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Internal magnetic field effect of lanthanoid ions on the photochemical reaction of naphthoquinone in a micelle
Volume 106. number 5
INTERNAL
MAGNETIC
CHEMICAL
PHYSICS
FIELD EFFECT OF LANTHANOID
ON THE PHOTOCHEMICAL
REACTION
4 May 1984
LETTERS
IONS
OF NAF’HTHOQUINONE
IN A MICELLE
Yoshio SAKAGUCHI and Hisaham HAYASHl 77~ Institute of Physical and QIemical Research. Wake. Saitkma 351. Japan Received 9 February 1984; in fiat form 23 February
1984
The effect of trivalent lanthanoid ions on the photochemical reaction of 1,4-naphthoquinone in a sodium dodecyl sulfate micelle is investkated in the presence of magnetic fields. The yields of the escaping semiquinone radicals observed at 0.1 and 1 T decreased with increasing the concentration of certain paramagnetic lanthanoid ions. This is considered to be due to the relaxation of the electron spins in tie triplet radical pair caused by the electron spins of the ions.
I_ Introduction
Recently [ 141, there have been considerable interests in the magnetic field effects on chemical reactions since they have not only physicochemical interests but also possible efficiencies to control chemical reactions and to separate isotopes. Especially the magnetic field effects on the dynamic behavior of photochemical reactions in micelles have been elucidated with the aid of nanosecond laser photolysis [ 1,2,4,5]. In a preceding paper [63 we investigated the photochemical reaction of naphthoquinone (NQ) in a sodium dodecyl sulfate (SDS) micelle and found that this reaction was influenced extraordinarily by a magnetic field below 134 T. In these studies, magnetic fields were applied from outside the reaction system. These effects are called “external magnetic field effects”. The magnetic isotopes in the component radicals of a radical pair also act as intramolecular magnetic fields. These effects are called “maguetic isotope effects”, which originate from the hyperfine coupling of isotopes. There is another possibility that a magnetic field is applied to a reaction system such as addition of paramagnetic species to the system. The magnetic species adjacent to a reaction center is considered to act as a strong magnetic field. The shift reagents used in NMR spectroscopy were developed along this idea [?‘I _As far as we know, however, ihe kffect of the magnetic fields of paramagnetic species on chemical reactions has never been studied. 420
Therefore, we investigate in this paper the effect of triv; lent lanthanoid (Ln3+) ions on the photochemical reaction of NQ in an SDS micelle.
2. Experimental 1,4-naphthoquinone (NQ) was recrystallized repeatedly from benzene and was further purified by sub limation. Sodium dodecyl sulfate (SDS) was purified by repeated recrystallizations from methanol-ethanol mixture. Water was deionized and distilled. Lanthanoid salts, JI.aC13-7H20 (above 995%. Mitsuwa Chemical Co.), NdCI,-6H,O, SmC13s6H,0, GdCI,-6H,O, DyCl3*6HzO, HoCI3-6H,O, and LuCl3-6HlO (99970, Soekawa Chemical Co.), were used without further purification. Laser-photolysis experiments were performed on degassed solutions at room temperature by using the fourth harmonic (266 nm) of a Quanta-Ray DCR-1 Nd : YAG laser as the exciting light source. The laserphotolysis apparatus and the measuring system were similar to those published elsewhere [S ,6] _Hereafter, we denote the experiments under the lowest magnetic field of our apparatus (less than 0.2 mT) as those in thf absence of magnetic field.
0 009-2614/84/S 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
4 May 1984
CHEkflCAL PHYSICS LE-ITERS
Volume 106, number 5 3. Results and discussion
The photochemical primary processes of NQ in an SDS micelle were described in a preceding paper as follows [6]; NQ +t~--*
‘NQ* -+ 3NQ*,
3NQ* + RH + 3(NQH3(NQH‘-3(NQH‘(NQH-
(1)
,R),
-R) + ‘(NQH-
(2)
-R),
(3)
*R) -+ NQH- + R-,
(4)
-R) + recombination
products,
(5)
NQH--+ NQ- + ii+.
(6)
By laser irradiation
the usual hydrogen abstraction reaction of triplet excited state of NQ (3NQ*) from a micelIar mofecule(RH) occurs in a micelle, forming a triplet radical pair of a naphthosemiquinone radical (NQH-) and the akyl one (R-) [reactions (1) and (3)]. The electron spin state of the radical pair changes between the triplet (T) and singlet (S) states [reaction (3)f _The radicals in the singlet and triplet radica) pairs are separated from each other and become escaping radicals [reaction (4)]. Only the radicals in the singlet state radical pair can recombine with each other and give recombination products [reaction (5)J _A part of NQH- dissociates to naphthoquinone anion (NQ-) and proton [reaction (6)3.
