Structural, Photophysical, and Water Sensing Properties of Pyrazolo[1,5-a]pyrimidine-Triphenylamine Hybrid Systems

Thermal, structural, and photophysical properties of five pyrazolo[1,5-a]pyrimidine-triphenylamine hybrid systems substituted at position 3 of the fused ring were studied (probe/R: D1/H, D2/I, D3/CCSiMe3, D4/CHO, D5/CN). The solvatofluorochromism of D4 and D5 showed that the solvent dipolarity (SdP) is mainly responsible for their spectral changes, which arose from an internal charge transfer (ICT) from the triphenylamine (TPA) group to the acceptor substituent at position 3. As a result, D5 was applied for the detection of water in organic solvents (i.e., AcOEt and THF); this probe also exhibited an intense light emission in the solid-state at 530 nm (phi=0.60). The five compound structures were solved and analyzed by X-ray crystallography (XRC), with a link found between their structural characteristics and their physicochemical properties. For example, although all the probes exhibited good thermostabilities, this property decreased in D3 and was explained by frontier molecular orbital calculations based on the XRC results. Crystallographic studies were also used to interpret the differences in solid-phase emission intensities in D1-D4 versus D5.