Synthesis of an aggregation-induced emission (AIE) active salicylaldehyde based Schiff base: Study of mechanoluminescence and sensitive Zn(II) sensing

Article abstract of DOI:10.1039/c7tc03046k

Syntheses of multi-functional Aggregation-Induced Emission (AIE) active molecules in a simple manner have been drawing great attention in current luminescence materials research. In this report a simple diamine molecule (N¹-tritylethane-1,2-dia

Full text of DOI:10.1039/c7tc03046k

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Fine-tuning of gold nanorod dimensions and plasmonic properties using
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Synthesis of ‘Aggregation Induced Emission (AIE)’ Active
Salicylaldehyde based Schiff Base: Study of
Mechanoluminescence and Sensitive Zn(II) Sensing
Received 00th January 20xx,
Accepted 00th January 20xx
Sheik Saleem Pashaa, Hare Ram Yadavb, Angshuman Roy Choudhuryb and Inamur Rahaman
Laskara*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Syntheses of multi-functional ‘Aggregation Induced Emission (AIE)’ active molecules in a simple manner have been drawing
a great attention in current luminescent material research. In this perspective, a simple diamine molecule (N1-tritylethane-
1,2-diamine(1)), is reacted with salicylaldehyde by Schiff-base technique which results a new AIE active organic molecule
[2-((2-(tritylamino)ethylideneamino)methyl)phenol (2)]. Computational calculations support the nature of transition is
intra-molecular charge transfer/twisted intramolecular charge transfer (ICT/TICT). The mechanism of AIE has been
attributed to restricted intramolecular rotation (RIR). Packing diagram supports the nature of aggregation which
corresponds to J-aggregation. The compound, 2 exhibits an irreversible mechanoluminescent (ML) property with a drastic
colour change from blue to green (λmax, 445nm → 512nm) upon grinding. But it undergoes a reversible transition with the
same colour change (blue → green) through applying pressure axially (using a hydraulic press). Again, the reversible
transition is observed by lowering the temperature of 2 to liquid nitrogen. The causes of such transitions showing emission
colour upon varying triggers have been investigated. In addition, the compound, 2 has been successfully tested in sensing
Zn(II) which shows a rare turn-on luminescent change and the mechanism behind it has been explored. The detection limit
of Zn(II) is determined to 0.064ppm.
The study of the property of mechanoluminescence (ML) of
luminescent materials results in a huge interest from the scientific
communities because of their importance in applications such as
Introduction
Luminescent materials which have been getting a great deal of
attention in research world since the last two decades, because of
their utility in various fields, such as in organic light emitting diodes,
1-3bio-imaging,4-10 chemo sensors11-14etc. There has a serious issue
in using these materials successfully for solid state applications
because of detrimental ‘Aggregation Caused Quenching’ (ACQ)
effect.15-20 In this case, the materials showing emission in solution or
molecular state but not in solid or aggregated state because of
formation of delocalized excitons. To overcome this effect, Tang
and his co-workers20 introduced a novel phenomenon in 2001 i.e.'
Aggregation Induced Emission' (AIE)15-20, materials which shows no
or weak emission in molecular state whereas emission intensity is
largely raised in aggregated state. This phenomenon opened up a
new avenue for applications of solid state luminescent materials.
pressure sensors, luminescent switches, memory devices,
healthcare, security inks etc.21-33 The ML property is resulted from
the external stimuli such as applied pressure, grinding, stress,
shearing, rubbing, milling and crushing of the solid state.21-33 In
1993 Gawinecki et. al34 reported low molecular weight mechano-
responsive pyridinium perchlorate compound, which is the first
report on ML. Mishra et. al reported35 4-(1,3-Dioxo-2,3-dihydro-1H-
phenalen-2-ylmethyl)-N-pyridin-3-yl-benzamide and 4-(1,3-Dioxo-
2,3-dihydro-1H-phenalen-2-ylmethyl)-N-pyridin-4-yl-
benzamide which are ML active with emission wavelength
difference of 31nm and 45nm, respectively. Yang et. al reported36
ML turn on of Zn(II) complex which was synthesized based on a new
Salen-type tetradentate N2O2 bisoxime chelate ligand (H2L) derived
from 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (PMBP) and 1,2-
bis(aminooxy)ethane. Fujihara et al also reported37 [Pt (ppy)L] [ppy
=
2-phenylpyridinato, L = Glycinato, Alaninato, Leucinato,
a.Department of Chemistry, Birla Institute of Technology and Science, Pilani Campus,
Pilani, India *E-mail: ir_laskar@pilani.bits-pilani.ac.in
Isoleucinato, Phenylalaninato and Sarcosinato] which exhibited ML
with a difference of wavelength 88nm, 68nm, 28nm, 20nm, 6nm
and 45nm, respectively.
