Structural, spectroscopic and electrochemical investigations on fluorinated meso -tetraaryl porphyrins

Article abstract of DOI:10.1142/S108842461750064X

Two series of meso-phenyl fluorinated porphyrins and their metal complexes, 5,10,15,20-tetrakis(2′,4′,6′-trifluorophenyl)porphyrin, MT(2′,4′,6′-TFP)Ps (2a-2d) and 5,10,15,20-tetrakis[3′,5′-bis(trifluoromethylphenyl)]porphyrin, MT(3′,5′-BTFMP)Ps (3a-3d); where M = 2H; Ni(II); Cu(II) and Zn(II) have been synthesized and characterized using various spectroscopic techniques including UV-visible, fluorescence and 1H NMR and mass spectrometry. Electronic absorption spectra of porphyrins show the typical Soret (B) and visible (Q) bands. Free ligands and zinc(II) derivatives display two well-defined emission bands around 600-660 nm and 650-720 nm upon exciting the Soret band. Porphyrins, 1d, 2a, 2b, 2c, 3a and 3d were structurally characterized by single crystal XRD analysis, and various intermolecular interactions present in them were quantified on the basis of Hirshfeld surfaces and 2D fingerprint plots. Electrochemical studies were performed and the HOMO-LUMO energy gap is high for all the porphyrins compared to MTPPs.

Full text of DOI:10.1142/S108842461750064X

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2017; 21: 622–631
DOI: 10.1142/S108842461750064X
Published at http://www.worldscinet.com/jpp/
Structural, spectroscopicand electrochemicalinvestigations
onfluorinatedmeso-tetraarylporphyrins
Chellaiah Arunkumar*◊, Fasalu Rahman Kooriyaden and Subramaniam Sujatha
Bioinorganic Materials Research Laboratory, Department of Chemistry, National Institute of Technology Calicut,
Kozhikode, Kerala, India — 673 601
Received 19 March 2017
Accepted 16 October 2017
ABSTRACT: Two series of meso-phenyl fluorinated porphyrins and their metal complexes,
5,10,15,20-tetrakis(2′,4′,6′-trifluorophenyl)porphyrin, MT(2′,4′,6′-TFP)Ps (2a–2d) and 5,10,15,20-
tetrakis[3′,5′-bis(trifluoromethylphenyl)]porphyrin, MT(3′,5′-BTFMP)Ps (3a–3d); where M = 2H;
Ni(II); Cu(II) and Zn(II) have been synthesized and characterized using various spectroscopic techniques
including UV-visible, fluorescence and 1H NMR and mass spectrometry. Electronic absorption spectra of
porphyrins show the typical Soret (B) and visible (Q) bands. Free ligands and zinc(II) derivatives display
two well-defined emission bands around 600–660 nm and 650–720 nm upon exciting the Soret band.
Porphyrins, 1d, 2a, 2b, 2c, 3a and 3d were structurally characterized by single crystal XRD analysis, and
various intermolecular interactions present in them were quantified on the basis of Hirshfeld surfaces and
2D fingerprint plots. Electrochemical studies were performed and the HOMO–LUMO energy gap is high
for all the porphyrins compared to MTPPs.
KEYWORDS: fluorinated porphyrins, spectroscopy, X-ray crystal structure, electrochemistry, Hirshfeld
surface analysis.
such as metabolic stability, tissue distribution and oral
INTRODUCTION
bioavailability as compared to fluorine-free analogues.
Synthetic porphyrins are of widespread interest
Hence, fluorinated drugs are plentiful in pharmaceutical
because they bear close resemblance to naturally
and chemical industries, and their cost-effectiveness
occurring tetrapyrrole pigments. These porphyrin
and social influence encourages more research in this
analogs find a variety of enhanced material applications
area. Several reports are available in the literature which
such as efficient catalysts for various chemical
highlight the importance of fluorinated porphyrinoids.
