Synthesis, photophysical, electrochemical and electrochemiluminescence properties of A2B2 zinc porphyrins: The effect of π-extended conjugation

Article abstract of DOI:10.1039/c6cp01926a

The synthesis of two A₂B₂ porphyrins, {5,15-bis-[4-(octyloxy)phenyl]-porphyrinato}zinc(ii) (4) and {5,15-bis-(carbazol-3-yl-ethynyl)-10,20-bis-[4-(octyloxy)phenyl]-porphinato}-zinc(ii) (9), is reported. Their photophysical properties were studied by steady-state absorption and emission. Substituting the carbazolylethynyl moieties at two of the meso positions results in a large bathochromic shift of all the absorption bands, a notable increase in the absorption coefficient of the Q(0,0) band, and higher fluorescence quantum yield compared to porphyrin 4, with two unsubstituted meso positions. Cyclic voltammetry and digital simulation show that electrogenerated radical ions of 9 are more stable than those of 4. The lack of substituents at the meso positions of 4 leads to dimerization reactions of the radical cation. Despite this, the annihilation reaction of 4 and 9 produces very similar electrogenerated chemiluminescence (ECL) intensity. Spectroelectrochemical experiments demonstrate that the electroreduction of 9 leads to a strong absorption band that might quench the ECL.

Full text of DOI:10.1039/c6cp01926a

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Ruvalcaba, Phys. Chem. Chem. Phys., 2016, DOI: 10.1039/C6CP01926A.
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Page 1 of 15 Physical Chemistry Chemical Physics
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DOI: 10.1039/C6CP01926A
Journal Name
ARTICLE
Synthesis,
photophysical,
electrochemical
and
electrochemiluminescent properties of A2B2 zinc porphyrins:
Received 00th January 20xx,
Accepted 00th January 20xx
effect of π–extended conjugation.
Elizabeth K. Galván-Miranda,a Hiram M. Castro-Cruz,a J. Arturo Arias-Orea,a Matteo Iurlo,b
DOI: 10.1039/x0xx00000x
Giovanni Valenti,b Massimo Marcaccio,b and Norma A. Macías-Ruvalcaba†a
www.rsc.org/
Synthesis of two A2B2 porphyrins, {5,15-bis-[4-(octyloxy)phenyl]-porphyrinato}zinc (II) (4) and {5,15-bis-(carbazol-3-yl-
ethynyl)-10,20-bis-[4-(octyloxy)phenyl]-porphinato}-zinc (II) (9), is reported. The photophysical properties were studied by
steady-state absorption and emission. Substituting the carbazolylethynyl moieties at two of the meso positions results in a
large bathochromic shift of all the absorption bands, a notable increase on the absorption coefficient of the Q (0,0) band,
and higher fluorescence quantum yield compared to porphyrin 4, with two unsubstituted meso positions. Cyclic
voltammetry and digital simulation show that electrogenerated radical ions of 9 are more stable than those of 4. The lack
of substituents at the meso positions of 4 leads to dimerization reactions of the radical cation. Despite this, annihilation
reaction of 4 and 9 produces very similar electrogenerated chemiluminescence (ECL) intensity. Spectroelectrochemical
experiments demonstrate that the electroreduction of 9 leads to a strong absorption band that might quench the ECL.
properties, the incorporation of ethyne- or butadiyne- linkers
has proved very successful. This strategy has been used in the
synthesis of porphyrin-yne-porphyrin dimers, trimers or
Introduction
The potential applications of porphyrins have made them
attractive molecules in a wide range of research areas. They
have been used for the development of a broad variety of
sensors,1 as catalysts,2 in organic electronic devices,3 in dye
sensitized solar cells 4 and in bulk heterojunction cells,5 where
porphyrins are used for light harvesting as well as in energy
and electron transfer.
Most of these applications rely on their rich photophysical and
electrochemical properties, which can be fine-tuned by
different substitution patterns on the ring periphery,
incorporation of different metal ions in the core, or by axial
ligand coordination to the incorporated metal.6,7 It is known
that aromatic substituents directly appended on the meso-
positions of the porphyrin have little effect on the electronic
properties of the molecule because the steric interactions
oligomers,7,9,10
tetrakis(arylethynil)porphyrins,11
and
bis(arylethynyl)porphyrins (A2B2 and A2BC types).12–15
Bis(arylethynyl)porphyrins are particularly interesting for
organic photovoltaics, OLED’s development, and, in general,
the organic electronics field.14–17 As these applications usually
involve electron transfer reactions, the study of porphyrins
radical ion species is of major importance for this area.
