Langmuir 1993,9, 1691-1697
1691
Electron Spin Resonance, Electron Spin Echo Modulation, and Electron Nuclear Double Resonance Studies on the Photolysis of Positively and Negatively Charged Alkylphenothiazines in Anionic Aerosol Dioctyl and Cationic Cetyltrimethylammonium Bromide/Hexanol Reverse Micelles Young So0 Kang and Larry Kevan' Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received February 5,1993. In Final Form: April 8,1993 Positively charged (phenothiazinylalky1)trimethylammonium bromides (PC,TAB, n = 4,6,9,12)and negatively charged sodium alkylphenothiazinesulfonates (PC,S, n = 3,6,9,12)incorporated into anionic aerosol diodyl (AOT) and cationic cetyltrimethylammonium bromide (CTAB)/hexanol reverse micelles have been photoionized at 77 K. The photoproduced radicals are identified as the alkylphenothiazine radical cation and are quantitated by electron spin resonance spectroscopy. The relative locations of the phenothiazine moiety relative to the reverse micelle interface with deuterated water are correlated with the photoionization efficiency by deuterium electron spin echo modulation (ESEM) and matrix proton electron nuclear double resonance (ENDOR) spectroscopies. PC,TAB and PC,S compounds both show the same U-shaped photoyield trends with increasing pendent alkyl chain length of the charged alkylphenothiazines. This is interpreted as alkyl chain bending to locate the phenothiazine moiety more toward the interface with longer alkyl chains. The photoyield decreases monotonically with increasing interior water pool size (WO)in anionic AOT reverse micelles and correlates with an increasingly negative interface charge of AOT due to increasing hydration of Na+ counterions. The decrease correlates with a greater distance for electron transfer with increasing w oas shown by the deuteron ESEM results. The photoyields and deuteron modulation depths of PC,TAB and PC,S compounds in cationic CTAB/hexanol reverse micelles show a U-shaped trend versus woexcept for PCdTAB and PGS. This suggests that more chain bending of the charged alkylphenothiazines occurs in cationic CTABIhexanol reverse micelles than in anionic AOT reverse micelles, perhaps because the surfactant alkyl chain packing is "opened up" by the hexanol cosurfactants. A higher photoyield for the same charged alkylphenothiazine is found in anionic AOT versus cationic CTAB/hexanol reverse micelles. This surprising result indicates that the interface charge is not the dominant aspect controlling the photoionization. Instead a shorter distance of the phenothiazine moiety to the interface water in CTAB/hexanol reverse micelles due to less a ordered interface structure dominates the photoionization efficiency. This shorter distance is indicated by both ESEM and ENDOR results.
Introduction Photoionization of solubilized molecules in organized molecular assemblies such as micelles, reverse micelles, and vesicles has been used as model systemsfor light energy storage.14 For effective light storage it is necessary to optomizethe photoionization efficiency. Aspecta for such optimization include changing the counterion,5*6surfactant headgroup, and alkyl chain length718and adding salb?Jo interface-active organic additives,'l-13 and electron ac(1) Kalyanaaundaram, K. Photochemistry in Microheterogeneous System; Academic: New York, 1987. (2) Fender, J. H. Acc. Chem. Res. 1980,13, 7. (3) Hurley, J. K.; Tollin, G. Sol. Energy 1982,28, 187. (4) Kevan, L. In Photoinduced Electron Transfer, Part B; Fox, M. A., C h o n , M., Eds.; Eleevier: Amsterdam, 1988; pp 329-384. (5) Szajdzinska-Pietak,E.; Maldonaldo,R.;Kevan, L.;Jones, R.R.M. J. Am. Chem. SOC.1984,106,4675. (6) Jones, R. R M.; Maldonaldo,R.;Szajdzinska-Pietek,E.; Kevan, L. J. Phys. Chem. 1986,90,1126. (7) H a , T.;Kevan, L. J. Phys. Chem. 1988,92,3982. (8)Hiff, T.;Kevan, L. J. Phys. Chem. 1989,93,2069. (9) Hiff,T.;Kevan, L. J. Phys. Chem. 1989,93,3227. (10) Fang, Y.; Tollin, G. Photochem. Photobiol. 1984,39,685. (11) Hiromitsu, I.; Kevan, L. J. Am. Chem. SOC.1987,109,4501. (12) Kang,Y. S.; Baglioni, P.; McManus, H. J. D.; Kevan, L. J. Phys. Chem. 1992, 96,10049. (13) Baglioni, P.; Rivara-Minten, E.; Kevan, L. J. Phys. Chem. 1988,
92, 4726. (14) h o t , M. P.; Kevan, L. J. Phys. Chem. 1989,93,5280. (15) Stenland, C.; Kevan, L. Radiat. Phys. Chem. 1991, 37, 423.
