Stavrou Kleitos, Danos Andrew, Hama Toshiki, Hatakeyama Takuji, Monkman Andrew
Department of Physics, Durham University, South Road, Durham, DH1 3LE, United Kingdom.
Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan.
ACS Appl Mater Interfaces. 2021 Feb 24;13(7):8643-8655. doi: 10.1021/acsami.0c20619. Epub 2021 Feb 8.
The photophysics of multiple resonance thermally activated delayed fluorescence molecule ν-DABNA is described. We show coupling of a 285 cm stretching/scissoring vibrational mode of peripheral phenyl rings to the S state, which dictates the ultimate emission full-width at half maximum. However, a separate high amplitude mode, 945 cm of the -biphenyl units, mediates the reverse intersystem crossing (rISC) mechanism. Concentration-dependent studies in solution and solid state reveal a second emission band that increases nonlinearly with concentration, independent of the environment assigned to excimer emission. Even at concentrations well below those used in devices, the excimer contribution affects performance. Using different solvents and solid hosts, rISC rates between 3-6 × 10 s are calculated, which show negligible dependence on environmental polarity or host packing. At 20 K over the first 10 ns, we observe a broad Gaussian excimer emission band with energy on-set above the S exciton band. An optical singlet-triplet gap (ΔE) of 70 meV is measured, agreeing with previous thermal estimates; however, the triplet energy is also found to be temperature-dependent. A monotonic increase of the exciton emission band full-width at half maximum with temperature indicates the role of hot transitions in forming vibrational excited states at room temperature (RT), and combined with an observed temperature dependency of ΔE, we deduce that the rISC mechanism is that of thermally activated reverse internal conversion of T to T ( ≥ 2) followed by rapid rISC of T to S. Organic light-emitting diodes with ν-DABNA as a hyperfluorescent emitter (0.5 wt % and 1 wt %) exhibit an increase of maximum external quantum efficiency, reaching 27.5% for the lower ν-DABNA concentration. On the contrary, a Förster radius analysis indicated that the energy transfer ratio is smaller because of higher donor-acceptor separation (>2.4 nm) with weak sensitizer emission observed in the electroluminescence. This indicates excimer quenching in 1 wt % devices.
描述了多共振热激活延迟荧光分子ν-DABNA的光物理性质。我们展示了外围苯环的285厘米拉伸/剪式振动模式与S态的耦合，这决定了最终发射的半高宽。然而，-联苯单元的一个单独的高振幅模式，945厘米，介导了反向系间窜越（rISC）机制。溶液和固态下的浓度依赖性研究揭示了一个与浓度非线性增加的第二发射带，其与分配给激基缔合物发射的环境无关。即使在远低于器件中使用的浓度下，激基缔合物的贡献也会影响性能。使用不同的溶剂和固体主体，计算出rISC速率在3 - 6×10 s之间，这表明其对环境极性或主体堆积的依赖性可忽略不计。在20 K下的前10 ns内，我们观察到一个宽的高斯激基缔合物发射带，其能量起始高于S激子带。测量得到光学单重态-三重态能隙（ΔE）为70 meV，与先前的热估计值一致；然而，三重态能量也被发现与温度有关。激子发射带半高宽随温度的单调增加表明热跃迁在室温（RT）下形成振动激发态中的作用，并且结合观察到的ΔE的温度依赖性，我们推断rISC机制是T到T（≥2）的热激活反向内转换，随后是T到S的快速rISC。以ν-DABNA作为超荧光发射体（0.5 wt%和1 wt%）的有机发光二极管表现出最大外量子效率的增加，较低ν-DABNA浓度时达到27.5%。相反，Förster半径分析表明，由于供体-受体间距较大（>2.4 nm）且在电致发光中观察到敏化剂发射较弱，能量转移率较小。这表明1 wt%器件中存在激基缔合物猝灭。