0
2
6
0
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Fig. 1. f(t) curves observed at 380 nm in the (A) absence and (B) presence of a magnetic field of 1 T for miceliar SDS solutions of NQ containing Cd3+ of (a) 0, (b) 0.37 X 10m3.(c) 0.93 x IO-“, and (d) 1.9 x 10m3 mol dm-3, respectiveIy.
%+
8 TIME&s
Fig. 2. f(t) awes observed 31 380 nm in the (A) absence and (B) presence of a magnetic field of I T for micellar SDS solulions of NQ containing Lu3+ of (a) 0, (b) 0.18 x 10a3. (c) 0.96 x iOT3, and (d) 1.9 x 10e3 mol dmm3, respectively_ ALI of the I(f) curves are overiapped with one another.
With the aid of the nanosecond laser-photolysis apparatus, we studied the dynamic behavior of the reactions of the aqueous micellar solution of SDS (8 X IOmol dmm3) containing NQ (5 X IO4 mol dm-3) and IanthanoiG chloride (0 - 3 X 10-s mol dm-3) in the absence and presezpe of a magnetic field of 0.1 or 1 T The decay curves of the transient absorption intensitie I(t), observed at 380 nm for the micellar solution of NQ containingGd3i (0 - 1.9 X IO-3 mol dm-3) in the absence and presence of a magnetic field of 1 T art shown in fig. 1. Fig. 2 shows the i(r) curves observed for the solutions containing Lu3+ (0 - i .9 X IO-3 mo dm-3)_ The absorption observed at 380 m-n arises mair ly from NQH - [6]. Thus the decaying part of each I(r) curve corresponds to the intersystem crossing and recombination of the triplet radical pair [reactions (3) and (5)] , and its remaining part to the escaping NQH. from the radical pair [reaction (4)]. There has been no plausible method to determine the positions of escaping radicals for reactions in miceIles. Either NQH- or R can esit from the micelle. In the present study the rela tive yield of the escaping radical (NQH -) can only be measured. It is represented by the ratio of the rransien absorption intensity observed at 6.5 ~.tsafter the excitation jI(6.5 PS)] normalized to the initial absorption intensity [f(O JIS)] [6]. As shown in fig. 1, the yield ok served in the absence of Gd3+ increased in the present of a magnetic field of I T through the retardation of 421
Votume 106, number 5
CHEMICAL
PHYSICS LETTERS
the intersystem crossing rate of the triplet radical pair [f&8]. In the absence of Gd3* ,I(65 ps)/ir(O gs) increased from 03 1 at 0 T to 0.7 1 at 1 T. When each of the lanthanoid salts was added to the micellar SDS solution in the absence of a magnetic field, the I(r) curve observed at 380 nrn for the solution did not change within the limit of experimental errors. The results for the Gd and Lu salts are shown in figs. LA and 2A, respectively. The decay of the escaping radical was also found to be unaffected by the lanthanoid salts. The I.,$+ ions are considered to be attracted to the negatively charged micellar surface. From the abovementioned results, the Ln3+ and Cl- ions are considered not to react with 3NQ*, NQH - in a radical pair, nor with escaping NQH -. When a diamagnetic Lns” ion (Las+ or Lu3+) was added to the micellar SDS solution in the presence of a magnetic field, its I(r) curve at 380 nm did not change as shown in tip. ZB. On the other hand, when a paramagnetic Ln3+ ion was added to the solution in the presence of a magnetic field, its I(t) curve at 380 nm changed remarkably from the I(r) curve observed in the absence of the ion. The results for Gd3+ are shown in fig. 1B. From fig. 1B, the decay rate at I T was found to increase and the yield of the escaping radical at 1 T to decrease with increasing the concentration of Gd3+. With Gd3* of 1 St X IO-’ mol dmm3, Z(6.5 &/I(0 ~~1s) at 1 T became 0.37, which was nearly the same as that
observed at ClT in the absence of Gd3+. On the contrary, the decay of the escaping radical was not changed by Gd3+, as observed at 0 T. This assures that Gd3f does not react with escaping NQH - in the presence of a magnetic field. Therefore, the effect of Gd3+ on the present reaction can be concluded to be due to some
4 hfay 1984
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1
2
ILn3’ll10dmoldm’J of the relative yield of the escaping naphrh semiquinone radical W6.S &/(O ~5)) on Ianthanoid ion concentration in the absence (m) and prrsencc of a magnetic ficId
Fig. 3. Dependence
ofO.lT(~)or
IT(o).
