J. Phys. Chem. 1992, 96, 10055-10060 (48) Chamulitrat, W.; Kevan, L. J. Phys. Chem. 1984,88, 5996. (49) F e n d e n , R. W.; Schuler, R. H.J . Chem. Phys. 1963, 39, 2147. (50) Cole, T.; Pritchard, H.D.; Davidson, N. R. Mol. Phys. 1958, I , 406. (51) Kevan, L. In Time Domain Elecrron Spin Resonance; Kevan, L., Schwartz, R. N., Eds.; Wiley-Interscicncc: New York, 1979; Chapter 8. (52) Kcvan, L. In Photoinduced Electron Transfec Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part B, pp 329-384.
tooss
(53) Mailliaris, A.; Moigne, J. L.; Stum, J.; Zana, R. J. Phys. Chem. 1985, 89, 2709. (54) Kevan, L.; Kispcrt, L. D. Elecrron Spin Double Resonance Spectroscopy; Wiley: New York, 1976; p 239. (55) Kurreck, H.;Kirste, B.; Lubitz, W. Electron Nuclear Double ResoMnce Spectroscopy of Radicals in Solution; VCH Publishers: New York, 1988; p 314.
An Electron Magnetic Resonance Study on the Photoionization of N-Alkylphenothiazines in DioctadecyidimethylammoniumChloride Frozen Vesicles: The Effect of Urea, 1,3-Dimethylurea, 1,3-DIethyiurea, and 1,1’,3,3‘-Tetramethyiurea Young So0 Kang, Hugh J. D. McManus, and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: April 2, 1992; In Final Form: June 24, 1992)
Photoionization of N-alkylphenothiazines was carried out in frozen dioctadecyldimethylammonium chloride (DODAC) vesicle suspensions to which various concentrations of urea, IJdimethylurea (DMU), 1Jdimethylurea (DEU), 1,1’,3,3’-tetramethylurea (TMU) had been added. The photoproduced cation radical yields were investigated by electron spin resonance (ESR). Electron spin echo modulation (ESEM) and electron nuclear double resonance (ENDOR) spectroscopies were used to probe the microenvironment of the photoproduced cation. The concentration of each of the urea derivatives was systematically varied. Urea, DMU, and DEU showed a similar trend in the photoyield versus concentration, with the highest yield obtained at 1 M additive. For TMU the photoyield of phenothiazine increased as a function of additive concentration. The different photoyield trends are explained in terms of the effect of urea on the DODAC vesicule interface. This interpretation is supported by the ESEM and ENDOR studies. Urea and its derivatives appear to interact with water through the “direct mechanism”. The ESR data for these species suggest that impregnation of the vesicle interface with urea decreases the strength of the photoproduced cation-water interactions which leads to a decrease in the photoyield. The ESE and ENDOR results for TMU suggest that this bulky additive opens up the vesicle interface to water penetration. The steady increase in the photoyield with TMU concentration together with the ESEM and ENDOR results support this interpretation of the ESR data.
