The electronic and magnetic properties of iron(III) derivatives of selected substituted meso-tetraphenyl porphyrins: ESR spectroscopic study

Article abstract of DOI:10.1016/j.jpcs.2004.07.008

The electronic and magnetic behaviour of the nonsolvated ferric complexes of: (a) meso-tetratolyl-, (b) meso-tetra-4-hydroxyphenyl-, (c) meso-tetra-4-carboxyphenyl-, and (d) 5-(4-carboxyphenyl)-10,15,20-tritolyl-porphyrins was studied by ESR spectroscopy in the temperature range between 300 and 4 K. The complexes exhibit predominant contribution from the low-spin iron species at high temperature and a drastic shift toward the high-spin iron species at the low-temperature limit. These paramagnetic species are in different proportions at different temperatures, as well as in porphyrins with different substituents. The examined porphyrins obtained as disordered solids contain iron complexes with continuous short-range disorder which is additionally sensitive to change of temperature.

Full text of DOI:10.1016/j.jpcs.2004.07.008

Journal of Physics and Chemistry of Solids 66 (2005) 91–97
www.elsevier.com/locate/jpcs
The electronic and magnetic properties of iron(III) derivatives of selected
substituted meso-tetraphenyl porphyrins: ESR spectroscopic study
a,
D. Skrzypek , I. Madejska , J. Habdas
a
b
*
aA. Chelkowski Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland
bInstitute of Chemistry, University of Silesia, Szkolna 7, 40-007 Katowice, Poland
Received 9 February 2004; revised 10 June 2004; accepted 12 July 2004
Abstract
The electronic and magnetic behaviour of the nonsolvated ferric complexes of: (a) meso-tetratolyl-, (b) meso-tetra-4-hydroxyphenyl-, (c)
meso-tetra-4-carboxyphenyl-, and (d) 5-(4-carboxyphenyl)-10,15,20-tritolyl-porphyrins was studied by ESR spectroscopy in the temperature
range between 300 and 4 K. The complexes exhibit predominant contribution from the low-spin iron species at high temperature and a drastic
shift toward the high-spin iron species at the low-temperature limit. These paramagnetic species are in different proportions at different
temperatures, as well as in porphyrins with different substituents. The examined porphyrins obtained as disordered solids contain iron
complexes with continuous short-range disorder which is additionally sensitive to change of temperature.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
PDT method. Meso-tetra-4-hydroxyphenylporphyrin is
applied since several years as approved for clinical use
photosensitizer [3]. We also decided to investigate these
porphyrins as a result of their chemical properties. This kind
of substituents linked with the phenyl rings makes them
very convenient intermediate to synthesize derivative of
porphyrins with compounds which show biological activity,
e.g. amino acids [4]. Carboxylic group allows forming an
amide bond which is very characteristic for proteins.
Methoxy- and hydroxyl-groups allow forming ether bonds
between a variety of compounds. The presence of such
substituents open the possibility to design compounds with
desired biochemical properties.
The iron porphyrins are important biological transients
whose intermediacy has been demonstrated in numerous
peroxidase and catalase enzymes. Therefore, they are the
most intensively studied from among metalloporphyrins.
A number of results of these investigations have been
summarized in excellent papers by Ann Walker [1,2]. The
ESR spectroscopy is a useful technique for determining
electronic and magnetic properties of the paramagnetic
complexes. It is considerably interesting for establishing
which factors affect the electronic and magnetic properties
and for understanding how these physical properties
may relate to each other. The aim of this work is to analyse
the influence of the porphyrin peripheral substituents and
the temperature effect on the electronic structure of ferric:
(a) meso-tetratolyl-, (b) meso-tetra-4-hydroxyphenyl-,
(c) meso-tetra-4-carboxyphenyl-, and (d) 5-(4-carboxyphe-
nyl)-10,15,20-tritolyl-porphyrins.