cesses. If the Ln3+ quenching of the semiqu~one
radical in the radical pair can be expressed by the product of its rate constant (kQ) and the Ln3+ ion concentration, the inverse of the yield of the escaping semiquinone radical from the pair should depend linearly OI the concentration. Indeed, it was found from the present study that the inverse of the relative yield [I(0 ns)/ Z(65 ~_ts)] showed a linear dependence on the Ln3+ ior concentration up to 1 X 10m3 mol dm-3. Therefore, the relative kQ values at 0.1 and 1 T were calculated b) this procedure for the investigated eight Lnjf ions (La3’, Nd3’, Sm3*,Gd3+, Dy3+, Ho3*, Er3+, and Lu3*) and the obtained values are plotted against the number of 4f electrons in fig. 4. As shown in fig. 4, the kQ value is the largest at Cds’ and decreases on each side. L_.n3+ions have been used for giving some petiurbations to the dynamic behavior of molecules. Among other things, Ln3+ complexes are the best known as
physical rather than chemical processes. Other pammagnetic Ln3” ions investigated in the present paper (Nd 3+, Sm3+, Dy3+, Ho3+, and Er3+) showed similar effects on the f(r) curves at 380 nm, but their changes were smaller than those observed for Gd3+. The observed concentration dependences of the five Ln3+ ions (Lax+, Sm3+, Gd3+, I@‘, and Lu3+) on the relative yield of the escaping radical in the absence and presence of a magnetic field of 0.1 or 1 T
are shown in fig. 3. With a simple kinetic scheme for the quenching
of a reaction intermediate, :he yield of a certain process. is expressed by the ratio of the rate constant for that process to the sum of the rate constants for all the pro422
Fig. 4. Relative values of the quenching of the yield of escaping naph~o~m~qu~one radical by Ln3+ ions ck@ observed at 0.1 T f-1 and 1 T iof. Alon:: the abcissa are shown the number of 4f electrons of each of the trivafent ions, its symbol, and electronic state.
Volume 106. number 5
-I hl3y 1984
CHEhlICAL PHYSICS LETTERS
shift reagents in NMR spectroscopy 171; Ln3+ compleses bring about shifts and line bloadenings in NMR spectral lines. The amount of the induced shifts in proton NhlR spectra is roughly ranked as follows [7]: (upfield shift) Dy > Tb,Ho > Pr > La,Nd,Sm,Gd, Lu (small or no shift) > Eu,Er,Yb > Tm (downfield shift), and the induced broadening is [7,9]: Gd > Tb,Dy,Ho,Er,Tm > Yb > Nd > Pr,Sm > Eu (very little broadening)_ The induced shifts in 14N or 170 NMR spectra are roughly ranked as follows [7, lo] : (upfield shift) Gd,Tb,Dy,Ho > Er,Eu 9 Ce.Nd,Yb, Pr,Sm (small downfield shift). Ln3+ ions have also been used for quenching escited molecules. For example, the quenching by some Ln3+ ions of the triplet excited state of phenanthrene was studied at 175 K and the order of the quenching ability was found to be as follows [ 1I ] : Nd > Pr > Ho > Eu > Tb > Yb > Gd,Ce (no detectable quenching). In addition, the magnetic susceptibilities of Ln3+ ions are [la]: Dy>Ho >Tb>Er>Gd>Tm>Yb>Nd>Pr > Eu > Ce > Sm, and the ionic radii decrease from La3+ to Lu3+: La> . ..>Lu. As mentioned above, the effects of the Lns’ ions observed in the present study can be considered to be due to some physical processes_ The observed order of their k Q values were obtained from fig. 3 as follows: Gd3+ > Sm3+ > Dy3+ > Nd3+ > Ho3’ > Ers+ > La3+ Lu3+ (nearly zero). This order is quite different from ihose mentioned above. At first sight, the diamag-
