Understanding Color Tuning and Reversible Oxidation of Conjugated

Article pubs.acs.org/JPCA

Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

Understanding Color Tuning and Reversible Oxidation of Conjugated Azomethines Sophie Bishop, Marie-Heĺ eǹ e Tremblay, Alexandra Gelle,́ and W. G. Skene* Laboratoire de caractérisation photophysique des matériaux conjugués, Département de Chimie, Université de Montréal, CP 6128 Centre-ville, Montreal, Quebec

J. Phys. Chem. A Downloaded from pubs.acs.org by UNIV OF ALABAMA BIRMINGHAM on 03/20/19. For personal use only.

S Supporting Information *

ABSTRACT: With the aim of achieving reversible oxidation and color tuning, the effect of the central aromatic on the spectroscopic, electrochemical, and spectrochemical properties of a series of electrochromic azomethine triads was investigated. The absorption of the alkylated thiophene derivatives was blue-shifted relative to their unalkylated counterparts when the central aromatic was either a bi- or terthiophene. It was further found that the alkylated thiophene derivatives had larger Stokes shifts than their unsubstituted counterparts. Theoretical calculations demonstrated that the torsion angles of these alkylated cores with respect to the flanking azomethines were responsible for the spectroscopic effects. While the electrochemical oxidation potential of the triads varied by only 100 mV, the reversibility of their anodic process was contingent on the central aromatic. The absorption of the electrochemically produced state red-shifted between 165 and 280 nm from its corresponding neutral state, leading to perceived color changes between orange and blue. Reversible color changes were chemically mediated with ferric chloride/hydrazine. The absorption of the chemically oxidized state shifted between 155 and 220 nm from the corresponding neutral state, contingent on the central aromatic. The palette of perceived colors that was possible with oxidation included orange, yellow, blue, and gray.

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INTRODUCTION Conjugated organic materials have received much attention owing to their potential uses in plastic electronics.1−3 Activity in the field continues with the development of new and enhanced synthetic methods that have resulted in improving the properties of conjugated materials.4,5 Optical property enhancement of organic materials is also possible by incorporating an intrinsic electron-withdrawing azomethine bond into the conjugated framework.6,7 Such functional and conjugated azomethines have been successfully used as active layers in photovoltaics8−12 and light-emitting devices13,14 to name but a few plastic organic-related devices. Although many azomethine derivatives have been prepared and extensively examined,15−18 their concise structure− property relationship remains relatively underexplored. This is particularly the case with conjugated thiophenes that are prepared from 2,5-diamino-3,4-diethylester thiophene, such as 1A and 5 (Figure 1).19,20 Understanding their structural effects on the optical and electrochemical properties is of particular importance for the design and subsequent preparation of azomethines that have desirable properties. Notably, reversible electrochemical oxidation and visibly detectable color changes with applied potential are key properties that are required for using azomethines in electrochromic applications. The exact structural requirements for achieving these two key properties, especially color tuning of both the neutral and oxidized states, are not completely understood. As a result, the palette of colors that can be achieved and the electrochemical stability of © XXXX American Chemical Society

azomethines lag behind their extensively studied all-carbon counterparts that have successfully been used as the color switching material in electrochromics.21 Accurately correlating the azomethine structure with color and electrochemical reversibility would provide pivotal knowledge for designing and preparing azomethines whose properties could challenge currently used electrochromic materials. To this end, we investigated the effect of varying the central thiophene of the azomethine triads 1 on their spectroscopic, electrochemical, and spectrochemical properties. More specifically, the effects of varying the aromatic core between the two thiophenoazomethines to include thiophene, bithiophene, terthiophene, thiophene vinylene, and their alkylated derivatives are herein reported to provide pivotal insight into the structure−property relationships of the conjugated azomethines.

