Effects of diamine bridge length and substituents on the spectral properties of N,N′-bis(α-substituted salicylidene)diamines in solution

Article abstract of DOI:10.1039/a807902a

Absorption and fluorescence spectra of thirteen N,N′-bis(α-substituted salicylidene)diamines in solution were investigated with the intention of investigating the role of the substituent and diamine bridge length on their optical properties. The fluorescence efficiency was improved by an increase in the electron-donating property of the substituents on the azomethine carbon accompanied by an increase of the n→π* transition absorption. However, the effect did not occur for the substituents on the azomethine nitrogen, in which no drastic changes in fluorescence efficiency could be observed. Through the investigation of the diamine bridge length effects, it was found that a diamine Schiff base seems to form neither an inter- nor an intramolecular dimer with any peculiar fluorescence in the solution even if it has a long methylene bridge. It was also suggested that the diamine Schiff base has a third fluorescence species in the excited state, which might be a pre-keto form, the existence of which is strongly affected by the hydrogen bond strength between the hydroxy and azomethine groups.

Full text of DOI:10.1039/a807902a

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Effects of diamine bridge length and substituents on the spectral
properties of N,NЈ-bis(á-substituted salicylidene)diamines in
solution
Toshio Kawasaki,a Toshihide Kamata,b Hirobumi Ushijima,b Mitsuhiro Kanakubo,b
Shigeo Murata,b Fujio Mizukami,*b Yuki Fujiia and Yoshiharu Usuia
a Graduate School of Science and Engineering, Ibaraki University, 2-1-1 Bunkyo, Mito,
Ibaraki 310-8512, Japan
b National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba,
Ibaraki 305-8565, Japan
Received (in Cambridge) 12th October 1998, Accepted 30th November 1998
Absorption and fluorescence spectra of thirteen N,NЈ-bis(α-substituted salicylidene)diamines in solution were
investigated with the intention of investigating the role of the substituent and diamine bridge length on their optical
properties. The fluorescence efficiency was improved by an increase in the electron-donating property of the sub-
stituents on the azomethine carbon accompanied by an increase of the n→π* transition absorption. However, the
effect did not occur for the substituents on the azomethine nitrogen, in which no drastic changes in fluorescence
efficiency could be observed. Through the investigation of the diamine bridge length effects, it was found that
a diamine Schiff base seems to form neither an inter- nor an intramolecular dimer with any peculiar fluorescence in
the solution even if it has a long methylene bridge. It was also suggested that the diamine Schiff base has a third
fluorescence species in the excited state, which might be a pre-keto form, the existence of which is strongly affected
by the hydrogen bond strength between the hydroxy and azomethine groups.
In a previous study, we have found that N,NЈ-bis(salicylidene)-
Introduction
butane-1,4-diamine and its analogues showed very strong fluor-
escence in the solid state under ultraviolet irradiation.6 In par-
ticular, their fluorescence intensities were extremely influenced
by the length of the bridge between the two chromophores. The
relationship between the bridge length and fluorescence inten-
sity was quite unusual and seemed to have no special rules.
Thus, it is of interest to investigate the origin of fluorescence for
diamine Schiff bases. In this work, we synthesized thirteen
N,NЈ-bis(α-substituted salicylidene)diamines and examined
their absorption, excitation and fluorescence properties in solu-
tion with the purpose of obtaining information about the influ-
ence of bridge length on their optical properties. The solvent
and substituent effects on the fluorescence intensity were also
investigated to obtain fundamental information about the
factors responsible for their optical properties.
Diamine Schiff bases such as N,NЈ-bis(salicylidene)ethylene-
diamine are quite familiar as tetradentate ligands in metal
complexes.1–3 They are different from monoamine Schiff bases
in having two chromophores bridged by a methylene chain in a
molecule, and thus the mutual interactions between the chromo-
phores appears to affect their chemical and physical properties.
It has been often reported that monoamine Schiff bases show
strong fluorescence through the proton transfer between the
hydroxy and azomethine groups in the chromophore.4,5 There-
fore, interesting optical properties derived from the interaction
between the two chromophores in the diamine Schiff bases
could be expected.
R
HO
(CH2)2
Results and discussion
Effect of the solvents
N
N
R
OH
1a
2
R = H
The chemical structures of the thirteen N,NЈ-bis(α-substituted
salicylidene)diamines synthesized are shown in Fig. 1. There
are two series for the compounds; one is for the investigation
of the substituent effects (1a, 2–4) and the other is for the
investigation of diamine bridge length effects (1a–1j). Their
optical properties were examined in solution.
