Single-compound libraries of organic materials: Parallel synthesis and screening of fluorescent dyes

Article abstract of DOI:10.1002/1521-3773(20011217)40:24<4677::AID-ANIE4677>3.0.CO;2-U

"Hits" with high quantum yields: The screening of the chromophores of a coumarin library for optical properties allowed the identification of "hits" with high fluorescence quantum yields (see picture) which could be used as fluorescence labels and laser dyes. The generation of the library was facilitated by an efficient method involving the parallel synthesis utilizing Pd-catalyzed cross-coupling reactions.

Full text of DOI:10.1002/1521-3773(20011217)40:24<4677::AID-ANIE4677>3.0.CO;2-U

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standing of the inherent structure
Single-Compound Libraries of Organic
Materials: Parallel Synthesis and Screening of
Fluorescent Dyes**
property relationships from which
specifically tailored materials can
be produced. Consequently, our
goal was the development of a fast,
parallel synthesis of highly pure
coumarins and screening them for
optical properties. Through sub-
stituent manipulation of the cou-
marin scaffold at positions 3, 4, 6
(Scheme 1) increased diversity is
introduced which crucially alters the electronic structure
and the associated dye properties.[11]
Starting from 3-bromocoumarin 1[12] we introduced sub-
stituents at the 3-position of the scaffold which extended the
p system of the parent chromophore; this was achieved by Pd-
catalyzed cross-coupling reactions. These types of reactions
involving coumarin have not been widely published.[13] The
efficiency of a combinatorial synthetic approach greatly
depends on the compatibility of the coupling components
and their suitability with the reaction conditions. Therefore,
the initial step requires the optimization of the synthetic
method for each individual reaction type. Scheme 2 shows the
Marc-Steffen Schiedel, Christoph A. Briehn, and
Peter B‰uerle*
The potential of combinatorial chemistry in the pharma-
ceutical industry has been demonstrated for lead-structure
identification and optimization withnumerous examples
published.[1] Similar change of paradigm has been seen in
the areas of materials and catalyst research.[2] To date the
principles of combinatorial chemistry have been applied in
the areas of inorganic solid-state materials (e.g. lumines-
cence[3]), homogenous and heterogeneous catalysts, and
polymeric materials. The combinatorial concept facilitates
the efficient generation of a large number of substances and
their screening for desirable properties. These properties are
dependent on numerous interdependent parameters.
A further field of research in materials science, where
structure property relationships are often not predictable
and an empirical identification process is required for the
elicitation of lead structures and their further development, is
that of organic materials. Consequently, we asked whether the
concepts of combinatorial chemistry are applicable to the
development of novel organic materials? We report here and
in the following correspondence[4] the efficient generation and
screening of single-compound libraries containing fluorescent
dyes[5] and p-conjugated oligomers; withthese studies we
show that the concepts of combinatorial chemistry can be
successfully applied to organic materials using rapid parallel
and mix-and-split synthesis in solution and also on solid
supports. For a successful screening and subsequent data
analysis highly pure compounds are of the utmost importance.
The analysis of the data sets encompassed the structure
property relationships[4] which should make a rational design
of novel organic materials withdesirable properties possible.
In this study we concentrate on fluorescent dyes of the
coumarin type which are of interest not only because of their
pharmacological activity[6] but also because of their applica-
tions as laser dyes,[7] fluorescent labels[8] (e.g. in biological
applications), emission layers in organic light-emitting diodes
(OLED),[9] and as optical brighteners.[10] The correlation
between optical properties (especially the fluorescence quan-
tum yield) and the molecular structure can currently be
described only empirically, since no detailed theoretical
predictions are possible. A large number of compounds in a
combinatorial library should correlate to a better under-
Scheme 1. Substituent
variation on the coumarin
scaffold.
8
À
coupling reactions of compound 1. Utilizing C C bond
formation reactions of Suzuki, Sonogashira Hagihara, and
Heck type withthe coupling components
B10, and C1 C10 (Scheme 3) facilitated the introduction
A1 A10, B1
of arene, ethynylene, and ethenylene moieties.
Scheme 2. Cross-coupling reactions with 3-bromocoumarin 1. The product
numbering is derived from the corresponding starting materials (e.g. 1A1 is
from 1 and A1). For the definitions of A1 C10 see Scheme 3.
