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- Results and Discussion
- Supporting Information
o-Quinodimethane (oQD) is a unique molecule that contains a cross-conjugating π-electron system. This hydrocarbon as well as its derivatives are considered versatile building blocks in synthetic chemistry due to their high reactivity in stereoselective Diels–Alder reactions.1 In the case of sterically hindered 7,7,8,8-tetraaryl-o-quinodimethanes (Ar4oQDs), in which four benzene rings are attached to exocyclic carbons, such intermolecular reactivities are suppressed due to steric congestion, which also causes the unique helical arrangement of π electrons, similar to the cases of helicenes consisting of ortho-condensed benzene rings. However, unlike many helicenes, Ar4oQDs are unstable. Spectroscopic characterization can be conducted only at a low temperature due to a facile isomerization to 1,1,2,2-tetraarylbenzocyclobutene (BCB) or 9,9a-dihydro-9,9,10-triarylanthracene (DHA) through electrocyclization (Scheme 1).2
If spontaneous transformation into cyclized isomers could be suppressed, Ar4oQDs would become available as thermodynamically stable species.3 Keeping this in mind, we designed 9,10-bis(diarylmethylene)phenanthrene (Ar4DMP, 1),4 because dibenzo annulation5 would destabilize the corresponding BCB and DHA isomers, but not Ar4DMP itself, by an additional steric repulsion.6 This idea was supported by the PM3 calculation for Ph4DMP, which has a much lower heat of formation than BCB- and DHA-type isomers (Figure S1 in the Supporting Information).
Because 1,1,4,4-tetraarylbutadienes7 are important members of violene/cyanine-hybrid-type7, 8 electrochromic systems,9 stabilized Ar4DMPs 1 may also serve as new chromic materials. If a suitable electron-donating group, such as an alkoxy group, is attached to each of the aryl rings of 1, the corresponding dications 22+ should become stable enough for isolation of their salts that exhibit strong absorption in the visible region, which is characteristic of triarylmethylium dyes.10
Because the redox-active chromophore is framed in the helically deformed skeleton of Ar4DMPs 1, a drastic change in geometry is expected upon electron transfer. Two of the four aryl rings in 1 are forced to overlap in proximity to cause a large steric repulsion,11 whereas two-electron oxidation would induce twisting of the diarylmethylium units around the exocyclic bonds [C9(10)C+] against the planarized phenanthrene core in 22+ so as to reduce the steric repulsion. Such a structural change is favorable for the construction of molecular response systems in terms of reversibility and bistability, which has been demonstrated in studies on “dynamic redox systems”.12 Another interesting feature of the redox pair of 1/22+ is the chiral element13 of the helicity of their skeletal frameworks, although the stereoisomers of the dications cannot be separated due to a rapid inversion of their helical sense by rotation around C9(10)C+ bonds [(P)-22+<=>(M)-22+]. It was found here that configuration of the helical sense in Ar4DMPs 1 is also unstable [(P)-1<=>(M)-1] (Scheme 2), similar to other DMP compounds.14
Herein, the successful generation and isolation of a series of (4-ROC6H4)4DMPs 1 a–1 d [RO=CH3O, C8H17O, (R)-C2H5CH(CH3)O, (R)-C6H13CH(CH3)O] as new isolable examples of Ar4oQD is reported.4 The helically deformed structure in 1 as well as the twisted geometry in the dication 22+ were demonstrated by low-temperature X-ray analyses. As was designed, quinodimethane donors 1 and phenanthrene-9,10-diyl dications 22+ constitute reversible electrochromic pairs that exhibit a vivid color change from yellow to violet. Notably, the point chirality of the chiral alkoxy group attached on the aryl rings is transmitted to a helicity preference15 to bias the diastereomeric ratio of 2 c2+ and 2 d2+, which enables us to newly construct electrochiroptical systems that give two kinds of spectral output [UV/Vis and circular dichroism (CD)] in response to electrochemical input (Scheme 3).16
Results and Discussion
- Top of page
- Results and Discussion
- Supporting Information
Preparation and helical geometry of 9,10-bis(diarylmethylene)phenanthrenes (Ar4DMPs, 1):
Synthetic strategy for 1: Sterically hindered molecules are generally difficult to obtain, and successful syntheses often adopt a common approach, in which they are generated from a less-hindered precursor by intramolecular reactions to minimize the disadvantage associated with entropy.17 Against this background, we pursued the formation of Ar4DMPs 1 from less-hindered dications 22+, which in turn would be obtained from 2,2′-bis(diarylethenyl)biphenyls 3 (Scheme 4). The oxidative cyclization of 3 to 22+ consists of several steps: 1) formation of a CC bond between the benzylidene carbons upon the two-electron oxidation of 3; 2) double deprotonation from the resulting butane-1,4-diyl dication18 to give 1; and 3) the further two-electron oxidation of 1 to furnish 22+.
