Poly(2,7-dibenzosilole): A blue light emitting polymer

Article abstract of DOI:10.1021/ja0508065

2,7-Disubstituted dibenzosilole monomers have been prepared by the selective trans-lithiation of 4,4′-dibromo-2,2′-diiodobiphenyl followed by silylation with dichlorodihexylsilane. Suzuki copolymerization of dibromo and bis(boronate) monomers afforded poly(9,9-dihexyl-2,7-dibenzosilole) which showed better efficiency than the corresponding polyfluorene in a single layer light emitting device. Preliminary studies demonstrated this to be a promising blue light emitting polymer. Copyright

Full text of DOI:10.1021/ja0508065

Published on Web 05/05/2005
Poly(2,7-dibenzosilole): A Blue Light Emitting Polymer
Khai Leok Chan,†,§ Mary J. McKiernan, Carl R. Towns, and Andrew B. Holmes*
‡
‡
,†,§
MelVille Laboratory for Polymer Synthesis, Department of Chemistry, UniVersity of Cambridge, Lensfield Road,
Cambridge, CB2 1EW, United Kingdom, Cambridge Display Technology Limited, Building 2020, Cambourne
Business Park, CB3 6DW, United Kingdom, and Bio21 Institute, UniVersity of Melbourne, ParkVille,
Victoria 3010, Australia
Received February 7, 2005; E-mail: aholmes@unimelb.edu.au
Scheme 1. Synthesis of Monomers 5 and 6a
Since the discovery of electroluminescence from conjugated
polymers, real progress has been made toward the development of
materials for commercially viable display devices.1 Polyfluorene
and derivatives have emerged as the dominant class of polymers
-3
4
for commercial applications. They have been shown to exhibit high
luminescence quantum efficiencies, thermal stability, and good
solubility. However, under certain circumstances, they develop an
unwanted long wavelength emission that has most recently been
5
attributed to the formation of carbonyl groups at the C-9 position.
In light of this, we decided to explore polymers in which the
vulnerable C-9 carbon in polyfluorene is replaced by a heteroatom,
such as silicon. A further attraction of such a polymer is the
expected enhancement of electron affinity owing to the σ*-π*
conjugation, as observed in analogous molecular and polymeric
siloles.6
In this Communication, we report the preparation of 2,7-
functionalized dibenzosilole monomers 5 and 6 and their Suzuki
cross-coupling to prepare poly(9,9-dihexyl-2,7-dibenzosilole) 7. The
physical and optoelectronic properties of polydibenzosilole are
compared with those of an analogous polyfluorene and an alternat-
ing copolymer of the two.
a
Reagents and conditions: (a) Cu, DMF, 125 °C, 88%; (b) Sn, HCl,
EtOH, 110 °C (bath temp.), 72%; (c) nitrosylsulfuric acid, concentrated
H2SO4, 0 °C, then aq. KI, -10 to 50 °C, 30%; (d) t-BuLi (4 equiv), THF,
-90 to -78 °C, then (C H ) SiCl , 25 °C, 52%; (e) t-BuLi, diethyl ether,
-78 °C, then 2-isopropoxy-4,4′,5,5′-tetramethyl-1,3,2-dioxaboralane, 25 °C,
86%.
6
13 2
2
The known Ullmann coupling product 2 was reduced to the
7
corresponding diamine 3 which was diazotized with nitrosylsulfuric
8
acid in concentrated sulfuric acid. Sandmeyer reaction of the
Scheme 2. Synthesis of the Homopolymer 7 and Copolymer 9a
resulting diazonium salt with concentrated KI solution gave 4,4′-
dibromo-2,2′-diiodobiphenyl 4 in 30% yield. Conventional diazo-
tization with nitrous acid followed by Sandmeyer reaction afforded
a much poorer yield (15%) of 4. The low yield for this reaction is
presumably a consequence of intramolecular cyclization of the first
formed iodo substituent onto the neighboring 2′-position by the
decomposition of the second diazonium substituent to give a
9
dibenzoiodonium iodide salt. This was minimized by use of a 10-
fold excess of KI.
The most direct route to the corresponding dibenzosilole would
be the selective trans-lithiation of the 2,2′-iodo substituents in the
presence of the apparently more accessible 4,4′-bromo substituents
and subsequent cyclization with a dialkyldihalosilane.10 In the event,
treatment of 4 with 4 equiv of t-BuLi in THF followed by addition
of dichlorodihexylsilane gave 2,7-dibromo-9,9-dihexyldibenzosilole
a
5
in 52% yield. The counterpart for the cross-coupling reaction,
Reagents and conditions: (a) Pd(OAc)2, tricyclohexylphosphine,
NEt4OH, toluene, 90 °C, then PhB(OH)2, then PhBr. Yield 7 (93%), 9
the dibenzosilole-2,7-diboronic ester 6, was readily prepared by
double lithiation of 5 followed by the formation of the bis(boronate)
ester 6, as shown in Scheme 1.
