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Near-IR electrochromism in electrodeposited thin films of cyclometallated complexes Andreea Ionescu, Iolinda Aiello, Massimo La Deda, Alessandra Crispini, Mauro Ghedini, Maria Penelope De Santo, and Nicolas Godbert ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01167 • Publication Date (Web): 26 Apr 2016 Downloaded from http://pubs.acs.org on May 2, 2016

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ACS Applied Materials & Interfaces

Near-IR electrochromism in electrodeposited thin films of cyclometallated complexes Andreea Ionescu,a Iolinda Aiello,a Massimo La Deda,a,b Alessandra Crispini,a,b Mauro Ghedini,a,b Maria Penelope De Santo,b,c Nicolas Godbert a,* a

MAT_INLAB (Laboratorio di Materiali Molecolari Inorganici), Centro di Eccellenza CEMIF.CAL, LASCAMM CRINSTM, Unità INSTM della Calabria, Dipartimento di Chimica e Tecnologie Chimiche, Università della Calabria, I-87036 Arcavacata di Rende (CS), Italy; [email protected]; Fax:+390984492066; Tel:+390984492881 b CNR Nanotec - Istituto di Nanotecnologia U.O.S. Cosenza – Università della Calabria, I-87036 Rende, CS, Italy c Dipartimento di Fisica– Università della Calabria, I-87036 Rende, CS, Italy KEYWORDS : Cyclometallated Metallopolymers, Pd(II) complexes, Pt(II) complexes, Pt-Pt interactions, Electropolymerization and Electrochromism.

ABSTRACT: Homogeneous thin films of controlled thickness obtained from cyclometallated complexes of general formula [(C^N)M(O^N)], where M= Pd(II) or Pt(II), H(C^N) = 2-phenylpyridine and, respectively, 2-thienylpyridine and H(O^N) = a triphenylamine functionalized Schiff base, have been deposited by oxidative electropolymerization. The films have been electrochemically and morphologically characterized. The metallopolymeric thin films present stable reversible redox behavior and typical cauliflower-like textures in agreement with a nucleation-growth electropolymerization mechanism. However, the film growth is greatly influenced by the nature of the metal center, with a higher tendency of the Pt complexes to promote the 3D growth. Furthermore, a complete spectroelectrochemical study has been performed on electrodeposited films showing Near -IR absorption in the oxidized state, high contrast ratios (up to 65%) and low response times.

1. INTRODUCTION Upon application of an external electric field, electrochromics undergo reversible color change induced by switching between their available redox states.1 Stable, fast-responsive, color-efficient and low-potential switching electrochromic materials are expected to be successfully applied in optoelectronics (displays, information storage devices, smart windows, biological sensors).2 Up to date, mainly organic conducting polymers34 and inorganic oxides5-7 have been studied as electrochromic materials. Nonetheless, organometallic complexes represent an important class of potentially efficient electrochromic materials due to the stability of their redox states, the readiness of visible region electronic transitions and the possibility to selectively tune the metal center and/or the peripheral ligands.8 Beside the first studies on the electrochromic Prussian Blue [Fe(Fe(CN)6]and phtalocyanines dyes,9-11 ruthenium(III) polypiridine12-13 and copper(I) pyridyl-pyrimidine14 organometallic electrochromic complexes have been synthesized and studied. In these organometallic complexes, electrochromism is generally induced by linkage isomerization. In order to take advantage of both the easy processing and high flexibility of organic polymers and the redox stability and high coloration of metal containing electrochromics, metallopolymers have been introduced as electrochromic materials.15 Furthermore, as a consequence of the incorporation of transition metals, a significant conductivity enhance has been observed in such electrochromic films.16-17

Three main different approaches have been followed for the deposition of electrochromic metallopolymeric films onto ITO coated substrates: i) solution-based deposition of coordination polymers of iron(II),18-19 copper(I)20 and zinc(II)21 based on tailored polypyridine bridging ligands; ii) self-propagating molecular based assemblies of polypyridyl osmium(II),22 zinc(II) and cadmium(II)23 complexes and iii) electropolymerization. Of the previously cited film deposition techniques, electropolymerization features the tremendous advantage of simultaneous polymer formation and deposition, thus limiting solubility issues and affording highly controllable film composition, thickness and homogeneous surface coverage. Through reductive electropolymerization of vinyl-substituted polypyridine complexes, ITO substrates have been coated with electrochromic metallopolymers. The obtained adhesive, stable and redox-active films incorporate either mono-24 or mixed-valent dimetallic ruthenium(II)25 monomers. Such mixed-valent dimetallic ruthenium(II) complexes are potentially applicable as near-IR electrochromic materials due to their modulable intervalence charge transfer electronic transitions.26 Near-IR (8002000 nm) extension of electrochromism makes these materials particularly useful as light-attenuators at the telecommunication wavelength (1310 and 1510 nm), but applicable also in biological imaging and solar cells.27 Although metal containing polymers in general could display the adequate properties to be applied in near-IR electrochromic devices, to the best of our knowledge only ruthenium based metallopolymers have been reported so far in this field.28-30

