The formation of a slipped cofacial dimer of zinc (II) tripyridylporphyrin in water-soluble polymer

Article abstract of DOI:10.1016/j.jphotochem.2011.03.020

A novel tripyridylporphyrin monomer, 5-[4-[2-(acryloyloxy)ethoxy]phenyl]- 10,15,20-tris(4-pyridyl)porphyrin (TrPyP), and its zinc (II) complex (ZnTrPyP) were synthesized. It was copolymerized with acrylamide to prepare water-soluble polymer P(ZnTrPyP-AM).

Full text of DOI:10.1016/j.jphotochem.2011.03.020

Journal of Photochemistry and Photobiology A: Chemistry 220 (2011) 64–69
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
The formation of a slipped cofacial dimer of zinc (II) tripyridylporphyrin in
water-soluble polymer
Kewei Dinga,b, Fei Wanga,b, Feipeng Wua,∗
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:
A
novel
tripyridylporphyrin
monomer,
5-[4-[2-(acryloyloxy)ethoxy]phenyl]-10,15,20-tris(4-
Received 15 December 2010
Received in revised form 25 February 2011
Accepted 20 March 2011
pyridyl)porphyrin (TrPyP), and its zinc (II) complex (ZnTrPyP) were synthesized. It was copolymerized
with acrylamide to prepare water-soluble polymer P(ZnTrPyP-AM). The aggregation behavior of
P(ZnTrPyP-AM) in aqueous solution was investigated by UV–visible absorption spectra. The red shift of
Q band and the significantly enhanced relative intensity (ε˛/εˇ) indicated that the ZnTrPyP was axially
coordinated. Meanwhile, Soret band of porphyrin split into two bands, suggesting exciton coupling of
aggregates. The porphyrin pendants of P(ZnTrPyP-AM) were supposed to form a slipped cofacial dimer
and the macromolecular matrix helped to create the well-defined porphyrin aggregates. This finding
was helpful for the assembly of artificial light-harvesting systems in aquatic environment to mimic the
dimeric structure in natural LHC.
Available online 27 March 2011
Key words:
Self-aggregation
Zinc tripyridylporphyrin
Water-soluble polymer
Exciton coupling
© 2011 Elsevier B.V. All rights reserved.
Slipped cofacial dimer
1. Introduction
Recently, lots of porphyrin aggregates induced by association
in some heterogeneous systems like dendrimers [6], LB films
Porphyrin assemblies are of fundamental importance since they
are not only used as building blocks for the construction of func-
tional molecular devices, but also to mimic the natural systems,
particularly the antenna complex and reaction center in the pho-
tosynthesis [1]. The simplest antenna system in nature is the core
light-harvesting antenna (LH1) of the photosynthetic purple bacte-
ria, in which bacteriochlorophyll-a (Bchl-a) molecules form highly
symmetric ring structure with the aid of coordination bonds from
peptide matrices and energy is stored by extremely rapid energy
migration within the ring [2]. The key functional unit of the LH1
consists of a Bchl-a dimer in a slipped cofacial orientation and these
dimers are further arranged into a macroring form [2,3].
Numerous porphyrin architectures aimed at mimicking LH
complexes have been prepared through either synthesis or self-
assembly [3,4] in organic solvents, but these media are largely
different from the microenvironments provided by protein matri-
ces in nature. Water is the only solvent in living systems and
surrounding medium for biological macromolecules [5]. Mean-
while, it is an effective pH buffer and a reactant in some biological
reactions, such as photosynthesis. For future application, water
is also a harmless solvent to conduct other photoredox reactions
driven by visible light.
[7], micelles [8] and so on [9] have attracted much attention,
among which the driving forces in aggregation process are always
hydrogen bonds, coordination bonds, electrostatic interactions,
etc. Water-soluble macromolecules could provide a matrix, an
environment more similar to the natural system, for side groups
to associate and the aggregation behavior of pendants can be
affected by main chain greatly. So water-soluble polymer contain-
ing porphyrin molecules as its pendant groups is a good scaffold
to research the aggregation and photophysical properties of por-
phyrin molecules for artificial photosynthesis, and construction of
noncovalent porphyrin complexes with a well-defined structure
within water-soluble macromolecules is a meaningful and chal-
lenging target.
