Green-to-Red Electrochromic Fe(II) Metallo ... - ACS Publications

Article pubs.acs.org/IC

Green-to-Red Electrochromic Fe(II) Metallo-Supramolecular Polyelectrolytes Self-Assembled from Fluorescent 2,6-Bis(2pyridyl)pyrimidine Bithiophene Sandesh Pai,† Michael Moos,‡ Maximilian H. Schreck,‡ Christoph Lambert,‡ and Dirk G. Kurth*,† †

Chemische Technologie der Materialsynthese, Julius-Maximilians-Universität Würzburg, Röntgenring 11, D-97070 Würzburg, Germany ‡ Center for Nanosystems Chemistry, Institut für Organische Chemie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany S Supporting Information *

ABSTRACT: The structure and properties of metallo-supramolecular polyelectrolytes (MEPEs) self-assembled from rigid 2,6-bis(2-pyridyl)pyrimidine and the metal ions FeII and CoII are presented. While FeL1-MEPE (L1 = 1,4-bis[2,6-bis(2pyridyl)pyrimidin-4-yl]benzene) is deep blue, FeL2- and CoL2-MEPE (L2 = 5,5′-bis[2,6-bis(2-pyridyl)pyrimidin-4yl]-2,2′-bithiophene) are intense green and red in color, respectively. These novel MEPEs display a high extinction coefficient and solvatochromism. Ligand L2 shows a high absolute fluorescence quantum yield (Φf = 82%). Viscosity and static light-scattering measurements reveal that the molar masses of these MEPEs are in the range of 1 × 108 g/mol under the current experimental conditions. In water, FeL1-MEPE forms a viscous gel at 20 °C (c = 8 mM). Thin films of high optical quality are fabricated by dip coating on transparent conducting indium tin oxide (ITO) glass substrate. Optical, electrochemical, and electrochromic properties of the obtained MEPEs are presented. Green to red and blue to colorless electrochromism is observed for FeL2-MEPE and FeL1-MEPE, respectively. The results show that the electrochromic properties are affected by the ligand topology. The Fe-MEPEs show a reversible redox behavior of the FeII/FeIII couple at 0.86 and 0.82 V versus Fc+/Fc and display an excellent redox cycle stability under switching conditions. FeL2-MEPE in its oxidized state exhibits a broad absorption band covering the near-IR region (ca. 1500 nm) due to the ligand-to-metal charge transfer transition originating due to charge delocalization in the bithiophene spacer.

■

INTRODUCTION Electrochromism results from the change of the electronic absorption upon switching between redox states by application of an electrical voltage.1,2 Recently, electrochromic materials (ECMs) have received an increasing attention due to the potential commercial applications, such as antiglare mirrors and glasses,3 protective eyewear,4 smart windows for automobiles and buildings, as well as electronic paper.5,6 A wide number of ECMs have been investigated including metal oxides (TYPE I),7−9 transition metal-complexes (TYPE II),10−13 and organic materials such as conducting polymers14,15 and viologens (TYPE III).16,17 Although these materials have been studied intensively, low switching speeds and prohibitive costs have led research toward more variable ECMs. Recently, an alternative ECM based on metallo-supramolecular coordination polyelectrolytes (MEPEs, TYPE IV) has been developed showing promising properties in terms of reversibility, performance, and fabrication efficiency.18−20 MEPEs are synthesized by metal ion induced self-assembly of organic ditopic ligands, typically based on terpyridine receptors that provide high binding constants and a well-defined coordination geometry. Therefore, straight © XXXX American Chemical Society

rigid bis-terpyridine ligands give rise to linear rigid rod-type macromolecular assemblies that are soluble in aqueous solutions and polar solvents such as alcohols. Redox-dependent transitions in the visible region of the electromagnetic spectrum such as d−d transitions, metal-to-ligand charge transfer (MLCT), and intervalence charge transfer give rise to rich electrochromic properties. The structure of MEPEs and their electrochromic property is readily varied by the rational choice of the metal ions and the design of the ligand. A number of MEPEs have been investigated based on the redox metal ions such as Fe, Ru, Co, Ni, Zn, Cu, Pt and ligands such as 1,4bis(2,2′:6′,2″-terpyridine-4′-yl)benzene (BTPY).18,19,21,22 Finally, because of the polymeric nature of the MEPE, wetchemical processing such as layer-by-layer deposition and dipcoating from aqueous media provides an added advantage for simplified and cost-efficient fabricating thin electrochromic films.23−25 Received: October 18, 2016

A

DOI: 10.1021/acs.inorgchem.6b02496 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Scheme 1. Synthesis of Ditopic 2,6-Bis(2-pyridyl)pyrimidine Ligands L1 and L2 with a Phenyl and 2,2′-Bithiophene Spacer, Respectively, and the Corresponding Monotopic Ligand L3a

a Reagents and conditions: (a) anhydrous THF, rt, 2 d, 71%. (b) KOH, EtOH, reflux, 12 h, 23%. (c) NaOH, H2O/EtOH (1:5), rt, 3 d, 22%. (d) SOCl2, 75 °C, 15 h, 81%. (e) Pd(PPh3)4, anhydrous toluene, 100 °C, 16 h, 40%. (f) anhydrous THF, rt, 2 d, 72%. (g) KOH, EtOH, reflux, 3 h, 45%.

morphology by scanning electron microscopy (SEM), static light scattering (SLS), viscosity data, and electrochromic properties.

In general, terpyridines exhibit a rich coordination chemistry with many elements of the periodic table and have an appropriate binding constant (with transition metal ions) corroborating dynamic macromolecular assembly. The binding constant K1 is considerably smaller than K2, where K1 is associated with the initial coordination of the metal ion and the ligand receptor, while K2 is the consecutive coordination step of the second ligand receptor, which constitutes the chain growth step. Hence, 1,4-bis(2,2′:6′,2″-terpyridine-4′-yl)benzene is the preferred ligand and shows outstanding electrochromic properties with FeII as central metal ion.23,26 The molar mass and chain length of such MEPE depends on the concentration, stoichiometry, de novo design, and the metal ion. Thus far, work on MEPE as ECMs has mostly relied on the metal ion induced self-assembly of BTPY ligand and its structural modification with electron-donating and accepting groups, affecting the properties of MEPEs and its electrochromism.18−20,27−29 The motivation for this study results from the fact that there are no reports so far showing the effect on the viscosity, molar mass, and optical and electrochromic properties of self-assembled MEPEs with pendant 2,6-bis(2pyridyl)pyrimidine (BPP) ligands. So, we envisioned that replacing the central pyridine ring of the BTPY ligand by a pyrimidine ring would favor a more rigid ligand possibly due to the interaction between the C−H and nitrogen lone pair.30 Additionally, the interaction favors a flat ligand topology with a large delocalized π surface bridging adjacent metal ions. In this report, we present the synthesis of 2,6-bis(2-pyridyl)pyrimidine ditopic ligand with phenyl or bithiophene spacer and its corresponding metallo-supramolecular coordination polyelectrolytes assembled using FeII and CoII as metal ions. In particular, we focus on the bithiophene-based BPP-Fe-MEPE, its optical and fluorescent properties, and its polymer structural

