Sterically encumbered bipyridyl-derivatized conjugated polymers and metallopolymers incorporating phenylenevinylene, phenyleneethynylene, and fluorenylene segments

Article abstract of DOI:10.1021/ma2025144

This work reports the preparation of a series of 2,2′-bipyridyl (bipy) modified π-conjugated polymers having an average of one or three monomer units (p-arylene ethynylene for PPE1 and PPE3 or 7,7-dihexylfluorene for PF1 and PF3) between metal-binding sites. Spectroscopic data demonstrate that strategic placement of sterically encumbered mesityl groups about the metal binding sites enforces a 1:1 metal to bipy binding ratio. This steric coordination control ensures that the metalated polymers remain solution processable rather than forming insoluble networks via coordinative cross-linking. The solution photophysical and electrochemical properties of metal-free and metalated materials are reported and compared with those of related conjugated polymers and conducting metallopolymers.

Full text of DOI:10.1021/ma2025144

Article
pubs.acs.org/Macromolecules
Sterically Encumbered Bipyridyl-Derivatized Conjugated Polymers
and Metallopolymers Incorporating Phenylenevinylene,
Phenyleneethynylene, and Fluorenylene Segments
Susan He, Ashley A. Buelt, Jessica M. Hanley, Brad P. Morgan, Andrew G. Tennyson,*
and Rhett C. Smith*
Department of Chemistry and Center for Optical Materials Science and Engineering Technologies (COMSET), Clemson University,
Clemson, South Carolina 29634, United States
S
* Supporting Information
ABSTRACT: This work reports the preparation of a series of 2,2′-bipyridyl
(bipy) modified π-conjugated polymers having an average of one or three
monomer units (p-arylene ethynylene for PPE1 and PPE3 or 7,7-
dihexylfluorene for PF1 and PF3) between metal-binding sites. Spectroscopic
data demonstrate that strategic placement of sterically encumbered mesityl
groups about the metal binding sites enforces a 1:1 metal to bipy binding ratio.
This steric coordination control ensures that the metalated polymers remain
solution processable rather than forming insoluble networks via coordinative
cross-linking. The solution photophysical and electrochemical properties of
metal-free and metalated materials are reported and compared with those of related conjugated polymers and conducting
metallopolymers.
relatively facile functionalization, stability and ability to bind a
large number of transition and lanthanide elements.8 The
versatility of the bipyridyl scaffold has consequently found
extensive use in materials and supramolecular chemistry.9 An
early study by Wang and Wasielewski10 demonstrated the effect
of an impressive number of metal ions on the photophysical
properties of bipy-modified CPs, including the effect of CP
spacer length between bipy units. Bipy-modified CMPs have
since found application in conjugated polymetallorotaxanes,11
electroluminescent devices,12 organic solar cells,13 and as
chemical sensors.4f,14 When one attempts to prepare CMPs
by metallating a bipy-functionalized CP, multiple bipy units can
bind to some metal ions. The result is coordinative cross-link
formation (Figure 1, left)15 and consequent production of a
highly insoluble network solid. The loss of solubility leads not
only to diminished capacity for processing the materials for
devices, but also to a loss of the ability to carry out solution
characterization.
The coordination chemistry of bipy derivatives can be
controlled by strategic distribution of sterically encumbering
units. The strategic distribution of sterically encumbering units
have found widespread utility in supramolecular and materials
chemistry,16 and has been exploited in the rational design of
elegant bipy-scaffolded supramolecular assemblies.17 We
recently reported that positioning sterically encumbering
groups about a bipy binding site can also provide steric
INTRODUCTION
■
Organic semiconductors such as π-conjugated polymers (CPs,
Chart 1A) have been targeted as active components of
optoelectronic devices due to their processability, tunability
from UV- to IR-range light absorption/emission, and potential
for metal-like conductivity in their doped states. These
spectacular properties have led to the use of CPs in applications
ranging from thin film solar cells and organic light-emitting
diodes (OLEDs)1 to printable circuits and batteries.2
Conducting metallopolymers (CMPs) are CPs that have
metal ions or atoms incorporated into them.3 Including metal
ions can augment the already impressive property profile of
CPs because metals can endow CMPs with a richer range of
bonding geometries, magnetic properties, redox chemistry and
photochemistry than may be possible from organic CPs alone.4
CMPs have proven especially successful is in OLEDs. Heavy
atoms like transition metals can improve the efficiency of
phosphorescence (emission from the triplet excited state) via
spin orbit coupling. Because up to 75% of excited states
generated in electroluminescent devices are triplets, efficient
phosphorescence could lead to up to four times the efficiency
of devices that would otherwise rely on fluorescence for light
emission. The majority of studies on CMPs for OLEDs to date
have focused on iridium5 and platinum6 containing CMPs.
Because of the added redox tunability and added photophysical
nuances endowed by metals, CMPs have also begun to draw
interest for use in solar cells.7
Although a wide range of ligands have been used to bind
metals in CMPs, 2,2′-bipyridyl (bipy) derivatives are among the
most common (Chart 1B). This is presumably because of their
Received: November 15, 2011
Revised: July 10, 2012
Published: July 31, 2012
© 2012 American Chemical Society
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Chart 1. CPs (A) and Bipy-Derivatized CPs (B)
Chart 2. Materials Featuring Sterically Enshrouded 2,2′-
Bipyridyl Units
while remaining fully soluble and thus easier to characterize. In
the current work, we report statistical copolymers of
polyfluorene and poly(p-phenylene ethynylene) (PPE) deriv-
atives incorporating different ratios of sterically enshrouded
bipy monomer units in their backbones. The photophysical and
electrochemical properties of select polymers, metal-free and
metalated, are reported.
