724
J. Phys. Chem. 1983, 87,724-726
Magnetic Field Effects on the Primary Photochemical Processes of Anthraquinones in SDS Micelles Yoshifumi Tanlmoto,’ Hlroko Udagawa, and Michlya Itoh Faculty of Pharmaceutical Sciences, Kanazawa University, Takara-Machi, Kanazawa 920, Japan (Received: August 26, 1982: I n Final Form: January 11, 1983)
External magnetic field effects on the laser flash photolysis of three anthraquinones (anthraquinone, anthraquinone-@-carboxylicacid, and anthraquinone-sodium @-sulfonate)were studied in sodium dodecyl sulfate (SDS) micelles (5800G). The triplet-singlet intersystem crossing (isc) rates of semiquinone radical-dodecyl sulfate ion radical pairs formed upon photolysis decrease from 2.5 X lo6 s-l at 0 G to 1.0 X lo6 s-l at 790 G for anthraquinone with increasing a magnetic field (5800 G), while the escape rates of the radicals from the pair do not change significantly. The magnetic field dependence of the isc rate is interpreted in terms of the To-S and T--S crossing mechanisms of the radical pair model.
Introduction Recently, magnetic field effects on primary photochemical processes have been extensively studied in micellar solutions by a laser flash spectroscopic Usually the effects are interpreted in terms of the radical pair model of CIDNP, where triplet-singlet intersystem crossing (isc) occurs by the electron-nuclear hyperfine interaction of the component radicals and is influenced by an external magnetic field.5 However, the study of the magnetic field effects on the isc rate itself is rather scanty. Scaiano and Abuin3 showed that the isc rate of the benzophenone ketyl radical-1,4-cyclohexadienylradical pair in sodium dodecyl sulfate (SDS) micelle is 6 X lo6 s-l at 0 G, though its magnetic field dependence is not clear. In the course of our studies of external magnetic field effects on the photochemistry of biologically significant molecules,4we have studied the magnetic field dependence of the triplet-singlet isc rate of the anthrasemiquinonedodecyl sulfate ion radical pairs in SDS micellar solutions by a nanosecond laser flash photolysis technique. The decay curves due to the transient absorption of the semiquinone radicals consist of a “fast” and a “slow” component. The kinetic analysis shows that the triplet-singlet isc rate gradually decreases with increasing magnetic field (5800 G), while the escape rate of the component radicals does not change significantly. The magnetic field dependence of the isc rate is different from the prediction of the To-S crossing mechanism of the radical pair model. Probably the T--S crossing also takes place in the present magnetic field strength. Experimental Section Anthraquinone (AQ), anthraquinone-@-carboxylic acid (AQC), and anthraquinone-sodium @-sulfonate (AQS), purchased from Nakarai Chemicals, were purified by recrystallization. The purest grade of sodium dodecyl sulfate (SDS) and distilled water (both Nakarai Chemicals) were used as supplied. (I) (a) Hayashi, H.; Sakaguchi, Y.; Nagakura, S. Chem. Lett. 1980, 1149. (b) Sakaguchi, Y.; Nagakura, S.; Minoh, A.; Hayashi, H. Chem. Phys. Lett. 1981,82, 213. (c) Sakaguchi, Y.; Hayashi, H. Ibid. 1982,87, 539. (2) Turro, N. J.; Chow, M.-F.; Chung, C.-J.; Tanimoto, Y.; Weed, G. C. J. Am. Chem. Sot. 1981, 103, 4574. (3) Scaiano, J. C.; Abuin, E. B. Chem. Phys. Lett. 1981, 81, 209. (4) (a) Tanimoto, Y.; Itoh, M. Chem. Phys. Lett. 1981, 83, 626. (b) Tanimoto, Y.; Udagawa, H.; Katauda, Y.; Itoh, M., submitted to J.Phys. Chem. (5) (a) Kaptein, R. J. Am. Chem. SOC.1972,94,6251. (b) Kaptein, R.; den Hollander, J. A. Ibid. 1972, 94, 6269. 0022-365418312087-0724$0 1.5010
