Schiff base bridged biporphyrin: Synthesis, characterization and spectral properties

Article abstract of DOI:10.1016/j.inoche.2014.03.017

Chromophoric molecules with mono-aminophenyl, 5-aminophenyl-10, 15, 20-triphenyl porphyrin, were chemically reduced from the precursors of 5-nitrophenyl-10, 15, 20-triphenyl porphyrin. The titled Schiff base bridged biporphyrins were furthermore prepared

Full text of DOI:10.1016/j.inoche.2014.03.017

Inorganic Chemistry Communications 45 (2014) 10–14
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Schiff base bridged biporphyrin: Synthesis, characterization and
spectral properties
Ya Hong Wu a, Ling Ling Hu a, Jing Zhang b, Jian Yu a, Shan Ling Tong a, Yan Yan a,
⁎
a
College of Chemical Engineering & Light Industry, Guangdong University of Technology, Guangzhou 510006, PR China
b
College of Chemical Engineering, Zhuhai Campus, Beijing Institute of Technology, Zhuhai 519085, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Chromophoric molecules with mono-aminophenyl, 5-aminophenyl-10, 15, 20-triphenyl porphyrin, were
chemically reduced from the precursors of 5-nitrophenyl-10, 15, 20-triphenyl porphyrin. The titled Schiff base
bridged biporphyrins were furthermore prepared by condensation of 5-aminophenyl-10, 15, 20-triphenyl
porphyrin with terephthalaldehyde. This novel conjugated Schiff base biporphyrin was well characterized by
spectral determinations. Experimental results indicated that the titled compound possessed fluorescence
enhancement in near infrared region. Compared with tetraphenyl porphyrin, both ultraviolet–visible absorption
and fluorescence spectra of the titled compound appeared with slightly red shift. Photo experiments revealed
that, after violet irradiation, the color of a chloroform solution with the titled compound changed sharply from
pale green to dark green along with sensitively spectral variation during ultraviolet–visible absorption and
electron paramagnetic resonance determinations. This sensitively photochromic mechanism was well explained
by molecular recognition of the titled compound towards hydrogen chloride from the decomposition of
the chloroform solvent. The titled compound was sensitive in recognizing towards hydrogen chloride.
© 2014 Elsevier B.V. All rights reserved.
Received 20 January 2014
Accepted 19 March 2014
Available online 27 March 2014
Keywords:
Schiff base
Porphyrin
Molecular recognition
Molecular wire
Halochromism
Photochromism
Metalloporphyrins and their derivatives, including hemoglobin,
vitamin B12, cytochrome and chlorophyll, closely relate to vital processes
occurring in living organisms, and therefore research on porphyrin
chemistry has been very active in molecular design, synthesis and func-
tional material development [1–4]. Based on their special configuration,
metalloporphyrins were broadly investigated as function devices
such as molecular wires, molecular switches and molecular rectifiers
[5–8]. For synthesizing novel molecular devices, a condensation
between 5-aminophenyl-10, 15, 20-triphenyl porphyrin (H2APTPP)
and terephthalaldehyde was designed to prepare a Schiff base bridged
biporphyrin (SBBBPor) with conjugation configuration. This special
conjugation system with energy transfer function is hopeful to be
designed as a functional material. Using nitric acid as nitrification
reagent, Kruper's nitrification-reduction was an efficient method to pre-
pare mono-aminophenyl porphyrin with lower yield (~60%) [9]. In this
work sodium nitrite was selected as nitrification reagent, and the yield
of H2NPTPP reached 80%. This nitrite method was more convenient for
the following reduction and condensation, finally the titled compound
with conjugation configuration was successfully prepared.
without further purification. 1H NMR spectra were recorded with
a Varian Mercury-Plus 300 FT-NMR (300 MHz) spectrometer in
chloroform-d with tetramethylsilane (Me4Si) as an internal standard.
Chemical shifts (δ) and coupling constants (J) are given in parts per mil-
lion and hertz respectively. UV–vis spectra were recorded on a
Shimadzu UV-2450 spectrophotometer, and IR determination was
performed with a Nicolet Avatar 370 FT-IR infrared spectrometer (KBr
pellets, 4000–400 cm−1). Fluorescence spectra were recorded on a
HORIBA Jobin Yvon Fluorescence-max4 luminescence spectrometer.
EPR spectra were acquired with a Bruker A200-9.5/12 spectrometer
(RF powers ranged from 200 to 400 W across the 7 MHz scanned
range, and microwave power ranged from 2 to 20 mW). Element micro-
analysis was carried out in air on a PerkinElmer 240 C elemental analyz-
er. The in situ experiments of ultraviolet irradiation (UVI) and visible
radiation (VR) were carried out by using a deuterium light source
(30 W, Lot Oriel Company, Germany) and a halogen-tungsten light
source (1000 W, Lot Oriel Company, Germany). In photochromic deter-
minations, the distance between the light source and the sample was set
at 20 cm (with 0.5 cm slit for UV–vis determinations and 0.2 mm slit for
EPR determinations). X-ray data collections and structure determina-
tions were performed on a Bruker SMART CCD. The data were collected
using graphite-monochromatic Mo-Kα radiation (λ = 0.71073 Å). The
crystal structure was solved by direct methods and refined by full-
matrix least-square calculation on F2 with SHELX-97 program package
[10]. All non-hydrogen atoms were treated anisotropically. Hydrogen
atoms were placed in calculated positions.
All reactions and processes were performed in air unless otherwise
noted. Pyrrole was freshly distilled before using. Other reagents and
solvents were commercially available and directly used as received
⁎
Corresponding author.
http://dx.doi.org/10.1016/j.inoche.2014.03.017
1387-7003/© 2014 Elsevier B.V. All rights reserved.

