Cystine-derived bis-naphthalimides as stimuli-responsive fluorescent gelators

Article abstract of DOI:10.1039/c4nj01755b

Two cystine-derived bis-naphthalimide gelators (L1, L2) were synthesised and characterised. Both L1 and L2 exhibited similar absorptions and emission spectra in solvents such as acetonitrile and DMF. The fluorescence spectra of both the compounds featured a distinct monomer and long-wavelength excimer emissions in the aforementioned solvents. It was found that the excimer emissions for the two compounds could be preferentially quenched by triethylamine, and subsequently restored with hydrofluoric acid. The stimuli-responsive nature of the excimer emissions was demonstrated using anion stimuli in solution and in the gel phase. Thus, the excimer emission for L1 (or L2) could be switched 'off' using fluoride anions, and subsequently re-activated using tetrafluoroborate anions as the chemical stimulus.

Full text of DOI:10.1039/c4nj01755b

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Cystine-derived bis-naphthalimides as
stimuli-responsive fluorescent gelators†
Cite this:DOI: 10.1039/c4nj01755b
Rupam J. Sarma* and Kakali Devi
Two cystine-derived bis-naphthalimide gelators (L1, L2) were synthesised and characterised. Both L1
and L2 exhibited similar absorptions and emission spectra in solvents such as acetonitrile and DMF. The
fluorescence spectra of both the compounds featured a distinct monomer and long-wavelength
excimer emissions in the aforementioned solvents. It was found that the excimer emissions for the
two compounds could be preferentially quenched by triethylamine, and subsequently restored with
hydrofluoric acid. The stimuli-responsive nature of the excimer emissions was demonstrated using anion
stimuli in solution and in the gel phase. Thus, the excimer emission for L1 (or L2) could be switched
‘off’ using fluoride anions, and subsequently re-activated using tetrafluoroborate anions as the
chemical stimulus.
Received (in Porto Alegre, Brazil)
8th October 2014,
Accepted 17th March 2015
DOI: 10.1039/c4nj01755b
www.rsc.org/njc
ions and self-aggregation.11,19 For instance, bis-naphthalimide
ligands that exhibit an unusual enhancement in the intra-
1. Introduction
The versatile luminescent characteristics of naphthalimide molecular excimer fluorescence upon metal ion coordination10,12
derivatives have evoked immense interest, which is evidenced could be developed as potential sensors for Cu2+, Hg2+ and Zn2+.
by the frequent use of this fluorophore in optoelectronic
Because naphthalimide emissions are also environment-
materials,1–4 as ionophoric sensors/probes5–9 and in stimuli- sensitive, this fluorophore has been utilised as luminescent
responsive systems.10,11 Typically, a stimuli-responsive system turn-on probes for studying ligand–cation interactions22,23 and
should be able to recognize an external stimulus (e.g. physical or in bio-sensing.24–26 However, compared to metal ions, the inter-
chemical input), evaluate the inputs, and then respond through actions of bis-naphthalimides with halide anions, particularly
changes in its physical and/or chemical characteristics.2,10,11
Among the various stimuli-responsive systems identified to excimer emissions remain relatively elusive.27–29
date, the importance of low molecular-weight gelators has grown An important question in this regard is the examination
fluoride, and the effect of such interactions on their monomer/
significantly owing to their potential applications as functional of how the bis-naphthalimide motif would respond to anionic
materials.12,13 In addition to being thermo-sensitive, these stimuli (e.g. fluoride anions), in solution and in the gel phase,
materials exhibit dynamic self-assembling characteristics, and whether these interactions could result in perceptible
which could be triggered by solvents,14 light,15 sound,16 pH17 changes in their fluorescent profiles. For this purpose, we syn-
and ionic species.18 An important challenge in this regard has thesized and characterised two L-cystine-based bis-naphthalimides,
been the development of stimuli-responsive gelators, which are L1 and L2 (Scheme 1) and then evaluated their fluorescent
capable of reversibly interacting with cations and anions, with responses with regard to acid–base and anion stimuli.
concomitant fluorescence switching.13,19
The presence of two naphthalimide motifs in close proximity
(i.e. bis-naphthalimides) can generate monomer and excimer
emissions, depending on the solvent, molecular environment
and distance separating the fluorophores.20,21 In addition to these
factors, the monomer/excimer emissions of bis-naphthalimide
derivatives can also be modulated by the coordination of metal
Department of Chemistry, Gauhati University, Guwahati, 781014, Assam, India.
