Self-assembly of novel tris(p-carboxyphenyl) porphyrin monomer and its copolymers with acrylamide in aqueous media

Article abstract of DOI:10.1016/j.dyepig.2011.03.008

The novel porphyrin monomer 5-(4-acryloyloxylphenyl)-10,15,20-tris(4- carboxyphenyl)porphyrinate zinc(II) (ZnAOTCPP) and its corresponding sodium salt (ZnAOTCPP-Na) were synthesized. The latter compound exhibited a new band in excitation spectra due to fo

Full text of DOI:10.1016/j.dyepig.2011.03.008

Dyes and Pigments 91 (2011) 199e207
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Self-assembly of novel tris(p-carboxyphenyl) porphyrin monomer and its
copolymers with acrylamide in aqueous media
,
,
,
Fei Wang a b, Kewei Ding a b, Feipeng Wu a
*
a Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
b Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The novel porphyrin monomer 5-(4-acryloyloxylphenyl)-10,15,20-tris(4-carboxyphenyl)porphyrinate
zinc(II) (ZnAOTCPP) and its corresponding sodium salt (ZnAOTCPP-Na) were synthesized. The latter
compound exhibited a new band in excitation spectra due to formation of porphyrin aggregates in water,
which were derived from its surface-activity when the concentration was higher than its critical asso-
ciation concentration (CAC). The porphyrins were copolymerized with acrylamide (AM) to prepare
water-soluble copolymers with random and micro-blocky structures, which all displayed very new
absorption and fluorescence emission bands in the long wavelength region compared with the porphyrin
monomer. Furthermore, the micro-blocky copolymer exhibited an additional new absorption band at
even longer wavelength region compared with the random copolymer. The experimental results and
analysis showed that the porphyrin units in the random copolymer chains self-assembled to form
Received 12 November 2010
Received in revised form
3 March 2011
Accepted 4 March 2011
Available online 15 March 2011
Keywords:
Porphyrin
Polyacrylamide
Porphyrin aggregates
Water-soluble copolymer containing
porphyrins
Amphiphilic copolymer
Light-harvesting
porphyrin association complexes by hydrophobic association and
pep stacking interactions, and cova-
lent restrictions of polymer chains in the micro-blocky copolymer.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
porphyrin moieties should be a feasible route to artificial LH,
though there are many difficulties to overcome [13e20]. Among
other challenges is the difficulty of assembling porphyrin molecules
into an arrangement that ensures efficient light-harvesting [8e12].
Another problem is the poor solubility of porphyrins in water,
which means that assembling a porphyrin-based artificial LH in
aqueous media is quite difficult [13e18,21,22].
Manyinvestigationsofsynthesisofwater-solublecopolymers with
porphyrin units have been reported, and two approaches have proven
to be successful. One approach is to copolymerize porphyrin mono-
mer with water-soluble monomers such as 2-(acrylamido)-2-meth-
ylpropanesulfonic acid (AMPS) [13e16] and sodium styrenesulfonate
(SSS) [17,18]; the other is to bind porphyrin molecules onto water-
solublepolymers[19,20]. Thecorrespondingphotophysicalproperties
of water-soluble copolymers with porphyrin units have been reported
[13e20].
Polyacrylamide (PAM) is very soluble in water, and is therefore
a good candidate for the construction of artificial LH systems in
aqueous media. In the present study, porphyrin monomer with
carboxyl groups was synthesized and copolymerized with AM to
form water-soluble copolymers. The effects of self-assembly of the
copolymers on the photophysical properties of the porphyrin units
were investigated via UVevis absorption spectra and fluorescence
emission spectra. The approach presented here is a new route to
The photosynthetic system, which converts solar light energy to
chemical energy with high efficiency, is the most elaborate nano-
scale biological device in nature, and of course, is the most
important for the life on earth [1e3]. Light-harvesting (LH)
complexes, the most abundant elements of photosynthetic system,
achieve absorption of solar light and efficient transport of excita-
tion energy in aqueous media [1e7]. In LH complexes, proteins are
employed as rigid scaffolds to anchor chlorophylls in a sophisti-
cated assembly [4e7]. Moreover, effective exciton delocalization in
chlorophyll assemblies generates absorption at long wavelength
region to afford a large cross section for light absorption, resulting
in the efficient and fast energy transfer [5e7].
Porphyrin is an analogue of chlorophyll; both molecules have
rigid core structures, intense absorption in the visible region, and
similar photochemical properties. Thus, porphyrin assemblies with
red-shifts in absorption spectra have attracted considerable
attention during the past decades for exploration of the mechanism
of LH [8e12]. In practice, using polymers as scaffolds to anchor
* Corresponding author. Tel.: þ86 10 82543569; fax: þ86 10 82543491.
E-mail address: fpwu@mail.ipc.ac.cn (F. Wu).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.03.008

200
F. Wang et al. / Dyes and Pigments 91 (2011) 199e207
Fig. 1. Synthesis of the porphyrins.
assembly of the porphyrin molecules with the assistance of poly-
mer chains in aqueous media.
(ACVA) was bought from Alfa Aesar. 2,20-Azobis(isobutyronitrile)
(AIBN), trifluoroacetic anhydride (TFAA), trifluoroacetic acid
(TFA), tetrahydrofuran (THF), acetic acid and other A.R. grade
reagents were purchased from Beijing Beihua Co., Ltd. Buffer
solutions were prepared from hexamethylenetetramine and
hydrochloric acid (HCl) for pH 5.37, disodium hydrogen phos-
phate (Na2HPO4) and sodium dihydrogen phosphate (NaH2PO4)
for pH 6.86, and potassium chloride (KCl) and sodium hydroxide
(NaOH) for pH 13.86.
