A Photochromic Zinc–Viologen Framework with a High-Contrast Nonlinear Optical Switchable Behavior

Abstract

As an important branch of nonlinear optical (NLO) switches, second-harmonic-generation (SHG) switches undergoing reversible SHG-on and SHG-off states upon external stimuli exposure have been the focus of intense scientific research due to their potential applications in photoelectric technologies. The high-contrast switching of SHG, a key issue for the potential applications of SHG optical switches, has been a challenge but is undoubtedly very interesting. Incorporating stimuli-responsive molecules into the SHG-active materials is an efficient approach to construct high-contrast SHG switches. In this paper, by using the ligand 1-(4-cyanobenzyl)-4,4′-bipyridinium (CBbpy) as a photosensitive group, a noncentrosymmetric photochromic zinc–viologen framework, {[(CBbpy)Zn3(TBC)(HTBC)(OH)]·NO3}n (1; H3BTC = 1,3,5-benzenetricarboylic acid), has been obtained. It shows an SHG intensity of 1.1 × KH2PO4 (KDP) due to the synergistic effect of the coplanar and parallel arrangement of NO3– anions, the asymmetric disposition of electron densities, and the large permanent dipole moment, as confirmed by Bader charge and dipole moment calculations. More importantly, an outstanding SHG switching contrast was found to be at least 23 times larger than that of the coloration sample under a 1064 nm laser beam, originating from the large difference in the dipole moment and the self-absorption effect before and after coloration. Such excellent SHG switching is reversible and can be cycled several times, indicating that 1 is a good candidate as a new solid-state SHG-switching material. This work should prompt researchers to explore novel high-performance SHG switches based on devisable stimuli-responsive groups.