J. Phys. Chem. 1995, 99, 14123-14128
14123
Magnetic Field Effects on the Dynamics of Nitroxide-Based Singlet Radical Pairs in Micelles E. C. Korolenko; F. L. Cozens, and J. C. Scaiano* Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada KIN 6N5 Received: April 14, 1995; In Final Form: July 5, 1995@
The nitroxide adduct l-benzyloxy-2,2,6,6-te~amethylpiperidine (3) undergoes rapid photocleavage to yield 2,2,6,6-tetramethylpiperidineN-oxide and benzyl radicals. Laser excitation (248nm) of 3 in aqueous micellar solution leads to the formation of singlet radical pairs which decay via a combination of separation and geminate processes. The singlet radical pair produced from 3 undergoes moderately slow recombination and as a result allows for the competition between spin evolution and radical escape. Application of an extemal magnetic field leads to an increase in the rate constant for the geminate radical reaction, reflecting that the magnetic field prevents the interconversion of the singlet radical pair to the unreactive T- and T+ triplet sublevels. In contrast, when the radicals are produced directly on the triplet surface by sensitization, application of a field leads to a slowdown of radical recombination, as normally observed for triplet radical pairs in heterogeneous media. The magnetic field effects are ascribed to the manifestation of the HFI mechanism. The kinetic analyses for the various cases possible are presented in detail.
Introduction Studies of magnetic field (MF) effects on radical pairs in aqueous micelles have been until now almost exclusively devoted to the behavior of geminate triplet radical pairs.'-5 Studies of singlet radical pairs in organized media6-* or of radical pairs produced as a result of random radical encount e r ~ ~have . ' ~ been rare. For an extemal magnetic field to have a significant effect on the recombination of a radical pair, spin evolution and separation of the radical partners must be competitive processes, while the spin selective chemical decay must occur over a sufficiently long period of time for the development of this competition. In the case of triplet radical pairs in organized systems, this competition is frequently assured by the fact that chemical reaction must be preceded by spin evolution (intersystem crossing) to yield a singlet radical pair which then decays rapidly to products. This is illustrated in Scheme 1, where the boxes represent micellar confinement. For singlet radical pairs MF effects often can be observed if the triplet state of the radical pair is reactive, and it is possible to monitor the triplet recombination yield as a function of magnetic field There are few examples of noticeable (> 10%) MF effects for singlet radical pairs recombining exclusively from the singlet (S) ~ t a t e . ~ . ~ Singlet radical pairs made up of typical carbon-centered radicals in micellar solution would be too short-lived for these types of studies, with rapid radical-radical geminate reactions being the anticipated mode of decay. In other words, the incage lifetime (z) of the radical pair would be determined mainly by geminate recombination, rather than by escape and spin evolution processes. Thus, the condition woz > 1 (where w ois the frequency of the singlet-triplet (S-To) transition and z is the lifetime of the geminate pair), necessary for magnetic field effects to be observable, will not be achieved. Clearly, observation of magnetic field effects on the behavior of a singlet radical pair will be expected if geminate recombina-
'
On leave from the Institute of Chemical Kinetics and Combustion, Russian Academy of Science, Novosibirsk, 630099 Russia. Abstract published in Advance ACS Abstracts, August 15, 1995. @
0022-365419512099-14123$09.OO/O
SCHEME 1
I
I Products
1
tion proceeds at a rate slower than or comparable with spin evolution. The reaction of many nitroxides with carbon-centered radicals, especially stabilized radicals, is much slower than the diffusion-controlled limit.I39l4 In particular, 2,2,6,6-tetramethylpiperidine N-oxide (TEMPO) is readily available and can be regarded as a distant analog of the NO radical that is very important as a result of its biological role. Further, the reactivity of TEMPO toward carbon-centered radicals is strongly solvent dependent; in a micellar system this property and its low aqueous solubility will essentially eliminate the possibility of reaction in the aqueous phase. For example, when the reaction partner is the benzyl radical, the rate constant for reaction 1 is 9 x lo7 M-I s-l in acetonitrile and 5 x lo8 M-' s-l in hexane.I4
1
2
3
In the alcohol-like polarity of the micellar environment one would expect the TEMPO-benzyl singlet radical pair to exhibit the desired low reactivity, necessary for the development of magnetic field effects. The present work concentrates on the study of the photochemistry of the adduct 3 in aqueous sodium dodecyl sulfate (SDS) micelles. Our study employs laser flash photolysis 0 1995 American Chemical Society
Korolenko et al.
14124 J. Phys. Chem., Vol. 99, No. 38, 1995 SCHEME 2 PhCHSOCHzPh 4
PhCH2CO
hv
k3
PhCH*
+
.
