H. Rahimi et al.
Journal of Photochemistry & Photobiology, A: Chemistry 407 (2021) 113049
types of fluorophore units like naphthyl and anthracenyl [37], benzo-
thiazole and benzimidazole [38] as well as thiadiazolyl [39] groups, in
order to recognize metal ions in a selective manner.
hexane 4:6 v/v, Rf value = 0.64). After completion of the reaction, 50
mL of saturated NaHCO3 (aq) was added to the reaction vessel and the
resulted mixture was filtered. The filtrate was concentrated under
reduced pressure to remove the solvent and then, the residue was
filtered off to afford the product 2 (1.60 g, 93 %) as transparent plates.
The crude product was used directly in the next step. M.p. 126–128 ◦C;
1H NMR (CDCl3, 400 MHz) δ 7.65 (d, J =7.6 Hz, 1 H), 7.58 (d, J =8.4 Hz,
1 H), 7.48 (d, J = 7.6 Hz, 1 H), 7.33 (td, J = 7.6 and J =0.8 Hz, 1 H), 7.21
(td, J = 7.6 and J =1.2 Hz, 1 H), 6.9 (d, J =1.2 Hz, 1 H), 6.73 (dd, J = 8.0
and J =2 Hz, 1 H), 3.83 (s, 2 H), 3.70 (br s, 2H, NH2) ppm (Fig. S2).
Taking into account the points listed above and in continuation of
our previous researches on fluorescent sensors using fluorene moieties
as fluorophore [40–45], herein we report the synthesis of a new and
effective pdca-based fluorescent probe bearing fluorene rings as fluo-
rophore which was efficiently employed for the detection of Pb2+ and
Cu2+ ions. Based on empirical results, this chemosensor demonstrated
remarkable selectivity and sensitivity towards copper (II) and lead (II)
ions.
2.3.3. The preparation of N,N′-bis(2-fluorenyl)-pyridine-2,6-
2. The experimental
dicarboxamide (3)
A mixture of pyridine-2,6-dicarboxylic acid (1.0 g, 6 mmol) and 2-
amino-9H-fluorene 2 (2.17 g, 12 mmol) were dissolved in 25 mL pyri-
dine and heated at reflux temperature with continuous stirring for 30
min under argon atmosphere. After that, P(OPh)3 (3.94 g, 12.7 mmol)
was added dropwise to the reaction mixture and it was stirred at 80 ◦C
for 12 h. After cooling to room temperature, the mixture was filtered off
and the resulting solid was washed with water and ethanol. Finally,
recrystallization of the residue from chloroform afforded the product 3
as yellow crystals. Yield: 2.2 g, 74 %; Rf value = 0.29 (ethyl acetate:
hexane, 3:7 v/v); M.p. 327–330 ◦C; Anal. calc. for C33H23N3O2: C, 80.31;
H, 4.70; N, 8.51 Found: C, 80.40; H, 5.00; N, 8.47; FT IR spectrum
(cmꢀ 1): 3301 (N–H), 1661 (C = O); UV/Vis spectrum (CH3CN): λmax 268
(2.7 × 104), 316 nm (2.9 × 104) ; ESI+-MS spectrum (CH3CN, m/z):
493.2 for [L3 + H+]; 1H NMR spectrum (500 MHz,CDCl3): 10.70 (s, 2 H),
8.14 (d, J =7.7 Hz, 2 H), 7.91 (s, 2 H), 7.82 (t, J =7.7 Hz, 1 H), 7.55 (d, J
=8.3 Hz, 2 H), 7.48 (d, J =8.15 Hz, 2 H), 7.42 (d, J =7.55 Hz, 2 H), 7.20
(d, J =7.45 Hz, 2 H), 7.02 (dt, J = 7.45 and J =7 Hz, 2 H), 6.93 (dt, J =
7.4 and J =7.35 Hz, 2 H), 3.62 (s, 4 H); 13C NMR spectrum (125 MHz,
CDCl3): 161.73, 149.11, 143.58, 142.85, 141.02, 138.83, 137.87,
136.75, 126.58, 126.15, 125.02, 124.78, 120.35, 119.59, 119.28,
118.16, 36.77 ppm (Fig. S3- S7).
2.1. Instrumentation
Measurement of melting points was carried out by an Electrothermal
Engineering IA9100 apparatus. 1H and 13C NMR spectra were assigned
by employing a Bruker Ultrashield 400 MHz Avance III spectrometer (1H
NMR at 400 MHz) or a Varian Unity Inova 500 MHz spectrometer (1H
NMR at 500 MHz and 13C NMR at 125 MHz) using CDCl3, as solvents and
tetramethylsilane (TMS) as the internal standard. A Costech-ECS 4010
CHNSO Analyzer was applied to evaluate the elemental analyses. The
UV–vis analyses were monitored by the use of SPEKOL 2000 Analytik
Jena spectrometer in acetonitrile as solvent. The fluorescence experi-
ments were screened by the JASCO FP-8300 spectrophotometer. Scan-
ning the mass spectra were performed by MSD Agilent 5975C. Fourier
transform infrared spectroscopy (FT-IR) spectra were recorded with KBr
pellets on a Bruker TENSOR 27 spectrometer. Analytical thin-layer
chromatography (TLC) was performed on silica gel plates (Merck, Kie-
selgel 60 Å, 0.25 mm thickness) with the F254 indicator. The pH mea-
surements were performed using a digital Metrohm 744 pH meter.
