Al3+ triggered aggregation induced emission of an anthracence based azine derivative in SDS medium

Article abstract of DOI:10.1039/d0nj00968g

An anthracene based unsymmetrical azine derivative, viz. 4-(anthracen-9-ylmethylene-hydrazonomethyl)-2-methoxy-phenol (L5), is synthesized and structurally characterized by single crystal X-ray diffraction (SC-XRD) analysis. The photo-physical interaction of L5 with Al³⁺ in sodium dodecyl sulfate (SDS) is highly selective, allowing recognition of nanomolar (62.8 nM) Al³⁺ without interference from common cations. The Job plot shows a 1 : 1 (mole ratio) interaction with an associated binding constant of 9.14 × 10⁴ M^-1. The "turn ON" fluorescence is attributed to aggregation induced emission (AIE). The Al³⁺ assisted particle size enhancement of L5 from 52.70 nm to 3413.06 nm (3.413 μM) observed through DLS studies corroborates the AIE mechanism. FESEM images also support the proposed mechanism. The cylindrical morphology of L5 changes to a cube shape due to aggregation of the [Al³⁺ + L5] adduct. Fluorescence lifetime decay (TCSPC) data indicate that the radiative decay of L5 is enhanced about ten times in the presence of Al³⁺. The average lifetime of L5 in the absence and presence of Al³⁺ is 0.98 ns and 1.45 respectively. In the presence of Na₂EDTA, the emission intensity of the [L5 + Al³⁺] system is significantly quenched due to chelation of Al³⁺ by EDTA. TDDFT studies indicate higher stability of the [L5 + Al³⁺] adduct over free L5.

Full text of DOI:10.1039/d0nj00968g

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Al3+ triggered aggregation induced emission of anthracence based azine
derivative in SDS medium
Somnath Khanra, Sabyasachi Ta, Milan Ghosh, Sudeshna Chatterjee, Pallabi Mukherjee and
Debasis Das*
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Department of Chemistry, The University of Burdwan, Burdwan, 713104, W.B., India
Abstract
An anthracene based unsymmetrical azine derivative, viz. 4-(anthracen-9-ylmethylene-
hydrazonomethyl)-2-methoxy-phenol (L5) is synthesized and structurally characterized by single
3
+
crystal X-ray diffraction (SC-XRD) analysis. Photo-physical interaction of L5 with Al in sodium
dodecyl sulfate (SDS) is highly selective to allow recognition of nanomolar (62.8 nM) Al3+
without interference from common cations. Jobs plot shows 1:1 (mole ratio) interaction with
4
-1
associated binding constant, 9.14×10 M . The “Turn ON” fluorescence is attributed to
aggregation induced emission (AIE). The Al3+ assisted particle size enhancement of L5 from
5
2.70 nm to 3413.06 nm (3.413 μM), observed through DLS studies corroborate the AIE
mechanism. FESEM images also support the proposed mechanism. The cylindrical morphology of
3
+
L5 changes to cube shape due to aggregation of [Al +L5] adduct. Fluorescence life time decay
3
+
(
TCSPC) data indicate radiative decay of L5 has enhanced about ten times in presence of Al .
3
+
The average life time of L5 in absence and presence of Al are 0.98 ns and 1.45 respectively. In
3+
presence of Na EDTA, the emission intensity of [L5+Al ] system significantly quenched due to
2
3
+
3+
chelation of Al by EDTA. TDDFT studies indicate higher stability of [L5+Al ] adduct over free
L5.
3+
Keywords: Al , SDS, AIE, FESEM, TCSPC.

