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Investigation of Triphenylamine−Thiophene−Azomethine Derivatives: Toward Understanding Their Electrochromic Behavior Marie-Hélène Tremblay, Thomas Skalski, Yohan Gautier, Grégory Pianezzola, and W. G. Skene* Laboratoire de caractérisation photophysique des matériaux conjugués Département de chimie, Université de Montréal, CP 6128, Centre-ville, Montreal, QC H3T 1J4, Canada S Supporting Information *

ABSTRACT: A series of thiophenoazomethines connected to a central triphenylamine were prepared. The effect of the type of aminothiophene used to prepare the conjugated azomethines and the number of azomethine bonds, ranging from one to three, flanking the triphenylamine on the spectral, electrochemical, and spectroelectrochemical properties were investigated. Both the absorption and fluorescence of the azomethines (6−8) derived from 2,5-diaminothiophene (1) were found to be contingent on the number of the azomethines. The spectral properties were redshifted with increasing the number of azomethine bonds. In contrast, the azomethines (9−11) derived from 2-aminothiophene (2) were not perturbed by the number of azomethines. However, their spectral properties were red-shifted relative to their triphenylamine counterparts derived from 1. The oxidation potentials of the azomethines were also contingent on structure. They were shifted to more positive potentials with increasing number of azomethines. The oxidation potentials of 6−8 were less positive than 9−11, owing to the electron-donating effect of the terminal amine. Reversible oxidation was observed with the triphenylamine having methyl groups in the 4′-positions. All the compounds examined underwent color changes upon both electrochemical and chemical oxidation. The oxidized state was shifted upward of 165 nm for the series 6−8 relative to the neutral state, whereas it was shifted at most by 85 nm for 9−11. The most pronounced color changes between the neutral and oxidized states occurred with the triphenylamine−thiophene−triphenylamine bisazomethine derivative (12).

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INTRODUCTION The interesting properties of triphenylamines have attracted much attention for their use in a wide range of applications.1−6 For example, triphenylamine derivatives are intrinsically fluorescent and have found many uses as fluorescence sensors.7 Meanwhile, their reversible oxidation has led to the use of triphenylamines in electrochemical devices.8,9 Owing to the significant color difference between the radical cation intermediate produced by electrochemical oxidation and its corresponding neutral state, triphenylamines have also been successfully used as the color switching layers in electrochromic devices.10−14 The benefits of using triphenylamines as the electrochromic layer in functioning devices is the persistent nature of the radical cation and its formation that can be at relatively low oxidation potentials. Other advantages of triphenylamines include color tuning of both the neutral and oxidized states and adjusting the oxidation potential. These are possible by substituting the triphenylamine with various aromatics and electronic groups.15 Conjugated azomethines have also attracted attention as suitable electrochromic materials for use in functioning devices. This is particularly the case with conjugated azomethines derived from the 2,5-diaminothiophene (1) in Figure 1.16−18 Similar to triphenylamines,19−24 the azomethines derivatives from 1 exhibit significant color differences between their neutral and oxidized states. Moreover, the colors of the neutral © 2016 American Chemical Society

and oxidized states can be modified contingent on the aromatics used for the azomethine preparation. Meanwhile, small molecule azomethines prepared from 1 exhibit reversible oxidation and oxidation potentials that are comparable to triphenylamines. Azomethines derived from 2-aminothiophene (2) have demonstrated biologically activity25,26 and include uses as antibacterial,27−30 antifungal31,32 agents, and antitumor properties. These aside, the electrochromic properties of azomethine derivatives of 2 have not been examined. In fact, to the best of our knowledge, triphenylamine azomethines derived from 2 have not been examined. We therefore prepared the derivatives 9−11 to evaluate their neutral and oxidized colored states, their oxidation potential, and their electrochemical reversibility. To better understand the effect of the thiophene without a terminal donating amine on the spectral and electrochemical properties, the corresponding azomethines derived from 1 were also prepared and both their experimental and theoretical properties were examined. The knowledge gained from such a structure− property study would be beneficial for designing and preparing new electrochromic materials that exhibit targeted colors. Received: February 18, 2016 Revised: April 8, 2016 Published: April 25, 2016 9081

DOI: 10.1021/acs.jpcc.6b01675 J. Phys. Chem. C 2016, 120, 9081−9087

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The Journal of Physical Chemistry C

