Proquinoid acceptors as building blocks for the design of efficient π- conjugated fluorophores with high electron affinity

Article abstract of DOI:10.1039/b002369h

The association of aromatic electron donor groups with proquinoid acceptors leads to π-conjugated fluorophores combining tunable emission wavelength at constant geometry, high fluorescence efficiency and high electron affinity.

Full text of DOI:10.1039/b002369h

Proquinoid acceptors as building blocks for the design of efficient
p-conjugated fluorophores with high electron affinity
Jean-Manuel Raimundo, Philippe Blanchard, Hugues Brisset, Said Akoudad and Jean Roncali*
Inge´nierie Mole´culaire et Mate´riaux Organiques, CNRS UMR 6501, Universite´ d’Angers, 2 Bd Lavoisier, 49045
Angers Cedex, France. E-mail: jean.roncali@univ-angers.fr
Received (in Cambridge, UK) 24th March 2000, Accepted 17th April 2000
The association of aromatic electron donor groups with
proquinoid acceptors leads to p-conjugated fluorophores
combining tunable emission wavelength at constant geome-
try, high fluorescence efficiency and high electron affinity.
procedure already described for the synthesis of 2,5-dithienyl-
benzo[c]thiophene.9 Reaction of phthaloyl chloride with 2-mer-
captopyridine led to 1,2-bis[S-(2-pyridyl)]benzenedithioate 6
which was then reacted with 3,4-ethylenedioxy-2-thienylmag-
nesium bromide to give diketone 5. Compound 2 was then
obtained by ring closure with the Lawesson reagent (L.R.).
Compounds 1–4 have been characterized by 1H and 13C NMR
spectroscopy, mass spectrometry and elemental analysis.†
Table 1 lists the cyclic voltammetric (CV) and optical data for
compounds 1–4. Literature data for terphenyls (3P) and
terthienyl (3T) have been included for comparison. Compounds
1–4 are reversibly reduced to their anion radicals and irreversi-
bly oxidized owing to electropolymerisation. The values of the
absorption maximum (lmax) and HOMO–LUMO gap, DE
(estimated from the absorption edge), for 3P and 1 show that
replacement of the median phenyl ring by the BTD group
produces a 100 nm red shift of lmax and a 1.10 eV decrease in
DE. Comparison of the anodic peak potentials (Epa) for 3P and
1 shows that the BTD group scarcely affects the oxidation
Conjugated fluorophores usable as active materials for light
emitting diodes (LEDs) should ideally combine high emission
quantum yields, tunable emission spectra and high electron
affinity in order to allow electron injection from stable metal
cathodes.1
The difficulty of meeting these various prerequisites with a
single compound has given rise to specific synthetic ap-
proaches. Thus, control of the emission wavelength has been
realized by adjusting the conjugation length of the fluor-
ophore,1,2 while increases in electron affinity have been
achieved by incorporation of electron deficient groups, such as
e.g. cyano or oxadiazole, in the p-conjugated system.1,3
It has been shown that the association of five-membered
heterocycles with electron-deficient systems such as thienopyr-
azine or benzo[c]thiadiazole (BTD) which have a strong
propensity to impose a quinoid geometry on the ground state of
the conjugated system, leads to polymers with small intrinsic
band gaps.4 As part of our continuing interest in band gap
engineering,5 we have synthesized a new series of tricyclic p-
conjugated systems 1–4 based on this approach. Besides their
use as precursors for low band gap polymers,6 these compounds
exhibit original electronic and optical properties which make
them potentially interesting for the design of photoluminescent
materials for LEDs.
Compounds 1, 3 and 4 were synthesized from 4,7-dibromo-
2,1,3-benzothiadiazole.7 Compound 1 was obtained by Suzuki
coupling with phenyl boronic acid and compounds 3 and 4 were
prepared by Stille coupling with the tributylstannyl derivatives
of 3,4-dihexyloxythiophene8 and 3,4-ethylenedioxythiophene,
respectively (Scheme 1). Compound 2 was prepared using a
Scheme 1
DOI: 10.1039/b002369h
Chem. Commun., 2000, 939–940
This journal is © The Royal Society of Chemistry 2000
939

