A Curious Case of Two-Photon Absorption in n-Helicene and n-Phenylene, n=6-10: Why n=7 is Different?

n-Helicenes and n-Phenylenes are interesting examples of twisted molecules, where although the atoms are connected through conjugated π ${\pi }$ -bonds, the π ${\pi }$ -conjugation is largely hindered by the twisted nature of the bonds. Such structures provide a unique opportunity to study the effect of twisted π ${\pi }$ -system on non-linear optical properties. In this work, we studied the two-photon absorption in donor-acceptor substituted n-helicenes and n-phenylenes employing the state-of-the-art RI-CC2 method and reported a unique feature we observed in n=7 systems. We found that both 7-helicene and 7-phenylene systems exhibit largest two-photon absorption than other members in their respective classes. Furthermore, using generalized few-state model, we provided a detailed microscopic mechanism of this unique observation involving participation of different transition dipole moment vectors and their relative orientations.

Optical Physical Mechanisms of Helicene Carbon Nanohoop with Möbius Topology

Optical and spectral properties of carbon nanohoop with Möbius topology is of great interest in nano-science and nano-technology. And it can be imagined that it has a lot of unexpected potential application prospects. However, theoretical calculations based on some figure-of-eight helicene carbon nanohoop with Möbius topology are still insufficient. Therefore, in this paper, we theoretically study the optical and spectral properties of figure-of-eight helicene carbon nanohoop with Möbius topology. Optical and spectral properties are analyzed with visualization method of transition density matrix and charge density difference, which reveal the unique characterization of carbon nanohoop with Möbius topology. Our results can not only deepen the understanding of the optical physical mechanisms of the nanorings with mobius carbons, but also provide deeper insight on optical properties and potential design on optical nanodevices.

Cyclic tris-[5]helicenes with single and triple twisted Möbius topologies and Möbius aromaticity

A number of singly (180°) twisted, largely single-stranded and thus conformationally rather fragile, Möbius molecules have been synthesized within the last 15 years, which are aromatic with 4n electrons, thus violating the Hückel rule. Annulenes with significantly higher twist (e.g. 540°) that retain a full cyclic conjugation path have been elusive, mainly because of the high strain and loss of orbital overlap. Recently, a topological strategy was devised to project the "twist" into "writhe", thus reducing the strain. However, orbital overlap was still severely reduced within the flexible building blocks. We now present a single and a triple twisted annulene with fully conjugated peripheries. They are unique in their pronounced band shape and conformational robustness as they are made up of three fully kata-condensed [5]helicene fragments. The triple twisted molecule exhibits a strong diatropic ring current in the outer periphery, even though the π system includes 4n electrons. The diatropic current is counterbalanced by a paratropic current in the σ system, resulting in no net manifestation of macrocyclic aromaticity. The key step of the synthesis of both Möbius compounds is a Perkin condensation of complementary bifunctional bismaleates leading to a flexible macrocycle containing alternating benzene and biphenyl fragments. Subsequent photocyclization yields a separable mixture of rigid diastereomeric tris-helicene macrocycles of the above topologies.

Solvent effects on static polarizability, static first hyperpolarizability and one- and two-photon absorption properties of functionalized triply twisted Möbius annulenes: a DFT study

Solvent effects on the polarizability (α), static first hyperpolarizability (β) and one- and two-photon absorption (OPA and TPA) properties of triply twisted Möbius annulenes.

New trans-stilbene derivatives with large two-photon absorption cross-section and non-linear optical susceptibility values – a theoretical investigation

In the present study, newly designed TSB derivatives decorated with suitable donor–acceptor groups have large TPA cross-sections and can be used as potential two-photon active materials.