EXPERIMENTAL SECTION
■
General Considerations. All air-sensitive manipulations were
carried out under a N2 atmosphere employing either an MBraun
Unilab glovebox or standard Schlenk techniques. Tetrahydrofuran was
made anhydrous/anaerobic by utilizing an MBraun solvent purification
system, in which the solvent was passed through alumina columns
under a N2 atmosphere. NMR spectra were obtained using either a
Bruker Avance 300 or Joel ECX-300, both operating at 300 MHz for
Figure 1. Schematic illustrating how insoluble network formation
(left) can be prevented by the steric coordination control strategy
(right).
coordination control in a π-conjugated small molecule model
compound (TAB, Chart 2)18 and in a bipy modified poly(p-
phenylenevinylene) derivative (PPV1, Chart 2).19 The steri-
cally enshrouded metal coordination sites enforce a 1:1
metal:bipy binding ratio (Figure 1, right), leading to impressive
improvements in the solubility/processability of metalated
polymers versus coordinatively cross-linked polymers that are
known to form upon metalation of nonsterically encumbered
analogues.15 Furthermore, PPV1 bound a variety of transition
elements with attendant changes in photophysical properties
1
proton and 75 MHz for carbon. Carbon-13 and H NMR Chemical
shifts were reported in parts per million (δ ppm). Reagents were
obtained from Aldrich Chemical Co., TCI America, Alfa Aesar, or
Acros and used without further purification.
General Spectroscopic Methods. All solvents were HPLC grade
or better and were degassed using nitrogen. The tetrahydrofuran was
purified by MBraun solvent purification system which was set up under
nitrogen and employed alumina columns. All UV−vis data was
collected using a Varian Cary-50 Bio spectrophotometer and the
fluorescence data was collected using a Varian Eclipse spectropho-
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tometer. Initial solutions for PL analysis were filtered through 0.2 μm
PTFE syringe filters prior to analysis.
128.2, 132.0, 132.8, 134.8, 135.1, 137.5, 138.8, 145.2, 191.8.2768;
HRMS (m/z): calcd for C25H25OI (M +,H)+, 468.0963; found,
468.0951. Mp: 160−161 °C.
General Considerations for Cyclic Voltammetry. Polymer was
dissolved in anhydrous CH2Cl2 containing 0.1 M (TBA)PF6 as
supporting electrolyte and 3-mm-diameter glassy carbon disk was used
as the working electrode, with Pt as counter electrode and Ag as
quasireference electrode. Ferrocene was added at the end of the
experiment as internal reference for the potential. Calculation of
ionization energies (IE) and electron affinities (EA) were carried out
according to the established convention.28
Metal Ion Selectivity Methods. A 3.0 mL aliquot of each
polymer in tetrahydrofuran solution was measured and added to an
optical path length of 1 cm quartz cell. Excess metal ions (10 or more
equiv) were added to the polymer solution. Absorbance changes were
measured. The experiment was repeated and followed by PL
spectroscopy.
Absorption and Photoluminescence Titrations of Polymers
with Metal Ions. A 3.0 mL aliquot of each polymer in
tetrahydrofuran solution was added to a quartz cell having an optical
path length of 1 cm. Aliquots, each containing 0.1 equiv of each metal
ion with respect to bipyridyl ligating units, were added to the polymer
solution and an absorption or photoluminescence spectrum was
collected after each addition.
Synthesis of ITABI. To a mixture of 3 (1.0 g, 2.14 mmol) and 4
(0.487 g, 0.107 mmol) in THF (20 mL), was added dropwise
potassium tert-butoxide (0.35 g, 3.12 mmol) in THF (20 mL). The
resultant dark brown solution was stirred for 40 h. An aliquot of
HCl(aq) (3.6 M, 4 mL) was then added dropwise to quench excess
potassium tert-butoxide. Volatiles were removed under reduced
pressure to yield a sticky yellow residue. Reprecipitation by addition
of a concentrated dichloromethane solution to pentane produced a
1
yellow powder that was ∼90% by H NMR spectroscopy. The final
product was obtained by washing the initial solid sequentially with
methanol and pentane (3 × 10 mL) and drying in vacuo to afford a
yellow powder, 0.75 g (65%). 1H NMR (300 MHz, CDCl3), δ: 2.01 (s,
24H; −CH3), 2.36 (s, 12H; −CH3), 5.81 (d, 2H; −CH2), 6.56 (d, 2H;
−CH2), 6.96 (s, 4H), 7.08 (d, 2H), 7.48 (s, 8H), 8.00 (d, 2H).13C
NMR (75.4 MHz, CDCl3), δ: 20.5, 21.1, 93.5, 120.4, 126.7, 128.2,
128.4, 132.9, 133.3, 133.4, 135.5, 136.8, 137.3, 137.8, 142.7, 147.6,
154.2. HRMS (m/z): calcd for C62H59I2N2 (M + H)+, 1085.2768;
found, 1085.2788. Mp: 226−227 °C.
