Hexaphenylbenzene based push-pull fluorophores displaying intriguing polarity-dependent fluorescence behavior, AIE(E) characteristics and mega-large Stokes shifts

摘要

In general, most donor-acceptor type fluorophores are known to exhibit solvatochromic behavior and obey the energy gap law simultaneously, showing decreased emission with increasing solvent polarity. In this work, four push-pull hexaphenylbenzene (HPB) derivatives 1-4 have been designed and synthesized by introducing methoxy and dicyanovinyl groups at the tip of aromatic blades, which behave exceptionally polarity-dependent emission properties. In the low to medium solvent polarity region, the four dipole fluorophores display gradually enhanced fluorescence, whereas a reverse trend is observed with the further increase of solvent polarity. Meanwhile, 1-4 are found to exhibit the strongest emission at the same wavelength remarkably, which appears to be somewhat relevant to the intrinsic characteristics of dicyanovinyl-phenyl acceptor. A mechanistic scheme containing a conical intersection (CI) has been proposed to explain the up-down emission of 1-4 with increasing solvent polarity. Large Stokes shifts (up to 254 nm) and AIE(E) characteristics have also been achieved in these four HPB fluorophores. Besides, fluorophores 1-4 exhibit obvious freezing-solution induced chromism (FSIC), demonstrating an enhancement of the local matrix polarity upon freezing solutions.

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