14359
J. Phys. Chem. 1995,99, 14359-14364
Spectroscopic Observation of Contact Ion Pairs Formed from Photolyzed TMPD and TMB in the Presence of Halogenated Compounds in Nonpolar Solvents Hiroshi Shimamori" and Hirofumi Musasa Fukui Institute of Technology, 3-6-1 Gakuen, Fukui 910, Japan
Received: March 8, 1995; In Final Form: July 13, 199P
Flash photolysis at 308 nm has been employed to detect contact ion pairs (CIP) formed from excited N,N,N ',N 'tetramethyl-p-phenylenediamine(TMPD) and N,N,N ',N '-tetramethyibenzidine (TMB) in CCL solvent and CzHsBr, Cd-€5I, C6H5Br, and C6H5in benzene solutions containing halogenated compounds AX (CC4, C~HSI, Cl). Transient absorption spectra in CCL solvent indicate the formation of CIPs for both TMPD and TMB. While the transient spectra for benzene solutions without AX show the formation of only the excited triplet state of TMPD or TMB, those with AX indicate the formation of the corresponding CIP with different mechanism between TMPD and TMB, from the excited triplet state for TMPD, and from direct excitation of weakly-bound heteromer for TMB. The rates and efficiencies for the CIP formation depend on the AX molecule. The present results support the mechanism of CIP formation suggested previously in the microwave dielectric absorption studies.
1. Introduction A possibility of the formation of a long-lived contact ion pair (CIP) in solution has been suggested from the time-resolved microwave conductivity measurement for photolysis of N,N,N ',N '-tetramethyl-p-phenylenediamine(TMPD) in CC4 solvent.' A recent study2, also using the time-resolved microwave dielectric absorption technique, has confirmed this and proposed the mechanism for the CIP formation along with its quantum yield and the dipole moment. It has also been suggested that the excited triplet state of TMPD can produce a CIP, TMPD+X- (X = halogen atom), in the presence of a halogenated compound AX (CC4, C2H5I, C&I, CsHsBr, and C6H5C1) in benzene or other nonpolar hydrocarbon solvent^.^ Similar ion-pair formation has been suggested for N,N,N ',N 'tetramethylbenzidine (TMB)$,5 for which the formation mechanism in CC4 solvent is the same as in the TMPD case, but in benzene solvent only the formation by direct excitation of weakly-bound heteromer (TMBAX)is possible. This difference has been ascribed to a higher ionization potential of TMB compared to TMPD. These studies are based solely on the measurements of polarity change of the solute molecule before and after the photolysis, and no clear spectroscopic evidence for the CIP formation has been given except for earlier findings of the formation of a very small amounts of TMPD cations in a steady-state photolysis of TMPD in various halogen-containing solvents6and a recent observation of the TMPD cation in CC4 solvent using a time-resolved resonance Raman spectroscopy.7 The spectroscopic observation, especially in a time-resolved way, is important not only in the identification of the radical cation or the ion pair formed but also in the substantiation of the kinetics of the ion-pair formation suggested by the dielectric absorption measurements. It is well recognized that the absorption spectra for the TMPD radical cation in solution resembles closely that for the excited triplet ~ t a t e ~and, .~ therefore, the distinction of the former from the latter when both are present simultaneously may not be an easy task with a conventional photoabsorption measurement. However, we should be aware that the cation in question is accompanied by @
Abstract published in Advance ACS Abstracts, September 1, 1995.
0022-3654/95/2099-14359$09.00/0
a halogen anion at a very close distance. Then, the absorption spectra could be different from the isolated cations. Furthermore, the kinetic behavior of the CIP revealed by the dielectric absorption measurements can be helpful for the differentiation of the CIP from the excited triplet state. On the basis of these considerations we have attempted to observe transient absorption spectra of cations involved in the CIP using the conventional laser flash photolysis method in order to substantiate the mechanism of the CIP formation proposed in previous microwave dielectric absorption studies.