The laser flash photolysis measurements were made for degassed solutions at room temperature, using a Molectron UV-12 nitrogen gas laser (337 nm, pulse width -10 ns) as an exciting light source. The details of the apparatus were reported el~ewhere.~ The decay curves of transients were analyzed by a nonlinear least-squares method for the following equation: I ( t ) = Ifexp(-kft) I, exp(-k,t) (1)
+
where I ( t ) is the absorption intensity at time t , Ifand I , are preexponential factors, and kf and k, are the decay rate constants of the “fast” and ‘‘slow’’ components, respectively. Results and Discussion Transient absorption spectra of the laser photolysis of anthraquinone (AQ) in SDS micellar solutions (0--22000 ns) show a characteristic strong absorption band in the 380-nm region. Since AQ is very insoluble in water and no appreciable transient was detected in AQ-saturated aqueous solution, the transient mentioned above is ascribed to a species in the micelles. From a comparison with the absorption spectra of AQ semiquinone radicals in the literature,6 the 380-nm band is assigned to the AQ semiquinone radicals in the micelles. The weak absorption band at -430 nm is probably due to the T-T absorption of AQ. Similarly, the strong transient absorption bands a t 380-390 nm of AQC and AQS were assigned to their semiquinone radicals. The reaction scheme for the interpretation of the data is given in Figure 1. The triplet radical pair (3AQH-S) is generated by the hydrogen abstraction of the excited triplet state of quinone (3AQ)from a dodecyl sulfate ion (HS). The triplet-singlet isc of a radical pair and the escape (separation) of the component radicals from the pair take place competitively. The adduct (AQH-S, cage product) of the semiquinone to the backbone methylene chain of the surfactant occurs via the singlet radical pair (1AQH-S).7 Escaped radicals in the micelle may recombine, undergo further reactions, and/or leave the micelle followed by reaction in the aqueous phase; hydroquinone (AQH,) is one of the main photoproducts. Figure 2 shows the external magnetic field effects on the decay of AQ semiquinone radicals observed at the 380-nm ( 6 ) Bridge, N. K.; Porter, G. Proc. R. SOC.London, Ser. A 1958,244, 259. (7) Swayambunathan,V.; Periasamy, N. J.Photochem. 1980,13,325.
0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No.
Letters 3
3AQ
I
A
&
kes-A Q H .
1 kisc
‘AQ
/
/k*
I
1
k1 AQH-S AQ CAGE PRODUCT
AQ
.S
FREE RADICALS
AQHS
hvl
+
5, 1983 725
+
JH S
\ \k3
\
AQH2 ESCAPE PROWCT
Figure 1. Pathways for photolysis of anthraquinones in an SDS micellar solution: AQ, ground state quinone; ’AQ, singlet-excited quinone; 3AQ, triplet-excited quinone; HS, dodecyl sulfate ion; AQH., semiquinone dodecyl sulfate ion radical; AQH,, hydroquinone; AQH-S, radical; 4, adduct of semiquinone-surfactant; 3AQH..S, triplet radical pair; ’AQH-S, singlet radical pair.
