Effect on shifting of phenyl ring in pyrazoline pyrene luminophore and their photophysical and electrochemical investigation

Abstract

Phenyl substitution in two different positions on pyrazoline ring bearing pyrene luminescent materials were synthesized (PP3M and PP5M) and their structure property relationship investigated. TGA results indicate good thermal stability up to 368 °C and 357 °C for PP3M and PP5M respectively. DSC and PXRD analysis confirm the existence between amorphous and crystalline nature. The small changes in position of molecular structure brought interesting emission properties in solution state as sky-blue (λem = 438 nm) and green (λem = 510 nm) color while emission in solid state as sky-blue (λem = 537 nm) and yellow (λem = 593 nm) color of PP3M and PP5M respectively. The PP3M exhibiting dual emission in solution and solid states are ascribed to twisted intramolecular charge transfer process. The decay of excited state displays shorter and longer values of 1.0 ns and 3.14 ns? The optical band gap decides color emission owing to conjugation length playing major role and their data suggesting semiconducting properties. With increasing polarity of the solvents the dipolemoment changes resulting in intramolecular charge transfer process and this is well understood from the large Stokes shift observed ascribed to solvatochromism. The emission color purities were compared in solution for PP3M and PP5M with standardized color purity values by NTSC. Third order nonlinear optical properties of luminophores were investigated by Z-scan studies refers saturable absorption and self-defocusing nature. Their HOMO-LUMO energy level values were experimentally calculated by electrochemical studies which are in accordance with theoretical values from DFT analysis. Further, DFT geometries reveal twisted molecular geometry of PP3M and PP5M in the ground state.