2. Experimental Section Flash photolysis was carried out with a 308-nm laser pulse from an excimer laser (Lambda Physik, LPX 205) using a rectangular quartz cell (13 x 13 x 47 mm) containing the sample solution. The light from a Xe flash lamp (Tokyo Instruments, 70 ps pulse duration, 80 W) passed through the cell (path length = 47 mm) and into a monochromator (JobinYvon, HR-320), and was monitored by a photomultiplier (Hamamatsu, R928). The laser light entered the wide surface (13 x 47 mm) of the cell. The signal was fed to a Tektronix 2430 digital oscilloscope with a 50 Q terminator. The timing for the trigger of the excimer laser with respect to the flash lamp was manipulated by a digital delay generator (Stanford Research Systems, DG 535). TMPD and TMB (Aldrich) were purified by vacuum sublimation. Carbon tetrachloride, benzene, and n-heptane (Wako Chemicals) were dehydrated by contact with Molecular-Sieve 3A. C2H51, C2H5Br, C6H51, C&Br, and C&C1 (Wako Chemicals) were used as received. Before irradiation all the samples were deaerated by bubbling with Ar gas for more than 40 min. Measurements were carried out at room temperature (298 K). 3. Results and Discussion
TMPD. The absorption spectra for TMPD in n-heptane and benzene solvents are shown in Figure 1. The present spectra in n-heptane is in excellent agreement with that observed in the same solvent at room temperature previously,'o which has been assigned to the T,, Ti absorption by the lowest excited triplet state of TMPD. The lifetimes determined from the
-
0 1995 American Chemical Society
Shimamori and Musasa
14360 J. Phys. Chem., Vol. 99, No. 39, 1995
TMPD
TMPD
,. . .
in benzene in n-heptane
/
.....
..... in C,H,OH
\ '.
in n-heptane ( Yokoyamn )
_-
..,
'\.
in CH,CN
1'. ,
500
550
600
I
.
,
*
650
550
600
Wavelength (nm) Figure 1. Transient absorption spectra observed immediately after
pulse irradiation of TMPD in benzene (-) and in n-heptane (- - -) at room temperature. Concentrations of TMPD are 0.32 and 0.41 mM for benzene and n-heptane solvents, respectively. The dotted line is the spectrum in n-heptane reported by Yokoyama (ref 10). The absorbance is in arbitrary units. observed exponential decays of the absorbances at 570 nm are 1.7 and 3.3 ps for n-heptane and benzene solvent, respectively. These values can be compared to a previous value 1.4 pus in n-heptane'O and are longer than in acetonitrile (0.5 p ) l l and much shorter than that in liquid paraffin (82 p ~ ) The . ~ spectrum in benzene resembles that in n-heptane and should be attributed to the excited triplet state. The difference between the lifetime obtained in the present study (or in ref 10) and that in the previous measurements in liquid paraffin9 cannot be explained clearly. YokoyamaIo has suggested that the difference in the lifetime between in n-heptane and in liquid paraffin may be due to the difference in the concentration of TMPD (3 mM in n-heptane and lo-* mM in liquid paraffin) and the viscosity of the solvents. If it is so, the self-quenching by TMPD is important in this system. However, the concentration of TMPD in the present study is 0.41 mM in n-heptane, yet the present lifetime is not much different from that observed in the Yokoyama's measurement. Thus a long lifetime observed in liquid paraffin may not be related directly to the triplet state. In the course of the present measurements it has been found that an excessive irradiation of photons, say eight shots, on the same sample solution with benzene solvent produces a nonexponential signal showing appearance of extra absorption with a longer lifetime as long as 10 ps. Such a dramatic change has not been observed for heptane solutions by irradiating with many shots, but we still notice a slight increase in the absorption on a longer time scale. Some products from the reactions involving impurities might be responsible for these. Similar effects could be related to the long lifetime reported previously, though the details are uncertain. Although it is possible that even the lifetime of 3.3 ps obtained from a single-shot irradiation on benzene solutions might have suffered from these effects because the value in benzene is somewhat longer than that in n-heptane, we regard at present the value of 3.3 pus as the lifetime for the triplet state in benzene solvent (see also below). Shown in Figure 2 are the transient spectra observed in CCk solvent. The spectrum in cc14 resembles that in benzene, but its peak is observed at 630 nm. In addition, the spectrum for the wavelengths between 560 and 610 nm is wide and flat compared to that in benzene and rather resembles to that for the TMPD cations observed in ethanol8t9and in acetonitrile" at room temperature, as shown in Figure 2. The time profile of the signal in CCld solvent is different from that in benzene (see Figure 3). The absorbance in CCh solvent does not
650
Wavelength (nm) Figure 2. Absorption spectra at 5 p s after the laser pulse for a solution of TMPD (0.19 mM) in CC14 at room temperature (solid line) in comparison with spectra for the TMPD radical cation observed in ethanol (- - -, refs 8 and 9) and in acetonitrile (- - -, ref 11).