transformations, dyes for solar cells, optical sensors,
In 2011, Goslinski and Piskorz reviewed the biomedical
etc. [1–5]. They have numerous biological applications
applications of fluorinated porphyrins [9]. To discover
especially in medicinal chemistry as photosensitizers
desirable new drugs, it is important to know their physico-
in photodynamic therapy to kill cancer cells [6, 7]. In
chemical and electrochemical properties. Furthermore,
recent years, fluorinated porphyrins have received much
it is essential to understand the weak intermolecular
attention due to the fact that the installation of fluorine
interactions present in the fluorinated compounds that
into the porphyrin periphery improves the oxidative
describes their structure-property relationship.
stability of the macrocyclic ring as well as decreases the
Inourcontinuedefforttodelineatetheroleoftheposition
ring nitrogen basicity [8]. Also, fluorinated porphyrins
engender enhanced biologically-important properties
of fluorine in the meso-phenyl groups on the spectral and
electrochemical properties of free base/metalloporphyrins,
two series of meso-substituted fluorinated porphyrins and
their metal derivatives, namely 5,10,15,20-tetrakis(2′,4′,-
SPP Full member in good standing
6′-trifluorophenyl)porphyrin (2a–2d), MT(2′,4′,6′-TFP)Ps
and 5,10,15,20-tetrakis[3′,5′-bis(trifluoromethylphenyl)]
porphyrin (3a–3d), MT(3′,5′-BTFMP)Ps; where M = 2H;
*Correspondence to: Chellaiah Arunkumar, e-mail:
arunkumarc@nitc.ac.in; Tel: 91 495 2285307; Fax: 91
495 2287250.
Copyright © 2017 World Scientific Publishing Company

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Fig. 1. Structures of fluorinated meso-tetraaryl porphyrins under study
Ni(II); Cu(II) and Zn(II) (Fig. 1) were synthesized and
as the supporting electrolyte. Single porphyrin crystals
were mounted on a glass capillary with appropriate
size and, data collections were performed on a Bruker
AXS Kappa Apex II CCD diffractometer with graphite
monochromated Mo Ka radiation (l = 0.71073 Å) at
298 K. Reflections with I > 2s(I) were employed for
structuresolutionandrefinement.TheSIR92(WINGX32)
program [14] was used for solving the structure by direct
methods. Successive Fourier synthesis was employed to
complete the structures after full-matrix least squares
refinement on |F|2 using the SHELXL97 software [15].
fully characterized. Note that the effect of change in
organic fluorine position on the structure and crystal
packing of porphyrins remains largely unexamined in the
literature [10]. In this context, structural features including
the various intermolecular interactions involving organic
fluorine present in selected freebase and metal complexes
were discussed in detail. Also, for comparison we have
taken 5,10,15,20-tetrakis(3′,5′-difluorophenyl)porphyrin
and its metalated analogs, MT(3′,5′-DFP)Ps, 1a–1d in
which the structural features of 1a, 1c and 1d are reported
earlier [11, 12].
Synthesis of porphyrins and their metal
complexes [1a–3d]
EXPERIMENTAL
Pyrrole (0.5 mL, 7.2 mmol) and 3,5-difluorobenz-
aldehyde (0.8 mL, 7.2 mmol) were taken in a 500 mL
two necked RB flask containing 300 mL of CH2Cl2 and
purged with N2 gas for 15 min followed by the addition of
Materials and instrumentation
Chemicals employed for synthesis and other studies
were commercially available reagents of analytical
grade and were used without further purification unless
mentioned. Solvents were purified using standard
procedures [13] prior to use. Optical spectra were
recorded at room temperature in CH2Cl2 using a
Systronics double beam 2202 spectrophotometer and
1H NMR spectra were taken in a Bruker Avance III
400 MHz spectrometer. Mass spectra were recorded
under ESI/HR-MS at 61,800 resolution using a Thermo
Scientific Exactive mass spectrometer (Thermo Fischer
Scientific, Bremen, Germany). Fluorescence spectral
data and quantum yield were acquired on a Perkin Elmer
LS 55 luminescence spectrophotometer. Electrochemical
studies were performed on a CH-instruments Inc., USA
(Model: CHI660E) potentiostat/galvanostat equipped
with a Fourier Transform AC Voltammeter. The electro-
chemical cell consisted of a three-electrode assembly: a
glassy carbon working electrode, Ag–AgCl as reference
electrode and a platinum wire as the auxiliary electrode.
