Synthesis of a series of zinc porphyrins and spectroscopic changes upon coordination reaction with imidazole derivatives

Article abstract of DOI:10.1002/bkcs.10550

Three metal-free porphyrins modified with Boc-L-threonine and their zinc analogs were synthesized and characterized by elemental analysis, ¹H NMR, UV/vis, and fluorescence spectroscopies. The binding of imidazole derivatives to these zinc porph

Full text of DOI:10.1002/bkcs.10550

Article
DOI: 10.1002/bkcs.10550
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S.-J. Wang et al.
KOREAN CHEMICAL SOCIETY
Synthesis of a Series of Zinc Porphyrins and Spectroscopic Changes upon
Coordination Reaction with Imidazole Derivatives
*
Shu-Jun Wang, Yu-Ling Peng, Cheng-Gen Zhang, Yong-Bing Li, and Chao Liu
College of Chemistry and Materials Science, Langfang Teachers University, Langfang 065000,
People’s Republic of China. *E-mail: d022036@mail.nankai.edu.cn
Received June 16, 2015, Accepted July 26, 2015, Published online October 4, 2015
Three metal-free porphyrins modified with Boc-L-threonine and their zinc analogs were synthesized and char-
acterized by elemental analysis, 1H NMR, UV/vis, and fluorescence spectroscopies. The binding of imidazole
derivatives to these zinc porphyrins was studied, with emphasis on the binding mechanism in CH2Cl2 solution,
by means of UV/vis spectroscopy and quantum chemical methods. Both experimental results and theoretical
calculations showed that a coordination reaction occurred between the zinc porphyrins and imidazole deriva-
tives. The association constants between the zinc porphyrins and imidazole derivatives decreased in the order
N-MeIm>Im. Increasing the temperature disfavored the interaction. Thermodynamic parameters calculated by
the van’t Hoff equation showed that the driving force for the reaction was the enthalpy change. The fluores-
cence changes associated with the interaction between the zinc porphyrins and imidazole derivatives were also
studied by fluorescence spectroscopy. The experimental results showed significant quenching between the var-
ious zinc porphyrins and imidazole derivatives.
Keywords: Zinc porphyrins, Coordination reaction, Thermodynamic parameters, Fluorescence quenching,
Theoretical calculation
Introduction
In this study, we have used Boc-L-threonine to synthesize
three kinds of free porphyrins and their zinc analogs with a
view to combining the biological activities of porphyrin and
threonine. The coordination reaction between zinc porphyrins
and imidazole derivatives has been studied by means of
UV/vis spectroscopy andquantum chemical methods. Associ-
ation constants have been determined at four different tem-
peratures; the binding mechanism in CH2Cl2 has been
determined; and the fundamental driving force for the coordi-
nation system has been elucidated. The quenching properties
have been studied by fluorescence spectroscopy. These exper-
imental results may be of value in a variety of areas, such as
drug design, protein engineering, and gene technology.
Porphyrins are often used as model compounds for some
important biological systems due to their unique roles in biol-
ogy and sensing systems, such as in imitating plants and the
reaction center of bacterial photosynthesis, or the active center
of cell pigment P-450.1–14 The axial coordination reaction
between a porphyrin and a small molecule containing a nitro-
gen atom has been widely used as a reaction model of hemo-
globin, myoglobin, cytochrome-c, and other biological
molecules. Upon such axial coordination, the conjugated
structure of the porphyrin molecule, characterized by alternat-
ing single and double bonds, greatly facilitates electron trans-
fer while also imparting good thermal stability and
photoelectric properties.15–20 Imidazole derivatives with
structures similar to that of the natural enzyme ligand are com-
monly used small molecules in axial coordination reactions.
The activation of metal porphyrins by coordination of imidaz-
ole derivatives not only enhances their reactivity but also cre-
ates the space and structural conditions for reaction. Thus,
imidazole derivatives play an important role in oxidation pro-
cesses, transferring an electron and enhancing the stability of
the whole system.21,22
Experimental
Materials and Methods. Pyrrole was distilled, and
p-nitrobenzaldehyde and imidazole (Im) were recrystallized
prior to use. Boc-L-threonine was purchased from Beijing
J&K Technology Co., Ltd. (Beijing, China), and was bio-
chemical reagent grade. Dry CH2Cl2 was obtained by redistil-
lation from CaCl2. N-methylimidazole (N-MeIm) and other
reagents were commercial chemicals of analytical grade and
were used as received without further purification.
Elemental analysis (C, H, N) was performed with a CE-440
elemental analyzer (Exeter Analytical, Inc., North Chelms-
ford, MA, USA). 1H NMR spectra were recorded at room tem-
perature on a Varian 400 MHz spectrometer (Varian, Inc.,
Palo Alto, CA, USA) from samples in CDCl3 solution. UV/vis
absorption spectra were measured on a Shimadzu 2550
The structures and functions of porphyrins modified with
amino acids are similar to those of natural porphyrins. Threo-
nine is a common amino acid and has many biological func-
tions. A protein kinase incorporating threonine and showing
antiviral activity has been studied by Venditto23 and Darbi-
nian24; this amino acid may provide a new route for develop-
ing novel drugs.
