Organic Polymorphs: One-Compound-Based Crystals with Molecular-Conformation- and Packing-Dependent Luminescent Properties

摘要

Luminescent organic crystals constructed by π-conjugated molecules have attracted great attention in the field of organic optoelectronics, including organic light-emitting diodes (OLEDs) and organic lasers, as their definite molecular conformation and packing can provide a visualized model to investigate basic intermolecular interactions and thus may precisely disclose the relationship between molecular packing modes and optoelectronic properties. At the same time, polymorphism of organic optoelectronic materials has received special attention as the different solid-state forms of one molecule can exhibit significantly different optical and electronic properties.Considering the difficult, as well as time-consuming, synthesis of optoelectronic materials, polymorphism is quite a convenient approach to tune the emission properties of organic solids based on one compound. Therefore, there is an urgent call for demonstrating the relationship between the molecular conformation/ arrangement and the emission properties of organic solids. Complementary to this research direction, a lot of experimental as well as theoretical research has been carried out. We studied the polymorphism of 3(5)-(9-anthryl)pyrazol and found that these polymorphs showed emission colors changing from deep blue to green when the intermolecular interaction between anthracene rings was enhanced. Yan and co-workers demonstrated that the emission colors of organic crystals can be finely modified from blue through green to yellow by supramolecular co-crystallization. Yuan and co-workers reported 2,7-diphenyl-fluorenone derivatives that exhibited reversible, stimuli-responsive, solid-state luminescence switching by altering the mode of the molecular packing. Most of the reported examples show different optical properties accompanied with simultaneous variations of the molecular arrangement and conformation. Therefore, it is hard to figure out which factor dominates the change in optical properties and a model system that separately discloses the effect of molecular conformation and packing on luminescence based on one organic compound has not yet been found. It is an important issue to develop luminescent polymorphs with the same or similar packing and different conformations, as well as polymorphs with different packing and the same or a similar conformation.