Theconcentrationoftheporphyrinsampleswereemployed
as ~1 mM and all measurements were performed at 25°C
in dichloromethane under N2 atmosphere using 0.1 M
tetrabutylammonium hexafluorophosphate (NBu4PF6)
.
BF3 ꢀOEt2 (0.1 mL, 2.5 M). The resultant solution was
stirred at room temperature for 1 h under inert atmosphere
and then 2,3-dichloro-5,6-dicyano-1,4-benzoquinone,
DDQ (550 mg) was added. The contents were stirred
further for one more hour at room temperature in air
and the solvent was removed by a rotary evaporator.
The residue was purified by column chromatography
using silica and hexane/chloroform as eluent, which
afforded the ligand, 1a in 31% yield (420 mg). The
zinc(II) and copper (II) complexes were synthesized in
a chloroform/methanol mixture using the corresponding
metal acetates, and the yields were found to be 88%
and 90% respectively. The nickel(II) complex was
prepared in DMF using nickel chloride and the yield
was 82%. UV-visible data of porphyrins in CH2Cl2, lmax
(log e/M-1 cm-1): 1a, 417 (5.67), 512 (4.45), 545 (3.82),
588 (3.94), 652 (3.57); 1b, 409 (5.54), 525 (4.64), 558
(4.45); 1c, 415 (5.75), 537 (4.51), 572 (3.91); 1d, 423
1
(5.67), 551 (4.84), 614 (4.04). H NMR data in CDCl3
(400 MHz): 1a: -2.96 (s, 2H, imino-H), 7.28–7.33
(m, 4H, p-phenyl-H), 7.75–7.76 (d, 8H, o-phenyl-H), 8.89
(s, 8H, b-pyrrole-H); 1b: 8.33 (s, 4H, p-phenyl-H), 8.53
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C. ARUNKUMAR ET AL.
(s, 8H, o-phenyl-H), 8.71 (s, 8H, b-pyrrole-H); 1d: 7.33
(m, 4H, p-phenyl-H), 7.77–7.79 (d, 8H, o-phenyl-H),
9.01 (s, 8H, b-pyrrole-H).
characterized by conventional spectroscopic methods
and single crystal X-ray diffraction analysis.
Porphyrins, 2a–2d and 3a–3d were synthesized
similarly as described above using appropriate aldehydes
and metal salts, and the yields were found to be
quantitative. UV-visible data of porphyrins in CH2Cl2,
lmax (log e/M-1 cm-1): 2a, 413 (5.71), 507 (4.83), 534
(4.60), 584 (4.36); 2b, 406 (5.67), 523 (4.59), 555 (4.38);
2c, 409 (5.71), 534 (4.46), 573 (3.98); 2d, 414 (5.70), 545
(4.42), 580 (3.49); 3a, 418 (5.70), 512 (4.54), 544 (4.13),
588 (4.13), 656 (4.22); 3b, 416 (5.68), 539 (4.48), 572
(3.45); 3c, 414 (5.54), 527 (4.34), 558 (3.77); 3d, 424
Spectral properties
The electronic absorption spectra of freebase por-
phyrins and their metal derivatives showed an intense
1
1
Soret (B) band which corresponds to a A1g → Eu
transition in the range of 413–418 nm and two or four
visible (Q) bands in dichloromethane. In the case of
nickel(II) porphyrins containing a di/trifluorophenyl ring
(1b and 2b), blue shifted absorption spectrum of 7–8 nm
was observed whereas only 2 nm blue shift was observed
forporphyrincontainingtrifluoromethylphenylgroup(3b
and 4b) compared to that of the free ligand [16] (Fig. 2).
The fluorescent spectra of porphyrins were also studied in
order to specify the effect of various fluorophenyl groups
on the electronic properties of the porphyrins in an excited
state, using toluene as the solvent. On excitation at the
Soret band, all the porphyrins exhibit two well-defined
emission bands around 600–660 nm and 650–720 nm.
