Protonation and subsequent intramolecular hydrogen bonding as a method to control chain structure and tune luminescence in heteroatomic conjugated polymers

Article abstract of DOI:10.1021/ja012409+

We report the effects of protonation on the structural and spectroscopic properties of 1,4-dimethoxy-2,5-bis (2-pyridyl) benzene (9) and the related AB coploymer poly{2,5-pyridylene-co-1,4-[2,5-bis(2-ethylhexyloxy)]phenylene} (7). X-ray crystallographic a

Full text of DOI:10.1021/ja012409+

Published on Web 05/03/2002
Protonation and Subsequent Intramolecular Hydrogen
Bonding as a Method to Control Chain Structure and Tune
Luminescence in Heteroatomic Conjugated Polymers
Andrew P. Monkman,*,† Lars-Olof Pålsson,† Roger W. T. Higgins,†
Changsheng Wang,‡ Martin R. Bryce,*,‡ Andrei S. Batsanov,‡ and
Judith A. K. Howard‡
Contribution from the Department of Physics and the Department of Chemistry,
UniVersity of Durham, South Road, Durham DH1 3LE, United Kingdom
Received October 22, 2001
Abstract: We report the effects of protonation on the structural and spectroscopic properties of
1,4-dimethoxy-2,5-bis(2-pyridyl)benzene (9) and the related AB coploymer poly{2,5-pyridylene-co-1,4-[2,5-
bis(2-ethylhexyloxy)]phenylene} (7). X-ray crystallographic analysis of 9, 1,4-dimethoxy-2,5-bis(2-pyridyl)-
benzene bis(formic acid) complex 10, and 1,4-dimethoxy-2,5-bis(2-pyridinium)benzene bis(tetrafluoroborate
salt) (11) establishes that reaction of formic acid with 9 does not form an ionic pyridinium salt in the solid
state, rather, the product 10 is a molecular complex with strong hydrogen bonds between each nitrogen
atom and the hydroxyl hydrogen in formic acid. In contrast, reaction of 9 with tetrafluoroboric acid leads to
the dication salt 11 with significant intramolecular hydrogen bonding (N-H‚‚‚O-Me) causing planarization
of the molecule. The pyridinium and benzene rings in 11 form a dihedral angle of only 3.9° (cf. pyridine-
benzene dihedral angles of 35.4° and 31.4° in 9, and 43.8° in 10). Accordingly, there are large red shifts
in the optical absorption and emission spectra of 11, compared to 9 and 10. Polymer 7 displays a similar
red shift in its absorption and photoluminescence spectra upon treatment with strong acids in neutral solution
(e.g. methanesulfonic acid, camphorsulfonic acid, and hydrochloric acid). This is also observed in films of
polymer 7 doped with strong acids. Excitation profiles show that emission arises from both protonated and
nonprotonated sites in the polymer backbone. The protonation of the pyridine rings in polymer 7,
accompanied by intramolecular hydrogen bonding to the oxygen of the adjacent solubilizing alkoxy
substituent, provides a novel mechanism for driving the polymer into a near-planar conformation, thereby
extending the π-conjugation, and tuning the absorption and emission profiles. The electroluminescence of
a device of configuration ITO/PEDOT/polymer 7/Ca/Al is similarly red-shifted by protonation of the polymer.
Introduction
Polymers with basic nitrogen atoms offer the possibility of
protonation or alkylation of the lone pair as a way of modifying
The ability to control the optical and electronic properties of
luminescent conjugated oligomers or polymers is a long-held
goal, which is currently an important issue in the design of new
materials for organic light-emitting diodes (LEDs).1 The great
attraction of organic materials as the emissive components in
large-area displays2 stems from their low cost, the versatility
of synthetic chemistry to fine-tune molecular structure and hence
physical properties, and the relative ease of device fabrication.
their properties.3 Recently, we have shown that polymers
containing pyridine repeat units are of considerable interest in
the areas of electron transport4 and new tailored luminescent
materials.5 It is important to investigate the role of protonation
in this class of polymers because poly(2,5-pyridylene) (PPY)
and some copolymers are soluble only in acids such as formic,
dichloroacetic, and sulfuric6 and the acidic alcohol, hexafluoro-
2-propanol.7 Formic acid is the most frequently used solvent
for PPY as it facilitates easy spinning of high-quality thin films
for device applications. Thus an important issue is whether
formic acid is retained in the thin films after spinning, and if
* Corresponding authors. E-mail: a.p.monkman@durham.ac.uk;
m.r.bryce@durham.ac.uk.