1 Ob
260
460
6bO
860
H i Gauss
Flgure 3. Magnetic field dependence of ki, and k , of anthraquinone semiquinone radicals in SDS micellar solutions. The concentration of SDS is 0.4 M. (A) Anthraquinone (AQ) semiquinone radical observed at 380 nm. The concentration of AQ is 2.7 X lo-‘ M. (B) Anthraquinone-p-carboxylic acM (AQC) semiquinone radical observed at 390 nm. The concentration of AQC is 3.1 X IOb4 M. (C) Anthraquinonesodium P-sulfonate (AQS) semiquinone radical observed at 385 nm. M. The concentration of AQS is 3.5 X 1
I
400
800
I
1200
1600
I
t Ins
Flgure 2. Magnetic field dependences of the decay of anthraquinone (AQ) Semiquinone radicals observed at 380 nm. The concentrations and 0.4 M, respectively. of AQ and SDS are 2.7 X
band. Decay profiles of the curve are somewhat complex. However, they were tentatively analyzed as a sum of two pseudo-fist-order decays shown in eq 1. The decay of the “slow” component is so slow that an accurate decay rate constant (k,) could not be determined by the present experimental setup (probably k, lo4 9-l). Thus the decay curves were analyzed by using eq 1,where k , is assumed to be lo4 s-l and invariant in a magnetic field (51000 G). At zero magnetic field, the decay rate constant of the “fast” component, kf, of the AQ semiquinone radicals is 3.4 X lo6 s-l and decreases with increasing magnetic field ( k f = 1.2 X los s-l at 790 G). The intensity ratio of the “fast” and “slow” components, R = I f / I , , is 2.8 at 0 G and also decreases with increasing magnetic field ( R = 2.0 at 790 G). When the rate of formation of the triplet radical pair is much larger than kf and k , is much smaller than k,, the intensity ratio, R, and the decay rate constant, k f , can be expressed by the following equation^:^ R = kisc/kes (2)
-
(3) k f = kisc + k e s Thus kkc and k,, were obtained by the combination of eq 2 and 3. Figure 3 shows the magnetic field dependence of Itisc and k,, for the AQ, AQC, and AQS semiquinone radicals. The rate constants, kko are clearly reduced with increasing magnetic field (5800 G). On the other hand, k , does not change significantly with the magnetic field; it is almost constant within experimental errors, as generally accepted in the radical pair model of CIDNP. A slight decrease of k , observed at low magnetic field (0-400 G ) may be due to the contamination of the signal by the absorption of photoproduct(s) (mostly the adduct of semiquinone-dodecyl sulfate ion), which makes k , slightly larger and kisc smaller. The obtained kisc values of AQ, AQC, and AQS semiquinone radicals are rather similar to each other, while the k , of AQS is three times larger than that of two other semiquinone radicals. This may be due
to the difference in the solubility of the quinones (or semiquinones). Since AQS has an ionic group, AQS is rather hydrophilic and soluble in water. However, AQ and AQC are very insoluble in water so that they prefer to stay in the micelles. Probably AQS is located on the surface of the micelles, and the escape of AQS semiquinone radicals from the pair is enhanced by a fast exit process out of the micelles. Scaiano and Abuin3 studied the magnetic field effects on the photoreduction of benzophenone by 1,4-cyclohexadiene in SDS micelles. They reported that kiscand k,, of the benzophenone ketyl-1,4-cyclohexadienylradical pair in the micelles are 6 X lo6 and 4 X lo6 s-l at 0 G, respectively, and kisc k,, is almost constant in the magnetic field between 0 and 2000 G. Since k,, is mostly magnetic field independent: this fact means that ki, may be magnetic field independent. However, since signals due to the “fast” decay component of the ketyl radicals were very weak, especially in the presence of a magnetic field, the accuracy of the analysis seems to be quite uncertain. In a previous paper,4 we estimated the kiscof the 2,5-dimethyl-p-benzoquinone-dodecyl sulfate ion radical pairs in SDS micelles to be 2.6 X lo6 s-l at 0 G and 1.0 X lo6 s-l at 950 G, assuming kes is magnetic field independent. In the present study, the signal intensity of the “fast” component is good enough for decay rate analysis, and rate constants, kiscand k,,, were directly obtained. The rate constants of anthraquinones are comparable to the values mentioned above. According to the radical pair model of CIDNP,5 the three triplet sublevels (T+, To,and T-) of a radical pair undergo electron-nuclear hyperfine-induced isc to the singlet state (S) at zero magnetic field. When the magnetic field is much higher than the magnitude (a)of the sum of the hyperfine coupling constants of the component radical^?^ T+and T- are split away from S, and T+-S and T--S crossings do not occur, though Toremains degenerate with S, and To-S crossing continues to take place (To-S crossing mechanism). Thus, it is expected that kisc shows
+
(8)Schulten, K.; Wolynea, P. G . J. Chem. Phys. 1978, 68, 3292. (9) Werner, H.-J.; Staerk, H.; Weller, A. J. Chem. Phys. 1978,68, 2419.
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The Journal of Physical Chemistry, Vol. 87, No.