2. Experimental procedure
2.1. Synthesis of compounds
The above-mentioned free base tetraphenylporphyrins
were chosen because they can be used as photosensitizers in
Meso-tetraphenylporphyrins and 5-(4-carboxyphenyl)-
10,15,20-tritolylporphyrin
were
prepared
using
the procedure of Adler and Longo [5]. Meso-tetratolylpor-
phyrin (TTP) was synthesied by heating 4-methylbenzalde-
hyde with pyrrole (molar ratio 1:1) in boiling propionic acid.
* Corresponding author. Tel./fax: C48 32 258 8431.
E-mail address: dskrzyp@us.edu.pl (D. Skrzypek).
0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2004.07.008

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D. Skrzypek et al. / Journal of Physics and Chemistry of Solids 66 (2005) 91–97
Overnight cooling gave purple crystals of meso-tetratolyl-
porphyrin which was separated from reaction mixture on
Buchner funnel, washed twice with water and finally with hot
water in order to remove traces of propionic acid and partially
formed tars. The obtained porphyrin was purified on
chromatography column (4!40 cm) filled with silica gel
(60–230 mesh). Dry chloroform was used as an eluent. The
same procedure was used for the preparation of meso-tetra-4-
hydroxyphenylporphyrin (THP) and meso-tetra-4-carboxy-
phenylporphyrin (TCP) using 4-hydroxybenzaldehyde and
4-carboxybenzaldehyde, respectively.
MS spectra were measured on MAS Spectrometer TSQ
700 Finigan-Mat using ESI technique. Molecular peaks
proved molecular mases of investigated metalloporphyrins:
(a) Fe(III)TTP-MZ760; (b) Fe(III)THP-MZ768; (c)
Fe(III)TCP-MZ880; and (d) Fe(III)MCP-MZ790.
IR spectra were recorded on IR 560 Magma Nicolet
spectrometer as KBr pellets in the range 450–4000 cmK1
.
Representative N–H stretching vibrations of free base
porphyrins in the region of 3300–3360 cmK1 are absent in
our spectra. This observation proves successful insertion
of iron(III) to porphyrin ring. Typical absorption band for
C–H, C–C stretching and deformation vibrations are in
agreement with literature [8]. The presence of substituents
of phenyl ring in p-position was observed. C–O stretching
vibration of carboxylic group absorbs at 1701 cmK1 for
TCP porphyrin and at 1723 cmK1 for MCP porphyrin. O–H
stretching mode of hydroxyl group occurring in the THP
porphyrin is presented in the spectra as a broad peak at
3413 cmK1. For TTP and TCP are observed typical
stretching mode for aliphatic hydrocarbons around
5-(4-carboxyphenyl)-10,15,20-tritolylporphyrin (MCP)
was prepared using 4-methylbenzaaldehyde, 4-carboxyben-
zaldehyde and pyrrole with molar ratio (1:3:4). The formed
six possible derivatives of tolylporphyrin were separated on
chromatography column (4!50 cm) filled with silica gel
(60–230 mesh) when mixture of chloroform:methanol, v/v
9:1 was applied as an eluent. The second fraction was the
desired product of 5-(4-carboxyphenyl)-10,15,20-
tritolylporphyrin.
2925 cmK1
.
The Fe(III) porphyrins were obtained by standard
method [6] of refluxing iron chloride (III) in DMF for 1 h.
The metalloporphyrins were precipitated by adding water
and then separated from solvent on Buchner funnel. The
formed porphyrins were checked by spectroscopic methods.