Synthesis of 1,4-Bis(hexyloxy)-2,5-diiodobenzene. Synthesis
of this compound followed the reported procedure.20 A mixture of 1,4-
bis(hexyloxy)benzene (1.11 g, 4.0 mmol), iodine (0.91 g, 3.6 mmol),
and potassium iodate (0.42 g, 2.4 mmol) was dissolved in sulfuric acid
(30%, 1.2 mL), CCl4 (1.6 mL) and acetic acid (7 mL) and refluxed for
3 h at 75 °C. The reaction was then placed on ice for 30 min to afford
pink crystals. The crystals were washed with methanol (20 mL × 4)
and dried in vacuo for 4 h to yield the final product (1.19 g, 56.1%) 1H
NMR (300 MHz, CDCl3), δ: 0.93 (t, 6H; −CH3), 1.36 (m, 8H;
−CH2), 1.54 (m, 4H; −CH2), 1.82 (m, 4H; −CH2), 3.94 (t, 4H,
OCH2), 7.19 (s, 2H, aromatic H).
Synthesis of (BrTABBr). To a mixture of 1 (0.82 g, 1.95 mmol)
and 4 (0.40 g, 0.884 mmol) in 100 mL of THF was added KOtBu
(0.218 g, 1.95 mmol) in 20 mL of THF dropwise and stirred for 40 h.
Upon reaction completion the solution was poured into saturated
aqueous sodium bicarbonate (400 mL) to wash any excess KOtBu.
The resulting mixture was extracted with dichloromethane and water.
Volatiles were removed by rotary evaporation. The product was
dissolved in dichloromethane/pentane and cooled to −10 °C. The
precipitate was collected by filtration and dried in vacuo to give 0.42 g
1
(47%) of the product as a white solid. H NMR (300 MHz, CDCl3),
Synthesis of 2,7-Diiodo-9,9-dihexylfluorene. Synthesis of this
compound followed the reported procedure.20 Iodine (0.585 g, 2.30
mmol), and potassium iodate (0.330 g, 1.54 mmol) were added to a
mixture of 9,9-dioctyl-9H-fluorene (1.0 g, 2.56 mmol) in sulfuric acid
(30%, 1.2 mL), carbon tetrachloride (1.6 mL) and acetic acid (7.0
mL). The reaction mixture was refluxed at 80 °C for 48 h then allowed
to cool to room temperature for 1 h. Upon cooling two layers were
observed, one light yellow and one dense orange. The yellow layer
(aqueous acid) was decanted away and then the dense orange layer
was added to a 5-fold volume of MeOH (10 mL), leading to formation
of a precipitate. Solvent was decanted away and the crude product was
dissolved in 4 mL of pentane. Slow evaporation of the pentane at −10
°C yielded pale yellow crystals that were dried in vacuo to provide the
δ: 2.01 (s, 24H; −CH3), 2.36 (s, 12H; −CH3), 5.80 (d, 2H; −CH2),
6.55 (d, 2H; −CH2), 6.97 (s, 10H), 7.09 (d, 2H), 7.99 (m, 4H).
HRMS (m/z): calcd for C62H59Br2N2 (M + H)+, 989.3045; found,
989.3045. Mp: 228−229 °C.
Synthesis of 2. A Dean−Stark distillation apparatus was set up
containing 1 (2.10 g, 4.98 mmol), ethylene glycol (1.55 g, 24.7 mmol),
and tosic acid (0.068 g, 0.399 mmol) dissolved in 180 mL of toluene.
The solution was heated at 135 °C until 10 mL of the solution was left
in the starting flask, at which point the heat was stopped and the flask
was left to cool to room temperature. Dichloromethane (25 mL),
saturated sodium bicarbonate (25 mL), and saturated brine solution
(25 mL) were added to the solution and the organic layer was
collected. The organic layer was rinsed with water (5 × 25 mL) and
volatiles were removed by rotary evaporation to yield a yellow solid.
The final product was obtained by washing the solid with methanol (6
mL) followed by pentane (10 mL) and drying in vacuo to yield a white
1
final product (1.0 g, 60%). H NMR (300 MHz, CDCl3), δ: 0.59
(virtual triplet, 6H; −CH3), 0.85 (m, 6H; −CH2), 1.21 (m, 18H;
−CH2), 1.92 (m, 4H; −CH2), 7.41 (d, 2H, aromatic H), 7.68 (t, 4H,
aromatic H).
1
solid (2.0 g, 87%). H NMR (300 MHz, CDCl3), δ: 2.03 (s, 12H;
Synthesis of [PdCl2(BrTABBr)]. To a solution of BrTABBr (50
mg, 0.0505 mmol) in 5 mL of THF was added PdCl2 (9.0 mg, 0.0507
mmol) in 2 mL of methanol and 3 mL of dichloromethane. The
−CH3), 2.33 (s, 6H; −CH3), 2.85 (t, 2H), 3.38 (t, 2H), 5.31 (s, 1H),
6.91 (s, 4H), 7.21 (s, 2H).13C NMR (75.4 MHz, CDCl3) δ: 20.8, 21.1,
64.9, 101.9, 123.3, 127.5, 132.0, 132.7, 136.4, 136.6, 136.7, 144.1.
