Abstract
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.
Highlights
Scientists’ searching for new and innovative materials with promising properties often finds their way back to the roots and fundamentals of nature
In order to establish a relationship between molecular topology, aromaticity and nonlinear optical (NLO) properties in hexaphyrin-based macrocycles, we first investigate the optoelectronic properties of unsubstituted [26] and [28]hexaphyrins
We discovered efficient nonlinear optical (NLO) switches with high βHRS contrasts by applying the Best-First
Summary
Scientists’ searching for new and innovative materials with promising properties often finds their way back to the roots and fundamentals of nature. Different modifications to the core structure of expanded porphyrins can be introduced, either by 1) replacing one or more pyrrole rings with other fivemembered heterocycles such as furan, thiophene or tellurophene (Chandrashekar and Venkatraman, 2003; Rath et al, 2005; Kumar et al, 2007; Chatterjee et al, 2017), or by 2) introducing different substituents at the meso-positions of the macrocycle (Marcos et al, 2014; Tanaka and Osuka, 2016; Woller et al, 2016; Torrent-Sucarrat et al, 2017b) These variations in the core structures have recently drawn a lot of attention as the modified structures exhibit outstanding properties for multidisciplinary applications such as bio-sensors, photodynamic therapy, photochemistry, catalysis and molecular electronics (Saito and Osuka, 2011; Sung et al, 2017; Cárdenas-Jirón et al, 2019)
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