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spectrophotometer (Shimadzu Corporation, Kyoto, Japan)
equipped with an HH-501 temperature controller in the region
375−750 nm using CH2Cl2 as solvent. Fluorescence measure-
ments were made on a Shimadzu F-4600 fluorescence
spectrometer (Shimadzu Corporation, Kyoto, Japan) at room
temperature.
G(d) for nonmetal atoms. Harmonic frequency analysis calcu-
lations were performed to characterize the optimized struc-
tures as true minima (no imaginary frequency). All standard
DFT calculations were carried out using the Gaussian
09 program.25–27
Synthesis. The synthetic route to the final complex is outlined
in Scheme 1. The structures of the imidazole derivatives are
shown in Scheme 2.
Porphyrin 1a: A mixture of 4-nitrobenzaldehyde (2.57 g,
0.017 mol), benzaldehyde (5.41 g, 0.051 mol), and pyrrole
(0.536 g, 8 mmol) in CH3CH2COOH (120 mL) was stirred
andheated to reflux for5 min. After the addition of further pyr-
role (4.29 g, 0.064 mol) in CH3CH2COOH (25 mL), the reac-
tion mixture was stirred for a further 20 min. The initially
colorless solution turned brown. The mixture was allowed
The Soret and Q bands in the UV/vis spectra of the free por-
phyrins and zinc porphyrins were measured using 2.5 and 25 μ
M solutions, respectively. Solutions of Im and N-MeIm were
added to 2.5 μM solutions of the three respective zinc porphyr-
ins in CH2Cl2, and the absorption changes at the Soret band
were monitored at four different temperatures. The value
n of the coordination reactions, the association constants Kθ,
the enthalpy change ΔrHmθ , and the entropy change ΔrSmθ were
calculated by means of Origin software (OriginLab
Corporation, Northampton, MA, USA). Similarly, solutions
of Im and N-MeIm were added to 2.5 μM solutions of the three
respective zinc porphyrins in CH2Cl2 at room temperature.
The excitation band width and emission band width were both
set at 10 nm. The excitation wavelength was 420 nm. The
scanning range was 450−750 nm.
CH3
HN
N
N
N
(Im)
(N-MeIm)
All structures were optimized at the B3LYP/BS level. BS
denotes a basis set combining LANL2DZ for Zn and 6-31 +
Scheme 2. Structure of imidazole derivatives.
R
R
CHO
CHO
NH
N
N
NH
N
SnCl2 H2O
HCl
NH2
R
NO2
R
+
+
3
4
HN
N
HN
N
H
NO2
R
R
R
2a: R=H
2b: R=Cl
2c: R=OCH3
1a: R=H
1b: R=Cl
1c: R= OCH3
R
R
CH3
CH3
CH
NH
N
N
HN
N
N
N
N
O
O
H
H
Zn (CH3COO)2
Boc-L-Thr
DCC
Zn
NH
NH
R
R
CH
OH
OH
NH
NH
3O
H3C
H3C
O
O
H3C
O
CH3
H3C CH
R
R
4a: R=H
4b: R=Cl
4c: R=OCH3
3a: R=H
3b: R=Cl
3c: R=OCH3
Scheme 1. Synthetic route of porphyrin complexes.
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Zinc Porphyrin Coordination Reaction
KOREAN CHEMICAL SOCIETY
to react at reflux under microwave conditions for 2 min and
then allowed to cool overnight. The CH3CH2COOH was dis-
tilled off under reduced pressure. The mixture including por-
phyrin 1a was obtained as a purple powder.28
in CHCl3 and purified on a column of silica gel. A red band
containing the product was collected. Yield 73.4%. Anal.
Calcd. for C53H46O4N6: C, 76.63; H, 5.54; N, 10.12.
1
Found: C, 76.41; H, 5.89; N, 9.82. H NMR (CDCl3, 400
Porphyrin 1b: The method used was the same as that
MHz): δ = 8.87−8.90 (m, 8H, pyrrole H), 8.23−8.25 (m, 8H,
5, 10, 15, 20 Ar-o-H), 7.96 (d, 2H, 5 Ar-m-H), 7.78−7.79
(m, 9H, 10, 15, 20 Ar-m-H and p-H), 5.79 (d, 1H, CO-NH),
4.70 (d, 1H, α H), 4.38 (d, 1H, β H), 3.38 (s, 1H, ArNH),
1.61 (s, 9H, C(CH3)3), 1.38 (d, 3H, CH3), −2.75 ppm (s,
2H, pyrrole NH).
