Synthesis and binding properties of arylethyne-linked porphyrin-zinc complexes for organic electronics applications

Article abstract of DOI:10.1166/jnn.2013.7251

Conjugated meso-alkynyl 5,15-dimesitylporphyrin metal complexes have been synthesized by Sonogashira coupling reaction in good yields. Alkynyl groups were chosen as a link at the meso positions in order to extend the π-conjugated length of porphyrin rings. These synthesized porphyrin derivatives were characterized by 1H NMR spectroscopy and MALDI-TOF mass spectrometry. Moreover, UV-visible spectroscopy and fluorescence spectroscopy were also used to investigate their photophysical properties. It has been demonstrated that central metal ions as well as meso substituents on porphyrin rings affected the electronic absorption and emission spectra of the compounds. Spectroscopic results revealed that alkyne-linked porphyrin metal complexes showed higher π-conjugation compared with porphyrin building blocks resulting in red shifts in both absorption and emission spectra. Coordination properties of synthesized porphyrins were preliminarily investigated by UV-visible absorption and fluorescence emission spectroscopic titration with pyridine as axial ligand. The formation of porphyrin-pyridine complexes resulted in significant red shifts in absorption spectra and decrease of fluorescence intensity in emission spectra. Moreover, the ¹H NMR titration experiments suggested that central metal ions play an important role to coordinate with pyridine and the coordination of porphyrin zinc(II) complex with pyridine occur in a 1:1 ratio. From these spectroscopic results, alkyne-linked porphyrin metal complexes offer potential applications as materials for optical organic nanosensors. Copyright

Full text of DOI:10.1166/jnn.2013.7251

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Journal of
Nanoscience and Nanotechnology
Vol. 13, 3617–3621, 2013
Synthesis and Binding Properties of Arylethyne-Linked
Porphyrin-Zinc Complexes for Organic
Electronics Applications
W. Reainthippayasakul1, B. Paosawatyanyong2ꢀ3, and W. Bhanthumnavin1ꢀ∗
1Faculty of Science, Department of Chemistry, Organic Synthesis Research Unit, Chulalongkorn University,
Bangkok 10330, Thailand
2Faculty of Science, Department of Physics, Chulalongkorn University, Bangkok 10330, Thailand
3ThEP Center, Commission of Higher Education, Bangkok 10400, Thailand
Conjugated meso-alkynyl 5,15-dimesitylporphyrin metal complexes have been synthesized by Sono-
gashira coupling reaction in good yields. Alkynyl groups were chosen as a link at the meso posi-
tions in order to extend the ꢀ-conjugated length of porphyrin rings. These synthesized porphyrin
derivatives were characterized by 1H NMR spectroscopy and MALDI-TOF mass spectrometry.
Moreover, UV-visible spectroscopy and fluorescence spectroscopy were also used to investigate
their photophysical properties. It has been demonstrated that central metal ions as well as meso
substituents on porphyrin rings affected the electronic absorption and emission spectra of the
compounds. Spectroscopic results revealed that alkyne-linked porphyrin metal complexes showed
higher ꢀ-conjugation compared with porphyrin building blocks resulting in red shifts in both absorp-
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tion and emission spectra. Coordination properties of synthesized porphyrins were preliminarily
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investigated by UV-visible absorption and fluorescence emission spectroscopic titration with pyri-
Copyright: American Scientific Publishers
dine as axial ligand. The formation of porphyrin-pyridine complexes resulted in significant red shifts
in absorption spectra and decrease of fluorescence intensity in emission spectra. Moreover, the
1H NMR titration experiments suggested that central metal ions play an important role to coordinate
with pyridine and the coordination of porphyrin zinc(II) complex with pyridine occur in a 1:1 ratio.
From these spectroscopic results, alkyne-linked porphyrin metal complexes offer potential applica-
tions as materials for optical organic nanosensors.
Keywords: Alkyne-Linked Porphyrin, Organic Nanosensor.
