Structural−Property Relationship in Pyrazino[2,3-g]quinoxaline Derivatives: Morphology, Photophysical, and Waveguide Properties

The objective of this contribution is to achieve crystalline one-dimensional organic micro/nanostructures by self-assembling for active waveguides and other optoelectronic nanodevices. To improve solid emitting performance for highly efficient waveguide fibers, our strategy here is to adopt nonplanar conjugated segments to minimize unfavorably intermolecular fluorescent quenching in solid state; meanwhile, long alkyl chains are added as structural group that controls molecular packing. With this intention, a series of molecules based on pyrazino[2,3-g]quinoxaline ( PyQ ) units were developed. The effects of the molecular geometry and symmetry on their optical properties, crystal dimensions, liquid crystals, and active waveguide properties were studied in detail to understand the structure−property relationship of organic conjugated molecules, which is of vital importance to rational design for organic self-assemblies toward optoelectronic applications. On a single molecular level, the molecular structures and their optical properties were correlated by computational methods. The combined results of morphology study, single crystal structure and two-dimensional wide-angle X-ray diffraction (2D-WAXD) indicated that the loss of the different molecular symmetry element affected the crystal dimensions. The microfibers of molecules 1 and 2 with their desirable crystal dimensions and surfaces were proved to be low-loss waveguide materials (0.02−0.05 dB/μm). Hence, we suggest that nonplanar aromatic core modified with proper side group may represent a promising approach to achieve crystalline optical waveguides. These structure−property investigations provide a deeper understanding for the structure and intermolecular interactions of such molecular aggregates.