described for 1a, but using 4-chlorobenzaldehyde.29
Porphyrin 1c: A mixture of 4-nitrobenzaldehyde (0.6 g, 4
m mol), p-methoxybenzaldehyde (1 mL, 0.051 mol), and pyr-
role (0.85 mL, 8 mmol) in acetic anhydride (12 mL) was stir-
red and then added to glacial acetic acid (100 mL). The
reaction mixture was heated under reflux for 60 min. It was
thenallowedtocool. After extractive work-up, thecrude mate-
rial was obtained as a purple powder.30
Porphyrin 3b: The method used was the same as that
described for 3a. Yield 77.2%. Anal. Calcd. for
C53H43O4Cl3N6: C, 68.13; H, 4.61; N, 9.00. Found: C,
1
Porphyrin 2a: A portion of the mixture including porphy-
rin 1a (30 mg) was dissolved in concentrated hydrochloric
acid (80 mL). Tin(II) chloride dihydrate (2.6 g) was added,
and the solution was heated at 65−70 ꢀC for 1.5 h. It was then
cooled and poured into water (100 mL), and the resulting mix-
ture was adjusted to pH 8 with concentrated ammonium
hydroxide solution. The aqueous phase was extracted with
chloroform. The organic phase was concentrated to a volume
of 50 mL ona rotary evaporator, and this solution was chroma-
tographed onasilicacolumneluting withchloroform. Thesec-
ond band eluted from the column contained the desired
product. Yield 18.4%. Anal. Calcd. for C44H31N5: C,
83.94; H, 4.93; N, 11.13. Found: C, 83.61; H, 5.17; N,
67.85; H, 4.30; N, 9.14. H NMR (CDCl3, 400 MHz): δ =
8.83−8.91 (m, 8H, pyrrole-H), 8.13−8.15 (m, 8H, 5, 10,
15, 20 Ar-o-H), 7.83−7.85 (m, 2H, 5 Ar-m-H), 7.75−7.78
(m, 6H, 10, 15, 20 Ar-m-H), 5.76 (d, 1H, CO–NH), 4.70 (s,
1H, α H), 4.37 (m, 1H, β H), 3.36 (d, 1H, ArNH), 1.60 (s,
9H, C(CH3)3), 1.39 (d, 3H, CH3), −2.82 ppm (s, 2H, pyr-
role NH).
Porphyrin 3c: The method used was the same as that
described for 3a. Yield 65.5%. Anal. Calcd. for
C56H52O7N6: C, 73.04; H, 5.65; N, 9.13. Found: C,
1
73.28; H, 5.25; N, 8.74. H NMR (CDCl3, 400 MHz): δ =
8.89−8.91 (m, 8H, pyrrole-H), 8.11−8.19 (m, 8H, 5, 10,
15, 20 Ar-o-H), 7.93−7.95 (m, 2H, 5 Ar-m-H), 7.28−7.32
(m, 6H, 10, 15, 20 Ar-m-H), 4.68 (s, 1H, α H), 5.81 (d, 1H,
CO–NH), 4.39 (m, 1H, β H), 4.11 (s, 9H, OCH3), 3.50 (d,
1H, ArNH), 1.60 (s, 9H, C(CH3)3), 1.44 (d, 3H, CH3),
−2.74 ppm (s, 2H, pyrrole NH).
1
11.44. H NMR (CDCl3, 400 MHz): δ = 8.83−8.95 (m, 8H,
pyrrole-H), 8.20−8.23 (m, 6H, 10, 15, 20 Ar-o-H), 8.00 (d,
2H, 5 Ar-o-H), 7.75−7.77 (m, 9H, 10, 15, 20 Ar-m-H and p-
H), 7.07 (d, 2H, 5 Ar-m-H), 4.04 (s, 2H, ArNH2), −2.76
ppm (s, 2H, pyrrole NH).
Porphyrin 4a: An excess of Zn(OAc)2 ꢁ 2H2O (20 mg,
0.11 mmol) was added to a solution of porphyrin 3a (100
mg, 0.074 mmol) in CH2Cl2/MeOH (20 mL, 10:1) and the
mixture was heated under reflux with exclusion of light for
1 h. Upon completion of the reaction, the solution was washed
withH2OanddriedoverNa2SO4. Themixture wasfiltered, the
filtrate was concentrated to dryness, and the remaining mate-
rial was purified on a column of silica gel (CHCl3/Et2O, 20:1,
as eluent) to give the product as a red amorphous solid. Yield
89.3%. Anal. Calcd. for C53H44O4N6Zn: C, 71.22; H, 4.93; N,
Porphyrin 2b: The method used was the same as that
described for 2a. Yield 21.2%. Anal. Calcd. for
C44H28Cl3N5: C, 72.08; H, 3.82; N, 9.56. Found: C,
1
71.88; H, 4.18; N, 9.14. H NMR (CDCl3, 400 MHz): δ =
8.83−8.87 (m, 8H, pyrrole-H), 8.16 (d, 6H, 10, 15, 20 Ar-o-
H), 8.00 (d, 2H, 5 Ar-o-H), 7.75−7.78 (m, 6H, 10, 15,
20 Ar-m-H), 7.06 (d, 2H, 5 Ar-m-H), 4.01 (s, 2H, ArNH2),
−2.78 ppm (s, 2H, pyrrole NH).
Porphyrin 2c: The method used was the same as that
described for 2a. Yield 15.3%. Anal. Calcd. for
C47H37O3N5: C, 78.44; H, 5.15; N, 9.74. Found: C,
1
9.41. Found: C, 71.28; H, 5.30; N, 9.14. H NMR (CDCl3,
400 MHz): δ = 8.97−8.99 (m, 8H, pyrrole H), 8.20−8.25 (m,
8H, 5, 10, 15, 20 Ar-o-H), 7.81−7.91 (d, 2H, 5 Ar-m-H),
7.54−7.79 (m, 9H, 10, 15, 20 Ar-m-H and p-H), 5.74 (d,
1H, CO–NH), 4.63 (s, 1H, α H), 4.30 (d, 1H, β H), 3.31 (s,
1H, ArNH), 1.58 (s, 9H, C(CH3)3), 1.48 ppm (d, 3H, CH3).
