Synthesis of meso-coumarin-conjugated porphyrins and investigation of their luminescence properties

Article abstract of DOI:10.1002/ejoc.200700475

A series of meso-coumarin-conjugated porphyrins 1a-e were designed and synthesized. Condensation of 4-chloroacetoacetate ethyl ester with m-cresol or resorcin afforded 4-chloromethylcoumarins, which were then hydrolyzed to give coumarin alcohols, followed by oxidation to provide coumarin aldehydes. The reaction of coumarin aldehydes with pyrrole under Adler or Lindsey conditions afforded the meso-coumarin-conjugated porphyrins 1a-e. Their UV/Vis absorption spectra and photoluminescent spectra were recorded both in dilute THF solution and as solid films. Analysis of the luminescence spectra indicate that the energy transfer from the coumarin substituents to the porphyrin core for 1a-e is more efficient in solid film than in solution, and the energy transfer from the coumarin substituent to the porphyrin core for 1d and 1e is more efficient than that of 1a, 1b and 1c in solid film. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejoc.200700475

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200700475
Synthesis of meso-Coumarin-Conjugated Porphyrins and Investigation of Their
Luminescence Properties
[a]
[a]
[a]
[a]
[a]
Weiying Lin,* Lingliang Long, Jianbo Feng, Bin Wang, and Cancheng Guo
Keywords: Porphyrinoids / Coumarin derivatives / Energy transfer / Luminescence
A series of meso-coumarin-conjugated porphyrins 1a–e were
both in dilute THF solution and as solid films. Analysis of the
designed and synthesized. Condensation of 4-chloroace- luminescence spectra indicate that the energy transfer from
toacetate ethyl ester with m-cresol or resorcin afforded 4-
the coumarin substituents to the porphyrin core for 1a–e is
chloromethylcoumarins, which were then hydrolyzed to give
more efficient in solid film than in solution, and the energy
coumarin alcohols, followed by oxidation to provide couma- transfer from the coumarin substituent to the porphyrin core
rin aldehydes. The reaction of coumarin aldehydes with pyr- for 1d and 1e is more efficient than that of 1a, 1b and 1c in
role under Adler or Lindsey conditions afforded the meso-
coumarin-conjugated porphyrins 1a–e. Their UV/Vis absorp-
tion spectra and photoluminescent spectra were recorded
solid film.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
Introduction
coumarin-conjugated porphyrins (Figure 1, 1a–e) as red
light-emitting materials. We envision that, upon excitation,
coumarin substituents (donor) may act as an antenna, ab-
sorbing the energy and then transferring it to the porphyrin
core (acceptor) to be reemitted by the porphyrin core as red
fluorescence. The electron-donating groups, such as methyl,
Light-emitting materials are the primary component
used in the preparation of organic light-emitting diodes
(OLEDs); these materials have attracted considerable scien-
tific and industrial attention because of their potential ap-
plications in large area flat-panel displays.[ Organic light-
emitting materials for three primary colors, such as blue,
green and red, are needed for full-color display. Among
them, green and blue light-emitting materials have been
1]
[
2]
fully developed, but the development of red light-emitting
materials is still at the early stage. Therefore, red light-emit-
ting materials are urgently sought in order to realize full-
color display of OLEDs.[ Porphyrins have a large conju-
gated skeleton and may emit fluorescence with wavelengths
around 650 nm; therefore, porphyrins are ideal candidates
for red light-emitting materials. The coumarins are an im-
portant class of fluorescent compounds with outstanding
optical properties, and they have been used as energy-trans-
fer donors in dendrimers, transferring energy to acceptor
3,4]
cores by long-range förster-type resonance energy trans-
fer.[
5–9]
In addition, it is known that electron-donating sub-
stituents in the 7-position of coumarins can make the cou-
marin fluorescent emission band redshift and enhance the
[
10]
emission intensity.
By taking advantage of the unique
photochemical properties of both porphyrins and couma-
rins, in this contribution, we provided an example of meso-
[
a] College of Chemistry and Chemical Engineering, Hunan Uni-
versity,
Changsha, Hunan 410082, P. R. China
Fax: +86-731-8821464
E-mail: weiyinglin@hnu.cn
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Figure 1. Molecular structure of meso-coumarin-conjugated por-
phyrins.