1. INTRODUCTION
ring, changing the central metal atom, expanding the size
of the macrocycle, and binding of the axial ligands. One of
the most interesting way to tune their optical and electronic
properties is the extension of ꢀ-electronic conjugation sys-
tem of porphyrin rings by linking conjugated connectors
to porphyrin ꢀ-systems. However, the modulation of the
electronic conjugation of aryl-, alkene-, and imine-linked
porphyrins has been reportedly limited because of steric
interactions between substituents on the linker and the por-
phyrin peripheries. On the other hand, alkyne linkers are
cylindrically ꢀ-symmetrical and allow planar ꢀ-overlap
with porphyrin rings without steric interactions. It is, there-
fore, envisaged that alkyne-linked porphyrins may be able
to show strong electronic communication and may find
utility as optoelectronic.10–12
Porphyrins are tetrapyrrolic macrocyclic conjugated mole-
cules which have a smaller energy gap between the
highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) compared
to other molecules. Porphyrins exhibit an extensive sys-
tem of delocalized ꢀ-electrons and display extraordi-
nary photophysical, photochemical, and electrochemical
properties.1 Due to these properties, porphyrins and their
derivatives have been widely investigated hitherto in rela-
tion to photocatalysts,2 photodynamic therapeutic agents,3
optical sensors,4ꢁ5 organic semiconductors,6 and opto-
electronic devices such as photovoltaic cells,7 nonlinear
optical materials,8ꢁ9 and organic light-emitting diodes.10
These properties can be tailored by connecting different
peripheral substituents at the meso- and ꢂ-positions of the
In this paper, we report the synthesis and character-
ization of alkyne-linked porphyrin metal complexes as
well as investigation of their photophysical properties.
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 5
1533-4880/2013/13/3617/005
doi:10.1166/jnn.2013.7251
3617

Synthesis and Binding Properties of Arylethyne-Linked Porphyrin Complexes for Organic Electronics
Reainthippayasakul et al.
Furthermore, the coordination properties of alkyne-linked
porphyrin metal complexes have been examined by titra-
tion with pyridine which resulted in changes in UV-visible,
5,15-dimesitylporphyrin (porphyrin 3) as the porphyrin
building block. Firstly, dipyrromethane (2) was prepared
from the reaction of paraformaldehyde with excess pyrrole
1
ꢀ
fluorescent, and H NMR spectra.
(1) in the presence of MgBr2 without solvent at 55 C
for 3 hours. Porphyrin 3 was synthesized by the reaction
of dipyrromethane with mesitaldehyde in the presence of
BF3 ·OEt2 and CHCl3 for 1 hour and then DDQ was added.
The obtained porphyrin 3 was easily metallated by reflux-
ing with Zn(OAc)2 ·2H2O in methanol for 1 hour to give
porphyrin 4 in high yield. The two meso-positions were
brominated by treatment of 4 with 2 equivalents of NBS
2. EXPERIMENTAL DETAILS
2.1. Synthesis and Characterization of Alkyne-Linked
Porphyrin Metal Complexes
Alkyne-linked porphyrin metal complexes have been syn-
thesized in many steps starting from pyrrole as shown in
Figure 1. The first two steps concerned the formation of
ꢀ
in CH2Cl2 at 0 C for 10 minutes to achieve the dibromi-
nated porphyrin 5. The palladium catalyzed cross-coupling
CHO
N
N
NH
N
N
N
N
.
(CH2O)x
Zn(OAc)2 2H2O
Zn
.
NH
HN
N
Mg Br2
55 oC, 3 h
36%
BF3 OEt2,
CHCl3, reflux, 1 h
98%
HN
H
CHCl3, 1 h
then DDQ
40%
1
2
3
4
NBS, CH2Cl2
o
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0
C, 10 min
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96%
N
N
N
N
N
N
N
N
TMS
TMS
Zn
Zn
Br
TMS
Br
Pd(PPh3)2Cl2, CuI, Et3N, THF, 24 h
88%
6
5
Pd(PPh3)4, CuI,
(i-Pr)2NH, THF
60 ºC, 24 h
TBAF, CH2Cl2
30 min
62%
44%
N
N
N
N
N
N
N
N
Zn
Zn
7
8
Fig. 1. Synthesis of alkyne-linked porphyrin metal complexes.
3618
J. Nanosci. Nanotechnol. 13, 3617–3621, 2013

Reainthippayasakul et al.
Synthesis and Binding Properties of Arylethyne-Linked Porphyrin Complexes for Organic Electronics
reaction of dibrominated porphyrin zinc complex with
equivalents of trimethylsilylacetylene containing
BF3 · OEt2 as Lewis acid and then DDQ was added in
order to oxidize product to obtain porphyrin 3 in moderate
yield (40%). An advantage of porphyrin synthesis using
dipyrromethane is the absence of undesired porphyrin
byproduct mixtures. Moreover, mesityl groups were cho-
sen as side chain groups at the meso-positions in order to
improve the solubility of the chromophores. The complex-
ation of porphyrin 3 with Zn(OAc)2 ·2H2O was completed
in high yield (98%). Bromination of porphyrin 4 with
2 equivalents of NBS proceeded with high regioselectivity,
exclusively affording only meso-disubstituted porphyrin 5
in 96% yield without formation of monosubstituted and
ꢂ-substituted porphyrin. The alkyne-linked porphyrin 6
was synthesized through the palladium catalyzed cross-
coupling reaction of porphyrin 5 with trimethylsilylacety-
lene in 88% yield. After that, the two trimethylsilyl
groups of porphyrin 6 were completely deprotected with
TBAF to yield porphyrin 7 in 62% yield. In the case
of phenylethyne-linked porphyrin metal complex (por-
phyrin 8), the palladium catalyzed coupling reaction of
porphyrin 5 and phenylacetylene could also be used to give
porphyrin 8 in 44% yield as depicted in Figure 1.
