Benzylidenemalononitrile bridged long-wavelength emission luminogens with “washing-free” properties for monitoring the dynamics of LDs

Understanding the physiological functions and monitoring the dynamics of lipid droplets (LDs) in living cells are crucial, because they play important roles in energy reservoir and as sources of lipid molecules, and can be potentially used as biomarkers in cancer diagnose. Organic small-molecule based luminogens with long-wavelength emission are advantageous imaging tools for visualizing the dynamics of LDs, as the long wavelength fluorescence has lower scattering and absorption by biomolecules, deeper penetration ability into tissues, and reduced interference effect. In this work, we report the design, synthesis and LDs imaging properties of two long-wavelength emitting organic luminogens (OLs) APBM and PyBM. The two materials have typical donor-acceptor-donor (D-A-D) structure with a benzylidenemalononitrile moiety as the acceptor, bridging different electron donors, i.e., N,N-dimethylamino phenyl for APBM, and pyrene for PyBM, respectively. The strong electron-withdrawing benzylidenemalononitrile moiety inducing intense intramolecular charge transfer effect, which endows the emissions of APBM and PyBM as far as 668 nm and 605 nm in the solid state, and on the other hand, creates their twisted molecular conformation. As a result, the OLs exhibit aggregation-induced-emission behaviors, due to the restriction of intramolecular motions, evolving the “washing-free” properties of the materials in bioimaging. APBM and PyBM both have low cytotoxicity and good photostability, and are able to light up HepG2 cells efficiently. In addition, APBM shows excellent LDs targeting ability, and it is successfully used to discriminate normal cells and cancer cells, and monitor the dynamics of LDs under different nutritional conditions, and the interactions with mitochondria.