ESIPT on/off switching and crystallization-enhanced emission properties of new design phenol-pyrazole modified cyclotriphosphazenes

A nucleophilic substitution reaction of N 3 P 3 Cl 2 (2,2′-dioxybiphenyl) 2 ( 1 ) with 2-(1 H -pyrazol-3-yl) phenol ( 2 ) was performed in order to synthesize molecules that may exhibit different photophysical properties on the cyclotriphosphazene scaffold. Bis-geminal phenol-pyrazole ( 3 ) and mono-spiro phenoxy-pyrazole ( 4 ) cyclotriphosphazene derivatives were obtained as a result of an effective single substitution reaction. The structures of the new cyclotriphosphazene derivatives ( 3 and 4 ) were illuminated by various spectroscopic techniques such as mass spectrometry (MALDI-TOF), 1 H, 31 P NMR, and elemental analysis, and also confirmed by single crystal X-ray diffraction. After the structural analysis, their photophysical properties in solution and the solid phase were examined by UV-Vis absorption and fluorescence spectroscopies. At the same time, the functionality of the excited state intramolecular proton transfer (ESIPT) mechanism and crystallization-enhanced emission (CEE), which are an important point, and their relationship with different substitution modes on the cyclotriphosphazene ring were investigated by steady-state fluorescence, time resolved-fluorescence and 3-D fluorescence measurements in solution and the crystalline state. The proton transfer properties of compounds 3 and 4 can lead to highly efficient ESIPT or inhibited-ESIPT and efficient crystallization-enhanced emission (CEE) via changing of the substitution mode. The phenol-pyrazole modified cyclotriphosphazenes ( 3 and 4 ) are the first examples where the ESIPT mechanism is active and inactive. Cyclophosphazene-based high-efficiency excited state intramolecular proton transfer (ESIPT) active and inactive molecules were prepared depending on the different bonding patterns of the difunctional phenol-pyrazol reagent.