Synthesis and Characterization of Acid-Responsive Luminescent Fe(II) Metallopolymers of Rigid and Flexible Backbone N-Donor Multidentate Conjugated Ligands

Article abstract of DOI:10.1021/acs.inorgchem.9b02985

In this report we have disclosed the syntheses and properties of two new conjugated organic moieties bearing the same coordination sites but possessing different backbone rigidities and rotational flexibilities. Two new metallopolymers have been synthesized from the corresponding ligands under identical reaction conditions, and they have been thoroughly characterized through different techniques to understand the effect of backbone rigidity on the evolution of different properties in these metallopolymers. A FESEM micrograph of the rigid metallopolymer confirms the formation of a rigid nanorod type structure, while long agglomerated nanofiber strands are visible on the substrate in the case of the flexible analogue. All of the newly synthesized materials are fluorescence active. An Fe(II) metallopolymer of the flexible ligand showed huge changes in emission properties in the presence of different acids and showed a possibility of it being used as a thin film acid vapor sensor. All of the materials showed reversible electrochemical activity, and both of these polymers have shown electroluminescence when an appropriate potential is applied.

Full text of DOI:10.1021/acs.inorgchem.9b02985

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Synthesis and Characterization of Acid-Responsive Luminescent
Fe(II) Metallopolymers of Rigid and Flexible Backbone N‑Donor
Multidentate Conjugated Ligands
Anil Kumar, Shubham Bawa, Kapil Ganorkar, Sujit Kumar Ghosh, and Anasuya Bandyopadhyay*
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ABSTRACT: In this report we have disclosed the syntheses and
properties of two new conjugated organic moieties bearing the same
coordination sites but possessing different backbone rigidities and
rotational flexibilities. Two new metallopolymers have been
synthesized from the corresponding ligands under identical reaction
conditions, and they have been thoroughly characterized through
different techniques to understand the effect of backbone rigidity on
the evolution of different properties in these metallopolymers. A
FESEM micrograph of the rigid metallopolymer confirms the
formation of a rigid nanorod type structure, while long agglomerated
nanofiber strands are visible on the substrate in the case of the flexible analogue. All of the newly synthesized materials are
fluorescence active. An Fe(II) metallopolymer of the flexible ligand showed huge changes in emission properties in the presence of
different acids and showed a possibility of it being used as a thin film acid vapor sensor. All of the materials showed reversible
electrochemical activity, and both of these polymers have shown electroluminescence when an appropriate potential is applied.
group/atom by changing their emission spectra sharply.
Extensive research has been carried out to synthesize several
fluorescent sensors to detect different metal ions such as
Hg(II),14 Fe(III),15 Cr(III),16 and Cu(II)17,18 in living systems
and chemicals such as nitric oxide,18−20 amino acids,21−23
saccharides,24 picric acid,25−28 ATP,29 ADP,29 D-glucose,30
sugar acids,23 cholic acid,31−33 etc. Recently, fluorescence
active polymers have drawn major interest over their organic
and inorganic small-molecule counterparts because of their
better mechanical, thermal, and film-forming ability34,35 which
is a direct consequence of the formation of very long chains by
a continuous combination of many monomer units. Alterable
electroluminescence36 is also observed in some of the
metallopolymer thin films when an appropriate potential is
applied, and it is initiated by a reversible electron push/pull
mechanism on transition-metal sites followed by fast
delocalization of the charge/electron cloud through a
conjugated organic backbone. Acid−base reactions in solution
are the most typical chemical reactions for switching systems,
as demonstrated for logic gates,37 molecular motors,38 and
machines.39 Acid−base gas-responsive luminescent materials
have potential applications in sensors,40 biochemical assays,41
INTRODUCTION
■
The combination of a multidentate organic moiety with
transition-metal ions through a complexation reaction has led
to the generation of a new field of supramolecular metal-
lopolymers. In these kinds of macromolecules, a metal−ligand
interaction is exploited to synthesize several new smart
functional materials for photophysical, electrochemical, and
spectroelectrochemical applications.1−3 The structural tuna-
bility of the organic moiety and the availability of several metal
ions for coordination purposes make these polymers tailor-
made functional materials, as tuning in either way will affect
the positions of the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital
(LUMO) states and the corresponding energy gap.4,5 Thus,
tuning of different properties such as solubility, stability,
optical properties, redox potential, and kinetics can be done
very easily. For this purpose, different organic moieties have
been synthesized with variation in both coordination sites and
coordination number to achieve several metallopolymers by
employing different metal ions to obtain various properties
such as thermochromism,6 solvatochromism,7 electrochrom-
ism,8 nonvolatile memory,9 fluorescence,10 etc. Terpyridine
type ligands are very popular for the fabrication of metal-
lopolymers because of their chelation ability toward various
metal ions and thus inducement of various properties in these
polymers.11−13
Received: October 10, 2019
Fluorescent chemosensors are an important class of
compounds, as they can easily detect a particular functional
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and display devices.42 However, only a few reports are available
on the vapoluminescent switching of materials triggered by the
presence of acid−base vapors in the solid state.43
vacuum sputtering. Time-resolved fluorescence decay measurements
were carried out by the time-correlated single-photon counting
(TCSPC) technique by using a PTI Pico Master TCSPC
spectrofluorometer with a PMH-100-4 detector. A 340 nm light-
emitting diode was used as the excitation source, and the lifetime was
recorded on a 150 ns scale. The Felix GX software was used for
fluorescence data collection and analysis. The goodness of fit was
judged by χ2 values, Durbin−Watson parameters, visual observations
of fitted lines, residuals, and autocorrelation function. For all lifetime
measurements, the fluorescence decay curves were analyzed by a
single- and double-exponential iterative fitting program. Ground state
(GS) and excited state (ES) optimizations of the synthesized
compounds RNL and FNL were performed by using density
functional theory (DFT) with the Gaussian 09 software package
and visualizing in GaussView. The optimization was done by using
Becke’s three-parameter exchange correlation functional Lee, Yang,
and Parr (B3LYP) functional with a 6-31G(d) basis set. The
molecular orbital optimizations (HOMO, LUMO) calculated by
using time-dependent density functional theory (TD-DFT) were used
for optimization of a geometry in the singlet excited state using the
mentioned basis set with the polarizable continuum model (PCM).49
The vertical excitations were computed using the linear response of
TD-DFT with the 12 lowest singlet−singlet transitions using the
aformentioned basis set and functional. The thermal stabilities of
synthesized homo- and copolymer powders (approximately 10 mg)
were investigated using thermogravimetric analysis (TGA) with a TA
Instruments TGA55 Discovery series instrument under an N2
atmosphere. The temperature was increased from 30 to 800 °C at a
scan rate of 10 °C min−1. The glass transition temperature (Tg) and
melting point (Tm) of the synthesized samples (3−4 mg) were
evaluated by using a differential scanning calorimeter (DSC) on a TA
Instruments DSC 25 Discovery series instrument under a N2
atmosphere. Samples (4 mg) were taken each time of the
corresponding polymer in an aluminum pan for the measurement.
