Rapid access to unusual solid-state luminescent merocyanines by a novel one-pot three-component synthesis

Article abstract of DOI:10.1021/ol200655s

A novel consecutive three-component coupling-enamine addition synthesis gives access to three types of diene merocyanines in a selective fashion and in good yields. Moreover, all these push-pull systems are intensely red or yellow emissive in the solid st

Full text of DOI:10.1021/ol200655s

ORGANIC
LETTERS
2011
Vol. 13, No. 10
2556–2559
Rapid Access to Unusual Solid-State
Luminescent Merocyanines by a Novel
One-Pot Three-Component Synthesis
,†
Christian Muschelknautz,† Walter Frank,‡,§ and Thomas J. J. Muller*
€
Institut fu€r Organische Chemie und Makromolekulare Chemie and Institut fu€r
€
Anorganische Chemie und Strukturchemie, Heinrich-Heine-Universitat Dusseldorf,
€
€
Universitatsstr. 1, D-40225 Dusseldorf, Germany
€
ThomasJJ.Mueller@uni-duesseldorf.de
Received March 11, 2011
ABSTRACT
A novel consecutive three-component couplingꢀenamine addition synthesis givesaccesstothree typesof diene merocyaninesin a selective fashion
and in good yields. Moreover, all these pushꢀpull systems are intensely red or yellow emissive in the solid state and display large Stokes shifts.
The quest for novel π-electron systems represents a
demanding challenge for organic chemists who cope with
the requisition for new functional materials, such as chro-
mophores, fluorophores, and electrophores.1 Especially
the venerable class of merocyanines,2 i.e. R-donor-ω-ac-
ceptorsubstitutedpolyenes, hasprovoked newinterest and
applications in various areas of science and technology,3
due to their tunable electronic distribution. In addition
merocyanines are also promising chromophores for
molecule-based nonlinear optical materials and in
photovoltaics.4 Over the years, general access to these
pushꢀpull chromophores always has been Knoevenagel
condensations5 or substitution reactions.6 However, recently
we have established and developed diversity-oriented synth-
eses of chromophores7 based upon transition-metal catalysis
as an entry to consecutive multicomponent8 and domino
reactions.9 These productive concepts open excellent
(6) (a) Kay, A. J.; Woolhouse, A. D.; Gainsford, G. J.; Haskell, T. G.;
Barnes, T. H.; McKinnie, I. T.; Wyss, C. P. J. Mater. Chem. 2001, 11,
Dedicated to Prof. Dr. Dr. h. c. mult. Rolf Gleiter on the occasion of his
75th birthday.
€
€
996–1002. (c) Wurthner, F. Synthesis 1999, 2103–2113. (c) Wurthner, F.;
Yao, S. J. Org. Chem. 2003, 68, 8943–8949. (d) Yao, S.; Beginn, U.;
† Institut f€ur Organische Chemie und Makromolekulare Chemie.
‡ Institut f€ur Anorganische Chemie und Strukturchemie.
§ X-ray structure analysis.
€
Gress, T.; Lysetska, M.; Wurthner, F. J. Am. Chem. Soc. 2004, 126,
8336–8346.
(7) For diversity-oriented syntheses of chromophores, see e.g.: (a)
(1) Functional Organic Materials; M€uller, T. J. J., Bunz, U. H. F., Eds.;
WILEY-VCH: Weinheim, 2007.
(2) (a) Hamer, F. M. The Cyanine Dyes and Related Compounds;
Interscience: New York, London, 1964. (b) Mishra, A.; Behera, R. K.;
Behera, P. K.; Mishra, B. K.; Behera, G. B. Chem. Rev. 2000, 100, 1973–
2011. (c) Kulinich, A. V.; Ishchenko, A. A. Russ. Chem. Rev. 2009, 78,
141–164.
(3) Peng, X. Z. Y.; Carroll, S.; Geise, H. J.; Peng, B.; Dommisse, R.;
Esmansc, E.; Carleer, R. J. Mater. Chem. 1996, 6, 1325–1333.
(4) Shirinian, V. Z.; Shimkin, A. A. Top. Heterocycl. Chem. 2008, 14,
75–105.
