Subtle structural variation in azine/imine derivatives controls Zn2+ sensitivity: ESIPT-CHEF combination for nano-molar detection of Zn2+ with DFT support

Article abstract of DOI:10.1039/c9ra03652k

Excited-state intra-molecular proton transfer (ESIPT)-active imine and azine derivatives, structurally characterised by XRD, and denoted L1, L2, L3 and L4, possess weak fluorescence. The interaction of these probes with Zn²⁺ turns ON the fluorescence to allow its nano-molar detection. Among the four ESIPT-active molecules, L2, L3 and L4 are bis-imine derivatives while L1 is a mono-imine derivative. Among the three bis-imine derivatives, one is symmetric (L3) while L2 and L4 are unsymmetrical. The lowest detection limits (DL) of L1, L2, L3 and L4 for Zn²⁺ are 32.66 nM, 36.16 nM, 15.20 nM and 33.50 nM respectively. All the probes bind Zn²⁺ (10⁵ M^-1 order) strongly. Computational studies explore the orbital level interactions responsible for the associated photo-physical processes.

Full text of DOI:10.1039/c9ra03652k

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Subtle structural variation in azine/imine
derivatives controls Zn2+ sensitivity: ESIPT-CHEF
combination for nano-molar detection of Zn2+ with
DFT support†
Cite this: RSC Adv., 2019, 9, 21302
Somnath Khanra, Sabyasachi Ta, Milan Ghosh, Sudeshna Chatterjee
*
and Debasis Das
Excited-state intra-molecular proton transfer (ESIPT)-active imine and azine derivatives, structurally
characterised by XRD, and denoted L1, L2, L3 and L4, possess weak fluorescence. The interaction of
these probes with Zn2+ turns ON the fluorescence to allow its nano-molar detection. Among the four
ESIPT-active molecules, L2, L3 and L4 are bis-imine derivatives while L1 is a mono-imine derivative.
Among the three bis-imine derivatives, one is symmetric (L3) while L2 and L4 are unsymmetrical. The
lowest detection limits (DL) of L1, L2, L3 and L4 for Zn2+ are 32.66 nM, 36.16 nM, 15.20 nM and 33.50 nM
respectively. All the probes bind Zn2+ (105 Mꢀ1 order) strongly. Computational studies explore the orbital
level interactions responsible for the associated photo-physical processes.
Received 15th May 2019
Accepted 30th June 2019
DOI: 10.1039/c9ra03652k
rsc.li/rsc-advances
with proteins,15 peptides and enzymes,16,17 it regulates functions like
gene expression, cellular metabolism, apoptosis, neurotransmis-
Introduction
sion, mammalian reproduction and so on. Moreover, zinc disorders
directly affect the metabolic activities of DNA.18–21 The US Food and
Nutritional Board recommends a daily dietary intake of Zn2+ from
2 mg (infants, 0–5 years) to 15 mg for adults.22 Its deciency reduces
the ability of the islet cells to produce insulin.23 Excess Zn2+ is
neurotoxic and can cause serious neurological disorders like Alz-
heimer's and Parkinson's disease.24–30 The WHO recommends up to
76 mM Zn2+ in drinking water.31,32 Among a signicant number of
pioneering Zn2+ sensors,33–36 a few recent reports deserve to be
mentioned here. For example, Yoon et al.37 developed both R and S
forms of a bis-binaphthol-pyrene conjugate for selective recognition
of Zn2+. Bhalla et al.38 have reported a hexaphenylbenzene derivative
to recognise Zn2+ in aqueous media. Out of several notable efforts at
Zn2+ sensing,39,40 however, few are based on the ESIPT approach.41,42
Herein, an imine (L1) and three azine (L2, L3 and L4) derivatives
have been synthesised and structurally characterised by single
crystal X-ray diffraction analysis. Out of the three azine derivatives,
one is symmetrical (L3) while other two (L2 and L4) are unsym-
metrical. Photo-physical interactions of all the molecules with Zn2+
allow selective determination of nano-molar Zn2+.
Design and development of organic molecules for selective recog-
nition of bio-relevant metal ions is a vibrant eld of research for its
extensive application in biology, ecology and endocrine systems.1–6
Meanwhile, the concept of excited-state intra-molecular proton
transfer (ESIPT) processes has been extensively applied in molecular
probes, luminescent materials, and molecular logic gates as it offers
large Stokes shis7 to exclude spectral overlap between absorption
and emission, highly desirable for uorescence sensing.8–10 The
basis of ESIPT is an intra-molecular hydrogen bond that allows
proton transfer from a relatively acidic region of a molecule
(hydrogen bond donor, –OH and NH2) to its basic region (hydrogen
bond acceptor, ]N– and C]O), leading to formation of a ve/six
membered ring. In most cases, this happens due to the existence
of keto–enol tautomerism in the system. In some cases, this leads to
dual uorescence, one at a shorter wavelength due to the enol form
and the other at a longer wavelength due to the proton-transferred
form.11 Knocking out this proton by a cation arrests this ESIPT
process, leading to uorescence enhancement.