Porphyrin 4b: The method used was the same as that
described for 4a. Yield 92.5%. Anal. Calcd. for
C53H41O4N6Cl3Zn: C, 63.82; H, 4.11; N, 8.43. Found: C,
1
78.01; H, 5.48; N, 9.52. H NMR (CDCl3, 400 MHz): δ =
8.85−8.99 (m, 8H, pyrrole-H), 8.13−8.16 (m, 6H, 10, 15,
20 Ar-o-H), 8.02 (d, 2H, 5 Ar-o-H), 7.28−7.32 (m, 6H,
10, 15, 20 Ar-m-H), 7.07 (d, 2H, 5 Ar-m-H), 4.09 (s, 9H,
OCH3), 4.07 (s, 2H, ArNH2), −2.70 ppm (s, 2H, pyrrole NH).
Porphyrin 3a: An excess of Boc-L-threonine (20 mg, 0.11
mmol) was added to a solution of porphyrin 2a (100 mg,
0.074 mmol) in CH2Cl2 (60 mL) and the mixture was stirred
in an ice bath for 10 min. N,N-Dicyclohexylcarbodiimide
(DCC) was added dropwise over a period of 20 min, and the
mixture was stirred for a further 12 h. The ice bath was then
removed, and the mixture was washed with 10% aqueous
Na2CO3 solution and water, and dried over Na2SO4. After
evaporation of the solvent, the resulting solid was redissolved
1
63.97; H, 4.44; N, 8.01. H NMR (CDCl3, 400 MHz): δ =
8.94−8.97 (m, 8H, pyrrole-H), 8.15 (s, 8H, 5, 10, 15, 20 Ar-
o-H), 7.76−7.86 (m, 8H, 5, 10, 15, 20 Ar-m-H), 5.63 (d,
1H, CONH), 4.44 (s, 1H, α H), 4.24 (m, 1H, β H), 3.20 (d,
1H, ArNH), 1.54 (s, 9H, C(CH3)3), 1.39 ppm (d, 3H, CH3).
Porphyrin 4c: The method used was the same as that
described for 4a. Yield 87.4%. Anal. Calcd. for
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C56H50O7N6Zn: C, 68.36; H, 5.09; N, 8.55. Found: C,
68.61; H, 5.54; N, 8.18. H NMR (CDCl3, 400 MHz): δ =
8.85−9.04 (m, 8H, pyrrole-H), 8.11−8.19 (m, 8H, 5, 10,
15, 20 Ar-o-H), 7.84−7.86 (d, 2H, 5 Ar-m-H), 7.28−7.32
(m, 6H, 10, 15, 20 Ar-m-H), 5.61 (s, 1H, CONH), 5.05 (m,
1H, α H), 4.45 (m, 1H, β H), 4.11 (s, 9H, OCH3), 3.39 (d,
1H, ArNH), 1.49 (s, 9H, C(CH3)3), 1.30 ppm (d, 3H, CH3).
and c are the UV/vis spectra of porphyrins 4a, 4b, and 4c,
respectively. The three kinds of zinc porphyrins each showed
twoQbands andoneSoret band. Compared to thoseof thecor-
responding free porphyrins, the Soret band and the Q bands of
the zinc porphyrins were somewhat shifted. (3) As shown in
Table 1, the value of molar extinction coefficient ε for 3a is
exceptionally large (~8 × 105 L/(M/cm)) compared to other
free porphyrins. Also, the value of ε in Soret band for 3a is lar-
ger (about twofold) than that of 3b and 3c, but the Q band is
smaller than that of 3c. For three zinc porphyrins, the value
of ε in Soret band for 4a is the highest, and the value of ε in
Q band for 4c is the smallest.
1
Results and Discussion
UV/vis Spectra. Figures 1 and 2 show the UV/vis spectra of
the free base porphyrins and zinc porphyrins in CH2Cl2, and
the data are listed in Table 1. Salient features can be summar-
ized as follows: (1) In Figure 1, traces A and a, B and b, and C
and c show the UV/vis spectra of porphyrins 3a, 3b, and 3c,
respectively. The three kinds of free-base porphyrins each
showed four Q bands and one Soret band. The Soret band
was at ~420 nm, and the Q bands were in the region 500
−700 nm. (2) In Figure 2, traces A and a, B and b, and C
It has been shown that the classic Goutermans four-orbital
model can explain the absorption properties of porphyrins.31
The UV/vis absorption bands of porphyrins are due totheelec-
tronic transitions from the ground state (S0) to the two lowest
singlet excited states S1 and S2. The S0 ! S1 transition gives
rise tothe weak Q bands invisibleregion, whereas the S0 ! S2
transition produces the strong Soret band in the near-UV
region. After metal ion incorporation into the porphyrin, the
number of Q bands decreases, and the absorption frequencies
are shifted due to the change in molecular symmetry.
