9658
J. Phys. Chem. 1996, 100, 9658-9663
Magnetic Properties of 1,5-Dimethylverdazyl Radical Crystals. Ferromagnetism in 3-(4-Chlorophenyl)-1,5-dimethyl-6-thioxoverdazyl Radical Crystal Kazuo Mukai,*,† Kensuke Konishi,‡,§ Kouichi Nedachi,† and Kazuyoshi Takeda‡ Department of Chemistry, Faculty of Science, Ehime UniVersity, Matsuyama 790, Japan, and Department of Applied Science, Faculty of Engineering, Kyushu UniVersity, Fukuoka 812, Japan ReceiVed: December 6, 1995; In Final Form: March 13, 1996X
Magnetic properties of organic radical crystals, 3-(4-chlorophenyl)-1,5-dimethyl-6-thioxoverdazyl (p-CDTV) and 3-(4-bromophenyl)-1,5-dimethyl-6-thioxoverdazyl (p-BDTV), have been studied by measuring the heat capacity and ac susceptibility above 0.13 K in external magnetic fields of 0-30 kOe. p-CDTV has been found to be a new ferromagnet with a Curie temperature of Tc ) 0.68 ( 0.02 K. p-CDTV behaves as a quasi-one-dimensional Heisenberg ferromagnet with the intrachain exchange interaction of 2J/kB ) +12.0 ( 0.6 K above the transition temperature Tc. The interchain exchange interaction zJ′/kB was estimated to be +0.21 ( 0.02 K, where z is the number of interchain bonds per spin. p-BDTV, on the other hand, behaves as a one-dimensional Heisenberg antiferromagnet with negative exchange interaction of 2J/kB ) -41.2 K.
Introduction The search for ferromagnetic organic radical crystals has been one of the most challenging problems in the field of material science. Recently, several examples of organic bulk ferromagnets composed only of light elements such as H, C, N, and O have been reported, e.g., the p-nitrophenylnitronyl nitroxide radical (Tc ) 0.60 K),1 a nitroxide biradical (Tc ) 1.48 K),2 two nitroxide monoradicals (Tc ) 0.18 and 0.4 K),3 and a nitronyl nitroxide radical with a hydroquinone moiety (Tc ) 0.5 K).4 The Curie (transition) temperatures (Tc) observed, however, are very low as compared to those of typical transition metals (Tc ) 1043 K for Fe, 1388 K for Co, and 627 K for Ni). Furthermore, the examples of organic ferromagnets are limited to nitroxide and nitronyl nitroxide radicals. Therefore, the search for new kinds of organic ferromagnets with a high Curie temperature remains to be a matter of high interest. The synthesis and ESR study of a series of 1,5-dimethyl-6oxo- and thioxoverdazyl radicals have been performed by Neugebauer et al.5 These verdazyl radicals have a delocalized π-electron system, differing from nitroxide and nitronyl nitroxide radicals with a π-electron system localized in the part of the NO group. Furthermore, these verdazyl radicals have a comparatively small molecular weight. Consequently, we can expect a strong intermolecular exchange interaction between neighboring radical molecules. In a previous paper, we measured the static magnetic susceptibilities, χM, of 3-(4-chlorophenyl)-1,5-dimethyl-6-thioxoverdazyl (p-CDTV) and 3-(4-bromophenyl)-1,5-dimethyl-6thioxoverdazyl (p-BDTV), which have similar molecular structures to each other, in the temperature range of 4-300 K, using a SQUID magnetometer.6 The χM of p-CDTV follows the Curie-Weiss law with a Curie constant of 0.381 (K emu)/mol and a positive Weiss constant of θ ) +3.2 ( 0.2 K, suggesting intermolecular ferromagnetic exchange interaction between neighboring radicals. On the other hand, the χM of p-BDTV follows the Curie-Weiss law with a Curie constant of 0.381 K * Author to whom correspondence should be addressed. † Ehime University. ‡ Kyushu University. § Present address: Department of Physics, Faculty of Science, Ehime University. X Abstract published in AdVance ACS Abstracts, May 15, 1996.
