J . Phys. Chem. 1989, 93, 6039-6043
6039
Photoionization of Alkylmethylviologens in Vesicles: Effects of the Alkyl Chain Length in Alkylmethylviologen and Radical Conversion to Surfactant Radicals Masato Sakaguchi and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: February 3, 1989)
Electron spin resonance spectroscopy has been applied to observe the photoionization yields of alkylmethylviologen (AV”) in rapidly frozen dipalmitoylphasphatidylcholine(DPPC), dioctadecyldimethylammonium chloride (DODAC), and dihexadecyl phosphate (DHP) vesicles. A singlet spectrum (g = 2.0030) due to the alkylmethylviologen cation radical (AV’) was observed from AV2+in each frozen vesicle system irradiated for 10 min with wavelength 240 < X < 410 nm. An alkyl radical from DODAC (DAC) was observed from AV2+in DODAC irradiated for 200 min and was assigned as CH3CHCH2(CH2)15N(CH3)2C18H37.DAC was produced by radical conversion from AV+ during photoirradiation with wavelength 240 < X < 300 nm. The photoionization yields were enhanced by a longer alkyl chain of AV2+. Electron spin echo modulation spectroscopy data support a correlation between the alkyl chain length and the photoionization yield. The highest photoionization yield was obtained in hexadecylmethylalkylviologenin DODAC which has the longest alkyl chain length. The highest photoionization yield is assisted by radical conversion from AV+ to DAC which impedes a back-reaction from AV+ to AV2+.
Introduction Light energy utilization involving molecular compartmentalization in micelles and vesicles is one of the basic models for artificial photaynthesi~.l-~The light utilization with such systems is enhanced by increasing the efficiency of the net charge separation by minimizing the rate of the back-reactions after the initial photoexcitation. Recent work shows that the photoionization is affected by controllable structural parameters of the micelle or h e a d g r o ~ p , and ~ . ~ hyvesicle, such as the type of co~nterion,~*’ drocarbon tail lengthlo of the amphiphiles, or by adding salts,” slightly water soluble alcohol^,^^-^^ and cholesterol.I6 Electron spin resonance (ESR) spectroscopy has been used to monitor the net photoionization in such systems.616 Electron spin echo modulation (ESEM) analysis has also been applied to correlate structural parameters of vesicles with the photoionization efficiency.”Jb18 In this study the photoionization yields of alkylmethylviologens (AV2+) in several types of vesicles are investigated in terms of the alkyl chain length variation in AV2+ and the location of the alkylviologen cation radical (AV’) with respect to the vesicle interface by using ESR and ESEM. Experimental Section m-a-Dipalmitoylphosphatidylcholine(DPPC) and dihexadecyl (1) Katz, J. J.; Hindman, J. C.; In Phorochemical Conuersion and Storage of Solar Energy; Connolly, J. S.,Ed.; Academic Press: New York, 1981; p 27. (2) Govindjee; Rabinowitch, E. Bioenergetics ofPhotosynthesis; Academic Press: New York, 1975; Chapter 4, p 34. (3) Legall, J.; Der Vartanian, D. V.; Peck, H. D. In Current Topics in Bioenergetics; Sanadi, D. R., Ed.; Academic Press: New York, 1979; Vol. 9, p 238. (4) Thomas, J. K. Acc. Chem. Res. 1977, 10, 133. (5) Gritzel, M.; Thomas, J. K. J . Phys. Chem. 1974, 78, 2248. (6) Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M.; Coleman, M. J. J . Am. Chem. SOC.1985, 107, 784. (7) Jones, R. R. M.; Maldonado, R.; Szajdzinska-Pietek, E.; Kevan, L. J . Phys. Chem. 1986, 90, 1126. (8) Plonka, A.; Kevan, L. J . Phys. Chem. 1985,89, 2087. (9) Hiromitsu, I.; Kevan, L. J. Phys. Chem. 1986, 90, 3088. (10) Narayana, P. A.; Li, A. S.W.; Kevan, L. .I.Am. Chem. SOC.1982, 104, 6502. (1 1) Plonka, A.; Kevan, L. J . Chem. Phys. 1985, 82, 4322. (12) Hiff, T.; Kevan, L. J . Phys. Chem. 1989, 93, 3227. (13) Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M. J . Am. Chem. SOC.1985, 107, 6467. (14) Baglioni, P.; Kevan, L. J. Phys. Chem. 1987, 91, 1516. (15) Baglioni, P.; Kevan, L. J . Phys. Chem. 1987, 91, 2106. (16) Hiff, T.; Kevan, L. J . Phys. Chem. 1989, 93, 2069. (17) Colaneri, M.; Kevan, L.; Thompson, D. H. P.; Hurst, J. K. J . Phys. Chem. 1987, 91, 4072. (18) Li, A. S. W.; Kevan, L. J. Am. Chem. SOC.1983, 105, 5752.