The importance of biological zinc needs no introduction.12,13 In
humans, its concentration varies from 12 mM (intracellular serum)
to ꢁ0.1–0.5 mM (brain and nerve tissue).14 As Zn2+ strongly interacts
Experimental
Materials and equipment
Department of Chemistry, The University of Burdwan, Golapbag, Burdwan, 713104,
West Bengal, India. E-mail: ddas100in@yahoo.com; Fax: +91-342-2530452; Tel:
+91-342-2533913 ext. 424
High purity bis–tris buffer, 4-diethylamino-2-hydroxy-
benzaldehyde, N1-phenyl-ethane-1,2-diamine, 2-hydroxy benz-
aldehyde, 4-methoxy benzaldehyde, 4-hydroxy benzaldehyde,
Zn(OAc)2, ZnCl2, ZnSO4, Zn(PO4)3, Zn(OCl)2 and other
† Electronic supplementary information (ESI) available: NMR, FTIR, UV-Vis,
uorescence spectra, X-ray single crystal data and DFT calculation data. CCDC
1484206, 1062760 and 1501269. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c9ra03652k
21302 | RSC Adv., 2019, 9, 21302–21310
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chemicals have been purchased from Sigma-Aldrich (India) and formula, C19H25N3O (MW, 311.20). Anal. found (%): C,
used as received. The spectroscopic and analytical grade 73.08; H, 7.69 and N, 12.89; calcd (%), C, 73.28; H, 8.09
solvents were used without further purication. Milli-Q Milli- and N, 13.49. ESI-MS (m/z): [M + H]+, 312.32; [M + Na]+,
pore 18.2 MU cmꢀ1 water was used as and when required.
334.32 [Fig. S1a, ESI†]. FTIR (KBr, cmꢀ1): 3370, n(O–H); 3038,
Absorption spectra were recorded with a Shimadzu Multi n(C–H, aromatic); 1608, n(C]N); 1512, n(C]C, stretch);
Spec 2450 spectrophotometer. Hitachi F-4500 spectro- 1331, n(C–N, stretch) 1250 and 1144, n(C–O, stretch);
A
uorometer was used to record steady state emission spectra. [Fig. S1b, ESI†]. 1H NMR (CDCl3, 400 MHz, J, Hz, d ppm,
The elemental analyses (C, H and N) have been performed on reference peak 7.26 ppm) [Fig. S1c, ESI†]: 10.730 (1H, s)
a PerkinElmer 2400 CHN elemental analyser. FTIR spectra were 9.499 (1H, s), 8.224 (1H, s) 7.970 (1H, s), 7.276 (1H, s), 7.205–
collected using a Shimadzu FTIR (model IR Prestige 21 CE) 7.160 (2H, m), 6.987–6.965 (1H, d, J ¼ 8.8 Hz) 6.736–6.700
spectrometer. Electrospray ionisation mass spectra (ESI mass) (1H, q), 6.643–6.614 (1H, m), 6.181–6.134 (1H, m), 3.715–
were recorded using a Thermo Fisher Scientic Exactive Mass 3.687 (1H, t, J ¼ 5.6 Hz), 3.571–3.541 (1H, t, J ¼ 6.0 Hz),
Spectrometer using HR (+/ꢀ) mode. The pH was measured 3.490–3.317 (3H, m), 1.288–1.213 (1H, q), 1.204–1.158 (2H,
1
using a Systronics digital pH meter (model 335). The H NMR m). The structure of L1 was conrmed by SC-XRD analysis.
spectra were collected using a Bruker 400 MHz spectrometer
1-[(4-Methoxy-benzylidene)-hydrazonomethyl]-naphthalen-
using CDCl3 and DMSO-d6 as solvent. The chemical shi is 2-ol (L2). First, 1-hydrazonomethyl-naphthalen-2-ol was syn-
expressed in ppm using the residual solvent peak as an internal thesised following a published procedure.47 This new amine
reference. Multiplicity is indicated as follows: s (singlet), (61 mg, 0.35 mmol) and 4-methoxy-benzaldehyde (48 mg, 0.35
d (doublet), t (triplet), q (quartet), m (multiplet). Coupling mmol) were mixed in 1 : 1 mole ratio and reuxed for 4 h
constants (J) are reported in Hertz (Hz).
(Scheme 1). The nal solution was kept for slow evaporation
Single crystal X-ray diffraction data were collected on and yellow crystals of L2 were collected aer one week in 91%
a Bruker X8 APEX-II CCD diffractometer at 100(2) K using yield. Molecular formula: C19H16N2O2 (MW ¼ 304.12). Anal.