2.0
A
1
1H NMR Spectra. H NMR spectra of porphyrins 2a−2c, 3a
−3c, and 4a−4c were measured in CDCl3 solution. The data
are listed in the synthesis section. Salient features can be sum-
marized as follows: (1) The 1H chemical shifts of NH2 in por-
phyrins 2a, 2b, and 2c are δ = 4.04, 4.01, and 4.07 ppm. In
porphyrins 3a, 3b, and 3c, due to the impact of the amino acid,
the 1H chemical shifts are δ = 3.38, 3.36, and 3.50 ppm, and
the amide protons of the Boc-amino acid give rise to signals
at δ = 5.79, 5.76, and 5.81 ppm. This confirmed the reaction
between the Boc-amino acid and porphyrins 2a, 2b, and 2c
1.5
C
1.0
c
a
0.5
0.0
b
B
1
to afford free porphyrin compounds. (2) The H chemical
400
450
500
550
600
650
700
750
shifts of the two protons inside the porphyrin ring in 3a,
3b, and 3c are −2.75, −2.82, and −2.74 ppm, respectively.
In porphyrins 4a, 4b, and 4c, these resonances are no
longer seen, confirming the formation of the zinc porphyrin
complexes.
wavelength ( nm)
Figure 1. UV–vis spectra of free base porphyrins. Soret band: 3a
(trace A); 3b (trace B); 3c (trace C). Q band: 3a (trace a); 3b (trace
b); 3c (trace c).
Isosbestic PointDiagram. Titrationsof porphyrin 4a with Im
and N-MeIm were studied by UV/vis spectrophotometry.
Inspection of Figure 3(a) reveals that, upon the addition of
Im to a solution of porphyrin 4a, the absorption intensity of
4a gradually diminished, while that of the product gradually
increased. The original Soret band of porphyrin 4a was red-
shifted from 419.8 to 429.5 nm. A clear isosbestic point in
the UV/vis spectra of the porphyrin 4a–Im system was
observed at 425.2 nm. Similar spectral changes were observed
for the porphyrin 4a–N-MeIm (Figure 3(b)), 4b–Im and 4b–
N-MeIm, as well as 4c–Im and 4c–N-MeIm systems.
The observed spectral changes, involving the consumption
of porphyrin 4a and the formation of isosbestic points, are
characteristic of axial coordination to a zinc porphyrin, con-
firming that the zinc porphyrins reacted with Im and N-MeIm.
Im and N-MeIm are strongly nucleophilic small molecules.
When coordinating to zinc porphyrins, they changed the elec-
tron density in the porphyrin ring.32–34 The position of
A
0.75
B
0.60
b
C
0.45
a
0.30
c
0.15
0.00
400
450
500
550
600
650
700
750
wavelength (nm)
Figure 2. UV–vis spectra of zinc porphyrins. Soret band: 4a
(trace A); 4b(trace B); 4c(trace C). Q band: 4a (trace a); 4b (trace
b); 4c (trace c).
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Zinc Porphyrin Coordination Reaction
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Table 1. Data of UV–vis spectra.
Q band, nm (ε × 10−4 L/(M/cm))
Porphyrins
Soret band, nm (ε × 10−5 L/(M/cm))
QI
QII
QIII
QIV
3a
3b
3c
4a
4b
4c
419.2(8.01)
419.2(4.83)
424.2(4.92)
419.8(3.25)
420.2(2.96)
425.9(1.96)
515.8(2.46)
516.0(1.78)
518.6(3.30)
551.4(1.14)
551.2(1.17)
555.2(1.63)
547.8(1.93)
548.6(1.98)
560.1(0.74)
590.8(0.74)
591.7(0.59)
593.8(1.02)
585.4(0.33)
586.9(0.40)
596.0(0.22)
646.4(0.91)
648.6(0.66)
651.2(0.91)
1.0
(b)
(a) 1.0
a
a
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
m
q
380
400
420
440
460
380
400
420
440
460
wavelength (nm)
wavelength (nm)
Figure 3. The isosbestic points of systems. (A) 4a–Im system at 5 ꢀC (a: 4a, 2.5 μM; b–m: cIm/c4a = 0.2, 1, 2, 3, 4, 5, 6, 8, 15, 20, 40, 50). (B) 4a–
N-MeIm system at 10 ꢀC (a: 4a, 2.5 μM; b-q: cN-MeIm/c4a = 0.4, 0.6, 0.8, 1, 1.4, 1.6, 2, 3, 4, 5, 6, 8, 10, 15, 30, 50, 300).
Table 2. Value n of the coordination reaction.
the maximum absorption peak in the axial coordination
system is not changed with increasing concentration of Im
or N-MeIm, illustrating that the coordination number of the
system remains unchanged.
n
Imidazole
Porphyrins derivatives 5 ꢀC 10 ꢀC 15 ꢀC 20 ꢀC naverage
4a
4b
4c
Im
N-MeIm
Im
N-MeIm
Im
1.04 1.03
1.12 1.11
1.09 1.08
1.09 1.07
1.00 0.99
0.99 0.99
1.02
1.09
1.09
1.07
0.96
0.99
1.03
1.08
1.06
1.06
0.97
1.00
1.03
1.10
1.08
1.08
0.98
0.99
The Value n and Association Constant Kθ. The value n and
association constant Kθ of a coordination system are given by
the equation35,36 ln[(A0 − Ae)/(Ae − A∞)] = n ln cL + ln Kθ. In
the present system, A0 is the absorbance of the zinc porphyrin
solution, Ae is the equilibrium absorbance of zinc porphyrin–
imidazole system, and A∞ is the equilibrium absorbance of the
zinc porphyrin–imidazole system after complete conversion
of the reactants to the complex. Here, n is a constant indicating
the ratio of imidazole derivative to zinc porphyrin in the sys-
tem, and Kθ is the association constant [Kθ = K/(mol/L)].