S0022-3654(95)03615-X CCC: $12.00
emu/mol and a negative Weiss constant of θ ) -25 ( 2 K above 80 K. At lower temperatures, the susceptibility deviates from the Curie-Weiss law and exhibits a broad maximum at 26.5 ( 0.5 K. This susceptibility can be explained by the onedimensional (1D) Heisenberg model with a negative exchange interaction of 2J/kB ) -41.3 K. In the present work, in order to clarify the low-temperature magnetic properties of p-CDTV and p-BDTV, the simultaneous measurements of heat capacity and ac magnetic susceptibility of these radical crystals have been performed above 0.13 K under the external field of 0-30 kOe.
Experimental Section Syntheses. 3-(4-chlorophenyl)-1,5-dimethyl-6-thioxoverdazyl (p-CDTV) and 3-(4-bromophenyl)-1,5-dimethyl-6-thioxoverdazyl (p-BDTV) radicals were prepared according to a procedure similar to that used by Neugebauer et al.5 to prepare 1,5-dimethyl-3-phenyl-6-thioxoverdazyl. Characterization data for new materials (p-CDTV, p-BDTV, and the precursors of p-CDTV and p-BDTV) are available as supporting information. Measurement of Heat Capacity. Heat capacities, Cp, were measured above 0.2 K in the external magnetic field of 0-30 kOe, using the usual type of adiabatic calorimeter, whose detailed description was previously reported.7 The data were taken on 0.3136 and 0.2940 g powder samples of p-CDTV and p-BDTV, respectively, and have been corrected for the heat capacity of the addenda. Measurement of ac Magnetic Susceptibility. In the temperature range of 1.7-300 K, the ac susceptibility (χ) measurements were performed using the Lake Shore 7110 ac susceptometer. The relaxation effects of the susceptibility were checked under various ac fields H(ν) and frequencies (ν), as will be discussed later. On the other hand, in the temperature range between 0.13 and 2.0 K, the ac susceptibility measurements were also performed by the use of a handmade Hartshorn © 1996 American Chemical Society
Magnetic Properties of p-CDTV and p-BDTV
J. Phys. Chem., Vol. 100, No. 23, 1996 9659
Figure 1. Heat capacity, Cp, of p-BDTV in the temperature range of 0.7-5 K. The solid line is the estimated lattice contribution, Cl. Inset shows the magnetic heat capacity, Cm, of p-BDTV calculated for the value of 2J/kB ) -41.2 K in the temperature range of 0.7-28 K.
Figure 2. Cp/T versus T2 plot of the heat capacity of p-BDTV.
bridge in the ac field of about 100 mOe at 164 Hz. The field dependence of the ac susceptibility was also studied under the external magnetic field of 0-2 kOe applied parallel to the ac field. Results Magnetic Property of p-BDTV. The heat capacity, Cp, of p-BDTV was measured under zero external field in the temperature range of 0.7-30 K. The result obtained is shown in Figure 1. The main contributions to the heat capacity come from the lattice and the magnetic systems. It is known that p-BDTV has the characteristics of a 1D Heisenberg antiferromagnet with S ) 1/2, as obtained from the result of dc magnetic susceptibility measurement.6 Consequently, at low temperatures the magnetic heat capacity, Cm, may vary as 0.35NkB2T/|J|, as expected for the 1D Heisenberg antiferromagnet in the temperature region T < 0.2|J|/kB.8-10 Then the total heat capacity in this temperature region may be expressed as
Cp ) Cm + Cl ) 0.35NkB2T/|J| + A(θD)T3
(1)
where N is Avogadro’s number and θD is the Debye temperature for the lattice contribution Cl. Numerical constants 0.35NkB2/ |J| and A(θD) are determined from the plot of Cp/T versus T2, as shown in Figure 2. From the straight line in Figure 2, eq 1 fits the experimental data with |J|/kB ) 20.6 K and A(θD) ) 7.4 × 10-3 J/(mol K4). Subtracting this magnetic contribution
Figure 3. Inverse magnetic susceptibility, 1/χ, of p-CDTV at low temperatures. Above 5 K, the observed values (open circles) obey the theoretical results for the 1D isotropic Heisenberg ferromagnet (HF) with J/kB ) +5.5 ( 0.5 K.8 The straight line corresponds to χ of the paramagnet with g ) 2.00 and S ) 1/2.