Figure 2. Resolved X-ray crystal structure of 10 shown from the top (top) and side-on (bottom) views.

were twisted by 64.5° with respect to each other. There is therefore limited conjugation between the two thiophene−N CH−phenyl arms of the triphenylamine. The derivatives 9−11 were targeted to provide information about the impact of the number of azomethine bonds on the spectroscopic and electrochemical properties of the central triphenylamine. To better understand the properties, the aminothiophene derivatives 6−8 were additionally investigated. The targeted 9−11 were expected to absorb in the visible owing to the conjugated π-donor−acceptor−donor arrangement of their conjugated thiophene−azomethine−phenylamine structure. The spectroscopic properties of the 6−8 were expected to be enhanced compared to their corresponding counterparts as a result of the pronounced donor effect of the terminal amine. Similarly, the effect of inverting the structure by replacing the central triphenylamine with the thiophene could be tracked by comparing 7 and 12. The collective structural and electronic differences were expected to lead to significant differences in both the absorption and fluorescence spectra. As expected, significant differences in the absorption spectra can be seen in Figure 3. The effect of the thiophene (1 vs 2) used to prepare the azomethines can be seen by comparing the spectral properties of 6 vs 9, 7 vs 10, and 8 vs 11. In the case of the monoazomethine, the absorption of 9 was red-shifted by 20 nm relative to 6 (Table 1). The shift was reduced by half for 10. In contrast, no appreciable difference was observed for the trisazomethines 8 and 11. The fluorescence difference between the two analogous series was consistent with the absorption, but with the exception of being enhanced. For example, the fluorescence was red-shifted by 45 and 25 nm respectively for the 6/9 and 7/10 pairs. No fluorescence shift was observed for the 8/11 set, similar to what was observed for their absorption. The spectral shifts of 9−11 are presumed to be from their electronic push−pull effect of the triarylamine and azomethine, respectively. The withdrawing effect of the azomethine is enhanced with its conjugated nitrile for 6−8, whereas the

Figure 1. Triphenylamine−azomethines prepared and examined in addition to their aminothiophene and triphenylamine precursors.

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RESULTS AND DISCUSSION The targeted 9−11 and their counterparts 6−8 and 12 were prepared by condensing the corresponding aldehyde with either 1 or 2. The desired products were confirmed by NMR according to the characteristic chemical shift of the imine proton in the 8−8.5 ppm region. In all cases, only one singlet was observed. This confirmed the formation of only one imine isomer, assumed to be the thermodynamically stable E-isomer. To confirm both the structure and isomer of the imine bond, crystallizations of 9−11 were attempted. Only in the case of 10 were crystals suitable for the X-ray diffraction obtained. The resolved crystal structure is shown in Figure 2. The absolute configuration (E) of the imine bond can readily be assigned from the crystal structure. Additional salient features of the resolved structure are the near coplanarity of the thiophene and phenyl segments with the imine bond (bottom, Figure 2). The planes described by the thiophene and phenyls connected by the imine bonds were found to be twisted by 8.8° in one arm and 24.5° in the other. The coplanarity of the aromatics connected to the imine is consistent with analogous azomethines derived from 2.33−35 The CN distance was measured to be 1.286(4) Å for one arm and 1.275(4) Å for the other arm. The bonds are shorter than the measured thiophene−N (1.386(4) Å) and phenyl−CH (1.495(4) Å) bonds, confirming their double-bond character. It is noteworthy that the planes of the two phenyls connected the azomethines 9082

