Langmuir 1994,10, 1129-1133
1129
Electron Spin Resonance and Electron Spin Echo Modulation Studies of the Photoionization of N-Alkyl-N,N,N-trimethylbenzidinesin Anionic, Zwitterionic, and Cationic Vesicles: Correlation of Photoproduced Cation Location with the Photoyield Chris Stenland and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received October 15,1993. In Final Form: December 13, 199P
N-Alkyl-NJV’JV-trimethylbenzidines(C,TMB, n = 1,4,8,12,16) were synthesized and photoionized in rapidly frozen anionic, zwitterionic, and cationic vesicles to demonstrate how control of the electron donor location by its alkyl chain length correlates with the photoyield for this new series of molecules. The relative photoyields of the cationic radicals were measured by electron spin resonance. Electron spin echo modulation spectroscopy was used to determine the relative location of the photoproduced cation radical with respect to the deuterated aqueous interface. These relative locations are correlated with the photoyields. The photoyield decreased as the C,TMB alkyl chain length increased. This photoyield trend correlates with deeper penetration of the benzidine moiety as a function of alkyl chain length as measured by electron spin echo modulation. Evidence for a relatively inefficient photoinduced radical conversion process in vesicular systems with electron donors is also found. Introduction There is considerable interest in studying the photoionization of electron donors witha low ionization potential in vesicular suspensions as possible energy storage and conversion media.lq Vesicles are surfactant dispersions which form a spherical lipid bilayer that contains an interior water pool. Investigation of the features which maximize the photoyield can be systematically studied by altering the structure of both the vesicle and the electron donor. In analogy to previous micelle workq the photoyield of solubilized electron donors is influenced by changing the vesicle surface chargesor the surfactant monomer chain length6 or by adding c~surfactants.~ The solubilization location of electron donors with respect to the aqueous interface in these systems has been shown to influence the relative photoyield as The N-alkyl-NJV’fl-trimethylbenzidine (C,TMB) electron donors with varying alkyl chain lengths control the solubilization location of the benzidine moiety in micelles but have little effect on the photoionization yield in micelles because of the orientation of the C,TMB molecules with respect to the micelle inte~face.~ Here, analogousstudies in vesicles demonstrate that such location control by variable alkyl chain lengths of the electron donor in vesicle assemblies is achieved and correlates with a changing net photoionization yield. In order to make a comparison between the alkyltrimethylbenzidines and other photoionizable molecules,”12 two different wavelenath ranaes are employed. .Abstract published in Advance ACS Abstracts, February 15, 1994. (1) Photochemical Conversion and Storage ojSolar Energy;Connolly, J. S., Ed.; Academic: New York, 1981. (2) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic: New York, 1975; Chapter 8. (3) Kevan, L. In Photoinduced Electron Trcmmjer Part B: Experi-
mental Techniques and Medium Ejjects; Fox,M . A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988, pp 329-384. (4) Stenland, C.; Kevan, L. J. Phys. Chem. 1993,97,5177. (5) Hiff, T.; Kevan, L. J. Phys. Chem. 1989,93, 2069. (6) Hiff, T.; Kevan, L. J. Phys. Chem. 1988,92, 3982. (7) Hiff, T.; Kevan, L. J. Phys. Chem. 1989,93,3227. (8) Kevan, L. Znt. Rev. Phys. Chem. 1990, 9,307. (9) Sakaguchi, M.; Hu, M.; Kevan, L. J. Phys. Chem. 1990,94,870. (10) Bratt, P.;Kang, Y. S.; Kevan, L.; Nakamura, H.; Mateuo, T. J. Phys. Chem. 1991,95,6399.