from the total heat capacity yields the lattice heat capacity, Cl, of p-BDTV in the temperature region T < 5 K, which is exhibited in Figure 1. The value of 2J/kB (-41.2 K) obtained shows good accordance with that (-41.3 K) obtained from the static magnetic susceptibility measurement.6 Consequently, the Cm above 5 K is expected to be given by the theoretical value for the 1D Heisenberg antiferromagnet with spin S ) 1/2 and 2J/kB ) -41.2 K,8 and this is plotted in the inset of Figure 1. The resultant lattice heat capacity curve for 5 < T < 28 K is obtained by subtracting the value of Cm(T) from the total Cp. The result of the present experiment shows that the Neel temperature TN of p-BDTV under the zero external field is lower than 0.7 K, suggesting strong one-dimensionality of the spin correlation in p-BDTV.9 Magnetic Properties of p-CDTV. The ac susceptibility of p-CDTV was measured in the temperature region between 1.7 and 300 K, using the Lake Shore 7110 ac susceptometer. For the check of the relaxation effects on the magnetic susceptibility, the measurements were performed by varying the ac fields H(ν) ) 0.1, 0.5, 2, 4, and 10 Oe (peak to peak) and frequencies ν ) 5, 25, 125, 500, and 1000 Hz. No relaxation effects were observed in the present experimental conditions down to 1.7 K. The data have been corrected for the diamagnetic contribution of χdia ) -0.133 × 10-3 emu/mol, calculated by Pascal’s method. Above 20 K, the ac susceptibility of p-CDTV follows the Curie-Weiss law with a positive Weiss constant of +3.0 ( 0.2 K, indicating dominant ferromagnetic exchange interaction between neighboring radical molecules. A plot of 1/χ against T is no longer linear at lower temperature ( 1.8 For lower temperatures (kBT/J < l), however, a depends on T (a f 4/5 for T f 0). In fact, above 5 K our data are well reproduced by eq 3 with J/kB ) +5.5 ( 0.5 K and g ) 2.00 as shown in Figure 3. At lower temperatures below 5 K, we have to consider the effects from the interchain interaction (J′) which induces the three-dimensional ordering at Tc ) 0.68 K. By the mean-field theory, the susceptibility of the quasi-one-dimensional (q-1D) system, χq-1D, is given by14-16
χq-1D ) χ1D/(1 - 2zJ′χ1D/Ng2µB2)
(4)
where χ1D is the susceptibility of the pure 1D Heisenberg model and z is the number of interchain bonds per spin. The threedimensional transition temperature Tc is approximated as the temperature at which the denominator of eq 4 becomes zero. In order to estimate J′ at low temperatures (kBT/J < 1), we need to use a more accurate χ1D than that obtained eq 3, which is valid for kBT/J > 1. The thermodynamical quantities, including χ1D, for the isotropic 1D Heisenberg ferromagnets with S ) 1/2 are studied on the basis of the modified spin wave theory and also of the Bethe-Ansats integral-equation method by Takahashi and Yamada (TY),17,18 where the susceptibility is expressed as
Magnetic Properties of p-CDTV and p-BDTV
J. Phys. Chem., Vol. 100, No. 23, 1996 9661
Figure 6. Inverse susceptibility of p-CDTV corrected for the isolated 1D system (open circle). Theoretical results for the isotropic 1D Heisenberg ferromagnet17,18 are drawn for J/kB ) 5.5 and 6.0 K.
