HUNGARIAN JOURNAL OF

INDUSTRY AND CHEMISTRY

Vol. 45(1) pp. 29–36 (2017)
hjic.mk.uni-pannon.hu
DOI: 10.1515/hjic-2017-0006

FORMATION, PHOTOPHYSICS, PHOTOCHEMISTRY AND QUANTUM

CHEMISTRY OF THE OUT-OF-PLANE METALLOPORPHYRINS
1* 1 1 2
ZSOLT VALICSEK, MELITTA P. KISS, MELINDA A. FODOR, MUHAMMAD IMRAN ,
1
AND O TTÓ H ORVÁTH

1
Department of General and Inorganic Chemistry, Institute of Chemistry, Faculty of Engineering,
University of Pannonia, Egyetem u. 10., H-8200 Veszprém, HUNGARY
2
Department of Chemistry, Baghdad-ul-Jadeed Campus, The Islamia University of Bahawalpur,
63100 Bahawalpur, PAKISTAN

Among the complexes of porphyrins, special attention has been paid to those possessing out-of-plane (OOP)
structures, for the formation of which the size, as well as the coordinative character of the metal center are re-
sponsible. In these coordination compounds, the central atom cannot fit coplanarly into the cavity of the ligand,
hence, it is located above the porphyrin plane, distorting it. Equilibria and kinetics of the complex formation,
spectrophotometric, photophysical and primary photochemical properties of post-transition and lanthanide OOP
metalloporphyrins were investigated, in addition electronic structural calculations were performed; hence, the
general OOP characteristics were determined.Meanwhile, few doubtful questions have attempted to be an-
swered concerning the categorization of metalloporphyrins, the borderline case complexes and hyperporphyrins.

Keywords: out-of-plane metalloporphyrins, formation kinetics, UV-Vis spectrophotometry, photo-

chemistry, borderline case complexes

1. Introduction activity and coordinative abilities of porphyrins can be

modified. Also, due to the distortion, the degree of
symmetry decreases, resulting in characteristic spectral
Porphyrins and their derivatives play important roles in changes in various ranges of the electromagnetic
several biochemical systems. Four pyrroles are spectrum. The most frequent types of distortions are
connected through methylidine bridges, forming the dome, saddle, ruffled and wave (chair-like) [3].
porphin ring. Its planar structure with an extended Overcrowded substitution on the periphery [3-4] or
conjugated π-electron system provides aromatic insufficiently short metal-nitrogen bonds due to the
characteristics and a special coordination cavity for the shrinkage of the coordination cavity can cause the
binding of metal ions of suitable radius [1-2]. ruffled or saddled deformation [2, 5-7]. If, however, the
Metalloporphyrins are the central parts of naturally M-N bonds are significantly longer than half the length
important compounds, e.g., magnesium(II) chlorins in of the diagonal N-N distance in the free-base porphyrin,
bacteriochlorophylls and chlorophylls; iron(II) dome deformation can occur. This happens if the radius
protoporphyrin in hemoglobin; and iron(III) of the metal center exceeds the critical value of about
protoporphyrins in myoglobin, cytochromes, oxidase, 75-90 pm (depending on the type of porphyrin ligand)
peroxidase, catalase, and oxoanion reductase enzymes. or square planar coordination is not preferred. Such
Ringed tetrapyrroles provide strong chelating effect metal ions are too big to fit into the ligand cavity. Hence
which can promote the hyperaccumulation of rare metal they are located above the plane of the pyrrolic
ions in living cells, and also in abiotic environments, nitrogens; forming sitting-atop (SAT) or out-of-plane
e.g. in kerogens. (OOP - see Fig.1) complexes, displaying
In porphyrins the conjugation favors a planar thermodynamic instability, kinetic lability, typical
structure. However, peripheral substituents or the metal photophysical features and photochemical reactivity [8-
center (originating from its size or axial ligand) can 9].
cause geometrical distortion. This certainly affects the In this work, we review our recent results
functions of enzymes, as well as the biosynthesis of regarding the formation, structure and photoinduced
metalloporphyrins. In chemical research, due to behavior of water-soluble OOP metalloporphyrins.
distortion, redox potentials, basicity, reactivity, catalytic These complexes with a diverse range of metal ions can
be more simply produced in aqueous systems than in
organic solvents. In this regard one of the most suitable
*Correspondence: valicsek@almos.uni-pannon.hu
free-base ligands is the anionic 5,10,15,20-tetrakis(4-
sulfonatophenyl)porphyrin (H2TSPP4– - see Fig.1) due

Brought to you by | University of Pannonia

Authenticated | valicsek@almos.uni-pannon.hu author's copy
Download Date | 5/2/18 3:53 PM
30 VALICSEK, KISS, FODOR, IMRAN, AND HORVÁTH

to its negative charge promoting the coordination of

positively charged metal ions. Besides, this ligand is the
most frequently used reagent among the free-base
porphyrins [1].