E-mail: rjs@gauhati.ac.in; Fax: +91 36125 70535; Tel: +91 98641 28514
† Electronic supplementary information (ESI) available: Details of synthesis,
characterisation; SEM images for L1; 1H NMR studies of L1 with NaBF4;
fluorescence titration plots of L1 and chloride, bromide and acetate. See DOI:
10.1039/c4nj01755b
Scheme 1 Synthesis of bis-naphthalimides L1 and L2.
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1
2. Experimental
2.1 Materials and methods
Yield 74%; H NMR (300 MHz, DMSO-d6/CDCl3) d 8.3 (d, 2H),
8.0 (d, 2H), 7.5–7.4 (t, 2H), 4.0–3.9 (t, 2H);13C NMR (75 MHz,
DMSO-d6/CDCl3) 174.53, 163.74, 133.75, 131.20, 130.80, 127.69,
126.62, 122.12, 40.51, 40.23, 39.95, 39.67, 39.39, 39.22, 39.11,
38.83, 31.44, 23.15.
All the chemicals were commercially available from Sigma-Aldrich
or Spectrochem (India) and were used as received. Solvents for
spectroscopic experiments were distilled under nitrogen atmo-
sphere before use. All the 1H and 13C NMR spectra were obtained
on a 300 MHz Bruker spectrometer, and reported in d per ppm.
The electronic absorption spectra were recorded on a Shimadzu
UV-vis spectrophotometer (Model UV-1800), and fluorescence
spectra were recorded using a Hitachi F2500 fluorimeter.
Bis-naphthalimide, L2: as described for L1. The recrystal-
lised product was obtained as a pale yellow solid. Yield 75%.
1H NMR (300 MHz, CDCl3) dH 8.54 (4H, d, J = 7.2 Hz), 8.15 (4H,
d, J = 7.5 Hz), 7.71 (4H, t, J = 7.5 Hz), 7.06 (2H, amide NH), 4.90
(2H, m, J = 4.2 Hz), 4.25 (2H, m, J = 4.5 Hz), 3.75 (3H, s), 3.24
(4H, d, J = 5.1 Hz), 2.39 (4H, t, J = 4.8 Hz), 2.14 (4H, m), 1.65
(residual H2O). 13C NMR (75 MHz, CDCl3): 172.33, 171.0,
164.31, 133.97, 131.43, 131.31, 128.0, 126.89, 122.36, 77.41,
2.2 Synthetic procedures
Synthesis of ND1 and bis-naphthalimide L1. ND1: naphthalene- 52.69, 51.72, 40.57, 39.50, 33.66, 24.14. FT-IR (cmÀ1): 3310
1,8-dicarboanhydride (2.011 g, 10 mmol) was mixed with b-alanine (amide NH), 1740 (ester CQO), 1693 (amide CQO), 1657,
(1.063 g, 10 mmol) in a round bottom flask equipped with a 1587; ES-MS: m/z 821.85, calc. for (M + Na+)
magnetic needle. To this mixture, DMF (10 mL) was added and
Gel formation with L1 and L2. In a typical gelation experiment,
the resulting mixture was allowed to stir at 80 1C for about 12 h. a known amount of the bis-naphthalimide gelator (3 mg mLÀ1 for
Initially, the colour of the mixture was brown, and after 12 h, a L1 and 5 mg mLÀ1 for L2) and solvent (1.00 mL) were mixed in a
homogeneous red solution was obtained. Subsequently, the screw-capped glass vial and maintained at 80 1C for 2 h. In the
reaction mixture was concentrated and cooled to 0 1C, which process a clear solution was obtained, which upon cooling to
produced ND1 as pale yellow crystals ( yield 69%). 1H NMR room temperature over a period of time (B2 h) led to gel
(300 MHz, DMSO-d6/CDCl3) dH 8.48 (d, 2H, J = 6.9 Hz), 8.16 formation. Moreover, the gelation of L2 in chloroform, aceto-
(d, 2H, J = 6.9 Hz), 7.67 (t, 2H, J = 7.5 Hz), 4.36 (t, 2H, J = 7.2 Hz), nitrile, DMF and DMSO could be triggered by ultra-sonic irradia-
2.6 (t, 2H); 13C NMR (75 MHz, DMSO-d6/CDCl3) d 173.20, 163.81, tion for 2–3 minutes. The stability of the organogel was tested
133.89, 131.42, 131.07, 127.98, 126.75, 122.35, 35.98, 32.32.
Bis-naphthalimide, L1: to a solution of ND1 (0.540 g,
2.0 mmol) in dichloromethane (10 mL), N-(3-dimethylamino-
propyl)-ethylcarbodiimide hydrochloride (EDCÁHCl) (0.400 g,
2.1 mmol) and 1-hydroxybenzotriazole (HOBT) (0.27 g, 2 mmol)
were added and the mixture was allowed to stir at 0 1C for 30 min.
