of a suitably synthesized fluorenone-containing model compound.

Full text of DOI:10.1016/j.tetlet.2008.01.126

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 2078–2082
Synthesis and optical behaviour of monodispersed
oligo(fluorenylidene)s
a
a
b
a,
*
Roberto Grisorio , Piero Mastrorilli , Giuseppe Ciccarella , Gian Paolo Suranna ,
Cosimo Francesco Nobile a
a Department of Water Engineering and Chemistry (DIAC), Polytechnic of Bari, via Orabona, 4 I-70125 Bari, Italy
b National Nanotechnology Laboratory (NNL) of INFM-CNR, Distretto Tecnologico ISUFI, via Arnesano, and Dipartimento di Ingegneria dell’Innovazione,
`
Universita del Salento, via Arnesano, 73100 Lecce, Italy
Received 8 January 2008; revised 25 January 2008; accepted 30 January 2008
Available online 2 February 2008
Abstract
The synthesis of the first series of monodispersed oligo(fluorenylidene)s is reported. The synthetic sequence envisages as key steps the
obtainment (a) of bis-pinacolboronate fluorenylidene dimer by Ni(cod)2 promoted homocoupling of the suitable
a
a
7-bromo-fluorenylidene boronic ester and (b) of the bis-pinacolboronate fluorenylidene trimer and tetramer by a Ni(cod)2 promoted
coupling of a 7-bromo-fluorenylidene boronic ester with 0.5 equiv of the corresponding 2,7-dibromofluorenylidene derivative. The
optical properties of the obtained oligomers were studied. In the case of OF3 the optical behaviour was compared to that of a suitably
synthesized fluorenone-containing model compound.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Fluorenylidene; Monodispersed oligomers; Ni(cod)2-promoted coupling; Fluorenone
The tuning of selective synthetic methods aimed at
obtaining well-defined oligomers is considered a valuable
aid for the organic chemist working on extensively conju-
gated materials. In this framework, the p-phenylene motif
is ubiquitous and has been investigated in an uncountable
number of variants. The introduction of a carbon atom
bridging two linked p-phenylene rings (to obtain a fluorene
structure) offered the possibility of decorating the biphenyl
building block with solubilizing alkyl chains.1 The sp3
hybridization of this carbon atom, however, forces the sub-
stituents to adopt an orthogonal disposition with respect to
the aromatic system.2 A completely different behaviour is
to be expected if the two phenyl rings are connected by
an sp2 hybridized C-9, a situation occurring in fluorenyl-
idene systems. In these molecules the C-9 carbon atom
forces the whole building block to assume a planar geo-
metry, so that the flattened structure should favour strong
intermolecular interactions in the solid state. The chemistry
of fluorenylidene-based materials is rather young. The first
proposed fluorenylidene-based materials have been poly-
mers conceived for applications in field effect transistors
(FET). Poly(fluorenylidene)s showed a good hole trans-
porting ability deriving from the close packing exhibited
in the solid state.3
The obtainment of monodispersed oligo(fluorenyl-
idene)s through a step-by-step convergent approach would
be an intriguing synthetic challenge owing to the reactivity
of the fluorenylidene double bond towards nucleophiles
and electrophiles, posing constrains on the preparation
sequence. Consequently, the strategies for the obtainment
of monodispersed oligomers envisaging halogenation
and/or metallation of the building blocks, commonly
used to obtain monodispersed oligofluorenes,4 should be
excluded in the case of a fluorenylidene-based material, in
*
Corresponding author. Tel.: +39 0805963603; fax: +39 0805963611.
E-mail address: surannag@poliba.it (G. P. Suranna).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.01.126

R. Grisorio et al. / Tetrahedron Letters 49 (2008) 2078–2082
2079
order to avoid unsatisfactory yields and difficult purifica-
tions of the obtained products. To date, no reports exist
on the synthesis and optical behaviour of monodispersed
oligo(fluorenylidene)s.
mentioned synthetic protocol to the preparation of reactive
bis-functionalized dimers, trimers and tetramers. To test
the width of its scope, we applied it to the preparation of
monodispersed fluorenylidene trimer, tetramer, pentamer
and hexamer. The synthesis of the four oligomers was
completed with a study of their optical properties both in
solution and in the solid state.