Table 1 CVa and opticalb data for compounds 1–4
mers leads to fluorophores with a unique combination of
enhanced electron affinity, high fluorescence efficiency and
ready control of the emission wavelength at constant chain
length. In addition to the intrinsic interest of these compounds
as a new class of efficient fluorophores, the above synthetic
strategy could open interesting perspectives for the design of
active molecular or polymeric materials for LEDs.
Compd. Epa/V
Epr/V
lmax/nm DE/eV
lem/nm ff
3P
1
3T
2
3
4
1.80c
1.82
1.10
0.56
1.06
0.92
22.70c,d 279e
3.97
2.87
3.05
2.33
2.29
2.19
339e
490
430
613
542
630
0.93e
21.36
22.00f
21.80
21.28
21.40
380
350
450
456
481
0.80
0.066g
0.92
0.73
0.75
Notes and references
a In 0.1 M NBu4PF6–MeCN, ref. SCE, 100 mV s21 b In CH2Cl2. c vs. Ag/
.
AgCl (ref. 11). d In NBu4Br–DMA (ref. 11). e In cyclohexane (ref. 12). f In
NBu4–DMF (ref. 13). g In dioxane (ref. 14).
† 1: yellow powder, mp 127–129 °C. dH(CDCl3): 7.97 (dd, 4H, 3J 8.39, 4J
3
3
1.17 Hz), 7.80 (s, 2H), 7.56 (t, 4H, J 7.51 Hz), 7.47 (t, 2H, J 7.42 Hz).
dC(CDCl3): 154.1, 137.4, 133.3, 129.3, 128.6, 128.4, 128.1. Anal: found
(calc.): C, 74.99 (74.98); H, 4.35 (4.20); N, 9.51 (9.72); S, 11.10
(11.15)%.
potential but produces a positive shift of the reduction peak
potential (Epr) by more than 1.30 V, indicating a considerable
increase in electron affinity.
As expected, thiophene-based compounds exhibit smaller
DE values than 1 owing to the lower resonance energy of
thiophene and the electron releasing effect of the alkoxy
groups.5
Comparison of the optical data for 3T and 2 shows that the
combined effects of replacement of the median thiophene by
benzo[c]thiophene and electron-releasing ethylenedioxy sub-
stituents produce a ca. 100 nm red shift of lmax associated with
a 0.70 eV decrease in DE. On the other hand, the 0.54 V
decrease in Epa and the 0.20 V positive shift of Epr, indicated by
the CV data, show that the gap reduction is essentially related to
raising of the HOMO level.
Further replacement of the benzo[c]thiophene in 2 by the
BTD group (3 and 4) produces a 6–31 nm red shift of lmax and
a small decrease in DE. However, the 0.40–0.50 V parallel
positive shift of both Epa and Epr shows that the BTD group
induces major changes in the HOMO and LUMO levels, with
again a large enhancement of electron affinity. The small
decrease in DE from 3 to 4 and the slight negative shift of Epa
reflects the stronger electron-donating effect of the ethylene-
dioxy bridge compared to dialkoxy chains.8
2: brown solid, mp 146–148 °C. dH(CDCl3): 7.96 (m, 2H), 7.13 (m, 2H),
6.42 (s, 2H), 4.32 (m, 4H), 4.27 (m, 4H). MS: m/z (%) 414 (M+; 100), 317
(60). HRMS: calc. 414.0058, found 414.0054. Anal: found (calc.): C, 57.58
(56.95); H, 3.49 (3.40); O, 16.02 (15.44); S, 22.96 (23.21)%.
3: orange solid, mp 79–80 °C. dH(CDCl3): 8.39 (s, 2H), 6.37 (s, 2H), 4.10
(t, 4H, 3J 6.60 Hz), 4.02 (t, 4H, 3J 6.60 Hz), 1.82 (m, 4H), 1.70 (m, 4H), 1.50
(m, 4H), 1.36 (m, 12H), 1.24 (m, 8H), 0.92 (m, 6H), 0.84 (m, 6H).
dC(CDCl3): 152.8, 150.3, 144.9, 127.6, 124.5, 120.6, 98.2, 72.9, 70.0, 31.5,
30.1, 29.1, 25.8, 25.6, 22.6, 22.5, 14.1, 14.0. MS: m/z (%) 700 (M+; 100),
