Polymeric ladderphanes

Article abstract of DOI:10.1021/ja9035362

A new class of polymers, which have a double-stranded polybinorbornene skeleton with multilayer planar oligoaryl linkers, defined as polymeric ladderphanes, are synthesized. The structures of these ladderphanes are determined by spectroscopic means. Photo

Full text of DOI:10.1021/ja9035362

Published on Web 08/12/2009
Polymeric Ladderphanes
Chih-Ming Chou,† Shern-Long Lee,† Chih-Hsien Chen,† Akkattu Thankappan Biju,†
Hsian-Wen Wang,† Yi-Lin Wu,† Guo-Fu Zhang,‡ Kuang-Wei Yang,†
Tsong-Shin Lim,§ Min-Jie Huang,† Po-Yu Tsai,† Kin-Chuan Lin,† Shou-Ling Huang,†
Chun-hsien Chen,*,† and Tien-Yau Luh*,†
Department of Chemistry, National Taiwan UniVersity, Taipei, Taiwan 106, Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences 354 Fenglin Lu, Shanghai 200032, China, and
Department of Physics, Tung Hai UniVersity, Taichung, Taiwan 407
Received May 1, 2009; E-mail: tyluh@ntu.edu.tw; chhchen@ntu.edu.tw
Abstract: A new class of polymers, which have a double-stranded polybinorbornene skeleton with multilayer
planar oligoaryl linkers, defined as polymeric ladderphanes, are synthesized. The structures of these
ladderphanes are determined by spectroscopic means. Photophysical studies and time-resolved fluores-
cence spectroscopic investigations reveal that there is a strong interaction between the chromophore linkers.
Thus, Soret band splitting in the absorption spectrum of the polymer with porphyrin linker (12e), significant
fluorescence quenching with oligoaryl linkers (12b-d), and excimer emission with a terphenylene-
diethynylene linker (12a) are characteristic photophysical properties of these polymers. Scanning tunneling
microscopy shows that polymers 12b and d exhibit a ladder-like morphology and form a supramolecular
assembly leading to a two-dimensional ordered array on a highly oriented pyrolytic graphite surface.
Introduction
A polymeric ladderphane is defined as multiple layers of
cyclophanes1,2 where the tethers are part of the polymeric
backbones (Figure 1a). Structurally, it can be considered as
a multiple stranded polymer with planar aromatic linkers. A
DNA molecule having a cofacial assembly of layers of base
pairs can be regarded as a special kind of ladderphane, and
electron hopping between these layers can readily take place.3
Indeed, interactions between two conjugated systems through
face-to-face π-π stacking have played an important role for
the extraordinary optoelectronic properties of aromatic
compounds in the solid state as well as in solution.4 Cofacial
arrangement of aromatic species (for example, pentacene)
† National Taiwan University.
‡ Shanghai Institute of Organic Chemistry.
§ Tung Hai University.
Figure 1. (a) Polymeric ladderphane; (b) assembly of ladderphanes to form
a two-dimensional array.
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devices.5 However, the orientation of conjugated molecules
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J. AM. CHEM. SOC. 2009, 131, 12579–12585 12579

A R T I C L E S
Chou et al.
Scheme 1a
Table 1. Photophysical Properties of Monomers 11 and Polymers
12
c
λem
f
g
λem
substrate Mn (PDI) λmaxa (ꢀ)b
Φfd
τ
(%wt)
11a
12a
1068 371 (31) 407, 428 0.82
46 000 371 (18) 407, 473 0.29
(1.7)
1020
50 (22%),
410-430
380-400
1060 (78%), 500-520
830
11b
12b
1006 379 (25) 449, 476 0.30
19 000 379 (22) 450, 480 0.10
(1.68)
1040 445 (87) 459, 488, 0.70
520
552
450-470
450-470
80 (78%),
550(22%)
12 400
11c
12c
11d
12d
450-470
27 000 445 (29) 459, 488, 0.11
140 (44%), 450-470
12 400 (56%)
(1.72)
520
1008 382 (53) 425, 451, 0.99
477
29 000 382 (37) 428, 454, 0.18
1080
440-460
130 (50%), 440-470
1010 (50%)
(1.74)
479
598
608
11e
12e
1304 421 (325)
31 000 422 (90)
(1.61)
0.04e
1520
590-610
0.01e 160 (70%), 590-610
1520 (30%)
a Absorption and fluorescence spectra were acquired at 10-5 M in
b
CH2Cl2 (λ in nm). Extinction coefficients (103 M-1 cm-1), based on
the molecular weight of the monomeric unit. c Excitation at λmax d The
.