Several polymers containing porphyrins in their side chains
have been studied [10], among which the derivatives of
tetraphenylporphyrin were used most frequently and they aggre-
gated randomly. Pyridine is a good candidate for coordination
and forming hydrogen bond and shows versatile solubility in
both polar and apolar solvents, such as, water and cyclo-
hexane [11]. As is well known, linking the large and rigid
groups to the polymer backbone via
a flexible spacer can
alleviate steric hindrance and reduce the entropy loss in
association [12]. Therefore, in this work, a novel tripyridylpor-
phyrin monomer, 5-[4-[2-(acryloyloxy)ethoxy]phenyl]-10,15,20-
tris(4-pyridyl)porphyrin (TrPyP), and its zinc (II) complex (ZnTr-
PyP) (Scheme 1) were synthesized. The monomer ZnTrPyP was
∗
Corresponding author. Tel.: +86 10 82543569; fax: +86 10 82543491.
E-mail addresses: fpwu@mail.ipc.ac.cn, keweiding@hotmail.com (F. Wu).
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.03.020

K. Ding et al. / Journal of Photochemistry and Photobiology A: Chemistry 220 (2011) 64–69
65
Scheme 1. Preparation of TrPyP and ZnTrPyP. (i) CH2Cl2, TEA, 4 h; (ii) propionic acid, Ac2O, reflux, 3 h; (iii) DMF, NaOH, 3 h, rt; (iv) DMF, Zn(OAc)2, 2 h, rt.
copolymerized with acrylamide to prepare amphiphilic copolymer
Poly(ZnTrPyP-co-acrylamide), which is abbreviated as P(ZnTrPyP-
AM). Similar copolymers P(TrPyP-AM) and P(ZnTrPyP-NVP) were
also synthesized for comparison. The spectral properties of
P(ZnTrPyP-AM) and aggregation behavior of porphyrin pendants
were discussed.
2.2.2. Synthesis of 5-[4-[(ethylcarbonyl)oxy]phenyl]-10,15,20-
tris(4-pyridyl)porphyrin
(2)
It was synthesized by slightly modified Adler–Longo method
[13] and confirmed by 1H NMR and MS. MS (ESI) calcd for M+
689.2539, found 690.2640 (M+1).
2.2.3. Synthesis of 5-[4-[2-(acryloyloxy)ethoxy]phenyl]-10,15,20-
tris(4-pyridyl)porphyrin
2. Experimental
(TrPyP)
2.1. Materials
To a solution of 2 (0.3 g, 0.43 mmol) in dry DMF (25 mL)
under nitrogen was added powdered sodium hydroxide (0.09 g,
2.2 mmol), and the mixture was stirred at room temperature for
about 1.5 h (the color of the solution changed from purple to green).
Formation of the phenolate form of 2 was checked by TLC. The
2-bromoethyl acrylate (1) (0.42 g, 2.3 mmol) was added to the solu-
tion and the mixture was stirred for 3 h at room temperature.
The pH of the reaction mixture was then adjusted to 7 with 1 M
HCl and the solvent was removed under reduced pressure. The
residue was dissolved in dichloromethane and the organic phase
was washed twice with water, dried over Na2SO4, and evaporated
to dryness. Products were purified by column chromatography on
silica with CH2Cl2 as the eluent and precipitated from a mixture
of dichloromethane–hexane to give TrPyP as a purple solid (0.24 g,
75%). 1H NMR (400 MHz, CDCl3) ı 9.08 (dd, J = 4.4, 1.5 Hz, 6H), 8.97
(d, J = 4.7 Hz, 2H), 8.87 (s, 4H), 8.84 (d, J = 4.8 Hz, 2H), 8.19 (dd, J = 4.4,
1.5 Hz, 6H), 8.14 (d, J = 8.6 Hz, 2H), 7.36 (d, J = 8.6 Hz, 2H), 6.59 (dd,
J = 17.3, 1.4 Hz, 1H), 6.31 (dd, J = 17.3, 10.4 Hz, 1H), 5.98 (dd, J = 10.4,
1.4 Hz, 1H), 4.75 (t, J = 6.1 Hz, 2H), 4.55 (t, J = 6.1 Hz, 2H), −2.86 (s,
2H). MS (ESI) calcd for M+ 731.2645, found 732.2731 (M+1).
