Understanding the Effect of Metal Centers on Charge Transport and

Article pubs.acs.org/Macromolecules

Understanding the Effect of Metal Centers on Charge Transport and Delocalization in Conducting Metallopolymers Minh T. Nguyen,* Richard A. Jones,* and Bradley J. Holliday* Department of Chemistry, The University of Texas at Austin, 105 E. 24th St., Mailstop A5300, Austin, Texas 78712-0165, United States S Supporting Information *

ABSTRACT: A series of conducting polymers, formed from an electropolymerizable Schiff-base ligand, N,N′-((2,2′dimethyl)propyl)bis(2-thiophenyl)salcylidenimine, and the corresponding metal complexes (i.e., Ni(II), Cu(II), V(IV)O, Co(II), and Zn(II)) have been prepared, characterized, and studied in detail. Our successful synthesis of the ligand polymer helps to make a direct comparison between the properties of metal-free conducting polymers and the corresponding metallopolymers. This enables the role of metal centers in these Schiff-base conducting metallopolymers (CMPs) in particular, and in Wolf type III CMPs in general, to be unambiguously elucidated. Vis−NIR absorption spectroelectrochemical studies show that longer distances for charge delocalization were found in the CMPs when compared to the metal-free counterpart, an indication of the contribution of the metal centers in extending the effective conjugation length of these electroactive polymers. The systematic use of both redox-active and redox-inactive first row transition metals helps to better understand the nature of charge transport and the specific role of the metal centers in these systems. Cyclic voltammetry and in situ conductivity show superior charge transport in the CMPs compared to the ligand polymer, especially in systems containing redox-active metal centers with redox potentials higher than, but similar to, that of the conjugated organic backbone. Our results indicate that inner-sphere charge transport within the organic backbone, which is serving as a hopping station, is the dominant mechanism of conductivity enhancement and favorable for efficient charge transport in Schiff-base CMPs.

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INTRODUCTION In recent years, there has been an increasing interest in conducting metallopolymers (CMPs) due to the potential for this class of materials to impact various applications including catalysis,1−6 sensing,7−11 light-emitting diodes,12−14 energy storage,15−19 and solid-state memory.20−24 Conducting polymers incorporated with metal centers exhibit multifunctional properties stemming from the introduction of the metal centers as well as the interactions between the metal ions and the conjugated organic backbone. Understanding electron transport processes in these polymers is important for the rational design of suitable systems for specific applications, especially for those utilizing the polymer conductivity (e.g., chemoresistive sensors).10,25 Electronic conductivity of CMPs depends on the dominant charge transport process which can be divided into three components: (i) the intrinsic property of the organic backbone, (ii) the redox behavior and electron transfer between metal centers, and (iii) the interaction between the metal ions and the organic backbone. In many cases, it is difficult to individually elucidate the contribution of each element on the overall conductivity due to overlapping effects.26,27 Consequently, systematic studies attempting to isolate each component are necessary to understand the influence each has on the electrochemical properties of CMPs. This requires investigation © XXXX American Chemical Society

of the charge transport properties of the isolated organic backbone, the role of the metal ions, and charge transport in systems that have metal redox potentials lower than, overlapping, and higher than that of the organic backbone to fully understand the precise role of the metal centers in CMPs. In a previous communication, we reported the effects of a vanadyl center on the metal-induced enhancement of conductivity in a Schiff-base CMP.28 The successful electrochemical polymerization of the ligand allowed independent characterization of the organic backbone in this polymer and a direct comparison with the corresponding metallopolymer. In this contribution, we report detailed investigations of the effects of different redox-active and redox-inactive metals on the charge transport properties of Schiff-base CMPs. The interaction of redox-active metals with the organic backbone has been studied by cyclic voltammetry, vis−NIR absorptionbased spectroelectrochemistry, and in situ conductivity. Computer modeling along with structural information from single-crystal X-ray diffraction analysis was used to help explain the electrochemical behavior of the CMP series. Received: October 31, 2016 Revised: January 3, 2017