As shown in Scheme 5, the reaction of 2,2′-dimethylbiphenyl with BuLi in the presence of TMEDA gave 2,2′-bis(lithiomethyl)biphenyl,19 which was then treated with 4,4′-dimethoxybenzophenone 5 a in THF to give bis[bis(4-methoxyphenyl)ethanol] derivative 4 a. Treatment with a catalytic amount of TsOH in benzene at reflux gave bis[bis(4-methoxyphenyl)ethenyl]biphenyl 3 a in 50 % yield over two steps. Next, oxidative cyclization11a was performed by the treatment of 3 a with four equivalents of NOBF4 in CH2Cl2. The dark purple powder of dicationic salt 2 a2+[BF4−]2 was obtained in 94 % yield. When a smaller amount of oxidant was used, the same salt was obtained, and the starting material 3 a was recovered, because 1 a generated in situ is more easily oxidized than the starting material 3 a (Scheme 4).
The dication exhibits a characteristic strong absorption band in the visible region [λmax=502 nm (log ε=4.76) in MeCN]. When 2 a2+[BF4−]2 was treated with excess Zn powder in dry DME, the deep purple color disappeared rapidly, and Ar4DMPs 1 a [λmax=325 nm (sh; log ε=4.04) in MeCN] was isolated as stable yellow cubes in 92 % yield after recrystallization. Its thermodynamic stability was demonstrated by quantitative recovery after heating at reflux for 24 h in toluene with no signs of electrocyclization to its isomers.
When other 4,4′-dialkoxybenzophenones 5 b–d were used in the reactions of 2,2′-bis(lithiomethyl)biphenyl, bis(diarylethenyl)biphenyls 3 b–d with different alkoxy groups were similarly obtained via diols 4 b–d in respective yields of 56, 58, and 55 % over two steps. The oxidative cyclization of 3 b–d was conducted by using four equivalents of (4-BrC6H4)3N+.SbCl6−. Without purification, the resulting dication salts of 2 b2+–d2+ were subjected to reduction with Zn to give Ar4DMPs as yellow crystals (1 c, 89 % yield) or yellow oils (1 b, 81 %; 1 d, 83 %). Again, they are thermodynamically stable and can be kept at room temperature under air, which validates our design concept for stabilizing Ar4oQD by dibenzo annulation.
Figure 1. a) Top view and b) side view of X-ray structures of (P)-1 a in the racemic crystal [1 a⋅(CHCl3)2] determined at 123 K. Thermal ellipsoids are shown at the 50 % probability level. c) Top view and d) side view of X-ray structures of (P)-1 c in the crystal of 1 c containing both diastereomers [(P)- and (M)-1 c] determined by X-ray analysis at 123 K. Thermal ellipsoids are shown at the 30 % probability level.
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In the 1H NMR spectrum of 1 a (300 MHz), methoxy protons on the aryl groups appeared as two sharp singlets (δ=3.50 and 3.30 ppm in [D5]bromobenzene) at 25 °C. The former resonance is assigned as that for the CH3O in the inner aryl groups, which is shifted downfield due to deshielding effects. When the temperature increases, the two peaks gradually coalesce (Tc=100 °C), and a sharp single resonance was then observed at higher temperature (Figure S2 in the Supporting Information), which shows that the inner and outer aryl groups are exchanged with an energy barrier of 18.4 kcal mol−1. This value also corresponds to the inversion of helicity [(P)-1 a<=>(M)-1 a]. Such a small barrier makes it impossible to perform the optical resolution of (P)- and (M)-1 a. This should also be the case for other Ar4DMPs 1 b–d. Although 1 c and 1 d with four chiral alkoxy groups [(R)-C2H5CH(CH3)O or (R)-C6H13CH(CH3)O, respectively] exist as mixtures of diastereomers with an opposite sense of helicity, they were treated as single entities due to facile interconversion.
Upon recrystallization, a single-crystal specimen of 1 c (P212121, Z=8, two independent molecules) was obtained. Interestingly, the crystal is composed of an equal amount of diastereomers [(P)-1 c for molecule-1, (M)-1 c for molecule-2], which is not a common crystallization phenomenon.15a,b X-ray analysis provided structural information on both diastereomers with the same point chiralities on the aryl rings, but with a different helical sense (Figures 1 c, d, and S3 in the Supporting Information). Despite the similar helical geometries of the (P)- and (M)-isomers, their structural parameters differ slightly and indicate larger deformation in the (M)-isomer. The torsion angle for the Ar2CCCCAr2 unit, the twisting angle of the biphenyl skeleton, and the twisting angles of the two exocyclic bonds are 59.7(8), 23.1(9), 4(1), and 2(1)° in the (P)-isomer and 69.4(6), 25.2(8), 10(1), and 8(1)° in the (M)-isomer, respectively.