An important factor that can control the photoionization efficiency is to add variable length alkyl chains to the photoionizable molecule to control ita location relative to the surfactant assembly interface. Such location control has been demonstrated with alkylviologens in micelles and vesicle^,^^^^ alkylporphyrins in and alkylphenothiazines in micelles,12*21*22 reverse micelles,2aand vesicles.24p25 Here, we demonstrate location control with charged alkylphenothiazines in anionic and cationic revem micelles. Electron spin resonance (ESR) is used to measure relative photoyields. Electron spin echo modulation (ESEM) spectroscopy is used to investigate the relative location of photoproduced cation radicals from deuterated water at the interface of various molecular assemblies.s2a A correlation of photoionization yield trends with interaction distance trends between the electron donor and (16) Sakaguchi, M.; Kevan, L. J. Phys. Chem. 1989,93,6039. (17) Colaneri, M. J.; Kevan, L.; Thompson, D. H. P.; Huret,.J. K. J. Phys. Chem. 1987,91,4072. (18)Sakaguchi, M.; Kevan, L. J. Phys. Chem. 1992,95,6996. (19) McManus, H. J. D.; Kevan, L. J. Phys. Chem. 1991,96,10172. (20) Chastenet de Castaing, E.; Kevan, L. J. Phys. Chem. 1991, 96, 10178. (21) Baglioni, P.; Hu, M.; Kevan, L. J. Phys. Chem. 1990,94,2686. (22) Kang,Y. 5.;McManus, H. J. D.; Kevan, L. J. Phys. Chem.1992, 96,7473. (23) Hu, M.; Kevan, L. J. Phys. Chem. 1990,94,5348. (24) Bratt, P.; Kang, Y. S.;Kevan, L. J. Phye. Chem. 1991, !36,6399. (25) Sakaguchi, M.; Hu, M.; Kevan, L. J. Phys. Chem. 1990,94,870.
Q743-7463/93/24Q9-1691$Q4.QQ/Q0 1993 American Chemical Society
Kang and Kevan
1692 Langmuir, Vol. 9, No. 7, 1993 interface water (DzO) as electron acceptor has generally been obtained which indicatesthat the interactiondistance usually controls the photoyield. Proton matrix electron nuclear double resonance (ENDOR) is used to obtain complementary information to ESEM. ENDOR gives information on the local proton density in the environment of the photoproduced paramagnetic species.% These studies are carried out in the frozen state because the lifetimes of photoproduced unstable species are very short at room temperature and the analysis and exploitation of ESEM and ENDOR require solid-state systems.26J" Hence the surfactant assembly solutions are rapidly frozen to make such an analysis possible. Numerous studies have demonstrated that micellar and vesicular structure is retained in rapidly frozen aqueous solutions; these include the following. (1)The long lifetime of photoreduced tetramethylbenzidine radical cations in micelles but not in bulk solution supports this.268 (2) Analogous photoionization trends for molecules embedded in micelles and vesicles are found.26b Since vesicles are considered to be more organized in their interiors than are micelles, this argues that micellar structure is retained in addition to vesicular structure in frozen solutions. (3) It has also been found that similar aggregation numbers, that is the number of surfactant molecules making up a micelle, are measured in liquid and frozen micellar solutions by luminescent quenching methods.2Bc (4) Micellar structure can also be directly detected in frozen micellar solutions by electron microscopy.za (5) It has been found that certain countercations such as the tetramethylammonium cation open up the surface structure of an anionic micelle such as sodium dodecyl sulfate (SDS) to increase the low concentration of water at the interface. It has been possible to study this effect in both liquid and solid media, and the results are similar in both cases.26a (6) It has also been possible to use ESEM methods to measure the partition coefficients of alcohols between the micellar interface and the bulk phase in frozen SDS micellar solutions.26f The values are found to agree with values determined by nuclear magnetic resonance or thermochemistry in SDS liquid micellar solutions. The photoionization of three negatively charged sodium alkylphenothiazinesulfonate (PC,S, n = 3,6,12) in anionic aerosol dioctyl (AOT) and cationic cetyltrimethylammonium bromide (CTAB)/hexanolreverse micelles has been recently studied with ESR.23 Likewise, two positively charged phenothiazinyltrimethylammonium bromides (PC,TAB, n = 4, 8) in AOT reverse micelles have been studied with ESR and ESEM,31 These limited studies suggest that controllingfactors for photoyield optimization in reverse micelles are the ratio of water to surfactant, usually termed WO,and to some extent the pendent alkyl chain length. (26) (a) Narayana, P. A.; Li, A. S.W.; Kevan, L. J. Am. Chem. SOC. 1981,103,3603. (b) Li, A. S.W.; Kevan, L. J.Am. Chem. SOC.1983,105, 5752. (c) Hashimoto,5.;Thomas, J. K. J. Am. Chem. Soc. 1983,105, 5230. (d) Bachman, L.; Dasch, W.; Kutter, P. Ber. Bunsen-Ces. Phys. Chem. 1981,85,883. (e) Hiromitau, I.; Kevan, L. J. Phys. Chem. 1986, 90,3088. (0Baglioni, P.; Kevan, L. J. Phys. Chem. 1987, 91, 1516. (27) Kevan, L. Radiat. Phys. Chem. 1991,37,629. (28) Kevan, L. In Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R. N., Ede.; Wiley: New York, 1979; Chapter 8. (29) Helbert, J.; Kevan, L.; Bales, B. L. J. Chem. Phys. 1972,57,723. (30) Kevan, L.; Kiepert, L. D. Electron Spin Double Resonance Spectroscopy; Wiley: New York, 1976; pp 165-253. Baglioni, P.; Kevan, L.; Matauo, T. J.Phys. Chem. (31) Nakamura, H.; 1991,96,1480.
