Synthesis and characterization of novel porphyrin-cinnamic acid conjugates

Article abstract of DOI:10.1016/j.saa.2019.117314

A series of novel porphyrin-cinnamic acid (porphyrin/CA) conjugates (4a-4c) have been synthesized by condensation of 5-(4-Hydroxyphenyl)-10,15,20-triphenyl- porphyrin (1) with different substituted cinnamic acids (CAs) through an alkyl linker due to the biological activities of CAs and the application of porphyrins in photodynamic therapy (PDT) of cancer. Their related zinc (II) complexes (5a-5c) were also prepared. These novel compounds have been fully characterized by ¹H NMR, Infrared (IR), Mass spectra (Ms) and Elemental analysis. The photophysical properties of these target molecules were studied by absorption and Fluorescence spectroscopy. In solution, the effect of pH, ionic strength in acid media and concentration on the aggregation behaviors of 4a has also been investigated by UV–Vis spectra. The broadened and red shifted Soret band indicated the formation of J-aggregates when the pH value was up to 2.0 in THF-aqueous solution. Furthermore, the higher ionic strength of 0.3 M NaCl in acid media resulted in the generation of J-aggregates in THF-aqueous solution. And the significant blue shift of Soret band also demonstrated the formation of H-aggregates of 4a at 1.1 × 10^?4 M in THF.

Full text of DOI:10.1016/j.saa.2019.117314

Accepted Manuscript
Synthesis and characterization of novel porphyrin-cinnamic acid
conjugates
Baojuan Zhao, Minzhen Wang, Xiang Wang, Pengfei Yu, Na
Wang, Fengjuan Li
PII:
S1386-1425(19)30704-8
DOI:
https://doi.org/10.1016/j.saa.2019.117314
Article
Number:
Reference:
117314
SAA 117314
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date:
Revised date:
Accepted date:
6 December 2018
15 June 2019
23 June 2019
Please cite this article as: B. Zhao, M. Wang, X. Wang, et al., Synthesis and
characterization of novel porphyrin-cinnamic acid conjugates, Spectrochimica Acta Part
A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117314
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ACCEPTED MANUSCRIPT
Synthesis and characterization of novel porphyrin-cinnamic acid
conjugates
Baojuan Zhao, ab Minzhen Wang,a Xiang Wang*a , Pengfei Yu,a Na Wang,c Fengjuan Lia
aSchool of Chemistry and Chemical Engineering, Tianjin Polytechnic University,
Tianjin, 300387, P. R. China
bTianjin Key Laboratory of Green Chemical Technology and Process Engineering,
Tianjin Polytechnic University, Tianjin, 300387, P. R. China
cTechnology R&D Center, China Tobacco Hubei Industrial Corporation, Wuhan,
430040, P. R. China
Corresponding address:
Xiang Wang, Associate Professor,
School of Chemistry and Chemical Engineering,
Tianjin Polytechnic University,
Tianjin, 300387, P. R. China
Tel: +86-22-83955966
Fax: +86-22-83955451
E-mail: tpuwx2008@163.com
1

ACCEPTED MANUSCRIPT
Abstract
A series of novel porphyrin-cinnamic acid (porphyrin/CA) conjugates (4a-4c) have
been synthesized by condensation of 5-(4-Hydroxyphenyl)-10,15,20-triphenyl-
porphyrin (1) with different substituted cinnamic acids (CAs) through an alkyl linker
due to the biological activities of CAs and the application of porphyrins in
photodynamic therapy (PDT) of cancer. Their related Zinc (II) complexes (5a-5c)
1
were also prepared. These novel compounds have been fully characterized by H
NMR, Infrared (IR), Mass spectra（Ms）and Elemental analysis. The photophysical
properties of these target molecules were studied by absorption and Fluorescence
spectroscopy. In solution，the effect of pH, ionic strength in acid media and
concentration on the aggregation behaviors of 4a has also been investigated by
UV-Vis spectra. The broadened and red shifted Soret band indicated the formation of
J-aggregates when the pH value was up to 2.0 in THF-aqueous solution. Furthermore,
the higher ionic strength of 0.3 M NaCl in acid media resulted in the generation of
J-aggregates in THF-aqueous solution. And the significant blue shift of Soret band
also demonstrated the formation of H-aggregates of 4a at 1.1×10-4 M in THF.
Keywords: Porphyrins; Cinnamic acids; Synthesis; Optical properties; Aggregation
2

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1. Introduction
One of the most important applications of porphyrin derivatives in biological
system is photodynamic therapy (PDT) for either cancer or microbial pathogens. The
common and effective way to acquire new porphyrin derivatives is to couple
porphyrins with various functional molecules [1-3]. By this means, many biological
porphyrin derivatives with improving efficiency and selectivity of photodynamic
treatment have been achieved. For example, glycoporphyrins have not only better
solubility in aqueous environment but also improved targeting [4]. In order to increase
tumoral targeting ability of porphyrins, folic acid targeted tetraphenylporphyrin have
been prepared [2]. Other related reports, such as a potential antibiotic of the
porphyrin-vancomycin conjugate for drug-resistant Gram-positive pathogens [1], a
potential PDT photosensitizer of porphyrin-profens derivative [5] and potential cancer
diagnosis agent of the chalcone-porphyrin conjugates [6], also specified similar
promising porphyrin derivatives. In addition, some reports pointed out that cinnamic
acid derivatives (CAs) had diverse biological activities, including antitumor[7, 8],
antimicrobial [9], antioxidant [10], antifungal [11] and so on. So far, porphyrin
3

ACCEPTED MANUSCRIPT
derivatives modified with CAs at their periphery have remained rather unexplored.