0022-3654/89/2093-6039$01.50/0
phosphate (DHP) were purchased from Sigma Chemical Co. and were used without further purification. Dioctadecyldimethylammonium bromide (DODAB) was purchased from Eastman Chemicals and purified by recrystallization from acetone. A methanol/chloroform (70:30 V/V)solution of DODAB was passed through an ion-exchange resin type AG2X8, 20-50 mesh, from Biorad Laboratories. The eluent containing DODAC was evaporated, and the solid residue was recrystallized two times from acetone/water ( 9 5 5 v/v). Tris(hydroxymethy1)aminomethane (Tris) and 2 N hydrochloric acid were obtained from Aldrich (Gold Label, 99.9+%) and Sigma Chemical Co. H20 was triply distilled as described elsewhere.I8 Deuterium oxide (DzO) was obtained from Aldrich (99.8 atom % D). Methylviologen dichloride hydrate (MV2+) was purchased from Aldrich and used without further purification. The alkylmethylviologens Nhexyl-N’-methyl-4,4’-bipyridinium dichloride (C6V2+),N-octylN’-methyL4,4’-bipyridinium dichloride (c8v2+),N-dodecyl-N’methyl-4,4’-bipyridinium dichloride (C12V2+),and N-hexadecyl-N’-methyl-4,4’-bipyridiniumdichloride (c16vz+) were generously provided by D. H. P. Thompson and J. K. Hurst of the Oregon Graduate Center. Sonications of vesicle solutions were carried out by using a Fisher Model 300 sonic dismembrator operated at 35% relative output power with a 4-mm-0.d. microtip under nitrogen atmosphere. DPPC vesicles in triply distilled water were prepared by sonication for 30 min at 53 f 2 “C. DODAC vesicles in triply distilled water and in D,O were formed by sonication for 15 min at 53 f 2 OC.I9 D H P vesicles in 20 mM Tris buffer solution adjusted to pH = 7.8 with hydrochloric acid were formed by sonication for 20 min at 71 f 2 0C.17,20 After the sonication, each AVZ+solution was added and the sample introduced into 2-mm4.d. by 3-mm-0.d. Suprasil quartz tubes, allowed to stand for 1 h at room temperature, and then rapidly frozen and stored in liquid nitrogen. These procedures after sonication were carried out within 2 h. The respective concentrations of AV2+ and surfactants were 0.07 and 9 mM, which amounts to a ratio of one AV2+ molecule per about 129 surfactant molecules. Photoirradiations were carried out at 77 K with a 300-W Cermax xenon lamp (LX 300UV) with a power supply from ILC Technology. The light passed through a IO-cm water filter and a glass filter (Corning 7-54 glass filter, 240 < X < 410 nm; Corion Shott WG320 glass filter, X > 300 nm; or Corning 3-70 glass filter, X > 490 nm). ESR spectra were recorded at X-band with a Bruker ER300 spectrometer with 100-kHz field modulation at 77 K and 0.2” (19) Lim, Y. Y.; Fendler, J. H. J. Am. Chem. SOC.1979, 101, 4023. (20) Hurst, J. K.; Thompson, D. H. P.; Connolly, J. S. J. Am. Chem. SOC. 1987, 109, 507.
