Cogan Semyon, Zilberg Shmuel, Haas Yehuda
Department of Physical Chemistry and the Farkas Center for Light Induced Processes, The Hebrew University of Jerusalem, Jerusalem, Israel.
J Am Chem Soc. 2006 Mar 15;128(10):3335-45. doi: 10.1021/ja0548945.
The origin of the dual fluorescence of DMABN (dimethylaminobenzonitrile) and other benzene derivatives is explained by a charge transfer model based on the properties of the benzene anion radical. It is shown that, in general, three low-lying electronically excited states are expected for these molecules, two of which are of charge transfer (CT) character, whereas the third is a locally excited (LE) state. Dual fluorescence may arise from any two of these states, as each has a different geometry at which it attains a minimum. The Jahn-Teller induced distortion of the benzene anion radical ground state helps to classify the CT states as having quinoid (Q) and antiquinoid (AQ) forms. The intramolecular charge transfer (ICT) state is formed by the transfer of an electron from a covalently linked donor group to an anti-bonding orbital of the pi-electron system of benzene. The change in charge distribution of the molecule in the CT states leads to the most significant geometry change undergone by the molecule which is the distortion of the benzene ring to a Q or AQ structure. As the dipole moment is larger in the perpendicular geometry than in the planar one, this geometry is preferred in polar solvents, supporting the twisted intramolecular charge transfer (TICT) model. However, in many cases the planar conformation of CT excited states is lower in energy than that of the LE state, and dual fluorescence can be observed also from planar structures.
基于苯阴离子自由基的性质，通过电荷转移模型解释了二甲基氨基苯甲腈（DMABN）和其他苯衍生物的双重荧光起源。结果表明，一般来说，这些分子预计有三个低电子激发态，其中两个具有电荷转移（CT）特征，而第三个是局域激发（LE）态。双重荧光可能源于这些状态中的任意两个，因为每个状态在达到最小值时都有不同的几何形状。苯阴离子自由基基态的 Jahn-Teller 诱导畸变有助于将 CT 态分类为具有醌型（Q）和反醌型（AQ）形式。分子内电荷转移（ICT）态是通过电子从共价连接的供体基团转移到苯π电子体系的反键轨道而形成的。分子在 CT 态下电荷分布的变化导致分子经历最显著的几何形状变化，即苯环扭曲为 Q 或 AQ 结构。由于垂直几何形状中的偶极矩大于平面几何形状中的偶极矩，这种几何形状在极性溶剂中更受青睐，这支持了扭曲分子内电荷转移（TICT）模型。然而，在许多情况下，CT 激发态的平面构象能量低于 LE 态，并且从平面结构中也可以观察到双重荧光。
