Chelation and fluorescence properties of tetraphenylporphyrin and 5,10,15,20-tetra(4-hydroxyphenyl)porphyrin in acetonitrile

Article abstract of DOI:10.1134/S0036024416120153

The kinetics of complex formation between zinc and 5,10,15,20-tetraphenylporphyrin and 5,10,15,20-tetra(4-hydroxyphenyl)porphyrin in acetonitrile is studied in the temperature range from 298 to 318 K. The fluorescent properties of these compounds are exam

Full text of DOI:10.1134/S0036024416120153

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2017, Vol. 91, No. 1, pp. 94–99. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © Yu.B. Ivanova, A.S. Parfenov, N.Zh. Mamardashvili, 2017, published in Zhurnal Fizicheskoi Khimii, 2017, Vol. 91, No. 1, pp. 93–98.
PHYSICAL CHEMISTRY
OF SOLUTIONS
Chelation and Fluorescence Properties of Tetraphenylporphyrin
and 5,10,15,20-Tetra(4-hydroxyphenyl)porphyrin in Acetonitrile
a
b
a
Yu. B. Ivanova *, A. S. Parfenov , and N. Zh. Mamardashvili
a
Krestov Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, 153045 Russia
b
Ivanovo State Medical Academy, RF Ministry of Healthcare, Ivanovo, 153012 Russia
*e-mail: jjiv@yandex.ru
Received December 9, 2015
Abstract—The kinetics of complex formation between zinc and 5,10,15,20-tetraphenylporphyrin and
,10,15,20-tetra(4-hydroxyphenyl)porphyrin in acetonitrile is studied in the temperature range from 298 to
18 K. The fluorescent properties of these compounds are examined, the emission in the red region of the
5
3
spectrum is measured, and the fluorescence quantum yields are determined. It is found that although the
electronic absorption spectra of the studied compounds are almost identical, hydroxyl substituents are
observed to have a considerable effect on the chelating ability of ligands. The rate constant of the formation
of ZnT(4-OH-Ph)P is thus approximately three times higher than that of ZnTPhP, with the energy consump-
–
1
tion being lower (about 20 kJ mol ). The calculated fluorescence quantum yields of H TPhP, H T(4-OH-
2
2
Ph)P, ZnTPhP, and ZnT(4-OH-Ph)P in acetonitrile are half those in toluene, while the ratio between the
quantum yields of ligands and their metal complexes is a constant equal to approximately 3 and does not
depend on which solvent is used.
Keywords: porphyrin, coordination properties, kinetics, fluorescence
DOI: 10.1134/S0036024416120153
INTRODUCTION
Zinc acetate of analytical grade was purified by
recrystallization from aqueous acetic acid and dehy-
drated at 380–390 K [8].
In this work, we investigate the kinetics of complex
formation between zinc acetate and 5,10,15,20-tetra-
phenylporphyrin (H TPhP) and 5,10,15,20-tetra(4-
2
hydroxyphenyl)porphyrin (H T(4-OH-Ph)P) in ace-
2
RESULTS AND DISCUSSION
tonitrile in the temperature range from 298 to 318 K.
The fluorescent properties of these compounds are
examined, the emission in the red region of the spec-
trum is measured, and the corresponding fluorescence
quantum yields are determined.
The formation of complexes between H TPhP,
2
H T(4-OH-Ph)P, and Zn(OAc) in system (1) was
2
2
examined spectrophotometrically [1, 9] in thermo-
statted quartz cuvettes with ground glass joints at tem-
peratures of 298 to 318 K, with at least three parallel
experiments being performed at each temperature
(Figs. 1–3, Table 1). The accuracy of temperature
control during an experiment was ±0.1 K. Isosbestic
points were clearly observed in all spectra of the reac-
tion systems (Figs. 1 and 3).
EXPERIMENTAL
Electronic absorption spectra (EAS) of solutions of
the studied ligands and their metal complexes were
recorded on a Cary 100 spectrophotometer (Varian). A
Shimadzu RF-5301 spectrofluorophotometer was
used to measure the fluorescence of solutions of
H TPhP and H T(4-OH-Ph)P zinc complexes in ace-
(
H TPhP
)
,
(
H T
(
4-OH-Ph
) )
P
2
2
(1)
−
Zn − CH CN.
( )
OAc 2
3
2
2
tonitrile. The experimental techniques and the meth- The formation of metal–porphyrin complexes (1) is
ods for processing the experimental data were similar characterized by a first order kinetics with respect to
to those described in [1–3]. Acetonitrile of high purity ligand [1]. Complex formation between doubly
containing no more than 0.03% water was used as a charged metal cations and porphyrins in nonaqueous
dipolar aprotic solvent. The spectra confirmed the solutions is described by the equation
molecular form of the initial porphyrins in acetoni-
Н P + [МХ2
(
Solv)n−2] → МР
2
trile. H TPhP and H T(4-OH-Ph)P were synthesized
2
2
(2)
according to the familiar procedures in [4–7].
( )
+ 2НХ + n − 2 Solv,
94

