Highly Bright Fluorescence from Dispersed Dimers: Deep-Red Polymorphs and Wide-Range Piezochromism

Near-infrared piezochromic materials (PCMs), which show a fluorescence response with a clear color difference and strong penetrability, have important potential applications, but few such organic luminophores have been reported. High-brightness donor (D)-acceptor (A) luminophores present great potential for preparing near-IR PCMs with clear color differences. However, the intermolecular pi-pi interactions are sharply enhanced at high pressures, which is not beneficial for the improvement of the piezochromic properties. Herein, two crystalline polymorphs (TPEB-alpha and TPEB-gamma) of a D-pi-A-type dye is presented, which emits deep-red fluorescence with a high emission efficiency (53.2% and 28.1%, respectively). In crystalline state, dispersed dimers are observed and arranged in a herringbone pattern. Interestingly, each dimer is surrounded by four pairs of twisted tetraphenyl ethylene units, which could disrupt the pi-pi interactions between adjacent dimers during compression. As a result, this luminophor with dispersed dimeric stacking demonstrates the deep-red to near-infrared piezochromic behavior with a remarkable redshift (212 nm) and high sensitivity (19.8 nm GPa(-1)).