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10Порфирины
Porphyrins
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Сообщение Communication
Effect of Solvent on Photooxidation of Protoporphyrin-IX d.m.e. by Singlet Molecular Oxygen
Evgeny A. Venediktov@ and Elena J. Tulikova
Institute of Solution Chemistry of Russian Academy of Sciences, 153045 Ivanovo, Russia @Corresponding author E-mail: eav@isc-ras.ru
The photooxidation of protoporphyrin - IX dimethyl ester by singlet molecular oxygen in various solvents was studied. It was shown that dielectric effect of a solvent is important in determining of the rate constant of this process.
Keywords: Protoporphyrin - IX d.m.e., photooxidation, dielectric effect of a solvent.
Introduction
Protoporphyrin - IX dimethyl ester (Figure 1, I) is one of the most important representatives of natural pigments,
the reactions of which with singlet oxygen (1O2, 1Ag) continue to be interesting because of the wide use of I in biomedical photochemical technologies.[1-4] Therefore the photochemical stability is fundamental and important characteristic of I.[4]
OCH
CHO
CO9CH3
COoCHo
__ C02CH3 CO2CH3 C02CH3
I II III
Figure 1. Structural formulae of protoporphyrin - IX dimethyl ester (I) and hydroxyaldehydes (II and III).
COoCH,
Dr. Evgeny Anatolievich Venediktov was born in 1951. After graduating from Ivanovo Institute of
Chemical Technology he started to study porphyrin chemistry in the laboratory of Professor B. D. Berezin. He prepared his thesis "Influence of molecular structure on physico-chemical interaction of porphyrins and metal porphy-rins with molecular oxygen" in the Institute of Biochemistry of USSR Academy of Sciences in Moscow wmth Prof. A.A. Krasnovskii Jr. and in 1980 received the Degree "Candidate of Chemical Sciences" (Ph.D.) in Physical chemistry from Ivanovo Institute of Chemistry and Technology. Since then he held positions at the Institute of Solution Chemistry of the Russian Academy of Sciences where he start to work in the laboratory heades by Prof. B.D. Berezin and currently is leading researcher. His research interests are connected with photochemistry of natural pigments and physical chemistry of singlet molecular oxygen.
Евгений Анатольевич Венедиктов родился в 1951 году. После окончания Ивановского химико-технологического института он начал заниматься химией порфиринов в лаборатории проф. Б.Д. Берёзина. Свою диссертацию "Влияние молекулярной структуры на физико-химическое взаимодействие порфиринов и металлопорфиринов с молекулярным кислородом" он подготовил в Институте биохимии АН СССР под руководством проф. А.А. Красновского мл. и в 1980 году получил степень кандидата химических наук по физической химии в Ивановском химико-технологическом институте. Он начал работать в в отделе, возглавляемом Б.Д.Берёзиным и В настоящее время он является ведущим научным сотрудником Института химии растворов Российской академии наук, где начал работать после защиты диссертации. Егно научные интересы связаны с фотохимией природных пигментов и физической химией молекулярного кислорода.
172
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Макрогетероциклы /Macroheterocycles 2009 2(2) 172-174
E.A. Venediktov and E.J. Tulikova
Earlier[3"10] it was demonstrated that I undergoes phototransformation in solution due to molecular oxygen. This reaction is highly specific. General oxidation products are hydroxyaldehydes II and III (Figure 1). The formation of these compounds formation involves the photochemical generation of 1O2 by energy transfer from the excited triplet molecules of I to the ground state of molecular oxygen and 1O2 transfer from photosensitizer to the ground state of I. Several reports[3A6-10] have postulated the concept of [4+2] cycloaddition of 1O2 to I which can be depicted as is shown below.
Results and Discussion
The irradiation of I in air-saturated solutions results in rapid spectral changes (Figure 2) which are identical with that observed previously.13-5-71 The key features are the decline of absorption attributed to I loss and the growth of the band at ~ 670 nm associated with II and III formation. As it can be seen from Figure 3 these changes depend on solvent.
The observed rate constants (k=krPO2]) for II and III formation were calculated from evaluation of the initial increment d[II, III]/d/ in Equation 1:
w = d[II, III]/dt = krPO2][I],
(1)
where [I] is the initial concentration of I. The kinetics of I photooxidation by molecular oxygen can be depicted by the simplified process shown below[8 -10]
(I) 1 (I) ^ 3 (I)
3 (I) + O2 (3 Z- g) ^ (I) + 1O2
1O2 ^ O2 (3 Z -) + hv (~1270 nm), kd
1O2 + Solv ^ O2 (3 S - ) + Solv, k
2 2 v g ' so
It was also shown[7,10] that the rate of I photooxidation by oxygen depends on solvent nature. Up to now, the discussion of this effect was limited to qualitative comparison. In this paper we demonstrate that in solution dielectric effect has a significant influence on the kinetics of I photooxidation by 1O2.
Experimental
I was obtained using the method outlined by Grinstein.[11] All solvents used in kinetic experiments were prepared as recommended in work.[12] Photolyses were conducted with a 70W halogen lamp equipped with SZS-21 glass filter (320< Xexc< 650 nm). Spectra were obtained on SPh Model 18 spectrophotometer with glass cells of 1.0 cm path length. The 1O2 lifetimes were measured with LIF Model 200 fluorimeter.[13] The initial optical density of I solutions was about 1 at the band maximum of 502-506 nm depending on the solvent. The initial concentration of I was ~ 7-10-5 M.