HRMS (m/z): calcd for C27H30O2Br (M + H)+, 465.1429; found,
465.1425. Mp: 189−190 °C.
ο
solution mixture was refluxed at 70 C for 15 h then the solution was
filtered through a 0.2 μm pore size PTFE membrane to remove
palladium particles. Hexane diffusion into the filtrate gave analytically
pure gold-yellow crystals (59 mg, 60%) that were also suitable for X-
ray diffraction. 1H NMR (300 MHz, THF-d8), δ: 2.02 (s, 24H;
−CH3), 2.38 (s, 12H; −CH3), 5.78 (d, 2H; −CH2), 6.77 (d, 2H;
−CH2), 7.02 (s, 8H), 7.37 (d, 6H), 7.82 (d, 2H), 8.78 (S, 2H). 13C
NMR (75.4 MHz, THF-d8), δ: 17.8, 18.6, 120.0, 120.6, 124.3, 126.6,
127.9, 129.7, 130.2, 132.5, 133.1, 134.2, 134.6, 135.9, 141.7, 146.9,
152.0. Anal. Calcd for C62H58Br2Cl2N2O2Pd·2H2O: C, 61.83; H, 5.19;
Synthesis of 3. In the glovebox 2 (2.19 g, 4.71 mmol) was
dissolved in THF (50 mL). The reaction flask was sealed, brought out
of the glovebox and cooled to −78 °C under nitrogen. Once cooled,
an aliquot of n-butyl lithium (2.5 M in hexanes, 2.2 mL, 5.65 mmol)
was added via syringe followed by stirring for 1 h. Iodine (2.4 g, 9.42
mmol) was then added to quench the reaction. The reaction contents
were allowed to warm to room temperature and stirred for a further 12
h. Excess iodine was quenched with saturated aqueous sodium sulfite
(30 mL). The organic layer was collected and HCl(aq) (9 M, 30 mL)
was added and the mixture was refluxed for 12 h. A yellow,
semicrystalline solid formed upon cooling. The solid was collected,
washed with pentane (2 × 10 mL) and dried in vacuo to afford the
ο
N, 2.33; Found: C, 61.79; H: 5.12; N: 2.35. Mp: 178−179 C.
Synthesis of PF1. Dimethylformamide (15.0 mL) was added to
ITABI (50.0 mg, 0.0461 mmol), 9,9-dihexylfluorene-2,7-bis-
(trimethyleneborate) (23.1 mg, 0.0461 mmol), cesium carbonate
(90.0 mg, 0.277 mmol) and tetrakis(triphenylphosphine)palladium (2
mg, 0.002 mmol). The reaction mixture was heated at 100 °C for 20 h,
and then allowed to cool to room temperature for 1 h. The organics
were extracted with dichloromethane, EDTA(aq) and brine (50 mL
1
target as a white solid (1.44 g, 65.6%). H NMR (300 MHz, CDCl3),
δ: 2.00 (s, 12H; −CH3), 2.34 (s, 6H; −CH3), 6.95 (s, 2H), 7.59 (s,
2H;), 9.60 (s, 1H). 13C NMR (75.4 MHz, CDCl3), δ: 20.6, 21.1, 101.2,
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each). The dichloromethane layer was collected and concentrated via
rotary evaporation to ∼3 mL. The concentrated solution was then
added dropwise to 20 mL of methanol to yield a fine green-yellow
precipitate that was subsequently dried in vacuo to give PF1 (30 mg,
the sterically encumbered materials to those of previously
reported materials lacking such bulk.
Preparation of TAB-modified PF and PPE derivatives
required a bipy-modified monomer capable of participating in
Sonogashira-Hagihara or Suzuki-Miyaura type coupling,
respectively. The initial monomer targeted was BrTABBr.
This monomer was readily prepared in 47% yield by Horner-
Wittig condensation of the m-terphenyl aldehyde 1 with
bis(phosphonate ester)-modified bipy derivative 4 as shown in
Scheme 1. Unfortunately, the degrees of polymerization for
1
55%). H NMR (300 MHz, CDCl3), δ: 0.69−2.38 (26H), 2.12 (s,
24H; −CH3), 2.42 (s, 12H; −CH3), 5.85 (d, 2H, −CH2), 6.74 (d, 2H,
−CH2), 6.98 (s, 2H), 7.05 (8H), 7.10−7.21 (4H), 7.54 (4H), 7.65−
7.76 (4 H), 8.06 (s, 2H). GPC: Mw/Mn = 14519/8026 = 1.8.