Y.H. Wu et al. / Inorganic Chemistry Communications 45 (2014) 10–14
11
5, 10, 15, 20-Tetraphenyl porphyrin (H2TPP) was synthesized by the
(KBr)/cm−1: 3314 (υN\H, pyrrole), 3024 (υC\H, phenyl), 1629 (υC_N
,
Schiff base), 1594 (υC_C, phenyl), 1167 (δC\H, pyrrole); Calc. for
C96H64N10: 84.93; H, 4.75; N, 10.32%; Found: C, 85.03; H, 4.82; N,
10.21%.
reported method [11]. Using sodium nitrite as the nitrification reagent
and tin dichloride as the reductant, H2TPP was conversed into 5-
aminophenyl-10, 15, 20-triphenyl porphyrin (H2TAPTPP) [12], followed
by condensation with terephthalaldehyde [13], and the Schiff base
bridged biporphyrin (SBBBPor) was finally prepared (Scheme 1). The
product was separated and purified by the reported method: [14] The
rough violet product was dissolved in dichloromethane, and purified by
column chromatography (for preventing dissociation of Schiff base
bond, the silica gel column was previously treated with alkaline
triethylamine) using a binary eluent of dichloromethane and acetone
(v:v = 300:1). The first red-violet band was collected. After vaporizing
and drying under vacuum, the Schiff base bridged biporphyrin was
finally obtained in yield of 20% based on the calculation of H2APTPP. 1H
NMR for SBBBPor (300 MHz, CDCl3) δ/ppm: 5.39–6.68 (16H, H\C in
pyrrole), 7.17 (d, J = 8.4 Hz, 14H, phenyl), 7.33–7.40 (m, 26H, phenyl),
8.64–8.66 (s, 2H, Schiff base), −2.72 (s, 4H, N\H in pyrrole); IR
In the synthetic process, the UV–vis spectra for the relative porphy-
rin products were recorded and their spectral data were listed in
Table 1. Compared with H2TPP, the Soret bands of its derivatives shifted
differently towards red direction. The Soret band and Q bands of
porphyrin derivatives are all originated from π–π* electron transition
of the conjugated configuration, but the peripherally phenyl rings
were not co-planar with the porphyrin ring, and therefore they were
not efficiently in conjugation with the porphyrin's π-system. As a result,
the slight red shift in Soret band peak (10 nm) of the product indicated
the conjugation form of the peripheral substitutes that was not clearly
enhanced.
Porphyrins and their derivatives have been developed as lumines-
cent materials [15–17]. Luminescent property is closely related to the
NH
N
N
TFA,NaNO2
Nitrobenzene
CHO
+
N
H
Lactic acid
HN
H2TPP
NH
N
N
NH
N
SnCl2,HCl
NO2
NH2
HN
N
HN
H2NPTPP
H2APTPP
NH
N
N
OHC
CHO
N
HN
C
CHO
+
H
Porphyrin-Schiff base
NH
N
N
N
HN
HC
CH
NH
N
N
HN
N
Schif fbase bridged biorphyrin
Scheme 1. Synthetic route for Schiff base bridged biporphyrin (SBBBPor).

