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The similar molecules [2.2]paracyclophane (22PCP) and 1,1,2,2,9,9,10,10-octafluoro[2.2]paracyclophane (8F22PCP) have both generated considerable synthetic interest since they were first prepared. In this work, the nonlinear optical properties of 22PCP, 8F22PCP, and the related Li-doped systems 22PCP-Li and 8F22PCP-Li (which have a Li atom above 22PCP and 8F22PCP, respectively) were investigated. An analysis of natural bond orbital charges showed that there is greater charge transfer from the Li atom to the benzene rings in 8F22PCP-Li than in 22PCP-Li. The variation in the calculated nucleus independent chemical shift (NICS) value as a function of the distance from the lower benzene ring towards the upper benzene ring was found to be W-shaped for both 22PCP and 22PCP-Li. Moreover, whereas all of the NICS values of 22PCP and 22PCP-Li were markedly negative, all of the NICS values of 8F22PCP and 8F22PCP-Li were either positive or only moderately negative. Calculations of the electro-optical properties of these systems showed that the first hyperpolarizability of 22PCP-Li was noticeably larger than that of 8F22PCP-Li. According to the two-level model, the larger first hyperpolarizability of 22PCP-Li is due to its smaller transition energy.

A series of hexaphyrins with different meso-carbon atoms and their protonated structures were investigated using density functional theory (DFT) and time-dependent DFT. Frontier molecular orbitals (FMOs), aromaticity, and electronic spectra were investigated systematically before and after protonation. The FMO energy gaps before and after protonation were different for the antiaromatic molecules, while they were only slightly different for the aromatic molecules. By analyzing the electronic spectra of the aromatic molecules, the absorption peaks in the Q-like and B-like bands were not significantly different before and after protonation. However, the absorption peaks of the antiaromatic molecules were clearly different before and after protonation in both the Q-like and B-like bands. [24]Hexaphyrin (1.0.1.0.1.0) has 24 π-electrons and is Huckel antiaromatic. However, the absorption spectrum of protonated [24]hexaphyrin (1.0.1.0.1.0) showed aromaticity. In addition, these conclusions were generally consistent with the FMOs, nucleus-independent chemical shifts, harmonic oscillator model of aromaticity, and absorption spectra. Although protonated [24]hexaphyrin (1.0.1.0.1.0) has 24 π-electrons and is Huckel antiaromatic, it has Mobius aromaticity because of the single-sided Mobius topological structure. This explains why [24]hexaphyrin (1.0.1.0.1.0) has diatropic ring currents in solvent. To the best of our knowledge, this system is the smallest Mobius aromatic molecule among the many uncoordinated extended porphyrins.