To eliminate background error for moisture and other impurities (if
any), a sequence of heating/cooling/heating (200 °C/60 °C/200 °C)
at a 10 °C min−1 scanning speed was carried out. The melting points
of different organic compounds were noted by a Digital melting point
apparatus. The high-resolution mass spectra (ESI-HRMS) of the two
ligands were recorded on a Bruker Daltronics microTOF-QII
spectrometer using ESI ionization. Samples were introduced into
the mass spectrometer ion source by direct infusion using a syringe
pump, and the instrument was operated in positive ion mode. The
capillary voltage was set at 4500 V. Elemental analysis was done with a
CHNS Elementar Vario micro cube analyzer.
In this context, we have first designed two different ligands
which bear the same coordination sites but have differences in
backbone flexibility which is purposefully introduced by
incorporating a fused aromatic ring on backbone of one
while the other enjoys rotational flexibility. The effect of
variation of coordination sites in the generation and tuning of
different properties has already been reported in many
reports,44−48 but the effect of backbone rigidity on the tuning
of properties is still an under-focused area. In this report we
have fabricated two novel Fe(II) metallopolymers by complex-
ation reactions with our newly synthesized multidentate,
ditopic Schiff base ligands. The motivation behind this work
is to develop new polymeric materials with differences in
properties as a result of the different extents of steric hindrance
on organic−inorganic direct interactions. The ligands and
polymers have all been thoroughly characterized by different
experiments to understand the structure−property relation-
ship.
EXPERIMENTAL SECTION
■
Materials. 1,2-Phenylenediamine was purchased from Sigma-
Aldrich and used after recrystallization. Thionyl chloride, bromine,
and sodium metal were purchased from LOBA (India). Bis-
(pinacolato)diboron and PdCl2(dppf) were purchased from Alfa
Aeser Pvt. Ltd. Potassium acetate (KOAc), potassium carbonate,
oxalyl chloride, and Fe(II) acetate were purchased from Sigma-
Aldrich India and used without any further purification. Phthalic acid,
acetic acid, sodium sulfite, and HBr were purchased from SRL Pvt.
Ltd.; Celite and sodium sulfate were purchased from HiMedia
(India). All acids (H2SO4, glacial CH3COOH, HNO3, HClO4) were
purchased from HiMedia (India). 1,4-Dioxane and THF were
purchased from SRL Pvt. Ltd. and dried over sodium metal and
benzophenone under a nitrogen atmosphere. All of the solvents were
AR grade and were purified by standard procedures wherever
required. All of the reactions were carried out under a dry nitrogen
atmosphere.
Caution! Concentrated acids are extremely corrosive materials and
should be handled carefully with proper precautions to avoid inhaling
of acid vapors.
Instrumentation and Characterization. ATR-IR (attenuated
total reflectance infrared) spectra of the ligands and polymers were
recorded on a PerkinElmer FT-IR C91158 spectrophotometer using a
Synthesis of Ligands. Synthesis of 2,1,3-Benzothiadiazole (2).
A 7.5 g portion (92.5 mmol) of 1,2-phenylenediamine was dissolved
in 225 mL of CH2Cl2, and then 39 mL (370 mmol) of triethylamine
was added to deprotonate the diamine solution. Freshly distilled
SOCl2 (10 mL, 184.9 mmol) was added dropwise to that solution,
and after complete addition of reactant, the mixture was refluxed for 6
h at 40 °C. The reaction was stopped, and the mixture was cooled.
The cooled reaction mixture was then neutralized with 2 M HCl
followed by extraction with CH2Cl2. The organic extract was washed
with brine solution and dried over anhydrous Na2SO4. The crude
product was purified by steam distillation, and a white crystalline solid
was obtained. Yield: 8.3 g (93%). Mp: 45 °C. ATR-IR (cm−1): 3087,
1
zinc selenide crystal, in the range of 4000−500 cm−1. H NMR and
13C NMR spectra were measured by 500 MHz Bruker instrument
with tetramethylsilane (TMS) as a reference. The wide-angle X-ray
diffraction (WXRD) patterns of the sample were collected with a
RIGAKU ULTIMA IV X-ray diffractometer, in the angle range of 2θ
= 5−80° at a scan rate of 4°/min, using Ni-filtered Cu Kα as a
radiation source. The absolute molecular weight of the polymer
sample was recorded by a static light scattering experiment (SLS) in
batch mode using a Brookhaven BI-200SM goniometer with the stock
solution and solvent filtered through a 0.2 μm Nylon filter. The
conductivity measurements were carried out using EUTECH
Instruments Model CON 700 instrument. The UV measurements
were measured on a Shimadzu UV spectrophotometer. Cyclic
voltammetry (CV) measurements were performed using a CH
Instruments CH1620E cyclic voltammeter at a scan rate of 50 mV/s.
Atomic force microscope (AFM) imaging of the polymer was carried
out on a freshly cleaved highly ordered pyrolytic graphene (HOPG)
substrate using an NT-MDT-INTEGRA microscope in tapping mode
at room temperature. Field emission scanning electron microscope
(FE-SEM) images of the sample were recorded on a TESCAN MIRA
3 instrument, by pouring a 10−5 M solution of the sample on glass
slides of area 1 cm2 mounted on stubs and sputtered by gold using
1
1611, 1511, 1478, 1328, 1133, 920, 843, 812, 745. H NMR (500
MHz, CDCl3): δ 8.03 (dd, 2H), δ 7.62 (dd, 2H).
Synthesis of 4,7-Dibromo-2,1,3-benzothiadiazole (3). A 6 g
portion (44 mmol) of 2,1,3-benzothiadiazole (2) was taken up in 80
mL of HBr (48%); 6.7 mL of Br2 (20.77 g, 130 mmol) was taken up
into 20 mL of HBr, and then it was added dropwise to the former
solution. After complete addition, the mixture was refluxed at 120 °C
for 6 h to obtain orange precipitates.
The reaction mixture was cooled to room temperature, and a
saturated solution of Na2SO3 was added. The resultant suspension
was filtered. A pale yellow solid was obtained as a precipitate, and it
was recrystallized from hexane to produce needlelike crystals. Yield:
10.62 g (82%). Mp: 188 °C. ATR-IR (cm−1): 3077, 1583, 1475, 1311,
B
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The solvent was removed with a rotary evaporator, and the product
was extracted by CH2Cl2. Finally, it was washed with brine solution
and dried over anhydrous Na2SO4. A yellow solid was obtained as the
final product. Yield: 866 mg (96%). Mp: 192 °C. ATR-IR (cm−1):
3419, 3295, 3028, 1677, 1450, 1402, 1335, 1220, 1126, 1014, 797,
Scheme 1. Schematic Representation of the Syntheses of the
Ligands
1
717, 615. H NMR (500 MHz, CDCl3): δ (ppm) 8.77 (s, 2H), 8.65
(d, 2H), 7.84 (d, 2H), 6.80 (s, 2H), 7.44 (t, 2H), 3.62 (s, 4H).