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RESULTS AND DISCUSSION Derivatives 1−4 were targeted in order to assess the impact of the central aromatic on each compound’s spectroscopic, electrochemical, and spectrochemical properties. For example, alkylation of the central thiophene (1B, 2B, and 3B) was expected to prevent cross-coupling of the oxidized intermediate in the thiophene 3,4-positions,24 resulting in reversible electrochemical oxidation. It was also expected to lower the Received: October 30, 2018 Revised: January 10, 2019

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DOI: 10.1021/acs.jpca.8b10593 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Figure 1. Azomethine triads prepared and examined (1B−4) along with their structurally inverted representative counterpart (5) and previously reported benchmark 1A.22,23

oxidation potentials and cause spectral shifts owing to its weak donating effect. Reducing the oxidation potential to less positive values along with increasing bathochromic spectral shifts were expected when progressing from 1 to 3. These variations were expected as a result of the enhanced degree of conjugation when increasing the number of thiophenes in the central aromatic. Significant spectral and electrochemical differences were also expected for the vinylene derivative (4) compared to its counterpart 2A. As expected, owing to their structural differences, significant spectral shifts were observed for 1−4 in both the absorption and fluorescence (Figure 2 and Table 1). Significant differences in the spectra are seen in the combined normalized spectra in Figure 2A and the data in Table 1. The absorption maximum of both the unalkylated bi- and terthiophene derivatives was red-shifted by 10 nm from that of the previously reported benchmark thiophene derivative 1A.22,23 This is consistent with a slight increase in the degree of conjugation by increasing the number of thiophenes placed between the two azomethines. The vinylene derivative (4) was additionally red-shifted by 10 nm compared to 2A, confirming its increased degree of conjugation. In contrast, the absorption of 2B and 3B was blue-shifted from 1A. This suggests that alkylation plays an important role in disrupting the conjugated framework, most likely by twisting the adjacent thiophenes out of co-planarity (vide infra). The absence of vibronic structure in the absorption spectra of 2B (Figure S3) and 3B further illustrates their reduced rigidity compared to their unalkylated counterparts. This confirms unstructured ground and excited states of the alkylated derivatives, most likely by rotation around the thiophene−thiophene bond. The same trend in spectral shifts as a function of the central aromatic observed for the absorption was also found for the fluorescence. Meanwhile, a vibronically structured emission was observed for only 2A and 3A (Figure 2B), suggesting a rigid structure in the excited state. As seen in Figure 2B, the low signal-to-noise ratio qualitatively illustrates the weak emission of the azomethines. In fact, the heteroatomic bond deactivates the excited state, with low emission yields (Φfl < 10−3) being reported for similar compounds.7 Important information about the ground- and excited-state structures can nonetheless be obtained from the Stokes shifts. As per Table 1, the calculated Stokes shift is larger for the alkylated derivatives than that for their unalkylated counterparts. This implies that the excited states of the alkylated derivatives are energetically more stable than those of their corresponding unalkylated

Figure 2. Normalized absorption (A) and normalized fluorescence (B) spectra of 1B (■), 2A (●), 3B (◆), and 4 (▲) measured in dichloromethane. Fluorescence spectra were measured by exciting at the corresponding absorption maximum in nitrogen-saturated solutions.

counterparts. This can occur by coplanarization of the bithiophene and terthiophene due to stretching of the thiophene−thiophene bond in the excited state. The collective excitation and emission spectra additionally provide useful information about the spectroscopic absolute energy level difference (ΔE). It can be seen from the data in Table 1 that the ΔE of the triads are consistently ca. 2.2 eV. 2B and 3B are the exceptions, having ΔE values 0.3 eV higher. Although 2B and 3B should have an extended conjugation length owing to the increased number of thiophenes, the B


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Table 1. Spectroscopic, Electrochemical, and Spectrochemical Data of the Azomethines Investigated in Dichloromethane compound

λAbs (nm)

λAbs (nm)a

λEm (nm)

ΔE (eV)b

Stokes shift (cm−1)c

Egopt (eV)d

Eox (V)e

Ered (V)f

1Ak 1B 2A 2B

495 505 505 440

465 535 502 450

595 620 630 580

2.2 2.2 2.2 2.5

3450 3700 (3800) 3960 (4100) 5670 (5940)

2.1 2.1 2.2 2.4

−1.2 −1.4 −1.2 −1.5

−5.2 −5.1 −5.1 −5.2

3A

505

516

630

2.2

3930 (4070)

2.1

−1.1

3B

430

432

575

2.5

5770 (6100)

2.5

4

515

534

650

2.2

4000 (4340)

2.1

0.89 0.86 0.87 0.92 1.1 1.3 0.84 0.92 1.1 0.94 1.2 1.3 0.86 1.3

oxidized λAbs (nm)j

EgEC (eV)

oxidized λAbs (nm)i

(−2.65) (−2.51) (−2.69) (−2.12)

