Spectral behavior and pH dependence of N-hexadecyl-5-iminomethyl-8-hydroxyquinoline

Article abstract of DOI:10.1016/S1386-1425(99)00272-3

A new kind of amphiphile derived from 8-hydroxyquinoline, N-hexadecyl-5-iminomethyl-8-hydroxyquinoline (HIHQ) was synthesized and characterized by different physical methods. The influence of pH value on UV-vis absorption, fluorescence spectra were investigated and the surface pressure-area isotherms were recorded under different pH subphase. The linear response range between the emission intensity and the concentration of HIHQ was 1-4×10^-6 mol L^-1. The strongest emission of HIHQ about 460 nm was obtained at pH 8.2 and the acid-base equilibrium of HIHQ in monolayer and solution was also discussed.

Full text of DOI:10.1016/S1386-1425(99)00272-3

Spectrochimica Acta Part A 56 (2000) 1399–1407
www.elsevier.nl/locate/saa
Spectral behavior and pH dependence of
N-hexadecyl-5-iminomethyl-8-hydroxyquinoline
Chaoyang Jiang a, Weijiang He a, Zihou Tai a,*, Jianming Ouyang b
a State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, Nanjing Uni6ersity,
Nanjing 210093, People’s Republic of China
b Department of Chemistry, Jinan Uni6ersity, Guangzhou, 510632, People’s Republic China
Received 8 June 1999; received in revised form 3 November 1999; accepted 3 November 1999
Abstract
A new kind of amphiphile derived from 8-hydroxyquinoline, N-hexadecyl-5-iminomethyl-8-hydroxyquinoline
(HIHQ) was synthesized and characterized by different physical methods. The influence of pH value on UV-vis
absorption, fluorescence spectra were investigated and the surface pressure–area isotherms were recorded under
different pH subphase. The linear response range between the emission intensity and the concentration of HIHQ was
1–4×10−6 mol L−1. The strongest emission of HIHQ about 460 nm was obtained at pH 8.2 and the acid–base
equilibrium of HIHQ in monolayer and solution was also discussed. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Amphiphile; pH dependence; Langmuir–Blodgett films; UV-Vis absorption; Fluorescence spectra
droxyquinoline and its derivatives have a rigid
headgroup with nitrogen and oxygen atoms,
which can coordinate with many metal ions.
These amphiphilic complexes would be used in
electroluminescence devices, oriented-crystalliza-
tion and other practical applications [5–9]. In the
present work, we have designed and synthesized
an amphiphilic ligand, N-hexadecyl-5-imi-
nomethyl-8-hydroxyquinoline (HIHQ), and inves-
tigated the characteristics of HIHQ monolayer at
the air–water interface with different acidity in
the subphase. The UV-visible spectra and fluores-
cence spectra of HIHQ in ethanol-water with
different acidity were recorded and its acid–base
equilibrium was also discussed.
1. Introduction
Since 1937, when Langmuir first reported the
amphiphilic property of stearic acid at air–water
interface [1], many kind of materials, such as
dioctadecyldithiocarbamate, dialkyldimethylam-
monium and amphiphilic azobenzene, etc., have
been studied in both theoretical and practical
fields [2–4]. Recently, a great deal of novel com-
pounds derived from 8-hydroxyquinoline were
synthesized and used in this field. Usually 8-hy-
* Corresponding author. Tel.: +86-25-3314502; fax: +86-
25-3314502.
E-mail address: sklc@netra.nju.edu.cn (Z. Tai)
1386-1425/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(99)00272-3