The XPS-technique confirmed the presence of Cl-species in
compounds. The absorption spectra at a room temperature
were measured by a JASCO UV-spectrometer using
chloroform solutions (with exception of Fe(III)THP where
the mixture of methanol/chloroform was used). In the
visible region (Q bands) two characteristic bands for metal
complexes of porphyrins are observed. Instead of the I, III
and II, IV bands which are present in free base porphyrin, a
and b bands have appeared. Their intensities are 10 times
smaller compared to Soret band. However, iron(III) ions
produce abnormal spectral types for the porphyrin which
belong to the ‘hyper spectrum’ class. This spectrum is
defined as a ‘shifted metalloporphyrin spectrum’ with a, b,
Soret and one more extra band. The ‘hyper’ type is caused
by charge transfer and other metal–ligand interactions. The
values of the optical parameters are summarized in Table 1.
The results of electronic spectroscopy data can be explained
by comparison with hemin studies in which the profile in the
visible region near 500 nm is the usual marker of a high-spin
ferric species [7]. On the other hand, the band shifted to
approximately 570 nm is characteristic of low-spin ferric
porphyrins.
The results obtained from TLC chromatography showed
higher RF values of metalloporphyrins in comparison with
free-base porphyrins.
The structural formula of iron(III) derivatives of meso-
tetra-phenylporphyrins is presented in Fig. 1.
2.2. Methods
The electron spin resonance spectra were recorded with a
standard EPR spectrometer operating at X-band (9 GHz)
frequency, using 100 kHz field modulation. The microwave
frequency was measured using Hewlett Packard 534
microwave frequency counter. The temperature dependence
measurements were performed in the temperature range
Table 1
The optical parameters of studied iron-porphyrins
Compound
Soret band (nm)
Q bands (nm)
Fe(III) TTP
Fe(III) THP
Fe(III) MCP
418
511 576 695
504 630 740
512 573 613
Fig. 1. General structure of iron(III) derivatives of meso-tetra-phenylpor-
421.5
417.5
phyrins. For samples: (a) R1,2,3,4ZCH3; (b) R1,2,3,4ZOH; (c) R1,2,3,4
COOH; and (d) R1,2,3ZCH3 and R4ZCOOH.
Z

D. Skrzypek et al. / Journal of Physics and Chemistry of Solids 66 (2005) 91–97
93
The crystalline structure was studied by powder X-ray
diffraction. The amorphous products were obtained by us, as
can be seen in Fig. 2, when exemplary diffraction pattern for
Fe(III)THP is shown. The diffraction patterns of the other
products are similar.
Simulated spectra were calculated with Bruker-Sympho-
nia program.
3. Results and discussion
Fig. 3(a)–(d) shows the ESR spectra of: (a) Fe(III)TTP,
(b) Fe(III)THP, (c) Fe(III)MCP, and (d) Fe(III)TCP
recorded at room temperature. For all compounds
ESR spectra are complex and contain the group lines
around gZ2 (denoted as B) and low-field signal with g-
factor near 6 (denoted as A). The appearance of these
features suggested that two paramagnetic species were
present: (i) A-type signals are due to the high-spin state (SZ
5/2) of Fe(III) ions in the tetragonal symmetry crystal field
Fig. 2. X-ray diffraction pattern of
hydroxyphenylporphyrin.
a
ferric-meso-tetra-4-
from 4 to 300 K with an Oxford Instruments ESR-900
continuous-flow helium cryostat.
All complexes studied in this work were used in the form
of solid samples.
Fig. 3. The comparison of the ESR spectra for: (a) Fe(III)TTP, (b) Fe(III)THP, (c) Fe(III)MCP, and (d) Fe(III)TCP recorded at TZ300 K. D-signal is the free
radical signal originated from base porphyrin.

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D. Skrzypek et al. / Journal of Physics and Chemistry of Solids 66 (2005) 91–97
Fig. 4. (a) The low-temperature ESR spectrum of Fe(III)TTP observed at TZ5 K. (Below) Simulated spectrum for the parameters which are listed in Table 2.