Figure 7. Field dependence of magnetic heat capacity, Cm, of p-CDTV. The solid line is the results of theoretical calculation for the isotropic 1D Heisenberg ferromagnet with J/kB ) 6.0 K. TY
χ1D
{
( )
( ) ( ) ( )}
Ng2µB2 2J 2 2J 1.5 ) 0.1667 + 0.581 + 8J kBT kBT 2J 1 2J +O 0.68 kBT kBT
0.5
(5)
Using χ1DTY for χ1D in the denominator of eq 4, we can estimate the interchain interaction, zJ′/kB. The value obtained is zJ′/kB ) +0.21 ( 0.02 K (or zJ′/J ) 3.8 × 10-2). Using the value of zJ′ obtained, we can evaluate the experimental χ1D corrected for the isolated chains again from eq 4 by using the value of zJ′ and the observed value of χq-1D, as shown in Figure 6. Due to this correction, χ1D becomes smaller than χq-1D as T becomes lower; although the correction is less than 1% at 5 K (kBT/J ) 1), it amounts to more than 20% at 1.7 K (kBT/J < 0.3). As shown in Figure 6, the value of χ1D estimated from experimental value is well reproduced by the TY theory with g ) 2.00 and J/kB ) +6.0 ( 0.3 K. The low-temperature heat capacity, Cp, of p-CDTV radical was measured in the external fields of 0-30 kOe. The magnetic heat capacity, Cm, was obtained by subtracting the lattice contribution which was evaluated by referring to the lattice heat capacity of the p-BDTV radical crystal with correction for the difference in the mass of Br and Cl atoms.19,20 Cm is larger than the lattice contribution below 5 K in zero field. The field dependence of the heat capacity (Figure 7) is qualitatively similar to that of the 1D ferromagnetic Ising system, (CH3)3NHCoCl3‚2H2O.7 In zero field and in the paramagnetic state, Cm of p-CDTV keeps the values comparable to Cm(max) )
Figure 8. Magnetic heat capacities of p-CDTV in the external fields H ) 0 and 20 kOe. The results of theoretical calculation for the isotropic 1D Heisenberg ferromagnet are given for gµBH/J ) 0 (sH ) 0, J/kB ) 6.0 K) and gµBH/J ) 0.5 (‚‚‚ H ) 20.5 kOe, J/kB ) 5.5 K. s H ) 22.3 kOe, J/kB ) 6.0 K).
0.134R ) 1.12 J/(mol K) above 2 K, which is a characteristic feature of the isotropic 1D Heisenberg ferromagnet.8 This is ascertained by a theoretical calculation of the magnetic heat capacity of a 1D ferromagnetic Heisenberg chain with J/kB ) 6.0 and J′/kB ) 0. The calculated heat capacity is shown by the solid curve in Figures 7 and 8. There is a small discrepancy between the observed values of Cm and the theoretical values for J/kB ) 6.0 K and J′/kB ) 0 K. This may be explained by the entropy consumption due to the three-dimensional ordering at low temperatures. The critical entropy Sm(Tc) up to Tc is only 25% of the total entropy R ln2, as described later. Another evidence for the one-dimensional interaction in this p-CDTV is more clearly disclosed in the heat capacity curve in high magnetic fields as discussed in the following (see Figures 7 and 8). In Figure 7 is shown the temperature dependence of the magnetic heat capacity of the p-CDTV as a function of applied field. In the external field of 5 kOe, which is much larger than the interchain exchange field Hex ) (2zJ′〈S〉)/(gµB) ∼ 1.5 kOe, the sharp λ-like peak observed in zero field disappears completely and the broad