Synthesis of the Rigid Neutral Ligand (RNL) 2,3-Bis(pyridin-2-yl)-
5,8-bis(pyridin-3-yl)quinoxaline (7). A 500 mg portion (1 mmol) of
3,6-bis(3-pyridyl)-1,2-phenylendiamine and 460 mg (1 mmol) of
pyridil were added to an ethanol/H2O (70/30) solvent mixture with a
catalytic amount of phthalic acid (5 mol %). The resultant reaction
mixture was stirred at room temperature for 10 min, and then the
temperature was increased to 60 °C and stirring was continued for 24
h. The course of the reaction was monitored by TLC. After
disappearance of the starting materials, the reaction mixture was
cooled to room temperature. A solid mass was obtained on cooling,
and it was filtered and washed several times with distilled water to give
a yellow product. Yield: 920 mg (95%), Mp: 232 °C. ATR-IR (cm−1):
3041, 1585, 1570, 1475, 1431, 1385, 1345, 1303, 1234, 1191, 1071,
1
1001, 916, 854, 796, 765, 750, 707, 645, 614. H NMR (500 MHz,
CDCl3): δ (ppm) 9.12 (s, 2H), 8.72 (d, 2H), 8.27 (d, 2H), 8.18 (dd,
4H), 8.02 (s, 2H), 7.87 (t, 2H), 7.51 (t, 2H), 7.23 (t, 2H). 13C NMR
(500 MHz, CDCl3): δ (ppm) 157.22, 152.01, 151.30,148.82, 148.12,
138.46, 138.02, 137.18, 136.96, 133.64, 130.36, 124.02, 123.16,
123.06 (see Figure S-1 in the Supporting Information). Anal. Calcd
for C28H18N6: C, 76.71; H, 4.12; N, 19.17. Found: C, 75.02; H, 4.04;
N, 18.82. C28H18N6 ESI-MS: m/z calcd, 438.004; found, 439.1795 (M
+ H+) (see Figure S-2 in the Supporting Information).
1189, 934, 875, 588. 1H NMR (500 MHz, CDCl3): δ (ppm) 7.72 (s,
2H).
Synthesis of the Flexible Neutral Ligand (FNL) (N1E,N2E)-N1,N2-
bis(pyridin-2-ylmethylene)-3,6-bis(pyridin-3-yl)benzene-1,2-dia-
mine (8). 3,6-Bis(3-pyridyl)-1,2-phenylendiamine (500 mg, 1 mmol)
was dissolved in an ethanol/H2O (70/30) solvent mixture. A solution
of 2-pyridinecarboxaldehyde 408 mg (2 mmol) in the aformentioned
solvent mixture was added to the above reaction mixture along with a
catalytic amount of phthalic acid (5 mol %). The reaction mixture
turned yellow immediately after addition of the second reactant, and it
was then stirred at room temperature for 10 min. After that it was
heated to 60 °C and stirring was continuing for 36 h. The reaction
progress was monitored from time to time by TLC. After
disappearance of the starting materials, the reaction mixture was
cooled to room temperature and filtered with crucible filter and then
the precipitate was washed several times with distilled water to
remove any unreacted materials. After several washings a light green
precipitate was collected as the final product and it was then dried in a
vacuum oven for 3 h at 45 °C. Yield: 696 mg (83%), Mp: 218 °C.
ATR-IR (cm−1): 3036, 1590, 1564, 1530, 1472, 1460, 1444, 1409,
1362, 1327, 1294, 1248, 1191, 1123, 1094, 1026, 994, 798, 744, 705,
660, 621, 585. 1H NMR (500 MHz, CDCl3): δ (ppm) 10.92 (s, 1H),
9.42 (s, 2H), 9.03 (s, 1H), 8.69 (d, 2H), 8.60 (t, 2H), 8.51 (d, 2H),
8.03 (d, 1H), 7.87 (t, 1H), 7.62 (t, 1H), 7.49 (m, 3H), 7.38 (t, 1H).
13C NMR (500 CDCl3) δ (ppm) 122.04, 122.24, 122.38, 123.35,
124.03, 124.10, 124.84, 129.15, 132.92, 133.67, 134, 134.34, 136.54,
137.20, 142.88, 147.86, 148.56, 148.96, 149.08, 149.16, 150.17,
157.79 (vide SI Figure S-3). Anal. Calcd for C28H20N6: C, 76.36; H,
4.55; N, 19.09. Found: C, 74.18; H, 4.46; N, 19.30. C28H20N6 ESI-
MS: m/z calcd.: 440.021; found: 439.1795 [M-H]+ (vide SI Figure S-
4).
Synthesis of Fe(II)-RNL poly (9). A 50 mg portion of RNL was
dissolved in 350 mL of acetic acid, and then 19.76 mg of Fe(OAc)2
was added under a nitrogen flow. The reaction mixture was refluxed
under nitrogen for 72 h, and the course of the reaction was monitored
with TLC. The reaction was stopped when all of the ligand was
consumed, and then the mixture was cooled to room temperature
followed by filtration with a cotton plug. The filtrate was kept in a
Petri dish for solvent evaporation. A dark brown brittle film was
obtained on complete evaporation of the solvent. Yield: 59.16 mg
(87%). Tm = 255 °C (by DSC; see Figure S-5a in the Supporting
Information). ATR-IR (cm−1): 2940, 2850, 1585, 1569, 1433, 1408,
1346, 1073, 1000, 791, 749, 709, 655, 613, 552. 1H NMR (500 MHz,
Synthesis of 4,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)-2,1,3-benzothiadiazole (4). A 1.0 g portion (3.4 mmol) of 4,7-
dibromo-2,1,3-benzothiadiazole (3), 1.9006 g (7.8 mmol) of
bis(pinacolato)diboron, 0.238 g (0.33 mmol) of Pd(PPh3)2Cl2, and
2.0 g (20 mmol) of KOAc were placed sequentially in a round-bottom
flask under a nitrogen flow, and then these solids were kept under
vacuum in round-bottom flask for 10 min. After that, freshly distilled
1,4-dioxane (20 mL) was added in the flask and the reaction mixture
was stirred at 110 °C for 20 h. After completion of the reaction the
mixture was cooled to room temperature and the solvent was
removed with a rotary evaporator. The crude solid mass was extracted
with ethyl acetate (100 mL), washed with brine, and dried over
anhydrous Na2SO4. The crude product was purified by column
chromatography, and a pale yellow solid was obtained as the final
product. Yield: 1.012 g (77%). Mp: 175 °C. ATR-IR (cm−1): 2985,
1677, 1550, 1486, 1374, 1327, 1303, 1270, 1189, 1138, 979, 882, 854,
1
700. H NMR (500 MHz, CDCl3): δ (ppm) 8.16 (s, 2H), 1.46 (s,
24H).
Synthesis of 4,7-Bis(3-pyridyl)-2,1,3-benzothiadiazole (5). A 500
mg portion (1.2 mmol) of 4,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxabor-
olan-2-yl)-2,1,3-benzothiadiazole (4), 407 mg (2.57 mmol) of 3-
bromopyridine, 52 mg (64 mmol) of PdCl2(dppf), and 890 mg (6.44
mmol) of K2CO3 were placed in a round-bottom flask and then kept
under a nitrogen flow followed by addition of freshly distilled 1,4-
dioxane (15 mL) to that flask. The resultant reaction mixture was
stirred at 110 °C for 36 h. Then it was cooled and passed through
Celite bed to destroy any catalyst if left. Then the collected filtrate was
evaporated in a rotary evaporator to obtain a crude mass. A yellow
solid product was obtained as the final product after column
chromatography of the crude mass using ethyl acetate as eluent.
Yield: 240 mg (65%). Mp: 216 °C. ATR-IR (cm−1): 3034, 1557,
1
1473, 1417, 1327, 1196, 1031, 705. H NMR (500 MHz, CDCl3): δ
(ppm) 9.19 (d, 2H), 8.73 (dd, 2H), 7.52 (dd, 2H), 8.40 (dt, 2H),
7.84 (s, 2H).