To this mixture, L-cystine methyl ester dihydrochloride (0.341 g,
1.0 mmol) and triethylamine (0.7 mL, 5.0 mmol) were added and
the mixture was allowed to stir for 12 h. After the reaction was
complete, the mixture was concentrated under vacuum, and a
viscous material was obtained. The addition of water afforded a
pale yellow solid, which was filtered, and rinsed with distilled
water. The solid product was recrystallised from acetone, and
using the ‘‘inversion’’ method.
3. Results and discussion
3.1 Synthesis and characterisation of L1 and L2
In a typical experiment, ND1, was synthesised by condensing
naphthalene-1,8-dicarboanhydride (1,8-NDA) and 3-amino-
propanoic acid, and then it was coupled to L-cystine methyl
ester using EDC–HOBT coupling strategies (Scheme 1); the
desired bis-naphthalimide L1 (or L2) was obtained as a solid,
1
and characterised using H/13C NMR and mass spectroscopy.
The UV-visible absorption spectra of L1 (Fig. 1a) and L2 were
markedly similar, both exhibited absorptions at 334 nm with a
shoulder at 346 nm. In addition, the fluorescence spectra of the
two compounds also displayed similar features. For instance,
when irradiated at 340 nm in acetonitrile, bis-naphthalimide, L1,
produced two emissions at 381 nm and 465 nm (Fig. 1b); of these,
1
dried in air. Yield 84%, H NMR (300 MHz, DMSO-d6) d 8.59
(2d, amide NH), 8.39 (4H, m), 7.82 (1H, t, J = 7.2 Hz), 4.55
(1H, bm), 4.22 (2H, s), 3.61 (3H, s), 3.336 (H2O), 3.050 (3H, s),
2.960 (3H, s), 2.933 (2H, m), 2.489 (2H, s). 13C NMR (75 MHz,
DMSO-d6): 171.79, 171.06, 164.17, 135.12, 132.12, 131.52,
128.22, 128.03, 122.96, 53.07, 52.14, 37.21, 34.14; FT-IR (cmÀ1):
3448 (amide NH), 3304, 1743 (ester CQO), 1701, 1649 (amide
CQO), 1587; ES-MS: m/z 793.81, calc. for (M + Na+)
Synthesis of ND2 and bis-naphthalimide L2. ND2: naphthalene-
1,8-dicarboanhydride (2.981 g, 15 mmol) was mixed with 4-amino-
butyric acid (1.556 g, 15 mmol) in a round bottom flask equipped
with a magnetic needle. To this mixture, DMF (30 mL) was
added and the resulting mixture was allowed to stir at 80 1C for
about 12 h. The colour of the mixture at the time of mixing was
brown. After about 12 h, a homogeneous yellow-brown solution
was obtained. The desired product, ND2, was crystallised after
the solution was placed in an ice bath. The solid compound was
Fig. 1 Concentration-dependent (a) UV-visible and (b) fluorescence spectra
filtered, and washed with EtOH–water to afford a yellow solid. of L1 in MeCN.
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the emission at 381 nm is characteristic of the naphthalimide
monomer, while the broad emission at 465 nm is attributed to the
intramolecular dimerisation of the naphthalimide motifs.20,30,31 The
formation of intramolecular naphthalimide dimers (i.e. excimers)
could be recognised from the relative intensities of the monomer–
dimer emissions at 381 nm and 465 nm, the ratio of which was
almost constant (I381/465 = 1.16 Æ 0.15) for concentrations up to
0.015 mM.30,31 Similarly, the irradiation of L2 at 340 nm in
acetonitrile produced two emissions at 382 nm and 467 nm,
which correspond to the naphthalimide monomer and excimer
emissions. Notably, the observation of intramolecular excimers
for L1 and L2 was in accordance with the previous reports of
bis-(1,8-naphthalimide) with flexible oligomethylene spacer
groups.20,32,33 The formation of intramolecular naphthalimide
dimers (i.e. L1 and L2) in the ground state was in agreement
with the fact that the excitation spectra obtained in both the
cases were similar.30
With this perspective, we inferred that the excimer emis-
sions for L1 and L2 originate from the intramolecular dimeri-
sation of the naphthalimide motifs in the ground state. As will
be discussed later, the formation of such excimers was possibly
favoured by intramolecular hydrogen bonds invoked by the
amide NH groups, and the hydrophobic interactions between
the naphthalimide groups, given the conformational flexibility
of the cystine disulfide group. Moreover, it can be pointed out
that the formation of intramolecular excimers between the two
naphthalimide motifs for L1 and L2 (412 C–C bonds) was
comparable to the previous reports for bis-naphthalimides
linked by oligoethylene spacers.21,27
Fig. 2 (a) Effect of triethylamine (1.5 mM) on the fluorescence emission of
L1 (0.001 mM, MeCN), followed by (b) hydrofluoric acid (aq., 3.0 mM) on
the L1–triethylamine mixture; (inset: plots of I/I0 vs. [triethylamine] and
[HF], for the emissions occurring at 381 nm and 465 nm).