The target molecules were prepared through a conver-
gent synthetic approach (Scheme 1). Starting from 2-bromo-
fluorene (1), the initial step was its condensation with
carbon disulfide in the presence of potassium tert-butylate,
followed by a reaction with methyl iodide, yielding the
ketene dimethyl dithioacetal 2. In the subsequent reaction
step, 2 was treated with 2 equiv of decylmagnesium
bromide to achieve the fluorenylidene derivative 3. Decyl
alkyl chains were chosen to ensure sufficient solubility to
the target molecules. An attempt to prepare the boronic
Following our interest in sharpening the tools of orga-
nometallic chemistry to obtain convenient synthetic meth-
ods for the selective synthesis of p-conjugated building
blocks, we have recently proposed a Ni(cod)2 promoted
homocoupling of bromofluorenes functionalized with
boronic esters as an efficient method for the preparation
of reactive bifluorene building blocks to employ for the
obtainment of quaterfluorenes.5,6 This synthetic approach
allows to bypass halogenation and metallation of a bifluo-
rene substrate, and selectively yields the 7,70-functionalized
bifluorene building block.
In this study we decided to tag the synthesis of well-
defined oligo(fluorenylidene)s by extending the above-
O
O
O
O
B
B
tBuOK
C
10H21MgBr
Li2CuCl4
O
B
Br
Br
Br
CS2 / MeI
PdCl2(dppf)
O
1
3
4
2
C10H21
C10H21
C10H21
C10H21
MeS
SMe
O
O
O
O
B B
O
Br
B
PdCl2(dppf)
O
O
O
B
5
6
C
10H21
C10H21
O
O
O
O
B
B
O
Ni(cod)2
O
O
Br
Br
Br
B
B
9
PdCl2(dppf)
O
O
O
C10H21
C10H21
C10H21
C10H21
C10H21
C10H21
7
8
C10H21
C10H21
O
O
B
O
B
10
O
C10H21
C10H21
C10H21
Ni(cod)2
C10H21
C10H21
7 +
8 (2 eq)
C
10H21
C10H21
C10H21
O
O
B
11
B
O
O
C10H21
C10H21
C10H21
C10H21
OF3
OF4
OF5
OF6
12
7 + 4 (2 eq)
9 + 3 (2 eq)
10 + 3 (2 eq)
11 + 3 (2 eq
7 + 6 (2 eq)
Reaction conditions: catalyst Pd(PPh3)4; K2CO3 (2M)/Toluene
)
Scheme 1. Synthesis of OF3–6 and 12.

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R. Grisorio et al. / Tetrahedron Letters 49 (2008) 2078–2082
ester 4 carried out by the metallation of 3 with n-BuLi and
subsequent reaction with 2-isopropoxy-4,4,5,5-tetramethyl-
[1,3,2]-dioxaborolane proved to be unsuccessful due to
the reactivity of the fluorenylidene moiety towards n-BuLi:
a strong red coloration of the solution was in fact
observed, indicating the formation of fluoren-9-yl anions
due to the attack of n-BuLi onto the double bond of fluor-
enylidene. Compound 4 was obtained in a very modest
yield (27%). On the other hand, the synthesis of 4 was
accomplished in a good (67%) yield by a PdCl2(dppf) cata-
lyzed reaction between 3 and bis-pinacoldiboron in the
presence of potassium acetate as base in DMF at 60 °C.
The synthesis of 8 was carried out analogously by a
PdCl2(dppf) catalyzed reaction between 7 and 1 equiv of
bis-pinacoldiboron.
purified by size exclusion chromatography with BioBeadsÒ
stationary phase. The building blocks 9, 10 and 11 were
used in the final reaction step consisting in the cross-
coupling reaction with 2 equiv of 3 catalyzed by Pd(PPh3)4
(Scheme 1). The reactions yielded oligomers OF4, OF5
and OF6, respectively, in satisfactory to good yields
(54–81%).
Trimer OF3, on the other hand, could be obtained by a
straightforward Suzuki reaction of 7 with 2 equiv of the
boronic ester 4. The reaction yielded OF3 with a 70% yield.
An analogous Suzuki cross-coupling of 7 with 2 equiv of
the fluorenone boronic ester 6 afforded the target molecule
12 in 74% yield. This compound has been used as a model
to mimic the oxidative degradation product of trimer OF3.
The OF3–6 oligomers have been eventually purified by size
exclusion chromatography with BioBeadsÒ stationary
phase. The target oligo(fluorenylidene)s are viscous solids
that had to be kept under inert atmosphere and in the dark,
due to slow decomposition, probably ascribable to the ten-
dency of the fluorenylidene double bond to undergo light
promoted [2+2] cyclization (see Fig. 1).
The key step of the synthetic procedure is the prepara-
tion of the fluorenylidene cores 9, 10 and 11. For the syn-
thesis of 9 we applied the conditions reported for the
preparation of reactive bifluorene building blocks,5 namely
the Ni(cod)2 promoted dimerization of 8 yielding the biflu-
orene bis-boronic ester 9 in 69% yield.