CHELATION AND FLUORESCENCE PROPERTIES
95
ln(С0 /С )
H2P
H2P
308 К
298 К
1
1
0
0
0
.2
.0
.8
.6
.4
318 К
(a)
(b)
А
1
.8
.5
.2
.9
.6
.3
1
1
0
0
0
0.2
0
3
60 380 400 420 440 460
0
500 1000 1500 2000
t, s
λ, nm
Fig. 1. (a) Changes in EAS during the coordination reaction between H TPhP and zinc acetate in system (1) at C
= 9.45 ×
2
porph
-
5
= 8.84 × 10-3 mol L-1 and a temperature of 298 K and (b) the dependence of ln(С 0 /СH2P) on t of the reaction
1
0 , C
Zn(OAc)2
H2P
-
5
= 1.84 × 10-3 mol L-1 and temperatures
of complex formation between zinc and H TPhP (1) at C
= 1.85 × 10 , C
2
porph
Zn(OAc)2
of 298, 308, and 318 K.
−logKeff
3.3
3.2
3.1
3.0
2.9
2.8
2.7
−
logKeff
(a)
(b)
2.9
2.8
2.7
2.6
2.5
2.4
2.3
2
.6 2.7 2.8 2.9 3.0 3.1 3.2
2
.8 2.9 3.0 3.1 3.2 3.3
−
logCZn(OAc)2 [mol/L]
−logCZn(OAc)2 [mol/L]
Fig. 2. (a) Dependence of –logkeff on –logCZn(OAc)2 of the reaction of ZnTPhP formation in system (1) at 298 K (the slope and
coefficient of determination are 1.01 and 0.999, respectively) and (b) the dependence of –logkeff on –logCZn(OAc)2 of the reaction
of ZnT(4-OH-Ph)P formation in system (1) at 298 K (the slope and coefficient of determination are 1.02 and 0.998, respectively).
where X is the acid ligand, Solv is the solvent mole-
The (n + 1)th order rate constants were found via
cule, n is the coordination number of the metal cation, Eq. (4):
H P is the porphyrin ligand (H TPhP or H T(4-OH-
2
2
2
n
2+
kn+1 = k /с
,
(4)
Ph)P), and M is the metal cation (Zn ).
eff
M(OAc)2
Our kinetic experiments were performed with a where n is the order of reaction (2) with respect to salt
in acetonitrile [10]); the
tenfold excess of Zn(OAc) with respect to porphyrin. (n = 1 in the case of Zn(OAc)
2
2
298
k
values are given in Table 2 as kv at a standard
The effective rate constants (k ) of complex forma-
n + 1
eff
temperature.
tion reaction (2) were calculated using the equation
Activation energy E was calculated in the studied
a
k = (1/t)ln[(A – A )/(A – A )],
(3)
eff
o
∞
∞
range of temperatures using the Arrhenius equation:
where A , A, and A are the absorbances of solution at
−E /RT
0
∞
a
k = Ae
= 19.1[(T
or
the initial time, to time t and at the end of the reaction,
(5)
respectively.
Е
а
1
T
2
)/(T
2
−T
1
( )
)]log k /k ,
2 1
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 91 No. 1 2017