R = CH3
3
R = C2H5
R = C6H5
4
HO
(CH2)n
N
N
Fig. 2 shows the absorption, fluorescence and excitation
spectra of N,NЈ-bis(salicylidene)butane-1,4-diamine (1c) in
chloroform. Three major bands appeared at 255, 320 and 410
nm in the absorption spectrum (solid line). According to the
literature7 these bands can be assigned as follows. The absorp-
tion band at 255 nm is most probably related to the π→π*
transition of the aromatic chromophore. The band at 320 nm
is assigned to the π→π* transition of the azomethine group.
The band at 410 nm is the n→π* transition involving the pro-
OH
1a
1b
1c
1d
1e
n = 2
n = 3
n = 4
n = 5
n = 6
1f
n = 7
1g n = 8
1h n = 9
1i
1j
n = 10
n = 12
Fig. 1 Molecular structure of N,NЈ-bis(α-substituted salicylidene)-
ethylenediamines and N,NЈ-bis(salicylidene)diamines.
J. Chem. Soc., Perkin Trans. 2, 1999, 193–198
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proton transfer
S1
S'1
S'0
S0
R
R
N
N
H
H
Fig.
2
Absorption (solid line), fluorescence (dotted line) and
O
O
excitation (broken line) spectra of 1c at a concentration of 1 × 10Ϫ4 mol
dmϪ3 in chloroform. Fluorescence spectrum was observed by 410 nm
excitation.
enol form
keto form
Scheme 1 Excited-state intramolecular proton transfer (ESIPT).
motion of a lone pair electron on the nitrogen atom to the
antibonding π-orbital associated with the azomethine group.
The fluorescence spectrum (dotted line) of 1c in chloroform,
which was obtained by 410 nm excitation, shows a broad and
strong peak at 510 nm. No phosphorescence was observed for
the present system. A weak shoulder around 460 nm can be
attributed to the Raman scattering of chloroform because peak
shifts without drastic intensity changes could be observed for
the band by changing the excitation wavelength.8 The observed
fluorescence peak wavelengths were not altered by varying the
excitation wavelength between 255 and 410 nm although their
intensities changed.
The excitation spectrum of 1c in chloroform probed at
510 nm (broken line) shows peaks at 320 and 410 nm. A
sharp peak at 440 nm originates in the Raman scattering. The
observed spectral pattern corresponds well to that of the
absorption spectrum.
In order to examine the existence of an intermolecular
interaction between the chromophores in solution, the concen-
tration dependence of the spectra of 1c was investigated
between 1 × 10Ϫ6 and 8 × 10Ϫ4 mol dmϪ3 in chloroform. It is
expected that this concentration range would not cause any
serious changes in the molecular structure and conformation of
the Schiff base itself. Although prepared samples were highly
soluble in the organic solvents to concentrations of higher
than 1 × 10Ϫ2 mol dmϪ3 the concentration dependence of
the absorption and fluorescence spectra was examined up to
1 × 10Ϫ2 mol dmϪ3. At 8 × 10Ϫ4 mol dmϪ3, the fluorescence
intensity starts to decrease because of the inner filter effect
under the present experimental conditions. In the whole
concentration region mentioned above, neither shifts of absorp-
tion and fluorescence maxima nor new peaks were observed.
Furthermore, the molar absorptivity was almost constant for
all absorption bands. These facts indicate that extensive
aggregation of the Schiff bases does not occur in the ground or
in the excited state at least in the concentration range of
1 × 10Ϫ6 to 8 × 10Ϫ4 mol dmϪ3. Therefore, all spectral measure-
ments were made at a concentration of 1 × 10Ϫ4 mol dmϪ3.
It has been known that proton transfer between the hydroxy
and azomethine groups in the Schiff base is dominant, giving
strong fluorescence (Scheme 1).9–11 Thus, the polarity of the
solvent must have a serious effect on the optical properties.
Here, the effects of solvents on the absorption, fluorescence and
excitation spectra of the Schiff base were examined.