We were pleased to find that the preparation of the supe-
rior homologues (trimer 10 and tetramer 11) could be
achieved by a similar reaction: the Ni(cod)2 promoted cou-
pling of 7 with 2 equiv of 8. The reaction permitted the
obtainment of 10 (41%) and 11 (23%) from the same
reaction mixture. The building blocks 10 and 11 could be
separated by column chromatography and eventually
The purity of 9–12 as well as that of OF3–6 has been
checked by size exclusion chromatography, coupled with
a UV detector operating at 340, 350 and 360 nm (Fig. 2).
ESI-MS provided further confirmation of the obtainment
of the molecular structures of OF3–5. No mass spectrum
could be obtained for OF6 under the conditions used for
the detection of the other monodispersed oligomers.
C10H21
C10H21
OF3
C10H21
C10H21
C10H21
C10H21
C10H21
C10H21
C10H21
C10H21
OF4
C10H21
C10H21
C10H21
C10H21
C10H21
C10H21
C10H21
C10H21
OF5
C10H21
C10H21
C10H21
C10H21
C10H21
C10H21
C10H21
C10H21
C10H21
C10H21
C10H21
C10H21
OF6
C10H21
C10H21
C10H21
C10H21
C10H21
C10H21
C10H21
C10H21
12
O
O
Fig. 1. Chemical structure of OF3–6 and 12.

R. Grisorio et al. / Tetrahedron Letters 49 (2008) 2078–2082
2081
300
350
400
450
500
550
600
9
10
11
A
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
12
OF3
OF4
OF5
OF6
OF3
OF4
OF5
OF6
4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.0 7.2 7.4
elution time (min)
300
300
350
400
450
500
550
600
700
wavelength (nm)
Fig. 2. GPC traces of 9–12 and OF3–6.
350
400
450
500
550
600
650
B
The target molecules OF3–6 have been characterized by
1H and 13C{1H} NMR. The very peculiar behaviour of
these materials, imparted by the sp2 hybridization of the
C-9 carbon was also apparent from their 1H NMR spectra:
the broadening of the signals of OF5 and OF6, with a con-
sequent misfit of the proton integral, can likely be ascribed
to the formation of aggregates even in a good solvent such
as chloroform. The absorption spectra of OF3–6 were
recorded in chloroform solutions (ꢀ10ꢁ5 M) and are
reported in Figure 3A. The absorption profile of the oligo-
mers are characterized by a peculiar shoulder, the intensity
of which increases passing from OF3 to OF6. This absorp-
tion profile is independent of the concentration of the sam-
ple and was observed also in extremely diluted solutions of
OF3–6. Therefore this feature cannot be ascribed to aggre-
gation phenomena. The analysis of the absorption behav-
iour of the fluorenone-containing 12 (Fig. 3C) provided
some insight in this aspect. In this molecule the p–p* charge
transfer transition band is located at a kmax of 440 nm and
the main absorption (kmax = 344 nm) related to the p–p*
transition of the p-conjugated backbone did not show
any transition ascribable to aggregation in solution, not-
withstanding the similarity of the structural architecture
of 12 and OF3.
This absorption shoulder is also present in the absor-
bance spectrum of the correspondenting poly[9-(1-decylun-
decylidene)-2,7-fluorene] obtained by Ni(cod)2 promoted
homocoupling of 7, both in solution and as thin film. In
solution, its absorption profile is independent of the con-
centration and is very similar to that obtained in the solid
state (see Supplementary data). On these basis we tenta-
tively attribute the shoulder in the absorption profile of
OF3–6 to a charge transfer from the p-conjugated back-
bone to the double bonds.
1.0
0.8
0.6
0.4
0.2
0.0
OF3
1.0
0.8
0.6
0.4
0.2
0.0
OF4
OF5
OF6
300
300
350
350
400
450
500
550
600
650
700
wavelength (nm)
400
450 500 550
600
650
700
750
C
OF3 film
1.0
1.0
0.8
0.6
0.4
0.2
0.0
OF3 annealed
12 solution
0.8
12 film
0.6
0.4
0.2
0.0
300
350
400
450
500
550
600
650
700
750
wavelength (nm)
Fig. 3. Optical features of OF3–6 and 12 in CHCl3 solution and in the
solid state.
OF6). Evidently the band gap saturation occurs for the
pentamer and higher oligomers.
In the solid state, the absorption spectra of OF3–6 are
quite broad (Fig. 3B). The increase of the shoulder at
longer wavelengths can be interpreted as the generation
of strong intermolecular interaction in the solid state.
