(9-Anthryl)vinyl(9-phenanthryl)vinylbenzene Position Isomers: Intriguing Effects of Molecular Configuration on Polymorphism, Solvatochromism, and Aggregation-Induced Emission Properties

Three [(9-anthryl)vinyl] [(9-phenanthryl)vinyl]benzene (APB) position isomers were synthesized and compared. The molecular configuration exhibits an extraordinary ability to affect polymorphism probability, unexpected solvatochromism, and aggregation-induced emission property. With the substitution changing from para-, ortho-, to meta-position, the polymorph number changes from 1, 2, to 3. Both 1,2-APB and 1,3-APB display a temperature-induced crystal-to-crystal phase transition. Furthermore, a pair of concomitant conformational polymorphs were obtained for 1,3-APB. Crystal structure analyses reveal that the steric hindrance between the two substituents leads to different molecular conformation and packing pattern. The unexpected solvatochromism is attributed to the strong electron-withdrawing ability of anthracene against phenanthrene, which produces permanent dipole moment. The solvatochromic degree is determined by the conjugative effect that varies with substitution position. 1,4-APB displays the most remarkable solvatochromic effect. Furthermore, it shows a totally different emission decay dynamics from 1,2-APB and 1,3-APB in polar solvents. Interestingly, the solution fluorescence quantum yields of these three isomers all increase with increased solvent polarity, displaying a negative solvatokinetic effect. Both 1,2-APB and 1,4-APB display an aggregation-induced emission enhancement. Although 1,3-APB is quenched in the solid state, it emits most efficiently among these three isomers either in solution or in solid state. The effect of the medium environment on the radiative process plays a vital role in determining their different aggregation-induced emission behaviors.