Cyanine–cyanine hybrid structure as a stabilized polyelectrochromic system: synthesis, stabilities, and redox behavior of di(1-azulenyl)methylium units connected with electron-accepting π-electron systems

This paper describes preparation of the two-types of cations composed of di(1-azulenyl)methylium units based on a new structural principle of cyanine–cyanine (C–C) hybrid for the design of polyelectrochromic materials. Voltammetric analysis of these cations revealed the idealized reversible two-step, two-electron redox properties. Two-step color changes presumed by their C–C hybrid structure were revealed by their electrochemical reduction. The scope of the creation of the stabilized polyelectrochromic materials taking the new structural principle with azulene skeletons is demonstrated by two-types of examples


Introduction
2] Stabilization of the redox cycle is very important in construction of potentially useful electrochromic materials, because the molecules utilized for the application require high redox-stability.Construction of organic molecules that contain multiple redox-active chromophores is required for the preparation of novel polyelectrochromic materials, which respond to different potentials with a variety of colors. 35][6][7] The hybrid is constructed by the violene-type redox system containing delocalized closed-shell polymethine (e.g., cyanine) dyes as one or two end groups.The hybrid is expected to provide a color of a cyanine dye by an overall two-electron transfer as illustrated by the general structure with a cyanine end group in both terminals (Scheme 1).However, multiple-color changes important for the polyelectrochromic materials will not be established by the hybrid structure.9] The cyanine dyes, themselves does not possess good electrochemical properties, because of their ionic structures.However, either one or two end groups of the cyanine-based polymethine dye are replaced by another cyanine unit, in that case, two-step redox activities with multiple-color changes should be induced.We call this molecular design cyanine-cyanine (C-C) hybrid.[12][13][14][15][16][17]

