Hydrophobicity-induced aggregation of N-alkyl-4-aminophthalimides in aqueous media probed by solvatochromic fluorescence

Article abstract of DOI:10.1039/a804631j

The fluorescence response of a series of n-alkanes of different chain length, all labelled with an environmentally sensitive fluorophore, is reported in aqueous media. It is shown that solvent-sensitive fluorescence properties of 4-aminophthalimide allow one to monitor subtle changes in the microenvironment of the fluorophore resulting from aggregation or self-coiling of the molecules. A new fluorescence that is observed in aqueous media for the higher homologues of the series has been unambiguously assigned to the aggregates of the molecules.

Full text of DOI:10.1039/a804631j

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Hydrophobicity-induced aggregation of N-alkyl-4-aminophthalimides
in aqueous media probed by solvatochromic Ñuorescence
G. Saroja and A. Samanta*
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India.
E-mail: assc=uohyd.ernet.in
Recei¿ed 18th June 1998, Accepted 12th August 1998
The Ñuorescence response of a series of n-alkanes of di†erent chain length, all labelled with an environmentally
sensitive Ñuorophore, is reported in aqueous media. It is shown that solvent-sensitive Ñuorescence properties of
4-aminophthalimide allow one to monitor subtle changes in the microenvironment of the Ñuorophore resulting
from aggregation or self-coiling of the molecules. A new Ñuorescence that is observed in aqueous media for the
higher homologues of the series has been unambiguously assigned to the aggregates of the molecules.
Hydrophobic interaction plays a crucial role in a number of
chemical phenomena such as the formation of micelles, lipid
bilayers, biological membranes, proteinÈamphiphile com-
plexes, and folding of globular proteins, and is one of the
basic forces responsible for hostÈguest interactions.1h7 It is
well known that in hydrophilic media, such as aqueous or
aqueousÈorganic binary solvents, hydrophobic interactions
force molecules with long hydrocarbon chains to form aggre-
gates even at low concentrations. Self-coiling can be regarded
as another simple way in which the hydrophobic forces are
manifested. While the aggregates represent intermolecular
clusters of neutral or charged species in equilibrium with the
monomers and with clusters of di†ering aggregate numbers, a
self-coiled species, on the other hand, represents a molecule
with coiled or folded Ñexible hydrocarbon chain.
Menger proposed the concept of intermolecular aggregation
to account for the rate of retardation of hydrolysis of certain
long-hydrocarbon chain esters.8 Knowles suggested that self-
coiling of long-chain molecules can also be the cause of such
rate retardation.9 Breslow and co-workers have used the
hydrophobic e†ect to accelerate reactions between lipophilic
compounds by placing them in water, thereby intentionally
bringing the reactants in close proximity.4 It has been re-
ported that self-coiling can be used to expedite the formation
of macrocyclic entities from molecules of Ñexible chains.10.11
A number of Ñuorescence studies have been carried out on
systems with long-chain hydrocarbon aggregates with a view
to understanding the molecular mobility, local polarity and
viscosity inside the aggregates.6,12h21 That the conformation
of a polymer chain can be a†ected by hydrophobic interaction
has been demonstrated by the observation of excimer emis-
sion in pyrene appended systems.17,18 Jiang et al. have shown
evidence of hydrophobicity-enforced chain folding and aggre-
gation by monitoring the excimer emission in naphthyl
appended systems.19 Using a Ñuorescence technique, Tung
and co-workers extensively studied several systems with long
polymethylene chains6,12h16 which not only demonstrate the
important role of the hydrophobic interactions in self-coiling
and aggregation but also highlight the potential of the Ñuores-
cence technique in probing these events. On the basis of the
dependence of the emission lifetime on the alkyl chain length,
intramolecular fold-back of the alkyl chain in a series of
[(bpy)Re(CO) NC(CH ) CH ]`
n \ 0È17) have been demonstrated by Demas et al.20,21
(bpy \ 2,2@-bipyridine,
3
2 n
3