† Department of Physics.
‡ Department of Chemistry.
(1) Reviews: (a) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem.,
Int. Ed. Engl. 1998, 37, 402. (b) Segura, J. L. Acta Polym. 1998, 49, 319.
(c) Mitschke, U.; Ba¨uerle, P. J. Mater. Chem. 2000, 10, 1471. (d) Segura,
J. L.; Mart´ın, N. J. Mater. Chem. 2000, 10, 2403.
(2) (a) Gustafsson, G.; Cao, Y.; Treacy, G. M.; Klavetter, F.; Colaneri, N.;
Heeger, A. J. AdV. Mater. 1992, 357, 477. (b) Gu, G.; Burrows, P. E.;
Venkatesh, S.; Forrest, S. R.; Thompson, M. E. Opt. Lett. 1997, 22, 172.
(c) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks,
R. N.; Taliani, C.; Bradley, D. D. C.; dos Santos, D. A.; Bre´das, J. L.;
Logdlund, M.; Salaneck, W. R. Nature 1999, 397, 121. (d) Visser, R. J.
Philips J. Res. 1998, 51, 467. (e) See the webpage of CDT: http://
www.cdtltd.co.uk.
(3) Wang, H.; Helgeson, R.; Bin, M.; Wudl, F. J. Org. Chem. 2000, 65, 5862.
(4) Dailey, S.; Halim, M.; Rebourt, E.; Samuel, I. D. W.; Monkman, A. P. J.
Condens. Matter 1998, 10, 5171.
(5) Wang, C.; Kilitziraki, M.; McBride, J. A. H.; Bryce, M. R.; Horsburgh, L.
E.; Sheridan, A. K.; Monkman, A. P.; Samuel, I. D. W. AdV. Mater. 2000,
12, 217.
(6) Yamamoto, T.; Murauyama, T.; Zhou, Z.-h.; Ito, T.; Fukuda, T.; Yoneda,
Y.; Begum, F.; Ikeda, T.; Sasaki, S.; Takezoe, H.; Fukuda, A.; Kubota, K.
J. Am. Chem. Soc. 1994, 116, 4832.
(7) Yamamoto, T.; Choi, B. K.; Takeuchi, M.; Kubota, K. Chem. Lett. 1999,
10, 1049.
9
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J. AM. CHEM. SOC. 2002, 124, 6049-6055
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A R T I C L E S
Monkman et al.
Chart 1. 1,4-Phenylene-2,5-pyridylene Copolymers 1 and 25
Scheme 1a
so, to what degree is the polymer protonated in the solid state.
Formic acid is not present in thin PPY films at concentrations
measurable by X-ray photoelectron spectroscopy (XPS), but the
addition of less volatile acids such as camphorsulfonic acid
(CSA) to the PPY formic acid solutions allowed protonated PPY
films to be cast.8 In these films changes occurred to both
absorption and emission properties of the PPY, and it was
possible to produce films with pseudo white light emission
which retained very respectable photoluminescence quantum
yields (PLQY).8
In the search for new electron transporting polymers 1,4-
phenylene-2,5-pyridylene copolymers 1 and 2 (Chart 1) were
studied.5 In contrast to other pyridine-containing polymers,6,9
polymers 1 and 2 were synthesized by Suzuki coupling10 to
achieve a regular AB structure. The solubility of polymers 1
and 2 in solvents ranging from neutral, e.g., chloroform, to
acidic, e.g., formic acid, allowed the effects of protonation to
be studied. A significant red shift was seen in the absorption
spectrum of polymer 1 in acidic solvents, but not for polymer
2.5 To explore this feature in detail we have now synthesized
the phenylene-pyridylene copolymer 7 and the model teraryl
compound 9. Their structural and photophysical properties
before and after protonation have been studied. In particular,
we now demonstrate that protonation of the pyridyl nitrogen
has an important structural effect: intramolecular hydrogen
bonding to the adjacent oxygen atom in the alkoxy substituent
planarizes the backbone of the molecules, which leads to red
shifts of both the optical absorption and emisson spectra. By
using different acids in combination with polymer 7 to control
this protonation/intramolecular hydrogen bonding process we
can tune the emission spectra in the solid state by ca. 0.5 eV.
