Incorporation of zinc(ii) porphyrins in polyaniline in its perigraniline form leading to polymers with the lowest band gap

Article abstract of DOI:10.1039/c1cc14026d

Conjugated copolymers built upon quinone diimine-zinc(ii) porphyrin units exhibit a very low lying charge transfer band at 800 nm and are strongly emissive from the S₂ and T₂ states.

Full text of DOI:10.1039/c1cc14026d

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COMMUNICATION
Incorporation of zinc(II) porphyrins in polyaniline in its perigraniline
form leading to polymers with the lowest band gapw
Simon Lamare, Shawkat M. Aly, Daniel Fortin and Pierre D. Harvey*
Received 5th July 2011, Accepted 22nd August 2011
DOI: 10.1039/c1cc14026d
Conjugated copolymers built upon quinone diimine-zinc(II)
porphyrin units exhibit a very low lying charge transfer band
at 800 nm and are strongly emissive from the S2 and T2 states.
by us before.12 Both 2a and 5a are then coupled (Sonogashira
coupling) with 3,13 expecting to form the oxidized and reduced
polymers. Surprisingly, only the oxidized copolymer 4a was
isolated. For X = Me, 2b was reduced to 5b with NaBH4, and
the subsequent coupling with 3 gives again the oxidized polymer
4b. Evidences for the oxidized form are the absence of n(N–H) IR
band and of d(N–H) resonance (addressable with the exchange
with D2O), and the presence of the n(CQN) IR bands for
polymer 4a although a very weak n(N–H) (and also perhaps
d(N–H)) signals are detected for copolymer 4b, indicating the very
small presence of diaminobenzene residues. The 1H NMR spectra
exhibit broad signals, which is consistent with the polymer
formation.
Conjugated metalloporphyrin-containing polymers have been
the subject of intense interest as they offer the possibility of
low-band gap excited states and many studies on their solar
cells have been reported.1–5 Similarly, these materials are also
found to be electroluminescent and light emitting diodes were
studied.6–8 Moreover, a series of conjugated Pt-polyyne-
containing polymers and oligomers were recently prepared
(Chart 1; top),9 and the design of solution-processed bulk
heterojunction polymer solar cells containing porphyrin
exhibiting the best photocurrent efficiency (1.04%) were
reported (Chart 1; bottom).10
During the course of this study, the X-ray structures of 2a,
5a and 5b were obtained. For 2a, the average angles between
the quinone and iodobenzene planes are 161.6 and 168.21
(Fig. 1). This feature indicates that p-conjugation is possible
between the two groups hence rendering the copolymer conju-
gated. The GPC data indicate that 4a and 4b are oligomers
with degree of polymerization of 7.7 and 4.4, respectively
(Table 1).
The smallest band gap reported for zinc(II) porphyrin
polymers, so far is for the copolymer of Chart 2.1 Indeed,
the broad low energy absorption band peaks at 640 nm and
extends all the way to 800 nm. The two fragments are obviously
interacting via charge transfer (CT) as those reported for the
quinone diimine-containing organometallic polymers shown
in Chart 3.11 The latter’s show broad CT bands peaking
at B600 nm with spectral gaps of over 220 nm between the
Pt-unit pp* and CT bands.
We now wish to report two conjugated polymers that exhibit
the smallest band gap in the zinc(II) porphyrin family (Chart 4).
These new materials also exhibit intense luminescence’s with
good total quantum yields of B23% for both S2-S0 and T2-S0
emissions. These polymers are substituted polyanilines (PANI)
into which luminescent porphyrin groups were incorporated.
The synthesis (Scheme 1) proceeds from a condensation of
the p-iodoaniline with the corresponding tetrasubstituted
quinone in the presence of TiCl4 and a base to form 2a and 2b.
For X = F, 5a was also formed and isolated. The presence of this
reduced form of quinone in this type of reaction has been noted
The UV-vis spectra and data of 4a and 4b (Fig. 2; Table 2)
exhibit broad absorptions spreading from the 300 to 850 nm
with large absorptivities. A B780 nm shoulder is also noted
(i.e. band gap = 1.59 eV). The features in the 300–700 nm are
reminiscent of the absorption spectrum of the zinc(II) porphyrin
unit.9c The Soret bands show a shoulder indicating the presence
of exciton couplings of B1200 cm ,
À1 9a illustrating communica-
tion between the porphyrins and hence conjugation. The low
energy bands appear as a CT (zinc(II) porphyrin-quinone
diimine),11,13 but the substituent effect alone cannot confirm
´
Departement de Chimie, Universite de Sherbrooke, 2550 Boulevard de
l’Universite, Sherbrooke, Quebec, Canada J1K 2R1.
´
´
E-mail: Pierre.Harvey@USherbrooke.ca; Fax: +1-819-821-8017;
Tel: +1-819-821-7092
w Electronic supplementary information (ESI) available: Experimental
section and DFT and TDDFT of the oxidized and reduced form of the
model compound for polymer 4a, and selected X-ray data for 2a, 5a,
and 5b. CCDC 832795–832797. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1cc14026d
Chart 1
c
10942 Chem. Commun., 2011, 47, 10942–10944
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Chart 2
Chart 3
Fig. 2 Absorption (black), emission (red) and excitation (blue)
spectra of 4a (up) and 4b (down) in 2MeTHF (298 K).
Chart 4
a p-system mainly concentrated on the quinone diimine frag-
ment. Hence, a HOMO–LUMO transition leads a zinc(II)
porphyrin - quinone diimine CT excited state. This CT was
verified by TDDFT on this same model, and on a model