b.Department of Chemical Sciences, Indian Institute of Science Education and Research
(IISER), Mohali, Sector 81, S. A. S. Nagar, Manauli PO, Mohali, India E-
mail: angshurc@iisermohali.ac.in
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Most of the reported ML active materials have been suffering from
ACQ effect and showing a limited wavelength difference before and
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after grinding part,21-33 which restricting them for useful crystal data were collected at 293K and 90K for 2 and at 293K for
applications. The ML property if it is observed in AIE active 2b.
materials can alleviate such problems. Additionally, the AIE active
materials are more susceptible to exhibit ML property. Tang et al Syntheses and characterizations
reported38
a
compound,
2-amino-3-{E- General Syntheses of N1-tritylethane-1,2-diamine, 151 In a two-
[4(dibutylamino)benzylidene]amino}maleonitrile (DBA) which was necked round-bottom flask (50 mL), trityl chloride (100 mg, 0.359
identified as AIE active and showing ML property. Recently, our mmol), Et3N (50 μL, 0.359 mmol), and ethylenediamine (27.0 μL,
group also reported16 Schiff-base based AIE active ML (E)-2- 0.359 mmol) were dissolved in 10ml THF and stirred overnight at
[(pyridin-2-ylimino)methyl]phenol with the observation of room temperature. After the solvent was evaporated under
maximum emission wavelength difference by 23nm. In continuation reduced pressure, the crude product was collected and product was
of our earlier work, we have designed and synthesized of an purified by column chromatography (60-120 mess of silica gel).
‘aggregation induced emission’ (AIE) active materials which Yield, 70%. 1HNMR (400 MHz, CDCl3) δ 7.55−7.48 (m, 5H), 7.35−7.27
possessing ML property in a simplified approach.
(m, 7H), 7.25−7.17 (m, 3H), 2.83 (t, J = 6.0 Hz, 2H), 2.23 (t, J = 6.0 Hz,
2H), 1.63 (s, 3H); ESI-HRMS calculated ([M + H]+) m/z 302.1783,
Zn(II) ion is a biologically important metal, which is involved in many
biological reactions. The deficiency of Zn(II) in children can
cause growth retardation, delayed sexual maturation, infection
susceptibility and diarrhea. Consumption of excess Zn(II) ion can
cause ataxia and lethargy.39-43 Thus, it is necessary to have a
convenient process to detect Zn(II) in environment. Although many
reports are there for luminescent based Zn(II) sensing44-46, but
found ([M + Na]+ )m/z 325.1764, ([M + TFA + H + Na]+ ) m/z
423.1753 (Fig S1, ESI).
General Synthesis of [2-(2-(tritylamino)ethylideneamino) methyl
phenol], 2. Salicylaldehyde (0.82mmol, 0.1g), N1-tritylethane-1,2-
diamine (0.82mmol, 0.247g) and catalytic amount of acetic acid
were dissolved in
a 5ml of methanol and stirred at room
sensing with AIE active compounds are rarely found47
.
temperature. After 30min of stirring, a light green precipitate was
separated out, filtered and dried under vacuum oven for 15 min.
