Fluorescence, Phosphorescence, or Delayed Fluorescence?—A Theoretical Exploration on the Reason Why a Series of Similar Organic Molecules Exhibit Different Luminescence Types

Abstract

In contrast to the traditional view that the small organic molecules emit fluorescence, more and more experiments manifest their special luminescence types, such as the thermally activated delayed fluorescence (TADF) and roomtemperature phosphorescence. Why the similar organic molecules exhibit different luminescence types is focused on in this work on the basis of density functional theory/timedependent density functional theory calculations on a series of small organic molecules with phenoxazine or carbazole as a donor and diphenyl-triazine as an acceptor. The deep analysis of the geometrical and electronic structures shows how the structure, especially for the donor−acceptor dihedral angle, determines the singlet−triplet energy gap and the property of excited states. The explorations on the electron−hole pairs of natural transition orbitals and the contribution of the key heteroatom (N) to different molecular orbitals reveal the distinct electron transition processes of excitation to singlet and triplet states, explain the reason for different energy-level distributions of excited states, and identify which pairs have more favorable intersystem crossing for these molecules, while the calculations of spin−orbit coupling and reorganization energy display the efficiency of the different luminescence types. Meanwhile, considering the potential application of TADF materials in organic light-emitting diodes, we also separately modified the phosphorescent molecule and the prompt fluorescent molecule through the introduction of methyls to increase the steric hindrance and realize the perpendicular orientation of donor and acceptor unit, and finally to screen the excellent TADF molecules theoretically.

Aggregation‐Induced Emission: New Vistas at the Aggregate Level

“…At the aggregate level, the regulation of fluorescence and phosphorescence can be achieved through suppression of non‐radiative decay and tuning of the ISC process. In addition, it is also possible to utilize both singlet and triplet excitons to realize delayed fluorescence by promoting reversed intersystem crossing . Organic luminogens with delayed fluorescence (DF) have aroused great interest owing to their high exciton utilization and low cost .…”

High Triplet Energy Host Materials for Blue TADF OLEDs—A Tool Box Approach

“…Except for mATRZ and ATRZ , all compounds have already been investigated in some part, either experimentally, or by calculations, or both (An et al, 2011 , 2015 ; Lin et al, 2016 ; Duan et al, 2018 ; Arjona-Esteban et al, 2019 ; Fan et al, 2019 ; Liu et al, 2019 ; Sharma et al, 2019 ). These previous investigations do not form a complete and consistent data set, and sometimes are even contradictory.…”

Organic Room‐Temperature Phosphorescence with Strong Circularly Polarized Luminescence Based on Paracyclophanes

“…[1b] Meanwhile,the radiative and nonradiative decay from T 1 to S 0 also needs to be effectively suppressed to improve the phosphorescence efficiency. [7] There are several strategies in terms of molecular design to realize organic RTP: 1) Introducing the carbonyl units, [4,8] heteroatoms, [9] heavy atoms, [10] or resonance-induced spin flipping [11] to subsequently induce the strong spin-orbit coupling (SOC) effect, which contributes to higher k ISC ,r esulting in the larger population of triplet excitons;2 )effectively suppressing the non-radiative decay of low-lying triplet excitons through inhibiting the molecular nonradiative decay,e ither through crystallization, [12] one of the most commonly adopted methods; [13] or via halogenbonding, [14] intramolecular H-aggregation, [15] aromatic guest inclusion in supramolecular host, [16] and so on.…”