(b) The ESR spectrum of Fe(III) MCP observed at a room temperature. (Below) Simulated spectrum for the parameters which are listed in Table 2 (A-type
signal is not included).
and (ii) B-type signals are associated with the low-spin state
(SZ1/2) of Fe(III) complexes in the rhombic symmetry
crystal field.
gseff Z gs
(3)
and when the magnetic field is applied perpendicularly to
the z-axis
The spin Hamiltonian for a paramagnetic ion with SZ5/2
in tetragonal symmetry may be written as:
2
K2
gteff Z 3gt½1 K2ðgtmBBÞ ð2DÞ
Š
(4)
H Z DfS2z K1=3SðS C1Þg CgsmBBzSz cos q
CgtmBBxSx sin q
From Eq. (4) we can see that when a zero-field-
parameter j2Dj is much greater than the Zeeman
splitting, the signal with effective g values gteffy6 is
observed. The spin-Hamiltonian (1) with the assumption
that D[hn was used to simulate the low-field part of
the experimental spectrum. Fig. 4(a) shows the exemp-
lary simulation obtained for Fe(III)TTP. The values of g-
factors for all compounds are listed in Table 2.
The B-type signal may be described by the rhombic
spin Hamiltonian including only the Zeeman inter-
action:
(1)
where the magnetic field B forms an angle q with the
z-tetragonal axis and D is zero-field splitting parameter.
For a large D value (D[hn), the ESR signal that can
be observed is the transition from mSZK1/2 to C1/2
states. The resonance condition for the ground-state
doublet is described with an effective g value so that [9]
hn Z geffðqÞmBB (2)
When the magnetic field is applied along the z-axis, one
has
H Z mBðgxBxSx CgyBySy CgzBzSzÞ
(5)
Table 2
g-Factors and crystal field parameters for Fe(III)TTP, Fe(III)THP, Fe(III)TCP, and Fe(III)MCP compounds
Compound
Low tem-
perature
g-factors
Room temperature spectra
g-Factors
V (cmK1
)
D (cmK1
)
V/D
Mixing coefficient
a
b
c
(I)
(II)
(I)
(I)
(I)
(I)
(I)
(I)
Fe(III)TTP
Fe(III)THP
Fe(III)TCP
Fe(III)MCP
TZ5 K
gtZ5.9
gjjZ1.99
TZ5 K
gtZ5.8
gjjZ1.99
TZ17 K
gtZ5.75
gjjZ1.99
TZ16 K
gtZ5.75
gjjZ2.00
gzZ2.71
gyZ2.27
gxZ1.96
gzZ2.39
gyZ2.05
gxZ1.85
1353
(II)
2936
(II)
0.46
(II)
0.41
0.36
(II)
0.35
0.05
(II)
0.04
0.02
(II)
0.02
1658
4054
gzZ2.34
gyZ2.08
gxZ1.83
1790
1186
1475
3016
1077
3697
0.60
1.10
0.40
0.35
0.37
0.36
0.04
0.07
0.05
0.02
0.05
0.02
gzZ2.30
gyZ2.10
gxZ1.50
gzZ2.35
gyZ1.97
gxZ1.75

D. Skrzypek et al. / Journal of Physics and Chemistry of Solids 66 (2005) 91–97
95
Crystal field parameters were estimated by using a
D=l Z Eyz KExy KðV=2Þ=l
method outlined by Taylor [10]. According to this
method the electron configuration of low-spin is
(dxy)2(dxz, dyz)3 and the energy separation between the
doubly degenerate dxz and dyz orbitals and the
nondegenerate dxy orbital is a measure of the tetragonal
splitting designated as D. V is designated the energy
separation between the dxz and dyz when lowering
symmetry removes degeneracy. The wave functions of
ferric ions are linear combination of the three states
with mixing coefficients a (for dyz), b (for dxz) and c
(for dxy). If the Zeeman operator is applied to these
functions the relationship between mixing coefficients
and the principal g values is as follows:
Z gx=ðgz CgyÞ Cgz=ðgy CgxÞ KðV=2Þ=l
(8)
(energies are in units of lZ400 cmK1, the spin–orbit
coupling constant).