■

RESULTS AND DISCUSSION Synthesis of Ligands and Metallo-Supramolecular Coordination Polyelectrolytes. Ligand L1 is synthesized by two condensation reactions. At first, the dienone 4 is synthesized by the aldol condensation of 1,4-diacetylbenzene and 2-pyridinecarboxaldhyde in the presence of an organic base, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). The final condensation of the dienone with pyridine-2-carboximidamide hydrochloride 5 affords ligand L1 with a phenyl spacer (Scheme 1). On the one hand, the utility of the DBU-mediated condensation is reported elsewhere and shows no formation of Michael addition product due to the elimination of metalcontaining inorganic bases such as KOH or NaOH.31 On the other hand, the fluorescent ligand L2 comprising the 2,2′bithiophene spacer is synthesized via Stille coupling of stannyl reagent 9 and 4-chloro-2,6-bis(2-pyridyl)pyrimidine 8. Compound 8 is synthesized by condensation of pyridine-2carboximidamide hydrochloride 5 and the β-keto ester 6 followed by the chlorination in the presence of thionyl chloride, according to the slightly modified method reported previously by Lafferty and Case.32 Ligand L3 is also synthesized by condensation of acetophenone and 2-pyridinecarboxaldhyde following the same protocol as that of L1. To synthesize the MEPEs in a reproducible fashion, conductometric titration is the method of choice used in this study and works promisingly for the charged assemblies. The conductivity of the solution is a function of the stoichiometry, y = [M]/[L], the ratio of the concentration of the metal ion and the ligand.33 In a typical experiment, an equimolar amount of B


Article

Inorganic Chemistry

Figure 1. Metal ion-induced self-assembly of metal ions FeII and CoII and 2,6-bis(2-pyridyl)pyrimidine-based ligands L1 and L2 results in MEPEs. The pseudo-octahedral coordination geometry is indicated by the wedges. 4-Phenyl-2,6-bis(2-pyridyl)pyrimidine (L3) forms the mononuclear metal complex [Fe(L3)2]2+ with FeII. The acetate counterions are omitted for clarity. The picture to the right show the colors of the solutions of FeL1MEPE (water, 80 μM, deep blue), FeL2-MEPE (ethanol, 80 μM, dark green), CoL2-MEPE (water, 80 μM, wine red), and FeL3 metal complex (water, 80 μM, purple).

Figure 2. Electronic absorption spectra of ligands L1−L3 in DCM (a), FeL1-MEPE, CoL2-MEPE, and FeL3 metal complex in water, and FeL2MEPE in ethanol (b).

Table 1. Ultraviolet−Visible Spectroscopic Properties of Ligands L1−L3a and Its Corresponding FeII and CoII-MEPEsa entries

π−π* λmax [nm];

MLCT λmax [nm];

ελmax [M−1 cm−1]b

ελmax [M−1 cm−1] H2O

L1 L2 L3 FeL1-MEPE FeL2-MEPE CoL2-MEPE FeL3-MC

328 413 308 340 437 281 321

(34 100), 281 (52 500) (53 200),c 277 (44 500), 241 (44 900) (11 900), 275 (28 800), 250 (25 600) (29 900), 285 (34 300) (35 600),c 279 (40 900) (34 900) (31 500), 285 (41 800)

626 (18 000) 510 (45 700) 585 (9700)

EtOH

599 632 457 586

(15 300) (26 800) (49 200) (8200)

MeOH

600 631 466 584

(13 200) (26 400) (51 100) (8400)

AcOH

624 655 470 585

(18 900) (29 600) (46 200) (9000)

Determined at room temperature. bThe λmax and εmax values refer to the π−π* transition for the ligands L1 (30 μM), L2 (25 μM), and L3 (60 μM) in DCM, to FeL1-MEPE (60 μM), CoL2-MEPE (25 μM), FeL3 metal complex (60 μM) in water, and to FeL2-MEPE (40 μM) in ethanol. cThe λmax and εmax values represent the transition from the thiophene-conjugated π-system in ligand L2 and FeL2MEPE. a

C


Article

Inorganic Chemistry

Figure 3. (a) Normalized absorption (solid line) and fluorescence (dotted line) spectrum of ligand L2 in DCM (λexc = 413 nm). (b) Change in fluorescence intensity of L2 upon titration with Fe(BF4)2 in MeOH (λexc = 420 nm). The concentration of L2 is 21.13 μM (black line), and the consecutive lines represent the addition of Fe(BF4)2 in a step of 0.2 equiv. The samples were stirred for 24 h for equilibration. (c) The photographic image shows the colors of L2, FeL2-MEPE, and its fluorescence at an excitation wavelength of 365 nm in DCM and MeOH, respectively. (d) Fluorescence decay curve of L2 in DCM upon excitation at 413 nm. Excitation energy: 24 200 cm−1.

The absorptions in the UV region (∼250 and ∼280 nm) of the spectrum are due to the π−π* and n−π* transitions inherent to BPP.30 The dominant absorption at 413 nm is tentatively attributed to the thiophene-conjugated π-system of L2. As is typically the case for FeII-based MEPEs, the UV−vis spectra of FeL1-, FeL2-, and CoL2-MEPEs and FeL3MC are dominated by MLCT transitions. All the MEPEs reported herein, including the metal complex, show absorption bands below 350 nm, attributed to 2,6-bis(2-pyridyl)pyrimidine ligand’s π−π* transitions. An intense absorption band at ∼630 nm is assigned to the MLCT bands of FeL1 and FeL2MEPE (Figure 2). On the one hand, FeL2-MEPE shows another strong absorption band at ∼440 nm, which is attributed to the π−π* transition of the conjugated thiophene spacer.34 On the other hand, CoL2-MEPE shows a distinct band centered at 510 nm, the origin of which is not yet clear. This band may be associated with a shift of the π−π* transitions of the thiophene spacer, d−d, or an MLCT transition. On the one hand, such ambiguity has also been reported in other Co-MEPEs and Co-metal complexes.35−37 On the other hand, FeL3 metal complex exhibits a characteristic absorption maximum centered at 585 nm due to the MLCT band. The absorption data of the previously reported Fe-MEPE based on BTPY ligand show a characteristic band

ligand in 75% acetic acid is condutometrically titrated by a solution of metal acetate (FeII and CoII) in 75% acetic acid, which results in a decrease of the conductivity and reaches a minimum at y = 1 (Figure S1). Approaching a stoichiometric ratio of 1:1 results in an increasing viscosity of the solution indicating the formation of macromolecular species. For simplicity, the following nomenclature will be used in this work to address the different MEPEs: FeL1-MEPE refers to FeII metal ion-induced self-assembly of ligand L1, FeL2-MEPE is the assembly of FeII and ligand L2, CoL2-MEPE is the assembly of CoII and ligand L2. The mononuclear [Fe(L3)2]2+ complex is also synthesized from FeII metal ion and ligand L3 and serves as a reference compound (Figure 1). Optical Spectroscopy: Ultraviolet/Visible Absorption and Fluorescence. The optical properties of ligands L1−L3 and their corresponding FeII- and CoII-MEPEs, as well as the FeL3 metal complex, are studied by UV−vis absorption spectroscopy. The electronic absorption spectra of the ligands and the corresponding MEPEs are shown in Figure 2, and the spectroscopic data with prominent absorption maxima are summarized in Table 1. While monotopic L3 and ditopic ligand L1 are off-white, ligand L2 containing a 2,2′-bithiophene spacer shows a distinct orange color. The redshift is apparent in the UV−vis spectra recorded in dichloromethane (DCM) solution. D


