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.