restore the excimer fluorescence at 465 nm. Accordingly, the
L1–triethylamine solution was titrated using hydrofluoric acid
(aq. HF, 3.0 mM); the emission spectra obtained during the acid
titration clearly revealed that the acid stimulus could switch ‘on’
the excimer emissions at 465 nm (Fig. 2b). Moreover, using the
same amine–acid combination as stimuli, we could induce multi-
ple cycles of reversible quenching and restoration of the 467 nm
emissions of L2 in acetonitrile.
The amine-induced quenching of the excimer emissions of
L1 and L2 could also be observed in solvents such as DMF and
DMF–water. This result is in accordance with the suggestion
that the fluorescence behaviour of substituted naphthalimides
can be influenced by protons, and/or by changes in pH.10,31
These features were complementary to two-step fluorogenic
switches, demonstrated for 21-amine derived, intramolecularly
quenched bis-naphthalimide systems, which could be activated
using protons as inputs.36,37
The effects of spacer groups on excimer fluorescence have been
illustrated earlier by the development of bis-naphthalimide ligands
for cation binding. In a related example, intramolecular excimer
emissions (at 470 nm) were reported for the complexes of Zn(OTf)2
with naphthalimide–imine ligands, in which excimer switching
could be observed, depending on the metal/ligand stoichiometry.34
3.3 Effects of anions on the fluorescence responses of L1 and L2
Again, intramolecular excimer formation was observed after the Notwithstanding the acid/base or proton-dependent switching
dimerisation of N-benzocrown derived naphthalimides, which was of naphthalimide fluorescence in L1 and L2,38,39 we were
apparently induced by the coordination of Ba2+ cations.11 Given curious to examine the effects of fluoride (i.e. TBAF) as anion
these spectral features, we were curious to examine the prospect of stimuli on the monomer/excimer fluorescence of the bis-
anion-induced switching of excimer emissions in relation to the naphthalimide derivatives, andÀhence compare the influence
monomer emissions of the bis-naphthalimides.
of halide anions, including BF4 anions.
Accordingly, we performed the fluorescence titrations of bis-
naphthalimide L1 with fluoride anions (i.e. TBAF) in acetonitrile.
3.2 Effects of acid–base stimuli on L1 and L2
In a related study, we noted that the addition of a 31-amine As shown in Fig. 3a, the incremental addition of fluoride (up to
(i.e. triethylamine) to bis-naphthalimides L1 (or L2) triggered 20 equiv.) to L1 caused the emission intensity at 465 nm to
a reduction in their emission profiles, particularly the long- gradually diminish, whereas the monomer emission at 381 nm
wavelength excimer emission. As shown in Fig. 2, the addition increased 2-fold.40 Notably, the changes in fluorescence at
of triethylamine to L1 led to the substantial quenching of the 381 nm and 465 nm for L1 with increasing fluoride concen-
465 nm emission, compared to the monomer emission at 381 nm. tration were such that I381/465 increased to 18.3. Although the
This result is understandable because the amino-groups have effects of other halide anions on the excimer fluorescence werÀe
been known to induce the quenching of long-wavelength emis- minor, we noted distinct fluorescence enhancements for BF4
sions via photo-induced electron transfer (PET), as reported earlier anions. Surprisingly, the addition of BF4À anion (B50 equiv.) to
for naphthalimide(-spermine) derivatives.30,35
L1/TBAF produced an enhancement in both the monomer and
Following this, we examined the influence of acid on the excimer emissions (as seen in Fig. 3b). This enhancement of
L1–triethylamine mixture, anticipating that the protonation of excimer fluorescence is unusual because it appeared to override
the 31-amine would block the incipient PET process, and hence the quenching effects of the fluoride anions, due to which the
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Fig. 3 Variations in the fluorescence emissions at 381 nm and 465 nm for
L1 (0.0005 mM; acetonitrile) (a) upon the addition of TBAF (inset shows rel.
changes in emissions, I/I0 vs. [TBAF], at 381 nm and 465 nm); (b) following
Fig. 4 (a) Gradual disappearance of the excimer emission of L2 (0.0004 mM;
acetonitrile) at 467 nm upon addition of TBAF; (b) excimer emission at 467 nm
À
‘restored’ following the addition of BF4 anions to the L2–TBAF solution;
À
the addition of BF4 anions to the L1–TBAF mixture (inset shows the rel.
