Complex formation and spectral properties of meso- phenyltetrabenzoporphyrins in pyridine and N,N-dimethylformamide

Article abstract of DOI:10.1023/B:RUGC.0000007663.33757.32

Contributions of structural (macroring distortion) and polarization (in asymmetrically substituted derivatives) effects into the reactivity and chromophoric properties of substituted porphyrins were revealed on the basis of the kinetics of complex formation of nona-, deca-, undeca-, and dodecasubstituted porphyrins (meso-phenyltetrabenzoporphyrins) with Zn(OAc) ₂ in pyridine and the electronic absorption spectra of the ligands and their complexes with Zn(II) and Cu(II) in pyridine and N,N-dimethylformamide (DMF). Dodecaphenyl substitution produces a weaker ring distortion in the more aromatic tetrabenzoporphyrin compared with porphyrins themselves. Irrespective of the degree of macroring nonplanarity, the Zn (II) and Cu complexes of tetrabenzoporphyrins with increasing degree of meso-phenyl substitution meet a spectral stability criterion.

Full text of DOI:10.1023/B:RUGC.0000007663.33757.32

Russian Journal of General Chemistry, Vol. 73, No. 8, 2003, pp. 1309 1314. Translated from Zhurnal Obshchei Khimii, Vol. 73, No. 8, 2003,
pp. 1383 1389.
Original Russian Text Copyright
2003 by Berezin, Toldina, Kudrik.
Complex Formation and Spectral Properties
of meso-Phenyltetrabenzoporphyrins in Pyridine
and N,N-Dimethylformamide
D. S. Berezin, O. V. Toldina, and E. V. Kudrik
Ivanovo State University of Chemical Technology, Ivanovo, Russia
Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, Russia
Received July 12, 2001
Abstract Contributions of structural (macroring distortion) and polarization (in asymmetrically substituted
derivatives) effects into the reactivity and chromophoric properties of substituted porphyrins were revealed on
the basis of the kinetics of complex formation of nona-, deca-, undeca-, and dodecasubstituted porphyrins
(meso-phenyltetrabenzoporphyrins) with Zn(OAc)2 in pyridine and the electronic absorption spectra of the
ligands and their complexes with Zn(II) and Cu(II) in pyridine and N,N-dimethylformamide (DMF). Do-
decaphenyl substitution produces a weaker ring distortion in the more aromatic tetrabenzoporphyrin compared
with porphyrins themselves. Irrespective of the degree of macroring nonplanarity, the Zn (II) and Cu com-
plexes of tetrabenzoporphyrins with increasing degree of meso-phenyl substitution meet a spectral stability
criterion.
Over the last two decades benzoporphyrins have
attracted interest due to the recognized propects for
their practical application [1], in particular, as ma-
terials with nonlinear optical properties, semicon-
ductors, photogenerators of singlet oxygen in photo-
dynamic therapy and engineering, as well as models
in studies on photosynthetic processes. Nevertheless,
until the present time benzoporphyrins seem to be the
least explored group of compounds of the entire class
of porphyrin (I) derivatives. This is caused by the
complexity of their synthesis and purification.
in terms of structure and properties [3]. This is proved
by quantum-chemical calculations [4], calculations of
bond -electron charges in molecule II [3], photoelec-
tron spectral data on the ionization potentials of nitro-
gen atoms in the molecules [5], and also data on their
coordination properties [6]. According to [4, 6], there
is a weak -electron interaction between the macro-
cyclic and fused benzene chromophores in molecule
II (the order of the C C
bond is as low as 0.386
[3, 4]). Therewith, the multicontour conjugated
system characteristic of porphyrins themselves [7, 8]
is disturbed. Thus, compound II and its derivatives
are rigid aromatic macrorings combining in their struc-
ure stable benzene and macrocyclic conjugated sys-
ems, which are almost independent of each other.
Tetrabenzoporphyrin (II) and its analogs III VI
present interest, since they occupy an intermediate
place between porphyrin (I) and phthalocyanine (VII)
R4
N
NH
N
N
NH
N
N
NH
N
N
R3
N
R1
N
HN
HN
HN
N
R2
II VI
I
VII
R1 = R2 = R3 = R4 = H (II), R1 = R2 = R3 = H, R4 = Ph (III), R1 = R3 = H, R2 = R4 = Ph (IV), R1 = H, R2 = R3 = R4 =
Ph (V), R1 = R2 = R3 = R4 = Ph (VI).
1070-3632/03/7308-1309$25.00 2003 MAIK Nauka/Interperiodica