12
Y.H. Wu et al. / Inorganic Chemistry Communications 45 (2014) 10–14
5x106
Table 1
UV–vis spectral data for H2TPP and its derivatives.
H2APTPP
Samples
λmax/nm(CHCl3)
4x106
3x106
2x106
Schiff base bridged biporphyrin
Porphyrin-schiff base
H2TPP
Soret
Q(IV)
Q(III)
Q(II)
Q(I)
H2TPP
H2APTPP
Porphyrin-Schiff base
SBBBPor
410.0
415.5
418.5
420.0
514.0
516.5
516.5
516.0
549.0
553.0
552.0
552.5
589.5
592.0
592.0
591.5
645.5
648.0
646.0
646.5
1x106
0
fluorescence of materials. Therefore the fluorescence (FL) determina-
tions for all prepared porphyrin derivatives were performed as described
in Figs. 1 and 2. Compared to the spectrum of H2TPP (λEx = 430 nm,
650
700
750
λ
/ nm
λEm = 651 nm), the emission peak of H2APTPP (λEx = 434 nm, λEm
=
655 nm) appeared with slightly red shift. The amino group in H2APTPP
donated electron to porphyrin ring, and it resulted in red shift in its emis-
sion spectrum. This electronic effect was similar to that in the FL deter-
mination of SBBBPor system (λEx = 439 nm, λEm = 655 nm); while in
the FL determination of porphyrin-Schiff base (Por-SB), the electron do-
nating from \C_N\ group and the electron accepting from \C_O
Fig. 2. Emission spectra of the synthetic porphyrin derivatives.
combine two protons to form a species of H4Por2+ cation with obvious
color change [23]. Furthermore the reversible color change for the syn-
thetic SBBBPor was realized by leading in dry HCl gas, and then followed
by adding base solvents of pyridine, triethylamine, or strong aqua am-
monia. Therefore this color change can be explained as the molecular
recognition of SBBBPor towards HCl molecules, and the HCl was
produced from chloroform's photodecomposition by ultraviolet irradia-
tion [24,25]. Therefore Fig. 4 provided a model for the metal free
porphyrin in molecular recognition towards hydrogen halides. This rec-
ognition towards HCl molecule is described in Scheme 2. In fact, nearly
all of the metal-free porphyrins in dichloromethane and chloroform so-
lutions exhibited the same coloring effect after ultraviolet irradiation,
and their reversible color change can be simulated by halochromic ex-
periments [26]. The detailed research and mechanism for this color
change of other metal-free porphyrin derivatives would be reported
elsewhere.
For the porphyrin chromophores, the delocated π-electron system in
conjugated ring and the radical species O2• from microwave irradiation
of oxygen in air resulted in unpaired electron appearance [27–29], and
these radical signals were easily detected by the EPR method [30,31].
But in a chloroform solution with SBBBPor, the unpaired electron was
not easy to be detected before UV irradiation. Under UV irradiation,
along with the molecular recognition towards decomposed HCl, the
group counteracted each other, then its emission peak position (λEx
=
440 nm, λEm = 651 nm) was similar to that of H2TPP. Therefore the elec-
tron donating effect from \C_N\ group in SBBBPor resulted in red shift
in its FL spectrum.
Violet irradiation experiments were carried out in a chloroform
solution with SBBBPor (1.0 × 10−3 mol/L). Before and after violet
irradiation, the UV–vis spectral variation was recorded (Fig. 3). At the
beginning, the UV–vis spectrum of SBBBPor was just the same to
those described in entries 5, Table 1; during 10 s' ultraviolet irradiation,
the original Soret band at 420 nm and Q (I–IV) were weakened along
with an isosbestic point at 434 nm, a new Soret band at 451 nm and a
new broad and strong Q band at 669 nm emerged; and finally, the
original Soret band and Q (I–IV) completely disappeared, and the new
Soret and Q bands reached their maximum absorption, and accompa-
nied color of the solution changed from pale green to dark green. This
color variation seems to adopt a photochromic mechanism. If this
color change was depended on the molecular structure change from
trans- to cis-configurations, SBBBPor would be the potential candidate
for molecular switch material [18–20], while the enhancement of Q
band in absorption width and intensity would exhibit itself in spectral
hole-burning materials [21].
What was the real reason for this color change? To seek an answer
several experiments and investigations were carried out. Our previous
molecular self-assembly indicated that H2TPP combined with two HCl
molecules to form H4TPPCl2 in solid state (Fig. 4) [22]. Therefore
metal-free porphyrins possess molecular recognition towards hydrogen
halides, while in aqueous solution metal-free porphyrin (H2Por) can
3.5
3.0
20s
2.5
5x106
H2APTPP
2.0
1.5
4x106
Schiff base bridged biporphyrin
H2TPP
Porphyrin-schiff base
0.0s
5s
3x106
2x106
1.0
0.5
0.0
1x106
0
420
400
500
600
700
430
440
/ nm
450
460
λ / nm
λ
Fig. 3. UV–vis spectral variation of SBBBPor before and after ultraviolet irradiation (λ =
Fig. 1. Excitation spectra of the synthetic porphyrin derivatives.
360 nm) in CHCl3 solution.

