Exploring the Influence of Engineering the Linker between the Donor and Acceptor Fragments on Thermally Activated Delayed Fluorescence Characteristics

Aftab Hussain; Farah Kanwal; Ahmad Irfan; Mehboob Hassan; Jingping Zhang

doi:10.1021/acsomega.3c01098

Exploring the Influence of Engineering the Linker between the Donor and Acceptor Fragments on Thermally Activated Delayed Fluorescence Characteristics

ACS Omega. 2023 Apr 20;8(17):15638-15649. doi: 10.1021/acsomega.3c01098. eCollection 2023 May 2.

Authors

Aftab Hussain¹, Farah Kanwal¹, Ahmad Irfan², Mehboob Hassan³, Jingping Zhang⁴

Affiliations

¹ School of Chemistry, University of the Punjab, Lahore 54590, Pakistan.
² Department of Chemistry, College of Science, King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia.
³ Department of Chemistry, University of Narowal, Narowal, Punjab 51600, Pakistan.
⁴ Faculty of Chemistry, Northeast Normal University, Changchun 130024, China.

Abstract

We have expounded the unique molecular design architecture for efficient thermally activated delayed fluorescence (TADF) materials based on a donor-linker-acceptor-linker-donor (D-L-A-L-D) framework, which can be employed as predecessors of organic light-emitting diode (OLED) devices. Different from traditional donor-acceptor-type (D-A-type) TADF scaffolds, the D-L-A-L-D structural design avoids direct coupling amid the D and A fragments allowing the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) to be spatially separated. It results in a reduced overlap between HOMOs and LUMOs, thus realizing fairly a slight singlet-triplet energy gap (ΔE _ST) and higher photoluminescence quantum yield (Φ). We revealed that manipulating a linker between D and A fragments in intramolecular charge transfer compounds is an auspicious approach for realizing small ΔE _ST. Herein, we report a group of organic electroluminescent D-L-A-L-D-type molecules with different electron-donating and electron-accepting moieties using density functional theory calculations and time-dependent density functional theory calculations. Two types of linkers, the π-conjugated phenylene (-C₆H₄-) and aliphatic alkyl chains or σ-spacer (-CH₂- and -CH₂-CH₂-), were exploited between D and A fragments. In principle, the conjugation in D-π-A-π-D-type molecules and hyperconjugation in D-σ-A-σ-D type molecules encourage the spatial separation of the HOMO-LUMO causing a reduction in the ΔE _ST. All the designed molecules show a blue-shift in the emission wavelengths (λ_em) over the directly linked parent molecules except DPA-DPS-C₆H₄ and BTPA-DPS-C₆H₄ which show a red-shift. Violet-blue to green-yellow (376-566 nm) λ_em was observed from all of the investigated molecules. Other important properties that affect the efficiency of emission quantum yields like frontier molecular orbital analysis, natural population analysis, electron excitation analysis, exciton binding energies, ionization potentials, electronic affinities, and reorganization energies of the designed molecules were also inspected. We are confident that our work will effectively give a straightforward and distinctive approach to building incredibly effective TADF-OLEDs and a new perspective on their structural design.