In the 1H NMR spectrum of 1 c at room temperature, the only one set of resonances that correspond to a single C2-symmetric species was observed, which shows that the two diastereomers [(P)- and (M)-1 c] exhibit indistinguishable chemical shifts. The slightly different steric interactions for the diastereomeric pair may or may not bias the equilibrium ratio in favor of one of the two diastereomers,20 however, this issue cannot be detailed experimentally in either 1 c or 1 d for the same reason.
Redox properties and interconversion between Ar4DMPs (1) and phenanthrene-9,10-diylbis(diarylmethylium)s (2): Due to not only by the electron-donating alkoxy groups, but also the observed skeletal deformation and intramolecular π–π interaction, Ar4DMP 1 a has an increased HOMO level, so that it undergoes a facile electrochemical oxidation (Eox=+0.61 V vs. Ag/Ag+ in CH2Cl2; two-electron process). Variation of the alkyl group seldom affects the donating properties (Eox=+0.66, +0.61, and +0.67 V, for 1 b–d, respectively; Figure 2). Upon treatment of 1 a–d with two equivalents of (4-BrC6H4)3N+.SbCl6−, the corresponding dication salts 2 a2+–d2+[SbCl6−]2 were isolated as deep purple crystals or amorphous compounds in high yields (93, 95, 98, and 97 %, respectively).21 The salt of 2 a2+ (see above) and those of 2 b2+–d2+ reproduced Ar4DMP 1 a–d upon reduction with Zn dust in high yields (92, 95, 100, and 97 %, respectively), as was confirmed by the reversible nature of the present redox pairs.
Figure 2. Cyclic voltammogram of 1 c recorded in CH2Cl2 (0.5 mM) containing Bu4NBF4 (0.1 M) as a supporting electrolyte (E/V vs Ag/Ag+, scan rate 100 mV s−1, Pt electrode).
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Interestingly, the reduction potentials of 2 a2+–d2+ were observed in the far cathodic region (Ered=+0.11, +0.07, +0.03 and +0.03 V for 2 a2+–d2+, respectively; two-electron process). Such a large shift of redox peaks as well as a one-wave two-electron oxidation process are commonly observed in dynamic redox pairs12 undergoing drastic structural changes and/or reversible formation/breaking of CC bonds upon electron transfer.7, 8, 14, 22 The separation by approximately 0.6 V indicates a high electrochemical bistability of the redox couples of 1/22+, which is favorable for realizing switching phenomena of redox active molecules.
Upon crystallization of 2 c2+[SbCl6−]2 from CH2Cl2/ether, a high-quality, single-crystal specimen was obtained, X-ray analysis of which (P21, Z=2) revealed that the phenanthrene core in 2 c2+ is nearly planar (Figure 3). The two diarylmethylium units are attached at the 9,10-positions with large twisting angles [68(1) and 63(1)°] to give a helical arrangement of the π system, the helical sense of which is similar to that in (P)-1 c. Although the helical inversion of (P)-2 c2+ can readily occur in solution through rotation about the C 9(10)C+ bonds, there are no diastereomeric dications with (M)-helicity in the crystal. Thus, the point chirality of the alkoxy group [(R)-C2H5CH(CH3)O] is transmitted to the (P)-helicity preference of the dication, in which the two chromophores are stacked nearly in parallel with a shortest CC contact of 3.05(1) Å. Such an asymmetric geometry is suitable for realizing exciton coupling of the two chromophores.23 Accordingly, the CD spectrum of 2 c2+[SbCl6−]2 salt, taken as a KBr tablet, showed the negative bisignated Cotton effects in the λ=500–600 nm region (Figure S4 in the Supporting Information).
Figure 3. Molecular structure of dication 2 c2+ in the SbCl6− salt determined by X-ray analysis at 153 K. All of the molecules in the crystal have the same configuration of (P)-helicity in terms of the twisting around C9(10)C+ bonds. Thermal ellipsoids are shown at the 50 % probability level.
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Chiroptical properties and a multi-input/multi-output response of 1 c/2 c2+ and 1 d/2 d2+: A large negative couplet was observed in the CD spectrum of 2 c2+[SbCl6−]2 salt recorded in CH2Cl2 [λext=591 nm (Δε=−23), 520 (+17)], showing that the transmission of point chirality to helicity15 also occurs in solution to induce a preference for (P)-helicity in 2 c2+. In the 1H NMR spectrum of 2 c2+[SbCl6−]2, only one set of resonances that corresponded to a single C2-symmetric species was observed, indicating that the two diastereomers [(P)- and (M)-2 c2+] interconvert so rapidly that we cannot determine the diastereomeric excess in terms of the helicity preference. The observed ellipticity in 2 c2+ is much larger than that in the reference monocation [4-(R)-C2H5CH(CH3)O-C6H4]2CPh+BF4− [λext=508 (Δε=−1.3), 411 (−0.83), 270 nm (−1.4)],15a which can be explained by the effective amplification of CD signals through exciton coupling of the two cationic chromophores in the preferred (P)-diastereomer. This is also the case for 2 d2+ having (R)-C6H13CH(CH3)O chiral auxiliaries on the aryl group [λext=589 (Δε=−18), 519 nm (+15) in CH2Cl2].