Electrochemical techniques are powerful and versatile tools to
investigate the formation and stability of radical cations and
anions. The electrochemistry of porphyrins has been
extensively studied, they usually show two one-electron
oxidation and two one-electron reduction processes
corresponding to the formation of radical cation, dication,
radical anion, and dianion.18 These radical ion species are fairly
stable, especially in metallated porphyrins.9,10,19 Such stability,
along with their known emission properties, makes the
between the porphyrin
not allow for an efficient
achieve the extended conjugation of porphyrin molecules,
which translates into modification of their electronic
β
-hydrogens and the aromatic unit do
π
overlap between the two.8 To
porphyrins
ideal
candidates
for
electrogenerated
chemiluminescence (also called electrochemiluminescence,
ECL) studies.
Electrochemiluminescence arises from electron-transfer
reactions of radical ion or ionic species generated at electrode
surfaces; such reactions produce excited state molecules that
emit light.20 In the past few years, ECL applications and
processes, including the mechanisms through which it takes
place, have been reviewed;21–23 yet, the investigation of some
fundamental aspects, such as the correlation between
structure and ECL properties, are still an active issue.24
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ARTICLE
Journal Name
Even though the first report of ECL from porphyrins, by Bard,
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DOI: 10.1039/C6CP01926A
Results and discussion
Synthetic procedures
dates back to 1972,25 there is a relatively small number of
publications on this area. Most of the porphyrin-ECL
publications involve highly phosphorescent PtII or PdII
metalloporphyrins,26–29 as well as to porphyrin-ruthenium
complexes.30,31 There are works carried out in aqueous
media,32,33 and more recently a zinc-porphyrin was used as a
coreactant to induce the ECL emission of singlet oxygen
Two porphyrins, {5,15-bis-(carbazol-3-yl-ethynyl)-10,20-bis-[4-
(octyloxy)phenyl]-porphinato}-zinc (II) (9) and {5,15-bis-[4-
(octyloxy)phenyl]-porphyrinato}zinc (II) (4) were synthesized
following the synthetic route depicted in Scheme 1 through
minor modification of the procedures from the indicated
references (see Experimental Section). Briefly, 5,15-bis-[4-
(octyloxy)phenyl]-porphyrin (3) was obtained by reacting
dipyrromethane (1) and 4-octyloxybenzaldehyde (2) in
presence of TFA and using DDQ to oxidize the porphyrinogen.
Then, porphyrin 3 was treated with Zn(OAc)2 to afford the Zn-
metallated porphyrin 4, a change in color, from deep purple to
bright pink, as well as a decrease in the number of Q bands of
the absorption spectrum indicated that metal incorporation
was successful. Once metallated, the two free meso positions
were brominated by treatment with NBS yielding porphyrin
(5). On the other hand, 3-ethynylcarbazole (8) was obtained
from carbazole by a succession of steps involving the
iodination of carbazole followed by a Sonogashira coupling
with trimethylsylil acetylene and desilylation of the latter. The
final step involved a copper-free Sonogashira coupling of 5 and
8 to obtain 9.
instead of being the emitter.34 Only the work of Sooambar et
35
al
compares the effect of the meso- substituents on a
porphyrin ring on its ECL properties.
In this work, we report the synthesis of {5,15-bis-(carbazol-3-
yl-ethynyl)-10,20-bis-[4-(octyloxy)phenyl]-porphinato}-zinc (II)
(9, Scheme 1) and a reference porphyrin without any
arylethynyl
substituents,
{5,15-bis-[4-(octyloxy)phenyl]-
porphyrinato}zinc (II) (4, Scheme 1)
A comprehensive study involving the optical characterization,
cyclic voltammetry, UV-vis spectroelectrochemistry and ECL
properties for these porphyrins is described. Digital simulation
of the voltammograms allowed us to demonstrate that
blocking the meso positions with the ethynylcarbazole
substituents improves the stability of the radical ion species,
but this does not necessarily translate into a more efficient ECL
emission as there are other factors that need to be taken into
consideration, such as the red-shifted absorption of the
extended conjugation porphyrin and its corresponding radical
species.