2. Experimental section
2.1. Materials
Acrylamide was purchased from Jiangxi Changjiu Biochemical
Engineering Corporation, and 4,40-azobis(4-cyanovaleric acid)

F. Wang et al. / Dyes and Pigments 91 (2011) 199e207
201
2.2. Synthesis of porphyrins
temperature. The solution was then evaporated under reduced
pressure, THF (0.100 dm3) was added to the residual solid, and the
mixture was ultrasonicated for 0.5 h to promote the dissolution of
zinc acetate in THF. The resulting insoluble ZnAOTCPP was sepa-
rated by filtration and washed three times with THF (0.100 dm3). A
red solid was obtained in 91% yield; the structure of product is
2.2.1. 5-(4-Hydroxyphenyl)-10,15,20-tris(4-
methoxycarboxyphenyl)porphyrin (MeHOTCPP) [23]
4-Hydroxylbenzaldehyde (0.020 mol) and 4-formyl benzoic acid
(0.060 mol) were dissolved in propionic acid (0.250 dm3). The
reactant mixture was brought to reflux and freshly distilled pyrrole
(0.080 mol) was added over a period of 0.4 h. The reactant mixture
was refluxed for further 2.5 h, and then cooled to room tempera-
ture and the solvent was removed by reduced pressure distillation.
A mixture of THF and methanol (3:2 in volume, 0.500 dm3) was
added into the residual solid, after which concentrated sulfuric acid
(0.015 dm3) was added dropwise in over a period of 0.4 h. This
reaction mixture was refluxed for 48 h, and then evaporated under
reduced pressure. The cold suspension was filtered to obtain the
purple crystals, which were washed with ethanol (0.250 dm3) three
times and dried under vacuum at 318 K. The dried crude product
was separated and purified by column chromatography using
chloroform/n-hexane mixture (90:10 in volume) as eluent. A purple
crystalline solid was obtained in 5% yield. 1H NMR (400 MHz,
shown in Fig. 1. 1H NMR (400 MHz, D2O-NaOH):
d
¼ 5.62e6.14
(m, 3H, CH2 ¼ CH-protons), 6.97 (d, J ¼ 8.0 Hz, 2H, CH2 ¼ CH-
phenyl-2,6-protons), 7.90 (d, J ¼ 8.0 Hz, 2H, CH2 ¼ CH-phenyl-3,5-
protons), 8.14 (d, J ¼ 8.0 Hz, 6H, COOH-phenyl-2,6-protons), 8.23 (d,
J ¼ 8.0 Hz, 6H, COOH-phenyl-3,5-protons), 8.75e9.06 (m, 8H,
b
-pyrrole). ESI-HRMS Found: m/z ¼ 879.14382 ([M þ H]þ), requires
m/z ¼ 879.14279. The content of Zn in ZnAOTCPP was 7.57 wt%
determined by inductively coupled plasma optical emission spec-
trometry (ICP-OES), which was close to the value calculated from its
chemical formula (7.40 wt%).
2.2.5. Sodium 5-(4-acryloyloxlphenyl)-10,15,20-tris(4-
carboxyphenyl)porphyrinate zinc(II) (ZnAOTCPP-Na)
ZnAOTCPP (0.002 mol) was dissolved in aqueous NaOH solution
(1 mol dmꢀ3, 0.010 dm3), and then the mixture was dropped
into acetone, immediately forming a grey-green precipitate. The
precipitate was filtered and vacuum-dried (318 K) overnight to
afford a brown solid powder, which was mixed with NaOH and
ZnAOTCPP-Na. The mixture was added in ethanol, and then
was ultrasonicated for 1 h to promote the dissolution of NaOH in
ethanol. The resulting insoluble ZnAOTCPP-Na was isolated by
filtration. These procedures were repeated several times until the
CDCl3):
d
¼ ꢀ2.78 (s, 2H, pyrrole NeH), 4.13 (s, 9H, methyl), 7.23
(d, J ¼ 8.0 Hz, 2H, OH-phenyl-2,6-protons), 8.08 (d, J ¼ 8.0 Hz, 2H,
OH-phenyl-3,5-protons), 8.30 (d, J ¼ 8.0 Hz, 6H, MeCOO-phenyl-
2,6-protons), 8.45 (d, J ¼ 8.0 Hz, 6H, MeCOO-phenyl-3,5-protons),
8.82-8.92 (m, 8H,
b-pyrrole). ESI-HRMS Found: m/z ¼ 805.26615
([M þ H]þ), requires m/z ¼ 805.26568.
2.2.2. 5-(4-Hydroxyphenyl)-10,15,20-tris (4-carboxyphenyl)
porphyrin (HOTCPP) [24]
pH of ZnAOTCPP-Na (with the concentration about 10ꢀ5 mol dmꢀ3
)
MeHOTCPP (0.002 mol) was dissolved in THF (0.060 dm3) and
a solution of potassium hydroxide (KOH, 10 wt%) in methanol
(0.090 dm3) was added. The reacting mixture was refluxed for 30 h,
and then the solvent was evaporated under reduced pressure.