The S1 → S0 fluorescence quantum yield of fluorinated
porphyrins was measured in toluene using H2TPP as
Ref. [19] (Ff = 0.11) and the values are around 0.05 to
0.06. The data reveals that the incorporation of fluoro
groups at the porphyrin periphery reduces the fluorescent
quantum yield [16].
1
(5.56), 553 (4.93), 598 (3.97). H NMR data in CDCl3
(400 MHz): 2a: -2.84 (s, 2H, imino-H), 7.16–7.18 (m,
8H, m-phenyl-H), 8.89 (s, 8H, b-pyrrole-H); 2d: 7.18–
7.22 (m, 8H, m-phenyl-H), 9.01 (s, 8H, b-pyrrole-H); 3a:
-2.87 (s, 2H, imino-H), 8.38 (s, 4H, p-phenyl-H), 8.69 (s,
8H, o-phenyl-H), 8.80 (s, 8H, b-pyrrole-H); 3b: 8.33 (s,
4H, p-phenyl-H), 8.52 (s, 8H, o-phenyl-H), 8.71 (s, 8H,
b-pyrrole-H); 3d: 8.41 (s, 4H, p-phenyl-H), 8.73 (s, 8H,
o-phenyl-H), 8.94 (s, 8H, b-pyrrole-H). ESI-mass data:
2a: 831.14 (calc: 830.62); 2d: 893.06 (calc: 893.99); 3a:
1159.15 (calc: 1158.15). Fluorescence data for Soret band
excitation of porphyrins in CH2Cl2 at 298 K: lem for 1a:
654 (m), 713 (s); 1b: 657 (w), 716 (w); 1d: 607 (m), 656
(s); 2a: 648 (m), 711 (s); 2b: 657 (w), 711 (w); 2d: 604
(m), 648 (s); 3a: 649 (m), 713 (s); 3b: 661 (w), 721 (w);
3d: 607 (m), 656 (s). For comparison, we discuss 4a–4d;
whose synthesis and characterization were reported by us
previously [16].
Structural descriptions on fluorinated porphyrins
Single crystal X-ray crystallography was used to
determine the molecular structures of porphyrins, 1d,
2a, 2b, 2c, 3a and 3d. Appropriate single crystals of
diffraction quality were grown by a slow diffusion
method using suitable solvents at room temperature
over a period of 7 days. Crystal structure data and the
refinement parameters are listed in Table 1.
The crystal system of the room temperature data
of 1d is orthorhombic, Pccn whereas the reported low
temperature data is found to be tetragonal, I -4 2d;
RESULTS AND DISCUSSION
Synthesis of fluorinated meso-tetraaryl porphyrin
ligandshavebeencarriedoutusingthemodifiedprocedure
of Lindsey et al. [17] and their metal complexes [Ni(II),
Cu(II) and Zn(II)] were prepared by the variant literature
method [18]. All the synthesized porphyrins were
Fig. 2. Overlaid (a) UV-visible (1 × 10-5 M) and (b) emission spectra (1 × 10-6 M) of 2a–2d in dichloromethane at 298 K
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Table 1. XRD crystal structure data of porphyrins, 1d, 2a, 2b, 2c, 3a and 3d
1d
2a
2b
2c
3a
3d
Empirical formula
C48H28F8N4
OZn
C44H18F12N4
C44H16F12N4Ni
C44H16CuF12N4
C52H22F24N4 C60H42F24N4
O5Zn
fw
894.11
1005885
Blue
828.61
1033434
Purple
Trigonal
R-3
887.32
1033436
Orange
Tetragonal
I-42d
892.15
1033435
Red
1158.74
1531386
Purple
Monoclinic
P21/n
1420.35
1033437
Pink
CCDC no.