(inset: rel. changes in emissions, I/I0 vs. [TBAF], and then [NaBF4] monitored at
382 nm and 467 nm); (c) fluorescence profiles of L2 (‘1’) following successive
addition of TBAF (‘2’) and NaBF4 (‘3’) in acetonitrile, as observed under
UV-illumination (365 nm). (d) Fluorescence emission of L2 in acetonitrile
solution and in the gel-state (5 mg mLÀ1, acetonitrile), when illuminated
with 365 nm light.
changes in fluorescence, I/I0 vs. [NaBF4], at 381 nm and 465 nm); (c) changes
in the emissions of L1 upon successive addition of TBAF, and NaBF4, as
viewed under UV-illumination (365 nm).
465 nm emission was ‘switched-off’. Furthermore, the changes
in the fluorescence output of L1 brought about by the addition of
fluoride anions, and subsequent addition of BF4À anion could be
monitored visually under 365 nm illumination (Fig. 3c).
Similarly, we monitored the fluorescence responses of L2, in
acetonitrile, subsequent to the addition of fluoride anions. As
shown in Fig. 4, the incremental addition of the fluoride anion
to L2 was accompanied by a gradual disappearance of the
excimer emission (467 nm), while the monomer emission at
382 nm increased 1.5 times, with saturation at 25 equiv. of the
anion. Furthermore, the addition of BF4À anions (B55 equiv.) to
the solution of L2–TBAF caused the excimer emission at 467 nm
to be restored, analogous to L1.
The abovementioned results, viz., quenching of the excimer
emissions of L1 and L2 in the presence of fluoride anions,
could be analysed as follows: first, the interaction of fluoride
anions with the amide NH groups (of L1 and L2) could disrupt the
formation of intramolecular excimers between the naphthalimide
motifs.40 Indeed, the quenching of the excimer fluorescence in
naphthalimides through fluoride-induced disruption of intra-
molecular hydrogen bonds has been reported earlier.41 Alterna-
tively, the anion–p interactions of the proximal fluoride with the
naphthalimide motif could facilitate a partial PET process, thereby
leading to the quenching of the excimer fluorescence.30,36
Second, the quenching effects of fluoride anions were rather
the effects of chloride, bromide and acetate anions on the bis-
naphthalimide fluorescence were relatively minor, although
acetate anions produced minor quenching of excimer emissions
(Fig. S14 and S15; ESI†) under identical conditions.42
Moreover, the enhancement of monomer emissions in both
À
the cases, L1 and L2, following the addition of fluoride and BF4
anions was notable. Again, compared to the highly electronegative
À
fluoride anion, the effects of BF4 anions were expected to be
relatively soft and unlikely to form strong hydrogen bonds.
Although the precise nature of the interactions of BF4À with thÀe
bis-naphthalimide was not clear, we inferred that the BF4
anion could assist the dimerisation of the naphthalimide motifs43
and impede the fluoride-induced PET process. This proposition
seems reasonable given the results obtained from the FTIR and
1H NMR investigations of the L2–NaBF4 system. The effect of the
fluoride anion on the emissions of L1 (or L2) was also illustrated
using NaF; despite its low solubility, the addition of NaF to bis-
naphthalimide induced quenching of the excimer emission, albeit
`
minor compared to TBAF. This quenching of the excimer vis-a-vis
monomer emissionÀs by NaF could subsequently be reversed upon
the addition of BF4 anions.44
3.4 Stimuli-responsive fluorescent gels from L1 and L2
supressed in polar solvents such as aqueous DMF and in the Interestingly, both the bis-naphthalimides, L1 and L2, pro-
presence of MeOH. This indicates that in protic solvents, such duced fluorescent gel phases in DMSO and DMF;45 in the case
as MeOH or aqueous DMF, the electronegative fluoride anions of DMSO, the critical gelation concentrations (CGC) were found
would preferably be hydrated, than the hydrogen bond to to be 3 mg mLÀ1 for L1 and 5 mg mLÀ1 for L2. Fig. 5a and b
the amide NH group of the bis-naphthalimide. In this context, show the scanning electron microscopic (SEM) images of the
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gels obtained from L1 and L2 in DMSO, which indicated the Similar self-assembled entangled networks could be visualised