The residue was redissolved in water (0.100 dm3). The pH of the
mixture was adjusted to about 7 with 1 mol dmꢀ3 aqueous HCl,
whereupon a green precipitate was formed. The precipitate was
filtered and washed three times with CHCl3 (0.150 dm3) and three
times with acetone (0.150 dm3), and then dried under vacuum
(318 K). A purple crystalline solid was obtained in 90% yield. 1H
in water decreased to about 7. A red solid powder was obtained in
70% yield, with the structure shown in Fig. 1. The 1H NMR and ESI-
HRMS spectra of ZnAOTCPP-Na in D2O were consistent with
ZnAOTCPP mixed with NaOH in D2O. The contents of Na and Zn in
ZnAOTCPP-Na determined by ICP-ES were 7.61 wt% and 6.96 wt%
respectively, which were close to the values in theory (7.29 wt% for
Zn and 6.86 wt%). Moreover, the Na/Zn ratio for ZnAOTCPP-Na
could be calculated and its value was about 3.0.
2.3. Preparation of copolymers
NMR(400 MHz, DMSO-d6):
d
¼ ꢀ2.92 (s, 2H, pyrrole NeH), 7.21
(d, J ¼ 8.0 Hz, 2H, OH-phenyl-2,6-protons), 8.01 (d, J ¼ 4.0 Hz, 2H,
Copolymer P-O was obtained by copolymerization of
ZnAOTCPP (5.26 ꢁ 10ꢀ3 mol dmꢀ3, 0.50 mol% in monomer) and
AM (1.05 mol dmꢀ3, 95.5 mol% in monomer) in a mixture of
dimethyl sulfoxide (DMSO) and acetic acid (3:2 in volume). AIBN
(1.05 ꢁ10ꢀ3 mol dmꢀ3) was used as initiator. Polymerization was
performed at 333 K for 12 h after purging with nitrogen for 0.4 h.
A green precipitate was formed during polymerization.
OH-phenyl-3,5-protons), 8.33e8.40 (m, 12H, COOH-phenyl-2,3,5,6-
protons), 8.84e8.89 (m, 8H,
b-pyrrole). ESI-HRMS Found:
m/z ¼ 763.21976 ([M þ H]þ), requires m/z ¼ 763.21873.
2.2.3. 5-(4-Acryloyloxyphenyl)-10,15,20-tris (4-carboxyphenyl)
porphyrin (AOTCPP)
HOTCPP (0.001 mol) and TFAA (0.001 dm3) were dissolved in TFA
(0.020 dm3), and the mixture was stirred under nitrogen atmosphere
for 5 h in an ice bath. The solution was evaporated under reduced
pressure, and the residual solid was filtered and washed three times
with water (0.100 dm3). A green solid was obtained in 92% yield. 1H
Copolymer P-W was prepared from ZnAOTCPP-Na (1.05 ꢁ 10ꢀ2
mol dmꢀ3
,
0.50 mol% in monomer) and AM (2.10 mol dmꢀ3
,
95.5 mol% in monomers) in water. ACVA (2.10 ꢁ 10ꢀ2 mol dmꢀ3
)
was used as initiator [25e27]. Polymerization was performed at
348 K for 4 h after purging with nitrogen for 0.4 h. A dark green
solution was obtained after polymerization.
NMR (400 MHz, DMSO-d6):
d
¼ ꢀ2.95 (s, 2H, pyrrole NeH),
6.26e6.73 (m, 3H, CH2 ¼ CH-protons), 7.63 (d, J ¼ 8.0 Hz, 2H,
CH2 ¼ CH-phenyl-2,6-protons), 8.25 (d, J ¼ 8.0 Hz, 2H, CH2 ¼ CH-
phenyl-3,5-protons), 8.25e8.38 (m, 12H, COOH-phenyl-2,3,5,6-
The procedure for the purification for P-O and P-W was the
same and the details are as follows. The reaction mixture after
polymerization was poured into excess of methanol (0.300 dm3)
and a green precipitate was formed. The precipitate was isolated by
filtration and washed three times with fresh methanol (0.150 dm3).
The solid product was dried under vacuum (318 K) overnight, and
then dissolved in the minimum volume of water about 0.010 dm3.
This purification procedure was repeated several times as described
above until the porphyrin content of the copolymer, which was
monitored by UVevis absorption spectrophotometer, became
constant. The yields of copolymers P-O and P-W were 85.7% and
75.2% respectively.
protons), 8.84 (d, J ¼ 4.0 Hz, 8H,
b-pyrrole). ESI-HRMS Found:
m/z ¼ 817.22929 ([M þ H]þ), requires m/z ¼ 817.23247.
2.2.4. 5-(4-Acryloyloxlphenyl)-10,15,20-tris(4-carboxyphenyl)
porphyrinate zinc(II) (ZnAOTCPP)
AOTCPP (0.001 mol) was dissolved in acetic acid (0.030 dm3),
followed by dropwise addition of a solution of zinc acetate (5 wt%)
in THF (0.025 dm3). The color of the reacting solution changed from
green to red. The reaction mixture was stirred for 4 h at room

202
F. Wang et al. / Dyes and Pigments 91 (2011) 199e207
3. Results and discussion
3.1. Self-assembly and photophysical properties of ZnAOTCPP-Na
monomer
Fig. 2 shows that the UVevis absorption spectra and fluores-
cence emission spectra of ZnAOTCPP-Na in water were very similar
to those of ZnAOTCPP, which means that the ionization of
ZnAOTCPP did not affect its photophysical properties. However, the
solubility of ZnAOTCPP-Na was more than 10ꢀ2 mol dmꢀ3 in water
at room temperature, whereas the saturation concentration of
ZnAOTCPP was only about 1.41 ꢁ10ꢀ5 mol dmꢀ3
.
