Monodisperse fluorene oligomers exhibiting strong dipolar coupling interactions

Article abstract of DOI:10.1039/b201414a

Well-defined fluorene oligomers (n = 1 to 6) were prepared step by step using Suzuki and Yamamoto couplings, while absorption and photoluminescence properties evidenced very large dipolar coupling interactions between fluorene moieties.

Full text of DOI:10.1039/b201414a

Monodisperse fluorene oligomers exhibiting strong dipolar coupling
interactions
Rémi Anémian,a Jean-Christophe Mulatier,a Chantal Andraud,*a Olivier Stéphanb and Jean-Claude
Vialb
a Laboratoire de Stéréochimie et Interactions Moléculaires, ENS-Lyon and CNRS, 46 Allée d’Italie, 69364
Lyon cédex 07, France. E-mail: Chantal.Andraud@ens-lyon.fr
b Laboratoire de Spectrométrie Physique, Université Joseph Fourier and CNRS, 140 rue de la physique, BP
87 38402 Saint Martin dAHères cédex, France
Received (in Corvallis, OR, USA) 8th February 2002, Accepted 29th May 2002
First published as an Advance Article on the web 26th June 2002
Well-defined fluorene oligomers (n = 1 to 6) were prepared
step by step using Suzuki and Yamamoto couplings, while
absorption and photoluminescence properties evidenced
very large dipolar coupling interactions between fluorene
moieties.
synthesis of the fluorene series has been reported. The notation
for our oligofluorene derivatives will consist in two parameters
N(n) in which N and n represent respectively the number of the
oligofluorene series and the number of fluorene units in the
series (Scheme 1). First of all, hexyl chains were introduced at
the C9 position of each fluorene unit to reach high solubility in
classical organic solvents.6 The compound 5(1) was directly
prepared in 97% yield by alkylation of the commercially
available fluorene with 1-bromohexane in the presence of
anhydrous potassium tert-butylate. Our general synthetic strat-
egy for longer oligomers was based on a step-by-step growth
methodology using cross-coupling reactions leading to the
formation of oligomers 5(n) up to n = 6 with yields ranging
from 58% to 89% (Scheme 1).
Oligomers 5(2), 5(4), 5(6) were obtained respectively from
the homocoupling of the 2-bromo-oligo(9,9-dihexyl)fluorenes
1(1), 1(2) and 1(3) using a Yamamoto procedure.8 Odd
oligomers 5(3) and 5(5) were prepared from the condensation of
the 2-oligo(9,9-dihexylfluorenyl)boronic acids 2(1) and 2(2)
with compounds 4a or 4b via a Suzuki cross-coupling reaction.9
Compounds 2(1) and 2(2) were obtained directly from the
2-bromo-oligo(9,9-dihexyl)fluorenes 1(1) and 1(2) after con-
version to their corresponding lithium derivatives and reaction
with triisopropoxyborane. The key compounds of this strategy
were the 2-bromo-oligo(9,9-dihexyl)fluorenes 1(1), 1(2) and
1(3); their synthesis is reported in Scheme 2.
Monodisperse systems are more than models providing struc-
ture–property relationships in order to rationalise behaviour of
high weight polymers.1 Thus, research focusing on conjugated
oligomers is also of current interest to investigate efficient
materials for electronics and photonics applications.2 In this
field, recent reports have shown the oligomer approach to be
promising in the design of optimised systems for two-photon
absorption (TPA) based applications. A theoretical study of
various conjugated oligomers has shown that their TPA
amplitude could arise from a large enhancement of the
transition dipole moments when compared to that of the
corresponding monomer.3 This special feature was ascribed to
coupling interactions between monomers as in the excitonic
model of Kasha.4 In this context we have already reported that
bifluorene derivatives exhibit very promising experimental
TPA induced optical limiting properties.5 An even more
pronounced effect was obtained with high molecular weight, i.e.
number-average molecular weight close to 20 000 Da, poly-
(dihexyl)fluorene (denoted 60-PDHF ; 60 fluorene units)
leading to tremendous TPA efficiencies in the visible range
when compared to other organic systems.6 Assuming that TPA
should result from dipolar coupling effects monodisperse
fluorene oligomers were prepared to validate the coupling
assumption.
Alkylation of the commercially available 2-bromofluorene 6
led to the first compound of the series 1(1), while higher
homologous 1(2) and 1(3) were obtained from the coupling of
the 2-oligo(9,9-dihexylfluorenyl)boronic acids 2(1) and 2(2)
respectively with 8 via a Suzuki reaction, in which the coupling
on the iodo site is significantly faster, leaving the bromo
unreacted.10 All compounds were characterised by NMR
Although monodisperse oligofluorenes have been already
isolated via a random approach by means of high pressure liquid
chromatography of an oligomeric mixture,7 no systematic
Scheme 1 Reagents and conditions: (i) P(Ph)3, Zn powder, 2,2A-dipyridyl, NiCl2, N,N-dimethylacetamide, 80 °C, 48 h; (ii) a. BuLi (2,5 M), TMEDA,
anhydrous toluene, rt, 2 h. b. B(OCH(CH3)2)3, anhydrous toluene, RT, 12 h. c. HCl (2 M), 30 min; (iii) Pd(P(Ph)3)4, Na2CO3 (2 M), anhydrous toluene, 80
°C, 2 days; (iv) t-BuOK, 1-bromohexane, anhydrous DMF, 40 °C, 18 h.
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This journal is © The Royal Society of Chemistry 2002