In the present study, a more extensive range of alkyl chain lengths of PC,S and PC,TAB compounda has been synthesized and photoionized in anionic AOT and cationic CTAEVhexanol reverse micelles. The photoproduced cation radicals were studied with ESR, ESEM, and proton matrix ENDOR to better establish the photoyield dependence on the pendent alkyl chain length of the charged alkylphenothiazinesand on the ratio of water to surfactant. These results are comparatively discussed with neutral N-alkylphenothiazinephotoionizationstudied in AOT and CTAB/hexanol reverse micelles.
Experimental Section SamplePreparation. The negativelychargedsodium alkylphenothiazineeulfonates (PC,S, n = 3,6, 9, 12) and positively charged (phenothiazinylalky1)trimethylammonium bromides (PC.TAB, n = 4,6,9,12) were synthesized according to previous literature procedures."*** AOT (99.976, bis(2-ethylhexyl) sulfoeuccinata used without further purification), CTAB (99.9%, purifiied by recrystallization three ti" in acetone), and D20 (99.9 atom 9%D) were obtained from Aldrich Chemical Co. D10 was used after deoxygenationby purging with nitrogen gas for 2Omin beforeuse. Anhydrous 2,2,6trimethylpentane(i"e, purity >99%), n-octane, and 1-hexanol were purchased from Aldrich Chemical Co. and used without further purification. Ten milliliters of AOT reverse micellar stock solutions with wo ([H2Ol/[surfactantl) = 5, 10, 20, and 30 were prepared by adding 0.09, 0.18, 0.36, and 0.54 mL of a0 into 0.1 M AOT/ isooctane solution. Then 70 pL of 1 x le2M of each charged alkylphenothiazine in D20 solution was introduced into 1mL of the AOT stock solution. CTAB/hexanol reverse micellar stock solutions with wo = 6,10,20, and 30 were prepared by dissolving 3.64 f of CTAB in 0.9-5.4 mL of D10 and adding a mixture of n-octane and 1-hexanolcosurfactant (10%hexanol by volume) until a volume of 50 mL was reached. A 70-mL portion of 1 X le2M of each charged alkylphenothiazine in D20 solution was added into 1 mL of the CTAB stock solution. The final concentrationof each chargedalkylphenothiazinewas 6.2 X 1W M, which was measured, with a Perkin-Elmer 330 optical absorption spectrophotometer (A- = 312 nm in CHCb; 1-ogE = 3.17).99 Then these AOT and CTAB/hexanol reverse micellar solutionswith charged alkylphenothiazines were kept for 2 h at 50 O C and 2 days at room temperature to ensure equilibration. The solutions turned clear, which indicated complete solubilization of the charged alkylphenothiazines. Argon gas was blown onto the surface of the reverse micellar solutions for 5 min to prevent contaminationfrom oxygen in the air. Then 100& of each sample solution was placed into 2 mm i.d. X 3 mm 0.d. Suprasil quartz tuba which had been We-sealed at one end. The samples were then sealed at the other end to prevent evaporation loss and contact with oxygen. Finally the sample t u b were shaken for 1min by hand to ensure equilibrationand then rapidly frozen by plunging into liquid nitrogen. Photoirradiation. Photoirradiation of the samplee was carried out at 77 K with a 300-W xenon lamp (ILC-LX300 UV). A 10-cm water fiiter was used to remove infrared light and a Coming fiiter No. 0-53 was used which passed radiation >280 nm. The photoyield of the photoinduced radical was saturated after a 10-min irradiation of the sample solutions. ESR experimentswere carriedout with lO-minphotoirradiation of allsample solutions. Photoinduced radical conversion from the alkylphenothiazine cation radical to surfactant radicalsw limited the optimum irradiation time for ESEM and ENDOR to 5 min. The Dewar holding the sample tube was rotated at 4 rpm during photolysis to ensure even irradiation of the sample. The light intensity at the aample position was measured with a YSIKettering Model 65 radiometer as 1.1 X 108 W-m-l. Magnetic Resonance Experiments. ESR spectra were recorded at X-band using a Bruker ESP 300 spectrometer with *
(32) Nakamura, H.; Fujii, H.; Sakaguchi, H.; Masato, T.;N N.; Yoshihara, K.; Ikeda, T.; Tazuke, S . J. Phys. Chem. 1 9 8 8 (33) Hanson, P.; Norman, R. 0. C.J. Chem. SOC.,Perkin Rans. 2 1973,264.