Therefore, inspired by the reports above, we initiated the research on porphyrin/CA
conjugates which might have the potential application in the field of medicine.
On
the
other
hand,
aggregation
behaviors
of
porphyrin
have received widespread attention in the porphyrin chemistry and their emergence
has a significant effect on the physicochemical properties of these molecules [12,13].
It even affects the efficacy of porphyrins in applications, for example, in the field of
medicine [14]. In recent years, much effort has been focused on the formation of
J-aggregates for their promising applications such as pH sensors [15,16], PDT [17],
nanophotonics [18,19].
In this work, we firstly report a series of novel unsymmetrical porphyrins bearing
CAs at the periphery and the investigation of their photophysical properties by
absorption and fluorescence spectroscopy. On the other hand, the influence of pH,
ionic strength in acidic media and concentration on aggregation behaviors of
porphyrin/CA conjugate 4a has also been investigated.
2. Experimental section
2.1. General procedures
1
The H NMR was recorded on a Bruker Avance 400/600 spectrometer with the
indicated solvents operating at 400.15 MHz or 600.62 MHz, respectively. Chemical
shifts (δ) are expressed in parts per million (ppm) and coupling constants (J) in Hertz
(Hz) using residual solvent peaks as internal standards. Mass spectra were recorded on
4

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a Finnigan-LCQ DECA XP MAXspectrometer. Infrared spectra (IR) were recorded
on a PerkineElmer SP2000 FT-IR spectrometer, using KBr disks technique. Elemental
analyses were performed by Elementar Vario EL CUBE (in Institution State Key Lab
Element Organic Chemistry, Nankai University, Tianjin, China).
Electronic absorption spectra were recorded on a Shimadzu UV-2450 spectrometer
using quartz cells with a width of 1 cm, employing THF and DMF as solvent.
Steady-state fluorescence spectra were recorded on a Hitachi F-7000 fluorescence
spectrophotometer with a Xe-900 lamp and the emission slit widths were of 5 nm.
Fluorescence quantum yields were determined as described in the previous reports
[20].These experiments were conducted at room temperature in a dark room to avoid
any impact of ambient light over the main light source.
2.2 Synthesis and purification
All reagents used in the synthesis were purchased from Sigma-Aldrich and used
without further purification. For anhydrous reactions, the solvents were dried
according to stated methods [21]. The dried solvents were then stored in flasks
containing molecular sieves 4Å under nitrogen atmosphere. Yields refer to
chromatographically pure compounds. 5-(4-Hydroxyphenyl)-10,15,20-triphenyl
-porphyrin (TPP-OH) (1) [22] and 5-(4-(2-Bromoethoxy)phenyl)-10,15,20-triphenyl
porphyrin (H2BTPP) (2) (SI-Fig. 1) [23, 24] are known compounds and their
spectroscopic data correspond to those reported previously.
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2.2.1 Synthesis of 3-phenyl-acrylic acid 2-[4-(10,15,20-triphenyl-porphyrin-5-yl)
-phenoxy]-ethyl ester (4a).
To a solution of cinnamic acid in distilled water was added dropwise a solution of
sodium hydroxide (1.0 mol/L) in distilled water to adjust pH to 8-9. Then the resulting
mixture was concentrated under vacuum and the crude product of sodium cinnamate
(3a) was used directly without further purification. A mixture of sodium cinnamate
(3a) (38.0 mg, 0.223 mmol) and 5-(4-(2-bromoethoxy)phenyl)-10,15,20-triphenyl
porphyrin (2) (150.0 mg, 0.203 mmol) in dry DMF (15 mL )was stirred at 75oC for 24
h. The reaction was monitored by thin layer chromatogrph (TLC) until it was
completed. After completing the reaction, the unreacted solid salt was separated by
filtering and the solvent was removed under vacuum, then the residue was dissolved
in CHCl3 and washed with H2O. Finally, the solvent was removed under vacuum and
the resulting residue was purified by silica gel chromatography with CH2Cl2/PE
(1:1-1:0) as mobile phase. The main purple band was collected to give the desired
product. Yield: 135.6 mg (83%); 1H NMR (600 MHz, DMSO-d6) (SI-Fig. 2) δH (ppm)
8.83 (m, 8H, β-pyrrole), 8.22 (d, J = 6.6 Hz 6H, o-phenyl), 8.13 (d, J = 7.8 Hz, 2H,
o-phenyl), 7.83 (m, 12H, o- and m-phenyl), 7.43 (m, 5H, p-phenyl and acryloyl-H),
6.82 (d, J = 15.6 Hz, 1H, acryloyl-H), 4.68(m, 2H, CH2), 4.58(m, 2H, CH2), -2.93 (s,
2H, NH-pyrrole). UV-Vis (DMF) λmax (log ε): 418.0 (5.46), 515.5 (4.05), 550.0 (3.76),
590.5 (3.52), 649.0 (3.43) nm; IR (KBr)/cm-1: 3319 (ʊN-H, pyrrole), 2924(ʊCH2
,
-CH2CH2-), 2850(ʊCH2, -CH2CH2-), 1709 (ʊC=O), 1248(ʊC-O-C), 964(δN-H, pyrrole); MS
(ESI): m/z calc. for [M+H]+, C55H41N4O3: 805.31, found: 805.49; elemental analysis
6

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calcd (%) for C55H40N4 (756.33): C 87.27, H 5.33, N 7.40; Found C 87.19, H 5.35, N
7.47.