Synthesis of 3,6-Bis(3-pyridyl)-1,2-phenylendiamine (6). A 1.00 g
portion (3.4 mmol) of 4,7-bis(3-pyridyl)-2,1,3-benzothiadiazole (5),
82 mg (0.34 mmol) of CoCl2·6H2O, and 657 mg (18.89 mmol) of
NaBH4 were added to an ethanol/THF (35/15) solvent mixture. The
resultant black suspension was heated to reflux at 80 °C for 2 h. Then
it was cooled to room temperature and passed through a Celite bed.
C
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CDCl3): δ (ppm) 9.16 (br, 2H), 8.77 (br, 2H), 8.21 (br, 6H), 8.01
backbone. These were designed in that way to understand the
effect of flexibility on coordination properties with the same
metal ion during the formation of metallopolymers by using
both ligands. RNL was synthesized with one unit of rigid 2,2′-
pyridil with our synthesized ligand precursor (6) bearing two
primary amines side by side. RNL is a very stiff ligand in nature
without much flexibility, as a condensation reaction for final
ligand formation did proceed through aromatic ring closure to
generate a new fused pyrazine ring. FNL was synthesized by
the reaction of the same ligand precursor (6) with two units of
pyridine-2-carboxaldehyde, and final ligand was formed by
formation of two aliphatic −CN bonds. As no ring closure
happened in this case, the newly formed imine bonds have
more rotational flexibility in FNL in comparison to that of
RNL. Two metallopolymers of iron were synthesized with the
two different ligands to understand the effect of the structural
rigidity of the organic moiety on the coordination behavior and
thus the generation of different properties in the two Fe(II)-
based metallopolymers. All of the characterization studies
related to both the ligands and corresponding metallopolymers
are discussed below.
(br, 2H), 7.87 (br 1H), 7.54 (br 2H), 7.25 (br, 2H). The molecular
weight as obtained by SLS was (3.32
Berry plot see Figure S-6 in the Supporting Information). Degree of
0.30) × 104 g/mol (for a
polymerization (molecular weight/repeating unit weight): n ≈ 54.
Scheme 2. Schematic Representation of the Possible Fe(II)-
RNL Poly Structure 9
Synthesis of Fe(II)-FNL poly (10). A 50 mg portion of FNL was
dissolved in 350 mL of acetic acid, and then 19.76 mg of Fe(OAc)2
was added under nitrogen flow and the reaction mixture was refluxed
under nitrogen for 72 h. The course of the reaction was monitored by
TLC, and when all the reactants were consumed, the reaction mixture
was cooled to room temperature and filtered with a cotton plug to
remove insolubles. The filtrate was placed in a Petri dish, and the
solvent was allowed to evaporate slowly to dryness. The dark brown
brittle film of the polymer was collected, and the product was dried
under vacuum overnight to obtain the purified polymer. Yield: 65.55
mg (95%), Tm = 235 °C (by DSC; see Figure S-7a in the Supporting
Information). ATR-IR (cm−1): 2980, 2926, 1585, 1566, 1411, 1139,
ATR-IR Studies. Overlaid ATR-IR spectra of both the
ligands and their corresponding polymers are collected in
Figure 1. The ATR-IR spectrum of RNL showed the
1
1075, 1000, 915, 880, 852, 795, 745, 702, 656, 551. H NMR (500
MHz, CDCl3): δ (ppm) 11.06 (br, 1H), 9.43 (br, 1H), 8.96 (br, 1H),
8.62 (br, 5H), 8.04 (br, 1H), 7.89 (br, 1H), 7.47 (br, 5H). The
molecular weight as obtained by SLS was (3.6 0.30) × 104 g/mol
(for a Berry plot see Figure S-8 in the Supporting Information).47
Degree of polymerization (molecular weight/repeating unit weight): n
≈ 59.
Figure 1. ATR-IR spectra of (a) RNL with Fe(II)−RNL poly and (b)
FNL with Fe(II)−FNL poly.
Scheme 3. Schematic Representation of the Possible Fe(II)-
FNL Poly Structure 10
characteristic bands at 3044 cm−1 for aromatic −C−H
stretching and 1585 and 1578 cm−1 bands for −CN
stretching of the pyridine ring and pyrazine ring, respectively,
while bands at 1475, 1431, and 1408 cm−1 were observed for
aromatic −CC stretching and 1345 cm−1 for −C−N
stretching. In the case of ATR-IR of FNL, the effect of the
presence of a more flexible backbone was also reflected in ATR
analysis of this ligand. The fingerprint regions of both of the
ligands are totally different, as they carry different backbone
structures. In the case of both polymers peak broadening was
observed in ATR-IR, which indicated the formation of long-
chain polymers. As both of the polymers were formed by the
coordination of ligand donating sites, viz. N,N,N, to Fe(II)
center metal ions continuously, the rotational flexibility of FNL
is lifted after coordination and this is also visible in the ATR
spectrum. Both polymers have the almost same type of ATR-
IR spectra with slight shifts in stretching vibrations. For Fe(II)-
RNL poly, the −CN stretching vibration was recorded at
1583 and 1569 cm−1 for the pyrazine moiety, similar shifts
were observed for −CC stretching at 1432 and 1405 cm−1,
and the −C−N stretching was also shifted to 1378 cm−1. For
Fe(II)-FNL poly, CN stretching (pyridine) was observed at
1583 cm−1, while 1566 cm−1 was observed for aliphatic −C
N, −CC stretchings were observed at 1444 and 1405 cm−1,
RESULTS AND DISCUSSION
■
The main objective of this research was to explore the effect of
the structural rigidity of the organic moiety on the
coordination properties of conjugated metallopolymers. We
have designed two different ditopic organic ligands bearing the
same coordination sites, viz. N,N,N, on each coordination half.
One ligand has a very rigid backbone and has the least
rotational degree of freedom as it carries a fused aromatic ring
in its backbone, while the other has more flexibility in its
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1
Figure 2. Overlaid H NMR spectra of RNL with Fe(II)-RNL poly in CDCl3. The inset shows the freestanding film of Fe(II)-RNL poly.
1
Figure 3. Overlaid H NMR spectra of FNL with Fe(II)-FNL poly in CDCl3. The inset shows the freestanding film of Fe(II)-FNL poly.
respectively, and the −C−N stretching was shifted to 1377
cm−1.
particular proton is not constant for a polymer and small
shifting of peaks from the original positions is observed on the
basis of its spatial environment. As a polymer is formed by
repetition of many monomer units where the spatial
orientation is not constant throughout the polymer chains,
this causes broadening of NMR peaks because of variable
magnetic equivalences and relaxation times of the same
functional group protons present in different repeating
units.50,51 In small coordination complexes of organic moieties
with metal ions, sharp NMR peaks have been observed as a
result of identical magnetic environments for one particular
proton present in two different ligands attached to metal
ions.48 Again, merging of several peaks was observed in the
NMR spectrum of Fe(II)-RNL poly between 8.4 and 8.1 ppm
and between 7.8 and 7.25 ppm due to slight shifting of ligand
peaks upon coordination with the metal center.