During the course of our studies on Ñuorescent 4-
aminophthalimide (AP) derivatives22h27 it was observed that
the Ñuorescence decay of AP-labelled detergent25 (APL) in
aqueous medium was bi-exponential with lifetimes of 1 and 7
ns for the major and minor components, respectively. Inter-
estingly, in the absence of the fatty acid chain, the Ñuorescence
decay was found to be clearly single-exponential (q B 1 ns for
AP).24 Because APL is an amphiphilic molecule it is quite
f
likely that the long-lived lifetime component arises due to
inter- or intramolecular interactions in hydrophilic media with
hydrophobic interactions as the driving force. The results of
preliminary experiments, carried out on this system to Ðnd
out whether aggregation or self-coiling gives rise to the long-
lived component, were inconclusive.25 To have insight into
this problem, we have examined the Ñuorescence behaviour of
a series of n-(4-aminophthalimido)alkanes derivatives (APn) in
which an environmentally sensitive Ñuorophore, AP, is cova-
lently attached with alkyl groups of varying chain length. The
idea was that any small changes in the polarity of the sur-
rounding environment of the Ñuorophore resulting from
aggregation or self-coiling could be conveniently monitored
by highly solvatochromic Ñuorescence of the AP moiety.
While excimer Ñuorescence is often monitored to probe the
formation of aggregate or self-coiling,12h19 to the best of our
knowledge no studies have been attempted where the solva-
tochromic Ñuorescence response of a Ñuorophore has been
exclusively utilised to monitor these processes. Since aggre-
gation or self-coiling is favoured in polar media, the studies on
these systems have been carried out in pure methanol, H O
and binary mixtures of dimethylsulfoxide (DMSO)Èwater.
2
surfactant-active
complexes
of
the
form
J. Chem. Soc., Faraday T rans., 1998, 94, 3141È3145
3141

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Hitachi-3100 spectroÑuorimeter. The Ñuorescence decay
curves were recorded using a single-photon counting spectro-
Ñuorimeter (IBH, Model 5000U). The instrument was oper-
ated with a thyratron-gated Ñash lamp Ðlled with hydrogen at
a pressure of 0.5 atm. The lamp was operated at a frequency
of 40 kHz and the pulse-width of the lamp under the oper-
ating conditions used was B1.2 ns. The lifetimes were esti-
mated from the measured Ñuorescence decay curves and the
lamp proÐle using a nonlinear least-squares iterative Ðtting
procedure.
Experimental
Materials
4-Aminophthalimide, obtained from Kodak, was recrystallised
from an ethanolÈwater mixture before use. Sodium dodecyl
sulfate (SDS; Aldrich) was recrystallised twice from ethanolÈ
water mixture before the experiments. The Ñuorescent systems,
AP1ÈAP15 were prepared according to Scheme 1.
Equimolar mixtures of AP and the amine in dimethyl-
formamide (DMF) were heated between 80È110 ¡C for several
hours. Water was poured into the cooled reaction mixture and
the product was extracted with ethyl acetate. Removal of the
solvent followed by puriÐcation by column chromatography
(silica gel column, hexaneÈethyl acetate mixture as eluent)
yielded yellowish crystalline solids. The desired products were
analysed by IR and 1H NMR spectroscopy. Analytical data
for various derivatives are provided in Table 1. Details of the
preparation have been described elsewhere.28
Results and discussion
Spectral behaviour in aqueous media
Absorption spectra. The absorption spectra of the AP-
derivatives (AP1ÈAP15) consist of a structureless charge trans-
fer band as the lowest energy transition both in methanol and
in aqueous media. Some representative spectra of the com-
pounds in methanol and water are shown in Fig. 1(a) and 1(b),
respectively. As can be seen, in methanol, there is no notice-
able change in the position of the spectral maximum or the
bandwidth as n varied from 1 to 15 indicating that the
environment around the Ñuorophore remains the same for all
the systems. In aqueous medium, even though no signiÐcant
di†erence in the spectral behaviour of the compounds, AP1 to
AP7 could be observed, as illustrated in Fig. 1(b), broadening
of the spectra along with B2È5 nm redshift of the absorption
band could be observed in the case of the higher homologues,
AP11 and AP15.
