Multinuclear Pd(II) oligomeric nanohoop complexes with 2,2'-bipyridyl-embedded cycloparaphenylenes ligands exhibiting ultrahigh third-order nonlinear optical performance

Unlike traditionally available linear frameworks, cyclic transition-metal architectures with cycloparaphenylenes (CPPs) ligands provide new access to develop modern nonlinear optical (NLO) materials. In this work, a sequence of "staircase" Pd(II) oligomeric nanohoop complexes with the 2,2'-bipyridyl-embedded CPPs ligand unit through "series connection" mode and "?-type" Pd(II) trimeric isomer have been systematically designed and investigated via density functional theory. Herein, we first report the dependence of third-order NLO responses on the number of bipyridine CPP building ligands in a quantitative way. Impressively, the Pd(II) trimeric complexes possess ultrahigh third-order NLO responses, which are ascribed to the large orbital delocalization and decreased energy gap. To our delight, the enlargement of the nanohoop size is beneficial to improving third-order NLO responses for Pd(II) oligomeric nanohoop complexes, and the maximum absorption spikes show the blue shift tendency. The effect of size on the third-order NLO responses becomes much larger with the addition of bipyridine CPPs ligand. Overall, we hope the current work will deliver a new quantitative structure-property relationship toward the design and synthesis of Pd(II) oligomeric nanohoop complexes with high third-order NLO responses.