1310
BEREZIN et al.
Table 1. Characteristics of the electronic absorption spectra of tetrabenzoporphyrins II VI in pyridine and DMF [
,
max
nm (log )].
Porphyrin
Solvent
DMF
Soret
3
2
2
2
1
II
413 (4.70),
428 (4.75)
416 (4.46),
431 (4.51)
417 (4.11),
432 (4.21)
421 (4.79),
434.5 (4.89)
422 (4.77),
432 (4.93)
422 (4.56),
434 (4.71)
423 (5.12),
436.5 (5.26)
426 (4.94),
440 (5.13)
462.5 (4.68)
569 (3.84)
598.5 sh
(4.13)
600 sh
(3.86)
601.5 sh
(3.49)
602.5 sh
(4.17)
602 sh
(4.16)
604 sh
(3.94)
604 sh
(4.48)
605 sh
(4.31)
605.5 (4.19)
613.5 sh
(4.18)
615.5
(3.96)
614 sh
(3.54)
616.5
(4.28)
616 sh
(4.22)
618.5
(4.06)
616 sh
(4.58)
618.5 sh
(4.38)
649.5 sh
(3.87)
661.5 (4.06)
664 (3.82)
664.5 (3.33)
666 (4.07)
665 (4.02)
666.5 (3.84)
667 (4.24)
666.5 (4.16)
695 (3.47)
697 (3.13)
Pyridine
DMF
570 (3.59)
608.5 (3.95)
608.5 (3.57)
611 (4.29)
609 (4.23)
611.5 (4.04)
612 (4.59)
613 (4.39)
631 (3.91)
637 (3.75)
III
IV
V
529.5 (2.47),
569 (2.97)
530 (3.30),
571.5 (3.73)
524 (3.41),
570.5 (5.24)
573 (3.55)
Pyridine
DMF
Pyridine
DMF
530 (3.64),
572 (5.10)
531 (3.53)
Pyridine
DMF
VI
584 (3.70)
622 sh
(3.89)
627 sh
(3.73)
Pyridine
465.5 (4.57)
543 (3.22),
590.5 (3.38)
651 sh
(3.71)
Dodeca substitution in molecule I, i.e. substitution
of all the twelve its peripheral hydrogen atoms, is one
of the most effective ways of distorting the planar
structure of the aromatic molecule [9]. Consecutive
meso-phenyl substitution in molecule II, leading, in
the limit, to dodeca-substituted tetraphenylbenzopor-
phyrin VI, too, disturbs the planar structure of the
parent molecule. This effect is opposite to benzo
substitution. The nonplanar structure of the Zn(II)
complex of compound VI follows from the X-ray
diffraction data [10]. By dynamic 1H NMR spec-
troscopy [11] it was shown that the distorted structure
of compound VI, its Zn complex, and acid dication
is preserved in solution.
spectrum of compound II differs considerably from
those of porphyrins themselves in band structure, they
are close to each other in the nature of electronic
transitions [7]. This can be considered as evidence for
the similarity of the chromophoric systems of com-
pounds I and II. Thus, bands 1 and 2 (Table 1,
*
Fig. 1) are formed by two
A1g
(A1g
B3u and
B2u) transitions, polarized at a right angle to
each other in the molecular plane, whereas bands 2
and 2 are assigned as their vibrational satellites (A1g
B2u and A1g
B3u). As found by polarization
measurements, both components of the Soret band of
II relate to two different electronic transitions
(A1g
B2u and A1g
B3u [12]), the long-wave
component being polarized similarly to band 1, and
The aim of the present work was to reveal specific
features of distortion of molecules more aromatic than
porphyrins themselves and also to estimate the elec-
tronic and structural contributions of substituents to
variation in the spectral and complex-forming pro-
perties of compound II upon its consecutive meso-
phenyl substitution.
the short-wave, similarly to the A1g
B2u transi-
tion (band 2 ) [7].
The
respective bands of porphyrins themselves, and the
quasiforbidenness of the A1g B3u and A1g
bands of II are much stronger than the
B2u transitions is already lifted as the first fused
benzene fragment appears (monobenzoporphyrin)
[13, 14]. Therewith, splitting of the Soret band into
components is a characteristic of tetrabenzopor-
phyrins only and does not take place in the case of
monobenzoporphyrin [13], cis- and trans-dibenzopor-
phyrins [14, 15], as well as their naphthyl analogs
When benzene rings are introduced into the por-
phyrin macroring, a tetraisoindole macroring II arises
instead of a tetrapyrrole macroring I. The macrocyclic
chromophore in II experiences a weak electron-
acceptor effect of fused benzene rings. It is supposed
that, despite the fact that the electronic absorption
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 8 2003

