meso-borneol- andmeso-carbazole-substituted porphyrins: multifunctional chromophores with tunable electronic structures and antitumor activities

Article abstract of DOI:10.1039/d0nj02954h

Herein, a series of fiveC₂symmetric H₂porphyrins withmeso-borneol andmeso-carbazole units have been synthesized and isolated. An analysis of the electronic structures was carried out by spectroscopic investigations and electrochemica

Full text of DOI:10.1039/d0nj02954h

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meso-borneol- and meso-carbazole-substituted
porphyrins: multifunctional chromophores with
tunable electronic structures and antitumor
activities†
Cite this: New J. Chem., 2021,
45, 2141
b
Bo Fu,‡a Xinyi Dong,‡b Xiaoxiao Yu,a Zhen Zhang,b Lei Sun,a Weihua Zhu,
ac
Xu Liang *b and Haijun Xu
*
Herein, a series of five C2 symmetric H2porphyrins with meso-borneol and meso-carbazole units have
been synthesized and isolated. An analysis of the electronic structures was carried out by spectroscopic
investigations and electrochemical illustrations. In addition, the chiral optical properties observed in the
CD spectra can be assigned to the interchromophore through-space coupling between the borneol
chiral center and the porphyrinic chromophore. Further investigation of the anticancer behaviors shows
that the modulation of the biological activity behaviors could be facilely achieved by introducing various
halogen atoms on the carbazole rings.
Received 11th June 2020,
Accepted 20th December 2020
DOI: 10.1039/d0nj02954h
rsc.li/njc
received considerable attention due to their unique molecular
structure and unprecedented properties. For example, chiral
Introduction
Natural borneol (NB), a terpene and bicyclic organic com- porphyrins could be widely applied as a chiral optical response,
pound, is found in the essential oils of medicinal plants and in chiral recognition and separation, chiral transformation and
is commonly used in traditional Chinese medicine for analge- expansion, and even chiral catalysis.9–12 Recently, the related
sia and anaesthesia.1,2 To date, NB has been extensively used studies were focused on the design, synthesis, and char-
in a wide range of applications in biomaterial chemistry.3 acterization of new chiral porphyrinic chromophores upon
Also, NB is sometimes used as the part of chiral ionic liquids modulating interchromophore through-space couplings by
that is applied in enantioselective sodium borohydride reduc- introducing various chiral substituents and/or building
tions of prochiral ketones to produce optically active blocks, and their physical and biomedical properties were also
alcohols.4 On the other hand, porphyrins, a group of highly investigated.13,14 Thus, the design and preparation of new
conjugated heterocyclic macrocyclic organic compounds, porphyrin–borneol hybrids becomes attractive, and several great
composed of four modified pyrrole subunits interconnected advantages as well as their potential applications could be
at their a carbon atoms via methine bridges have been rationally expected. Carbazole-based organic materials have
investigated in various fields during past decades, such as been widely employed as active ingredients in electronic devices
synthetic chemistry,5 catalysis,6 medicinal materials,7 and due to their unique charge-transporting properties and pro-
coordination chemistry.8 As an important branch of por- nounced thermal stability.15–18 On the other hand, we found
phyrinic chromophores, chiral porphyrins have specifically that halogen atoms shew an obvious influence on the photo-
physical properties of porphyrin derivatives,19 and these cases
greatly raise the spirits of exploiting the potential application of
carbazole–porphyrin–borneol hybrids with multiple targets,
which will provide an important step forward in the photo-
a Jiangsu Co-innovation Center of Efficient Processing and Utilization of Forest
Resources, College of Chemical Engineering, Jiangsu Key Lab of Biomass-based
Green Fuels and Chemicals, Nanjing Forestry University, Nanjing 210037,
physical and medicinal field. Thus, when porphyrin was
tested as a large p-conjugated moiety, the chiral optical and
biomedical performance of natural borneol, carbazole and
porphyrin may be significantly enhanced upon combining all
advantages among them, and the rational design of multi-
functional chromophores with tunable electronic structures
and antitumor activities could be expected.