The values of n and the association constants Kθ for the
coordination systems are summarized in Tables 2 and 3,
respectively. Plots of ln[(A0−Ae)/(Ae−A∞)] vs. ln cL con-
structed from the experimental results showed good linear cor-
relations (r > 0.995). As shown in Table 2, the values of n were
all close to 1 at various temperatures, indicating the formation
of 1:1 complexes between porphyrins 4a, 4b, and 4c and Im or
N-MeIm. Zn(II) has a d10 electron configuration, and accord-
ing to ligand field theory generally prefers to reside in a tetra-
hedral field. When a numberof ligands or ions interact with the
zinc porphyrin, the zinc ion is slightly displaced fromthe plane
N-MeIm
of the porphyrin, and its increased electronegativity allows the
binding of a fifth ligand. That is to say, besides the four N
atoms of the porphyrin ring, the Zn2+ is also coordinated by
an imidazole derivative, resulting in a five-coordinate system.
From Table 3, we can state the following: (1) For the same
porphyrin and the same imidazole derivative, the association
constants Kθ decreased with increasing temperature, illustrat-
ing that the coordination reaction was spontaneous. (2) For the
same porphyrin and different imidazole derivatives, the asso-
ciation constants for axial coordination decreased in the order
K(N-MeIm) > K(Im). The superior coordinating ability of N-
MeIm can be attributed to increased electron density on the
coordinating nitrogen atom due to the electron-donating
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meta-methyl substituent. (3) For the different porphyrins with
the same imidazole derivative, the system porphyrin 4b–N-
MeIm showed the highest association constant. This is
because the electron-withdrawing chloro substituents in 4b
increased the electropositivity of the zinc ion in the porphyrin
plane, therebyenhancingthecoordination ability. Conversely,
the electron-donating methoxy group in 4c decreased the elec-
tropositivity of the zinc ion in the porphyrin plane, thereby
weakening the coordination ability of 4c.
was negative, indicating a decrease in the degree of freedom
of the components upon complexation, which could be
ascribed to the restriction of translational, rotational, and inter-
nal rotational motions upon binding.39 (4) The most negative
values of ΔrGmθ and ΔrHmθ for the 4b–N-MeIm system indi-
cated the strongest interactions.
Theoretical Calculations. The properties of these molecules
mainly depend on their geometrical structures. Consequently,
it was very important to determine the structures of the sys-
tems. The optimized geometries of systems 4a–Im and 4a–
N-MeIm are given in Figure 4. Table 5 lists the total energies,
the Mulliken charges ontheZn atoms, theZn–Nbondlengths,
and bond energies for each of the systems.
We consider first 4a, 4b, and 4c. Compared to the charge on
Zn2+ in the structure of 4a (1.071 e), the charge on Zn2+ in 4b
waslarger(1.074e),whereasthatontheZn2+ in4cwassmaller
(1.068 e). The results indicated that the Cl substituents
increased the charge on Zn2+, whereas the methoxy substitu-
ents decreased the charge on Zn2+.
When Im was bound to the zinc porphyrins (4a, 4b, and 4c),
the charges on Zn2+ were reduced to 1.041, 1.042, and 1.039 e,
respectively. The bond lengths for 4a–Im, 4b–Im, and 4c–Im
were determined as 2.242, 2.238, and 2.244 Å, respectively,
and the corresponding bond energies were 62.2, 65.3, and
61.1 kJ/mol. The results indicated that the Cl substituents
increased the bond energy, such that the zinc porphyrin–Im
complex was more tightly bound. However, the methoxy
ThermodynamicParameters. Inordertogainfurtherinsight
into the interaction mechanism, we determined the thermody-
namic parameters for the present coordination reactions
according
to
the
van’t
Hoff
equation37,38
lnKθ = −ΔrHmθ =RT + ΔrSmθ =R, where R is the universal gas
constant and T is the temperature in degrees kelvin. Accord-
ingly, the enthalpy and entropy changes associated with these
reactions were obtained from the slope and intercept, respec-
tively, of linear plots of lnKθ versus T−1. The Gibbs free ener-
gies ΔrGmθ were calculated according to the equation
ΔrGmθ = ΔrHmθ −TΔrSmθ . The values of ΔrGmθ , ΔrHmθ , and
ΔrSmθ are listed in Table 4. From Table 4, the following conclu-
sions can be drawn: (1) The ΔrGmθ values showed that the com-
plex formation was spontaneous. (2) The magnitude of
−ΔrHmθ was larger than that of −ΔrSmθ for each system, indicat-
ing that the enthalpy change was the main driving force of the
coordination process. (3) The value of ΔrSmθ for each system
Table 3. Association constants Kθ of the coordination reaction.
