T. Daniel Thangadurai et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 211 (2019) 132–140
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device have also been developed, which could impersonate the function
of a security keypad lock on chronological addition of cation analytes.
for chemical trace detection due to its simplicity and high sensitivity
[17]. Receptors undergoing fluorescence changes, even at low concentra-
tion of analyte, are particularly attractive because of their high degree of
specificity and low detection limit, especially for real-time and on-line
analysis of analytes [18]. Designed chemosensor 1 was prepared by the
reaction between 2, 3-diisocynatophenazine (4) and HCl salt of 6-
amino-2, 3-dihydrophthalazine-1, 4-dione (8) (1:2 M ratio) in dry
CH3CN as pale yellow microcrystals (Scheme 1).
2. Experimental
2.1. Materials and Methods
All the reagents and solvents involved in synthesis were analytical
grade and used without any further purification. The chemicals 2-
aminobenzophenone, o-phenylenediamine, palladium acetate, p-
nitroaniline, trichloromethyl chloroformate, phthalimide and hydrazine
hydrate were purchased from Sigma Aldrich, India. The other chemical
dimethyl-2,3-dithiole-4,5-dicarboxylate was purchased from TCI
Chemicals, India. Reactions were carried out under an inert atmosphere
(N2 or Ar) and monitored by TLC. Column chromatography was per-
formed on silica gel (60–120 mesh) to purify the compounds. The struc-
ture of all the synthesized compounds was confirmed by standard
physiochemical techniques. NMR (1H- and 13C-) spectra were recorded
in DMSO-d6 (Bruker) and chemical shifts (δ) are expressed in ppm with
tetramethylsilane (TMS) as an internal standard and coupling constant
(J) values are given in Hz. Infrared spectra were recorded on Bruker
ATR-FTIR spectrometer (Bruker Alpha – T, Germany). UV–visible ab-
sorption was carried out on Analytik Jena spectrophotometer (Model
SZ-100, Germany). Fluorescence experiments were recorded on Agilent
Technologies spectrofluorophotometer (G9800A) at 298 K. Mass spec-
tra was recorded on Agilent 6330 Ion Trap LC/MS system.
3.2. Detection of Cations and Anions
The cation and anion sensing ability of 1 was studied by its changes
in fluorescence emission intensity in the presence of various guests
(1:50 equiv. ratio) in water (Fig. 1). Fascinatingly, 1 showed dual fluo-
rescence behavior at 580 nm (enhancement for Fe2+ and AcO− ions;
quenching for Sr2+ and Cu2+ ions) upon addition of chloride salt of cat-
ions and tetrabutylammonium (TBA) salt of anions. On the other hand,
exposure of receptor 1 to other cationic and anionic solutions, did not
lead to any conspicuous change in fluorescence intensity, i.e., there
was no evidence of either exciplex or excimer emission upon addition
of other entities, suggesting no binding or very weak binding for these
ionic species to the receptor 1.
3.3. Selectivity and Sensitivity Studies
To study the selectivity and sensitivity of the receptor 1 towards Fe2
+, Sr2+, Cu2+ and AcO− ions, we performed series of fluorescence titra-
tions using chloride salt of cations and TBA salt of acetate anion (Fig. 2).
Addition of chloride salt of Fe2+ and TBA salt of AcO− to 1 induces fluo-
rescence intensity enhancement (switch on) at 580 nm (Fig. 2a and d).
To our surprise, we found that fluorescence intensity was steadily de-
creased (switch off) at 580 nm as the concentration of Sr2+ and Cu2+ in-
creased (Fig. 2b and c) [19]. Noteworthy to mention, upon increasing
the concentration of guest molecules, the emission intensity peak of 1
at 580 nm showed slight blue shift and red shift (~2 nm) when under-
going enhancement and quenching, respectively. This indicates that
the entire π-system of 1 was perturbed due to an increase in rigidity,
which confirms the involvement of the phenol oxygen atom and urea-
NH protons in binding [16c,20]. The manifestation of distinct isosbestic
point at 563 nm clearly reveals that only one guest molecule can (i) bind
through the deprotonation of two\\OH protons of phthalazine moiety
(guest: Fe2+ or Cu2+ or Sr2+) and (ii) form hydrogen bonds with\\NH
protons of urea moiety (guest: AcO−) during the titration (Fig. SI 19)
[21].
2.2.
Synthesis
of
1,
1′-(Phenazine-2,3-diyl)bis(3-(1,4-
dihydroxyphthalazin-6-yl)urea) (1)
To
a boiling acetonitrile solution (5 mL) of 4 (2, 3-
diisocynatophenazine; 0.04 mmol, 10 mg) was added drop-wise solu-
tion of 8 (HCl salt of 6-amino-2, 3-dihydrophthalazine-1, 4-dione;
0.08 mmol, 14 mg). The reaction mixture was refluxed at 85 °C for
8–10 h. After completion of the reaction, the reaction mixture was
cooled to room temperature; the precipitate was collected and dried.
(Color: pale yellow solid; Yield: 50 mg (74%)). FTIR-ATR (ν, cm−1):
3113 (N\\H), 2992 (OH), 1702 (C_O), 1041 (C\\N), and 642 (C\\H);
UV–Visible (H2O, λmax, nm): 229, 257, and 395; Fluorescence emission
(H2O, λex 395 nm): λem 580 nm; 1H NMR (DMSO‑d6, 500 MHz, ppm):
δ 7.36 (m, 2H, ArH), 7.49 (m, 4H, ArH), 7.61 (m, 6H, ArH), 8.72 (s, 2H,
\\NH), 9.14 (s, 2H, \\NH), 11.11 (s, 4H, \\OH); 13C NMR (DMSO-d6,
100 MHz, ppm): δ 168.9, 161.3, 160.4, 151.8, 143.3, 136.7, 133.7,
133.4, 133.1, 131.8, 130.2, 128.4, 127.3, 122.2, 116.0, 112. 7, 111.4; MS
(m/z) = 616.6.
3.4. Plausible Sensing Mechanisms
3. Results and Discussion
Water mediated excited-state intramolecular proton transfer
(ESIPT) mechanism is accountable for the dual behavior of 1 at particu-
lar fluorescence emission intensity wavelength (580 nm) upon addition
of cations such as Fe2+, Sr2+ and Cu2+. The proton transfer takes place
from the acidic (hydroxyl proton) to the basic site (aromatic nitrogen)
3.1. Design and Synthesis
In the midst of optical sensors, fluorescent systems offer many advan-
tages. Fluorescence measurement has become increasingly interesting
Scheme 1. Synthesis of 1, 1′-(phenazine-2,3-diyl)bis(3-(1,4-dihydroxyphthalazin-6-yl)urea) (1).