quantum yield was determined in CH2Cl2 (a-d) by employing
coumarin-1 (Φf ) 0.99 in EtOAc) (ref 19) as the reference. e The
quantum yield was determined in CH2Cl2 by employing ZnTPP (Φf )
0.033 in toluene) (ref 20) as the reference. f Lifetime (ps) was measured
in CH2Cl2. g Monitored wavelength in time-resolved fluorescence
spectroscopy.
We recently reported that the ferrocene moieties in sym-
metrical or unsymmetrical double-stranded polybisnorbornenes
1 form a linear array with an Fe-Fe distance of ∼5.5 Å, the
spacing occupied by each of the monomeric units in these
polymers.7-12 Because of such a short distance, the ferrocene
moieties in 1 and related single- and double-stranded ferrocene
appended polynorbornenes may interact strongly with each other
as revealed by the electrochemical and magnetic studies.12 We
have established that ring-opening metathesis polymerizations
(ROMPs) of the endo-N-aryl-pyrrolidine-fused norbornene
derivatives catalyzed by Grubbs-I catalyst give the correspond-
ing polynorbornenes in homogeneous tacticity with all double
bonds in the trans configuration.7-12 Interactions between these
pending aryl groups might take place during the course of the
polymerization and would be responsible for the stereoselectivity
of the polymerization reaction. Indeed, we have recently shown
a Reaction conditions: (a) 4-bromobenzyl alcohol, Pd(PPh3)4, CuI, iPr2NH,
toluene, 80 °C, 84%; (b) POCl3, DMF, 90 °C, 69%; (c) 6, Pd(PPh3)2Cl2,
DMF, 100 °C, 43%; (d) NaBH4, MeOH, THF, 0 °C, 94%; (e) (i)
LiCt CCH2OTBS, (ii) SnCl2 ·2H2O /50%HOAc, (iii) TBAF, THF, rt, 39%.
constrict the charge mobility.6 Multilayered cyclophanes
offered an interesting model for efficient transannular
π-electronic interactions between aromatic rings.1,2 To the
best of our knowledge, polymeric ladderphanes using planar
aromatic moieties as linkers that are cofacially assembled in
an array are unprecedented.
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Polymeric Ladderphanes
A R T I C L E S
Figure 2. 13C NMR spectra of (a) 11d and (b) 12d.
that a replacement of the endoaryl moiety by a cyclohexyl group
(e.g., 2) results in an atactic polynorbornene with a mixture of
cis and trans double bonds.11 In addition, several single-stranded
polynorbornenes have recently been found to be useful for
replication reactions to give the corresponding complimented
polymers.9,13 This observation further indicates that all pending
groups in these single-stranded polynorbornenes should be
coherently aligned toward the same direction and the corre-
sponding double-stranded polymers will be formed in these
replication reactions.
oriented pyrolytic graphite (HOPG) to form an ordered two-
dimensional array.8,9 Presumably, π-π stacking among terminal
groups in the longitudinal direction and van der Waal interac-
tions between the polymeric backbones in the horizontal
dimension may account for this highly oriented morphology. It
is envisaged that the replacement of the ferrocene linker by an
aromatic species would lead to a polymeric ladderphane that
could serve as a molecular module to form a highly ordered
two-dimensional aggregate, such as the one illustrated in Figure
1b, that could be considered as cofacial packing of aromatic
moieties on a surface. We now wish to report the synthesis and
properties of a series of polymeric ladderphanes based on
double-stranded polybisnorbornenes using a range of different
planar aromatic linkers.
Results and Discussion
The syntheses of aromatic linkers 3a-c are summarized in
Scheme 1, and the details are described in the Experimental
Section in the Supporting Information. Linkers 3d14 and e15
were obtained according to literature procedures. The incorpora-
tion of alkyl substituents in these linkers would increase the
solubility of the corresponding polymers 12. Esterification of 3
with 10a under Mitsunobu conditions or with 10b in the
presence of Et3N afforded the corresponding diesters 11. ROMP
of 11 using Grubbs-I catalyst16 yielded the corresponding
polymeric ladderphanes 12. The Mn values and polydispersities
of 12 are summarized in Table 1.
