Designing Efficient Azobenzene and Azothiophene Nonlinear Optical Photochromes

The present contribution constitutes an extensive density functional theory (DFT) investigation of the nonlinear optical (NLO) properties of a large number of molecules belonging to the azobenzene and azothiophene families of photochromic compounds which can act as NLO switches. Toward the design of systems simultaneously presenting both large total nonlinear response values, β tot, and large contrast, β ratio, between the cis and trans isomers, we have focused not only on the monomers but also on azobenzene dimers, the latter containing two N═N bonds along the molecular backbone. After it was established that the inclusion of implicit solvation is not important in drawing qualitative conclusions on the NLO switching ability for the investigated systems, gas-phase calculations have shown that for the asymmetric push–pull azobenzene and azothiophene candidates, the combination of strong donating groups such as NPh 2, N(Ph-OMe) 2, and N(Ph-NMe 2 ) 2 with the dicyanoethene group CH═C(C≡N) 2 as an acceptor delivers large β trans (150–217 × 10 –30 esu) and non-negligible β cis (18–55 × 10 –30 esu) values as well as substantial contrast, β ratio (3.9–8.7). For the investigated double azobenzenes, it is found that, with a careful choice of donor and acceptor groups, the contrast, β ratio, can be significantly increased compared to that of the monomers while maintaining large β values that facilitate their detection with standard experimental techniques (e.g., electric-field-induced second-harmonic generation). Our results set the stage on which further theoretical and experimental studies can be based in the search for efficient and versatile NLO switches.