2. Experimental

Analytical grade tetrasodium 5,10,15,20-tetrakis(4-

sulfonatophenyl)porphyrin (C44H26N4O12S4Na4·12H2O
= Na4H2TSPP·12H2O) (Sigma–Aldrich) and simple
metal salts such as nitrate, sulfate, chloride or
perchlorate were used for the experiments. The solvent
was double-distilled water purified with a Millipore
Milli-Q system. The pH of the majority of the
a)
metalloporphyrin solutions was adjusted to 8 by
application of a borate buffer, whilst maintaining the
ionic strength at a constant value of 0.01 M. In a few
cases, the pH was regulated to 6, and the ionic strength
to 1 M, by an acetate buffer, to hinder hydrolysis.
The absorption spectra were recorded and the
spectrophotometric titrations were monitored by using a
Specord S-100 or a Specord S-600 diode array
spectrophotometer. For the measurement of
fluorescence spectra, a Perkin Elmer LS-50B or a
Horiba Jobin Yvon FluoroMax-4 spectrofluorometer
was applied. The latter piece of equipment
supplemented with a time-correlated single photon
counting (TCSPC) accessory was utilized to determine
fluorescence lifetimes, too. UV-Vis spectrophotometric
b)
data (molar absorption, fluorescence quantum yields
and lifetimes) of the free-base porphyrin were used as Figure 1. Structure of an in-plane metalloporphyrin
references for the determination of those of {MTSPP=metallo-5,10,15,20-tetrakis(4-
metalloporphyrin complexes [1]. sulfonatophenyl)porphyrin} (a);
For the determination of photochemical properties and that of an out-of-plane complex (b) [9].
via continuous irradiations, a piece of AMKO LTI
photolysis equipment (containing a 200W Xe–Hg lamp generally with intensities of one order of magnitude
and a monochromator) was applied. less. These latter bands in free-base ligands split due to
For the electronic structural calculations, the the presence of protons on two diagonally situated
B3LYP Density Functional Theory (DFT) method and pyrrolic nitrogens, to be more precise, as a result of the
the LANL2DZ basis set were used. On the basis of our reduced symmetry (because of the disappearance of the
earlier quantum chemistry experiences, the four-fold rotation axis) compared to the metallated or
sulfonatophenyl substituents exhibit negligible effects deprotonated forms. This split is not detectable in the
on the coordination of the metal center in the cavity; Soret range, hence, these two types of bands in the
thus, the anionic porphyrin (H2TSPP4−) can be modeled visible region are remarkably different [1].
on the unsubstituted porphin (H2P) [4, 10]. In the Soret region, compared to the corresponding
free-base ligands, the typical in-plane metalloporphyrins
(e.g. Fe3+, Au3+, Cu2+, Pd2+) exhibit blueshifts because
3. Results and discussion
the atomic orbitals of their metal centers which are
covalently bonded in the plane can overlap more
3.1. UV-Vis spectrophotometry strongly with the highest occupied molecular orbitals
(HOMO) of the ligand, resulting in a stronger reduction
Porphyrins and their derivatives belong to the strongest in energy; whereas the lowest unoccupied molecular
light-absorbing materials (both natural and artificial), orbitals (LUMO) do not change. Thus, the energy gaps
therefore, ultraviolet-visible spectrophotometry is one of between the excited and ground states become greater.
the most fundamental, in addition, most informative In the OOP complexes, the atomic orbitals of the more
spectroscopic techniques in porphyrin chemistry. They weakly bonded metal ions (e.g. Cd2+, Hg2+, Tl3+) may
possess two ππ* electronic transitions in the visible slightly affect the unoccupied MOs and to a lesser
range of the electromagnetic spectrum: B- or Soret band extent the occupied ones, leading to a reduction of the
at about 350-500 nm, usually with a molar absorbance energy gaps, i.e. an increase in the corresponding
of 105 M-1cm-1 (Fig.2), and Q bands at 500-750 nm wavelengths (Scheme 1) [1, 9].
Hungarian Journal of Industry and Chemistry