2.2.2 Synthesis of 3-(4-chloro-phenyl)-acrylic acid 2-[4-(10,15,20-triphenyl-
porphyrin- 5-yl)-phenoxy]-ethyl ester (4b)
Sodium 3-(4-chloro-phenyl)-acrylate (3b) was prepared following a similar
procedure to that described for the synthesis of 3a. A mixture of sodium
3-(4-chloro-phenyl)-acrylate
(3b)
(45.6
mg,
0.223
mmol)
and
5-(4-(2-bromoethoxy)phenyl)-10,15,20-triphenyl porphyrin (2) (150.0 mg, 0.203
mmol) in dry DMF (15 mL ) was stirred at 75oC for 8 h. The reaction was monitored
by TLC until it was completed. After completing the reaction, the reaction mixture
was poured into the solution of ice-water/MeOH(10:1) and a purple suspension was
obtained which was further filtrated to give the purple crude product. Finally, the
resulting crude product was purified by silica gel chromatography with CH2Cl2/PE
(1:1-1:0) as mobile phase. The main purple band was collected to give the desired
product. Yield: 126.8 mg (74.5%); 1H NMR (600 MHz, DMSO-d6) (SI-Fig. 4) δ (ppm)
8.86~8.84 (m, 8H, β-pyrrole), 8.22 (d, J = 6.6 Hz, 6H, o-phenyl), 8.13 (d, J = 7.8 Hz,
2H, o-phenyl), 7.79~7.75 (m, 10H, o- and m- phenyl and acryloyl-H), 7.50 (d, Jo-m
=
8.4 Hz, 2H, m-phenyl), 7.36 (d, J = 8.4 Hz, 2H, m- and p-phenyl), 7.31 (d, Jm-p = 7.8
Hz, 2H, p-phenyl), 6.56 (d, J = 15.6 Hz, 1H, acryloyl-H), 4.74 (s, 2H, CH2), 4.53 (d, J
= 3.6, 2H, CH2), -2.78(s, 2H, NH- pyrrole). UV-Vis (DMF) λmax (log ε): 418.0 (5.10),
515.5 (3.67), 550.5 (3.36), 590.5 (3.06), 645.5 (2.94) nm; IR (KBr)/cm-1: 3236 (ʊN-H
,
7

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pyrrole), 2926(ʊCH2, -CH2CH2-), 2852(ʊCH2, -CH2CH2-), 1716(ʊC=O), 1243ʊC-O-C),
966(δN-H, pyrrole); MS (ESI): m/z calc. for [M+H]+, C55H40ClN4O3: 839.27, found:
839.47; elemental analysis calcd (%) for C55H39N4 (755.32): C 87.39, H 5.20, N 7.41;
Found C 87.42, H 5.17, N 7.42.
2.2.3 Synthesis of 3-(4-methoxy-phenyl)-acrylic acid 2-[4-(10, 15, 20-triphenyl-
porphyrin-5-yl)-phenoxy]-ethyl ester (4c)
Sodium 3-(4-methoxy phenyl) acrylate (3c) was prepared following a similar
procedure to that described for the synthesis of 3a. A mixture of sodium 3-(4-methoxy
phenyl)
acrylate
(3c)
(44.0
mg,
0.223
mmol)
and
5-(4-(2-bromoethoxy)phenyl)-10,15,20-triphenyl porphyrin (2) (150.0 mg, 0.203
mmol) in dry DMF (15 mL ) was stirred at 60oC for 5 h. The reaction was monitored
by TLC until it was completed. After completing the reaction, the unreacted solid salt
was separated by filtering and the solvent was removed under vacuum, then the
residue was dissolved in CHCl3 and washed with H2O. Finally, the solvent was
removed under vacuum and the resulting residue was purified by silica gel
chromatography with CH2Cl2/PE (1:1-1:0) as mobile phase. The main purple band
1
was collected to give the desired product. Yield: 120 mg (71.2 %); H NMR (600
MHz, CDCl3) (SI-Fig. 6) δH (ppm) 8.85 (m, 8H, β-pyrrole), 8.21 (d, J = 6.6 Hz, 6H,
o-phenyl); 8.13 (d, J = 7.8 Hz, 2H, o-phenyl), 7.76 (m, 10H, o-,m-phenyl and
acryloyl-H), 7.53 (d, Jo-m = 9.0 Hz, 2H, m-phenyl), 7.31 (d, Jm-p = 8.4 Hz, 2H, m- and
p-phenyl), 6.90 (d, Jm-p = 8.4 Hz, 2H, p-phenyl), 6.47 (d, J = 15.6 Hz, 1H, acryloyl-H),
8

ACCEPTED MANUSCRIPT
4.74(t, 2H, CH2), 4.52(t, J = 4.2 Hz, 2H, CH2), 3.82( s, 3H, -OMe), -2.78 (s, 2H,
NH-pyrrole); UV-Vis (DMF) λmax (log ε): 418.5 (5.38), 516.0 (3.99), 550.0 (3.73),
592.0 (3.59), 646.0 (3.52) nm; IR (KBr)/cm-1: 3240 (ʊN-H, pyrrole), 2924(ʊCH2
,
-CH2CH2-), 2850(ʊCH2, -CH2CH2-), 1712(ʊC=O), 1250(ʊC-O-C), 966(δN-H, pyrrole); MS
(ESI): m/z calc. for [M+H]+, C56H43N4O4: 835.32, found: 835.54; elemental analysis
calcd (%) for C56H42N4 (770.34): C 87.24, H 5.49, N 7.27; Found C 87.26, H 5.43, N
7.32.