All reagents not explicitly referenced were obtained from
commercial sources and used as received. 2-Bromo-1-ethanol,
pyridine-4-carboxaldehyde and acryloyl chloride were purchased
from Alfa Aesar Company. Pyrrole was purchased from Sinopharm
Chemical Reagent Co., Ltd. and distilled before use. Acrylamide
(AM) was purchased from Jiangxi Changjiu Biochemical Engi-
neering Corporation. Acetic anhydride, p-hydroxybenzaldehyde,
n-propanoic acid, triethylamine (TEA), 2,2ꢀ-azobis (isobutyroni-
trile) (AIBN), pyridine, N,N-dimethyl formamide (DMF), dimethyl
sulfoxide (DMSO), zinc (II) acetate dehydrate and other A.R. grade
reagents were obtained from Beijing Chemical Works. The aqueous
solution was prepared by deionized water.
2.2. Synthesis of porphyrin monomers
2.2.1. Synthesis of 2-bromoethyl acrylate (1) [11]
To a CH2Cl2 (25 mL) solution of 2-bromo-1-ethanol (4.25 mL,
60 mmol) and triethylamine (8 mL, 60 mmol), a CH2Cl2 (35 mL)
solution of acryloyl chloride (4.9 mL, 60 mmol) was added dropwise
at 0 ◦C and then the reaction mixture was stirred at room tempera-
ture for 4 h. The white suspension (triethylamine hydrochloride)
appeared in the reaction was filtered off and the light yellow
filtrate was washed with water. The organic phase was dried
over magnesium sulfate and concentrated under reduced pres-
sure, which was then subjected to column chromatography (SiO2,
CHCl3/hexane = 1:3) to give 1 as yellow oil (6.7 g, 62%). 1H NMR
(400 MHz, CDCl3) ı 6.46 (dd, J = 17.3, 1.3 Hz, 1H), 6.15 (dd, J = 17.3,
10.5 Hz, 1H), 5.88 (dd, J = 10.5, 1.3 Hz, 1H), 4.46 (t, J = 6.2 Hz, 2H),
3.55 (t, J = 6.2 Hz, 2H).
2.2.4. Synthesis of 5-[4-[2-(acryloyloxy)ethoxy]phenyl]-10,15,20-
tris(4-pyridyl)porphyrinate zinc (II)
(ZnTrPyP)
TrPyP (0.2 g, 0.27 mmol) was dissolved in a mixed solution of
CHCl3 (15 mL) and methanol (5 mL). Zinc (II) acetate dehydrate
(0.31 g, 1.4 mmol) was added to the solution and the reaction
mixture was stirred at room temperature for 2 h. A little of tri-
ethylamine was added and the reaction was continued for 2 h. The
organic phase was washed three times with water and dried over
Na2SO4. After removal of CHCl3, the residue was purified by recrys-
tallization (CHCl3/hexane) to give ZnTrPyP as a purple solid (0.2 g,

66
K. Ding et al. / Journal of Photochemistry and Photobiology A: Chemistry 220 (2011) 64–69
Fig. 2. UV–vis absorption spectra of P(ZnTrPyP-AM) in aqueous solution in different
Fig. 1. FT-IR spectra of ZnTrPyP and Poly-ZnTrPyP in KBr.
concentration (solid line: 8.0 × 10−2; dashed line: 3.2 × 10−1; dotted line: 4.0 g/L and
measured in 1 mm cuvette). Inset: UV–vis absorption spectra of P(TrPyP-AM) and
P(ZnTrPyP-NVP) in aqueous solution at 8.0 × 10−2 g/L.