The crude product was recrystallized by methanol which produced
a solid product. Yield, 90%. 1H NMR (400 MHz, CDCl3) δ 13.41 (s,
1H), 8.44 (s, 1H), 7.53 – 7.44 (m, 6H), 7.32 (dddd, J = 10.2, 7.9, 5.0,
1.6 Hz, 8H), 7.24 – 7.15 (m, 3H), 6.94 (ddd, J = 17.1, 8.5, 4.6 Hz, 2H),
3.74 (t, J = 5.5 Hz, 2H), 2.54 – 2.45 (t, 2H) : 13C NMR (101 MHz,
CDCl3) δ 165.88, 161.19, 145.92, 132.26, 131.29, 128.57, 127.91,
126.34, 118.79, 118.57, 117.04, 70.81, 60.40, 44.18.
Herein we reported an AIE active simple organic molecule 2 which
has been synthesized by a simple technique This compound exhibits
reversible and irreversible ML property depending upon the
shearing or axial stress acting upon the compound resulting a wide
separation of emission wavelength. A thorough study of the
mechanism of AIE and ML property of 2 has been carried out.
Additionally, the selective sensing of Zn(II) with 2 has been
elaborated.
Preparation of solution of mixtures of MeOH / H2O solution of 2
for PL measurement: 50 ml 10-5 M stock solution of 2 was prepared
in methanol. Ten 5 ml glass tubes were taken and labeled them as
0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% and 90%. 0.5 mL of
stock solution was added to each of the flask. Then, volume was
filled to 5 ml through addition of 0 ml, 0.5 ml, 1ml, 1.5 ml, 2ml, 2.5
ml, 3.0 ml , 3.5 ml, 4 ml and 4.5 ml water to 0%,10%, 20% 30%, 40%,
50%, 60% ,70%, 80% and 90% labelled flasks, respectively.
Experimental
Materials: Salicyldehyde, ethylenediamine and tritylchloride were
purchased from Sigma Aldrich Chemical Company Ltd. The UV grade
solvents (DCM, Hexane, 1,4-dioxane, chloroform, acetone,
methanol, DMSO, DMF and acetonitrile), NaCl, triethylamine and
acetic acid were procured from Merck Company. The other used
solvents and salts were procured from Merck and Spectrochem
Chemical Company, respectively.
Method of preparation of solution 2 for Zn(II) sensing: 50 ml 10-5
M stock solution of 2 was prepared in methanol. Thirteen 5 ml glass
tube were taken and labelled them as Blank, Mg2+, Zn+2, K+, Na+,
Hg+2, Cd+2, Ca+2, Cu+2, Ba+2, Bi+3, Co+2 and Pb+2. Then 3mL of stock
solution and 10equivalent of corresponding metal salt was added to
the labelled glass tube.
Characterization: H NMR and 13C NMR spectra were recorded in a
1
400 MHz Brucker NMR spectroscope. UV-Vis absorption spectra
were recorded in a Shimadzu Spectrophotometer (model UV-1800
and 2550). Steady state photoluminescence (PL) spectra were
recorded on Horiba Jobin Yvon Spectro fluorometer (FluoroMax-4).
Particle sizes of the nano-aggregates were determined on
a
Computational study: The Density Functional Theory (DFT) based
calculations were performed using Gaussian 09 program suit.52
Becke’s three-parameter hybrid exchange functional with Lee–
Yang–Parr gradient-corrected correlation (B3LYP functional) 53 and
the basis set 6-31G(+) level 54 were used in both the DFT and TD-
DFT methods. All calculations were performed in methanol
(ε = 32.7) using the Integral Equation Formalism-Polarizable
Continuum Model (IEF-PCM) for the solvent.
Malvern Zetasizer (MAL1040152). Powder X-Ray Diffraction (XRD)
were recorded with a Rigakuminiflexll, λ =1.54, Cu Kα.