2.2.4 Synthesis of zinc(II) 3-phenyl-acrylic acid 2-[4-(10,15,20-triphenyl-porphyrin-
5-yl)-phenoxy]-ethyl ester (5a)
A mixture of Zn(OAc)2 • 2H2O (2.66 g, 11.3 mmol) and the free 3-Phenyl-acrylic
acid 2-[4-(10,15,20-triphenyl-porphyrin-5-yl)-phenoxy]-ethyl ester (4a) (100.0 mg,
o
0.124 mmol) in DMF (25 ml) was heated to 70 C under nitrogen for ca. 3h. The
reaction was monitored by TLC until it was completed. The solvent was then removed
in vacuo and the residue was subjected to chromatography on Al2O3 column using
PE/CH2Cl2 (1:5) as eluent. Repeated chromatography gave pure target compound as a
light purple compound. Yield: 83.9 mg (78%); 1H NMR (600 MHz, CDCl3) (SI-Fig. 3)
δH (ppm) 8.84 (d, J = 15.0, 8H, β-pyrrole), 8.15 (d, J = 5.4 Hz 6H, o-phenyl), 8.06 (d,
J = 6.6 Hz, 2H, o-phenyl), 7.74 (d, J = 15.6 Hz, 1H, acryloyl-H), 7.66 (d, J = 6.6 Hz,
9H, o- and m-phenyl), 7.52 (s, 2H, m-phenyl), 7.33 (s, 3H, m- and p-phenyl), 7.23 (d,
Jm-p = 6.6 Hz, 2H, p-phenyl), 6.52 (d, J = 15.6 Hz, 1H, acryloyl-H), 4.67(s, 2H, CH2),
4.47(s, 2H, CH2); UV-Vis (DMF) λmax (log ε): 426.5 (5.46), 559.0 (4.00), 599.0 (3.71)
9

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nm; IR (KBr)/cm-1: 2919(ʊCH2, -CH2CH2-), 2853(ʊCH2, -CH2CH2-), 1708(ʊC=O),
1242(ʊC-O-C), 1169(δCβ-H), 992(ʊN-Zn); MS (ESI): m/z calc. for [M + H]+,
C55H39N4O3Zn: 867.23, found: 867.43; elemental analysis calcd (%) for C55H38N4
(754.31): C 87.50, H 5.07, N 7.42; Found C 87.56, H 4.98, N 7.45.
2.2.5 Synthesis of zinc (II) 3-(4-chloro-phenyl)-acrylic acid 2-[4-(10,15,20-triphenyl-
porphyrin-5-yl)-phenoxy]-ethyl ester (5b)
A mixture of Zn(OAc)2 • 2H2O (3.03 g, 11.3 mmol) and 3-(4-chloro-phenyl)-
acrylic acid 2-[4-(10,15,20-triphenyl-porphyrin-5-yl)-phenoxy]-ethyl ester (4b) (100.0
mg, 0.124 mmol) in DMF (25 ml) was heated to 120 oC under nitrogen for ca. 2h. The
reaction was monitored by TLC until it was finished. The solvent was then removed
in vacuo and the residue was subjected to chromatography on Al2O3 column using
PE/CH2Cl2 (1:3) as eluent to give pure the product as a purple compound. Yield: 83.8
mg (75%); 1H NMR (600 MHz, CDCl3) (SI-Fig. 5) δH (ppm) 8.95 (d, J =10.8 Hz, 8H,
β-pyrrole) 8.22 (d, J = 3.6 Hz, 6H, o-phenyl), 8.13 (d, J = 6.6 Hz, 2H, o-phenyl), 7.75
(d, J = 6.0 Hz, 9H, o- and m-phenyl), 7.62 (d, J = 16.2 Hz, 1H, acryloyl-H), 7.47 (d,
Jo-m = 7.8 Hz, 2H, m-phenyl), 7.35 (d, J = 7.2 Hz, 2H, m- and p- phenyl), 7.29 (d, Jm-p
= 6.6 Hz, 2H, p-phenyl), 6.47 (d, J = 15.6 Hz, 1H, acryloyl-H), 4.63(s, 2H, CH2),
4.46(s, 2H, CH2); UV-Vis (DMF) λmax (log ε) : 426.5 (5.58), 559.0 (4.18), 598.5 (3.96)
nm; IR (KBr)/cm-1: 2926(ʊCH2, -CH2CH2-), 2854(ʊCH2, -CH2CH2-), 1716(ʊC=O),
1243(ʊC-O-C), 1166(δCβ-H), 993(ʊN-Zn); MS (ESI): m/z calc. for [M + H]+,
C55H38ClN4O3Zn(isotope): 901.19, found: 901.30; elemental analysis calcd (%) for
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C55H37N4 (753.30): C 87.62, H 4.95, N 7.43; Found C 87.65, H 4.89, N 7.46.