Brought to you by | University of Pannonia

Authenticated | valicsek@almos.uni-pannon.hu author's copy
Download Date | 5/2/18 3:53 PM
STUDY OF THE OUT-OF-PLANE METALLOPORPHYRINS 31

Figure 2. Absorption spectra of the free base

(H2TSPP4–); the highly distorted, octabrominated free
base (H2TSPPBr84–); a typical in-plane (PdIITSPP4–);
and a typical out-of-plane metalloporphyrin
(HgIITSPP4–) within the Soret range [1].
Scheme 1. Simplified energy level diagram for the
change of the porphyrin’s molecular orbitals in
different types of complexes [9].
tetradentate, as well as protonated structure. The
formation of an out-of-plane complex of a large metal
Beside electronic factors, due to the rigidity of the ion is usually at least two orders of magnitude faster
porphyrins’ ringed structure, steric effects also influence than that of an in-plane one since a smaller metal ion is
the spectra: the redshift of absorption bands is one of not able to coordinate to all four pyrrolic nitrogens of
the most common spectroscopic consequences of the the reaction intermediate, in the cavity of which the two
non-planarity of porphyrin [3]. Octabrominated free- protons also remain {H2-P-M}. Therefore, dissociation
base porphyrin, H2TSPPBr84–, was applied to investigate of the metal ion is more favorable than that of the
the spectrophotometric effects of the macrocycle’s protons. Besides the insertion of a smaller metal ion, its
highly saddle-distorted structure (Fig.2). In porphyrins dissociation from the in-plane complex of the end-
with aryl substituents, this distortion can lead to the product may be kinetically hindered due to the rigidity
extension of delocalization by the twisting of aryl of the macrocycle [1].
substituents from a nearly perpendicular orientation Formation of the in-plane complexes used to be
closer to the porphyrin plane (Fig.1) [4]. enhanced by the addition of a small concentration of a
The larger, post-transition metal ions, e.g. metal ion with larger ionic radius (e.g. Cd2+, Hg2+, Pb2+)
thallium(I), lead(II) and bismuth(III) ions, can cause a to the solution of the smaller one because the insertion
similarly large redshift of the porphyrins’ absorption of the larger metal ion into the ligand cavity is much
bands. Since their complexes possess the most highly faster. However, the OOP complex is considerably less
dome-distorted structures, also a ruffled-like stable. In its dome-distorted structure, two diagonal
deformation of the periphery superposes on this high pyrrolic nitrogens are more accessible from the other
degree of doming. Considering the spectral effects side of the ligand, owing to the enhancement of their sp3
(bathochromic or not quite exactly hyperchromic hybridization, hence, the metal center can be easily
effects), the complexes possessing such highly exchanged for the smaller one [1-2, 9].
redshifted absorption bands used to be referred to as This accessibility makes the realization of
hyperporphyrins; depending on the highest occupied dinuclear out-of-plane monoporphyrins (2:1 complexes)
electron subshell of the metal center, p- or d-type possible if the metal ion possesses a low (single)
hyperporphyrins. Previously in terms of this positive charge and is large, i.e. its charge density is
categorization of metalloporphyrins, only the electronic small enough, e.g. mercury(I), silver(I) and thallium(I)
effects of the metal ion (through its electron ions [1, 8-9].
configuration) were taken into consideration and not Moreover, the out-of-plane position of the metal
steric (distorting) effects [1]. Nevertheless, in the typical center, together with the dome-distorted structure
d-type hyperporphyrins, e.g. the low-spin (owing to the twisting of aromatic substituents from a
chromium(III), manganese(III), nickel(II) and nearly perpendicular position closer to the porphyrin
cobalt(III) porphyrins, the radius of the metal center, plane) may promote the formation of so-called
and thus, the metal-nitrogen bonds are too short, sandwich complexes of various compositions, in which
resulting in the contraction of the coordination cavity, two metal ions can coordinate to one macrocycle, and,
along with the ruffled deformation of the macrocycle [2, reversely, one metal ion can concomitantly coordinate
5-7]. to two ligands, (Fig.3) [1-2, 9]. Lanthanide(III) ions
form typical examples of metallo-oligoporphyrins
3.2. Equilibrium and kinetics of complex because they are inclined to form complexes of higher
formation coordination number (8-12). However, they are hard
Lewis acids, hence, their insertion into the coordination
Porphyrins are peculiar ligands in terms of cavity of the softer N-donor porphyrin ligand is a slow
complexation due to their planar, cyclic, rigid, aromatic, and complicated process in aqueous solutions. This
45(1) pp. 29–36 (2017)