First, bromocoumarin 1 was coupled in a parallel synthe-
sizer[14] with the (hetero)aromatic boronic esters A1 A10,
with[Pd(PPh )4] as catalyst and CsF as base. The generation
of the boronic esters was accomplished using reported
procedures.[15] The coupling products between these boronic
3
[*] Prof. P. B‰uerle, Dipl.-Chem. M.-S. Schiedel, Dr. C. A. Briehn
Abteilung Organische Chemie II (Organische Materialien und
Kombinatorische Chemie)
esters and compound 1 were pre-purified by parallel filtration
Universit‰t Ulm
[16]
withSPE-columns
followed by further purification and
Albert-Einstein-Allee 11, 89081 Ulm (Germany)
Fax : (‡49)731 5022840
analysis by sequential automated high-pressure liquid chro-
matography-mass spectrometry (HPLC-MS).[17] From these
parallel reactions we obtained 3-(hetero)aryl substituted
coumarins 1A1 1A10 in yields of 67 97% withgreater
than 99% purity. The Sonogashira Hagihara coupling reac-
tions of coumarin 1 and terminal acetylenes B1 B10 utilizing
E-mail: peter.baeuerle@chemie.uni-ulm.de
[**] This work was made possible with funds from Fonds der Chemischen
Industrie. We also thank Prof. V. Austel and Dr. E. Mena-Osteritz,
University of Ulm, for valuable discussions and general input; Dr. G.
Gˆtz, University of Ulm, for photography and also Boehringer
Ingelheim, Biberach, for apparatus support.
Angew. Chem. Int. Ed. 2001, 40, No. 24
¹ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
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A1 A10, B1 B10, and C1 C10 and the resulting synthetic
method employed for the generation of all possible combi-
nations.[19] From a possible 240 coumarin derivatives 151
(63%) were isolated in >99% purity[20] and of these 127
(84%) were previously unpublished compounds. The remain-
ing compounds could not be isolated in sufficient purity for
screening because of low conversions or separation difficul-
ties.
The purified library members were screened for the desired
optical properties. The use of a microplate reader allowed the
accelerated recording of fluorescence and excitation spectra
of the coumarins in ethanol. The determined absorption
maxima (lmax ˆ 314 430 nm) and the emission maxima
(lmax ˆ 400 569 nm) covered a wide range of values. The
quantity of the acquired data was sufficient for a detailed
correlation between the geometrical and electronic structures
of the dyes to be deduced. These results should allow the
rational design of dyes which contain specifically desired
properties and results pertaining to this will be published in
due course.
While the structure property relationships can be deduced
for absorption and fluorescence, the correlation between
these factors with respect to the fluorescence quantum yield is
still not totally understood. To estimate the quantum yield of
the individual compounds, ethanolic solutions of the coumar-
ins were irradiated in microplates witha UV lamp (Figure 1).
From the coumarin library 34 derivatives with the most
intensive fluorescence (in different emission ranges) were
Scheme 3. Coupling components for the Pd-catalyzed parallel syntheses of
3-bromocoumarins 1 8.
the catalytic system of [Pd(PPh3)2Cl2]/CuI furnished the
3-ethynylcoumarins 1B1 1B10 in yields of 74 95%. Sim-
ilarly, the Heck coupling with alkenes C1 C10 utilizing the
catalytic system [Pd2(dba)3]/AgOAc (dba ˆ dibenzylidene-
acetone, OAc ˆ acetate) gave isomerically pure 3-(E)-ethe-
nylcoumarins 1C1 1C10 in yields of 30 94%. The effi-
ciency of the synthetic methods is shown in that all 30 prod-
ucts were isolated withhighpurity and in yields sufficient for
further screening.
Additionally, seven 3-bromocoumarins 2 8 which incorpo-
rated functional groups at the 4-, 6-, 7-, and 8-positions were
synthesized for the parallel generation of further coumarin
Figure 1. Screening of a coumarin library in a microtiter plate. In the row
A
F the coumarins are sorted with respect to their absorption properties.
selected and the fluorescence quantum yield of each was
determined by standard procedures (Table 1).[21] The follow-
ing ™hits∫ were identified (Figure 2): 5A9 (ff ˆ 0.18, labs
ˆ
373 nm, lem ˆ 535 nm), 8A2 (ff ˆ 0.90, labs ˆ 395 nm, lem
ˆ
478 nm), 7C4 (ff ˆ 0.62, labs ˆ 393 nm, lem ˆ 480 nm), and
7B1 (ff ˆ 0.98, labs ˆ 397 nm, lem ˆ 455 nm). Among these
™hits∫ the 3-phenylethynyl substituted coumarin 7B1 exhib-
ited a higher fluorescence quantum yield than the comparable
libraries.[18] The aforementioned reaction conditions of the
three cross-coupling reactions were optimized for selected
combinations of coumarins 2 8 and the coupling components
4678
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Table 1. Absorptions and emission properties of selected coumarins based
on the fluorescence quantum yields.
