Synthesis and luminescent properties of branched oligophenylenefluorenes

Article abstract of DOI:10.1016/j.mencom.2011.01.004

The spectrophotometric and luminescence studies on branched oligophenylenefluorenes have shown that the incorporation of n-octyl groups to the 9-position of the fluorene moiety does not interrupt the conjugation of internodal luminophore moieties, and there is also no aggregation or excimer emission both for functionalized and nonfunctionalized macromolecules in solution.

Full text of DOI:10.1016/j.mencom.2011.01.004

Mendeleev
Communications
Mendeleev Commun., 2011, 21, 9–11
Synthesis and luminescent properties of branched
oligophenylenefluorenes
a
a
a
b
Aleksei I. Kovalev, Aleksei V. Shapovalov,* Evgeniya V. Sukhorukova, Alexander M. Sergeev,
a
a
a
Alexander S. Peregudov, Alexander L. Rusanov and Irina A. Khotina
a
b
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow,
Russian Federation. Fax: +7 499 135 5085; e-mail: Shapovalov-alex@yandex.ru
L. Ya. Karpov Institute of Physical Chemistry, 105064 Moscow, Russian Federation
DOI: 10.1016/j.mencom.2011.01.004
The spectrophotometric and luminescence studies on branched oligophenylenefluorenes have shown that the incorporation of n-octyl
groups to the 9-position of the fluorene moiety does not interrupt the conjugation of internodal luminophore moieties, and there is also
no aggregation or excimer emission both for functionalized and nonfunctionalized macromolecules in solution.
Currently, a search is under way for new polymers intensely
emitting light and possessing properties that allow specific practical
problems to be solved. Poly-p-phenylenes are among the most
we performed a comparative study of branched oligomers, both
containing and not containing functional n-octyl groups.
A few syntheses of branched phenylene oligomers in which
1,3,5-substituted benzenes are used as branching centres have
been reported. However, the longest conjugated fragments in
such oligomers either consist of eight or more benzene rings or
have a variable length, which makes it more difficult to find out
correlations between the macromolecular structure and lumine-
scent properties. For this reason, we synthesised oligophenylene-
fluorenes whose luminophore moieties have a fixed length and
consist of seven conjugated benzene rings.
To synthesize branched oligomers, we first obtained trifunc-
tional monomers 3a and 3b (Scheme 1). 2-Bromo-9,9-dioctyl-
fluorene 1a was obtained by dialkylation of bromofluorene with
1-bromooctane in the presence of NaOH and catalytic amounts
of triethylbutylammonium chloride by a standard technique.8
Monoacetyl derivatives of fluorene and 9,9'-dioctylfluorene 2a,b
were obtained by the Friedel–Crafts reaction of the [MeCO∙AlCl4]
1
efficient blue luminophores since they display good working
characteristics in the production of plastic scintillators, solid-
phase lasers and various electroluminescent devices, and also
have high quantum yields of fluorescence and molar absorption
6
,7
2
coefficients. Fluorene and indenofluorene, as well as poly-
mers based on them, are the most interesting and well studied
3
phenylene luminophores. The main problems in the develop-
ment of polymeric luminophores include fluorescence quenching
due to p-stacking of conjugated chains and low solubility. A way
to solve these problems is to functionalize oligomers by n-alkyl
groups and synthesize branched systems that have higher solubi-
lities than their linear analogues. However, n-alkyl groups can
favour the interruption of the internodal moieties conjugation
due to an increase in torsion angle between adjacent benzene rings
4
,5
leading to a decrease in fluorescence quantum yield. Thus,
X
X
O
X
ii
X
iii
3
3
b, 31%
d, 29%
X
1
b,d
2b, 59%
d, 79%
2
R
R
i
a, b X = Br
c, d X = H
R = n-octyl
X
R
R
R
R
R R
O
X
ii
X
iii
X
R
R
1
1
a, 66%
c, 64%
2a, 74%
2c, 67%
3a, 69%
3c, 62%
X
Scheme 1 Reagents and conditions: i, 1-bromooctane, NaOH, TEBA, DMSO, 25°C, 12 h; ii, AcCl, AlCl , DCE, 0°C, 1 h; iii, HC(OEt) , HCl (gas),
3
3
benzene, 25°C, 4 h.
©
2011 Mendeleev Communications. All rights reserved.
– 9 –