Brought to you by | University of Pannonia

Authenticated | valicsek@almos.uni-pannon.hu author's copy
Download Date | 5/2/18 3:53 PM
32 VALICSEK, KISS, FODOR, IMRAN, AND HORVÁTH

phenomenon partly originates from the stability of their

aqua complexes. Due to the consequence of their
Pearson-type hard character, they coordinate rather to
the peripheral substituents of porphyrin (instead of the
pyrrolic nitrogens), i.e. to the ionic group ensuring
water-solubility if they possess similarly hard O-donor
atoms (e.g. carboxy or sulfonatophenyl groups). At
lower temperatures, under kinetic control, the early
lanthanide(III) ions are not able to coordinate into the
cavity, rather to the periphery; resulting in the formation
of the tail-to-tail dimer of free-base ligands (as the tail
used to be referred to as the periphery). Higher a)
temperatures and thermodynamic control are also
necessary for the insertion of metal ions into the cavity
produced by four pyrrolic nitrogens; resulting in the
formation of typical metalloporphyrin complexes. After
the discovery of the possible coordination bonds
between lanthanide ions and sulfonato substituents, the
formation of lanthanide bisporphyrins may be realized
as a tail-to-tail dimerization of two metallo-
monoporphyrin complexes through a metal bridge;
deviating from the head-to-head connection as in the
case of typical sandwich complexes (head refers to the
cavity; see Fig.3). On the basis of our previous b)
experiences, the coordination position of lanthanide ions Figure 3. Potential structures of 3:2 bisporphyrin:
was influenced by the change in temperature [1-2], [11- (a) head-to-head or (b) tail-to-tail [2].
13].
During the investigation of the formation of
“typical p-type hyperporphyrin” complexes (e.g. Tl+, 3.3. Photophysics
Pb2+, Bi3+), the species possessing highly redshifted
absorption bands are the end-products of metalation Porphyrins represent one of the most interesting groups
only in hydrophobic solvents, since they can appear in of compounds in terms of photophysical properties and
aqueous solutions as intermediates with shorter or biological significance. Due to their rigid structure and
longer lifetimes depending on the metal ion. The aromatic electronic system, they display two types of
absorption spectra of the end-products of these fluorescence: beside their relatively strong singlet-1
transformation reactions are very similar (less fluorescence in the range of 550–800 nm, weak and rare
redshifted) to those of the typical, common out-of-plane singlet-2 luminescence is observable between 400 and
metallo-monoporphyrins (e.g. HgII-porphyrin in Fig.2). 550 nm upon excitation of the Soret band (Scheme 1)
This phenomenon may be accounted for to the [1].
considerable coordination ability or the polarizing effect The quantum yields of S2-fluorescence are about 3
of water molecules, which can enable the complex to orders of magnitude lower than those of S1-fluorescence
overcome the kinetic energy barrier towards the in the case of free-base porphyrins, especially ~1200-
formation of the more stable structure, in which the fold for H2TSPP4- (6.3×10-5 vs. 7.5%). However, this
metal center is located closer to the ligand plane, ratio decreases with metalation, mainly in the case of
resulting in decreases in distortion, as well as redshift. the formation of out-of-plane complexes. Since the
Furthermore, “hyperporphyrins” can appear as structure of S2-excited porphyrins may be close to that
intermediates in smaller amounts during the formation of the dome-distorted OOP complexes that are already
of typical, common out-of-plane metallo- in the electronic ground state. Another consequence of
monoporphyrins as well [1, 14]. this structural similarity (namely small Stokes shift) is
In the case of “d-type hyperporphyrin” complexes that the directions of the shifts of S2-fluorescence bands
(e.g. Mn3+, Co3+, Ni2+), the low-spin and ruffled invert (according to Soret absorption) between the in-
complex with a contracted cavity can exist in a spin- plane (redshifted) and out-of-plane (blueshifted)
isomerization equilibrium with the high-spin and planar complexes when compared to the free-base ligand [1].
forms, which not only exhibits less redshift, but rather Singlet-1 fluorescence bands exhibit blueshifts in
blueshifted absorption bands compared to those of the both types (in-plane and out-of-plane) of
free-base ligand. This reaction can be influenced by the metalloporphyrins, as a consequence of the
strength of the M-N bonds (owing to the electronic aforementioned split in free-base ligands because of the
effects of peripheral or axial substituents), due to the presence of two protons, as well as their reduced
size of the coordination cavity (owing to the substitution symmetry (Scheme 1). Furthermore, almost all
or saturation of methylidene bridges or pyrrolic complexes exhibit similarly large Stokes shifts, as well
carbons) [1-2, 5-7]. as lifetimes and quantum yields. In the case of in-plane