0743-7463/94/2410-ll29$04.50/0
The relative distance of the photoproduced alkyltrimethylbenzidine radical to the aqueous interface of the vesicle is determined by electron spin echo modulation (ESEM) spectroscopy. This technique employs a contrast in local nuclear environments by suspending surfactant assemblies, which are composed of hydrocarbons, in deuterium oxide. ESEM can be used to determine the relative distance to the nearest deuterons by measuring the strength of the unpaired electron to deuteron dipolar coupling,which is manifested as modulations at the nuclear Larmor frequency in the electron spin echo decay curve. Measurement of dipolar couplingsrequires frozen solutions so that such couplings are not averaged to zero. Considerable previous work318 has shown that micellar and vesicular structure is retained in rapid frozen aqueous solutions. This information is then correlated with the photoionization efficiencymeasured by double integration of the electron spin resonance (ESR) signal of the radical cation. Experimental Section Materials. Dihexadecyl phosphate (DHP),99%, and tris(hydroxymethyl)aminomethane, 99.9%, were obtained from also called diAldrich. Dipalmitoyl-DL-a-phosphatidylcholine hexadecylphosphatidylcholine (DHPC), 99% , was purchased from Sigma Chemical Co. These surfactants were used as received. Dioctadecyldimethylammonium bromide (DODAB) was obtained from Eastman Kodak and recrystallized from acetone. A methanol/chloroform (7030v/v) solutionof DODAB was passed though the chloride form of AG2X8 20-50-mesh ion exchange resin (Biorad Laboratories) three times to exchange bromide for chloride since the chloride salt makes more stable vesicles. The solvent containing dioctadecyldimethylammonium chloride (DODAC)waa removed on a rotary evaporator,and the DODAC was recrystallized from acetone/water (965v/v) and then dried under vacuum. Deuterium oxide (99.9% D) was obtained from Cambridge IsotopeLabs. MilliporeMilliQtreated deionized water with a conductivityof 18 MO cm-l was used for solutions. Chloroform and acetone were reagent grade from Fisher Scientific. Methylenechlorideand methanol were reagent (11) Bratt, P.; Kang, Y. S.; Kevan, L. J. Phys. Chem. 1992, M,6629. (12) Kang, Y. S.; McManus, H. J. D.; Kevan, L. J.Phys. Chem. 1992, 96,7473.
0 1994 American Chemical Society
1130 Langmuir, Vol. 10, No. 4, 1994
Stenland and Keuan function of interpulse time is often modulated. This echo intensity modulation arises from relatively weak electron-nuclear hyperfiie interaction to which the paramagneticspins are exposed as they undergo dephasing and rephasing. A complete analysis of this modulation gives information about the number and distance of magnetic nuclei surrounding the paramagnetic center.l3 Since the structure of surfactant assemblies such as vesicles is complex and rather disordered, it has been found to be most useful to analyze the modulation in such systems in terms of the normalized modulation depth at the first minimum in the modulation pattern. The normalized modulation depth is the modulation depth from the extrapolated unmodulated curve divided by the overall depth to the baee1ine.l' In the work described here the modulation is always caused by interaction with deuterium nuclei from deuterated water at the vesicle interface. The normalized deuterium modulation depth is a function of both the distance and number of surrounding deuterium nuclei from the paramagnetic species, but since the number of deuteriums at the interface is large and approximately constant, changes in the deuterium modulation depth are interpreted mainly in terms of relative distance changes between the radical cation and deuterium at the vesicle interface. The detection of deuterium modulation is possible for physically reasonable numbers of deuterium nuclei based on molecular density out to a distance of about 0.6 nm. Two-pulse electron spin echo signals were obtained at 4.2 K on a home-built spin echo spectrometer operating at X-band using 40-11s exciting pulses. The ESE signal was obtained for each C,TMB in each vesicle studied and recorded for at least four different seta of samples in order to calculate average deviations. The ESE data were transferred to an IBM PC for lMalySiS. The microwave frequency of the Bruker ESR and the ESE spectrometers was measured by Hewlett-Packard Model 6360B and 5342A microwave frequency counters, respectively. The magnetic field was measured using a Varian Model E501 NMR gaussmeter.