˚
graphite-monochromated Mo-Ka radiation (0.71073 A) at 150 K. found (%), C, 74.78; H, 5.11 and N, 9.39; calcd (%), C, 74.98; H,
Data were processed and corrected for Lorentz and polarisation 5.30 and N, 9.20. ESI-MS (m/z): [M + H]+, 305.29; [M + Na]+,
absorption effects. Crystal structures were solved by standard 327.28 [Fig. S2a, ESI†]. FTIR (KBr, cmꢀ1): 3214.83, n(O–H);
direct methods using SHELXS43 and rened by full-matrix least- 3012.81 and 2923.69, n(C–H, aromatic); 1612.19, n(C]N);
squares with SHELXL44 and OLEX2 soware.45 Signicant 1507.98, n(C]C, stretch); 1246.49 and 1172.28, n(C–O, stretch);
1
crystal parameters and renement data are presented in Table 1098.13, n(N–N, stretch). [Fig. S2b, ESI†]. H NMR (CDCl3, 400
S1 (ESI†). All non-hydrogen atoms are rened with anisotropic MHz, J, Hz, d ppm, reference peak, 7.26 ppm) [Fig. S2c, ESI†]:
thermal displacements. Hydrogen atoms are included in the 13.406 (1H, s), 9.707 (1H, s), 9.646 (1H, s), 8.740–8.634 (1H, d, J
structure factor calculation in geometrically idealised positions, ¼ 1.6 Hz), 8.574 (1H, s), 8.202–8.181 (2H, d, J ¼ 8.4 Hz), 7.879–
with thermal parameters depending on the parent atom, using
a riding model. Images were generated by Mercury soware.46
Synthesis
Fig. 1 shows the molecules (L1, L2, L3 and L4) that have been
synthesised simply by reuxing mixtures of aldehyde and amine
(1 : 1 or 1 : 2, mole ratio) as per requirement.
5-Diethylamino-2-[(2-pheylamino-ethylimino)-methyl]-
phenol (L1).
A
mixture of 4-diethylamino-2-hydroxy-
benzaldehyde (80.876 mg, 0.41 mmol) and N1-phenyl-
ethane-1,2-diamine (57 mg, 0.41 mmol) in methanol was
reuxed for ꢁ4 h (Scheme 1). Slow evaporation of the solvent
resulted in yellow crystals of L1 in 94.5% yield. Molecular
Fig. 1 ChemDraw structures of new molecules.
Scheme 1 Synthetic protocol of L1, L2, L3 and L4.
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7.808 (5H, m), 7.585–7.567 (1H, d, J ¼ 7.2), 7.430–7.396 (1H, d, J and yellow-white solids corresponding to the adducts, viz. L1-
¼ 6.4 Hz), 7.283–7.261 (1H, d, J ¼ 9.2), 7.032–7.003 (3H, t, J ¼ 5.8 Zn(OAc)2 (Ad1), L2-Zn(OAc)2 (Ad2), L3-Zn(OAc)2 (Ad3), and L4-
Hz), 3.902–3.842 (5H, t, J ¼ 12 Hz). The structure of L2 was Zn(OAc)2 (Ad4), were found aer two days (Fig. 5).
conrmed by SC-XRD analysis.
Ad1: molecular formula, C21H27N3O3Zn; anal. found (%), C,
N,N0-Bis-(2-hydroxo-benzylidene)-hydrazine (L3). To a meth- 57.41; H, 6.17 and N, 8.30; calcd (%), C, 58.00; H, 6.26 and N,
anol solution of 2-hydroxy-benzaldehyde (122 mg, 1 mmol), 9.66. ESI-MS (m/z): [Ad1 + MeOH + Na]+, 486.79 [Fig. S5a, ESI†].
a slight excess of hydrazine was added and reuxed for 4 h FTIR (KBr, cmꢀ1): 3107.81, n(C–H, aromatic); 1560.60, n(C]N);
(Scheme 1). Finally, the solution was kept for slow evaporation 1437.17, n(C]C, stretch); 1247.28 and 1148.75, n(C–O, stretch)
and yellow crystals of L3 were collected aer one week in 96.3% [Fig. S5b, ESI†].
yield. Molecular formula: C14H12N2O2 (MW ¼ 204.09). Anal.
Ad2: molecular formula, C21H19N2O4Zn: anal. found (%), C,
found (%), C, 69.48; H, 4.80 and N, 11.37; calcd (%), C, 69.99; H, 58.41; H, 5.07 and N, 6.01; calcd, C, 58.82; H, 4.47 and N, 6.53.
