Anthracene Based AIE Active Probe for Colorimetric and Fluorimetric Detection of Cu2+ Ions

Article abstract of DOI:10.1515/zpch-2018-1215

A novel aggregation induced emission (AIE) active anthracene based dihydroquinazolinone derivative (probe 1) has been synthesized and characterized by means of spectroscopic methods. The photophysical properties of this probe have been investigated in solvents of different polarity display that fluorescence states are of intramolecular charge transfer (ICT) character. Probe 1 show clear AIE behavior in water/THF mixture on reaching water fraction 95%. The AIE behavior of probe 1 have been exploited for the detection of metal ions in aqueous solution which reveals high selectivity and sensitivity towards Cu2+ ions by colorimetrically and function as a chemosensor in a remarkable turn-off fluorescence manner. Further, the experimental results were investigated by computational means by optimizing the ground state geometries of probe 1 and probe 1-Cu complex using density functional theory (DFT) at B3LYP/6-31G and B3LYP/6-31G (LANL2DZ) levels of theory. Intra-molecular charge transfer was observed in probe 1 while ligand to metal charge transfer (LMCT) for probe 1-Cu complex.

Full text of DOI:10.1515/zpch-2018-1215

Z. Phys. Chem. 2018; aop
Abdullah G. Al-Sehemi, Ahmed Irfan, Mehboobali Pannipara*,
Mohammed A. Assiri and Abul Kalam
Anthracene Based AIE Active Probe for
Colorimetric and Fluorimetric Detection
of Cu2+ Ions
https://doi.org/10.1515/zpch-2018-1215
Received April 23, 2018; accepted October 10, 2018
Abstract: A novel aggregation induced emission (AIE) active anthracene based
dihydroquinazolinone derivative (probe 1) has been synthesized and character-
ized by means of spectroscopic methods. The photophysical properties of this
probe have been investigated in solvents of different polarity display that fluores-
cence states are of intramolecular charge transfer (ICT) character. Probe 1 show
clear AIE behavior in water/THF mixture on reaching water fraction 95%. The AIE
behavior of probe 1 have been exploited for the detection of metal ions in aque-
ous solution which reveals high selectivity and sensitivity towards Cu2+ ions by
colorimetrically and function as a chemosensor in a remarkable turn-off fluores-
cence manner. Further, the experimental results were investigated by computa-
tional means by optimizing the ground state geometries of probe 1 and probe 1-Cu
complex using density functional theory (DFT) at B3LYP/6-31G** and B3LYP/6-
31G**(LANL2DZ) levels of theory. Intra-molecular charge transfer was observed
in probe 1 while ligand to metal charge transfer (LMCT) for probe 1-Cu complex.
Keywords: AIE active; anthracene; Cu2+ ions
*Corresponding author: Mehboobali Pannipara, Department of Chemistry, Faculty of
Science, King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia; and Research
Center for Advanced Materials Science (RCAMS), King Khalid University, P.O. Box
9004, Abha 61413, Saudi Arabia, Tel.: +966 553956503, Fax: +0096672418426,
e-mail: mpannipara@kku.edu.sa, mhbb25@gmail.com
Abdullah G. Al-Sehemi, Ahmed Irfan, Mohammed A. Assiri and Abul Kalam: Department of
Chemistry, Faculty of Science, King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia;
and Research Center for Advanced Materials Science (RCAMS), King Khalid University, P.O. Box
9004, Abha 61413, Saudi Arabia
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1 Introduction
Design and fabrication of stimuli responsive organic fluorescent materials have
been receiving tremendous attention in recent years in the field of science and
technology as its optical properties can be tuned for numerous applications such
as sensors, displays, switches, security inks, and optoelectronic devices [1–4].
The tunable optical and electrical properties of these π-conjugated molecules for
such applications arise from the non-covalent interaction associated with molec-
ular packing or self-assembly of molecules in the condensed state [5]. Hence,
developing such high performance smart switching materials with desirable pho-
tophysical properties could be achieved by altering the molecular packing of the
fluorophore by controlling the intermolecular interactions [6].