2.2.6 Synthesis of zinc (II) 3-(4-methoxy-phenyl)-acrylic acid 2-[4-(10, 15,
20-triphenyl-porphyrin-5-yl)-phenoxy]-ethyl ester (5c)
A mixture of Zn(OAc)2 • 2H2O (2.66 g, 11.3 mmol) and the free
3-(4-methoxy-phenyl)-acrylic acid 2-[4-(10, 15, 20-triphenyl-porphyrin-5-yl)-
phenoxy]-ethyl ester (4c) (100.0 mg, 0.199 mmol) in DMF (25 ml) was heated to 70
oC under nitrogen for ca. 4h. The reaction was monitored by TLC until it was
completed. The solvent was then removed in vacuo and the residue was subjected to
chromatography on Al2O3 column using PE/CH2Cl2 (1:5) as eluent. Repeated
chromatography gave pure target compound as a light purple compound. Yield: 76 mg
1
(71%); H NMR (600 MHz, CDCl3) (SI-Fig. 7) δH (ppm) 8.96 (m, 8H, β-pyrrole and
phenyl), 8.22 (d, J = 6.6 Hz, 6H, o-phenyl); 8.13 (d, J = 7.2 Hz, 2H, o-phenyl), 7.75
(m, 9H, o- and m-phenyl), 7.61 (d, J = 16.2 Hz, 1H, acryloyl-H), 7.47 (d, Jo-m = 7.8
Hz, 2H, m-phenyl), 7.29 (d, J = 7.8 Hz, 2H, m- and p-phenyl), 6.86 (d, Jm-p = 7.8 Hz,
2H, p-phenyl), 6.34 (d, J = 15.6 Hz, 1H, acryloyl-H), 4.60(m, 2H, CH2), 4.44(m, 2H,
CH2), 3.80( s, 3H, -OMe); UV-Vis (DMF) λmax (log ε) : 426.0 (5.65), 559.5 (4.19),
598.5 (3.92) nm. IR (KBr)/cm-1: 2926(ʊCH2, -CH2CH2-), 2852(ʊCH2, -CH2CH2-),
1716(ʊC=O), 1243(ʊC-O-C), 1163(δCβ-H), 991(ʊN-Zn); Elemental analysis calcd (%) for
C56H40N4 (768.33): C 87.47, H 5.24, N 7.29; Found C 87.44, H 5.26, N 7.30.
2.3 Aggregation studies
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The concentration of porphyrin/CA conjugates 4a in the THF-aqueous solution
was varied from 1.0 to 110 μM. The pH variation was carried out by addition of 0.1
M HCl or NaOH concentrated stock solutions. The pH values were measured with a
digital pH-meter Corning 430. The ionic strength was 0.1, 0.2, and 0.3 M NaCl
respectively and changes in ionic strength were made by addition of NaCl in dry form.
The resulted solutions were then measured by means of UV-Vis absorption spectra
to study the aggregation behaviors of the porphyrin/CA conjugate 4a. As previously
described [25, 26], porphyrins self-assembling behaviors were evaluated through
changes of the UV-Vis spectra including the maximum absorption wavelength,
intensity and the band shape of the Soret bands under different measurement
conditions.
3. Results and discussions
3.1 Synthesis
A series of porphyrin/CA conjugates have been prepared by coupling TPP-OH (1)
with cinnamic acid derivatives through alkyl linker (Scheme 1). Although attempts to
directly couple TPP-OH with cinnamic acid derivatives were made, desired products
failed to be obtained either by using DCC/DMAP as condensing agent or converting
cinnamic acids into corresponding acyl chlorides. All this maybe resulted from the
lower nucleophilicity of hydroxyl group of TPP-OH. We then opted to couple the
cinnamic acid derivatives with TPP-OH through an alkyl linker and prepare the
building
block
5-[4-(2-bromo-ethoxy)-phenyl]-10,15,20-triphenyl-porphyrin
12

ACCEPTED MANUSCRIPT
(H2BTPP) (2). For comparisons, we chose cinnamic acid derivatives substituted at
4-position with chloro- (-Cl), methoxy (-OCH3) groups and without any substitutents
(-H). The cinnamic acid derivatives were then transformed into corresponding sodium
cinnamates. Having both H2BTPP and sodium cinnamate 3a as starting materials, the
coupling reaction was performed using DMF as solvent at 75oC to afford the desired
product 4a in an excellent yield 83%. Compounds 4b, 4c were also synthesized with
similar methods in yields 75%, 71%, respectively [27].