0 1989 American Chemical Society
6040
The Journal of Physical Chemistry, Vol. 93, No. 16, 1989
Sakaguchi and Kevan
I
DHP MV'
cy+ c,v+
c
t 2 mT
H
hV
/i
I / '
H
g = 2.0030
Figure 1. ESR spectra at 77 K from AV' in DPPC vesicles after 10-min photoirradiation: (a) MV', (b) C6vt, (c) CsV', (d) c,zV', and (e)
CI6Vt. The outer parts of the singlet spectra are shown with 4 times higher gain.
9.2.0030
Figure 3. ESR spectra at 77 K from AV' in DHP vesicles after 10-min
photoirradiation: (a) MV', (b) C6vt, (c) CBV', (d) clZv', and (e) CL6V+.The outer parts of the singlet spectra are shown with 4 times higher gain.
1
DODAC
cy+
c
ClzV'
d
g 2,0030 I
Figure 2. ESR spectra at 77 K from AV' in DODAC vesicles after 10-min photoirradiation: (a) MV', (b) C6vt, (c) CBV', (d) cl2V+,and (e) C16V+.The outer parts of the singlet spectra are shown with 4 times
higher gain. microwave power to avoid power saturation. The magnetic fields were measured with a Varian E-500 nuclear magnetic resonance gaussmeter, and the microwave frequencies in the 9-GHz range were directly measured with a Hewlett-Packard 5350B microwave frequency counter. Two-pulse electron spin echo (ESE) spectra were observed at 4.2 K, 9 GHz, and about 330 mT with a home-built spectrometer.21 Results The ESR spectra from frozen solutions of AV2' in DPPC, DODAC, and DHP vesicles were obtained after photoirradiation with a Corning 7-54 glass filter at 77 K for 10 min (Figures 1-3). The singlet spectrum from each irradiated sample had g = 2.0030, (21) Ichikawa, T.; Kevan, L.; Narayana, P. A . J . Phys. Chem. 1979.83, 3378.
MV+/DOD~C
Figure 4. ESR spectra at 77 K from AV' in vesicle or in bulk after photoirradiation: (a) MV' in DODAC vesicles irradiated for 20 min; (b) MV' in bulk irradiated for 40 min; (c) cl6v' in bulk irradiated for 20 min.
and the sample color was pale blue. Small lines superposed on each side of the singlet spectrum are shown by arrows in the ESR spectrum of C16v' in DPPC (Figure le). The lines are more clearly indicated in the ESR spectra from AV' in DODAC vesicles (Figure 2) and coincide in magnetic field positions with those of CI6Vt in DPPC. The intensity of the lines increases with the alkyl chain length in AV2'. The lines comprise an octet spectrum with a hyperfine coupling of about 2.2 mT and a slightly smaller g value than the singlet spectrum (Figure 2e). No such lines were observed in the ESR spectrum from AV' in DHP, even for C16V+ (Figure 3). The lines from MV' in DODAC irradiated for 20 min (Figure 4a) are more clear than when irradiated for 10 min. However, no such lines were observed from MV+ in bulk, even for 40-min irradiation (Figure 4b) or for c16v+ in bulk for 20-min irradiation (Figure 4c). The ESR spectra from cl6v+in DODAC were observed versus increasing irradiation time (Figure 5). A clear octet spectrum was observed after 200-min irradiation. The octet spectrum has equal hyperfine couplings of 2.2 mT with intensity ratios
Photoionization of Alkylmethylviologens in Vesicles
The Journal of Physical Chemistry, Vol. 93, No. 16, 1989 6041
A
0
100 t, min.
200
Figure 7. Relative ESR intensity at 77 K versus irradiation time: (0) cI6v+and DAC in DODAC vesicles irradiated with Corning glass filter 7-54 (240 < X < 410 nm); (A)c,6v+only in DODAC vesicles irradiated with a Corion Shott glass filter WG320 (A > 300 nm); ( 0 )DAC only in DODAC vesicles without AVZ+ irradiated with Corning glass filter 7-54.
H
Figure 5. ESR spectra at 77 K from cl6v+ in DODAC vesicles after photoirradiation for (a) 10 min, (b) 72 min, (c) 200 min; (d) the background spectrum from DODAC vesicles without AV2+ irradiation for 200 min. The stick diagram in (c) indicates hyperfine splitting of the octet spectrum.