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DOI: 10.1021/acs.macromol.6b02349 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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NMR (400 MHz, CDCl3): 8.35 (s, 2H), 7.56 (dd, 2H, J = 2.3, 8.6), 7.46 (d, 2H, J = 2.3), 7.18 (dd, 2H, J = 1.2, 5.1), 7.15 (dd, 2H, J = 1.2, 3.6), 7.04 (dd, 2H, J = 3.5, 5.1), 6.99 (d, 2H, J = 8.6), 3.51 (s, 4H), 1.09 (s, 6H). 13C{1H} NMR: compound is not sufficiently soluble in any common solvent to obtain a spectrum. FTIR (neat powder on ATR): 1610 cm−1 (CN). UV−vis (λmax (ε), CH2Cl2): 434 nm (6440 cm−1 M−1), 325 nm (48 400 cm−1 M−1), 270 nm (49 000 cm−1 M−1). HRMS (CI+) m/z calculated for C27H24N2O2S2Ni 530.0633; found 530.0636. Elemental analysis calculated (found): C, 61.04 (59.98); H, 4.55 (4.75); N 5.27 (5.09). Crystals suitable for singlecrystal X-ray diffraction analysis were grown via slow evaporation of a dichloromethane (CH2Cl2) solution. CCDC deposition number 1513050. CuLI. This complex was prepared in a manner similar to VOLI using Cu(OAc)2; yield = 98%, bright green solid, mp >250 °C. FTIR (neat powder on ATR): 1616 cm−1 (CN). UV−vis (λmax (ε), CH2Cl2): 400 nm (10 300 cm−1 M−1), 320 nm (54 800 cm−1 M−1), 274 nm (51 000 cm − 1 M − 1 ). HRMS (CI + ) m/z calculated for C27H24N2O2S2Cu 535.0575; found 535.0584. Elemental analysis calculated (found): C, 60.48 (60.24); H, 4.51 (4.62); N 5.22 (5.26). Crystals suitable for single-crystal X-ray diffraction analysis were grown via slow evaporation of a CH2Cl2 solution. CCDC deposition number 1513049. ZnLI. This complex was prepared in a manner similar to VOLI using Zn(OAc)2·2H2O; yield = 77%, bright yellow solid, mp >250 °C. 1H NMR (400 MHz, CDCl3): 8.37 (s, 2H), 7.56 (dd, 2H, J = 2.3, 8.6), 7.47 (d, 2H, J = 2.3), 7.19 (dd, 2H, J = 1.2, 5.1), 7.16 (dd, 2H, J = 1.2, 3.6), 7.03 (dd, 2H, J = 3.5, 5.1), 6.98 (d, 2H, J = 8.6), 3.51 (s, 4H), 1.09 (s, 6H). 13C{1H} NMR: compound is not sufficiently soluble in any common solvent to obtain a spectrum. FTIR (neat powder on ATR): 1625 cm−1 (CN). UV−vis (λmax (ε), CH2Cl2): 380 nm (7100 cm−1 M−1), 311 nm (26 800 cm−1 M−1), 269 nm (31 200 cm−1 M−1). HRMS (CI+) m/z calculated for C27H24N2O2S2Zn 536.0571; found 536.0569. Elemental analysis calculated (found) for (ZnLI)2· Zn(AcO)2: C, 55.31 (55.33); H, 4.32 (4.44); N 4.45 (4.31). Crystals suitable for single-crystal X-ray diffraction analysis were grown via slow evaporation of a DMF solution. CCDC deposition number 1513052 and 1513053 for monomeric and dimeric trinuclear complexes, respectively. X-ray Structure Determination. Single-crystal X-ray diffraction data were collected on a Rigaku AFC12 diffractometer, a Saturn 724+ CCD, or a Rigaku SCX-Mini diffractometer with a Mercury CCD using a graphite monochromator with Mo Kα radiation (α = 0.710 70 Å). Absorption corrections were applied using Multiscan. Data reduction was performed using the Rigaku Americas Corporation’s Crystal Clear version 1.40.29 The structures were solved by direct methods using SIR9730 and refined anisotropically using full-matrix least-squares methods with the SHELX 9731 program package. The coordinates of the non-hydrogen atoms were refined anisotropically, while hydrogen atoms were included in the calculation isotropically but not refined. Neutral atom scattering factors and values used to calculate the linear absorption coefficient are from the International Tables for X-ray Crystallography (1992).32 Electrochemistry. Electrochemical studies were preformed in a drybox under a nitrogen atmosphere utilizing a Metrohm Eco Chemie Autolab PGSTAT30 potentiostat/galvanostat with a FRA2-module for electrochemical impedance studies and a bipotentiostat module. The software interface utilized to obtain and process the data was the General Purpose Electrochemical Software (GPES) supplied by the potentiostat manufacturer. All the electrochemical experiments were carried out in a three-electrode cell with Ag/AgNO3 reference electrode (silver wire dipped in a 0.01 M silver nitrate solution with 0.1 M [(n-Bu)4N][PF6](TBAPF6) in CH3CN), a Pt working electrode, and Pt wire coil counter electrode. Potentials were relative to this 0.01 M Ag/AgNO3 reference electrode. Ferrocene was used as an external reference to calibrate the reference electrode before and after experiments were performed, and that value was used to correct the measured potentials. All electrochemistry and electropolymerizations were performed in dry CH2Cl2 or dry acetonitrile (CH3CN) using 0.1 M TBAPF6 as the supporting electrolyte. The TBAPF6 was purified by