After chelating with zinc acetate dihydrate (Zn(OAc)2·2H2O) [28, 29], compound
4a was converted into its zinc complex (5a). And the other target complexes 5b, 5c
were obtained in a similar way.
3.2 1H NMR spectral Analysis
1H NMR spectra of compounds 4a~4c and their zinc complexes of 5a~5c were
shown in Fig 1. As seen from Fig 1, all of the free bases 4a~4c gave the characteristic
signals at -2.8 ppm which were attributed to NH-pyrrole of the porphyrin ring.
However, as to the zinc complexes 5a~5c, the corresponding signals disappeared
indicating the formation of N-metal bond of the metalloporphyrins. Moreover, all the
compounds gave doublets at about 6.5 ppm with typical coupling constants of 15.6 Hz
which suggested the double bond of acryloyl group in E-configuration. And the
multiplets at about 8.8 ppm were assigned to the hydrogens of β-pyrrole.
3.3 IR Spectral analysis
13

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FT-IR spectra of free bases of 4a~4c and their zinc complexes of 5a~5c, with the
appearance of C=O vibration around 1710 cm-1, signified the successful coupling of
CA derivatives with HBTPP. Furthermore, the disappearance of N-H vibration
around 3236, 3540 cm-1 and the emergence of metal-dependent band at around
990-1030 cm-1 for the divalent metal derivatives for the tetraphenylporphines implied
that zinc complexes were also successfully prepared[30, 31]. The IR spectra of 4b and
its zinc complex 5a well showed the characteristic signals in Fig 2.
3.4 Electronic Spectral analysis
Similar to the spectra of starting material TPP-OH (λmax=418 nm) and the
intermediate H2BTPP (2) (λmax=418 nm) (Fig. 3), the UV-Vis spectra of free base
porphyrin/CA conjugates 4a~4c showed intense Soret bands at 418 nm, arising from
S0→S2 transition, followed by a series of low intensity bands (Q bands) at 516, 550,
592, 646 nm (Table 1, Fig. 4) due to the S0→S1 transition. Remarkably, the electronic
effect from –OCH2CH2Br and –OCH2CH2-CA groups was comparable to that of the
hydroxyl group in TPP-OH although CA derivatives showed their typical absorption
bands, respectively (CA λmax: 274 nm；4-OCH3-CA λmax: 299 nm；4-Cl-CA λmax: 279
nm) (SI-Fig.11). That was to say, the CA moieties had little effect on π-electron
transferring in the conjugated porphyrin rings. Furthermore, the similar absorption
spectra for 4a-4c also indicated that the outermost substituents, including -H, -OCH3,
-Cl, almost had no influence on the π→π* transitions in the obtained porphyrin/CA
conjugates.
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The similarities of the absorption spectra between compounds 4a-4c, H2BTPP and
TPP-OH might result from the saturated alkyl linker between the CA moieties and
the porphyrin rings. A large conjugate system between the porphyrin ring and the CA
moieties failed to form because of the introduction of the saturated alkyl linker. And
no electronic effect from CAs or -OCH2CH2Br could be passed onto the porphyrin
chromophores.
In addition, the absorption spectra of zinc complexes of 5a-5c all exhibited a Soret
band at 426 nm, rooting from the S0→S2 transition, followed by two Q bands at 559
and 598 nm, caused by S0→S1 transition (Table 1). The electronic spectra of 5a-5c
were also similar to each other (Fig. 5). Moreover, all the absorption bands of the
metallated complexes 5a-5c showed a red shift compared with those of corresponding
ligands 4a-4c. The metallated complexes 5a-5c also exhibited two Q bands instead of
four, due to their higher symmetry.
3.5 Fluorescence spectral analysis
The main fluorescence emission of these porphyrin/CA conjugates was the usual
1
1(π ←π*) or also was described as (S0←S1) transition [32]. The peripheral
substituents around the porphyrin chromophores almost had no electronic effect on
the emission spectra of 4a-4c, H2BTPP and TPP-OH (Fig. 4 and Fig. 3) because they
nearly had no influence on the UV-Vis spectra of 4a-4c, H2BTPP and TPP-OH [31].
And the emission spectra of 4a-4c exhibit two emission peaks of 656 and 723 nm
(excited at 418 nm), respectively (Table 2) (Fig. 4). As CA derivatives showed their
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emission peaks as follows: 620 nm (CA, excited at 274 nm)；580 nm (4-OCH3-CA,
excited at 299 nm); 646 nm (4-Cl-CA, excited at 279 nm) (SI-Fig.11), the mutual
overlap effect could only be found in the emission spectrum of 4b. Furthermore,
fluorescence quantum yields were calculated compared with the standard H2TPP,
irradiating at 513 nm in deaerated DMF. Results in Table 2 showed that compounds
4a-4c had lower Φf (0.13-0.15) which suggested that the singlet excited state was
deactivated largely by processes other than fluorescence [20].