I
0.4
20.31 0.2
AV2+
Figure 8. Relative ESR intensities after 10-min irradiation at 77 K of the octet surfactant radical, R,,, versus the alkyl chain length of AVz+ in (A) DODAC, (0)DPPC, and ( 0 )DHP vesicles.
1, min.
Figure 6. Relative ESR intensity at 77 K versus irradiation time 7 for c)6v+in DODAC vesicles: (A) total ESR intensity; (0)octet spectrum from DAC; ( 0 )singlet spectrum from c16v+. 1:5:l1:16:l6:l1:4:1 and a gvalue of 2.0024 (Figure 5c). The background spectrum from DODAC without AV2+ for 200-min irradiation is shown in Figure 5d. The relative ESR intensities of the singlet and octet spectra from c16v+in DODAC are plotted as a function of irradiation time in Figure 6. The total intensity, obtained by double integration, versus irradiation time remained constant. Also, the total intensity at 200 min (Figure 6) was due entirely to the octet spectrum. Relative intensities of the octet spectrum at earlier times were measured as shown in Figure 5c normalized to the intensity at 200 min. The normalized intensity of the singlet spectrum is then obtained by subtraction. The intensities arising from c16v+in DODAC vesicles irradiated with the Corion Shott WG320 glass filter (A > 300 nm) and with the Corning 7-54 glass filter (A > 240 nm) are plotted against irradiation time in Figure 7. With X > 240 nm, as shown above, the c16v+radical is converted to the DAC radical with increasing photolysis time. But with X > 300 nm only the c16v+ radical is observed, even at long photolysis times, and no DAC radical is observed if only DODAC vesicles are photolyzed. Also no ESR signal is observed from c16v+in DODAC irradiated with the Corning 3-70 glass filter (A > 490 nm). No ESR pattern change was observed in the irradiated samples stored at 77 K in the dark for 3 weeks, indicating no thermal conversion of AV+ to DAC. All subsequent photoirradiation experiments were carried out using the Corning 7-54 filter (240 < X < 410 nm). The normalized intensities from AV+ in DODAC vesicles were obtained in the same manner as for c16v+. The intensity R,, of the octet spectrum from AV+ in DODAC irradiated for 10 min is plotted against the alkyl chain length of AV2+in Figure 8. The
0.5
1.0
1.5
2.0
2.5
TVPs
Figure 9. Two-pulse ESE spectra at 4.2 K from photoirradiated AV2+ in DODAC vesicles prepared in DzO: (a) C8V+and (b) c16v+ from 10-min photoirradiation; (c) DAC from 200-min photoirradiation.
intensity was normalized to the intensity of the octet spectrum of c16v2+in DODAC vesicles irradiated for 200 min. The relative intensity increases with an increase in the alkyl chain length in DODAC and DPPC vesicles. No octet spectrum was observed from AVZ+in DHP. ESE spectra from C8V+ and c16v+in DODAC vesicles prepared in D 2 0 irradiated for 10 min are shown in Figure 9a,b. The ESE spectrum of the DAC radical arising from cl6v+in DODAC vesicles irradiated for 200 min is shown in Figure 9c. All the echo spectra exhibit one modulation with a period of 0.08 ps that is related to weak hyperfine interactions of AV+ with hydrogens from the surfactant alkyl chains and a second modulation with a period of 0.5 p s resulting from interactions of AV+ with deuterium in
6042
The Journal of Physical Chemistry, Vol. 93, No. 16, 1989
AV2+
Figure 10. Normalized deuterium modulation depth versus alkyl chain length in AV2+: (A) AV+ in DODAC vesicle irradiated for 10 min; (0) DAC from C16V2+in DODAC vesicle irradiated for 200 min. ESE data for AV+ in D H P vesicles are from ref 17. c
a
MV~+
c#+ c8vp+ c,,v2+
c,b+
AV2'
Figure 11. Photoirradiation yield at 77 K from AV2+ in vesicles for IO-min irradiation versus alkyl chain length in AV+: (A) DODAC, (0) DPPC, and (0)DHP.