Color
Crystal system
Space group
a, Å
Orthorhombic
Pccn
Trigonal
R-3
Triclinic
P-1
16.6722 (13)
33.394 (3)
14.3560 (10)
90.0
19.5492 (11)
19.5492 (11)
24.7749 (18)
90.0
15.2487 (5)
15.2487 (5)
15.4058 (7)
90.0
19.697 (3)
19.697 (3)
24.690 (3)
90.0
9.650 (2)
18.479 (5)
28.370 (7)
90.0
8.546 (4)
14.753 (7)
14.800 (7)
109.321 (19)
106.717 (16)
101.424 (15)
293 (2)
0.71073
1595.2 (13)
1
b, Å
c, Å
a, (deg)
b, (deg)
90.0
90.0
90.0
90.0
97.669 (9)
90.0
g, (deg)
90.0
120.00
296 (2)
0.71073
8199.7 (9)
9
90.0
120.00
293 (2)
0.71073
8296 (2)
9
T (K)
296 (2)
0.71073
7992.6 (10)
8
296 (2)
0.71073
3582.2 (2)
4
296 (2)
0.71073
5014 (2)
4
l, Å
Volume (Å3)
Z
Dcalcd (mg/m3)
No. of unique reflections
1.486
1.510
1.645
1.607
1.535
1.478
7036
3576
2422
4746
8830
9854
No. of parameters refined 605
275
138
332
958
594
GOF on F2
R1a
wR2b
1.063
0.979
1.094
1.043
0.972
1.358
0.0472
0.0917
0.0474
0.1209
0.0370
0.0884
0.0525
0.1340
0.0605
0.0605
0.1262
0.3357
aR1 = S||Fo|–|Fc||/S|Fo|; Io > 2s (Io); bwR2 = [Sw(Fo2–Fc2)2/Sw(Fo2)2]1/2
.
and the R factor is 4.72 and 3.34 respectively. One
molecule of porphyrin presents in the asymmetric unit
of 1d and the zinc(II) center is five coordinated with a
THF molecule at the apex position, resulting in a domed
structure, with the zinc atom situated above the plane at
a distance of 0.2472 Å. The solvent molecule THF shows
an envelope confirmation as reported earlier [16] and the
Zn–O distance (2.214 Å) is also comparable with the five
coordinatedzincporphyrin.Theporphyrincorewasfound
to be non-planar in nature with a mean plane deviation of
0.0876 Å and dihedral angles of 58.96, 57.05, 76.35
and 74.59°. The observed non-planarity in 1d is
minimal compared to that of reported low temperature
data of ZnT(3′,5′-DFP)P complex ( 0.218 Å) which
is four coordinated [12] in nature. The molecules are
interconnected through various intermolecular inter-
distance is found to be 2.453 Å, which is comparable
with the reported data [20], and the molecular crystal
packing is through only (ph)C-H F(tfmp) interaction
(Fig. 3d and Table 2).
…
The asymmetric unit of 2a and 2c consists of a half
molecule of porphyrin crystallized in a trigonal system
with an R-3 space group with a similar way of packing
with various intermolecular interactions (Figs 4c, 4f and
Table 2) indicating their isostructural behavior [10, 20].
This is further supported by the Hirshfeld surfaces and
2D fingerprint plots analyzed and discussed later in this
paper. The porphyrin core in both structures is found to
be planar in nature.
The non-planar nature of the porphyrin core in the
nickel(II) complex, 2b is presented in Fig. 5 in which
the nickel(II) atom with four core nitrogen atoms features
distorted square planar geometry with a (N–Ni–N)adj
angle of 90°, but the (N–Ni–N)opp angle deviates slightly
(179.0°). The average Ni–N distance is 1.912 Å in
2b which is lesser than the reported Ni(II) porphyrin
complexes [10, 21] and this leads to a distortion of the
porphyrin core. Moreover, the mean plane deviation of
…
…
actions, namely (ph/pyr/sol)C-H F(ph)
,
(ph/pyr)C-H C(ph/pyr)
…
C
and (ph/sol)
F(ph) (Fig. 3b and Table 2).
However, the asymmetric unit of 3d consists of a
half molecule of porphyrin with a THF molecule at the
apex position resulting in a hexa-coordinated zinc atom.
Hence the porphyrin core is planar in nature. The Zn–O
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C. ARUNKUMAR ET AL.