Fluorescence based molecular probes has witnessed a tremendous growth
over the last decades for the selective and sensitive detection of heavy metal ions
as the method of operation is rapid, convenient and simple [7, 8]. Stimuli respon-
sive molecular material are considered to be ideal candidates for the use for chem-
ical sensor as the change in molecular configuration brought out by the external
stimulus or analyte can be detected either by colorimetric orfluorimetric approach
[9, 10]. Compared with the procedures based on expensive equipment, sensing of
metals with the help of easily synthesized receptor and nominal instrumental sup-
port with the help of small organic molecule-derived probes established on color
or fluorescence changes has the benefits of low cost, high sensitivity and appro-
priate operation. A colorimetric probe will stimulate color changes on addition of
the target ion which can be observed by the naked eyes. A fluorometer or even
a portable UV lamp can be used to analyze the fluorescent changes imparted by
fluorescent probes [11]. Designing and developing such a probe involves linking a
recognition unit to signal reporting chromophore that can act as efficient binding
site [12].
The explorations of novel AIE-active materials are currently attracting signifi-
cant attention especially for to design the ligands having selective and significant
binding abilities towards various metal ions. The toxic effects of heavy metal ions
in the environment have been well-documented, and a large number of selective
and sensitive sensors for the determination of heavy metals in varying chemi-
cal systems have been continually devised. Among them, AIE-active molecular
switches with multi-stimuli-responsive features with potential binding units such
as SH, OH, and NH towards various metal ions are currently attracting signifi-
cant attention for to design the ligands for the selective and significant detection
in a broad range of environments [13, 14]. Recently, we have reported AIE active
Schiff base derivative fluorescent probes for the rapid detection of Cu2+ ions via
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Anthracene Based AIE Active Probe
| 3
a simple and effective method [15]. AIE-active fluorescent probes, as compared
to other fluorescent probes that are available in the literature for sensing mostly
work only in organic solvents, enjoy great advantages as it mostly works in aque-
ous media as water is responsible for aggregation in most cases and the detection
limit is too low. The current work report on the photophysical properties of an AIE
active anthracene appended dihydroquinazolinone derivative (probe 1). The AIE
behavior of probe 1 was further exploited for the selective and sensitive determi-
nation of Cu2+ ions through remarkable turn-off fluorescence manner colorimet-
ric response towards Cu2+ along with colorimetric response under physiological
conditions.
2 Experimental
2.1 Materials and methods
Analytical grade chemicals and solvents used in this study were used without
further purification. Reaction was monitored by thin layer chromatography (TLC)
with the aid of UV light. 2-amino benzamide, 9-Anthracenecarboxaldehyde, sul-
fanilic acid and metal salts were purchased from Sigma–Aldrich and Merck. Stock
solution of the title compound and its dilutions were prepared for UV–visible and
fluorescent study at different concentrations. Stock solutions of the salts of Ag+,
Co2+, Cu2+, Ni2+, Hg2+, Zn2+, Mn2+, Cd2+and Pb2+ were prepared in distilled
water and test solutions for metal ion detection were prepared by diluting appro-
priate aliquot of each metal ion stock and compound stock solution to desired con-
centration. Twenty micromole solutions of probe 1 in THF or water/THF mixtures
with different water fractions were prepared to analyze the AIE characteristics.
2.2 Spectral measurements
Gallenkamp melting point apparatus was used to determine the melting point and
the infrared (IR) spectra were recorded on Shimadzu FT-IR 8400S infrared spec-
trophotometer using KBr pellets. The NMR (1H and 13C) spectra were recorded on
a Bruker DPX-600 at 600 MHz and 150 MHz, respectively, using TMS as the inter-
nal standard. The chemical shift values are documented on δ scale and coupling
constants (J) in Hertz; Splitting patterns were entitled as follows: s: singlet; d:
doublet; m: multiplet. PG UV-160A spectrophotometer was used to record the UV-
Vis electronic absorption spectra, and the steady-state fluorescence spectra were
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A. G. Al-Sehemi et al.
