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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Twisting dynamics in the excited singlet state of Michler's ketone
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Twisting dynamics in the excited singlet state of Michler's ketone

机译：Michler酮的激发单重态下的扭转动力学

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摘要

Ultrafast relaxation dynamics of the excited singlet (SI) state of Michler's ketone (MK) has been investigated in different kinds of solvents using a time-resolved absorption spectroscopic technique with 120 fs time resolution. This technique reveals that conversion of the locally excited (LE) state to the twisted intramolecular charge transfer (TICT) state because of twisting of the N,N-dimethylanilino groups with respect to the central carbonyl group is the major relaxation process responsible for the multi-exponential and probe-wavelength-dependent transient absorption dynamics of the S, state of MK, but solvation dynamics does not have a significant role in this process. Theoretical optimization of the ground-state geometry of MK shows that the dimethylanilino groups attached to the central carbonyl group are at a dihedral angle of about 51 degrees with respect to each other because of steric interaction between the phenyl rings. Following photoexcitation of MK to its S, state, two kinds of twisting motions have been resolved. Immediately after photoexcitation, an ultrafast "anti-twisting" motion of the dimethylanilino groups brings back the pretwisted molecule to a near-planar geometry with high mesomeric interaction and intramolecular charge transfer (ICT) character. This motion is observed in all kinds of solvents. Additionally, in solvents of large polarity, the dimethylamino groups undergo further twisting to about 90 degrees with respect to the phenyl ring, to which it is attached, leading to the conversion of the ICT state to the TICT state. Similar characteristics of the absorption spectra of the TICT state and the anion radical of MK establish the nearly pure electron transfer (ET) character of the TICT state. In aprotic solvents, because of the steep slope of the potential energy surface near the Franck-Condon (FC) or LE state region, the LE state is nearly nonemissive at room temperature and fluorescence emission is observed from only the ICT and TICT states. Alternatively, in protic solvents, because of an intermolecular hydrogen-bonding interaction between MK and the solvent, the LE region is more flat and stimulated emission from this state is also observed. However, a stronger hydrogen-bonding interaction between the TICT state and the solvent as well as the closeness between the two potential energy surfaces due to the TICT and the ground states cause the nonradiative coupling between these states to be very effective and, hence, cause the TICT state to be weakly emissive. The multi-exponentiality and strong wavelength-dependence of the kinetics of the relaxation process taking place in the S, state of MK have arisen for several reasons, such as strong overlapping of transient absorption and stimulated emission spectra of the LE, ICT, and TICT states, which are formed consecutively following, photoexcitation of the molecule, as well as the fact that different probe wavelengths monitor different regions of the potential energy surface representing the twisting motion of the excited molecule.
机译：使用具有120 fs时间分辨率的时间分辨吸收光谱技术，在不同种类的溶剂中研究了Michler酮（MK）的激发单重态（SI）态的超快弛豫动力学。这项技术表明，由于N，N-二甲基苯胺基相对于中心羰基的扭曲，导致局部激发（LE）态向分子内电荷转移（TICT）态的转化是导致多官能团的主要弛豫过程-S，MK状态的指数和探针波长相关的瞬态吸收动力学，但溶剂化动力学在此过程中没有重要作用。 MK的基态几何结构的理论优化表明，由于苯环之间的空间相互作用，连接至中心羰基的二甲基苯胺基彼此之间的二面角约为51度。在将MK光激发到其S状态后，解决了两种扭曲运动。在光激发之后，二甲基苯胺基团的超快速“抗扭曲”运动将预扭曲的分子恢复为具有高介晶相互作用和分子内电荷转移（ICT）特征的近平面几何形状。在各种溶剂中都可以观察到这种运动。另外，在大极性的溶剂中，相对于其连接的苯环，二甲基氨基基团进一步扭曲至约90度，导致ICT状态转化为TICT状态。 TICT态的吸收光谱和MK的阴离子基团的相似特征建立了TICT态的近乎纯电子传递（ET）特性。在非质子溶剂中，由于在Franck-Condon（FC）或LE状态区域附近的势能表面陡峭，因此LE状态在室温下几乎是无辐射的，仅从ICT和TICT状态观察到荧光发射。或者，在质子溶剂中，由于MK与溶剂之间的分子间氢键相互作用，LE区更加平坦，并且还观察到了从该状态激发的发射。但是，由于TICT和基态，在TICT状态与溶剂之间更强的氢键相互作用以及两个势能面之间的紧密性导致这些状态之间的非辐射耦合非常有效，因此导致TICT的状态要弱发射。在S，MK状态下发生的弛豫过程的动力学具有多种指数性和强烈的波长依赖性，这是由于多种原因引起的，例如LE，ICT和TICT的瞬态吸收与受激发射光谱的强烈重叠分子的光激发之后连续形成的一种状态，以及不同的探针波长监视代表被激发分子扭转运动的势能表面的不同区域这一事实。

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