9
6
IVANOVA et al.
ln(C0 /C )
H2P
H2P
А
423
298 К
0
0
0
0
0
0
0
.7 (a)
2.5
2.0
1.5
1.0
0.5
0
(b)
308 К
418
.6
.5
.4
.3
.2
.1
3
18 К
0
700
1400 t, s
3
80 390 400 410 420 430 440
λ, nm
Fig. 3. (a) Changes in EAS during the coordination reaction between H T(4-OH-Ph)P and zinc acetate in system (1) at C
=
2
porph
1
.01 × 10–5, CZn(OAc)2 = 1.84 × 10–3 mol L–1 and a temperature of 298 K and (b) the dependence of ln(С 0 /СH2P) on t of the
H2P
reaction of complex formation between zinc and H T(4-OH-Ph)P (1) at C
= 1.01 × 10–5, CZn(OAc)2 = 1.84 × 10–3 mol L–1
porph
2
and temperatures of 298, 308, and 318 K.
and the entropy of the formation of the transition state
The kinetic parameters of the coordination reac-
tion between H TPhP, H T(4-OH-Ph)P porphyrins
≠
(
ΔS ) was determined using the equation
2
2
and zinc acetate in system (1) are summarized in
(6) Table 2. The processes of the partial destruction and
deformation of the coordination sphere of the salt sol-
≠
ΔS = 19.1 × log k2 + Е /T – 253.
98
а
Samples of 40–50 experimental points from the linear vate complex, e.g., the detachment of two Solv mole-
regions of the corresponding dependences of absor- cules from metal cation and the stretching of other
0
bonds (M–Solv and M–X), are main contributors to
the activation energy [1]. The lengths of N–H bonds
of porphyrin change in the transition state; this also
undoubtedly affects the process energy [1, 11]. The
bance ln(С /СH2P ) on t (s) were used to determine
H2P
keff values for each of the three parallel experiments at
temperatures ranging between 298 and 318 K. The
298
average values were then used to calculate the kv val- electronic effects of peripheral substituents are weaker
ues at a confidence level of 0.90 and an accuracy of but contribute to the destabilization of N–H bonds,
≠
±
0.03. The E and ΔS values were determined for thereby also affecting the complex formation rate.
a
each of the parallel experiments using Eqs. (5) and (6),
Electron-acceptor substituents reduce the electron
with the average values being recorded as final results density on tertiary nitrogen atoms and contribute to
and the errors being calculated as deviations from the the stretching of N–H bonds and the detachment of
arithmetic means.
protons. Electron-donor groups have the opposite
Table 1. Spectral characteristics of the molecular forms of H TPhP and H T(4-OH-Ph)P porphyrins and their zinc com-
2
2
plexes prepared in system (1)
Porphyrin Impurities
λ4(log ε)
λ3(log ε)
λ2(log ε)
λ1(log ε)
H TPhP
ZnTPhP
413(4.09)
420(4.34)
417(4.73)
423(4.84)
511(3.02)
545(2.87)
555(3.42)
553 (3.60)
556(3.75)
589(2.82)
594(3.08)
593(3.37)
598(3.66)
646(2.83)
2
H T(4-OH-Ph)P
516 (3.72)
649(4.41)
2
ZnT(4-OH-Ph)P
H TPhP is 5,10,15,20-tetraphenylporphyrin; H T(4-OH-Ph)P is 5,10,15,20-tetra(4-hydroxyphenyl)porphyrin; ZnTPhP is the zinc
2
2
–1
–1
complex of H TPhP; ZnT(4-OH-Ph)P is the zinc complex of H T(4-OH-Ph)P; and ε, mol L cm , is the molar extinction coeffi-
2
2
cient. The error of determination in three parallel experiments ranged from 1 to 3%.
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CHELATION AND FLUORESCENCE PROPERTIES
97
Table 2. Kinetic parameters of the coordination reaction between H PhP and H T(4-OH-Ph)P porphyrins and zinc ace-
2
2
tate in system (1)
CZn(OAc)2 , mol L–1
kv298, L mol–1 s–1
Еа, kJ mol–1
–ΔS , J mol K–1
≠
–1
Porphyrin
H TPhP
1.84 × 10–3
1.84 × 10–3
0.302 ± 0.029
0.875 ± 0.018
70 ± 2
60 ± 3
46 ± 2
53 ± 2
2
H T(4-OH-Ph)P
2
effect. Bulk meso-substituents change the conforma-
4. Rayleigh scattering was calculated by varying the
tion of porphyrin molecules and can distort the planar excitation wavelengths in the range of ±40 nm until
structure of macrocycles [12, 13]. Analysis of the stable coordinates of the fluorescence peaks of sam-
obtained data shows that although the EASes of the ples were achieved (the fluorescence bands should not
studied compounds were almost identical, the change according to the excitation wavelength [16]).
hydroxyl substituents significantly affect the chelating
5
. The resulting fluorescence spectra of the studied
ability of ligands. In other words, the rate constant of
formation of ZnT(4-OH-Ph)P is approximately three
times higher than that of ZnTPhP, and the energy con-
samples in acetonitrile were compared with the litera-
ture data [15], and the corresponding integral fluores-
cence intensities were calculated.
–1
sumption is lower (about 20 kJ mol ). Introducing
four electron-donor groups into the tetraphenylpor-
phyrin molecule seems to increase the electron density
on the tertiary nitrogen atoms and accelerate complex
formation (1). This conclusion agrees with earlier data
on the total inductive and resonance effects of elec-
6
. The fluorescence quantum yields of the studied
acetonitrile solutions of porphyrins and metal-por-
phyrin complexes were calculated using the standard
technique [17] by the equation
Ix Ar
Ir Ax
Q =
Qr,
(7)
tron-donor substituents (‒OH and ‒NH ) in position
x
2
4
of meso-aryl groups of tetraphenylporphyrins, lead-
ing to a substantial increase in the basicity of tetrapyr- where Q and Q are the quantum yields of the test and
x
r
role ligands, compared to unsubstituted tetraphenyl- reference samples, respectively; A and A are the
x
r
≠
porphyrin [14]. The drop in the ΔS values in the case absorbances of these samples at the excitation wave-
of ZnTPhP formation agrees with the obtained values length, respectively; and I
and I are their integral
r
x
of the constants, since the increase in the concentra- intensities.
tion of molecules of the formed zinc-ligand complex
depends on the number of collisions between mole-
cules (the degree of disorder) in the chemical process.
The recorded fluorescence spectra of acetonitrile
solutions of H T(4-OH-Ph)P, ZnT(4-OH-Ph)P,
2
H TPhP, and ZnTPhP, normalized to the maximum
2
The fluorescence of acetonitrile solutions of
H T(4-OH-Ph)P, ZnT(4-OH-Ph)P, H TPhP and
2
2
I
ZnTPhP was measured with a Shimadzu RF-5301
1
spectrofluorophotometer. H TPhP and its zinc com-
2
1.0
652.2
657.4
plex, known to be characterized by quantum yields of
0
.11 and 0.033 (measured in toluene) [15], were used
2
as references.
0.8
Fluorescence was measured as follows.
1
. A solution of the test sample (H T(4-OH-Ph)P, 0.6
2
ZnT(4-OH-Ph)P, H TPhP, or ZnTPhP) in acetoni-
2
trile was placed in an optical quartz cuvette with an
optical path length of 1 cm, the absorbance at the exci-
tation wavelength being A < 0.1 to avoid fluorescence
quenching.
0
.4
.2
0
2. A correction for dark current (the photomulti-
plier output current in the absence of incident light)
was introduced to avoid error in determining the
intensity of the sample solution’s fluorescence.
6
00
650
700
750
λ, nm
3. The intensity of the pure solvent’s (acetoni-
trile’s) fluorescence, measured at the same parameters
of the spectrofluorophotometer, was subtracted to
allow for the effect the solvent had on the intensity of
the test sample’s fluorescence.
Fig. 4. Fluorescence spectra of (1) H TPhP (1.01 ×
2
‒
5
–1
10
0
mol L ) and (2) H T(4-OH-Ph)P (1.50 ×
2
‒5
–1
1
mol L ) in acetonitrile at 298 K and λex = 555 nm.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 91 No. 1 2017