Article

Inorganic Chemistry Table 2. Fluorescence Data of L2 and FeL2-MEPE FL excit, λmax [nm]; ν̃ [cm−1]

FL emiss, λmax [nm]; ν̃ [cm−1]

Stokes shift Δλmax [nm]

Φfd

L2 FeL2-MEPEb

413 (24 200) 420 (23 800)

462 (21 600) 491 (20 400)

49 71

0.82 0.04

2,2′-bithiophenec

302 (33 100)

361 (27 700)

59

0.02

entries a

τ [ns]e 0.44 (0.55) 1.15 (0.45) 0.07 (0.34) 0.59 (0.47) 1.32 (0.19) nr

τ̅ [ns]f 0.59 (0.77) 0.54 (0.75) nr

Fluorescence values in DCM. bFluorescence values in methanol. cFluorescence data were obtained from ref 39. dThe absolute fluorescence quantum yield determined using an integrating sphere by Edinburgh Instruments FLS 980 in DCM and MeOH for L2 and FeL2-MEPE, respectively. eMultiexponential fit of fluorescence decays measured by TCSPC. The relative amplitudes are given in brackets. fLifetimes acquired by stretched exponential analysis of fluorescence decays measured by TCSPC. Stretching exponent are given in brackets. nr: not reported. a

similar to the FeL3 metal complex.18 The repulsion of the lone pairs of the heteroatoms favors an anti-conformation, which is further stabilized by intermolecular C−H···S and C−H···N interactions.38 The thiophene unit acts as a π-conjugated bridge with higher delocalized π surface giving rise to the red shift of the MLCT bands of these MEPEs compared to the earlier reported Fe-MEPEs with a phenyl spacer in the ditopic ligand. A subtle effect of the ligand structure on the optical properties of the resulting metallo-supramolecular assemblies is clearly recognized by the distinct colors displayed by these MEPEs. In solution, the FeL1-MEPE is deep blue, FeL2-MEPE is dark green, CoL2-MEPE is wine red, and FeL3MC is purple in color (see Figure 1). Fluorescence. The fluorescence measurement of ligand L2 is performed in DCM, and that of FeL2-MEPE is performed in methanol. The corresponding spectra are displayed in Figure 3, and the fluorescence data are summarized in Table 2. Ligand L2 shows a distinctive fluorescence due to the presence of bithiophene linker, similarly to the previously published ditopic triazine ligand.39 The fluorescence spectrum of L2 (Figure 3a) shows a broad emission band with a maximum centered at 462 nm (∼21 600 cm−1) with a Stokes shift of 2600 cm−1; this band can be assigned to the emission from the intraligand π−π* transition. For comparison, 2,2′-bithiophene shows an emission at 361 nm (Table 2) indicating the influence of the BPP on the optical properties.40,41 FeL2-MEPE, upon excitation (λexc = 420 nm; Figure S3), shows an intense and broad fluorescence band at 491 nm (∼20 400 cm−1) in methanol with an apparent Stokes shift of 3400 cm−1. The absolute fluorescence quantum yield of L2 is 82% (Φf = 0.82) and is significantly higher in comparison to 2,2′-bithiophene (Φf = 0.02; Table 2) due to the electron-deficient BPP unit in the ligand. This increase in quantum yield is mainly caused by an intramolecular photoinduced electron transfer in the excited states.40,42 However, the Φf of pyrimidine is slightly higher than that of the pyridine unit (Φf = 0.78). Compared to the ligand solution, the absolute quantum yield of FeL2-MEPE solution is significantly lower (Φf = 0.04). As shown in Figure 3b, the fluorescence intensity of a solution of L2 (λem = 462 nm) decreases upon addition of Fe(BF4)2 solution in methanol. However, even at an excess of Fe(II) metal ion (1:2) a weak fluorescence of L2 is still detected. We suggest that the remaining fluorescence is due to the uncoordinated chain ends. This result is in agreement with the reported results for Fe(II) quenching, due to ligand-tometal charge transfer (LMCT) mechanism leading to a nonradiative deactivation of the excited fluorophore L2.43−46 To understand the emission properties, decay profiles of the fluorescence properties of the ligand and FeII-MEPE were measured in a nanosecond time regime by time-correlated single-photon counting (TCSPC) photomultiplier tube (PMT) technique at an excitation wavelength of 413 nm for L2 (Figure

3d) and 420 nm for FeL2-MEPE, using a picosecond laser diode. Both ligand L2 and FeL2-MEPE show multiexponential decays with similar amplitudes (Table 2). Additionally, a stretched exponential fit was performed using Kohlrausch equation with the exponent β close to 0.8 (Figure 3d and Table 2).47 Lifetimes acquired by stretched exponential analysis are in the range of sub-nanosecond. The lifetime is slightly higher than those of the previously reported BTPY ligands.48 Viscosity of MEPE Solutions. Viscosity properties reveal the details about the macromolecular nature of MEPEs in solution and play a crucial role for the fabrication of thin films by wet-chemical coating procedures.25 Because viscosity is a function of chain length, it is a direct measure of the molecular weight, for example, as a function of stoichiometry and concentration.49 In the following, we present the dynamic and kinematic viscosity of FeL1-MEPE, FeL2-MEPE, and CoL2-MEPE as a function of concentration in water and ethanol, respectively (Figure 4). The viscosity η of FeL1-MEPE (Figure 4a) in water increases exponentially from 1.63 ± 0.01 to 14.37 ± 0.14 mPa·s, when moving from a concentration of 5 to 8 mM under the same sample equilibration time (∼24 h). Unlike the previously reported data on MEPEs, wherein the viscosity values are reported in 75% acetic acid and ethanol, the present MEPE reported herein shows a high viscosity in water. The increasing viscosity can be conclusively attributed to the high molecular mass (Table 3). Next we study the viscosity of solutions containing FeL1MEPE and Polyox WSR N-80 [a commercially available poly(ethylene oxide) (PEO)]. Polyox is known for its excellent film-forming characteristics, good electrochemical stability, and compatibility with lithium salts.50−53 Thus, it is a potential candidate as polymer hosts in electrochromic devices. In the presence of 1% polyox, FeL1MEPE shows almost a twofold increase in the viscosity with η = 26.47 ± 1.68 mPa·s at a concentration of 8 mM (Figure 4b). Apart from the contribution of polyox to the viscosity of the solution, additional interactions between the water molecules, polyox, and MEPE may further enhance the viscosity. This is evident by the formation of gel by FeL1-MEPE (without polyox) in water (8 mM) when placed overnight at 20 °C (Figure S4). Next, we investigated the time-dependent viscosity of FeL1MEPE in water both in presence and absence of Polyox WSR N-80 at a concentration of 8 mM. Over a period of 18 h, the FeL1-MEPE displays an increase in viscosity from 14 to 24 mPa·s in water (Figure 4c). However, the viscosity showed a plateau at η = 18 mPa·s, when measured in a time interval of 15 min, but increased gradually to η = 24 mPa·s when the time interval was increased to 30 min. This result is important because it suggests that the equilibration time is an important factor. Even if the individual bond formation between the metal ion and the ligand is fast, the large number of binding events in E