This mechanism is distinctly different from that reported during
the course of this work by Meijer et al.11 in which hydrogen
bonding in a neutral oligomer/copolymer facilitates coplanarity
of the adjacent aryl/heteroaryl units.
a Key: (i) 2-ethylhexyl bromide, DMF, K2CO3, 100 °C. (ii) Dry THF,
BuLi then triisopropyl borate, followed by H2O and HCl. (iii) Compound
5, THF, Pd(PPh3)4, Na2CO3, Reflux.
mohydroquinone 3 to afford 412 in 74% yield. Conversion to
the diboronic acid 5 proceeded in 38% yield under standard
conditions. Suzuki-type polymerization10 of an equimolar
mixture of 5 and 2,5-dibromopyridine 6 in tetrahydrofuran
solutions afforded copolymer 7 as a yellow solid in high yield,
with good solubility in a range of organic solvents. Gel
permeation chromatography indicated Mw ) 12 000, Mn
)
7 400, and a polydispersity of 2.46 (in THF solution with
polystyrene standard).
The model compound 9 was similarly obtained in 89% yield
from the diboronic acid 85 and 2-bromopyridine (Scheme 2).
Dissolution of 9 in ethyl acetate followed by addition of formic
acid and evaporation of the solution gave the di(formic acid)
complex 10 as the only product. The bis(tetrafluoroborate) salt
11 was obtained by adding fluoroboric acid in ether to a solution
of 9 in methanol.
Absorption and Emission Spectroscopy. The solid-state
absorption and emission spectra of polymer 7 as a function of
protonation level are shown in Figures 1 and 2, respectively,
which depict the general spectroscopic behavior of this class
of AB polymers upon protonation of thin films. In the neutral
state the lowest energy absorption maxima is observed at 3.25
eV. On protonation with methanesulfonic acid (MSA) this band
red shifts considerably reaching a minimum of 2.8 eV. The shift
depends to some degree on the strength of the acid used. A
weak second absorption at 4.1 eV in the neutral polymer also
red shifts to 3.85 eV. Whereas the low-energy feature decreases
in relative oscillator strength upon protonation, the higher energy
feature increases. Previous theoretical calculations13 have
Results and Discussion
Synthesis. The synthesis of polymer 7 is shown in Scheme
1. Solubilizing 2-ethylhexyloxy chains were added to dibro-
(8) Monkman, A. P.; Halim, M.; Samuel, I. D. W.; Horsburgh, L. J. Chem.
Phys. 1998, 109, 10372.
(9) (a) Epstein, A. J.; Blatchford, J. W.; Wang, Y. Z.; Jessen, S. W.; Gebler,
D. D.; Lin, L. B.; Gustafson, T. L.; Wang, H.-L.; Park, Y. W.; Swager, T.
M.; MacDiarmid, A. G. Synth. Met. 1996, 78, 253. (b) Yamamoto, T.;
Sugiyama, K.; Kushida, T.; Inoue, T.; Kanbara, T. J. Am. Chem. Soc. 1996,
118, 3930. (c) Marsella, M.; Fu, D.-K.; Swager, T. M. AdV. Mater. 1995,
7, 145. (d) Tian, J.; Wu, C.-C.; Thompson, M. E.; Sturm, J. C.; Register,
R. A.; Marsella, M. J.; Swager, T. M. AdV. Mater. 1995, 7, 395. (e)
Yamamoto, T.; Zhou, Z.-h.; Kanbara, T.; Shimura, M.; Kizu, K.; Muruyama,
T.; Nakamura, Y.; Fukuda, T.; Lee, B.-L.; Ooba, N.; Tomaru, S.; Kurihara,
T.; Kaino, T.; Kubota, K.; Sasaki, S. J. Am. Chem. Soc. 1996, 118, 10389.
(f) Epstein, A. J.; Wang, Y. Z.; Gebler, D. D.; Fu, D. K.; Swager, T. M.