grade from Mallinckrodt. Absolute ethyl alcoholwas from Aaper Chemicals. These solvents were used as received. The N-alkyl-NJV'JV-trimethylbenzidines (C,TMB, where n = 4, 8, 12, 16) were synthesized and purified as previously described.' C1 TMB was obtained from Kodak and purified as the synthesized materials. Sample Preparation. Thin films of the C,TMB and surfactant were deposited from chloroform or methylene chloride solutions in a test tube by blowing a stream of nitrogen gas over the surface of the solution. DHPC thin films required further solvent removal for at least 2 h on a rotary evaporator in a water bath at 60 OC to remove chloroform which was not removed by the Nz stream alone. The thin films were then stored under vacuum overnight to ensure removal of residual chloroform. DHPC and DHP films were dissolved in 1mL of fresh 2.0 X le2 M Tris buffer in DzO and heated for 10 min at 74 f 3 "C. After a gel had formed, the sample was sonicated under a positive pressure of nitrogen gas in the same water bath with a Fisher Model 300 sonic dismembrator with a 4-mm microtip operating at 35% power. The sonication time varied from 20 min to 1h in order to achieve optically clear (slightly blue tinted) DHP solutions and slightly turbid DHPC solutions. On cooling, the turbidity increased a bit for both of these vesicle systems. DODAC vesicles were made in a similar manner, but without buffer and sonication at 55 f 3 "C. The DODAC suspensions after 15 min of sonication were optically clear even after cooling to room temperature. The final concentration of C,TMB was 3 X 1 W M in 1.2 X 10-2M DHP, DHPC, or DODAC. The C,TMB concentration was verified from its extinction coefficient. The ESR and ESEM samples were composed of 50 pL of each solution in 2-mm-i.d. by 3-mm-0.d. quartz Suprasil tubes sealed with Parafilm and plunged into liquid nitrogen to rapidly freeze the sample. Duplicate samples were made from two separate solutions for each C,TMB and vesicle type. Photoirradiation. Photoirradiation was carried out at 77 K in a quartz finger dewar rotated approximately four times per minute to uniformly irradiate the sample. The light source was a Cermax ILC Technologies300-W UV-enhancedxenon arc lamp (ILC-LX 300 UV). This lamp was energized by an ILC Technology Model PS300-1 power supply operating at a lamp current of 10 A. The light was filtered though a 10-cm water filter and a band-pass filter. Two different band-pass filters were used. The first was a Corning 7-60 filter with transmittance in the range of 300-400 nm with a maximal transmittance of 70 % at 365 nm and with typical light flux intensity at the sample of 1.4 X lo2W m-2 (YSI Kettering Model 65 radiometer). The second was a Corning 7-54 filter which passes light from 240 to 400 nm with a maximum transmittance of 80% centered at 310 nm and with typical light flux intensity at the sample of 1X 103 W m-2. The light intensity was constant with time. In routine photoyield studies the samples were irradiated for 10 min. The vesicles with and without electron donor were irradiated for prolonged times to investigate the production of secondary radicals. Tris buffer solution alone was also irradiated to investigate the presence of background radicals. Magnetic Resonance Experiments. ESR spectra were acquired on a Bruker ESP 300 X-band spectrometer with a 100kHz Zeeman field modulation with a 1.1-G modulation amplitude at 77 K. The incident microwave power was 2.0 mW which did not saturate the spectra. The photoyields were determined by doubly integrating the first-derivative absorption spectra using the ESP 300 software. The photoyield data were an average of at least two different sets of samples. In a paramagnetic system an electron spin echo (ESE)lS is produced in response to a two-pulse sequence in which the first 90" pulse causes focusing of the spin system and after a time 7 , during which the spin system dephases, a second 180" pulse causes the initiation of refocusing which after another time 7 after the second pulse results in the generation of a burst of microwave energy called an echo. The ESE intensity is measured as a function of the interpulse decay time. In solid systems in which the dipolar electron-nuclear hyperfine interaction is not averaged out by rapid tumbling of molecules, the echo intensity as a
