Alam Md Mehboob, Misra Ramprasad, Ruud Kenneth
Hylleraas Centre for Quantum Molecular Sciences, Department of Chemistry, The University of Tromsø - The Arctic University of Norway, Tromsø, Norway.
Department of Physical Chemistry, Indian Association for the Cultivation of Science, Kolkata 700032, India.
Phys Chem Chem Phys. 2017 Nov 8;19(43):29461-29471. doi: 10.1039/c7cp05679f.
Channel interference plays a crucial role in understanding the physics behind multiphoton absorption processes. In this work, we study the role of channel interference and solvent effects on the two-photon absorption in aryl-substituted boron dipyrromethene (BODIPY) dyes, a class of intramolecular charge-transfer (ICT) molecules. For this purpose, we consider fourteen dyes of this class with various donor/acceptor substitutions at the para position of the phenyl ring and with or without methyl (-CH) substitution on the BODIPY moiety. The presence of a methyl group on the BODIPY moiety affects the dihedral angle significantly, which in turn affects the one- (OPA) and two-photon absorption (TPA) properties of the molecules. Among the molecules studied, the one having the strong electron-donating dimethylamino group and no methyl substitution at the BODIPY moiety is found to have the highest TPA cross section. Our few-state model analysis shows that the large TPA activity of this molecule is due to the all positive contributions from different channel interference terms. Change in dielectric constant of the medium is found to have a profound impact on both the magnitude and sign of the channel interference terms. The magnitude of destructive channel interference gradually decreases with decreasing solvent polarity and becomes constructive in a low-polarity solvent. We also study the effect of rotating the phenyl ring with respect to the BODIPY moiety on the TPA activity. In the gas phase and in different solvents, we found that channel interference is changed from destructive to constructive on twisting the molecule. These results are explained by considering different dipole-, energy- and angle-terms appearing in the expression of a two-state model.
通道干涉在理解多光子吸收过程背后的物理机制中起着至关重要的作用。在这项工作中，我们研究了通道干涉和溶剂效应在芳基取代的硼二吡咯亚甲基（BODIPY）染料中的双光子吸收中的作用，这类染料属于分子内电荷转移（ICT）分子。为此，我们考虑了这类中的十四种染料，它们在苯环的对位具有各种供体/受体取代，并且在BODIPY部分有或没有甲基（-CH）取代。BODIPY部分上甲基的存在显著影响二面角，这反过来又影响分子的单光子吸收（OPA）和双光子吸收（TPA）性质。在所研究的分子中，在BODIPY部分具有强供电子二甲基氨基且没有甲基取代的分子具有最高的TPA截面。我们的少态模型分析表明，该分子的大TPA活性归因于不同通道干涉项的所有正贡献。发现介质介电常数的变化对通道干涉项的大小和符号都有深远影响。随着溶剂极性降低，破坏性通道干涉的幅度逐渐减小，并在低极性溶剂中变为建设性的。我们还研究了相对于BODIPY部分旋转苯环对TPA活性的影响。在气相和不同溶剂中，我们发现扭曲分子时通道干涉从破坏性变为建设性。通过考虑二态模型表达式中出现的不同偶极、能量和角度项来解释这些结果。