~
~
Photolysis of Alkylphenothiazines
Langmuir, Vol. 9, No. 7, 1993 1693
100lrHzfield modulation. The irradiated samplecell was placed in a quartzESR Dewar (WilmadGlass Co.) which was filled with cavity. The loaded Q liquid nitrogen and secured in a TEIOZ factor of this cavitywae measured ae about 1700. The microwave power was 1.97 mW. The microwave frequency was measured with a Hewlett-Packard 5350B frequency counter, and the magneticfield was monitored with a Bruker ER 032M Hall effect field controller. The standard ESR spectrometer settings were 0.281mT modulationfield amplitude, 20 mT sweepwidth, seven scau accumulations,56 s scan time constant,microwavefrequency 9.495GHz,and 1.25X 106receiver gain. The photoinducedradical yield wae determined by double integration of the ESR spectra using the ESP 1600 software. Each relative photoyield value is an average of triple determinations and normalized by dividing by the photoyield of the PC,&AOT/D2O/wo = 5 sample. Two-pulse ESE signals were recorded at 4.2 K with a homebuilt spin echo spectrometer which was operated at X-band.aP= The microwave pulse sequences and data acquisition processes were controlled by a Nicolet 12/80minicomputer which was interfaced to the ESE spectrometer. The typical external microwave pulse widths used in the experimentswere 40 and 80 ns. Once obtained, the ESE data were transferred to an IBMcompatible 486-based microcomputer for analysis. Each signal was scanned 10 times. A deuteron modulation appeared with about 0.5-paperiodicity and its depth at the first minimum wae normalized by dividing by the height of the extrapolated unmodulated echo to the baseline at that minimum.26s9BThe reported value is the mean of three separate determinations. Proton matrix ENDOR spectra were recorded at 141 K using a Bruker ESP 350ENDOR unit. A Bruker ER 4111VT nitrogen flow variable temperature unit was used to control and monitor the temperature in the ENDOR cavity. The radiofrequency (rf) power was constant at 100 W and wae frequency modulated at a rate of 12.5 kHz. The resulting microwave response at the center of the deaaturated ESR line was synchronouslydetected at the rf modulation frequency, resulting in a first derivative presentation of the ENDOR spectrum.s7 Each spectrum was scanned 64 times. The ENDOR l i e width was obtained by measuringthe peak-bpeak distanceof the first derivativesignal. The ENDOR line widths are an average of three experimenta.
PCnTABIAOTID20li-octane
.-cE
h
wo = 5 10 20 30
0.3
2
4
6
8
10
12
14
Alkyl Chain Length (nj
Figure 1. Normalized photoyields at 77 K of PCnTAB in AOT/ DzO/ieooctane reverse micelles with wo = 5 (O), 10 (A),20 (MI, and 30 (0) versus the pendent alkyl chain length after 10 m m of photoirradiation at 77 K. PCnSIAOTID20/i-octane Wg = 5 I
0.9-
10 20
30
0.7
0.5-
Results Samples which do not include a charged alkylphenothiazine show no ESR signal. This indicates that the charged alkylphenothiazine is the only photosensitive material absorbing ultraviolet light in the X > 280 nm range. A pink color appears in the photoirradiated samples which is characteristic of alkylphenothiazine cation radicals. The color intensity reaches a maximum after about 10 min of photoirradiation. The doubly integrated ESR signal intensity at g = 2.0051 also reaches a plateau after 10 min of photoirradiation. Photoyields. The noflIlliliZed photoyields of the PC,TAB and PC,S compounds in AOT and CTAB/hexanol reverse micelles with D2O versus the pendent alkyl chain length are shown in Figures 1 to 4. They show U-shaped photoyield dependenceson the pendent alkyl chain length. The normalized photoyields of PC,TAB (Figure 5) and PC,S (not shown) generally decrease with increasing wo in AOT reverse micelles. The photoyields of PC,TAB in CTAB/hexanol reverse micelles are somewhat U-shaped as shown in Figure 6. The same trend is observed for PC,S in CTAB/hexanol reverse micelles. (34) Ichikawa, T.;Kevan, L.; Narayana, P. A. J. Phys. Chem. 1B79,83, 3378. (35)Narayaua, P.A.; Kevan, L.; Szajdziika-Pietek, E.; Jones, R. R. M.J. Chem. Phys. 1984,81,3986. (36)Kevan,L.InModernfilsedand Continuous-WaueElectronSpin Resonance: Kevan.. L... Bowman,M. K., Eda.: Wilev: New York, 1990: Chapter 5.. (37) Hyde, J. S.;Riet, G. H.; Erickson, L. G. J. Phys. Chem. 1968,72, 4269.
0.3
u 4 6 8 10 1 2 14
2
Alkyl Chain Length (n) Figure 2. Normalized photoyields at 77 K of PCnS in AOT/ Ddisooctane reverse micelles with wo = 5 (O), 10 (A),20 (m), and 30 (0) versus the pendent alkyl chain length after 10 min of photoirradiation at 77 K.