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C. Jiang et al. / Spectrochimica Acta Part A 56 (2000) 1399–1407
2. Experimental
by addition of isopiestically distilled HClO4 or
NaOH solution. All work was carried out in a
dust-free box at 1491°C and the experiments
were repeated at least twice.
2.1. Materials
8-Hydroxyquinoline
(Shanghai
Chemical
Reagents) and hexadecylamine (Fluka Chemical)
were used without any further purification. All
solutions were purified by standard procedures.
Deionized purified water from a Milli-Q System
(Millipore) was used in all experiments.
2.4. UV-6isible spectra and fluorescence studies
The studies of UV-visible absorption spectra
and fluorescence spectra, were carried with an
ethanol-water (V/V=1:9) solution. The results
were obtained at room temperature and the pH
values of solution were adjusted as above men-
tioned. The fluorescence spectra were recorded
using the Perkin–Elmer Fluorescence Data Man-
ager software. Both excitation slit and emission
slit were kept at 2.5 nm. The scan speed was 400
nm min−1. The excitation monochromator was
chosen at 257 nm. A cutoff filter of 290 nm was
usually used in front of the emission monochro-
mator to remove the second order scattering.
2.2. Apparatus
C, H, N analyses were performed using a
1
Perkin-Elmer Model 240C elemental analyzer. H
NMR spectra were obtained using a Bruker Am-
500 NMR spectrometer. Infrared spectra were
recorded on a Nicolet Model 170 SX FTIR spec-
trometer and the UV-Vis spectra were measured
with a Shimadzu Model 3100 UV-VIS-NIR
recording spectrophotometer. Fluorescence spec-
tra were obtained from a LS 50B luminescence
spectrometer controlled by a personal computer.
The pH of solutions and subphases were deter-
mined by the pH sensor attached with the Lang-
muir trough.
3. Results and discussion
3.1. Synthesis and conformation of title
amphiphile
2.3. Measurement of surface pressure–area
The synthesis procedure for the amphiphilic
isotherms
ligand,
N-hexadecyl-5-iminomethyl-8-hydroxy-
quinoline (HIHQ) is illustrated in Scheme 1. This
route involves the preparation of 5-formyl-8-hy-
droxyquinoline [10], and its condensation reaction
with long chain alkylamine.
8-Hydroxyquinoline (10 g), ethanol (50 ml) and
aqueous sodium hydroxide (20 g in 25 ml of
water) were refluxed while chloroform (14 g) was
added dropwise during 50 min. After refluxing for
12 h, ethanol and excess chloroform were distilled
off, and the separated solid was dried and ex-
Measurement of surface pressure–area (y–A)
isotherms was carried out with a KSV5000 Lang-
muir trough system. Solution of HIHQ (ca. 1×
10−3 mol l−1 in chloroform) was added dropwise
to the clean subphase surface by syringe. After the
chloroform had evaporated for about 15 min, the
monolayer was compressed by two Teflon barriers
at 20 mm min−1, and y–A isotherm was recorded
automatically. Acidity of subphase were adjusted
Scheme 1. Synthesis procedure of the amphiphilic molecule HIHQ.

C. Jiang et al. / Spectrochimica Acta Part A 56 (2000) 1399–1407
1401
Fig. 1. Surface pressure–area (y–A) curves for HIHQ on aqueous subphases of different pH: a, 5.0; b, 6.0; c, 7.0; d, 8.3; e, 10.4.
tracted continuously with light petroleum (b.p.
60–90°C). The extract furnished 5-formyl-8-hy-
droxyquinoline (FHQ) as straw-coloured needles
(m.p. 177–178°C). Analysis calculated for
C10H7NO2: C, 69.36; H, 4.07; N, 8.09%. Found:
C, 69.13; H, 4.10; N, 7.86%. FTIR (KBr), w
(cm−1): 3188 (OH); 1663(CꢀO). UV-Vis, u (nm):
388; 330; 262; 241. 1H NMR (CDCl3): 10.14
(1/2H, OꢁH); 9.68 (1H, ArꢁH); 8.88 (1H, ArꢁH);
8.00 (1H, ArꢁH); 7.68 (1H, ArꢁH); 7.48 (1H,
HꢁCꢀNꢁ).
3.2. Formation of HIHQ monolayer on the
subphases of different pH 6alues
Fig.
1
shows the surface pressure–area
isotherms of monolayer of HIHQ on the subphase
of different pH values from 5.0 to 10.4. On a
subphase of pH 7.0, the monolayer collapses
when the surface pressure is increased to ca. 67
mN m−1. The y–A isotherm has no plateau and
only a condensed region appears. This indicates
that neither a phase change nor a change in
orientation of the head group fragment (quinoline
ring) of HIHQ occurs in this situation. According
to a space-filling molecular (CPK) model, the
8-hydroxyquinolinol ring can be approximated as
5-Formyl-8-hydroxyquinoline (0.34 g) and hex-
adecylamine (0.75 g) were added in 40 ml ethanol
and refluxed for about 10 min. Then solution was
kept below 0°C. After evaporating of the solvent,
yellow-green needles were obtained. The product
gave a satisfactory analysis (m.p. 76–77°C). Anal-
ysis calculated for C26H40N2O: C, 78.70; H, 10.11;
N, 7.03%. Found: C, 78.71; H, 10.11; N, 7.03%.
a rectangular block with dimensions of about
3
,
8.4×7.4×3.6 A . A limiting area of about 26.5
2
,
A per molecule of HIHQ was obtained by ex-
w
(cm−1): 3188 (OH); 2916,
trapolating the surface pressure to zero in the
FTIR (KBr),
compression region. Therefore, this value corre-
2849(CH2, CH3); 1646 (CꢀN); 786( (CH2)n). UV-
Vis, u (nm): 427, 336, 265, 246. 1H NMR
(CDCl3): 10.14 (1/3H, OꢁH); 9.73 (1H, ArꢁH);
8.80 (1H, Ar–H); 8.60 (1H, ArꢁH); 8.00 (1H,
ArꢁH); 7.67 (1H, ArꢁH); 7.54 (1H, ArꢁH); 3.65
(2H, NꢁCH2); 1.76 (4H, NꢁCꢁCH2ꢁCH2); 1.26
(24H, (CH2)n); 0.89 (3H, CH3).
2
,
sponds to the side area (7.4×3.6 A ) of the
8-hydroxyquinolinol ring of HIHQ.
The y–A isotherm of HIHQ depends remark-
ably on the pH value of the aqueous subphase.
The pH dependence of the amphiphile should be
attributed to the weakly acidic character of the