Introduction Organized molecular systems, such as micelles, vesicles, and reversed micelles have been the focus of much recent attention. Such systems have been found to be useful in supporting the catalysis of various chemical reactions and as a simple model for solar energy conversion processes. Of particular importance is the enhancement in the photoionization efficiency of a substrate solubilized in a cationic surfactant.’-‘ In such a system the photoprodud cation is sequestered in the surfactant hydrocarbon bilayer interior, whereas the electron is ejected into the bulk water phasc. This segregation of photoproducts results in more efficient charge separation when compared to a homogeneous solution. Advances in this area of research depend on understanding the detailed structure of a surfactant aggregate. Intimate knowledge about the interfacial structure, particularly the extent of water penetration into a micellar or vesicular interface, is of critical importance. Electron spin echo modulation (ESEM) is a powerful technique for probing the microenvironment of a radical in a The extent of water penetration into the molecular bilayer can be obtained by the deuteron modulation depth which results from dipolar hyperfine interactions between the radical and the surrounding water (D,O). Hydration of the interface can be further increased or demasedby the axpolubilization of organic additives such as alcohol,’ crown ether! cholesterol?JOand ureal1 or by changing the counterion of the s~rfactant.~z’~ Photoionizable porphyrins,15viologen~,~~ substrates such as tetram~thylbenzidine,~~ or phenothiazines” have been introduced into the micellar or vesicular surfactant suspensions to study the optimization of the photoionization within these organized molecular assemblies. In these studies, the photoinduced cation yield was correlated with the relative location of the radical with respect to the hydrocation/water interface. The photoyield depends on a number of factors including the surface charge of the aggregate, the chain 0022-3654/92/2096-10055$03.00/0
length of the surfactant, and the micropolarity at the radical site. The location of the chromophore is determined by a balance of lipophilic and hydrophilic forces. The lipophilic character of a photoionizable molecule can be manipulated by appending an 1chain, effecting solubilization deeper into the hydrocarbon region of the aggregate.1°J8 As stated above, the analysis and exploitation of ESEM and matrix ENDOR requires a rigid system. Hence the surfactant assemblies are rapidly frozen in order to make ESEM and ENDOR analysis possible. This poses no real limitation because numerous studies have demonstrated that in relatively rapidly frozen aqueous solutions the vesicular structure is retained.1g*20 In addition, many ion-solvation studies demonstrate retention of the solvation shell in rapidly frozen aqueous solutions.21 In a previous study, an investigation was made on the effect of urea on the interfacial structure of a micelle.” Also, recent work has focused on the effect which added urea has on the photoyield of a series of N-alkylphenothiazines solubilized in In both cases urea replaced the interfacial micellar water, which is referred to as the direct mechanism for the action of urea in an aqueous solution. The trend in the photoyield was supported by ESEM and electron nuclear double resonance (ENDOR) studies on both the location of the chromophore and the degree of hydration at the micellar interface. In the present work a comparative study on the effect of urea, IJ-dimethylurea, 1,3diethylurea, or 1,1’,3,3’-tetramethylureaon the photoionization efficiency of N-alkylphenothiazines in an aqueous dioctadecyldimethylammonium chloride (DODAC) vesicle suspension is carried out. The degree of water penetration into the bilayer due to the distorting effect of urea and its alkyl derivatives on the vesicle is investigated with ESEM. ENDOR spectrampy is used to probe the environment of the phenothiazine within the hydrocarbon bilayer of the vesicle. Q 1992 American Chemical Society
Kang et al.
10056 The Journal of Physical Chemistry, Vo1. 96, No. 24, 1992