purification the screening for optical properties identified
several ™hits∫ with high fluorescence quantum yields. The
development of structure property relationships based on
the collective data sets should allow the rational design of
coumarin dyes. This study demonstrates the successful
application of a combinatorial approachstrategy encompass-
ing the design of the lead structure, development of synthetic
routes, efficient library synthesis and purification techniques,
screening, and data analysis.
[a]
[b]
[a]
[b]
Coumarin labs
lem
ff[c]
Coumarin labs
lem
ff[c]
[nm] [nm]
[nm] [nm]
1A4
1A8
1A10
1C1
1C2
1C4
1C10
2A7
2C2
2C4
2C7
7A2
7A5
7A7
7A9
7B1
7C6
337
355
334
355
354
360
371
341
346
352
360
360
370
378
363
397
369
439
442
438
447
446
452
463
439
435
441
466
449
469
481
442
455
455
0.94
0.91
0.72
0.67
0.66
0.72
0.73
0.25
0.32
0.38
0.57
0.35
0.26
0.40
0.70
0.98
0.38
4A7
4A8
4B4
5A9
5B1
5B4
6A5
6A6
6A10
7C4
7C7
8A2
8A6
8A7
8A8
8A9
8C7
385
394
394
373
389
387
382
364
361
393
396
395
395
411
405
394
427
553
564
578
535
549
548
565
532
530
480
485
478
479
495
506
474
496
0.02
0.02
0.03
0.18
0.10
0.09
0.12
0.16
0.10
0.62
0.58
0.90
0.77
0.73
0.55
0.84
0.58
Received: July 25, 2001 [Z17596]
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[a] Absorption maxima in ethanol (longest wavelength transition).
[b] Maxima of the corrected emission spectra in ethanol. [c] External
standard: 9,10-diphenylanthracene (ff ˆ 0.95, in cyclohexane).[20]
[4] C. A. Briehn, M.-S. Schiedel, E. M. Bonsen, W. Schuhmann, P.
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Figure 2. ™Hits∫ identified by the qualitative and subsequent quantitative
screening of the fluorescence quantum yield: 8A2, 7C4, 5A9, and 7B1.
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ˆ
354 nm, lem ˆ 435 nm in ethanol).[22] In view of possible
applications of these compounds (e.g. as fluorescent labels
in biological systems) the NH2 substituted derivatives 7B1,
7C4, and 5A9 are of interest since these dyes can be linked to
a biomolecule through their amino groups. Also, the following
step of the combinatorial development process, namely the
application of these novel chromophores as active layers in
organic light-emitting diodes and their optimization should be
possible. The application of combinatorial strategies for the
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emitting diodes was recently reported by Schmidt et al.[23]
This work clearly demonstrates that the concepts of
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synthetic method for the parallel execution of Pd-catalyzed
cross-coupling reactions yielded a large number of unpub-
lished fluorescent coumarin dyes. Following the automated
¬
100, 5541 5553; d) A. Adronov, S. L. Gilat, J. M. Frechet, K. Ohta,
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Leising, G. Froyer, L. Athouel, Adv. Mater. 1997, 9, 33 36.
[10] A. E. Siegrist, H. Hefti, H. R. Meyer, E. Schmidt, Rev. Prog.
Coloration 1987, 17, 39 55.
[11] Note: Substituent variation at the 3-, 4-, 6-, and 7-positions exhibit an
especially strong influence on the electronic properties of the
chromophore. a) Organic Luminescent Materials (Eds.: B. M. Kraso-
vitskii, B. M. Bolotin), Wiley-VCH, Weinheim, 1988; b) R. M. Chris-
tie, Rev. Prog. Coloration 1993, 23, 1 18; c) N. A. Kuznetsova, O. L.
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¹ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
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[13] a) A. K. Mitra, A. De, N. Karchaudhuri, J. Mitra, J. Chem. Res. (S)
1998, 766 767; b) G. M. Boland, D. M. X. Donnelly, J.-P. Finet, M. D.
Rea, J. Chem. Soc. Perkin Trans. 1 1996, 2591 2597.