Fig. 3 (a) Absorption, (b) fluorescence and (c) excitation spectra
of 1c at a concentration of 1 × 10Ϫ4 mol dmϪ3 in several solvents.
i, Cyclohexane; ii, 1,4-dioxane; iii, dichloromethane; iv, chloroform;
v, acetonitrile; vi, propan-2-ol; vii, methanol. Excitation wavelength
for the fluorescence spectra was 320 nm for the cyclohexane solution,
and 410 nm for other solutions.
Fig. 3(a) shows the absorption spectra of 1c in several solv-
ents. It is found that the appearance of the absorption bands is
influenced by the solvent polarity. For instance, only the π→π*
transition bands were observed at 255 and 320 nm in cyclo-
hexane (— - - — - line). Cyclohexane is a non-polar solvent, thus
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barely interacts with either the azomethine or the hydroxy
groups of the Schiff base, resulting in a weak intramolecular
hydrogen bond being formed between the hydroxy and azome-
thine groups of the Schiff base. This may be related to the result
that the n→π* transition band could not be observed in cyclo-
hexane solution. On the other hand, not only the π→π* transi-
tion bands but also the n→π* transition band (410 nm) could
be observed in polar solvents such as chloroform. The n→π*
transition band increased its intensity and was slightly blue
shifted with an increase in the polarity of the solvents, although
the π→π* transition bands were almost independent of the
solvent’s polarity. Furthermore, a new band appeared at 280
nm in alcohol, which had the highest polarity of the solvents
used. This new band is probably associated with the phenolic
chromophore, because the intramolecular hydrogen bond is
usually broken in the alcohol solution.7
Fig. 3(b) shows the fluorescence spectra of 1c obtained by
the excitation wavelength of 410 nm in several solvents except
for cyclohexane. Light absorption of 1c at 410 nm in cyclo-
hexane was extremely small, thus the 320 nm excitation wave-
length was used for the fluorescence measurements of the
cyclohexane solution. A broad fluorescence peak appeared at
ca. 510 nm in the various solutions, except for alcohol. The
alcohol solution brought about much stronger fluorescence
than the other solutions, but the fluorescence peak appeared at
450 nm. Polar solvents tended to give stronger fluorescence
than non-polar solvents. The spectral patterns were indepen-
dent of the excitation wavelength of 255, 320 or 410 nm for all
samples.
Fig. 3(c) shows the excitation spectra of 1c in several solvents
at each fluorescence maximum. The spectral patterns in the
aprotic solvents were similar to those of the corresponding
absorption spectra. On the other hand, the spectral pattern in
the alcohol solution was quite different from those in the other
solvents; the excitation peaks appeared at 280 and 350 nm.
While the 280 nm band is compatible with an absorption band
due to the phenolic chromophore, the 350 nm band is in conflict
with the absorption spectra.
Fig. 4 Fluorescence intensities normalized with the absorptivity vs.
the oriented polarizability f (ε, n) of several solvents. (a) 320 nm
excitation, (b) 410 nm excitation. Triangle: cyclohexane, square: 1,4-
dioxane, star: dichloromethane, circle: chloroform, pentagon: aceto-
nitrile, inverted triangle: propan-2-ol, diamond: methanol. f (ε, n) =
n2 Ϫ 1
ε Ϫ 1
2ε ϩ 1 2n2 ϩ 1
Fluorescence intensity compared with 1c in chloroform.
ͩ
Ϫ
ͪ
, ε = relative permittivity, n = refractive index.
molar absorptivities which vary depending on the polarity of
the solvent.
If the solvent can compete with the hydrogen atom of the
hydroxy group in forming a hydrogen bond with the nitrogen
lone pair, it would be expected that the intramolecular hydrogen
bond tends to be weakened in polar solvents such as alcohol.
Previously, Charette et al. have observed 280 and 350 nm bands
in the absorption spectra of the N-salicylidenepropan-2-amine
in acid solution, and suggested that these originated from its
protonated form.12 Sharm et al. reported that the acid dissoci-
ation constant pKa of a Schiff base was 15.4 in the excited
state and 8.51 in the ground state, indicating that Schiff bases
could easily accept protic solvents in the excited state.13 From
these facts, it can be considered that the Schiff base in a protic
solvent such as alcohol becomes solvated, and the structure or
conformation around the azomethine groups might cause a
big change by the strong interaction between the azomethine
nitrogen and alcohol molecule. The fluorescence species for the
excited state in alcohol seems to be different from that in an
aprotic solvent.
