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Статья Paper
Protonation of 5,10,15,20-Tetrakis(4-sulfonatophenyl)porphine in Water
Vladimir B. Sheinin,a@ Sergey A. Shabunin,a Elena V. Bobritskayab and Oscar I. Koifmanab
aInstitute of Solution Chemistry of Russian Academy of Sciences, 153045 Ivanovo, Russia ^Ivanovo State University of Chemistry and Technology, 153000 Ivanovo, Russia @Corresponding author E-mail: vbs@isc-ras.ru
Protonation equilibriums of the porphyrin core H2P in H2P(PhSO3)4 with perchloric acid in water are investigated by methods of spectropotentiometry and computer chemistry (DFT/B3LYP and PM3). It is shown that leveling of step constants is caused by formation of aquacomplex [Hp2+ (PhSO3) ](H2O)2 of diprotonated porphyrin platform showing properties of anion-molecular receptor. Values of lgK1 (4.85 ± 0.03), lg(K2KJ(1.22 ± 0.03) are defined, and rough value oflgK2 (-0.23) for reactions Hp + H+ <=> H3P+ (1); H3P+ + H+ <=> H4P2+ (2); Hf2+ + 2H2O <=> [Hf2+](H2O)2 (3) is calculated. Molecular parameters [H4P2+(PhSO3) ](H2O)2 are calculated and it is established that receptor Hp2+ possesses very high complementarity concerning two water molecules.
Keywords: 5,10,15,20-Tetrakis(4-sulfonatophenyl)porphine, basicity, J-aggregates.
Introduction
Some porphyrins are perspective tectons for creation of supramolecular nanostructured materials on technology "from below upwards". Nanoscale porphyrin fibers, rods, ribbons, sheets, hollow hexagonal prisms, filled and hollow spheres, tubes, wheels and biomorphous crystals of the various form are received and actively investigated by this time.[1] Their firm nature and unique properties provide access to a new class of nanomaterials with potential possibilities of use in such important areas, as solar elements, photocatalysis, hydrogen power, electronics, nonlinear optics and chemosensors.
One of the perspective compounds is water-soluble 5,10,15,20-tetrakis(4-sulfophenyl)porphine H2P(PhSO3H)4.
Molecule of H2P(PhSO3H)4 consists of hydrophobic porphyrin platform-chromophore H2P, and four peripheral hydrophilic sulfophenyl substituents -PhSO3H, the combination of which provides its unique properties.
Zwitterion H4P2+(PhSO3-)4 is tecton for pH-operated ionic self-assemble of supramolecular polymers {H4P2+(PhSOA} ,
named J-aggregates (in honor of Edvin Jelley[2]), organized as a bricklaying[3,4] which is formed as a result of interaction of phenylsulfonate groups -PhSO3- ("tail") with diprotonated porphyrin platform H4P2+ ("head"), showing properties of anion-molecular receptor.[5] In acidic water solution J-aggregates {H4P2+(PhSO3-)4}n form solid one-wall tubes in diameter about 25 nanometers and length to 1 micron,[6] which drop out in a deposit. As formation of J-aggregates is kinetically hindered, it is possible to study the protonation equilibriums of porphyrin platform H2P(PhSO3-)4. Protolytic transformations of H2P(PhSO3H)4 depending on pH of water solution are shown in Scheme 1.[4,7,8]
H2P(PhSO3-)4 H4P2+(PhSO3-)4
pH 2.6
ptf 2.6
H4P2+(PhSO3-)2(PhSO3H)2 ( vH 11 > H4P2+(PhSO3H)4
Scheme 1. Transformations of H2P(PhSO3H)4 depending on pH of water.
Published data testify that protonation equilibriums of H2P(PhSO3-)4 in a water solution have close values of step
HChS
SO,H
ROsS
so3H
HP(PhSO3H)4
H,0
J
ХЛ
Q3SPh—^/kjp^Z—PhSQ3" ^
^ j I
pÛ3SPh—y/ii^Py^-—PhSQ3~| n
^ . , ^ ■ -jn <£■
7X x4
Ô3SPh—//H4P+V—PhS03-
7\
HO
H4P2+(PhSO3-)4
zwitterion-tecton
J-aggregate (n >100000)
constants which have not been measured till now, and the reason of their leveling is not found out. For completion of this blank the study of protonation reactions of H2P(PhSO3-)4 in water was carried out by spectropotentiometric method and by methods of computer chemistry (DFT/B3LYP and PM3) in a hypothetical ideal gas phase.