Synthesis of PF3. Dimethylformamide (18 mL) was added to
ITABI (50.0 mg, 0.0461 mmol), 9,9-dihexylfluorene-2,7-bis-
(trimethyleneborate) (51.5 mg, 0.0922 mmol), 2,7diiodo-9,9-dioctyl-
9H-fluorene (29.6 mg, 0.0461 mmol), cesium carbonate (90.0 mg,
0.277 mmol) and tetrakis(triphenylphosphine)palladium (2.1 mg,
0.002 mmol). The reaction mixture was heated at 95 °C for 18 h and
then allowed to cool to room temperature for 2 h. The organics were
extracted with dichloromethane (2 × 15 mL) and washed with EDTA
(2 × 25 mL) and a brine solution (2 × 25 mL). The dichloromethane
layer was collected and concentrated to ∼3 mL by reduced pressure
and then added dropwise to 20 mL of methanol, which afforded a fine
green-yellow precipitate, subsequently dried in vacuo for 18 h to yield
PF3 (51 mg, 60%). 1H NMR (300 MHz, CDCl3), δ: 0.65−1.11, 1.98−
2.39, 5.81 (d, 2H, −CH2), 6.71 (d, 2H, −CH2), 6.94−7.12, 7.50−7.81,
8.02. GPC: Mw/Mn = 24714/7536 = 3.28.
Scheme 1. Preparation of Bulky Bipy Monomers
Synthesis of PPE1. A mixture of 1,4-diethynyl-2,5-bis(hexyloxy)-
benzene (13.0 mg, 0.0433 mmol) in 4.0 mL of diisopropylamine was
added dropwise to a separate solution containing BrTABBr (50.0 mg,
0.0433 mmol), copper iodide (0.33 mg, 0.002 mmol) and tetrakis-
(triphenylphosphine)palladium (2.0 mg, 0.002 mmol) in 5.0 mL of
toluene. The reaction mixture was stirred at room temperature for 25
h then refluxed at 50 °C for 24 h. The contents were allowed to cool
to room temperature, and then concentrated by rotary evaporation
which gave a polymer film. Dichloromethane (3 mL) was added to
dissolve the film and the resultant solution was added dropwise to 20
mL of methanol to afford a fine orange-yellow precipitate. The crude
product was subsequently dried in vacuo to yield PPE1 (29 mg, 60%).
1H NMR (300 MHz, CDCl3), δ: 0.79−1.61 (22H), 2.02 (s, 24H;
−CH3), 2.36 (s, 12H; −CH3), 4.01 (t, 4H; −OCHH2), 5.82 (d, 2H;
−CH2), 6.66 (t, 2H), 6.93 (s, 4H), 6.98−7.14 (6H), 7.30 (2H), 7.48
(s, 8H), 8.01 (s, 2H). GPC: Mw/Mn = 38716/9369 = 4.1.
polymers produced using BrTABBr were unsatisfactory. To
solve this problem, we prepared ITABI, whose aryl iodide units
make it more active in Pd-catalyzed coupling compared to
BrTABBr. Preparation of ITABI required a more involved
synthetic approach than did BrTABBr (Scheme 1). The first
step was protection of the aldehyde functionality in 1 as an
acetal to give 2 (87%). This was followed by lithium−bromine
exchange of 2 with n-butyl lithium, subsequent quenching with
molecular iodine, and finally an acid work-up to deprotect the
aldehyde, giving 3 (66%). Compound 3 was then employed in
a Horner-Wittig condensation with 4 to give ITABI (65%).
Although spectroscopic studies had verified the 1:1 metal to
ligand ratio for TAB derivatives,18,19 it was of interest to isolate
and structurally characterize metal complexes in order to
unequivocally demonstrate the geometry of metal binding sites
available from the sterically encumbered ligand. In this vein,
[PdCl2(BrTABBr)] was prepared and characterized by single
crystal X-ray diffraction (Figure 2). Despite the global steric
protection about the bipy unit, the distribution of bulky groups
provides a pocket of free space about the Pd center such that a
standard pseudosquare planar geometry is observed, with no
apparent distortions. The average Pd−N (2.03 Å) and Pd−Cl
(2.27 Å) bond lengths in [PdCl2(BrTABBr)] compare well to
the Pd−N (2.02 Å) and Pd−Cl (2.29 Å) lengths in
unencumbered [PdCl2(2,2′-bipyridine)].21 The N−Pd−N
(80.95(17)°) and Cl−Pd−Cl (90.15(7)°) angles in
[PdCl2(BrTABBr)] are also similar to analogous N−Pd−N
(80.6(1)°) and Cl−Pd−Cl (89.94(5)°) for [PdCl2(2,2′-
bipyridine)].21
Synthesis of PPE3. To a mixture of BrTABBr (50.0 mg, 0.0433
mmol), copper iodide (0.35 mg, 0.002 mmol) and tetrakis-
(triphenylphosphine)palladium (2.0 mg, 0.002 mmol) toluene (10
mL) was added 1,4-diethynyl-2,5-bis(hexyloxy)benzene (27.8 mg,
0.092 mmol) and 1,4-bis(hexyloxyl)-2,5-diiodobenzene (24.4 mg,
0.046 mmol) in 5.0 mL of diisopropylamine. The reaction was stirred
at room temperature for 25 h then refluxed at 80 °C for 24 h. Once the
reaction contents cooled to room temperature the volatiles were
removed by rotary evaporation and a polymer film remained.
Dichloromethane (3.0 mL) was added to dissolve the film and then
was added dropwise to 20 mL of MeOH which gave an orange-yellow
precipitate. The organics were decanted and the resulting solid was
1
dried in vacuo to yield PPE3 (48 mg, 65%). H NMR (300 MHz,
CDCl3), δ: 0.81−1.86 (66H), 2.02 (s, 24H; −CH3), 2.38 (s, 12H;
−CH3), 4.03 (t, 12H; −OCHH2), 5.83 (d, 2H; −CH2), 6.77 (d, 2H),
6.93 (s, 4H), 6.98−7.14 (6H), 7.30 (6H), 7.48 (6H), 8.03 (s, 2H).
GPC: Mw/Mn = 9757/3471 = 2.81.