netic La3+ and Lu3’ are shown to have no quenching ability. Among Ln3* ions, Las+ is the lightest and largest ion and Lu3+ is the heaviest and smallest. Therefore, the effect of ionic strength and the external heavy atom effect by Ln3+ rons _ surrounding a micelle can be concluded to be negligible. In -tie next place, Cd;+ and Sm3+ showed the largest and the second largest quenching abilities in the present study, respectively. Alrhougb Gd3+ belongs to the top group of the broadening of proton NMR spectra and of the shift of 14N or 170 NMR spectra, Sm 3+ shows very weak effects on the broadening and shift. Then the mechanisms which induce the broadening and shift on NMR spectra [7,9, IO] cannot be applied to our results. As stated above, no Ln3+ quenching of 3NQ* was observed in the present study. Moreover, the energy transfer from the trip-
let radica: pair to Ln 3+ ions is also unlikely because of the very small energy separation between the triplet and singlet radical pairs and among the Zeeman splittir of the triplet one. Accordingly, the effect of Ln3+ ions on kQ is considered to be due to their magnetic effects to induce th relasation of the electron spins in the triplet radical pa According to the relaxation mechanism [g]. the relasa tions from the T+, and T_, sublevels of the triplet rad. ical pair to the To sublevel of the singlet pair in the presence of a magnetic field control the decay of NQH in a radical pair and the yield of escaping NQH.. The observed effects of paramagnetic Lnj+ ions on the decay and yield can be considered to be due to the rnhancement of the relaxation rates. The pammagnetic ions tumbling around the micelle would induce the relaxation of the electron spins of the triplet radical pair Therefore, this effect can be called “the internal magnetic field effect” by Ln s+ ions. In the absence of a magnetic field these relaxations cannot be considered. because there is no Zeeman splitting for the triplet radi cal pair. This may be the reason why the internal magnetic field effect by Ln j+ ions could not be observed i the absence of a magnetic field. At first sight, this Lnj+ effect might be considered to be related with the magnetic susceptibility. However, the observed order of kQ does not agree with that of magnetic susceptibility. which is due to the combination
of the orbital
of If electrons,
and spin magnetic
but only with
moment
that of
e!ccrron spin multiplicity. as shown in fig. 4. As far as we know, thi! is the first observation of the effecr of spin multiplicir: in Ln3+ ions on the dynamic behavior of molecules. It may be safe to consider that the electron spins in Lns’ ions would induce the relasation of the triplet radical pair. although the detailed mechanism of this effect is not clear at rhis stage. For a more detaiJed analysis of the internal magnetic field effccr by I_n3+ ions which was shown to be present in the present study. the results of all rhc Lnj+ ions and the kinetic anslysis of /(I) curves would be necessary. These experiments are now in progress.
Acknowledgement Grateful acknowledgement is made by k’s for a Pos doctoral Fellowship of Japan Society for Promotion o
Volume 106. number 5
CHEMICAL PHYSICS
Science. This study was partially supported by a grant on Solar Energy Conversion at the Institute of Physical and Chemical Research (Rikagaku Kenkyusho).
Y. Sakaguchi. S. Nagakura and H. Hayashi,Chem. Phys. Letters 72 (1980) 420; Y. Sakaguchi. H. Hayashi and S. Nagakura, J. Phys. Chem. 86 (1982) 3177. Y.Tanimoto.M.Takashima and M. Itoh,Chcm. Phys. Letters 100 (1983) 442; Y. Tanimoto, H. Udagawa and hl. ltoh, J. Phys. Chem. 87 (1983) 724. N. Hata and N. Nishida.Chem.Zette.rs(l983) 1043; NJ.Turro andG.C. Weed.J.Am.Chem.Soc. 105 (1983) 1861; N. Periasamy and H. Linschitz.Chem. Phys. Letters 64 (1979) 281:
424
1984
T. Ulrich. U-E. Steiner and R.E. Piill, J. Phys. Chem. 87 (1983) 1873; H. Fischer, Chem. Phys. Letters 100 (1983) 255. 141 J.C. Smiano. E.B. Abuin and L.C. Stewart, J. Am. Chcm Sot. 104 (1982) 5673. [S] Y. Sakaguchi and H. Hayashi,Chem.
References
4 hfay
LETTERS
Phys.
Letters
87
(1982) 539. 161 Y. Sakzquchi and H. Hayashi.