Strong CD signaling in 2 c2+ and 2 d2+ is advantageous for their use as an electrochiroptical material, because an electrochemical input can be transduced into two kinds of outputs, namely, UV/Vis and CD spectral changes. Thus, upon electrochemical oxidation of 1 c to 2 c2+, both UV/Vis and CD spectra changed drastically, as shown in Figure 4. The presence of several isosbestic points indicates a clean conversion between 1 c and 2 c2+ as well as a negligible steady-state concentration of the intermediary cation radical species. A two-way-output response was similarly observed in the case of 1 d/2 d2+ (Figure S5 in the Supporting Information). The observation validates our molecular design concept for constructing novel electrochiroptical systems16 based on Ar4DMPs by attaching chiral auxiliaries on the aryl groups.
Figure 4. Changes in a) UV/Vis and b) CD spectra upon constant current (27 µA) electrolysis of 1 c (3.5 mL, 1.8×10−5 M) in CH2Cl2 containing Bu4NBF4 (0.05 M) as a supporting electrolyte (every 5 min).
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Through overlap of the two cationic chromophores, the electrostatic destabilization of 22+ can be reduced by π–π interaction,24 which is more important when the salt is dissolved in a less-polar solvent without enough stabilization by solvation.25 Thus, the UV/Vis and/or CD spectra for some helical dicationic dyes often exhibit spectra with interesting solvent-dependent feature.15a, 16i This falls true for the case of the present dications 22+. The solvatochromic behavior can be demonstrated more easily by using 2 b2+ with long alkyl chains due to their higher solubility in a variety of solvents including less-polar ones, such as benzene. As shown in Table 1 and Figure S5 in the Supporting Information, the strongest absorption band of 2 b2+ in MeCN appeared at λ=504 nm, whereas when recorded in benzene, it is at 523 nm. Although the shift is not so spectacular, continuous changes were observed in THF, CH2Cl2, and CHCl3 with intermediate polarity. Such solvent dependency is likely originated from the stacking geometry allowing π–π interaction between two cationic chromophores, because the Davydov splitting of the band is evident in all cases. Similar solvatochromism was observed for 2 d2+ with chiral auxiliaries, in which not only UV/Vis, but also CD spectra change according to the solvent polarity (Tables 1, 2, and Figure 5). The shift of λext in CD spectra simply corresponds to the different absorption maximum in each solvent. The dependence of CD amplitude, as was verified by the A value for the first couplet, is more interesting, because the value in MeCN (−45) is three times as large as that in benzene (−14), which suggests higher diastereomeric excess26 in polar solvents with keeping preference for the (P)-helicity. The longer alkyl group in the chiral auxiliary in 2 d2+ may play an important role to realize chirosolvatochromism by solvophobic effects, because sec-butyl derivative 2 c2+ does not exhibit such drastic and polarity-dependent change in the A value.27
Table 1. Solvent-dependent UV/Vis spectra[a] of [SbCl6−]2 salts of 2 b2+ and 2 d2+.
|Solvent||Dielectric constant||λ [nm] (log ε)|
| || ||2 b2+||2 d2+|
|MeCN||37.5||504 (4.92)||508 (4.94)|
| || ||560 (sh; 4.60)||565 (sh; 4.62)|
|CH2Cl2||9.1||517 (4.93)||518 (4.96)|
| || ||575 (sh; 4.62)||580 (sh; 4.62)|
|THF||7.6||513 (4.86)||514 (4.92)|
| || ||567 (sh; 4.62)||565 (sh; 4.62)|
|CHCl3||4.9||519 (4.93)||520 (4.98)|
| || ||573 (sh; 4.63)||580 (sh; 4.59)|
|benzene||2.3||523 (4.83)||523 (4.91)|
| || ||572 (sh; 4.58)||576 (sh; 4.59)|
Table 2. Solvent-dependent CD spectra[a] of 2 d2+[SbCl6−]2 in various solvents.
|Solvent||Dielectric constant||λext [nm] (Δε)|
|MeCN||37.5||581 (−27), 508 (+18)|
|CH2Cl2||9.1||589 (−18), 519 (+15)|
|THF||7.6||586 (−16), 514 (+13)|
|CHCl3||4.9||589 (−14), 520 (+11)|
|benzene||2.3||587 (−6), 527 (+8)|