For the same alkylphenothiazines, higher photoyields are observed in anionic AOT reverse micelles than in cationic CTAB/hexanol reverse mitelles. A slightlyhigher photoyield is obtained from PC,S versus PC,TAB compounds for the same alkyl chain length and reverse micelle type. Compare Figures 1 to 4. Deuteron Modulation Depths. The n o " d deuteron modulation depths of PC,TAB and PC,S compounds in both AOT and CTAB/hexanol reverse micelles with D2O versus the pendent alkyl chain length are shown in Figures 7 to 10. They show U-shaped trends similar to the corresponding photoyields. The deuteron modulation depths of PCnTAB in AOT and CTAB/hexanol reverse micelles in D2O versus woare shown in Figures 11 and 12,respectively. The same trends of deuteron modulation depths versus woare obtained for PC,S in both AOT and CTAB/hexanol reverse micelles. For alkylphenothiazines with a short alkyl chain of three or four carbons, the deuteron modulation depths decreaae monotonically with increasing w o from 5 to 30. For the alkylphenothiazines with a longer alkyl chain of 6,9,or
Kang and Kevan
1694 Longmuir, Vol. 9, No. 7, 1993 0.40
.3 4
PCnTABICTAB/HexOH/ DzOln-octane
n
4
e q
.-h 0 0
C
0 0
c
n
.--It: m
0.25
P)
s
0.20
0.: 2
4
6
8
10
12
14
Figure3. Normalized photoyields at 77K of PC,TAB in CTAB/ l-hexanol/D2O/n-octanereverse micelleswith wo= 5 (O), 10(A), 20 (m), and 30 (0)versus the pendent alkyl chain length after 10 min of photoirradiation at 77 K. 0.6
Q)
c
3
0.5
4s
0.30
-.U
PC4TA B PC12TAB
P)
c
0.4
C
0
2 -0 .--It:
0
E
PC,TABICTABIHexOHI D 20In- o c t a n e
c
c n
=
40
.-C
c
.--Nm
30
20
n
c
4 s s .-
10
Interior Water Pool Size (wo) Figure 5. Normalized photoyields at 77K of PC4TAB (a),PCS TAB (A),PGTAB(D), and PCllTAB ( 0 )in AOT/DzO/n-octane reverse micelles versu woafter 10min of photoirradiation at 77 K. 0.35
PC,SICTABIHexOHI DnOIn-octane
h
3
0.5
E0
Alkyl Chain Length (n)
.-e
0.7
P)
0.30
C
-.-Nm
0.9
3
0.35
5
c n u
= .C
c
.-a
PCnTABIAOTIDzO/l-octane
0.3
0.25
m
z 0.2
4
6
8
10
12
14
Alkyl Chaln Length (n) Figure 4. Normalized photoyields at 77 K of PC,S in CTAB/ l-hexanol/D2O/n-octanereverse micelleswith wo= 5 (O),10(A), 20 (m), and 30 (0)versus the pendent alkyl chain length after 10 min of photoirradiation at 77 K.
12 carbons,the deuteron modulation depths decrease with increasing wo from 5 to 20 and then increase slightly for
wo = 30. For the same alkylphenothiazine, larger normalized deuteron modulation depths are observed in AOT reverse micelles than in CTAB/hexanol reverse micelles. Also larger deuteron modulation depths are obtained from PC,S versus PC,TAB compoundsfor the same alkyl chain length and reverse micelle type. Proton Matrix ENDOR Line Widths. Almost constant proton matrix ENDOR line widths for the photoproduced alkylphenothiazine cation radicals in AOT and CTAB/hexanol reverse micelles versus the pendent alkyl chain length are observed. The proton matrix ENDOR line widths of PC,TAB in AOT reverse micelles decrease with increasing woas shown in Figure 13. The same trends are observed for PC,S in AOT reverse micelles and for PC,S and PC,TAB in CTAB/hexanol reverse micelles. Slightlylarger ENDOR line widths are observed in CTAB/ hexanol reverse micelles than in AOT reverse micelles for the same alkyl chain length and PC,TAB compounds show
0.20
. 10
20
-
T I
30
40
Interior Water Pool Size (w,) Figure 6. Normalizedphotoyieldeat 77K of PC4TAB (a),Pc6TAB (A),PCeTAB (m), and PCIZTAB( 0 )in CTAB/l-hexanol/ DzO/n-octane reverse micelles versus woder 10 min of photoirradiation at 77 K. larger ENDOR line widths than PC,S compounds for the same alkyl chain length and reverse micelle type.
