C3 Substituted Naphthalimide Derivatives as Promising TADF Emitters: Effect of Donor Strength Revealed by Computational Studies

Abstract

Thermally activated delayed fluorescence (TADF) emitters are pivotal in enhancing the electroluminescence efficiency of organic light-emitting diodes (OLEDs) by enabling effective utilization of triplet excitons. Emitters based on naphthalimide (NI) have not received much attention, particularly the C3 substituted variants. In this study, a potential TADF molecule NI-AZB featuring 10-mesityl-5,10-dihydrodibenzo[b,e][1,4]azaborinine (AZB) as a donor is shortlisted after a rigorous consideration of several similar derivatives possessing donors such as carbazole, dimethylacridine, phenoxazine, and phenothiazine. Computational analyses indicate that NI-AZB exhibits a small singlet-triplet energy gap, promising radiative decay rates, moderate spin-orbit coupling, and substantial reverse intersystem crossing (rISC) rates. The S₁ state of NI-AZB is charge-transfer (CT) in nature, while the T₁ state exhibits localized excitation (LE), facilitating enhanced spin-orbit coupling and rISC rates. Additionally, NI-AZB absorbs in the UV region, suggesting its potential as a blue-emitting material for OLED devices. Furthermore, it is observed that substitution at the C4 of the naphthalimide core enhances CT character, leading to higher rISC rates but reduced radiative rates. Conversely, substitution at the C3 diminishes CT character, resulting in increased radiative rates while maintaining moderate rISC rates. These insights underscore the importance of C3 substitution in optimizing TADF properties of naphthalimide-based emitters for OLED applications.