ACS Paragon Plus Environment


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In this context, with the aim of developing novel near-IR electrochromic stable and high quality metallopolymeric films and encouraged by our recently reported results on the electropolymerization of photoconductive square-planar cyclometallated compounds (Chart 1),31-32 we investigated the potential electrochromic properties of Pd(II) and Pt(II) complexes (Scheme 1). Differently from the bipyridine ruthenium(II) based materials reported so far which were deposited by reductive vinyl polymerization, the electrodeposition in this case is allowed by the introduction onto the ancillary Schiff base ligand of a triphenylamine moiety (TPA) as a oxidative electropolymerizable group. The choice of the TPA fragment has been driven by its high hole-transport properties,33 high electrochemical stability and the isotropic charge transport of the resulting cross-linked conjugated electropolymer.32,34-37 Noteworthy, triphenylaminebased organic-only polymeric materials have recently been reported for their near-IR electrochromism.38

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complex 2 was obtained instead from the relative cycloplatinated mononuclear intermediate complex B by microwave assisted synthesis.40 The bridge cleavage of A and the ligands substitutions of B with the Schiff base H(O^N)tpa, afforded the final complexes in good yields (90% for 1, 60% for 2 respectively). The Schiff base H(O^N)tpa was obtained by condensation of 5-methoxy-2-hydroxybenzaldehyde with an opportunely substituted triphenylamino derivative as previously reported.32

Scheme 1. Synthesis of complexes 1 and 2. Reagents and conditions: i) Pd(CH3COO)2, , CH3COOH, 50°C, 3.5 h.; ii) K2PtCl4, 2-EtOEtOH/H2O: 3:1 (v:v), 250W, 1 m.; iii) H(O^N)tpa, EtOH, r.t., 24 h; iv) H(O^N)tpa, 2-EtOEtOH, Na2CO3, 80°C, overnight.

Chart 1. The photoconductive cyclometallated complexes (I and II),31 the electropolymerizable photoconductive cyclometallated complexes (III and IV)32 and the electropolymerizable Schiff base H(O^N)tpa

2. RESULTS AND DISCUSSION

Solution Electrochemistry. The solution electrochemical data for complexes 1 and 2 were obtained by cyclic voltammetry (CV) and estimated HOMO energy values were determined by using ferrocene as an internal standard. Data are collected in Table 1 together with oxidation potentials of the reference complexes III and IV , and the corresponding free Schiff base ligand H(O^N)tpa for direct comparison. The CV measurements were performed in dichloromethane, using tetrabutylammonium hexafluorophosphate(0.1M) as supporting electrolyte, a Pt disk working electrode, a Pt wire counter electrode and an Ag wire as pseudo-reference electrode. Table 1. Cyclic voltammetry data. Complex

Synthesis To probe the electrochromic properties of photoconductive cyclometallated Pd/Pt complexes of general formula [(C^N)M(O^N)], 2-phenylpyridine (H(PhPy)) was chosen as archetype cyclometallating ligand and complexes III and IV were synthesized following the previously reported procedure.32 To complete the series, two new cyclometallated complexes (1 and 2) comprising 2-thienylpyridine H(ThPy) as cyclometallated ligand and the identical H(O^N)tpa Schiff base as ancillary ligand have been synthesized (Scheme 1). The H(ThPy) ligand has been chosen in order to: i) test the versatility of the H(O^N)tpa Schiff base in promoting electropolymerization processes in square-planar Pd(II)/Pt(II) complexes bearing different (C^N) ligands; ii) tune the absorption of the potentially electrochromic deposited thin film. The synthesis of complexes 1 and 2 (Scheme 1) was achieved in two steps. Pd(II) complex 1 was obtained through the formation of its corresponding cyclopalladated acetato bridged binuclear intermediate A, intermediate obtained by cyclometallation of the H(ThPy) ligand with palladium(II) acetate.39 Pt(II)

H(O^N)tpa 1 2 III IV

Eox1 (V)a +0.47 (Irr.) +0.36 (Irr.) +0.38 (Rev.) +0.34 (Irr.)b +0.35 (Rev.)b

Eox2 (V)a +0.50 (Rev.) +0.57 (Rev.) +0.48 (Rev.) +0.52 (Rev.)b +0.51 (Rev.)b

HOMO (eV) -5.7 -5.2 -5.3 -5.2b -5.2b

Ddiff (cm2.s-1) 1.39.10-5 1.24.10-5 1.18.10-5 1.21.10-5 1.11.10-5

a

Potentials are given versus ferrocene/ferrocinium (Fc/Fc+). Rev.: reversible wave, Irr.: Irreversible wave. Measurements performed in dichloromethane at a 100 mV/s scan rate. b from Ref. 32. Eox = (Epa+Epc)/2 for Rev. processes, Eox = Epa for Irr. processes.