Eur. J. Org. Chem. 2007, 4301–4304
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4301

W. Lin, L. Long, J. Feng, B. Wang, C. Guo
SHORT COMMUNICATION
hydroxy, and diethylamino, in the 7-position of the couma- tions[11]). Coumarin 5a was also treated with pyrrole to give
rin moiety may promote the energy transfer from the cou- porphyrin 1b by using the same Lindsey conditions. Be-
marin substituent to the porphyrin core by electron-donat- cause coumarin aldehyde 5b is poorly soluble in chloro-
ing effects. In this communication, we presented the synthe- form, it was treated with benzaldehyde and pyrrole in re-
[
12]
sis and luminescence studies of these novel red light-emit- fluxing propionic acid (Alder conditions ) to prepare por-
ting materials.
phyrin 1c.
The synthesis of porphyrin 1d was initially attempted by
treating 5c with benzaldehyde and pyrrole under the stan-
dard Lindsey conditions by using BF ·OEt as the catalyst,
but no desired product porphyrin 1d was observed and
most of 5c remained intact. This could be due to the reac-
Results and Discussion
3
2
The synthetic route of meso-coumarin-conjugated por-
phyrins (1a–e) is outlined in Scheme 1. Condensation of 4-
chloroacetoacetate ethyl ester with m-cresol (2a) or resorcin
tion of the strong lewis acid BF ·OEt2 with the dieth-
3
ylamino group of 5c to form a quaternary ammonium salt
(2b) in concentrated sulfuric acid at –5 °C afforded 4-chlo-
[13]
instead; therefore the BF ·OEt catalyst became inactive.
3
2
romethyl-7-methylcoumarin (3a) or 4-chloromethyl-7-hy-
droxycoumarin (3b) in 72 or 85% yield, respectively. 4-
Chloromethyl-7-hydroxycoumarin (3b) was hydrolyzed in
boiling water overnight to give 7-hydroxy-4-(hy-
droxymethyl)coumarin (4b) in 92% yield. However, couma-
rin 3a has limited solubility in water, so it was hydrolyzed
in water/DMF (1:1) instead, and coumarin 4a was obtained
in 72% yield. Oxidation of 4a or 4b with manganese dioxide
in refluxing THF for 3 d afforded 4-formyl-7-methylcouma-
rin (5a) or 4-formyl-7-hydroxycoumarin (5b) in yields of 81
or 41%, respectively. By contrast, 7-diethylamino-4-formyl-
coumarin (5c) was prepared from commercially available 7-
diethylamino-4-methylcoumarin (4c), which was oxidized
by selenium dioxide in refluxing p-xylene.
Thus, the standard Adler method was then employed to
prepare porphyrin 1d by treating 5c with benzaldehyde and
pyrrole in refluxing propionic acid, and the desired product
1d was successfully obtained. The success of the synthesis
of porphyrin 1d by employing the Adler conditions
prompted us to attempt the preparation of porphyrin 1e by
using the same conditions; however, surprisingly, when 5c
was treated with pyrrole in refluxing propionic acid, a com-
plex mixture was obtained. Alternatively, we decided to em-
ploy the Lindsey conditions by using TFA as a catalyst to
prepare 1e. Fortunately, after treating 5c with pyrrole and
TFA at room temperature for 4 h in the dark, we were able
to isolate the desired product porphyrin 1e.
Because the porphyrins synthesized as the potential emit-
ting materials will ultimately be used in solid films in
OLEDs, the absorption and photoluminescent spectra of
1
(
a–e were recorded both in dilute THF solutions
5ϫ10–6 mol/L) and as solid films. The solid films of 1a–e
on quartz plates were spin-coated with 8 mg/mL dichloro-
methane solutions at 1000 rpm, and the thickness of these
solid films is in the range of 100–120 nm.