Scheme 1: Synthetic route for probe 1.
measured using Varain Cary Eclipse spectrofluorophotometer using a rectangular
quartz cell of dimensions 0.2 cm × 1 cm.
2.3 Synthetic procedure for probe 1
Probe 1 was synthesized by the sufanilic acid catalyzed reaction between 2-amino
benzamide and 9-Anthracenecarboxaldehyde in aqueous ethanol (Scheme 1)
[15]. A mixture of 2-amino benzamide (10 mmol), 9-Anthracenecarboxaldehyde
(10 mmol) and sufanilic acid (1 mmol) in 50% aq ethanol (25 mL) was stirred
at 70 °C and the reaction progress was monitored by TLC. After the comple-
tion of the reaction as indicated from TLC, the reaction mass was allowed
stand overnight at room temperature. The obtained product was collected
by simple filtration and washed with 50% aqueous ethanol and purified
by crystallization using aqueous ethanol to get the product as off green
solid. The structure of the compound was confirmed by IR, 1H NMR and
13C NMR. Melting point: 202–204 °C;1H-NMR (600 MHz, d6-DMSO) δ: 4.18
(s, 1H, NH), 6.61 (d, 1H), 6.76 (dd, 1H), 7.29 (dd, 1H), 7.35 (dd, 1H), 7.36 (dd,
1H), 7.38 (dd, 1H), 7.4 (dd, 1H), 7.73 (d, 1H), 7.80 (d, 1H), 7.8 (d, 1H), 7.82
(d, 1H), 7.90 (1H, d), 8.10 (s, 1H), 8.20 (s, 1H, NH); 13C-NMR (150 MHz, d6-
DMSO) δ: 65.35, 112.33, 116.72, 124.49, 124.74, 125, 125.2, 125.8, 126.74, 128.2, 128.62,
128.70, 131.56, 132.30, 132.72, 132.3, 132.2, 134.2, 143.6, 168.42; FT-IR (KBr) (cm−1);
2950–3300, 1730.2, 1139.9, 1197.1, 1250.9, 1289.82, 1365.58, 1448.9.
3 Results and discussion
3.1 Photophysical and aggregation-induced emission
properties
The solvent effect on the photophysical properties of probe 1 (20 µM) has been elu-
cidated by applying UV-vis absorption and emission measurements. As inferred
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Anthracene Based AIE Active Probe
| 5
from Figure 1a, absorption spectra show typical features of anthracene chro-
mophore with characteristic vibronic patterns between 350 and 390 nm corre-
sponding to the π–π* transition without any shift in the spectra on changing the
polarity of the solvents which rules out the possibility of charge transfer char-
acter in the ground state. As seen in Figure 1b, emission spectra of probe 1 is
found to be markedly influenced by the polarity of the solvents. On exciting at
365 nm, it shows more structured emission spectra in aprotic solvents with sig-
nificant emission intensity which may arise due to attainment of more coplanar
structure during excitation whereas weakened light emission with significant red
Fig. 1: Electronic absorption spectra (a) and emission spectra (b) of receptor 1 (20 µM) in
different solvents.
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shift is observed on increasing polarity of solvents suggesting involvement of
photo induced intramolecular charge transfer (ICT) in the excited state [16]. The
emission intensity of probe 1 in dramatically decreases on increasing the solvent
polarity with disappearance of its vibrational structure, which could be attributed
to the specific solute solvent interaction such as hydrogen bonding or due to the
stabilization of the twisted intramolecular charge transfer (TICT) state by the sol-
vating effect of the polar solvents [17]. However, it is interesting note that, an
exception for this general trend was observed in the case of ethylene glycol and
water. The emission profile of ethylene glycol is slightly blue shifted ∼24 nm from
that of toluene with significant emission enhancement and somewhat structured
band, whereas a broad red shifted (56 nm) band with remarkable emission inten-
sity was observed in case of water. The slight blue shift in ethylene glycol with
fine structure could easily be explained in terms of high viscosity of the solvent in
which the emission arises from the state close to Franck-Condon state as observed
in the case of nonpolar solvents due to restriction of intramolecular rotation that
hinder the TICT state by favoring the planar aggregated state [18]. Though probe 1
is non fluorescent in alcoholic solvents, it emits strongly in water, ethylene glycol
and in solid state suggesting the presence of aggregation induced (AIE) character.