Results and Discussion
Structural Principle.Electronic nature of the terminal groups constituted the cyanine end group is very important for the design of the C-C hybrid with high redox stability.The hybrid structure in Scheme 2 represents the hybrid constructed by the two cyanine units composed of equal-electronic nature, in which a cyanine substructure should be induced in either reduced or oxidized form.In contrast, the hybrid structure could be constructed by the two cyanine units with push and pull electronic nature.The general structure illustrated in Scheme 3 represents such an example.In this case, the cyanine structure A in Scheme 3 will be transform into structure C by two-electron transfer via a neutral radical state B. Expected species A and C in Scheme 3 should represent the closed shell systems.9][20] Therefore, the three-colored states will be stabilized by the C-C hybrid constructed by two cyanine units with push and pull electronic nature represented in Scheme 3. Di(1-azulenyl)(6-azulenyl)methylium ions (1a-c + ) would exemplify this type of the C-C hybrid, in which cyanine substructure should be induced in both reduced and oxidized forms (Scheme 4). 219] In contrast, the C-C hybrid structure constructed by two cyanine end groups with push and pull electronic nature should avoid such disadvantage in the conjugation in the core cyanine unit as illustrated in Scheme 5.Both the colored species D and F in Scheme 5 should also represent the closed shell systems.Therefore, the stabilized three-colored states will be achieved by the twotypes of the C-C hybrid structure constructed by the cyanine units with push and pull electronic nature illustrated by the general structures in Schemes 3 and 5 with oxidation levels varying from 1+ to 1−.
Scheme 5. General structure of the C-C hybrid prepared by four terminal groups (X and Y) with push and pull electronic nature.The presumed cyanine substructures generated by the two-electron transfer are represented in bold line in the Scheme.
For the construction of the new hybrid system represented the general structures in Schemes 3 and 5, it is very important to select highly colored polymethine end groups with high stability.][12][13][14][15][16][17] Two di(1-azulenyl)methylium units with bis(3,6-di-tert-butyl and 3-methoxycarbonyl) substituents were applied to examine the substituent effect on the azulene rings (Schemes 6-9).Introduction of the tert-butyl group into the azulene rings should increase their thermodynamic stabilities and also reversibility of both electrochemical reduction and oxidation states.3-Methoxycarbonyl derivatives might increase the stability in their presumed anionic state to decrease the strong electron-donating character of the 1-azulenyl groups.
Analogously, preparation of the corresponding bis(3-methoxycarbonyl) derivatives 4b and 7b for the precursors of 5b + and 8b + was accomplished via the reaction of methyl azulene-1-carboxylate (3b) with 2 and 6 in refluxing acetic acid 84% and 11% yields, respectively.The yield of 7b has been further improved to 35% by performing the reaction in a boiling mixture of acetic acid and toluene, probably due to the increasing solubility of 6 in the mixed solvent.Furthermore, reaction of 3a with β-phenylcinnamaldehyde (9a) in acetic acid at room temperature afforded 3,3-bis(3,6-di-tert-butyl-1-azulenyl)-1,1-diphenylpropene (10a) in 76% yield (Scheme 8).Preparation of the corresponding bis(3-methoxycarbonyl) derivative 10b for the precursor of 11b + was accomplished by the similar reaction of 3b with 9a in refluxing acetic acid in 27% yield.
The UV-vis spectra of 11a + and 11b + in visible region were characterized by two strong absorption bands, whereas, 5a + and 5b + exhibited an absorption band in this region.The longest wavelength absorption of 11a + (722 nm) and 11b + (655 nm) showed bathochromic shift by 41 nm and 39 nm, respectively, compared with that for 15a + and 15b + .Electron-withdrawing substituents on the terminal phenyl groups showed a bathochromic shift by, 2 nm in the fluoro derivative 11c + , 14 nm in nitro derivative 11d + , and 7 nm in the fluorenilidene derivative 14 + compared with that of 11a + .As expected, 3-methoxycarbonyl-1-azulenyl groups exhibited hypsochromic shift in the longest wavelength absorption band compared with that of tert-butyl derivative 11a + .The extinction coefficients of these cations 11a-d + and 14 + are approximately as large as those of cations 5a + , 5b + , 8a + , and 8b + .Thermodynamic stability.As a measure of the thermodynamic stability, the pK R + value of these cations was determined spectrophotomerically in a buffer solution prepared in 50% aqueous acetonitrile.The K R + scale is defined by the equilibrium constant for the reaction of a carbocation with a water molecule ) indicates higher stability of the carbocation.However, the neutralization of these cations was not completely reversible attributable to the instability of the neutralized products under the highly basic conditions for the pK R + measurement.After the measurement, acidification of the alkaline solutions with HCl regenerated the characteristic absorption in the visible region in 1-89% (Table 2).Therefore, the reported values correspond to the decomposition point started by the reaction with water molecule.The values are summarized in Table 2.  Cation 5a + (pK R + = 9.5) is not strongly destabilized by the 4-nitro substituent compared with that of 15a + (pK R + = 12.4), 10 in spite of the existence of the strong electron-withdrawing nitro substituent as an end group.
The pK R + value of 11a + (pK R + = 12.1) and 11b + (pK R + = 5.3) is almost equal to that of 15a + and 15b + (pK R + = 3.4), 10 in spite of the existence of two phenyl substituents as end groups with allylic cationic system.As expected by destabilization by the electron-withdrawing substituents as the end groups, bisfluorophenyl substituted cation 11c + (pK R + = 11.7),bisnitrophenyl substituted cation 11d + (pK R + = 8.7), fluorenylidene substituted cation 14 + (pK R + = 9.3) is less stable than 11a + , as similar to the results on the cations between 5a + and 15a + owing to the destabilization by the electron-withdrawing groups.
Redox potentials.Subsequently, the redox behavior of 5a + , 5b + , 8a + , 8b + , 11a-d + , and 14 + was examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV).The first and second redox potentials (in volts vs Ag/AgNO 3 ) of these cations are summarized in Table 2.The redox potentials observed at higher potentials are summarized in the Supporting Information.The first and second reduction waves of 5a + , 8a + , 11a + , and 11d + are shown in Figures 3 and 4 and those of the others are summarized in the Supporting Information.