EXPERIMENTAL SECTION

Materials and Methods. Air- and moisture-sensitive reactions were carried out in oven-dried glassware using standard Schlenk techniques under an inert nitrogen atmosphere. All chemicals were purchased from commercial sources and used as received. Dry solvents were obtained using an Innovative Technology, Pure Solv solvent purifier with a double purifying column. 1H NMR (400 MHz) and 13 C{1H} NMR (100 MHz) spectra were obtained on a Varian (400 MHz) spectrometer and were referenced to residual solvent peaks. All peak positions are given in ppm, and coupling constants are reported in hertz. Low- and high-resolution mass spectrometry was carried out using a Thermo Finngan TSQ 700 and Waters Autospec Ultima, respectively. Melting points were recorded with an OptiMelt automated melting point system with digital image processing technology from Stanford Research System (SRS, Sunnyvale, CA), and the uncorrected values are reported. Elemental analyses were performed by Midwest Microlab, Indianapolis, IN. X-ray photoelectron spectroscopy (XPS) was carried out on a PHI 5700 XPS system equipped with dual Mg X-ray source and monochromatic Al Xray source complete with depth profiling and angle-resolved capabilities. Synthesis of Monomers. Ligand H2LI. To a solution of 5-(2thiophenyl)-2-hydroxybenzaldehyde26 (0.64 g, 1.3 mmol) in CHCl3/ EtOH (30 mL, v/v = 1/2) was added 1,3-diamino-2,2-dimethylpropane (0.068 g, 0.66 mmol) in EtOH (10 mL). The reaction mixture was heated to 60 °C for 2 h and then cooled to room temperature, and CHCl3 was removed in vacuo resulting in a yellow suspension in EtOH. The precipitate was collected by vacuum filtration to give 0.60 g (88%) of desired product (mp 131 °C). 1H NMR (400 MHz, CDCl3): 13.57 (s, 2H), 8.37 (s, 2H), 7.56 (dd, 2H, J = 2.3, 8.6), 7.47 (d, 2H, J = 2.3), 7.20 (dd, 2H, J = 1.2, 5.1), 7.16 (dd, 2H, J = 1.2, 3.6), 7.02 (dd, 2H, J = 3.6, 5.1), 6.99 (d, 2H, J = 8.6), 3.52 (s, 4H), 1.09 (s, 6H). 13C{1H} NMR (100 MHz, CDCl3): 165.6, 160.9, 143.7, 130.3, 128.7, 128.0, 125.6, 123.9, 122.1, 118.6, 117.6, 68.0, 36.3, 24.4. FTIR (neat powder on ATR): 1633 cm−1 (CN). UV−vis (λmax (ε), CH2Cl2): 345 nm (6050 cm−1 M−1), 294 nm (32 850 cm−1 M−1). HRMS (CI+) m/z calculated for C27H24N2O2S2 474.1436; found 474.1436. Elemental analysis calculated (found): C, 68.33 (68.44); H, 5.52 (5.67); N 5.90 (5.93). VOLI. Ligand H2LI (0.125 g, 0.122 mmol) was dissolved in a mixture of CHCl3 and EtOH (20 mL, 1:1), and the solution was purged with nitrogen for 5 min. To this solution, a nitrogen-purged solution of vanadyl acetylacetonate (32.3 mg, 0.122 mmol) in EtOH (10 mL) was added. The reaction mixture was stirred under nitrogen at 60 °C for 1 h and then cooled to room temperature. CHCl3 was removed in vacuo resulting in a suspension in EtOH. The light orange solid was isolated by vacuum filtration then further dried under vacuum to give 0.11 g of a yellow-orange solid (83%); mp >250 °C. FTIR (neat powder on ATR): 1617 cm−1 (CN). UV−vis (λmax (ε), CH2Cl2): 401 nm (6400 cm−1 M−1), 313 nm (44 200 cm−1 M−1), 282 nm (52 100 cm−1 M−1). HRMS (CI+) m/z calculated for C27H24N2O3S2V 539.0668; found 539.0663. Elemental analysis calculated (found): C, 60.10 (59.92); H, 4.48 (4.45); N, 5.19 (5.08). Crystals suitable for singlecrystal X-ray diffraction analysis were grown via slow evaporation of a dimethyl sulfoxide (DMSO) solution under a nitrogen atmosphere. CCDC deposition number 1513051. (CoLI)2·Co(AcO)2. This complex was prepared in a manner similar to VOLI using Co(OAc)2·4H2O; yield = 63%, yellow-brown solid, mp >250 °C. FTIR (neat powder on ATR): 1618 cm−1 (CN). UV−vis (λmax (ε), CH2Cl2): 390 nm (4000 cm−1 M−1), 315 nm (19 500 cm−1 M−1), 269 nm (18 900 cm−1 M−1). HRMS (CI+) m/z calculated for C27H24N2O2S2Co 531.0611; found 531.0618. Elemental analysis calculated (found) for (CoLI)2·Co(AcO)2: C, 56.17 (56.79); H, 4.34 (4.53); N 4.52 (4.46). Crystals suitable for single-crystal X-ray diffraction analysis were grown via slow evaporation of a N,Ndimethylformaldehyde (DMF) solution. CCDC deposition number 1513048. NiLI. This complex was prepared in a manner similar to VOLI using Ni(OAc)2·4H2O; yield = 98%, dark green solid, mp >250 °C. 1H B


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Macromolecules Scheme 1. Synthesis of Ligand and Metal Complex Monomers

recrystallization three times from hot ethanol before being dried for 3 days at 100−150 °C under dynamic vacuum prior to use. Electrosyntheses of polymer films were performed using 0.5 and 1.0 mM solutions of the ligand and metal complexes, respectively, by continuous cycling between −0.75 and 1.25 V at 100 mV s−1. The films obtained were then repeatedly washed with fresh CH2Cl2 before continuing on to the next set of experiments. Spectroelectrochemistry. The in situ vis−NIR absorption-based spectroelectrochemical measurements were performed using the cell arrangement described immediately above with a polymer film electrochemically deposited on indium−tin−oxide (ITO)-coated glass substrate as the working electrode, a platinum mesh as the counter electrode, and a Ag/AgNO3 reference electrode. Experiments were carried out in an optical cuvette inside an inert atmosphere (N2) glovebox. Absorption spectra were recorded on a Varian Cary 6000i UV−vis−NIR spectrophotometer within the NIR/vis spectral region (1600 ≥ λ ≥ 400 nm) under several applied potentials. In Situ Conductivities. The conductivities of polymer films were determined using an interdigitated array electrode (IDA) purchased from CH Instrument (CH 012126) with 10 μm interdigitated electrode spacing, 129 gaps, and 0.2 cm electrode length. Film thickness was obtained on a Dektak 3 surface profilometer. Conductivity profiles were recorded with a 0.04 V applied offset potential and a scan rate of 10 mV/s in CH2Cl2/electrolyte solution. Calculations. Geometry optimizations of neutral and +1 forms of the ligand and metal complexes were performed with density functional theory (DFT) using the Gaussian 03 program,33 employing the B3YLP functional in conjunction with the SDD basis set. All geometries were fully optimized using the default optimization criteria of the program. Orbital analyses were completed with the GaussView 5.0 program.

on the monomers were determined by combustion-based elemental analysis and single-crystal X-ray diffraction analysis, respectively. The results of single-crystal X-ray diffraction studies of metal complexes with LI are shown in Figures 1−4,

Figure 1. ORTEP view of molecules of NiLI (A) and CuLI (B), drawn with the thermal ellipsoids at the 50% probability level. Hydrogen atoms and solvent molecules have been omitted for clarity.