formation of fibrous networks involving the gelator molecules. in the case of the L2/DMF gel (Fig. 5c and d), and the L2/
acetonitrile gel, which, on close inspection, revealed fibres with
dimensions 42 mm. The critical gelation concentrations for L1
and L2 in DMF were found to be 3 g mLÀ1 and 6 mg mLÀ1
,
respectively. Moreover, L2 produced thermo-reversible gels in
chloroform and acetonitrile, which could also be triggered by
ultra-sonic treatment.45
Preliminary insights into the nature of the intermolecular
`
interactions in the gel phase vis-a-vis solution could be obtained
from fluorescence experiments. For instance, the L2/DMF organo-
gel was characterised by emissions at 399 nm and 468 nm,
respectively, whereas the corresponding emissions for L2
(0.004 mM) in dilute DMF solution occurred at 382 nm and
460 nm (Fig. S11, ESI†). Such red-shifting of the fluorescence
emissions were evident for both L1 and L2 in the gel phases, as
compared to the solution. In particular, the red-shifts observed
in the emission spectra for L2 in the gel state are consistent
with the weak aromatic–p interactions of the naphthalimide
motifs.13,42 L1 and L2 exhibit red-shifted excimer emissions,
particularly in the gel state,46 which also provided support to the
possible intramolecular dimerisation of the pendent naphthalimide
groups.47,48
We propose that the gelation process involving bis-
naphthalimide, L2, (or L1) could be driven by multiple supra-
molecular interactions, viz., intermolecular hydrogen bonding
involving the amide and naphthalimide groups and p–p inter-
actions between the naphthalimide motifs. While the red-shifting
of fluorescence emissions in the gel state suggested aromatic–p
stacking between the naphthalimide motifs, we reasoned that
FTIR and 1H NMR studies could provide valuable insights into the
driving forces for gel formation.
In order to examine the effect of hydrogen bonding inter-
actions on the gelation process, we performed the FTIR analysis
of L2 in chloroform/acetonitrile. In dilute solution, the FTIR
spectra of L2 (0.1 mg mLÀ1) indicated a distinct amide NH
absorption at 3303 cmÀ1; the absorption at 3303 cmÀ1 remained
relatively unaffected within the gelator [L2] = 1.0 mg mLÀ1, and
emanated from intramolecular hydrogen bonds involving the
amide NH groups. However, at [L2] = 2.0 mg mLÀ1, a broad
absorption appeared at 3470 cmÀ1 in chloroform/acetonitrile,
which shifted further to 3440 cmÀ1 with substantially broadening
upon gel formation (Fig. S16, ESI†), indicating the formation of
extensive hydrogen bonded networks.
The crucial role of hydrogen bonding during the gelation
process was also investigated using 1H NMR in chloroform-d
and acetonitrile-d3. In this regard, the interactions of fluoride
with bis-naphthalimide as described in previous sections could
also be substantiated, particularly for L2 in chloroform-d and
acetonitrile-d3.
As indicated by 1H NMR, increasing the concentrations of L2
from 0.3 mg mLÀ1 to 2 mg mLÀ1 in chloroform-d did not
significantly affect either the naphthalimide CH or amide NH
resonances (Fig. S19, ESI†). Given that the hydrogen bonding
tendency of chloroform under these conditions is limited, we
anticipated that the intra-molecular hydrogen bonds between
Fig. 5 SEM images of the dried organogel obtained from: (a) L1 in DMSO;
(b) L2 in DMSO; (c), (d) L2/DMF gel under different magnifications.
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the amide NH groups would persist. These features were consis- 8.42 ppm, emanated from the solvated L2 molecules. In other
tent with the observations made in the FTIR spectra for L2 in words, the two sets of resonances produced by the naphthalimide
chloroform. At a higher concentration of L2 (CGC = 5 mg mLÀ1
,
CH groups in acetonitrile (cf. in chloroform the signals were minor),
with ultra-sonic irradiation) gel formation was observed, and the were reminiscent of the inequivalence of the naphthalimide motifs
47
`
naphthalimide CH resonances at 7.72, 8.17, and 8.57 ppm shifted in the solution/sol state vis-a-vis the gel phase.