Experimeatal Section MateMs. Three N-alkylphenothiazines(PC,, n = 1, 6, 16) were synthesized according to the procedures described earlier.23 M stock solution of the N-alkylphenothiazine in A 1X chloroform was prepared. The exact concentration of the chromophore was determined through optical absorption spectroscopy with a Perkin-Elmer 300 spectrophotometer (A- = 312 in CHC13, Urea (99+% A.C.S. reagent grade), log c = 3.71 M-' 1,3-dimethylurea (99%) [DMU], 1,3-diethylurea (97%) [DEU], 1,1',3,3/-tetramethylurea (99%) [TMU] were obtained from Aldrich Chemical Co.and were used without further purification. Deuterium oxide (D,O 99 atom % D) was obtained from Aldrich Chemical Co. and was deoxygenated by bubbling with nitrogen for at least 20 min before use. DODAC Aqwous Vesicle Suspensions. Dioctadecyldimethylammonium bromide (DODAB) was purchased from Eastman Chemicals and purified by recrystallization from acetone. A methanol/chloroform (7030 v/v) solution of DODAB was passed through a chloride ion exchange resin type AG2X8,20-50 mesh from Biorad Laboratories. The eluent containing DODAC was evaporated, and the solid residue was recrystallized twice from acetone/water (955 v/v). A 30 mM DODAC stock solution was prepared in chloroform. Preparation of Samples. A 70-pL volume of each N-alkylphenothiazine and 0.6 mL of the DODAC stock solution were transferred into a clean 16 X 125 mm Fisher test tube. The solvent was evaporated under a stream of nitrogen gas which resulted in the formation of a thin film on the test tube walls. Then 1 mL of deoxygenated D 2 0 was added to the thin film. The mixture was sonicated for 30 min at 53 f 2 OC using a Fisher model 300 sonic dismembrator operated at 35% relative output power through a 4-mm 0.d. tungsten microtip. The sonication was carried out under a nitrogen atmosphere. The concentration of the resulting suspension was verified by optical absorption spectroscopy using a Perkin-Elmer Model 330 spectrophotometer. The respective concentrationsof N-alkylphenothiazineand DODAC were 601 pM and 18 mM. Argon gas was blown onto the surface of the vesicular aqueous suspensions for 10 min, after which the mixtures were thermostat4 at 60 f 2 OC for 3 h. The resulting clear suspensions, which indicated complete solubilization of the material, were introduced into 2 mm i.d. X 3 mm 0.d. Suprasil quartz tubes which were flame sealed at one end. The samples were rapidly frozen by plunging the tubes into liquid nitrogen, followed by storage at 77 K. Since vesicles retain their shape upon rapid freezing,2I the information gained at cryostatic temperatures provides insight into the features of the microenvironment of a membrane mimetic system which enhances photoinduced charge separation at room temperature. Photoirradiation and Data Acquisition. Photoirradiation was camed out at 77 K with a 300-W Cermax Xenon lamp (CX 300 UV). The power supply used was from ILC Technology. The photolysis light passed through a 10-cm water filter, a Corning filter (No. 7-54, 86% transmittance at 320 nm, 240 nm < X < 410 nm), and a Pyrex Corning filter (70% transmittance at X > 270 nm). This filter combination provided light in the range 270 nm < A < 420 nm. The light intensity was measured with a YSI-Ketteringmodel 65 Radiometer at the same location at which the photoirradiated samples were located; the intensity was 1.1 X lo3 W m-2. The irradiation Dewar was rotated at 4 rpm to ensure even illumination of the sample. Electron spin resonance (ESR) spectra were recorded at 77 K with a Bruker ESP 300 spectrometer operated at X-band with 100-kHz magnetic field modulation. The microwave power was set at 1.97 mW to avoid saturation. Each ESR spectrum was accumulated over seven scans. The magnetic field was monitored with a Bruker ER 032M Hall effect field controller. The microwave frequency, which was in the 9-GHz range, was measured directly with a Hewlett-Packard 5350B microwave frequency center. The photoyield of the resulting spectrum was then determined by double integration. Each photoyield was normalized to the yield for the highest value which was obtained from the PCI/DODAC/D20/1 M DMU system.
DODAC
I
(CH2)" I
0 II
CH3- NH- C - NH - CH3 UREA
1,3 -DMU
1,1',3,3'-TMU
Figure 1. The structures of various compounds used.
Two-pulse electron spin echo deuteron modulation signals were recorded at 4.2 K on a home-built spectrometer using 40- and 80.1~excitation pulse^?^,^ The deuteron modulation depths were nonnalized by dividing the depth at the first modulation minimum from an extrapolated unmodulated echo decay by the depth to the base line at same interpulse time. ENDOR spectra were recorded at 141 K using a Bruker ESP 300 spectrometer interfaced with a Bruker ENDOR unit. Frequency modulation was performed at constant magnetic field which resulted in a first derivative presentation of the ENDOR spectrum. Each spectrum was accumulated over 96 scans. Typical experimental conditions were microwave power 1.97 mW, microwave frequency 9.46 GHz, and radiofrequency power 100 W. A Bruker ER 41 1 variable temperature unit was used to monitor and control the temperature in the microwave cavity. The proton matrix ENDOR line widths were obtained by measuring the peak-to-peak distance in the first derivative ENDOR spectra. The ESR, ESEM, and ENDOR results reported are the average of three separate experiments.