4
Pd(PPh3)2Cl2, CuI, and Et3N in THF under a N2 atmo-
sphere for 24 hours provided porphyrin 6. The two
trimethylsilyl groups were easily removed when react-
ing with TBAF in CH2Cl2 for 30 minutes to afford
porphyrin 7. Similarly, phenylethyne-linked porphyrin
derivative (porphyrin 8) could be prepared through a pal-
ladium catalyzed cross-coupling reaction of dibrominated
porphyrin derivatives with phenylacetylene. All of these
synthesized porphyrin derivatives were characterized by
1H NMR spectroscopy and MALDI-TOF mass spectrom-
etry. All 1H NMR spectra were obtained in CDCl3 or
DMSO-d6 using a 400 MHz Varian Mercury plus 400.
MALDI-TOF mass spectroscopic data were performed
on a Bruker instrument and dithranol was utilized as the
matrix.
2.2. Investigation of Photophysical Properties
Photophysical properties of synthesized porphyrin deriva-
tives were investigated by using UV-visible spectroscopic
and fluorescence spectroscopic techniques. UV-visible
spectra were recorded on a Varian Cary 100 Bio UV-Visible
Spectrophotometer. Fluorescence spectra were acquired
using a Varian Cary Eclipse Fluorescence Spectrophotome-
ter. The UV-visible and fluorescence spectra of the por-
phyrin compounds were measured at 10 M in THF.
3.2. Investigation of Photophysical Properties
Photophysical properties of synthesized porphyrin deriva-
tives were investigated by using UV-visible spectroscopy
and fluorescence spectroscopy. These investigations
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demonstrated that metal ions such as Zn2+ ion as well as
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substituents at the meso-positions, especially alkyne sub-
stituents, affected the UV-visible absorption and fluores-
cence emission spectra as shown in Table I.
2.3. Investigation of Coordination Properties
Coordination properties were examined by titration of
porphyrin metal complexes with pyridine. In titration mea-
surements, pyridine was added to a solution of porphyrin
metal complexes and the resulting solutions were sub-
jected to UV-visible absorption, fluorescence emission, and
1H NMR spectroscopy.
UV-visible spectra of alkyne-linked porphyrin metal
complexes (porphyrins 6 and 7) and phenylethyne-linked
porphyrin metal complex (porphyrin 8) exhibited red-
shifted, broad and well split Soret bands and intense
red-shifted Q bands compared with porphyrin building
block 4, features of which were more pronounced as the
ꢀ-conjugated system was extended. In addition, porphyrins
6, 7, and 8 showed remarkably strong fluorescence intensity
and red shifted spectra compared with porphyrins 3 and 4.
The surprising splitting of Soret bands and the clearly red
shifts in both Soret bands and Q bands are strong evidences
3. RESULTS AND DISCUSSION
3.1. Synthesis and Characterization of Alkyne-Linked
Porphyrin Metal Complexes
The first step in the synthesis of alkyne-linked porphyrin
metal complexes is the preparation of dipyrromethane 2
from pyrrole 1. In this step, the starting equivalent ratio
of pyrrole 1 and paraformaldehyde was 25:1. MgBr2 was
chosen as Lewis acid catalyst because in its presence,
the reaction gave higher yield, fewer byproducts, and rel-
atively small amount of darkened products compared to
that when trifluoroacetic acid (TFA) was used. The prod-
uct mixtures including unreacted pyrrole, dipyrromethane,
tripyrrane, and other byproducts were separated by col-
umn chromatography to provide pure dipyrromethane (2)
in 36% yield. Dipyrromethane could be reacted with mesi-
taldehyde through an acid catalyzed condensation using
Table I. Absorption and emission wavelength of porphyrins at 10−5
in THF.
M
Absorption (ꢃmax, nm)
Porphyrin
Soret
Q
Emission (ꢃmax, nm)
Porphyrin 3
Porphyrin 4
Porphyrin 5
Porphyrin 6
Porphyrin 7
Porphyrin 8
404
410
427
438 446
432 441
447
500 531 575 630
544 578
633 700
584 636
—
642 702
630 687
660 720
a
566 606
582 634
573 624
597 651
Note: aNo fluorescence emission.
