Structural tailoring of p-terphenyl scaffold: Towards advanced plastic scintillator

Abstract

A comprehensive series of fifteen p-terphenyl derivatives has been designed and prepared using facile synthetic approaches (Suzuki-Miyaura and Friedel-Crafts reactions). Various alkyl, alkoxy or trifluoromethyl groups have been attached to p-terphenyl to gain both symmetrical and unsymmetrical derivatives. Symmetry breaking accounts for improved solubility of mono-substituted derivatives. Thermal analysis revealed a reversible formation of several crystalline phases and very high thermal robustness. Reduction and oxidation of the parent p-terphenyl π-system affected by the electronic and steric effects of the substituents were recorded by cyclic voltammetry. When compared to parent p-terphenyl, a narrowed HOMO–LUMO gap has been measured/calculated for most of the prepared derivatives. The absorption and emission maxima ranged from 286 to 293 and 341–360 nm, respectively. The latter is affected by the substitution more significantly, the fluorescence quantum yields are within the range of 0.52–0.75. Stokes shifts up to 6400 cm−1 were measured pointing to a large structural rearrangement (planarization) upon excitation. PTP derivatives were further used as primary and secondary luminophores for scintillation. 4-Methyl-p-terphenyl proved to be a superior luminophore with a straightforward synthesis, easy isolation, large solubility and excellent luminescence characteristic. Based on this derivative, a plastic scintillator has been manufactured that showed superior performance as compared to standard and widely used unsubstituted p-terphenyl. Thus, a relatively minor structural change (appending single methyl group) altered scintillation properties significantly.