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Introduction
Photo-physical interaction studies with conventional fluorescent molecules such as
fluorescein, rhodamine, and cyanine derivatives generally suffer from aggregation caused
quenching (ACQ) via non-radiative pathway at excited state1-5 i.e. it experiences self-
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quenching at higher concentration. Tang et al. added a new dimension by introducing
aggregation induced emission enhancement (AIEE or simply AIE) that overcomes ACQ.
Restricted intra-molecular rotation and vibration are primarily responsible for the process.7
On the other hand hydrophobic core of micelles are considered as one of the versatile
containers, for studying various photo-physical process including fluorescence sensing
8
mechanism. Moreover surfactant medium is very commendable media in chemical biology
than that of aqueous media as it can exert direct effect on biological system during drug
9
transport across the cell membrane. Thus photo-physical interaction studies in surfactant
medium leading to AIE is an area immense research interest10-14
.
3
+
Al , a non-essential element enters into human body through several processes, viz.
medicines, food additives, water purification etc. and interferes physiological processes
2+
15-26
including Fe sorption in blood.
In this context, attempts have been made to synthesise a molecule involving hydrophobic
anthracence moiety viz. anthracene-9-carbaldehyde that may have AIE characteristics in
3+
surfactant media. To facilitate Al binding, the anthracence unit is anchored to a hard donor
viz. 4-hydroxy-3-methoxy-benzaldehyde using hydrazine spacer leading to 4-(anthracen-9-
ylmethylene-hydrazonomethyl)-2-methoxy-phenol (L5). X-ray structurally characterized
3
+
unsymmetrical L5 selectively interacts with Al in sodium dodecyl sulfate (SDS) medium
resulting AIE. The hydrogen bond assisted π-π stacking among adjacent anthracene moieties
may be attributed to this AIE process.
Materials and measurements
High purity anthracene-9-carbaldehyde, 4-hydroxy-3-methoxy-benzaldehyde, hydrazine,
HEPES buffer and Al(NO ) , 9H O were purchased from Sigma−Aldrich (India) and used
3
3
2
without further purification. The spectroscopic grade solvents and analytical reagent grade
−
1
chemicals were used without further purification. Milli-Q Millipore (18.2 MΩ cm ) water
was used whenever required. Shimadzu Multi Spec 2450 spectrophotometer and Hitachi F-
7000 spectrofluorimeter were used to measure absorption and steady state emission
spectroscopic studies. Shimadzu FTIR (model IR Prestige 21 CE) spectrometer was used to
record FTIR spectra. The elemental analysis (C, H and N) was performed using PerkinElmer
2
400 CHN elemental analyser. Systronics digital pH meter (model 335) was used for pH

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measurement. High resolution mass spectra were recorded using Thermo Fisher scientific
Exactive Mass Spectrometer using HR positive/ negative mode. NMR spectra were recorded
on a Bruker 400 MHz spectrometer using DMSO-d as solvent. Multiplicity is indicated as
6
follows: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet). Coupling constants (J, s)
are reported in Hertz (Hz). Dynamic light scattering (DLS) experiments were performed
using Malvern Zetasizer NANO instrument. Time-resolved fluorescence lifetime
measurements were performed using picosecond pulsed diode laser-based time-correlated
single-photon counting (TCSPC) spectrometer (IBH, UK, λ = 373 nm, λ = 400 nm)
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ex
em
coupled to MCP-PMT detector (model FL-1057). Field emission scanning electron
microscope (FESEM) images were collected using ZEISS Gemini Sigma 300 instrument.
Perkin Elmer LS55 instrument was used for solid state fluorescence measurement.
Single crystal X-ray data were collected on a Bruker X8 APEX-II CCD diffractometer at
100(2) K using graphite-monochromated Mo-K radiation (0.71073Å) at 150K. Data were
α
processed and corrected for Lorentz and polarization absorption effects. Crystal structure was
2
7
solved by standard direct methods using the SHELXS and refined by full-matrix least-
28
29
squares with SHELXL and OLEX2 software. All non-hydrogen atoms were refined with
anisotropic thermal displacements. Hydrogen atoms were included in the structure factor
calculation in geometrically idealized positions, with thermal parameters depending on the
parent atom, using a riding model. Images were generated by Mercury software.30
Synthesis
4
-(Anthracen-9-ylmethylene-hydrazonomethyl)-2-methoxy-phenol (L5) (Scheme 1)
In an ethanol solution of anthracene-9-carbaldehyde (206.26 mg, 1.0 mmol), few drops of
hydrazine were added under stirring condition followed by reflux for about 8h.31
The crude was purified by crystallization to yield 64.2% anthracen-9-ylmethylene-hydrazine
(HYHN).
Methanol solution containing HYHN (110.13 mg, 0.5 mmol) and 4-hydroxy-3-methoxy-
benzaldehyde (76.0 mg, 0.5 mmol) was refluxed for 4h. The reaction mixture was kept for a
week for slow crystallization. Yellow crystals of L5 were collected in 71.5% yield. Anal.
found, C, 77.90; H, 4.72 and N, 7.32; calcd., C, 77.95; H, 5.12 and N, 7.90. ESI-MS (m/z):
+
1
[
L5+H] , 355.27 (Figure S1, ESI). H NMR (DMSO-d , 400 MHz, J, Hz, δ ppm, reference
6
peak = δ 2.5 ppm) (Figure S2, ESI): 11.475 (1H, s) 9.764 (1H, s), 9.456 (1H, s) 8.043 (4H,
s), 7.372-7.353 (2H, t, J = 4.6 Hz), 7.193 (1H, s) 6.988-6.966 (1H, d, J = 8.8 Hz), 6.150 (1H,
−
1
s), 3.905 (3H, s). FTIR (KBr, cm ): 3603.00, υ (O-H); 3169.91, υ (C-H, aromatic); 2849.11,