and the selected bond lengths and angles are listed in Table 1. The coordination environment around the metal centers in NiLI and CuLI are distorted square planar with more distortion found in CuLI (Figure 1). The dihedral angle between the two iminophenol rings in NiLI and CuLI is 14.22° and 35.03°, respectively. The thiophene rings in both complexes were found to be twisted relative to the iminophenol rings. In CuLI, the two thiophene rings are crystallographically equivalent and are twisted by 23.32° from the iminophenol rings. One of the two thiophene rings in NiLI was modeled with static disorder and is twisted at a larger angle (32.21°) to the connected iminophenol ring than the other thiophene (11.39°). The VOLI complex, on the other hand, crystallizes with a distorted octahedral geometry around the vanadium(IV) center which is made up of the addition of one weakly coordinated DMSO solvent molecule as an ancillary ligand, one oxide oxygen atom of the vanadyl ion, and four coordination sites (O1N1O2N2) from the salpen ligand (salicyaldehyde connected through a 1,3-propylenediamine backbone, Figure 2). In contrast to the monomeric structures of NiLI, CuLI, and VOLI, the solid-state structure of the complex between Co(II) and ligand LI,

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RESULTS AND DISCUSSION Synthesis and Structural Properties of the Ligand and Metal Complexes. The ligand monomer (H2LI) was synthesized from the condensation reaction of 2,2-dimethylpropanediamine and 5-(thiophen-2-yl)salicylaldehyde (Scheme 1). Metal complexes (MLI) were prepared by the reaction of H2LI with metal acetate or acetylacetonate salts in a 1:1 mixture of CHCl3:EtOH at 60 °C. First-row transition metals with redox-active and redox-inactive behaviors were studied in order to provide information on the effect of various metals on polymer properties. The use of 2,2-dimethylpropanediamine helps to increase the solubility of the ligand and metal complexes compared to those prepared from ethylenediamine or unsubstituted propylenediamine. All characterization data recorded for the ligand and metal complex monomers are fully consistent with the proposed structures. Additionally, the purity and structural information C


Article

Macromolecules Table 1. Selected Bond Lengths and Angles of Metal Complexes NiLI

CuLI

M(1)−O(1) M(1)−O(2) M(1)−N(1) M(1)−N(2) M(1)−O(3)

1.864(3) 1.857(2) 1.882(3) 1.878(3)

1.915(2) 1.915(2) 1.961(2) 1.961(2)

O(1)−M(1)−O(2) O(1)−M(1)−N(1) O(1)−M(1)−N(2) O(2)−M(1)−N(1) O(2)−M(1)−N(2) N(1)−M(1)−N(2)

84.67(11) 93.06(11) 169.74(12) 168.98(12) 93.30(11) 90.78(12)

91.09(12) 94.07(9) 154.76(9) 154.76(9) 94.07(9) 91.72(13)

VOLI bond lengths (Å) 1.966(2) 1.964(2) 2.089(2) 2.108(2) 1.610(2) bond angles (deg) 85.89(9) 88.35(9) 163.48(10) 162.10(10) 89.69(9) 91.07(9)

CoLI

ZnLI

ZnLI-trinuclear

2.0189(18) 2.0337(18) 2.040(2) 2.060(2) 2.0985(19)

2.024(2) 2.029(2) 2.093(3) 2.090(2) 1.993(2)

2.069(4) 2.111(4) 2.106(5) 2.087(5) 2.004(4)

82.89(7) 91.63(8) 170.84(8) 171.83(8) 89.02(8) 95.93(9)

94.64(9) 86.98(9) 169.07(9) 148.23(9) 86.76(9) 89.65(10)

80.58(17) 89.20(18) 156.9(2) 147.2(2) 87.21(19) 90.6(2)

five- or six-coordinate geometries and from acetate metal salts.34,35 For ZnLI two crystal structuresone monomeric and one dimeric trinuclearwere obtained under the same conditions (Figure 4). In the ZnLI mononuclear structure,

Figure 2. ORTEP view of molecule VOLI with one DMSO coordinating molecule, drawn with the thermal ellipsoids at the 50% probability level. Hydrogen atoms and other solvent molecules have been omitted for clarity.

(CoLI)2·Co(AcO)2, features a dimeric trinuclear core with a metal-to-ligand ratio of 3:2 (Figure 3). The two outer Co(II)

Figure 4. ORTEP view of molecule ZnLI·H2O (A) and ZnLI-trinuclear (B) drawn with the thermal ellipsoids at the 50% and 30% probability level, respectively. Hydrogen atoms and solvent molecules have been omitted for clarity.

Figure 3. ORTEP view of molecule (CoLI)2·Co(AcO)2, drawn with the thermal ellipsoids at the 50% probability level. Hydrogen atoms and solvent molecules have been omitted for clarity.

the apical coordination site is occupied by a water molecule, and the coordination geometry of the Zn(II) ion in this complex is distorted square pyramidal, defined by two N atoms and two O atoms of the Schiff-base ligand as the basal plane and by an apical O atom of the water molecule. The Zn(II) ion sits in the body of the pyramid and is 0.379 Å from the basal O1N1O2N2 plane. The structure of the dimeric trinuclear ZnLI complex ((ZnLI)2·Zn(AcO)2) is similar to that of (CoLI)2· Co(AcO)2 with bridging acetate ligands except that there is no

ions have distorted octahedral coordination geometries with the salpen ligand occupying the four base coordination sites, one oxygen atom from a bridging acetate ligand occupying one apical site, and one oxygen atom from a bound DMF solvent molecule occupying the final coordination site. In our experience, this is a common structural motif in complexes of this type when they are synthesized from metal ions that prefer D


