Motley Tyler C, Troian-Gautier Ludovic, Brennaman M Kyle, Meyer Gerald J
Department of Chemistry, University of North Carolina at Chapel Hill , Chapel Hill, North Carolina 27599, United States.
Inorg Chem. 2017 Nov 6;56(21):13579-13592. doi: 10.1021/acs.inorgchem.7b02321.
The synthesis, electrochemistry, and photophysical characterization are reported for 11 tris(bidentate) cyclometalated ruthenium(II) compounds, [Ru(N^N)(C^N)]. The electrochemical and photophysical properties were varied by the addition of substituents on the 2,2'-bipyridine, N^N, and 2-phenylpyridine, C^N, ligands with different electron-donating and -withdrawing groups. The systematic tuning of these properties offered a tremendous opportunity to investigate the origin of the rapid excited-state decay for these cyclometalated compounds and to probe the accessibility of the dissociative, ligand-field (LF) states from the metal-to-ligand charge-transfer (MLCT) excited state. The photoluminescence quantum yield for [Ru(N^N)(C^N)] increased from 0.0001 to 0.002 as more electron-withdrawing substituents were added to C^N. An analogous substituent dependence was observed for the excited-state lifetimes, τ, which ranged from 3 to 40 ns in neat acetonitrile, significantly shorter than those for their [Ru(N^N)] analogues. The excited-state decay for [Ru(N^N)(C^N)] was accelerated because of an increased vibronic overlap between the ground- and excited-state wavefunctions rather than an increased electronic coupling as revealed by a comparison of the Franck-Condon factors. The radiative (k) and non-radiative (k) rate constants of excited-state decay were determined to be on the order of 10 and 10-10 s, respectively. For sets of [Ru(N^N)(C^N)] compounds functionalized with the same N^N ligand, k scaled with excited-state energy in accordance with the energy gap law. Furthermore, an Arrhenius analysis of τ for all of the compounds between 273 and 343 K was consistent with activated crossing into a single, fourth MLCT state under the conditions studied with preexponential factors on the order of 10-10 s and activation energies between 300 and 1000 cm. This result provides compelling evidence that LF states are not significantly populated near room temperature unlike many ruthenium(II) polypyridyl compounds. On the basis of the underlying photophysics presented here for [Ru(N^N)(C^N)], molecules of this type represent a robust class of compounds with built-in design features that should greatly enhance the molecular photostability necessary for photochemical and photoelectrochemical applications.
本文报道了11种三（双齿）环金属化钌（II）化合物[Ru(N^N)(C^N)]的合成、电化学和光物理表征。通过在2,2'-联吡啶（N^N）和2-苯基吡啶（C^N）配体上添加具有不同供电子和吸电子基团的取代基，改变了其电化学和光物理性质。这些性质的系统调节为研究这些环金属化化合物快速激发态衰变的起源以及探究从金属到配体电荷转移（MLCT）激发态到解离配体场（LF）态的可及性提供了巨大机会。随着更多吸电子取代基添加到C^N上，[Ru(N^N)(C^N)]的光致发光量子产率从0.0001增加到0.002。对于激发态寿命τ也观察到了类似的取代基依赖性，在纯乙腈中其范围为3至40 ns，明显短于其[Ru(N^N)]类似物。通过比较弗兰克 - 康登因子发现，[Ru(N^N)(C^N)]的激发态衰变加速是由于基态和激发态波函数之间的振动重叠增加，而不是电子耦合增加。激发态衰变的辐射（k）和非辐射（k）速率常数分别确定为10和10^-10 s量级。对于用相同N^N配体功能化的[Ru(N^N)(C^N)]化合物组，k根据能隙定律与激发态能量成比例。此外，对所有化合物在273至343 K之间的τ进行阿仑尼乌斯分析，结果与在所研究条件下活化穿越到单一的第四MLCT态一致，预指数因子为10^-10 s量级，活化能在300至1000 cm之间。该结果提供了令人信服的证据，即与许多钌（II）多吡啶化合物不同，在室温附近LF态没有显著填充。基于本文给出的[Ru(N^N)(C^N)]的基础光物理，这类分子代表了一类具有内在设计特征的稳健化合物，这些特征应极大地增强光化学和光电化学应用所需的分子光稳定性。