RESULTS AND DISCUSSION
■
Design and Synthesis. Because the steric capsule about
the bipy unit in TAB (Chart 2)18 enforces a 1:1 metal:bipy
ratio in PPV1,19 we have chosen to incorporate this same motif
into the polyfluorene (PF) and poly(p-phenylene ethynylene)
(PPE) derivatives (Chart 2) for the current study. The goal was
to explore how the photophysical and electrochemical proper-
ties of metal-free and metalated materials are related to the
identity and length of the π-conjugated spacer between ligand
monomer units. Another goal was to compare the properties of
Whereas BrTABBr had performed poorly in Pd-catalyzed
coupling polymerization attempts, polymerization reactions
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Figure 2. ORTEP drawing (50% probability ellipsoids) of [PdCl2(BrTABBr)]. H atoms are excluded for clarity. Refinement details are provided in
the Supporting Information.
Scheme 2. Preparation of Bulky Bipy Polymers
employing ITABI (Scheme 2) produced polymers with
reasonable molecular weights. A 1:1 ratio of ligand to spacer
monomer was used to produce alternating copolymers PF1 and
PPE1. A 1:3 ratio of ligand to spacer monomer was used to
produce statistical copolymers PF3 and PPE3. It is worth
noting that, although ITABI features steric hindrance about the
bipy binding site, it is not encumbered at the iodo substituent,
and thus no detrimental influence of reaction rate was
anticipated, an assumption supported by the reasonable
polymer masses attained. These particular polymers were
selected so that a comparison of copolymers PF3 and PPE3 to
PF1 and PPE1 (as well as to previously reported PPV1) could
be undertaken to reveal how properties change with (1) spacer
identity selected from common CP components and (2) the
average spacer length between ligating sites.
All of the polymers are readily soluble in common polar
organic solvents such as chlorobenzene, dichloromethane,
chloroform, acetone, and tetrahydrofuran (THF). Visually,
PF1 and PF3 were yellow to yellow-green solids, while PPE1
and PPE3 were isolated as bright yellow solids. All of the
polymers are also brightly photoluminescent in solution upon
irradiation with a hand-held UV light (λexcit = 365 nm).
Photophysical Properties of Metal-Free Polymers.
Select photophysical data for PF1, PF3, PPE1, PPE3, PPV1
and some related bipy-modified CPs from the literature (Chart
1B) are provided in Table 1. The π-conjugated backbone of
PF1 is identical to that in CP1c,4f with the only structural
difference being the presence of flanking mesityl groups as
steric shields in PF1.
a
Table 1. Select Photophysical Properties of Polymers
a
λabs (nm) λemit (nm) Stokes shift (nm)
Φ
Io/ICu(II)
PF1
400
402
390
410
380
416
437
462
449
452
485
448
461
430
468
505
540
515
52
83
58
51
50
52
68
78
66
0.49
0.44
0.62
0.32
0.41
0.32
0.37
0.30
0.34
239
310
154
57
CPlc
PF3
PPE1
CPlb
PPE3
PPV1
CPla
CP2a
58
37
31
6.2
NA
a
Photoluminescent quantum efficiencies (Φ) are repored with an
accuracy of 0.05.
Predictably, the absorption maxima (λmax) for the two
polymers are nearly identical. A significant difference between
the materials is that the Stokes shift of CP1c (83 nm) is
significantly greater than that of PF1 (52 nm) so the emission
maxima (λem) of CP1c (485 nm) and PF1 (452 nm) differ by
33 nm. The magnitude of a Stokes shift is related to the
difference in geometry of the ground versus excited state,22 so a
smaller Stokes shift can reflect a more rigid CP backbone and
its greater resistance to geometric distortion.23 The sterically
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Table 2. Photophysical Response to Metal Ion Coordination
no Mn+
Zn2+
Cd2+
Hg2+
Co2+
Cu2+
λmax
λem
λmax
λem
Io/I
λmax
λem
Io/I
λmax
λem
Io/I
λmax
λem
Io/I
λmax
λem
Io/I
PF1
PF3
408
400
410
416
437
452
450
461
468
505
408
400
410
416
437
452
452
460
468
502
1.3
1.1
1.3
3.6
1.8
408
400
410
416
437
453
452
460
468
504
1.0
3.1
1.3
1.2
4.5
408
400
410
416
437
453
452
460
468
505
1.1
1.3
1.2
1.1
1.7
408
400
410
416
437
453
452
460
468
505
17
51
408
400
410
416
473
453
452
460
468
500
239
162
57
PPE1
PPE3
PPV1
32
201
37
7.6
31
Figure 3. Absorption (left) and photoluminescence (right) responses of PPE1 (A) and PF1 (B) to addition of metal ions. The band at 535 nm in
the PPE1 + Co2+ absorption spectrum is due to the excess cobalt salt used in the titration.
encumbering groups in PF1 are likely to diminish the
conformational flexibility of PF1 and may thus be the origin
of the smaller Stokes shift. A similar Stokes shift is observed in
the other sterically encumbered derivative PF3 (58 nm),
although its λmax is shifted to the blue by ∼10 nm versus CP1c
or PF1 because fluorenylene units comprise a higher
percentage of the polymer chain in PF3. The photo-
luminescence quantum efficiency (Φ) of PF1 (0.49) and
CP1c (0.44) are identical within error, whereas that of PF3
(0.62) is significantly higher, suggesting that incorporation of
bipy units into the backbone is what leads to the diminished
photoluminescence efficiency.