Results Figure 1 shows the structures of the compounds used in this study for easy reference. Electron Spin Resonmce. No ESR signal was obtained from a frozen, photoirradiated sample with and without urea and its derivatives to which N-alkylphenothiazines had not been added. Further, none of the PC, samples showed an ESR signal before photoirradiation. These results imply that the Phenothiazine chromophore is the only photosensitive material in the systems studied. The photoirradiated samples were pink in color, and the g-factor of the N-alkylphenothiazine was determined to be 2.0052. These results are consistent with previous studies on this syst e m . 1 7 m a The ESR spectra of PC,/DODAC/D20 in 2 M urea, DMU, DEU, and TMU are shown in Figure 1. A secondary surfactant or DAC radical and a methyl radical are also present in this figure. DAC is a radical conversion product and has been previously observed in this system.29 The normalized photoyield values of PC,, Pc6, and E 1 6 in DODAC/D20/additives versus concentration of the additives showed similar trends. One of them, PC,/DODAC/D20/additives, is shown in Figure 2. The normalization procedure used was described in the preceding section of this paper. The photoyield trends for urea, DMU, and DEU are similar with a maximum at 1 M of the additive, followed by a steady decrease
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 10057
Photoionization of N-Alkylphenothiazines I’CI/DODAC/DZO/Z M ADDITIVES
1.2
-
PCnlDODACID2012 M OF
U :UREA
A
A :DMU U :DEU 0 :TMU
.E 1.0
a
n
5 0.8 -P.-al
$ 0.6
c)
c P
U (DMU)
.--w E
0.4 0.2
z 0.0
6
1
16
Alkyl Chain Length (n) 4. Normalized photoyield values of PC,/DODAC/D,O measured a t 77 K after 10-min photoirradiation versus the pendant alkyl chain length of phenothiazines with 2 M concentration of urea (O), 1,3methylurea (A), 1,3-diethylurea ( O ) , and 1,1’,3,3’-tetramethylurea (0). F i i
A=23G
+--
= 2.0041
1
Figure 2. First derivative X-band ESR spectra of PCI/DODAC/D20 with 2 M (A) urea, (B), 1,3-dimethylurea, (C) 1,3-diethylurea, and (D) 1,1’,3,3’-tetramethylurea at 77 K after 10-min photoirradiation.
o-.
1’C i/I>OL)AC/Dz0/2M ADDL’I’IVES
I
0
1
2
3
4
5
6
7
I
0
2 7,
Conc. of Additives (Mole) Figure 3. Normalized photoyield values of PCl/DODAC/D20 measured at 77 K after 10 min photoirradiation versus concentration of urea (O), 1,3-dimethylurea (A), 1,3-diethylurea ( O ) , and 1,1’,3,3’-tetramethylurea
PS
Figure 5. Two-pulse X-band ESEM signals of PCl/DODAC/D20 with 2 M (A) urea, (B) 1,3-dimcthylurea, (C) 1,3-dicthylurea, and (D) 1,1’,3,3’-tetramethylureaobtained at 4.2 K after 5-min photoirradiation at 77 K. The base lines are offset vertically to facilitate comparison.