Experimental
Synthesis. The tetraammonium salt of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphine was prepared according to the known procedure.191
Spectropotentiometry. The investigation of basicity of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphine was carried out by the spectropotentiometric method in system (I) at 298 K, as it was described earlier.1101 Electronic absorption spectra were obtained in water using AvaSpec-2048-2 for UV-visible spectroscopy.
RP(PhSO3NH4)4 - HOO4
- H2O
(I)
Calculation methods. Basicity constants of H2P(PhSO3-)4 were calculated by the method of fitting parameters using program SigmaPlot® software provided by Systat Software Inc. (SSI). Geometry optimizing calculations were carried out using DFT/ B3LYP method with 3-21G** basis set. All calculations were performed with the Gaussian 98 program suite.[11]
Results and Discussion
Aquacomplex [Hf2+(PhSO3-) J(H20)2
Earlier, it has been shown that diprotonated porphyrin platform, showing properties of anion-molecular receptor-chromophore, forms steady complexes [H4P2+](AN)2, [H4P2+] (AN)(HO), [H4P2+](H2O)2, [H4P2+](AN)(Hal-) and [H^2+] (Hal-)2. Formation constants of these complexes have been measured by spectropotentiometric method in acetonitrile (AN).[5,81213] Geometry of anion-molecular complexes H4P2+ and also enthalpy of guest linkages were studied by methods of computer chemistry DFT/B3LYP/3-21G**, PM3 and MMX.[5'13'14]
Results of DFT calculations of aquacomplex geometry [H4P2+(PhSO3")4](H20)2 are shown in Figure 1.
atoms of opposite NH-groups in such manner that their projections to 1.3-alternate mesoplane form angle of 90°. Elastic 1,3-alternate adapts to 2 molecules of water therefore the initial capture angle increases to 102°. In structure of aquacomplex, receptor H4P2+(PhSO3-)4 gets almost ideal geometrical complementarity concerning two oxygen atoms, which is estimated by angles of hydrogen bonds N-H-O[5] of 178° (180 ° at ideal complementarity). Distance between the mesoplane of 1,3-alternate and oxygen atoms amounts 1.93 A. Charge transfer (Mulliken charge) from oxygen atom on H4P2+ makes about 20 %, and enthalpy of each molecule of water is 6.5 kcal/mole.
Protonation of H2P(PhSO3-)4 in Water
The diprotonated platform of zwitterion in water solution is in a kind of aquacomplex [H4P2+(PhS03-)4](H20)2 (Figure 1) that it is necessary to consider at interpretation of spectropotentiometric experiment results. To exclude possibility of anionic complexes formation chloric acid was used in the work, since its anion is indifferent to H4P2+.[10]
Changes of an UV-vis spectrum of H2P(PhS03-)4 at p^ = 2-8 (Figure 2) are in equilibrium and pH-reversible. Full reversibility of spectral changes has been checked up specially in system H2P(PhS03NH4)4 - HCl04/K0H - H20. Time of achievement of equilibrium in experimental conditions is extremely fast and is limited only by rate of solution hashing.
With increase in acidity of solution the UV-vis spectrum of H2P(PhS03-)4 is consequentially transformed into that of H3P+(PhS03-)4 and [H4P2+(PhS03-)4](H20),[316] and the corresponding one-stage spectropotentiometric curves are observed (Figure 3).
In extreme range of acidity (4.3> pH> 5.3) in the UV-vis spectrum of system (I) two families of isosbestic points (421, 474, 560 nm and 425, 495 and 619 nm) are observed. The corresponding linear sections of the correlation dependence of optical density vs different absorption maxima are shown in Figure 4. These facts indicate coupled equilibria (1) and (2) between two pairs of light-absorbing centers H2P/H3P+ and H P+/H,P2+ with close values of K h K*.[17]
Figure 1. Geometry of solvatocomplex [H4P2+(PhS03")4](H20)2 (phenyl rings are not shown).