Taking 12d as an example, the chemical shifts in 13C NMR
spectrum were assigned based on 13C-1H correlation spectra
of 11d and 12d (Figures S13b and S24b). The 13C NMR signal
for the olefinic carbons shifted from δ 135.8 ppm in 11d to δ
131.4 ppm in 12d (Figure 2).17 The 13C signals for each of the
monomeric units in 12d appear to be similar, if not identical.
As can be seen from Figure 2, the 13C NMR chemical shifts of
12d matched nicely with those of 11d.
Scanning tunneling microscopy (STM) investigations on
double-stranded polymers 1 and related polybisnorbornenes
indicate that these polymers can facilely self-assemble to a
submicrometer structure along the symmetric axes of highly
(13) (a) South, C. R.; Weck, M. Macromolecules 2007, 40, 1386. (b) Lo,
P.-K.; Sleiman, H. F. J. Am. Chem. Soc. 2009, 131, 4182.
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A R T I C L E S
Chou et al.
Figure 3. STM images of (a) 12b and (b) 12d on HOPG. Panels a2 and b2 were acquired by using an impedance smaller than that of panels a1 and b1 to
unveil the underlying HOPG. Imaging conditions of Ebias, itunneling, and image sizes: (a1) 0.90 V, 40 pA, 45 × 45 nm2; (a2) 0.10 V, 0.20 nA, 4 × 4 nm2; (a3)
0.90 V, 30 pA, 4 × 4 nm2; (b1) 0.50 V, 10 pA, 15 × 15 nm2; (b2) 0.10 V, 0.20 nA, 4 × 4 nm2; (b3) 0.50 V, 10 pA, 4 × 4 nm2.
In general, the cis olefinic protons of polynorbornene
appeared at higher field than those of the trans alkenyl protons
selective ROMP of bisnorbornenes 11 leading to double-
stranded polymers 12. Again, the branched structure for 12
could be ruled out.
11,18
1
in the H NMR spectra.
When polymers contain both
cis and trans double bonds, two signals are commonly
observed (e.g., 2).11,18 It is noteworthy that a symmetric broad
1
peak was observed for the olefinic protons of 12a-d in H
NMR spectra. This observation may imply that the double
bonds in these polymers would have the same configuration.
1
As can be seen from the H and 13C NMR spectra of 12
(Figures S17-S26), no residual olefinic proton and carbon
signals due to the unreacted norbornene moiety were
observed. These results indicate that all norbornene moieties
may have undergone an ROMP reaction under the polym-
erization conditions. If there was any branching in 12 (such
as 12′), the polymers would be expected to have a globular-
like three-dimensional structure, which was not consistent
with STM (scanning tunneling microscopy) results as de-
scribed later.
STM Imaging. Figure 3 shows STM images of 12b and d
dropcast on HOPG. Similar to the case for ferrocene-linked
double-stranded polybisnorbornenes, the films of 12b and d
formed an ordered two-dimensional assembly on HOPG due
to π-π attractions between end groups (vinyl and styryl) along
the longitudinal axis and van der Waals interactions between
the neighboring polymeric backbones along the second
dimension.8,9 For the assemblies of 12b and d, the nominal
widths of these polymers were ca. 3.2 and 3.5 nm, respectively,
which correlated quite well with the widths of the corresponding
monomeric units. The spacing between each of the linkers in
12b and d was ∼0.5 nm which was the typical spacing occupied
by each of the monomeric units.10a The bright features of the
STM images were attributed to the oligoaryl linkers of
thiophene-containing 12b and benzene-furan pentaaryl 12d. The
distance between the tips of the two terminal thiophene rings
in the linkers of 12b would be ∼1.4 nm21 which was consistent
with the width of the bright feature of the STM image of 12b.
(14) Lee, C.-F.; Liu, C.-Y.; Song, H.-C.; Luo, S.-J.; Tseng, J.-C.; Tso, H.-
H.; Luh, T.-Y. Chem. Commun. 2002, 23, 2824.