615 (20), 531 (20). HRMS: calc. 700.3402, found 700.3412.
4: brown solid, mp 314 °C. dH(CDCl3): 8.4 (1s, 2H), 6.55 (s, 2H), 4.44
(t, 4H), 4.31 (t, 4H). dC(CDCl3): 152.3, 141.6, 140.2, 126.6, 123.6, 113.7,
101.9, 64.9, 64.3. Anal: found (calc.): C, 51.77 (51.91); H, 3.04 (2.90); N,
6.62 (6.73); O, 15.28 (15.37); S, 23.29 (23.09)%.
1 A. Kraft, A. C. Grimsdale and A. B. Holmes, Angew. Chem., Int. Ed.,
1998, 37, 402.
2 B. S. Kang, M.-L. Seo, Y. S. Jun, C. K. Lee and S. C. Shin, Chem.
Commun., 1996, 1167; R. Gomez, J. L. Segura and N. Martin, Chem.
Commun., 1999, 619.
3 N. C. Greeham, S. C. Moratti, D. C. C. Bradley, R. H. Friend and A. B.
Holmes, Nature, 1993, 365, 628; X.-C. Li, A. B. Holmes, A. Kraft, S. C.
Moratti, G. W. Spencer, F. Cacialli, J. Gru¨ner and R. H. Friend, J. Chem.
Soc., Chem. Commun., 1995, 2211; W.-L. Yu, H. Meng, J. Pei, Y.-H.
Lai, S.-J. Chua and W. Huang, Chem. Commun., 1998, 1957; U.
Mitschke, E. Mena Osteritz, T. Debaerdemaeker, M. Sokolowski and P.
Ba¨uerle, Chem. Eur. J., 1998, 4, 2211.
4 S. Kitamura, S. Tanaka and Y. Yamashita, Chem. Mater., 1996, 8, 570;
S. Akoudad and J. Roncali, Chem. Commun., 1998, 2081.
5 J. Roncali, Chem. Rev., 1997, 97, 173.
The fluorescence properties of compounds 1–4 have been
analyzed in CH2Cl2 and the emission quantum yields (ff) have
been determined against anthracene in 95% EtOH for 1, and
perylene in 95% EtOH for 2–4. Introduction of BTD in the 3P
system produces a 150 nm red shift of the emission maximum
(lem) and a 50 nm increase in the Stokes shift. Whereas the
effect of BTD on ff is difficult to evaluate due to the different
experimental conditions, the measured value of 0.80 for 1
shows that ff remains high.
6 J.-M. Raimundo, P. Blanchard, S. Akoudad and J. Roncali, unpublished
work.
7 K. Pilgram, M. Zupan and R. Skiles, J. Heterocycl. Chem., 1970, 7,
629.
8 S. Akoudad, P. Fre`re, N. Mercier and J. Roncali, J. Org. Chem., 1999,
64, 4267.
Comparison of the data for 3T and 2–4 reveals a large red
shift of lem (up to 200 nm for 4) and a 70–80 nm increase in the
Stokes shift for 2 and 4. Furthermore, the three compounds
exhibit a considerable increase in ff which reaches a value as
high as 0.92 for 2. The significant increase in ff compared to the
analog of 2 based on unsubstituted thiophene (ff = 0.67),10
underlines the strong effect of the ethylenedioxy substituents on
the fluorescence efficiency.
9 D. Vangeneugden, R. Kiebooms, P. Adriaensens, D. Vanderzande, J.
Gelan, J. Desmet and G. Huyberechts, Acta Polym., 1998, 49, 687.
10 P. Ba¨uerle, G. Go¨tz, P. Emerle and H. Port, Adv. Mater., 1992, 9,
564.
11 K. Meerholtz and J. Heinze, Electrochim. Acta, 1996, 41, 1839.
12 H. Du, R. A. Fuh, J. Li, A. Corkan and J. S. Lindsey, Photochem.
Photobiol., 1998, 68, 141.
13 D. Jones, M. Guerra, L. Favaretto, A. Modelli, M. Fabrizio and G.
Distefano, J. Phys. Chem., 1990, 94, 5761.
To summarize, we have shown that the insertion of
proquinoid acceptor groups in short-chain p-conjugated oligo-
14 R. S. Becker, J. Seixas de Melo, A. L. Maçanita and F. Elisei, Pure Appl.
Chem., 1995, 67, 9.
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Chem. Commun., 2000, 939–940