I
C
m ij
Q
$
TMPD - CCl4
0.2 -
Q 0.1
-
1w-
0.0
Time (1ps/div) Figure 3. Time dependence of absorbance at 570 nm for photolyzed TMPD in C6H6 and CC4. Concentrations of TMPD are 0.32 and 0.19 mM for C6H6 and CCl4 solvents, respectively.
decrease for a long time, with the lifetime being longer than 100 ps. From this result, it is reasonable to regard that the spectra observed in CC4 solvent correspond to the absorption by the contact ion pair TMPD+Cl- formed from the reaction of the excited state, probably in the singlet state, of TMPD with the solvent CC14. TMPD hv TMPD* TMPD*
+ CC1, (solvent) - TMPD'Cl- + CC1,
(1)
(2)
The red shift of the spectrum compared to those in ethanol or acetonitrile may be due to the presence of C1- in the vicinity of the cation. The time profile of the dielectric absorption signal in CCL solvent2 shows an initial rise followed by a secondorder decay to reach a flat level above the base line. These features have been interpreted as being due to the formation of the CIPs and their successive dimerization. TMPD'CI-
+ TMPDfCl- - (TMPD'Cl-),
(3)
Contact Ion Pairs Formed from Photolyzed TMPD and TMB
J. Phys. Chem., Vol. 99, No. 39, 1995 14361
0.4
TMPD-CCI, in Benzene
0.1
0.3 0.0
8 c
2
0.2
6 u)
5!
8C
0.1
Q
0.1
0.0'
"
550
"
"
'
600
"
"
'
> e n 8 10.0 -
650
Wavelength (nm) Figure 4. Transient absorption spectra for photolyzed Th4PD in the presence of CC4 in benzene solvent at room temperature. Concentrations; [TMPD] = 0.36 mM, [CCl4] = 0.063 mM.
0.0
0.52 m M
t
I
However, the time profile of the photoabsorption observed Time (1 ps/div) here shows no such decays apparently, suggesting that the Figure 5. Time dependence of absorbance at 570 nm for photolyzed absorption coefficient of the cation in the CIP dimer is not much TMPD in benzene solvent at different concentrations of added CC4. different from that in the CIP monomer. The transient absorption spectra for a benzene solution containing TMPD and CCb are shown in Figure 4. The absorbance shows a decay but the two peaks at 570 and 610 0.012mM o.2: 0.1 nm tend to shift to longer wavelengths. Comparing with the ry spectra shown in Figures 1 and 2, this result can be rationalized 0.0 M. by the mechanism that the lowest triplet state of TMPD decays by the reaction with CC4 to produce the CIP. The proposal of Q) 0.2 the TIstate as the precursor for the CIP is supported by the 0 C 0.024mM result that the addition of oxygen mostly quenched the signals 0.1 at around 570 nm and 630 nm. It has been found that at a high concentration of CC4 (>1 mM) there remain unquenched amplitudes and they increase with the CC4 concentration. Such O.O a phenomenon has also been observed in the microwave 0.2 I 1 dielectric absorption measurements for TMPD containing CC4 0.096mM or C&Br in benzene solution in an attempt to examine the effect of oxygen on the CIP yields (see results of additional measurements described in ref 5). This has been interpreted as 0.0 indicating the CIP formation from direct excitation of (TMPDAX) complex, which is the same mechanism as in TMB, which Time (1p/div) increases with the AX concentration. Thus, as long as the Figure 6. Time dependence of absorbance at 570 nm for photolyzed concentration of CC4 is not too high, the precursor of the CIP TMPD in benzene solvent at different concentrations of added C2HsI. is the triplet TMPD in benzene solvent. Since the signal-to-noise ratio at 630 nm was relatively low, for CCL. Typical features for C2H5I are shown in Figure 6. mainly due to the lower intensity of Xe flash lamp at 630 nm, For C,&Br, C&Cl, and C2H5Br, on the other hand, concentrathe signals at 570 nm have been used for kinetic analyses tions higher than 10 mM were required to observe a noticeable described below. We expect that at the wavelength of 570 nm flat level corresponding to CIPs. For C&F and C6F6 there the triplet state and the CIP can be detected simultaneously. was no indication of the CIP formation even at concentrations of 20 mM. All these results are consistent with the observation Indeed the signal at 570 nm shows an initial peak followed by a decay reaching to a flat level, as shown in Figure 5. As the of the dielectric absorption measurements. These observations indicate that the following reactions should be involved in the concentration of CCb increases the decay becomes faster and observation for the halogenated compound AX (X = a halogen the flat level increases. The previous dielectric absorption study3 atom). has shown that the efficiency and the rate for the CIP formation varies considerably depending on the halogenated compound present in the solution. The order of the rate for the CIP TMPD hv 'TMPD 3TMPD (4) formation was CC4 > C6H51> C2H5I > C6H5Br =- C6HsC1, kd and no CIP formation was evident for CzHsBr, CsHsF, and (3%. 3TMPD TMPD energy (5) These differences have been correlated with the electron capture capability of the compound that is inferred from the data of 3TMPD A X TMPD'XA (6) dissociative electron attachment in the gas phase. In order to where k's are the rate constants. Reaction 6 is efficient for examine the validity of these results the photoabsorption signals have been measured for all these compounds. The observed AX = CC4, C6H5I, and C2H51but very inefficient for CsHsBr, signals for C6H51and C2H51at 570 nm were similar to those C,&C1, and C2H5Br.