Generally, the aggregated state of compounds was brought about by dissolv-
ing it in pure solvent and then adding incremental amount of a poor solvent.
Herein, the AIE characteristics of probe 1 were investigated by dissolving it in
pure THF and then adding different volumes of water fractions (fw, vol %) from
0 to 98% by keeping the probe concentration fixed (20 µM). Figure 2 represents
the photographic images of probe 1 (20 µM) at different THF-water mixture sys-
tems taken under UV 365 nm illumination and the corresponding absorption and
emission spectra recorded at selected increment of water fraction (fw) are shown
in Figures 3 and 4. These figures clearly demonstrate AIE behavior of probe 1.
As evident from Figure 3, increasing water fractions have profound effect on the
absorption spectra of probe 1. Absorbance of probe 1 remains unchanged up to
the water fractions 90%, but a decrease in absorbance was observed on further
addition of the water fractions accompanied by the appearance of level-off tails
at higher wavelength region that could be attributed to the well-known Mei scat-
tering effect, which has been well documented in the literatures for AIE-active
molecules [19]. The reduction in absorbance at higher water fraction (fw > 95%)
may arise due to decrease in solubility of the probe 1 at higher water fraction
(fw > 95%) resulting in more insoluble particles [20].
Probe 1 (20 µM) is found to be practically non-emissive in pure THF solution
on exciting at 365 nm. However, as depicted from Figure 4, on progressive addi-
tion of water fraction to probe 1 in THF solution does not show any spectral change
on the photoluminescence features up to fw = 90%, but an abrupt fluorescence
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Fig. 2: Luminescent images of probe 1 in different water fraction (fw = 0–98%) under 365 nm
UV lamp.
Fig. 3: Electronic absorption spectra of probe 1 (20 µM) in H2O/THF mixtures with different
water fractions (fw).
amplification was observed with intense bright cyan emission at ∼472 nm on
reaching the water fraction of 95%. The emission intensity of at fw = 95% shows
a 390-fold enhancement (expressed as I–I0/I0) in comparison with pure THF solu-
tion indicating a clear observation of aggregation induced emission (AIE) at this
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A. G. Al-Sehemi et al.
Fig. 4: Emission spectra of probe 1 (20 µM) in H2O/THF mixtures with different water fractions
(fw).
water fraction. The emission enhancement behavior of probe 1 at higher water
fraction could be attributed to the formation of nanoaggregates that effectively
suppress the intramolecular rotation and C=N imine bond isomerization effec-
tively which in turn inhibit the non-radiative relaxation channels and activates
radiative decay of the excited state molecule leading to aggregated state emis-
sion enhancement [21, 22]. It was found that addition of more water fraction
(fw > 95%) results in dropping off the emission intensity; a characteristic phe-
nomenon often observed with AIE luminogens. Though the reason for this phe-
nomenon remains unclear, we speculate that the hydrophobic molecules of probe
1 adopt to aggregate into larger particles and precipitate rapidly at higher water
fractions which may cause the decrease in the number of emitting molecules in
the system and thus lowers fluorescence enhancement [23].
3.2 Colorimetric detection of metal ions
The interaction of metal ions with the probe 1 (20 µM) have been investigated at
water fraction 95%. As seen in Figure 5, probe 1 shows visual color change from
colorless to pale green upon addition of Cu2+ ions (10 equiv), whereas other met-
als ions such as Ag+, Co2+, Ni2+, Hg2+, Zn2+, Mn2+, Cd2+and Pb2+ did not display
any significant visual change in color under similar experimental condition signi-
fying the interesting feature of the probe molecule for to detect Cu2+ without any
other instrumental support. The color change may arise due to the partly filled d
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Fig. 5: Color changes of probe 1 (20 µM) after the addition respective metal ions (10 equiv.) in
THF: H2O mixtures, fw = 95%.
orbitals in Cu2+ ion where light is absorbed as electrons move from one d orbital
to another [24].