Similarly, the fluorescence emission spectra of zinc complexes 5a-5c (Fig. 5) also
showed same emission peaks of 611 and 662 nm which demonstrated a significant
blue shift compared with those of free bases 4a-4c, separately. In the case of zinc-ion
( II ), the insertion of metal ion could improve the coplanarity of porphyrins which
ascends the energy difference between HOMO and LUMO and makes emission peaks
blue shift [33]. Their fluorescence quantum yields (Φf = 0.08-0.10) were also lower
than that of H2TPP (Φf = 0.12).
3.6 Aggregation studies
3.6.1 Influence of the acid medium
As previously reported, most of free porphyrin bases showed significant
aggregation behaviors in acid aqueous solutions with changes of the position, the
shape and the intensity of the characteristic absorption bands [34].
The changes appeared in the UV-Vis spectra of 4a in THF-aqueous solution at
different values of pH have been studied as exhibited in Fig. 6. When the acidity of
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the solutions was increased, the protonation of the inner nitrogen atom of the
porphyrin ring was formed and partial positive charge was induced in the central part
of the molecule. Compared with the broadened and unresolved curve at pH 5.5 in Fig.
6, the curve at pH 4.5 slightly splitted into the main peak at 417.5 nm and a shoulder
band at 440.0 nm which showed a tendency to form aggregates. Among them, the
band at 417.5 nm was unequivocally assigned to the monomer.
It was shown from Fig. 7, by increasing the acidity of the solution, the absorption
of Soret bands of the monomer at 417 nm was gradually decreased while the
absorption of shoulder bands related to the formation of aggregates was gradually
increased. The formation of aggregates was further evidenced by a broadened and
bathochromically shifted from 417 nm to 430 nm Soret band when the value of pH
was up to 2.0. Moreover, the Q bands didn’t show any obvious changes at different
values of pH.
As formerly presented, diprotonation of meso-tetraarylporphyrins led to the
saddling of the porphyrin core and to an enhanced coplanarity of the meso-aryl groups
with the porphyrin core [35]. As a consequence of the saddling of the porphyrin core,
because of greater stabilization of eg orbital with respect to a1u orbital, a bathochromic
shift of the Soretband was displayed [34].
According to the excitonic splitting theory, side-to-side J-aggregates are
characterized by a broadened and red shifted absorption band [34, 36].
3.6.2 Influence of the ionic strength in acid media
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It's known that changes of the ionic strength in acid media could affect the
aggregation of porphyrins [13, 37].
Studies have been carried out on the changes in the UV-vis spectra of 4a in
THF-aqueous solutions with different ionic strength in acid media.
As seen from Fig. 8, the curve (1) showed a broadened and slightly split Soret band
without NaCl at pH 4.5, but with addition of 0.1 M NaCl at pH 4.5, the curve (2)
showed a remarkable tendency to split into two bands at 417.5 and 453.5 nm
respectively.
By increasing the ionic strength with varying concentration of NaCl from 0.1 M to
0.3 M with the fixed concentration of porphyrin 4a, the generation of aggregates was
achieved and confirmed by the clear splitting of the broadened Soret band into two
individual Lorentzian bands located around 417 nm and 456 nm, respectively (Fig. 9,
curve (2) and (3)). Obviously, the band at 417 nm could be exactly assigned to the
monomer while the red shift of the Soret band to 456 nm should be attributed to
formation of side-by-side J-aggregates.
As expeted, the addition of salt could reduce the solubility of porphyrins in the
acidic THF-aqueous media through salting-out effect and change the
micro-environment in which the porphyrin existed. Then the hydrophobic-lipophilic
interaction among hydrophobic molecules of porphyrin was improved which
contributed to aggregation of porphyrins. As already presented, in the aggregation
complex, the adjacent porphyrin molecules interact by hydrophobic interaction or
electrostatic attraction [13, 38].
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3.6.3 Influence of the concentration
It can be observed that, from Fig. 10, with increasing the concentration of 4a, the
maximum of Soret band was blue shifted from 417.0 to 398.5 nm, and a shoulder
band [39] appeared around 422.0 nm, while the four Q bands did not show significant
changes in shape and location. And the significant blue shift was not present until the
concentration was up to 1.093×10-4 mol/L.
According to Kasha’s exciton theory [40], the obvious blue shift with respect to
the Soret monomer band resulted from the formation of face-to-face H-aggregates,
while the red shift arised from the generation of side by side J-aggregates. Moreover,
experimental data on the dihedral angle between the porphyrin planes indicated that
the hypsochromic shift component of the Soret band should have a dihedral angle of
0o and manifest a dipole-forbidden transition [41].
4. Conclusions
In this paper, a new series of porphyrin/CA conjugates 4a-4c and their zinc
complexes 5a-5c were synthesized and fully characterized. These new compounds
exhibited the typical bands of the porphyrin core in the absorption and emission
spectra which were not significantly affected by the introduction of the alkyl linker
and CA moieties. The characteristic Soret band of the porphyrin/CA conjugates 4a-4c
was at ca. 418 nm followed by four Q bands situated at 516, 550, 591 and 646 nm..
Furthermore, the Soret bands of metallated porphyrins 5a-5c were red-shifted to ca.
