40 research outputs found
Comprehensive understanding of multiple resonance thermally activated delayed fluorescence through quantum chemistry calculations
Molecules that exhibit multiple resonance (MR) type thermally activated delayed fluorescence (TADF) are highly efficient electroluminescent materials with narrow emission spectra. Despite their importance in various applications, the emission mechanism is still controversial. Here, a comprehensive understanding of the mechanism for a representative MR-TADF molecule (5, 9-diphenyl-5, 9-diaza-13b-boranaphtho[3, 2, 1-de]anthracene, DABNA-1) is presented. Using the equation-of-motion coupled-cluster singles and doubles method and Fermi’s golden rule, we quantitatively reproduced all rate constants relevant to the emission mechanism; prompt and delayed fluorescence, internal conversion (IC), intersystem crossing, and reverse intersystem crossing (RISC). In addition, the photoluminescence quantum yield and its prompt and delayed contributions were quantified by calculating the population kinetics of excited states and the transient photoluminescence decay curve. The calculations also revealed that TADF occurred via a stepwise process of 1) thermally activated IC from the electronically excited lowest triplet state T₁ to the second-lowest triplet state T₂, 2) RISC from T₂ to the lowest excited singlet state S₁, and 3) fluorescence from S₁
振電相互作用密度解析とその応用
京都大学0048新制・課程博士博士(工学)甲第16044号工博第3367号新制||工||1509(附属図書館)28623京都大学大学院工学研究科分子工学専攻(主査)教授 田中 一義, 教授 佐藤 啓文, 教授 梶 弘典学位規則第4条第1項該当Doctor of Philosophy (Engineering)Kyoto UniversityDFA
Correlated Triplet Pair Formation Activated by Geometry Relaxation in Directly Linked Tetracene Dimer (5,5′-Bitetracene)
Singlet fission (SF) materials have the potential to overcome the traditional external quantum efficiency limits of organic light-emitting diodes (OLEDs). In this study, we theoretically designed an intramolecular SF molecule, 5, 5′-bitetracene (55BT), in which two tetracene units were directly connected through a C–C bond. Using quantum chemical calculation and the Fermi golden rule, we show that 55BT undergoes efficient SF induced by geometry relaxation in a locally excited singlet state, ¹(S0S1). Compared with another high-performing SF system, the tetracene dimer in the crystalline state, 55BT has advantages when used in doped systems owing to covalent bonding of the two tetracene units. This feature makes 55BT a promising candidate triplet sensitizer for near-infrared OLEDs
Theoretical Determination of Rate Constants from Excited-States: Application to Benzophenone
A cost-effective
method of theoretically predicting electronic transition rate constants from the
excited-states of molecules is reported.
This method is based on density functional theory calculations
of electronic states and quantitative rate constant determination with the
Fermi golden rule
A boron-containing molecule as an efficient electron-transporting material with low-power consumption
We theoretically propose a boron-containing molecule, hexaboracyclophane (HBCP), as an electron-transporting (ET) material with low-power loss. We calculate the vibronic coupling of HBCP, comparing them with those of other ET materials, tris-(8-hydroxyquinoline) aluminum(III) (Alq3) and tris[3-(3-pyridyl)mesityl]borane (3TPYMB). Using the nonequilibrium Green's function method to evaluate their single molecular ET properties, we show that HBCP exhibits more efficient and lower-power consumption than Alq3 and 3TPYMB. HBCP has suitable highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels as an electron-transport layer when Alq3 is employed as an emitter
Quantitative prediction of rate constants and its application to organic emitters
Gaussian 16 input and output files.ORCA input and output files.</p
Conformation Control of Iminodibenzyl-Based Thermally Activated Delayed Fluorescence Material by Tilted Face-to-Face Alignment With Optimal Distance (tFFO) Design
In organic light-emitting diodes (OLEDs), all triplet excitons can be harvested as light via reverse intersystem crossing (RISC) based on thermally activated delayed fluorescence (TADF) emitters. To realize efficient TADF, RISC should be fast. Thus, to accomplish rapid RISC, in the present study, a novel TADF emitter, namely, TpIBT-tFFO, was reported. TpIBT-tFFO was compared with IB-TRZ, which contains the same electron donor and acceptor segments, specifically iminodibenzyl and triazine moieties. TpIBT-tFFO is based on a recently proposed molecular design strategy called tilted face-to-face alignment with optimal distance (tFFO), whereas IB-TRZ is a conventional through-bond type molecule. According to quantum chemical calculations, a very large RISC rate constant, kRISC, was expected for TpIBT-tFFO because not only the lowest triplet state but also the second lowest triplet state were close to the lowest excited singlet state, as designed in the tFFO strategy. IB-TRZ has two different conformers, leading to dual emission. Conversely, owing to excellent packing, the conformation was fixed to one in the tFFO system, resulting in single-peaked emission for TpIBT-tFFO. TpIBT-tFFO displayed TADF type behavior and afforded higher photoluminescence quantum yield (PLQY) compared to IB-TRZ. The kRISC of TpIBT-tFFO was determined at 6.9 × 106 s−1, which is one of the highest values among molecules composed of only H, C, and N atoms. The external quantum efficiency of the TpIBT-tFFO-based OLED was much higher than that of the IB-TRZ-based one. The present study confirms the effectiveness of the tFFO design to realize rapid RISC. The tFFO-based emitters were found to exhibit an additional feature, enabling the control of the molecular conformations of the donor and/or acceptor segments
Promoting Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence via Heavy-Atom Effect
Thermally activated delayed fluorescence (TADF) molecules are promising for realizing durable organic light-emitting diodes in all color regions. Fast reverse intersystem crossing (RISC) is a way of improving the device lifetime of TADF-based organic light-emitting diodes. To date, RISC rate constants (kRISC) of 10^8 s−1 have been reported for metal-free TADF molecules. Here, we report the heavy-atom effect on TADF and a molecular design for further promoting RISC. First, the RISC mechanism of a sulfur-containing TADF molecule (with kRISC of 10^8 s−1) was comprehensively investigated via density functional theory. The role of the heavy-atom effect on the rapid RISC process was clarified. Our calculations also predicted that much larger kRISC (>10^10 s−1) will be obtained for selenium- and tellurium-containing TADF molecules. However, a polonium-containing molecule promotes phosphorescence without exhibiting TADF, indicating that too strong heavy-atom effect is unfavorable for achieving both rapid RISC and efficient TADF