(0). 0 :UREA A :DMU U :DEU 0:TMU
as the urea concentration is increased further. The trend for the TMU data is completely different from the other urea derivatives. In samples to which TMU was added, the photoyield increases monotonically as a function of concentration of the additive. The normalized photoyields of PC,/DODAC/D20/2 M additive versus alkyl chain length of phenothiazine are shown in Figure 3. Similar trends were obtained from 1 and 6 M additive concentrations. The photoyield for K ! 6 is the smallest for all additives. The lowest photoyield from all samples was obtained from those to which DEU was added. EIectraaSpia Eeho Moduhtioa. Representative ESEM signals of PCl/DODAC/D20/2 M additive are shown in Figure 4. The normalized deuteron modulation depth of PCI, Pc6, and PCi6/ DODAC/D20 with urea, DMU, DEU, and TMU versus additlve concentration showed similar trends of results. Representative resUltS Of PC6/DODAC/D@ are Shown h Figure 5. The w e s t modulation depth was obtained from samples to which 1 M DMU had been added. The deuteron modulation depth for urea, DMU, and DEU is largest at 1 M and decreases as the concentration of the additive is increased further. In contrast, the deuteron modulation for samples to which TMU was added show a monotonic increase with additive concentration. The normalized deuteron modulation depth of PC,/DODAC/D,O/ 1 M additive
0.5
C
0 .-c
-ma U
P0
0.0
1.--
0
1
2
3
4
5
6
7
Conc. of Additives (Mole)
Figure 6. Normalized deuteron modulation depth of PC,/DODAC/D,O versus concentration of urea (0),1,3-dimethylurca (A), 1,3-diethylurca ( O ) , and 1,1’,3,3’-tetramethylurea( 0 ) .The data were recorded at 4.2 K after 5-min photoirradiation a t 77 K.
versus alkyl chain length of the N-alkylphenothiazineis shown in Figure 6. Similar trends for the deuteron modulation depth
10058 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
Kang et al.
0.6
I’C,JDODACIDzOII M 01‘
0 :UREA A :DMU U :DEU
0 :TMU
0.5.
0
lk
w
0.2.
N
I
r
0.4-
5
‘0
c
0.3. 0.2-
0.4
U
Q 2
0.1
o.o-.
0.6
.--I
c1
C6
-.-
C16
Alkyl Chain Length
FIgm 7. Normalized deuteron modulation depth of PCJDODAC/D,O
versus pendant alkyl chain length of phenothiazines with 1 M concentration of urea (0),1,3-dimethylurea (A), 1,3-dimethylurea (a), and 1,1’,3,3’-tetramethylurea(0). The data were recorded at 4.2 K after 5-min photoirradiation at 77 K.
’5
Discusdon In DODAC vesicles, radical conversion from the phenothiazine cation radical to form either the surfactant DAC radical, the methyl radical, or a urea-type radical was observed in all systems. Thus, the total integrated ESR intensity is a measure of the total photoionization yield even though several different radicals contribute to this intensity. The ESR data showed that the intensity o f t h e p i l e ” eradkalsdecrrasadwithplongcdirradiation, whereas the intensity of the conversion products increased. The
4
5
6
7
PCnlDODACID2OI6 M OF
5 0
were obtained from 1 and 6 M additive concentrations. The modulation depth reaches a minimum for PC6 after which it increases. The lowest modulation depths were obtained from samples to which 6 M DEU had been added. The photoyield results with imwiugalkyl chain length showed the lowest value at hexyl. It was already reported that the great solubilization of neutral N-alkylphenothiazines in the molecular assunbibpushes phenothiazine moiety out from the medium alkyl chain length of phenothiazines. Consequently, near location from the interface as methyl- and hexadccylphenothiazines resulted in higher photoyield. l?ktron Nudcu Double Remmux. Figure 7 shows the matrix ENDOR spectrum of PCI6/DODAC/D20/1 M TMU. The ENDOR line width of PC,(in DODAC/D,O with urea, DMU, DEU, or TMU vmus the concentrationof the additives are shown in Figure 8. A similar trend of the ENDOR line width versus concentration of the additive was obtained for all PC, chromophores. The greatest absolute change in line width was observed for samples to which 6 M TMU was added. The ENDOR line width of PC,/DODAC/D20/6 M additive versus the pendant alkyl chain length of the phenothiazine is presented in Figure 9. The line widths show a similar dependence on the pendant alkyl chain length. Samples to which TMU was added give the narrowest line width in all cas-.
3
1.2-
Pr 0.8.