P. R. China. E-mail: xuhaijun@njfu.edu.cn
b School of Chemistry and Chemical Engineering, Jiangsu University,
Zhenjiang 212013, P. R. China. E-mail: liangxu@ujs.edu.cn
c School of Chemistry and Chemical Engineering, Henan Normal University,
Xinxiang 453007, P. R. China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0nj02954h
‡ These authors equally contributed to this work.
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Experimental
Synthesis of 5,15-bis-(4-(3,6-dibutyl-9-(4–N-benzoylphenyl))
carbazole)phenyl)-10,20-(bornyl hydroxybenzoate)porphyrin
(4a)
Synthesis of 5,15-bis-(4-(3,6-dibromo-9-(4-N-benzoylphenyl))
carbazole)phenyl)-10,20-(bornyl hydroxybenzoate)porphyrin
(7b)
The general synthetic procedure is similar to that of 4a, except that
5-(4-(3,6-dibromo-9-(4-N-benzoylphenyl))carbazole)phenyl)-
dipyrromethane 6b was used instead; the target compound was
obtained as a deep-red solid state compound in 9% yield. High-
resolution ESI-mass: m/z = 1617.2578 (calcd [M]+ = 1617.2421).
1H NMR (600 MHz, CDCl3; ppm): 9.07–9.02 (m, 4H), 8.93–8.88
(m, 4H), 8.52–8.49 (m, 7H), 8.38–8.33 (m, 7H), 8.22 (d, J = 7.2 Hz,
1H), 8.01–7.96 (m, 4H), 7.85 (d, J = 7.8 Hz, 1H), 7.74–7.66
(m, 6 H), 7.63 (t, J = 7.2 Hz, 1H), 7.44 (t, J = 7.8 Hz, 1H),
5.35–5.32 (m, 2H), 2.67–2.63 (m, 2H), 2.36–2.31 (m, 2H),
1.96–1.89 (m, 2H), 1.86–1.85 (m, 2H), 1.54–1.43 (m, 4H), 1.33
(d, J = 13.8 Hz, 2H), 1.07 (s, 12H),1.00 (s, 6 H), À2.72 (s, 2H).
Elemental analysis: found: C, 66.78; H, 4.52; N, 5.11; O, 3.87;
calcd: C, 66.68; H, 4.48; N, 5.18; O, 3.95. UV-vis (CH2Cl2): UV-vis
(CH2Cl2, e  105 L molÀ1 cmÀ1; nm): 422 (3.270), 514 (0.144),
552 (0.062), 592 (0.037), 672 (0.127).
p-(Bornyl hydroxybenzoate)aldehyde (0.57 g, 2.00 mmol) and
5-(4-(3,6-dibutyl-9-(4-N-benzoylphenyl))carbazole)phenyl) dipyrro-
methane (3a) (1.00 g, 2.00 mmol) were dissolved in 250 mL of
dry CH2Cl2 in the dark under an argon atmosphere, and TFA
(0.15 mL, 1.95 mmol) was added dropwise into the above
solution. After the reaction mixture was stirred at room tem-
perature for 14 h, 2.00 mmol of DDQ (0.454 g) was added and
stirred for another 1 h. After removal of the organic solvent, the
residue was purified by silica gel column chromatography
using a mixed solvent (CH2Cl2 : hexane = 1 : 1, v/v) as an eluent
to give the pure deep-red solid state compound in 14% yield. High-
resolution ESI-mass: m/z = 1529.8505 (calcd [M + H]+ = 1529.8592).