Article

Inorganic Chemistry

Figure 4. Dynamic and kinematic viscosity in water as a function of concentration of FeL1-MEPE (a) and in the presence of Polyox WSR N-80 (b). For comparison, the viscosity of water, and water with 1% Polyox WSR N-80 is shown (gray line). Time-dependent viscosity in water of FeL1MEPE (c) and in the presence of 1% Polyox WSR N-80 (d) at a concentration of 8 mM. Viscosity of FeL2-MEPE (e) and CoL2-MEPE (f) in ethanol and water, respectively, at different concentration. The data on Y-axis at c = 0 mM represents the viscosity of ethanol. The viscosity data points are an average of three independent measurements performed at 20 °C.

Table 3. Molecular Parameters for FeL1-, FeL2-, and CoL2-MEPE from Static Light Scattering MEPEs

M̅ w [g/mol]a

Rg [nm]b

Rh [nm]c

A2 [mol·L g−2]d

ρe

DPwf

FeL1-MEPEg FeL2-MEPEh CoL2-MEPEg

7.27 × 108 29.97 × 108 0.17 × 108

168.20 166.63 154.33

267.38 157.56 242.49

0.62 × 10−9 0.16 × 10−9 26.86 × 10−9

0.63 1.06 0.64

10.14 × 105 37.24 × 105 0.21 × 105

a

Weight-average molar mass of MEPEs. bRadius of gyration. cHydrodynamic radius. dSecond virial coefficient. eRatio of radii of gyration and hydrodynamic radii (Rg/Rh). fWeight-average degree of polymerization. gSLS data obtained in water. hSLS data obtained in ethanol.

F


Article

Inorganic Chemistry MEPEs require sufficient equilibration time to reach the full chain length. Not surprisingly, the addition of 1% polyox results in almost a fourfold increase of the viscosity from η = 30 to 110 mPa·s (Figure 4d). Notably, the dynamic viscosity of FeL1-MEPE is significantly lower than that of FeL2-MEPE even at concentrations of 20− 40 mM. The viscosity of FeL1-MEPE in ethanol (Figure S6) reaches only 3.3 mPa·s, whereas FeL2-MEPE gives a viscosity of η = 29 mPa·s in ethanol (Figure 4e). Similarly, the viscosity of CoL2-MEPE in ethanol increases exponentially and reaches 44 mPa·s at 14 mM (Figure 4f). Apparently, the presence of the bithiophene spacer significantly increases the interaction of L2 with the metal ion. Finally, we conclude that through the ligand design, the assembly conditions, and the choice of solvents it is possible to control the viscosity of MEPE solution. Light Scattering. On the one hand, SLS technique provides a useful account of molar mass Mw, radius of gyration Rg, and second virial coefficient A2 of polymers and particles. The dynamic light scattering (DLS), on the other hand, provides information about the size distribution.54−56 Thus, the combination of static and dynamic light scattering methods is used to investigate the dynamics, size, and architecture of FeL1-, FeL2-, and CoL2-MEPE in solution. The lightscattering experiments are performed at 23 °C in diluted solutions of 1−5 mg·ml−1. The data are evaluated by Guinier− Zimm plot (Figure S12) using eq 1,57,58 where K is optical constant, Rθ is the Rayleigh ratio, M̅ w is the weight-average molar mass of the polymer, Rg is the radius of gyration, q is the scattering vector, A2 is the second virial coefficient, and c is the concentration of the MEPE polymeric solution (for details concerning theory of SLS, see Supporting Information). ⎛ ⎞ ⎛ Kc ⎞ 1 ln⎜ ⎟ = ln⎜⎜ + 2A 2 c ⎟⎟ 2 2 −1/3R g q ⎝ Rθ ⎠ ) ⎝ M̅ w (e ⎠

Figure 5. A plot of log Rg vs log M̅ w of FeL1-, CoL2-, and FeL2MEPE in water and ethanol, respectively, at a temperature of 23 °C according to eq 2. The slope represents the coefficient ν.

Thin Films and Surface Morphology. Films are prepared by dip coating of solution of FeL2-MEPE in ethanol onto an ITO (indium tin oxide) coated glass substrate (1.6 cm × 2.5 cm). Here, we decided to use ethanol as solvent because it evaporates more quickly than water and we know from previous studies that films cast from ethanol have very good optical properties. The effect of the withdrawing speed (50 and 100 mm·min−1) and the concentration of the dipping solution (10, 12, 14, 16 mM) on the film properties are also studied, since these parameters affect the film thickness according to the Landau−Levich equation.60,61 The SEM images of FeL2MEPE films (Figure 6) show a smooth homogeneous and defect-free surface. At lower concentrations, the images reveal fiberlike structures (Figure 6A,E), but the films evidently become smoother at higher concentrations. Nonetheless, the homogeneity of the films prepared at different withdrawing speed is retained. The UV−vis spectra of the fabricated thin films prepared from FeL1-, FeL2-, and CoL2-MEPE are also measured. Both the FeL1-MEPE (ε644 nm = 12 300 M−1 cm−1) and FeL2-MEPE (ε642 nm = 25 700 M−1 cm−1) films reveal a red shift in the MLCT band (Figures S8 and S9) with a transmittance value of less than 10%. Electrochemical and Spectro-Electrochemical Properties. The electrochemical properties of the MEPEs are investigated by cyclic voltammetry. The MEPE-coated ITO glass substrate was used as working electrode in a threeelectrode setup with a leak-free Ag/AgCl as reference electrode and Pt wire as the counter electrode. All voltammograms were referenced to ferrocene by measuring a blank ITO electrode against Ag/AgCl. The cyclic voltammograms (CVs) of the MEPEs recorded in 0.2 M tetrabutylammoniumn hexafluorophosphate (TBAH) in anhydrous DCM are displayed in Figure 7, and its redox values are summarized in Table 4. The FeII-MEPEs exhibit a reversible redox reaction of the FeII/FeIII couple at 0.86 and 0.82 V. There is no significant effect of the spacers (phenyl vs bithiophene) in the redox potentials of FeL1- and FeL2-MEPE. The difference in the anodic and cathodic peak potentials, Δφp, is 0.14 and 0.35 V for FeL1- and FeL2-MEPE, respectively. Under ideal conditions one would expect the anodic wave to be a mirror image of the cathodic wave. The nonideal behavior reflects the high thickness of the films, which leads to high uncompensated resistance and hence to peak splitting and deviation from symmetric shapes (Figure 7c,f and Figures S13 and S14).62 However, the ratio of the anodic to cathodic peak currents ia/ic is ∼1 indicating a close to