3.3 Metal binding study
The sensing properties of probe 1 against various metal ions have been inves-
tigated by applying absorption and fluorescence spectral measurements in
water/THF mixtures by keeping water fraction 95% to maintain the AIE behav-
ior. At first, in order to check recognition abilities of probe 1 with different metal
ions, metal binding test of probe 1 was evaluated using absorption measurements.
The absorption profile of probe 1 was investigated upon addition of various metal
ions (10 equiv) such as Ag+, Co2+, Cu2+, Ni2+, Hg2+, Zn2+, Mn2+, Cd2+and Pb2+
in H2O/THF (fw = 95%) solvent system. The effect of metal ions on probe 1 shows
that the complex is much sensitive to Cu2+ ions by showing changes in the absorp-
tion band. As expected, coordination of Cu2+ to probe 1 resulted in the formation
of red shifted broad absorption spectra at 473 nm with synchronous increase in
the absorption band as seen Figure 6. The increase in absorption band of probe 1
on interaction with Cu2+ may be due to the ligand-to-metal charge transfer com-
plex formation. Further, the interaction of Cu2+ with probe 1 has resulted with the
disappearance of the structured band at 350–390 nm. It is interesting to note that
other competitive cations did not show any obvious spectral changes suggesting
the selectivity of probe 1 for Cu2+ in the presence of other metal ions.
The changes in emission intensity of probe 1 (20 µM) towards various metal
ions was investigated by treating it with 10 equiv of varying metal ions namely
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Fig. 6: Absorption spectra of probe 1 (20 µM) probe 1 upon addition of Ag1+, Cd2+, Co2+
Cu2+, Mn+2, Pb2+ 2+, Hg2+, Zn2+ and Ni2+ (10 equiv.) in THF: H2O mixtures, fw = 95%.
,
,
Ag+, Co2+, Cu2+, Ni2+, Hg2+, Zn2+, Mn2+, Cd2+ and Pb2+ by keeping water frac-
tion 95%. As seen from Figure 7, dramatic decrease in the emission spectra was
observed on addition of ten equivalents Cu2+ ions, whereas no such changes
Fig. 7: Emission spectra of probe 1 (20 µM) probe 1 upon addition of Ag1+, Cd2+, Co2+
,
Cu2+, Mn+2, Pb2+ 2+, Hg2+, Zn2+ and Ni2+ (10 equiv.) in THF: H2O mixtures, fw = 95%,
,
(λex = 365 nm).
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were observed with other metal ions. In case of Cu2+, the emission intensity
suffered significantly and resulted in almost quenching of fluorescence inten-
sity compared to other tested metal ions which clearly demonstrates that probe 1
have higher selectivity towards Cu2+ ions over other tested metals. The observed
quenching efficiency is found to be about 93.83% (expressed in (I0-I0)/I0 × 100)
by the addition Cu2+ as shown in Figure 8. The observed quenching of emission
intensity by Cu2+ ions may be ascribed due to the paramagnetic nature of Cu2+
which result in chelation-enhanced quenching (CHEQ) by metal to ligand charge
transfer reaction between probe 1 and Cu2+ [25].
Fluorescence titration experiment was carried out to have more insight into
the sensing properties of probe 1 towards Cu2+ by changing the concentration of
metal ions. As see in Figure 9, the intense fluorescence of probe 1 around 474 nm
was gradually decreased on increasing the concentration of Cu2+ ions on exciting
at 365 nm. In fact, treatment of even with 1 equiv of Cu2+ ions (20 µM) resulted
in the quenching fluorescence effectively by 44% of the original value and almost
93% of quenching of emission intensity was obtained with 10 equiv of metal ions
suggesting highly selective sensing ability of receptor 1 towards the detection of
Cu2+ ions. Further, compared to other fluorescent probes that are available in the
literature for Cu2+ sensing mostly work only in organic solvents whereas the probe
1 work in aqueous media for detection of copper ions as water is responsible for
aggregation and a very low amount of probe (20 µM) is needed for quite efficient
quenching.