2.1 1.6 1.5 1.6

750 785 605

735 725 690 595

−5.2 (−5.04)

−3.1 (−2.68)

1.6

632l

690

−0.81

−5.3(−5.23)

−2.8 (−2.15)

1.4

595

−1.2

−5.2 (−5.11)

−3.1 (−2.78)

1.7

690 (900)l

HOMO (eV)g (−5.18) (−5.12) (−5.14) (−5.31)

LUMOg,h (eV) −3.1 −3.0 −3.0 −2.8

a Theoretically calculated values using TD-DFT B3LYP/6-31G(d). bAbsolute energy gap determined from the intercept between the normalized absorption and emission spectra. cValues in parentheses are calculated using λ2 emission. dEnergy gap calculated from the absorption onset. e Oxidation potential for the forward scan vs SCE. fReduction potential for the forward scan vs SCE. gValues in parentheses are the theoretically derived values by DFT using B3LYP/6-31G(d). hLUMO energy level value calculated according to Egopt + HOMO. iMaximum absorption of the electrochemically produced state. jMaximum absorption of the chemically produced state with FeCl3. kLiterature values.22,23,34 lShoulder, no dinstinct maximum.

Figure 3. Top-down view (left) and side-on view (right) of the calculated optimized geometries of 2A (top) and 2B (bottom). The hydrogens have been removed for clarity.

known to accurately calculate molecular geometries and their corresponding energy levels for the type of molecules examined in this study.32,33 The theoretical calculations were done using dichloromethane as a continuum model using the optimized geometry to correlate the calculated data with the experimentally measured values. Vibration calculations were also done on the optimized geometries. Positive frequencies were calculated for all of the optimized structures, confirming that the structures were indeed optimized geometries and not transition states. Valuable insight into the structural conformations was derived from the theoretically optimized geometries. The calculated geometries of the azomethines 1A and 1B were found to be similar. In these cases, the plane described by the thiophene was coplanar with the adjacent azomethines. The central thiophenes of 2A (Figure 3) and 3A (Figure S11) were also coplanar with respect to each other. The vinylene moiety of 4 was similarly coplanar with both of its adjacent thiophenes and the azomethine bonds. The HOMO frontier orbitals were found to be equally distributed across the molecules, proving

effective conjugation length is smaller than that of the corresponding counterpart 1B. This is possibly due to twisting of the thiophene−thiophene bonds that is caused by steric hindrance from the alkyl chains. Only one geometric isomer was observed for the azomethines evaluated. This was according to the characteristic 1 H NMR singlet in the ca. 8.2 ppm region. The thermodynamically stable E isomer was assumed to be preferentially formed based on crystallographic work of similar conjugated azomethines.9,25 Unequivocal evidence for this isomer and the rotational conformations responsible for the observed spectroscopic differences can be obtained from X-ray crystal structures. However, single crystals suitable for X-ray diffraction could not be obtained for any of the azomethines studied. We therefore relied on theoretical calculations to provide insight into the conformational differences. The molecular geometries and their corresponding energy levels for π-electron conjugated molecules were calculated using DFT methods with the B3LYP functional and the 6-31G(d) basis set.26−31 This level of theory was chosen because it is C


