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空间位阻较大的Cu(I) MLCT激发态的瞬态吸收动力学。

Transient absorption dynamics of sterically congested Cu(I) MLCT excited states.

作者信息

Garakyaraghi Sofia, Danilov Evgeny O, McCusker Catherine E, Castellano Felix N

机构信息

Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States.

出版信息

J Phys Chem A. 2015 Apr 2;119(13):3181-93. doi: 10.1021/acs.jpca.5b00901. Epub 2015 Mar 18.

Abstract

Subpicosecond through supra-nanosecond transient absorption dynamics of the homoleptic Cu(I) metal-to-ligand charge transfer (MLCT) photosensitizers including the benchmark Cu(dmp)2 (dmp =2,9-dimethyl-1,10-phenanthroline) chromophore, as well as Cu(dsbp)2 (dsbp =2,9-di(sec-butyl)-1,10-phenanthroline and Cu(dsbtmp)2 (dsbtmp =2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline) were investigated in dichloromethane and tetrahydrofuran solutions. Visible and near-IR spectroelectrochemical measurements of the singly reduced Cu(dsbp)2 and Cu(dsbtmp)2 species were determined in tetrahydrofuran, allowing for the identification of redox-specific phenanthroline-based radical anion spectroscopic signatures prevalent in the respective transient absorption experiments. This study utilized four different excitation wavelengths (418, 470, 500, and 530 nm) to elucidate dynamics on ultrafast times scales spanning probe wavelengths ranging from the UV to the near-IR (350 to 1450 nm). With the current time resolution of ∼150 fs, initial excited state decay in all three compounds was found to be independent of excitation wavelength. Not surprisingly, there was little to no observed influence of solvent in the initial stages of excited state decay in any of these molecules including Cu(dmp)2, consistent with results from previous investigators. The combined experimental data revealed two ranges of time constants observed on short time scales in all three MLCT chromophores and both components lengthen as a function of structure in the following manner: Cu(dsbtmp)2 < Cu(dsbp)2 < Cu(dmp)2. The molecule with the most inhibited potential for distortion, Cu(dsbtmp)2, possessed the fastest ultrafast dynamics as well as the longest excited state lifetimes in both solvents. These results are consistent with a small degree of excited state distortion, rapid intersystem crossing, and weak vibronic coupling to the ground state. The concomitant systematic variation in both initial time constants, assigned to pseudo-Jahn-Teller distortion and intersystem crossing, suggest that both processes are intimately coupled in all molecules in the series. The variability in these time scales illustrate that strongly impeded structural distortion in Cu(I) MLCT excited state enables more rapid surface crossings in the initial deactivation dynamics.

摘要

研究了同配体Cu(I)金属到配体电荷转移(MLCT)光敏剂的亚皮秒至超纳秒瞬态吸收动力学,包括基准Cu(dmp)₂(dmp = 2,9 - 二甲基 - 1,10 - 菲咯啉)发色团,以及Cu(dsbp)₂(dsbp = 2,9 - 二(仲丁基)-1,10 - 菲咯啉)和Cu(dsbtmp)₂(dsbtmp = 2,9 - 二(仲丁基)-3,4,7,8 - 四甲基 - 1,10 - 菲咯啉),在二氯甲烷和四氢呋喃溶液中进行了研究。在四氢呋喃中对单还原的Cu(dsbp)₂Cu(dsbtmp)₂物种进行了可见和近红外光谱电化学测量,从而确定了在各自的瞬态吸收实验中普遍存在的基于菲咯啉的氧化还原特异性自由基阴离子光谱特征。本研究利用四个不同的激发波长(418、470、500和530 nm)来阐明跨越从紫外到近红外(350至1450 nm)探测波长的超快时间尺度上的动力学。以目前约150 fs的时间分辨率,发现所有三种化合物的初始激发态衰减与激发波长无关。不出所料,在包括Cu(dmp)₂在内的任何这些分子的激发态衰减初始阶段,几乎没有观察到溶剂的影响,这与先前研究者的结果一致。综合实验数据揭示了在所有三种MLCT发色团的短时间尺度上观察到的两个时间常数范围,并且两个组分都以以下方式随结构而延长:Cu(dsbtmp)₂<Cu(dsbp)₂<Cu(dmp)₂。具有最受抑制扭曲潜力的分子Cu(dsbtmp)₂,在两种溶剂中都具有最快的超快动力学以及最长的激发态寿命。这些结果与小程度的激发态扭曲、快速的系间窜越以及与基态的弱振动耦合一致。归因于赝 Jahn - Teller 扭曲和系间窜越的两个初始时间常数的伴随系统变化表明,这两个过程在该系列的所有分子中紧密耦合。这些时间尺度的变化说明,Cu(I) MLCT激发态中强烈受阻的结构扭曲使得在初始失活动力学中能够更快地进行表面穿越。

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