J. Phys. Chem.. to be published. [ 71 A.F. Cockerill. G.L.O. Davies, R.C. Harden and DM. Rackham.Chem. Rev. 73 (1973) 553. 181 H. Hayashi and S. Nagakrra. Bull. Chem. Sot. Japan 87 (1984) 322. 19) P_D. Bums and CN. la Mar, J. Magn. Reson. 46 (1982) [ IO] iti. Lewis, J_A. Jackson, J.F. Lemons and H.Taube, J. Chcm. Phys. 36 (1962) 694; J. Reuben and D. Fiat. J. Chem. Phys. 5 1 (1969) 4909. ( 111 E-l. Marshall and MJ. Pilling. J. Chem. Sot. Faraday Trans. II 74 (1978) 579. [ 13) JD. Lee, A new concise inorganic chemistry. 3rd Ed. (Van Nostrand.Princeton, 1977).
INTERNAL
MAGNETIC
CHEMICAL
PHYSICS
FIELD EFFECT OF LANTHANOID
ON THE PHOTOCHEMICAL
REACTION
4 May 1984
LETTERS
IONS
OF NAF’HTHOQUINONE
IN A MICELLE
Yoshio SAKAGUCHI and Hisaham HAYASHl 77~ Institute of Physical and QIemical Research. Wake. Saitkma 351. Japan Received 9 February 1984; in fiat form 23 February
1984
The effect of trivalent lanthanoid ions on the photochemical reaction of 1,4-naphthoquinone in a sodium dodecyl sulfate micelle is investkated in the presence of magnetic fields. The yields of the escaping semiquinone radicals observed at 0.1 and 1 T decreased with increasing the concentration of certain paramagnetic lanthanoid ions. This is considered to be due to the relaxation of the electron spins in tie triplet radical pair caused by the electron spins of the ions.
I_ Introduction
Recently [ 141, there have been considerable interests in the magnetic field effects on chemical reactions since they have not only physicochemical interests but also possible efficiencies to control chemical reactions and to separate isotopes. Especially the magnetic field effects on the dynamic behavior of photochemical reactions in micelles have been elucidated with the aid of nanosecond laser photolysis [ 1,2,4,5]. In a preceding paper [63 we investigated the photochemical reaction of naphthoquinone (NQ) in a sodium dodecyl sulfate (SDS) micelle and found that this reaction was influenced extraordinarily by a magnetic field below 134 T. In these studies, magnetic fields were applied from outside the reaction system. These effects are called “external magnetic field effects”. The magnetic isotopes in the component radicals of a radical pair also act as intramolecular magnetic fields. These effects are called “maguetic isotope effects”, which originate from the hyperfine coupling of isotopes. There is another possibility that a magnetic field is applied to a reaction system such as addition of paramagnetic species to the system. The magnetic species adjacent to a reaction center is considered to act as a strong magnetic field. The shift reagents used in NMR spectroscopy were developed along this idea [?‘I _As far as we know, however, ihe kffect of the magnetic fields of paramagnetic species on chemical reactions has never been studied. 420
Therefore, we investigate in this paper the effect of triv; lent lanthanoid (Ln3+) ions on the photochemical reaction of NQ in an SDS micelle.
2. Experimental 1,4-naphthoquinone (NQ) was recrystallized repeatedly from benzene and was further purified by sub limation. Sodium dodecyl sulfate (SDS) was purified by repeated recrystallizations from methanol-ethanol mixture. Water was deionized and distilled. Lanthanoid salts, JI.aC13-7H20 (above 995%. Mitsuwa Chemical Co.), NdCI,-6H,O, SmC13s6H,0, GdCI,-6H,O, DyCl3*6HzO, HoCI3-6H,O, and LuCl3-6HlO (99970, Soekawa Chemical Co.), were used without further purification. Laser-photolysis experiments were performed on degassed solutions at room temperature by using the fourth harmonic (266 nm) of a Quanta-Ray DCR-1 Nd : YAG laser as the exciting light source. The laserphotolysis apparatus and the measuring system were similar to those published elsewhere [S ,6] _Hereafter, we denote the experiments under the lowest magnetic field of our apparatus (less than 0.2 mT) as those in thf absence of magnetic field.
0 009-2614/84/S 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
4 May 1984
CHEkflCAL PHYSICS LE-ITERS
Volume 106, number 5 3. Results and discussion
The photochemical primary processes of NQ in an SDS micelle were described in a preceding paper as follows [6]; NQ +t~--*
‘NQ* -+ 3NQ*,
3NQ* + RH + 3(NQH3(NQH‘-3(NQH‘(NQH-
(1)
,R),
-R) + ‘(NQH-
(2)
-R),
(3)
*R) -+ NQH- + R-,
(4)
-R) + recombination
products,
(5)
NQH--+ NQ- + ii+.