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Macromolecules

were synthesized via electropolymerization onto various electrode surfaces (i.e., Pt buttons, ITO-coated glass, stainless steel, etc.) using cyclic voltammetry. The monomer solution of the ligand or metal complexes in CH2Cl2 was oxidatively polymerized by sweeping the potential of the working electrode between −0.75 and 1.25 V vs Fc/Fc+ at a 100 mV/s scan rate. The electrochemical reactions and representative polymerizations of H2LI and NiLI are shown in Scheme 2 and Figure 5,

additional coordinating solvent molecule and the geometries of the two outer Zn(II) ions are square pyramidal. The trinuclear structures found in (CoLI)2·Co(AcO)2 and (ZnLI)2·Zn(AcO)2 have also been observed in other salen complexes of these metals.36−42 Overall, various geometries were observed for the metal centers in the complexes of nickel(II), copper(II), vanadium(IV), cobalt(II), and zinc(II) ions with H2LI (Figure S1, Supporting Information). The coordination geometries around nickel(II) and copper(II) are distorted squared planar while the geometries around vanadium(IV) and cobalt(II) are octahedral. Zinc(II) complexes, on the other hand, adopt a 5-coordinate environment with a square pyramidal geometry. At first blush, the solid-state structural information determined for these metal complexes suggests that there is potential for rich structural diversity when the complexes are incorporated into CMPs via electropolymerization. Based upon this information, copper(II) and nickel(II) CMPs may not necessarily undergo major structural changes upon polymerization due the square planar geometry of the metal centers while cobalt(II), vanadyl(IV), and zinc(II) polymers may undergo changes in order to satisfy the metal coordination during electropolymerization. However, gas-phase (MS) and solution structures (NMR), when possible, of the complexes in this and previous studies34,35 indicate a monomeric structure for complexes of this type when fully dissolved. This is important because the CMPs are formed from relatively dilute monomer solutions in electrolyte/solvent mixtures. It is expected that these conditions would lead to fully dissociated monomeric structures like those depicted in Schemes 1 and 2. It is Scheme 2. Electropolymerization of Ligand and Metal Complex Monomers

Figure 5. Electropolymerization of H2LI (A) and NiLI (B) in CH2Cl2 using a 0.5 and 1.0 mM monomer solutions, respectively. Insets show the linear relationship between current at peak oxidation/reduction potentials and number of scans.

respectively (for electropolymerization of other complexes see Figure S2).43 It is important to note that by varying the concentration of the H2LI solution used in the electropolymerization,28 we were able to obtain the metal-free polymer from a thiophene-substituted Schiff-base ligand, allowing a direct comparison between the properties of the ligand polymer and the corresponding metallopolymers. This should allow for a detailed investigation of the role of the metal ions in the system. It is interesting to note that unsuccessful attempts to electropolymerize thiophene-26,27 or bithiophenesubstituted44 Schiff-base ligands to yield metal-free polymers have been reported. The presence of the metal centers with oxidation states matching the monomers as well as the elemental composition of the ligand polymer and CMPs was confirmed by X-ray photoelectron spectroscopy (XPS) analysis. The relative atomic ratios of metal:nitrogen:sulfur (M:N:S) in these polymers are listed in Table 2, which shows good agreement with the theoretical ratios in the proposed ligand polymer and CMP structures (for full XPS spectra, see Figure S3). The absorption coefficient (α) of these polymers was determined from thin films deposited on ITO-coated glass using eq 145

important to note that the five MLI solution structures are expected to be monomeric under electropolymerization conditions as the systematic study of the CMPs that follows (vide inf ra) will be most meaningful in the context of small variations in the polymer structure allowing differences in the bulk material properties to be directly attributed to the presence and identity of the metal ions. Electrochemical Synthesis and Polymer Characterization. Ligand polymer and CMPs (poly-H2LI and poly-MLI) E


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Macromolecules

CMPs in both CH3CN (Figure S4) and CH2Cl2 (Figure S6) electrolyte solutions, and polymer redox potentials in CH3CN are selected for comparison and shown in Table 3.

Table 2. XPS and UV−Vis Absorption Data XPS elemental analysisa poly-VOL

I

poly-CoLI poly-NiLI poly-CuLI poly-ZnLI poly-H2LI

M:N:S ratiob

M peaksc

λmax (nm) (α (cm−1))d

1.00:2.01:2.34 (1:2:2) 1.00:2.26:2.18 (1:2:2) 1.00:2.00:2.35 (1:2:2) 1.00:1.85:2.02 (1:2:2) 1.00:2.04:1.90 (1:2:2) 0.00:1.00:0.91 (0:1:1)

516.2 (2p3/2) 523.6 (2p1/2) 782.0 (2p3/2) 797.2 (2p1/2) 855.4 (2p3/2) 872.8 (2p1/2) 934.7 (2p3/2) 954.9 (2p1/2) 1022.6 (2p3/2) 1045.4 (2p1/2)

403 (87 400) 386 (93 200) 401 (24 600) 399 (60 100) 384 (20 100) 402 (5160)

Table 3. Electrochemical and Conductivity Data cyclic voltammetrya poly-VOLI poly-CoLI poly-NiLI

poly-CuLI

Measured from thin film on ITO-coated glass. Based on the total integration of metal, nitrogen, and sulfur peaks; calculated values given in parentheses. cMetal peaks are taken after correcting the C 1s peak to 284.8 eV. dAbsorption coefficient (α) was calculated from eq 1. a

b

α=

−ln(10−A) T

poly-ZnLI poly-H2LI

conductivityb

Ep,a (V)

Ep,c (V)

Emax (V)

σ (mS/cm)

0.29 0.77 0.57 0.67 0.39 0.69 1.19 0.48 0.65 0.49 0.69 0.47 0.77

0.22 0.65 0.42 0.59 0.33 0.65 1.11 0.3 0.62 0.37 0.58 0.33 0.65

0.50

6.7

0.47

1.1

0.57 1.12

27.0

0.34

11.2

0.73

0.28

0.94

0.11

a

Volts vs Fc/Fc+, Pt working electrode, CH3CN electrolyte solution. Emax is potential at maximum conductivity measured in CH2Cl2. All conductivity is corrected to poly(3-methylthiophene) which has a known value of 60 S/cm.47,48 b