hump (rounded peak) appears at around 1.8 K. The hump grows gradually and shifts to the higher temperatures as the external field is increased, as shown in Figure 7. The field dependence of Cm of this kind has not been reported because of the lack of real 1D Heisenberg ferromagnets with S ) 1/2. The theoretical calculation of the field dependence of the magnetic heat capacity, Cm, has been performed for the ideal 1D Heisenberg ferromagnet by Bonner11 for some discrete fields gµBH/J ) 0.5, 1.0, 1.5, and 2.0. In Figure 8, we compare the Cm of p-CDTV for H ) 20 kOe, which corresponds to gµBH/J ) 0.45, with that obtained by theoretical calculation (gµBH/J ) 0.5). It can be seen that the calculated curve reproduces satisfactorily the experimental curve (Figure 8). The temperature dependence of the magnetic entropy Sm(T) was obtained from the relation
Sm(T) ) ∫0 (Cm/T)dT T
(6)
The low-temperature part of Sm(T) is determined by extrapolating the experimental value of Cm down to 0 K with the threedimensional spin-wave approximation, namely, with a series expansion starting from T3/2 terms. At any rate, Sm(T0) at the lowest T0 for the present data points is fairly small for Sm(∞) ) R ln 2. About 80% of the magnetic entropy (R ln 2 for S ) 1/2) is consumed up to 10 K, and the remaining 20% is expected
9662 J. Phys. Chem., Vol. 100, No. 23, 1996
Mukai et al. temperature of Tc ) 0.68 K. Further, the p-CDTV behaves as a quasi-one-dimensional Heisenberg ferromagnet with the intrachain exchange interaction of 2J/kB ) +12.0 K above Tc. Therefore, X-ray structure analyses of p-BDTV and p-CDTV are an interesting method for clarifying how replacement of the bromo atom in p-BDTV by a chloro atom affects the crystal structure and thus the magnetic behavior of the radicals. However, to our regret, we have not succeeded in obtaining single crystals suitable for X-ray structure analyses to date. 1,5Dimethyl-6-thioxoverdazyl radicals show lower chemical and thermal stability in organic solvents than that of 1,3,5-triphenylverdazyl radicals, although these verdazyl radicals can be isolated as solvent-free pure radical solids.
Figure 9. Temperature dependence of the magnetic entropy, Sm, of p-CDTV.
at higher temperatures, as shown in Figure 9. This implies that there is one unpaired electron in each of the p-CDTV molecules in the crystal. The critical entropy Sm(Tc) up to Tc is about 25% of the total entropy R ln 2. This is another reflection of the low-dimensionality of the magnetic interaction in pCDTV: the value of Sm(Tc) is nearly 50% for the twodimensional localized spin systems, and in the usual threedimensional systems, Sm(Tc) is rather close to R ln 2.9,21 A one-dimensional (1D) magnet with localized spins is one of the simplest systems of many-body problems from which exact or correct physical quantities have been derived. For 1D Heisenberg antiferromagnets with S ) 1/2, the exact ground state energy, energy dispersion relation, and various thermodynamical quantities are obtained.22 The physical properties expected of these antiferromagnets have been experimentally checked with abundant real magnetic compounds. The