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υ (C-H, aliphatic); 1628.97, υ (C=N, stretch); 1428.73, υ (C=C); 1272.34, υ (C-O, stretch);
1130.38, υ (N-N) (Figure S3, ESI).
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+
[
L5+Al ] adduct (Al1)
In methanol solution of L5, solution of Al(NO ) in methanol was added drop-wise and
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stirred for 1h. The mixture was filtered and the filtrate was kept for slow evaporation
whereby Al1 was isolated in 41% yield. The mass spectrum shows molecular ion peak at m/z
3
+
-
+
5
87.88 that corresponds to [L5+Al +2NO +2MeOH+H O] (Figure S4, ESI). The FTIR
3 2
−
1
data (KBr, cm ): 3416.93, υ (H O); 2998.28, υ (C-H, aromatic); 2781.73, υ (C-H, aliphatic);
2
1
608.38, υ (C=N, stretch); 1421.56, υ (C=C); 1331.69, υ (C=N) and 1092.69, υ (N-N)
(Figure S5, ESI).
Scheme 1 Synthesis of L5
Result and discussion
Single crystal X-ray structure of L5
L5 is well characterized by spectroscopic techniques while its structure is confirmed by
single crystal X-ray diffraction analysis (Figure 1). Its molecular view with atom labelling
scheme is shown in Figure S6 (ESI).The crystallographic data, refinement parameters
(
Table S1) and bond lengths, angles (Table S2) are presented in ESI. The L5 is triclinic with
space group P21/c (CCDC 1483217) having a(Å), 9.4580(12); b(Å), 9.6384(12); c(Å),
3
20.324(3); α(°),90; β(°),100.752(9); γ(°),90; volume (Å ), 1820.2(4) and Z = 4. It adopts E
configuration in solid state. The bond lengths N5 C00C, 1.270(3)Å and N004 C009,

