From π-expanded coumarins to π-expanded pentacenes

Article abstract of DOI:10.1039/c4cc03078h

The synthesis of two novel types of π-expanded coumarins has been developed. Modified Knoevenagel bis-condensation afforded 3,9-dioxa-perylene-2, 8-diones. Subsequent oxidative aromatic coupling or light driven electrocyclization reaction led to dibenzo-1

Full text of DOI:10.1039/c4cc03078h

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From p-expanded coumarins to p-expanded
pentacenes†
Cite this: Chem. Commun., 2014,
50, 9105
Marek K. We˛cławski,a Mariusz Tasior,a Tommy Hammann,b Piotr J. Cywinski*b
´
and Daniel T. Gryko*a
Received 25th April 2014,
Accepted 15th June 2014
DOI: 10.1039/c4cc03078h
www.rsc.org/chemcomm
The synthesis of two novel types of p-expanded coumarins has been
developed. Modified Knoevenagel bis-condensation afforded
3,9-dioxa-perylene-2,8-diones. Subsequent oxidative aromatic
coupling or light driven electrocyclization reaction led to dibenzo-
1,7-dioxacoronene-2,8-dione. Unparalleled synthetic simplicity,
straightforward purification and superb optical properties have
the potential to bring these perylene and coronene analogs towards
various applications.
p-Expanded coumarins have attracted considerable attention in recent
years. The growing number of their possible applications, including
two-photon fluorescence microscopy,1 OLEDs,2 dye-sensitized solar
cells,3 energy and electron transfer systems4 and fluorescent probes,5
significantly accelerates the progress in the synthesis of coumarins.
Noticeably, the extension of coumarin p-systems can be achieved
through multiple methods. Recent activity in this area proved that
fused rings introduced as structural elements constitute a powerful
approach to control photophysical properties of these chromophores,
therefore, a large number of simple benzocoumarins have been
Scheme 1
Klimenko and co-workers discovered that reaction of
studied over years,6 including heteroatom analogues.7 Among 1-acetoxyanthraquinone with phenylacetonitrile led to the
coumarins p-expanded at positions 4 and 5, many 4-oxa-5- mixture of a p-expanded furan derivative and a compound bearing
oxopyrene analogs have also been synthesized,8 and isolated from a coumarin unit.12 In our approach, we reasoned that starting
orchids.9 Recently the first total synthesis has been reported for from the corresponding 1,5-dibenzoyloxyanthraquinone (1) an
santiagonamine isolated from stems and branches of South American analogous reaction would lead to coumarin 7 (Scheme 1).
shrub Berberis darwinii.10 Moreover, many benzocoumarins exhibit Unfortunately, a complex mixture of highly coloured products
strong biological activity.11 Encouraged by great applicability of was formed under the original reaction conditions. Thus, we
these aromatic compounds, we decided to examine the synthesis decided to use phenylacetic acid methyl ester (2) instead of
and photophysical properties of a previously unknown family of nitrile, which led to the formation of a strongly fluorescent
p-expanded coumarins – namely new hybrids constructed from two yellow compound, which after simple work-up and NMR
coumarin units linked in a head-to-tail manner.
and MS analysis was confirmed to be desired bis-coumarin 7
(73% yield, Scheme 1). The same procedure could easily be applied
to phenylacetic acid esters bearing both electron-donating and
electron-accepting groups, as well as to the heterocyclic analogs
3–6, leading to bis-coumarins 8–11 in yields of 48–83% (Scheme 1).
Five different perylene analogues have been synthesized, which
were subsequently subjected to oxidative aromatic coupling.
It is important to emphasize that all attempts to perform this
condensation from esters of aliphatic acids failed. Reactions of
a Institute of Organic Chemistry PAS, Kasprzaka 44/52, 01-224 Warsaw, Poland.
E-mail: dtgryko@icho.edu.pl
b NanoPolyPhotonics, Fraunhofer Institute for Applied Polymer Research,
Geiselbergstr. 69, 14476 Potsdam-Golm, Germany.
E-mail: piotr.cywinski@iap.fraunhofer.de
† Electronic supplementary information (ESI) available: Experimental details of
compounds 7–11 and 14, 1H NMR, 13C NMR spectra, powder XRD and fluores-
cence decays. See DOI: 10.1039/c4cc03078h
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Scheme 2
compound 1 with substrates such as methyl propionate, ethyl
acetoacetate and diethyl malonate failed to lead to any identifiable
products. We believed that the fusion of additional two benzene
rings, resulting in further elongation of the p-system, would have
a substantial effect on the photophysical properties.
Interestingly, we found that compound 7 is unstable in solution
under sunlight or UV irradiation at 365 nm. Both natural (sunlight)
and artificial irradiation led to exactly the same product 12, but
with slightly lower yield in the case of UV irradiation (Scheme 2).
Dye 12 however proved to be very weakly soluble in organic
solvents. Such light-driven dehydrogenation is generally known