5.03 and N, 11.66. ESI-MS (m/z): [M + H2O]+, 257.22 (100%; calcd ESI-MS (m/z): [M
+
Na]+, 451.50 [Fig. S6a, ESI†]. FTIR
258.10) [Fig. S3a, ESI†]. FTIR (KBr, cmꢀ1): 3373.91, n(O–H); (KBr, cmꢀ1): 3102.54, n(C–H, aromatic); 1557.86, n(C]N);
3030.25, n(C–H, aromatic); 1597.11, n(C]N); 1474.46, n(C]C, 1451.65, n(C]C, stretch); 1249.97 and 1169.97, n(C–O, stretch);
stretch); 1251.60, n(C–O, stretch); 1122.08, n(N–N, stretch). 1066.20, n(N–N, stretch) [Fig. S6b, ESI†].
[Fig. S3b, ESI†]. 1H NMR (DMSO-d6, 400 MHz, J, Hz, d ppm,
Ad3: molecular formula, C18H18N2O6Zn2: anal. found (%), C,
reference peak, 2.5 ppm) [Fig. S3c, ESI†]: 10.450 (1H, s), 7.930 43.68; H, 4.23 and N, 4.97; calcd (%), C, 44.20; H, 3.71 and N,
(1H, s), 7.203–7.179 (1H, q), 7.130–7.091 (1H, q), 6.825–6.789 5.73. ESI-MS (m/z): [M + H]+, 486.18 [Fig. S7a, ESI†]. FTIR
(3H, t, J ¼ 7.2 Hz). The structure of L3 was further conrmed by (KBr, cmꢀ1): 3098.60, n(C–H, aromatic); 1601.91 and 1574.94,
SC-XRD analysis.
N-(2-Hydroxo-benzylidene)-N0-(4-hydroxo-benzylidene)-
hydrazine (L4). Methanol solution of 4-hydroxy-
n(C]N); 1438.32, n(C]C, stretch); 1312.55 and 1171.95, n(C–O,
stretch); 1135.28, n(N–N, stretch) [Fig. S7b, ESI†].
Ad4: molecular formula, C16H15N2O4Zn: anal. found (%), C,
benzaldehyde (122 mg, 1 mmol) was added drop-wise to 52.85; H, 3.92 and N, 6.76; calcd (%), C, 52.69; H, 4.15 and N,
hydrazine
with
continuous
stirring
to
get
4- 7.68. ESI-MS (m/z): [M + H]+, 363.07 [Fig. S8a, ESI†]. FTIR
hydrazonomethyl-phenol (L4a) in 84.3% yield (Scheme 1). (KBr, cmꢀ1): 3102.54, n(C–H, aromatic); 1614.49 and 1559.52,
Molecular formula: C7H8N2O (MW, 136.06). Anal. found (%), n(C]N); 1450.67, n(C]C, stretch); 1286.86 and 1197.53, n(C–O,
C, 61.38; H, 5.80 and N, 19.97; calcd (%), C, 61.75; H, 5.29, N, stretch); 1111.63, n(N–N, stretch). [Fig. S8b, ESI†].
20.58. ESI-MS (m/z): [M + H]+, 136.49 (100%; calcd 137.07)
[Fig. S4a, ESI†]. FTIR (KBr, cmꢀ1): 3481.02, n(N–H); 3316.83,
n(O–H); 3176.50, n(C–H, aromatic); 1600.97, n(C]N);
Results and discussion
Single crystal X-ray diffraction analysis of L1, L2 and L3
1508.14, n(C]C, stretch); 1253.26 and 1158.12, n(C–O,
stretch); 1100.24, n(N–N, stretch). [Fig. S4b, ESI†]. 1H NMR L1, L2 and L3 were well characterised by spectroscopic tech-
(DMSO-d6, 400 MHz, J, Hz, d ppm, reference peak, 2.5 ppm) niques and their stereo-chemical structures were conrmed by
[Fig. S4c, ESI†]: 10.726 (1H, s) 9.500 (2H, s), 8.547 (1H, s), single crystal X-ray diffraction analysis (Fig. 2). Their molecular
7.693–7.671 (2H, d, J ¼ 8.8 Hz), 7.615 (1H, s), 7.305–7.278 views along with atom labelling schemes are shown in Fig. S9a–
(2H, q) 6.863–6.842 (2H, d, J ¼ 8.4 Hz), 6.725–6.704 (2H, q), c (ESI†) while their packing patterns are shown in Fig. S10a–c
6.369 (2H, s).
(ESI†). The crystallographic data and renement parameters are
Then L4 was synthesised by reuxing a mixture of L4a presented in Table S1 (ESI†) while selected bond lengths and
(68 mg, 0.5 mmol) and 2-hydroxy-benzaldehyde (61 mg, 0.5 angles are listed in Table S2a–c (ESI†).
