Pyrazoline-based organic emitters as color-conversion materials for led applications

In this study, in the first step, some novel pyrazoline derivatives comprising aryl groups at positions 1 and 3, and a vinyl-1,2,4-oxadiazole group at position 5, were successfully synthesized. The molecular structures of the compounds were elucidated and confirmed using infrared (IR), magnetic nuclear resonance (NMR), and mass spectroscopy techniques. In the second part, the photophysical properties of fluorescent pyrazoline-based compounds were investigated by linear absorption, steady-state fluorescence, and time-resolved fluorescence (TRF) spectroscopies. The results showed that the electron-withdrawing ability of the substituent at position 3 has a crucial role in the spectral position of the fluorescence (FL) emission wavelength of the compounds in the solution. However, there is no significant contribution to the FL emission by the substituents on the 1,2,4-oxadiazole group at position 5. In addition, the pyrazoline-based compound having -CN group at position 3 has been found to have the highest fluorescence quantum yield (FL-QY) reaching about 86% in the solution. It was also found that the compounds are strongly affected by the polarity of the solvent, which results in a redshift in fluorescence emission with decreasing FL-QYs. These results reveal the intramolecular charge transfer (ICT) characteristics of the compounds. On the other hand, the synthesized compounds exhibit photoluminescence (PL) emission in the solid state, covering the regions from blue to red of the electromagnetic spectrum. It was found that all compounds have low PL QYs in the solid state, in which the highest PL QY was measured to be 18.2%. The analysis of the TRF results has revealed that the nonradiative decay processes dominantly govern the emission kinetics of the compounds depending on the substituents. Furthermore, the monolith structure of some of the compounds, which were prepared under 80 kN to investigate color-conversion application, exhibited highly mechano-fluorochromic (MFC) properties. Here, it was found that their PL emission shifted about 70 nm to the longer-wavelength side of the spectrum. Finally, as a proof-of-concept demonstration, the synthesized pyrazoline derivatives were employed as a color-converter material on a light-emitting diode (LED).