Table 2. Intermolecular short contact distances (in Å) for the different types of interactions in
the crystal packing of porphyrins
Interactionsa
1db
2ab
2bb
2cb
3ab
3db
(ph)C-H∙∙∙F(ph)
(pyr)C-H∙∙∙F(ph)
(sol)C-H∙∙∙F(ph)
(pyr)C-H∙∙∙F(tfmp)
(ph)C-H∙∙∙F(tfmp)
(ph)C-H∙∙∙C(ph)
(pyr)C-H∙∙∙C(pyr)
(pyr)C∙∙∙F(ph)
2.46–2.66 (8)
2.48–2.62 (6)
2.46 (2)
—
2.48 (4)
—
—
2.58 (4)
—
—
2.49 (4)
—
—
—
—
—
—
—
—
—
—
—
2.45 (4)
—
—
—
—
—
—
2.61 (4)
—
2.79 (2)
2.88 (2)
—
—
—
—
—
2.80 (4)
3.13 (4)
—
—
2.80 (4)
3.14 (4)
3.15 (4)
—
—
—
3.00 (4)
—
—
—
(ph)C∙∙∙F(ph)
3.07 (2)
3.11 (2)
—
—
—
(sol)C∙∙∙F(ph)
—
—
—
—
(pyr)N∙∙∙F(ph)
2.87 (4)
2.30 (4)
—
—
—
(ph)F∙∙∙F(ph)
—
—
—
2.93 (4)
—
—
aDifferent types. bValue in parenthesis gives the number of interactions of each types per molecule.
Fig. 3. (a, c) ORTEP diagrams and (b, d) molecular packing diagrams of 1d and 3d viewed down “c” axes. Atom colors: gray,
carbon; white, hydrogen; blue, nitrogen; green, fluorine; red, oxygen; brown, zinc
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Fig. 4. ORTEP diagrams of 2a and 2c (a, d) top view and (b, e) side view; molecular packing diagrams of 2a and 2c (c, f) viewed
down “c” axes. Atom colors: gray, carbon; white, hydrogen; blue, nitrogen; green, fluorine; brown, copper
Fig. 5. (a, b) ORTEP diagrams of 2b; (c) mean plane deviation of atoms in Å and (d) molecular packing diagram of 2b viewed down
“a” axis showing non-planarity. Atom colors: gray, carbon; white, hydrogen; blue, nitrogen; green, fluorine; brown, nickel
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C. ARUNKUMAR ET AL.
Fig. 6. (a, b) ORTEP diagrams of 3a and (c, d) molecular packing diagrams viewed down “a” and “b” axis respectively. Atom colors:
gray, carbon; white, hydrogen; blue, nitrogen; green, fluorine
atoms is greater ( 0.1262 Å) than that of the reported
nickel(II) complex ( 0.0521 Å) [10] indicating that the
extent of doming of the porphyrin core is higher in 2b.
The asymmetric unit of 3a consists of two half
molecules of porphyrin, crystallized in a monoclinic
system with space group P21/n. The ORTEP diagrams
of 3a are presented in Figs 6a and 6b. Interestingly, the
molecules are packed through (pyr)C-H∙∙∙F(tfmp) interaction
forming a cluster of trimers (Fig. 6c) which is viewed
down “a” axis and depicted in Fig. 6d.
The FPs of the compounds feature spikes of various
length and thickness, and the most dominant one is the
…
…
F
F
H contact (30–47%) except for 3a in which 39% of
F interaction is observed. The interactions involving
fluorine are 43.4% for 1a; 60–65% for 2a-2c; around
75% for 3a and 3d which play a key role in directing
the molecular crystal packing of porphyrins (Fig. 7b).
…
…
In addition, C H, H H contacts are also seen and the
distribution of individual intermolecular interactions is
given in Fig. 8.
Hirshfeld surface analysis
Electrochemical studies
To analyze the intermolecular interactions quantita-
tively, Hirshfeld surfaces (HSs) and 2D fingerprint plots
(FPs) were generated using Crystal Explorer 3.1 [22].