COMPLEX FORMATION AND SPECTRAL PROPERTIES
100
1311
A
4.0
(a)
2
50
1
2.0
0
0
(b)
100
400
500
600
700
, nm
150
200
1
2
3
4 n
Fig. 2. Changes in (a) E and (b)
S
upon complex
a
Fig. 1. Electronic absorption spectra of (1) meso-phenyl-
tetrabenzoporphyrin (III) and (2) its zinc complex
in pyridine.
formation along the series of meso-phenytetrabenzopor-
phyrins II VI. n is the number of meso-phenyl groups
in tetrabenzoporphyrin.
[16]. In spite of the obvious symmetry of molecule
II, apparently, there is some nonequivalence of iso-
indole (N H bonds) and isoindolenine ( N= atoms)
fragments, which reveals itself in anomaly in certain
photophysical properties of the molecule [7].
stability criterion whose physical sence has been
described in the monograph [3] applies to metal com-
plexes of II. According to this criterion, the thermo-
dynamic stability of metal complexes in a series of
similar compounds is the higher, the larger is the
hypsochromic shift of band 1 of metal porphyrin (MP)
in comparison to band 1 of the porphyrin ligand (H2P)
We established for the first time that consecutive
meso-phenyl substituion in compound II makes the
components of the Soret band closer together until
they coalesce in the spectrum of compound VI.
Apparently, the Soret band in the latter spectrum is
also split, but this splitting is not evident because of
the higher conformational flexibility of the molecule
(the bands are more diffuse). All bands in the visible
region of the electronic absorption spectrum of com-
pound II shift bathochromically with meso-phenyl
substitution (the Soret band and band 1 by 36 and 34
nm, respectively, for compound VI compared to com-
pound II), the strongest changes produced by introduc-
MP
MP
1
(
=
H P). Based on the basis of the
1 2
elec1tronic absorption spectra and kinetic data, we can
contend that the spectral stability criterion applies to
all the tetrabenzoporphyrin complexes studied in the
present work, irrespective of the distortion degree.
Thus, band 1 of the more covalent Cu(II) complexes
[3] exhibits a larger hyprochromic shift compared to
the respective band for the less stable Zn(II) complex
whose M N bonds are more ionic (Table 2).
The electronic absorption spectra of tetrabenzo-
porphyrins II VI in pyridine and DMF (Table 1)
revealed a very weak solvatochromic effect in these
solvents for the whole series of compounds. Analysis
of the log values for benzoporphyrin ligands II VI
(Table 1) shows that band 1 in the spectra of
asymmetrically substituted ligands III, V is always
stronger than in the spectra of symmetrical ligands II,
IV, and VI, which points [3] to polarization effects in
III and V.
tion of the fourth phenyl group [transition V
VI
(26 and 28 nm, respectively)].
Complex formation of tetrabenzoporphyrins II VI
with 3d-metal salts is accompanied by a cardinal
change in their electronic absorption spectra: The
spectra of the complexes contain no other bands than
the Soret band and two visible bands (Fig. 1)
belonging, supposedly, to purely electronic transitions
[7]. The simplification of the electronic spectrum is
associated with increase in the molecular symmetry
and also, probably, with suppression of vibrational
states on complex formation. The positions of bands
in the electronic absorption spectra of metal com-
plexes is determined by the type and strength of M N
chemical bonds.
We have studied the kinetics of complex formation
of phenylbenzoporphyrins II VI with Zn(OAc)2 in
pyridine (Py). The salt was taken in a 100-fold molar
excess compared to porphyrin (H2P), which allowed
us to describe the reaction by a pseudo-first-order
equation. The linear character of the dependences
log (c0H P/cH P) = f( ) (Fig. 2) points to a first reaction
2
2
Karavaeva et al. [17] established that the spectral
order in porphyrin. The true rate constant (kv,
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 8 2003