Single crystals Analysis: Suitable single crystals of C28H26N2O (2) and
C56H52N4O2Zn (2b) were collected. The single crystal was mounted
on a Rigaku Mercury 375R (2x2 bin mode) diffractometer. The
structure was solved by direct methods with using Olex248 and
SHELXT49 structure solution program. It was refined with the
SHELXL50 refinement package using Least Squares minimisation. The
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structured blue-shifted emission was observed with λmax, ~407nm
(Fig. S4, ESI). These results imply that the nature of transition in
Detection limit calculation:55-56 To determine the Signal/Noise ratio,
2
shows intramolecular charge transfer/twisted intramolecular
charge transfer (ICT/TICT) transition.57-59 To support the fact, the
geometry of
the emission intensity of
2 was measured by 10 times and the
standard deviation of blank measurements was determined. The
detection limit was then calculated with the following equation,
detection limit = 3σ/m; where σ is the standard deviation of blank
measurements, m is the slope between the plot of PL intensity
versus sample concentration.
2
was optimized by Gaussian 09. 52 The results exhibits
that the HOMO states are mainly localized one of the appended
phenyl rings of trityl group and LUMO states are mainly localized on
salicylaldehyde phenyl ring (Fig. 1). Hence, the localization of
HOMO on the donor part of the molecule (trityl) and localization of
LUMO on the acceptor part of the molecule (salicyldehyde)
ensuring the ICT/TICT transition. The calculated band gap is found
to 3.2eV which corresponds to the experimentally obtained band
gap (3.6eV). The TD-DFT calculation shows a good agreement with
the experimentally obtained absorbance band with λmax at 315nm
(ε=11157 Lmol-1cm-1) (λcalc~290nm, f=0.5148), 255nm (ε=36313.5
Lmol-1cm-1) (λcalc~236 nm, f=0.0224), 228nm (ε=82137 Lmol-1cm-1)
(λcalc~228 nm, f=0.3064) in DCM (Fig S5 & Table S4, ESI). The
Results and discussion
Trityl chloride was reacted with ethylenediamine which produced
tritylsubstituted ethylenediamine48
,
1
.
This compound,
reacted with salicylaldehyde by using Schiff-base synthetic
methodology and generated
(Scheme 1). 1H NMR and the mass
spectrum of support the formation of the compound, (Fig. S1,
ESI). The solid product, was purified by recrystallization and
1 was
2
structured emission was observed by
2 in non-polar solvents with a
1
1
distinct two peaks at λmax, 407 and 432nm (Fig. S4, ESI). The
separation of these peaks corresponds to ~1421cm-1 which is
attributed to the vibrational progressions related to the stretching
frequency of C–C double bonds at the aromatic ligands.60
2
characterized by 1H, 13C NMR and single crystal XRD.
Scheme1. Synthetic route and chemical structures of 1 and 2
1H NMR spectrum of
2 shows aromatic proton signals in the range
δ= 7.21−7.51ppm, the four protons of (CH2-CH2) are observed as
double triplets at δ = 2.51 and 3.74ppm, -OH and HC=N protons are
observed as singlet at δ = 13.42 and 8.44ppm, respectively (Fig. S2,
a
ESI). The 13C NMR spectra of
2 shows aromatic carbon signals in the
range δ = 116.93−161.19ppm, the three aliphatic carbon signals at
44.18, 60.25, 70.69ppm and C=N carbon signal observed at δ Fig
.
1
(a) X-ray Single crystal structure of
2
; (b) DFT based ground
state optimized HOMO and LUMO orbitals of
2
165.68ppm (Fig. S3, ESI). The single crystal of
slowly evaporating solvent at room temperature. The crystal
structure of is triclinic system with space group P−1. Each unit cell
2 was obtained by
2
In the crystal structure of 2, the packing diagram shows head to tail
contains two molecules which are oriented in anti-parallel to each
other. There has one intra-molecular hydrogen bonding (N1....H1-
O1, 1.895Å) as shown in the packing diagram (Fig 2a).
arrangement of two molecules and that extends in one-dimension
(Fig. 2c and S7, ESI). The packed molecules in one-dimension
looking like to form a ladder type arrangement which substantiate
the formation of J- aggregation (Fig. 2c and S7, ESI).61 Furthermore,
spectroscopic study shows red-shifted absorption (λmax, 360nm) and
Aggregation Induced Emission (AIE) property
emission (λmax
aggregation (Fig. S6, ESI).62
Compound is well soluble in methanol, whereas it is insoluble in
, 445nm) band supporting the formation of j-
2
shows low energy absorbance bands with λmax, at 402nm and
411nm, respectively after dissolving in polar methanol and DMSO.