Figure 8. Proton matrix ENDOR spectrum of PCI6/DODAC/D2O/l M 1,1’,3,3’-tetramethylurea. The data were recorded at 141 K after 5 min-photoirradiation at 77 K.
2
1,1’,3,3’-tetramethylurea (0). The data were rccorded at 141 K after 5-min photoirradiation at 77 K.
1.o.
l-4
1
Conc. of Additives (Mole) Figure 9. ENDOR line widths of PCI6DODAC/D20versus concentration of urea (0), 1,3-dimethylurea (A), 1,3-dimethylurea (O), and
PC16/DODAC/DzO/l M TMU
0.7MHz
0
0.6.
C
1 0.4.
Bz
0 :UREA A :DMU U :DEU 0 :TMU
-
c-
-
s
1
6
16
w 0.2. 0.0-
Alkyl Chain Length (n) F i p e 10. ENDOR line widths of PC,/DODAC/D20 versus pendant alkyl chain length of phenothiazines with 6 M concentration of urea (O), 1,3-dimethylurea(A), 1,3-diethylurea(O), and 1,1’,3,3‘-tetramethylurta ( 0 ) . The data were recorded at 141 K after 5-min photoirradiation at 77 K.
intensity of the pink color of the photoirradiated sample, which is characteristic of the phenothiazine radical cation, decreased as the radiation time increased. The spectra of these secondary radicals are illustrated in Figure 1. It was previously reported that this secondary alkyl (DAC) radical is produced through hydrogen abstraction by the initial photoproduced species from the surfactant alkyl chain, followed by migration of the radical site along the surfactant chain to the The moat likely source of the methyl radical penultimate is the dimethylamint h e a d p u p of the DODAC surfactant. The g factor and hyperfine coupling constant of the methyl radical were measured as g = 2.0025 and A = 2.3 mT, which are in reasonable agreement with previous reports on this ~ p s c i e s . ~ J ~ The methyl radical was previously observed during the photoirradiation of N-alkylphenothiazines in dodtcyltrimethylamine bromide micelles?’ In addition to the DAC and methyl radicals, radicals ascribed to conversion products of urea, DMU, DEU, and TMU were also observed. The assignment and production mechanism of these species is being further studied with y-radiation of urea, 1,3-DMU, 1,3-DEU, and 1,1’,3,3’-TMU.32 Urea and the various derivatives used in this investigation (DMU, DEU, and TMU) have different physical properties. Examples of thee differences include viscosity and the degree of interaction with water,35their stability in aqueous
Photoionization of N-Alkylphenothiazines
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 10059
solution,36and their surface and protein denaturing a b i l i t ~ . ~ ~ , in ~ ~conjunction to determine the structural modifications of an organic aggregate which promotes efficient charge separation. These differences are due to the different substituents on the urea The photoyield data for urea, DMU, and DEU illustrate the molecule. Consequently, each of the derivatives should interact effect of urea in disrupting the interface of the vesicle. Initially, with the vesicular interface in a distinct manner. the photoyield grows as the concentration of urea is increased to The interaction of urea with a hydrophobic solute dissolved in 1 M. The ESEM data in Figures 5 and 6 show that the polarity water is believed to occur through one of two different mechaof the interface increases since greater water penetration occurs. n i s m ~ .In~ the ~ indirect mechanism, urea is involved in the hyThis water penetration effectively lowers the mean phenodrogen bonding of water reducing the long range structure of the thiazine/water interaction distance. It has been shown previously solvent. Nuclear magnetic resonance experiments on urea/water that the phenothiazine radical is stabilized by photoproduced solutions have supported this "structure breaking" interpretation.4o cation/water interaction^.^'*^ Since these interactions increase In the direct mechanism,urea replaces some of the water molecules with decreasing phenothiazine/water sepatation, water penetration in the solvation shell of a solute. into the vesicular interface promotes electron transfer to the bulk Alternative mechanisms have been reported. Ahluwalia et al. phase. As the concentration of urea is increased still further, both have reported a comparative study of urea, thiourea, 1,3-dithe photoyield and the deuteron modulation depth decrease. In methylurea, and 1,1',3,3'-tetramethylurea regarding their effect an earlier study on the effect of urea in micellar suspensions, this on disrupting the structure of water without any other added same behavior in the ESR and ESEM data was observed.28 These experiments were carried out by measuring the The results were interpreted in terms of the direct mechanism