Even though the solvents, DMF, DMSO and methanol,
were of spectral grade (obtained locally), extreme care was
taken to purify and dry them, particularly those used for spec-
troscopic measurements (methanol, water, DMSO) in view of
the solvatochromic nature of the spectral bands of the com-
pounds studied. Each solvent was puriÐed by following a stan-
dard procedure.29 The extent of dryness of each solvent was
checked by measuring the E (30) value of the solvents, using
Betaine dye.30
T
Instrumentation
Fluorescence and excitation spectra. The room temperature
Ñuorescence spectra of APn are broad and their locations are
governed by the polarity of the medium. This behaviour is in
agreement with the photophysical behaviour of AP.26 The
The IR and 1H NMR spectra for the characterisation of the
Ñuorescent systems were recorded on JASCO FTÈIR/5300
and Bruker ACF-200 (200 MHz) instruments, respectively.
The absorption spectra were recorded on a JASCO 7800 spec-
trophotometer. The Ñuorescence spectra were recorded on a
Ñuorescence spectra of the systems in methanol and H O are
2
shown in Fig. 2(a) and 2(b), respectively. The location of the
emission maximum (observed between 520 and 522 nm) and
the bandwidth (fwhm B2900 cm~1) remain more or less the
same in methanol as n varied from 1 to 15, indicating that
neither intra- nor intermolecular interactions are dominant in
this medium. In water, the Ñuorescence maxima for systems
AP1 to AP7 are Stokes-shifted relative to those in methanol
by 22È23 nm, which is in agreement with the solvatochromic
response of AP.26 No noticeable change in the location of the
emission maximum or the bandwidth could be observed
Scheme 1
Table 1 Analytical data of the N-alkyl-AP derivatives
System
AP1
M.p/¡C
IR (KBr)/cm~1
1H NMR (CdCl )(d)
Yield (%)
60
3
159È160
3452, 3356, 2939,
1759, 1689, 1612,
1502
1.2 (t, 3H); 3.7 (t, 2H); 6.8 (dd, 1H); 7.0 (d, 1H);
7.5 (d, 1H); 4.4 (s, 2H)
AP2
AP3
AP5
AP7
AP11
AP16
120È121
118È120
È
3479, 3373, 2955,
2872, 1749, 1685,
1630, 1612
3483, 3375, 2753,
1753, 1689, 1626,
1502
3440, 3368, 2930,
2858, 1759, 1699,
1616, 1502
3476, 3362, 2922,
2851, 1759, 1695,
1630, 1502
3487, 3379, 2918,
2849, 1763, 1689,
1614, 1494
3495, 3373, 2918,
2849, 1763, 1689,
1614, 1406
1.0 (t, 3H); 1.7 (m, 2H); 3.6 (t, 2H); 6.8 (dd, 1H);
7.0 (d, 1H); 7.6 (d, 1H); 4.3 (s, 2H)
65
80
55
70
75
65
0.9 (t, 3H); 1.4 (m, 2H); 1.6 (t, 2H); 3.7 (t, 2H);
6.8 (dd, 1H); 7.0 (d, 1H); 7.6 (d, 1H); 4.4 (s, 2H)
0.9 (t, 3H); 1.3 (m, 6H); 1.6 (t, 2H); 3.6 (t, 2H);
6.7 (dd, 1H); 7.0 (d, 1H); 7.6 (d, 1H); 4.3 (s, 2H)
52È55
72È74
82È84
0.8 (t, 3H); 1.3 (m, 10H); 1.6 (t, 2H); 3.7 (t, 2H);
6.8 (dd, 1H); 7.0 (d, 1H); 7.6 (d, 1H); 4.4 (s, 2H)
0.8 (t, 3H); 1.3 (m, 18H); 1.6 (t, 2H); 3.6 (t, 2H);
6.8 (dd, 1H); 7.0 (d, 1H); 7.6 (d, 1H); 4.4 (s, 2H)
0.9 (t, 3H); 1.3È1.6 (m, 28H); 3.6 (t, 2H); 6.8 (dd), 1H);
7.0 (d, 1H); 7.6 (d, 1H); 4.3 (s, 2H)
3142
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Fig. 1 Absorption spectra of AP-labelled derivatives in methanol
(a) (ÈÈ) AP3; (É É É É É) AP7; (È È È) AP11; (É È É È É) AP15, and in
aqueous solution (b) (ÈÈ) AP2; (É É É É É) AP3; (È È È) AP7; (É È É È É)
AP11; (ÉÉ È ÉÉ È ÉÉ) AP15.
