Department of Chemistry, University of Calcutta , 92 A. P. C. Road, Kolkata - 700 009, India.
Acc Chem Res. 2014 May 20;47(5):1604-12. doi: 10.1021/ar500083f. Epub 2014 Apr 23.
The two-photon absorption (TPA) process is the simplest and hence the most studied nonlinear optical phenomenon, and various aspects of this process have been explored in the past few decades, experimentally as well as theoretically. Previous investigations have shown that the two-photon (TP) activity of a molecular system can be tuned, and at present, performance-tailored TP active materials are easy to develop by monitoring factors such as length of conjugation, dimensionality of charge-transfer network, strength of donor-acceptor groups, polarity of solvents, self-aggregation, H-bonding, and micellar encapsulation to mention but a few. One of the most intriguing phenomena affecting the TP activity of a molecule is channel interference. The phrase "channel interference" implies that if the TP transition from one electronic state to another involves more than one optical pathway or channel, characterized by the corresponding transition dipole moment (TDM) vectors, the channels may interfere with each other depending upon the angles between the TDM vectors and hence can either increase (constructive interference) or decrease (destructive interference) the overall TP activity of a system to a significant extent. This phenomenon was first pointed out by Cronstrand, Luo, and Ågren [Chem. Phys. Lett. 2002, 352, 262-269] in two-dimensional systems (i.e., only involving two components of the transition moment vectors). For three-dimensional molecules, an extended version of this idea was required. In order to fill this gap, we developed a generalized model for describing and exploring channel interference, valid for systems of any dimensionality. We have in particular applied it to through-bond (TB) and through-space (TS) charge-transfer systems both in gas phase and in solvents with different polarities. In this Account, we will, in addition to briefly describing the concept of channel interference, discuss two key findings of our recent work: (1) how to control the channel interference by chemical means, and (2) the role of channel interference in the anomalous solvent dependence of certain TP chromophores. For example, we will show that simple structurally induced changes in certain dihedral angles of the well-known betaine dye (TB type) will help fine-tune the constructive channel interference and hence increase the overall TP activity of molecules with this general TP channel structure. Another intriguing result we will discuss is observed for a tweezer-trinitrofluorinone complex (TS type) where, on moving from polar to essentially nonpolar solvents, the nature of the channel interference switches from destructive to constructive, leading to a net abnormal solvent dependence of the TP activity of the system. The present Account highlights the usefulness of the channel interference effect and establishes it as a new and unique way of controlling the TP transition probability in different types of three-dimensional molecules.
双光子吸收（TPA）过程是最简单的，因此也是研究最多的非线性光学现象，过去几十年，人们从实验和理论两方面对这个过程的各个方面进行了探索。之前的研究表明，可以调整分子系统的双光子（TP）活性，目前，通过监测共轭长度、电荷转移网络的维度、供体-受体基团的强度、溶剂的极性、自组装、氢键和胶束包封等因素，很容易开发出性能可调的 TP 活性材料。影响分子的 TP 活性的最有趣现象之一是通道干扰。“通道干扰”一词是指，如果从一个电子态到另一个电子态的 TP 跃迁涉及多个光通道或通道，其特征是相应的跃迁偶极矩（TDM）矢量，那么通道之间可能会相互干扰，这取决于 TDM 矢量之间的角度，因此可以在很大程度上增加（建设性干扰）或减少（破坏性干扰）系统的整体 TP 活性。这一现象首先由 Cronstrand、Luo 和Ågren [Chem. Phys. Lett. 2002, 352, 262-269] 在二维系统（即，仅涉及跃迁矩向量的两个分量）中指出。对于三维分子，需要扩展这个想法。为了填补这一空白，我们开发了一个用于描述和探索通道干扰的广义模型，该模型适用于任何维度的系统。我们特别将其应用于气相和不同极性溶剂中的键间（TB）和键间（TS）电荷转移系统。在本报告中，除了简要描述通道干扰的概念外，我们还将讨论我们最近工作的两个关键发现：（1）如何通过化学手段控制通道干扰，以及（2）通道干扰在某些 TP 发色团异常溶剂依赖性中的作用。例如，我们将展示如何通过简单的结构诱导变化来调整某些熟知的甜菜碱染料（TB 型）的某些二面角，从而帮助微调建设性通道干扰，并因此提高具有这种通用 TP 通道结构的分子的整体 TP 活性。我们将讨论的另一个有趣结果是在 Tweezer-trinitrofluorinone 配合物（TS 型）中观察到的，当从极性溶剂移动到几乎非极性溶剂时，通道干扰的性质从破坏性变为建设性，导致系统的 TP 活性出现净异常溶剂依赖性。本报告强调了通道干扰效应的有用性，并将其确立为控制不同类型三维分子中 TP 跃迁概率的新的独特方法。