partial molar heat capacitiesin aqueous solution with corroborative for the action of urea in water. At low concentrations,urea "opens evidence obtained from proton chemical shifts in the NMR up" the interface permitting greater water penetration into the spectrum of water.4I The data showed that urea and its derivatives hydrocarbon bilayer of the vesicle. As the concentration of urea could act as either a "structure maker" or a "structure breaker". is increased, the additive replaces water at the micellar interface. A structuremaking solute dissolved in water increased the ice-like This substitution effectively increases the mean water/phenonature of the solvent.42 Consequently, melting the ice required thiazine interaction distance, which results in a lower photoyield greater energy which was measured as an excess partial molal and a shallower modulation depth. The same mechanism appears heat capacity. The converse is true for a solute exhibiting to be at work for the urea, DMU, and DEU additives in this study. structure-breaking properties. Both NMR and calorimetric data at 1 and 2 M concentrationsof the solutes showed that the water A different trend in the photoyield data was obtained when structure was enhanced by the presence of DMU and TMU; TMU was used as an additive. This difference is most likely due however, urea acted as a weak structure breaker. The ability of to the hydrophobic and bulky dimethyl substituent on the urea the urea derivatives to effect this change decreased in the order molecule. TMU is also hydrocarbon-like; therefore, it should TMU > DMU > urea. At higher concentrationsTMU perturbed solubilize in the hydrocarbon region of the bilayer most effectively. the hydrogen bonding structure of an aqueous solution to a much As the TMU is added to the vesicle suspension, the additive larger extent than the other derivatives. The conclusion of this impregnates the surface of the vesicle which serves to disrupt the work is that substitution of hydrogens by nonpolar methyl groups interface. This disruption of the integrity of the vesicle interface in urea serves to assist the formation of the water structure in their allows greater water penetration into the nonpolar region of the microenvironment. aggregate. The ESE modulation depth data show that the mean phenothiazine/water interaction distance decreases with added In an infrared investigation of water/urea solutions, Swenson reported that urea does not perceptibly alter the structure of water, TMU. The deuterium-modulation depth shows a monotonic while 1,3-ditnethylurea enhances the hydrogen bonding ~tructure.4~ increase as more TMU is added to the system. This decrease in Once again, the ability of TMU to act as a denaturant was asthe interaction distance is accompanied by an increase in the cribed to the presence of the methyl groups. photoyield. Further support for this interpretation of the TMU results is given during the discussion below of the ENDOR data. Sakaki and Arakawa concluded from their ultrasonic studies on aqueous solutions of methylated ureas that the order of deDEU should also penetrate into the vesicle bilayer; however, creasing structure enhancing propensities is TMU > DEU = this additive is not as effective in opening up the interface as the DMU > These results are in reasonable agreement with more bulky TMU. The ESEM data do show a steady decrease the conclusions reached by Ahluwalia and co-workers?' in the deuterium modulation depth, which implies an increase in the phenothiazine/water interaction distance. This interpretation All of the studies mentioned thus far were camed out in aqueous is consistent with the steady decrease in the photoyield with added solution. In surfactant suspensions, the factors which affect the DEU (Figure 2). disruption of the hydrogen bonding structure might also influence the structure of the hydrocarbon/water vesicle interface. In the Electron Nuclear Double Resonance. In solid disordered systems, the ENDOR spectrum of radicals often shows a single present study, the photoyield values of PC, in DODAC/D20 This spectrum is due to the presence of weak dipolar interactions suspensions displayed different trends depending on whether the of an unpaired electron with surrounding matrix nuclei located additive was urea, DMU, DEU, or TMU. When urea, DMU, or DEU was added, similar trends in the photoyield data of the within a distance of about 0.75 nm1.4749 The line width of the PC,were o k e d . The photoyield was the largest at 1 M additive matrix ENDOR spectrum is inversely