Two-way chromic interconversion of the 2,2 0 - biphenol-6,6 0 -diyl dication with 5 H ,10 H -dioxapyrene or 9 H ,10 H -4,5-dihydroxyphenanthrene †

Two-proton or two-electron transfer of the title biphenolic dication proceeds nearly simultaneously to induce 2,6 0 /2 0 ,6-or 6,6 0 -bond formation to give dioxapyrene or dihydrophenanthrene derivatives, respectively, with vivid changes in color (halochromism and electrochromism).

5H,10H-Dioxapyrene (diopy; 5H,10H-[1]benzopyrano [5,4,3-cde][1]benzopyran) is a less-studied heterocyclic skeleton 1 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(sp 3 )-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 (C 2+ ) 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 C 2+ , so that double protonation/ deprotonation between A and C 2+ 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. 2When the cationic part in C 2+ is endowed with a strong absorption in the visible region, interconversion between A and C 2+ is accompanied by halochromism, 3 since diopy A shows absorptions only in the UV region.
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 2 2+ 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 2 2+ .Another interesting point is that, upon reduction, dication 2 2+ would be transformed into a dihydrophenanthrene (DHP) derivative 3 accompanied by C(sp 3 )-C(sp 3 ) bonding through ''dynamic redox (dyrex)'' behavior, 6 and the interconversion between 2 2+ and 3 would also exhibit characteristic color and structural changes.Thus, 1, 2 2+ and 3 can serve as a novel motif for multi-input molecular response systems. 7ere we report the preparation and X-ray structures of 1 and 2 2+ along with their chromic behavior during the interconversion between 1 and 2 2+ (halochromic 3 pair) as well as 2 2+ and 3 (electrochromic 8 pair).
This journal is © The Royal Society of Chemistry 2015 Based on the results of an X-ray analysis 11 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: 01).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)1], as found in other structurally-related molecules. 12In contrast, the two molecular halves in dication 2 2+ are largely twisted in the crystal of (BF 4 À ) 2 salt 11 (Fig. 2 and Fig. S2, ESI †).The dihedral angle of the biphenyl unit is 68.8(1)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 0 -positions are separated by a distance of 3.050(2) Å, intermolecular H-bonding is not effective in 2 2+ (typical distance for the H-bonded OÁ Á ÁO: 2.75 AE 0.2 Å).The p-p 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)1] with the shortest CÁ Á ÁC contact of 3.284(3) Å (sum of van der Waals radii: 3.40 Å).
Diopy 1 is colorless, with absorptions only in the UV region [l max /nm: 339 (4.30) in CH 2 Cl 2 ], whereas 2 2+ exhibits a yelloworange color [358 (3.92) in MeCN] due to the characteristic absorptions of acridinium (Fig. 3a).Although 10-methylacridinium itself is highly fluorescent, 2 2+ is non-fluorescent due to the chargeshift-type quenching of the excited state by the electron-donating biphenol unit.Upon the aliquot addition of TfOH to a DMSO-d 6 solution of 1, a clean conversion to 2 2+ was observed (Fig. S4, ESI †).The resulting spectra showed the presence of only two species (1 and 2 2+ ), 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 Â 10 À5 M), followed by the addition of Et 3 N (200 microL) to the solution of as-generated 2 2+ .By monitoring the color change using UV-vis spectroscopy, we could confirm the reversibility of the present halochromism (Fig. 3b and Fig. S5, ESI †).
In this work, we have demonstrated the reversible halochromic and electrochromic interconversion of 2,2 0 -biphenyl-6,6 0 -diyl dication with two kinds of neutral molecules (diopy and DHP).This is the first example of concomitant but independent twoproton 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.

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