On recording emission spectra in the same solvents, shows a
2
2
water. The combinations of these two solvents have been used to
examine the AIE property. After this experiment, three notable
observations are shown: firstly, the emission intensity increases
from 0% to 50% water concentration, secondly, PL intensity
broad emission band with λmax 449nm and 451nm, respectively (Fig.
S4, ESI). In non-polar solvents (THF and DCM), it was not observed
any noticeable absorption in the range, 400nm-450nm and a
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gradually decreases from 50% to 90% and thirdly, the emission
wavelength shifts from 449nm to 497nm (from 0% to 50%) (Fig. ). It
3
has been performed several experiments to find out the causes of
such variations of PL intensity.
Fig
. 2 (a) and (c) show packing diagram of 2 at room temperature; (b) and (d) show packing diagrams of 2 at liq. N2 temperature
(interaction shown in Figure in Å unit)
A set of solutions have been prepared of
2 with different solvent the mechanism of AIE activity of 2 is mainly operated through
mixture of methanol and polyethyleneglycol (PEG, viscous solvent) restricted intramolecular rotation (RIR). In the packing diagram (Fig.
in the range of 0-90% (to a fixed amount of methanol, with 2a and 2c), it is clearly observed of several intermolecular short
gradually increasing order of PEG) which lead to the enhancement contacts where the rotating unit, the phenyl is involved and
of emission intensity (Fig. S8, ESI). This experiment proposes that become responsible for showing restricted intramolecular rotation
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and blocking of non-radiative channels and subsequent support of such transition has been obtained from differential
enhancement in emission intensity.
scanning calorimetry study. Both 2 and 2a show the same melting
point at ~1040C, whereas only 2a showing glass transition
temperature at 490C (Fig. S11, ESI). The ground compound, 2a
remains stable in solid phase for long days (checked up to 15 days).
The 50% and 90% water fraction solutions were centrifuged,
filtered and the residue at each fraction were collected. Then these
were characterized by powder XRD. At 50% water fractions, the
observed diffraction peaks are sharp and intense indicating the
crystallinity whereas at 90% fractions, broad peaks are observed
It is reverted into the original compound,
2 after dissolving it into
dichloromethane followed by recrystallization.
indicating amorphous nature (Fig. S9, ESI). Although RIR effect On applying pressure with a hydraulic press (10ton pressure kept
should increases the PL intensity with increasing concentration of
water, but oppositely acting the transformation from crystallinity to
amorphous texture reduces the PL intensity. Hence at 50% water
fraction the PL intensity gets reduced and decreases further with
increasing concentration of water.
for 5min),
= 445nm→ 520nm). In the following case, 2b is slowly reverted into
the pristine form, (10h). On lowering the temperature of to the
temperature of liquid nitrogen (78K), the emission colour of is
changed into bluish-green ( max = 445nm → 490nm) (Fig. 6) which is
transformed reversibly into the original compound.
2 is transformed into a green emitting complex, 2b (λmax
2
2
2
λ
Because of increment in the polarity from 0% (only MeOH) to 50%
(MeOH & Water mixture more polar than 0% water in methanol)
excited state of
2 interacts with dipoles of solvent molecules
leading to a stabilization of ICT/TICT excited state with lowering
energy resulting a red-shifted emission (Fig. 3). The formation of
aggregation was confirmed by DLS study and the average size of the
aggregates is 359 nm at fw = 50% (Fig. S10, ESI).
Fig.4(a) Shows luminescent images of as-synthesized and ground
sample of (photograph taken under 365 nm UV illumination); (b)
Emission spectra of as-synthesized solid and ground sample of ; (c)
Maximum emission wavelength change upon repeated grinding-
recrystallization cycles of
2
2
2.
Fig.3(a) Luminescent images of
mixed solvents with the concentration kept at 2 × 10-5 M; (b) PL
spectra of in MeOH/water mixed solvents with different fw with
2 (λexc, 365nm) in water-MeOH
2
excitation at 365nm; (c) The changes of PL peak intensity with
different fw (λmax at 497 nm).