17
H2P + h+ <==!=» h3p+
K9
HP+ + H+ < > H,P+
K,
H4P++ + 2HO [H4P++KH2O)
it
H3P+ + H+ [H4P++](H2O)
(1) (2) (3)
H3P+ + H+ + 2H2O ( > [H4P++](H2O)2 (2, 3)
(4)
Initial diprotonated platform in structure of H4P2+(PhSO3-)4 represents a symmetric 1,3-alternate with capture angles[15] a = 94° in each site of linkage. Each molecule of water is coupled by two hydrogen bonds with
Calculation of Kl and K2-K3 was performed by selecting the parameters in the equation (5) which connects p^ value and current optical density AT of solution at the analytical wavelength L[17]
Figure 2. UV-vis spectra of system (I) at p^ = 2-8, at porphyrin concentration 2.07-10-5 mol/l (1) and 1.23-10-4 mol/l (2): j H2P(PhS03-)4, t [H4P2+(PhS03-)4](H20)2.
Figure 3. Spectropotentiometric curves in system (I).
Figure 4. Correlation dependence of optical densities at 645 and 515 nm in system (I).
(aH2P + aH P+ • k • 10-ph + aH P++ • k • k2 • k3 • s2 • 10-2ph ) • *
2 1 f\-2pH
Aa -
.t^t —
1+k •10-ph + k • K2 • K3 • s2 40
2 .1 D-2Ph
(5)
where AH
a»
ah,p+
ao
h4p++ (h20)2
- optical densities analytical concentration of porphyrin Co (ao = ei -1 - Co), of solution components at concentration equal to the S - concentration of water in water at 298 K.
о
■1
2
1
n
1
о
3 37
pH
Figure 5. Dependence of lg(7) on pH for H2P(PhSO3-)4 in system (I).
Figure 6. Distribution of equilibrium concentrations of H2P, H3P+ and [H4P2+](H20)2 in system (I).
Constants of H2P(PhSO3")4 protonation in water were calculated on a titration curve at 645 nm (Figure 3). As a result of averaging of calculations on three independent experiments, values of lg^ = 4.85 ± 0.03 and lg(^2-^3) = 1.22 ± 0.03 have been received. The correlation coefficient R of experimental and modeling dependences in all cases wasn't worse 0.9999. With a glance of work results[8] rough value of lgK2 makes nearby -0.23 (lgK3 = 1.45 for dimethyl ether of mesoporphyrin IX in AN at 298К), and full size of leveling effect in water nearby 4.94 (2lgS + lgK3). Due to this circumstance, value of lgK2* (a conditional constant[18] of the second step of protonation) which is usually calculated from spectropotentiometric titration results of porphyrins on the equation (4), makes 4.71 ± 0.03, that is practically coincides with lgKr The experimental dependence (6) shown in Figure 5, is linear (R = 0.999) and characterized by high steepness (coefficient 2.82 vs 1 for one-proton reaction) that allows to use function рЯ as the effective switch of self-assemble of J-aggregates {H4P2+(PhSO3-)4}n.
lg
/ \ Ao -A
h2p t
at ah4p++
= 2.82xpH-13.11; R =0.999 (6)
where AT - current value of optical density on analytical wavelength of 645 nm.
For the description of the equilibrium mixture H2P, H3P+ and [H4P2+](H20)2 in system (I) the inverse problem was solved and the current values of component concentrations in range of p^ 3-7 (Figure 6) were calculated. The Figure 6 shows that at experimental conditions a maximum value of H3P+ concentration reaches only 37 %. All UV-vis spectra
in рЯ range are the superposition of three light-absorbing centers of H2P, H3P+ and H4P2+, and isosbestic points are formed when concentration of one of three absorbing centers falls below 7 %.
Conclusions
Leveling of protonation step constants of H2P(PhSO3-)4 is caused by formation of aquacomplex [H4P2+(PhSO3-)4](H2O)2 of diprotonated porphyrin platform showing properties of anion-molecular receptor.
The value of leveling effect is determined by multiplication of aquacomplex formation [H4P2+(PhSO3-)4] (H2O)2 constant and a square of concentration of water in water.
Diprotonated porphyrin platform of H4P2+(PhSO3-)4 possesses very high complementarity concerning two water molecules.
Acknowledgements. This work was supported by the Russian Foundation for Basic Research (No. 09-03-97530) and the Program of fundamental investigations of Chemistry Department of the Material Sciences RAS "Chemistry and physical chemistry of supramolecular systems and atomic clusters".
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Received 22.05.2011 Accepted 23.06.2011
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Макрогетероциклы /Macroheterocycles 2011 4(2) 80-84
