A Hybridized Local and Charge-Transfer Excited State for Highly Efficient Fluorescent OLEDs: Molecular Design, Spectral Character, and Full Exciton Utilization
A Hybridized Local and Charge-Transfer Excited State for Highly Efficient Fluorescent OLEDs: Molecular Design, Spectral Character, and Full Exciton Utilization
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高效荧光 OLED 的混合局部和电荷转移激发态：分子设计、光谱特征和充分激子利用
DOI：
10.1002/adom.201400154
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发表时间：
2014-09-01
影响因子：
9
通讯作者：
Ma, Yuguang
中科院分区：
文献类型：
--
作者：
Li, Weijun;Pan, Yuyu;Ma, Yuguang
对于供体-受体（D-A）分子，其低激发态（S-1）有三种可能的情况：π-π * 态（定域电子态）、电荷转移（CT）态（离域电子态）和π-π * 和CT的混合或杂化态（在此称为杂化局域和电荷转移（HLCT）态）。HLCT态是设计下一代有机发光二极管（OLED）材料的重要激发态，其具有高的光致发光（PL）效率和在电致发光（EL）中大部分的单重态激子产生。根据量子化学中的态混合原理，设计并合成了一系列扭曲D-A分子，并通过荧光溶致变色实验和量子化学计算验证了其HLCT态的性质。在HLCT态中，随着D-A链段扭转角的减小或D-A链段强度的增加，对分子光学性质影响很大的CT组分增强。在有机电致发光器件中，使用这些HLCT化合物作为发光层，最大激子利用效率可达93%。令人惊讶的是，在一些HLCT化合物中，基于激发态能级的分析和低温光谱的测量，发现了Kasha规则的例外：从高位三重态（T-2）到低位三重态T-1的受限内转换（IC），以及从T-2到S-1或S-2的重新开放的反向系间穿越（RISC）路径。从T-2到S-1（S-2）的RISC作为“热激子”通道被认为有助于大比例的辐射单重态激子。
For a donor-acceptor (D-A) molecule, there are three possible cases for its low-lying excited state (S-1): a pi-pi* state (a localized electronic state), a charge-transfer (CT) state (a delocalized electronic state), and a mixed or hybridized state of pi-pi* and CT (named here as the hybridized local and charge transfer (HLCT) state). The HLCT state is an important excited state for the design of next-generation organic light-emitting diode (OLED) materials with both high photoluminescence (PL) efficiency and a large fraction of singlet exciton generation in electroluminescence (EL). According to the principle of state mixing in quantum chemistry, a series of twisting D-A molecules are designed and synthesized, and their HLCT state characters are verified by both fluorescent solvatochromic experiments and quantum chemical calculations. The CT components in the HLCT state, which greatly affect the molecular optical properties, are found to be enhanced with a decrease of the twist angle of the D-A segment or an increase of the D-A intensity in these twisting D-A molecules. In OLEDs, using these HLCT compounds as the emitting layer, the maximum exciton utilization efficiency is harvested up to 93%. Surprisingly, an exception of Kasha's rule is revealed in some HLCT compounds: restricted internal-conversion (IC) from the high-lying triplet state (T-2) to the low-lying triplet T-1, and a reopened path of reverse intersystem crossing (RISC) from T-2 to S-1 or S-2, based on the analysis of the excited-state energy levels and the measurement of the low-temperature spectrum. RISC from T-2 to S-1 (S-2) as a "hot exciton" channel is believed to contribute to the large proportion of the radiative singlet excitons.