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