As it is shown in Table 2, the characteristic feature of the cations with electron-accepting nitrophenyl groups was small E 1 red -E 2 red values, compared with those without the electron-withdrawing group.Indeed, 5a + showed a reversible two-step, two-electron reduction wave at −0.59 V and −0.94 V on CV due to the formation of a radical and an anionic species.The first reduction potentials of 5a + are slightly less negative compared with that of 15a + ; this indicates the electrochemical destabilization of the methyl cation by the 4nitro substituent as similar to the results on the pK R + measurements.The less negative second reduction potential of 5a + by 0.70 V, compared with that of 15a + , corresponds to the stabilization of anionic state by the 4-nitro substituent.The potential difference between the first and the second reduction waves (0.35 V) is significantly small as compared with that of 15a + (0.86 V).These results should correspond to the characteristic features of the C-C hybrid structure.The electrochemical oxidation of 5a + showed a reversible wave at 0.94 V on CV, due to the oxidation of an azulene ring to give a dication radical.The oxidation potential is almost equal to that of 15a + (E 1 ox = 0.88 V). 10 The first reduction potential of bis(3-methoxycarbonyl) derivative 5b + is slightly less negative compared with that of 5a + , as similar to the results on those of cations 15a + and 15b + (E 1 red = −0.49V). 10 The oxidation of 5b + also exhibited a wave at 1.38 V on CV ascribed to the oxidation of an azulene ring to generate a dicationic species, but without reversibility.The reduction of 8a + and 8b + showed same tendencies for 5a + and 5b + except for the slightly more negative reduction potentials as expected by the stabilization by the thienylene insertion.
The electrochemical reduction of 11a + showed two-step, two-electron reduction wave at −0.67 V and −1.48 V on CV due to the formation of a radical and an anionic species.The reduction potentials of 11b + are slightly less negative compared with those of 11a + ; this indicates the electrochemical destabilization of the methyl cations by the methoxycarbonyl substituents as similar to the results on 5a + and 5b + .
The characteristic feature of the cations with electron-accepting groups 11c + , 11d + , and 14 + was small E 1 red -E 2 red values, compared with those without the electron-withdrawing group, although difluoro derivative 11c + showed almost same value with that of 11a + .The less negative second reduction potential of 11d + by 0.62 V, compared with that of 11a + , corresponds to the increase of electron affinity of the cation by the two 4nitrophenyl substituents.The high reversibility of the CV waves and small potential difference (0.36 V) of 11d + indicate the characteristic features of the presumed C-C hybrid structure.The electrochemical oxidation of 11a-c + and 14 + showed almost similar tendency on CV with those of 5a + , 5b + , 8a + , and 8b + , due to the oxidation of an azulene ring to give a dication radical.Electrochromic analysis.Visible spectra of the two-types of C-C hybrids were monitored to clarify the color changes under the electrochemical reduction conditions.A constant-current reduction was applied to the solutions of 5a + , 5b + , 8a + , 8b + , 11a-d + , and 14 + with a platinum mesh for working and a wire counter electrodes.
When the visible spectra of 5a + were monitored in benzonitrile containing Et 4 NClO 4 (0.1 M) as a supporting electrolyte at room temperature under the electrochemical reduction conditions, the strong absorption band at 710 nm in the visible region was gradually decreased along with the development of new absorption band at around 590 nm (Figure 5).Further reduction developed a broad absorption band around the new absorption band (525 nm).Thus, the spectral changes should correspond to the two-step, two-electron reduction wave on CV arising from the formation of a radical and an anionic species.The color of the solution of 5a + gradually changed from dark green to deep purple during the electrochemical reduction.Absence of a clear isosbestic point during the electrochemical reduction suggests some decomposition of the reduced species of 5a + under the spectrophotometric measurements.Indeed, reverse oxidation of the purple-colored solution did not regenerate the spectrum of 5a + (regeneration 33%), completely, although the CV analysis revealed good reversibility in the two step reduction.Instability of the reduced species might be attributable to the reduction of nitro function under the electrochemical conditions.
Similar two-step spectral changes were observed during the electrochemical reduction of 5b + (see the Supporting Information).The dark green color of the solution of 5b + also changed to purple one during the electrochemical reduction.Reverse oxidation of the purple-colored solution regenerated the UV-vis spectra of the deep colored 5b + in 30%.Therefore, the two step color changes of 5a + and 5b + should be concluded to the formation of presumed radical and anionic species with some instability under the electrochemical conditions.We also tried electrochemical reduction of 8a + and 8b + under visible spectral monitoring (Figure 5 for 8a + and see the Supporting Information for 8b + ).We anticipated that the formation of the thienoquinoid forms during the redox reaction might improve their reversibility.In these cases, however, we also observed reversibility for the reduction of 8a + (regeneration 24%) and 8b + (regeneration 51%), insufficiently.© ARKAT USA, Inc The strong absorption band of 11a + at 732 nm in the visible region was gradually decreased during the electrochemical reduction (Figure 6).Absence of a clear isosbestic point suggests some instability of the reduced species of 11a + under the spectrophotometric measurements.The electrochemical reduction of 11b + also caused gradual color change of the solution from purple to pink (see the Supporting Information).Reverse oxidation of the reduced solution did not regenerate the spectrum of 11a + (regeneration 57%) and 11b + (regeneration 45%), completely.
We also tried electrochemical reduction of 11c + , 11d + , and 14 + under visible spectral monitoring (Figure 6 for 11d + and see the Supporting Information for 11c + and 14 + ).We anticipated that the electron-withdrawing nature of the end groups should improve the two-step color changes.The purple colored solution of 11c + exhibits color change to pink one under the reduction conditions.The spectral features of 11c + under the electrochemical reduction resembled to those of 11a + and the reversibility was still low in this case (regeneration 39%).In the case of 11d + , the two-strong absorption bands at 509 nm and 741 nm in the visible region was gradually decreased along with the development of new absorption band at around 580 nm under the electrochemical reduction conditions (Figure 6).Further reduction developed a broad absorption band spread into the wide range of visible region.As expected, the spectral changes should correspond to a twostep color changes arising from the formation of a radical and an anionic species.Absence of a clear isosbestic point during the electrochemical reduction suggests some decomposition during the two-step reduction of 11d + under the spectrophotometric measurements.Reverse oxidation of the green-colored solution did not regenerate the spectrum of 11d + (regeneration 48%), completely, although the CV analysis revealed good reversibility in the two step reduction.Similarly, color change was observed during the electrochemical reduction of 14 + (see the Supporting Information), but still low reversibility (regeneration 61%).

