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

Abstract

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 in 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.

Rational Design of Dual‐State Emission Luminogens with Solvatochromism by Combining a Partially Shared Donor–Acceptor Pattern and Twisted Structures

“…[5][6][7][8][9] Dual-state emissionc an also be attained by incorporating bulk substituents into molecular structurest op rohibit p-p stacking in the solid state. [10] A zero-twist donor-acceptor molecule was also found as ad ualstate emitter, [11] and ar ecent study showed that D-p-A structures can be used to achieve DSE properties. [12] However,t here is no valid design strategy for designing DSEgens with solvatochromism because of the lack of an investigation into the relationship between structure and DSE property.…”

Dipolar vinyl sulfur fluorescent dyes. Synthesis and photophysics of sulfide, sulfoxide and sulfone based D–π–A compounds

“…[12][13][14][15][16][17] In general, optical materials take important roles in optoelectronics, as part of transistors, OLEDs, solar cells, lightening agents and light modulators. 18,19 These usual D-p-A molecular architectures play an important role in their photophysics because they not only show high uorescent quantum yields owing to the effective radiative decay of their excited intramolecular chargetransfer (ICT) state but also possess impressive bipolar chargetransporting properties for their constitutive hole-and electrontransporting moieties, [20][21][22][23][24][25] that can be modulated through tuning the groups electron donor and/or acceptor strengths. [26][27][28][29][30] Concerning those functional groups, the organosuldes allow a tuneable electronic behavior that can be easily modulated through a single step oxidation to the respective sulfoxides or sulfones, which are effective electron acceptor groups.…”

Unsymmetrical Hexafluorocyclopentene-Linked Twisted π-Conjugated Molecules as Dual-State Emissive Luminophores

“…However, the development of DSE luminophores is difficult due to the distinct molecular design parameters. To date, reports on the molecular design of DSE luminophores have incorporated the following elements: (i) π-conjugation with a zero-twist donor-acceptor moiety to facilitate emission in solution state [20], (ii) twisted molecular structure to avoid fluorescence quenching in the solid state induced by π/π stacking, and (iii) a V-shaped configuration to enhance intramolecular charge transfer (ICT) and potentially suppress short-range π/π intermolecular interactions [21].…”