1H NMR (600 MHz, CDCl3; ppm): 9.09 (d, J = 26.4 Hz, 4H), 8.90
(d, J = 29.4 Hz, 4H), 8.52–8.44 (m, 8H), 8.37 (t, J = 9.6 Hz, 4H), 8.27
(s, 4H), 8.03–8.00 (m, 4H), 7.80 (d, J = 9.0 Hz, 4H), 7.64 (d, J = 9.0 Hz,
4H), 5.33 (d, J = 9.0 Hz, 2H), 2.67–2.63 (m, 2H), 2.36–2.33 (m, 2H),
1.93–1.89 (m, 2H), 1.85 (t, J = 10.2 Hz, 2H), 1.55 (s, 36H), 1.47–1.42
(m, 4H), 1.34 (d, J = 13.8 Hz, 2H), 1.08 (s, 12H), 1.00 (s, 6 H), À2.69
(s, 2H); elemental analysis: found: C, 83.27; H, 7.20; N, 5.40; O,
Synthesis of 5,15-bis-(4-(3,6-diiodo-9-(4-N-benzoylphenyl))
carbazole)phenyl)-10,20-(bornyl hydroxybenzoate)porphyrin
(7c)
4.12; calcd: C, 83.21; H, 7.11; N, 5.49; O, 4.18. UV-vis (CH2Cl2, The general synthetic procedure is similar to that of 4a, except that
e  105 L molÀ1 cmÀ1; nm): 422 (1.632), 515 (0.110), 553 (0.059), 5-(4-(3,6-diiodo-9-(4-N-benzoylphenyl))carbazole)phenyl)-dipyrro-
591 (0.040), 683 (0.082).
methane (6c) was used instead; the target compound was
obtained as a deep-red solid state compound in 11% yield.
High-resolution ESI-mass: m/z = 1809.1867 (calcd [M + H]+
=
Synthesis of 5,15-bis-(4-(3,6-dichoro-9-(4-N-benzoylphenyl))
carbazole)phenyl)-10,20-(bornyl hydroxybenzoate)porphyrin
(4b)
1809.1873). 1H NMR (600 MHz, CDCl3; ppm): 9.11–9.06 (m, 4H),
8.96–8.87 (m, 4H), 8.56–8.49 (m, 8H), 8.36 (t, J = 8.4 Hz, 4H),
8.18 (d, J = 7.8 Hz, 4H), 7.99 (dd, J = 8.4, 20.4 Hz, 4H), 7.76
The general synthetic procedure is the same as that of 4a, except that (d, J = 9.0 Hz, 4H), 7.56 (d, J = 7.2 Hz, 4H), 5.36–5.33 (m, 2H),
2.70–2.62 (m, 2H), 2.36–2.31 (m, 2H), 1.95–1.91 (m, 2H),
1.86–1.85 (m, 2H), 1.52–1.44 (m, 4H), 1.34 (dd, J = 3.6,
14.4 Hz, 2H), 1.08 (s, 12H),1.00 (s, 6H), À2.71(s, 2H); elemental
analysis: found: C, 59.85; H, 4.11; N, 4.55; O, 3.46; calcd: C,
59.75; H, 4.01; I, 28.06; N, 4.65; O, 3.54. UV-vis (CH2Cl2,
e  105 L molÀ1 cmÀ1; nm): 422 (2.582), 516 (0.129), 553
(0.060), 592 (0.027), 682 (0.1118).
5-(4-(3,6-dichloro-9-(4-N-benzoylphenyl))carbazole)phenyl)-dipyrro-
methane 3b was used instead. The target compound was finally
obtained as a deep-red solid state compound in a 12% yield.
High-resolution ESI-mass: m/z = 1441.4384 (calcd [M + H]+
=
1
1441.4442). H NMR (600 MHz, CDCl3; ppm): 9.07–9.01 (m, 6 H),
8.94–8.92 (m, 2 H), 8.53–8.50 (m, 2 H), 8.37 (d, J = 7.6 Hz, 2 H), 7.97
(dd, J = 7.6, 25.2 Hz, 6H), 7.85 (d, J = 8.4 Hz, 4H), 7.60 (d, J = 8.4 Hz,
4H), 5.36–5.33 (m, 2H), 2.68–2.63 (m, 2H), 2.36–2.32 (m, 2H),
1.94–1.89 (m, 2H), 1.87–1.86 (m, 2H), 1.51–1.45 (m, 4H), 1.36–1.33
(m, 2H), 1.08 (s, 12H), 1.00 (s, 6 H), À2.67 (s, 2H); elemental analysis:
found: C, 74.95; H, 5.1; N, 5.80; O, 4.30; calcd: C, 74.89; H, 5.03; N,
5.82; O, 4.43. UV-vis (CH2Cl2, e  105 L molÀ1 cmÀ1; nm): 421
(3.008), 516 (0.135), 553 (0.058), 591 (0.033), 672 (0.079).