1312
BEREZIN et al.
Table 2. Electronic absorption spectra of tetrabenzoporphyrins II VI and their complexes with Cu(II) and Zn(II) in
pyridine ( max, nm)
Porphyrin
(metal
a
Soret
3
2
2
2
1
1
porphyrin)
II
415.0, 430.0
428.0
433.0
422.0, 434.0
433.5
438.0
423.0, 434.0
435.0
436.0
428.0, 440.0
438.5
444.5
570.0
529.0
573.0
534.5
600.0 sh
605.5 sh
615.5
663.0
II(Cu)
II(Zn)
III
III(Cu)
III(Zn)
IV
IV(Cu)
IV(Zn)
V
V(Cu)
V(Zn)
VI
625.0
631.5
668.0
628.0
633.5
667.5
629.0
633.5
669.0
632.5
637.0
697.0
652.0
656.0
38.0
31.5
584.0
617.0 sh
605.0 sh
605.0 sh
605.0 sh
627.0 sh
611.0
612.0
613.0
636.0
40.0
34.5
586.0
617.5
583.5
592.0
38.0
34.0
617.5 sh
36.5
32.0
593.0
652.0
sh
466.0
448.0
445.0
542.0, 589.0
VI(Cu)
VI(Zn)
45.0
41.0
612.0
a
MP
1
MP
is the bathochromic shift of band 1 in the electronic spectrum of tetrabenzoporphyrins upon complex formation (
).
=
1
l
H P
1 2
Table 3. Kinetic parameters of complex formation of
tetrabenzoporphyrins II VI with Zn(OAc)2 in pyridine
asymmetrical meso substitution favors the process.
Thus, Ea decreases by 7 kJ/mol and S decreases by
1
15 J mol 1 K in passing from II to III. When the
5
3
(298 K, cII VI 2.6 10 , cZn(OAc) 2.6 10 M)
2
second phenyl group is introduced in III trans to the
first phenyl group, Ea and S return to values cha-
racteristic of II. Presumably, the symmetrical trans-
diphenyl substitution in IV results in symmetrization
Por-
phyrin
,
kv 102,
Ea,
kJ/mol
S ,
exp
nm
a
1
1
l mol 1 s
Jmol 1 K
II
631.5
633.5
633.5
637.0
656.0
0.69
2.56
1.38
4.71
10.03
50.8 0.1
43.2 2.6
50.5 1.7
29.6 1.1
38.5 0.7
124 6.0
139 2.0
120 6.0
179 9.0
143 2.5
of
the
coordination
cavity
[18],
and,
III
IV
V
consequently, of the electron density in the macroring.
In passing from IV to V, Ea and S decrease again
and grow again in passing to VI. These changes may
be associated with polarization of the macroring in II
by asymmetrical meso-phenyl substitution. Similar
effects has been observed with meso- phenylocta-
ethylporphyrins on their complex formation in DMF
and Py [18] and also with meso-alkyloctaethylpor-
phirins in acetonitrile [19].
VI
a
Obtained by extrapolation.
1
l mol 1 s ) was calculated as kv = kapp/cs (kapp is the
1
apparent rate constant, s , and cs is the molar con-
centration of salt, M). Consecutive meso-phenyl
substitution inceases the rate and decreases the activa-
tion energy (Ea) and S (enhanced solvation of the
transition state) of reaction (1) (Table 3). However, as
seen from Table 3, there is no regular trend in the
complex-forming ability of compounds II VI.
Decrease in Ea of reaction (1) always entails de-
crease in S (Fig. 2). However, in force of structural
distinctions between compounds II VI, we failed to
reveal a compensation kinetic effect [3] in the com-
plex formation. Conventionally speaking, II, IV, VI,
on the one hand, and II, III, V on the other, form two
groups of molecules differing by the character of elec-
tron density distribution and macroring distortion. It
should be noted that no correlation was found between
changes in reaction rate and band shifts in the elec-
tronic absorption spectrum on consecutive meso-
H2P + M(OAc)2(Py)n 2
[H2P Zn(OAc)2(Py)n 2
2)Py.
]
ZnP + 2HOAc + (n
Even though the rate of reaction (1) changes rather
steadily (Fig. 2), the activation parameters show that
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 8 2003