e s 2
1I
L
1-b
-
+
-
+
+
Shimamori and Musasa
14362 J. Phys. Chem., Vol. 99, No. 39, 1995
10
-
I
0‘ 0
0‘ 0
2
4
10
Figure 8. Plots of reciprocal amplitude of the flat level of the
Figure 7. Plots of the decay rate in the absorbance as a function of the concentration of the halogenated compound AX (CC4, c&I, CzH51,
-
TMB in CC1,
C6H5C1) added to the solution of TMPD in benzene (see text).
TABLE 1: Kinetic Parameters for the Ion-Pair Formation Processes for TMPIP
cc1.i
C6HsI C2H5I CsHjC1
kx
(M-’ s-I 1 11 (10.4) 7.0 (7.1) 4.4 (5.0) 1.3 (1.5)
300
absorbance as a function bf reciprocal concentration of CC14.
[AX] ( 10-5M )
compd
200
[CC141-’ (mM-’)
I
a
6
100
kd
X
1 ps
- 5
8 C m
(S-’)
% 9
3.0 (30) 3.0 (38) 2.5 (40)
v)
2.5 (31)
Values in the parentheses correspond to those determined by the time-resolved microwave dielectric absorption method (ref 3). The decay rate for the triplet state (kd) in the parenthesis may suffer from the effect of residual oxygen (see text). a
Let us make a kinetic analysis for the observed signals on the basis of the above mechanism. The time dependence of the absorbance (OD) can be expressed by
I
400
[CIPI, = w w [ T l d ( k ,
+ k[AXI)
(8)
which corresponds to the flat level of the signal. The slopes of plots of ln(0D - ECIP[CIP],) vs time give values of decay rates, k[AX], as a function of [AX]. This type of plot has been made for CC4, CsHsI, C2H5I, and CsHsC1, which is shown in Figure 7. The rate parameters obtained from the slopes and the intercepts are listed in Table 1. For comparison the values obtained from the dielectric absorption measurements are also listed. The present values of the rate constants for the CIP formation are in good agreement with those from the dielectric absorption measurements. Although the lifetime of the triplet state (=llkd) is in good agreement with 3.3 ps obtained from the decay observed in the TMPD-benzene system (Figure 31, they are about an order of magnitude longer than those obtained previously. The shorter lifetime in the previous study was not explained clearly. We have found in a recent dielectric absorption measurement for TMPD-CCL in benzene that an extensive period of bubbling of AIfor the sample solution gives a decay rate much slower than that obtained previously. Thus the previous short lifetime may be due to insufficient removal of oxygen. We conclude that the lifetime of the lowest excited state of TMPD in benzene is 3.3 ps.