Synthesis of 5,-15-bis-(4-(3,6-dichoro-9-(4-N-benzoylphenyl))
carbazole)phenyl)-10,20-(bornyl hydroxybenzoate)porphyrin
Zn(II) (4b)
5 mL of a CHCl3/MeOH (v/v = 4 : 1) mixture containg 2 mg of 4b
and 10 mg of Zn(CH3COO)2 was stirred and refluexed under N2
for 1 h. After removal of the organic solvents, purification by
silica gel column chromatography gave the pure deep-red solid
state compound in a 75% yield (1.5 mg). MALDI-TOF-mass:
m/z = 1503.48 (calcd [M + H]+ = 1503.35). 1H NMR (400 MHz,
CDCl3; ppm): 9.06 (m, 6 H), 8.53–8.50 (m, 2 H), 8.38 (d, J = 7.6 Hz,
2 H), 8.18 (d, J = 7.6 Hz, 2 H), 7.97 (dd, J = 7.6, 25.2 Hz, 6H), 7.75
(d, J = 8.4 Hz, 4H), 7.60 (d, J = 8.4 Hz, 4H), 5.35 (m, 2H), 2.64
(m, 2H), 2.33 (m, 2H), 1.92 (m, 2H), 1.83 (m, 2H), 1.50 (m, 4H),
1.33 (m, 2H), 1.08 (s, 12H),1.00 (s, 6 H). UV-vis (CH2Cl2,
e  105 L molÀ1 cmÀ1; nm): 420 (4.320), 551 (0.184), 590 (0.091).
Synthesis of 5,15-bis-(4-(9-(4-N-benzoylphenyl))carbazole)
phenyl)-10,20-(bornyl hydroxybenzoate)porphyrin (7a)
The general synthetic procedure is similar to that of 4a,
except that 5-(4-(9-(4-N-benzoylphenyl))carbazole)phenyl)-dipyrro-
methane 5a was used instead; the target compound was
obtained as a deep-red solid-state compound in a 16% yield.
High-resolution ESI-mass: m/z
= =
1291.5392 (calcd [M]+
1291.6165).
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Results and discussion
Synthesis and structural characterization
synthesized. Similar high-resolution ESI mass spectra also con-
firmed that [2+2] type H2porphyrins were isolated successfully
(Fig. S2–S5, see ESI†). 1H NMR spectra of [2+2] type H2porphyrins
were also recorded to confirm the characteristic peaks of the
bornyl-substituents between 1.0–3.0 ppm and around 5.2 ppm.
All other protons could be assigned to b-protons of the porphyri-
nic core and meso-aryl and/or carbazole substituents.
The synthesis of the porphyrin derivatives 4a–b and 7a–c was
shown in Scheme 1. Firstly, p-(bornyl hydroxybenzoate)aldehyde
was synthesized according to the literature-reported procedure.