Mater. Res. Soc. Symp. Proc. 1998, 488, 75. (g) Kim, J. K.; Yu, J. W.;
Hong, J. M.; Cho, H. N.; Kim, Y.; Kim, C. Y. J. Mater. Chem. 1999, 9,
2171. (h) Irvin, D. J.; DuBois, C. J., Jr.; Reynolds, J. R. Chem. Commun.
1999, 2121. (i) Liu, Y.; Ma, H.; Jen, A. K.-Y. J. Mater. Chem. 2001, 11,
1800.
1
shown that the (π-π*) transition is of lower energy than the
1(n-π*) transition in PPY, thus we ascribe the lowest energy
transition observed in 7 to the 1(π-π*) transition. As the higher
energy feature also red shifts and increases in oscillator strength
on protonation we also ascribe it to a 1(π-π*) transition, as an
(12) For an alternative synthesis of 4 from 3 (34% yield) see: Irvin, J. A.;
Schwendeman, I.; Lee, Y.; Abboud, K. A.; Reynolds, J. R. J. Polym. Sci.
Part A: Polym. Chem. 2001, 39, 2164.
(13) Vaschetto, M. E.; Springborg, M.; Monkman, A. P. J. Mol. Struct. (Theo.
Chem.) 1999, 468, 181.
(10) Review: Schluter, A. D. J. Polym. Sci. A: Polym. Chem. 2001, 39, 1533.
(11) (a) Pieterse, K.; Vekemans, J. A. J. M.; Kooijman, H.; Spek, A. L.; Meijer,
E. W. Chem. Eur. J. 2000, 6, 4597. (b) Delnoye, D. A. P.; Sijbesma, R. P.;
Vekemans, J. A. J. M.; Meijer, E. W. J. Am. Chem. Soc. 1996, 118, 8717.
9
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Effects of Protonation on Spectroscopic Properties
A R T I C L E S
Scheme 2a
a Key: (i) 2-bromopyridine, THF, Pd(PPh3)4, Na2CO3, reflux. (ii) HCO2H, EtOAc. (iii) HBF4, MeOH.
Figure 1. Absorption spectra of thin films of polymer 7 protonated with
Figure 2. Normalized photoluminescence spectra of the polymer films
shown in Figure 1.
a range of acids showing that the red shift is dependent on acid strength.
Abbreviations: methanesulfonic acid (MSA); camphorsulfonic acid (CSA);
dichloracetic acid (DCA).
energy sites is effectively quenched. These excitation profiles
1
(n-π*) transition should blue shift upon protonation.13 Proto-
nation relaxes both these states, i.e., they become more
delocalized.
also show that light absorbed by the higher energy (π-π*)
transition decays at the same energy as light absorbed by the
1
lower energy (π-π*) supporting the hypothesis that the final
state of these two transitions is the same, i.e., the π* state.
Dissolution of polymer 7 in formic acid gives red shifted
absorption, consistent with protonation in solution, but this shift
is not observed in the spectra of thin films spun from these
solutions. This is consistent with the crystallographic studies
on 10 reported below. In chloroform solution, polymer 7
protonates effectively upon addition of CSA: the spectra show
the same red shift as observed in the solid state. Spectra were
also measured as a function of concentration in the range 10-3
to 10-6 M (in polymer repeat units). At very low concentration
the absorption spectrum of the salt is very similar to that of the
unprotonated polymer, although a low-energy knee is still
observed on the lowest energy absorption band. This may point
to an aggregation effect at high concentration. Very similar
The red shifts observed in the absorption spectra are also
mirrored in the PL spectra (Figure 2). In the case of CSA we
observed that the PL band shifted from 2.55 eV to ca. 2.2 eV.
The PLQY of the polymer salt films is not adversely affected
by protonation with weak acids. In the case of DCA it increases
from 0.20 to 0.34, whereas the stronger acids such as CSA have
little effect (0.19). Only MSA causes the PLQY to drop
significantly to 0.08. To determine whether the protonation
process is homogeneous, excitation spectra were measured. The
excitation profiles show that emission emanates from both
protonated and unprotonated sites on the polymer backbone,
indicating that both types of site coexist within the films. This
implies that excitons do not efficiently migrate to the lowest
energy sites such that luminescence from unprotonated higher
9
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A R T I C L E S
Monkman et al.