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(16) (a) Schwab, P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996,
118, 100. (b) Hamilton, J. G. In Handbook of Metathesis, Vol. 3;
Grubbs, R. H., Ed.; Wiley-VCH: Weinheim, 2003; p 143. (c) Slugovc,
C. Macromol. Rapid Commun. 2004, 25, 1283.
(17) The signal at δ 131.4 ppm in 12d is overlapping with absorptions
due to aromatic carbons ortho to the ester group as revealed by 13C-
1H correlation spectrum (Figure S24b).
Methanolysis of 12d with NaOMe afforded the corre-
sponding single-stranded isotactic 13 (Mn ) 6800, PDI )
1.1) and linker 3d in 83% and 85% yield, respectively.
Moreover, all double bonds in 13 were in the trans
configuration.10a,b The narrow dispersity of 13 and similar
degree of polymerization of 13 (25 monomeric units) and
12d (28 monomeric units) suggest that the polynorbornene
moieties in 12d should have a chain length similar to that of
13. These results provided additional evidence for the double-
stranded structure of 12d and solid support for the stereo-
(18) (a) Gilliom, L. R.; Grubbs, R. H. J. Am. Chem. Soc. 1986, 108, 733.
(b) Liaw, D.-J.; Tsai, J.-S.; Wu, P.-L. Macromolecules 2000, 33, 6925.
(c) Bielawski, C. W.; Benitez, D.; Morita, T.; Grubbs, R. H.
Macromolecules 2001, 34, 8610. (d) Nomura, K.; Sagara, A.; Imanishi,
Y. Macromolecules 2002, 35, 1583.
(19) (a) Lakowicz, J.-R. Principles of Fluorescence Spectroscopy, 2nd ed.;
Plenum Publishers: New York, 1999. (b) Fery-Forgues, S.; Lavabre,
D. J. Chem. Educ. 1999, 76, 1260. (c) Demas, J.-N.; Crosby, G.-A. J.
Phys. Chem. 1971, 75, 991.
(20) (a) Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.; Kearney, P. C.;
Marguerettaz, A. M. J. Org. Chem. 1987, 52, 827. (b) Lindsey, J. S.;
Wagner, R. W. J. Org. Chem. 1989, 54, 828.
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Polymeric Ladderphanes
A R T I C L E S
Figure 4. Absorption and emission profiles of 11 and 12 in CH2Cl2.
As such the linkers in 12b might orient toward a concave
upward conformation so that the butyl substituents may be
directed away from the graphite surface.22
The images of 12d were less self-explanatory. The separation
of the two bright features (dashed rectangle in panel b3 of Figure
3) was ∼1.4 nm, which might be attributed to the benzene
moieties at both ends of the oligoaryl linkers, while the dark
region in between would be the central furan-benzene-furan
module which might be away from the substrate and thus yield
a smaller tunneling probability.22,23
The X-ray structure of an alternating benzene-furan pentaaryl
anti-3d showed an S-shape anti-conformation having two
terminal benzene moieties (center-to-center) separated by ∼1.5
nm (Figure S35).14,24 The center-to-center distance between two
benzene rings in syn-3d could be estimated to be ∼1.3 nm which
(21) Melucci, M.; Favaretto, L.; Bettini, C.; Gazzano, M.; Camaioni, N.;
Maccagnani, P.; Ostoja, P.; Monari, M.; Barbarella, G. Chem.sEur.
J. 2007, 13, 10046.
(22) At this stage, the presence of possible anti-conformations for the linkers
used in polymers 12b and d could not be ruled out from the polymer
products. Presumably, the interaction of these oligoaryl linkers having
anti-conformation with the graphite substrate would be less favored
than those having syn-conformation with HOPG, because the alkyl
substituents in anti-conformation would be on both sides of the linkers.
(23) Samori, P.; Severin, N.; Simpson, C. D.; Mu¨llen, K.; Rabe, J. P. J. Am.
Chem. Soc. 2001, 123, 11462.
(24) Lee, C.-F. Ph.D. thesis, National Taiwan University, 2001.
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A R T I C L E S
Chou et al.
Scheme 2b
is comparable with the spacing between the two bright features
for each of the monomeric units of 12d in the STM images
(panels b1 and b3 of Figure 3).
Since the space between the linkers was ∼0.5 nm and the
linkers are connected to the polymeric backbones via single
bonds, rotation about these bonds might lead to multiple
conformations resulting in obscure images measured by STM.
