Rehybridization of Nitrogen Atom Induced Photoluminescence Enhancement under Pressure Stimulation

The excited state properties of organic fluorescent materials are crucial for their photoelectronic performance. Here, a study on the highly efficient electrofluorescent material 4-(2-(4'-(diphenylamino)-[1,1'-biphenyl]-4-yl)-1H-phenanthro[9,10-d]imidazol-1-yl) benzonitrile (TBPMCN) is carried out, focusing mainly on its crystal structure and photophysical properties under pressure stimulation. The special triangular-cone (TC) configuration of triphenylamine group in TBPMCN crystal exhibits charge-transfer (CT)-dominated excited state property in TBPMCN, which gives rise to a blue-shifted emission in the crystal. Theoretical calculations prove that the TC conformation is a dynamically metastable state, which is higher in energy than the three-blade-propeller (TBP) configuration. In a further piezochromic experiment, a unique rehybridization-induced emission enhancement phenomenon is found in this crystal, which is essentially different from the aggregation-induced emission enhancement (AIEE) mechanism. It can be assigned to the change of excited state property from a CT-dominated state to a hybridized locally excited and charge-transfer state, as a result of the rehybridization of nitrogen atom upon the increased external pressure. This work provides deep insight into the relationship between molecular structure and excited state properties in crystal by means of the pressure stimulation and further enriches the AIEE mechanism. Additionally, the large red-shifted piezochromic phenomenon of this CT material is stressed.