DOI: 10.1021/acs.jpcc.6b01675 J. Phys. Chem. C 2016, 120, 9081−9087

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absorption and emission were red-shifted with increasing number of azomethines. The absorptions for 7 and 8 were red-shifted by 35 and 45 nm, respectively, relative to 6. The shifts were less for their fluorescence, being 5 nm for 7 and 30 nm for 8 compared to 6. In contrast, the absorption and fluorescence of 9−11 varied little with the number of azomethine bonds. As seen in Figure 3B, the fluorescence spectra are undefined. This is in contrast to the absorption spectra of 7−11 that have vibronic bands. The latter imply a rigid configuration is adopted in their ground state, unlike the excited state, whereas the large Stokes shift (Table 1) of 6, 9, and 12 confirm a pronounced stabilization of their excited state compared to their ground state. Although fluorescence could be measured for all the compounds, their quantum yield was too low to be measured by absolute means. The weak fluorescence can qualitatively be seen in the inset of Figure 3B. The fluorescence deactivation modes are assumed to occur by intramolecular photoinduced electron transfer and E → Z photoisomerization.36−38 The fluorescence quenching pathway can be partially suppressed by chemically oxidizing the azomethine with ferric chloride. While the exact mechanism for the fluorescence enhancement with oxidation has not yet been confirmed, the effect is obvious, as seen in the photographs in the inset of Figure 3B. To better understand the spectroscopic observations, the theoretical properties of the azomethines were calculated. The X-ray data of 10 were used as the starting geometry to accelerate the DFT calculations. The geometries of the fully optimized compounds were in good agreement with the experimentally measured X-ray data. As seen in Table 1, the theoretically derived absorbance from time dependent DFT are consistent with the measured values. The principal transitions involved in the absorbance were calculated to be from the HOMO directly to the LUMO levels (Figure 4). These are consistent with the vibrational structures in the absorption spectra. The calculated LUMO energies for the two series (6−8 and 9−11) were similar (Table 2). Only a slight stabilization, represented by a lower LUMO energy level, was calculated with the addition of a second azomethine bond on the central triphenylamine. In contrast, no additional stabilization was gained with a third azomethine bond. This is consistent with the spectroscopic data. The absence of stabilization with the third azomethine is owing to the lack of molecular orbital overlap of the azomethine with the triphenylamine. The same trend of increasing stabilization with increasing number of azomethine bonds was calculated for the HOMO level, albeit only a small net stabilization. The calculated energy difference between the HOMO and LUMO levels is consistent with the experimentally derived values. Similar to the spectral properties, significant differences in both the oxidation potential and stability of the oxidized intermediate were expected contingent on structure. For example, irreversible oxidation was expected for 6, 7, 9, and 10, owing to radical cation coupling reactions occurring with the unsubstituted phenyl.39 Also, the oxidation potentials for 6−8 were expected to be less positive than their corresponding 9−11 because of the electron-donating effect of the terminal amine. The enhanced donor character of the 6−8 series relative to the 9−11 was also supported by the theoretical calculations (vide supra). Differences in the oxidation potentials were also expected contingent on the number of azomethines.

Figure 3. Normalized absorption (A) and fluorescence (B) spectra of 6 (black), 7 (red), 8 (blue), 9 (green), 10 (orange), 11 (navy), and 12 (wine) measured in dichloromethane. Inset: photographs of 6 with varying mole fraction of ferric chloride in dichloromethane irradiated with a UV lamp from left to right: 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.

Table 1. Spectroscopic Values of the Azomethines Investigated in Dichloromethane compd

λabsa (nm)

λabsb (nm)

λem (nm)

Stokes shift (cm−1)

6 7 8 9 10 11 12

420d 455d, 405sh 465d 440 465, 415sh 460 502

446 448 470 464 489 485 474

510 515 540 555 540 537 640

4200 2560 2985 4710 2990 3120 4300

ΔEc (nm) 460 485 490 490 500 495 575

(2.69) (2.56) (2.53) (2.53) (2.48) (2.50) (2.16)

a

d = doublet with the center value being reported; sh = shoulder. Theoretically calculated values using with B3LYP/6-31G(d). c Absolute energy difference calculated from the intercept between the normalized absorbance and emission spectra. Values in parentheses are in eV. b

electronic pull effect of the azomethine bond of 6−8 is dampened by the other substituents on the thiophene. Evidence of conjugation between the central triphenylamine and azomethine can be had from the X-ray structure. The structure shows the central amine of the triphenylamine adopts a trigonal planar configuration (Figure 2, bottom panel). This configuration is supported by the three angles of the triphenylamine that sum to 359.7(6)°. The near trigonal planar configuration is most likely a result of collective electronic and steric effects. This a side, flanking the central thiophene with two triphenylamines in 12 led to the most pronounced spectral changes. In fact, the absorption and fluorescence of 12 were shifted by 47 and 125 nm, respectively, compared its structurally inverted counterpart 7. Only in the case of 6−8 were spectral differences observed contingent on the number of azomethine bonds. Both the 9083

DOI: 10.1021/acs.jpcc.6b01675 J. Phys. Chem. C 2016, 120, 9081−9087

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The Journal of Physical Chemistry C

Figure 4. Calculated HOMO and LUMO frontier orbitals and their corresponding energy difference.