The Suzuki polycondensation of the dibromide 5 with the
diboronic ester 6 (Scheme 2) was carried out under basic conditions
(94%).
hydroxide11 with phenyl group endcapping to afford poly(9,9-
dihexyl-2,7-dibenzosilole) 7 in 93% yield; the molecular weight
of 7 was determined by GPC analysis with polystyrene standards,
2
with the Pd(OAc) /tricyclohexylphosphine catalyst system in a
biphasic system of toluene and aqueous tetraethylammonium
M
w
) 220 000 and M
n
) 31 000 (DP ≈ 90). Copolymers can
similarly be prepared between 1 or 2 and their correspondingly
functionalized aromatic partners. For instance, the alternating
copolymer 9 was prepared in 94% yield from the dibromide 5 and
†
Melville Laboratory for Polymer Synthesis.
‡
Cambridge Display Technology Limited.
§
Bio21 Institute.
7662
9
J. AM. CHEM. SOC. 2005, 127, 7662-7663
10.1021/ja0508065 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. Electroluminescence emission spectra of polymer 7 (s) and PF8
- - -).
(
the fluorene bis(boronate) 8 (Scheme 2) under similar reaction
conditions; M ) 425 000 and M
polymers 7 and 9 are completely soluble in common organic
w
n
) 109 000 (DP ≈ 160). Both
Figure 2. Effect of thermal aging on PL spectrum of 7 and PF8 under air
and ambient light.
solvents, such as chloroform and THF.
The optical properties of polymer 7 are remarkably similar to
those of poly(9,9-dioctyl-2,7-fluorene) (PF8). The absorption
maximum in the UV-vis spectrum of a thin film of 7 (λmax ) 390
nm) is comparable with that of PF8 (λmax ) 389 nm), and the optical
band gaps, determined from the λ0-0 band edges, are 2.93 eV for
both polymers. The photoluminescence (PL) emission maximum
Acknowledgment. We thank the Gates Cambridge Trust and
the Universities UK (scholarships to K.L.C.), the Engineering and
Physical Sciences Research Council (UK), the European Commis-
sion, the Australian Research Council and VESKI for generous
financial support. We thank Drs. S. Watkins, I. Grizzi, C. Murphy,
A. Goodsell, and S. Goddard (CDT) for their interest in this work.
(excitation at 325 nm) of a film of polymer 7 at 425 nm and its
two vibronic sidebands at 449 and 482 (CIE coordinates x ) 0.15,
y ) 0.11, PL efficiency ) 62%) are all within 3 nm of the
corresponding bands for PF8 (PL efficiency ) 60%). These
observations suggest that the extent of π-conjugation within the
two polymers is comparable and indicate the similarity of the poly-
p-phenylene backbones as chromophores.
Supporting Information Available: Experimental procedures and
characterization of 2-7 and 9, UV-vis, PL, EL, DSC, and TGA data
for 7 and 9. This material is available free of charge via the Internet at
http://pubs.acs.org.
References
Preliminary results for the electroluminescence emission spec-
trum of the homopolymer 7 were measured on a single layer device
in the configuration ITO/PEDOT/polymer(65 nm)/Ba/Al and
showed very promising device characteristics with emission maxima
at 431 and 451 nm and an efficiency slightly superior to that of
the corresponding device fabricated with a single layer of PF8
(
1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay,
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To test the thermal stability, films of polymer 7 (T ) 149 °C)
g
(
and PF8 were annealed under air and ambient light, and the PL
emission spectra were recorded (Figure 2). Films of PF8 degraded
as evidenced by the appearance of a broad green band centered at
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(
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tained its color integrity even after annealing at 250 °C for 16 h.
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determined by cyclic voltammetry to be -5.77 and -2.18 eV,
respectively. These are about 0.1 eV lower than the corresponding
levels for PF8 when measured under the same conditions with a
potential advantage of lowering the energy barrier for electron
injection.
In summary, the successful synthesis of 2,7-functionalized
dibenzosilole building blocks has opened the door to a new class
of high energy gap light emitting polymers with the potential to
avoid development of unwanted long wavelength emission under
operation.
(
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(
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(
12) We thank Drs. C. Murphy and I. Grizzi (CDT) for these data.
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