96%). 1H NMR (400 MHz, DMSO) ı 9.01 (d, J = 5.7 Hz, 6H), 8.88 (d,
J = 4.7 Hz, 2H), 8.83 (s, 4H), 8.81 (d, J = 4.7 Hz, 2H), 8.22 (d, J = 5.8 Hz,
6H), 8.09 (d, J = 8.5 Hz, 2H), 7.39 (d, J = 8.5 Hz, 2H), 6.47 (dd, J = 17.3,
1.4 Hz, 1H), 6.34 (dd, J = 17.3, 10.2 Hz, 1H), 6.06 (dd, J = 10.2, 1.4 Hz,
1H), 4.64 (t, J = 4.8 Hz, 2H), 4.56 (t, J = 4.8 Hz, 2H). MS (ESI) calcd for
M+ 793.1780, found 794.1950 (M+1).
The copolymers displayed very similar UV–vis absorption spec-
tra to the corresponding porphyrin monomers in H2O–DMSO (1/9
v/v) mixed solvent. So the contents of porphyrin units in the copoly-
mers can be estimated from quantitative absorption spectra setting
the molar extinction coefficient of Soret band of corresponding
monomers in H2O–DMSO (1/9 v/v) mixed solvent as a standard.
The plot of Abs versus concentration of TrPyP and ZnTrPyP in
H2O–DMSO (1/9 v/v) was fitted as A = 4.0 × 106 C and A = 6.4 × 106 C
(A, Abs; C, concentration), respectively. The molecular weights
of the polymer were also investigated by static light scattering
(SLS). The laser was 690 nm in wavelength where the absorption
of copolymer is nearly zero (Fig. 2).
2.3. Preparation of homopolymer of ZnTrPyP
A solution of ZnTrPyP and 2,2ꢀ-azobis (isobutyronitrile) (AIBN) in
DMSO was placed in a glass tube, degassed, sealed and polymerized
at 60 ◦C for 24 h. The amount of AIBN was 2 wt% corresponding to
the monomer. After polymerization, the DMSO was removed under
reduced pressure. The residuum could partially dissolve in chloro-
form. After washed three times by chloroform under ultrasonics,
the homopolymer Poly-ZnTrPyP was obtained by centrifugal sep-
aration. The IR bands of the monomer at 1636 cm−1 are assigned
to the vibration of C C bonds [14], which disappeared in that of
homopolymer Poly-ZnTrPyP as shown in Fig. 1. In addition, another
peak appeared when polymerized at 1225 cm−1 presumably corre-
sponds to the skeleton vibration of carbon–carbon chain. The 1H
NMR of Poly-ZnTrPyP in D-substituted pyridine also showed the
vanish of H peaks of H2C CH–. The results indicated that ZnTrPyP
can be polymerized via the double bonds on acryloyl groups.
2.5. Measurement
1H NMR spectrum was obtained on a Bruker DPX400 spectrom-
eter. Mass spectrum was carried out on Bruker APEX-IV. Static
light scattering (SLS) measurement of copolymers was performed
by the DAWN EOS (Wyatt). FT-IR spectra were obtained from
Excalibur 3100 (Varian, USA) infrared spectrometer in KBr disks.
UV–vis absorption spectrum was recorded with a Hitachi UV-3900
UV–vis spectrophotometer. Fluorescence emission spectrum was
performed on a Hitachi F-4500 fluorescence spectrophotometer.
1 mm colorimetric ware was applied in the test of spectroscopic
properties of sample with high concentration.
2.4. Preparation of copolymers
The copolymerization of TrPyP and ZnTrPyP with acrylamide
(AM) and N-vinyl-2-pyrrolidone (NVP) was carried out in dimethyl
sulfoxide (DMSO) (Table 1). The total concentration of monomers
was 0.4–0.45 mol/L, respectively, and the molar ratio of M1 (AM
or NVP) and M2 (TrPyP or ZnTrPyP) was 200:1. 4.5 mmol/L of AIBN
(1 mol% corresponding to the whole amount of acrylamide and por-
phyrin) was added as initiator. The polymerization was performed
in a sealed tube at 60 ◦C for 24 h after purging with nitrogen for
20 min.
After the polymerization, the resultant solution keeps homo-
geneous and it was poured dropwise into excess of acetone with
stirring to precipitate the polymers. The precipitates formed in this
process were filtered off and dried in vacuum at 45 ◦C. The poly-
mers were redissolved gradually in a little amount of H2O–DMSO
(1/9 v/v) mixed solvent with stirring, and then precipitated again.
