Solid state light emitting polyaromatic luminogens containing pyrazoline chromophore

Abstract

A series of polyaromatic luminophores based on pyrene pyrazoline containing donor-acceptor π-conjugation extended through naphthalene (PPN), anthracene (PPA) and pyrene (PPP) were designed and synthesized. They exhibited high glass transition temperatures and high thermal stability. Fluorescence shifted to longer wavelength while moving from solution state emission (blue to greenish yellow) to solid state and thin film emission (yellow, orange, red), further, electron accepting ability also increased with polyaryl substituents. PPN exhibited dual emission in solution, solid state and thin film. The purity of color emission was confirmed from CIE values. The luminogens reveal typical AIE (PPN, PPA) and AIBSE (PPP) features of twisted molecular conformation into aggregation of molecules arranged in J and H type aggregations. Solvatochromism observed as prominent blue to yellow emission shift, large Stokes shift in D-A of twisted molecular geometry and intramolecular charge transfer in various solvents. They demonstrate dual emission in nonpolar solvents with twisted intramolecular charge transfer process. The lifetime analysis of TCSPC revealed longer and shorter lifetime values in the excited state. Powder X-ray diffraction and differential scanning calorimetry disclosed phase transition between crystalline and amorphous states. The single beam Z-scan analysis revealed third order nonlinear optical properties of the materials. They exhibited nonlinear absorption owing to saturable absorption process while negative nonlinear refraction led to self defocusing nature. DFT and TD-DFT calculations established that these luminogens are highly twisted in nature. Charge transport properties were calculated through reorganization energy and reveals that these molecules could act as electron transporting materials. Computed IP and EA values along with reorganization energy reveal that they could be used as trifunctional materials (electron, hole and emitter). The changing of polyaromatic groups in 3, 5 position of pyrazoline ring was demonstrated that it could lead to new solid state light emitters that these molecules have potential in organic optoelectronics as OLED and down conversion solar cell applications.