ꢀ
mmol) in methanol for 3 h (Scheme 1). The nal solution was
kept for slow evaporation and yellow crystals of L4 were 10.8682(5) (A); b, 11.1818(6) (A); c, 16.0637(8) (A); a ( ), 88.542(3);
observed aer one week in 92.7% yield. Molecular formula: b ( ), 73.254(2); g ( ), 70.084(3); volume (A ), 1754.84(15); Z ¼ 4
L1 is triclinic having space group P1 (CCDC 1484206): a,
ꢂ
˚
˚
˚
3
ꢂ
ꢂ
˚
C
14H12N2O2 (MW, 240.09). Anal. found (%), C, 70.12; H, 4.83 (Table S1 (ESI†)). It exists as the enol form in the solid state. The
˚
˚
and N, 11.02; calcd (%), C, 69.99; H, 5.03 and N, 11.66. ESI-MS bond lengths N003–C00J, 1.322(2) A and N005–C00H, 1.295(2) A
(m/z): [M + H]+, 241.79 [Fig. S4d, ESI†]. FTIR (KBr, cmꢀ1): indicate the double bond character of C–N whereas N004–C00Z,
˚
˚
˚
3319.09, n(O–H); 3027.96, n(C–H, aromatic); 1616.41, n(C]N); 1.447(3) A, N004–C00W, 1.462(2) A, N3–C00Q, 1.448(3) A, and
˚
1478.60, n(C]C, stretch); 1216.17 and 1154.94, n(C–O, stretch); N3–C016, 1.536(4) A indicate the single bond character of C–N
1100.71, n(N–N, stretch). [Fig. S4e, ESI†]. 1H NMR (DMSO-d6, bonds in the N-ethyl amino group. The bond lengths O001–
˚
˚
400 MHz, J, Hz, d ppm, reference peak, 2.5 ppm) [Fig. S4f, ESI†]: C00C, 1.305(2) A and O002–C00M, 1.291(2) A indicate the single
11.427 (1H, s), 11.051 (1H, s) 8.993 (1H, s), 7.940 (1H, s), 7.696– bond character of C–O in the phenol moiety. The structure is
7.677 (1H, d, J ¼ 7.6 Hz), 7.418–7.379 (1H, t, J ¼ 7.8 Hz) 7.222– stabilised by intra-molecular H-bonding. The bond angles
7.203 (1H, t, J ¼ 3.8), 7.155–7.009 (1H, q), 6.997–6.907 (2H, m), C00J–N003–C00V,
121.68(18)ꢂ
and
124.16(16)ꢂ indicate the sp2 nature of the N003 and N005
[Probe-Zn2+] adducts (Ad1, Ad2, Ad3 and Ad4). To four sets of centres (Fig. S9a and Table S2a, ESI†).
magnetically stirred methanol solutions of L1, L2, L3 and L4, L2 is monoclinic having space group P1 (CCDC 1062760): a,
methanol solutions of Zn(OAc)2 were added drop-wise for 10– 7.1822(10) (A); b, 14.0258(18) (A); c, 15.7770(19) (A); a ( ),
C00H–N005–C00T,
6.873–6.804 (4H, m).
ꢀ
ꢂ
˚
˚
ꢂ
˚
3
ꢂ
˚
15 min. Aer ltration, the ltrate was kept for slow evaporation 89.987(9); b ( ), 90.007(10); g ( ), 77.128(10); volume (A ),
21304 | RSC Adv., 2019, 9, 21302–21310
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Fig. 2 ORTEP views of L1, L2 and L3 and DFT optimised structure of L4.
1549.4(4); Z ¼ 4 (Table S1, ESI†). It exists as the enol form in the ions, viz. Na+, K+, Ca2+, Al3+, Hg2+, Pb2+, Cr3+, Mn2+, Fe3+, Co2+,
solid state. The bond lengths N007–C00I, 1.284(6) A and N003– Ni2+, Cu2+, Cd2+, Sn2+ and Ag+, do not signicantly affect their
˚
˚
C00R, 1.288(6) A indicate the double bond character of C–N of emission intensities (Fig. 3a–d). Moreover, Zn salts other than
˚
the aldimine group whereas N006–C00D, 1.280(6) A and N008– Zn(OAc)2, viz. ZnCl2, ZnSO4, Zn(PO4)3, and Zn(OCl)2, show
C00T, 1.279(6) A indicate the C–N bond on the p-methoxy similar uorescence enhancements in response to Zn2+. In
˚
˚
benzaldehyde side. The bond lengths N003–N006, 1.405(6) A addition, the emission intensity of L1-Zn(OAc)2 (Ad1) is
˚
and N007–N008, 1.403(6) A indicate the single bond character of maximum over the pH range 7 to 10 (Fig. S11a–d, ESI†). Hence,
N–N from the hydrazine moiety. The bond lengths O1–C00J, the near-biological pH 7.4 was chosen for all experiments. As L1
˚
˚
1.347(6) A and O2–C00K, 1.348(6) A indicate the single bond emits at two different wavelengths, a ratiometric change is
character of C–O in the phenol moiety. The structure is stabi- observed upon gradual addition of Zn(OAc)2 to L1.
lised by intra-molecular H bonding. The bond angles C00R–
The emission intensity decreases at 316 nm while it
N003–N006, 114.2(5)ꢂ, C00D–N006–N003, 113.0(5)ꢂ, C00I– increases at 414 nm through an iso-emissive point at 371 nm
N007–N008, 113.6(5)ꢂ, and C00T–N008–N007, 112.7(5)ꢂ indicate (Fig. 4a). In all other cases (excluding L1), the emission intensity
the sp2 nature of the N003, N006, N007 and N008 centres at the respective wavelength increases upon gradual addition of
(Fig. S9b and Table S2b, ESI†).