Herein, we should mention that the introduction of a chloride
atom and tert-butyl-substituents should be obtained by use of
the carbazole unit (1a and 1b), but the introduction of bromine
and iodine atoms should be finished based on the carbazole-
aldehyde structure (5b and 5c). On the other hand, TFA was used
as the catalyst for the macrocyclic reaction. We should mention
here that all solvents and chemicals should be carefully dried
before the reactions to ensure that the target molecules were
obtained in satisfactory yields. All these [2+2] type porphyrins
could be obtained in isolated yields of between 10% and 15%,
except for 7a in the absence of any substituents at the carbazole
moiety. The lack of isolation of 7a could be attributed to the poor
solubility that results in difficulties during purification. All
bornyl- and carbazole-based precursors were also confirmed by
1H NMR spectra (Fig. S1–S16, see ESI†). In addition, high-
resolution ESI mass spectra of 4a contains an intense parent
peak at m/z = 1530.8625 (calcd [M]+ = 1530.0295) (Fig. S17, see
ESI†) providing direct evidence that the target meso-borneol
substituted C2 symmetric A2B2 type porphyrin was successfully
Electronic structure
To investigate the electronic structure of these meso-borneol-
substituted C2 symmetric A2B2 type porphyrins, spectroscopic
characterizations, as well as electrochemical measurements, were
carried out. As shown in Fig. 1, 4a with tert-butyl-substituents on
the carbazole rings exhibited Soret band absorptions at l = 424 nm,
and four Q-band absorption peaks appeared at l = 518, 554, 594,
and 650 nm. Compared with regular tetraphenyl porphyrins,20 the
clear red-shifts of the main absorption bands were revealed. This
phenomenon could be assigned to the perturbation of electron
donating carbazole-substituents at the 10- and 20-meso-positions to
destabilization of the HOMO orbitals by meso-electron-donating
substituents. When various halogen atoms were introduced to the
carbazole rings, negligible spectroscopic changes were observed,
which could be illustrated as the weak influence of halogen atoms
on the electronic structures themselves. Since borneol-substituents
Scheme 1 Synthesis of meso-borneol- and meso-carbazole-substituted porphyrins 4a–b and 7a–c.
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Fig. 1 UV-vis absorption (bottom) and circular dichroism spectra (top) of 4a–b, and 7b–c in CH2Cl2.
have their own chiral centers, the excitation coupling between
meso-borneol-substituents and the porphyrin core could be inves-
tigated by circular dichroism spectra. As shown in Fig. 1, the ÆCD
sequences appeared at around l = 420 nm that related to the Soret-
band absorptions of the porphyrins. It also showed that the points
of intersection of the CD curve and x-axis were certainly approx-
imate to the largest absorption value of the Soret-band. On the
other hand, the Zn(II)4b (Fig. S22, see ESI†) has an intense Soret
band absorption at l = 420 nm and two broad Q-band absorptions
at l = 551 and 590 nm, respectively. Also, negative CD signals were
observed in the Soret-band region. The significant changes in the
CD signals could be explained as the enhanced molecular symme-
try upon introducing Zn ions at the porphyrin core. Steady-state
fluorescence spectra of the porphyrins 4a, 4b, 7b, and 7c in CH2Cl2
are shown with normalized intensities upon excitation at the lmax
of the Soret band in Fig. 2 and the emission peak values corres-
Fig. 2 Normalized fluorescence spectra of 4a–b and 7b–c in CH2Cl2
(5 Â 10À6 mol LÀ1) at 298 K.
ponding to Q(0,0) and Q(0,1) are provided in Table 1. The fluo-
rescence spectra of the porphyrins (4a, 4b, 7b, and 7c) show two
very similar emission bands at around 654 and 718 nm, which
present a little difference in intensity. In addition, the fluorescence
quantum yields (fF) and singlet excited-state lifetimes (ts) for 4a,
4b, 7b, and 7c were also measured and are presented in Table 1. All
voltammetry and the similar characterizations at different
scan speeds. As shown in Fig. 3 and Table 2, these C2 symmetric
the compounds exhibit similar fluorescence yields (0.039–0.057) meso-borneol- and meso-carbazole-substituted porphyrins have
and fluorescence lifetimes (8.08–8.40). The results indicate that the similar redox potentials in both the oxidation and reduction
different substituent groups on the carbazole have almost little parts, which indicated the mirror influence of halogen atoms
effect on the physical properties of the porphyrins.
on the electronic structures. On the other hand, the scan-speed-
To gain insight into the perturbation of the electronic dependent cyclic voltammetry measurements (Fig. S23,
structures by the meso-substituents, especially the energy levels see ESI†) were also performed, and the clear linear correla-
of the HOMOs and LUMOs of 4a–b and 7b–c were compared tions indicated that these molecules were electrochemically
by the electrochemical characterization methods of cyclic diffusion-controlled.