The H(O^N)tpa Schiff base presents two consecutive oxidation processes that are observed at potential of ca. + 470 mV and + 500 mV, respectively, with respect to Fc/Fc+. The first oxidation wave can be attributed to the oxidation of the TPA fragment to form the radical cation TPA+• as already reported for substituted TPA.41 The second oxidation process can be attributed to the Schiff base moiety31 leading to the formation of the dication H(O^N)tpa2+. For both complexes 1 and 2, two analogous consecutive oxidation processes are observed. Identical features have been previously observed for the parent complexes III and IV.32 These oxidation processes relative to the


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oxidation of the (O^N)tpa ligand are slightly perturbed by the chelation to the metal center. In particular, the first oxidation is mainly affected and shifted at lower potential (of ca. 100 mV) with respect to the free ligand. Cyclic voltammograms obtained at a standard scan rate of 100 mV.s-1 of H(O^N)tpa and complexes 1 and 2 are reported in Figure 1. Cyclic voltammograms of complexes III and IV are reported in ESI.

small chemical and structural differences between the complexes with molecular weights varying from ca. 730 (III), 736 (1), 819 (IV) and 825 g.mol-1 (2), diffusion coefficients are very similar, of the order of 1,2x10-5 cm2 s-1 (Table 1). These values are in agreement with the slightly higher diffusion coefficient determined for the free ligand H(O^N)tpa (1.4x10-5 cm2 s-1) and for ferrocene reported in dichloromethane 1.67x10 -5 cm2 s-1.42 The obtained Randles–Sevcik plots are reported in ESI.

a)

Current Intensity (A)

4 3 2 1 0 -1 -2 -0,2

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

E (V) vs. Fc/Fc+

Electropolymerization Processes. Mono-substitution of TPA by an electron withdrawing group, (EWG)TPA, promote the electropolymerization process. Upon a potential application in oxidation, the TPA fragment is oxidized. One electron from the nitrogen atom is removed, leading to a delocalized radical cation (EWG)TPA+• which dimerizes to produce the corresponding tetraphenylbenzidine (EWG)2TPB. The introduction of the EWG group in para-postion of TPA, favors the electropolymerization process of TPB.44 (EWG)2TPB is oxidized at lower potentials than (EWG)TPA, and upon cycling oxidative potentials, chain extension occurs with formation of a branched polymeric network (Scheme 2).45-46

b)


4 3 2 1 0 -1 -2 -0,2

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

E (V) vs. Fc/Fc+ b) 4


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3

2

1

0

Scheme 2. Electropolymerizzation process of substituted TPA fragment (modified from Refs. 45-46).

-1

-2

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

E (V) vs. Fc/Fc+

Figure 1. Cyclic voltammograms of H(O^N)tpa (a), complexes 1 (b) and 2 (c) obtained in dichloromethane at a 100 mV.s-1 scan rate. [Plots are reported vs. Fc/Fc+ taking into account +0.46 V vs. SCE for Fc/Fc+ in dichloromethane.43] For all complexes, 1, 2, III and IV, the diffusion coefficient in dichloromethane solution has been determined measuring the variation of the current intensity of the first anodic peak (Ipa) at different scan rates (ν). The slope of the linear dependences of the Ipa = f(ν1/2), observed for all complexes, allowed through the Randles–Sevcik equation to estimate the diffusion coefficient (Ddiff) of each complex in solution. As expected, due to the

Upon repetitive oxidation cycling, the free ligand H(O^N)tpa can be electropolymerized onto the working electrode (Pt disk or ITO covered glass) according to scheme 2. The Schiff base moiety is therefore acting for the TPA fragment as an electronwithdrawing group and such electronic character, although decreased, is preserved upon chelation of the Schiff base to the metal centre. Consequently, as already observed for complexes III and IV,32 complexes 1 and 2 can also be electropolymerized. The cyclic voltammograms of the electropolymerization processes observed for the free ligand H(O^N)tpa, complexes 1 and 2 are reported in Figure 2. Film Stability. The stability of the resulting electrodepositied films of complexes 1 and 2 was probed in freshly distilled electrolytic dichloromethane or tetrahydrofuran solutions. Upon 50 repetitive oxidation cycles performed at 100 mV.s-1


ACS Applied Materials & Interfaces scan rate in both solvents, no significant decrease of oxidation peaks intensities has been recorded showing the high stability of the electropolymerized thin films as previously observed for electrodeposited films of complexes III and IV.32 The corresponding cyclic voltammograms are reported in ESI.

a)


80 60 40 20

Film thickness and morphology. The thickness of electrodeposited films can be finely controlled either by modifying the complex concentration or by changing the scan rate value used during the electropolymerization process.47 Starting from a 5x10-4 M dichloromethane solution and effectuating 50 repetitive oxidation cycles, the influence of the scan rate onto the film thickness has been probed in conjunction with a morphological study of the film surface realized by Atomic Force Microscopy (AFM) and the determination of the root-mean square roughness (RRMS). This study has been performed for H(O^N)tpa and all the complexes 1, 2, III and IV. Results obtained are reported in Table 2. Table 2. Film thickness and roughness.