From the observed results, it can be considered that the
solvents that can strongly interact with the azomethine or
hydroxy group bring about strong fluorescence of the com-
pound. Fig. 4 shows normalized fluorescence intensity with the
molar absorptivity plotted against the orientation polarizability
f (ε, n) of the solvent.14–16 As is shown in Fig. 4, fluorescence
efficiency is not dependent on the polarity of the solvent except
for alcohol. Alcohol gave high fluorescence efficiency only
in the case of 320 nm excitation. However, as is described
above, protonation can occur in protic solvents and induce
changes of fluorescence species in the excited states. Thus, it
is not appropriate to compare the fluorescence efficiency in
alcohol with other solutions. Consequently, it can be said that
the fluorescence intensity of 1c is simply dependent upon the
Effect of the substituent on the azomethine carbon
As the transition bands for the azomethine group seem to be
mainly responsible for the fluorescence, a change in the electron
density of the azomethine group by several substituents
must bring about certain changes in both the absorption and
fluorescence. Thus, the substituent effect of the carbon of the
azomethine group upon the absorption and fluorescence
spectra was examined here.
Fig. 5(a) shows the absorption spectra of N,NЈ-bis(α-
substituted salicylidene)ethylenediamine in chloroform. Three
absorption bands of the π→π* (255 and 320 nm) and n→π*
(410 nm) transitions appeared in the respective spectra. While
the peak wavelengths and intensities of the π→π* transition
bands were almost independent of the substituents, the
n→π* transition was sensitive to the substituents. The molar
absorptivities of the n→π* transition band of 2, 3 and 4 were
larger than that of 1a.
Fig. 5(b) shows the fluorescence spectra of N,NЈ-bis(α-
substituted salicylidene)ethylenediamine in chloroform. The
peak at 460 nm is due to Raman scattering of chloroform as
mentioned above.8 2, 3 and 4 showed a fluorescence peak at
510 nm which was independent of the excitation wavelength.
On the other hand, 1a gave a fluorescence peak at 510 nm in
the case of 255 or 320 nm excitation, but at 450 nm by 410 nm
excitation for which the spectral pattern was similar to that of
1c in the alcohol solution described in the previous section. The
excitation spectrum of 1a at 450 nm was also similar to that of
1c in the alcohol solution, namely peaks appeared at 280 and
350 nm.
The lone pair electrons on the nitrogen atom can form a
J. Chem. Soc., Perkin Trans. 2, 1999, 193–198
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Fig. 6 Relationships between fluorescence intensity and the chemical
shift of the hydroxy group (δOH). ᭺ is the case of the 320 nm excitation,
ᮀ is the 410 nm excitation. Fluorescence intensity was normalized by
the molar absorptivity. Fluorescence intensity compared with 4.
been reported that the fluorescence peak of 2-(2Ј-hydroxy-
phenyl)benzothiazole appears at 410 nm for its enol form
1
and at 510 nm for its keto form.18 According to the H NMR
measurement, the intramolecular hydrogen bonding ability of
1a was weaker than other Schiff bases. As is mentioned above,
1a and 1c in the alcohol solution give similar fluorescence
spectra with a 450 nm peak and excitation spectra with 280
and 350 nm peaks. In both cases, proton transfer from the
enol to the keto form is more difficult than for the other
compounds. However, it was reported that the enol form of
the Schiff base shows only very weak fluorescence,4,9,10 and the
observed 450 nm fluorescence for 1a is also different from
the reported 410 nm fluorescence for the enol form. Thus, it can
be considered that there is a third fluorescence species in the
excited state, which is neither the usual keto form nor the
enol form but is probably a pre-keto form suggested by Barbara
et al.19
Fig. 5 (a) Absorption and (b) fluorescence spectra of N,NЈ-bis-
(α-substituted salicylidene)ethylenediamine at a concentration of 1 ×
10Ϫ4 mol dmϪ3 in chloroform. Fluorescence spectra were observed by
410 nm excitation.
hydrogen bond with the hydroxy group on the benzene ring in
the Schiff base molecule (see Scheme 1), thus the hydrogen
bond should be affected by the substituents. This effect was
examined through 1H NMR measurements. The chemical shifts
of hydroxy protons, δOH, were observed at 13.2 ppm for 1a,
15.9 ppm for 2, 16.1 ppm for 3 and 15.2 ppm for 4. Previously, it
has been reported that δOH of 2-hydroxyacetophenone, which
had a carbonyl group at the ortho-position to the hydroxy
group, is 12.05 ppm, much larger than that of phenol, 7.54
ppm.17 This deshielding shift is attributable to the formation of
an intramolecular hydrogen bond. Considering this result, we
expect that δOH can be a good measure of the intramolecular
hydrogen bond in the Schiff base. The obtained δOH values for
the compounds indicate that the hydrogen bond strength is in
the order of 1a < 4 < 2 < 3.
