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zhou14579新虫 (小有名气)
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The genesis of organometallic complexes and polymers took place over a century ago and their great diversity has made them applicable in many different areas of materials applications since the initial discovery [9–11]. Metal complexes have unique advantages in electronic and photonic applications, since the electronic states can be changed in a controlled fashion within easily accessible ranges [12]. Spin states may also be controlled by the strength and symmetry of the ligand field and the redox states of metal ions. Fig. 1 describes the difference in the energy level and elec-tronic transition involved in organic and organometallic emitters. In organic molecules (with negligible spin–orbit coupling), they emit only from the lowest excited singlet S 1 state. Because T 1 →S 0 emission is extremely weak, triplet excitation is lost through radiationless decay rather than photon emission and consequently the maximum emission quantum yield is limited to 25%. However, in triplet harvesting, electron–hole recombination leads to excitations having a triplet-to-singlet population ratio of 3:1 [13]. Excitation cascades down to the triplet and singlet manifolds, in which internal conversion (IC) and intersystem crossing (ISC) ultimately give states residing in the lowest triplet metal-to-ligand charge transfer, 3 MLCT (T 1 ), which then emit photons. Facilitated ISC from a singlet excited state to other multiplet states (e.g. the triplet state) in the presence of heavy metal atom may be utilized to tune the excited state energetics, lifetimes, emission spectra and efficiencies [9]. Many of the transition metal complexes exhibit a sufficient excited state lifetime that permits various processes to occur, such as charge and energy transfer [14]. An additional merit in the use of metal complexes is the ability to organize organic ligandsinawell-definedandpredictablegeometry[9].Therefore,itis important to study the structural and electronic nature of the unit |
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zhou14579: 金币+100, ★★★很有帮助 2018-03-29 14:51:12
zhou14579: 金币+100, ★★★很有帮助 2018-03-29 14:51:12
| 人们于100多年前发现有机金属化合物和有机金属聚合物的成因。自发现以来，因其多样性，可应用于材料的多个领域[9-11]。因为电子状态可以在易接近的范围内控制改变，因而金属化合物在电子和光子应用中具有独特的优势[12]。自旋态也可能通过配体场的强度和对称性以及金属离子的氧化还原态来控制。图1描述了有机发射体和有机金属发射体中能级和电子跃迁的差异。在有机分子中（自旋轨道耦合忽略不计），它们仅仅从激发最低单态S1态发射。由于T1→S0的发射非常弱，所以通过无辐射衰减而不是光子发射，会导致三重态激发损失，因而最大发射量子产率限制在25％以内。然而，在三重态收获中，电子-空穴重组会导致激发三重态与单重态的比例为3：1[13]。激发级联到三重态和单重态流形，最终内部转换（IC）和系间窜越（ISC）会使其状态处于最低三重态金属间配体电荷转移3MLCT（T1），而后发射光子。在重金属原子存在的条件下，促进系间窜越从单重激发态到其他多重激发态（例如三重态）可用于调整激发态能量、寿命、发射光谱和效率[9]。很多过渡金属化合物表现出足够的激发态寿命，允许各样过程发生，如电荷和能量转移[14]。使用金属化合物的另一个优点是能够以良好的定义和可预测的几何结构来组织有机配体[9]。因此，研究单位的结构和单位的电子性质十分重要。 |
2楼2018-03-29 10:52:41
