Two-way chromic interconversion of the 2,2′-biphenol-6,6′-diyl dication with 5H,10H-dioxapyrene or 9H,10H-4,5-dihydroxyphenanthrene

Abstract

Two-proton or two-electron transfer of the title biphenolic dication proceeds nearly simultaneously to induce 2,6′/2′,6- or 6,6′-bond formation to give dioxapyrene or dihydrophenanthrene derivatives, respectively, with vivid changes in color (halochromism and electrochromism).

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5H,10H-Dioxapyrene (diopy; 5H,10H-[1]benzopyrano[5,4,3-cde][1]benzopyran) is a less-studied heterocyclic skeleton¹ in contrast to its 5,10-dione analogue. We envisaged that the flattened framework of diopy provides a unique opportunity for the development of new bistable molecular response systems, in which an external stimulus induces the cleavage of two C(sp₃)–O bonds to transform into the corresponding biphenyl derivatives with a twisted conformation (Scheme 1). Upon acid treatment of diopy (A) with cation-stabilizing substituents at 5,10-positions, the biphenolic dication (C²⁺) would be generated via the monocationic intermediate (B⁺). When B⁺ suffers from severe steric repulsion at the bay-region, this amphiprotic species easily undergoes acid–base disproportionation to A and C²⁺, so that double protonation/deprotonation between A and C²⁺ would occur nearly simultaneously. Such a simplified pseudo-two-state switching is favored for the construction of promising molecular response systems with a sharp ON/OFF threshold.² When the cationic part in C²⁺ is endowed with a strong absorption in the visible region, interconversion between A and C²⁺ is accompanied by halochromism,³ since diopy A shows absorptions only in the UV region.

Scheme 1 Interconversion of diopy A and biphenolic dication C²⁺ upon double protonation/deprotonation via intermediate B⁺.

To generate and isolate the dicationic state as a stable entity despite the presence of hydroxy groups within the molecule, the cationic subunit should have a large pK_R+ value, which prompted us to select the 10-methylacridinium chromophore^4,5 (Scheme 2). Due to the bulkiness of the chromophore, the biphenol skeleton in 2²⁺ would have a large torsion angle, whereas the diopy skeleton in 1 would be nearly planar, since the spiro(10-methylacridan) units do not induce any steric hindrance. Such a drastic structural change would realize the two-state halochromic interconversion between 1 and 2²⁺. Another interesting point is that, upon reduction, dication 2²⁺ would be transformed into a dihydrophenanthrene (DHP) derivative 3 accompanied by C(sp₃)–C(sp₃) bonding through “dynamic redox (dyrex)” behavior,⁶ and the interconversion between 2²⁺ and 3 would also exhibit characteristic color and structural changes. Thus, 1, 2²⁺ and 3 can serve as a novel motif for multi-input molecular response systems.⁷

Scheme 2 Multi-input chromic behavior of diopy 1, biphenolic dication 2²⁺, and DHP 3.

Here we report the preparation and X-ray structures of 1 and 2²⁺ along with their chromic behavior during the interconversion between 1 and 2²⁺ (halochromic³ pair) as well as 2²⁺ and 3 (electrochromic⁸ pair).

6,6′-Dibromo-2,2′-biphenol 4⁹ was first reacted with methoxymethyl (MOM) chloride/NaH in DMF to give MOM-protected biphenol 5¹⁰ in 77% yield. The dilithio derivative derived from 5 and 4 equiv. of ^tBuLi in THF was then reacted with 10-methyl-9(10H)-acridone to give bis(hydroxy)base 6¹⁰ in 70% yield. Upon treatment of 6 with HBF₄ in MeOH–CHCl₃ at reflux afforded the desired 2²⁺(BF₄⁻)₂¹⁰ as yellow-orange crystals in 95% yield. The reaction of 2²⁺(BF₄⁻)₂ with Et₃N in MeCN gave colorless crystals of diopy 1¹⁰ in 88% yield (Scheme 3).

Scheme 3 Preparation scheme for 1, 2²⁺(BF₄⁻)₂ salt, and 3.

Based on the results of an X-ray analysis¹¹ at 150 K, the diopy core in 1 is nearly planar (largest deviation of an atom from the mean plane: 0.23 Å), although the pyran rings adopt a very shallow twist-chair form (Fig. 1 and Fig. S1, ESI†). The two benzene rings are coplanar (dihedral angle: 0°). To this core are attached the spiro(10-methylacridan) units, which are slightly deformed into a butterfly-shape [dihedral angle between two benzene nuclei of acridan: 21.3(2)°], as found in other structurally-related molecules.¹² In contrast, the two molecular halves in dication 2²⁺ are largely twisted in the crystal of (BF₄⁻)₂ salt¹¹ (Fig. 2 and Fig. S2, ESI†). The dihedral angle of the biphenyl unit is 68.8(1)° (syn-form), and there are no signs to indicate coordination of the hydroxy groups to the acridinium chromophores. If we consider that the two oxygen atoms at the 2,2′-positions are separated by a distance of 3.050(2) Å, intermolecular H-bonding is not effective in 2²⁺ (typical distance for the H-bonded O⋯O: 2.75 ± 0.2 Å). The π–π interaction between two acridinium units must be the major directing force to give the observed syn-form (Fig. S2 and S3, ESI†),¹³ and thus the chromophores are stacked nearly in parallel [dihedral angle: 3.92(3)°] with the shortest C⋯C contact of 3.284(3) Å (sum of van der Waals radii: 3.40 Å).