(5) (a) Kovtun, Y. P.; Prostota, Y. O.; Shandura, M. P.; Poronik,
Y. M.; Tolmachev, A. I. Dyes Pigm. 2004, 60, 215–221. (b) Kovtun,
Y. P.; Prostota, Y. O.; Tolmachev, A. I. Dyes Pigm. 2003, 58, 83–91. (c)
Yagi, S.; Maeda, K.; Nakazumi, H. J. Mater. Chem. 1999, 9, 2991–2997.
€
Muller, T. J. J. In Functional Organic Materials. Syntheses, Strategies,
€
and Applications; Wiley-VCH: Weinheim, 2007; pp 179ꢀ223. (b) Muller,
T. J. J.; D’Souza, D. M. Pure Appl. Chem. 2008, 80, 609–620. (c) Yi, C.;
Blum, C.; Liu, S.-X.; Frei, G.; Neels, A.; Stoeckli-Evans, H.; Leutwyler,
S.; Decurtins, S. Tetrahedron 2008, 40, 9437–9441. (d) For combinatorial
syntheses of chromophores, see e.g.: Samanta, A.; Vendrell, M.; Dasa,
R.; Chang, Y.-T. Chem. Commun. 2010, 46, 7406–7408. (e) Vendrell, M.;
Lee, J.-S.; Chang, Y.-T. Curr. Opin. Chem. Biol. 2010, 14, 383–389. (f)
Main, M.; Snaith, J. S.; Meloni, M. M.; Jauregui, M.; Sykes, D.;
Faulkner, S.; Kenwright, A. M. Chem. Commun. 2008, 5212–5214. (g)
€
Briehn, C. A.; Bauerle, P. Chem. Commun. 2002, 1015–1023. (h) Briehn,
€
C. A.; Schiedel, M.-S.; Bonsen, E. M.; Schuhmann, W.; Bauerle, P.
Angew. Chem., Int. Ed. 2001, 40, 4680–4683.
(8) (a) Willy, B.; Muller, T. J. J. Curr. Org. Chem. 2009, 13, 1777–
€
€
1790. (b) Willy, B.; Muller, T. J. J. ARKIVOC 2008, Part I, 195ꢀ208.
r
10.1021/ol200655s
Published on Web 04/14/2011
2011 American Chemical Society

synthetic access to luminescent indolone terminated
pushꢀpull dienes, pyrazoles, benzodiazepines, furans, and
pyrroles by multicomponent reactions10,11 and to highly
emissive spirocycles in a domino fashion.9 Furthermore,
we devised a straightforward, versatile three-compo-
nent enaminone synthesis based upon a one-pot cou-
plingꢀMichael addition sequence.12 Therefore, we became
intrigued in scouting the vinylogous extension with respect
to the nucleophile, i.e. the transformation of enamines in the
concluding Michael addition step.13 In addition to scouting
the scope of the synthetic methodology we set out to open a
new synthetic access to pushꢀpull chromophores with
unique luminescence. Here we communicate our findings
on the diversity-oriented and highly selective three-compo-
nent synthesis of a new class of merocyanines in a one-pot
fashion. In addition we report the unusual luminescence
behavior, which is strongly affected by the substitution
pattern.
Scheme 1. Consecutive One-Pot Three-Component Syntheses
of Merocyanines 5, 6, and 7
Commencing with the smooth coupling of benzoyl chlor-
ides 1 and terminal alkynes 2 at room temperature under
modified Sonogashira conditions,12 the anticipated alky-
nones (the conversion was monitored by TLC) were trans-
formed without isolation by addition of ethanolic solutions
of 1,3,3-trimethyl-2-methylene indoline (Fischer’s base) (3)
or 1,2-dimethyl benzothiazolium iodide (4) and triethyla-
mine. After heating to 80 °C in a sealed reaction tube for 16
h three different types of diene merocyanines 5, 6, and 7
were isolated in moderate to good yields (Scheme 1,
Table 1) as crystalline solids with a red (5), an orange (6)
or a bright yellow hue (7). The structures of the novel
merocyanines were unambiguously assigned by spectro-
scopic characterization and combustion analysis and later
corroborated by X-ray crystal structure analyses for com-
pounds 6b and 7b (Figure 1, top and bottom, respective-
ly).14 Whereas the pushꢀpullbutadiene systems 5 and 6 are
obtained in a stereoselective fashion, as supported by
structural assignments from the crystallography and the
occurrence of single sets of signals in all 13C NMR spectra,
the vinyl substituted merocyanines 7 are formed as 1:1
mixtures of double bond isomers.