COMPLEX FORMATION AND SPECTRAL PROPERTIES
1313
phenyl substitution in II. This result suggests that
chromophoric and coordination properties are dif-
ferently affected by structural and electronic factors.
phenyl rings, whereas in the similar species involving
compound II it is basically localized in the C12N4
macroring [26]. Apparently, in view of the decreased
electron density in the macroring in tetrabenzopor-
phyrin II compared with porphyrin I, the meso-phenyl
substituent exerts a very weak electron-donor effect
in the former. Therefore, in the case of symmetrical
tetra-meso-phenyl substitution, the polarization factor
is insignificant and the increased complex-forming
ability of compound II is defined by the distorting
effect of phenyl groups on the aromatic macroring.
In the case of asymmetrical substitution, as, for
example, in V, the polarization factor shows up in the
coordination properties of phenylbenzoporphyrins.
Thus, the energy of the process is most strongly af-
fected by the asymmetrical triphenyl substitution in
compound V for which the activation energy of reac-
tion (1) is the lowest (Table 3, Fig. 2), rather than the
dodecaphenyl substitution in compound VI whose
macroring is distorted to the greatest extent [10, 11].
Apparently, such an asymmetrical substitution results
in that the molecule can adopt one of the most re-
active asymmetrically distorted conformations with a
higher degree of of N H bond polarization. Evidence
for the low symmetry of the conformation of com-
1
The H NMR spectral data point to a strong deshield-
ing of NH protons in VI ( 1.2 ppm) [21] compared to
I ( 3.76 ppm) [27]. Nevertheless, the reaction rate
with Zn(OAc)2 in pyridine increases in passing from
II to VI by a factor of 15 only, which may result from
a weak macroring distortion compared to dodeca-
substituted porphyrins themselves, where kv increased
several orders of magnitude [18]. In fact, according to
X-ray diffraction data, the macroring in the Zn com-
plex of octaethyltetraphenylporphyrin is distorted
stronger than in metal-free compound VI. The devia-
tion of C atoms from the macroring mean plane is
more than 1 in the former [28] and is only 0.765
in the latter [10]. Therewith, both the macrorings have
a saddle-type of distortion. The moderate macroring
distortion in the Zn(II) complex of compound VI is
attributable to the aromaticity, i.e. resistance to
deformations, of the parent macroring II.
1
pound V comes from the observation in the H NMR
spectrum of nonequivalence of endocyclic NH protons
already at 298 K [20]. The substitution by phenyl
groups of three meso-hydrogen atoms in molecule II
accelerates reaction (1) almost 7 times and decreases
its Ea by more than 20 kJ/mol and
S
by
1
55 J mol 1 K . The symmetric tetraphenyl substitu-
tion in compound VI appears to be less effective in
this regard. Though the reaction rate increases com-
pared to unsubstituted tetrabenzoporphyrin II by a
factor of 15, the Ea and S values decrease by as
1
little as 13 kJ/mol and 19 J mol 1 K , respectively.
We are the first to study the complex-forming
properties of meso-phenyltetrabenzoporhyrins, and, in
particular, compound VI. As follows from published
data [21], compound VI synthesized by the procedure
in [22] and studied in a number of works, for example,
[23], was a mixture of meso-substituted tetrabenzo-
porphyrins, mainly IV and V [21]. Compound VI as a
single product can be obtained by the procedure in
[24]. We found that, under comparable conditions,
individual porphyrin VI reacts with Zn(OAc)2 in
pyridine almost 2.5 times faster compared with the
mixture in [23], with lower Ea (by 10 kJ/mol) and
EXPERIMENTAL
Spectral and kinetic measurements were carried out
on Specord M-40 and Hitachi U-2000 spectrophoto-
meters. The technique of kinetic measurements has
been described in [23].
Phenylbenzoporphyrins III VI were obtained by
the procedures in [20, 24, 29, 30].
Purity control
was performed by 1H NMR and electronic spec-
troscopy, as well as by TLC on Silica F-60, eluent
CHCl3 Me2CO.
1
S (by 25 J mol 1 K ), i.e. with a stronger solvation
Tetrabenzoporphyrin II was synthesized as de-
scribed in [31]. N,N-Dimethylformamide and pyridine
(chemical grade) were dehydrated before use by reflux-
ing over MgSO4 and KOH, respectively, followed by
distillation. Zinc and copper acetates (analytical grade)
were recrystallized from glacial AcOH and handled in
a desiccator over concentrated H2SO4.
of the transition state.
We can contend that two main factors, electronic
and steric, are responsible for the enhanced complex-
forming ability of meso-substituted benzoporphyrins.
meso-Phenyl substituents, as established in studies on
their acid base and coordination properties [3, 25],
act as electron acceptors in relation to the porphyrin
macroring. In the case of benzoporphyrin II deri-
vatives, meso-phenyl groups, too, actively participate
in electron density distribution. Thus, the spin elec-
tron density in the -radical cation of the zinc com-
plex of VI is substantially redistributed into meso-
ACKNOWLEDGMENTS
The work was financially supported by the
Ministry of Education of the Russian Federation
(grant no. E00-5.0-129).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 8 2003

1314
BEREZIN et al.
Zh. Prikl. Spektrosk., 1980, vol. 32, no. 2, p. 512.
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