(1)

The weight-average molar mass M̅ w of FeL1-, FeL2-, and CoL2-MEPE as determined by SLS is found to be 7.3 × 108, 30 × 108, and 0.2 × 108 g/mol, respectively (Table 3). The hydrodynamic radii of the MEPE chains amount to be 267, 157, and 242 nm. The weight-average molar mass M̅ w of FeL1-, FeL2-, and CoL2-MEPE as determined by SLS is found to be 7.3 × 108, 30 × 108, and 0.2 × 108 g/mol, respectively (Table 3). Assuming a linear MEPE chain, the weight-average degree of polymerization (DPw) for the FeL1-, FeL2-, and CoL2MEPEs is in the range of 1 × 105. The second virial coefficient, A2, provides information about the interaction of MEPEs with the solvent. The experimentally determined values of A2 > 0 indicate that the interaction between MEPEs and solvent molecules are favored over those among MEPEs. Hence, the solvents (ethanol and water) under study can be referred to as “good solvent”. Moreover, the high DPw results in an increasing M̅ w and decreasing A2, which is a common phenomenon in polymer chemistry. In other words, an increase in worm/coillike structure is expected with increasing M̅ w.54 This is also evident from the ρ values obtained and is significant for FeL2MEPE. Further, using the linearized eq 2, the dependence of M̅ w versus Rg is shown in Figure 5 and results in the coefficient ν = 0.16 ± 0.01 for FeL1-, 0.18 ± 0.02 for FeL2-, and 0.14 ± 0.01 for CoL2-MEPE. This value is much lower compared to other polymers such as PEO most likely due to its high M̅ w (∼1 × 108) and excluded volume effect.59 log(R g ) ≈ ν·log M̅ w

(2) G


Article

Inorganic Chemistry

Figure 6. SEM images of FeL2-MEPE film surface on ITO-coated glass prepared by dip coating with a solution concentration of 10, 12, 14, and 16 mM in ethanol, at a withdrawing speed of 50 mm·min−1 (A−D) and 100 mm·min−1 (E−H). The photographic image (I) of FeL1-MEPE (deep blue, 30 mM), FeL2-MEPE (dark green, 14 mM), and CoL2-MEPE (red, 14 mM) films prepared by dip coating at a withdrawing speed of 100 mm· min−1 on an ITO-coated glass (dimension: 1.6 cm × 2.5 cm).

appear at ca. 530 and 1500 nm (Figure 8c). The broad intense band at 530 nm is assigned to the red-shifted π−π* transition of the bithiophene ligand together with the hidden d−d transition. The less intense band in the near-IR region at ca. 1500 nm is assigned to the low-energy LMCT transition originating due to the charge delocalization in the bithiophene chain resulting in the energy level of the thiophene donor to lie close to the FeIII acceptor upon oxidation. Such behavior has been reported in ferrocene−thiophene-based monomers, which are commonly known for enhancing charge-transfer (CT) states.64−66 In addition, the recovery of the MLCT band shows no loss in the absorption value indicating good reversibility of FeL2-MEPE (Figure 8d).

reversible redox couple. Hence, we predict that the equilibration between the oxidized and reduced species in the MEPE film is slowed and also scan-rate dependent consistent with quasi-reversible redox reactions in thin films. ECMs should possess a good redox cycle stability under switching conditions. The CVs of the Fe-MEPE films on ITO-coated glass is monitored over 50 redox cycles (Figure 7d and Figure S15). Although a slight change in the initial CV envelop is observed, indicating a formation process, the CV envelop remains unchanged throughout. The electrochromic nature and the switching process of the MEPEs is examined by monitoring the absorbance change (λmax) in the visible wavelengths as a function of applied potential. The in situ spectro-electrochemical measurements were performed by using a transmission quartz cell in a threeelectrode compartment with MEPE-coated ITO as working electrode, AgCl-coated silver wire as reference, and gold-coated V2A as a counter electrode. All spectra were recorded in 0.2 M TBAH in anhydrous DCM. Upon stepping the potential from 0.0 to +1.6 V, the absorbance of the MLCT band of FeL1MEPE (Figure 8a) at 620 nm decreases and almost disappears (FeII→FeIII). Subsequently stepping back the potential results in the recovery of the MLCT band (Figure 8b). The recovery of the MLCT band is slightly slower, most likely due to the optical memory effect of the Fe-MEPE layers, and is also observed in previously reported MEPE films.63 Similarly, the MLCT band of FeL2-MEPE at 636 nm completely disappears upon increasing the voltage incrementally, while two new bands

■

CONCLUSION In search of new metallo-supramolecular polyelectrolytes as attractive ECMs, three distinct MEPEs based on FeII and CoII were self-assembled from a novel BPP ligand with a phenyl or bithiophene spacer. The investigation is motivated to favor a rigid and flat ligand topology with a large delocalized π surface bridging adjacent metal ions. This has resulted in the red shift of the dominant MLCT bands of the MEPEs displaying distinct deep blue, dark green, and wine red colors for FeL1-, FeL2-, and CoL2-MEPE, respectively, with high extinction coefficient. As a proof of principle, we have investigated the photophysical and electrochemical properties of the presented MEPEs. The introduction of bithiophene spacer leads to tunable absorption and emission properties, with high absolute quantum yield (Φf H


Article

Inorganic Chemistry

Figure 7. Cyclic voltammograms of FeL1-MEPE (a) and FeL2-MEPE (b) recorded from thin films on ITO-coated glass (dimension: 0.7 cm × 2.5 cm) at room temperature with a scan rate of 20 mV·s−1. Electrolyte: 0.2 M TBAH in anhydrous DCM. Counter electrode: platinum wire. Reference electrode: Ag/AgCl. φa is the anodic and φc is the cathodic peak potential. Cyclic voltammogram of FeL2-MEPE film at different scan rates (c) and redox stability (d) after 1, and 50 redox cycles at a scan rate of 20 mV·s−1. Linear increase of the anodic and cathodic peak currents as a function of increased scan rate in FeL2-MEPE film (e). Anodic and cathodic peak currents of FeL2-MEPE film as a function of square root of the scan rate (f).

