Exploring an Emissive Charge Transfer Process in Zero-Twist Donor-Acceptor Molecular Design as a Dual-State Emitter

摘要

The present work reports a new concept on how to diminish dark twist intramolecular charge transfer (TICT) states with the zero-twist D-A systems in order to design,frameworks with dual solution and solid-state emission property. The Study began with theoretical calculations to understand the structural needs followed by the chemical synthesis of conceptually new two molecular designs, 1 and 2, with zero-twist angle between electron donor and acceptor units linked through a covalent bond and finally their applications in OLED devices. Oxazole was used as an acceptor hi combination with the phenothiazene core as the donor, and the effect of enhanced electron donation was studied using methyl and anisole donor groups. DFT studies indicated a partial segregation of HOMO-LUMO levels in molecular designs, and the photophysics of these planar charge transfer molecules have been investigated. Natural transition orbital (NTO) calculations were carried out to understand excited-state transition character in these D-A molecules. Molecular level studies through single-crystal analysis revealed the importance of steric factor in controlling other molecular parameters, particularly short-range molecular forces. The synthesized compounds were eventually utilized in green-emitting OLED devices as a pristine emitting layer. Compound 2 showed better device efficiency than 1 in unoptimized devices largely due to the presence of the anisole group which prevented stacking of molecules. Solution-state emission and electroluminescence data of fabricated devices using 1 and 2 pointed out that molecular modification helped to enhance emission efficiency of 2 without shifting the emission wavelength.