The HSs of 1a, 2a, 2b, 2c, 3a and 3d were mapped over
a dnorm range of -0.30 to 1.46 Å (Fig. 7a) in which the
red spots show the presence of close contacts whereas
areas without close contacts are revealed as blue spots.
In all the cases, the close contacts involving fluorine, i.e.
F∙∙∙C/H/F are observed as intense red spots on the HSs.
As predicted [20], the shape of the FPs of porphyrins
in 2a and 2c are comparable, indicating that they are
isostructural in nature which is further supported by the
geometrical data discussed earlier (Table 2).
In order to understand and study the effect of different
fluorinated meso-phenyl substituents on the porphyrin
p-system, electrochemical studies were performed by
cyclic voltammetric technique using dichloromethane
with 0.1 M tetrabutylammonium hexafluorophosphate
(NBu4PF6) as supporting electrolyte in presence of N2
atmosphere at a scan rate of 100 mV/s, and the data
are summarized in Table 3. For comparison, TPPs
were also analyzed under similar conditions (Table 3)
and their electrochemical redox data were comparable
with reported values [23]. As anticipated, almost all the
ligands and metal derivatives (Ni, Cu and Zn) show two
reversible reductions and oxidations except in few cases.
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Fig. 7. (a, c) HS of 1d, 2a, 2b, 2c, 3a and 3d mapped with dnorm ranging from -0.30 Å (red) to 1.46 Å (blue); (b, d) 2D-FP of 1d, 2a,
2b, 2c, 3a and 3d with di and de ranging from 1.0 to 2.8 Å
Figure 9 represents the overlaid cyclic voltammograms
(CVs) of freebase porphyrins bearing different fluorinated
meso-phenyl substituent(s) along with H2TPP. Insertion
of fluorophenyl moiety at the porphyrin macrocycle
(1a–4a) results in a positive shift in redox potentials
[23–25] which shows the electron-deficient nature of the
fluorinated porphyrins under study. In the case of 1a, a
positive shift of 0.33 V of the first oxidation potential and
a shift of 0.16 V of the first reduction potential compared
to H2TPP were observed. Similar trends were observed
in other freebase porphyrins and their metal derivatives.
Thus, all the compounds are harder to oxidize and easier
to reduce compared to MTPPs.
The potential difference between the first ring-
centered oxidation and reduction was identified as the
Fig. 8. Intermolecular interactions (in %) for porphyrins, 1d,
2a, 2b, 2c, 3a and 3d on the basis of HSs
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630
C. ARUNKUMAR ET AL.
Fig. 9. Overlaid cyclic voltammograms of 1a, 2a, 3a and 4a; (a) oxidation, (b) reduction in CH2Cl2 containing 0.1 M NBu4PF6 as
the supporting electrolyte with a scan rate of 0.1 V/s under N2 at 25°C
Table 3. Half-wave electrochemical redox data (inV vs. Ag/AgCl) and calculated HOMO and LUMO energy levels for 1a–4d
Sl. No.