The optical band gap of the four oligomers was estimated
by the onset of the absorption spectra in solution as
2.92 eV (OF3), 2.86 eV (OF4) and 2.79 eV (OF5 and
In chloroform solution, OF3–6 shows a structured blue-
emission (kmax = 414–415 nm). The emission spectra
shown in Figure 3A had to be obtained from solutions of
ꢀ0.05 absorbance. For more concentrated solutions, a low-
ering of the intensity of the first vibronic replica was
observed for all compounds, due to self-absorption pheno-
mena caused by the small Stokes shift of the compounds

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R. Grisorio et al. / Tetrahedron Letters 49 (2008) 2078–2082
in solution. The emission of OF3–6 in the solid state
(Fig. 3B) is strongly red-shifted and shows very close max-
ima (kmax = 536–548 nm) together with the loss of the
vibronic structure. This behaviour can be attributed to
the formation of aggregates and/or excimers in the solid
state. A similar emission behaviour was observed for the
poly[9-(1-decylundecylidene)-2,7-fluorene], showing an
ter- and quater-fluorenylidene, respectively. Their optical
properties in the solid state are consistent with a strongly
aggregating behaviour.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2008.01.126.
emission centred in the blue region in solution (kem
=
445 nm) and a low energy emission at 529 nm dominating
the solid state emission (see Supplementary data).
Since the emission profiles of OF3–6 and of the polymer
in the solid state resemble that of fluorenone-containing
polyfluorenes,7 a further experiment was carried out to rule
out the influence of the possible formation of the fluore-
none acceptors in the fluorescence of OF3–6. These defects,
which can form by oxidative degradation processes,
can affect the emission of a fluorene-based material even
in amounts undetectable by the common analytical
techniques.
References and notes
1. Scherf, U.; List, E. J. W. Adv. Mater. 2002, 14, 477; Grimsdale, A. C.;
Mullen, K. Macromol. Rapid Commun. 2007, 28, 1676; Neher, D.
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Macromol. Rapid Commun. 2001, 22, 1365.
2. Leclerc, M. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2867;
Culligan, S. W.; Geng, Y.; Chen, S. H.; Klubek, K.; Vaeth, K. M.;
Tang, C. W. Adv. Mater. 2003, 15, 1176; Geng, Y.; Culligan, S. W.;
Trajkovska, A.; Wallace, J. U.; Chen, S. H. Chem. Mater. 2003, 15,
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3. Heeney, M.; Bailey, C.; Giles, M.; Shkunov, M.; Sparrowe, D.;
Tierney, S.; Zhang, W.; McCulloch, I. Macromolecules 2004, 37, 5250.
4. Geng, Y.; Culligan, S. W.; Trajkovska, A.; Wallace, J. U.; Chen, S. H.
Chem. Mater. 2003, 15, 542.
5. Grisorio, R.; Mastrorilli, P.; Nobile, C. F.; Romanazzi, G.; Suranna,
G. P.; Meijer, E. W. Tetrahedron Lett. 2004, 45, 5367.
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Mastrorilli, P.; Cosma, P.; Acierno, D.; Amendola, E.; Ciccarella,
G.; Nobile, C. F. Tetrahedron 2006, 62, 627.
7. Becker, K.; Lupton, J. M.; Feldmann, J.; Nehls, B. S.; Galbrecht, F.;
Gao, D. Q.; Scherf, U. Adv. Funct. Mater. 2006, 16, 364; Kulkarni, A.
P.; Kong, X.; Jenekhe, S. A. J. Phys. Chem. B 2004, 108, 8689;
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8. Bliznyuk, V. N.; Carter, S. A.; Scott, J. C.; Kla¨rner, G.; Miller, R. D.;
By carrying out, however, a photostability test8 on the
spectral stability of OF3 exposing a film of the molecule
to an UV-irradiation under air for 30 min it was observed
that the fluorescence of pristine OF3 was red-shifted of
20 nm and its emission wavelength matched that of the
fluorenone-containing model molecule 12, thus suggesting
(a) that a fluorenone moiety is incorporated into OF3 only
after the degradation process, and (b) that the solid state
emission recorded for OF3–6 (Fig. 3B) definitely derives
from non-oxidatively degraded molecular structures.
In conclusion, the preparation of the first series of mono-
dispersed oligo(fluorenylidene)s (trimer, tetramer, penta-
mer and hexamer) was achieved by
synthetic approach founded on the facile obtainment of
2,70-, 2,700- and 2,7000-bis-boronic ester derivatives of bi-,
a
convergent
Miller, D. C. Macromolecules 1999, 32, 361; List, E. J. W.; Guentner,
R.; Scandiucci de Freitas, P.; Scherf, U. Adv. Mater. 2002, 14, 374;
Kulkarni, A. P.; Jenekhe, S. A. Macromolecules 2003, 36, 5285.
¨