2.2. Chemicals and materials
3. Results and discussion
The chemicals and also solvents were provided from Merck Company
and were utilized without further purifications. Zn(ClO4)2⋅6H2O, Pb
(ClO4)2, Hg(ClO4)2⋅3H2O, Cu(ClO4)2⋅6H2O, Ba(ClO4)2, Co(ClO4)2⋅6H2O,
Mg(NO3)2⋅6H2O, Mn(ClO4)2⋅6H2O, Al(NO3)3⋅9H2O, Cr(NO3)3⋅9H2O, Fe
(ClO4)2, LiClO4, AgClO4, NaClO4 and KClO4 were employed as sources
for metal ions.
The cation receptor 3 was successfully synthesized as depicted in
Scheme 1. Accordingly, nitration of fluorene was accomplished by the
use of concentrated HNO3 in glacial acetic acid which led to the for-
mation of 2-nitro-9H-fluorene 1. Next, the reduction of 2-nitro-9H-fluo-
rene 1 to 2-amino-9H-fluorene 2 was performed using iron powder and
ammonium chloride in aqueous ethanol. Eventually, the final product 3
was obtained through the coupling reaction between pyridine-2,6-
dicarboxylic acid and 2-amino-9H-fluorene 2 in the presence of tri-
phenyl phosphite (74 % yield). The structure of these compounds was
identified using FT-IR, 1HNMR, 13CNMR, ESI+-MS and elemental
analyses.
2.3. The synthetic procedures and characterizations
2.3.1. The preparation of 2-nitro-9H-fluorene (1)
The 2-nitro-9H-fluorene (1) was prepared based on the procedure
reported in the literature [46]. 9H-fluorene (4.9 g, 30 mmol) was dis-
solved in 45 mL of glacial acetic acid at 60 ◦C and subsequently, nitric
acid 65 % (7 mL) was added in a dropwise manner over 15 min with
vigorous stirring. During the addition, the color of the solution slightly
turns yellow, along with a little precipitation. The resulting mixture was
allowed to stir at 80 ◦C. The progress of the reaction was screened via
TLC (ethyl acetate: hexane 1:9 v/v, Rf value = 0.67). After completion of
the reaction, the mixture was poured into 300 mL of water and then, the
crude product was filtered off, washed with water and recrystallized
from 150 mL ethanol to afford the pure product 1 (5.78 g, 91 %) as
slightly-yellow needles. M.p. 155–157 ◦C; 1H NMR (CDCl3, 400 MHz) δ
8.43 (s, 1 H), 8.33 (d, J = 8 Hz, 1 H), 7.91–7.88 (m, 2 H), 7.65 (d, J = 6.4
Hz, 1 H), 7.50–7.46 (m, 2 H), 4.04 (s, 2 H) ppm (Fig. S1).
FT IR spectrum of 3 exhibits characteristic peaks at 3301 and 1661
cmꢀ 1 attributed to N H and C = Oamide stretching vibrations, respec-
–
tively. The 1H NMR spectrum displays a signal at 10.70 ppm related to
–
the N H group, as well as aromatic protons that emerged at the range of
6.92–8.14 ppm. Furthermore, the observed signal at 3.62 ppm indicates
the presence of two methylene groups of fluorene rings in the chemo-
sensor structure. The ESI+ mass spectrum of receptor 3 reveals the
molecular ion peak at 493.2, representing the existence of [3 + H+].
3.1. Cation binding studies
Evaluation of the cation-recognition efficiency of chemosensor 3 was
performed by employing spectral techniques including UV–vis and
spectrofluorimetry methods. UV–vis absorption measurements of re-
ceptor 3 upon addition of several metal ions as their perchlorate salts
were accomplished within the range of 220ꢀ 650 nm in CH3CN
employing a quartz cell (1 cm) at room temperature (25.0 ± 0.1 ◦C).
Moreover, the fluorescence emission spectra were accomplished in λem
2.3.2. The preparation of 2-amino-9H-fluorene (2) [41,47]
To a mixture of 2-nitro-9H-fluorene 1 (2.0 g, 9.5 mmol) in aqueous
ethanol (90 mL of ethanol and 25 mL of water), iron powder (1.59 g,
28.51 mmol) and NH4Cl (1.02 g, 19 mmol) were added and the reaction
mixture was stirred under argon atmosphere at reflux temperature for 4
h. The reaction progress was checked out through TLC (ethyl acetate:
2