The π-conjugated backbone of PPE1 is identical to that in
CP1b, again with the only structural difference being the
mesityl groups in PPE1. The absorption maxima (λmax) for the
two polymers differ by about 20 nm, while the Stokes shift for
sterically encumbered PPE1 (51 nm) and PPE2 (52 nm) are
nearly identical to unencumbered CP1b (50 nm), and Φ is
slightly higher for CP1b (0.41) versus PPE1 (0.32) and PPE3
(0.32). As noted previously,4f the difference in how
incorporating bipy units influences the properties of PF versus
PPE derivatives stems from the difference in energy matching
between the spacer (2,7-fluorenylene of phenylene ethynylene,
respectively) and the bipy unit. Energy matching is an
important characteristic when determining delocalization of π
electrons, where the better the energy match (smaller band
gap) the more delocalization is enhanced. The 2,5-dialkox-
yphenyleneethynylene units in the PPE derivatives PPE1 and
PPE3 are significantly more electron rich than are the 7,7-
dialkylfluorenylene units in PF derivatives PF1 and PF2,
leading to a poorer energy match between the spacer and the
metal-binding subunits of the polymer. Such considerations
regarding energy matching between the conjugated system and
the embedded metal complex3 are another motivation for
determining relative orbital energies by electrochemical
measurements, as will be discussed further in the following
sections.
The three PPV derivatives listed in Table 1 all feature the
same π-conjugated backbone, but each has a different number
of hexyloxy side chains, and PPV1 additionally features bulky
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c
Table 3. Electrochemical and Band Gap Data
b
ionization energy (eV)
electron affinity (eV)
bandgap Eg (eV)
+Zn2+
no Mn+
+Zn2+
+Cu2+
no Mn+
+Zn2+
+Cu2+
no Mn+
(2.64)
+Cu2+
a
PF1
PF3
5.35
5.42
5.80
4.55
5.12
5.90
−
5.16
NA
4.57
−
−
5.10
NA
4.56
−
2.71
−
2.70
−
(2.25)
(2.46)
NA
(2.38)
(2.38)
NA
a
a
2.86
2.90
2.76
2.72
NA
3.20
2.56 (2.70)
2.90
PFO
PPE1
PPE3
PPE
NA
3.26
−
1.79
1.31
1.36
a
2.54
−
NA
(2.58)
3.40
(2.41)
NA
(2.38)
NA
NA
NA
2.50
NA
a
b
c
These data are estimated using the optical bandgap. Numbers in parentheses are bandgaps estimated from onset of absorption. Values for
ionization energies and electron affinities are quoted as values below vacuum level.
mesityl substituents (structures are provided in Chart 1).
Alkoxy-substituted PPV derivatives exhibit bathochromically
shifted absorption and emission versus derivatives lacking such
electron donating substituents.24 Similarly, the λmax values for
PPV derivatives in Table 2 predictably increase as the number
of electron-releasing hexyloxy substituents increases from PPV1
(437 nm) to CP2a (449 nm)25 to CP1a (462 nm). The Φ
values for PPV derivatives (0.30−0.37) are similar to one
another and to those of the PPE derivatives discussed above
(0.32−0.42). The Stokes shift of PPV derivatives are also
similar to one another (66−78 nm) and only slightly higher
than those of PPE derivatives (50−52 nm), reflecting the
additional conformational flexibility afforded by the double
bonds in PPV derivatives versus triple bonds in PPE derivatives.
The influence of the spacer on photophysical properties is
apparent upon comparison of absorption and emission maxima.
The general trend that emerges is that the absorption and
emission maxima are progressively red-shifted upon changing
from PF to PPE to PPV-based spacers. This is not surprising
because the same trend is observed for photophysical
parameters of the bipy-free parent polymers comprised of
7,7-dialky-2,7-fluorenylene, 2,5-dialkoxyphenyleneethynylene,
or 2,5-dialkoxyphenylenevinylene repeat units.
Photophysical Response of Polymers to Metal Ion
Coordination. Previous work with TAB and PPV1 confirmed
that the dissociation constants of these sterically encumbered
ligands for metals such as Cu2+, Fe3+, and Zn2+ are essentially
the same as they are for simple, unencumbred 2,2′-bipyridyl.18
Job analysis derived from titrations followed by absorption and
photoluminescence spectroscopy confirmed the 1:1 metal:bipy
binding mode, and the results of a single crystal X-ray
diffraction structure determination of [PdCl2(BrTABBr)]
(Figure 2) demonstrate the ability of the core to shield the
metal-bound π-conjugated construct from external π−π
interactions or coordinative cross-links. In the current study,
the photophysical response of the polymers to select metal ion
coordination was examined by absorption and photolumines-
cence spectroscopy. The spectroscopic changes observed upon
excess metal ion (10−20 equiv) addition to PF1 and PPE1 are
provided in Figure 3. Corresponding data for PF3 and PPE3
were similar, and their spectra are thus provided in the
Supporting Information. Titrations of each polymer with each
metal ion were also carried out and followed by both
absorption and photoluminescence spectroscopy in order to
observe the gradual change of the spectra as up to one equiv of
each metal ion was added. The effects of excess metal ion
addition on absorption and emission properties are summarized
in Table 2, while spectra for all titrations are provided in the
Supporting Information.