PhCH2CO
1
PhCHz*
(2)
5
+
CO
(3)
combined with static magnetic fields ranging from 0 to 137 mT. Our work allows the determination of the rate constant for recombination of benzyl and TEMPO radicals in SDS micelles, as well as of the exit rate of the radicals from the micelle. Study of the model compound 3 is also interesting from the point of view of recent developments in the techniques of free radical polymeri~ation.'~~'~ In the presence of TEMPO, radical polymerization leads to TEMPO-capped polymer chains. Polymerization can be reinitiated by heating in the presence of additional monomer; Le., the mild heating in the presence of polar substances is sufficient for the TEMPO terminal group to come off in a reversible process." Polymers produced in this manner show remarkably low polydispersity. The analogy between radical pairs and biradicals is a frequent one. We note that magnetic field effects have been extensively studied in singlet and triplet derived biradicals (or "linked" radical pairs).Is To the best of our knowledge, no nitroxide systems have been studied except for the stable dinitroxide biradicals, where the work concentrated on their ESR spectroscopy.'9 In 1970, CIDNP effects in recombination product of benzyl and dimethyl nitroxide radicals were reported.20 A recent study examines the effect of added nitroxide on triplet radical pairs.21 It concludes that the dominant effect is assisted intersystem crossing, rather than radical scavenging. The effect is the same that has been long established in the case of biradical quenching by nitroxides and other paramagnetic species.22
Experimental Section Preparation of l-Benzyloxy-2,2,6,6-tetramethylpiperidine (3). Compound 3 was synthesized by a modification of the procedure reported by Robbins and E a ~ t m a n . ~ ~ excess An amount of dibenzyl ketone (4)was photolyzed in hexane in the presence of the 2,2,6,6-tetramethyl-1-piperidinyloxy radical (TEMPO) (Scheme 2). The concentration of TEMPO was kept sufficiently low (less than 0.013 M) to allow for decarbonylation (reaction 3) of the phenylacetyl radical to occur prior to radical trapping by TEMPO (reaction 4). Thus, the formation of product 6 could be suppressed (see Scheme 2). Dibenzyl ketone, 4,(Aldrich, 99%), was recrystallized from petroleum ether prior to use. TEMPO, 2 (Aldrich, 98%), was used as received. A 0.38 g sample of 4 was dissolved in 40 mL of hexane and placed into a quartz photolysis tube. TEMPO was added to the solution in three 0.08 g increments. After each addition of TEMPO, the solution was deaerated for 45 min by bubbling the sample with oxygen-free nitrogen. The sample was then irradiated with eight 300 nm lamps in a Rayonet reactor until the pink color disappeared. To ensure the complete consumption of TEMPO, the last irradiation was continued for 5 min after the disappearance of the pink color. After evaporation of solvent, the residue was subjected to
preparative TLC with a Chromatotron 7924T instrument (Harrison Research) using a plate covered by a 2 mm thick layer of silica gel PF-254 with Cas04 (Merck) and a mixture of hexane and ethyl acetate (10: 1 v/v) as the eluent. The chromatograph gave two distinctly separated bands. The two bands were independently collected, and the solvent was evaporated. From the first fraction compound 3 was obtained as a pale oil (0.2 g). The second fraction contained unreacted starting material. The products were identified by their IR and 'H NMR spectra. Spectral data for 3: IR (film) vlcm-l 1469, 1452, 1373, 1360, 1133, 1046, 1028, 734, and 696; NMR d~(cDCl3)1.15 and 1.255 (2 x s, 12), 1.50 (br m, 6), 4.82 (s, 2), and 7.32 (br m, 5). Spectroscopic properties for 3 agree well with those reported in the l i t e r a t ~ r e . ~Sample ~ . ~ ~ 3 contains a small (ca. 6%) amount of 1,2-diphenylethane, which does not influence the photochefistry of 3. Preparationof Samples. Sodium dodecyl sulfate (SDS) was obtained from Fluka (Biochemika Microselect for molecular biology) and used as received. Aqueous solutions of SDS (0.15 M) were prepared using Millipore purified water. The samples were contained in 7 x 7 mm2 cells made from Suprasil quartz tubing. The samples were prepared by injecting -1 pL of 3 into a laser cell containing 2 mL of the SDS solution. For experiments carried out in homogeneous solution, -1 p L of 3 was injected into a cell containing 2 mL of acetonitrile (BDH, Omnisolv). The samples typically had an absorbance of -1.2 at 248 nm. For absorption spectra measurements, a Hewlett-Packard 8451 diode array spectrometer was used. The change of the absorbance at 248 nm after the photolysis was