Sakaguchi and Kevan shown that substitutions in the viologen moiety have relatively little effect on the unpaired spin distribution in the viologen cation r a d i ~ a l We . ~ ~conclude ~ ~ ~ that the singlet spectra shown in Figures 1-3 are assigned as alkylmethylviologen cation radicals: MV', CsV', csv+,CI~V',and c,6v+. Figures 1-4 indicate that the lines arising from the octet spectrum are produced by photoirradiation in the presence of DODAC or DPPC vesicles. Figure 5c clearly shows the octet spectrum from cl6v+in DODAC vesicle for 200-min photoirradiation. This spectrum has a g value of 2.0024 and equal hyperfine couplings of 2.2 mT with intensity ratios of 1:5:11 : 16:16: 1 1:4:1. The blue color completely disappeared after 200 min of photoirradiation. Therefore, the octet spectrum is assigned to an alkyl free radical. The McConnell relation26 for the hyperfine coupling of (3protons ( A , = 4.6 cos2 B mT) indicates that if the angle between the C,-H, bond axis and the p-orbital on C, is 13' or 47O, the (3-proton hyperfine coupling is about 4.4 or 2.2 mT. For a CH,CHCH2-alkyl radical, if one a-proton and four @-protons (three methyl protons and one @-proton)have equal hyperfine couplings of 2.2 mT, and the other (3-proton has a hyperfine coupling of 4.4 mT, the predicted ESR spectrum has eight lines split by 2.2 mT and intensity ratios of 1:5:l1:15:l5:l1:5:1 as observed. The intensity ratios and the hyperfine coupling from the octet spectrum (Figure 5c) coincide with the eight-line spectrum from this model of the alkyl radical. Therefore, the octet spectrum is assigned as an alkyl radical with the structure CH3CHCH2-. No lines due to the octet were observed from AV' without vesicles (Figure 4). MV2+cannot form a CH,CHCH2- radical; however, octet lines were observed from MV' in DODAC vesicles (Figure 4a). DODAC has alkyl chain tails in its molecular structure. We conclude that the octet spectrum (Figure 5c) is due to the following radical from an alkyl chain in DODAC which will be termed as the DAC radical. CH3CHCH2(CH2)1S,N
yCH3
CH3(CH2),7'
D 2 0 located at the vesicle i n t e r f a ~ e . ~The ~ , ~variations ~ of the normalized deuterium modulation depth of AV2+ in DODAC vesicles, computed by the graphic method,23are plotted against the alkyl chain length of AV2+ in Figure IO. This is compared with data for AV+ in DHP vesicles." The value of the normalized modulation depth of AV+ in DODAC vesicles is always smaller than that of AV' in DHP vesicles. The photoionization yields of AV2+ in DODAC, DPPC, and DHP vesicles are plotted against the alkyl chain length of AV2+ in Figure 1 I . The photoionization yield was obtained from double integration of the ESR spectrum after 10-min irradiation at which the yield plateaus. All yields were normalized to the yield of c16v2+ in DODAC irradiated for 10 min. All yields increase with an increase in the alkyl chain length. The yields from the DODAC vesicles are higher than those in the DPPC or DHP vesicles. The highest yields in DODAC, DPPC, and DHP vesicles were obtained with c16v2+as 1.OO,0.68, and 0.57, respectively. The highest yield obtained from c,6v2+in DODAC vesicles is more than 2 times greater than the yield (0.43) of MV2+ in DHP vesicles. Discussion
Radical Conversionfrom A F to a CH3CHCH2-AlkylRadical from the Surfactant. The g value (2.0030) of the singlet spectra coincides with that of AV+ in DHP vesicle^.'^ All irradiated samples appeared blue in color. The blue color is a characteristic feature of AV+.24,25 Before irradiation, no ESR signal was observed from AV2+ in the vesicles. Previous ESR studies have (22) Hiff, T.; Kevan, L. J . Phys. Chem. 1988, 92, 3982. (23) Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Berr, S. S.; Jones,
R. R. M. J . Phys. Chem. 1985, 89, 1547. (24) Johnson, C. S.; Gutowsky, H. S. J . Chem. Phys. 1963, 39, 58. (25) Evans, A. G . ; Evans, J. C.; Baker, M. W. J . Chem. Soc., Perkin Trans. 2 1917. 1787.