Figure 4. Normalized photoluminescence spectra of compound 9 protonated
with CSA at various dilutions in CHCl3.
Figure 3. Absorption spectra of compound 9 protonated with CSA at
various dilutions in CHCl3.
behavior is observed for the PL in solution, with the emission
showing a strong red shift at high concentrations, which is
ameliorated at low concentrations. Using a stronger acid, the
position becomes clearer. The absorption spectra of polymer 7
protonated with MSA remains red shifted in the presence of
the acid even at 10-6 M. This behavior is also seen in the
emission spectra of 7 protonated with MSA. As with CSA, the
small degree of vibronic structure on the emission spectra of
the parent polymer is lost upon protonation.
The solution spectra of polymer 7 and compound 9 are
directly comparable. (Thin films of 9 could not be obtained by
spin casting.) Figure 3 shows the absorption spectra of 9 with
CSA as a function of concentration, which was varied by
dilution of a stock (protonated) solution. Again, two absorption
bands appear in the range 2.5-4.5 eV, attributed to the two
lowest lying 1(π-π*) transitions; both red shift on protonation.
For unprotonated 7 these bands occur at 3.6 and 4.3 eV,
respectively. We also observe an isosbestic point at 3.4 eV,
indicating that dilution causes a simple A f B disproportion-
ation. Compound 9 shows very similar behavior to the polymer,
as confirmed by the emission spectra (Figure 4). To investigate
the homogeneity of protonation in solution, excitation spectra
were measured. Figures 5 and 6 show excitation spectra for 9
protonated with CSA, monitored at 3.0 and 2.15 eV, respec-
tively. Emission at 2.15 eV arises from absorption of light by
either of the absorption bands of 9. Observing emission at 3
eV we see that at high concentration (10-3 M) the protonated
compound shows only a narrow band at 3.6 eV with little
emission due to absorption at higher energies (Figure 5). This
somewhat unusual excitation spectrum probably arises from the
highly concentrated solution absorbing all the excitation light
at the front surface of the cuvette; only light in the low-energy
tail of the absorption band excites the whole of the solution
leading to efficient collection of the emitted light. When
monitoring the emission at 2.15 eV, however, it can clearly be
seen that in the highly protonated state the emission comes
predominantly from light absorbed at 2.8 eV, the lowest lying
Figure 5. Excitation profiles of compound 9 protonated with CSA at
various dilutions in CHCl3. Emission was monitored at 3.0 eV.
absorption band in the protonated state (Figure 6). With MSA,
9 behaves in a fashion very similar to 7.
Absorption and emission spectra of 9 were also measured in
methanol solutions. We observe that with CSA methanol
effectively reduces the protonation level. This is also seen with
MSA, which is a stronger acid. This provides clear evidence
that solvation by methanol interferes with the protonation
mechanism of 9.
To aid our understanding of the protonation mechanism with
CSA, 10-5 M solutions of both 7 and 9, protonated in a 1:1
ratio of CSA to pyridyl repeat unit, were prepared and their
absorption spectra recorded. Excess CSA was then added to
the solutions and their spectra measured again. These spectra
9
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Effects of Protonation on Spectroscopic Properties
A R T I C L E S
Figure 7. The ordered molecule of compound 9 in the crystal (50%
displacement ellipsoids).
Chart 2. 1H NMR Chemical Shifts of Compound 9 and Its
Diprotonated Salt 11, Assigned by H-H COSY and NOE
Experiments
Figure 6. Excitation profiles of compound 9 protonated with CSA at
various dilutions in CHCl3. Emission was monitored at 2.15 eV.
are shown in the Supporting Information. In both cases the
excess acid caused the full red shift of the absorption band,
showing that upon dilution, the CSA equilibrium is pushed
toward nondissociation and so the CSA deprotonates in the
dilute regime.