When the concentration of ZnAOTCPP-Na was changed from
1.48 ꢁ 10ꢀ3 to 1.72 ꢁ 10ꢀ2 mol dmꢀ3 in water, the UVevis absorp-
tion spectra and fluorescence emission spectra were analogous to
the corresponding spectra in Fig. 2. However, when the concen-
tration reached 4.49 ꢁ 10ꢀ3 mol dmꢀ3 a new band (labeled Ex-A3)
appeared at 623 nm in the excitation spectrum (Fig. 3a). The
structure of ZnAOTCPP-Na comprises a hydrophobic tatrapyrrole
macrocycle and hydrophilic carboxylate anions. Consequently,
ZnAOTCPP-Na self-aggregates in water, resulting in the electronic
interactions of ZnAOTCPP-Na molecules leading to the formation
of the Ex-A3 band. CAC determined by this method was
4.49 ꢁ 10ꢀ3 mol dmꢀ3 at room temperature (about 298 K). This
value was close to the value, 5.65 ꢁ10ꢀ3 mol dmꢀ3, that was
obtained by analyzing the relationship between the surface tension
and the concentration of ZnAOTCPP-Na aqueous solution (Fig. 3b)
Fig. 2. (a) UVevis absorption spectra (inset: expanded spectra from 520 nm to
670 nm), and (b) fluorescence emission spectra (lex ¼ 422 nm); of ZnAOTCPP and
ZnAOTCPP-Na
in
water.
(cZnAOTCPP ¼ 5.24 ꢁ10ꢀ6 mol dmꢀ3
and
cZnAOTCPP-
¼ 6.02 ꢁ 10ꢀ6 mol dmꢀ3).
Na
UVevis absorption spectra of the copolymers in water dis-
played the characteristic absorption bands of the monomers at
422 nm. The content of porphyrin units in the copolymers was
calculated from the spectra [19,20]. The molecular weights of the
copolymers were measured by static light scattering (SLS). The
fluorescence quantum yields (4f) of porphyrin monomer and
porphyrin units in copolymers, in water, were determined by
using tetraphenyl porphyrin (TPP, 4f ¼ 0.11 in benzene) as a stan-
dard [28].
2.4. Methods
1H NMR spectra were obtained with a Bruker DPX400 spec-
trometer. Mass spectra were determined with a Buker APEX-IV
instructment. UVevis absorption spectra were recorded with
a
Hitachi UV-3900 UVevis spectrophotometer. Fluorescence
emission spectra and excitation spectra were performed on
a Hitachi F-4500 fluorescence spectrophotometer. Surface tensions
were measured at 298 K using a Drop Shape Analysis System KRüSS
DSA100. Static light scattering of copolymers was performed at
308 K with a DAWN EOS (Wyatt) instrument in 0.5 mol dmꢀ3
NaNO3 aqueous solution with excitation at 690 nm. Elemental
analysis of Na and Zn was performed on an inductively coupled
plasma optical emission spectrometry (ICP-OES) instrument (Var-
ian 710-ES).
Fig. 3. (a) Excitation spectra (lem ¼ 680 nm), and (b) the variation of surface tension at
298 K for ZnAOTCPP-Na in water.

F. Wang et al. / Dyes and Pigments 91 (2011) 199e207
203
Table 1
Photophysical comparison of porphyrin monomers and copolymers in their dilute
aqueous solutions.
Absorption bands (nm)a
Fluorescence
emission bands
(nm)a
3Q1/B
4fc
b
B
Q1
422 557 597 ed
ZnAOTCPP-Na 422 557 597 ed
P-O
P-W
Q2
A3
A4
ed
ed
E1
E2
E3
ZnAOTCPP
607 655 ed
0.039 0.078
0.038 0.079
607 655 ed
422 557 597 622 ed
605 654 624 0.040 0.054
422 566 606 622 752 605 654 624 0.077 0.068
a
The bands labeled as B, Q1, Q2, A3, A4, E1, E2 and E3 as shown in Figs. 2 and 5.
The ratio of molar absorption coefficient of Q1 band to B band.
The fluorescence quantum yield (4f) of porphyrins in water using TPP in
b
c
benzene as a standard (4f ¼ 0.11) [28].
d
No signal in this wavelength region of the spectrum.
addition of acetone, which is a poor solvent for ZnAOTCPP-Na. This
was done to enhance the aggregation of ZnAOTCPP-Na and hence
the associated spectral effects. Obvious changes were observed in
the UVevis absorption spectra (Fig. 4a) by adding acetone to
ZnAOTCPP-Na aqueous solution. As the volume of acetone in the
solution was increased, the absorbance of ZnAOTCPP-Na decreased.
However, when the ratio of acetone was increased to 50%, a new
red-shift absorption band at 625 nm (labeled A3 in Fig. 4a)
emerged, as a result of the formation of porphyrin association
complex [30]. Moreover, that band became stronger and more
Fig. 4. (a) UVevis absorption spectra, (b) fluorescence emission spectra (lex ¼ 422 nm,
inset: fluorescence emission spectrum of A-90), and (c) excitation spectra
(
lem ¼ 680 nm); of ZnAOTCPP-Na in the mixtures of acetone and water. (cZnAOTCPP-
¼ 5.56 ꢁ 10ꢀ4 mol dmꢀ3; A-N: Acetone/(Acetone þ Water) ¼ N vol%).
Na
[29]. Moreover, the excitation spectrum exhibited a clear new peak
at 625 nm when the concentration reached 1.72 ꢁ 10ꢀ2 mol dmꢀ3
,
which means that the aggregation of porphyrins was helpful in
decreasing their environmental polarity. However, the interactions
between ZnAOTCPP-Na were very weak, because the related signal
could only be observed in excitation spectra.
To confirm the observation of the electronic interactions
between porphyrins shown by the spectra in Fig. 3a, an experiment
to reduce the solubility of ZnAOTCPP-Na in water was performed by
Fig. 5. (a) UVevis absorption spectra (inset: expanded spectra from 520 nm to
670 nm), and (b) fluorescence emission spectra (lex ¼ 422 nm); of copolymers in water.