The ESR g-values, mixing coefficient, tetragonal and
rhombic splitting parameters for all examined complexes
are shown in Table 2.
The B-type signals of investigated porphyrins, presented
in Fig. 3(a)–(d), are dissimilar. The B-type spectrum of
Fe(III)TTP is interpreted as two overlapping rhombic
spectra with g-factors whose values are listed in Table 2.
A common feature of ESR in amorphous solids is the
broadening of absorption lines. Because of the randomness
in the local atomic surrounding of paramagnetic ions in
disordered solids, the ESR spectra in these materials are
explained by the statistical distribution of ESR parameters.
The ESR spectrum of Fe(III)TTP is characterized by smaller
linewidth in comparison with DB for other obtained
porphyrins. Under these conditions, the high-field ESR
spectrum for meso-tetratolylporphyrin at a room tempera-
ture is characterized by two features with g-values of about
a Z ðgz CgyÞ=4p; b Z ðgz KgxÞ=4p;
Z ðgy KgxÞ=4p
c
(6)
with pZðgx Cgy CgzÞ=2.
The crystal field parameters V and D may be expressed
as:
V=l Z Eyz KExz Z gx=ðgz CgyÞ Cgy=ðgz KgxÞ
(7)
Fig. 5. The temperature evolution of the ESR spectra of: (a) Fe(III)TTP, (b) Fe(III)THP, (c) Fe(III)MCP, and (d) Fe(III)TCP.

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D. Skrzypek et al. / Journal of Physics and Chemistry of Solids 66 (2005) 91–97
Fig. 6. The relative spin susceptibility of: (a) Fe(III)TTP, (b) Fe(III)THP, (c) Fe(III)MCP, and (d) Fe(III)TCP versus temperature.
gavZ2.09 and 2.31. At the same time, more symmetric B-
spectrum, characteristic for the others complexes, means
that the features of rhombic spectrum are smeared out, as the
amount of short-range disorder is increased [11]. The
exemplary simulation referring only to B-type species is
presented in Fig. 4(b).
where D and E are zero-field-splitting parameters and
SZ5/2.
Assuming jDjRhn and jD/Ejy3, the following
expression was obtained for the middle Kramers doublet
[11]:
geff Z ð30=7Þ½1 Cð4=7Þqðl2z Kly2Þ Cð4=49Þð4p2
C15q2Þðl2y Clz2Þ Kð2=49Þð5p2 C4q2Þðl2y
The temperature evolution from the room temperature
to TZ4 K of ESR spectrum of ferric complexes is
displayed in Fig. 5(a)–(d). The inspection of overlapping
spectra shows a sequential build-up of the high-spin
species with g-values present near gZ6 (see Table 2).
The two paramagnetic species are in different pro-
portions at different temperatures. For Fe(III)TTP there
was no evidence to support the existence of the low-
spin component over the lowest temperature range,
however, for remaining porphyrins the low-spin com-
ponent is still present but much less intense than at
room temperature.
Kl2z Þ2 Kð24=343Þð4p2 C17q2ފ
(10)
where pZgmBB=ðDCEÞ and qZðDK3EÞ=ðDCEÞ.
The C-signal with gZ4.3 implies the presence in
examined compounds the high-spin ferric species in the
rhombic symmetry crystal field.
Additionally, for explanation of the character of
observed ESR spectra we estimated the temperature
dependence of the spectrum intensity. In Fig. 6 temperature
evolution of intensity of ESR spectrum (calculated as
double integration of the spectrum—DI) is shown. This
defined intensity should be proportional to the spin
susceptibility of the sample.