0

Scan Rate (mV s-1)

-20 -40 -60 -0,2

100 0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

250

E (V) vs. Ag wire

500 b)

800 Current Intensity (A)

80 60

H(O^N)tpa

1

72 (4.5) 35 (2.8) 16 (2.2) 6.0 (2.5)

62 (9.1) 44.0 (5.5) 22.0 (2.1) 9.0 (2.4)

Film thickness (nm) (RRMS) 2 III 2100 (59.8) 350 (21.9 ) 115 (12.0) 12 (3.2)

96.2 (6.7) 43.8 (7.8) 23.0 (3.3) 4.0 (6.4)

IV 2303 (325) 443 (60.4) 129 (5.5) 52 (6.9)

Films were deposited onto ITO covered glass electrodes from a 5x10-4M dichloromethane solution, using 50 repetitive oxidation cycles. Thin thickness were determined by sampling three different zones and R RMS values were determined by probing four different ca. 20 m2 squares of surfaces.

40 20 0 -20 -40 -60 -80 -0,2 0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

E vs. Ag wire

c) 200 Current Intensity (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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150 100 50

A first striking feature is the difference of thickness between the Pt(II) generated thin films and the films formed by H(O^N)tpa and the Pd(II) complexes. Indeed, the electropolymerization process conducted in identical conditions (concentration, temperature and scan rate) lead to thicker film deposition for both Pt(II) complexes (2 and IV) with respect to their Pd(II) counterparts (1 and III) and this difference is accentuated with the diminution of scan rate. While thin films of Pd(II) complexes obtained at 100 mV s-1 scan rate, for example, are less than 100 nm thick, their Pt (II) analogues present a thickness of ca. 25 times higher (> 2300 nm). The difference between the two complexes is clearly visible on the photograph of an electrodeposited thin film of Pt(II) complex 2 compared with a thin film of Pd(II) complex 1, both presented in Figure 3. The two films were obtained in identical experimental conditions (temperature, concentration and scan rate).

0 -50 -100 -150 -0,2

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

E (V) vs. Ag wire

Figure 2. Cyclic voltammograms of H(O^N)tpa (a), complexes 1 (b) and 2 (c) obtained on a ITO covered glass electrode upon 50 repetitive scans at a 100 mV s-1 scan rate in a 5x10-4 M dichloromethane solution using tetrabutylammonium hexafluorophate (0.1M) as supporting electrolyte, a Pt wire as counter electrode and an Ag wire as pseudo-reference electrode. Note that plots are shown every 5 cycles.

Figure 3. Photograph of an electrodeposited thin film on ITO covered glass electrode of complex 2 (a) and 1 (b) obtained in identical conditions (5x10-4 M dichloromethane solution, 50 repetitive oxidation cycles and 100 mV s-1 scan rate).


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A part from the difference in thickness, the morphology study performed on all films, reveals another important diverse feature highly noticeable for films obtained at low scan rates (< 500 mV s-1). When electrodepositions of Pt(II) complexes are performed at low scan rate, high roughness surface are observable by AFM microscopy with a cauliflower-like texture similar in features to films of TPA obtained through solid state electropolymerization.48 The RRMS roughness reaches consequently high values (ca. 300 nm for 100 mV s-1). On the opposite, thin films of Pd(II) complexes do not present these topographical characteristics, being much more smoother with RRMS values of the order of 5-10 nm independently of the scan rate used during electropolymerization, similarly to what obtained for the H(O^N)tpa generated thin films. When depositions of Pt(II) complexes are effectuated at higher velocity ( > 500 mV s-1), this disparity between Pt(II) and Pd(II) thin film morphology disappears, similar RRMS values and texture are obtained. AFM micrographs of surfaces of thin films of H(O^N)tpa, complexes III and IV obtained at different scan rates that illustrate such observations are presented in Figure 4. Electrodeposited H(O^N)tpa

1.0µm

a) 100 mV.s-1

d) 100 mV.s-1

g) 100 mV.s-1

1.0µm

1.0µm

b) 250 mV.s-1 c) 500 mV.s-1 Electrodeposited Complex III

e) 250 mV.s-1 f) 500 mV.s-1 Electrodeposited Complex IV

h) 250 mV.s-1

i) 500 mV.s-1

Figure 4. AFM micrographs of electrodeposited thin films of H(O^N)tpa (a-c), complexes III (d-f) and IV (g-i) on ITO covered glass substrates from 5x10-4 M dichloromethane solution at different scan rates 100, 250 and 500 mV s-1. The possible reason for the observed difference in the film thickness and morphology of electropolymerized Pd(II) and Pt(II) complexes, is not easily identifiable. Since all complexes display a similar diffusion coefficient (Table 1), difference in mass transport in solution between Pd(II) and Pt(II) complexes