(cPor ¼ 5.78 ꢁ 10ꢀ6 mol dmꢀ3 for P-O and cPor ¼ 5.87 ꢁ 10ꢀ6 mol dmꢀ3 for P-W).

204
F. Wang et al. / Dyes and Pigments 91 (2011) 199e207
prominent with the increase in the proportion of acetone. Inter-
estingly, there was a new band at 627 nm labeled E3 in the fluo-
rescence emission spectrum of ZnAOTCPP-Na solution with 90%
acetone (Fig. 4b), which was also caused by the association complex
[31e34]. The excitation spectra of ZnAOTCPP-Na with a low
concentration (5.56 ꢁ 10ꢀ4 mol dmꢀ3) in mixtures of acetone and
water are shown in Fig. 4c. The spectra for solution with more than
30% acetone were similar to the spectrum of 1.72 ꢁ 10ꢀ2 mol dmꢀ3
ZnAOTCPP-Na aqueous solution in Fig. 3a, in which the Ex-A3 band
was also at 625 nm. The more acetone added in ZnAOTCPP-Na
aqueous solution, the greater should be the tendency for porphyrin
association complexes to form. Thus, the Ex-A3 band displayed in
Fig. 4c increased in intensity with increasing proportion of acetone.
All of these results indicated that weak electronic interactions
existed between the porphyrin molecules in an aqueous solution of
ZnAOTCPP-Na, and they could be tuned by addition of a poor
solvent.
3.2. Self-assembly and photophysical properties of the copolymers
In the P-O polymerization system, the monomers AM and
ZnAOTCPP distributed uniformly in a mixture of DMSO and acetic
acid. Consequently, during the polymerization the initiator AIBN
initiated any kind of monomers encountered in solution randomly,
i.e. either AM or ZnAOTCPP, to form propagating radicals of poly-
merized AM and porphyrin units until they were terminated. Under
these polymerization conditions nonionic porphyrin units became
distributed randomly in the P-O copolymer chains.
In the P-W polymerization system, ZnAOTCPP-Na aggregates
were formed in water due to its initial concentration
(1.05 ꢁ10ꢀ2 mol dmꢀ3) being higher than CAC. In the polymerizing
process of P-W, the water-soluble initiator ACVA initiated AM
monomer to polymerize in aqueous media and form propagating
radicals of PAM at first. When PAM radicals encountered ZnAOTCPP-
Na aggregates, they copolymerized with several ZnAOTCPP-Na
molecules, which resulted in micro-blocks of anionic porphyrin
units being incorporated in the propagating radicals [25e27].
The propagating radicals polymerized with AM monomer again after
they had departed from the aggregates of ZnAOTCPP-Na. These steps
were repeated many times until the propagating radicals were
terminated. Therefore, P-W had micro-blocky sequences of anionic
porphyrin units in the copolymer chains. Moreover, there were
electrostatic repulsions between the P-W propagating radicals with
incorporated anionic porphyrin units and ZnAOTCPP-Na monomer,
which resulted in copolymer P-W having a smaller content of
porphyrin units (0.34 mol%) than P-O (0.67 mol%), for the same
porphyrin monomer/AM feed ratio. Furthermore, PAM has a larger
solubility in water than in a mixture of DMSO and acetic acid, so
P-W had a higher molecular weight (3.0 ꢁ 106 g molꢀ1) than P-O
(5.4 ꢁ105 g molꢀ1).
When copolymer P-O was dissolved in water, the nonionic
porphyrin units anchored in polymer scaffolds would try to
aggregate due to their hydrophobicity. Therefore, hydrophobic
microdomains were formed by the amphiphilic copolymer chains,
where the most of the porphyrin units should be located to escape
from water [15e17]. When some of them were located close enough
and in favorable orientations, they could interact with each other
by
pep stacking interactions to form porphyrin association
complexes [30e34]. Consequently, compared with monomer at
Fig. 6. UVevis absorption spectra for (a) P-O (inset: expanded spectra from 520 nm to
760 nm), (b) for P-W (inset: expanded spectra from 520 nm to 900 nm); and fluorescence
emission spectra (lex ¼ 422 nm) for (c) P-O, and (d) for P-W in various buffer solutions.
(cPor ¼ 5.03 ꢁ 10ꢀ6 mol dmꢀ3 for P-O and cPor ¼ 5.12 ꢁ 10ꢀ6 mol dmꢀ3 for P-W).

F. Wang et al. / Dyes and Pigments 91 (2011) 199e207
205
approximately equal concentrations of porphyrin molecules
(Table 1), copolymer P-O exhibited a red-shift absorption band at
622 nm (labeled A3 in Fig. 5a). This was similar to the band of the
solution of ZnAOTCPP-Na with up to 50% acetone (Fig. 4a). In a like
manner, there was a new emission band at 624 nm (labeled E3 in
Fig. 5b) that was similar to that formed in the solution of
ZnAOTCPP-Na with 90% acetone (Fig. 4b). Thus, copolymer P-O
enlarged the range of spectral absorption, which was meaningful in
the effort to set up an artificial LH system in aqueous media. As
observed in absorption and emission spectra (Fig. 5), some of the
porphyrin units in P-O did not form association complexes, and
displayed the photophysical characteristics of monomers, such as
the location of Q1 and Q2 as well as the value of 3Q1/3B, as listed in
Table 1.