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The Journal of Physical Chemistry A the extended conjugation with efficient delocalization across the framework owing to the coplanar configuration (see the Supporting Information). The coplanar configuration of the central thiophenes with the azomethines is apparent in the side-on view of 2A in Figure 3. In contrast, the alkyl groups of 2B and 3B caused significant degrees of twisting between the adjacent thiophenes in the central heterocycles. More specifically, the torsional angle between the central bithiophenes in 2B was ca. 84° (Figure 3). The azomethine was also twisted by 14° from the plane described by the terminal thiophenes. The outer thiophenes of the terthiophene moiety of 3B were twisted by ca. 72 and 75° from the plane described by the central thiophene. The significant torsional angle between both of the heterocycles of bithiophene and the outer thiophenes for the butylated derivative 2B is clearly seen in the bottom panel of Figure 3. The blue shift observed in the absorption spectra for the two alkylated derivatives 2B and 3B relative to their unsubstituted counterparts was expected to be from the nonzero torsion angle between the central aromatic and its connected azomethines. This twisting limits the overall degree of conjugation. The effect of twisting the azomethine−central aromatic bond on the spectral properties was further evaluated by the TD-DFT calculations. Although true absolute values cannot be theoretically calculated for a given compound, the relative values in a series of similar compounds can be accurately derived. As expected, the same trend in absorption shift contingent on structure was observed for both the experimental and theoretical values (Table 1). More specifically, the theoretically calculated absorption maximum of the alkylated derivatives 2B and 3B was blue-shifted by 52 and 84 nm, respectively, relative to their unalkyated counterparts. The consistent theoretical and experimental values indirectly validate the theoretically optimized structures. The data further demonstrate that alkylation of the central thiophenes affects the spectroscopic properties. The electrochemical reversibility of the azomethine thiophene derivatives is known to be contingent on structure. For example, derivatives of 5 undergo reversible oxidation with substituents in the 2,2′-positions of the external aromatics.34,35 1A was previously found to also exhibit a reversible behavior.22,23 The oxidation potential of the azomethines is further known to be contingent on the structure, with the redox potential depending on the π-donor strength of the aromatics.34 The effects of the thiophene linking the two azomethine connections on both the oxidation potential and electrochemical oxidation reversibility were therefore investigated by cyclic voltammetry. Equimolar ferrocene was added to the various samples to serve as an internal reference for easy comparison of the different voltammograms based on the reversible Fc/Fc+ couple. To further facilitate direct comparisons of the redox potentials of the azomethines studied, regardless of electrochemical reversibility, the forward waves for the electrochemical oxidation (Eox) and reduction (Ered) are reported (Table 1). The anodic processes were preferentially examined because thiophenes are known for their oxidation behavior. As seen in Figure 4, there are appreciable differences in the anodic cyclic voltammograms of the azomethines. Regardless of the type of aromatic core, the first anodic process consistently occurred at ca. 880 mV. Taking the previously reported 1A as a benchmark, the effect of the different cores on the Eox is apparent.22,23,34 The Eox of 1B decreased by 30 mV relative to

Figure 4. Anodic cyclic voltammograms 1A (□), 1B (■), 2A (●), 2B (○), 3A (◊), 3B (◆), and 4 (▲) measured in dichloromethane at 100 mV/s with TBAPF6 as the supporting electrolyte. Ferrocene was added as an internal reference.

that of 1A, owing to the weak donating effect of its alkyl groups. In contrast, the Eox values of 2B and 3B were more positive than those of their corresponding unalkylated counterparts. The reduced overall degree of conjugation of these azomethines (vide supra) is a result of the large torsion angle between the bi- and terthiophenes. The Eox of the unalkylated derivatives became less positive with the increasing number of thiophenes in the central moiety. This trend is consistent with the increased π-donating character of the biand terthiophenes. The similar Eox values of 2A and 4 implies that the latter adopts a coplanar arrangement and it is fully conjugated. The adopted configuration is consistent with the known X-ray structures for the derivatives of 2A and 4.22,25 The compounds also underwent electrochemical reduction ranging between −800 mV and −1.2 V. The observed cathodic process was irreversible, regardless of the central thiophene structure. In the case of 1A, 2A, and 1B, only one oxidation process was observed. This is in contrast to 2B, 3A, and 3B that exhibited three oxidations, whereas two processes were observed for 4. Of particular interest is the reversibility of the anodic process, indicating the stability of the electrochemically produced state. A desired reversible oxidation was observed for only 1A34 and 1B. This behavior is consistent with other alkylated derivatives of 1B.34,39 The absence of reversible oxidation for the other triads was surprising given that alkylated thiophenes are known to increase the stability of radical cations by preventing their cross-coupling. This suggests that the oxidized intermediate of the alkylated derivatives reacts via other modes.36,37 This aside, the spectroelectrochemistry of the azomethines was examined to determine the effect of the aromatic core on the color of the electrochemically generated state. For these measurements, a constant potential slightly greater than the corresponding Eox was applied, and the resulting absorption spectra were recorded at given intervals. Representative spectroelectrochemical spectra are found in Figure 5. For 2B, applying a potential slightly more positive than its Eox, measured by cyclic voltammetry, caused the absorption of the neutral state to bleach, with the concomitant growth in absorption of the oxidized state at 605 nm. Applying a potential of −100 mV reduced the oxidized state to the neutral state. This gave rise to the absorption bleaching at 605 nm with D


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Figure 6. Absorption spectra of 3B measured in dichloromethane (black ), chemically oxidized with FeCl3 (blue ), followed by neutralization with hydrazine hydrate (red ). Inset: photographs showing the color of 1B (A), 2A (B), 2B (C), 3A (D), 3B (E), and 4 (F) in dichloromethane (left), with the addition of FeCl3 (middle) and subsequent addition of hydrazine hydrate (right).