can be ruled out. On the other hand, TPA electropolymerization process is governed by the formation of the radical cation TPA+•.35-36 However, a difference in sticking coefficient onto the surface electrode between the radical species generated from the Pd(II) or the Pt(II) complexes can reasonably be precluded. Indeed, the large physical spacing between the TPA fragment and the metal center should prevent any significant influence on such parameter. As in many electropolymerization processes, in the early cycles, direct formation of electroactive radical occurs on the bare surface electrode surface producing 2D islands growth. Successively, by reaction with new monomers/oligomers a lateral growth takes place starting from the boundaries of these 2D islands, producing a 3D growth.49-50 The high tendency of Pt(II) complexes to form cauliflowerlike texture and thicker film deposits must be therefore correlated to the capacity of Pt(II) complexes to undergo greater 3D growth than their Pd(II) analogues. Some hypothesis can be formulated to explain such differences invoking for example the higher probability of Pt(II) complexes to form Pt-Pt interactions which might enhance the 3D growth. This is also corroborated by the fact that the cauliflower texture is only observed for low scan rate values, when enough time is given to the system to establish intimate intermolecular interactions. Such metal-metal interactions are particularly known for Pt(II) complexes and are often the driving force of the formation of efficient stacking assemblies either in solid state,51-53 in mesophase54-55 or in solution.56 Indeed, is has been recently observed that such Pt-Pt interactions are even able to promote in solution the formation of micrometer long chains of end-to-end self assemblies of Gold nanorods decorated on both extremities by square planar Pt(II) complexes.57 However, further studies which are currently into working progress have to be performed to elucidate, at a molecular scale, the exact mechanism of film growth. Electrochromic Properties. Spectroelectrochemical measurements were performed on electrodeposited thin films of 1, 2, III and IV on ITO coated glass substrates immersed in a ca. 0.1 M electrolyte dichloromethane solution. The measurements were performed in a quartz cell, using the electrodeposited film on ITO covered glass as working electrode immersed in an electrolyte solution, a Pt wire as counter electrode and an Ag wire as pseudo-reference electrode. Films were obtained from a 5x10-4 M dichloromethane solution. In order to properly compare the spectroelectrochemical performances of all materials, films of the same thickness (ca. 100 nm) were prepared by opportunely choosing the scan rate of electrodeposition. The absorption spectra of the neutral (-0.2 V) and oxidized (1.6 V) states of electrodeposited films of 1, 2, III and IV are presented in Figure 5. Upon application of an oxidizing potential, the (O^N)tpa fragments are fully oxidized. Light irradiation leads to an excitation from the dication ground state towards the first excited state, probably involving a transition from the metalcontaining 5-membered cyclometallated ring to the (O^N)tpa fragments. This originates an absorption band in the NIR range, centered at  (nm) : 886 (1), 864 (2), 860 (III) and 834 (IV) respectively. The contribution of these cyclometallated parts of the metallopolymers to the generation of this NIR absorption band during oxidation is confirmed by the absence of electrochromic property characterizing the electrodeposited film of the free ligand H(O^N)tpa. Indeed, H(O^N)tpa electropolymerized thin films do not show any change in absorption upon oxidation (See ESI).


ACS Applied Materials & Interfaces b) 0,4

a) film of 1 neutral state film of 1 oxidized

max = 886 nm

film of 2 neutral state film of 2 oxidized

Absorbance

Absorbance

0,4

0,2

0,0

max = 864 nm 0,2

0,0

350

600

850

1100

1350

1600

350

Wavelength (nm)

d)

850

1100

0,4

1350

1600

max= 860 nm 0,2

Table 3. Electrochromic performances

max= 834 nm

Complex  (nm) T% a

0,2

0,0 350

600

850

1100

Wavelength(nm)

1350

1600

0,0 350

600

850

1100

1350

1600

f)

700

800

900

1000

Bleached-Dark Dark-Bleached a

1 886 24%

2 864 65%

III 860 24%

IV 834 28%

20 4.0

20 10.5

3.4 3.0

3.5 3.1

at the second redox step

g)

1 2 III IV

600

tsd (s) tsb (s)

Wavelength (nm)

e)

500

ing of contrast with time can be noticed from the second potential switch (See Figure 6c). On the opposite, films of complex 1, 2 and IV exhibited more stable electrochromic behavior (Figure 6 a-b, d). The contrast (T%) was measured considering the difference in transmittance between the neutral and the oxidized states under the first application of the step potential. Moderate contrast values were obtained for electrodeposited films of complexes III-IV and 1, while the film obtained from 2 presented a very high contrast (Table 3).

film of IV neutral state film of IV oxidized

film of III neutral state film of III oxidized

Absorbance

Absorbance

0,4

600

Wavelength (nm)

c)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1100

Wavelength (nm)

Figure 5. Absorption spectra of ca. 100 nm thick films of complexes 1 (a) and 2 (b),III (c), IV (d), in the oxidized state (red line) vs. neutral state (black line). Comparison between the absorption spectra of the oxidized states of electrodeposited films of 1, 2, III and IV (e). Picture of ca. 100 nm thick thin film of 1 in the neutral state at Eappl. = 0 V (f) and in the oxidized state at Eappl. = 1.6 V (g). Considering the metal center, Pd(II) based films (complexes 1 and III) show higher wavelengths absorption in the oxidized state with respect to films comprising a Pt(II) center. The incorporation of a (ThPy) moiety into the metal coordination sphere alters the spectroelectrochemical properties of the films. In fact, the absorption of electrodeposited films of 1 and 2 is tuned further towards the NIR spectral region, when compared to films of III and IV comprising the (PhPy) ligand (Figure 5e) suggesting a more effective charge transfer from the cyclometallated fragment to the oxidized (O^N)tpa in the case of (ThPy) complexes with respect to their (PhPy) analogues. The electrochromic color switch reversibility of each film was checked by measuring the transmittance (at max of the absorption in the oxidized state) in function of time, under repeated application of a step potential (Figure 6). The potential step absorptiometry experiments (30 step potential applications) were performed on the ITO-glass substrates coated with the ca. 100 nm thick electrodeposited films of 1, 2, III and IV in a 0.1 M NBu4PF6 electrolytic dry and degassed dichloromethane solution. The potential was stepped between -0.2 V and 1.6 V and maintained for 20 seconds. Reversibility was partially lost only for the electrodeposited films of complexes III for which slight and progressive decreas-