The solution photophysics of both 7 and 9 establishes that
energy transfer occurs from excited nonprotonated chain seg-
ments or molecules to lower energy, planar, protonated segments
(molecules).14 For 9 there is good overlap between the absorp-
tion band of the protonated molecules and the emission band
of the nonprotonated molecules, suggesting that Fo¨rster energy
transfer (ET) will be efficient.15 This is further supported by
the dilution studies where at low concentrations less low-energy
emission is observed consistent with molecules being separated
by a greater distance on average. Both polymer and molecular
excitation profiles support this hypothesis where it is clearly
seen that exciting anywhere in the broad absorption bands of
either material gives rise to emission in the red, i.e., ET from
localized twisted regions (molecules) to planar more delocalized
protonated segments (molecules). In thin films of 7 this process
still occurs but is not as efficient as in solution. This could be
due either to a poorer spectral overlap or shorter singlet lifetime,
which competes more effectively with EET.
1H NMR Spectroscopy. Evidence for protonation of 9 in
solution comes not only from electronic spectra but also from
1H NMR data. Chart 2 shows the chemical shifts of neutral
compound 9 and its diprotonated form (as the dichloride salt)
in (CD3)2SO solution. The significant downfield shifts of all
the peaks from the 2-pyridyl groups, especially that of He which
shifted by 0.56 ppm, can be explained by the protonation of
the nitrogen atoms. However, the methoxy protons show a
negligible shift after protonation. It is likely that the hydrogen
bonds observed in the solid state (Figure 9) in compound 11
are broken in DMSO solution. This competition for hydrogen
bonding interactions is also evident in the UV-vis spectra of 9
with both CSA and MSA in methanol solution.
X-ray Crystal Structures of 9, 10, and 11. X-ray diffraction
analysis of crystals of 9 grown from chloroform solution shows
an asymmetric unit comprising two neutral molecules. One of
them is ordered (Figure 7); the two pyridyl rings are not coplanar
with the central benzene ring, but inclined in opposite directions,
through a twist of 35.4° and 31.4° around the C(2)-C(12) and
C(5)-C(22) bonds, respectively. (For PPY, a torsion angle of
ca. 30° has been calculated.13) The pyridyl rings have mutually
anti orientations of their nitrogen atoms, and each of the latter
is also in an anti position relative to the adjacent methoxy group.
The major conformation of the second, disordered, molecule is
also twisted around the C(2)-C(12) and C(5)-C(22) bonds by
16° and 22°, respectively.
(14) (a) Jenehke, S. A.; Osaheni, J. A. Science 1994, 265, 765. (b) Grell, M.;
Bradley, D. D. C.; Ungar, G.; Hill, J.; Woodhead, K. S. Macromolecules
1999, 32, 5810. (c) Nguyen, T. Q.; Doan, V.; Schartz, B. J. J. Chem. Phys.
1999, 110, 4068.
When formic acid is added to the chloroform solutions we
observe that protonation occurs (see above). However, in crystals
grown from ethyl acetate we isolated a molecular complex 10
(15) Forster, T. Discuss. Faraday Soc. 1951, 27, 7.
9
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A R T I C L E S
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Figure 10. Electroluminescence spectra of protonated (CSA) and neutral
polymer 7.
Figure 8. X-ray molecular structure of 10 (50% displacement ellipsoids).
Primed atoms are generated by an inversion center. Selected distances
(Å): C(10)-O(2) 1.308(2), C(10)-O(3) 1.198(2), O(2)-H(O) 1.10(3),
H(O)‚‚‚N 1.50(3), O(2)‚‚‚N 2.593(2), H(9)‚‚‚O(3) 2.50(2). Angles (deg):
O(2)-H(O)-N 175(3), C(9)-H(9)-O(3) 134(2).
in a dense stacking motif stabilized by electrostatic interactions
with the anions, while 9 packs as dimers and 10 as looser stacks
of twisted molecules.
Electroluminescence Studies. To establish if the red shift
of the emission band in polymer 7 could also be observed in
electroluminescence (EL) devices were made from protonated
and nonprotonated polymer with a standard device configura-
tion,16 i.e., ITO/PEDOT/polymer 7/Ca/Al. Thin films of 7 (100
nm thickness) were obtained by spin coating from chlorobenzene
solutions with and without 1 molar equiv of CSA added to the
solution. When CSA was added the spinning solution had to
be diluted to enable comparable film thicknesses to be obtained
due to the increased viscosity of the protonated polymer solution.