Results The vesicular suspensions rapidly froze into a white polycrystalline solid. These samples, though exposed to ambient light, were ESR silent. The C,TMB molecules all have an absorption maximum near 310 nm. Ultraviolet irradiation at 77 K gave a green phosphorescence observable with the unaided eye after the light beam was shut off. After irradiation the sample was yellow which indicates the cation radical of C,TMB;4 the ESR intensity correlatedwith the degree of yellow color. The ESR signal was predominately a single line with a line width of about 23 G and a g-factor of 2.004 identifiable as the alkylbenzidine cation radical4 Secondary radicals are also observed but are typically minor in intensity. The vesicle solutions do not show adsorption bands above 240 nm but do have some weak adsorption tail extending to a t least 300 nm. Photoyield. The photoyields for the C,TMB electron donors were investigated as a function of vesicle charge, electron donor alkyl chain length, and photoexcitation spectrum. Figure 1shows the total photoyield for three vesicle systems as a function of C,TMB alkyl chain length, photoirradiated with 300-400-nm light. Figure 2 shows the same systems for 240-400-nm irradiation which is about 7 times more intense. Figures 1and 2 both show that the photoyield increases in order of anionic to zwitterionic to cationic vesicular charge. The zwitterionic yields in Figure 1are approximately halfway between the cationic and anionic yields, but in Figure 2 the zwitterionic yields are closer to the anionic yields. The photoyields for 240-400-nm light are greater than the yields with 3 W
113) Kevan, L.In Time Domain Electron Spin Resononce; Kevan,L., Schwartz, R. N., Eds.; Wiley: New York, 1979; pp 279-341.
(14) Szajdzinska-Pietek,E.;Maldonado, R.; Kevan,L.;Jones, R. R. M. J. Am. Chem. SOC.1985,107,6467.
ESR and ESEM Studies of CnTMB
Langmuir, Vol. 10, No. 4, 1994 1131
CnTMB / Vesicles (300-400 nm)
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CIBTMB Total
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t DODAC
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Figure 1. Relative photoyield at 77 K of CnTMB electrondonors versus C, alkyl chain lengthin DODAC,DHPC, and DHP vesicles, irradiated for 10 min with 300-400-nm light. CnTMB / Vesicles (240-400nm)
DODAC
%!'ol 8
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Alkyl Chain Length (n) Figure 2. Relativephotoyield at 77 K of CnTMBelectron donors versus C, alkyl chain lengthin DODAC,DHPC, and DHP vesiclee, irradiated for 10 min with 240-400-nm light. CiTMB
0
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2
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Figure 3. Relative photoyield at 77 K of ClTMB and C4TMB in DHP vesicles irradiated for 10 min with 300-400-nm light. 400-nm light which is consistent with the greater light intensity. The two photoexcitation spectra also give different relative amounts of secondary radicals (see below). The photoyields as a function of electron donor alkyl chain length generally decrease as the added alkyl chain increases in length, but the trend is more pronounced with the lower energy 300-40O-nm light. With 240400-nm light the photoyield decreases from ClTMB to C8TMB and increases slightly for longer CnTMB chain lengths. Photoyield versus Electron Donor Concentration. The photoyield as a function of electron donor concentration is shown in Figure 3 for DHP vesicles for 300400-nm light. The photoyield plateaus for concentrations above 2 mM. Secondary Radicals. Secondary radicals are observed, but in most cases these are minor components. The
C4TMB*
60
120 DODAC 180 only ITME+
Time (min) Figure 4. Relative photoyield at 77 K of C4TMB and CleTMB in DODAC vesicles (open symbols) irradiated with 24MOo-nm light. The singletESR component which represents the C,TMB cation radical is plotted with analogous solid symbols. The yield of DODAC alkyl radicals without CnTMB is also shown (+).