(1)

where A is the polymer absorbance and T is the film thickness measured by profilometry. All the CMPs show much higher absorption coefficients than that of the ligand polymer (Table 2), which is an important characteristic for applications in optoelectronics, such as photovoltaic devices.46 Polymer Electrochemistry and Conductivity. Electrochemical properties of the ligand polymer and the CMPs were investigated by cyclic voltammetry (CV) in both CH2Cl2 and CH3CN electrolyte solutions. CVs of the electrode-confined CMP films in CH3CN reveal more detail and greater resolution of electrochemical features when compared to those recorded in CH2Cl2, possibly be due to the stabilization of oxidized species by the high dielectric CH3CN solvent and/or by the coordinating nature of CH3CN. For example, the CV of polyNiLI in CH3CN shows three reversible waves while only two broad features were observed in CH2Cl2 (Figure 6). We therefore studied and report the CVs of the ligand polymer and

A cyclic voltammogram of the ligand polymer (poly-H2LI) in CH3CN shows two quasi-reversible oxidative peaks at 0.47 and 0.77 V. As discussed in our previous work for a similar system,28 we assign the first peak at 0.47 V as the process for the formation of phenoxyl radical on the salpen ligand,49 while the other peak at 0.77 V is assigned as the oxidation of the thiophene backbone to form radical cations or polarons. These two processes are observed in all corresponding CMPs but at lower potentials (Figure S4 and Table 3). The second electrochemical process occurs at about the same potential ca. 0.7 V for all polymers, an indication that the metal centers have little effect on the oxidation potential of the thiophene backbone. Interestingly, there is an additional distinctive peak found at 1.19 V in the CV of poly-NiLI. The CVs in both CH2Cl2 and CH3CN seem to reveal a one-electron process for this wave. However, the differential pulse voltammetry (in CH3CN, Figure 6) shows a 2:1:2 ratio for these three peaks at 0.39, 0.69, and 1.19 V, respectively, which indicates that there is more than one electron involved in the redox process occurring at 1.19 V. In combination with the conductivity studies (vide inf ra), we assign this wave to the combination of the Ni2+/3+ redox process and an additional oxidation of the organic portion of the CMP leading to the formation of a bipolaron on the thiophene backbone. CVs of other CMPs show only two oxidative and two reductive waves with a small shoulder in the 1.05−1.20 V range, assigned to the formation of a bipolaron on the thiophene backbone in each material (Figure S4). The charge transport properties of the ligand polymer (polyH2LI) and the CMPs (poly-MLI) were then studied by CV scan-rate dependence and in situ conductivity measurements. CV studies of all polymers in pure electrolyte solution at different scan rates from 10 to 500 mV/s revealed a linear relationship of observed peak oxidation and reduction currents with the scan rate (Figure S5). This linear dependence is a characteristic observation of a strongly absorbed electroactive film in which current is not limited by the diffusion of counterions and gives initial indications of both the ionic and

Figure 6. CVs of poly-NiLI taken in CH3CN (solid, red) and in CH2Cl2 (dotted, blue) with polymer film grown from 1.0 mM solution in CH2Cl2; scan rate = 50 mV/s. Inset shows differential pulse voltammogram of the same film in CH3CN electrolyte solution. F


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Macromolecules electronic conductivity behavior of the polymers.50 There is a significant increase in the linearity of oxidative and reductive current versus scan rate curves of CMPs compared to that of poly-H2LI (Figure S5), revealing an increase in electroactivity of the CMP films.50 The absolute conductivity of the polymers was then determined by in situ conductivity using 10 μm Pt interdigitated electrode arrays. This method was first described by Wrighton and co-workers51 to measure the conductivity of polypyrole and was then employed widely to determine the redox conductivity of conducting polymers such as polyanilines or polythiophenes by Murray52−55 and Swager.56−62 The basic principle of this measurement is based on Ohm’s law: as an electric field is applied inside one material, it will cause an electric current to flow. Conductivity (σ), which is the inverse of resistivity (ρ), is defined as the ratio of current density (J) to the applied electric field (E).

σ = 1/ρ = J /E

(2)

In the redox conductivity measurements using interdigitated electrode arrays (IDAs), a conducting polymer is deposited on and in between the fingers of the two interdigitated electrode arrays ensuring that any observed current flow is a result of the electrochemical properties of the polymer being tested. CVs are then recorded with a small offset potential (Vd) between the two electrode arrays. The magnitude of Vd is usually small (40 mV in our experiments) to ensure a linear relationship between the current induced by this voltage and the value of the electrical field, E (eq 2). E is determined as the voltage offset (Vd) divided by the distance (D) between the two sets of electrodes in the array. The measured current in the CV, Id, when obtained from arrays with two sets of interdigitated electrodes is composed of two components: the CV of the polymer and the current flowing between the two sets of electrodes in the array caused by the offset potential. In most cases, the latter current, which reflects the conductivity of the polymer at certain doping states corresponding to the applied oxidative or reductive potential, is much higher than the current from the CV of the polymer, and by approximation the drain current, Id, can be used to calculate the redox conductivity.51−55 It follows that the current density is then calculated from eq 3 J = Id /(nLT )

Figure 7. Cyclic voltammetry (red) and in situ conductivity profile (blue) of ligand polymer, poly-H2LI (A), and nickel complex polymer, poly-NiLI, in 0.1 M TBAPF6/CH2Cl2 solutions.

conductivity profile of poly-NiLI is assigned to the increase in redox conductivity when a Ni2+/3+ redox event creates a mixedvalence state within the CMP which enables electron transfer between the two metal oxidation states facilitated by polaron charge carriers on the oxidized organic bridge. The much higher conductivity of the CMPs compared to the corresponding ligand polymer indicates the crucial role of the metal centers to the charge transport properties of CMPs in general (Figure 8). Electron transport in the ligand polymer and the CMPs is governed by two processes: the interchain and the intrachain charge migration mechanisms. Kingsborogh et al. reported that increasing the sterics of the polymer backbone of copper(II) Schiff-base CMPs leads to the decrease of the redox conductivity in these polymers due to the reduction of interchain interactions.63 Therefore, in the studies reported herein, an organic backbone with relatively small branching substituents on the propylenediamine linker (i.e., −CH3) was selected to result in materials with relatively high redox conductivity enabling more facile studies. To ensure that the effects of interchain interactions are consistent across the series of CMPs studied herein, thus making any observed differences solely the result of intrachain interactions and creating a situation where elucidation of the role of the metal center is possible, the organic component of the CMPs was unchanged in poly-MLI (where M = H2, V(O), Co, Ni, Cu, Zn). In intrachain electron transfer, charge carriers migrate along the polymer chain and are affected by the effective polymer conjugation length and the interaction between the metal