and has been previously used for the stilbene-phenanthrene
phototransformation,8f,13 however, the only example of cou-
marins resembling an oxacoronenone structure was described
by Zinke in 1953.14 The same conditions applied to oxidation of
compounds 8 and 9 led again to a dramatic decrease in
solubility of the product. The NMR spectra were difficult to
interpret unambiguously and the mass spectra were the only
evidence for the existence of new coronene analogues. Thus, we
decided to replace methoxy substituents in 9 with hexyloxy
groups, which should break p–p interaction due to higher steric
hindrance. After treatment of 9 with BBr3, 4-fold demethylation
took place and the resulting phenol was immediately transformed
into hexyl ether 13 under standard Williamson conditions in good
yield (Scheme 3). The obtained product was used for photocycliza-
tion. After irradiation in the presence of air (UV-lamp, 365 nm) we
obtained compound 14 in 54% yield. As expected, compound 14
has moderate solubility in selected solvents, which allowed for
Scheme 3
full characterization using 1H NMR, 13C NMR, COSY, HSQC, alkoxyl groups to planar dye 12, is due to the stronger influence
HMBC and MS. Subsequently, we performed the transformation of oxygen lone electron pairs with planar chromophores. The
of 13 into 14 under oxidative aromatic coupling (FeCl3, BF3ÁOEt2 electronic spectra of new heterocycles differ in shape. Due to a
and CH2Cl2),15 which resulted in higher yield (87%). Both more rigid structure, compounds 12 and 14 are characterized
reactions are regioselective and lead to the product with lower by a well resolved structure, while dyes 7–11 show unstructured
steric hindrance.
absorption bands. Fluorescence quantum yields determined for
The spectral characteristics were examined for compounds dyes 7–11 are moderate (Ffl = 10–30%) and the Stokes shifts are
7–11 and 14 and were subsequently compared to those of perylene, rather large (5000–7000 cmÀ1).
pentacene and 7-hydroxycoumarin (Table 1 and Fig. 1). The
It is reasonable therefore to assume that aryl substituents in
analysis carried out for the absorption spectra taken for com- dyes 7–11 do not overlap with the p-system of the main
pounds 7–11 showed that the introduction of electron-donating chromophore in the ground state (due to steric hindrance with
substituents into a new 3,9-dioxaperylene-2,8-dione system influences hydrogens at the bay position) but their geometry alters in the
both the absorption and emission properties of these heterocycles excited state. The drastic change in fluorescence quantum yield
(bathochromic shift B50 nm). A very strong red-shift of absorption between dyes 7–11 and fully fused compound 14 (Ffl = 90%)
(dye 14 – 142 nm) and emission, observed upon addition of four can be associated with the effect of a more flexible molecular
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Table 1 Photophysical properties (in CHCl3) determined for synthesized
Still, the advantages of compounds 12 and 14 are much more
straightforward synthesis and typically stronger emission.
In summary, we report the discovery of two unprecedented
p-expanded heterocyclic scaffolds bearing two coumarin cores.
The proposed synthetic method is operationally simple and
leads to analogs of perylene and pentacene in 2–3 steps. The
examination of spectroscopic properties of all new compounds,
which are formally p-expanded coumarins, showed that they
displayed intense fluorescence. The smooth photochemical
transformation of 3,9-dioxa-perylene-2,8-diones into dibenzo-
pentacenes under open-air-conditions opens new possibilities
towards the synthesis of large p-expanded heterocyclic analogs
of PAHs. The use of the presented approach can lead to a wide
range of previously unknown heterocycles, which can serve as
ideal platforms in diverse areas such as molecular electronics
and fluorescent imaging.
compounds and their analogsa
l
abs/nm
l
em/nm
tb
(ns) (108 sÀ1 d
kf
knr
Compd
(e  10À3
)
(Ffl)b,c
)
(108 sÀ1
)
7
8
9
10
385 (24.5) 527 (0.16)
433 (18.5) 613 (0.25)
450 (16.0) 565 (0.10)
1.1 1.5
2.4 1.1
1.6 0.6
7.6
3.1
5.6
8.6
3.2
—
0.2
—
—
445 (15.0) 535, 610 (0.31) 0.8 3.9
385 (30.0) 535 (0.15) 2.7 0.6
378 460, 472
11
12e
—
—
14
520 (60.0) 571, 615 (0.90)e 4.8 1.9
7-HC f
B[ f ]C f
B[ g]C f
Perylene f
325 (4.11)e 410 (0.07)e
275 (10.2) 425 (0.08)
321 (19.0) 429 (0.11)
439 (4.55) 446, 504 (0.94)
—
—
—
—
—
—
—
—
—
—
—
—
—
Pentacene f 580
595 (0.08)
a
b
Concentrations of compounds were in the range of 5–7 mM. lex
=
c
d
e
320 nm, Æ0.1. Measured using integrating spheres. Æ0.1. Due to
extremely low solubility of 12 only qualitative absorption and emission
f
The work was financially supported by the Polish Ministry of
Science and Higher Education from the funds for the studies in
the years 2012–2016 as a part of ‘‘Diamond Grant’’ programme,
statutory research project no. DI2012000742 and MISTRZ
programme (FNP).
spectra were recorded. Ref. 16.
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