magnetic properties of p-BDTV studied in the present work, belong to this group, and can be explained by the one-dimensional Heisenberg model with negative exchange interaction of 2J/kB ) -41.2 K. However, for 1D ferromagnetic systems, the situation rather differs from the case of antiferromagnets, because we can rarely get the 1D quantum ferromagnetic substances. Consequently, only a few examples of a 1D Heisenberg ferromagnet, such as (C6H11NH3)CuCl3 (CHAC),23 K2Cu0.59Zn0.41F4,24 and the γ-phase of p-NPNN,1b have been reported. As described above, the present experiments have given prototype thermodynamical behavior expected of the most isotropic Heisenberg ferromagnet in one dimension, including the magnetic susceptibility and the field dependence of magnetic heat capacity. 1,3,5-Triphenylverdazyl (TPV) radical is well-known as one of the stable organic free radicals. Consequently, the magnetic properties of TPV radical and its derivatives have been studied extensively, indicating the antiferromagnetic intermolecular interaction in these radicals, as observed for usual organic free radical solids.25-28 A positive Weiss constant (θ ) +1.6 K) was observed for one of the verdazyl radicals (1,5-diphenyl-3(4-nitrophenyl)-6-phenylverdazyl).29 However, the presence of antiferromagnetic coupling between ferromagnetic chains has been suggested, because the magnetic susceptibility, χM, exhibits a maximum at 2.15 K. Furthermore, X-ray structure analyses have been performed for these verdazyl radicals, in order to find a relation between crystal structure and observed spinspin exchange interactions, e.g., antiferromagnetic and ferromagnetic behavior at low temperature.27-30 As described above, p-BDTV behaves as a one-dimensional Heisenberg antiferromagnet with a negative exchange interaction of 2J/kB ) -41.2 K. The p-CDTV, on the other hand, has been found to be a new and bulk ferromagnet with a Curie
Acknowledgment. We are very grateful to Professor F. A. Neugebauer for his helpful discussions for the synthesis of p-CDTV and p-BDTV radicals. We are very grateful to Mr. Mitsuhide Matsubara for the synthesis of p-CDTV. This work was partly supported by the Grant-in Aid for Scientific Research on priority area Molecular Magnetism (Area No. 228/04242104) from the Ministry of Education, Science and Culture, Japan. Supporting Information Available: Characterization data for p-CDTV, p-BDTV, and the precursors of p-CDTV and p-BDTV; real and imaginary components (χ′ and χ′′) of the ac magnetic susceptibility of p-CDTV (Figure 10S) (4 pages). Ordering information is given on any current masthead page. References and Notes (1) (a) Tamura, M.; Nakazawa, Y.; Shiomi, D.; Nozawa, K.; Hosokoshi, Y.; Ishikawa, M.; Takahashi, M.; Kinoshita, M. Chem. Phys. Lett. 1991, 186, 401. (b) Nakazawa, Y.; Tamura, M.; Shirakawa, N.; Shiomi, D.; Takahashi, M.; Kinoshita, M.; Ishikawa, M. Phys. ReV. 1992, B46, 8906. (c) Kinoshita, M. Mol. Cryst. Liq. Cryst. 1993, 232, 1. (2) (a) Chiarelli, A.; Rassat, A.; Rey, P. J. Chem. Soc. Chem. Commun. 