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a remarkable decrease with increasing magnetic field up to a field strength corresponding to CY,and is almost constant at higher magnetic fields (up to several thousand Gauss). For the AQ semiquinone radical-dodecyl sulfate ion radical pair, CY is estimated to be -106 G.l0J1 I t is expected that kkc decreases rapidly in the magnetic field between 0 and -200 G and is almost constant above -200 G. However, Figure 3 shows a continuous decrease of kisc above -200 G. Actually, at intermediate magnetic field strengths (several hundred to several thousand Gauss) where T+-S crossing is not possible, T--S crossing may still occur, since S is usually energetically lower than Toby an electron exchange interaction (T--S crossing mechanism). On the other hand, the magnetic field dependence of the CIDNP signals of the diradicals has been discussed in ~.~~ the terms of a T--S crossing m e c h a n i ~ m . ~Therefore, present magnetic field dependence of kisc above -200 G may be ascribed to a T--S crossing mechanism as mentioned above. Steiner14studied magnetic field effects (54000 G) on the radical yield of electron transfer reactions between triplet thionine and heavy-atom-substituted anilines in methanol, and interpreted his results by a triplet mechanism in which spin-selective sublevel depopulation of an exciplex is influenced by an external magnetic field. Sakaguchi et al.’ studied the magnetic field effects on the benzyl and benzophenone ketyl radicals generated in SDS micelles. They observed that the intensity ratio of the “fast” and “slow” decay component still decreases at a magnetic field above 1000 G, and suggested the occurrence of another triplet mechanism of CIDEP,15in which the spin-selective pop~~
(IO) Krusic, P. J.; Kochi, J. K. J . Am. Chem. SOC. 1968,90,7155. (11)Wong,S.K.;Sytnyk, W.;Wan,J. K. S.Can. J. Chem. 1972,50, 3052. (12)(a) Closs, G . L.; Doubleday, C. E. J . Am. Chem. SOC.1972,94, 9248. (b) Closs, G.L.; Doubleday, C. E. Ibid. 1973,95,2735. (13)Kaptein, R.;Van Leeuwen, P.W.N. M.;Huis, R. Chem. Phys. Lett. 1976,41, 264. (14)(a) Steiner, U.Z . Nuturforsch. A 1979,34,1093.(b) Chem. Phys. Lett. 1980,74,108. (15)Wong,S. K.; Hutchinson, D. A.; Wan,J. K.S.J. Chem. Phys. 1973,58,985.
Letters
ulation of the precursor leads to a spin polarization of the radical pair. According to these mechanisms, however, no magnetic field effect on the kisc rate is expected, though the intensity ratio of the two decay components may probably be influenced by the magnetic field. Therefore, triplet mechanisms seem to be inadequate for the interpretation of the present results. The appearance of a magnetic field effect in the present reaction completely depends on the rates of the competing two processes from the triplet radical pair, i.e., escape (separation) of the radicals and isc to the singlet state followed by the rapid recombination (see Figure 1). When the rate of the former is comparable to the latter, the change of the latter induced by an external magnetic field gives a significant effect on the decay of semiquinone radicals. In the classical picture of triplet-singlet ~ s c , ~ J ~ the To-S crossing rate is proportional to a weighted sum of the hyperfine coupling constants of the radicals (typically 50-100 G), while the T,-S crossing rate is proportional to the individual hyperfine coupling constant (typically 2-10 G). Thus the To-S crossing rate and the T,-S crossing rate are of the orders of 107-108s-l and 106-107 s-l, respectively. Since, in SDS micellar solution, the escape rate is of the order of lo6 s-l, the magnetic field induced change of the T--S crossing rate may also take an important role in the appearance of the effects. On the other hand, in homogeneous solution where the escape rate is 108-109 s-l, the isc rate is estimated to be 107-108 s-l which is attributable to a To-S crossing me~hanism.’~ Therefore, the isc mechanism in micelles may be different from the one in homogeneous solution because of the difference in the rate of the competing escape process. Studies of magnetic field effects on steady-state photochemistry and the quantum mechanical treatment of the reaction in micelles are in progress for a complete understanding of the effect observed in micelles and will be reported in the near future. (16)Buchachenko, A. L.Russ. Chem. Reu. 1976,45,375. (17)Werner,H.-J.; Schulten, Z.;Schulten, K. J. Chem. Phys. 1977,67, 646.