= 0.82). The MEPEs reported herein show a high viscosity in water and ethanol due to the increasing interaction of the ligand and the metal ion. With the aid of static and dynamic light scattering, the weight-average molar mass M̅ w of the MEPEs was determined and is in the range of 1 × 108 g/mol. The Fe-MEPE films show excellent electrochromic properties with regard to applied potential, optical contrast, switching, and long-term stability. In summary, our study confirms that a

careful and controlled design of ligand (pyrimidine vs pyridine) offers the possibility to tune and improve the optoelectronic and electrochromic properties of metallo-supramolecular polymers. Currently, efforts are being made to prepare a variety of novel ligands derived from electron-donating and withdrawing groups based on BPP and will now investigate the structure−property relationship of these MEPEs. I


Article

Inorganic Chemistry

MHz; 13C: 75.48 MHz) at ambient temperature in chloroform-d, dimethyl sulfoxide-d6 (DMSO-d6), and methanol-d4. Chemical shifts (δ) are given (in ppm) downfield from tetramethylsilane and are referenced to the solvent signal (1H: CDCl3: δ 7.26 ppm, (CD3)2SO: 2.50 ppm, CD3OD: 3.31 ppm; 13C: CDCl3: δ 77.16 ppm, (CD3)2SO: 39.52 ppm, CD3OD: 49.00 ppm).68 Coupling constants J are given (in Hz). Individual peaks are marked as singlet (s), doublet (d), doubletof-doublet (dd), triplet (t), or multiplet (m). When necessary, assignments were determined by HSQC (1-bond 1H−13C correlation) and HMBC (2/3-bond 1H−13C correlation) two-dimensional experiments. Mass spectra were recorded with a Bruker Daltonics microTOF focus electrospray ionization (ESI) mass spectrometer with a nebulizer pressure of 0.3 bar, a dry gas flow of 4 L/min, a dry gas temperature of 180 °C, and a solvent flow rate of 4−5 μL/min. Only characteristic fragments are given for the most abundant isotope peak. Data were recorded in positive ion mode. IR spectra were recorded as pure solid samples by using a Jasco FT/IR-4100 spectrometer equipped with a Pike MIRacle Micro ATR unit. Elemental analysis (C, H, N) was performed with a Vario MICRO cube Elemental Analyzer. UV−vis absorption spectra were measured with a Varian Cary 50 UV−vis spectro-photometer in quartz cuvettes (d = 1 cm). The SEM was performed with a field emission scanning electron microscope Crossbeam 340 (Zeiss, Oberkochen, Germany). Because of rather low acceleration voltage (1 kV) imaging could be performed without preceding gold coating.

Table 4. Electrochemical Properties of FeL1- and FeL2MEPEa MEPEs

Eoxid [V]

Ered [V]

E1/2 [V]b

FeL1-MEPE FeL2-MEPE

0.93 1.00

0.79 0.65

0.86 0.82

a CVs recorded from thin films coated on an ITO glass at room temperature in an electrolyte solution of 0.2 M TBAH in anhydrous DCM using platinum wire as counter electrode and Ag/AgCl as a reference electrode. Scan rate: 100 mV·s−1. bE1/2 = 0.5(Eoxid + Ered), where Eoxid and Ered are the oxidation and reduction peak potentials, respectively. The potentials are referenced to ferrocene (Fc) by measuring a blank ITO substrate against Ag/AgCl.

■

EXPERIMENTAL SECTION

Materials and Methods. Reactions were performed in oven-dried Schlenk glassware under a dry and oxygen-free argon atmosphere when necessary. Solvents were dried with molecular sieves (activated for 24 h at 200 °C) overnight and degassed prior to use. All reagents were purchased from Sigma-Aldrich, Acros Organics, and abcr GmbH and used without further purification. 4-Hydroxy-2,6-bis(2-pyridyl)pyrimidine (7),32 4-chloro-2,6-bis(2-pyridyl)pyrimidine (8),32 4phenyl-2,6-bis(2-pyridyl)pyrimidine (L3),31,32 and Fe(OAc)2,67 were synthesized according to modified literature procedures. NMR spectra were recorded with a Bruker Fourier 300 spectrometer (1H: 300.18

Figure 8. Spectro-electrochemical measurements showing changes at visible wavelengths of FeL1-MEPE (a, b), and FeL2-MEPE (c, d) thin films on ITO-coated glass (dimension: 1.6 cm × 2.5 cm) at room temperature at various applied potentials in 0.2 M TBAH in anhydrous DCM. The corresponding photographs display the color of the thin films in its oxidized and reduced state (inset). J