C ' H,
The intensity of the DAC radical increases with irradiation time. However, the total ESR intensity is unaltered with an increase of the photoirradiation time (Figure 6). These results indicate that most of the DAC radical is produced by radical conversion from cl6v+which is induced by photoirradiation. If DODAC vesicles without c16v2+are irradiated with X > 240 nm, the DAC radical is produced directly but is about 4 times weaker than with c16v2+ present. When the Corion filter is used so that X > 300 nm, no DAC radical is produced when only DODAC vesicles are irradiated. When C16V2+is added to DODAC, c,6v+is produced by X > 300 nm with accidently the same yield as for DAC produced from DODAC by X > 240 nm. These results indicate that the radical conversion of AV+ to DAC is induced by photoirradiation in the wavelength range of 240 < X < 300 nm. It has been reported27that MV+ in DHP vesicles has absorption bands around 380 and 550 nm. However, in our system, those bands seem to be ineffective for inducing the radical conversion. A similar radical conversion is observed from C6V+,c8v+,and C12V+in DODAC vesicles. These results suggest that the radical conversion by photoirradiation is a general phenomenon for AV+ in DODAC vesicles. The scheme of the radical conversion is proposed as follows.
A V
+ DODAC-H
240 C h
< 300 om
The ESR lines from CsV', CI2V+,or
*
AV+-H
c16v+in
+ DAC
(I)
DPPC (Figure
1) coincide in magnetic field with those from the DAC radical. These results suggest that radical conversion occurs from AV+
(26) Heller, C.; McConnell, H. M . J . Chem. Phys. 1960, 32, 1535. (27) Patterson, B. C.; Thompson, D. H.; Hurst, J. K. J . Am. Chem. Soc. 1988, 110, 3657.
J. Phys. Chem. 1989, 93, 6043-6051 to also form a CH3CHCH2-radical (PC) in DPPC as follows. CHSCHCH2(CH2)12coocH2>CHCH3P64CH2CH2~(CH3)3 CH3(CH2)14C00CH2
The DAC radical intensity in DODAC vesicles after 10-min irradiation increases with an increase in the alkyl chain length of AV2+ (Figure 8). This is consistent with the increased alkyl chain length locating the AV2+deeper into the vesicle interface which promotes the hydrogen abstraction from DODAC. The identity of the DAC radical as a secondary alkyl radical suggests that radicals are initially produced by hydrogen abstraction from various carbons on the DODAC alkyl chain followed by hydrogen migration to convert a primary radical to a more stable secondary radical.28 This proposed idea for DAC radical production via hydrogen migration is supported by the ESE patterns in Figure 9 and the modulation depth data in Figure 10. The normalized deuterium modulation depth reflects a degree of interaction between AV+ and the water deuteriums at the vesicle interace. This dipolar interaction depends on the interaction distance as well as on the average number of interacting water molecules. The vesicle interface structure and hence the number of interacting waters should be the same in identical vesicle systems after photoirradiation. The normalized modulation depths of c16v+(0.37) and DAC (0.18) indicate different locations within the vesicle for the two radicals and that the DAC radical is located deeper in the vesicle than is cl6v+. We conclude that primary surfactant radicals are produced by hydrogen abstraction and are converted by hydrogen migration to a more stable secondary alkyl radical to result in the DAC radical. No radical conversion was observed from AV' in DHP (Figure 1). The normalized modulation depth of AV+ in D H P vesicles is always higher than in DODAC vesicles (Figure 10). This indicates that AV' is less hydrated in DODAC versus DHP. This suggests that the alkyl chain effect for deeper insertion in vesicles for the alkylviologens is stronger than the vesicle interface charge (28) Shimada, S.; Kashiwabara, H.; Sohma, J. J . Polym. Sci., Part A-2 1970, 8, 129 1.