The purification process was repeated several times until the por-
phyrin in filtrate could hardly be detected by UV–vis absorption
spectrum, and IR band of C C bond (1636 cm−1) of the unsaturated
ester was not found in the IR spectra of copolymers.
3. Results and discussion
3.1. Absorption spectra of monomers
Table 2 (the first five lines) lists the steady-state absorption
spectral data of monomer TrPyP and its zinc complex ZnTrPyP
in some solvents with different coordination ability and polarity.
TrPyP exhibits typical spectroscopic features of free base por-
phyrin and the property of solvent has little influence on it. For the
monomer ZnTrPyP, it shows properties of four or five-coordinate
zinc tripyridylporphyrin, respectively in different conditions. By
increasing the concentration of ZnTrPyP in chloroform, some
pyridyl substituents of porphyrin ring could axially coordinate to
the central metal ion of others [15] and therefore the new Soret
band at 429 nm becomes larger and larger, and the Q band is red
shifted and the relative intensity (ε˛/εˇ) increases [16]. Similarly,
the fluorescence spectrum is also red shifted and I˛/Iˇ increases.
In the coordinating solvents, such as pyridine and DMSO, the sol-

K. Ding et al. / Journal of Photochemistry and Photobiology A: Chemistry 220 (2011) 64–69
67
Table 1
Copolymerization of AM or NVP (M1) with TrPyP or ZnTrPyP (M2).
Copolymer
M1
M2
In feed
In copolymer
[M2] wt%
Mw × 10−4 g/mol
[M] mol/L
[M1]:[M2]
[M1]:[M2]
P(TrPyP-AM)
P(ZnTrPyP-AM)
P(ZnTrPyP-NVP)
AM
AM
NVP
TrPyP
ZnTrPyP
ZnTrPyP
0.40
0.45
0.45
200:1
200:1
200:1
2.3
2.7
2.2
433:1
394:1
314:1
7.13
9.72
49.6
vent molecules coordinate to the zinc ion and the solution exhibits
typical spectroscopic features of five-coordinate zinc tripyridylpor-
phyrin.
gated species and monomeric species in water–pyridine mixture.
The equilibrium shifts toward monomeric species as the addition
of pyridine.
Aggregated species (displaying split B band)
3.2. Absorption spectra of the copolymer P(ZnTrPyP-AM)
↔ Monomeric species
(1)
Fig. 2 illustrates the steady-state absorption spectra of the
copolymer P(ZnTrPyP-AM) in aqueous solution at a variety of con-
centration. It is interesting that the Soret band is split to B1 (421 nm)
and B2 (445 nm) and the profile of them keeps the same over a wide
range of concentration. There are also two Q-bands with ˇ band at
567 nm and ˛ band at 609 nm and the intensity ratio of ˛ band over
ˇ band (ε˛/εˇ) is up to 0.59 (see Table 2). Compared with that of the
four-coordinate ZnTrPyP (in CHCl3), the Q bands of the P(ZnTrPyP-
AM) in aqueous solution are red shifted by about 20 nm and ε˛/εˇ is
increased considerably, which suggest that the zinc tripyridylpor-
phyrins in copolymer P(ZnTrPyP-AM) are axially coordinated by a
certain kind of ligand in aqueous solutions [16].
The fluorescence emission spectra in this process are shown in
Fig. 3B. As the volume fraction of pyridine increases, the whole
intensity of fluorescence rises greatly due to reducing quench-
ing caused by aggregation, and the intensity ratio of 628 nm over
609 nm decreases continuously as shown in the inset of Fig. 3B.
Hence, the fluorescence of monomeric species with Q (0, 0) at
609 nm and Q (0, 1) at 659 nm is enhanced quickly, whereas the
relative fluorescence intensity of the aggregated species (628 nm)
becomes lower. The above phenomena are reversible and indicate
that the intermolecular forces in the aggregated species can be
destroyed by the addition of pyridine.
