Hysteretic Tricolor Electrochromic Systems Based on the Dynamic Redox Properties of Unsymmetrically Substituted Dihydrophenanthrenes and Biphenyl-2,2'-Diyl Dications: Efficient Precursor Synthesis by a Flow Microreactor Method

A series of biphenyl-2,2'-diylbis(diarylmethanol)s 3, which have two kinds of aryl groups at the bay region, were efficiently obtained by integrated flow microreactor synthesis. The diols 3NO/NX are the precursors of unsymmetric biphenylic dications 2NO/NX2+, which are transformed into the corresponding dihydrophenanthrenes 1NO/NX via 2NO/NX+• upon reduction, when they exhibit two-stage color changes. On the other hand, the steady-state concentration of the intermediate 2NO/NX+• is negligible during the oxidation of 1NO/NX to 2NO/NX2+, which reflects unique tricolor electrochromicity with a hysteretic pattern of color change [color 1→color 2→color 3→color 1].


Introduction
Electrochromism [1] is a representative function of organic redox systems, by which electrochemical input is reversibly transduced into UV-Vis spectral output. A vivid color change is a desirable feature of these systems, and thus the stable cationic dye moieties such as triarylmethyliums [2] have often been adopted for this purpose. During the course of our studies on "dynamic redox systems" [3,4] that undergo reversible C-C bond-formation/-breaking upon electron transfer, we found that 9,9,10,10-tetraaryl-9,10-dihydrophenanthrenes (DHP) 1 and biphenyl-2,2'-diylbis(diarylmethylium)s 2 2+ constitute a novel series of electrochromic pairs.

Scheme 2. Preparation of precursor diols 3 under the macro batch conditions.
On the other hand, some of us recently demonstrated that monolithiation of dibromobiphenyl can be successfully conducted under the flow microreactor conditions [22,23]. By taking advantage of this process, we succeeded in the sequential introduction of two diarylmethyl units by reaction integration using flow microreactor synthesis [24][25][26] (Figure 1) thanks to fast micromixing and precise temperature control. The best result was obtained when 0.1 M 2,2'-dibromobiphenyl in THF (flow rate 6.0 mL/min) was reacted with 0.5 M BuLi in hexane (1.2 mL/min) for 0.06 s at 24 °C to generate 2-lithio-2'-bromobiphenyl, which was sequentially reacted with ketone 4O (0.2 M in THF, 3.0 mL/min), BuLi (0.5 M in hexane, 1.44 mL/min), and another ketone 4N (0.1 M in THF, 7.2 mL/min). This sequence of reactions proceeded in a short period of 15.5 s, and, unsymmetric diol 3NO with 4-dimethylaminophenyl and 4-methoxyphenyl groups was generated in high NMR yield of 92% and isolated in 73% yield after chromatography. Under similar conditions, 3NX with 4-dimethylaminophenyl and xanthenyl groups was prepared in 81% NMR yield and isolated in 61% yield. In both 3NO and 3NX, the two diarylmethyl units differ significantly in terms of their electron-donating properties, so that the combination would be suitable for suppressing the disproportionation of the cation radical intermediate (2NO +• and 2NX +• ) during the electrochemical interconversion of 1 and 2 2+ . In addition to its value in preparing the precursors of tricolor electrochromic systems shown above, the flow microreactor method can also produce another group of materials by adopting other bulky diarylketones with long alkoxy chains as an electrophile [e.g., 3,3',4,4'-tetrakis(octyloxy)-or tetrakis(hexadecyloxy)benzophenones, 4O 8 or 4O 16 ]. Unsymmetrically substituted diols 3OO 8 and 3OO 16 were obtained in good isolated yields of 68% and 62%, respectively, and could be used to generate unique dicationic dyes that are soluble in a hydrocarbon solvent [7]. The results shown above clearly demonstrate that the flow microreactor system is very effective for the sequential introduction of two bulky substituents at the bay region of the biphenyl skeleton.
According to an X-ray analysis of 1NX (Figure 2), the C9-C10 bond of the DHP unit [1.643(6) Å] is longer than standard (1.54 Å) [27], which is due to the "front strain" [28] among the aryl groups at C9 and C10, as in the case of other polyarylated cyclic compounds [29][30][31][32]. The DHP skeleton adopts a half-chair conformation with a dihedral angle of 16.2° for two benzene nuclei, which endows the molecule with an asymmetric element of helicity (P/M). In a single crystal of 1NX (space group: P2 1 2 1 2 1 ), all of the molecules adopt the same helicity, which shows that spontaneous resolution occurs. Based on the result of a VT-NMR experiment, however, the spectrum indicates C1-symmetry only at low temperature (N-methyl protons: 2.95 and 2.80 ppm in CDCl 3 ), and Cs symmetry is attained at room temperature (Tc = −40 °C). P/M-1NX readily undergoes racemization due to facile ring-flip in solution G ‡ = 11.4 kcal mol −1 at −40 °C) (Scheme 4), while this process is prohibited in the crystal.