The switching time of darkening (tSd) was calculated considering the time needed for the transmittance to reach its minimum value (in the oxidized state), after switching the applied potential to Eappl. = 1.6 V; the switching time of bleaching (tsb) taking into account time required to reach the maximum value (in the neutral state) after switching Eappl. to 0 V (See Figure 6, right column). Films of III and IV, containing a H(PhPy) moiety as a cyclometallated ligand, present the same fast-response behavior, while films of complexes 1 and 2, comprising the H(ThPy) cyclometallated ligand present a slower response, especially when returning to the neutral state. Such behavior would suggest slower charge mobility within the films of 1 and 2 attributed to the presence of the thienyl ring onto the cyclometallated ligand which probably acts as a hole trapping site.58-59 The electrochromic performances of 1, 2, III and IV were compared to those of metallopolymers. All the four electrodeposited films are highly stable. Even if they feature switching times one order of magnitude higher than reported values observed for self-assembled and coordination metallopolymers, 21, 23 the performances are similar to electrodeposited NIR electrochromic Ru(III) complexes.25 Considering the measured contrast, while III, IV and 1 show moderate T% (ca. 25%), electrodeposited films of complex 2 present a high color contrast (65%) between the neutral and the oxidized state. This is a remarkable result considering the performances reached by electrochromic metallopolymers obtained by all the above cited deposition methods.


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3. CONCLUSION

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In order to investigate the spectroelectrochemical properties of cyclometallated Pd/Pt complexes of general formula [(C^N)M(O^N)], 2-phenylpyridine H(PhPy) was chosen as an archetype cyclometallating ligand and a triphenylamine comprising Schiff base as an electropolymerizable H(O^N) ancillary ligand. Thus, films of complexes III and IV have been electrodeposited and completely characterized. To elucidate the role of the cyclometallating ligand on tuning the electrochromic absorption of electrodeposited films of Pd/Pt, two new cyclometallated complexes (1 and 2) containing 2-thienylpyridine H(ThPy) as cyclometallated ligand and the identical Schiff base as ancillary ligand have been synthesized. Electrodeposited high quality films of 1 and 2 have been obtained and have shown red-shifted electrochromic absorption with respect to the electropolymerized films of III and IV. Considering the spectroelectrochemical features of the electrodeposited films, moderate to high contrast ratios (up to 65%) have been measured. Switching times between the neutral and oxidized states are low (ca. 3 s) for films comprising H(PhPy), while longer responses (4-20 s) have been registered for the films containing H(ThPy), probably due to the presence of a hole-trapping site onto the thienyl ring. Different film thicknesses have been measured for Pd(II) and Pt(II) complexes under identical experimental conditions with thicker films obtained from Pt(II) complexes. Moreover, the AFM morphological characterization revealed that for films of Pt complexes electrodeposited at low scan rate (< 250 mV.s-1), a highly rough cauliflower-like texture is observed, while thin films of Pd(II) complexes do not present such topographical characteristics. Such difference, absent for higher scan rates can hypothetically be attributed to Pt-Pt intermolecular interactions taking place at molecular scale, which might enhance the 3D growth contribution of the electropolymerization mechanism when enough time is given for their establishment. Further studies are currently in progress to shed the light on this issue. However, the present work confirms the use of TPA embedded Schiff bases as efficient ancillary ligands to promote electropolymerization processes in organometallic complexes, leading to the deposition of high quality electroactive metallopolymer thin films.

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Figure 6. Potentials step absorptiometry on the modified ITO electrodes coated with ca. 100 nm thick electrodeposited films of: a) 1 at = 886 nm and b) 2 at = 864 nm c) III at = 860 nm; d) IV at = 834 nm. On the right a zoom detail on a single sequence Eappl. = 1.6V (20s) – Eappl. = 0 V (20s)- Eappl. = 1.6 V (20s).