secondary radicals depend on the photoexcitation spectrum, the type of vesicle, and the presence of Tris buffer. In DHP vesicles, the ESR signal was in most samples a 23-G-widesingle line. In some cases a narrower 7-G single line at g = 2.002 was observed. Tris buffer has been shown to be an electron donor.ls To check the possible presence of a radical derived from Tris, a 2.0 X 1P2M Tris solution (pH 7.8) was irradiated with 240-400-nm light for 10min at 77 K. It shows a 9-G single line at g = 2.002. In DHPC vesicles secondary radicals are slightly more abundant. Here a weak four-line signal overlaps the benzidine cation line with intensities of 1:3:3:1, a hyperfine coupling constant of 23 G, and a g-factor of about 2.0024 which is identified as a methyl r a d i ~ a l .For ~ DODAC vesicles, the abundance of secondary radicals is largest. DODAC vesicles show a methyl radical signal and a weak eight-line signal with binomial line intensities and a coupling constant of about 23 G. This radical has been identified as an octadecyl radical from the DODAC surfactant.ls The yield of secondary radicals increases in the order of anionic to zwitterionic to cationic vesicles. Deconvolution4 of the ESR spectra to determine the C,TMB+ component in vesicles irradiated with 300-400-nm light indicates that the secondary radicals in anionic vesicles are less than 5 % , in zwitterionic vesicles about 9%, and in cationic vesicles about 20% of the total relative photoyield. The ratio of secondary radicals to CnTMB+ is also influenced by the photoexcitation spectrum used. When irradiated with 240-400-nm light, the yields of secondary radicals in DHP and DHPC are only slightly higher, but the yield in DODAC vesicles is significantly larger (-35 % of the total relative photoyield). Radical Conversion. A photoinduced radical conversion process has been proposed to take place in vesicular systems with electron donors? DODAC vesiclescontaining C4TMB and CuTMB were irradiated with 240-400-nm light for various time intervals totaling 3 h at constant light intensity. The ESR signal after each irradiation time interval is doubly integrated and plotted versus time in Figure 4. Also shown in Figure 4 is the yield versus time of the deconvoluted CnTMB+component of the totalESR signal. The deconvolution employed the predominately C,TMB signal at early irradiation times (20 8). For C4TMB and CleTMB in DODAC vesicles, the total yield rises very rapidly, plateauing at about 30 min. The (15) McManue, H. J. D.; Kang, Y. S.; Kevan, L. J . Phys. Chem. 1992, 96,2214. (16) Sakaguchi, M.; Kevan, L. J . Phys. Chem. 1989,93,6039.
1132 Langmuir, Vol. 10,No. 4,1994 CnTMB I Verlcles / D 2 0 (300-400nm) I T
f
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Alkyl Chain Length (n) Figure 5. Normalized deuteron modulation depth from twopulse ESEM at 4 K for CnTMB+versus C n in DHP, DHPC, and DODAC vesicles. The photoexcitation spectrum was 300-400nm light.
C,TMB+ component plateaus at about 10 min. The intensity of the C,TMB+ slowly decreases with a corresponding increase in the total spin concentration for long irradiation times. The loss of the C,TMB+ component is at most 20% of the maximal yield. An experiment was also performed with DODAC vesicles with no C,TMB as shown in Figure 4. The yield of DODAC surfactant alkyl radical is very low even after prolonged irradiation times, consistent with very little adsorption by DODAC for 240-400-nm light. Comparingalkyl radical yields in vesicles with and without C,TMB, it is clear that the presence of the electron donor significantly enhances the yield of the secondary radicals. This is consistent with a photoinduced radical conversion process. There also appears to be a C,TMB alkyl chain dependence on the yield of alkyl surfactant radicals. The longer alkyl chain CnTMB donors give higher yields of the surfactant radicals; after 3 h of irradiation C4TMB gives about 40% while ClsTMB gives about 50% surfactant radicals of the totalrelative photoyield in DODAC vesicles. ESEM Results. Figure 5 shows the normalized deuteron modulation depth versus C,TMB alkyl chain length for the three vesicle systems. The general trend shows a decrease in the normalized deuteron modulation depth as the alkyl chain increases in length. The normalized modulation depth decreasesin the order of DHP, DHPC-, to DODAC vesicles. The irradiation spectrum was 300400-nm light, so secondary radicals are minimized.
