Synthesis and photophysical properties of charge transfer cocrystals based on TCNB and fluorene and its derivatives

A series of charge transfer cocrystals with fluorescent properties involving 1,2,4,5-tetracyanobenzene (TCNB, C10H2N4) as the acceptor, fluorene (FR, C13H10) and its bromide 2-bromofluorene (2-BrFR, C13H9Br), carbazole (CZ, C12H9N) and its bromide 2-bromocarbazole (2-BrCZ, C12H8BrN) as donors were synthesized via cocrystal design strategies. The formation mechanism of the crystal structure was explored by density functional theory at molecular level. The stacking structure of the cocrystal was formed by the combined interaction of charge transfer interactions, π-π interactions and hydrogen bonding. N atom of donors in TCNB-CZ and TCNB-2-BrCZ could enhance the charge transfer, appear as a red shift of the fluorescence emission wavelength. Conversely, Br atom in TCNB-2-BrFR and TCNB-2-BrCZ weaken the charge transfer, giving a blue shift in fluorescence emission spectrum. The four new cocrystals exhibit tunable luminescence from green (529 nm) to yellow (552 nm) and to red (604 nm and 614 nm). It was found that the introduction of N and Br atoms in the assembly units can not only regulate the luminescence range of the cocrystals, but also adjust the luminescence efficiency, which provides a strategy for the development of promising organic luminescent materials.