Understanding the Control of Singlet-Triplet Splitting for Organic Exciton Manipulating: A Combined Theoretical and Experimental Approach

Abstract

Developing organic optoelectronic materials with desired photophysical properties has always been at the forefront of organic electronics. The variation of singlet-triplet splitting (ΔEST) can provide useful means in modulating organic excitons for diversified photophysical phenomena, but controlling ΔEST in a desired manner within a large tuning scope remains a daunting challenge. Here, we demonstrate a convenient and quantitative approach to relate ΔEST to the frontier orbital overlap and separation distance via a set of newly developed parameters using natural transition orbital analysis to consider whole pictures of electron transitions for both the lowest singlet (S1) and triplet (T1) excited states. These critical parameters revealed that both separated S1 and T1 states leads to ultralow ΔEST; separated S1 and overlapped T1 states results in small ΔEST; and both overlapped S1 and T1 states induces large ΔEST. Importantly, we realized a widely-tuned ΔEST in a range from ultralow (0.0003 eV) to extra-large (1.47 eV) via a subtle symmetric control of triazine molecules, based on time-dependent density functional theory calculations combined with experimental explorations. These findings provide keen insights into ΔEST control for feasible excited state tuning, offering valuable guidelines for the construction of molecules with desired optoelectronic properties.

3.2: Singlet‐Triplet Splitting Energy Management via Acceptor Substitution: Complanation Molecular Design for Deep‐Blue Thermally Activated Delayed Fluorescent Organic Light‐Emitting Diodes

“…The classic mechanism electrostatic interaction (described by K) is corrected by electron exchange energy (J). (8,17) Assuming the same half-filled orbital configuration of S1 and T1, the matrix element for J has the form of Equation (4) and ΔEST can be evaluated by 2J which is directly proportional to FMOs overlap of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). The Columbic interaction operator, e 2 /(r1 -r2), also plays a role in manipulating J.…”

Isomeric Thermally Activated Delayed Fluorescence Emitters for Color Purity-Improved Emission in Organic Light-Emitting Devices

“…12 With an extremely small singlet-triplet splitting (∆E ST ), TADF emitter can realize efficient upconversion of non-radiative triplet excitons to radiative singlet excitons via reverse intersystem crossing (RISC) process. [13][14][15][16] Thus, compared with PhOLEDs, the TADF OLEDs can also deliver a full exciton harvesting theoretically, but without heavy metals. 18-21 3 To realize the key point of extremely small ∆E ST s for TADF emitters, minimizing the overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the molecules is essential.…”

“…18-21 3 To realize the key point of extremely small ∆E ST s for TADF emitters, minimizing the overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the molecules is essential. 15,[17][18][19][20] Thus, TADF emitters are generally constructed by linking the electron-donor (D) segment and electron-acceptor (A) segment with a nearly vertical dihedral angle. 17,22 Such subvertical D-A structure can suppress the conjugation between D and A segments, and strictly isolate the HOMO and LUMO orbitals on D and A segments, respectively.…”

Unraveling the Microscopic Origin of Triplet Lasing from Organic Solids

“…This suggests efficient population gain of the T 1 state, satisfying the fundamental condition for triplet lasing, and simultaneously avoiding any possibility of TADF. 26 To drive population transfer from the T 1 state, there still exist two more challenges: a long lifetime of the triplet states and a strong T 1 →T n absorption. To estimate the lifetime (τ) of the T 1 state of S-BF 2 , we have evaluated the radiative (k r ) and nonradiative (k nr ) rate constants, as τ is given by the expression1/(k r +k nr ).…”