Discussion The broad singlet at g = 2.0051 is identified by ESR as a photoproduced alkylphenothiazinecation radical. This is consistent with g = 2.0052 for N-ethylphenothiazine,S8 g = 2.0053 for N-methylphenothiazine,as g = 2.0053 for 10-H-phenothiazine,asand g = 2.0053 for 2-methoxyphenothiazine."O Also each photoirradiated sample shows a pink color which is characteristic of alkylphenothiazine cation r a d i t x d ~ . ~ ~ ~ ~ ~ Effect of Alkylphenothiazine Chain Length. The location of the phenothiazine moiety from the interface (38)Clark,D.; G ilbert,B.C.; Hanson, P.;Kirk, C . M. J. Chem. Soc., Perkin Tram.2 1W8,8, 1103. (39) Chiu, M.F.;Gilbert, B. C.; Hanson, P.J. Chem.Soc. B 1970,1700. (40)Clark,D.; Gilbert, B.C.; Hanson, P.J. Chem. SOC.,Perkin 2'"u 2 1977, 7, 517. (41) Hovey, M. C. J. Am. Chem. SOC.1982,104,4196. (42) Fujihara, H.; Fuk, S.;Yoehihara, M.;Maeahima, T.Chem. Lett. 1981, 1271.
Photolysis of Alkylphenothiazines PCnTABIA0TID20li-oct0~e
0.51
PCnTABlCTABlHexOHl DzOIn-octane
5 1 8 0.4
I
C
0.14
4
2
6
8
10
12
14
Alkyl Chain Length (n)
Figure 7. Normalized deuteron modulation depths at 4.2 K of PCnTABin AOT/D2O/iaooctane reverse micelles with wo = 5 (e),10 (A), 20 (m), and 30 (0)versus the pendent alkyl chain length after 5 min of photoirradiation at 77 K. 0.41
z
0 . 1 ~ " ' " ' 2 4 6 8
'.I]
=5
fn
'0
.--N P,
m
j!
10
=
10
12
14
PCnSICTABIHexOHI
DzOIn-octane
5n
s
Wa
Alkyl Chain Length (n) Figure 9. Normalized deuteron modulation depths at 4.2 K of PCnTABin CTAB/l-hexanol/D&/n-odane revemmicelleewith wo = 5 (e),10 (A), 20 (m), and 30 ( 0 )versus the pendent alkyl chain length after 5 min of photoirradiation at 77 K.
PCnSIAOTID20lI-octane wo
\
Wa
0.3-
0.3-
0.2-
0.2-
5
I
I - I
zb
0.1
2
4 6 8 10 12 Alkyl Chain Length (n)
14
Figure 8. Normalized deuteron modulation depths at 4.2 K of PC,S in AOT/DaO/ieooctanereverse micelles with wo = 5 (a),10 (A), 20 (m), and 30 ( 0 )versus the pendent alkyl chain length after 5 min of photoirradiation at 77 K.
water (DzO)which acta as an electron acceptor in reverse micelles is a critical factor for the photoionization efficiency. Such relative distances can be monitored by the normalized deuteron modulation depth. A greater normalized deuteron modulation depth indicates a shorter distance between the phenothiazine moiety to interface water. The similar photoyield trends of PC,S and PC,TAB in both AOT and CTAB/hexanol reverse micelles versus the pendent alkyl chain length (see Figures 1to 4)show that the highest photoyield is observed with PC3S and PGTAB. Their short chains locate the phenothiazine moiety close to the interface water which provides efficient electron transfer from the phenothiazine moiety to interface water. The increased alkyl chain length of PC6S and PCsTAB locates the phenothiazine moiety further from the interface and decreases the photoyield due to the increased distance for electron transfer from the phenothiazine moiety to interface water. For Pens and PClzTAB the photoyields increase significantly. This indicates that the longer alkyl chain bends to locate the phenothiazine moiety more toward the interface and
0.1
2
6 8 10 12 14 Alkyl Chain Length (n)
4
Figure 10. Normalized deuteron modulationdepth at 4.2 K of PCnS in CTAB/l-hexanol/D2O/n-odane revme micelles with wo = 5 (e),10 (A), 20 (W, and 30 (0)versus the pendent alkyl chain length after 5 min of photoirradiation at 77 K.
decrease the electron transfer distance. These resulta extend the incomplete photoyield trends reported for a more limited series of PC,S compounds in A O T and CTAB/hexanol reverse micelles.23 Such U-shaped photoyield trends are also found for PC,S and PC,TAB compounds in anionic, neutral, and cationic vesicles." The above interpretation of alkyl chain bending for longer alkyl chains in charged alkylphenothiazines is confirmed by the U-shaped trends in the normalized deuteron modulation depths (see Figures 7-10). The matrix proton ENDOR line widths are expected to show an inverse U-shaped trend with increasing alkyl chain length of the charged alkylphenothiazines since they measure the local proton density which increases further from the interface into the alkane phase. Since no trend is seen, it is concluded that this effect is toosmall to obeerve in this case. The alkyl chain length effect for neutral N-alkylphenothiazines (PC,, n = 1,3,6,9,12,16) in AOT and CTABI (43)Kang,Y.S.;McManus, H.J. D.;Kevan, L.J. Php. Chsm. 1998, 97, 2027.
Kang and Keuan
1696 Langmuir, Vol. 9,No. 7,1993
5 n
a
A
2
.-c 0
5
m
3
4 .--w
E
=I
0
0
A : PC6TAB : PCgTAB 0 : PC12TAB
0 : PC4TAB
z.