Article

Inorganic Chemistry Synthesis of 1,4-Bis[3-(pyridine-2-yl)-propene-1-one]-phenylene (4). 1,4-Diacetylbenzene 1 (5.21 g, 32 mmol) and DBU 3 (11.96 mL, 80 mmol) were added to a solution of 2-pyridinecarboxaldehyde 2 (10.65 mL, 112 mmol) in anhydrous tetrahydrofuran (THF; 100 mL). The mixture was stirred at room temperature for 48 h under argon. The pale yellow product that precipitated was filtered off, washed with tetrahydrofuran (3 × 20 mL), and dried under vacuum. Yield: 7.65 g (22.47 mmol, 71%). 1H NMR (300.18 MHz, CDCl3): δ 8.71 (m, 2H, H6, H6′), 8.20 (s, 4H, H11, H12, H11′, H12′), 8.15 (s, 1H, H8), 8.10 (s, 1H, H8′), 7.83 (s, 1H, H7), 7.78 (s, 1H, H7′), 7.76 (dt, 3J = 7.7 Hz, 2H, H4, H4′), 7.50 (d, 3J = 7.8 Hz, 2H, H3, H3′), 7.32 (ddd, 3J = 4.7 Hz, 4J = 1.1 Hz, 2H, H5, H5′) ppm. 13 C NMR (75.48 MHz, CDCl3): δ 190.11 (C9, C9′), 153.09 (C2, C2′), 150.43 (C6, C6′), 143.78 (C7, C7′), 141.20 (C10, C10′), 137.10 (C4, C4′), 129.07 (C11, C11′, C12, C12′), 125.82 (C3, C3′), 125.39 (C8, C8′), 124.82 (C5, C5′) ppm. IR (ν/cm−1, ATR): 3045 (m, C C−H), 1657 (s, CO), 1605 (s, alkene CC), 1323 (s), 1213 (s), 983 (s), 761 (s). MS (ESI+, CH3CN): m/z 341.12 [M + H]+. Anal. Calcd for C22H16N2O2: C 77.63, H 4.74, N 8.23. Found: C 77.44, H 4.95, N 8.22%. Synthesis of 4-Hydroxy-2,6-bis(2-pyridyl)pyrimidine (7).32 Ethylpicolinoyl acetate 6 (1.28 g, 6.45 mmol) and sodium hydroxide (0.26 g, 6.34 mmol) were added to a solution of pyridine-2carboximidamide hydrochloride 5 (1.02 g, 6.34 mmol) in ethanol (25 mL). The reaction mixture was stirred at room temperature for 72 h and was concentrated to 10 mL under reduced pressure. Then, water (40 mL) was added, and the product was crystallized from alcohol overnight upon cooling. The precipitate was filtered, washed with water (2 × 5 mL), and dried under vacuum to afford 7 as white solid. Yield: 0.35 g (1.41 mmol, 22%). 1H NMR (300.18 MHz, DMSO-d6): δ 12.27 (s, 1H, OH), 8.78 (m, 1H, H3), 8.74 (m, 1H, H3′), 8.59 (d, 3J = 7.9 Hz, 1H, H6), 8.52 (d, 3J = 7.9 Hz, 1H, H6′), 8.11 (td, 3J = 7.8 Hz, 4J = 1.7 Hz, 1H, H5), 8.03 (td, 3J = 7.7 Hz, 4J = 1.8 Hz, 1H, H5′), 7.69 (ddd, 3J = 4.8 Hz, 4J = 1.1 Hz, 1H, H4), 7.56 (ddd, 3J = 4.7 Hz, 4J = 1.1 Hz, 1H, H4′), 7.28 (s, 1H, H9) ppm. 13C NMR (75.48 MHz, DMSO-d6): δ 162.05 (C7), 159.27 (C10), 154.89 (C2′), 152.70 (C2), 149.75 (C3′), 149.18 (C3), 148.48 (C8), 138.07 (C5′), 137.53 (C5), 126.92 (C4′), 125.56 (C4), 122.81 (C6′), 121.79 (C6), 109.97 (C9) ppm. IR (ν/cm−1, ATR): 3310 (s, O−H), 1686 (s), 1603 (s), 1544 (s), 1460 (s), 997 (s), 739 (s). MS (ESI+, CH3OH): m/z 251.09 [M + H]+. Anal. Calcd for C14H10N4O: C 67.19, H 4.03, N 22.39. Found: C 67.54, H 3.94, N 21.75%. Synthesis of 4-Chloro-2,6-bis(2-pyridyl)pyrimidine (8). 4Hydroxy-2,6-bis(2-pyridyl)pyrimidine 7 (0.88 g, 3.52 mmol) was added slowly to the freshly distilled thionyl chloride (30 mL) under an inert atmosphere. The reaction mixture was heated to reflux and stirred for 20 h. Thionyl chloride was removed by distillation, and 10% sodium hydroxide (100 mL, w/v) was added. The mixture was stirred at room temperature for 3 h, after which the product was extracted with diethyl ether (3 × 30 mL). The combined ether extracts were dried over anhydrous sodium sulfate and filtered, and the solvent was removed under reduced pressure to give the product as an off-white solid. Yield: 0.76 g (2.84 mmol, 81%). 1H NMR (300.18 MHz, CDCl3): δ 8.89 (m, 1H, H3), 8.74 (m, 1H, H3′), 8.68 (d, 3J = 7.9 Hz, 1H, H6), 8.62 (d, 3J = 8.0 Hz, 1H, H6′), 8.44 (s, 1H, H9), 7.90 (m, 2H, H5, H5′), 7.45 (m, 2H, H4, H4′) ppm. 13C NMR (75.48 MHz, CDCl3): δ 165.17 (C8), 164.18 (C7), 163.38 (C10), 153.90 (C2′), 152.97 (C2), 150.44 (C3′), 149.82 (C3), 137.37 (C5′), 137.11 (C5), 126.14 (C4′), 125.48 (C4), 124.32 (C6′), 122.66 (C6), 117.06 (C9) ppm. IR (ν/cm−1, ATR): 3050 (w), 1560 (s), 1520 (s), 1370 (s), 1320 (s), 990 (m), 825 (s). MS (ESI+, CH3OH): m/z 269.05 [M + H]+. Anal. Calcd for C14H9ClN4: C 62.58, H 3.38, N 20.85. Found: C 61.78, H 3.59, N 20.67%. Synthesis of 2-Pyridalacetophenone (11).31 Acetophenone 10 (3.85 g, 32 mmol) and DBU 3 (4.79 mL, 32 mmol) were added to a solution of 2-pyridinecarboxaldehyde 2 (6.08 mL, 64 mmol) in anhydrous tetrahydrofuran (80 mL). The mixture was stirred at room temperature for 48 h under argon. The tetrahydrofuran was then removed, and the residual oil was purified by column chromatography on silica (petroleum ether/ether 2:1) to give 11 as pale yellow