Fig. 2 Fluorescence spectra of AP-labelled derivatives in methanol
(a) (ÈÈ) AP3; (É É É É É) AP7; (È È È) AP11; (É È É È É) AP15, and in
aqueous solution (b) (ÈÈ) AP2; (É É É É É) AP3; (È È È) AP7; (É È É È É)
AP11; (ÉÉ È ÉÉ È ÉÉ) AP15. j \ 370 nm.
exc
among di†erent compounds. This is in accordance with the
observation that the alkyl chain of these systems does not
inÑuence the photophysics of the AP moiety. Interestingly, in
the case of AP11 and AP15, the emission spectra are found to
be drastically di†erent [Fig. 2(b)] from those obtained for the
lower chain systems when the measurements are performed in
the same concentration range (B1 ] 10~5 M). While the Ñuo-
rescence maximum of AP and the lower homologues of APn is
observed at around 540È545 nm in water, for AP11 and AP15
the Ñuorescence spectra are signiÐcantly blue-shifted. The
spectral behaviour of AP11 ad AP15 remained the same when
the experiments were performed at the lowest permissible con-
centrations (B1 ] 10~6 M). The peculiar behaviour of the
higher homologues of APn can be due to intra- or intermo-
lecular interactions (or both) induced by the hydrophobic
forces of the long alkyl chains. Since the concentration range
at which the unusually blue-shifted Ñuorescence spectra have
been observed is rather low it may appear that the intramole-
cular interactions (presumably self-coiling) are responsible for
the observed e†ect rather than intermolecular aggregation.
However, one has to take note of the reports that the critical
aggregation concentration of the substrates of alkyl chain
length of 12 carbons or above can be as low as 10~7 M.8a,31
Therefore, the unusual spectral behaviour of AP11 and AP15
can be due to aggregation as well. Since aggregation is a func-
tion of the concentration of the probe molecule whereas self-
coiling is not, one can distinguish the two by studying the
changes in the spectral behaviour as a function of the concen-
tration of systems. However, as stated above, even when the
experiments were performed at 1 ] 10~6 M, the spectra of
AP11 and AP15 remained the same. Practical limitations did
not allow us to carry out experiments with concentrations
lower than this. This is owing to the low molar absorption
coefficient of these systems at the absorption maximum
(e B 3700 mol~1 dm3 cm~1) and to the low Ñuorescence
380
yield in aqueous medium. Hence, from the concentration
dependent experiments, it was not possible to Ðnd the origin
of the blue-shifted Ñuorescence of the two higher systems of
APn.
As an aggregate is likely to have an absorption spectrum
di†erent from that of the monomer, the excitation spectra of
the various systems in aqueous media have been carefully
measured, monitoring the respective Ñuorescence bands. Fig. 3
shows some representative spectra of the systems. As can be
seen, while there is hardly any variation in the position of the
spectral maxima of AP1ÈAP7, for AP11 and AP15 the excita-
tion maxima are signiÐcantly red-shifted.
