Conformation twisting induced orientational disorder, polymorphism and solid-state emission properties of 1-(9-anthryl)-2-(1-naphthyl)ethylene

Polymorphism is important to study the structure-property relationship with a minimum number of variables. Compound 1-(9-anthryl)-2-(1-naphthyl) ethylene (ANE) can form three polymorphs by controlling the crystallization conditions, which belong to the P2(1)2(1)2(1) space group for ANE-a, P2(1)/c space group for ANE-b and P(1) over bar space group for ANE-c, respectively. Orientational disorder was found in ANE-b and ANE-c polymorphs. The emission maximum of the three polymorphs are 496, 470 and 490 nm, respectively. Each is blue-shifted from that of a concentrated solution. The formation of orientational disorder and the blue-shift behavior in emission are closely related to the degree of conformational twisting. A large dihedral angle between the anthracene and naphthalene rings is helpful to induce orientational disorder and blue-shift in the solid emission maximum. The emission efficiency of ANE solution and polymorphs is 42% (CH2Cl2 solution), 28% (ANE-a), 22% (ANE-b) and 47% (ANE-c), respectively. The ANE-c polymorph emits even more efficiently than the solution. This is attributed to the different packing environments, which produce distinct PL decay dynamics. For the three polymorphs, the decay behaviors are all double-exponential and the fluorescence efficiency is enhanced as the proportion of the short-lived component increases. This study shows that compound ANE is a kind of tunable solid-state fluorescent material and the molecular conformation plays an important role in the orientational disorder, aggregate packing and solid-state emission.