Table 2. Electrochemical and Spectroelectrochemical Values of the Azomethine Derivatives compd

Eoxa (mV)

6 7 8 9 10 11 12

340 365 455 635 680 755 415

HOMOb (eV) −5.03 −5.06 −5.05 −5.30 −5.36 −5.38 −5.14

(−5.28) (−5.01) (−5.00) (−5.46) (−5.29) (−5.33) (−5.07)

LUMOc (eV)

λElec/oxd (nm)

λChem/oxe (nm)

Δλf (nm)

Tg (%)

−2.17 −2.29 −2.38 −2.47 −2.40 −2.58 −2.20

585 (blue) 585 (blue) 615 (green) 525, 610sh (purple) 532 (broad; red), 625sh 540 (broad; transparent) 700 (black)

580 590 610 520 540 545 640

165 130 150 85 67 80 198

22 16 44 7 26 53 15

a

Oxidation potential for the forward scan relative to the reversible ferrocene (Fc/Fc+) redox couple. bCalculated from the electrochemical data. Values in parentheses are theoretically calculated by DFT means. cTheoretically derived values by DFT using B3LYP/6-31G(d). dAbsorption maximum observed when applying an oxidation potential slightly more positive than Eox. Observed color by the standard user is listed in parentheses; sh = shoulder. eAbsorption maximum observed with ferric chloride. fAbsorption difference between the neutral and electrochemically oxidized states. gMaximum transmission % of the oxidized state monitored at the corresponding maximum absorption when applying a given potential for 1 min.

Significant differences in both the oxidation potential and reversibility of the oxidation processes can be seen from the collective anodic cyclic voltammograms in Figure 5. Notably, 6,

9, 10, and 12 undergo reversible oxidation. This is not surprising for 12, owing to the substitution in each of the 4′ phenyl positions that is expected to prevent radical cation coupling. In contrast, the oxidation of 7 and 8 was irreversible, whereas the oxidation of 11 was quasi-reversible. Although the oxidation potentials of 9−11 are higher than their counterparts 6−8, they are more interesting as electrochromic candidates, owing to their reversible-like oxidation Taking 6 as benchmark, the effect of the number of azomethine bonds and the thiophene derivative on the oxidation potential can be seen from the data in Table 2. Each azomethine added to the central triphenylamine increases the oxidation. This is consistent with the intrinsic electronaccepting character of the azomethine. The oxidation potentials of the azomethines derived from 1 were consistently lower than their counterparts derived from 2. This is owing to the electron-donating effect of the terminal amine of 6−8. The spectroelectrochemistry of the azomethines was examined to further assess the impact of structural modification on the color of the oxidation state. All the azomethines examined underwent significant spectral changes upon electrochemical oxidation. A representative change in absorption with electrochemical oxidation and the resulting color differences between the neutral and oxidized states are seen in Figure 6A. In the case of 12, the original red color was bleached, and it was replaced with a blue color when oxidized. The original color of the neutral state could be restored by reducing the oxidized state by applying a zero potential. As seen in Figure 6A, when

Figure 5. Cyclic voltammograms of 6 (black), 7 (red), 8 (blue), 9 (green), 10 (orange), 11 (navy), and 12 (wine) in degassed dichloromethane with ferrocene used as an internal reference, [C] = 10−3 M, and TBAPF6 as the electrolyte, [C] = 0.1 M measured at 100 mV/s. 9084

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with hydrazine. The similar absorptions observed with chemical and electrochemical oxidation suggest the same intermediate is produced, which is assumed to be the radical cation. The absorption was further red-shifted upon adding an excess of ferric chloride (Figure 6C). The gas phase optimized energy levels of the neutral, oxidized, and doubly oxidized states were calculated to validate the spectroelectrochemical results for 6. It can be seen in Figure 7 that upon oxidation both the HOMO and SOMO levels are

Figure 6. (A) Spectroelectrochemistry of 12 measured in dichloromethane with increasing applied potentials of 0 (black), 0.1 (red), 0.2 (blue), 0.4 (green), and 1.1 V (orange). (B) L*a*b* CIE coordinate representation of the neutral and oxidized states of 6. (C) Absorption spectra of 8 with increasing (0 to 20) equivalents of ferric chloride. Inset: photographs showing the change in color from the undoped (left) and doped (right) states of 6 (A), 7 (B), 8 (C), 9 (D), 10 (E), and 11 (F) with ferric chloride.