It may be noted in this context that, unlike the Ñuorescence
spectrum, the absorption spectrum of AP is rather insensitive
to the polarity of the medium.26 In the case of a change in the
local environment of the Ñuorophore resulting from the fold-
back of the hydrocarbon chain, the microenvironment of the
Ñuorophore was expected to be less polar and, hence, a small
blue-shift of the absorption or the excitation spectrum was
expected.26 Obviously, a large red-shift of the excitation
maximum can not be accounted for in terms of the self-coiling
of the hydrocarbon chain around the Ñuorophore. This clearly
indicates that the blue-shifted Ñuorescence of AP11 and AP15
(for which the excitation maxima are red-shifted) originates
from intermolecular aggregates of the systems which are
formed at a very low concentration (below 1 ] 10~6 M). The
excitation spectral data show that the aggregates of AP11,
which emit at B480 nm, absorb at around 412 nm and the
same is true for aggregates of AP15, which emit at B470 nm
and absorb at B397 nm.
J. Chem. Soc., Faraday T rans., 1998, 94, 3141È3145
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Fig. 5 Fluorescence excitation spectra of AP11 in DMSOÈH O mix-
2
tures. The / values are (ÈÈ) 1; (È È È) 0.4. Spectra were obtained by
monitoring the wavelength corresponding to the maximum Ñuores-
cence intensity.
Fig. 3 Fluorescence excitation spectra of AP-labelled derivatives in
aqueous medium: (ÈÈ) AP2; (É É É É É) AP3; (È È È) AP7; (É È É È É) AP11;
(ÉÉ È ÉÉ È ÉÉ) AP15. Spectra were obtained by monitoring the respective
Ñuorescence maxima.
phobic interaction of the long alkyl chain give rise to the
unusual Ñuorescence of AP11 and AP15.
Spectral behaviour in water–organic binary mixtures
Having established that the unusual Ñuorescence and absorp-
tion of AP11 and AP15 in aqueous solution arise from aggre-
gates, further experiments have been carried out to
substantiate our conclusion. Fig. 4 shows the Ñuorescence
InÑuence of the surfactants on the spectral behaviour
To conÐrm the formation of the aggregates AP11 and AP15
in aqueous media, the e†ect of the surfactants on the Ñuores-
cence spectra of the systems has been studied. Since the micel-
les are capable of breaking the aggregates into monomers, if
the unusual Ñuorescence were due to the aggregates, in micel-
lar media, normal monomer Ñuorescence of the AP moiety
should appear with the disappearance of the unusual aggre-
gate Ñuorescence. Fig. 6 shows the e†ect of SDS, which readily
forms micelles, on the Ñuorescence spectra of AP11
(B3.5 ] 10~5 M) in aqueous media. It should be noted that
binding of AP with a relatively nonpolar micellar interface
spectra of dilute solutions of AP11 in DMSOÈH O mixtures.
2
In pure DMSO the Ñuorescence spectrum of the system is
quite similar to that of AP. However, with an increase in the
fraction (v/v) of water (/), a hypsochromic shift of the Ñuores-
cence maximum, indicating the onset of aggregation, could be
observed. At lower / values, most of the molecules are present
as monomers as the emission corresponds closely to that of
the Ñuorophore in pure organic medium. Had there been no
aggregation or self-coiling, the spectral maxima would have
been red-shifted with increasing / values, as water addition
increases the polarity of the medium. Both aggregation, as
stated earlier, and self-coiling that creates a relatively non-
polar environment as a result of the fold-back of the long
hydrocarbon chain onto the Ñuorophore are expected to give
rise to a blue-shift of the Ñuorescence maximum. One should
take note of the fact that, unlike the aggregates which absorb
at longer wavelengths in this case, a self-coiled AP Ñuoro-
phore would have had an absorption maximum at a shorter
wavelength. Since the excitation maximum corresponding to
the unusual blue-shifted Ñuorescence appears to be red-shifted
(Fig. 5), it is clear that the aggregates formed by the hydro-
leads to
a blue-shift of the Ñuorescence maxima.24,25
However, addition of SDS to an aqueous solution of AP11
leads to a red-shift of the maximum along with an enhance-
ment of the intensity of the Ñuorescence. This observation can
only be rationalised in terms of deaggregation of the clusters
of the host molecules by the surfactant molecules. With
increasing amounts of SDS, as more and more AP11 mol-
ecules are deaggregated, the emission approaches that of AP in
Fig. 6 Fluorescence spectra of AP11 in aqueous solution containing
various amount of SDS. SDS concentrations (mM) for the spectra
labelled 1È5 are 0, 3.6, 12, 22 ad 63, respectively. The probe to micelle
concentration ratios for the spectra labelled 3È5 are 1 : 2, 1 : 6 and
1 : 25, respectively.