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.235(3)Å indicate double bond character of aldimine moiety whereas the bond length, N5
DOI: 10.1039/D0NJ00968G
N004, 1.414(3)Å indicates single bond feature of N-N. The bond lengths O001 C007,
.355(3)Å indicate single bond feature of C-O of phenol moiety. On the other hand O002
1
C005, 1.367(3)Å and O002 C00O, 1.397(4)Å represent respective aromatic and aliphatic C-O
bonds of methoxy moieties. Bond angles, C00C N5 N004, 112.0(2)° and C009 N004 N5,
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114.4(2)° indicate sp nature of the N004 and N5 respectively whereas N004 C009 C006,
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23.9(3)° and N5 C00C C008, 126.3(3)° indicate sp nature of the C009 and C00C atoms
respectively (Figure S6, Table S2, ESI). Photo induced electron transfer (PET) and -C=N
isomerisation processes, responsible for weak fluorescence of L5 is portrayed in Figure 2.
Figure 2 PET and -CH=N- isomerisation in L5
Figure 1 Mercury view of L5
Spectroscopic studies
Emission studies
The L5 exhibits weak emission with associated quantum yield, Φ = 0.024 at 400 nm (λ =
ex
3
+
326 nm) in SDS medium due to PET effect (Figure S7, ESI). Upon gradual addition of Al ,
a sharp increase of emission intensity along with 49 fold fluorescence enhancement (Figure
a) is observed while Φ enhanced to 0.412 (~17-fold). Moreover interaction of L5 with Al3+
3
2
+
+
2+
2+
2+
2+
3+
is highly selective over other common cations viz. Zn K , Ca , Mg , Hg , Pb , Cr ,
2
+
2+
3+
2+
2+
2+
2+
+
Mn , Fe , Fe , Co , Ni , Cu , Cd and Ag (Figure 3b). The changes in emission
3
+
intensities for L5 and [L5+Al ] systems with pH (Figure S8, ESI) indicate that the emission
intensity of L5 remains almost same over a wide range of pH whereas it increases in presence
3
+
of Al (pH 6- 9) allowing to perform the interaction studies at physiological pH, 7.
3
+
In a competitive environment of interfering cations, the emission intensity of [L5+Al ]
system remains almost unchanged (Figure S9, ESI). The emission intensity of L5 increases
3+
exponentially with added Al (Figure S10, ESI). The limit of detection (LOD) is 62.8 nM
(Figure S11, ESI). The binding constant, determined using Benesi-Hildebrand equation is
4
-1
9
.14×10 M (Figure S12, ESI). Job’s method reveals 1:1 (mole ratio) interaction between
3+
L5 and Al (Figure S13, ESI).

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Figure 3a Changes in emission intensity of L5 (20 Figure 3b Changes in the emission spectra of L5 (20
µM) in SDS media (0.1M HEPES buffer, pH 7.4) upon µM) in presence of common cations (1000 µM) in SDS
3
+
gradual addition of Al (0-1500 μM; λex = 326 nm).
medium (0.1M HEPES buffer, pH 7.4; λex = 326 nm.
The photo-physical interaction studies have also been performed in different surfactant media
viz. anionic SDS, cataionic CTAB (cetrimonium bromide) and neutral TX-100 (triton X-100).
The emission profile (λ = 326 nm) of free L5 is almost same in each of the surfactant
ex
medium except TX-100 where a new weak emission at 535 nm is observed (Figure S14,
3
+
ESI). Interestingly, the mode of interaction of Al with L5 varies in different surfactant
media. The studies in SDS medium are already discussed supra. In case of CTBA, the
maximum emission intensity is observed at 397 nm instead of 400 nm for SDS. On the other
hand, for TX-100, the maximum emission intensity observed at 385 nm in addition to
disappearance of the emission at 535 nm. Hence, SDS medium has chosen for the interaction
studies.
Absorption studies
UV-Vis. spectrum of L5 exhibits two bands at 278 nm and 310 nm, attributed to π–π*
transition whereas the 353 nm band is attributed to n-π* transition (Figure 4a).32 Upon
3
+
gradual addition of Al , the absorbance of both wavelength decreases along with shift of the
53 nm peak to 360 nm, attributed to metal to ligand charge transfer (MLCT). At saturation,
3
the 310 nm peak slightly shifted to 304 nm while the 278 nm peak almost disappeared with
enhancement of 360 nm peak. An isobestic point is observed at 340 nm. The selectivity of L5
towards Al3+ in presence of common cations viz. Zn , K , Ca , Mg , Hg , Pb , Cr ,
2+
+
2+
2+
2+
2+
3+
2+
2+
3+
2+
2+
2+
2+
+
Mn , Fe , Fe , Co , Ni , Cu , Cd and Ag is presented in Figure 4b.
AIE studies
The presence of both hydrophobic anthracenece moiety and polar hydrophilic hydroxy group
made the L5 unique allowing its solubilisation both in micelle as well as organic solvent. The
AIE characteristics of L5 in a mixture of good and poor solvent at different proportions have

