Md. Amin Hasan et al. / Polyhedron 50 (2013) 306–313
307
solutions of the cations to be detected (5 ꢁ 10ꢀ3 M), were prepared
similarly as prepared for their titrations using absorption measure-
ments. Luminescence titrations were performed by maintaining the
concentration of the host molecules (1 and 2) at 10ꢀ4 M while the
concentrations of the guest molecules (cations to be detected) were
varied within (0–200)ꢁ10ꢀ6 M, and the fluorescence spectra were
measured until the fluorescence intensity reached a maximum.
The fluorescence intensity was measured at kex 370 nm. The maxi-
mum emission was observed at kem 458 and 459 nm for complexes
1 and 2, respectively.
2. Experimental
2.1. Materials and methods
Reagents of A.R. grade were purchased from Sigma–Aldrich and
Merck, and were used without further purification. Solvents were
dried and distilled using standard procedures [28]. Elemental anal-
ysis was carried out using a Carbo-Erba elemental analyzer 1108,
IR spectra were recorded as KBr pellets using a Varian 3100 FT-IR
spectrometer and 1H NMR spectra were recorded on a JEOL AL
300 MHz spectrometer using DMSO-d6 as the solvent and TMS as
an internal reference. A Shimadzu UV-1701 spectrophotometer
was used to record UV–Vis spectra and emission spectra were re-
corded in Tris–HCl buffer [pH 7–8; (DMSO/water 1:9; v/v)] at room
temperature using a Shimadzu UV-1601 spectrometer and a Perkin
Elmer LS-45 luminescence spectrometer. The time-resolved fluo-
rescence delay was measured on a single-photon counting spec-
trometer equipped with pulsed nanosecond LED excitation heads
at 280 nm (HORIBA, Jobin Yvon, IBH Ltd., Glasgow, UK), run in re-
verse mode. This experiment was also performed at room temper-
ature. The fluorescence lifetime data were measured to 10000
counts in the peak, unless otherwise indicated. The instrumental
response function was recorded sequentially using a scattering
solution and a time calibration of 114 ps/channel. Data were ana-
lyzed by using a sum of exponentials, employing a non-linear least
squares reconvolution analysis from HORIBA, Jobin Yvon, IBH Ltd.
The pH values of the solutions were measured on a CyberScan
pH/mV/°C/F metre with MFRS (Toshniwal Instruments, MFG Pvt.
Ltd.), using the reported method [29]. Stock solution (25 mL) of
the complexes (1 ꢁ 10ꢀ3 M) were prepared initially in DMSO as
they were sparingly soluble in water. To 1.0 mL of the stock solu-
tions of complexes in DMSO, 9.0 mL of 0.5 M aqueous HCl was
added to get a 10 mL stock solutions of 1 ꢁ 10ꢀ4 M concentration.
The pH of the solution was varied between 2 and 12 by the addi-
tion of a calculated amount of aqueous 1.0 M NaOH solution
2.2. Synthesis of the ligands
The ligands L1H2 [31] and L2H2 [32] were synthesized and char-
acterized by the reported methods. A solution of the diazonium
salt was prepared under cooling (0–5 °C) from the respective ani-
line (20 mmol) in hydrochloric acid (3 N, 40 mL) and a conc. aque-
ous solution of sodium nitrite (1.37 g, 20 mmol), according to the
standard procedure [33]. A cold solution of the diazonium salt
was added under cooling (0 °C) and stirring to a mixture composed
of pentane-2,4-dione (2.1 mL, 2.04 g, 20 mmol), sodium acetate
(8.2 g, 100 mmol), methanol (160 mL) and water (160 mL). The
mixture was then warmed to room temperature and stirred for
1 h. The corresponding precipitate was collected, washed with
water and recrystallized from ethanol. The details for each com-
pound are given below.
2.2.1. Synthesis of L1H2
2-Aminobenzoic acid (2.74 g, 20 mmol) was used for the syn-
thesis of L1H2 and provided 3.9 g (78%) as a yellow powder, soluble
in DMSO, methanol, ethanol and acetone, but insoluble in water.
M.p.: 245 °C. Anal. Calc. for [C12H12N2O4]: C, 58.06; H, 4.87; N,
11.29. Found: C, 57.93; H, 5.03; N, 11.27%. IR (KBr pellets, cmꢀ1):
3482
m
m(NH), 1680v(C@O), 1633 m(C@O), 1605 (C@O--H), 1518
(C@N), 1H NMR (DMSO-d6, 300 MHz, ppm) d: 15.103 (s, 1H, –
(ꢂ10
l
L), consequently forming a B/BH+ type buffer system. The
COOH), 13.707 (s, 1H, –NH), 7.983 (d, 2H, –Ph), 7.653 (d, 2H, –
Ph), 2.510 (s, 3H, –CH3), 2.483 (s, 3H, –CH3). 13C NMR (DMSOd6,
ppm) d: 195.324 (C@O), 193.626 (C@O), 166.831 (COOH),
145.411 (C@N), 134.846 (Ph–H), 130.964 (Ph–H), 126.934 (Ar–H),
115.750 (Ph–C–COOH), 31.267 (CH3), 26.338 (CH3).