To investigate the reason of the splitting of Soret band in
aqueous solution of P(ZnTrPyP-AM), the reference copolymers
P(TrPyP-AM) and P(ZnTrPyP-NVP) were prepared in the same way
as P(ZnTrPyP-AM), and their absorption spectra in aqueous solu-
tion are shown in the inset of Fig. 2. The Soret band of copolymer
P(TrPyP-AM) becomes broad in aqueous solution relative to that of
the TrPyP due to their interaction in hydrophobic microdomain,
but it is not split. In addition, in the case of reference copoly-
mer P(ZnTrPyP-NVP), the Soret band is not split too. These results
demonstrate that the central metal ions in tripyridylporphyrins as
well as the polyacrylamide main chain of copolymer play important
roles in originating the split Soret band.
Fig. 3A shows the changes of absorption spectra after pyridine
was added to the solution. The absorption bands of B1 and B2
decrease and a new band between them appears rapidly, which
becomes very sharp and finally locates at 430 nm when vol-
ume fraction of pyridine is up to 10%. This new band is due to
five-coordinate zinc tripyridylporphyrin. This result indicates that
neither B1 nor B2 is resulted from the self-coordination. Contrarily,
both of them should be due to aggregated species. Furthermore, the
dissociation progress gives isosbestic points in the UV–vis titration
spectra, which suggests that there is equilibrium between aggre-
3.3. Structure of the porphyrin aggregates in P(ZnTrPyP-AM)
Some structural information aboutthe porphyrin aggregates can
be deduced from the change of absorption spectra. The splitting or
shift of absorption bands of covalently linked porphyrin dimer or
oligomer models has been explained very well by exciton inter-
actions [18]. According to Kasha’s exciton theory [19], when the
dipole transition moments of monomer molecules are aligned par-
allel (“side-by-side”) to the line joining the molecular centers in
the aggregate, the absorption band will red shift relative to that
of the monomer and this kind of aggregate is called J aggregate.
On the other hand, for the H aggregate, the monomers’ dipole
transition moments are perpendicular to the line of centers (“face-
to-face”) leading to a blue-shift electronic transition with respect
to the absorption band of the monomer. The dipole–dipole exciton
splitting energy, ꢀE, in coplanar geometry is given by the equation
[19]:
2
2|ꢁM
|
ꢀE =
(1 − 3 cos2 Â)
(2)
r3
Table 2
Absorption spectral data of porphyrin monomers and copolymers in different solvents.
Solvent
Absorption
a
Soret band (nm)
Half-bandwidth (nm)
Q bands (nm)
Peak ratio ε˛/εˇ
TrPyP
CHCl3
Pyridine
CHCl3
CHCl3
Pyridine
DMSO
H2O
H2O
1H2O:9 pyridine
H2O
419
421
419
14
16
12
22
10
10
36
44
11
31
515,550,590,645
516,551,591,647
547,586
TrPyP
ZnTrPyPb
0.13
0.41
0.37
0.36
ZnTrPyPc
421,429
429
428
416
422,445
430
562,605
562,603
560,600
ZnTrPyP
ZnTrPyP
P(TrPyP-AM)
P(ZnTrPyP-AM)
P(ZnTrPyP-AM)
P(ZnTrPyP-NVP)
520,556,591,651
567,609
0.59
0.39
560,600
566,607,630
431
a
The ratio of the molar extinction coefficients of the intensity maxima of ˛: Q (0, 0) and ˇ: Q (1, 0) absorption bands [17].
Obtained from a dilute solution (8.0 × 10−7 mol/L).
b
c
Obtained from a concentrated solution (8.0 × 10−5 mol/L).

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K. Ding et al. / Journal of Photochemistry and Photobiology A: Chemistry 220 (2011) 64–69
Fig. 3. UV–vis absorption (A) and fluorescence emission (B) spectra of P(ZnTrPyP-AM) in different pyridine volume fractions in water–pyridine mixtures (1.0 × 10−1 g/L).
Inset (B): dependence of intensity ratio 628:609 on volume fraction of pyridine.
where ꢁM is the transition dipole moment of the monomer, r is
the center to center distance of the two dipoles, and  is the angle
made by the dipole with the line of molecular centers. The transi-
tion dipole moment used for the calculations can be obtained by
integration of the absorption band [20].
It is clear that the formation of the dimer in copolymer
P(ZnTrPyP-AM) is dependent on some intermolecular interac-
tions. As discussed previously, zinc tripyridylporphyrins are
five-coordinated and polyacrylamide main chain is a key factor
in originating the split Soret band. So it is safe to consider that
amino-groups on the polymer backbone are the fifth coordinates.