In the case of NMR spectrum of FNL, a greater number of
signals was observed, which clearly indicates lifting of
symmetry in the ligand backbone. In Figure 4, energy-
minimized structures of both the ligands are collected; energy
minimization was done by the MOPAC program. In the case
of FNL, it was evident from the energy-minimized structure
(see Figure 4b) that the rotation of the two upper pyridine
NMR Studies. To understand the structure−property
relationship, NMR characterizations of both the ligands and
the respective Fe(II) polymers were carried out, and overlaid
NMR spectra are collected in Figures 2 and 3. RNL is
symmetric in structure, and it showed proton signals for only
1
half of the ligand. H NMR of RNL showed two singlets at
9.12 and 8.02 ppm due to the C-1 and C-6 protons,
respectively. Three distinguished triplets were also observed
at 7.23, 7.51, and 7.87 ppm for the C-8, C-9, and C-4 protons,
respectively. The mass spectrum (see Figure S-2 in the
Supporting Information) of RNL also validated the structure of
the ligand proposed from NMR and ATR-IR findings. The
corresponding Fe(II)-RNL poly showed broadening of all
peaks, as the synthesized polymers are not pure molecules but
a collection of molecules with the same repeating units but
different chain lengths. In the case of synthetic polymers, the
spatial distribution of the same functional group protons of
different monomer units is not constant throughout the chain,
as the ligand is not attached to a metal ion every time during a
self-assembly reaction with an identical orientation. As a result
of that, magnetic equivalence and the relaxation time of a
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Figure 4. Energy-minimized structures of (a) RNL and (b) FNL by
MOPAC, using ball and stick models.
Figure 5. (a, b) FESEM images of Fe(II)-RNL poly on a glass
substrate. (c) An agglomerated polymer strand of Fe(II)-RNL poly
obtained from AFM. (d, e) FESEM images of Fe(II)-FNL poly on a
glass substrate. (f) Polymer nanofibers of Fe(II)-FNL poly obtained
from AFM, while Fe(II)-FNL poly showed flexible nanofiber type
orientation on either a glass or HOPG substrate.
rings A and A′ is difficult due to steric hindrance between the
imine hydrogen (red circle) with hydrogen on C-2 of the
corresponding pyridine rings and this actually slowed down the
free rotation (C−C) of the upper two pyridine rings with
respect to the NMR time scale; the final result is that both of
the pyridine rings give signals in NMR spectrum by losing their
symmetry. The lower two pyridine rings (B and B′) attached
to imine group do not experience any steric hindrance during
their C−C bond free rotation, and as a result they will have
their symmetry intact while the upper two pyridine ring (A and
A′) lose it because of very slow rotation (high energy barrier)
around the −C−C bond. FNL showed three singlets at 10.92,
9.42, and 9.03 ppm due to C-17, C-1, and C-12 protons. Five
doublets were observed at 8.60, 8.56, 8.51, 8.03, 8.03, and 7.62
ppm due to C-3, C-5, C-16, C-8, and C-10 protons,
respectively. Two triplets were visible at 7.87 and 7.38 ppm
due to C-4 and C-9, respectively, while C-13, C-14, and C-15
peaks were merged together to form a multiplet. In Fe(II)-
FNL poly, all of the peaks similar to those of ligand were
visible but peaks became broader as a consequence of
polymerization. The mass spectrum of FNL (see Figure S-4
in the Supporting Information) is also in complete agreement
with the proposed structure, as a molecular peak is found with
mass 439.1795. which is in correspondence with fragmentation
of one imine proton (M − H)+.
poly showed a flexible nanofiber type orientation on either a
glass or HOPG substrate.
XRD Studies. To observe the crystalline phase of ligands
and polymers, the patterns of XRD were obtained by using an
X-ray diffractometer. The overlaid XRD images of ligands and
corresponding polymers are collected in Figure 6. In the case
Figure 6. XRD graphs of (a) RNL and Fe(II)-RNL poly and (b) FNL
and Fe(II)-FNL poly.
Surface Morphology. The surface morphology of thin
films of both polymers was studied with the help of FESEM
and AFM techniques. For the FESEM analysis of a polymer
sample, it was dissolved in methanol and sonicated for 30 min.
A thin film of the polymer solution (10−5 M) was developed by
drop-casting this solution on a glass substrate (1 cm2) and then
drying properly under vacuum overnight. The micrographs of
Fe(II)-RNL poly and Fe(II)-FNL poly are collected in Figure
5. Both polymers show a long fibrous network of nanofibers
which are formed by the agglomeration of many individual
polymer strands. Empirical calculations with help of the MM2
energy minimization method of the octamer of similar
polymers of Fe(II)-RNL poly and Fe(II)-FNL poly were
carried out, and the thicknesses of individual polymer chains
are found to be 1.14 and 1.40 nm, respectively. The polymeric
structure of Fe(II)-RNL poly and Fe(II)-FNL poly were also
confirmed with the help of AFM. Both of the polymers were
dissolved in methanol, and the sample solutions were poured
on freshly cleaved highly oriented pyrolytic graphite (HOPG)
surface to form a thin film. The image obtained from the AFM
analysis of Fe(II)-RNL poly showed aggregated polymer
chains with 45 and 217 nm thickness in Figure 5c, while the
analysis of Fe(II)-FNL poly showed polymer fibers in the
range of 35−70 nm thickness in Figure 5f. A greater rigidity
and rodlike orientation of agglomerated polymer strands were
observed in the case of Fe(II)-RNL poly, while Fe(II)-FNL
of XRD images of both ligands, sharp peaks were observed,
which indicate the crystalline nature of the ligands, while in the
case of Fe(II)-RNL poly the sharp peaks were reduced in both
number and intensity, which indicates that the crystallinity is
reduced drastically on polymer formation. Fe(II)-FNL poly
showed very broad peaks, which showed its amorphous
nature.52 Broadness in peaks was expected, as synthesized
polymers can never be 100% crystalline because of huge
entanglements originating from long chain lengths restricting
that particular portion of chain(s) in participating in a compact
crystalline arrangement and amorphous regions are formed by
those entangled chains.34,35 Thus, it is confirmed from XRD
analysis that semicrystalline polymers are formed on complex-
ation of the metal salt with the respective organic ligands.
Study of Photophysical Properties. UV−vis spectra of
both the ligands and corresponding polymers are collected in
Figure 7. The UV spectrum of RNL showed three major bands
in the UV region. The first band was observed at 342 nm
(near-UV), while the second band came at 281 nm (mid-UV)
and the third band was at 258 nm (far-UV). The first two
bands were observed because of π−π* transitions of aromatic
rings, and the third band corresponds to an n−π* transition of
the imine moiety. Similarly, FNL also showed three major
absorption bands in the near-, mid-, and far-UV regions at
wavelengths of 320, 272, and 232 nm, respectively, because of
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Effect of Different Acids on UV−Vis Spectra. The effect
of addition of different acids in equivalent amounts to both
polymer solutions was monitored through UV−vis experi-
ments. Very dilute (10−5 M) solutions of both polymers were
prepared in methanol. Then 2 mL each of Fe(II)-RNL poly
and Fe(II)-FNL poly solution were placed in four different
cuvettes. A 2 μL portion of different acids (namely
CH3COOH, HNO3, H2SO4, and HClO4) was added to each
solution, and the corresponding changes in UV spectra were
recorded. Figure 8 shows the effect of addition of different
Figure 7. Absorption spectra of 10−5 M solutions of (a) RNL with
Fe(II)-RNL poly and (b) FNL with Fe(II)-FNL poly in methanol.
similar modes of transitions but a blue shift was observed
because of more flexibility incurred in the FNL structure in
comparison to the RNL structure. After polymerization of both
RNL and FNL with same Fe(II) metal salt, respectively, slight
changes in UV spectra were observed. In the case of Fe(II)-
RNL poly, peaks at 258 and 281 nm disappeared. This
happened because of the coordination of ligand electrons to
the center metal ion to form a metallopolymer. Similarly, in the
case of Fe(II)-FNL poly the absorption band at 272 nm
disappeared with a small blue shift in UV spectra. UV peaks
appeared at wavelengths of 320, 272, and 232 nm, respectively,
because of similar modes of transitions, but a blue shift was
observed because of more flexibility incurred in the FNL
structure in comparison to the RNL structure.