Fig. 8: Emission intensity of probe 1 in presence of different metal ions (λex = 365 nm).
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Fig. 9: PL spectra of probe 1 as a function of [Cu2+]. [1] = 20 µM in THF:water mixtures,
fw = 95% (λex = 365 nm).
To gain insight into the nature of quenching mechanism and the extent
of quenching, Stern-Volmer plot was constructed by applying Stern-Volmer
equation [26],
Io
I
= 1 + Ksv Cu
[
]
where KSV is the Stern–Volmer quenching constant, Io and I are the original flu-
orescence intensity and quenched intensity in presence of the quencher concen-
tration [Cu]. The KSV value obtained by plotting I0/I values versus the concen-
trations of Cu2+ ions were found to be 2.4 × 104 M−1 with correlation coefficient
(R2) equal to 0.92 (Figure 10). The higher values for KSV designate the efficient
interaction between the Cu metal ions and fluorophore. The detection limit was
calculated from fluorescence titration experiment, using the IUPAC definition i.e.
3 σ/slope where σ is the standard deviation of 10 blank solutions of the sensor. The
changes in the fluorescence intensity were recorded as a function of the change
in concentration of Cu2+ ions. Detection limit calculated from the fluorescence
titration data by varying the concentration of Cu2+ was found to be 2.14 µM.
3.4 Computational details
The density functional theory (DFT) is found to be consistent to reproduce
the experimental data. Moreover, it is reliable to predict various properties
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Fig. 10: Stern-Vollmer plot for probe 1 (20 µM) at various concentration of Cu2+ ions.
of interests. Previous studies showed that B3LYP is healthier choice to reproduce
and predict the electronic properties [27–32]. Moreover, various semiconducting
properties were studied by using B3LYP level [33, 34]. All calculations were exe-
cuted with 6-31G** basis set except for Cu where LANL2DZ basis set was applied
which was established accurate and rational for metal comprising compounds
[35–37]. In the current study, ground state geometries of probe 1 and probe 1-Cu
complex were optimized at B3LYP/6-31G** and B3LYP/6-31G** (LANL2DZ) level,
respectively. Various properties of interests were calculated using Gaussian 09
package [27].
3.5 Binding energies
The binding energy ∆E of probe 1-Cu complex was estimated by using the follow-
ing equation:
∆E = E AB − E A − E B
( ) ( )
(
)
where E(AB) is the energy of optimized probe 1-Cu complex whilst E(A) and E(B)
are the optimized energies of probe 1 and Cu, respectively. The deprotonated
hydrogen energy has also been added to probe 1-Cu complex to attain the reli-
able and accurate ∆E values. A positive ∆E value is specifying stable adsorption,
see Table 1.
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Tab. 1: The binding energies ∆E of probe 1-Cu complex computed
at B3LYP/6-31G** and B3LYP/6-31G**(LANL2DZ) level.
Probe 1
Metal
Reactant
Complex
H
Product
∆E Kcal/mol
−1032.7639686
−196.1168077
−1228.880776
−1228.2889114
−0.50027
−1228.789181
57.48
3.6 Electronic properties
As inferred from Figure 11, three frontier molecular orbitals (FMOs), highest occu-
pied molecular orbital (HOMO) and unoccupied molecular orbitals (LUMO and
LUMO+1) of probe 1 and its Cu-complexe have been showed. In probe 1, charge
density of the HOMO and LUMO is distributed on anthracene moiety whereas
LUMO+1 is localized at dihydroquinazolinone. The intra-molecular charge trans-
fer (ICT) was found from anthracene of HOMO to dihydroquinazolinone unit
(LUMO+1). In probe 1-Cu complex, most of the HOMO is distributed on dihydro-
quinazolinone with some charge density at anthracene. The LUMO is localized
Probe 1
HOMO
LUMO
LUMO+1
Probe 1-Cu complex
HOMO
LUMO
LUMO+1
Fig. 11: Ground state frontier molecular orbitals of probe 1 and its Cu-complexes (contour value
0.05).