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been studied by varying their volume (%). Thus L5 is soluble in organic solvents like
dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol and water. Therefore,
emission spectra of L5 were recorded by varying water content, keeping L5 concentration
unaltered. In present study, DMF is used as good solvent while water as poor solvent. The
very weak emission of L5 (λ , 326 nm) in pure DMF (0% H O) gradually increases with
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increasing water content (Figure 5a), a fact that probably indicates aggregation of L5 upon
introduction of H O in the medium.
2
Figure 4a Variation of absorption spectra of L5 (20 Figure 4b Absorption spectral changes of L5 (20 µM)
µM) upon gradual addition of Al (0-1500 μM) in in presence of common cations (1000 µM) in SDS
SDS media (0.1 M aqueous HEPES buffer, pH 7.4) medium (0.1 M HEPES buffer, pH 7.4)
3
+
The DMF solution of L5 being homogenous, allows free rotation around N-N bond, in
addition to the existing PET process.33 Upon gradual addition of H O, hydrophobic
2
anthracene moiety of L5 come closer to each other to initiate aggregation. As a result, the
rigidity of the system increases leading to fluorescence enhancement. Corresponding changes
in the absorption spectra is shown in Figure 5b.
However, the emission intensity of aggregated L5 in DMF-H O (2:98, v/v) is less than
2
3
+
[L5+Al ] aggregate in SDS medium. This is attributed to further enhancement of rigidity due
3
+
to chelation of Al by L5.
3+
Comparison of L5 with reported Al probes18, 26, 33-42
3
+
The efficiency of L5 is compared with other reported Al selective probes. The AIE based
amphipathic L5 enables to detect nano-molar Al3+ in surfactant media with a binding
4
-1
18, 26, 34-36
3+
constant, 9.14×10 M . Thus, Table 1
compares different pioneering Al probes
while Table 233, 37-42 compares AIE based Al probes.
3+

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1
1
1
1
1
1
1
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2
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2
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3
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3
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3
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0
1
2
3
4
5
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 5a Changes in emission intensity of L5 (20 Figure 5b Changes in absorption spectra of L5 (20
µM) in DMF upon increasing volume of water (0- µM) in DMF upon increasing volume of water (0-
9
8%, λex = 326 nm).
98%).
Table 1 Comparison among several pioneering Al3+ selective probes
Sensing process
DMSO/Tris buffer (2:8, v/v)
Mechanism
ICT/ CHEF
ESIPT/ CHEF
ESIPT/ CHEF
PET/ CHEF
ICT/ CHEF
LOD
Binding constant
Ref.
34
35
18
36
0.29 μM
0.83 nM
0.224 μM
0.11 μM
0.29 μM
-
6
-1
-1
CH
CH
3
OH/HEPES buffer (1/4, v/v)
OH/H O (4/1, v/v), pH 7.2
OH/H O (4:1,v/v), pH 7.2
O/CH OH (1:1, v/v)
1.3×10 M
7.1×10 M
7.033×10 M
3.1×10 M
4
3
2
3
-1
C
2
H
5
2
8
-1
H
2
3
26
Table 2 Comparison among several pioneering AIE based Al3+ probe
Solvent System
THF/H O (25:75, v/v)
THF/Water (9:1, v/v)
MeOH/H O ( (9:1, v/v), pH 7.3
HEPES buffer (10 mM), pH 7.4
LOD
Binding constant Reference
2
1.2 μM
0.33 μM
2.8 μM
0.621 μM
0.4 μM
0.27 μM
0.15 μM
62.8 nM
-
-
-
-
37
38
39
2
33
40
41
3
-1
-1
THF/H
MeOH
ACN
2
O (4:1, v/v)
8.4×10 M
1.6×10 M
4
-
42
4
-1
SDS (0.1M HEPES buffer), pH 7.4
9.14×10 M
This study
(
0.062 μM)
Sensing mechanism
Due to presence of PET and -CH=N- isomerisation (Figure 2), L5 is almost non-fluorescent.
In surfactant medium, its fluorescence intensity significantly enhances upon addition of Al3+.
3
+
The Al induced enhanced rigidity through inhibition of -CH=N- isomerization and π-π
stacking between neighbouring anthracene moiety, assisted by hydrogen bonding are
responsible for fluorescence enhancement (Scheme 2). Moreover, the entire process further
propagates to AIE in surfactant media. Oxygen rich donors appended to anthracene moiety
via azine spacer43 is probably responsible for Al3+ induced AIE process in surfactant
media.33,39