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C. Jiang et al. / Spectrochimica Acta Part A 56 (2000) 1399–1407
phenolic group and the weakly basic character of
the tertiary amine group. The addition of a small
account of HClO4 solution results in a great alter-
ation of the y–A isotherm. For example, at pH
6.0, the collapse pressure is decreased to about 20
mN m−1. Further decrease in the subphase pH
leads to further reduction of collapse pressure. At
pH 5.0, the y–A isotherm is a typical expanded
isotherm with a very low collapse pressure of 13
mN m−1. On the contrary, in basic subphases,
with increasing pH values, the isotherm of HIHQ
have a tendency to increasing the condensed
phase, but the collapse pressure does not obvi-
ously change.
This means that HIHQ can form stable mono-
layers on neutral and basic subphases, but on
acidic subphase the y–A isotherms are of the
liquid expanded type and HIHQ cannot form
stable monolayers. Usually, the imine group is
unstable in basic solution, but the imine group of
HIHQ confers an unusual stability on the mono-
layer on basic subphases, showing the difference
between the air–water interface and the bulk so-
lution. In 1951, Davies suggested that the surface
pressure could be conceived of as the sum of two
components, an electrostatic term and a nonelec-
trostatic term [11]. HIHQ exists as a deprotonated
species in basic subphases, which can form a
stable, diffuse electric double layer at air–water
interface with the counterion (Na+). Therefore,
the electric double layers with a net zero charge
reduce the electrostatic repulsion between the am-
phiphiles and facilitate the formation of a close-
packed condensed film [12]. The protonated
species cannot form a stable monolayer on acidic
subphase. Increase of acidity in subphase would
causes large change of collapse pressure. This may
be explained by the increase of solubility of
HIHQ, namely, the protonation of the nitrogen
atom in the quinoline, which prevents HIHQ
from forming a condensed film [13].
The collapse pressure (ymax) of an HIHQ
monolayer at different pH values of aqueous sub-
phases can be obtained from the y–A isotherms.
Plotting the collapse pressure as a function of pH
value of the subphase, (Fig. 2) shows the pro-
nounced dependence of ymax on the pH values in
the subphases. With increasing pH values, the
ymax increases gradually, then a steep increase is
observed at ca. pH 8.0. But the further increase of
the pH value leads to a small decrease of ymax
.
From this result, we can see that HIHQ can be
used as a material to form LB film only on the
subphases with pH greater than 5.5, in which an
HIHQ monolayer with a surface pressure greater
than 20 mN m−1 can be obtained.
As mentioned above, in the pH range of 5–10,
a small change of the pH values of the subphase
can cause a large modification of the properties of
the HIHQ monolayer. It can thus be deduced that
the association-dissociation of two coordination
groups N and OH of HIHQ with the H+ ion
occurs in the subphase pH range 5–10.
3.3. Influence of acidity on the absorption of
HIHQ solution
The UV-visible spectra of HIHQ in ethanol at
different medium are shown in Fig. 3. The most
important features are the presence of three bands
at ca. 255, 292 and 380 nm. They are assigned to
1
1
1
the Bb, Lb and La bands of the quinoline ring,
respectively, and are due to p“p* transitions.
Although the position of the 1Bb band hardly
1
changes on going from low to high pH, the La
1
and Lb bands are more sensitive both in terms of
frequency and intensity. For example, the absorp-
tion of the 1La band of HIHQ at various pH
values is quite different (see Fig. 4). The absorp-
tion maximum at 381 nm was bathochromically
Fig. 2. Plot of collapse pressure versus pH values of subphase.