composed of a central diamino-tetrafluorobenzene flanked
by two zinc(II) porphyrins (ESIw). The calculated spectrum
(vide infra) exhibits a strong absorption at 850 nm (no vibronic
progression can be calculated with this method). TDDFT
computes exclusively a HOMO-LUMO transition, hence
corroborating the assignment. The difference between calcu-
lated (850) and observed shoulder values (at B780 nm; accept-
ing it is the 0–0 peak) of B70 nm is consistent with previous
reported differences on other porphyrin-containing materials
using this method.9c
As a verification, the reduced version of this model using its
optimized geometry of 1,4-diaminobenzene instead of quinone
diimine is also studied (ESIw). The computed spectrum
exhibits a lowest energy transition at 598 nm corresponding
to a Q-band, whereas a strong peak is calculated at 459 nm,
which is associated with the Soret band. In other words, there
is no band calculated above 600 nm. The conclusion is that in
its 1,4-diaminobenzene reduced form, the polymer behaves as
a series of independent chromophores. This conclusion is
perfectly in line with the X-ray structure of 5b where the
aromatic groups are placed far from planarity. To the best of
our knowledge, polymers 4 and 6 exhibit the lowest band gaps
ever reported so far in the porphyrin-containing materials.
The luminescence spectra of 4a and 4b exhibit features at
B680 (weak) and 820 nm that fall on top of the broad CT
absorption (Fig. 2). These cannot be assigned to this CT state
despite that the excitation spectra superpose well the absorp-
tion ones (ESIw). The time-resolved spectra exhibit two species
one of which decays under the ms time scale as illustrated by
the disappearance of the shoulder at 675 nm for 4b. Indeed, the
emission lifetimes for 4a and 4b at 675 nm are B1.2 Æ 0.1 ns
indicating fluorescence (Table 2). The long-lived species at
820 nm decay with 16 and 35 ms kinetics for 4a and 4b,
respectively, indicating phosphorescence. The B680 nm band
and fluorescence lifetimes, tF, of B1.2 ns compare favourably
Scheme 1 General reaction paths.
Fig. 1 ORTEP representation of 2a; only one of the two independent
molecules in the lattice is shown. The H-atoms are removed for clarity
and the thermal ellipsoids are shown at 50% probability.
Table 1 GPC data for 4a and 4b in THF
Polymer
Mn
Mw
PD
DP
4a
4b
18050
10280
24140
13780
1.34
1.34
7.7
4.4
the assignment as the CT band for X = F should be red shifted
with respect to X = Me). Here no shift is observed.
DFT computations (B3LYP) using a model for 4a contain-
ing a central tetrafluoroquinone diimine flanked by two zinc(II)
porphyrin residues are used to address the frontier MOs
(Fig. 3). The HOMO exhibits a p-system spreading all over
the model which confirms the conjugation deduced by the large
exciton couplings. The LUMO exhibits atomic contributions of
c
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Table 2 Spectrocopic and photophysical data of 4a and 4ba
lmax (nm) (e (MÀ1cmÀ1))
tF (ns)
b
Fe
Polymer
4a
4b
464 (13200), 493 (63100), 586 (7380), 680 (26700), 747 (29000)
463 (181400), 490 (101300), 583 (12300), 672 (30200), 745 (45800)
1.32 Æ 0.02
1.19 Æ 0.02
0.22
0.23
a
b
In 2MeTHF at 298 K. Reference used is rhodamine 6G (FF = 0.95; R.F. Kubin and A.N. Fletcher J. Lumin. 1982, 27, 455.
CT band in the red region of the spectrum lead to the lowest
band gap ever observed for zinc(II) porphyrin-containing
materials. The use of quinone diimine units in the polymer
backbone is prone to generate low-lying CT states, and in
some cases with large absorptivities. Therefore such type of
conjugated polymers represents good candidates for solar cell
design. Moreover, the intense phosphorescence makes them
also candidates for PLED applications in the near-IR region.
This research was supported by the Natural Sciences
and Engineering Research Council of Canada (NSERC), the
´
Centre d’Etudes sur les Mate
l’Universite de Sherbrooke (CEMOPUS), and the Fonds
Quebecois pour la Recherche en Sciences Naturelles et
´
riaux Optiques et Polymeres de
´
´
´
Technologie (FQRNT).
Notes and references
Fig. 3 HOMO and LUMO representations for 4a.
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with the literature data for similar zinc(II) porphyrin compounds.9c
The phosphorescence of the porphyrin unit, which was never
observed before for the bisethynyl derivatives, such as 3, falls at the
right range for zinc(II) porphyrins.14 Interestingly, the 680 nm
band in 4b decays with two exponentials (1.2 ns and 35 ms)
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transitions, hence violating Kasha’s rule. This appears unusual
at first glance but in fact it is not for conjugated polymers
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mers investigated so far (see those of Chart 3 and others)11,13
exhibit exactly S2 - S0 and T2 -S0 emissions with no
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detection limit. Interestingly, the total emission quantum
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Polymers 4a and 4b are built upon quinone diimines
exhibiting a structure resembling to PANI in its perigraniline
(fully oxidized) form in which a luminescent aromatic group,
here zinc(II) porphyrin is incorporated. They also exhibit a
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c
10944 Chem. Commun., 2011, 47, 10942–10944
This journal is The Royal Society of Chemistry 2011