have been applied as substrates (1) to enhance the
electrophilicity of the ynone in the terminal enamine
addition step and (2) to increase the dipole moment of
the desired pushꢀpull system. Scouting methodological
studies have also revealed that conventional enamines
gave unselective product formation arising from solvo-
lysis of the enamines and direct addition of the second-
ary amino component to the ynone. With respect to
chromophore syntheses we therefore continued with
1,3,3-trimethyl-2-methylene indoline (Fischer’s base) (3)
Table 1. Three-Component Syntheses of Merocyanines 5, 6,
and 7
merocyanines
benzoyl
enamine
(isolated
yield)a
entry chloride 1
alkyne 2
(precursor)
1
R1 = H
(1a)
R2 = SiMe3 (2a)
3
5a (34%)a
2
3
1a
2a
4
3
5b (53%)a
5c (58%)a
The catalytic generation of the intermediate ynones
occurs with expected efficiency.8,12 In this study only
electroneutral and electron-deficient aroyl chlorides
R1 = CF3 2a
(1b)
4
5
1b
2a
4
3
5d (73%)a
5e (45%)a
R1 = CN 2a
(1c)
€
(9) (a) D’Souza, D. M.; Kiel, A.; Herten, D. P.; Muller, T. J. J.
Chem.;Eur. J. 2008, 14, 529–547. (b) D’Souza, D. M.; Rominger, F.;
Muller, T. J. J. Angew. Chem., Int. Ed. 2005, 44, 153–158.
6
7
8
9
1c
1a
1b
1c
2a
4
4
4
4
4
3
3
3
3
5f (45%)a
6a (48%)
6b (47%)
6c (53%)
6d (67%)
7a (73%)
7b (76%)
7c (69%)
7d (91%)
€
R2 = p-MeC6H4 (2b)
€
(10) (a) Braun, R. U.; Muller, T. J. J. Synthesis 2004, 2391–2406. (b)
R2 = Ph (2c)
€
€
Willy, B.; Dallos, T.; Rominger, F.; Schonhaber, J.; Muller, T. J. J. Eur.
€
2b
J. Org. Chem. 2008, 4796–4805. (c) Willy, B.; Muller, T. J. J. Eur. J. Org.
Chem. 2008, 4157–4168.
(11) D’Souza, D. M.; Muschelknautz, C.; Rominger, F.; Muller,
10 1c
11 1a
12 1a
13 1a
14 1a
R2 = p-MeOC6H4 (2d)
€
2c
T. J. J. Org. Lett. 2010, 3364–3367.
2b
€
(12) (a) Karpov, A. S.; Muller, T. J. J. Org. Lett. 2003, 5, 3451–3454.
R2 = p-ClC6H4 (2e)
R2 = p-MeO2CC6H4
(2f)
€
(b) Karpov, A. S.; Muller, T. J. J. Synthesis 2003, 2815–2826.
(13) For the addition of methylene active compounds to aryl ethynyl
ketones, see e.g.: Johnson, A. W. J. Chem. Soc. 1947, 1626–1631.
(14) Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary publication
nos. CCDC-816762 (6b) and -816763 (7b). Copies of the data can be
obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (Fax: þ 44-1223/336-033; E-mail: depos-
it@ccdc.cam.ac.uk).
15 1a
16 1c
R2 = p-O2NC6H4 (2g)
2b
3
3
7e (88%)
7f (87%)
a The substituent R2 of all merocyanines 5 (see Scheme 2) is H as a
consequence of concomitant desilylation in the addition step.