Hungarian Journal of Industry and Chemistry

Brought to you by | University of Pannonia

Authenticated | valicsek@almos.uni-pannon.hu author's copy
Download Date | 5/2/18 3:53 PM
STUDY OF THE OUT-OF-PLANE METALLOPORPHYRINS 33

metal centers, the spin-orbit coupling, as an electronic

quenching effect, may be dominant. Whereas for typical
out-of-plane metalloporphyrins, the distortion, as a
steric effect, can enhance their non-radiative decay.
The highly distorted (d- and p-type)
hyperporphyrins, the paramagnetic in-plane complexes
(e.g. FeIIITSPP3-), as well as the head-to-head-type OOP
bisporphyrins {e.g. HgII3(TSPP)26-} do not exhibit
significant levels of luminescence at room temperature.
Conversely, the paramagnetic out-of-plane complexes
(e.g. LnIIITSPP3-) possess similar fluorescence
properties to that of the diamagnetic ones because a
paramagnetic metal ion can cause the disappearance of
fluorescence by spin-orbit coupling only if it is located
in the plane. In the OOP position, it is not able to
perturb as efficiently the molecule orbitals of the
macrocycle that result in the common absorption and
emission out-of-plane characteristics [1-2, 9].
Lanthanide(III) bisporphyrins {LnIII3(TSPP)23-}
have many similarities in terms of absorption and
emission properties to those of monoporphyrin
complexes (LnIIITSPP3-). These may only originate from
the very weak π–π interactions between the macrocycles
in the tail-to-tail-type aggregations (Fig.3) [1-2, 11-13].

3.4. Photochemistry
Porphyrin derivatives are the main components of
photosynthesis, synthetically as well. Since the overall
quantum yield of fluorescence and intersystem crossing Scheme 2. Simplified demonstration of the mechanism
resulting in the formation of triplet states is in excess of for the inner-sphere photoredox reaction of an out-of-
95%, merely a slight proportion of excitation energy is plane metalloporphyrin [8].
dissipated as heat from singlet states. This ratio is the
major reason why porphyrins are efficient in terms of
hence, the coordinative bonds can easily split. The
optical sensations and photosensitizations. Free-base
reduced metal ion can leave the cavity, primarily in
and kinetically inert in-plane metalloporphyrins may be
polar solvents, and induce further redox reactions. The
appropriate candidates to be applied in photocatalytic
latter processes strongly depend on the stability of the
systems based on outer-sphere electron transfer. D-type
reduced metalion in the actual medium. The oxidized
hyperporphyrins can be particularly promising from this
and metal-free (cat)ionic radical of porphyrin is a very
viewpoint owing to their distorted structure which may
strong base: it is immediately protonated and forms the
enhance the (photo)redox reactivity of these
free-base radical, which is a long-lived and rather strong
coordination compounds. In the presence of a suitable
electron acceptor, especally in deaerated solutions.
electron acceptor (methylviologen, MV2+) and donor
Since it would only oxidize water to oxygen at higher
(e.g. triethanolamine, TEOA), these complexes proved
pHs, a slightly more efficent reducer (such as alcohols
to be efficaciousl photocatalysts that transfer electrons
or aldehydes of low molecular weight) is needed, from
between the ground-state reactants through an outer-
which useful byproducts can be produced in terms of
sphere mechanism, generating the MV•+ radical cation.
photocatalytic hydrogen generation. In the absence of
This system can be applied for the production of
any electron donor that promotes the regeneration of the
hydrogen from water [2, 5-7].
porphyrin, it undergoes the primary photochemical
Contrarily, the inner-sphere photoredox reactions
processes; an overall four-electron oxidation involving a
are characteristic of the out-of-plane metalloporphyrins
ring-cleavage, the end-product of which is a dioxo-
because of this special coordination (Scheme 2): an
tetrapyrrole derivative (bilindione). This ring-opening
irreversible photoinduced charge-transfer from the
process can be followed by spectrophotometry owing to
ligand to the metal center (ligand-to-metal charge
the disappearance of the Soret band, as well as the
transfer, LMCT) improves the efficiency of charge
typical change in the region of Q bands [2, 4, 8-16].
separation, which allows their utilization as catalysts in
Photochemical quantum yields of this ring-opening
cyclic processes for the synthesis of chemicals capable
reaction (without regeneration) are about 2-3 orders of
of conserving light energy, hopefully in terms of the
magnitude higher for the out-of-plane complexes
photochemical cleavage of water. Due to photoinduced
(10-4 – 10-2) than for the free-base and in-plane
LMCT the charge of the metal center decreases and its
metalloporphyrins (10-6 – 10-5). In addition, in the case
size increases, overall its charge density diminishes,
of out-of-plane complexes, photoinduced dissociation in
45(1) pp. 29–36 (2017)