Zn(OAc)2 (for L2, 491 nm, for L3, 431 nm and for L4, 495 nm)
L3 is monoclinic having space group P21/c (CCDC 1501269): (Fig. 4b–d). Competitive experiments indicate no signicant
a, 16.524(3) (A); b, 5.9394(12) (A); c, 13.232(2) (A); a ( ), 90; b ( ), interference from common metal ions during sensing of Zn2+ in
ꢂ
ꢂ
˚
˚
˚
3
ꢂ
˚
113.560(11); g ( ), 90; volume (A ), 1190.4(4); Z ¼ 4 (Table S1, the form of Zn(OAc)2 by L1–L4 (Fig. S12a–d, ESI†). The change
ESI†). It exists as the enol form in the solid state. The bond in emission intensities of L1 at two different wavelengths
lengths N2–C14, 1.277(7) A and N1–C7, 1.286(7) A indicate the (414 nm and 316 nm) upon addition of Zn2+ is shown in
double bond character in C–N of the aldimine group whereas Fig. S13a (ESI†). Similar plots for L2, L3 and L4 are shown in
˚
˚
˚
˚
N004–C00Z, 1.447(3) A, N1–N1, 1.395(9) A and N2–N2, 1.409(9) Fig. S13b–d (ESI†). The linear regions of the plots allow deter-
˚
A indicate the single bond character of N–N in the hydrazine mination of unknown concentrations of Zn2+. The lowest
moiety. The bond lengths O1–C1, 1.356(7) A and O2–C8, detection limits (DL) of Zn2+ by L1, L2, L3 and L4 are 32.66 nM,
˚
˚
1.350(7) A indicate the single bond character of C–O in the 36.16 nM, 15.20 nM and 33.50 nM respectively (Fig. S14a–d,
phenol moiety. The structure is stabilised by intra-molecular H ESI†). These values lie well below the WHO recommended
bonding. The bond angles C14–N2–N2, 113.3(6)ꢂ, N2–C14–C13, acceptable level of Zn2+ in drinking water (76 mM). The DL of L3
121.7(6)ꢂ, C7–N1–N1, 113.1(6)ꢂ, and N1–C7–C6, 121.3(6)ꢂ indi- for Zn2+ is the lowest in the series, indicating stronger interac-
cate the sp2 nature of the N003, N006, N007 and N008 centres tion of L3 with Zn2+, probably due to the presence of two –OH
(Fig. S9c and Table S2c, ESI†).
groups. Fluorescence titration enables determination of the
association constants (K) of the probes for Zn2+ using the Ben-
esi–Hildebrand equation (Fig. S15a–d, ESI†).
Spectroscopic studies
The respective association constants are 2.146 ꢃ 105 Mꢀ1
,
Emission properties. The presence of both acidic (–OH) and
basic (NHR and –C]NR) moieties makes L1, L2, L3 and L4 pH
sensitive. Consequently, their emission properties upon inter-
action with cations, anions and biomolecules have been inves-
tigated at different pH. Interestingly, their weak emission (L1:
2.407 ꢃ 105 Mꢀ1, 3.28 ꢃ 105 Mꢀ2 and 1.692 ꢃ 105 Mꢀ1 for L1,
L2, L3 and L4 respectively. The fact that L3 has the highest K is
due to the same reason as its having the lowest DL. From the Job
plots (Fig. S16a–d, ESI†), ESI mass spectra (Fig. S5a, S6a, S7a
and S8a, ESI†), FTIR spectra (Fig. S5b, S6b, S7b and S8b, ESI†),
and 1H NMR titration (Fig. 7), plausible binding modes of L1–L4
with Zn2+ are proposed in Scheme 2 and the corresponding
adducts (Ad1–Ad4) are presented in Fig. 5. Scheme 2 shows the
lem, 414 nm, lex, 287 nm; L2: lem, 491 nm, lex, 436 nm; L3: lem
,
431 nm, lex, 353 nm and L4: lem, 495 nm, lex, 400 nm) is
enhanced signicantly in the presence of Zn2+: specically, the
enhancement is maximum for Zn(OAc)2. Other common metal
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Fig. 3 Changes in the emission spectra of L1 (a), L2 (b), L3 (c) and L4 (d) in presence of different metal ions in bis–tris buffered DMSO/H2O (1 : 1,
v/v) media, pH 7.4.
