Molecular Marriage via Charge Transfer Interaction in Organic Charge Transfer Co-Crystals toward Solid-State Fluorescence Modulation

A set of three new binary-component charge transfer (CT) complexes (Ia, Ib, and Ic) based on anthracene derivatives 1-acetyl-3-naphthyl-(9-anthryl)-2-pyrazoline (ANNP)/1-acetyl-3-(4-methoxyphenyl)-5-(9-anthryl)-2-pyrazoline (AMAP)/1-acetyl-3-thiophene-5-(9-anthryl)-2-pyrazoline (ATAP) as donors (D) and 1,2,4,5-tetracyanobenzene (TCNB) as acceptor (A) were fabricated via molecular self-assembly and comprehensively characterized. Crystal structural analysis revealed that acceptor molecules become sandwiched in a face-to-face manner between the anthracene units through charge transfer, hydrogen bond, and pi...pi interactions forming CT organic co-crystals with a unique mixed stacking of D..A..D..A..D (Ib and Ic) and DAD...DAD (Ia) arrangement. All of these crystals displayed significant enhancement and red-shift in fluorescence and color-tunable emission such as blue to orange in the solid state. Remarkably, the solid state luminescence efficiency of these CT co-crystals has been improved in comparison to pristine donors. On the basis of structural analysis, the mixed stacking mode between pi-stacked donors and acceptor results in a CT transition state that should be responsible for these behaviors of crystals. The present study demonstrates that using a co-crystal strategy could provide these unique mixed stacking CT assemblies, which may have a high potential in optoelectronic applications considering their high solid-state luminescence efficiency.