Org. Lett., Vol. 13, No. 10, 2011
2557

Table 2. Selected Optical Properties of Merocyanines 5, 6, and 7
absorption
emission
λ
max,abs [nm]
(ε [L mol/cm])
solution
λmax,abs
[nm]
λ
max,em[nm]
(Φf [%])a
solution
entry
merocyanine
(film)
1
5a
5b
5c
5d
5e
5f
450 (53 400)
473 (51 800)
464 (47 000)
486 (51 200)
474 (42 400)
496 (41 500)
492 (53 600)
504 (65 800)
513 (43 400)
516 (28 900)
402 (12 700)
400 (19 400)
401 (12 300)
400 (14 100)
395 (13 300)
417 (16 800)
452
482
ꢀ
512 (3)
2
523 (1)
3
528 (4)
4
ꢀ
544 (3)
5
ꢀ
564 (e1)
6
ꢀ
574 (e1)
7
6a
6b
6c
6d
7a
7b
7c
7d
7e
7f
509
522
535
540
401
402
403
400
393
421
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Figure 1. ORTEP plots of compounds 6b (top) and 7b (bottom).
8
9
10
11
12
13
14
15
16
or 1,2-dimethyl benzothiazolium iodide (4) as an enamine
precursor. Most remarkably, a slight electronic differ-
ence of the Fischer’s base over the 4 derived S,
N-ketene acetal accounts for the selective formation of
the unexpected addition products 7. The connectivity of 7
suggests that formally a (2 þ 2)-cycloaddition with sub-
sequent electrocyclic cyclobutene ring opening has occur-
red. The latter step is also supported by the occurrence of
the 1:1 isomeric mixture arising from a conrotatory ring
opening in either direction with almost equal preference.
Therefore, the consecutive three-component synthesis of
merocyanines 5, 6, and 7 can be mechanistically rationa-
lized as follows (Scheme 2). After formation of the ynone 8
by Sonogashira coupling from acid chloride 1 and alkyne 2,
Fischer’s base (3) or the in situ generated S,N-ketene acetal
9 (in situ generated from the benzothiazolium salt 4) most
likely attacks the ynone as an enamine nucleophile at the
β-position furnishing the zwitterions 10. The zwitterionic
intermediates 10 undergo either prototropy assisted by
ethanol to give the pushꢀpull dienes 5 and 6 or 1,4-dipolar
cyclization generating the cyclobutene 11, which upon
electrocyclic ring opening furnishes the unexpected mero-
cyanine 7.
a Recorded in dichloromethane (c = 10ꢀ6 M) upon irradiation at
λmax,abs ( 10 nm with coumarine 6 (Φf = 78% in ethanol) as a standard.
Scheme 2. Mechanistic Rationale for the Formation of the
Merocyanines 5, 6, and 7
Most interesting are the optical properties of title com-
pounds 5, 6, and 7 (Table 2). The absorption maxima of all
substances were recorded in dichloromethane at room
temperature. Furthermore, thin films of the merocyanines
5a and 5b, 6, and 7 were prepared by slow evaporation
from dichloromethane solutions to additionally determine
absorption in the amorphous solid state. Compounds 5cꢀf
showed a considerable tendency to crystallize instead of
forming films. The comparison of the longest absorption
wavelengths of substance class 5 clearly reveals the possi-
bility of fine-tuning the electronic structure by the choice of
the starting materials (Table 2, entries 1ꢀ6, 450ꢀ496 nm).
Electron-withdrawing substituents R1 enhance the di-
polar character of the π-system 5 causing a bathochromic
shift in the absorption spectra. In addition, on the donor
part the polarization and polarizability can be increased by
switching from a Fischer base to the more electron-rich
3-methyl-2-methylene-2,3-dihydro-benzo-[d]-thiazole, again
accompanied by a bathochromic shift. A similar trend can
be observed for the donorꢀacceptor dienes 6, where
stronger electron-withdrawing substituents R1 cause a
red shift of the longest absorption wavelength bands
(Table 2, entries 7ꢀ10, 492ꢀ516 nm). Apparently, the
substituent R2 does not affect the absorption maxima
(Table 2, entries 8ꢀ9). Expectedly, due to the shorter
conjugated π-chromophore the merocyanines 7 display
shorter absorption wavelengths (Table 2, entries 11ꢀ16,
2558
Org. Lett., Vol. 13, No. 10, 2011

395ꢀ417 nm). Again, the electron-withdrawing substituent
R1 shifts the absorption maximum bathochromically
(Table 2, entry 16), whereas R2 exerts only a minor
influence (Table 2, entries 11ꢀ15). With respect to the
oscillator strengths, as reflected by the extinction coeffi-
cients, the shorter chromophore 7 expectedly shows smal-
ler values than those for more extended dienes 5 and 6.