O(CH2)7CH3
O(CH2)7CH3
H
N
i
NH
HN
HN
N
N
N
N
N
H
H
Zn
N
(1)
iv
iii
NH
OH
O(CH2)7CH3
+
CH3(CH2)7Br
ii
CHO
(2)
O(CH2)7CH3
(3)
O(CH2)7CH3
O
(4)
v
O(CH2)7CH3
O(CH2)7CH3
H
(8)
N
H
N
Zn
N
N
N
N
N
NH
Zn
Br
Br
HN
vi
N
N
O(CH2)7CH3
(5)
O(CH2)7CH3
(9)
Scheme 1 Synthesis of porphyrin 9. i) Paraformaldehyde, InCl3, 50
°
C, N2 atmosphere. ii) K2CO3, DMF, 100
°
C. iii) TFA, CH2Cl2, N2 atmosphere, dark, then DDQ and TEA.
iv) Zn(OAc)2, DMF, reflux. v) NBS, CHCl3, 0
°
C. vi) Pd(dba2)3, P(o-tol)3, TEA, THF, 30 C, N2 atmosphere.
°
2 | J. Name., 2012, 00, 1-3
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Voltammetric Analysis
ARTICLE
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DOI: 10.1039/C6CP01926A
The electrochemical behavior of 4 and 9 was studied by cyclic
voltammetry in Bu4NPF6 0.1 M /THF at –5 °C. The voltammetric
curves of the two porphyrins, recorded at 0.1 V/s, are shown
in Fig. 1 and 2. On the anodic scan, two oxidation processes
are observed for both 4 and 9 (Fig. 1). Voltammograms
encompassing only the first oxidation peak show that it is a
reversible or quasi-reversible process; this signal has been
attributed to the one-electron oxidation yielding the
corresponding radical cation. The potential of the first
oxidation peak of 9 is about 0.12 V less positive than that of 4,
and qualitatively the reversibility of the process suggests that
the radical cation of 9 is more stable than that of 4. These
observations can be explained by the extended π-conjugation
of 9. On the other hand, the second oxidation, which leads to
the formation of the dication, is an irreversible process for 4,
and slightly reversible for 9. On the negative potential sweep,
two reduction signals are observed for each porphyrin (Fig. 2),
at approximately –2.01 and –2.43 V for 4 and –1.69 and –2.19
V for 9. These peaks are attributed to consecutive one-
electron reductions, yielding the corresponding radical anion
and dianion.
Fig. 2 Experimental cathodic scan of: 1.21 mM 4 (top) and 1.19 mM 9 (bottom)
encompassing only the first (solid), and first and second (dashed) reduction
processes at 0.1 V/s in THF containing 0.1 M Bu4NPF6 at –5
working electrode.
°C at a glassy carbon
Experimental voltammograms encompassing only the first
oxidation peak were recorded for scan rates ranging from 0.1
to 1.0 V/s and concentrations between about 0.4 and 1.2 mM,
and the voltammograms were analyzed by digital simulation.
The analysis of this peak was based in terms of the reversible
oxidation of the porphyrin to its radical cation, reaction 1. In
order to account for the loss of reversibility, a bimolecular
dimerization of the radical cation was initially tested (reaction
2). This reaction scheme allowed quite acceptable agreement
at all scan rates and concentrations for 4, but not for 9. The
latter needed large increases in the value of kf,2 as the
concentration of porphyrin decreased (200 M-1s-1, 1.19 mM;
500 M-1s-1, 0.67mM; 700 M-1s-1, 0.4 mM). This nonconstant
rate constant means that the proposed reaction scheme is
inadequate for 9; therefore, reaction 2 was replaced by a first-
order decomposition of the radical cation (reaction 3). This
mechanism was found to fit well for all of the voltammetric
data of 9.
P → Pl+ + e−
2 Pl+ → PP2+
Pl+ → Prod
E°1, ks,1, α1
K2, kf,2, kb,2
K3, kf,3, kb,3
(1)
(2)
(3)
Fig. 1 Experimental anodic scan of: 1.21 mM 4 (top) and 1.19 mM 9 (bottom)
encompassing only the first (solid), and first and second (dashed) oxidation
processes at 0.1 V/s in THF containing 0.1 M Bu4NPF6 at –5
working electrode.
°C at a glassy carbon
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