(6)
By laser irradiation
the usual hydrogen abstraction reaction of triplet excited state of NQ (3NQ*) from a micelIar mofecule(RH) occurs in a micelle, forming a triplet radical pair of a naphthosemiquinone radical (NQH-) and the akyl one (R-) [reactions (1) and (3)]. The electron spin state of the radical pair changes between the triplet (T) and singlet (S) states [reaction (3)f _The radicals in the singlet and triplet radica) pairs are separated from each other and become escaping radicals [reaction (4)]. Only the radicals in the singlet state radical pair can recombine with each other and give recombination products [reaction (5)J _A part of NQH- dissociates to naphthoquinone anion (NQ-) and proton [reaction (6)3.
0
2
6
0
T&IS
Fig. 1. f(t) curves observed at 380 nm in the (A) absence and (B) presence of a magnetic field of 1 T for miceliar SDS solutions of NQ containing Cd3+ of (a) 0, (b) 0.37 X 10m3.(c) 0.93 x IO-“, and (d) 1.9 x 10m3 mol dm-3, respectiveIy.
%+
8 TIME&s
Fig. 2. f(t) awes observed 31 380 nm in the (A) absence and (B) presence of a magnetic field of I T for micellar SDS solulions of NQ containing Lu3+ of (a) 0, (b) 0.18 x 10a3. (c) 0.96 x iOT3, and (d) 1.9 x 10e3 mol dmm3, respectively_ ALI of the I(f) curves are overiapped with one another.
With the aid of the nanosecond laser-photolysis apparatus, we studied the dynamic behavior of the reactions of the aqueous micellar solution of SDS (8 X IOmol dmm3) containing NQ (5 X IO4 mol dm-3) and IanthanoiG chloride (0 - 3 X 10-s mol dm-3) in the absence and presezpe of a magnetic field of 0.1 or 1 T The decay curves of the transient absorption intensitie I(t), observed at 380 nm for the micellar solution of NQ containingGd3i (0 - 1.9 X IO-3 mol dm-3) in the absence and presence of a magnetic field of 1 T art shown in fig. 1. Fig. 2 shows the i(r) curves observed for the solutions containing Lu3+ (0 - i .9 X IO-3 mo dm-3)_ The absorption observed at 380 m-n arises mair ly from NQH - [6]. Thus the decaying part of each I(r) curve corresponds to the intersystem crossing and recombination of the triplet radical pair [reactions (3) and (5)] , and its remaining part to the escaping NQH. from the radical pair [reaction (4)]. There has been no plausible method to determine the positions of escaping radicals for reactions in miceIles. Either NQH- or R can esit from the micelle. In the present study the rela tive yield of the escaping radical (NQH -) can only be measured. It is represented by the ratio of the rransien absorption intensity observed at 6.5 ~.tsafter the excitation jI(6.5 PS)] normalized to the initial absorption intensity [f(O JIS)] [6]. As shown in fig. 1, the yield ok served in the absence of Gd3+ increased in the present of a magnetic field of I T through the retardation of 421
Votume 106, number 5
CHEMICAL
PHYSICS LETTERS
the intersystem crossing rate of the triplet radical pair [f&8]. In the absence of Gd3* ,I(65 ps)/ir(O gs) increased from 03 1 at 0 T to 0.7 1 at 1 T. When each of the lanthanoid salts was added to the micellar SDS solution in the absence of a magnetic field, the I(r) curve observed at 380 nrn for the solution did not change within the limit of experimental errors. The results for the Gd and Lu salts are shown in figs. LA and 2A, respectively. The decay of the escaping radical was also found to be unaffected by the lanthanoid salts. The I.,$+ ions are considered to be attracted to the negatively charged micellar surface. From the abovementioned results, the Ln3+ and Cl- ions are considered not to react with 3NQ*, NQH - in a radical pair, nor with escaping NQH -. When a diamagnetic Lns” ion (Las+ or Lu3+) was added to the micellar SDS solution in the presence of a magnetic field, its I(r) curve at 380 nm did not change as shown in tip. ZB. On the other hand, when a paramagnetic Ln3+ ion was added to the solution in the presence of a magnetic field, its I(t) curve at 380 nm changed remarkably from the I(r) curve observed in the absence of the ion. The results for Gd3+ are shown in fig. 1B. From fig. 1B, the decay rate at I T was found to increase and the yield of the escaping radical at 1 T to decrease with increasing the concentration of Gd3+. With Gd3* of 1 St X IO-’ mol dmm3, Z(6.5 &/I(0 ~~1s) at 1 T became 0.37, which was nearly the same as that
observed at ClT in the absence of Gd3+. On the contrary, the decay of the escaping radical was not changed by Gd3+, as observed at 0 T. This assures that Gd3f does not react with escaping NQH - in the presence of a magnetic field. Therefore, the effect of Gd3+ on the present reaction can be concluded to be due to some
4 hfay 1984
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1
2
ILn3’ll10dmoldm’J of the relative yield of the escaping naphrh semiquinone radical W6.S &/(O ~5)) on Ianthanoid ion concentration in the absence (m) and prrsencc of a magnetic ficId
Fig. 3. Dependence
ofO.lT(~)or
IT(o).