Single-Compound Libraries of Organic
Materials: From the Combinatorial Synthesis of
Conjugated Oligomers to Structure Property
Relationships**
[14] Quest 210 Synthesizer, Fa. Argonaut Technologies.
[15] compare witha) W. Kˆnig, W. Scharrnbeck, J. Prakt. Chem. 1930, 128,
153 170; b) M. S. Wong, J.-F. Nicoud, Tetrahedron Lett. 1993, 34,
8237 8240; c) Y. Kobayashi, R. Mizojiri, E. Ikeda, J. Org. Chem.
1996, 61, 5391 5399; d) A. Finch, J. C. Lockhart, J. Chem. Soc. 1962,
3723 3728. The boronic acids A7, A9, and A10 are unpublished.
[16] SPE ˆ Solid-phase extraction; for filtration columns with RP-18
material was used in conjunction witha ™VacMaster SPE Processing
Station∫ (Separtis IST); mobile phase: acetonitrile/water.
[17] Purification and mass detection were performed on an HPLC-MS
(Waters) fitted witha LC/MS detector (Micromass ZMD; electro-
spray ionization (ESI) mode). Chromatography performed on Xterra
RP-18 (analytic: 3.5 mm, 4.6 Â 100 mm; semi-preparative: 7.0 mm,
19 Â 150 mm) columns (Waters) withan acetonitrile water gradient.
[18] The following references were utilized for the synthesis of bromo-
coumarins 2 4, 7, and 8: 2: F. Peters, H. Simonis, Chem. Ber. 1908, 41,
830 837; 3: K. Takagi, M. Hubert-Habart, Bull. Soc. Chim. Fr. 1980, 2,
444 448; 4: a) B. B. Dey, K. K. Row, J. Chem. Soc. 1923, 123, 3375
3384; b) B. B. Dey, T. R. Seshadri, Chem. Zentralbl. 1926, 97, 1648
1649; 7, 8: N. A. Gordeeva, M. A. Kirpichenok, N. S. Patalakha, I. I.
Grandberg, Chem. Hetreocycl. Compd. (Engl. Transl.) 1990, 26,
1329 1337; Bromocoumarins 5 and 6 were available by selective
nitration and subsequent reduction starting from coumarin 2 and 3:
KNO3 (1.22 g, 12 mmol) was added to a solution of coumarin 2 or 3
(10 mmol) in 96% sulfuric acid (60 mL) at 08C. After stirring at 08C
Christoph A. Briehn, Marc-Steffen Schiedel,
Eva M. Bonsen, Wolfgang Schuhmann, and
Peter B‰uerle*
The development process of novel materials is often
encumbered by the time-consuming ™one-at-a-time∫ process
of material synthesis and evaluation. This situation is partic-
ularly true for p-conjugated oligomers which serve as model
compounds for conducting polymers and are recognized as
materials in their own right.[1] Accordingly, there is a need for
novel methods that provide for both rapid compound
generation and subsequent evaluation. Combinatorial meth-
odologies that were developed for the high-speed synthesis
and high-throughput screening of pharmaceuticals could help
to overcome these bottlenecks in the materials development
process.[2] Moreover, the rapid generation of data sets
provided by combinatorial methods and their subsequent
translation into structure property relationships may enable
the rational design of new materials.
for 2.5 h, the solution was poured into crushed ice forming
a
precipitate. This precipitate was collected and washed successively
withdiluted Na 2CO3 solution. The pure 6-nitrated derivatives were
obtained by recrystallizing from ethanol/water (95/5). For the
reduction, the nitro compounds (10 mmol) were dissolved at 1008C
in 80% acetic acid (200 mL) and treated portionwise withiron powder
(3.35 g, 60 mmol). After 10 min the solution was poured into ice and
extracted several times with chloroform. The solvent was subsequent-
ly removed and the crude product purified by column chromatog-
raphy on silica (hexane/ethylacetate 50/50; overall yields: 66% (5),
63% (6)).