Mendeleev Commun., 2011, 21, 9–11
Br
R
R
Br
O
O
O
O
B
B
3
a or 3b, i
R
R
R
R
R
R
R
R
n
OPF-1 (from 3b) R = H, 32%
OPF-2 (from 3a) R = n-octyl, 45%
R
O
B
O
R
Br
Scheme 2 Reagents and conditions: i, Pd (dppf), 2 m K CO , toluene, 90°C, 48 h.
2
3
9
complex with compounds 1a,b in dichloroethane. Monomers
a,b were synthesized by trimerization cyclocondensation of
compounds 2a,b in the presence of ethyl orthoformate and dry
The absorption spectrum of compound 4 is a wide structureless
band with a maximum at 289 nm. At the same time, for 3c, the
main absorption maximum was observed at 315 nm and a shoulder
peakat296nm[Figure1(a)].The3dabsorptionbandbatochromic
shift in comparison with compound 4 can be explained by the
fact that the structure of the phenylfluorene moiety in cyclotrimer
3
10
gaseous HCl by the reported procedure. The products were
characterised by H NMR spectroscopy, mass spectrometry (EI),
1
and elemental analysis.†
Branched oligophenylenefluorenes OPF-1 and OPF-2 were
obtained from trifunctional monomers by the ‘A + B ’ type Suzuki
reaction, i.e., the reaction of compounds 3a,b with phenyl-bis-
3d in the ground S state is more planar than that of the p-terphenyl
0
moiety of compound 4. The absorption spectra of compounds 3c
and 3d virtually coincide. This suggests that the conjugated phenyl-
fluorene moieties of both trimers have almost the same conforma-
3
2
pinacoldiborate in the presence of the Pd ferrocenyl complex and
1
1
2
m aqueous K CO (Scheme 2). The resulting oligophenylene-
tion in the ground S state, i.e., the n-octyl groups do not cause
2
3
0
1
fluorenes were characterised by H NMR spectroscopy and elemental
analysis. The OPF-1 oligophenylenefluorene is only soluble in
the rotation of fluorene moieties with respect to the branching
centre. The small (by 3–4 nm) bathochromic shift of the 3c absorp-
tion band in comparison with 3d one may be due to the electron-
donating nature of the n-octyl substituents.
The fluorescence spectra of compounds 4, 3d and 3c obtained
by excitation in absorption maxima are wide structureless bands
with peaks at 362, 366 and 365 nm, respectively. This is in a good
†
dichloromethane and chloroform, whereas OPF-2 is well soluble
even in nonpolar solvents such as benzene and light petroleum.
The adjacent benzene rings in p-phenylene luminophores are
somewhat tilted in the ground electronic S state due to steric
0
factors (repulsion of o-hydrogen atoms), but they are arranged
1
2
in almost the same plane in the excited electronic S state. The
1
adjacent rings of the fluorene moieties stay in the same plane in
both states since they are additionally linked by a methylene bridge.
In view of this, it was important to find out whether the branching
Table 1 Spectral characteristics of model compounds 3c,d, 4 and oligo-
mers OPF-1 and OPF-2 in dichloromethane solutions at 293 K (the main
maximum is given in italic).
1
,3,5-trisubstituted benzene ring is part of the p-phenylene lumino-
abs
fl
jfl
Compound
l
/nm
l
/nm
max
max
phore moiety, since conjugation between the fluorene groups
and 1,3,5-substituted benzene can be lacking due to planarity
destruction. This is most likely to occur in oligomer OPF-2, in
which the torsion angle between the specified moieties may be
higher due to the bulky side groups. In order to clarify this issue,
we synthesized (Scheme 1) model cyclotrimers without bromine
atoms 3c,d that simulate the arrangement of substituted and
non-substituted fluorene moieties with respect to the branching
centre, i.e., the 1,3,5-trisubstituted benzene. For these model
compounds and, for comparison, for 1,3,5-tri(p-diphenylyl)-
3
3
4
c
d
296, 315
299, 319
289
325
329
365
366
362
369, 391
401, 422
0.31
0.41
0.32
0.40
0.58
OPF-1
OPF-2
‡
Absorption spectra were recorded with a Cary 100-Scan spectrophoto-