cesses. If the Ln3+ quenching of the semiqu~one
radical in the radical pair can be expressed by the product of its rate constant (kQ) and the Ln3+ ion concentration, the inverse of the yield of the escaping semiquinone radical from the pair should depend linearly OI the concentration. Indeed, it was found from the present study that the inverse of the relative yield [I(0 ns)/ Z(65 ~_ts)] showed a linear dependence on the Ln3+ ior concentration up to 1 X 10m3 mol dm-3. Therefore, the relative kQ values at 0.1 and 1 T were calculated b) this procedure for the investigated eight Lnjf ions (La3’, Nd3’, Sm3*,Gd3+, Dy3+, Ho3*, Er3+, and Lu3*) and the obtained values are plotted against the number of 4f electrons in fig. 4. As shown in fig. 4, the kQ value is the largest at Cds’ and decreases on each side. L_.n3+ions have been used for giving some petiurbations to the dynamic behavior of molecules. Among other things, Ln3+ complexes are the best known as
physical rather than chemical processes. Other pammagnetic Ln3” ions investigated in the present paper (Nd 3+, Sm3+, Dy3+, Ho3+, and Er3+) showed similar effects on the f(r) curves at 380 nm, but their changes were smaller than those observed for Gd3+. The observed concentration dependences of the five Ln3+ ions (Lax+, Sm3+, Gd3+, I@‘, and Lu3+) on the relative yield of the escaping radical in the absence and presence of a magnetic field of 0.1 or 1 T
are shown in fig. 3. With a simple kinetic scheme for the quenching
of a reaction intermediate, :he yield of a certain process. is expressed by the ratio of the rate constant for that process to the sum of the rate constants for all the pro422
Fig. 4. Relative values of the quenching of the yield of escaping naph~o~m~qu~one radical by Ln3+ ions ck@ observed at 0.1 T f-1 and 1 T iof. Alon:: the abcissa are shown the number of 4f electrons of each of the trivafent ions, its symbol, and electronic state.
Volume 106. number 5
-I hl3y 1984
CHEhlICAL PHYSICS LETTERS
shift reagents in NMR spectroscopy 171; Ln3+ compleses bring about shifts and line bloadenings in NMR spectral lines. The amount of the induced shifts in proton NhlR spectra is roughly ranked as follows [7]: (upfield shift) Dy > Tb,Ho > Pr > La,Nd,Sm,Gd, Lu (small or no shift) > Eu,Er,Yb > Tm (downfield shift), and the induced broadening is [7,9]: Gd > Tb,Dy,Ho,Er,Tm > Yb > Nd > Pr,Sm > Eu (very little broadening)_ The induced shifts in 14N or 170 NMR spectra are roughly ranked as follows [7, lo] : (upfield shift) Gd,Tb,Dy,Ho > Er,Eu 9 Ce.Nd,Yb, Pr,Sm (small downfield shift). Ln3+ ions have also been used for quenching escited molecules. For example, the quenching by some Ln3+ ions of the triplet excited state of phenanthrene was studied at 175 K and the order of the quenching ability was found to be as follows [ 1I ] : Nd > Pr > Ho > Eu > Tb > Yb > Gd,Ce (no detectable quenching). In addition, the magnetic susceptibilities of Ln3+ ions are [la]: Dy>Ho >Tb>Er>Gd>Tm>Yb>Nd>Pr > Eu > Ce > Sm, and the ionic radii decrease from La3+ to Lu3+: La> . ..>Lu. As mentioned above, the effects of the Lns’ ions observed in the present study can be considered to be due to some physical processes_ The observed order of their k Q values were obtained from fig. 3 as follows: Gd3+ > Sm3+ > Dy3+ > Nd3+ > Ho3’ > Ers+ > La3+ Lu3+ (nearly zero). This order is quite different from ihose mentioned above. At first sight, the diamag-