By adjusting the tunneling conditions, panels a2 and b2 of
Figure 3 unveiled the unit cell vectors of the underlayer HOPG
and thus the uniaxial commensuration of the corrugations of
j
both 12b and d with the [1340] direction of the HOPG substrate,
demonstrating the contribution of substrate to the formation of
the array assembly along the longitudinal axis. Attempts to
obtain the similar STM images for other polymeric ladderphanes
(12a, c, and e) were unsuccessful. It is noteworthy that long
chain alkyl substitutents are present on both sides of the linkers
in these ladderphanes.22 It seems likely that interaction of these
aliphatic chains with the graphite surface might not be strong
enough to enable the formation of the assembled pattern as 12b
and d as shown in Figure 3.
Photophyscial Properties. The absorption profiles for 12a-d
were similar to those of the corresponding monomers 11a-d
(Figure 4 and Table 1). The profiles for these polymers were
essentially concentration independent. The extinction coefficients
for these monomers 11 were larger than those contributed by
each of the monomeric units of the corresponding polymers 12.
Because of the proximal distance between neighboring linkers
(∼0.5 nm) as described above, interactions between these
chromophores may lead to a decrease in extinction coefficients.
A similar observation was found in single-stranded polynor-
bornenes with coherently aligned pending chromophores.10,11
It is striking to note that the Soret band for 12e split into two
bands at 407 and 421 nm, whereas there was only one band at
421 nm for 11e (Figure 4e).25 A similar observation was found
in single-stranded polynorbornenes having the same porphyrin
b Reaction conditions: (a) 3a or 3c, and 10a, PPh3, DIAD, THF, rt (11a:
63%, 11c: 21%); (b) 3b or 3d or 3e, and 10b, DMAP, Et3N, CH2Cl2, rt
(11b: 38%, 11d: 87%, 11f: 38%); (c) ZnOAc, MeOH, CH2Cl2, rt, 94%; (d)
(Cy3P)2Cl2Ru)CHPh, CH2Cl2, rt (12a: 87%, 12b: 87%, 12c: 81%, 12d:
64%, 12e: 80%).
chromophore 14 coherently aligned as pendants.11 These results
suggest that there might be exciton coupling between neighbor-
ing porphyrin linkers in 12e, even though the spacings occupied
by each of the monomeric units were ∼5.5 Å.11 It is known
that intramolecular interactions between two porphyrin moieties
leading to the splitting of the Soret band may depend on the
distance and the dihedral angle between these two chromo-
phores.11,26,27 Cofacial alignment of porphyrin chromophores
in oligomers26 and polymers27 may occasionally facilitate such
exciton coupling. In general, the exciton interaction between
two porphyrins is relatively weak when the face-to-face distance
(25) The absorbances of 11e were linearly related with concentration
(10-6-10-5 M) and profiles remained unchanged, indicating that there
would be no aggregation under these conditions (Figures S33, S34).
These results suggest that the Soret band splitting in 12e might have
arisen from the locally high concentration due to the proximal distance
between the porphyrin chromophores.
(26) For porphyrin containing oligomers, see: (a) Osuka, A.; Maruyama,
K. J. Am. Chem. Soc. 1988, 110, 4454. (b) Deng, Y.; Chang, C. K.;
Nocera, D. G. Angew. Chem., Int. Ed. 2000, 39, 1006. (c) Osuka, A.;
Liu, B.-l.; Maruyama, K. J. Org. Chem. 1993, 58, 3582. (d) Cho, S.;
Yoon, M.-C.; Kim, C. H.; Aratani, N.; Mori, G.; Joo, T.; Osuka, A.;
Kim, D. J. Phys. Chem. C 2007, 111, 14881. (e) Nagata, T.; Osuka,
A.; Maruyama, K. J. Am. Chem. Soc. 1990, 112, 3054.
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Polymeric Ladderphanes
A R T I C L E S
of porphyrins in cofacial bisporphyrins was more than 0.5 nm.28
A slight flexibility in the linker in 12e might bring the
neighboring porphyrin moieties closer leading to exciton
coupling.11,29
porphyrin pending groups in single-stranded polynorbornenes.12
These results are consistent with the double-stranded structure
having all neighboring linkers in close proximity.