Discussion ESEM. As described earlier, ESEM is sensitiveto weak electron-nuclear dipolar hyperfine interactions that are not typically resolvable by continuous-waveESR. ESEM can determine the type and distribution of magnetic nuclei by the analysis of the modulation found in the electron spin echo decay curve. In disordered systems,like vesicles, a relative measure of the distance from the free radical to nearby nuclei is made by measuring the normalized modulation depth. This measurement is possible when a contrast in the local nuclear environment is created by dispersing vesicles composed of hydrocarbon surfactants in deuterium oxide. The ESEM nuclear modulation depth in the spin echo decay curve is directly related to the number of interacting nuclei and inversely related to the average distance to the nearest nuclear distribution. The number of deuterium nuclei at the vesicle interface is about constant for different CnTMB. The average distance may be defined from the center of the spin distribution to the center of the nearest nuclear distribution of interest. It has been demonstrated4
Stenland and Kevan
that the spin distribution in C,TMB for n 1 2 is not significantly different from that for C1TMB. So the comparisonof the normalized modulation depths between different C,TMBs is valid. The deeper the deuterium modulation, the closer the cation radical is to the deuterated aqueous interface. C,TMB Alkyl Chain Effect. Figure 5 shows the normalized deuterium modulation depth as a function of C,TMB alkyl chain length and vesicle charge type. The normalized modulation depth decreasesas the alkyl chain increases. This is interpreted as the benzidine moiety being located deeper into the lipid bilayer as the CnTMB alkyl chain gets longer. This trend supports that observed in the neutral N-alkylphenothiazines in vesicles12 and in photoreduced alkylmethylviologens in ~esic1es.l~ The ESEM normalized modulation depths show a variation as a function of vesicle charge type. The difference in the ESEM normalized modulation depth correlates with the overall length of the vesicle monomer. The length of the vesicle monomer should influence the thickness of the lipid bilayer formed when the vesicle is assembled. A thicker bilayer will have deeper regions available to solubilize a nonpolar electron donor. Cation radicals that are formed and localized farther from the deuterated aqueous interface will exhibit shallower deuterium modulations. A DODAC monomer has two Cle alkyl chains which is longer than a DHP monomer which has two CISalkyl chains. Although DHPC also has two c 1 6 alkyl chains, its large head group effectively makes it longer than DHP. The lipid bilayer as a function of vesicle monomer chain length is expected to grow in thickness, increasing from DHP to DHPC to DODAC. So the ESEM deuteron modulation depth as a function of vesicle size is expected to increase in the order of DODAC to DHPC to DHP. While this qualitativelyexplainsthe trend observed, it doesn’t adequately quantify the magnitude of the vesicletype effect. The ESEM deuterium modulation depth has been explored as a function of the vesicle monomer alkyl chain length for a series of dialkylphosphatidylcholine vesicles with solubilized chlorophyll and tetrabromobenzoquinone.6 These data suggest that the alkyl chain length effect is too small to explain the difference between DHP and DODAC. The water structure at the interface has also been suggested to be an important factor. The large modulation seen in DHP has been attributed to the strong hydration of the phosphate head groups by D ~ 0 . l ~ The photoyield is shown to decrease as the CnTMB alkyl chain gets longer. ESEM indicatesthat the electron donor moiety is located deeper in the bilayer for longer C,TMB chain lengths. These two results indicate that the probability of producing net charge separation, independent of interface charge, decreases as the benzidine moiety is located deeper into the vesicle. This is consistent with an electron-transfer rate constant that decreases exponentially with distance.l8 InterfaceCharge Effect on the Relative Photoyield. The motion of the electron is not solely influenced by the barrier presented by the lipid bilayer; the photoelectron’s motion is also governed by the local electric field generated by the surfactant head group charge. The charge of the interface determines whether the passage of the electron will be assisted by the cationic head groups or hindered by the anionic head groups. Ideally, a neutral interface should neither assist nor hinder the ejection of the (17) Colaneri, M. J.; Kevan, L.;Thompson, D.H.P.;Hum& J. K.J. Phys. Chem. 1987,91,4072-4077. (18)Gratzel, M.Heterogeneous Photochemical Electron Ticmsfer; CRC: Boca Raton, FL, 1988.