Thermal Analysis. Thermal analysis of both polymers was
carried out by DSC and TGA techniques to get an idea of the
thermal behavior and stability of both polymers. Thermograms
are collected in SI Figures S-5 and S-7 in the Supporting
Information for Fe(II)-RNL poly and Fe(II)-FNL poly,
respectively. The glass transition (Tg) temperature for
Fe(II)-RNL poly was observed at 115 °C, while that of
Fe(II)-FNL was at 110 °C. As the polymers are semicrystalline
in nature, Tc(crystalline temperature) values are observed for
both of them at 200 and 190 °C, respectively. The Tm
(crystalline melting point) value was observed at 255 °C for
Fe(II)-RNL poly (see Figure S-5b in the Supporting
Information) while the other polymer showed this value at
235 °C (see Figure S-7a in the Supporting Information). As
Fe(II)-RNL poly has a fused aromatic ring on its backbone, its
Tg and Tm values are higher than those of the other polymer
because of the stiffness of its backbone.34,35
In a TGA analysis of Fe(II)-RNL poly (see Figure S-5a in
the Supporting Information) stepwise thermal degradation was
observed, as it showed an initial 20% mass loss when the
temperature reached 300 °C. Then a very steep degradation
curve was observed within the temperature range of 335−400
°C with a mass loss of 75%. A third step of degradation was
observed between 410 and 490 °C, which left metal oxide
residues on the aluminum pan.
In case of Fe(II)-FNL poly a two-step degradation curve was
observed in the thermogram (see Figure S-7b in the
Supporting Information), where an initial 20% mass loss
occurred within 300 °C. A steep second step of degradation
was observed in the temperature range 400−560 °C with a
total mass loss of 90%, leaving metal oxide residues. Fe(II)-
RNL poly contains a quinoxalene moiety, which has better
thermal stability in comparison to that of pyridine and benzene
rings, and because of this Fe(II)-RNL poly has shown a three-
step degradation while Fe(II)-FNL poly made with benzene
and pyridine organic moieties showed a two-step degradation.
Figure 8. UV−vis spectra of (a) Fe(II)-RNL poly with 2 μL of the
acids CH3COOH, HNO3, H2SO4, and HClO4 and (b) Fe(II)-FNL
poly with 2 μL of the acids CH3COOH, HNO3, H2SO4, and HClO4.
acids on two different dilute polymer solutions. Both polymers
have counteranions closely associated with each Fe(II) metal
center to satisfy primary valency, as both ligands are neutral in
nature and satisfy the secondary valency of metal ions. The
presence of a counteranion is again confirmed with a solution
state conductivity study of both polymers in the solution state
with varying concentrations (see Table S-1 in the Supporting
Information). The conductivity of the both polymers in the
solution state decreased with decreasing concentration, which
indicates the presence of a counteranion and polymeric cation.
If the acetate anions are present in the coordination sphere,
then they will satisfy both primary and secondary valencies of
metal ions and the polymers will be neutral in nature and their
conductivity will be unaffected by variation of the concen-
tration.
The effect of addition of very dilute solutions of acid may
change the counterion from acetate to the corresponding acid
anion. In case of Fe(II)-RNL poly, addition of 2 μL of
CH3COOH made a pronounced difference. A new band is
generated which is confirmed as pure acetic acid band in
comparison with a blank acetic acid solution of the same
concentration in methanol. A hyperchromic shift was observed
with no change in position of absorption bands with respect to
blank Fe(II)-RNL poly. When 2 μL of HNO3 was added to a
second batch of same polymer solution, a huge hyperchromic
shift was observed. When the same amount of H2SO4 was
added to the third batch, the intensity of the absorption
spectrum was almost the same but a small bathochromic shift
was visible at 281 nm, while an equal amount of HClO4 caused
a hypsochromic shift to a slight extent of the fourth batch of
Fe(II)-RNL poly. When similar experiments were repeated
with the same concentration for Fe(II)-FNL poly, remarkable
changes were observed in the UV spectra. On addition of 2 μL
of CH3COOH to the first batch of Fe(II)-FNL poly solution,
the same new band appeared as in the case of Fe(II)-RNL
poly, a band at 234 nm showed a hypsochromic shift, and a
hyperchromic shift appeared at 208 nm. On addition of 2 μL of
HNO3 to a second batch of the polymer solution, the UV
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spectrum showed a slight bathochromic shift and maximum
hyperchromic shift at 233 nm with generation of a new
absorption band at 273 nm. In case of addition of H2SO4, a
bathochromic shift was observed for the absorption band at
234 nm (blank polymer solution), which was shifted to 244
nm and an absorption band at 320 nm in the polymer solution
shifted to 347 nm with no change in intensity of absorption
spectra. On addition of 2 μL of HClO4 to the last batch of the
same polymer solution, a new band at 330 nm was observed
with a slight hypsochromic shift. This study showed that effect
of addition of a 2 μL aliquot of different acids in Fe(II)-RNL
poly solution is not very pronounced, as the spectra remain
unaltered with only a small hyperchromic shift but Fe(II)-FNL
poly showed pronounced changes in spectral behavior on
addition of 2 μL of HNO3 and CH3COOH. Addition of
different acids to both polymer solutions probably induces
replacement of parent acetate counteranions associated with
each metal center by the corresponding acid anions, and as a
result spectral changes were observed on addition of acid.
Fe(II)-FNL poly shows greater spectral changes in comparison
to those of Fe(II)-RNL poly, as the flexibility of the backbone
of FNL allows it to change its shape to fit larger/smaller size
counterions in the coordination cavity accordingly when it is
itself coordinated with two different metal centers.
corresponding emission peaks were found at 409 nm with a
decrease in intensity of Fe(II)-FNL poly with respect to FNL.
For both polymers, the decrease in intensity of the emission
peaks was observed as coordination complexes sometime favor
relaxation through other channels over the emission of photons
that come from the lowest singlet excited state.
Apart from the quenching in emission intensity, there is not
much change in the emission properties of metallopolymers in
comparason to the corresponding ligands. Time-resolved
fluorescence decay analysis of all of them sheds more light
on the nature of excited states. Time-resolved decays of both
ligands and polymers are collected in Figure 11 and Table 1.
The time-resolved fluorescence decay profile for both ligands
as well as their metallopolymers are best fitted by a double-
exponential fitting parameter with the least χ2 (statistic) value.