(3)

where the denominator is the total cross-sectional area of polymer between the two sets of electrodes in the array. The total area is calculated from the area of one single crosssectional gap determined by the multiplication of polymer thickness, T, by the length of one finger which is then integrated across the entire device by multiplying by the total number of gaps, n, created by the two sets of interdigitated electrodes. The redox conductivity is obtained by combining eqs 2 and 3 to obtain eq 4. σ=

Id D Vd nTL

(4)

The results of redox conductivity of our polymer series using the aforementioned method are summarized in Table 3. The conductivity profiles of poly-H2LI and all CMPs exhibit one peak at the potential corresponding to the formation of polarons on the thiophene backbone (Figure 7 and Figure S6). Interestingly, there are two peaks in the conductivity profile of poly-NiLI, appearing at the same positions as the second and the third oxidative events in the CV. The second peak in the G


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insulated ligand binding site (Figure 9A). The farther distance for the electron to hop results in less effective electron transfer and lower conductivity of the polymer. This effect is more obvious in the conductivity profile of CMPs when the redox potential of the metal center is much lower than that of the organic backbone. There is no metal-based redox conductivity found in these CMPs due to the long undoped insulated segments of organic backbone between metal centers.26 CMPs that have only one peak in the in situ conductivity profile contain either redox-inactive metal centers (Cu2+ and Zn2+) or redox-active metals (Co2+ and V(O)2+) with no redox matching65 between the redox potential of the organic backbone and that of the metal centers. The intrachain charge transport in these CMPs follows a mechanism described in Figure 9B through electron hopping between organic segments, as in the ligand polymer, and through superexchange over the metal centers. The lower conductivity of poly-CoLI and polyZnLI compared to those of poly-CuLI and poly-VOLI could be due to the unsatisfied coordination sphere contributing from the salpen center in the monomeric forms of CoLI and ZnLI formed in solution under the polymerization conditions. In the monomers this is evidenced by the tendency of these two complexes to form dimeric trinuclear structures as observed in the solid-state crystal structures in order to satisfy the coordination around the metal center. In the CMPs (polyCoLI and poly-ZnLI), the available coordination sites are most likely filled with coordinated solvent or electrolyte molecules therefore complicating the redox processes and potentially leading to less efficient chain packing and concomitant decreases in the degree of interchain interactions in the Coand Zn-containing CMPs. In poly-NiLI, the first maximum in the conductivity profile is associated with the oxidation potential of the organic backbone as in other CMPs. However, this polymer shows an additional conductivity peak which corresponds to a potential where a metal-based redox event takes place. In this oxidized state, the organic backbone is doped and could act as a hopping station, or highly conductive bridge, for charge transport between a mixed-valence state (Ni2+/3+) of the metal centers (Figure 9C). Both strong interchain interaction and intrachain charge transport could account for the highest conductivity observed for the poly-NiLI CMP series. Vis−NIR Absorption Spectroelectrochemistry. To further understand the polymer excited states upon oxidation, spectroelectrochemical analysis was conducted which directly characterizes the properties of the highly conductive states by doping the polymers and interrogating the material by vis−NIR absorption spectroscopy. Figure 10A shows the spectroelectrochemical data of poly-H2LI in the 400−1600 nm region. Upon oxidation, two peaks at 630 and 856 nm grow in and are blue-shifted at higher applied potential. These two peaks are attributed to the formation of radical cations on the thiophene backbone. The phenoxyl radical of the salpen ligand is not expected to have an active transition in the 400−800 nm window.66−69 Further oxidation resulted in a new peak growing in at 770 nm while the two peaks at 630 and 856 nm diminish in intensity. The increase of the 770 nm band and the continuous decrease of the 856 nm band provide support for the conversion of polarons to bipolarons. In the spectroelectrochemical spectra of poly-ZnLI, shown in Figure 10B, polaron and bipolaron bands are red-shifted by about 20 nm relative to poly-H2LI and exhibit peaks at 877 and 795 nm for polaron and bipolaron bands, respectively. The red-

Figure 8. Conductivity profiles of ligand polymer and CMPs in CH2Cl2 electrolyte solution. Scan rate 10 mV/s, 40 mV offset potential.

centers and the conjugated organic backbone. These processes directly reflect the unique behaviors of the different metal centers incorporated into the CMPs and therefore highlight the roles of the metal ions in determining the observed electroactive properties of the CMPs. Based on the conductivity results (Figure 8 and Table 3), the CMPs studied in this work can be classified into three groups: (i) poly-CoLI and poly-ZnLI; (ii) poly-CuLI and poly-VOLI; and (iii) poly-NiLI with maximum conductivity of 1 order of magnitude, 2 orders of magnitude, and 240 times higher than that of poly-H2LI, respectively. The proposed mechanisms for intrachain electron transfer in the ligand polymer and CMPs are shown in Figure 9. In the ligand polymer, the only possible manner for intrachain electron transfer is the hopping of charge carriers from one conjugated segment to another over an

Figure 9. Mechanisms of intrachain electron transfer in Wolf type III64 CMPs: (A) charge transfer via hopping over insulated segments in ligand polymer (top) and CMPs that have metal redox potential lower than that of the organic backbone (bottom); (B) electron transfer in CMP with redox-inactive metals via hopping (top) and superexchange (bottom) mechanisms; (C) electron transfer via superexchange (top) and hopping (bottom) between mixed-valence states of metal centers with higher, but similar, redox potential than that of the organic backbone. H


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Figure 10. Vis−NIR absorption spectroelectrochemistry of poly-H2LI (A), poly-ZnLI (B), and poly-VOLI (C) measured on ITO-coated glass in CH2Cl2 electrolyte solution at various applied potentials between −0.25 and 1.2 V vs Fc/Fc+.

