Teoretisk Kemi, Kemisk Institut, Aarhus Universitet, Langelandsgade 140, 8000 Aarhus C, Denmark.
Istituto per i Processi Chimico-Fisici del Consiglio Nazionale delle Ricerche (IPCF-CNR), Area della Ricerca, via G. Moruzzi 1, I-56124 Pisa, Italy.
J Chem Theory Comput. 2008 Mar;4(3):457-67. doi: 10.1021/ct700329a.
Using a recently derived origin-invariant quadratic response approach combined with time-dependent density functional theory, four representative helicenes are shown to present a very strong two-photon circular dichroism (TPCD) response, which makes them candidates for the first experimental observation of a TPCD effect. The large response is attributed to the unique combination of chirality and electron delocalization. Comparison with electronic circular dichroism and two-photon absorption (TPA) shows that the three effects exhibit complementary features for unravelling the molecular structures. In particular, for the four (M)-helicenes studied here, the first, i.e., low-energy, dominant Cotton band is always negative, whereas for TPCD it is positive. From an analysis of the frontier orbitals describing most of the one-electron excitation vectors, the largest TPCD response of tetramethoxy-bisquinone-dithia-[7]-helicene has been attributed to the charge-transfer character of the excited state, like for the parent TPA effect. Moreover, the TPCD intensities are found to be mostly governed by the electric and magnetic dipole contributions, while the electric quadrupole terms are, on a relative basis, less important.
利用最近推导的原点不变二次响应方法结合含时密度泛函理论，我们展示了四个具有代表性的螺旋化合物具有很强的双光子圆二色性（TPCD）响应，这使它们成为第一个实验观察 TPCD 效应的候选者。这种强响应归因于手性和电子离域的独特组合。与电子圆二色性和双光子吸收（TPA）的比较表明，这三种效应对于揭示分子结构具有互补的特征。特别是对于这里研究的四个（M）螺旋化合物，第一个，即低能量，主导的 Cotton 带总是负的，而对于 TPCD 则是正的。通过分析描述大多数单电子激发矢量的前线轨道，可以将四甲氧基-双醌-二硫-[7]-螺旋化合物的最大 TPCD 响应归因于激发态的电荷转移特性，就像母体 TPA 效应一样。此外，发现 TPCD 强度主要由电偶极和磁偶极贡献决定，而相对而言，电四极项的贡献较小。