Brought to you by | University of Pannonia

Authenticated | valicsek@almos.uni-pannon.hu author's copy
Download Date | 5/2/18 3:53 PM
34 VALICSEK, KISS, FODOR, IMRAN, AND HORVÁTH

the absence of a redox reaction can occur, originating According to our quantum chemical experience,
from their lability, and structural transformations to the value of the critical radius became ~100 pm instead
another complex form or conformer were observed in of the experimentally suggested ~75-90 pm as a
some cases as a photoinduced change of the type or consequence of the significant expansion of the
measure of distortion (e.g. d- and p-type coordination cavity to coplanarly incorporate the metal
hyperporphyrins) [2, 4, 8-10, 14-16]. ions. The proportion of borderline cases, i.e. complexes
Besides the typical post-transition metal ions, with questionable structures (somewhere between in-
lanthanide(III) ions were also applied for out-of-plane plane and out-of-plane), increased with further post-
coordination because their contraction makes the fine- transition metal ions (e.g. Ag2+ [15], Cd2+ [4], Tl3+ [16])
tuning of the out-of-plane distance possible, and their that possess ionic radii of ~90-95 pm. Calculated bond
high negative redox potentials promote the lengths (M-N) and atomic distances (N-N) considerably
photoinduced cleavage of water. Photochemical deviate from the expected ones supposed on the basis of
activities of their complexes confirm that the redox the values of the deprotonated porphyrins (P2-). To
potentials of the metal centers are not the main describe this phenomenon, an axial ligand was applied
determining factor, rather their out-of-plane distances to these metal centers to extract them out of the cavity.
[2, 11-13]. Consequently, expansion stopped, and the out-of-plane
Deviating from the OOP complexes of post- distance increased dramatically together with the degree
transition metal ions, another stable photoproduct was of dome distortion and redshifts of absorption bands.
observable during the photolysis of lanthanide(III) From this point of view, two possible explanations can
porphyrins. It displays a typical absorption band in the be supposed for the borderline-case complexes: the
Q range (at ~600 nm), which may be assigned as a experimentally observed common OOP characteristics
charge transfer between the metal ion and open-chain, may originate from this expansion, tension; and small
dioxo-tetrapyrrole derivative (bilindione, see Scheme 2). perturbations (e.g. the axial coordination in the
Its oxo-groups, as donor atoms, may coordinate with the calculation or photoexcitation in the experiments) may
lanthanide ions, as a consequence of their similar facilitate the metal center to adopt an out-of-plane
Pearson-type hard characteristics, contrary to the softer position, too. Another possibility is that the method of
post-transition metal ions [2, 11]. calculation strongly prefers planar structures.
During the photolysis experiments, only small In our time-dependent density functional theory
differences appeared between lanthanide(III) mono- and (TD-DFT) calculations, the correlation found between
bisporphyrin complexes, which might confirm a special the measured and calculated shifts associated with the
type of aggregation through the peripheral sulfonato position of the metal center was not totally linear, but
substituents with weak π-π interactions (tail-to-tail, see nevertheless acceptable. The main exceptions were the
Fig.3) [2, 11-13]. Deviating from these observations, borderline cases (high-spin Mn2+, Fe2+ and Zn2+) and the
the differences are much more significant in the case of d-type hyperporphyrins (because their structures were
the most typical, post-transition metallo-bisporphyrin determined to be totally planar), as well as the p-type
compared to the monoporphyrin equivalent; namely hyperporphyrins (because a ruffled-like deformation did
between HgII3(TSPP)26- and HgIITSPP4-. The overall not superpose on their dome-like structure).
quantum yield is ~2 orders of magnitude higher and the The regression of correlation was much worse
photoinduced dissociation of a metal ion became the within the Soret band than in the case of the Q bands.
dominant reaction in the head-to-head sandwich The Soret band was also split in the calculations, which
complex as a consequence of the strong π-π interactions cannot be detected experimentally. On the basis of fur-
[9-10]. ther experimental observations and doubts in the litera-
ture, the validity of the theoretical model in use at pre-
3.5. Quantum chemical calculations sent is questionable. Hence, the development of a more
suitable one is in progress.
The main aims of our electronic structural calculations
were to determine the primary consequences for the out-
of-plane position of a metal center, and confirm the 4. Conclusion
experimentally observed correlation between the UV-
Vis spectral shifts and the coordination position of the In conclusion, it can be declared that the categorization
metal center (in-plane or out-of-plane). In the light of of metalloporphyrins was complemented by the role of
these aspects, the unsubstituted porphin (H2P, C20H14N4) their distortion, which is primarily responsible for their
was used as a model for the calculations, instead of the spectral features, whereas the electronic structure of
tetrakis(sulfonatophenyl)porphyrin (H2TSPP4–, their metal centers is a secondary factor, with a
4-
C44H26N4O12S4 ). On the basis of the few comparative considerable level of emphasis on the in-plane
calculations that were conducted, the phenyl-, as well as complexes. The position of the metal center (in-plane or
the sulfonatophenyl substituents have negligible effects out-of-plane) in the monoporphyrin complexes, as well
on the coordination of the metal center in the cavity. as the type (head-to-head or tail-to-tail) of the
However, they can significantly influence the formation bisporphyrin complexes can be determined on the basis
of bisporphyrin complexes, even in the case of head-to- of their UV-Vis absorption and emission properties.
head structures [4, 10].