The fluorescence intensity of these compounds increases as
the molar absorptivity increases, as can be seen in Fig. 5. In
order to examine the efficiency of the fluorescence, relative
fluorescence intensities normalized by the molar absorptivity
were plotted against the δOH value, see Fig. 6. It is found that
not only the fluorescence intensity increases but the fluores-
cence efficiency is also improved for 2, 3 and 4 as the δOH value
shifts to lower field, that is, as the hydrogen bond strengthens. It
has been reported that the keto form fluoresces strongly but the
enol form fluoresces weakly.9–11 Then, if the keto form is more
stabilized than the enol form, especially at the transition state,
the Schiff bases would strongly fluoresce. As the electron
density on the nitrogen atoms is increased by the substituent
electron-donating groups, the population of the keto form at
the transition state would increase, resulting in an improvement
of the fluorescence efficiency of the fluorescence.
Effect of the diamine bridge length
As reported in the previous paper, unusual fluorescence
behavior depending on the diamine bridge length could be
observed in the solid state.6 It seemed to be due to structural
effects of the Schiff bases such as inter- or intramolecular inter-
action between chromophores. In order to investigate the
effects, a series of Schiff bases with several bridge lengths were
prepared, and their absorption and fluorescence spectra in the
chloroform solution were investigated.
Fig. 7(a) shows the absorption spectra of diamine Schiff
bases with several bridge lengths. Three absorption bands of
the π→π* (255 and 320 nm) and n→π* (410 nm) transitions
appeared in each spectrum. The absorption peak wavelengths
are almost the same for all the samples used here. While the
peak intensities of the π→π* transition bands are almost
independent of the length of diamine bridge, that of the n→π*
transition band becomes stronger as the bridge length becomes
longer.
Fig. 7(b) shows the fluorescence spectra of diamine Schiff
bases with several bridge lengths, excited at 410 nm. The
fluorescence peak was observed at ca. 510 nm except for 1a. As
is described above, 1a shows a fluorescence peak at 450 nm by
410 nm excitation, and seems to be an exceptional case among
the series. Fluorescence intensities also increase as the bridge
length becomes longer.
Considering the fact that absorption and fluorescence peak
wavelength were almost independent of the diamine bridge
length, the possibility that the fluorescence results from the
intramolecular dimer between the chromophores can be
On the other hand, 1a gave a different fluorescence spectrum,
and thus seems to be an exceptional case in the series. It has
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Fig. 8 Relationship between the fluorescence intensity and number of
methylene unit of diamine bridge. ᭺ is the case of the 320 nm excita-
tion, ᮀ is the 410 nm excitation. Fluorescence intensity was normalized
by the molar absorptivity. Fluorescence intensity compared with 1c.
gated. Their optical properties were considerably influenced by
the polarity of solvents and substituents on the chromophore.
It was shown that the diamine Schiff bases in a protic solvent,
such as alcohol, were in a solvated form to give different excit-
ation bands and fluorescence spectra. While substituted groups
on the azomethine carbon influenced the fluorescence efficiency
of the fluorescence in the solution, those on the azomethine
nitrogen did not have any serious effects. Although details
about the phenomena are still not clear, it seems to be related
to a transition process of excited species concerning the azo-
methine group. On examining the optical properties of diamine
Schiff bases with several bridge lengths in solution, neither
inter- nor intramolecular interaction could be observed, despite
the diamine bridge being up to twelve methylene units long. It
seems to be difficult to form a stacking structure between two
chromophores for diamine Schiff bases. From these results, it is
suggested that there are at least three fluorescence species of
diamine Schiff bases in the excited state: enol, keto and possibly
pre-keto forms, of which the lifetimes are closely related to the
electron density of the azomethine nitrogen.
Fig.