Porphyrin
Oxidation
Reduction
Energy
Level (eV) Gap (eV)
Band
HOMOb
LUMOb
I
II
I
II
1
2
H2TPP
NiTPPa
0.94
1.01
0.98
0.81
1.27
1.07*
1.20
1.01
1.39
1.31
1.32
1.17
1.38
1.37*
1.35
1.12
1.39
1.03
1.16
0.98
1.28
1.31
1.35
1.13
1.51
1.35*
1.44
1.28
1.70
1.57
1.71*
1.44
1.56
1.86*
1.52
1.36
1.47
1.30
1.42
1.25
-1.23
-1.31
-1.28*
-1.34*
-1.07
-0.92*
-1.21
-1.15
-0.93
-1.00
-1.06
-1.36
-0.94
-1.05
-1.06
-1.06
-1.10
-1.32*
-1.19*
-1.43*
-1.55
—
-5.34
-5.41
-5.38
-5.21
-5.67
-5.47
-5.60
-5.41
-5.79
-5.71
-5.72
-5.57
-5.78
-5.77
-5.75
-5.52
-5.79
-5.43
-5.56
-5.38
-3.17
-3.09
-3.12
-3.06
-3.33
-3.48
-3.19
-3.25
-3.47
-3.40
-3.34
-3.04
-3.46
-3.35
-3.34
-3.34
-3.30
-3.08
-3.21
-2.97
2.17
2.32
2.26
2.15
2.34
1.99
2.41
2.16
2.32
2.31
2.38
2.53
2.32
2.42
2.41
2.18
2.49
2.35
2.35
2.41
3
CuTPP
-1.70*
-1.55*
-1.39
-1.38*
-1.58
-1.49
-1.37
-1.61
-1.53
-1.53
-1.26
-1.44
-1.45
-1.39
-1.41
-1.77*
-1.57*
-1.61*
4
ZnTPP
5
H2(3′,5′-DFP)P, 1a
Ni(3′,5′-DFP)P, 1b
Cu(3′,5′-DFP)P, 1c
Zn(3′,5′-DFP)P, 1d
H2(2′,4′,6′-TFP)P, 2a
Ni(2′,4′,6′-TFP)P, 2b
Cu(2′,4′,6′-TFP)P, 2c
Zn(2′,4′,6′-TFP)P, 2d
H2(3′,5′-BTFMP)P, 3a
Ni(3′,5′-BTFMP)P, 3b
Cu(3′,5′-BTFMP)P, 3c
Zn(3′,5′-BTFMP)P, 3d
H2(4′-TFMP)P, 4a
Ni(4′-TFMP)P, 4b
Cu(4′-TFMP)P, 4c
Zn(4′-TFMP)P, 4d
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
aTaken from Chang D, Malinski T, Ulman A, Kadish K. M, Inorg. Chem. 1984; 23: 817; bEHOMO = -(EOxdn +ꢀ4.4) eV and ELUMO = -(ERedn +ꢀ4.4)
eV taken from Lohrman J, Zhang C, Zhang W, Ren S, Chem. Commun. 2012; 48: 8377; *Value taken from DPV. Error: ꢀ30–50 mV.
HOMO–LUMO energy gap and consequently, the band
gap values are calculated in eV for all compounds and
listed in Table 3. It is noted that the band gap is found to
be increased for all the fluorinated freebase porphyrins
(1a–4a) compared to H2TPP. Also, for the metal com-
plexes, the band gap is found to be greater than that
of MTPPs except for 1b, and the value is as high as
2.53 V for 2d.
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STRUCTURAL, SPECTROSCOPIC AND ELECTROCHEMICAL INVESTIGATIONS ON FLUORINATED
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Advances in the Biology, Therapy and Manage-
ment of Melanoma, Davids L (Ed.); 2013; ISBN:
CONCLUSIONS
Meso-tetraaryl fluorinated porphyrins and their
metal complexes, MT(2′,4′,6′-TFP)Ps and MT(3′,5′-
BTFMP)Ps [M = 2H; Ni(II); Cu(II) and Zn(II)] have
been synthesized and characterized by conventional
spectroscopic methods. Structural characterization of
porphyrins, 1d, 2a, 2b, 2c, 3a and 3d were performed by
single crystal XRD analysis and the interactions involving
fluorine play a significant role in directing the molecular
crystal packing. Electrochemical studies showed a
positive shift in redox potentials, and the energy band
gap is increased for all the porphyrins relative to MTPPs.
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Acknowledgments
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the financial support. The authors would like to thank Dr.
Babu Varghese, SAIF, IIT Madras, Chennai, Tamilnadu
for the data collection, structure solution and refinement.
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APPENDIX A. SUPPLEMENTARY DATA
Crystallographic data of the porphyrins, 1d, 2a, 2b,
2c, 3a and 3d have been deposited with the Cambridge
Crystallographic Data Centre with the CCDC numbers
1005885, 1033434, 1033436, 1033435, 1531386 and
1033437. These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge, CB2 1EZ, UK; Fax: + 44 123
336 033; or e-mail: deposit@ccdc.cam.ac.uk.
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