The copolymer P-W was an anionic copolymer and readily
soluble in water, and the porphyrin units were arranged in micro-
blocky sequences. In the UVevis absorption spectra of P-W shown
in Fig. 5a, there is a 9 nm red-shift in Q bands (Table 1), which
means that the energy level of S0eS1 of the porphyrin units in the
copolymer was lower than that of monomer. Hence there was
a larger transition probability between ground state and excited
state of porphyrin units in P-W compared to monomer, because the
value of 3Q1/3B in P-W was 0.076 while the value for monomer was
0.039 [35,36]. Moreover, there was a shoulder at 622 nm (labeled
A3 in Fig. 5a) and a new emission band at 624 nm (labeled E3 in
Fig. 5b), which arose from the porphyrin association complexes
similar to those of P-O in water, and those of ZnAOTPP-Na in
mixtures of acetone and water. Interestingly, another new red-shift
absorption band appeared at 752 nm (labeled A4 in Fig. 5a) for P-W
but not for P-O, which indicated that there was another kind of
porphyrin association complex formed in the system. It is note-
worthy that this kind of long wavelength absorption by a water-
soluble polymer containing porphyrins has not been reported to
the best of our knowledge. However, the origin of A4 could not be
ascertained in details in this study. In addition, when the concen-
trations of porphyrin units (cPor) of P-O and P-W were approxi-
mately equal, P-W had larger cross section of absorption
(400e800 nm) and higher molar absorption coefficient 3Q1/3B
(0.076) than P-O (400e650 nm and 0.041 respectively). These
results indicated that the copolymer P-W can more efficiently in
absorbing light, compared to P-O, to accomplish light-harvesting in
water.
When copolymer P-O was dissolved in buffer solution at pH 5.37,
ionization of the carboxylic acid groups of the porphyrin units was
reduced, hence the hydrophobicity of porphyrin units increased,
and electrostatic interactions between them decreased. These
changes favored the formation of porphyrin association complex.
However, the carboxylic groups of the porphyrin units were ionized
in pH 13.86 buffer, so the solubility and electrostatic interactions of
porphyrin units of P-O were increased. As a result, the lower the pH
of the buffer solution in which P-O is dissolved, the stronger the
absorption and emission of porphyrin association complexes
becomes (Fig. 6a and c). In case of copolymer P-W, the micro-blocky
sequences of porphyrin units in the copolymer chains, the covalent
restrictions of polymer chains, played a more important role in the
formation of porphyrin association complexes. Therefore, the
spectral changes with pH were less noticeable for P-W (Fig. 6b and
d) than for P-O, although the effects of pH on porphyrin units were
similar for these two copolymers. The different effects of pH of the
Fig. 7. The UVevis absorption spectra ((a) P-O, (b) P-W; insets: the variation of the
ratio of absorption bands (Rabs) with cPor), and fluorescence emission spectra ((c) P-O,
(d) P-W; insets: the variation of the ratio of fluorescence emission bands (Rfl) with
cPor), for the copolymers in water.

206
F. Wang et al. / Dyes and Pigments 91 (2011) 199e207
aqueous solution on the photophysical properties of P-O and P-W
indicated that the formation of porphyrin association complex was
caused by weak interactions in P-O while strong interactions in P-W.
As shown in Fig. 7, the solubility of porphyrin units in copoly-
mers in water was improved to 10ꢀ3 mol dmꢀ3 with the aid of the
water-soluble polymer chain, thus these copolymers overcome one
of the main problems in setting up an artificial LH system in
aqueous media. Spectroscopic methods can give good insight into
the self-assembly behavior of molecules, especially for molecules
that have characteristic photophysical properties. These polymers
have porphyrin units that are either aggregated or non-aggregated,
with corresponding differences in absorption and fluorescence
emission spectra. The non-aggregated porphyrin units in copol-
ymer P-O shows Q1 and Q2 bands in the UVevis absorption spectra,
and E1 and E2 bands in fluorescence emission spectra, while a new
band A3 in UVevis absorption spectra and band E3 in the fluores-
cence emission spectra was for one kind of porphyrin association
complex. In copolymer P-W, there was another kind of porphyrin
association complex with absorption band A4. The intensity ratios
for absorption or fluorescence emission between the aggregated
porphyrin units and non-aggregated porphyrin units in the copol-
ymers can be used to monitor the aggregation of copolymer in the
aqueous media [37e40]. In the spectra of P-O (Figs. 7a and c), Rabs of
A3/Q1 and Rfl of E3/E1 increased rapidly with cPor for
In copolymers, the solubility of porphyrin was greatly improved by
water-soluble AM units in the polymer chains. Moreover, porphyrin
association complexes with absorption and emission at long wave-
length region were anchored in the polymer scaffolds, especially for
the micro-blocky copolymer. To the best of our knowledge, this is the
first observation of such absorption or emission of porphyrin asso-
ciation complexes in water-soluble polymers. These results suggest
that the water-soluble copolymers containing porphyrin units
should be promising materials for the construction of artificial LH
systems in aqueous media.
References
[1] McDermott G, Prince SM, Freer AA, Hawthornthwaite-Lawless AM, Paplz MZ,
Cogdell RJ, et al. Crystal structure of an intergral membrane light-harvesting
complex from photosynthetic bacteria. Nature 1995;374:517e21.
[2] Kühlbrandt W, Wang DN, Fujiyoshi Y. Atomic model of plant light-harvesting
complex by electron crystallography. Nature 1994;367:614e21.
[3] Roszak AW, Howard TD, Southall J, Gardiner AT, Law CJ, Isaacs NW, et al.
Crystal structure of the RC-LH1 core complex from rhodopseudomonas pal-
ustris. Science 2003;302:1969e72.