Figure 7. Calculated HOMO, HOMO(−1), SOMO, and LUMO energy levels of 6.

stabilized. The net effect is a decrease in the energy gap. As per the calculated energy levels, 6 is expected to be red-shifted when oxidized to the radical cation, assuming the principal transition in the visible spectrum is the SOMO → LUMO transition. Similarly, the dication is expected to be further redshifted. While the absolute values cannot be directly compared between the experimental and theoretical values, the relative values between the neutral and oxidized states for the measured and calculated are consistent.

12 is oxidized, the absorbance at 550 nm bleaches concomitant with new absorbances at both 350 and 700 nm. The 198 nm difference between the neutral and oxidized states of 12 leads to a stark color difference between the two states. In solution, the reversible switching was limited to a few cycles before the onset of color fatigue. This aside, the maximum transmission % of the oxidized state was found to be contingent on structure, with the maximum color change being observed for the trisazomethines (8 and 11; Table 2). For 6−8, their oxidized state was red-shifted by up to 165 nm from their corresponding neutral state. The associated color upon oxidation was intense blue. Whereas, red-shifts between the neutral and oxidized states of 9−11 were only ca. 75 nm. Quantitatively, the color change can be tracked by the CIE L*a*b* color space (Figure 6B). The L* parameter relates to the color lightness, with a low value corresponding to dark color, whereas the a* and b* values refer to the red-green and yellow/blue components, respectively. The L* value of the oxidized state of all the examples studied decreased with the exception of 12. This confirms that the color darkens upon oxidization. The a* parameter of 6−9 and 12 decreased when oxidized, confirming their increased apparent green color. In contrast, the a* value of 10 and 11 increased upon oxidization. This is associated with an increase in the red component. The b* parameter of all the compounds decreased when oxidized, resulting in their perceived blue color. Similar absorptions and colors were also observed when oxidizing the azomethines with ferric chloride (inset Figure 6C). In the case of chemical oxidation, the absorption and apparent color of the original neutral state could be restored upon reducing the oxidized state

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CONCLUSION In conclusion, a series of triphenylamines conjugated with thiophenes via azomethines were prepared. These served as model compounds to investigate the effect of the type of thiophene, number of azomethine bonds, and placement of the triphenylamine on the spectroscopic, electrochemical, and spectroelectrochemical properties. While the absorption of the neutral state varied only by ca. 80 nm contingent on structure, the absorption of the oxidized intermediate was markedly shifted from the neutral state. In fact, the absorption of the oxidized state could be shifted upward of 198 nm from its corresponding neutral state, contingent on structure. The resulting colors observed for the oxidized states were red, blue, green, mauve, and gray. The large absorption difference between the neutral and oxidized states is a coveted property for electrochromic materials. An additional property desired with electroactive materials is a reversible oxidation. Such a process was possible with the 4′-methyl-substituted triphenylamine derivative. The collective spectroscopic, electrochemical, and spectroelectrochemical properties concomitant with the significant color differences between the neutral and oxidized states demonstrate the suitability of triphenylaminoazomethine derivatives as electrochromic materials. The data further demonstrate that significant property modulation of azomethines is possible through structural modification. 9085

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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.jpcc.6b01675. Additional details of materials, experimental methods, spectroelectrochemical data, and CIE coordinates of the neutral and oxidized compounds (PDF) Structure of C38H31N5S2 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W.G.S.). Author Contributions

M.-H.T. and T.S. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge both the Natural Sciences and Engineering Research Council Canada and Canada Foundation for Innovation for operating and equipment grants, respectively. M.-H.T. acknowledges the Fonds de recherche du Québec−Nature et technologies for a graduate scholarship. WestGrid and Compute Canada/Calcul Canada are acknowledged for access to software and computational resources. The assistance of Dr. M. Simard is also greatly appreciated for assistance with the X-ray diffraction analysis.

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DOI: 10.1021/acs.jpcc.6b01675 J. Phys. Chem. C 2016, 120, 9081−9087

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DOI: 10.1021/acs.jpcc.6b01675 J. Phys. Chem. C 2016, 120, 9081−9087