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Addition of EDTA to [L5+Al ] results significant quenching due to complexation of EDTA
3
+
3+
with Al (Figure S15, ESI) which reverts back upon addition of Al to the media. This is
reflected in the FTIR spectrum (Figure S3, S5, ESI) where the –OH stretching of L5 at 3603
-1
3+
-1
cm disappears upon addition of Al with the appearance of a broad band at 3416 cm for
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
-1
-1
-1
H O. In addition, -C-O and C=N peaks at 1272 cm and 1628 cm of L5 shifted to 1331 cm
2
-1
3+
and 1608 cm indicating its interaction with Al (Figure S4, ESI). The molecular ion peak
3
+
3+
-
(
m/z) at 587.88 in the ESI-MS spectrum of [L5+Al ] adduct is assigned to [L5+Al +2NO
3
3
+
+
2MeOH+H O]. Jobs plot indicates 1:1 (mole ratio) interaction between L5 and Al (Figure
2
1
S13, ESI). The proposed mechanism of interaction is also corroborated by H NMR titration,
DLS studies, TCSPC experiment and SEM images.
Scheme 2 Proposed AIE mechanism
1HNMR titration
1H NMR titration that has been performed for in-depth understanding of the binding mode,
3
+
indicates a significant change upon addition of Al to L5 (Figure 6). Upon addition of 0.5
3
+
equiv. of Al , highly de-shielded “a” proton (-OH) at 11.47 ppm slightly up-filed shifted to
4
4
3+
11.41 ppm, indicating coordination of phenol oxygen to Al . In addition, the peak at 9.76
ppm (b proton) has been shifted up-field to 9.75 ppm and the peak at 9.45 ppm (c proton)
splits. The methoxy hydrogen (d proton) at 3.90 ppm also splits. Upon addition of 1.0 equiv.
3
+
Al (saturation) a proton fully disappears while both b and d proton further up-filed shifted
along with splitting of methoxy proton. The ring protons also shift and split as expected.
DLS studies
3+
Figure S16 (ESI) reveals that the average particle sizes (Z ) of L5 (20 μm) and Al are
av
52.70 nm and 87.414 nm in SDS medium. However, a significant enhancement of particle