netic La3+ and Lu3’ are shown to have no quenching ability. Among Ln3* ions, Las+ is the lightest and largest ion and Lu3+ is the heaviest and smallest. Therefore, the effect of ionic strength and the external heavy atom effect by Ln3+ rons _ surrounding a micelle can be concluded to be negligible. In -tie next place, Cd;+ and Sm3+ showed the largest and the second largest quenching abilities in the present study, respectively. Alrhougb Gd3+ belongs to the top group of the broadening of proton NMR spectra and of the shift of 14N or 170 NMR spectra, Sm 3+ shows very weak effects on the broadening and shift. Then the mechanisms which induce the broadening and shift on NMR spectra [7,9, IO] cannot be applied to our results. As stated above, no Ln3+ quenching of 3NQ* was observed in the present study. Moreover, the energy transfer from the trip-
let radica: pair to Ln 3+ ions is also unlikely because of the very small energy separation between the triplet and singlet radical pairs and among the Zeeman splittir of the triplet one. Accordingly, the effect of Ln3+ ions on kQ is considered to be due to their magnetic effects to induce th relasation of the electron spins in the triplet radical pa According to the relaxation mechanism [g]. the relasa tions from the T+, and T_, sublevels of the triplet rad. ical pair to the To sublevel of the singlet pair in the presence of a magnetic field control the decay of NQH in a radical pair and the yield of escaping NQH.. The observed effects of paramagnetic Lnj+ ions on the decay and yield can be considered to be due to the rnhancement of the relaxation rates. The pammagnetic ions tumbling around the micelle would induce the relaxation of the electron spins of the triplet radical pair Therefore, this effect can be called “the internal magnetic field effect” by Ln s+ ions. In the absence of a magnetic field these relaxations cannot be considered. because there is no Zeeman splitting for the triplet radi cal pair. This may be the reason why the internal magnetic field effect by Ln j+ ions could not be observed i the absence of a magnetic field. At first sight, this Lnj+ effect might be considered to be related with the magnetic susceptibility. However, the observed order of kQ does not agree with that of magnetic susceptibility. which is due to the combination
of the orbital
of If electrons,
and spin magnetic
but only with
moment
that of
e!ccrron spin multiplicity. as shown in fig. 4. As far as we know, thi! is the first observation of the effecr of spin multiplicir: in Ln3+ ions on the dynamic behavior of molecules. It may be safe to consider that the electron spins in Lns’ ions would induce the relasation of the triplet radical pair. although the detailed mechanism of this effect is not clear at rhis stage. For a more detaiJed analysis of the internal magnetic field effccr by I_n3+ ions which was shown to be present in the present study. the results of all rhc Lnj+ ions and the kinetic anslysis of /(I) curves would be necessary. These experiments are now in progress.
Acknowledgement Grateful acknowledgement is made by k’s for a Pos doctoral Fellowship of Japan Society for Promotion o
Volume 106. number 5
CHEMICAL PHYSICS
Science. This study was partially supported by a grant on Solar Energy Conversion at the Institute of Physical and Chemical Research (Rikagaku Kenkyusho).
Y. Sakaguchi. S. Nagakura and H. Hayashi,Chem. Phys. Letters 72 (1980) 420; Y. Sakaguchi. H. Hayashi and S. Nagakura, J. Phys. Chem. 86 (1982) 3177. Y.Tanimoto.M.Takashima and M. Itoh,Chcm. Phys. Letters 100 (1983) 442; Y. Tanimoto, H. Udagawa and hl. ltoh, J. Phys. Chem. 87 (1983) 724. N. Hata and N. Nishida.Chem.Zette.rs(l983) 1043; NJ.Turro andG.C. Weed.J.Am.Chem.Soc. 105 (1983) 1861; N. Periasamy and H. Linschitz.Chem. Phys. Letters 64 (1979) 281:
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T. Ulrich. U-E. Steiner and R.E. Piill, J. Phys. Chem. 87 (1983) 1873; H. Fischer, Chem. Phys. Letters 100 (1983) 255. 141 J.C. Smiano. E.B. Abuin and L.C. Stewart, J. Am. Chcm Sot. 104 (1982) 5673. [S] Y. Sakaguchi and H. Hayashi,Chem.
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