proportional to the mean and then decreased as the urea concentration was increased interaction distance between the unpaired electron and the surfurther. For TMU the photoyield grew monotonically as the rounding nuclei. Proton matrix ENDOR can be used to obtain concentration of the urea was increased. information on the environment of the solubilized radical.1° A clearer explanation of the ESR photoyield experiments is In the present study, ESEM shows the interaction distance of obtained when these data are interpreted in conjunction with the the photoinduced phenothiazine cation radical from interface ESEM results. Electron spin echo modulation is a powerful water. Proton matrix ENDOR can give complementary infortechnique which may be used to correlate the microenvironment mation on the penetration of the chromophore into the hydroof a paramagnetic species with its photoyield. ESEM gives incarbon surface of the vesicle. The proton density in the viscinity formation about the degree of interaction of an unpaired electron of the phenothiazine cation radical is inversely proportional to with the surrounding paramagnetic nuclei located within a sphere the interaction distance of the photoproduced species from the of about 0.6 nm radius."*4s This modulation arises from electron surfactant alkyl chains or the urea additives because the dipolar dipolenuclear dipole interactions, which are averaged out in interaction is related with distance of phenothiazine cation radical solution due to rapid tumbling of the radical. The modulation with proton rich surfactant alkyl chain or urea additives. The depth depends on both the number of and distance to these nuclei. ENDOR line widths in Figures 8 and 9 show a slight decrease So a change in modulation depth can result from either a change for the samples to which urea, DMU, or DEU is added. The line in the average interaction distance or number of nuclei, or both. width changes for the samples to which TMU is added is much In this work, ESEM and ESR are complementary techniques used more pronounced. The ESEM data indicated that the addition
lo060 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 of TMU caused the interface of the vesicle to "open up". As the interface becomes hydrated with deuterated water, the mean proton density in the vicinity of the phenothiazine moiety decreases. This claim is supported by the strong decrease in proton matrix ENDOR line width with increasing TMU concentration. The change in line width is not as dramatic for samples to which urea, DMU, or DEU has been added. These additives are not as bulky as the tetramethylurea derivative and do not disrupt the interface to the same extent. Further, the ENDOR line width is not affected as much as in the case of TMU, since there is less water (D20) penetration. This claim is supported by the fact that the ESEM data show that urea, DMU, and DEU are not as effective as TMU in opening up the vesicle interface.
Conclusions The effect of the pendant alkyl chain length in N-alkylphenothiazines and the addition of urea, DMU, DEU, and TMU to DODAC vesicles on the N-alkylphenothiazine photoionization efficiency was studied with ESR, ESEM, and ENDOR spectroscopies. Urea and its various derivatives appear to interact with the vesicular interface through a direct mechanism. There is no evidence in this work to suggest that either the structure enhancing or structure breaking properties of urea and its derivatives are important in effecting modifications of the interfacial structure of the DODAC vesicles. The ESEM and ENDOR data show that TMU is most effective in disrupting the vesicle interface. TMU appears to penetrate into the hydrocarbon bilayer region of the aggregate, allowing greater water penetration into the vesicle surface. Urea, DMU, and DEU disrupt the interface to a lesser degree than TMU. Each of these derivatives replaces water at the vesicle surface which increases the mean water/phenothiazine distance. The photoyield is reduced due to a decrease in the water/cation interaction as the average separation is increased. The ENDOR data for urea, DMU, and DEU show a slight decrease in line width as the concentration of the additive is increased. The most likely explanation for the change in ENDOR line width is that ma, DMU, and DEU are less proton dense than the surfactant chains in the host DODAC vesicle. Acknowledgment. This research was supported by the Division of Chemical Science, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy.
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