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2
2
2
2
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3
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3
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size from 52.70 nm to 260.20 nm is observed in [L5+Al ] adduct (Al1), indicating aggregate
4
5
formation in SDS medium. Interestingly, upon addition of EDTA to the system, the particle
3
+
size reduces by 50% to 124.54 nm indicating partial removal of Al from Al1 by EDTA.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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9
0
1
2
3
4
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1
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3
4
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9
0
1
2
3
4
5
6
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9
0
1
3+
Figure 6 HNMR titration of L5 by Al in DMSO-d
6
Solid state fluorescence
Solid state photoluminescence (Figure S17, ESI) of L5 exhibits an intense emission at 403
nm (λ = 326 nm), indicating higher emission of L5 in solid state than in the solution,
ex
46
generally known as crystal induced emission enhancement. In other words, aggregation of
L5 in crystalline phase may be responsible for more intense emission than in solution. The
packing diagram of L5 also corroborates the fact (Figure S18, ESI). In addition, the emission
3+
intensity of L5 in presence of Al (λ = 402 nm, λ = 326 nm) is higher in solid state than
em
ex
in solution, supporting the AIE process.
FESEM analysis
3
+
FESEM analysis of free L5 and its Al adduct (Al1) have been presented as additional
support for the proposed Al3+ assisted AIE of L5 (Figure 7). Moving towards higher
resolution, a distinct difference in morphology is observed between L5 and Al1. The FESEM
images clearly indicate the enhancement of particle size from L5 to Al1, suggesting Al3+
assisted aggregation.42 Moreover, cylindrical particles of L5 turn to cube shape in the
aggregated Al1.

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0
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0
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2
3
4
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8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 7 FESEM images of dried sample: L5 (4 µm, 2 µm, 1 µm and 0.2 µm resolution) and A11 (4 µm, 1 µm,
.4 µm and 0.2 µm resolution).
0
TCSPC measurement
3
+
The fluorescence life time decay of L5 in absence and presence of Al has been studied in
SDS medium using nano-LED excitation source (Figure 8). The Table S3 (ESI) reveals that
the average fluorescence lifetime (<τ>) of L5 increases from 0.98 ns to 1.45 ns in Al1. The
4
7
-1
value of k /k , determined from the standard equation, τ = k + k and kr = Φ/τ (where
r
nr
r
nr
k is the radiative rate constant and k is total non-radiative rate constant) enhances from
r
nr
0
.0246 for L5 to 0.69 in A11. This fact suggests that the probability of radiative decay of L5
3
+
increase in presence of Al .
Table 3
9
-1
9 -1
Sample <τ> (ns)
Φ
0.024
0.412
k
r
(10 s )
k
nr (10 s )
0.996
L5
Al1
0.98
1.45
0.0245
0.284
0.406
Figure 8
Fluorescence life time decay of L5 and Al1 in SDS media (λem = 400 nm): profile (Figure 8) and data (Table 3)
Density functional theoretical (DFT) studies
For DFT studies, TD-SCF/DFT/B3LYP/SDD basis set has been used. For A11, the
-
monomeric unit representing [Al (L5) (NO ) (OH ) ] is used (NO is used as counter anion).
2
2
3 4
2 2
3
The HOMO-LUMO energy gap lowered about 5.7 times in Al1 over L5 (from 0.13021eV for
L5) to 0.0229eV (Al1 monomer) (Figure 9a). The electron transition energies for principal
absorption band of L5 and Al1 monomer are presented in Table S4 (ESI). The DFT
optimised structures of L5 and Al1 monomer are shown in Figure S19 (ESI). In case of L5,

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electron density on HOMO is localized on anthracene unit whereas for LUMO it is
distributed over whole molecule. On the other hand, for Al1 the electron densities on both
HOMO and LUMO are localized at coordination zone. This indicates flow of electron from
L5 to Al3+ in the Al1 monomer. The DFT optimised structure of Al1 clearly indicates
existence of π-π stacking among neighbouring anthracene moieties assisted by intermolecular
hydrogen bond (Figure 9b).
0
1
2
3
4
5
6
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8
9
0
1
2
3
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2
3
4
5
6
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8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 9a Frontier molecular orbitals of L5 and Al1.
Figure 9b Intermolecular hydrogen bond assisted π-π
stacking in Al1 aggregate observed from DFT
studies.
Logic gate construction
3+
The L5 may be employed to construct binary logic gate considering “Al and EDTA” as
input and “turn on fluorescence” as output. For input, the presence and absence of both Al3+
and EDTA are assigned as 1 (on-state) and 0 (off-state) respectively. For output,
corresponding assignments are, turn-on fluorescence as 1 (on-state) and quenched/ weak
fluorescence as 0 (off-state) (Figure 10a, Table 4). Thus the logic agate function represents
4
8
AND gate having an inverter as one of its input (Figure 10b). Hence the fluorescence
3
+
changes at 400 nm with input signal Al and EDTA may be interpreted as a monomolecular
circuit showing an INHIBT logic operation.49
Real water analysis
3
+
Thus efficiency of L5 to determine Al in real water has been utilized through development
of a calibration plot as presented in Figure S20 (ESI). For this purpose, water (both drinking
water viz. D1 and D2 at two different point and industrial water viz. IW) collected from