While most of the combinatorial approaches in materials
science concentrate on the development of inorganic solid-
state materials, polymeric materials, and catalytic systems,[3]
we report here and in the preceding correspondence[4] the
development of combinatorial strategies for the generation of
organic materials. The focus of this study is the combinatorial
synthesis and subsequent screening of oligothiophenes which
are one of the most examined classes of p-conjugated
oligomers.[5] The strategy covers all stages of the combinato-
rial discovery process: design of the lead structure, elabo-
ration of the synthetic route, generation of the library and
purification, screening, and data analysis. We focused on a
regioregular head-to-tail coupled quater(3-arylthiophene) as
the lead structure (Scheme 1). Because of their defined
structure these aryl substituted oligomers, together with the
already intensively investigated oligo(3-alkylthiophene)s, are
outstanding model compounds for the parent (polydisperse)
poly(3-arylthiophene)s and poly(3-alkylthiophene)s.[6] The
[19] Representative example for a parallel Suzuki cross-coupling: syn-
thesis of 7-amino-4-methyl-3-p-tolyl-chromen-2-one (7A2): Couma-
rin 7 (25.4 mg, 0.1 mmol), boronic ester A2 (35.2 mg, 0.2 mmol), CsF
(121 mg, 0.8 mmol), and [Pd(PPh3)4] (5.78 mg, 5 mol%) were heated
at 908C under argon in dry dioxane for 16 h. The solvent was
evaporated and the residue was dissolved in acetonitrile/water. This
solution was then filtered through a SPE (RP-18) column and purified
by automated HPLC-MS affording the pure product (21.2 mg, 80%).
M.p. 260 2628C; 1H NMR (400 MHz, [D6]DMSO, 258C): d ˆ 7.44 (d,
J ˆ 8.7 Hz, 1H), 7.20 (d, J ˆ 7.9 Hz, 2H), 7.12 (d, J ˆ 7.9 Hz, 2H), 6.60
(dd, J ˆ 8.7, 1.9 Hz, 1H), 6.45 (d, J ˆ 1.9 Hz, 1H), 6.07 (broad s, 2H),
2.33 (s, 3H), 2.13 (s, 3H); 13C NMR (100 MHz, [D6]DMSO, 258C):
d ˆ 160.8, 154.6, 152.8, 148.6, 136.6, 132.5, 130.5 (2C), 128.7 (2C),
126.8, 119.8, 111.6, 109.4, 98.5, 21.0, 16.3; UV/Vis (ethanol): lmax
(e [LmolÀ1 cmÀ1]) ˆ 360 nm (22500); EI-MS (70 eV): m/z (%) : 265
‡
‡
(88) [M ], 237 (100) [M À CO].
[*] Prof. Dr. P. B‰uerle, Dr. C. A. Briehn, Dipl.-Chem. M.-S. Schiedel
Abteilung Organische Chemie II (Organische Materialien und
Kombinatorische Chemie)
[20] For further structure validation 20% of the library members were
characterized spectroscopically (1H NMR).
[21] Optically dilute measurements withrefractive-index corrections,
maximal absorption of the solutions ꢀ0.04. Calibrated spectrometer
(Perkin Elmer LS 50B); measurements referenced to 9,10-dipheny-
lanthracene (ff ˆ 0.95); a) Principles of Fluorescence Spectroscopy,
2nd ed. (Ed.: J. R. Lakowicz), Kluwer Academic/Plenum Publishers,
New York, 1999; b) J. N. Demas, G. A. Crosby, J. Phys. Chem. 1971, 75,
991 1024.
[22] T. Besson, G. Coudert, G. Guillaumet, J. Heterocycl. Chem. 1991, 28,
1517 1523.
[23] a) C. Schmitz, P. Pˆsch, M. Thelakkat, H.-W. Schmidt, Phys. Chem.
Chem. Phys. 1999, 1, 1777 1781; b) C. Schmitz, M. Thelakkat, H.-W.
Schmidt, Adv. Mater. 1999, 11, 821 826.
Universit‰t Ulm
Albert-Einstein-Allee 11, 89081 Ulm (Germany)
Fax : (‡49)731-502-2840
E-mail: peter.baeuerle@chemie.uni-ulm.de
Dr. E. M. Bonsen, Prof. Dr. W. Schuhmann
Analytische Chemie–Elektroanalytik and Sensorik
Ruhr-Universit‰t Bochum, 44780 Bochum (Germany)
[**] This work was made possible with funds donated from Fonds der
¬
Chemischen Industrie, the BMBF (Kekule grant to C.A.B) and the
DFG (Schu 929/5-1). We thank Prof. V. Austel, the University of Ulm,
for valuable discussions and also Boehringer Ingelheim Pharmaceut-
icals Inc., Ridgefield (USA), for apparatus support.
Supporting information for this article is available on the WWW under
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Angew. Chem. Int. Ed. 2001, 40, No. 24