crystalline solid. The product was further crystallized from ether/ hexane (1:1, v/v). Yield: 4.80 g (23.04 mmol, 72%). 1H NMR (300.18 MHz, CDCl3): δ 8.69 (dd, 3J = 7.6 Hz, 4J = 0.7 Hz, 1H), 8.10 (m, 3H), 7.74 (m, 2H), 7.58 (d, 3J = 7.3 Hz, 1H), 7.49 (m, 3H), 7.29 (ddd, 3J = 4.8 Hz, 4J = 1.1 Hz, 1H) ppm. 13C NMR (75.48 MHz, CDCl3): δ 190.60, 153.36, 150.32, 142.91, 137.98, 137.00, 133.19, 128.86, 128.78, 125.72, 125.50, 124.54 ppm. IR (ν/cm−1, ATR): 3048 (w), 1660 (s), 1610 (s), 1575 (s), 1430 (s), 1324 (s), 1213 (s), 970 (s), 754 (s). MS (ESI+, CH3CN): m/z 210.09 [M + H]+. Anal. Calcd for C14H11NO: C 80.36, H 5.30, N 6.69. Found: C 80.25, H 5.40, N 6.64%. Synthesis of 1,4-Bis[2,6-bis(2-pyridyl)pyrimidin-4-yl]benzene (L1). Pyridine-2-carboximidamide hydrochloride 5 (1.39 g, 8.79 mmol) and 1,4-Bis[3-(pyridine-2-yl)-propene-1-one]-phenylene 4 (1.50 g, 4.40 mmol) were dissolved in ethanol (85 mL). To this solution, potassium hydroxide (0.75 g, 13.20 mmol) was added and heated to reflux for 15 h. When it was cooled to room temperature, the precipitate was filtered and washed with water (2 × 20 mL) and ethanol (2 × 10 mL). The product was further dissolved in chloroform and filtered to remove trace amounts of salt. The solvent was removed under reduced pressure and dried under vacuum to afford L1 as offwhite powder. Yield: 0.55 g (1.01 mmol, 23%). 1H NMR (300.18 MHz, CDCl3): δ 8.94 (d, 3J = 4.0 Hz, 4H, H3), 8.80 (m, 6H, H9, H6), 8.58 (s, 4H, H12, H13), 7.95 (m, 4H, H5), 7.47 (dd, 3J = 6.6 Hz, 3J = 4.9 Hz, 4H, H4) ppm. 13C NMR (75.48 MHz, CDCl3): δ 165.18 (C8), 164.70 (C7), 164.02 (C2′), 155.58 (C10), 154.33 (C2), 150.31 (C3′), 149.64 (C6′), 139.55 (C11), 137.36 (C4′), 137.08 (C4), 128.29 (C12, C13), 125.72 (C5′), 124.97 (C5), 124.38 (C6), 122.67 (C9), 112.53 (C3) ppm. IR (ν/cm−1, ATR): 3055 (w), 1564 (s), 1532 (s), 1470 (m), 1363 (s), 993 (m), 757 (s). MS (ESI+, CH3CN): m/z 543.20 [M + H]+. UV−vis (λmax in DCM): 328, 281. Anal. Calcd for C34H22N8: C 75.26, H 4.09, N 20.65. Found: C 75.15, H 4.07, N 20.32%. Synthesis of 5,5′-Bis[2,6-bis(2-pyridyl)pyrimidin-4-yl]-2,2′bithiophene (L2). A solution of 4-chloro-2,6-bis(2-pyridyl)pyrimidine 8 (0.75 g, 2.80 mmol), 5,5′-bis(tributylstannyl)-2,2′bithiophene 9 (0.86 mL, 1.39 mmol) and Pd(PPh3)4 (0.16 g, 0.14 mmol) in degassed toluene (40 mL) were heated for 16 h under an inert atmosphere at 100 °C. The mixture was cooled to room temperature, and the precipitate that formed was filtered off and washed with toluene (2 × 20 mL), hot hexane (2 × 10 mL), and diethyl ether (2 × 10 mL). The solid was dried under vacuum to give the product L2 as orange solid. Yield: 0.35 g (0.56 mmol, 40%). 1H NMR (300.18 MHz, CDCl3): δ 8.99 (d, 3J = 7.9 Hz, 2H, H3), 8.88 (d, 3 J = 7.9 Hz, 2H, H6), 8.76 (m, 4H, H3′, H6′), 8.69 (s, 2H, H8), 8.01 (m, 4H, H5′, H12), 7.94 (t, 3J = 7.7 Hz, 2H, H5), 7.52 (m, 2H, H4), 7.45 (m, 4H, H4′, H13) ppm. 13C NMR (75.48 MHz, CDCl3): δ 164.39 (C9), 163.26 (C2′), 160.27 (C7), 154.67 (C10), 154.00 (C2), 149.85 (C3), 149.53 (C3′), 142.42 (C14), 141.69 (C11), 137.71 (C12), 137.42 (C5), 129.24 (C5′), 126.00 (C4′), 125.78 (C13), 125.18 (C4), 124.50 (C6′), 122.87 (C6), 110.41 (C8) ppm. IR (ν/ cm−1, ATR): 3053 (w), 1560 (s), 1526 (s), 1470 (s), 1436 (s), 1370 (s), 1227 (m), 1015 (m), 790 (s), 759 (s). MS (ESI+, CH3CN): m/z 631.14 [M + H]+, 653.13 [M + Na]+, 1283.77 [2 M + Na]+. UV−vis (λmax in DCM): 413, 277, 241. Anal. Calcd for C36H22N8S2: C 68.55, H 3.52, N 17.77, S 10.17. Found: C 68.96, H 3.55, N 17.44, S 9.63%. Synthesis of 4-Phenyl-2,6-bis(2-pyridyl)pyrimidine (L3). Pyridine-2-carboximidamide hydrochloride 5 (0.56 g, 3.58 mmol) and 2-pyridalacetophenone 11 (1.50 g, 7.16 mmol) were dissolved in ethanol (75 mL). To this solution, potassium hydroxide (0.40 g, 7.16 mmol) was added and heated to reflux for 4 h. After it cooled to room temperature, the solution was concentrated in vacuo to 20 mL, and water (100 mL) was added. The product was crystallized from alcohol overnight upon cooling. The precipitate was then filtered, washed with water (2 × 10 mL), and dried under vacuum to afford L3 as pale brown powder. Yield: 0.51 g (1.63 mmol, 45%). 1H NMR (300.18 MHz, CDCl3): δ 8.92 (d, 3J = 4.2 Hz, 1H, H3′), 8.87 (s, 1H, H9), 8.77 (m, 3H, H3, H6, H6′), 8.39 (m, 2H, phenyl), 7.92 (t, 3J = 7.7 Hz, 2H, H5, H5′), 7.56 (dd, 3J = 5.2 Hz, 1.9 Hz, 3H, phenyl), 7.45 (ddd, 3J = 4.7 Hz, 4J = 1.1 Hz, 2H, H4, H4′) ppm. 13C NMR (75.48 MHz, CDCl3): δ 112.24 (C9), 122.59 (C6), 124.31 (C6′), 124.85 (C4), K


Article

Inorganic Chemistry

under argon in a step of 0.1 mL to the ligand solution, while the conductivity was monitored every 30 s. After each titration step, the solution was stirred to give a constant conductivity value with an accuracy of ±0.1 μs. Addition of Fe(OAc)2 solution was accompanied by a corresponding decrease in the conductivity, until a 1:1 stoichiometry was achieved. Further addition of Fe(OAc)2 solution results in the increase in conductivity indicating the presence of an excess of metal ions in the solution (Figure S1). Dip-Coating Process. A commercially available ITO-coated glass (Rs = 15−25 Ω) with a dimension of 2.5 cm × 2.5 cm × 1.1 mm was used for dip coating. Before the dip-coating process the substrates were rinsed with ethanol and dried under compressed air. Solid MEPE was dissolved either in deionized water or absolute ethanol depending upon the solubility. The dip-coating process was followed with the aid of a custom-built dip coater (LCTM, Universität Würzburg) at a withdrawing speed of 50 and 100 mm·min−1 using the solution concentration of 14 and 30 mM. The MEPE films were dried at 50 °C for 24 h and are in the reduced state. The reduced state is very stable at ambient temperature and does not oxidize in air. Viscosity. Viscosity measurements were performed using an Anton-Paar Lovis 2000 M (Ostfildern, Germany) capillary viscometer at 20 °C based on the rolling ball viscosity method employing a steel ball in a 1.8 diameter glass capillary. The density of the solution analyte was determined with an Anton-Paar density meter DMA 4100 M (Ostfildern, Germany) to obtain dynamic and kinematic viscosity. For viscosity measurements, the MEPE concentrations between 5 and 16 mM were used by dissolving the MEPE in either ethanol, methanol, or doubly distilled water (conductivity

Green-to-Red Electrochromic Fe(II) Metallo ... - ACS Publications

Recommend Documents