ESR and ESEM Studies of CnTMB photoelectron. This is what is observed experimentally; the relative photoyields in frozen vesicles are highest in cationic vesicles and lowest in anionic vesicles, while the neutral (actuallyzwitterionic) vesicles give an intermediate photoyield. Radical Conversion. A radical conversion process has been convincingly demonstrated for the photoreduction of alkylmethylviologens in DODAC vesicles.16 Most notable about this process is that the spin concentration remains constant as a function of irradiation time. The photoreduced alkylmethylviologen cation radical is quantitatively converted into the secondary octadecyl radical. A photoinducedradical conversion process has also been suggested to occur for alkylphenothiazine cation radicals embedded in DODAC vesicle^.^ However, the evidence is not as convincing since the total intensity increases somewhat as a function of irradiation time, and the relative ratio of the cation radical to secondary radical appears to decrease slightly. For CnTMB in vesicles, the spectra are similar to those of the alkylphenothiazined a h g A detailed deconvolution of the data as a function of irradiation time suggests that the rate of radical conversion is slower than that observed with alkylviologens; see Figure 4. The decrease in the radical cation componentamounts to about 20% at 3 h. This contrasts with 100%conversion of the alkylmethylviologen radical cation at 2.5 h. There is an alkyl chain length dependence on the yield of the secondary octadecyl radical. The longer chain length alkylmethylviologens and alkyltrimethylbenzidines produce greater yields of the secondaryoctadecyl radical than their shorter chained analogs. The photoinduced production of the secondary surfactant alkyl radicals depends on the interaction of the surfactant with an excited state of a radical cation solubilized in the lipid bilayer. The yield of alkyl radcials in DODAC vesicles irradiated with 240-400-nm light without added CnTMB is less than 10% of that with C,TMB added. The production of the octadecyl chain
Langmuir, Vol. 10, No. 4,1994 1133
radicals by photoexcited C,TMB, which is an electron donor, is probably generated by a different process than by photoexcited alkylmethylviologen which is an electron acceptor. One possible process is the ejection of an electron by CnTMB, and the capture of the photoejected electron by a cationic surfactant molecule,lg yielding secondary alkyl radicals. Conclusions The photoyields correlate with the relative locations of the CnTMB electron donorsrelative to the vesicle interface. The radical cation yield decreases as the electron donor moiety locates deeper into the vesicle. The photoyield dependence on the surfactant head group charge also influences the yield of the C,TMB radical cation as also observed in alkylphenothiazine/vesicle systems. The production of secondary alkyl radicals derived from the surfactant monomers depends on the vesicle charge, the photoexcitation spectrum, and the C,TMB alkyl chain length. The secondary radical yield increases when the vesicle charge is changed from anionic to zwitterionic to cationic and when the electron donor moiety locates deeper into the vesicle. The total radical yield increases with irradiation time in addition to a slow radical conversion process. Acknowledgment. This research was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S.Department of Energy. (19) In DTAC, a cationic micelle with solubilized TMB,irradiated with 300-400-nm light shows the production of methyl radicals.' If thew methyl radicals are produced by electron attachment to the cationic surfactants, then addition of a fast electron scavenger like nitrate should atop the production of these methyl radicals. This ia what is obrved. The Secondary radicals are NO2 radicals and not methyl radicals after addition of nitrate.