The EL spectra obtained from both types of device are shown
in Figure 10 and it can be seen that the electroluminescence
behavior mirrors that of the PL upon protonation, with the EL
peak shifting from 2.4 to 2.15 eV. Efficiency measurements
reveal little difference between the neutral and protonated films;
typically an efficiency of 0.02% was measured. This is very
similar to values obtained for other pyridine-containing poly-
mers5 and is rather low due to the poor match between the ITO
and HOMO levels of pyridine-containing systems.
Figure 9. X-ray molecular structure of 11 (50% displacement ellipsoids).
Primed atoms are generated by an inversion center. Selected distances (Å):
N-H 0.90(3), H‚‚‚O(1) 1.90(3), N‚‚‚O(l) 2.599(2), H‚‚‚F(1) 2.29 (3). Angles
(deg): N-H-O(1) 132(2), N-H-F(1) 135(2).
Conclusion and Outlook
We have established a new way of tuning the emission energy
of a luminescent polymer by increasing the planarity of the
system by protonation of pyridine nitrogen. The steric hindrance
of the required solubilizing side groups can be overcome by
intramolecular hydrogen bonding, which forces the molecules
into a planar configuration. This strategy is, therefore, clearly
distinct from the intramolecular hydrogen bonding mechanism
in neutral 2,5-diarylpyrazine systems studied by Meijer et al.11
and the interchain hydrogen bonding that can planarize and
optically tune soluble polydiacetylenes and poly(p-phenylene)s.17
As alkoxy side chains are widely used to provide solubility,
our approach may be a way of producing readily soluble
polymers where efficient green emission can be shifted into the
longer wavelength region, which offers many challenges for
displays. From photophysical measurements it has been deduced
that Fo¨rster transfer is an efficient mechanism in systems 7 and
of (neutral) 9 with two molecules of formic acid, rather than a
diformate salt. The teraryl molecule in 10 is situated at a
crystallographic inversion center, with two formic acid mol-
ecules linked to it by very strong hydrogen bonds (Figure 8).
The hydrogen atom in the latter is located (as found in the
electron density map and confirmed by successful refinement)
at the O(2) atom, with no appreciable residual electron density
at the N atom. The C-N-C angle of 119.5(1)° is more typical
of neutral, rather than protonated, pyridine [cf. 117.8(2)o for
the ordered molecule in the crystal of 9]. The pyridine ring forms
a dihedral angle of 43.8° with the benzene ring and of 15.3°
with the carboxyl group.
On the contrary, with HBF4 compound 9 is clearly diproto-
nated in the crystal and the product (11) is ionic. The dication
also has crystallographic Ci symmetry. The benzene and
pyridinium rings form a dihedral angle of only 3.9° and are
held in this conformation by intramolecular hydrogen bonds
(Figure 9). Here, the C-N-C angle is increased to 124.0(2)°,
in accordance with the VSEPR theory. The dications are packed
(16) Higgins, R. W. T.; Nothofer, H.-G.; Scherf, U.; Monkman, A. P. Appl.
Phys. Lett. 2001, 17, 837.
(17) (a) Brown, A. J.; Rumbles, G.; Phillips, D.; Bloor, D. Chem. Phys. Lett.
1988, 151, 247. (b) Basker, C.; Lai, Y.-H.; Valiyaveettil, S. Macromolecules
2001, 34, 6255.
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A R T I C L E S
9 enabling excitation energy to migrate efficiently to the lowest
energy, i.e. the most planar regions or molecules of the system.
In molecularly doped polymer systems, this has been shown to
be an effective method for the production of highly efficient
saturated color devices.18 Further studies on related heterocyclic
polymers are in progress.
terization for 4, 5, 7, 9, 10, and 11; absorption spectra of polymer
7 protonated with CSA in the stoichiometric ratio (1:1) at 10-5
dilution in CHCl3 and the effect on the spectrum of the addition
of excess CSA; figures showing the disordered molecule of 9
in the crystal, and the crystal packing of 11 (PDF); X-ray
crystallographic files (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
Acknowledgment. We thank EPSRC for funding this work.
JA012409+
Supporting Information Available: Description of experi-
mental instrumentation used and synthetic details and charac-
(18) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nature 2000, 403, 750.
9
J. AM. CHEM. SOC. VOL. 124, NO. 21, 2002 6055