Kθ
Porphyrins
Imidazole derivatives
Im
5 ꢀC
10 ꢀC
15 ꢀC
20 ꢀC
4a
(1.26 ꢂ 0.03) × 105
(9.21 ꢂ 0.02) × 104
(6.68 ꢂ 0.01) × 104
(4.74 ꢂ 0.04) × 104
r = 0.9992
r = 0.9988
r = 0.9995
r = 0.9989
N-MeIm
Im
(5.46 ꢂ 0.01) × 105
r = 0.9991
(3.43 ꢂ 0.03) × 105
r = 0.9989
(2.28 ꢂ 0.03) × 105
r = 0.9993
(1.59 ꢂ 0.02) × 105
r = 0.9996
4b
4c
(2.43 ꢂ 0.02) × 105
(1.64 ꢂ 0.04) × 105
(1.12 ꢂ 0.04) × 105
(7.62 ꢂ 0.03) × 104
r = 0.9989
r = 0.9995
r = 0.9991
r = 0.9991
N-MeIm
Im
(1.01 ꢂ 0.01) × 106
r = 0.9997
(6.38 ꢂ 0.02) × 105
r = 0.9992
(3.65 ꢂ 0.04) × 105
r = 0.9998
(2.37 ꢂ 0.04) × 105
r = 0.9998
(1.04 ꢂ 0.03) × 105
(7.92 ꢂ 0.01) × 104
(6.15 ꢂ 0.04) × 104
(4.59 ꢂ 0.01) × 104
r = 0.9988
r = 0.9992
r = 0.9993
r = 0.9992
N-MeIm
(2.22 ꢂ 0.04) × 105
r = 0.9990
(1.67 ꢂ 0.03) × 105
r = 0.9992
(1.23 ꢂ 0.03) × 105
r = 0.9992
(9.30 ꢂ 0.04) × 104
r = 0.9990
r is the linear correlation coefficient.
Table 4. Thermodynamic data of the coordination reactions.
Porphyrins
Imidazole derivatives
ΔrHmθ (kJ/mol)
ΔrSmθ (J/(mol/K)
ΔrGmθ (kJ/mol)
r
4a
Im
N-MeIm
Im
N-MeIm
Im
N-MeIm
−44.12
−55.72
−50.73
−66.43
−36.82
−39.56
−60.86
−90.62
−79.32
−123.80
−36.25
−39.82
−25.97
−28.70
−27.08
−29.52
−26.01
−27.69
0.9993
0.9991
0.9995
0.9990
0.9993
0.9999
4b
4c
r is the linear correlation coefficient.
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Zinc Porphyrin Coordination Reaction
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(a)
(b)
Figure 4. The structures of system-optimized geometries. (a) 4a–Im and (b) 4a–N-MeIm).
Table 5. Quantum chemistry results of system.
140
120
100
80
Combination
ChargeZn
(e)
energy
(kJ/mol)
Bond length
(Å) (Zn–N)
a
Porphyrins
4a
4b
4c
4a-Im
1.071
1.074
1.068
1.041
1.042
1.039
1.040
1.041
1.038
60
62.2
2.242
2.238
2.244
2.235
2.231
2.237
4b-Im
65.3
61.1
64.2
67.6
63.0
40
k
4c–Im
4a–N-MeIm
4b–N-MeIm
4c–N-MeIm
20
i
0
500
550
600
650
700
750
wavelength (nm)
Figure 5. Fluorescence spectra of the system 4a–N-MeIm. (a: 4a,
substituents decreased the bond energy, leading to a less
tightly bound complex, in agreement with the experimental
results.
2.5 μM; b–k: cN-MeIm 4a
/
= 2, 4, 6, 10, 15, 20, 30, 40, 100, 300; i:
N-MeIm, 7.50 × 10−4 M).
The combination energies for 4a–N-MeIm (64.2 kJ/mol),
4b–N-MeIm (67.6 kJ/mol), and 4c–N-MeIm (63.0 kJ/mol)
were calculated to be larger than those for 4a–Im, 4b-Im
and 4c–Im by ~2.0 kJ/mol. The Zn–N bond lengths for 4a–
N-MeIm (2.235 Å), 4b–N-MeIm (2.231 Å), and 4c–N-MeIm
(2.237 Å) were calculated to be shorter than those for 4a–Im,
4b–Im, and 4c–Im by about 0.07 Å. The results supported the
conclusion that the bonding between the zinc porphyrins and
N-MeIm was stronger than that between the zinc porphyrins
and Im.
Fluorescence Spectra. The coordination reactions between
zinc porphyrins and imidazole derivatives were also studied
on the basis of fluorescence measurements at room tempera-
ture. Upon excitation of the Soret band of zinc porphyrins at
420 nm, the fluorescence emission spectra in CH2Cl2 are
shown in Figures 5–7.40
There appears fluorescence of the S2 (Soret band) and the S1
(Q bands) in the porphyrin complexes. The fluorescence
of Soret band is attributed to the transition from the
second excited state S2 to the ground state S0, which is
much weaker than for the S1 ! S0 transition of the Q band
emission. And then, the S1 state, whose fluorescence lifetime
is the nanosecond scale, is in a dominant position in the pho-
tochemical reaction of porphyrin molecules.41–43 For 4a, 4b,
and 4c, the fluorescence of S1 consisted of two bands. From
inspection of Figures 5−7, the following conclusions can be
drawn:
a
60
40
20
0
k
500
550
600
650
700
750
wavelength (nm)
Figure 6. Fluorescence spectra of the system 4b–N-MeIm. (a: 4b,
2.5 μM; b–k: cN-MeIm/c4b = 1.5, 2.4, 3, 3.4, 5, 5.4, 7, 8,8.4, 10).