Hysteretic Redox Behavior of Unsymmetrically Substituted Redox Pairs
The cyclic voltammograms of C2-symmetric DHPs are similar to each other, with an irreversible 2e-oxidation peak at +0.77 (1NN), +1.47 (1OO) or +1.42 (1XX) V vs. SCE in CH 2 Cl 2 , respectively [5,6]. The return wave is largely shifted to the cathode, and was assigned to the 2e-reduction peak of 2NN 2+ (−0.45 V), 2OO 2+ (+0.18 V), or 2XX 2+ (+0.50 V), respectively (Table 1). Similarly, the present unsymmetric DHPs 1NO and 1NX undergo irreversible 2e-oxidation ( Figure 3). Their oxidation potentials are close to that of 1NN, indicating that the HOMO level of 1NO or 1NX is close to that of 1NN due to the dimethylaminophenyl groups with strong electron-donating properties. The irreversibility of the oxidation wave suggests that the as-generated cation radical, 1NO +• or 1NX +• , readily isomerizes to 2NO +• or 2NX +• by C9-C10 bond fission. In the return cycle of the voltammogram of 1NO or 1NX, two cathodic peaks were seen: such behavior is quite different from that of C2-symmetric compounds.  . The reduction peaks are absent when the voltammogram is first scanned cathodically. As shown by the dotted line, the first reduction wave at +0.07 V is reversible when the scanning is reversed at −0.10 V.
Independent measurements of 2NO 2+ and 2NX 2+ confirmed that the two cathodic peaks are due to two-stage 1e-reduction processes of the unsymmetric dications. The first process is completely reversible, and corresponds to the reduction of bis(4-methoxyphenyl)methylium in 2NO 2+ or the xanthenylium moiety in 2NX 2+ . Furthermore, after scanning of the irreversible second 1e-reduction wave of 2NO 2+ and 2NX 2+ , the anodic peak due to the oxidation of 1NO or 1NX appears in the far anodic region of the voltammograms. Such redox properties can only be accounted for by assuming the reaction mechanism shown in Scheme 1, where the elongated C9-C10 bond in DHP is cleaved after just 1e oxidation of 1NO to 1NO +• [33] whereas two-fold 1e-reduction of 2NO 2+ to 2NO •• is necessary before the ring closure. 2NO +• produced from 1NO +• is more easily oxidized than 1NO [E ox (1NO) = +0.83 V; E ox (2NO +• ) = E 1 red (2NO 2+ ) = +0.10 V], and thus the steady-state concentration of 2NO +• is negligible during the electrochemical oxidation of 1NO, although the same specimen is a long-lived intermediate in the reduction of 2NO 2+ due to suppression of disproportionation. The same is true for another series of compounds, Thanks to the hysteretic interconversion in redox reactions, unique tricolor electrochromic systems could be constructed using the present unsymmetric derivatives, as shown below.

Hysteretic Tricolor Electrochromicity of Unsymmetrically Substituted Redox Pairs
Upon the electrochemical oxidation of colorless 1NO in CH 2 Cl 2 , both the blue and red chromophores grow simultaneously to develop a violet color for 2NO 2+ (Figure 4(a), isosbestic point at 310 nm). On the other hand, the red chromophore predominantly disappears in the first stage of the electrochemical reduction of 2NO 2+ (Figure 4(b), 295 nm), and the blue cation radical 2NO +• is then converted to colorless 1NO (Figure 4(c), 290 nm) even under constant-current electrolytic conditions (Scheme 5). A similar behavior, but with different colors, was observed for the xanthene derivative. Thus, colorless donor 1NX was transformed directly into green 2NX 2+ (isosbestic point: 309 nm), whereas reduction is a two-stage process; i.e., green 2NX 2+ changes to blue 2NX +• (248, 270, 300 nm) and blue 2NX +• changes to colorless 1NX (296 nm), which shows the generality of the unique pattern of the color change.
We are now developing a new series of tricolor chromic system, which also afford chiroptical properties (e.g., circular dichroism (CD)) as an additional signal, to construct the multi-output response systems (Scheme 6) [3,4,7,[52][53][54]. While the triarylmethyliums attached with chiral substituents only exhibit very small ellipticity to be used as an output signal (|| < 1.5), huge enhancement (|| > 100) could be realized by intramolecular transfer of the point chirality to the axial chirality in the biphenyl-diyl dications 2 2+ (Scheme 7) [54], in which the two triarylmethylium units are suitably arranged for effective exciton coupling [52,53]. Studies along this vein are now in progress and the results will be reported in due course.

