Scholz Reinhard, Kleine Paul, Lygaitis Ramunas, Popp Ludwig, Lenk Simone, Etherington Marc K, Monkman Andrew P, Reineke Sebastian
Dresden Integrated Center for Applied Physics and Photonic Materials (IAPP) and Institute for Applied Physics , Technische Universität Dresden , 01062 Dresden , Germany.
Leibniz Institute of Polymer Research Dresden , P.O. Box 120 411, 01005 Dresden , Germany.
J Phys Chem A. 2020 Feb 27;124(8):1535-1553. doi: 10.1021/acs.jpca.9b11083. Epub 2020 Feb 19.
Emitters showing thermally activated delayed fluorescence (TADF) in electroluminescent devices rely on efficient reverse intersystem crossing (rISC) arising from small thermal activation barriers between the lowest excited triplet and singlet manifolds. A small donor-acceptor compound consisting of a demethylacridine donor and a methylbenzoate acceptor group is used as a model TADF emitter. The spectroscopic signatures of this system are characterized using a combination of photoluminescence and photoluminescence excitation, and the photoluminescence decay dynamics are recorded between delays of 2 ns and 20 ms. Above = 200 K, our data provide convincing evidence for TADF at intermediate delays in the microsecond range, whereas triplet-triplet annihilation and slow triplet decay at later times can be observed over the entire temperature range from = 80 K to room temperature. Moreover, close to room temperature, we find a second and faster up-conversion mechanism, tentatively assigned to reverse internal conversion between different triplet configurations. An interpretation of these experimental findings requires a calculation of the deformation patterns and potential minima of several electronic configurations. This task is performed with a range-separated hybrid functional, outperforming standard density functionals or global hybrids. In particular, the systematic underestimation of the energy of charge transfer (CT) states with respect to local excitations within the constituting chromophores is replaced by more reliable transition energies for both kinds of excitations. Hence, several absorption and emission features can be assigned unambiguously, and the observed activation barriers for rISC and reverse internal conversion correspond to calculated energy differences between the potential surfaces in different electronic configurations.
在电致发光器件中表现出热激活延迟荧光（TADF）的发光体依赖于最低激发三重态和单重态之间较小的热激活势垒所产生的高效反向系间窜越（rISC）。一种由脱甲基吖啶供体和苯甲酸甲酯受体基团组成的小型供体 - 受体化合物被用作模型TADF发光体。该系统的光谱特征通过光致发光和光致发光激发相结合的方法进行表征，并在2 ns至20 ms的延迟时间内记录光致发光衰减动力学。在T = 200 K以上，我们的数据为微秒范围内中间延迟时的TADF提供了令人信服的证据，而在从T = 80 K到室温的整个温度范围内，后期可以观察到三重态 - 三重态湮灭和缓慢的三重态衰减。此外，接近室温时，我们发现了第二种更快的上转换机制，初步认为是不同三重态构型之间的反向内转换。对这些实验结果的解释需要计算几种电子构型的变形模式和势阱。这项任务是使用范围分离的混合泛函来完成的，其性能优于标准密度泛函或全局混合泛函。特别是，相对于构成发色团内的局部激发，电荷转移（CT）态能量的系统性低估被两种激发更可靠的跃迁能量所取代。因此，可以明确地确定几个吸收和发射特征，并且观察到的rISC和反向内转换的激活势垒对应于不同电子构型中势面之间的计算能量差。