As shown in Figure 4a, the emission of 12a appears at a
longer wavelength (473 nm) than those of 11a (407, 428 nm).
The lifetime of emission at 473 nm was 830 ps owing to the
possible formation of an excimer-like species (Figure 4a). It is
known that the orientation of participating chromophores would
play an important role in the excimer formation.30 The
conjugated linker in 12a is essentially linear, and the neighboring
chromophores in 12a might be nearly in an eclipsed relationship.
No excimer-like emission was, however, observed for 12b-e.
Conclusions
As shown in Figure 4 and Table 1, the intensities of the
fluorescence spectra and the quantum yields (Φf) of 12 were
much weaker than those of the corresponding monomers 11.
Except for 12a, the emission profiles of 12b-e are essentially
similar to those of the corresponding monomers 11b-e. Self-
quenching might take place in polymers 12, where intrachain
interaction between chromophores might occur. These results
are consistent with the double-stranded nature of 12.
Time-Resolved Experiments. Time-resolved fluorescence
decays for both monomers 11 and polymers 12 in CH2Cl2 were
measured, and the results are also outlined in Table 1. Two-
exponential fittings were used to obtain the fluorescence
lifetimes (Figures S28-32). In addition to the longer lifetimes
(τ) for 12b-e which were comparable to those of 11b-e, the
shorter lifetimes were attributed to the self-quenching between
neighboring chromophores. It is interesting to note that the
lifetimes and their relative weights for 12e were, for example,
comparable with those of 14 which have coherently aligned
In summary, we have demonstrated a new class of polymers
which have a double-stranded polybisnorbornene skeleton with
multilayer planar oligoaryl linkers and are named as “polymeric
ladderphanes”. Because of the ladder-like structure, all linkers
are coherently aligned perpendicular to the longitudinal axis of
the polymer. Strong interactions between these chromophore
linkers have led to distinct photophysical characteristics. It is
particularly noteworthy that these polymers can easily self-
assemble to form an ordered pattern as shown in Figure 3. The
use of this strategy for possible materials exploitation is in
progress.
Acknowledgment. We thank the National Science Council and
the National Taiwan University for support. T.Y.L. and G.F.Z. thank
the Shanghai Institute of Organic Chemistry for support. In memory
of the late Professor Nien-chu C. Yang.
Supporting Information Available: Experimental Section, 1H
and 13C NMR spectra of all new compounds, time-resolved
fluorescence decay profiles of 11 and 12, and the X-ray data of
3d. This material is available free of charge via the Internet at
http://pubs.acs.org.
(27) For porphyrin containing polymers, see: (a) Ogawa, K.; Kobuke, Y.
Angew. Chem., Int. Ed. 2000, 39, 4070. (b) Takei, F.; Onitsuka, K.;
Kobayashi, N.; Takahashi, S. J. Polym. Sci., Part A: Polym. Chem.
2006, 44, 585. (c) Fletcher, J. T.; Therien, M. J. J. Am. Chem. Soc.
2002, 124, 4298. (d) Takei, F.; Onitsuka, K.; Kobayashi, N.; Takahashi,
S. Chem. Lett. 2000, 914. (e) Takei, F.; Hayashi, H.; Onitsuka, K.;
Kobayashi, N.; Takahashi, S. Angew. Chem., Int. Ed. 2001, 40, 4092.
(f) de Witte, P. A. J.; Castriciano, M.; Cornelissen, J. J. L. M.; Scolaro,
L. M.; Nolte, R. J. M.; Rowan, A. E. Chem.sEur. J. 2003, 9, 1775.
(g) Tabei, J.; Shiotsuki, M.; Sanda, F.; Masuda, T. Macromolecules
2005, 38, 9448.
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(30) (a) Sen, S.; Gra¨slund, A. Eur. Biophys. J. 1995, 23, 399. (b) Ikeda,
T.; Kurihara, S.; Tazuket, S. J. Phys. Chem. 1990, 94, 6550. (c)
Berberan-Santos, M. N.; Canceill, J.; Brochon, J.-C.; Jullien, L.; Lehn,
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(28) Faure, S.; Stern, C.; Guilard, R.; Harvey, P. D. J. Am. Chem. Soc.
2004, 126, 1253.
(29) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965,
11, 371.
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