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Tab. 2: The computed HOMO energies (EHOMO), LUMO energies (ELUMO), LUMO+1 energies
(ELUMO+1), HOMO-LUMO energy gap (EH–L) and HOMO-LUMO+1 energy gap (EH–L+1) for probe 1
at B3LYP/6-31G** and probe 1-Cu complex at B3LYP/6-31G**(LANL2DZ) levels of theory.
Systems
EHOMO
ELUMO
ELUMO+1
EH–L
EH–L+1
Probe 1
Probe 1-Cu complex
−5.51
−5.27
−2.01
−2.24
−0.89
−1.78
3.50
3.03
4.62
3.49
on the Cu metal while LUMO+1 at anthracene moiety. The charge density dis-
tribution pattern on probe 1-Cu complex is illuminating ligand to metal charge
transfer (LMCT) from probe 1 (HOMO) to the metal ion (LUMO). This LMCT in
probe 1-Cu complex is playing important role to sense the Cu ions by probe 1. In
Table 2, we have displayed the calculated HOMO energies (EHOMO), LUMO ener-
gies (ELUMO), LUMO+1 energies (ELUMO+1), HOMO-LUMO energy gap (EH–L) and
HOMO-LUMO+1 energy gap (EH–L+1) for probe 1 at B3LYP/6-31G** and probe 1-
Cu complex at B3LYP/6-31G** (LANL2DZ) levels of theory. It can observed that
after binding the Cu ion to the probe 1 EHOMO increases while ELUMO and ELUMO+1
decreases resulting reduced EH–L and EH–L+1 in probe 1-Cu complex as compared
to the probe 1. This reduced energy gap would be favorable in probe 1-Cu complex
to tune the absorption spectrum towards longer wavelength.
4 Conclusion
In summary, an anthracene appended dihydroquinazolinone derivative have
been designed and synthesized. The solvent effect on the photophysical proper-
ties of probe molecule reveals that the emission profile is greatly influenced by
the polarity of the solvents; suggesting involvement of photo induced intramolec-
ular charge transfer (ICT) in the excited state. The AIE properties of the probe
molecule has been elucidated using water/THF mixtures with different water vol-
ume fractions and found that the molecule emits strongly on reaching the water
fraction 95%. Further, by exploiting the AIE property, the sensing ability of the
probe molecule towards various metal ions unravel that it could act as a selective
and sensitive sensor for Cu2+ ions through a turn-off fluorescence signaling mech-
anism as well as by colorimetric response. Theoretical studies further support
intra-molecular charge transfer (ICT) from anthracene (HOMO) to dihydroquina-
zolinone unit (LUMO+1) in probe 1 where as in probe 1-Cu complex ligand to metal
charge transfer (LMCT) from probe 1 (HOMO) to the metal ion (LUMO) was noticed.
Moreover, the decreased energy gap after binding the Cu ion to the probe 1 resulted
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A. G. Al-Sehemi et al.
tuned the absorption of probe 1-Cu complex towards longer wavelength. To con-
clude, the reported probe molecule enjoy greater practical interest as it works
in aqueous media for detection of copper ions where the color change could be
clearly observed by the naked eyes as well as the fluorescence changes imparted
by the metal ion could be visualized with the help of a 365 nm UV lamp. Hence,
the current direction of research will enhance the contribution to the global field
of sensing technology by making cheap and efficient smart material to detect the
heavy metal ions in solution and solid supported materials (test kits).
Acknowledgement: The authors extend their appreciation to the Deanship of
Scientific Research at King Khalid University for funding this work through
Research Group Project under grant number (R.G.P.1/15/38).
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