Fluorescence Spectral Study of Wavelength Shifters for Scintillation Plastics

An investigation of the fluorescence spectra of a series of 1,3,5-triaryl-2-pyrazoline indicated that substituents in the 3-phenyl ring shifted the fluorescence emission wavelength linearly with Brown's σp+. On the basis of this evidence the excited state is thought to have a partial positive charge located on carbon-3 of the pyrazoline ring. The shift in the absorption spectral bands of 1,3,5-triphenyl-2-pyrazoline with concentration is presented as evidence of a dimer or an associated complex. The fluorescence quenching at higher concentrations may be due to the energy dissipating effect of the dimers or complexes. The high extinction coefficient of the uv maximum and yet the similarity of the fluorescence intensity of 4,4′-bis-[5-(1,3-diphenyl-2-pyrazolinyl)] benzene to 1,3,5-triphenyl-2-pyrazoline is explained on the basis of an internal quenching mechanism. The efficiency of these two compounds as wavelength shifters in plastic scintillators are approximately equal.