Conclusions
The scope of the creation of stabilized polyelectrochromic materials containing azulene skeletons has been demonstrated by the two-types of the C-C hybrid structure 5a + , 5b + , 8a + , 8b + , 11a-d + , and 14 + constructed by the cyanine units with push and pull electronic nature illustrated by the general structures in Schemes 3 and 5 with oxidation levels varying from 1+ to 1−.9] The two-types of C-C hybrids exhibit the presumed reversible two-step, two-electron redox properties on CV in the cases of the hybrids with nitro function.Two-step color changes presumed by their C-C hybrid structure were revealed by their electrochemical reduction, although the reversibility of the redox reaction on electrochromic measurements was still low for all cases.Thus, the electrochemical behavior was not ideal for the presumed C-C hybrid, since the stabilization in the anionic state is insufficient by the introduction of nitro substituents, but the use of di(1azulenyl)methylium units as a stabilized redox active polymethine units would be highly effective in the point of their high stability and their strong absorption in visible region with their redox activity.Preparations of the other-types of hybrid structures by using the di(1-azulenyl)methylium units as cyanine end groups are now in progress in our groups.

Experimental Section
General.Melting points were determined on a Stuart Scientific melting point apparatus SMP3 or a Yanagimoto micro melting point apparatus MP-S3 and are uncorrected.Mass spectra were obtained with a Bruker APEX II instrument under ESI conditions or a Bruker Daltonics autoflex III TOF/TOF instrument under MALDI conditions.IR and UV/Vis spectra were measured on a BIO-RAD FTS-30 or a JUSCO FT/IR-6100 infrared spectrophotometer, and a JASCO V-670 spectrophotometer, respectively. 1H NMR spectra ( 13 C NMR spectra) were recorded on a JEOL ECA500 or a JEOL ECZR500 spectrometer at 500 MHz (125 MHz).The peak assignment of 1 H and 13 C NMR spectra reported was accomplished by HH COSY, NOE, DEPT, HMQC, and/or HMBC experiments. 1 H chemical shifts in CDCl 3 are reported in parts per million (ppm) downfield from internal tetramethylsilane. 13C chemical shifts in CDCl 3 are referred by the solvent signals as 77.0 ppm.Chemical shifts in (CDCl 2 ) 2 are referred by the solvent signals as 5.90 ppm in 1 H and 74.2 ppm in 13 C. Gel permeation chromatography (GPC) was performed on a JAI LC-9110 NEXT with JAIGEL-1H-3H using CHCl 3 as an eluent.Column chromatography on silica gel and Al 2 O 3 was performed using MERCK Silica gel 60 Art.7734 or Cica Silica gel 60 Art.37564 and MERCK Aluminium oxide 90 Art.1097, respectively.Elemental analyses were performed at the Instrumental Analysis Center of Hirosaki University.

Scheme 1 .
Scheme 1.General structure of the Hünig's V-C hybrid system.The presumed cyanine substructures generated by the two-electron transfer are represented in bold line in the Scheme.

Scheme 2 .Scheme 3 .
Scheme 2. General structure of the C-C hybrid prepared by three terminal groups (X) with same electronic nature.The presumed cyanine substructures generated by the two-electron transfer are represented in bold line in the Scheme.

Scheme 4 .
Scheme 4.An example of the C-C hybrid prepared by the three terminal groups by azulenyl groups.

acetonitrile. 10
Regenerated absorption maxima (%) of the cations in visible region by acidification of the alkaline solution with HCl are shown in parentheses.b Redox potentials were measured by CV and DPV [V vs. Ag/AgNO 3 , 1 mM in benzonitrile containing Et 4 NClO 4 (0.1 M), Pt electrode (ID: 1.6 mm), scan rate 100 mV s -1 , and Fc/Fc + = 0.15 V]. c The peak potentials measured by DPV are shown in parentheses.d Potentials were the value measured in acetonitrile.e The wave was accompanied by second peak at −0.57V probably due to the deposition of reduced species on the electrode in acetonitrile.

4a, 4b, 7a, 7b, 10a-d, and 13 with
DDQ at room temperature and obtained as hexafluorophosphates by the treatment with hexafluorophosphoric acid.The reaction of