Materials and Methods. All commercially available chemicals were purchased from Aldrich Chemical Co. and were used without further purification. 1H-NMR spectra were recorded either on a Brucker WH-300 spectrometer in deuterated solvents. IR spectra (KBr pellets) were recorded on a Spectrum One Perkin-Elmer FT-IR spectrometer. Elemental analyses were performed with a Perkin-Elmer 2400 analyzer CHNS/O. Melting points were determined with a Leica DMLP polarising microscope equipped with a Leica DFC280 camera and a CalCTec (Italy) heating stage. Absorption spectra of deposited films were registered on a JASCO V-570 UV/Vis/NIR Spectrophotometer. Atomic Force Microscopy measurements were performed using a Bruker Multimode 8 equipped with a Nanoscope V controller. Data were acquired in tapping mode in air, using silicon cantilevers (model TAP150, Bruker). Electrochemistry and Electropolymerization. All potentials were measured using an Epsilon electrochemical analyser or on an Autolab Potentiostat/Galvanostat. Cyclic voltammetry



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

experiments were performed in a 3 mL cell of dry, freshly distilled, and degassed (Ar) dichloromethane and tetrahydrofuran solutions using tetrabutylammonium hexafluorophosphate (0.1 M) as supporting electrolyte, a Pt disk working electrode, a Pt wire counter-electrode and an Ag wire as pseudoreference electrode. Electropolymerizations were carried out by cyclic voltammetry, performed with an Autolab Potentiostat-Galvanostat controlled by the NOVA 1.10 software. A conventional three-electrode cell was employed, with a Pt wire as counter-electrode, an Ag wire as pseudo-reference electrode and a ca. 1 cm wide ITO coated glass as working electrode. Tetrabutylammonium hexafluorophosphate (0.1 M) was used as supporting electrolyte and experiments were performed in a dry, freshly distilled and degassed (Ar) dichloromethane/tetrahydrofuran solution. The electropolymerization process was achieved by continuously cycling the applied potential around the oxidation potential of the complex at a constant scan rate. Films thicknesses were measured with a stylus profilometer (Veeco, Dektak 8). After electropolymerization, the resulting films were aged overnight in a dichloromethane vapor saturated atmosphere before being washed with freshly distilled dichloromethane. Spectroelectrochemical Measurements. The experiments were carried out in a home-made spectroelectrochemical cell consisting of a three-electrode setup immersed in a quartz spectroscopic cell filled with a 0.1 M tetrabutylammonium hexafluorophosphate electrolytic solution. Ca. 1 cm wide ITO glasses coated with the electrodeposited films of synthesized complexes were used as working electrode with a Pt wire counter electrode and a Ag wire as a pseudoreference. Absorption spectra of electrodeposited films were registered under oxidizing and reducing potentials. Transmittance spectra were measured upon application of a cycling oxidizing step potential controlled by a ad hoc written code with the NOVA 1.10 software. Synthesis. H(O^N)tpa, complexes III and IV were prepared according to the reported procedure.32 The 2-thienylpyridine cyclopalladated binuclear intermediate A was obtained by cyclometallation of 2-thienylpyridine with palladium acetate as previously described,39 while the mononuclear cycloplatinated intermediate B was obtained by reaction of 2-thienylpyridine with potassium tetrachloroplatinate.40 Data for H(O^N)tpa: Orange solid; m.p. 165-166 °C; 1HNMR (300 MHz, CDCl3, 25°C, TMS): δ (ppm): 12.86 (s, 1H), 8.65 (s, 1H), 7.63 (d, J=7.63 Hz, 1H), 7.49 (d, J=9.06 Hz, 1H), 7.35 (d, J=8.52 Hz, 2H), 7.30 (s, 1H), 7.28-7.26 (m, 5H), 7.167.12 (m, 6H), 7.04 (t, J=7.11 Hz, 3H), 6.99-6.95 (m, 3H), 6.91 (d, J=2.61 Hz, 2H), 3.82 (s, 3H); FT-IR (KBr, cm-1): 3400, 3014, 2950, 1323; elemental analysis calculated for C32H26N2O2 (470,56 g/mol): C 81.68%, H 5.57%, N 5.95%, found: C 81.72%, H 5.60%, N 6.17%. Data for III: Orange solid, m.p. 241-242°C (amorphous on cooling); Tdec=270 °C 1H-NMR (300 MHz, CDCl3, 25°C, TMS): δ (ppm): 9.30 (dd, J=5.7 Hz, J=0.9 Hz, 1H), 8.12 (s, 1H), 7.82 (td, J=8.1 Hz, J=1.5 Hz, 1H), 7.66-7.48 (m, 6H), 7.36 (dd, J=7.8 Hz, J=1.5 Hz, 1H), 7.31-7.22 (m, 5H), 7.15 (m, 6H), 7.077.02 (m, 4H), 3.47 (d, J=9.3 Hz, 1H), 6.8 (td, J=7.2 Hz, J=0.9 Hz, 1H), 6.71 (d, J=3 Hz, 1H), 6.58 (t, J=7.2 Hz, 1H), 5.87 (d, J=6.1 Hz, 1H), 3.89 (s, 3H); FT-IR (KBr, cm-1): 3057, 3029, 2929, 2833, 1624, 1583, 1531, 1490, 1468, 1325, 1271, 1215, 1159, 829, 753, 698; elemental analysis: calculated (%) for C43H33N3OPd (730.16 g/mol): C 70.73, H 4.56, N 5.75, found C 70.25, H 4.55, N 6.01.