Hungarian Journal of Industry and Chemistry

Brought to you by | University of Pannonia

Authenticated | valicsek@almos.uni-pannon.hu author's copy
Download Date | 5/2/18 3:53 PM
STUDY OF THE OUT-OF-PLANE METALLOPORPHYRINS 35

Hyperporphyrin spectra can appear, owing to the [4] Valicsek, Z.; Horváth, O.; Lendvay, G.; Kikaš, I.;
peripheral substitution (octabromination) of free-base Škorić, I.: Formation, photophysics, and photo-
ligands. Furthermore, the high degree of redshift may chemistry of cadmium(II) complexes with
disappear during the spin isomerization of d-type 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin
metalloporphyrins or the transformation of p-type ones. and its octabromo derivative: the effects of bro-
Consequently, the real origin cannot be an electronic but mination and the axial hydroxo ligand, J. Photo-
rather a steric effect, namely the measure of distortion, chem. Photobiol. A, 2011 218, 143–155 DOI:
which can confirm the absence of their fluorescence. 10.1016/j.jphotochem.2010.12.014
In terms of photochemical activity, several [5] Fodor, M.A.; Horváth, O.; Fodor, L.; Grampp, G.;
dissimilarities were found between the in-plane and out- Wankmüller, A.: Photophysical and photocatalytic
of-plane metalloporphyrins; the most remarkable of behavior of cobalt(III) 5,10,15,20-tetrakis(1-
them was the mechanism of their photoredox reactions: methylpyridinium-4-yl)porphyrin, Inorg. Chem.
outer-sphere electron transfer is typical of the previous Commun., 2014 50, 110–112 DOI:
10.1016/j.inoche.2014.10.029
ones, while the inner-sphere equivalent is most
prevalent for the latter ones. As a further consequence [6] Fodor, M.A.; Horváth, O.; Fodor, L.; Vazdar, K.;
of the OOP position of the metal center, photoinduced Grampp, G.; Wankmüller, A.: Photophysical and
dissociation and transformation reactions can occur photochemical properties of manganese complexes
within their complexes. with cationic porphyrin ligands: Effects of alkyl
In our electronic structural calculations, the substituents and micellar environment, J. Photo-
number of borderline-case complexes expanded, on the chem. Photobiol. A, 2016 328, 233–239 DOI:
10.1016/j.jphotochem.2016.06.011
basis of which common OOP characteristics that can be [7] Major, M.M.; Horváth, O.; Fodor, M.A.; Fodor, L.;
experimentally observed may acquire a novel Valicsek, Z.; Grampp, G.; Wankmüller, A.: Photo-
explanation. physical and photocatalytic behavior of nickel(II)
5,10,15,20-tetrakis(1-methylpyridinium-4-yl) por-
Acknowledgement phyrin, Inorg. Chem. Commun., 2016 73, 1–3 DOI:
10.1016/j.inoche.2016.09.001
[8] Horváth, O.; Valicsek, Z.; Harrach, G.; Lendvay,
This research was supported by the Széchenyi 2020 G.; Fodor, M.A.: Spectroscopic and photochemical
Fund under the GINOP-2.3.2-15-2016-00016 and properties of water-soluble metalloporphyrins of
EFOP-3.6.1-16-2016-00015 projects. distorted structure, Coord. Chem. Rev., 2012 256,
Assistance with quantum chemical calculations 1531–1545 DOI: 10.1016/j.ccr.2012.02.011
provided by Professor György Lendvay (Research [9] Horváth, O.; Huszánk, R.; Valicsek, Z.; Lendvay,
Centre for Natural Sciences, Hungarian Academy of G.: Photophysics and photochemistry of kinetically