J. Nanosci. Nanotechnol. 13, 3617–3621, 2013
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Synthesis and Binding Properties of Arylethyne-Linked Porphyrin Complexes for Organic Electronics
Reainthippayasakul et al.
which confirm the extension of ꢀ-conjugated system.13ꢁ14
(a)
In fact, higher conjugation causes splitting in the ꢀ and
ꢀ∗ levels which results in a reduced HOMO–LUMO gap.
Hence, the simplest manifestations of this are red shifts and
broadening in the electronic spectra.15 Based on this fact,
it can be concluded that alkyne linkers indeed allow planar
ꢀ-overlap with porphyrin ring and may be able to exhibit
strong electronic communications between them. The flu-
orescence results have indeed provided useful insight into
the novel luminescent materials.
(b)
3.3. Investigation of Coordination Properties
Coordination properties of synthesized porphyrin zinc
complexes were investigated by UV-visible and fluores-
cence titration of porphyrins with pyridine. This examina-
tion has shed light on the manner by which axial ligands
induce changes in the optical absorption characteristics of
porphyrin metal complexes.16 Typical UV-visible absorp-
tion spectra of porphyrins 4 and 6 in THF solution on
addition of increasing amounts of pyridine are illustrated
in Figures 2(a) and (b), respectively. Moreover, fluores-
cence titration spectra of porphyrins 4 and 6 with pyridine
are depicted in Figures 3(a) and (b), respectively.
Fig. 3. Fluorescence titration spectra of porphyrin 4 (a) and porphyrin 6
(b) at 10−5 M in THF with pyridine.
Moreover, the titration experiments strongly suggested that
the absorption bands of alkyne-linked porphyrin (por-
phyrin 6) displayed remarkably greater changes, especially
at Q bands, compared to those of porphyrin 4. This exam-
As shown in Figure 2, both Soret band and Q bands
in the absorption spectra were induced to the red shifted
absorption bands upon the addition of pyridine. The new
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ination indicated that alkyne-linked porphyrin metal com-
red shifted bands are characteristics of porphyrin-pyridine
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plexes exhibit higher potential for binding materials in
optical sensors than their porphyrin building blocks.
In the case of fluorescence titration spectra, the intensity
of fluorescence emission continually decreased with a con-
comitant of slight red shifts upon the addition of pyridine
as shown in Figure 3. However, there are no significant dif-
ferences in the changes of titration spectra of porphyrins 4
and 6 with pyridine.
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complexes. The spectra presented clear isosbestic points
which are indicatives of sequential two-state equilibria.
(a)
Furthermore, coordination properties of synthesized por-
phyrins were also studied by 1H NMR titration with
pyridine. The most interesting result in the titrations of
porphyrin 4 and 6 with pyridine is the changes of proton
chemical shifts of pyridine. When the amounts of pyri-
dine increased, the chemical shifts of protons, especially
at ortho-positions, of pyridine significantly shifted down-
field. The signals of ortho-protons of pyridine exhibited
the largest changes because these protons are closest to
the core of porphyrin rings which can be affected violently
by the induced ring current. However, at this stage there
is an inadequate amount of data to predict the binding
stoichiometry of porphyrin-pyridine complexes. Therefore,
the continuous variation method (Job’s plots analysis) was
evaluated as an alternative to determine the binding stoi-
chiometry of the complexes. The Job’s plots of complexa-
tion of porphyrins 4 and 6 in Figure 4 exhibited relatively
symmetrical curves with maximum points at 0.5 mole frac-
tion. It concerned the formation of 1:1 porphyrin-pyridine
complexes.
(b)
Fig. 2. UV-visible titration spectra of (a) porphyrin 4 and (b) por-
phyrin 6 at 10−5 M in THF with pyridine.
3620
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Reainthippayasakul et al.
Synthesis and Binding Properties of Arylethyne-Linked Porphyrin Complexes for Organic Electronics
(a)
porphyrin metal complexes may find potential applications
as materials for optical organic nanosensors as well as
optoelectronic devices.
Acknowledgment: This research was partially sup-
ported by Chulalongkorn University Graduate Scholarship
to Commemorate the 72nd Anniversary of His Majesty
King Bhumibol Adulyadej.
References and Notes
(b)
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4. CONCLUSION
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Conjugated meso-alkynyl 5,15-dimesitylporphyrin metal
complexes have been synthesized by Sonogashira coupling
reactions in good yield (44–88%). It has been demon-
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Received: 4 November 2011. Accepted: 7 February 2012.
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