^ + (I) ^ O2 (3 Z - g) + (I), kq 'O2 + (I) ^ oxidation products, k, where k , k ., k and k are the rate constants of elementary
rad solv q r J
processes.
c-10 , M
8 r
4 -
t, s
0
10000
20000
D
1,0
450
550
650 X, hm
Figure 2. Examples of absorption spectra of protoporphyrin - IX in acetone measured after 0, 15, 30, 45, 60 h 75 min of exposition to light (a - absorption spectrum of hydroxyaldehydes).
Figure 3. Examples of increase of II and III concentration under protoporphyrin - IX photooxidation by 1O2 measured at 670 nm in diethyl ether (1), benzene (2), acetone (3) and dimethylformamide (4).
According to this kinetic scheme the rate of II and III formation can be expressed in a generalized form as
d[II, III]/dt
WM l + *ß[l]x
(2)
Here kr is the rate constant of I oxidation; y is the 1O2 quantum yield; I is the light absorption intensity; t = 1/(krad+ kjSolv]) is the 1O2 lifetime in pure solvent; kQ is the rate constant of 1O2 deactivation by I. Assuming that kQ is less than 2107 M-1s-1 [91014] Equation 2 can be simplified as follows
I
Î
w
0
Макрогетер0циmbl /Macroheterocycles 2009 2(2) 172-174
173
Photooxidation of Protoporphyrin-IX d.m.e.
Table. Kinetic parameter of protoporphyrin - IX photooxidation by Ю lifetime of Ю2 in various solvents and their characteristics.
Solvent к / kza r r.bz s[12] и[12] n*[18]
Diethyl ether 30 0.15 4.23 1.3526 0.27
Toluene 29.5 0.73 2.38 1.4961 0.54
Benzene 31 1.00 2.27 1.5011 0.59
Acetone 50 1.03 20.56 1.3588 0.71
Pyridine 17 3.8 12.4 1.5095 0.87
Dimethylformamide 18 6.42 36.7 1.4303 0.88
aErrors of t and kr / k.z values are about 5 and 15 %, respectively.
w = к у/т [I]
(3)
Connection between k and kr is derived from Equatins 1 and 3
k= krylT (4)
Thus k is dependent on y I and t terms. As shown in works,[1517] the quantum yield of 'O2 photogeneration by I is relatively constant in different solvents and has the value from 0.56 to 0.77. Therefore the relative change of kr can be approximated by Equation 5
к/к,
■ к/к.
(5)
where к,, к, and т. are the kinetic parameters of reaction
bz' r,bz bz i
and the Ю2 lifetime in benzene as standard solvent. Using experimental data of к and т computed from the decay of Ю2 luminescence,1131 the relative rate constant may be calculated. The к /к, values of this reaction in various solvents are
r r,bz
summarized in Table.
As it can be seen from the Table, reaction rate constant is sensitive to solvent. Thus, in diethyl ether the к /к , value
' J r r,bz
is approximately 43 times smaller than in dimethylforma-mide. These data demonstrate the important role of solvent in I photooxidation. What is the origin of the solvent effect in this reaction? Early studies19-141 indicated that an exciplex intermediate formation is a fundamental process of 1O2 interaction with porphyrins. In this system exciplex can occur through partial charge transfer from porphyrin to 1O2 and therefore it can possess polar character. From here we can assume that solvent kinetic effect reflects electrostatic interactions. However, the к /к , parameter does not exhibit
' r r,bzi
dependence on solvent refractive index, n, and (or) solvent
& Ö
7t
Figure 4. Dependence of the logarithm of relative rate constant of protoporphyrin - IX photooxidation by 1O2 on the solvent dipolarity/polarizability parameter.
permeability, e. At the same time it was observed the increasing of kjkrbz with the growth of empirical parameter of solvent dipolarity/polarizability, n*, [18] characterizing the ability of solvent to stabilize a charge by dielectric effect, and this correlation is linear (Figure 4).
So it can be concluded that solvent dielectric effect is important in determining of the rate constant of this process.
References
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4. Bonnett R., Martinez G. Tetrahedron 2001, 57, 9513-9547.
5. Gurinovich I.F., Gurinovich G.P., Sevchenko A.N., Tauger S.M. Dokl. Akad. NaukSSSR 1965, 164, 201-204 (in Russ).
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11. Gurinovich G.P., Sevchenko A.N., Solov'ev K.N. Spectroscopy of Chlorophyll and Related Compounds. Minsk: Nauka i Tekhnika, 1968, 516 p. (in Russ).
12. Gordon A.J., Ford R.A. The Chemist's Companion. New York-London-Sydney-Toronto: John Wiley and Sons, 1972, 541 p.
13. Venediktov E.A., Tokareva O.G. Kinet. Katal. 2000, 41, 166 - 169.
14. Aveline B., Delgado O., Brault D. J. Chem. Soc. Faraday Trans. 1992, 88, 1971-1976.
15. Venediktov E.A., Krasnovskii A.A. (jr.) J. Appl. Spectr. 1982, 36, 152 - 154.
16. Fernandez J.M., Bilgin M.D., Grossweiner L.I. J. Photochem. Photobiol. B. 1997, 37, 131-140.
17. Egorov S.Ju., Krasnovskii A.A. (jr.) Safronova I.A., Bystrova M.I., Krasnovskii A.A. Dokl. Akad. Nauk SSSR 1988, 299, 1266-1270 (in Russ).
18. Kamlet M.J., Abboud J.-L.M., Abraham M.H., Taft R.W. J. Org. Chem. 1983, 48, 2877-2887.
Received 25.06.2009 Accepted 10.07.2009
т. /т
bz
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