When the temperature is lowered the ESR spectra of
Fe(III)THP and Fe(III)TCP exhibit the signal with gZ4.3
(denoted as C). The ESR features with geffZ4.3 have been
observed for Fe3C in various disordered solids and were
interpreted by Kliava [11] on the basis of the rhombic spin-
Hamiltonian:
(a) For Fe(III)TTP, susceptibility increases monotoni-
cally up to about TZ50 K, below this temperature the rise is
more rapid; (b) Fe(III)THP susceptibility increases up to
H Z gmBBS CD½S2z K1=3SðS C1ފ CEðSx2 KS2yÞ
(9)

D. Skrzypek et al. / Journal of Physics and Chemistry of Solids 66 (2005) 91–97
97
TZ190 K, next in 40 K!T!190 K range there is a plateau
and below TZ40 K rapid increase is observed; (c) for
Fe(III)MCP DI-values monotonically increase as the
temperature is reduced from 300 to 4 K; (d) at the same
time, spin susceptibility of Fe(III)TCP does not change up to
TZ100 K and next increases to higher values.
the high temperature the samples exhibit predominantly
the presence of the low-spin ferric species in the rhombic
local symmetry and the high-spin ferric species in the (i)
tetragonal and (ii) rhombic symmetries when the tempera-
ture is lowered. These species are in different proportions at
different temperatures, as well as in the porphyrins with
various substituents. The examined porphyrins obtained as
disordered solids contain iron complexes with continuous
short-range disorder which is additionally sensitive to
change of temperature. It is worth noting that observed
effects appear under solvent-free conditions. Therefore, the
ESR investigations provide convincing arguments to
influence of the temperature on electronic and magnetic
properties of iron-porphyrin complexes.
It is known from the ferriheme protein investigations [1]
that two different state ferric species arise from two six-
coordinated conformers, differing in the position of Fe with
respect to the ligands and to the heme plane. According to
the crystal field theory changing axial field strength leads
low-spin to high-spin states. The characteristic features of
disordered solids are distributions of the short-range order
parameters, namely bond lengths and bond angles. We
believe that the examined porphyrins obtained by us as
disordered solids contain iron complexes with different
axial ligands and the different local symmetry around them.
As can be seen in Fig. 6, DI(T) curves exhibit influence of
temperature on electronic and magnetic properties of
obtained complexes. When temperature is lowered the
ESR spectra exhibit predominant contribution of high-spin
state of ferric ions. It means the change of ferric ion position
and stiffness of Fe(III) complexes. For porphyrins with
different substituents this process takes place with various
rates in different temperature ranges. We assume therefore
that the temperature sensitivity of the complexes is
dependant upon the substituents.
References
[1] F. Ann Walker, in: K.M. Kadish, K.M. Smith, R. Guilard (Eds.), The
Porphyrin Handbook, vol. 5, Academic Press, New York, 2000,
pp. 81–183.
[2] F. Ann Walker, Coord. Chem. Rev. 185/186 (1999) 471–534.
[3] R. Bonnett, J. Chem. Soc. 1995; 5.
[4] J. Habdas, Ann. Pol. Chem. Soc. 1 (2003) 88.
[5] A.D. Adler, E.R. Longo, W. Shergalis, J. Am. Chem. Soc. 86 (1964)
3145.
[6] J.W. Buchler, G. Eikellmann, J. Puppe, K. Rohback, H. Schneehage,
D. Weck, Justus Liebigs Ann. Chem. 745 (1971) 135.
[7] M. Gouterman, in: D. Dolphin (Ed.), The Porphyrins, vol. 3,
Academic Press, New York, 1978 (Chapter 1).
4. Conclusions
[8] L.J. Boucher, J.J. Katz, J. Am. Chem. Soc. 89 (1967) 1340.
[9] E.S. Kirkpatrick, K.A. Muller, R.S. Rubins, Phys. Rev. 135 (1964) 86.
[10] C.P.S. Taylor, Biochim. Biophys. Acta 491 (1977) 137–149.
[11] J. Kliava, Phys. Status Solidi (b) 134 (1986) 411–455.
The observed EPR spectra of iron-phenylporphyrin
complexes with various substituents are complex; at