0.3-
C
o'81
Q
PC4TAB PCsT A B PCgTAB
0.2-
0
.--cI K
f
0.7
I
sz W
B L = 0.1
10
5
20
30
Interior Water Pool Size (wo) Figure 11. Normalized deuteron modulation depths at 4.2 K of PCiTAB (O), PCeTAB (A),PCgTAB (W), a d PCiiTAB ( 0 )in AOT/DzO/ieooctane reverse micelles versus wo after 5 min of photoirradiation at 77 K.
0.6 0
10
20
30
40
Interior Water Pool Size (wo) Figure 13. Proton matrix ENDOR l i e widths at 141 K of PCdTAB (O),PCeTAB (A),PGTAB (W), and PCiiTAB (0)in AOT/ DzO/ieooctane reverse micelles versus wo after 5 min of p h e toirradiation at 77 K.
creasing wo increases the interface charge by increasing hydration of the Na+ counterion which impedes electron escape through the interface and thus decreases the photoyields. The deuteron modulation d e p t h also de5n crease with w~ which indicate that the phenothiazine moiety penetrates deeper into the interface region with 0.3wo which will also decrease the photoyield. Probable C reasons for deeper solute molecule penetration with increasing w o include "opening up" the interface by increased water which also better solvates the charged end of the charged alkylphenothiazines, decreased alkyl chain bending of the charged alkylphenothiazines due to "opening up" the interface, and decreased packing density of the surfactant alkyl chains. For PCnS and PCnTAB with n = 9 and 12 there is a small increase in the deuteron modulation depth for wo = 30 which indicates some alkyl chain bending. However, the photoyields for these cases 2 0.1' ' 0 10 20 30 40 still decrease indicating the continued dominance of the interface charge for controlling the photoyield. Interior Water Pool Size ( wo) The matrix proton ENDOR line widths decrease with Figure 12. Normalized deuteron modulation depths at 4.2 K of increasing WO. This seems inconsistent with deeper PCdTAB (O), PceTAB (A),PCgTAB (m), a d PCizTAB (0)in penetration of the charged alkylphenothiazines into the CTAB/l-hexanoVD?O/n-octane reverse micelles versus woafter 6 min photoirradiation at 77 K. interface indicated by deuteron ESEM. However, since woincreases DzO penetration into the interface hexanol reverse micelles has also been s t ~ d i e d .A~ ~ ~ increasing ~ and decreases the packing density of the surfactant alkyl similar U-shaped photoyield trend is observed which is chains, the net result could be a decrease in ENDOR line also supported by the normalized deuterium modulation width as observed. depths. A trend of decreasing photoyield for only two PC,TAB Effect of Interior Water Pool Size (WO).Increasing compounds in anionic AOT reverse micelles versus w o the interior water pool size, characterized by WO,in reverse found previously3l is corroborated by the more extensive micelles decreases the packing density of the surfactant resulta reportad here. Also, neutral N-alkylphenothiazines alkyl chains in the interfaceregion and increasesthe degree (PCn, n = 1,3,6,9,12,16) in anionic AOT reverse micelles of counteriondissociation.The packing density affecta show decreasing photoyields versus w o even though the solubilization site of incorporated molecules and the increasing deuteron modulation depths are observed due degree of counterion dissociation changes the interface to more DzO penetration into the interface.44 All these charge. resulta cons&tentlyindicate that an increasingly negative The photoyields of PC,S and PCnTAB compounds in interface for AOT reverse micelles with wois the dominant AOT reverse micelles decrease with increasing WO. Infactor decreasing the photoyields. (44) Kang, Y. s.; McManus, H. J. D.; Kevnn, L. J. Phys. Chem. 1992, The photoyields of PCnS and PCnTAB compounds in 96,8647. CTAB/hexanol reverse micellesversus wo show a U-shaped (45) Kang, Y. S.; Kevan, L. J. Chem. SOC.,Faraday Trans.,in press. trend except for PC3S and PCaTAB which show more of (46)Vos, K.; Laane, C.; Vissa, A. J. G. Photochem. Photobiol. 1987, 45,863. a monotonic decrease. These photoyield trends are Thomas,J. K.; Nowak,J. J.Am. Chem. SOC.1977,99, (47) Wong, M.; supported by a U-shaped trend of the deuteron modulation 4730. depths with WO. These trends are interpreted in the same (48)Rekker, R. F.The Hydrophobic Fragmental Constant; Elsevier: New York, 1987; Chapter 3. way as in AOT reverse micelles except that there is more PC,TABICTABIHexOHl DzOln-octane
a
a
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Photolysis of Alkylphenothiazines alkyl chain bending in the PCnS and PC,TAB compounds in CTAB/hexanolreverse micelles. Perhaps this is because the surfactant alkyl chain packing is decreased by the hexanol cosurfactant. The photoyield trends of PCnS and PCnTAB in cationic CTAB/hexanol reverse micelles versus w o are different from the trends in anionic AOT reverse micelles. This difference results from the different interface charges of AOT and CTAB/hexanol reverse micelles. The cationic interface charge of CTAB/hexanol reverse micelles is not varied much by increasing wo since the bromide counterion is less affected by increasing hydration compared to a smaller sodium ion. Consequently, the photoyields are controlled more by the location of the phenothiazine moiety relative to interface water in cationic CTABI hexanol reverse micelles than by interface charge changes. This is supported by the better correlation of the photoyield trends with the deuteron