Facile intra- and intermolecular charge transfer control for efficient mechanofluorochromic material

A series of donor-acceptor-donor (D-A-D) pyrene (Py) 2,7-position-based compounds ( CN , F , H , Me , and OMe ) were designed and synthesized to demonstrate facile intra- and intermolecular charge transfer control by an electron push-pull effect for an efficient mechanofluorochromic (MFC) material. An examination of the photophysical properties of CN-OMe revealed that the locally excited (LE) and intramolecular charge transfer (ICT) state of the compounds were modulated finely by the electron push-pull substituent effect. In addition, it is noteworthy that the CN emission originated only from the LE state despite the D-A-D molecular system. Moreover, we confirmed that the selective LE and/or ICT state modulation affects intra- and intermolecular charge transfer control in the solid-state emission. In particular, this intra- and intermolecular charge transfer control is directly dominated by MFC phenomena, which means that the molecular electron push-pull substituent effect is dependent on MFC behavior. As a result, this study shows that as the electron-withdrawing group ability increases, the intramolecular interactions become insufficient and induce strong intermolecular D-A interactions for stabilizing molecules, leading to high MFC efficiency that is reversibly repeated several times. The mechanofluorochromism is related to the optical properties according to the electron push-pull substituent effect. As the intramolecular charge transfer decreases, strong intermolecular interactions occur, leading to mechanofluorochromism.