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426 nm followed by two Q bands located at 559 and 599 nm. The emissive bands of
the free bases appeared at ca. 656 and 722 nm while the bands of the metallated
porphyrins are blue-shifted to ca. 611 and 662 nm. Moreover, the aggregation
behaviors of porphyrin/CA conjugates 4a in solutions were explored under different
conditions by UV-vis spectra, icluding: different pH values, various ionic strengths in
the acidic media and diverse concentrations. As pH values were decreased with fixed
concentration of 4a in THF-aqueous solution, UV-vis bands showed obvious changes
in the shape and location. J aggregates were clearly present when pH value was up to
2.0. Additionally, increase of ionic strength under acidic conditions led to the splitting
of broadened Soret band into two distinct Lorentzian bands which suggested the
formation of J aggregates. When the concentration was increased to 1.093×10-4 mol/L,
the maximum of Soret band was obviously blue shifted from 417.0 nm to 398.5 nm
which indicated the formation of H aggregates. Furthermore, the investigation of the
photodynamic biological activity of these compounds is currently under investigation
in our laboratories at present.
Acknowledgements
This work was supported by grants from the National students' training program for
innovation and entrepreneurship (No. 201510058026 and No. 201710058057).
Pengfei Yu thanks the grants from the Municipal Natural Science Foundation of
Tianjin (No. 15JCQNJC13900).
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NH
N
Br
NH
N
DMF, 75oC
K2CO3, DMF
BrCH2CH2Br
OH
O
N
HN
N
HN
R
NaO
O
2
1
3a R=H
3b R=Cl
3c R=OMe
R
R
O
O
NH
N
O
O
N
N
N
M
O
O
M(OAc)2
THF or DMF, 60oC
N
HN
N
5a M = Zn ( II ), R=H
5b M = Zn ( II ), R=Cl
5c M = Zn ( II ), R=OMe
4a R=H
4b R=Cl
4c R=OMe
Scheme 1. Synthesis of CA / porphyrin conjugates 4a-4c and their Zn complexes 5a-5c.
Table 1
UV-Vis spectral data for TPP-OH, H2BTPP, and compounds 4a-4c, 5a-5c a
Compounds
Soret band/nm( ε ) Qy(1,0)/nm ( ε )
Qy(0,0)/nm (ε)
Qx(1,0)/nm (ε )
Qx(0,0)/nm ( ε )
1 ( TPP-OH )
418.0（3.37×105） 516.0（1.6×104） 550.0（8.0×103）
418.0（3.59×105） 516.0（1.2×104） 550.5（5.0×103）
591.0（4.0×103）
591.0（1.8×103）
590.5（3.3×103）
590.5（1.15×103）
592.0（3.87×103）
599.0( 5.1×103)
647.0（4.13×103）
646.0（1.5×103）
646.0（2.7×103）
645.5（0.87×103）
646.0（3.3×103）
2 ( H2BTPP )
4a ( R = H))
418.0（2.87×105） 515.5(1.1×104）
550.0（5.7×103）
4b ( R = Cl )
418.0（1.26×105） 515.5（4.6×103） 550.5（2.3×103）
418.5（2.42×105） 516.0（9.8×103） 550.0（5.4×103）
4c ( R = OMe)
5a (M= Zn, R= H )
5b ( M = Zn, R= Cl )
5c ( M = Zn, R= OMe)
426.5（2.91×105)
426.5（3.84×105）
426.0（4.47×105）
559.0(9.97×103)
559.5（1.53×104）
559.5（1.56×104）
598.5(9.03×103)
598.5（8.3×103）
a A solution of DMF was used in UV-Vis spectral determinations. The concentrations for the porphyrins and their
derivatives were 3×10-6 mol/L.
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Table 2 Emission data for H2TBPP and compounds 4a-4c, 5a-5c.a
Φf(513nm)b
Φf(513nm)b
Compounds
Q(0,0)/nm
(intensity)
656 (4762)
656 (3572)
656 (4734)
657(4963)
Q(0,1)/nm
Compounds
Q(0,0)/nm
(intensity)
611 (2138)
611 (2052)
611 (2144)
657(4785)
Q(0,1)/nm
(intensity)
662 (1195)
662 (1338)
662 (1198)
721(1279)
(intensity)
4a
722（1310）
722（981）
722（1284）
722(1248)
0.15
0.13
5a
0.09
0.08
0.10
0.10
4b
5b
4c
0.15
5c
TPP-OH
0.12
H2TBPP
a A solution of DMF was used in fluorenscence emission spectral determinations. The concentrations for the
porphyrin-CA conjugates and the intermediates were 3×10-6 mol/L.
b Using H2TPP in DMF (Φf = 0.12) as a reference.
THF:CH2Cl2 30:70 (blue line), THF: water 30:70 (magenta line) and THF (red line)at room temperature (conc =
1μM).
Fig. 1. 1H NMR spectra of prophyrins/CA conjugates 4a~4c and their zinc complexes 5a~5c
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Fig. 2. IR spectra of free base of 4b and its zinc complex of 5b
Fig. 3. Normalized Absorption (solid) and Fluorescence spectra (dotted) of compounds TPP-OH, H2BTPP in
DMF.
Fig. 4. Normalized Absorption (solid) and Fluorescence spectra (dotted) of the metal free porphyrins 4a-4c in
DMF.
29