In fact, since the amphiphilic copolymer contains only a few zinc
tripyridylporphyrins, the amount of amino-groups on backbone
is numerous compared with the limited pyridyl moieties on por-
phyrins. Hence, we suppose that the pyridine moieties on zinc
tripyridylporphyrins in P(ZnTrPyP-AM) interact with the coordi-
nated amino-groups in the manner of N–H· · ·N(py) intermolecular
hydrogen bands which help to form and stabilize the slipped cofa-
cial dimer of zinc tripyridylporphyrin (Scheme 2). This was similar
to the reported dimeric structure of aquamagnesium phthalo-
cyanine and protonated meso-tetraphenylporphine, which were
stabilized by hydrogen bonds of the bound water molecules [22].
Due to the complexity of macromolecules, it is hard to optimize the
proposed structure by calculation. However, according to general
bond length and bond angle, the proposed structure well coincides
with the above parameters and therefore the conjecture is reason-
able. So the water-soluble macromolecular matrix PAM makes the
ZnTrPyP display dimeric absorption spectrum in aqueous solution.
It is like that Bchl-a molecules are coordinated by peptide matri-
ces and thus their position and orientation are fixed accurately in
light-harvesting complex (LHC).
Taking the absorption spectra of P(ZnTrPyP-AM) in aqueous
solution into account, the most amounts of zinc tripyridylpor-
phyrins are supposed to be in a slipped cofacial orientation as
shown in Scheme 2. In a slipped cofacial dimer [3,21], the neigh-
boring molecular planes are parallel, while the molecular centers
are displaced. Soret band of zinc (II) porphyrin monomer has
two perpendicular transition dipole moments, Bx and By, that are
degenerated in a simple monomer. In the dimer we proposed
above, Bx dipole moments are arranged in head-to-tail orienta-
tion (the angle between dipole and the line of molecular centers
is Âx), whereas By dipole moment interacts face to face (Ây is 90◦).
And then, Bx and By couple, respectively and the Soret bands
of dimer are split into a red-shifted Bx component and a blue-
shifted By components (Scheme 2). The total splitting energy ꢀE
in this work corresponds to 1281 cm−1, from which the relation-
˚
ships between geometrical parameters, r (A) and Â, can be obtained.
According to similar works reported earlier, the interplanar dis-
˚
tance of the dimer is fixed at 3.5 A [8d]. So geometrical parameters
of the slipped cofacial dimer can be obtained approximately: the
˚
center-to-center distance◦ is 9.9 A, the slipping displacement is
˚
9.3 A, and  is about 20.8 . We do not observe the phenomenon
derived from the coupling of the Q-bands due to their relative
smaller transition dipole moments compared with that of the Soret
band.
The experimental phenomena can be interpreted well by the
proposed structure. For copolymer P(ZnTrPyP-NVP), since tertiary
amine on NVP cannot generate hydrogen band, the NVP could not
Scheme 2. Proposed structure of slipped cofacial dimer and the exciton coupling model.

K. Ding et al. / Journal of Photochemistry and Photobiology A: Chemistry 220 (2011) 64–69
69
serve as the similar bridge. In the case of P(ZnTrPyP-AM), because
there are two N–H· · ·N(py) intermolecular hydrogen bands in every
one of the dimers, the dimer has a large association constant,
which explains why the dimer still exist even in very dilute aque-
ous solution. Besides, the four-coordinate zinc tripyridylporphyrin
will accept one and only one amino-group to form five-coordinate
complex, so this type of aggregate is limited to dimer rather than
extends to larger oligomers. With the increase of concentration,
it is only the increase in the number of dimers in large aggre-
gates and there are not strong electronic interactions even some
extent of hydrophobic association may exist between these dimers.
As a result, except the increase of intensity, no other changes
are observed in absorption spectra in concentrated aqueous solu-
tion of P(ZnTrPyP-AM). The addition of pyridine could destroy
the N–H· · ·N(py) intermolecular hydrogen bands by replacing the
coordinated amino-groups, and make the amphiphilic copolymer
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