Saini Chetan, Gouthaman Siddan, Justin Thomas K R
Organic Materials Laboratory, Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee - 247667, India.
Phys Chem Chem Phys. 2025 Jan 15;27(3):1327-1338. doi: 10.1039/d4cp02636e.
Thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) are most promising processes for harvesting triplet excitons in organic light-emitting diodes. In this work, the effect of the linkage between the carbazole (Cz) donor (D) and the naphthalenediimide (NDI) acceptor (A) on the TADF and RTP propensities is elucidated using density functional theory computations employing D-A, D-A-D, D-π-A, and D-π-A-π-D structural designs. The effects of the dihedral angle between the donor and acceptor units on the energy difference between the singlet and triplet excited states (Δ), the spin-orbit coupling (SOC) constants, and radiative (), intersystem crossing () and reverse intersystem crossing () rates are unravelled. The molecules possessing a direct linkage between Cz and NDI exhibit large Δ values due to substantial overlap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). However, the insertion of a phenyl spacer between the Cz and NDI units led to disjoint HOMO and LUMO and consequently resulted in a small Δ. Furthermore, the presence of two donors with and without a phenyl spacer on NDI resulted in a high-lying triplet state (T) that is energetically lower than the lowest singlet excited state (S), hence providing additional channels to the TADF and RTP processes. Also, the orientation of Cz and NDI in the -positions of the phenyl unit resulted in a T state with dominant LE character which led to moderate spin-orbit coupling constants and highest rates compared to the analogous - and -linked derivatives. Thus, the -derivatives possessed small Δ, charge transfer dominated S, joint holes and electrons for the T state, characteristic of local excitation, high SOC, and promising rISC and rates. Overall, the phenyl linked derivatives possess TADF characteristics, while the directly linked analogues show RTP propensity.
热激活延迟荧光（TADF）和室温磷光（RTP）是有机发光二极管中捕获三重态激子最有前景的过程。在这项工作中，利用密度泛函理论计算，采用D - A、D - A - D、D - π - A和D - π - A - π - D结构设计，阐明了咔唑（Cz）供体（D）与萘二酰亚胺（NDI）受体（A）之间的连接对TADF和RTP倾向的影响。揭示了供体和受体单元之间的二面角对单重态和三重态激发态之间的能量差（Δ）、自旋 - 轨道耦合（SOC）常数以及辐射（）、系间窜越（）和反向系间窜越（）速率的影响。在Cz和NDI之间具有直接连接的分子由于最高占据分子轨道（HOMO）和最低未占据分子轨道（LUMO）之间的大量重叠而表现出大的Δ值。然而，在Cz和NDI单元之间插入苯基间隔基导致HOMO和LUMO不连续，因此导致小的Δ。此外，在NDI上存在有和没有苯基间隔基的两个供体导致一个高能三重态（T），其能量低于最低单重态激发态（S），因此为TADF和RTP过程提供了额外的通道。而且，Cz和NDI在苯基单元的 - 位的取向导致具有主导LE特征的T态，与类似的 - 和 - 连接的衍生物相比，其导致适度的自旋 - 轨道耦合常数和最高的速率。因此， - 衍生物具有小的Δ，电荷转移主导S，T态的空穴和电子联合，具有局部激发、高SOC以及有前景的rISC和速率的特征。总体而言，苯基连接的衍生物具有TADF特性，而直接连接的类似物表现出RTP倾向。
