ACQ-to-AIE Transformation: Tuning Molecular Packing by Regioisomerization for Two-photon NIR Bioimaging.

: The traditional molecule's strategic designs for highly bright solid-state luminescent materials rely on weakening the intermolecular π-π interactions, which may limit diversity when developing new materials. Herein, we propose a strategy of tuning molecular packing mode by regioisomerization to regulate the solid-state fluorescence. TBP- e -TPA with a molecular rotor in the end position of planar core adopts a long-range cofacial packing mode, of which in the solid state is almost non-emissive. By shifting molecular rotors to the bay position, the resultant TBP- b -TPA possesses a discrete cross packing mode, giving a quantum yield of 15.6 % ± 0.2%. These results demonstrate the solid-state fluorescence efficiency's relationship with the molecule's packing mode. Thanks to the good photophysical properties, TBP- b -TPA nanoparticles exhibit excellent two-photon deep brain imaging. This molecular design philosophy provides a new way of designing highly bright solid-state fluorophores.