Computational design of spiropyran-merocyanine nonlinear optical photoswitches: from isolated molecules to model aggregates

Marilù G. Maraldi1,2, Angela Dellai1, Frédéric Castet1

1 Institut des Sciences Moleculaires (ISM), Université de Bordeaux, Talence (France)
2 Molecular Chemistry, Materials and Catalysis Division (MOST), Institute of Condensed Matter and Nanosciences (IMCN), Université Catholique de Louvain, Louvain-la-Neuve (Belgium).


For optoelectronic and photonic devices, designing photoresponsive materials that enable remote reversible switching of their electrical or optical properties is a significant challenge1. Extensive research has been done on organic photochromic compounds for a variety of applications, including logic gates and optical memories2. The majority of these optical devices, however, rely on linear optical properties of the systems both for writing/erasing and reading the information onto the material, which may result in a destructive reading process because the photochromes’ state is switched upon light exposure. To solve this problem, it is possible to use second harmonic generation (SHG) as non-destructive readout technique, which exploits the nonlinear (NLO) response of molecules3 to an optical stimulus. Moreover, functionalization of self-assembled monolayers (SAMs)4 constitutes the most effective strategy for introducing NLO chromophores into a solid-state device in view of maximizing its macroscopic second-order optical susceptibility. In this context, this work provides a computational workflow for the design of new photonic devices based on 2D-NLO molecular materials. The procedure intends to fill the gap between the in-solution photoswitches and the final material while providing design insights by examining the relationships between the chemistry of each individual photoswitch and its optical properties. The linear and nonlinear optical properties of monosubstituted spiropyran (SP) - merocyanine (MC) systems5 are characterized using Density Functional Theory6. Representative compounds are first investigated in solution; then the effect of aggregation is questioned by investigating interacting dimers, which allows to evaluate, at the quantum mechanical (QM) level, the effect of aggregation in small model systems as well as the optimal intermolecular distance to maximise the first hyperpolarizability. Finally, the problem of simulating larger supramolecular aggregates, more representative of SAMs, using an all-atom QM computation is tackled.


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