1. The characteristic peaks of the S1 states of 4a, 4b, and 4c
are 597.0 and 644.8 nm, 599.1 and 645.9 nm, 603.2 and
649.8 nm, respectively. Compared to those of 4a, the
emission peaks of 4b and 4c show bathochromic shift.
This indicates that the porphyrin macrocycle conjuga-
tion is affected by Cl and methoxy substituents.
2. The characteristic fluorescence peak of N-MeIm is at
652.7 nm (line i in Figure 5) and the fluorescence inten-
sity is very low. When different concentrations of N-
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a
4.0
3.5
3.0
2.5
2.0
1.5
1.0
20
a
15
10
5
b
c
d
e
k
f
0
500
550
600
650
700
750
0.00000
0.00002
0.00004
0.00006
0.00008
wavelength (nm)
[imidazole derivatives] (M)
Figure 7. Fluorescence spectra of the system 4c–N-MeIm. (a: 4c,
2.5 μM; b–k: cN-MeIm/c4c = 0.8, 2, 3, 5, 6, 7, 10, 15, 20, 30).
Figure 8. The Stern–Volmer curves. (a: 4c–N-MeIm; b: 4b–N-
MeIm; c: 4c–Im; d: 4a–N-MeIm; e: 4b–Im; f: 4a–Im).
MeIm were added to a solution of zinc porphyrin of a
certain concentration, the fluorescence spectrum of
the mixed system changed considerably, which was
not a simple superposition of the spectra of the zinc por-
phyrin and N-MeIm. This indicated that the configura-
tion of the complex was changed after the coordination
of N-MeIm, with the symmetry of the molecule chan-
ging fromtheoriginal D4h toC4v. Theluminescent prop-
erties of the zinc porphyrin were thus changed, as borne
out by the UV spectroscopic results.
Table 6. Quenching rate constant Kq at room temperature.
Systems
F0/F – [Q]
Kq (L/(mol/s))
r
4a–N-MeIm
1.0101 + 18 414[Q]
(1.84 ꢂ
0.9992
0.001)×1012
(1.11 ꢂ
4a–Im
1.0311 + 11 094[Q]
0.9701 + 22 836[Q]
0.9729 + 16 676[Q]
1.0265 + 36 555[Q]
1.0045 + 18 920[Q]
0.9997
0.9994
0.9991
0.9990
0.9992
0.001)×1012
(2.28 ꢂ
4b–N-MeIm
4b–Im
0.004)×1012
(1.67 ꢂ
3. With increasing concentration of N-MeIm, the emission
intensity of 4a, 4b, and 4c decreased, which may be due
to efficient photoinduced electron transfer between zinc
porphyrin and N-MeIm. When cN-MeIm/c4a, cN-MeIm/c4b,
0.003)×1012
(3.66 ꢂ
4c–N-MeIm
4c–Im
0.005)×1012
(1.89 + 0.002)
×1012
c
N-MeIm/c4c is 300, 10, 30, respectively, the intensity of
the spectra was almost unchanged, indicating that fluo-
rescence quenching was induced by the complex.42,44
This was clearly the consequence of the interaction
between zinc porphyrin and imidazole derivative. The
imidazole N atom bore lone pair electrons before the
coordination reaction. On one hand, this N atom formed
a σ-bond with Zn2+ upon coordination, while, on the
other, the Zn2+ ion engaged in π-backbonding with
the N atom through its d electrons, the π-backbonding
changed the electron density of the porphyrin ring,
and the distribution of the electron cloud was more une-
ven over the whole conjugation system. The symmetry
ofthemoleculewastherebyreduced, whichchangedthe
vibrational mode of the S1 state. Thus, nonradiative
transition from Sl to Tl was strengthened, and the fluo-
rescence intensity of the complex decreased. That is to
say, fluorescence quenching occurred between the zinc
porphyrins and the imidazole derivatives. Similar
changes in the fluorescence spectra were observed for
the various zinc porphyrins with Im.
r is the linear correlation coefficient.
are listed in Table 6. The plots revealed a certain degree
of slope, indicating the occurrence of efficient quench-
ing between the zinc porphyrin and imidazole
derivatives.
As shown in Table 6, the values of the quenching rate con-
stants Kq between the various zinc porphyrins and imidazole
derivatives were all much larger than 2.0 × 1010 L/(mol/s) (the
constant of maximum diffusion collision quenching rate),
which was caused by formation of the complex before excita-
tion occurred. The 4c–N-MeIm system showed the highest
quenching rate constant Kq. In other words, the chemical reac-
tion between zinc porphyrins and imidazole derivatives at dif-
ferent concentrations led to a quenching effect.
4. In order to qualitatively explore the quenching mechan-
ism, we obtained the Stern−Volmer quenching
constant Ksv and the quenching rate constant Kq accord-
Conclusion
ing to the equation F0/F = 1 + Kqτ0[Q] = 1 + Ksv[Q].45
A
Porphyrins and threonine have important biological functions.
In order to combine their biological activities, we have
synthesized three kinds of free porphyrins modified with
linear relationship was obtained by plotting and fitting
F0/F against [Q], as shown in Figure 8. The values
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Zinc Porphyrin Coordination Reaction
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