9
8
IVANOVA et al.
I
1
656.0
1
0
0
0
0
.0
6
57.6
2
.8
.6
.4
.2
600
650
700
λ, nm
Fig. 5. Fluorescence spectra of (1) ZnTPhP (1.76 × 10–6 mol L–1) and (2) ZnT(4-OH-Ph)P (1.86 × 10–6 mol L–1) in acetonitrile
at 298 K and λex = 555 nm.
of the fluorescence intensity, are shown in Figs. 4 and mation, but also allow us to stabilize the fluorescent
5
. The fluorescence quantum yields of ZnTPhP and properties of compounds.
ZnT(4-OH-Ph)P (Fig. 5) in toluene and acetonitrile
are close at 0.033 [15], 0.032 and 0.017, 0.016, respec-
ACKNOWLEDGMENTS
tively. The fluorescence quantum yields of H TPhP
and H T(4-OH-Ph)P (Fig. 4) in acetonitrile are 0.059
2
Equipment at the Upper Volga Regional Center of
Physical and Chemical Research was used in our spec-
trophotometric studies.
2
and 0.048, respectively. Fluorescence was measured at
room temperature (295 K) with an accuracy of
approximately 10%. Our data show that the hydroxy
groups of ligands have a slight affect on their own flu-
orescence and that of the zinc complex, while the
nature of the solvent makes the main contribution to
changes.
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Translated by D. Lonshakov
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 91 No. 1 2017