meter (Varian). Fluorescence spectra were measured using a Fluorolog 3-221
spectrofluorimeter (Horiba JobinYvon) in the reflection mode. The fluore-
scence spectra were obtained for the non-deaired dichloromethane solu-
tions (C ≈ 10–6 m) and corrected for nonuniformity of detector spectral
sensitivity. The fluorescence quantum yields were calculated with respect
13
benzene 4, absorption and emission spectra in dichloromethane
solution were obtained.‡
fl
15
Ar
to a diphenylanthracene solution in dichloromethane (j = 0.9) by the
Vavilov equation
Ar =
Aref Ii
fl
fl
ji = jref
,
Ar
Ar
AiIref
4
where i is the test solution, ‘ref’is the reference solution, j is the quantum
yield, A is absorbance, and I is the area under the curve of the fluorescence
spectrum.
†
For characteristics of compounds 1a,b, 2a–d, 3a–d, OPF-1 and OPF-2,
see Online Supplementary Materials.
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Mendeleev Commun., 2011, 21, 9–11
of the OPF-1 oligomer has a main maximum at 391 nm and a
(
a)
1.2
shoulder at 369 nm, as well as two inflection points in the region
of the long-wave edge (at 415 and 447 nm). The spectrum of
oligomer OPF-2 is shifted towards longer waves by about 8 nm
in comparison with OPF-1 and its vibrational structure elements
differ: there is an inflection point in place of the first maximum
1
0
0
0
.0
.8
.6
.4
.2
1
2
3
1
'
3'
(at 377 nm) and a shoulder (at 422 nm) in place of the inflection
point at 415 nm. The observed small bathochromic shift of
OPF-2 absorption and fluorescence maxima in comparison with
OPF-1 may be, as in cyclotrimers case, due to the electron-
donating nature of the n-octyl substituents. The absence of a
hypsochromic shift for OPF-2 absorption and fluorescence bands
confirms the assumption that internodal moieties in both oligo-
mers are in the same conformation forms, i.e. n-alkyl groups do
not favour the interruption of the internodal moieties conjugation.
It is also in a good agreement with fluorescence quantum yields
for OPF-1 and OPF-2 (see Table 1). Like the fluorescence spectra of
the models, the spectra of both oligomers show no fluorescence
at 500–550 nm. This allows one to conclude that both functionalized
and nonfunctionalized branched oligophenylenefluorenes do not
form excimers or supramolecular structures in solution.
Thus, the results obtained show that the absence of luminophore
moieties aggregation or excimer luminescence makes branched
oligophenylenefluorenes advantageous over linear analogues. The
model compounds and oligomers obtained in this study are
promising materials for using them as blue luminophores in
electroluminescent devices.
2
'
0
0
.0
2
50
300
350
400
450
500
l/nm
(
b)
1.2
1
0
0
0
.0
.8
.6
.4
.2
4
5
4' 5'
0
0
.0
2
50
300 350
400
450 500
550
l/nm
Figure 1 (1)–(5) Absorption spectra and (1')–(5') fluorescence spectra of
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.mencom.2011.01.004.
(
(
a) model compounds and (b) oligophenylenefluorenes in dichloromethane:
1) 4, (2) 3d, (3) 3c, (4) OPF-1, and (5) OPF-2.
agreement with the statement that 1,3,5-trisubstituted benzene
rings conjugate with biphenyl or fluorine moieties. The planarity
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3
4
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4
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oligomer are harder than those in OPF-2.
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The fluorescence spectra of oligomers OPF-1 and OPF-2 are
bathochromically shifted with respect to the spectra of models
3
d and 3c by 25 and 35 nm, respectively, and they do not depend
on the excitation wavelength (300, 320 or 340 nm). Unlike the
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contain vibrational structure elements. The fluorescence spectrum
Received: 7th July 2010; Com. 10/3561
–
11 –