Fluorescence in Rhoda- and Iridacyclopentadienes Neglecting the Spin鈥揙rbit Coupling of the Heavy Atom: The Ligand Dominates

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• 作者：Andreas Steffen ; Karine Costuas ; Abdou Boucekkine ; Marie-H茅l猫ne Thibault ; Andrew Beeby ; Andrei S. Batsanov ; Azzam Charaf-Eddin ; Denis Jacquemin ; Jean-Fran莽ois Halet ; Todd B. Marder
• 刊名：Inorganic Chemistry
• 出版年：2014
• 出版时间：July 7, 2014
• 年：2014
• 卷：53
• 期：13
• 页码：7055-7069
• 全文大小：668K
• ISSN：1520-510X

文摘

We present a detailed photophysical study and theoretical analysis of 2,5-bis(arylethynyl)rhodacyclopenta-2,4-dienes (1a鈥?b>c and 2a鈥?b>c) and a 2,5-bis(arylethynyl)iridacyclopenta-2,4-diene (3). Despite the presence of heavy atoms, these systems display unusually intense fluorescence from the S₁ excited state and no phosphorescence from T₁. The S₁ 鈫?T₁ intersystem crossing (ISC) is remarkably slow with a rate constant of 10⁸ s^鈥? (i.e., on the nanosecond time scale). Traditionally, for organometallic systems bearing 4d or 5d metals, ISC is 2鈥? orders of magnitude faster. Emission lifetime measurements suggest that the title compounds undergo S₁ 鈫?T₁ interconversion mainly via a thermally activated ISC channel above 233 K. The associated experimental activation energy is found to be 螖H_ISC = 28 kJ mol^鈥? (2340 cm^鈥?) for 1a, which is supported by density functional theory (DFT) and time-dependent DFT calculations [螖H_ISC(calc.) = 11 kJ mol^鈥? (920 cm^鈥?) for 1a-H]. However, below 233 K a second, temperature-independent ISC process via spin鈥搊rbit coupling occurs. The calculated lifetime for this S₁ 鈫?T₁ ISC process is 1.1 s, indicating that although this is the main path for triplet state formation upon photoexcitation in common organometallic luminophores, it plays a minor role in our Rh compounds. Thus, the organic 蟺-chromophore ligand seems to neglect the presence of the heavy rhodium or iridium atom, winning control over the excited-state photophysical behavior. This is attributed to a large energy separation of the ligand-centered highest occupied molecular orbital (HOMO) and lowest unoccupied MO (LUMO) from the metal-centered orbitals. The lowest excited states S₁ and T₁ arise exclusively from a HOMO-to-LUMO transition. The weak metal participation and the cumulenic distortion of the T₁ state associated with a large S₁鈥揟₁ energy separation favor an 鈥渙rganic-like鈥?photophysical behavior.