Figure 11. Molecular orbitals of neutral and one-electron-oxidized state of H2LI (A), ZnLI (B), and VOLI (C).

chemical process happens at a potential around 1.1 V and generates more positive charges on the poly-NiLI backbone, causing charge repulsion and reduced delocalization distances of the bipolarons. Spectroelectrochemistry of poly-VOLI, shown in Figure 10C, has the most significant red-shifting of the bipolaron band (ca. 882 nm). The presence of the oxide ligand may cause a better mixing of the orbitals of the vanadium metal center with those of the organic backbone to give a more extended effective conjugation length across which the bipolaron charge carriers can delocalize. Density Functional Theory (DFT) Calculations. In order to provide deeper insights into charge delocalization in the polymer systems, DFT calculations were performed using the B3LYP/SDD basis set to calculate the optimized conformation and the spin distribution in neutral and one-electron-oxidized forms of the ligand and metal complex monomers. Figure 11

shifting in both bands is consistent with contributions from the metal centers to the overall effective conjugation length of the polymer. These bands are shifted farther in other CMPs (Figure S7). However, the bands are overlapping and could not be distinguished from each other. For comparison, we use the well-defined bipolaron band at high potential, ca. 1.2 V. The bipolaron band of poly-NiLI, poly-CuLI, and poly-CoLI appears at longer wavelength (ca. 870 nm), and the shapes are broader than those in the ligand polymer, which are indications of highly delocalized charge carriers in these polymers, a result of extended effective conjugation length in these systems. The spectroelectrochemistry of poly-NiLI shows a dramatic decrease in the intensity of the polaron band and a change in shape of the bipolaron, which becomes much sharper. This could be evidence for the formation of the Ni2+/3+ mixed-valence state with partial oxidation of the Ni metal centers. This electroI


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Macromolecules shows the HOMOs/SOMOs and the LUMOs of H2LI, ZnLI, and VOLI in the neutral and +1 forms (DFT results for other metal complexes can be found in Figure S8). The HOMOs of the ligand molecule show a break in orbital overlap in both the oxidized and neutral forms while continuous conjugation is found in the metal complexes with some contribution from the metal orbitals. The incorporation of the metal centers also helps to planarize the structure and supports extended conjugation, as observed by the red-shifting of the bipolaron band in the spectroelectrochemistry (vide supra). The different features in the spectroelectrochemistry, as well as the low redox conductivity of poly-ZnLI, are explained when carefully considering the molecular orbitals of this complex. Both the HOMO of the neutral form and the SOMO of the +1 form show the majority of electron density on the metal center and the imine−phenoxy moiety, indicating a highly localized radical cation when formed. Unlike the ZnLI system, VOLI shows the distribution of density of states across both the vanadyl center and the organic backbone in the SOMO and HOMO of the neutral and +1 forms, respectively. A significant contribution from the vanadium metal center could explain the increased red-shifting in the vis−NIR spectroelectrochemistry compared to that of the other CMPs (vide supra).

particular, which is crucial for the rational design of functional materials from this compelling class of polymeric materials.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02349. Single crystal X-ray crystallographic data, additional electropolymerization, XPS results, electrochemical and scan-rate dependence data, spectroelectrochemistry, and DFT calculations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.T.N.). *E-mail: [email protected] (R.A.J.). *E-mail: [email protected] (B.J.H.). ORCID

Richard A. Jones: 0000-0003-4174-6530 Present Address

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M.T.N.: Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3113.

CONCLUSIONS In summary, we have prepared, characterized, and studied a series of electropolymerizable complexes incorporating a thiophene-functionalized Schiff-base ligand. CMPs prepared from these monomers allow us to systematically study charge transport and delocalization using in situ conductivity and vis− NIR absorption spectroelectrochemistry. By using metals with different redox properties relative to the redox properties of the organic portion of the polymers, intrachain interactions between the metal center and the organic backbone in CMPs were investigated. Our studies have revealed that the charge transport is most effective in CMPs with redox-active metals that have oxidation potentials higher than but still matching that of the organic backbone (poly-NiLI). The redox matching in such systems facilitates the electron transport process between conjugated organic segments and the metal centers, especially when in a mixed-valence state. CMPs with redoxinactive metals (poly-CuLI and poly-ZnLI) and redox-active metals without the appropriate redox matching (poly-CoLI and poly-VOLI) show electrochemical properties similar to those of the metal-free organic conducting polymer as the charge transfer processes in these materials are dominated by the organic components. Any observed increase in bulk electrochemical properties is most likely due to the structural role of the metal centers (i.e., planarization of the structure or increased intermolecular packing of polymer chains). In all cases, CMPs show much higher redox conductivity than the ligand polymer, poly-H2LI (up to a 240-fold increase in the case of poly-NiLI), due to the contribution of the metal centers to the overall charge carrier delocalization and, in some cases where the incorporated metal ions are judiciously chosen, to the creation of beneficial redox-matching situations which utilize metal center mixed-valence states to efficiently shuttle charge throughout the material. Our charge transport studies of CMPs could be extended to any general Wolf Type III CMPs where the metal centers act as a part of the effective conjugation pathway. The insights provided here should help in understanding the structure−property relationships of CMPs in general and the role the metal centers play in CMPs in

Notes

The authors declare no competing financial interest. B.J.H.: Unaffiliated.

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ACKNOWLEDGMENTS The authors are grateful to the Welch Foundation (F-1631) and National Science Foundation (CHE-0847763) for support of this research.

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REFERENCES

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