Solution luminescence could only be measured for the
merocyanines 5, whereas the emission of compounds 6 and
7 is essentially quenched in solution (Figure 2).
Figure 3. Normalized absorption (;) and emission spectra (---)
of compound 5e (recorded in CH2Cl2 at 298 K and at c(5e) =
10ꢀ3 M (absorption) and c(5e) = 10ꢀ6 M (emission)).
significantly higher dipole moment due to the enhanced
charge-transfer character. As a consequence the effect of
the solvent polarity on the emission encompasses a red shift
of 860 cmꢀ1 upon increasing the solvent polarity from
methyl cyclohexane to benzonitrile. As supported by the
X-ray structure analysis of compound 6b the anti-parallel
alignment and self-organization by πꢀπ-stacking of the
merocyanines 5 and 6 enhance the polar microenvironment.
In conclusion we have disclosed a novel consecutive
three-component couplingꢀenamine addition sequence
selectively giving rise to three types of diene merocyanines
witha flexible substitution pattern. The occurrenceof vinyl
substituted pushꢀpull alkenes 7 accounts for a dipolar
addition intermediate, which is sterically and electronically
sensitive either toward 1,4-dipolar cyclization with subse-
quent cyclobutene ring opening giving rise to the forma-
tion of the short pushꢀpull systems 7 or toward
prototropy furnishing the pushꢀpull dienes 5 and 6. Most
remarkably all merocyanines are red (5 and 6) or yellow (7)
emissive in the solid state. Further studies addressing the
three-component methodology and more detailed photo-
physical and computational investigations to elucidate the
electronic structure are currently underway.
Figure 2. Solid state (left) and solution (in dichloromethane)
luminescence of compounds 5e, 6d, and 7b (right) (irradiation at
λexc = 366 nm).
The emission quantum yields Φf of compounds 5aꢀf are
found in the low range between 1 and 4%. Nevertheless,
these dienes take in a unique position, not only in compar-
ison to all other pushꢀpull systems 6 and 7 but also with
respect to indolone terminated pushꢀpull dienes,11 which
are nonluminescent in solution as well. Interestingly, upon
transition from solution to the solid state all pushꢀpull
chromophores 5, 6, and 7 evidently display considerable
luminescence (for the naked eye). A closer inspection of the
solvent polarity effects on the optical properties reveals
positive absorption and emission solvatochromism of
merocyanine 5e (Figure 3). Linear correlations of the
longest wavelength absorption (r2 = 0.92) and emission
(r2 = 0.97) maxima can be obtained as well as Stokes shifts
(r2 = 0.99) with solvents of variable polarity by plotting
against Reichardt’s ET(30) values.15
The correlation of the Stokes shifts to the empirical
ET(30) values show an excellent goodness of fit (r2 =
0.99), whereas the correlation to LippertꢀMataga polarity
parameters16 of the solvents is relatively poor (r2 = 0.90),
caused by the poor fit of dioxane. The empirical LFER
indicates a low dipole moment of the electronic ground
state accompanied by a minor charge transfer in the
excited FranckꢀCondon state by an S0ꢀS1 transition.17
However, the vibrationally relaxed S1 state displays a
Acknowledgment. The work was supported by the
Fonds der Chemischen Industrie. The authors are grateful
€
to B. Sc. Marie Tschage (University of Dusseldorf) for
experimental assistance and to the BASF SE for generous
donations of chemicals.
Supporting Information Available. Experimental and
analytical details for the three-component synthesis of
merocyanines 5, 6, and 7; NMR, absorption and emission
spectra of compounds 5, 6, and 7; solvatochromicity data
of compound 5e; crystallographic data; figures; and crystal
packing of compounds6band 7b. This material is available
free of charge via the Internet at http://pubs.acs.org.
(15) Reichardt, C. Angew. Chem., Int. Ed. Engl. 1979, 18, 98–110.
(16) (a) Lippert, E. Z. Elektrochem. 1957, 61, 962–975. (b) Mataga,
N.; Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1956, 29, 465–470.
(17) Franco, L.; Pasimeni, L.; Ponterini, G.; Ruzzi, M.; Serge, U.
Phys. Chem. Chem. Phys. 2001, 3, 1736–1742.
Org. Lett., Vol. 13, No. 10, 2011
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