Coordination Polymers Based on Carbazole-Derived Chromophore Linkers for Optimized Multiphoton Absorption: A Structural and Photophysical Study

Multiphoton absorption (MPA), as a subgroup of non-linear optical effects, is of high interest in modern materials research since it has a great applicability in optoelectronics. However, most of the commonly used materials featuring MPA properties are chromophore molecules, which are limited by their thermal stability and uncontrolled aggregation in high-concentration solutions. A prominent material class which could in principle overcome these problems are metal-organic frameworks and coordination polymers (CPs) as they can be modularly tuned to possess chemical and thermal stability. In addition, by incorporating chromophores as linkers in the framework, their molecular properties can be retained or even enhanced. In this article, we report the synthesis and characterization of three new and highly MPA-active CPs, Zn₂(sbcd)(DMAc)₂(H₂O)_1.5, Sr(fbcd)(DMAc)_0.25(H₂O)_3.5, and Ba(fbcd)(DMAc)_2.5(H₂O)_1.5, based on two carbazole-containing chromophore linkers: a previously reported 9,9′-stilbene-bis-carbazole-3,6-dicarboxylic acid (H₄sbcd) and the new 2,7-fluorene-9,9′-dimethyl-bis-carbazole-3,6-dicarboxylic acid (H₄fbcd). Single-crystal structure analysis of the zinc-based CP reveals a sql network, whereas the barium- and strontium-based CPs are isostructural, showing a 4,8-c network topology. Z-scan analysis of the networks shows large two-photon absorption cross-sections σ⁽²⁾of 2100 to 33,300 GM, which is an enhancement of up to 3 orders of magnitude in comparison to the solvated linker and is also one of the highest MPA-cross-sections reported for CPs up to date.