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Table 1 Photophysical properties of compounds 4a–b and 7b–c in CH2Cl2
Cpds
labs [nm]
e [105 MÀ1 cmÀ1
]
lem [nm]
FF
tF [ns]
4a
4b
7b
7c
422, 515, 553, 591, 683
421, 516, 553, 591, 672
422, 514, 552, 592, 672
422, 516, 553, 592, 682
1.63, 0.11, 0.06, 0.04, 0.08
3.01, 0.14, 0.06, 0.03, 0.08
3.27, 0.14, 0.06, 0.04, 0.13
2.58, 0.13, 0.06, 0.03, 0.11
655, 720
654, 718
653, 718
654, 718
0.039
0.052
0.049
0.057
8.08
8.40
8.17
8.08
c = 5 Â 10À6 mol LÀ1; excitation at the lmax of the Soret band.
Fig. 3 Cyclic voltammograms of 4a–b and 7b–c in o-dichlorobenzene (c = 5 Â 10À2 mol LÀ1).
Table 2 Redox potentials of 4a–b and 7b–c in o-dichlorobenzene
(o-DCB)
OXII
OXI
RedI
RedII
4a
4b
7b
7c
—
1.66
—
—
À0.73
À0.71
À0.69
À0.71
À1.00
À0.99
À1.03
À1.01
1.39
1.66
1.59
—
Fig. 4 Inhibition against the proliferation of HepG2 cells in vitro and the
corresponding IC50 values of borneol and compounds 4a–b and 7b–c.
Antitumor activity evaluation
The in vitro antitumor activities of the porphyrin derivatives
were evaluated against human cancer HepG2 cells by the MTT
assay. Firstly, the stability of these synthetic borneol–porphyrin antitumor activities than pure borneol, tetraphenylporphyrin
hybrids was evaluated. The stability was also confirmed by the (TPP) and carbazole except for 4b. In addition, compound 7c
UV-vis absorption spectra, when borneol–porphyrin hybrids 4a, shows better antitumor activity against HepG2 cells in contrast
4b, 7b, and 7c were measured in a mixture solvent of DMSO and to the other four compounds. The cell inhibition rate of
pH 7.4 PBS from 0 h to 48 h at 37 1C (Fig. S29–S32, see ESI†), but compound 7c was up to 70% by MTT assay at 100 mM. Then
no significant changes were observed under the current condi- borneol–porphyrin hybrids 4a, 4b, 7b, and 7c were dissolved in
tions indicating that the satisfactory stability was confirmed. the mixture solvent of DMSO and pH 7.4 PBS at 37 1C over 48 h,
1
Then, the in vitro antitumor activities of the porphyrin deriva- and H NMR spectra were tested (all solvents were completely
tives were rationally evaluated. As shown in Fig. 4, the IC50 removed before measurements). As shown in Fig. S33–S36
values of complexes 4a, 4b, 7b, and 7c are 13.09 mM, 4100 mM, (ESI†), the major change to 4b, minor change to 7b, and little
15.27 mM, and 9.87 mM, respectively, and they present better to no change in 4a and 7c may provide a better explanation
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regarding the differences in the IC50 values that were observed Notes and references
during MTT measurements, and 7c has higher chemical stability
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Conclusions
A series of five C2 symmetric A2B2 type porphyrins with meso
borneol-substitutions have been successfully synthesized and
characterized. A detailed analysis of the electronic structures
was carried out by comparing the results obtained via UV-vis
and CD spectroscopy. This study demonstrates that A2B2 type
borneol–porphyrin hybrids could be applied as enhanced antitumor
activating reagents, and their tunable properties were modulated by
different halogen atoms. Considering that multifunctional chiral
porphyrinic chromophores have great potentials in various applica-
tions, these molecules will be potentially applied in the future, such
as biomedical applications.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was financially supported by the National Natural Science
Foundation of China (21701058), National Key R&D Program of 15 S. Choi, T. Chatterjee, W. J. Choi, Y. You and E. J. Cho,
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