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In the search for new nonlinear optical (NLO) switching devices, expanded porphyrins have emerged as ideal candidates thanks to their tunable chemical and photophysical properties. Introducing meso -substituents to these macrocycles is a successful strategy to enhance the NLO contrasts. Despite its potential, the influence of meso -substitution on their structural and geometrical properties has been scarcely investigated. In this work, we pursue to grasp the underlying pivotal concepts for the fine-tuning of the NLO contrasts of hexaphyrin-based molecular switches, with a particular focus on the first hyperpolarizability related to the hyper-Rayleigh scattering ( β HRS ). Building further on these concepts, we also aim to develop a rational design protocol. Starting from the (un)substituted hexaphyrins with various π -conjugation topologies and redox states, structure-property relationships are established linking aromaticity, photophysical properties and β HRS responses. Ultimately, inverse molecular design using the best-first search algorithm is applied on the most favorable switches with the aim to further explore the combinatorial chemical compound space of meso -substituted hexaphyrins in search of high-contrast NLO switches. Two definitions of the figure-of-merit of the switch performance were used as target objectives in the optimization problem. Several meso -substitution patterns and their underlying characteristics are identified, uncovering molecular symmetry and the electronic nature of the substituents as the key players for fine-tuning the β HRS values and NLO contrasts of hexaphyrin-based switches., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Desmedt, Woller, Teunissen, De Vleeschouwer and Alonso.)

This study addresses one of the most important drawbacks inherently related to molecular searches in chemical compound space by greedy algorithms such as Best First Search and Genetic Algorithm, i.e., the large computational cost required to optimize one or more quantum-chemical properties. Significant speed-ups are obtained by initial property screening via predictive techniques starting already from very small databases. It is shown that the attainable acceleration depends heavily on the molecular properties, the predictive model, the molecular descriptor, and the current size of the database. We discuss the implementation and performance of predictive techniques in molecular searches based on a fixed molecular framework with a selection of sites to be filled with groups from a chemical fragment library. It is shown that for some properties speed-ups of a factor of 5 to even 20 can be obtained, while inverse design procedures on more complex properties still reach speed-ups of a factor of 2 without losing performance.

Functionalized diamondoids show great potential as building blocks for various new optoelectronic applications. However, until now, only simple mono and double substitutions were investigated. In this work, we considered up to 10 and 6 sites for functionalization of the two smallest diamondoids, adamantane and diamantane, respectively, in search for diamondoid derivatives with a minimal and maximal HOMO-LUMO energy gap. To this end, the energy gap was optimized systematically using an inverse molecular design methodology based on the best-first search algorithm combined with a Monte Carlo component to escape local optima. Adamantane derivatives were found with HOMO-LUMO gaps ranging from 2.42 to 10.63 eV, with 9.45 eV being the energy gap of pure adamantane. For diamantane, similar values were obtained. The structures with the lowest HOMO-LUMO gaps showed apparent push-pull character. The push character is mainly formed by sulfur or nitrogen dopants and thiol groups, whereas the pull character is predominantly determined by the presence of electron-withdrawing nitro or carbonyl groups assisted by amino and hydroxyl groups via the formation of intramolecular hydrogen bonds. In contrast, maximal HOMO-LUMO gaps were obtained by introducing numerous electronegative groups.

The B20(2-) cluster is predicted to exhibit a planar sheet-like structure with a circular circumference. Orbital plots and energy correlations demonstrate the close correspondence between the electronic structure of B20(2-) and the Bessel functions describing the waves of a quantum mechanical particle confined to a disk. The π-band of B20(2-), and its B19(-) congener, contains 12 π-electrons, forming a (1σ)(2)(1π)(4)(1δ)(4)(2σ)(2) configuration, which corresponds to a "disk aromaticity" electron count. The analogy not only applies to the π-band, but also extends to the 50 valence σ-electrons. The occupied σ-orbitals are assigned on the basis of radial and angular nodes of the scalar disk waves. The magnetic response of the cluster was examined by Nucleus Independent Chemical Shift (NICS) values and current density calculations based on the ipsocentric model. B20(2-) is found to exhibit a remarkable inner paratropic current in the σ-channel and an outer diatropic current in the π-channel. The orbital excitations responsible for the antiaromaticity in σ and the disk-aromaticity in π are identified.