The double-exponential fluorescence decay suggests the
existence of two different species in the excited state even
for the ligand alone. These two species may be ascribed to the
normal and charge transfer species of the molecule, which may
be substantiated by DFT- and TD-DFT-based quantum
chemical calculations of both ligands. During aformentioned
quantum chemical calculations, both ligands show significant
charge transfer in their excited state, as is evident from their
HOMO−LUMO diagrams (see Figure S-9 in the Supporting
Information). The fluorescence lifetimes of both species almost
remain unaltered along with the respective contributions in
forming metallopolymers (especially in the case of FNL).
The effects of addition of different acids in the same amount
to four different batches of polymer solutions with the same
concentrations, as was done in the case UV−vis spectroscopy
were observed with a spectrofluorimeter, and the correspond-
ing changes in the decay of excited species was observed
through time-resolved fluorescence decay analyses. Corre-
sponding figures for both polymers are collected in Figures 10
and 11, and all data are collected in Table 1. In Figure 10a,
changes in fluorescence spectra were visible due to addition of
the same amounts of different acids to the Fe(II)-RNL poly
solution. Addition of acetic acid resulted in a minor increase in
fluorescence intensity, probably due to reduction of non-
radiative decay processes, and in this case the populations of
both excited states are comparable. Addition of H2SO4 brought
bifurcation in the emission spectrum. In this case also
population differences in both excited species was observed
as seen previously. The population difference of excited states
is probably induced by replacement of the counteranion by the
corresponding acid anions.
Fluorescence Study. The emission spectra of both ligands
(RNL and FNL) and their corresponding polymers (Fe(II)-
RNL poly and Fe(II)-FNL poly) are shown in Figure 9. Both
Figure 9. Fluorescence spectra of (a) RNL and Fe(II)-RNL poly at
λex 340 nm and (b) FNL and Fe(II)-FNL poly at λex 320 nm.
RNL and Fe(II)-RNL poly solutions (10−5 M) in methanol
were excited at a 340 nm wavelength, and the emission peaks
were found at a 435 nm wavelength. The fluorescence intensity
of Fe(II)-RNL poly was quenched with respect to that of the
ligand. Similarly FNL and Fe(II)-FNL poly solutions (10−5 M)
in methanol were excited at a 320 nm wavelength and
Table 1. Fluorescence Lifetimes of Fe(II)-RNL and Fe(II)-FNL Poly with Different Acids in Methanol
sample
λem (nm)
χ2
τ1 (ns)
α1 (%)
τ2 (ns)
α2 (%)
av lifetime (ns)
RNL
Fe(II)-RNL poly
435
435
462
435
436
434
409
409
470
474
453
431
1.00
1.02
1.22
1.03
1.02
0.97
0.98
0.97
1.24
1.31
1.13
1.06
0.79
1.26
1.12
1.12
1.05
0.31
0.93
0.48
2.06
1.98
1.82
1.24
73.39
74.87
92.02
94.48
98.49
51.83
55.53
53.94
28.18
45.66
79.51
75.71
1.99
3.46
4.07
2.29
3.44
1.45
2.02
1.88
7.06
6.53
5.84
2.65
26.60
25.13
7.98
5.52
1.5
48.16
44.57
46.06
71.82
54.34
20.49
24.29
1.10
1.18
1.35
1.18
1.08
0.85
1.41
1.12
5.65
4.50
2.88
2.15
Fe(II)-RNL poly + H2SO4
Fe(II)-RNL poly + HNO3
Fe(II)-RNL poly + HClO4
Fe(II)-RNL poly + CH3COOH
FNL
Fe(II)-FNL poly
Fe(II)-FNL poly + H2SO4
Fe(II)-FNL poly + HClO4
Fe(II)-FNL poly + HNO3
Fe(II)-FNL poly + CH3COOH
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fluorescence sensing data. The solid state spectral changes of
Fe(II)-FNL poly are collected in Figure S-10 in the Supporting
Information. To understand the reversibility and stability of
the polymer Fe(II)-FNL, a thin film of this polymer had casted
on a glass slide was continuously exposed to a saturated vapor
(5 min) of glacial acetic acid and then the polymer-coated glass
slide was kept in an open atmosphere under a hood (5 min) to
evaporate the adsorbed acid molecules from the polymer
surface. This experiment with repeated acid exposures/
removals was continued for 2 h, and after each acid exposure
and acid removal, the corresponding OD values at 335 nm
were noted. A plot of OD at 335 nm vs time is collected in
Figure S-10b in the Supporting Information, which shows
unchanged OD values for the polymer after each exposure
followed by removal; this is a confirmation that the polymer
molecule can keep its coordination environment intact in the
presence and removal of acid over repeated times. We have
also checked the changes in absorbance when the polymer film
was exposed for a long time to acid vapors, and the absorbance
value was noted every 12 h. The plot of absorbance vs time is
shown in Figure S-10c in the Supporting Information, which
shows the stability of polymers in the presence of acid vapor
for a long duration (red line) plotted against a reference plot of
a virgin polymer film kept in an open environment for the
same duration. Both the stability plot (Figure S-10c in the
Supporting Information) and reversibility plot (Figure S-10b in
the Supporting Information) show that the Fe(II)-FNL
polymer can withstand a long exposure of acid vapors without
a change in coordination environment along with reversible
exposure (exposure followed by complete removal).
Figure 10. Fluorescence spectra of (a) Fe(II)-RNL poly alone and
with 2 μL of the acids CH3COOH (c-2), HNO3 (c-3), H2SO4 (c-4),
and HClO4 (c-5) and (b) Fe(II)-FNL poly alone and with 2 μL of the
acids CH3COOH (d-2), HNO3 (d-3), H2SO4 (d-4), HClO4(d-5) (c)
Emission colors of fluorescent Fe(II)-RNL poly alone and with the
acids CH3COOH (2), HNO3 (3), H2SO4 (4), HClO4 (5) acids,
respectively. (d) Emission colors of Fe(II)-FNL poly alone and with
the acids CH3COOH (2), HNO3 (3), H2SO4 (4), and HClO4 (5),
respectively.
Electrochemical Properties. The electrochemical proper-
ties of the ligands RNL and FNL and their corresponding
polymers Fe(II)-RNL poly and Fe(II)-FNL poly were studied
by cyclic voltammetry (CV) experiments with a glassy-carbon
working electrode, Pt wire as the counter electrode, and an Ag/
AgCl reference electrode in dichloromethane solution
containing 0.1 M TBAP with a scan rate of 100 mV s−1.
The voltammograms for the polymers are shown in Figure 12,
and those of the ligands are collected in Figures S-11 and S-12
in the Supporting Information.
Figure 11. Time-resolved fluorescence decay of (a) Fe(II)-RNL poly
with different acids and (b) Fe(II)-FNL poly with different acids.
The effect of addition of different acids to Fe(II)-FNL poly
was observed by the same process, and huge changes in
fluorescence intensity and lifetime were observed with addition
of 2 μL of each of the acides H2SO4 and HClO4. The addition
of acids in this case also resulted in a change in counteranion
from the acetate to the corresponding acid anion, and as FNL
has a flexible backbone, the coordination cavity around each
central metal ion can be adjusted accordingly to accommodate
a new counteranion. Sulfate is dianionic in nature, and one
sulfate anion can replace two acetate anions at a time for
charge balancing of one Fe(II) center. Thus, addition of
sulfuric acid is actually inducing huge changes at the
coordination site of the Fe(II) center and it would probably
stabilize both excited species; as a result, their lifetimes were
increased by 4 times in comparison to original polymer. This
enhanced stabilization of excited states of a metallopolymer is
reflected in increased emission intensity as a greater number of
lowest singlet excited states are relaxed through emission of
photons rather than a nonradiative thermal decay process.