solution was stirred for 3–5 min and the pH was recorded with
the help of a digital pre-calibrated pH metre, and then absorption
and luminescence spectra were measured at a particular pH. For a
typical titration experiment, the stock solutions of 1 and 2
(1.0 ꢁ 10ꢀ5 M) were prepared separately using spectroscopic grade
DMSO and triply distilled H2O (1:9, v/v) in 0.01 M Tris–HCl buffer.
2.2.2. Synthesis of L2H2
The solutions of nitrate salts of Li+, Na+, K+, Ca2+, Mg2+, Ag+, Cu2+
,
4-Aminobenzoic acid (2.74 g, 20 mmol) was used in the synthe-
sis and gave 3.75 g (75%) of L2H2 as a yellow powder. M.p.: 216–
218 °C. Anal. Calc. for [C12H12N2O4]: C, 58.06; H, 4.87; N, 11.29.
Found: C, 57.73; H, 4.93; N, 11.09%. IR (KBr pellets, cmꢀ1): 3451
Fe3+, Hg2+, Ni2+, Co2+ and Zn2+ were prepared by dissolving them
in distilled water (5 ꢁ 10ꢀ4 M). Solutions (2.0 mL) of complexes 1
and 2 (1.0 ꢁ 10ꢀ5 M) were taken separately in a quartz cell of
10 mm path length, then the solutions of the metal ions were
added (0–2.0 equiv) gradually in the cell. The spectra were re-
corded after equilibration for 5 min, allowing the complexes to
bind with the cations. The absorption, if any, by the cations (guest
molecules) was eliminated initially by keeping their equal quanti-
ties separately in the hosts (1 and 2) and a reference solution. From
the absorption data, the intrinsic association constant Ka was
m
(NH), 1680 m(C@O), 1633 m(C@O), 1605 (C@O--H), 1514 m(C@N).
1H NMR (DMSO-d6, 300 MHz, ppm) d: 15.201 (s, 1H, –COOH),
13.692 (s, 1H, –NH), 7.962 (d, 2H, J = 9.0 Hz, –Ph), 7.630 (d, 2H,
J = 9.0 Hz, ꢀPh), 2.483 (s, 3H, –CH3), 2.441 (s, 3H, –CH3). 13C NMR
(DMSOd6, ppm) d: 194.337 (C@O), d: 193.574 (C@O), 166.160
(COOH), 144.867 (C@N), 134.623 (Ph–C), 130.881 (Ph–C),
115.651 (Ph–C), 31.308 (CH3), 26.437 (CH3).
determined from a plot of [guest]/(ea
[30] Eq. (1)
ꢀ
ef) versus [guest] using
2.3. Synthesis of [Zn(L1H)2(bpy)] 1
ef Þꢃꢀ1
;
ð1Þ
A solution of Zn(bpy)(NO3)2ꢄ2H2O (0.363 g, 1 mmol) [34] dis-
solved in EtOH:DMF (3:1) (8 mL) was added dropwise to a solution
of L1H2 (0.496 g, 2.0 mmol) in EtOH:DMF (3:1) (16 mL) over half an
hour at 50 °C with stirring. The mixture was then heated under re-
flux for 18 h. After cooling to room temperature, it gave a yellow
solid. This solid was recrystallized from EtOH/DMF (4:1), giving
yellow crystals. Yield: 0.801 g (57%). M.p.: > 280 °C. Anal. Calc. for
[C34H30N6O8Zn]: C, 57.03; H, 4.22; N, 11.74. Found: C, 57.10; H,
4.11; N, 11.45%. IR (KBr pellets, cmꢀ1): 3430(w), 1686(vs),
1606(vs), 1524(s), 1383(vs), 1317(s), 1261(s), 1202(m), 1027(w),
½guestꢃ=ðea
ꢀ
ef Þ ¼ ½guestꢃ=ðeb
ꢀ
ef Þ þ ½Kaðeb
ꢀ
where [guest] are the metal ions to be detected. The apparent
absorption coefficients ea ef and eb correspond to Aobsd/[1] or Aobsd
,
/
[2], the extinction coefficient of the free 1 or 2 and the extinction
coefficient of 1 or 2 in the fully bound form, respectively. The value
of Ka (association constant) is given by the ratio of the slope to the
intercept. The binding constants were calculated in duplicate, and
an average is reported. For the fluorescence measurements, solu-
tions of 1 and 2 (1.0 ꢁ 10ꢀ4 M) separately, together with the