6043
effect for interaction with the alkylviologen. Photoionization Yield of Alkylviologens in Vesicles. We have suggested that the surfactant secondary alkyl radicals (DAC and PC) are produced by a radical conversion which is induced by photoirradiation. According to this proposal, the total intensity of the ESR spectrum composed of AV+ and DAC or PC reflects a net photoionization yield. The radical yield versus alkyl chain length (Figure 8) shows a striking parallel to the photoionization yield (Figure 11). Since the photoionization yield and the radical yield are normalized as in the preceding section, we can directly compare the values from Figures 8 and 1 1. Subtraction of the radical yield in D H P from that in DODAC and from that in DPPC gives 0.34 and 0.10. The subtraction of the photoirradiation yield in DHP from that in DODAC and from that in DPPC gives 0.43 and 0.1 1, respectively. The reasonable agreement between the net radical yield and the net photoionization yield indicates that the back-reaction of AV+ is impeded by reaction 1 and that the photoionization yield is increased by the radical conversion. The ESE data in Figure 10 show that the interaction of AV' with water at DODAC or D H P vesicle interfaces decreases with increasing alkyl chain length of AV2+ and that the interaction in DODAC vesicle is weaker than in DHP vesicle. It is concluded that the radical conversion is promoted in a less hydrated location which also results in an increase in the photoionization yield. Conclusions We have studied the photoionization yield of AV2+ in vesicles. This study shows that the radical conversion from AV' to DAC or PC radicals is induced by photoirradiation with wavelength 240 < X < 300 nm. The yield of AV+ increases with an increase in the alkyl chain length of AVZ+and with the cationic DODAC vesicle interface versus the anionic D H P interface. The highest photoionization yield correlates with the highest radical yield. This occurs with c16v2+in DODAC vesicles. The highest yield is promoted by impeding the back-reaction of AV' to AV2+.
Acknowledgment. This research was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, US.Department of Energy. We thank D. H. P. Thompson and J. K. Hurst for the alkylviologens.
Theoretical Study of Conformations of Some Heterocyclic Transition-Metal Complexes Ming-Der Su and San-Yan Chu* Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China (Received: January 30, 1989)
The conformations for heterocyclic transition-metal sandwich complexes involving five-membered rings (C4H4X2)Mwith X = P, As, and Sb, six-membered rings such as (CSH6B)*Mand (C5H5N),M, and some five-membered polyheterocyclic rings are studied by use of the extended Huckel method. The calculations are in good agreement with the observed results. The main purpose of this work is to identify the key orbitals for the observed dihedral angles. The general pattern emerging from this study is that the metal (dxz, dyz)pair favors 90" conformation, for it allows them to interact independently with ring orbitals. The metal (d,,, d,+,.z) pair favors the 45' and 135' conformations. Therefore, the conformation is determined by these two major competing factors.
Introduction The chemistry of heterocyclic transition-metal complexes 1 has In the present theoretical been actively developed (1) (a) Ashe, A. J.; Drone, F. J. Organometallics 1985, 4, 1478, and references therein. (b) Epiotis, N. D.; Cherry, W. J. Am. Chem. Soc. 1976, 98,4365. (c) Ashe, A. J.; Diephous, T. R. J . Organomet. Chem. 1980,202, C95. (d) Mercier, F.; Goff, C. H.-L.; Mathey, F. Organometallics 1988, 7 , 955. (e) Ashe, A. J. Top. Curr. Chem. 1982, 105, 125.
0022-365418912093-6043$01 SO10
study, we will focus our attention on symmetrical sandwich complexes involving five-membered and six-membered heterocyclic (2) (a) Mathey, F. J. Organomet. Chem. 1975, 93, 377; Ibid. 1977, 139, 77. (b) Lauzon, G.; Deschamps, B.; Fischer, J.; Mathey, F.; Mitschler, A. J. Am. Chem. SOC.1980, 102,944, and references therein. (c) Lemoine, P.;
Gross, M.; Braunstein, P.; Mathey, F.; Deschamps, B.; Nelson, J. H. Organometallics 1984, 3, 1303. (3) Chiche, L.; Galy, J.; Thiollet, G.; Mathey, F. Acta Crystallogr. 1980, 836, 1344, and references therein.
0 1989 American Chemical Society