Perchloric acid addition induces a slightly higher population of
the second excited species but the emission intensity is lower
than that of sulfuric acid. When an equal amount of nitric acid
was added to Fe(II)-FNL poly, then the emission intensity was
increased in comparison to that of the metallopolymer but this
increment in emission intensity was of a lesser extent in
comparison to the previous two acids. Hence the time-resolved
fluorescence decay analysis is in line with steady state
Figure 12. Cyclic voltammograms of (a) Fe(II)-RNL poly and (b)
Fe(II)-RNL-poly in dichloromethane containing 0.1 M TBAP at 100
mV/s with a glassy-carbon working electrode.
RNL showed two cathodic responses at −0.42 and −1.40 V,
respectively, due to the one-electron reduction of pyridine and
two-electron reduction of the quinoxalene moiety. In the case
of FNL, three reductions at −0.35, −0.80, and −1.30 V,
respectively, were observed with a slight left-side shift (lower
potential), which indicates easy reduction of FNL in
comparison to RNL. The effect of flexibility is also observed
in a voltammogram of this ligand. When the first imine group
was reduced at −0.80 V, then that message was conveyed
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through the conjugated backbone to a second imine function
present in the same FNL moiety and the second imine group
was reduced at even higher potential. Both Fe(II)-RNL poly
and Fe(II)-FNL poly exhibited three reversible cathodic
responses in cyclic voltammograms. Three cathodic responses
for the Fe(II)-RNL poly came at −0.75, −1.50, and −0.95 V,
while the other iron polymer showed the same at −0.95,
−1.30, and −0.71 V. As the functional groups responsible for
electrochemical changes in both polymers are almost the same,
the types of responses are similar, though there is a shift in
cathodic peak position because of an inherent difference in
backbone rigidity of the polymers. The first cathodic reduction
in both polymers is attributed to the pyridine moiety
electrochemical response, while the next two are due to
stepwise reduction of the quinaxolene moiety in Fe(II)-RNL
poly and two aliphatic imine groups in Fe(II)-FNL poly.
Shifting of all the cathodic responses in both polymers in
comparison to both ligands are expected, as the metal ion is
present in the +2 oxidation state and the presence of an
electron-withdrawing center is known to increase reduction
potential;53 this also give hints of strong dπ−pπ back-donation
from the metal center to the ligand.46
Spectroelectrochemical Characterization. A reversible
change in the optical properties of the polymeric solutions with
the applied voltage has been observed by spectroelectrochem-
istry experiments. For this experiment, dilute solutions of
polymers were prepared in dichloromethane solvent and 0.1 M
TBAP was used as an electrolyte using a three-electrode cell
assembly with Pt mesh as the working electrode, Pt wire as the
counter electrode, and Ag wire as the reference electrode.
For Fe(II)-RNL poly and Fe(II)-FNL poly, solution state
spectroelectrochemical experiments were performed in di-
chloromethane. For Fe(II)-RNL poly the reduction potential
−1.50 V was divided into 10 segments of 0.15 V potential and
each potential was applied for 60 s. The change in color of the
solution was observed from royal blue to greenish white
fluorescence in the UV region. As the applied potential
increased, the polymeric solution started to become reduced.
The change was also reflected in UV spectra with a gradual
increase in intensity of π−π* transitions at 342 and 281 nm
and the n−π* transition at 258 nm up to −1.50 V, as is shown
in Figure13a. For Fe(II)-FNL poly, the potential was applied
in a similar manner and the solution emission color changes
from royal blue to cyan (see Figure S-13 in the Supporting
Information). A change in the UV spectrum was also observed
at 320 and 272 nm for π−π* transitions and at 232 nm for the
n−π* transition with a gradual decrease in the intensity of
spectra up to −1.35 V, as is shown in Figure13b.
CONCLUSION
■
In this study, we have successfully synthesized two novel,
multidentate, NNN donor ligands with a difference in
flexibility of their backbones. The aim of this research was to
understand how the effect of backbone rigidity of an organic
moiety is conveyed to metallopolymers by observed variations
in different properties when both ligands have identical
coordination sites for complexation. Two corresponding Fe(II)
metallopolymers were synthesized and characterized thor-
oughly to understand their structure−property relationship. All
of the characterizations indicate a stronger coordination
complex formed by a flexible ligand in comparison to that of
a rigid ligand, and both polymers showed very good
fluorescence properties. All of the properties generated in
these polymers due to the coordination of two organic ligands
have variations, and this is a useful indication for achieving
property tuning in metallopolymers by introduction of
rotational rigidity on the ligand backbone, with the
coordination sites kept identical. Fe(II)-FNL poly showed a
good variation in fluorescence spectra upon addition of
different acids, and it shows the potential to be applied as a
broad-range luminescent acid sensor, as it changed its emission
nature along with the lifetime of excited species on addition of
different acids. This is an important finding, as most of the
fluorescent sensors are good for only one or two chemicals,
and there are also some reports about the realization of broad
pH range sensors by MOFs, but report on multiacid sensors
are scarce in the literature. All of the materials are redox-active,
and both polymers showed electroluminescence when an
appropriate potential is applied to the corresponding polymer
solution. In the future, we want to extend this study to other
metal ions also to check property variations in them.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b02985.
Molecular weight determination procedure of Fe(II)-
RNL poly and Fe(II)-FNL poly by SLS, 13C NMR and
CV of RNL and FNL ligands, and DSC and TGA data of
Fe(II)-RNL poly and Fe(II)-FNL poly polymers (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Anasuya Bandyopadhyay − IIT Roorkee, Saharanpur,
India; orcid.org/0000-0003-0054-0341;
Email: abptsfpt@iitr.ac.in
Other Authors
Anil Kumar − IIT Roorkee, Saharanpur, India
Shubham Bawa − IIT Roorkee, Saharanpur, India;
orcid.org/0000-0001-5394-9971
Kapil Ganorkar − Visvesvaraya National Institute of
Technology, Nagpur, India
Sujit Kumar Ghosh − Visvesvaraya National Institute of
Technology, Nagpur, India; orcid.org/0000-0002-
6336-9441
Figure 13. Spectroelectrochemistry of (a) Fe(II)-RNL poly (inset
shows the color change at 254 nm from 0 V to −1.5 V) and (b)
Fe(II)-RNL poly (inset shows the color change at 254 nm from 0 V
to −1.35 V).
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.9b02985
J
https://dx.doi.org/10.1021/acs.inorgchem.9b02985
Inorg. Chem. XXXX, XXX, XXX−XXX

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Article
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support received from the Science and Engineering
Research Board, DST New Delhi (under the grant EMR/
2016/001994) is acknowledged. A.K. is grateful to the MHRD,
New Delhi, for his fellowship. S.B. is thankful to the Science
and Engineering Research Board, DST New Delhi (under the
grant EMR/2016/001994), for his fellowship.
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