[4] Liu ZF, Yan HC, Wang KB, Kuang TY, Zhang JP, Gui LL, et al. Crystal structure of
spinach major light-harvesting complex at 2.72
Å resolution. Nature
2004;428:287e92.
[5] Grondelle RV, Novoderezhkin V. Dynamics of excitation energy transfer in the
LH1 and LH2 light-harvesting complexes of photosynthetic bacteria.
Biochemistry 2001;40(50):15057e68.
[6] Pullerits T, Sundström V. Photosynthetic light-harvesting pigment-protein
complexes: toward understanding how and why. Accounts of Chemical
Research 1996;29:381e9.
[7] Brixner T, Stenger J, Vaswani HM, Cho M, Blankenship RE, Fleming GR. Two-
dimensional spectroscopy of electronic couplings in photosynthesis. Nature
2005;434:625e8.
[8] Okada S, Segawa H. Substituent-control exciton in J-aggregates of protonated
water-insoluble porphyrins. Journal of the American Chemical Society
2003;125:2792e6.
[9] Andrade SM, Costa SMB. Spectroscopic studies of water-soluble porphyrins
with protein encapsulated in bis(2-ethylhexyl)sulfosuccinate (AOT) reverse
micelles: aggregation versus complexation. Chemistry e A European Journal
2006;12:1046e57.
[10] Gonçalves PJ, Boni LD, Barbosa Neto NM, Rodrigues Jr JJ, Zílio SC,
Borissevitch IE. Effect of protonation on the photophysical properties of
meso-tetra(sulfonatophenyl) porphyrin. Chemical Physics Letters 2005;
407:236e41.
[11] Agostiano A, Catucci L, Colafemmina G, Scheer H. Role of functional groups
and surfactant charge in regulating chlorophyll aggregation in micellar solu-
tions. The Journal of Physical Chemistry B 2002;106:1446e54.
[12] George S, Goldberg I. Self-assembly of supramolecular porphyrin arrays by
hydrogen bonding: new structures and reflections. Crystal Growth & Design
2006;6(3):755e62.
[13] Konishi T, Ikeda A, Asai M, Hatano T, Shinkai S, Fujitsuka M, et al. Improve-
ment of quantum yields for photoinduced energy/electron transfer by isola-
tion of self-aggregative zinc tetraphenyl porphyrin-pendant polymer using
cyclodextrin inclusion in aqueous solution. The Journal of Physical Chemistry
B 2003;107:11261e6.
[14] Morishima Y, Aota H, Saegusa K, Kamachi M. Long-lived porphyrin cation
radicals protected in unimer micelles of hydrophobically-modified poly-
electrolytes. Macromolecules 1996;29:6505e9.
[15] Morishima Y, Saegusa K, Kamachi M. Anomalously blue-shifted fluorescence
and phosphorescence of zinc (II) tetraphenylporphyrin in highly constraining
microenvironments in hydrophobically modified polysulfonates. Macromol-
ecules 1995;28:1203e7.
[16] Morishima Y, Saegusa K, Kamachi M. Photophysical behavior of zinc(II) tet-
raphenylporphyrin in highly constraining microenvironments. Anomalously
long-lived excited-triplet in the hydrophobic clusters of amphiphilic poly-
sulfonates. The Journal of Physical Chemistry 1995;99:4512e7.
[17] Nowakowska M, Karewicz A, Klos M, Zapotocny S. Synthesis and properties of
water-soluble poly(sodium styrenesulfonate-block-5-(4-acryloyloxyphenyl)-
10,15,20-tritolylporphyrin) by nitroxide-mediated free radical polymeriza-
tion. Macromolecules 2003;36:4134e9.
c
Por < 8.72 ꢁ 10ꢀ4 mol dmꢀ3. However, those ratios changed slowly
for cPor > 8.72 ꢁ 10ꢀ4 mol dmꢀ3. These results indicated that there
were two stages of self-assembly of P-O in water, and
8.72 ꢁ 10ꢀ4 mol dmꢀ3 could be taken as the critical concentration
of P-O to reach a balance of the self-assembly. Similar behavior was
shown by copolymer P-W (Figs. 7b and d), for which its critical
concentration thus determined was 2.04 ꢁ10ꢀ4 mol dmꢀ3. The first
stage of variations of Rabs and Rfl represents the intermolecular
associations of the copolymer chains containing porphyrins in the
aqueous media, during which a large amount of porphyrin associ-
ation complexes is formed [39,40]. The second stage corresponded
to the range of cPor in which further addition of copolymer does not
greatly change the local environment of porphyrin units to reach
a balance of the self-assembly in water, although assemblies of
copolymers should continue to grow [37]. P-W had a lower critical
concentration than P-O, even though the content of porphyrin units
of P-W was much smaller, which indicates that the micro-blocky
sequences of porphyrin units in P-W played a primary role in its
aggregation in water [39]. The values of Rabs and Rfl for P-O
increased about 2-fold with the increasing cPor, while those for P-W
increased not more than 1.2-fold. Thus, there were larger effects of
cPor on the formation of porphyrin association complex of P-O than
of P-W, suggesting that the complexes are formed by weaker
interactions in P-O than in P-W. It is worthy of note that the
increase of porphyrin association complex absorbing at 752 nm
(1.56-fold) was enhanced much more than the complex that
absorbs at 624 nm. That suggests that the increasing cPor of P-W in
water is an effective way to afford a large cross section of absorption
at longer wavelength region. In general, P-W with micro-blocky
sequences of porphyrin units in the copolymer chains is a better
candidate for building an artificial LH system in aqueous media due
to the porphyrin association complex with absorption at the longer
wavelength region.