1992, 1081. (b) Chiarelli, R.; Novak, M. A.; Rassat, A.; Tholence, J. L. Nature 1993, 363, 147. (3) (a) Nogami, T.; Tomioka, K.; Ishida, T.; Yoshikawa, H.; Yasui, M.; Iwasaki, F.; Iwamura, H.; Takeda, N.; Ishikawa, M. Chem. Lett. 1994, 29. (b) Ishida, T.; Tsuboi, H.; Nogami, T.; Yoshikawa, H.; Yasui, M.; Iwasaki, F.; Iwamura, H.; Takeda, N.; Ishikawa, M. Chem. Lett. 1994, 919. (4) Sugawara, T.; Matsushita, M. M.; Izuoka, A.; Wada, N.; Takeda, N.; Ishikawa, M. J. Chem. Soc., Chem. Commun. 1994, 1723. (5) (a) Neugebauer, F. A.; Siegel, R. Angew. Chem., Int. Ed. Engl. 1983, 22, 320. (b) Neugebauer, F. A.; Fisher, H.; Siegel, R. Chem. Ber. 1988, 121, 815. (6) Mukai, K.; Nedachi, K.; Jamali, J. B.; Achiwa, N. Chem. Phys. Lett. 1993, 214, 559. (7) Takeda, K.; Wada, M. J. Phys. Soc. Jpn. 1981, 50, 3603. (8) Bonner, J. C.; Fisher, M. E. Phys. ReV. 1964, A315, 649. (9) Takeda, K.; Yoshino, Y.; Matsumoto, K.; Haseda, T. J. Phys. Soc. Jpn. 1980, 49, 162. (10) Blo¨te, H. W. J. Physica 1975, 79B, 427. (11) Bonner, J. C. Thesis, University of London, 1968. (12) Takeda, K.; Deguchi, H.; Hoshiko, T.; Konishi, K.; Takahashi, K.; Yamauchi, J. J. Phys. Soc. Jpn. 1989, 58, 3361. (13) Takahashi, M.; Turek, P.; Nakazawa, Y.; Tamura, M.; Nozawa, K.; Shiomi, D.; Ishikawa, M.; Kinoshita, M. Phys. ReV. Lett. 1991, 67, 746. (14) Watanabe, T. J. Phys. Soc. Jpn. 1962, 17, 1856. (15) McElearney, J. N.; Losee, D. B.; Merchant, S.; Carlin, R. L. Phys. ReV. 1973, B7, 3314. (16) Losee, D. B.; McElearney, J. N.; Shankle, G. E.; Carlin, R. L.; Cresswell, P. J.; Robinson, T. Phys. ReV. 1973, B8, 2185. (17) Takahashi, M. Phys. ReV. Lett. 1987, 58, 168. (18) Yamada, M.; Takahashi, M. J. Phys. Soc. Jpn. 1986, 55, 2024. (19) Takeda, K.; Uryu, N.; Ubukoshi, K.; Hirakawa, K. J. Phys. Soc. Jpn. 1986, 55, 727. (20) Bouvier, M.; Lethuillier, P.; Schmitt, D. Phys. ReV. 1991, B43, 13137. (21) de Jongh, L. J.; Miedema, A. R. AdV. Phys. 1974, 23, 135. (22) See, for example: Steiner, M.; Villan, J.; Windsor, C. G. AdV. Phys. 1976, 25, 87. (23) Haines, D. N.; Drumheller, J. E. Phys. ReV. Lett. 1987, 58, 2702. (24) Takeda, K.; Okuda, Y.; Yamada, T.; Haseda, T. J. Phys. Soc. Jpn. 1981, 50, 1917.
Magnetic Properties of p-CDTV and p-BDTV (25) (a) Mukai, K.; Oishi, K.; Ishizu, K.; Azuma, N. Chem. Phys. Lett. 1973, 23, 522. (b) Azuma, N.; Ohya-Nishiguchi, H.; Yamauchi, J.; Mukai, K.; Deguchi, Y. Bull. Chem. Soc. Jpn. 1974, 47, 2369. (c) Azuma, N.; Ishizu, K.; Mukai, K. J. Chem. Phys. 1974, 61, 2294. (26) (a) Kopf, P.; Morokuma, K.; Kreilick, R. J. Chem. Phys. 1971, 54, 105. (b) Brown, I. M.; Kreilick, R. W. J. Chem. Phys. 1975, 62, 1190. (27) Azuma, N. Bull. Chem. Soc. Jpn. 1982, 55, 1357. (28) Nakamura, T.; Awaga, K.; Inabe, T.; Matsumoto, M.; Maruyama, Y. Mol. Cryst. Liq. Cryst. 1993, 233, 105.
J. Phys. Chem., Vol. 100, No. 23, 1996 9663 (29) (a) Allemand, P.-M.; Srdanov, G.; Wudl, F. Synth. Met. 1991, 4143, 3245. (b) Allemand, P.-M.; Srdanov, G.; Wudl, F. J. Am. Chem. Soc. 1990, 112, 9391. (30) (a) Azuma, N.; Deguchi, Y.; Marumo, F.; Saito, Y. Bull. Chem. Soc. Jpn. 1975, 48, 819. (b) Azuma, N.; Deguchi, Y.; Marumo, F.; Saito, Y. Bull. Chem. Soc. Jpn. 1975, 48, 825. (c) Azuma, N.; Tsutsui, K.; Miura, Y.; Higuchi, T. Bull. Chem. Soc. Jpn. 1981, 54, 3274.
JP9536155