Fig. 4 Fluorescence spectra of AP11 in DMSOÈH O mixtures. The
2
/ values for the spectra labelled 1 to 6 are 1, 0.80, 0.60, 0.50, 0.45 and
0.40, respectively. j \ 370 nm.
exc
3144
J. Chem. Soc., Faraday T rans., 1998, 94, 3141È3145

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Table 2 Fluorescence lifetimes of N-alkyl-AP derivatives in aqueous
aminophthalimide Ñuorophore in probing subtle changes in
its microenvironment resulting from inter- or intramolecular
interactions.
medium at room temperature
Probe
Concentration/lM
q /ns
q /ns
B a
1
2
1
The research described herein is supported by a grant from
the Department of Science and Technology, Government of
India. G.S. is thankful to the Council of ScientiÐc and Indus-
trial Research for a fellowship.
AP15
AP11
AP7
AP5
AP3
AP2
4.4
2.2
43
110
1.8
130
0.6
0.6
1.2
1.2
1.0
1.2
4.1
3.7
9.8
8.4
3.1
4.4
0.72b
0.61b
0.96
0.91
0.90
0.97
References
a Fluorescence decay curves were analysed using I(t) \ B e~t@q1
1
] B e~t@q2. b For AP15 and AP11 a third component could be
2
1
2
C. Tanford, T he Hydrophobic E†ects, Wiley, New York, 1980.
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of the exciting Ñash lamp was 1.2 ns, lifetimes lower than 1 ns indicate
only the trend.
3
W. Blokzijl and J. B. F. N. Engberts, Angew. Chem., Int. Ed.
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the same solvent. Since AP emits at a wavelength (540È545
nm) much longer than that observed for the aggregates, a red-
shift is observed on SDS addition. At a higher concentration
of SDS (63 mM), when deaggregation is more or less complete
(as the probe to micelle concentration ratio is closer to 1:25),
no further shift of the spectral maximum could be observed
with increasing concentration of the surfactant. The Ñuores-
cence enhancement at high concentration of SDS is presum-
ably due to the micellisation of AP11 monomers. It is inter-
esting to note that the position of the Ñuorescence maximum
of AP11 at the highest concentration of SDS is almost identi-
cal to that observed for AP in a micellar environment.24,25
7
8
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As stated earlier, the present investigation was undertaken
with a view to understanding the origin of an unusual long-
lived second component in the Ñuorescence decay of APL.
Since this component could only be observed for systems with
a long polymethylene chain, it was apparent that hydrophobic
interaction, induced by the long hydrocarbon chain is
responsible for the appearance of the second component.
However, it was not clear immediately whether intermolecular
interactions (leading to aggregation) or intramolecular inter-
actions (leading to self-coiling) give rise to the long-lived com-
ponent. The results carried out on APn clearly show that the
long-lived component has to be assigned to aggregates of the
probe molecules. The Ñuorescence decay curves of all the
systems have been measured in aqueous solution and analysed
using a bi-exponential decay function to obtain the best pos-
sible Ðts. The results are collected in Table 2. It can be seen
that all the systems contain a second component with a rela-
tively long lifetime. However, the relative weight of this com-
ponent to the total decay is relatively small for AP1ÈAP7.
Interestingly, as expected, the second component is present to
an appreciable extent for even fairly dilute aqueous solutions
of AP11 and AP15.
1381.
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Conclusions
30 C. Reichardt, Solvents and Solvent E†ects in Organic Chemistry,
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VCH, Weinheim, 1988, ch. 7.
31 J. P. Guthurie, Can. J. Chem., 1973, 51, 3494.
Paper 8/04631J
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3145