Fig. 1 ORTEP drawing of diopy 1 determined by X-ray analysis at 150 K.

Fig. 2 ORTEP drawing of 2²⁺ in 2²⁺(BF₄⁻)₂ salt determined by X-ray analysis at 150 K.

Diopy 1 is colorless, with absorptions only in the UV region [λ_max/nm: 339 (4.30) in CH₂Cl₂], whereas 2²⁺ exhibits a yellow-orange color [358(3.92) in MeCN] due to the characteristic absorptions of acridinium (Fig. 3a). Although 10-methylacridinium itself is highly fluorescent, 2²⁺ is non-fluorescent due to the charge-shift-type quenching of the excited state by the electron-donating biphenol unit. Upon the aliquot addition of TfOH to a DMSO-d₆ solution of 1, a clean conversion to 2²⁺ was observed (Fig. S4, ESI†). The resulting spectra showed the presence of only two species (1 and 2²⁺), which demonstrated that the steady-state concentration of the intermediary monocationic derivatives is negligible. The halochromic response was examined by the repeated addition of TfOH (100 microL) to a DMSO solution of 1 (1.2 × 10⁻⁵ M), followed by the addition of Et₃N (200 microL) to the solution of as-generated 2²⁺. By monitoring the color change using UV-vis spectroscopy, we could confirm the reversibility of the present halochromism (Fig. 3b and Fig. S5, ESI†).

Fig. 3 (a) UV-vis spectra of 1 (solid line) in CH₂Cl₂ and 2²⁺(BF₄⁻)₂ (dashed line) in MeCN. (b) UV-vis spectral changes at 440 nm for halochromic switching from 1 [1.2 × 10⁻⁵ M; 3 mL] to 2²⁺ upon addition of TfOH (100 microL) in DMSO. The reverse conversion was accomplished upon addition of Et₃N (200 microL). The halochromic cycles could be repeated without significant loss of response.

According to the results of a voltammetric analysis,¹⁴3 undergoes irreversible two-electron oxidation at an anodic peak potential (E_pa) of +0.32 V in CH₂Cl₂/MeCN (4 : 1) vs. SCE (Fig. S6a, ESI†). The return peak was observed in the far cathodic region (E_pc = −0.23 V), which corresponds to the reduction process of dication 2²⁺ (Fig. S6b, ESI†). In fact, Zn-reduction of 2²⁺(BF₄⁻)₂ induced C(sp₃)–C(sp₃) bonding at the C6 and C6′ positions to give DHP 3. Colorless crystals of 3 [λ_max/nm: 285 (4.37) in CH₂Cl₂] were isolated in 91% yield, and regenerated 2²⁺(BF₄⁻)₂ in 87% yield upon treatment with 2 equiv. of ferrocenium tetrafluoroborate in CH₂Cl₂/MeCN. In this way, reversible redox interconversion between 2²⁺ and 3 accompanied by C–C bond formation/cleavage (“dyrex” behavior) was confirmed. Due to the dynamic geometrical changes,¹⁵ two-electron transfer occurs nearly simultaneously, which was confirmed by the negligible steady-state concentration of the intermediary cation radical upon the electrochemical conversion of 3 to 2²⁺ (Fig. 4).

Fig. 4 A continuous change in the UV-vis spectra of 3 [2.1 × 10⁻⁵ M; 3 mL] to 2²⁺ in CH₂Cl₂/MeCN (4 : 1) containing 0.05 M Bu₄NBF₄ upon constant-current electrochemical oxidation on a Pt electrode (30 microA, every 1 min).

In this work, we have demonstrated the reversible halochromic and electrochromic interconversion of 2,2′-biphenyl-6,6′-diyl dication with two kinds of neutral molecules (diopy and DHP). This is the first example of concomitant but independent two-proton or two-electron transfer with a negligible concentration of the intermediates. A drastic structural change is the key to this novel feature, which may represent a new molecular design concept for multi-input response systems with advanced features.

Notes and references

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10. Experimental details and selected spectral data are given in the ESI†.
11. CCDC deposition numbers are as follows: 1 [P21/c, Z = 2] 1061368; 2²⁺(BF₄⁻)₂ [Pbca, Z = 8] 1061367; MOM-2²⁺(BF₄⁻)₂-(CH₃CN)_0.5 [P21/c, Z = 8] 1061369.
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14. Cyclic voltammetry was conducted in CH₂Cl₂/MeCN (4 : 1) containing 0.1 M Bu₄NBF₄ as a supporting electrolyte (E/V vs. SCE, Pt electrode, scan rate 100 mV s⁻¹). Ferrocene undergoes 1e-oxidation at +0.53 V under similar conditions.
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Footnote

† Electronic supplementary information (ESI) available: Experimental procedure and characterization data. Supplementary figures of X-ray single-crystal structure analyses, halochromic titration, and cyclic voltammograms. CCDC 1061367, 1061368 and 1061369. For crystallographic data in CIF or other electronic format, see DOI: 10.1039/c5cc06338h

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