C. Jiang et al. / Spectrochimica Acta Part A 56 (2000) 1399–1407
1403
Fig. 3. Absorption of HIHQ in acidic (A), basic (B) and neutral (N) ethanol-water solution.
Fig. 4. Absorption of the 1La band of HIHQ in different pH values.
shifted about 2.5 nm in the basic medium of pH
12.3, and the intensity of this peak is also raised
with the increasing of the pH. Nevertheless, in acid
medium, the p“p* band at 380 nm was decreased
greatly. According to Murrell’s opinion [14,15],
there is strong mixing of a charge-transfer transi-
tion, from the ꢁCH2ꢁ group to the ring, with the
1La transition. Similarly, in the case of the alkaline
media we can show empirically that the transition
labeled La also has considerable charge-transfer
character, and the charge transfer is from the ꢁOꢁ
to the heterocyclic, nitrogen-containing ring.
1

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C. Jiang et al. / Spectrochimica Acta Part A 56 (2000) 1399–1407
sponse range of the emission intensity versus the
concentration of HIHQ would be obtained when
the concentration is smaller than 4×10−6 mol
3.4. Fluorescence spectra of HIHQ solution at
different pH 6alues
The fluorescence emission spectra of HIHQ so-
lution are shown in Fig. 5. Detected at 460 nm, a
maximum excitation wavelength about 257 nm is
obtained and choosen to excite the sample in the
following experiments. Only a sign at ca. 460 nm
is investigated in the emission spectrum. Usually,
a high value of fluorescence efficiency is generally
associated with a relatively rigid structure [15].
Therefore, the emission of HIHQ is much greater
than that of 8-hydroxyquinoline in our experi-
ments. The concentration dependence on the
emission intensity of HIHQ is also examined and
shown in Fig. 5. From our repeated experiments,
we have seen that, as the concentration of HIHQ
increases, the fluorescence intensity versus concen-
tration becomes increasingly curved. However,
the emission band does not enlarge when the
dm−3
.
Fig. 6 shows the fluorescence emission spectra
of HIHQ at various pH values ranging from 2.6
to 12.2. Although the fluorescence peaks do not
change a lot in the HIHQ solutions with different
pH values, the intensity of fluorescence emission
band at ca. 460 nm changes remarkably. At pH
2.6, only a broad fluorescence emission about
400–500 nm is observed and the intensity of
emission is not enhanced greatly until the pH
value is higher than 5.3. A decrease of solution
acidity leads to an enhancement of the emission.
When the pH value is around 7, obvious changes
are not observed neither in shapes nor intensity.
On the other hand, the addition of the alkali also
results in the decrement of the emission band.
When the pH value is higher than 10.4, only a
weak band is observed.
concentration is higher than 4×10−6 mol dm−3
.
The main reason for the additional curvature,
observed at high concentration, is that each layer
of the sample absorbs some of the incident radia-
tion. Then we could deduce that the linear re-
A plot of the fluorescence peak area (fluores-
cence intensity) as a function on the pH values is
shown in Fig. 7. This figure shows that when a
HIHQ solution of pH 8.2 is examined, an emis-
Fig. 5. Fluorescence emission spectra of HIHQ in ethanol solution. Inset: emission intensity as a function of HIHQ concentration
in ethanol.

C. Jiang et al. / Spectrochimica Acta Part A 56 (2000) 1399–1407
1405
Fig. 6. Fluorescence emission spectra of HIHQ at various pH values.
Fig. 7. Plot of area of fluorescence peak versus pH values of the solution.
tion of the pH dependence of the fluorescence is
dependent upon the prototypic form initially pre-
sented. But according to the opinions of Goldman
and his co-workers [16], the neutral molecule is
the fluorescent species while the cation is a nonfl-
uorescent one. Perhaps their suggestion can be
used to explain our experiments.
sion band with very strong intensity is obtained at
the same experimental condition. Its strength is
about 15 times higher than that of pH 2.6 and the
emission intensity is diminished with the increas-
ing of the pH value. It means that the strongest
fluorescence emission is obtained when the pH
value is maintained at 8.2. In general, the explana-

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C. Jiang et al. / Spectrochimica Acta Part A 56 (2000) 1399–1407
Fig. 8. Plot of FWHM (nm) versus pH of the solution.
Another interesting result found in our experi-
Acknowledgements
ments is that the FWHM (Full width at half
maximum) of the fluorescence emission at 460nm
changes regularly with the change of the pH
value. As shown in Fig. 8, the pH dependence of
W1/2 corresponds to the following equation,
The authors would like to thank for the finan-
cial support from the National Natural Science
Foundation of China and the Natural Science
Foundation of Jiangsu Province of China.
X−2.74
/
1/2=62.03+29.77 e−
2.31
W
References
where W1/2 is FWHM of the emission band and X
is the pH value of the HIHQ solutions. This
phenomenon may be considered as the pH depen-
dence of the energy of excited singlet. Further
details will be taken up in the future.
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In this paper, a novel amphiphilic N-hexadecyl-
5-iminomethyl-8-hydroxy-quinoline was synthe-
sized and characterized by IR, UV-vis spectra and
1H NMR. The y–A isotherm of HIHQ depends
remarkably on the pH values of the aqueous
subphase. The fluorescence of HIHQ changes
greatly with different pH values and the strongest
emission band at 460 nm is obtained at pH 8.2.
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