Luminescent Charge-Transfer Platinum(II) Metallacycle

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• 作者：Fei Hua ; Solen Kinayyigit ; Aaron A. Rachford ; Elena A. Shikhova ; Sé ; bastien Goeb ; John R. Cable ; Christopher J. Adams ; Kristin Kirschbaum ; A. Alan Pinkerton ; Felix N. Castellano
• 刊名：Inorganic Chemistry
• 出版年：2007
• 出版时间：October 15, 2007
• 年：2007
• 卷：46
• 期：21
• 页码：8771 - 8783
• 全文大小：409K
• 年卷期：v.46,no.21(October 15, 2007)
• ISSN：1520-510X

文摘

The photophysical and electrochemical properties of a platinum(II) diimine complex bearing the bidentate diacetylideligand tolan-2,2'-diacetylide (tda), Pt(dbbpy)(tda) [dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine] (1), are compared withtwo reference compounds, Pt(dbbpy)(CCPh)₂ (2) and Pt(dppp)tda [dppp = 1,3-bis(diphenylphosphino)propane](3), respectively. The X-ray crystal structure of 1 is reported, which illustrates the nearly perfect square planarityexhibited by this metallacycle. Chromophore 2 possesses low-lying charge-transfer excited states analogous to 1,whereas structure 3 lacks such excited states but features a low-lying platinum-perturbed tda intraligand tripletmanifold. In CH₂Cl₂, 1 exhibits a broad emission centered at 562 nm at ambient temperature, similar to 2, but witha higher photoluminescence quantum yield and longer excited-state lifetime. In both instances, the photoluminescenceis consistent with triplet-charge-transfer excited-state parentage. The rigidity imposed by the cyclic diacetylide ligandin 1 leads to a reduction in nonradiative decay, which enhances its room-temperature photophysical properties. Bycomparison, 3 radiates highly structured tda-localized triplet-state phosphorescence at room temperature. The 77K emission spectrum of 1 in 4:1 EtOH/MeOH becomes structured and is quantitatively similar to that measured for3 under the same conditions. Because the 77 K spectra are nearly identical, the emissions are assigned as ³tdain nature, implying that the charge-transfer states are raised in energy, relative to the ³tda levels in 1 in thelow-temperature glass. Nanosecond transient absorption spectrometry and ultrafast difference spectra were determinedfor 1-3 in CH₂Cl₂ and DMF at ambient temperature. In 1 and 2, the major absorption transients are consistent withthe one-electron reduced complexes, corroborated by reductive spectroelectrochemical measurements performedat room temperature. As 3 does not possess any charge-transfer character, excitation into the * transitions ofthe tda ligand generated transient absorptions in the relaxed excited state assigned to the ligand-localized tripletstate. In all three cases, the excited-state lifetimes measured by transient absorption are similar to those measuredby time-resolved photoluminescence, suggesting that the same excited states giving rise to the photoluminescenceare responsible for the absorption transients. ESR spectroscopy of the anions 1^- and 2^- and reductivespectroelectrochemistry of 1 and 2 revealed a LUMO based largely on the * orbital of the dbbpy ligand. Time-dependent density functional theory calculations performed on 1-3 both in vacuum and in a CH₂Cl₂ continuumrevealed the molecular orbitals, energies, dipole moments, and oscillator strengths for the various electronic transitionsin these molecules. A SCF-method-derived shift applied to the calculated transition energies in the solvent continuumyielded good agreement between theory and experiment for each molecule in this study.