The absorption spectra of 1a–e in dilute THF solutions
and solid films are shown in Figure 2. The absorption spec-
tra of 1a–e in solid films were almost identical to those in
solutions except that they were redshifted by about 12–
20 nm. The absorption spectra of 1a–e both in solid film
and THF solution show the main features of free-base por-
phyrins including an intense soret band and four weak Q
bands. Interestingly, the Q bands of 1e are redshifted, but
the soret band of 1e is blueshifted relative to those of 1a,
Scheme 1. Synthetic route of meso-coumarin-conjugated porphy- 1b, 1c, and 1d. In addition, the soret band of 1e appears
rins. Reaction conditions: (1) Concentrated H SO , –5 °C; (2) for
a: DMF, H O, 100 °C; for 4b: H O, 100 °C; (3) for 5a: MnO
THF, reflux; for 5b: MnO , ethyl acetate, reflux; (4) SeO , p-xylene,
reflux; (5) for 1a: benzaldehyde, pyrrole, 5a, BF ·OEt , CHCl
room temp., 4 h, then p-chloranil, 61 °C, 2 h; for 1b: pyrrole, 5a, the 7-diethylaminocoumarin substituent.
2
4
significantly broader both in solution and film than that of
a, 1b, 1c, or 1d. This is probably due to the overlap of the
absorption soret band of the porphyrin core with that of
4
2
2
2
,
1
2
2
3
2
3
,
BF
for 1c: benzaldehyde, pyrrole, 5b, propionic acid, reflux, 45 min; for
d: benzaldehyde, pyrrole, 5c, propionic acid, reflux, 45 min; for 1e:
pyrrole, 5c, TFA, CHCl , room temp., 4 h, then p-chloranil, 61 °C,
h.
3
·OEt
2
, CHCl
3
, room temp., 4 h, then p-chloranil, 61 °C, 2 h;
The photoluminescent spectra of 1a–e in dilute THF
solutions were recorded by exciting, in the soret band, the
1
highest Q (0,1) transition or the corresponding coumarin-
y
3
substituent absorption maxima, and all photoluminescent
spectra exhibited characteristics of the porphyrin chromo-
2
For the synthesis of porphyrin 1a, coumarin 5a was phore with intense red fluorescence emission: a peak maxi-
treated with pyrrole and benzaldehyde in a proportion of mum at about 650 nm and a weak shoulder at about
:5:4 in dry chloroform by using BF ·OEt as the catalyst 710 nm. Furthermore, in the photoluminescent spectra
1
3
2
under a nitrogen atmosphere in the dark (Lindsey condi- (Supporting Information, Figure S7) of 1a–e obtained by
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Eur. J. Org. Chem. 2007, 4301–4304

meso-Coumarin-Conjugated Porphyrins
SHORT COMMUNICATION
characteristics when compared with those in THF solution
(Figure 3). Firstly, in the photoluminescent spectra of 1d
and 1e in solid film, besides the apparent porphyrin chro-
mophore emission at around 670 nm in the red range, there
is almost no residual emission of the coumarin substituents
from 400 to 600 nm. This is in sharp contrast with the pho-
toluminescent spectra of 1d and 1e in THF solution, which
still, to some extent, exhibit emission of the coumarin sub-
stituents from 400 to 600 nm. To exclude the factor of the
self-quenching effect of 7-diethylaminocoumarin substitu-
ent in the solid film, we further investigated the photolumi-
nescent spectra of 7-diethylamino-4-formylcoumarin 5c in
the solid film. As shown in Figure S9 (Supporting Infor-
mation), there is an apparently strong emission peak
around 600 nm in the photoluminescent spectra of 7-dieth-
ylamino-4-formylcoumarin 5c, this clearly indicates that 7-
diethylamino-4-formylcoumarin 5c does not self-quench in
the solid film. Therefore, the lack of emission of the couma-
rin substituents in the photoluminescent spectra of 1d and
1e in solid film demonstrates that there is a good energy
transfer between the 7-diethylaminocoumarin substituent
and the porphyrin core. The energy absorbed by the couma-
rin antenna was apparently efficiently transferred to the
porphyrin core and reemitted by the porphyrin chromo-
phore as intense red fluorescence light. Secondly, the por-
phyrin chromophore emission peaks at around 670 nm in
the photoluminescent spectra of 1d and 1e in solid film are
markedly redshifted and broader relative to those in THF
solution. This further supports that there is good energy
transfer from the coumarin moiety to the porphyrin core
Figure 2. The normalized UV/Vis spectra of meso-coumarin-conju- for 1d and 1e in solid film, and the energy transfer for 1d
gated porphyrins in THF solution (top) and solid film (bottom).
