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

Abstract

A nucleophilic substitution reaction of N₃P₃Cl₂(2,2′-dioxybiphenyl)₂ (1) with 2-(1H-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), ¹H, ³¹P 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.