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Data for IV: Orange solid, m.p. 270-271°C (amorphous on cooling); Tdec=280°C; 1H-NMR (300 MHz, CDCl3, 25°C, TMS): δ (ppm): 9.53 (dd, JPt=45 Hz, J=8 Hz, 1H), 8.22 (s, 1H), 7.80 (t, J=7.8 Hz, 1H), 7.62 (q, J=6 Hz, 4H), 7.47 (d, J=8 Hz, 2H), 7.3-7.1 (m, 15 H), 7.1-7.0 (m, 2H), 6.85 (t, J=5.7 Hz, 1H), 6.75 (d, J=1.5 Hz, 1H), 6.55 (t, J=5 Hz, 1H), 5.85 (dd, JPt=45 Hz, J=8.1 Hz, 1H); FT-IR (KBr, cm-1): 3056, 3031, 2928, 1625, 1607, 1584, 1534, 1490, 1471, 1384, 1323, 1271, 1216, 1164, 1041, 82, 754, 698; elemental analysis: calculated (%) for C43H33N3O2Pt (818.82 g/mol): C 63.07, H 4.06, N 5.13, found C 63.10, H 4.10, N 5.25. Synthesis of 1. Intermediate binuclear complex A (100 mg, 0.153 mmol) and 2 eq. of H(O^N)tpa (144 mg, 0.306 mmol) were solubilized in ethanol and stirred at r.t. for 24 h. The yellow precipitated solid was filtered off, washed with ethanol and dried under vacuum. Recrystallization was performed in dichloromethane/ethanol solution. Yellow solid; yield 90% (221 mg); m.p. 235°C; 1H-NMR (300 MHz, CDCl3, 25°C, TMS): δ (ppm): 9.12 (d, J=5.7, 1H), 7.99 (s, 1H), 7.71 (td, J=7.5 Hz, J=1.6 Hz, 1H), 7.62 (d, J=8.6 Hz, 2H), 7.52 (t, J=9.6 Hz, 3H), 7.31-7.29 (m, 6H), 7.18-7.14 (m, 5H), 7.10-7.05 (m, 3H), 7.016.98 (m, 2H), 6.88 (d, J=4.5 Hz, 1H), 6.69 (d, J= 2.97 Hz, 1H), 5.08 (d, J=4.92 Hz, 1H), 3.76 (s, 3H); FT-IR (KBr, cm-1): 1584, 1530, 1488, 1322, 1275, 1219, 1163, 1044, 834, 753, 695; elemental analysis: calculated (%) for C41H31N3OPdS (720,19 g/mol): C 68.38, H 4.34, N 5.83, found C 68.52, H 4.04, N 5.90. Synthesis of 2. Intermediate mononuclear complex B (50 mg, 0.091 mmol), 1 eq. of H(O^N)tpa (43 mg, 0.091 mmol) and 8 eq. of Na2CO3 (100 mg, 0.728 mmol ) were solubilized in ca. 30 mL of 2-ethoxyethanol. The reaction mixture was stirred at 80°C overnight to give an orange precipitate. The solid was filtered off and washed with cold ethanol. Afterwards, the solid was solubilized in dichloromethane and filtrated on a celite short column to remove the inorganic salts. Recrystallization was performed from dichloromethane/ethanol. Orange solid; yield 60% (40 mg); m.p. 261°C; 1H-NMR (300 MHz, CDCl3, 25°C, TMS): δ (ppm): 9.33 (dd, JPt-Pt=41.7 Hz, J=5.3 Hz, 1H), 8.33 (s, 1H), 7.65 (d, J=8.4 Hz, 3H), 7.52 (q, J=8.8 Hz, 4H), 7.31-7.29 (m, 5H), 7.19-7.14 (m, 6H), 7.10-7.04 (m, 4H), 6.96 (d, J=4.92 Hz, 1H), 6.75 (d, J=1.8 Hz, 1H), 4.92 (d, J=4.9 Hz, 1H), 3.76 (s, 3H); FT-IR (KBr, cm-1): 1586, 1488, 1470, 1318, 1280, 1219, 1165, 1042, 835, 695; elemental analysis: calculated (%) for C41H31N3OPtS (808,85 g/mol): C 60.88, H 3.86, N 5.20, found C 60.90, H 4.00, N 5.51. Acknowledgments. This work was supported by MIUR ELIOTROPO project-(PON03PE_00092)

ASSOCIATED CONTENT Supporting Information Available: Cyclic voltammograms of H(O^N)tpa and all of studied complexes 1-2, III and IV, Randles-Sevcik plots, film stability measurements and spectroelectrochemical properties of H(O^N)tpa.

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0V

1,6 V

0,4

-0,2

M = Pd or Pt

0,0

0,2

0,4

0,6

0,8

E vs. Ag wire

Pt

1,0

1,2

1,4

1,6

Absorbance

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0V 1,6 V

0,2

Pd 0,0 500

1000

1500

Wavelength (nm)


11


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Graphical Abstract 199x81mm (150 x 150 DPI)


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Near-IR Electrochromism in Electrodeposited Thin ... - ACS Publications

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