Bent π-Conjugated System Composed of Two Dibenzocyclooctatetraene Units: Multifunctional Properties of Dynamic Molecular Tweezers in Solution and the Solid State

Abstract

Abstract A series of bent π-conjugated compounds, 2a, 2b, 3a, and 3b, composed of two tub-shaped dibenzocyclooctatetraene (DBCOT) units with two cyano substituents on each cyclooctatetraene (COT) ring were designed and synthesized by using improved synthetic procedures. The anti- and syn-conformers of 2a, 2b and 3a, 3b, which coexist in equilibrium in solution, were independently observed by using 1H NMR spectroscopy at room temperature, and the signals of the anti- and syn-forms coalesced at 130 °C to give signals assigned to a D2h symmetric molecule. The ratio of the anti- and syn-isomers of these dynamic molecular tweezers (DMTs) were dependent on the position of the cyano substituents on the COT rings due to the difference in either torsional strain or dipole moments between the anti- and syn-isomers (2a, 2b: syn > anti; 3a, 3b: syn < anti). The dynamic guest incorporation ability of these DMTs in solution was investigated by using 1H NMR and UV–vis spectroscopic titration experiments and evaluated on the basis of the calculated affinity for a guest molecule. We found that 2a, 2b and 3a, 3b exhibited unique aggregation-induced emission in solution and crystalline-state emission induced by incorporated solvents in the crystal lattices of the DMTs. The emission wavelength and quantum yield of the aggregation-induced and crystalline-state emissions depended on their restricted flexibility and syn–anti ratio. Theoretical calculations confirmed that the dihedral angle between the central and terminal benzene rings had the largest effect on the emission wavelength in the crystalline state.