7
(a) Absorption and (b) fluorescence spectra of N,NЈ-
bis(salicylidene)diamines with several bridge lengths at a concentration
of 1 × 10Ϫ4 mol dmϪ3 in chloroform. Fluorescence spectra were
observed by 410 nm excitation.
excluded. The bridge length of 12 methylene units seems to
be long enough to form an intramolecular dimer between the
chromophores by the alkyl chain twisting if desirable. There-
fore, it can be concluded that diamine Schiff bases do not form
an intramolecular dimer in the solution even though they have a
long methylene bridge.
These obtained results are not directly related to the unique
fluorescence phenomena of solid state diamine Schiff bases
observed in our previous study, but seem to be helpful in under-
standing it, giving fundamental information about their optical
properties. Further investigations into the relationship between
the fluorescence properties and crystal structures or the life-
times of excited species are necessary to fully understand the
fluorescence phenomena.
In order to investigate the fluorescence efficiency of fluor-
escence, the relative fluorescence intensity normalized with the
molar absorptivity was plotted against the diamine bridge
length for various compounds in Fig. 8. As is shown in the
figure, the fluorescence efficiency of the fluorescence seems to
be almost independent of the diamine bridge length for both
320 and 410 nm excitation. It has been known that the basicity
of diamines increases with an increase in the number of
methylene units between the two amino groups.20 Then, at
least the methylene units should behave as an electron donating
substituent on the azomethine group. Actually, the effect seems
to be reflected in the absorption spectra as the change of
absorptivity of the n→π* transition band. The longer diamine
bridge gave the higher electron density on the nitrogen atom,
resulting in an increase of the molar absorptivity of the n→π*
transition band due to the substituent effect on the azomethine
nitrogen. However, the fluorescence efficiency of the fluores-
cence was not influenced by the diamine bridge length. This is
contrary to the results of the substituent effect on the azome-
thine carbon. Namely, it is found that there is an essential
difference in the substituent effect of the azomethine carbon
and nitrogen upon the fluorescence although the absorption
spectra are not so much influenced by them.
Experimental
Materials
Thirteen N,NЈ-bis(α-substituted salicylidene)diamines were syn-
thesized using the following procedure. N,NЈ-bis(salicylidene)-
butane-1,4-diamine (1c) was synthesized in the following
manner. 0.4 mol of salicylaldehyde in 60 cm3 of hot ethanol was
added to 0.2 mol of butane-1,4-diamine in 50 cm3 of ethanol.
The solution was then stirred for 30 min at 60 ЊC (water bath),
and the yellow microcrystals produced were cooled to room
temperature. Recrystallization from ethanol gave yellow plate-
like crystals. Other diamine Schiff bases were also synthesized
in a similar manner, and recrystallized from ethanol or chloro-
form to give yellow crystals.
Measurements
Conclusion
The UV-VIS absorption spectra of diamine Schiff bases were
recorded on a SHIMADZU UV-3100 spectrophotometer in
the wavelength range of 200–1000 nm. Weighed samples were
Absorption and fluorescence spectra of two series of N,NЈ-
bis(α-substituted salicylidene)diamines in solution were investi-
J. Chem. Soc., Perkin Trans. 2, 1999, 193–198
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dissolved in chloroform, methanol, propan-2-ol, acetonitrile,
dichloromethane, 1,4-dioxane, and cyclohexane which were
distilled before use. Fluorescence spectra of diamine Schiff base
compounds were measured with a 1 cm3 quartz cell using
a HITACHI Model 850 fluorescence spectrophotometer at
25 ЊC. Fluorescence was monitored at a 90Њ angle to the ex-
citation light. The fluorescence and excitation spectra were not
corrected for spectral response. This does not cause serious
errors, because the change of fluorescence spectra of the
diamine Schiff bases is small for different chain lengths. The
relative fluorescence efficiency is defined as the relative fluor-
escence intensity divided by the absorbance of the sample at
the excitation wavelength. In the study of the substituent effect,
4 was selected as the fluorescence efficiency standard with unit
fluorescence efficiency, while in the study of the effect of
diamine bridge length, 1c was selected as a standard. 1H NMR
spectra were taken on a BRUKER AMX-500 spectrometer
operating at 500 MHz, using the sample dissolved in CDCl3 in
the presence of tetramethylsilane as the internal reference.
17 R. M. Silverstain, G. C. Bassler and T. C. Morrill, Spectrometric
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Acknowledgements
This work was partly supported by scientific Research Grant-
Aid No. 09440224 from the Ministry of Education, Science and
Culture.
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