Qian Zhou,Huitian Du,Zhen Chen,Yuan Yu,Junfeng Ren,Shenghao Han,Jihui Fan,Zhiyong Pang
Near-infrared two-photon absorption (TPA) molecules designed based on intramolecular charge-transfer (CT) interactions have been widely investigated during the last few decades; however, studies on supramolecular TPA materials based on intermolecular CT interactions are quite rare. In this work, cocrystals of acridine and 1,2,4,5-tetracyano-benzene were successfully prepared by self-assembly. The cocrystals show yellow emission centered at ∼540 nm with a solid-state photoluminescence quantum yields (PLQY) of 22.48%. Two-photon excited fluorescence (TPEF) measurements show that the cocrystals display wide-window TPEF that is absent in their individual components. Optical absorption analysis indicate that the fluorescence is originated from excitation from the ground states to CT exciton states instead of exciton states of donor or acceptor molecules. This work may facilitate further two-component nonlinear optical material design and applications.
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.
Qian Zhou,Yuan Yu,Huitian Du,Chenghao Liu,Junfeng Ren,Shenghao Han,Zhiyong Pang
Organic charge-transfer cocrystals exhibit fantastic physical properties owning to their multi-component synergistic and collective effect. In spite of the abundant material supplies, the cocrystals people prepared so far are far less than expectation. Here, we reported the preparation, structure and optical properties of a mixed stacked cocrystal of 2,2':6′,2″-terpyridine and 1,2,4,5-tetracyanobenzene. The structural and optical properties were analyzed by using single-crystal X-ray diffraction (SCXRD), FTIR, Raman, optical absorption and emission spectra experimentally and the molecular electrostatic potential (MEP), Hirshfeld surface , the reduced density gradient (RDG), frontier molecular orbitals (FMO) theory, inter-fragment charge transfer (IFCT) analysis and time dependent density functional theory (TD-DFT) theoretically. The TTC sample exhibits strong up-conversion luminescence that is absent in their individual components. Theoretical analysis indicates that the NLO properties of TTC may be due to a significant increase in two-photon transition probabilities δ TPA related to intermolecular charge transfer (CT) interactions. Our results may be helpful for a better understanding of NLO mechanism in ground state CT cocrystals.
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