Y.H. Wu et al. / Inorganic Chemistry Communications 45 (2014) 10–14
13
N
H
H2O, HCl
N
N
H
N
Hydrothermal
Self-assembly
Fig. 4. Self-assembly of H2TPP with HCl by hydrothermal method.
planar configuration of SBBBPor was partially destroyed, as a result the
unpaired π-electrons were detected, and a weak EPR signal was
detected around 3520 G (Fig. 5). Therefore molecular configuration
change resulted in a different distribution state of π-electrons. And
this change can also be monitored by EPR determinations.
Porphyrin's derivatives were favored sub-units as used in the
electronic luminescent and devices. These applications are mainly
based on their special molecular configurations: (1) with cyclic rigid
structure of π-conjugations; (2) small energy difference between
HOMO and LUMO resulted in red emission; and (3) easy to be modified
in the molecular periphery, and they can conveniently ligate different
metal centers in their complexes, and furthermore to be constructed
as a whole conjugated molecule to transmit electrons and photons
[32–34]. Hereby a Schiff base bridged biporphyrin (SBBBPor) was pre-
pared successfully. This novel derivative with conjugation system of
porphyrin-Schiff base–Schiff base-porphyrin configuration possessed
red emission property and it can easily recognize HCl molecules from
the decomposition of chloroform. This supramolecular SBBBPor with
conjugated structure would be a potential candidate in the design and
assembly of molecular wires.
Acknowledgment
The authors acknowledge financial support from the National Scien-
tific Foundation of China (No. 20771073), and the Key Project of Educa-
tion Office from Guangdong Province, China (No. 2012CXZD0023).
Ph
Ph
Appendix A. Supplementary data
NH
N
N
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2014.03.017.
Ph
Ph
NH
N
N
HN
Ph
Ph
HN
Ph
Ph
SBBBPor In CHCl3
UV Irradiation
1.0x106
UVI 60 s
λ
= 360 nm, 20s
Ph
UVI 10s
UVI 0.0s
Ph
NH
HN
Cl
HN
Cl
NH
Ph
Ph
NH
HN
Ph
Cl
NH
Ph
Cl
HN
3480
3500
3520
3540
Ph
Magnetic flux density / Tx104
Ph
Fig. 5. EPR spectra of Schiff base bridged biporphyrin (SBBBPor) before and after
ultraviolet irradiation (UVI, λ = 360 nm) in CHCl3 solution.
Scheme 2. Molecular configurations of SBBBPor before and after UVI in CHCl3 solution.

14
Y.H. Wu et al. / Inorganic Chemistry Communications 45 (2014) 10–14
[20] K. Baberschke, Magnetic switching of Fe-porphyrin molecules adsorbed on surfaces:
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