Scheme 2 Reagents and conditions (i) I2, HIO3, H2SO4, CCl4, 80 °C, 4 h; (ii) t-BuOK, 1-bromohexane, anhydrous DMF, 40 °C, 18 h; (iii) Pd(P(Ph)3)4,
Na2CO3 (2 M), anhydrous toluene, 80 °C, 2 days.
Table 1 Optical properties of polyfluorenes 5(n)
n
labs (nm)a
llum (nm)b
e (L mol21cm21
)
mn (D)c
t (ns)d
2
3
4
5
6
330
352
364
370
375
370
400
411
416
418
53570
90975
124350
147050
173870
6.6
9.2
10.7
12.3
13.1
1.030
0.812
0.760
0.650
0.604
a In dichloromethane. b In CHCl3; zero phonon peak. c mn was derived from absorption spectra. d A short rise time followed by exponential decay was
obtained from the laser femtosecond mode with a time resolution of 50 ps.
spectroscopy, elemental analysis and/or mass spectrometry and
found to be in good agreement with expected structures.
All oligomers 5(n) show an intense absorption in the UV
region, while photoluminescence spectra, obtained by excita-
tion at 310 nm from the second harmonic of a Ti laser, consist
in structured bands (vibronic spacing about 1200 cm21).
Optical properties of these polyfluorenes depend strongly on the
length of the molecule (Table 1) and could be rationalised in the
following in terms of coupling interactions. Absorption ener-
gies decrease with n following the quantum-mechanical law
eqn. (1) of Davidov
shift of w (Table 1), the strong decrease of t with n can be
ascribed to the high enhancement of mn (eqn. (1)).
In conclusion, coupling interactions were shown strongly to
control physical properties of polyfluorene-based materials
such as absorption and luminescence decay times by generation
of large transition dipole moment values. Although energy
transfer processes have been previously assumed in dendrimers,
no direct correlation with physical properties of these systems
was established.12 Recent calculations on optical properties of
other different oligomer families could allow the generalization
of this behaviour.13 This trend makes the oligomer approach
very attractive in several nonlinear optic fields according to the
possible optimisation of nonlinearity–transparency trade-off
assuming that all studied polyfluorenes (up to 60-PDHF) absorb
in the visible. In that context, the synthesis of longer oligomers
(n > 6) is in progress to complete the series. The TPA
properties of polyfluorenes 5(n) will be also investigated in
detail.
(1)
based on monomer interactions in oligomers, in which A and M
are respectively the fluorene energy and the interaction
matrix.11 Moreover, the experimental energy value obtained for
the 60-PDHF polyfluorene (3.20 eV)6 is compatible with the
infinite extrapolation energy (3.19 eV) found when using the
relationship (1) and led to a conjugation effective length of n =
11 in good agreement with previous data.7 An exponential
dependence of type given in eqn. (2)
Notes and references
† The difference between the Kasha and the experimental exponent values
arises from conjugation effects, which should be weak due to the non-
coplanarity of the different fluorene moities.
mn = nam1
(2)
between the transition dipole moments mn of the oligomer 5(n)
and m1 of the fluorene unit was determined from absorption
spectra with a = 0.7 in excellent correlation with the Kasha
aggregate model (a = 0.5).4† Moreover photoluminescence
intensities were found to decay exponentially (100 femtosecond
excitation pulse at 360 nm; detection at llums) due to the well-
defined structure of polyfluorenes with very short lifetimes in
the sub-nanosecond range. The oligofluorenes high lumines-
cence quantum efficiency QE ranging from 0.9 to 1, measured
by means of an etalon, allows the approximation eqn. (3)
1 R. E. Martin and F. Diederich, Angew. Chem., Int. Ed., 1999, 38,
1350.
2 Electronic Materials: The Oligomer Approach, eds. K. Müllen and G.
Wegner, Wiley VCH, Weinheim, 1998.
3 C. Andraud and P. L. Baldeck, Nonlinear Optics, 2000, 25, 133.
4 E. G. McRae and M. Kasha, J. Chem. Phys., 1958, 28, 721.
5 Y. Morel, A. Irimia, P. Najechalski, O. Stephan, P. L. Baldeck and C.
Andraud, J. Chem.Phys., 2001, 114, 5391.
6 P. Najechalski, Y. Morel, O. Stéphan and P. L. Baldeck, Chem. Phys.
Lett., 2001, 343, 44.
7 G. Klaerner and R. D. Miller, Macromolecules, 1998, 31, 2007.
8 T. Yamamoto, Y. Hayashi and A. Yamamoto, Bull. Chem. Soc. Jpn.,
1978, 51, 209.
9 N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457.
10 G. W. Gray, M. Hird, D. Lacey and K. J. Toyne, J. Chem. Soc., Perkin
Trans. 2, 1989, 2041.
(3)
QE = Wr/(Wr + Wnr) ≈ 1
and leads to relate directly their fast decay times t to the
radiative rate Wr according to eqn. (4)
11 A. S. Davidov, Zh. Eksp. Teor. Fiz., 1948, 18, 515; Theory and
Applications of Ultraviolet Spectroscopy, eds. H.H. Jaffé and M.
Orchin, Wiley, NY, 1964.
(4)
12 M. Drobizhev, A. Rebane, C. Sigel, E. H. Elandaloussi and C. W.
Spangler, Chem. Phys. Lett., 2000, 325, 375.
13 R. Anémian, P. Baldeck, N. Plé, and C. Andraud: manuscript on
theoretical TPA properties of various oligomers in preparation.
in which w is the emission frequency. It is noteworthy that the
nonradiative rate Wnr was neglected. Due to the bathochromic
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