probable sensing mechanism, assigned to the combined effect corresponding data for Ad3 indicate 1 : 2 (mole ratio) stoichi-
of ESIPT and chelation enhanced uorescence (CHEF).
ometry (Fig. S16c, ESI†). Thus chelation of L1–L4 to zinc ion
Absorption properties. The absorption spectrum of L1 leads to CHEF through inhibition of ESIPT and –CH]N– iso-
1
exhibits two peaks, viz. 242 nm and 363 nm, assigned to p–p* merisation (Scheme 2 and Fig. 5). H NMR titration of L1–L4
and n–p* transitions.48 Upon gradual addition of Zn(OAc)2 to with Zn(OAc)2 further supports the proposed mechanism. The
L1, the absorbance at both wavelengths increases (Fig. 6a) while FTIR data of the adducts also corroborate the binding mode
for the rest (L2–L4), a ratiometric change is observed. For L2, the (Fig. S5b, S6b, S7b and S8b, ESI†). It was observed that the n(C]
absorbance decreases at 380 nm but increases at 439 nm N) band of free L1, L2, L3 and L4 at 1608, 1612, 1597 and
through an isosbestic point at 402 nm (Fig. 6b). For L3, the weak 1616 cmꢀ1 shied to 1560, 1557, 1574, 1559 cmꢀ1 in the
absorbance at 346 nm increases while the absorbance at 308 nm respective adducts Ad1, Ad2, Ad3 and Ad4, in addition to their
decreases with the appearance of four isosbestic points at original peaks in some cases. This indicates involvement of
256 nm, 279 nm, 296 nm, and 321 nm (Fig. 6c). The situation for imine-N in chelation. Moreover, the respective n(O–H) bands at
L4 is almost the same as that of L2. The absorbance at 354 nm 3370, 3214, 3373 and 3319 cmꢀ1 almost disappear in the case of
decreases with the emergence of a new band at 404 nm through Ad1, Ad2 and Ad4 while it partially disappears for Ad3. Thus
an isosbestic point at 377 nm (Fig. 6d). Changes in the ESIPT protons disappear upon binding of phenol-O to Zn(OAc)2.
absorption spectra of L1–L4 in the presence of common cations
are presented in Fig. S17a–d (ESI†).
1H NMR titration
The 1H NMR titration results unveil the binding mode of L1
Sensing mechanism
with Zn(OAc)2 (Fig. 7a). Upon addition of 0.5 equiv. of Zn2+, the
The ESI mass spectra of Ad1, Ad2 and Ad4 along with the Job highly de-shielded proton (–OH) at 10.73 ppm almost disap-
plots (Fig. S16a, b and d, ESI†) support 1 : 1 (mole ratio) binding pears, indicating interaction of phenol-O with Zn2+, while the
stoichiometry between the probe and Zn(OAc)2 whereas the imine proton is slightly up-eld shied from 9.49 ppm to
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Fig. 4 Changes in emission intensity of L1 (a), L2 (b), L3 (c) and L4 (d) upon gradual addition of Zn(OAc)2; inset: plot of emission intensities vs.
added Zn(OAc)2 at respective emission wavelengths. Media and pH are same as in Fig. 3.
L2, the –OH proton at 13.41 ppm gradually disappears upon
gradual addition of Zn(OAc)2 while the imine proton experi-
ences a slight up-eld shi (Fig. 7b). However, upon addition of
1.0 equiv. Zn(OAc)2 to L3, the –OH peak at 10.45 ppm does not
Scheme 2 Binding mode of L1, L2, L3 and L4 towards Zn2+ salt.
disappear fully, probably due to the presence of two symmetric
–OH units that can equally interact with two molecules of
Zn(OAc)2. Interestingly, upon addition of two equiv. of
Zn(OAc)2, the –OH peak almost disappears and the imine
proton is shied up-eld (Fig. 7c). In L4, two –OH protons at
11.427 and 11.051 ppm have been observed (Fig. S4f, ESI†).
Upon addition of 0.5 equiv. Zn(OAc)2, the –OH peak ortho to the
imine group tends to disappear while the other –OH at the para
position of the other ring is slightly up-eld shied from 11.427
to 11.344 ppm. At saturation point both –OH peaks almost
disappear (Fig. 7d).
Density functional theoretical (DFT) studies
Fig. 5 Binding mode of L1, L2, L3 and L4 with Zn(OAc)2 forming
adduct Ad1, Ad2, Ad3 and Ad4 respectively.