slightly upfield (Dd B 0.01 ppm), which was reminiscent of the
With a decrease in the temperature, the set of naphthalimide
weak intra-molecular interactions between the naphthalimide CH resonances at 7.82, 8.34 and 8.54 ppm gradually gained
motifs. Though the amide NH resonance appeared to be invariant prominence. Thus, the intensities of the two sets resonances
up to 1.0 mg mLÀ1, gelation caused this resonance to shift slightly became almost comparable at 25 1C, and concomitantly gelation
downfield from 7.07 ppm to 7.09 ppm, possibly indicating an was initiated. The gradual changes observed in the naphthalimide
extended hydrogen bonding situation. Thus, a clear correlation CH resonances as a function of temperature (Fig. 6), reflects the
can be seen in the behaviour of the amide NH groups, with regard importance of the aromatic–p interactions during the gelation
to intra-molecular hydrogen bonding, both from the FTIR and process. It was noted that gel formation was accompanied by the
1H NMR studies. Therefore, we inferred that the formation of the appearance of an additional signal due to the aliphatic CH groups
L2/chloroform gel was driven by aromatic p-stacking interactions, connected to the naphthalimide motif (Fig. S21, ESI†). However,
assisted by hydrogen bonding interactions between the amide NH the effect of gelation on the amide NH resonances of L2 was
groups. Further support to the hydrogen bonding condition of minor, with a gradual shift from 7.07 ppm in solution (50 1C) to
amide NH groups could be obtained by the gradual addition of 7.12 ppm in the gel phase. With an increase in temperature, the
TBAF (up to 3 equiv.) to the L2/chloroform gel, which caused a amide NH resonance (of L2) was shifted upfield; such changes
downfield shift of the amide NH resonance from 7.09 to 7.23 ppm could be explained by the breaking of the intermolecular hydrogen
(Fig. S20, ESI†) with partial dissolution of the gel.
bonds associated with the amide NH group, viz. with the solvent
Following this, we sought to examine the effect of gelation and neighbouring gelator molecules.
on the naphthalimide CH and amide NH groups of the
As illustrated in Scheme 2, the emergence of downfield
L2/acetonitrile gel using 1H NMR spectroscopy. Because L2 shifted resonances for the naphthalimide CH protons at 7.82,
caused the rapid gelation of acetonitrile, it seemed pertinent 8.34 and 8.54 ppm following gel formation was illuminating
to carry out temperature-dependent 1H NMR analysis of the because it corroborated the presence of L2 both as solvated
L2–acetonitrile-d3 system.
species and in the gel phase. Moreover, noteworthy was the
As shown in Fig. 6, the solution of L2 in acetonitrile-d3 at splitting/broadening of these resonances due to the cys-methyl
50 1C produced a distinct set of resonances for the naphthalimide ester group, which could be correlated to the proximal inter-
CH protons at 7.70, 8.20 and 8.42 ppm along with a minor set of actions of this residue with the naphthalimide motifs groups
resonances at 7.82, 8.34 and 8.54 ppm, respectively; the integrated during the formation of the L2/acetonitrile organogel (Fig. S21,
ratios for the major and minor signals were found to be B9:1. At ESI†). The temperature-dependent self-assembling process of L2
50 1C, the minor resonances were attributed to the L2 molecules in acetonitrile clearly reflected the importance of the aromatic–p
in the incipient gel,47 while the major resonances at 7.70, 8.20 and stacking interactions in its gelation behaviour, which was sup-
plemented by hydrogen bonds between the gelator molecules.
Based on the results of the 1H NMR experiments, and given the
red-shifted emissions observed for L2 in the gel phase, it seems
reasonable that the naphthalimide motifs interact in a head-to-
tail manner in the gel phase.13,46
Moreover, as monitored by 1H NMR, the onset of gelation for
L2/acetonitrile-d3 (or chloroform-d) was marked by lower inten-
sity signals, compared to those in solution. This is understand-
able because the formation of the L2/acetonitrile gel restricted
a larger proportion of L2 gelator molecules to enter the gel
structure, thereby reducing the thermal motions of the gelator
molecules.48
On the basis of these observations, it is inferred that the self-
assembly of the gelator molecules of L2, in acetonitrile or
chloroform is driven by aromatic–p stacking between the
Fig. 6 Partial 1H NMR spectra of L2 (5 mg mLÀ1) showing the temperature-
naphthalimide motifs and supplemented by hydrogen bonds
dependent variations of the naphthalimide CH and amide NH resonances in involving the amide NH groups (Scheme 2). As mentioned earlier,
acetonitrile-d3: (a) 50 1C; (b) 45 1C; (c) 35 1C; (d) 25 1C; (e) 25 1C after 12 h;
(the two sets of naphthalimide CH resonances could be attributed to the
the FTIR analysis indicated the presence of intramolecular
hydrogen bonds in L2, which in turn could explain the observa-
tions of excimer emission in the fluorescence spectra. This
intramolecular nature of the incipient hydrogen bonds was also
reflected in the 1H NMR spectra, with the amide NH resonances
inequivalence of the naphthalimide motifs in the solution/sol state, designated
as ‘m’ vis-a-vis the gel phase, indicated as ‘’’). The increase in temperature
`
could lead to disruption of hydrogen bonds involving the amide NH groups,
which accounted for the gradual upfield shifting of the resonances.