[18] Nowakowska M, Karewicz A, Loukine N, Guilet JE. Study of the antenna effect
in polymer molecules. 29. Synthesis and properties of poly[sodium styrene
sulfonate-co-(4-acryolyloxyphenyl)-10,15,20-tritolylporphyrin] in aqueous
solution. Polymer 2002;43:2003e9.
4. Conclusions
[19] Wöhrle D, Paliuras M. Polymer-bound porphyrins and their precursors, 10a)
Syntheses and photoredox properties of water-soluble polymers with cova-
lently bonded zinc tetraphenylporphyrin. Die Makromolekulare Chemie
1991;192:819e32.
The novel porphyrin monomer ZnAOTCPP and its salt ZnAOTCPP-
Na were synthesized and their corresponding copolymers with AM,
named P-O and P-W respectively, were also prepared. There were
electronic interactions between ZnAOTCPP-Na molecules in water,
when its concentration was above CAC or when acetone was added.
[20] Wöhrle D, Krawczyk G, Paliuras M. Polymeric bound porphyrins and their
precursors, 6a) Syntheses of water-soluble, negatively or un-charged polymers

F. Wang et al. / Dyes and Pigments 91 (2011) 199e207
207
with covalently bound moieties of porphyrin derivatives. Die Makromoleku-
lare Chemie 1988;189:1001e11.
[21] He XZ, Zhou YL, Wang LX, Li TK, Zhang MH, Shen T. Photophysical properties
of amphiphilic porphyrins in different media. Dyes and Pigments
1998;39(3):172e82.
[22] Kandaz M, Özkaya AR, Koca A, Salih B. Water and alcohol-soluble octakis-
metalloporphyrazines bearing sulfanyl polyetherol substituents: synthesis,
spectroscopy and electrochemistry. Dyes and Pigments 2007;74:483e9.
[23] Milgrom LR, O’Neill F. Towards synthetic-porphyrin/monoclonal antibody
conjugates. Tetrahedron 1995;51(7):2137e44.
[24] Milgrom LR. Synthesis of some new tetra-aryl porphyrins for studies in solar
energy conversion. Part 2*. Asymmetric porphyrins. Journal of the Chemical
Society, Perkin Transactions 1984;1:1483e7.
[31] Andrade SM, Teixeira R, Costa SMB, Sobral AJFN. Self-aggregation of free base
porphyrins in aqueous solution and in DMPC vesicles. Biophysical Chemistry
2008;133:1e10.
[32] Andrade SM, Teixeira C, Togashi DM, Costa SMB, Sobral AJFN. Self-association
of free base porphyrins with aminoacid substituents in AOT reverse micelles.
Journal of Photochemistry and Photobiology A: Chemistry 2006;178:225e35.
[33] Togashi DM, Costa SMB, Sobral AJFN, Gonsalves AMd’AR. Self-aggregation of
lipophilic porphyrins in reverse micelles of aerosol OT. The Journal of Physical
Chemistry B 2004;108(31):11344e56.
[34] Chandrashekar TK, Willigen HV, Ebersole MH. Optical and electron spin
resonance study of cation and cation-crown ether induced dimerization of
tetrakis(4-sulfonatophenyl)porphyrin. The Journal of Physical Chemistry
1984;88(19):4326e32.
[25] Peiffer DG. Hydrophobically associating polymers and their interactions with
rod-like micelles. Polymer 1990;31:2353e60.
[35] Kasha M, Rawls HR, Ashraf El-Bayoumi M. The exciton model in molecular
spectroscopy. Pure and Applied Chemistry 1965;11:371e92.
[26] Schulz DN, Kaladas JJ, Maurer JJ, Bock J, Pace SJ, Schulz WW. Copolymers of
acrylamide and surfactant macromonomers: synthesis and solution proper-
ties. Polymer 1987;28:2110e5.
[27] HillA, CandauF, Selb J.Properties ofhydrophobically associatingpolyacrylamide:
influence of the method of synthesis. Macromolecules 1993;26:4521e32.
[28] Seybold PG, Gouterman M. Porphyrins XIII1: fluorescence spectra and
quantum yields. Journal of Molecular Spectroscopy 1969;31:1e13.
[29] Dionísio M, Sotomayor J. A surface chemistry experiment using an inexpen-
sive contact angle goniometer. Journal of Chemical Education
2000;77(1):59e62.
[36] McRae EG, Kasha M. Physical processes in radiation biology. New York:
Academic Press; 1964. p. 23.
[37] Neumann MG, Schmitt CC, Prieto KR, Goi BE. The photophysical determination
of the minimum hydrotrope concentration of aromatic hydrotropes. Journal of
Colloid and Interface Science 2007;315:810e3.
[38] Miguel MG, Burrows HD, Lindman B. Polymeresurfactant association as seen
by fluorescence. Progress in Colloid and Polymer Science 2002;120:13e22.
[39] Ezzell SA, Hoyle CE, Creed D, McCormick CL. Water-soluble copolymer. 40.
photophysical studies of the solution behavior of associative pyr-
enesulfonamide-labeled polyacrylamide. Macromolecules 1992;25:1887e95.
[40] Nowakowska M, White B, Guillet JE. Studies of the antenna effect in polymer
molecules. 13. preparation and photophysical studies of poly(sodium styr-
enesulfonate-co-2-vinylnaphthalene). Macromolecules 1989;22:3902e8.
[30] Balaban TS, Linke-Schaetzel M, Bhise AD, Vanthuyne N, Roussel C. Green self-
assembling porphyrins and chlorines as minics of the natural bacteriochlo-
rophylls c, d, and e. European Journal of Organic Chemistry; 2004:3919e30.