and 1e in solid film is much more efficient than in THF
solution. This is probably because, in the solid state, the
stacking porphyrins reduce the torsion angle between the
exciting in the corresponding coumarin-substituent absorp- porphyrin core and the coumarin substituent, forcing them
[
15]
tion maxima, besides the porphyrin chromophore emission to be coplanar.
Thus, it enhances the conjugated extent
at around 650 nm in the red range, there is still the residual of the porphyrin core and the coumarin substituent, facili-
emission of the coumarin substituents from 400 to 600 nm.
This indicates that there is some extent of energy transfer
from the coumarin substituent to the porphyrin core but
that it is inefficient. The fluorescence quantum yields (ΦF)
of porphyrins 1a–e in THF were measured by using tetra-
[14]
phenylporphyrin (Φ = 0.11 in benzene ) as a reference,
F
and their fluorescence quantum yields range from 0.15 to
0.22.
The photoluminescent spectra of 1a–e in solid film was
also excited in the soret band, in the highest Q (0,1) transi-
y
tion or in the coumarin-substituent absorption, and all the
photoluminescent spectra in solid film still exhibited the red
fluorescence emission, the characteristics of the porphyrin
chromophore. When compared with the emission bands of
the photoluminescent spectra of 1a–e in THF solution,
those of 1a–e in solid film exhibit a slight redshift. This is
consistent with the redshift of the Q (0,0) absorption band
X
in solid film.
Figure 3. The normalized photoluminescent spectra of 1d, 1e in
THF solution and solid film. The spectra were obtained upon exci-
tation at the absorption of 7-diethylamino-4-formylcoumarin
The photoluminescent spectra of 1d and 1e in solid film,
obtained by exciting in the corresponding coumarin-substit-
uent absorption maxima, exhibit two significantly different (380 nm).
Eur. J. Org. Chem. 2007, 4301–4304
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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W. Lin, L. Long, J. Feng, B. Wang, C. Guo
SHORT COMMUNICATION
tating the energy transfer from the coumarin substituent to data for all compounds synthesized and some photoluminescent
spectra of the porphyrin compounds.
the porphyrin core through bonds. Another line of evidence
of efficient energy transfer between the 7-diethylaminocou-
marin substituent and the porphyrin core of 1d and 1e in
the solid film comes from the comparison of photolumines-
Acknowledgments
cent excitation spectra of 1d and 1e in the solid film with
their absorption spectrum (Supporting Information, Fig-
ures S10–S13).
Funding was partially provided by the Scientific Research Founda-
tion for the Returned Overseas Chinese Scholars, State Education
Ministry (2007-24), Hunan University “985 project” funds, and re-
The photoluminescent spectra of porphyrins 1a–c in so-
lid film (Supporting Information, Figures S1–S3) still show
the emission peaks of the corresponding coumarin substitu-
ents, but with the intensity of the emission peaks of the
corresponding coumarin substituents decrease significantly
relative to those in solution. In addition, the porphyrin [2] H. Nakamura, C. Hosokawa, T. Kusumoto, Inorganic and Or-
chromophore emission peaks at around 670 nm of 1a–c in
solid film are broader and redshifted relative to those in
THF solution. These lines of evidence indicate that, for 1a–
c, the energy transfer efficiency is still higher in solid film
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Conclusions
A series of novel meso-coumarin-conjugated porphyrins,
a–e, were designed and synthesized as red fluorescent emit-
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(
Φ ) of porphyrins 1a–e in THF solution are higher than
F
[14]
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and 1e in organic light-emitting diodes (OLEDs) are in pro-
gress in our laboratory.
[
2
Supporting Information (see footnote on the first page of this arti-
Received: May 26, 2007
cle): Detailed experimental procedures and full characterization
Published Online: July 25, 2007
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2007, 4301–4304