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Fig. 7 (a) Partial 1H NMR of L2/acetonitrile-d3 gel at room temperature;
and (b) disruption of hydrogen bonds involving the amide NH groups,
induced by the addition of 1.0 equiv. of fluoride anions.
gelator molecules, apparently initiated by the coordination of
the fluoride anion to the amide NH groups (Fig. 7).
Further support for the effects of TBAF on the gel-to-sol
transformation (Scheme 2), and the stimuli-responsive nature
of the bis-naphthalimides, L1 and L2, could be obtained from
1H NMR studies. As evident from the 1H NMR spectra, the
addition of TBAF (1.0 equiv.) to the L2/acetonitrile gel caused
the shifting of the amide NH resonance from 7.15 to 6.87 ppm,
along with splitting (Fig. 7). The addition of 10 equiv. of TBAF
was accompanied by a gradual dissolution and collapse of the
organogel network. This effect of TBAF on the hydrogen bond-
ing network of the L2–chloroform system could be identified by
the disappearance of the absorption at 3304 cmÀ1 in the FTIR
spectra. Moreover, the FTIR spectra of L2 exhibited distinct
features due to intra- and intermolecular hydrogen bonding
interactions involving the amide NH in solution and in the gel
phase absorption. The absorption featured at 3303 cmÀ1 in the
FTIR spectra is consistent with intra-molecular hydrogen bonds,
and was slightly affected in the gel phase. Accordingly, the
1H NMR spectra of the L2–TBAF system in chloroform revealed
distinct downfield shifts for the amide NH resonances, which
were in accordance with the formation of a hydrogen bonded
Scheme 2 Formation of intramolecular hydrogen bonds for L2 in solvents
such as acetonitrile, showing the plausible interaction of the FÀ anions
(i.e. TBAF) with the bis-naphthalimide; The importance of aromatic–p and
hydrogen bonding interactions between the gelator molecules during
gel formation and fluoride-induced gel-to-sol transformation have been
illustrated (inset: photographic images of the L2/acetonitrile gel and solution
[L2] = 5 mg mLÀ1).
showing only minor downfield shifts. Considering these aspects,
we inferred that hydrogen bonding between the amide NH groups
were favored when the gelator molecules adopted bent confor-
mations. Previous studies have shown the importance of intra-
molecular hydrogen bonding interactions during the formation of
self-assembled gels, and how such interactions were favored when
the gelator molecules adopted bent conformations.47 Again, these
polar interactions facilitated the hydrophobic packing of the
naphthalimide groups (Scheme 2), such that the gelator mole-
cules slowly self-assembled into entangled fibrous networks.47
In fact, the influence of the hydrophobic aromatic–p inter-
1
L2–anion complex (Fig. S20, ESI†). Subsequent H NMR studies
of the interactions of NaBF4 with L2 in acetonitrile-d3 and in
dimethyl sulfoxide-d6 revealed minor upfield shifts for the
naphthalimide CH resonances at 7.77 and 8.32 ppm. Such featureÀs
have often been attributed to anion–p interactions between BF4
anions and the naphthalimide motifs.43
4. Conclusions
actions between the naphthalimide groups and segregation of In conclusion, we have synthesised and characterised two cystine-
hydrophilic interactions during the gel formation process was based bis-naphthalimides which form fluorescent gels in aceto-
recently reported.46,48
nitrile, DMF and DMSO. Remarkably, both bis-naphthalimides
À
Similar gel-to-sol transformations were noted for the L2/ exhibit fluoride-induced quenching and BF4 induced restoration
acetonitrile gels, upon the addition of TBAF, which was also of the excimer emissions, which could be visualized under 365 nm
accompanied by the quenching of the excimer fluorescence. We UV-illumination. Such ‘off/on’ switching of excimer emission for
reasoned that the gel-to-sol transformations emanated from the the bis-naphthalimides could also be achieved by the successive
disruption of the hydrogen bonded network involving the addition of 31-amine and hydrofluoric acid. 1H NMR studies
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Acknowledgements
The authors gratefully acknowledge Department of Science &
Technology, India (DST-SR/FTP/CS-102/2007) and University Grants
Commission (UGC), India for funding and infrastructure facilities,
and CIF-IIT Guwahati for assistance with the SEM analysis.
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