+
500
Wavelength (nm) Figure 9. Transient absorption spectra for photolyzed TMB in CC14 at room temperature in comparison with spectra of the lowest triplet state TMB* (- - -, ref 12) and the TMB radical cation (- -, ref
14).
where T and CIP denote the triplet state and the contact ion pair, respectively, E represents an effective photoabsorption efficiency proportional to the extinction coefficient, [TI0 is the initial concentration of T, and
450
-
We can expect from eq 8 that ECIP[CIP]..(or the flat level of the signal) increases with [AX] and saturates at a high [AX]. This alternatively suggests that the plots of ~/EC[P[CIP]..vs l/[AX] show a straight line, the slope of which gives the ratio kdlk by the combination with the intercept. This type of plot for AX = CCl4 is shown in Figure 8. The ratio kdlk obtained M-I, which is comparable to the value 2.7 x is 3.2 x M-’ calculated from the values in Table 1. Although detailed analysis could not be made for the other AX compounds because of large uncertainties involved in the values of ECIP[CIP],, they were qualitatively consistent with the results shown in Table 1. We should note that at a high concentration of CCk (> 1 mM) the amplitude of the flat level becomes approximately the same as the initial amplitude. Thus at the wavelength of 570 nm the extinction coefficient for the TMPD cation in the CIP must be close to that of the triplet TMPD. TMB. The absorption spectra observed in the present study for a benzene solution containing TMB show a peak at 470 nm and are very similar to that for the excited triplet state in c-CgH12 observed previously by Alkaitis and Graetzel.I2 The absorbance at 470 nm was observed to have decay kinetics, with a lifetime of 4 ps, which is shorter than 10 ps observed previously in acetonitrile s~lution.’~ The transient absorption spectra for TMB in CCL over the wavelengths between 400 and 500 nm are shown in Figure 9. They are clearly different from the absorption spectrum for the triplet stateI2 but resemble closely that for the TMB cation observed in a~etonitri1e.I~The signal at 470 nm giving the peak absorbance showed a rapid growth immediately after the photoirradiation and simply decayed with
Contact Ion Pairs Formed from Photolyzed TMPD and TMB
I
TMB - CCla in Benzene
I
J. Phys. Chem., Vol. 99, No. 39, 1995 14363 1
0.1 I
0.1
1
i
Time (2pddiv)
..
450
500
550
Wavelength (nm) Figure 10. Transient absorption spectra for photolyzed TMB in the presence of CCld in benzene solvent at room temperature. Concentrations; [TMB] = 0.023 mM, [CC14] = 0.125 mM.
the lifetime of about 20 ps. The presence of oxygen did not change the signal at all. We can see from Figure 9 that the spectra observed at earlier times are much like that for the TMB cation observed in acetonitrile, but they show the presence of isosbestic point at about 483 nm. The dielectric absorption study4 for TMB in CC4 has indicated that the contact ion pairs ( T M B T - ) are formed by reaction of the excited state of TMB with CC4, as in the reactions 1 and 2 in the TMPD case, and they subsequently undergo dimerization like reaction 3. The present results definitely support the formation of the CIP, but the occurrence of the dimerization process is not certain. The increase in the absorbance at around 500 nm could correspond to the absorption by the CIP dimers. The transient spectra for solutions of TMB containing CC4 in benzene solvent are shown in Figure 10. The peak around 470 nm decays but still remains at an appreciable level on a longer time scale, whereas the absorbance at wavelengths longer than 490 nm disappears rather quickly. There is again an isosbestic point at about 485 nm. The time variation of the signal at 470 nm shows a decay that is not exactly exponential (see Figure 1l), and the dependence of the time profile on the C C 4 concentration is clearly different from that for TMPDCC14-C6H6 system shown in Figure 5. An apparent lifetime estimated from the long-decay portion is about 20 ps, which is the same as that for the CIP observed in the TMB-CCL system. The addition of oxygen quenches the signal to some extent, but the shape of the residual signal is similar to that of the original. The higher the CCL concentration, the lesser quenching of the signal by oxygen. The observed time variation can be explained by assuming that both the excited triplet state of TMB and the CIP are formed initially, and the former decays with a shorter lifetime (4 ps), while the latter survives relatively long. We can also suggest that the precursor of the CIP is not the triplet TMB,as opposed to the TMPD case. Thus the spectra shown in Figure 10 can be interpreted as follows. The absorbance at around 470 nm involves contributions from both the excited triplet state and the CIP. Since the triplet decays
Figure 11. Time dependence of absorbance at 470 nm for photolyzed TMB in benzene solvent at different concentrations of added CC4.