modulation depth trends in cationic CTAB/hexanol reverse micelles compared to anionic AOT reverse micelles. It is of interest to compare the above trends with w oand their interpretation with similar studies on the photoionization of neutral N-alkylphenothiazines (PC,, n = 1, 3, 6, 9, 12, 16) in A O F 4 and C T A B / h e x a n ~ lreverse ~~ micelles. In AOT reverse micellesthe photoyield decreases with increasing w o which is the same trend as found for the PCnS and PCnTAB charged alkylphenothiazines. However the deuterium modulation depth increases with w o for the PCn compounds which is an opposite trend to that found for PCnS and PCnTAB compounds. For PCn in AOT reverse micelles the matrix proton ENDOR line width decreases which is consistent with the deuterium modulation depth trend. Thus with increasing wo the PCn compounds move more toward the interface although the driving force remains unclear. Nevertheless, since the photoyield decreases with WO, it seems clear that the photoyield trend is dominated by the increasinglynegative interface in contrast to the location changes. The PCn trends with w o in CTAEVhexanol reverse micellesG show decreases in the photoyields, deuterium modulation depths, and proton matrix ENDOR line widths. These contrast with the trends in AOT reverse micellestrends for PC, compounds but are the same trends as for PCnS and PCnTAB compounds in AOT reveree micelles and can be similarly interpreted. So overall, the photoionization of PCn compounds in reverse micelles shows somewhat different trends with w o from those for the PC,S and PC,TAB compounds in the same reverse micellar system. Effect of Reverse Micelle Interfacecharge. Higher photoyieldsof PC,S and PCnTAB compounds are observed in anionic AOT reverse micelles compared to cationic CTAB/hexanol reverse micelles (see Figures 1-4). This is surprising compared to previous photoionization studies on the interface charge effect in micelles and vesicles12920f21924,a147 where the photoionization yield was always higher in cationic surfactant assemblies. However, the higher photoyields in anionic AOT reverse micelles are consistent with the deuteron modulation and proton matrix ENDOR data which both indicate that the phenothiazine moiety penetrates deeper into CTAB/hexanol reverse micellesthan in AOT reverse micelles. This deeper penetration can be rationalized by considering the interface
Langmuir, Vol. 9, No. 7, 1993 1697 structure to be less organized and less compact for CTAB/ hexanol reverse micelles which have bulky trimethylammonium headgroups than for AOT reverse micelles which have more compact sulfonate headgroups. Also, the hexanol may serve to somewhat sopen up” or disorder the interface of the CTAB reverse micelles. Since the higher photoyields in AOT versus CTAB/ hexanol reverse micelles correlate with a shorter distance of the phenothiazine moiety to interfacewater, the location of the phenothiazine moiety dominates the interface charge effect for electron escape during the photoionization process. This shows the necessity of doing experimenta to measure the photoyield and the distance differences for electron transfer independently. Effect of Alkylphenothiazine Charge. PC,S compounds show slightly larger photoyields than PCnTAB compounds in the same reverse micelles. This can be interpreted as due to a greater hydrophobic character of the trimethylammonium headgroup of PCnTAB compared to the sulfonate group of PCnS, which results in deeper solubilization of PCnTAB into the hydrocarbon region. This is consistent with a larger hydrophobic fragmental constant for the trimethylammonium group compared to the sulfonate group.* Similar results have been found for PCnTAB and PCnS photoionization in anionic, neutral, and cationic vesicles.a The above interpretation is supported well by the deuteron modulation depths and the matrix proton ENDOR line widths. PCnS compounds show slightly higher deuteron modulation depths and lower matrix proton ENDOR line widths than PCnTAB compounds for the same reverse micelle type and carbon number of the charged alkylphenothiazines. Soboth deuteron ESEM and proton matrix ENDOR indicate deeper penetration of PCnTAB compounds into the hydrocarbon phase of reverse micelles.
Conclusions The photoyield trends of PCnTAB and PCnSCompounds show a good correlation with deuteron modulation depth trends on the alkyl chain length of charged alkylphenothiazines and on WO. This indica- that the photoyields in reverse micelles are primarily affected by the location of the charged alkylphenothiazine molecules in the surfactant assemblies which is modified by variation of the alkyl chain length and by WO. The unexpectedly more efficient photoionization in AOT than in CTAEVhexanol reverse micelles in interpreted by a shorter distance of charged alkylphenothiazinesin AOT from interface water which dominates any interface charge effect. The leeser penetration of PCnS versus PCnTAB compounds into reverse micelles probably arises from the lesser hydrophobicity of the sulfonate headgroup compared to the trimethylammonium headgroup. This correlates with more efficent photoionization for the PCnS compounds which again shows the importance of the photoionizable molecule location within a surfactant assembly. 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.