DFT studies have been performed using the TD-SCF/DFT/
B3LYP/6-311G level of theory. Only the HOMO and LUMOs are
provided to demonstrate the stability of the adducts compared
to the free probes. The lowering of the HOMO–LUMO energy
gaps in Ad1–Ad4 compared to free L1–L4 indicates stabilisation
of the former over the latter. In each case, the oscillator strength
(fb) for the S0 / S1 transition of the adduct increases compared
9.41 ppm, suggesting involvement of imine-N in chelation.57 At
1.0 equiv. of Zn(OAc)2 the phenol proton completely disappears,
while the imine proton is further shied from 9.41 ppm to
9.40 ppm. Similar facts are observed for L2 and L4. In the case of
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Fig. 6 Changes in the absorption spectra of (a) L1, (b) L2, (c) L3 and (d) L4 upon gradual addition of Zn(OAc)2.
Fig. 7 1H NMR titration of L1 (a), L2 (b), L3 (c) and L4 (d) with Zn(OAc)2 in DMSO-d6.
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Fig. 8 Frontier molecular orbitals of L1 and its adduct Ad1 (a), L2 and its adduct Ad2 (b), L3 and its adduct Ad3 (c), and L4 and its adduct Ad4 (d).
to the free probes. This indicates increased probability of relaxation is more pronounced in the case of Ad3 than L3. The
absorption/emission as observed in the experimental ndings. respective values of the HOMO–LUMO energy gap of L3 and Ad3
The electron transition energies for the predominant absorp- are 0.14254 eV and 0.10751 eV (Fig. 8c). Here also the main
tion of the free probes and corresponding Zn(OAc)2 adducts are transition S0 / S1 (HOMO ꢀ 1 / LUMO, HOMO / LUMO)
depicted in Table S3a–d (ESI†).
occurs with a very low value of fb (0.0032) for L3, as seen from
For L1, the HOMO–LUMO energy gap was reduced by the MO prole. Meanwhile, for Ad3 the transition S0 / S1
0.04855 eV from 0.16836 eV (free L1) to 0.11987 eV (Ad1) (HOMO ꢀ 1 / LUMO, HOMO / LUMO) occurs with fb
¼
(Fig. 8a). The S0 / S1 transition (HOMO / LUMO) at 363 nm 0.1602. In the case of L4, the HOMO–LUMO energy gap
has a low fb, 0.006, which indicates intra-molecular charge decreases from 0.13004 eV to 0.113695 eV for the Ad4 adduct
transfer (ICT) from phenol moieties to the amide. A hyp- (Fig. 8d). The respective fb values for L4 and Ad4 are 0.0010 and
sochromic shi to 477 nm is observed with a signicant 0.0978.
increase of fb (0.0237) for Ad1 (S0 / S1 transition, HOMO /
LUMO, HOMO / LUMO + 1), suggesting formation of rigid
Conclusion
Ad1.49 Another HOMO / LUMO transition along with HOMO
/ LUMO + 1 have been observed for S0 / S2 with fb ¼ 0.0095.
The HOMO–LUMO energy gap in L2 is 0.11953 eV while it is
0.110807 eV in Ad2 (Fig. 8b). Here again the main transition S0
/ S1 (HOMO / LUMO, HOMO / LUMO + 1) occurs with
a very low value of fb (0.0011), while Ad2 has a higher value of fb
(0.2214) for the same electronic transition. Again one inter-
esting observation is that the transition S0 / S2 (HOMO ꢀ 1 /
LUMO, HOMO / LUMO + 1, HOMO ꢀ 1 / LUMO + 2) at
371.56 nm for L2 is well matched with the experimental value of
the absorption peak at 380 nm, and S0 / S2 (HOMO / LUMO,
HOMO / LUMO + 1) for Ad2 at 430.78 nm is also well matched
with the experimental value at 439 nm. We have performed
a DFT study of L3 and Ad3, and again observed that energy
Four simple organic molecules (Schiff base and azine deriva-
tives) have been synthesised and structurally characterised by
SC-XRD analysis. Due to the existence of an ESIPT process,
a weak uorescence is observed in the molecules. It is found
that the molecules selectively interact with the Zn2+ ion as
monitored by uorescence and absorption spectroscopic
studies. The association constants of L1, L2, L3 and L4 with
Zn(OAc)2 are 2.146 ꢃ 105 Mꢀ1, 2.15 ꢃ 105 Mꢀ1, 3.28 ꢃ 105 Mꢀ2
,
and 1.692 ꢃ 105 Mꢀ1 respectively. The enhancement of uo-
rescence is due to the arrested ESIPT with the formation of
a rigid adduct, leading to a CHEF process. The lowest detection
limit is found to be in the nano-molar range. A DFT study
elucidates the experimental results. Thus, a new class of ESIPT-
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CHEF probes for recognition of Zn(II) is established for its ultra- 24 E. U. Akkaya, M. E. Huston and A. W. Czarnik, J. Am. Chem.
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