fast but the CIP survives long, the total signal shows a relatively long decay. The absorbance at the wavelengths longer than 490 nm should mostly be due to the triplet state, since the decay is fast. The reformation of the absorbance at a time longer than 10 ,us resembles that observed in the TMB-CCL system (see Figure 9). It may be a result of the decay of the CIP, possibly due to the formation of the CIP dimers. Consequently, the following mechanism that was proposed in the dielectric absorption study should hold in the present case. TMB 4-CC1, (complex)
hv
Kc F=+
(complex)*
(complex)
-
TMB
(9)
+ CC1,
-TMB+C~-
+ cci,
(10) (11)
where (complex) is a weakly-bound van der Waals complex with both molecules in the ground state, Kc is the equilibrium constant for the complex formation, and (complex)* denotes an exciplex (TMB'CCL) formed by photoabsorption. It should be noted that the mechanism (9)-( 11) holds for TMB even at very low concentration of CC4, but, as mentioned above, similar mechanism can be applied to the TMPD case only when [CCL] is very high. As in the case of TMPD we have extended the measurement to other TMB-halogenated compound-benzene systems, where the halogenated compounds are C6H51, C2H5I, C&Br, and C&15Cl. It has been found that all the time dependencies of the absorbance at 400-500 nm show the same feature that the amplitude monotonically decays with the lifetime close to that of the triplet state (about 4 ps) and the long-decay portion at 470 nm which corresponds to the CIP becomes noticeable at a very high concentration of the halogenated compound. By comparing the magnitude of the long-decay portion at the same solute compound, the relative efficiency for the CIP formation is found to be cc4 > > C6H5I > CzH5I > C6HsBr =- C6H5C1. These are also consistent with the results by the dielectric absorption measurements5
Shimamori and Musasa
14364 J. Phys. Chem., Vol. 99,No. 39, 1995 4. Conclusion The CIP formation for TMPD and TMB as proposed by the time-resolved microwave dielectric absorption studies has been confirmed by the present spectroscopic observation. The formation mechanism as well as the effects of the halogenated compounds on the efficiency of the CIP formation have been confirmed. It seems that the spectroscopic observation cannot give by itself adequate assignments of transient species and processes that we are observing, in particular for the special case where the spectra of the T-T absorption and the radical cation are very similar. The existence of the dielectric absorption data definitely helped the interpretation of the present spectroscopic results. The dimerization and the clustering of the ion pairs proposed in the dielectric absorption studies have not been substantiated by the present measurements for both TMPD and TMB. This may be due to the resemblance of the absorption spectra and the extinction coefficients between the cation in the CIP and that in the CIP dimer.
Acknowledgment. This work was partly supported by a Grant-in-Aid on Priority-Area-Research “Photoreaction Dynamics” from the Ministry of Education, Science and Culture, Japan (06228230).
References and Notes (1) Warman, J. M.; Visser, R.-J. Chem. Phys. Lett. 1983, 98, 49. (2) Shimamori, H.; Uegaito, H. J. Phys. Chem. 1991, 95, 6218. (3) Shimamori, H.; Hanamuro, K.; Tatsumi, Y. J. Phys. Chem. 1993, 97, 3545. (4) Shimamori, H.; Tatsumi,Y. J. Phys. Chem. 1993, 97, 9408. (5) Shimamori, H.; Okuda, T. J . Phys. Chem. 1993, 98, 2576. (6) Meyer, W. C. J. Phys. Chem. 1970, 74, 21 18. (7) Isaka, H.; Suzuki, S.; Ohzeki, T.; Sakaino, Y.; Takahashi, H. J. Photochem. 1987, 38, 167. (8) Tsubomura, H.; Yamamoto, N.; Kimura, K; Sato, J.; Yamada, H.; Kato, M.; Yamaguchi, G.; Nakato, Y. Bull. Chem. SOC. Jpn. 1965,38,2021. (9) Yamamoto, N.; Nakato, Y.; Tsubomura, H. Bull. Chem. SOC.Jpn. 1966, 39, 2603. (10) Yokoyama, K. Chem. Phys. Lett. 1982, 92, 93. ( 1 1 ) Nakamura, S.; Kanamaru, N.; Nohara, S.; Nakamura, H.; Saito, Y.; Tanaka, J.; Sumitani, M.; Nakashima, N.; Yoshihara, K. Bull. Chem. SOC. Jpn. 1984, 57, 145. (12) Alkaitis, S. A.; Graetzel, M. J. Am. Chem. SOC. 1976, 98, 3549. (13) Das, P. K.; Muller, A. J.; Griffin, G. W. J. Org. Chem. 1984, 49, 1917. (14) Vemois, M.; Friedmann, G.; Brini, M.; Federlin, P. Bull. Chem. SOC. Fr. 1971, 1794.
Jp9506853
