Cho Yang-Jin, Kim So-Yoen, Cho Minji, Wee Kyung-Ryang, Son Ho-Jin, Han Won-Sik, Cho Dae Won, Kang Sang Ook
Department of Advanced Materials Chemistry, Korea University (Sejong), Sejong, 30019, South Korea.
Department of Chemistry, Daegu University, Gyeongsan, 38453, South Korea.
Phys Chem Chem Phys. 2016 Jun 1;18(22):15162-9. doi: 10.1039/c6cp02087a.
To gain new insights into ligand-to-ligand charge-transfer (LLCT) dynamics, we synthesised two heteroleptic Ir(3+) complexes: (Ir(dfppy)2(tpphz)) and (Ir(dfppy)2(dpq)), where dfppy, tpphz, and dpq are 2-(4,6-difluorophenyl)pyridine, tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine, and 2,3-bis-(2-pyridyl)-quinoxaline, respectively. The tpphz and dpq ligands have longer π-conjugation than dfppy. Therefore, the excited ligand-centred (LC) state and the metal-to-ligand charge transfer (MLCT) state of dfppy are higher than those of tpphz and dpq. The LLCT dynamics from dfppy to tpphz (or dpq) was probed using femtoscond transient absorption (TA) spectroscopy after the selective excitation of dfppy. The TA band for the LC/MLCT state of dfppy is observed at 480 nm. Because of the LLCT process, the TA bands related to the MLCT states of tpphz and dpq ligands increased, whereas those of dfppy decreased. The time constants for the LLCT process were 17 ps for Ir(dfppy)2(tpphz) and 5 ps for Ir(dfppy)2(dpq). The MLCT emission of Ir(dfppy)2(tpphz) showed strong temperature dependence, indicating that the LLCT process has a significant energy barrier. In comparison, the temperature weakly influenced the emission of Ir(dfppy)2(dpq), and thus, its LLCT process may have a smaller barrier. The anomalous rigidochromism and photodynamic behaviours can be explained in terms of the barrier between cyclometalating and ancillary ligands.
为了深入了解配体间电荷转移（LLCT）动力学，我们合成了两种异质金属铱（III）配合物：（Ir(dfppy)2(tpphz)）和（Ir(dfppy)2(dpq)），其中dfppy、tpphz和dpq分别是2-(4,6-二氟苯基)吡啶、四吡啶并[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]菲嗪和2,3-双-(2-吡啶基)-喹喔啉。tpphz和dpq配体的π共轭比dfppy长。因此，dfppy的激发配体中心（LC）态和金属到配体电荷转移（MLCT）态高于tpphz和dpq。在对dfppy进行选择性激发后，使用飞秒瞬态吸收（TA）光谱探测了从dfppy到tpphz（或dpq）的LLCT动力学。dfppy的LC/MLCT态的TA带在480 nm处观察到。由于LLCT过程，与tpphz和dpq配体的MLCT态相关的TA带增加，而dfppy的TA带减少。Ir(dfppy)2(tpphz)的LLCT过程的时间常数为17 ps，Ir(dfppy)2(dpq)的为5 ps。Ir(dfppy)2(tpphz)的MLCT发射表现出强烈的温度依赖性，表明LLCT过程具有显著的能垒。相比之下，温度对Ir(dfppy)2(dpq)的发射影响较弱，因此其LLCT过程可能具有较小的能垒。异常的刚性变色和光动力学行为可以用环金属化配体和辅助配体之间的能垒来解释。