Sciences) is gratefully acknowledged. labile, water-soluble porphyrin complexes, Coord.
Finally, this manuscript is dedicated to the Chem. Rev., 2006 250, 1792–1803 DOI:
memory of Professor János Liszi, who, as the head of 10.1016/j.ccr.2006.02.014
the Doctoral School for Chemistry at the University of [10] Valicsek, Z.; Lendvay, G.; Horváth, O.: Equilibri-
Veszprém, praised the corresponding author’s PhD um, photophysical, photochemical and quantum
dissertation of a similar title in 2007. chemical examination of anionic mercury(II)
mono- and bisporphyrins, J. Phys. Chem. B, 2008
112(46), 14509–14524 DOI: 10.1021/jp804039s
[11] Imran, M.; Szentgyörgyi, C.; Eller, G.; Valicsek,
Z.; Horváth, O.: Peculiar photoinduced properties
REFERENCES of water-soluble, early lanthanide(III) porphyrins,
Inorg. Chem. Commun., 2015 52, 60–63 DOI:
10.1016/j.inoche.2014.12.016
[1] Valicsek, Z.; Horváth, O.: Application of the elec- [12] Kiss, M.P.; Imran, M.; Szentgyörgyi, C.; Valicsek,
tronic spectra of porphyrins for analytical purpos- Z.; Horváth, O.: Peculiarities of the reactions be-
es: the effects of metal ions and structural distor- tween early lanthanide(III) ions and an anionic
tions, Microchem. J., 2013 107, 47–62 DOI: porphyrin, Inorg. Chem. Commun., 2014 48, 22–25
10.1016/j.microc.2012.07.002 DOI: 10.1016/j.inoche.2014.08.001
[2] Horváth, O.; Valicsek, Z.; Fodor, M.A.; Major, [13] Valicsek, Z.; Eller, G.; Horváth, O.: Equilibrium,
M.M.; Imran, M.; Grampp, G.; Wankmüller, A.: photophysical and photochemical examination of
Visible light-driven photophysics and photochem- anionic lanthanum(III) mono- and bisporphyrins:
istry of water-soluble metalloporphyrins, Coord. the effects of the out-of-plane structure, Dalton
Chem. Rev., 2016 325, 59–66 DOI: Trans., 2012 41, 13120–13131 DOI:
10.1016/j.ccr.2015.12.011 10.1039/C2DT31189E
[3] Shelnutt, J.A.; Song, X.-Z.; Ma, J.-G.; Jia, S.-L.; [14] Valicsek, Z.; Horváth, O.; Patonay, K.: Formation,
Jentzen, W.; Medforth, C.J.: Nonplanar porphyrins photophysical and photochemical properties of wa-
and their significance in proteins, Chem. Soc. Rev., ter-soluble bismuth(III) porphyrins: the role of the
1998 27, 31–41 DOI: 10.1039/A827031Z charge and structure, J. Photochem. Photobiol. A,
2011 226, 23– 35 DOI: 10.1016/j.jphotochem.2011.10.011
45(1) pp. 29–36 (2017)

Brought to you by | University of Pannonia

Authenticated | valicsek@almos.uni-pannon.hu author's copy
Download Date | 5/2/18 3:53 PM
36 VALICSEK, KISS, FODOR, IMRAN, AND HORVÁTH

[15] Harrach, G.; Valicsek, Z.; Horváth, O.: Water- [16] Valicsek, Z.; Horváth, O.: Formation, photophysics
soluble silver(II) and gold(III) porphyrins: the ef- and photochemistry of thallium(III) 5,10,15,20-
fect of structural distortion on the photophysical tetrakis(4-sulphonatophenyl)porphyrin: New sup-
and photochemical behavior, Inorg. Chem. Com- ports of typical sitting-atop features, J. Photochem.
mun. 2011 14, 1756–1761 DOI: Photobiol. A, 2007 186, 1–7 DOI:
10.1016/j.inoche.2011.08.003 10.1016/j.jphotochem.2006.07.003

Hungarian Journal of Industry and Chemistry

Brought to you by | University of Pannonia

Authenticated | valicsek@almos.uni-pannon.hu author's copy
Download Date | 5/2/18 3:53 PM