Influence of Chromophore Packing on Multiphoton Absorption in Carbazole-Based Pillar-Layered Coordination Polymers

Coordination polymers (CP) and their subgroup metal-organic frameworks (MOF) are promising classes of modular multiphoton-absorption active materials. However, a detailed knowledge of the structure-property relationship or generalized design principles remains elusive. This study examines how various packings of the chromophore linker 9,9′-stilbene-bis-carbazole-3,6-dicarboxylic acid in three synthesized zinc-based CPs affect their MPA activity. Different spatial chromophore arrangements are achieved by the so-called “pillar-layer” synthesis approach, using the chromophore and two different additional pillar linkers (4,4′-bipyridine and 1,2-bis(4-pyridyl)ethane) for CP formation. Two novel pillar-layered CPs, Zn_2n(sbcd)(bpy)(DMAc)_2n(H₂O)_3n and Zn_2n(sbcd)(bpe)(DMAc)_3n(H₂O), are reported and examined in their two-photon-absorption-induced photoluminescence and compared to a previously synthesized CP Zn_2n(sbcd)(DMAc)_2n(H₂O)_1.5n, containing the same chromophore but no pillars. The comparison shows significant differences for the two-photon absorption cross-sections of the materials, improving it by incorporating the pillar. Our findings point toward the significance of controlling the chromophore orientation to tailor the nonlinear optical properties of the materials. These insights pave the way toward an aim-directed development of MOFs for advanced photonic applications.