Figure 5. Change in absorption spectra of the electrochemically oxidized 2B measured at 1 min intervals with increasing applied potentials from −100 to −300 mV vs Ago in dichloromethane with TBAPF6 as the supporting electrolyte.

the concurrent formation of the original absorption of the neutral state. A concise isosbestic point can be seen at ca. 500 nm in Figure 5, confirming the interdependence of the two states. Similar reversible spectroelectrochemical behavior was observed for 1A34 and 1B, whose oxidized states were bathochromically shifted by 255 and 280 nm, respectively, from their corresponding neutral states. In contrast, no spectroelectrochemically induced reversible color change was observed for the other derivatives. Although spectroelectrochemistry provides insight into the color of the oxidized state, it does not provide any information about the stability of the intermediate. This is partly due to the limited diffusion in the narrow optical path cuvette used for the measurements. We therefore turned to chemical oxidation using ferric chloride.38 The stability of the azomethine can further be evaluated by comparing the original absorption spectrum with that obtained after chemically reducing the oxidized species with hydrazine. Any spectral differences between the original and the chemically reduced oxidized state spectra would confirm degradation of the azomethine. As seen in Figure 6, the absorption of 3B is red-shifted by 165 nm when oxidized with ferric chloride. The original absorption spectrum with an identical intensity was restored when the oxidized state was reduced with hydrazine. The other azomethines exhibited similar reversible spectral changes when chemically oxidized and neutralized with ferric chloride and hydrazine, respectively. Even though no significant color change was observed for 3A, 3B, and 4 during electrochemical oxidization, their chemical oxidation resulted in visible spectral changes. In general, the absorption of the triads examined was red-shifted between 155 and 220 nm when oxidized with ferric chloride. The resulting reversible color changes when oxidized and neutralized with ferric chloride and hydrazine, respectively, are represented in the inset of Figure 6.

shifted relative to their unalkylated counterparts, owing to the alkylated cores twisting from the planes described by the terminal aminothiophenes and their adjacent azomethines. From the collective spectral and electrochemical measurements, it can be concluded that simple modification of the central thiophene significantly impacts the reversibility of the oxidation process and the color of both the neutral and oxidized states. Although azomethines of extended degrees are possible with multiple aromatics, suitable electrochemical and color properties for electrochromic use result from a single thiophene core. This knowledge is valuable for preparing new generations of azomethines having tailored optical and electrochemical properties. Notably, the structural effects for desired electrochemical reversibility and absorption tuning are pivotal for designing and developing electrochromic materials that can reversibly switch their colors in the visible with applied potentials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.8b10593. Experimental and synthetic details, cyclic voltammograms, calculated optimized geometries, and NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

W. G. Skene: 0000-0002-0002-1704 Notes

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The authors declare no competing financial interest.

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CONCLUSIONS A series of conjugated triads were prepared having different central thiophenes inserted between two aminothiophene azomethines. Their spectroscopic, electrochemical, and spectrochemical properties were contingent on the number of the central thiophenes and their alkylation. Notably, the absorption of the alkylated thiophene derivatives was red-

ACKNOWLEDGMENTS The authors acknowledge both the Natural Sciences and Engineering Research Council Canada and Canada Foundation for Innovation for operating and equipment grants, respectively. Both S.B. and M.-H.T. acknowledge the Fonds de recherche du Québec − Nature et technologies for graduate E


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scholarships. Dr. M. Ettaoussi is also acknowledged for assistance in preparing 3,3′3″,4,4’4″-hexabutyl-2,2′:5′2″-terthiophene. Mr. P.-O. Roy is also thanked for contributing to the manuscript. WestGrid (www.westgrid.ca) and Compute Canada Calcul Canada (www.computecanada.ca) are acknowledged for providing access to computational resources and software.

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