Mechanoluminescence (ML)
The blue emitting compound,
2 (λmax = 445nm) in solid state is
transformed into a green emitting form, 2a (λmax = 512nm) upon
grinding (Fig. 4) with a quite large shifting of emission wavelength
(67nm). Powder XRD pattern of
2 shows a sharp and intense peaks
which indicates the crystalline nature, whereas the observation of
relatively lower intense peaks of the ground sample, 2a, pointing
less / or semi crystalline nature (Fig. 5). The peak position for the
ground sample has been changed from the original one. Additional
Fig. 5 Powder XRD of as-synthesized and ground sample of 2.
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To investigate the mechanism behind different luminescence AIE active
behaviour with several external triggers [temperature, axial force the presence of O, N coordinating sites with appropriate ring-size
(using hydraulic press) and shearing force], crystal structures have ligands. The emission intensity of in methanol was very weak (λmax
2 has a great potential to sense metal cation, because of
2
been obtained from the crystal data collecting at room temperature 449nm; λexc 365nm) because of cis-trans isomerisation of 2 with
and low temperature (78K, Fig.2b & 2d, Fig. S7b, ESI) of the same
crystals of 2. The packing diagram obtained from room temperature
crystal data showing two molecules are placed in antiparallel
orientation forming a molecular pair held by several interactions
(two H-bonding and three short contacts) (Table S5, ESI). A long
range one-dimensional chain type pattern is generated through
interactions of each of these molecular pairs to the neighbouring
pair held with a total of four interactions and it extending in a single
dimension (Fig.2c, Table S5). Each of these one dimensional chains
is again involved in interacting sideways (through a total eight
interactions) to another one which is sited parallel to it and the
pattern extends in both sides forming a two-dimensional sheet (Fig.
S7a, ESI).
It is to be noted that the complex
2 exhibiting ML by transforming
reversibly under applying a normal force (by hydraulic) and by
lowering temperature at 78K, too whereas it is transformed
irreversibly under applying shearing force (grinding in a mortar
pestle). From the crystal structure at the room temperature, it is
observed that the presence of several short contacts bind the two
molecules which leading to form a molecular pair positioned in a
symmetric fashion. On comparison of this structure with the one
recorded at low temperature (90K), there has no notable changes
observed (including short-contacts) within the molecular pair.
Hence there has no appreciable change observed on the structure
of molecular pairs by lowering temperature. Furthermore,
comparison to the next level of structure where the molecular pair
unit itself involved to interact with the neighbouring molecular pair
leading to one-dimensional molecular chain (two inequivalent short
contacts are present, 2.882 and 2.752Å, between two molecular
pairs, Fig. 2c). But on comparative study of it with the one whose X-
ray structural data is recorded at liq. N2 temperature, there has no
significant structural changes observed (four new molecular
interactions observed) (Fig. 2d). This minor structural change on
lowering temperature results a change in electron distribution and
Fig.6(a) Shows luminescent images of
(λexc = 365nm); (b) Emission spectra of as-synthesized solid, under
liq.N2 and under hydraulic pressure of ; after removal of triggers
(hydraulic press / rising of temperature) the emission spectra
(blue/red) is reverted to the original one (black); (c) Change of
maximum emission wavelength upon changing condition of room
2 under various conditions
2
temperature and liq.N2 of
2 for several times (room temperature,
λmax, 445nm and liq N2 temperature, λmax= 490nm); (d) Maximum
emission wavelength change upon repeated under pressure to
normal condition cycles of 2.
