Resemblances of experiment and theory on aryl substituted luminogenic polypyrazolines

Abstract

Polyarylpyrazolines (PPB, PPA, PPT, PPBt) containing various aryl substituents emit light in a broad color range from orange to blue, making them suitable for optoelectronics. These polymers have been synthesized by the Claisen–Schmidt condensation, followed by the Suzuki cross-coupling polycondensation. The photophysical and electrochemical properties of these polyarylpyrazolines have been established by varying the side arms in the polymer backbone. The polymers are designed to work as difunctional charge carriers both as hole and electron transport materials, which are useful in polymer light-emitting diodes (PLEDs). Pyrazolines with monomer units of polymers were used as templates with various substituents to deduce their optoelectronic properties and photophysical properties, and to understand their electronic origin via the density functional theory (DFT), time-dependent density functional theory (TD-DFT) and Tamm–Dancoff approximation (TDA) methodology. By computing the thermally activated delayed fluorescence (TADF) properties of the polyarylpyazolines, their suitability for better PLED performance were analyzed. Frontier molecular orbitals (FMOs) and natural transition orbitals (NTOs) analyses reveal that the donor group (phenylene, anthracene and thiophene) and the acceptor group (benzothiadiazole) affect the electronic distribution and transitions. The analyses also reveal that the transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) transition is due to an intramolecular charge transfer (ICT). Notably, the HOMO of MPBt (monomer of PPBt) is localized on donor moieties (phenyl ring) and the LUMO is localized on an acceptor moiety (benzothiadiazole); such molecules lead to the smallest singlet and triplet energy gap, and eventually have a high propensity for TADF. The TD-DFT and corresponding TDA calculations proved that MPBt has the smallest vertical (TD-DFT method, 0.448 eV and TDA method, 0.245 eV) and adiabatic (TDA method, 0.358 eV) singlet and triplet energy gap. Its fluorescence efficiency increases via a reverse intersystem crossing (RISC). Likewise, a high emission value (588 nm) was observed for the PPBt polymer through experimental studies. Compared to the experimental and theoretical results of four polymers under scrutiny, the photophysical features of PPBt suggested it to be a better optoelectronic material (TADF emitter) than PPB, PPT and PPA. This study clearly reveals that with suitable donor and acceptor substituents, the singlet–triplet energy gap can be fine-tuned with enhanced TADF behavior for designing high performance PLED materials.