Confirmation that the metal ions are binding to the bipy sites
in the polymers is provided by the observed red-shift in
absorption maxima for each of the polymer/metal combina-
tions (Table 2), similar to the trend noted for TAB.18 The
specific manner in which different metals impact photo-
luminescence, however, varies. One observation that applies
generally to all of the polymers in Table 2, though, is that they
exhibit significantly less photoluminescence quenching upon
binding the diamagnetic Zn2+, Cd2+, and Hg2+ ions, all of which
have a closed-shell d10 electronic configuration, compared to
Co2+ and Cu2+, both of which have only partially filled d
electron shells. This is similar to the trend observed for small
molecule model TAB18 as well as related bipy-modified CPs
such as those shown in Chart 1.4f,10,14b,d
The trends in photoluminescence quenching efficacy induced
by binding are quite different for Cu2+ versus Co2+. First, Cu2+
induces a greater quenching effect the more electron deficient
the polymer host; quenching follows the trend PF > PPE >
PPV. The Co2+ induced quenching follows the opposite trend,
with the greatest quenching for the most electron rich polymer
host, PPV1. With this trend in mind, it is not surprising that the
presence of a greater percentage of electron-withdrawing
bipyridyl units in PF1/PPE1 leads to greater Cu2+ induced
quenching of either of these versus their less bipy-laden
analogues PF3 and PPE3, respectively. Predictably, then, Co2+
induces greater quenching in PF3 and PPE3 versus more
electron deficient PF1 or PPE1, respectively. We hypothesize
that the origin of the disparity between the quenching induced
by Cu2+ versus Co2+ is that Cu2+ is in its highest readily
accessible oxidation state under the experimental conditions
employed, while Co2+ can attain a Co3+ oxidation state through
release of an electron to the ligand host. Unfortunately, efforts
to confirm this hypothesis by observing the metal to ligand
charge transfer (MLCT) band in the absorption spectrum of
the PF1−Co2+ complex in Figure 2A (left), was complicated by
the presence of the band at 535 nm that is attributable to the
Co2+ complex used in the titration.
Electrochemistry of Polymers and Metallopolymers.
The electrochemical properties of the TAB-modified CPs along
with Zn2+ and Cu2+ metallopolymer derivatives were probed by
cyclic voltammetry (CV) with the aim of elucidating the
HOMO (ionization energy, IE) and LUMO (electron affinity,
EA) levels. Electrochemical data are summarized in Table 3,
along with data for PFO26 (Chart1A) and unmodified PPE27
for comparison. In some cases, either EA, IE or both could not
be determined by CV. In these cases the band gap estimated
from the onset of absorption spectra were used to fill in data
where possible.
Predictably, as more of the bipy-containing monomer units
of metal free CPS were replaced by fluorenylene (from PF1 to
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PF3 to PFO) or phenyleneethynylene units (from PPE1 to
PPE3 to PPE), the EA, IE and band gap all approached those
of the bipy-free parent CPs for metal-free materials.
Incorporating bipy units had the effect of raising the IE closer
to the vacuum level versus the IE in the bipy-free materials
which is presumably due to the diminished stability of positive
charge on the electron deficient bipy-appended systems. Bipy
incorporation had a greater impact on the IEs of the PPE
derivatives than on those of the PF derivatives because the
fluorenylene units are already significantly more electron
deficient compared to the 2,5-dialkoxyphenyleneethynylene
units in the PPE derivatives. The EAs were less effected than
the IEs by incorporating bipy units, but this is not altogether
surprising given that organic CPs tend to be much more readily
oxidatively doped than reductively doped; incorporating bipy
units does not seem to significantly alleviate this difficulty.
Because the steric coordination control strategy allows CMP
derivatives of our polymers to remain soluble and free of
coordinative cross-links, they were amenable to solution cyclic
voltammetric studies that are not possible for CMPs prepared
by metalation of traditional bipy-CPs. When PF derivatives
bind metal ions, which are positively charged and thus electron-
withdrawing, the effect is similar to incorporating electron-
withdrawing bipy units. First, and consistent with the observed
red shift in absorption spectra, the band gap decreases upon
metal ion binding. Metal coordination to the polymers
additionally leads to raising the IE toward the vacuum level.
For the PPE derivatives, the large effect of bipy incorporation
remains relatively the same even after metal ions bind. The EA,
however, is relatively constant for PF derivatives before and
after metal binding, whereas the EA sinks lower below vacuum
level in the PPE derivatives. Unfortunately, however,
observations pertaining to the EA values must be viewed with
caution because the EA values for metal-free and metalated
polymers were only electrochemically determinable (without
using less-reliable optical band gap estimation) for one polymer
(PF1). Nonetheless, the data that were obtainable suggest that
the family of soluble and easily metalated materials studied may
have potential for independently tuning the EA and IE to
desired levels for device applications through judicious selection
of polymer host and metal complex guest energy levels.3
spectra for titrations of polymers with metal ions, refinement
details, and .cif files for single crystal X-ray diffraction data for 2
and [PdCl2(BrTABBr)], and an ORTEP drawing for 2. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: rhett@clemson.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Clemson University and a
National Science Foundation CAREER Award (CHE-
0847132).
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CONCLUSIONS
■
Through judicious monomer design, a motif was accessed that
provides a 2,2′-bipyridyl unit that provides steric shielding from
undesired cross-linking/network formation, while simultane-
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ASSOCIATED CONTENT
■
S
* Supporting Information
Proton and 13C NMR spectra for polymers, monomers and
monomer precursors, absorption and photoluminescence
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