Scheme 6.
Multi-output response based on the novel tricolor chromic systems.

Scheme 7.
Interconversion of the enantiomers of axially chiral dications 2 2+ by rotation to give a diasteromerically biased mixture due to the transfer of point chirality to axial chirality.

Preparation of 9,9-Bis(4-Dimethylaminophenyl)-10,10-Bis(4-Methoxyphenyl)-9,10-Dihydrophenanthrene 1NO
To a suspension of dication salt of 2NO 2+ (BF 4 − ) 2 (104 mg, 0.129 mmol) in THF (10 mL) was added triethylamine (3 mL) followed by SmI 2 (0.1 mol dm −3 in THF, 5.0 mL, 0.50 mmol) over 10 min at rt. The violet suspension gradually turned to ocher, and then the blue color of SmI 2 remained persistent during the addition. After stirring for 1 h and removal of THF and amine, the residue was suspended in water and extracted with benzene. The organic layer was washed with water and brine, and dried over K 2 CO 3 . Evaporation of solvent followed by chromatographic separation (

Preparation of 2-[Bis(4-Dimethylaminophenyl)Hydroxymethyl]-2'-[Bis(4'-Methoxylphenyl)Hydroxymethyl]Biphenyl 3NO via Flow Microreactor Method
An integrated After diluted with H 2 O, the whole mixture was extracted with Et 2 O. The combined organic layers were washed with water and brine, and dried over anhydrous Na 2 SO 4 . After filtration, solvent was concentrated under reduced pressure. The residue was purified by column chromatography on silica gel (hexane/EtOAc = 3) to give 3NO (291 mg) as a colorless solid in 73% yield.
THF was evaporated, and the residue was extracted with benzene. The organic layer was washed with water and brine, and dried over Na 2 SO 4 . Evaporation of the solvent gave 3.66 g of oily material containing unsymmetric diol 3NX. Two symmetric diols, 3NN and 3XX, were also formed in this reaction. Chromatographic separation on SiO 2 (AcOEt/hexane, 1/4-1/2) followed by crystallization from MeOH gave 3NX as colorless crystals (250 mg, y. 8.6%). The resulting solution was passed through R4 ( = 1000 μm, l = 200 cm). After a steady state was reached, the product solution was collected for 120 s and was treated with H 2 O to quench the reaction.

Preparation of 3,3',4,4'-Tetrakis(Octyloxy)Benzophenone 4O 8
To a solution of 4-bromo-1,2-bis(octyloxy)benzene (4.14 g, 10.0 mmol) in 50 mL of dry ether was added BuLi in hexane(1.57 M, 6.40 mL, 10.0 mmol) at 22 °C under Ar, and the mixture was stirred for 1 h. To the suspension was added N-carboethoxypiperidine (770 µL, 4.99 mmol) and the mixture was stirred at 23 °C for 20 h. After diluted with 3 M HCl aq., the whole mixture was extracted with CH 2 Cl 2 . The combined organic layers were washed with 3 M HCl aq. and 1 M NaOH aq., and dried over anhydrous MgSO 4 . After filtration, solvent was concentrated under reduced pressure. The residue was purified by column chromatography on silica gel (CH 2 Cl 2 /hexane = 2) to give 4O 8

Preparation of 3,3',4,4'-Tetrakis(Hexadecyloxy)Benzophenone 4O 16
To a solution of 4-bromo-1,2-bis(hexadecyloxy)benzene (5.01 g, 7.85 mmol) in 25 mL of dry ether and 25 mL of dry hexane was added BuLi in hexane (1.57 M, 5.00 mL, 7.85 mmol) at 23 °C under Ar, and the mixture was stirred for 1 h. To the suspension was added N-carboethoxypiperidine (600 µL, 3.89 mmol) and the mixture was stirred at 24 °C for 23 h. After diluted with 3 M HCl aq., the whole mixture was extracted with CH 2 Cl 2 . The combined organic layers were washed with 3 M HCl aq. and 1 M NaOH aq., and dried over anhydrous MgSO 4 . After filtration, solvent was concentrated under reduced pressure. The residue was purified by column chromatography on silica gel (CH 2 Cl 2 /hexane = 1) to give 4O 16 (3.

Measurement of Redox Potentials
All the redox potentials (E ox and E red ) were measured under argon atmosphere by cyclic voltammetry in CH 2 Cl 2 containing 0.1 M Bu 4 NBF 4 as a supporting electrolyte. All the values are reported in E/V vs. SCE, and Pt wire was used as the working electrode. In the case of irreversible waves, half-wave potentials were estimated from the peak potentials as E ox = E pa (anodic peak potential) −0.03 V, and E red = E pc (cathodic peak potential) +0.03 V.