respect to C=N moiety and rotation of triphenyl group in solution
state. The weakly emissive solution of is transformed into a bright
2
blue solution in presence of Zn(II) salts. We have studied the
selectivity test and found that there has no change of emission
colour in the presence of different metal ions such as Mg2+, Zn+2, K+,
Na+, Hg+2, Cd+2, Ca+2, Cu+2, Ba+2, Bi+3, Co+2 and Pb+2 (Fig.7a & 7b). At
room temperature reaction, none of the congeners of Zn(II) [Mg(II),
Cd(II) and Hg(II)] results any emission after addition of 2 into their
respective salts [Mg(NO3)2, Cd(NO3)2 and HgCl2] in methanol. It is
hence the emission colour. But the overall structures regain the observed that Mg(II) and Cd(II) salts result green and blue emission,
respectively if the reaction mixture is heated at 70°C with stirring for
~15min (Fig. S12, ESI). But Hg(II) salt doesn’t produce any
emission even upon heating for longer time. While Zn(II) salt shows
the following emission instantaneously on adding of 2 at room
temperature. It can be explained based on the general trend of
reactivity of Zn(II) which is expected to be higher as compared to its
congeners. Furthermore, Hg(II) and Cd(II), being softer cann’t easily
react with the hard ligated donors N and O in 2. The emission
intensity was found to ~23 times higher in presence of Zn(II) metal
ion as compared to blank solution (other metal ions do not interfere
original symmetry and hence the stable form with gradual rising of
temperature. This phenomenon can be extended to the case which
rationalizes the reversible transformation that occurring by applying
normal stress (hydraulic press) and this fact can be related to the
elastic mechanoluminescent. From these studies, it is speculated
that reason behind the irreversible transformation as observed
under shearing stress (grinding) can be rationalized by the
permanent deformation occurring into the molecular pairs from
their regular arrangements. The crystal data for the compounds
2
(at 293K and 90K) and 2b (at 293K) have been given in Table S1, S2
into the emission spectra). The absorption spectra of
2 shows two
and S3, respectively.
peaks at 314 and 402nm, which are gradually decreasing and
absorption at 352nm is gradually increasing with addition of Zn(II)
metal ion (Fig. S13, ESI), in addition it is observed two isosbestic
points at 337nm and 373nm. It indicates Zn(II) metal ion is
Sensory Application
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interacting with 2. Titration experiment was performed by the help
rationalized. Furthermore, it selectively senses Zn(II) through
turn-on emission signal with detection limit 0.064ppm and the
of steady state fluorescence spectrophotometer. Gradual addition
of Zn(II) metal ion into the methanolic solution of
2
, emission cause behind it has been rationalized.
intensity rises up at λmax, 410nm and 446nm (Fig. S14, ESI). A linear
relationship was found between concentration of Zn(II) metal ion (0
Acknowledgements
We thank the ‘Department of Science and Technology (DST),
Govt. of India’ for financial support under the project SB/S1/IC-
13/2014 and Council of Scientific and Industrial Research
(CSIR) no. 01/2551/12/EMR-II. We also Acknowledge ‘UGC-
sponsored Special Assistance Programme (F.540/14/DRS/2007,
SAP-I)’ and DST-FIST for the instrumental support. We are also
thankful to physics department, BITS Pilani, India for X-ray
powder diffraction facility sponsored by DST-FIST. We are
thankful to Prof. S. K. Saha, Department Of Chemistry, BITS
Pilani, Rajasthan, India for helpful discussion.
to 12
indicating sensitivity of
limit was calculated as ~0.064ppm, with the help of 3
µ
M) with PL intensity of
towards Zn(II) metal ion. The detection
/m, where
2 (Fig. S14, ESI) at 410nm and 446nm,
2
σ
σ
is a standard deviation of the blank measurements and m is the
slope. Furthermore, to evaluate the selectivity for Zn(II), the
fluorescence spectra of
2 was recorded in presence of Zn(II) and the
various metal ions [Mg(II), Zn(II), K(I), Na(I), Hg(II), Cd(II), Ca(II),
Cu(II), Ba(II), Bi(III), Co(II) and Pb(III)] was added and found that
there has no detectable change of emission (Fig. S15, ESI). To grow
single crystals, 10 equivalents of Zn(II) salts was added into the
methanolic solution of
2 and kept aside for overnight and obtained
the single crystals of Zn(II) complex (2b). The analysed single crystal
structure of the complex was found to form bis complex where the
ligand is coordinated in chelating mode forming a six-membered
ring (Fig.7c & Table S3, ESI ).
Notes and references
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