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Asansol-Durgapur industrial area (West Bengal, India) are analysed using the developed
5
0
method and compared with the values available in literature (Table 5).
Table 4 Truth table
Input A (Al ) Input B (EDTA)
3
+
Output (400 nm)
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
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9
0
1
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3
4
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9
0
1
2
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8
9
0
1
2
3
4
5
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9
0
1
2
3
4
5
6
7
8
9
0
0
1
0
1
0
0
1
1
0
1
0
0
Figure 10a Output signal (at 400 nm) of the
logic gate in presence of different inputs.
Figure 10b General representation of an INHIBIT gate.
Table 5 Determination of Al3+ in industrial water
Sample
Al3+ added (µM) Emission intensity Al3+ found (µM) Recovery (%)
Drinking water (D1)
05
09
-
572.01
879.46
978.21
4.38
8.12
9.33
87.60
90.22
-
Drinking water (D2
Industrial water (IW)
Conclusion
Single crystal X-ray structurally characterized anthracene appended unsymmetrical azine
3+
derivative (L5) shows Al assisted aggregation induced emission in SDS medium. The weak
emission of L5, attributed to PET and -CH=N isomerisation, intensifies upon addition of Al3+
3
+
in solution. The emission intensity of the [L5+Al ] adduct (Al1) further enhances in solid
3+
state, a phenomenon that corroborates aggregation as proposed Al sensing mechanism. The
π-π stacking among neighbouring anthracene units, assisted by intermolecular hydrogen
3
+
bonding between [L5+Al ] adduct (Al1) is the driving force of AIE. The probe offers LOD
3
+
4
-1
3+
and binding constant for Al as 62.80 nM and 9.14×10 M respectively. In presence of Al ,
the average fluorescence lifetimes (<τ>) of L5 enhances from 0.98 ns to 1.45 ns. The DLS
3
+
studies reveals that addition of Al significantly increases the average particle size of L5
indicating aggregation. Moreover, cylindrical shaped L5 turns into cube shaped aggregate in

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presence of Al . DFT studies support Al assisted aggregation of L5. A combination of
NOT and AND gate is used to construct the logic function to describe ON/ OFF fluorescence
3
+
switching of L5 upon sequential addition of Al and EDTA. The L5 may successfully be
3
+
used to determine Al concentration in water samples.
0
1
2
3
4
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9
0
Acknowledgement
SK is grateful to CSIR, New Delhi for fellowship. Financial assistance from DST (Inspire
fellowship), New Delhi is greatly acknowledged (SC and PM). We acknowledge USIC, BU
for extending SC-XRD and FESEM facilities. We sincerely thank T. K. Das, UGC-DAE,
Kolkata for helping with DLS and TCSPC studies.
Supporting Information
Contains ESI-MS, NMR, FTIR, UV-Vis. and fluorescence spectra, single crystal X-ray
diffraction and DFT data.
Author information
Department of Chemistry, The University of Burdwan, Golapbag, Burdwan, India, Fax: +91-
3
42-2530452; Tel: +91-342-2533913 (ext. 424); E-mail: ddas100in@yahoo.com
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