A nitrophenyl-carbazole based push-pull linker as a building block for non-linear optical active coordination polymers: A structural and photophysical study

Non-linear optical effects (NLO) such as multi-photon absorption, second harmonic generation (SHG) etc. have a wide range of applications. Nevertheless, the performance of many NLO-active organic dyes is limited by their thermal stability and photobleaching. These problems can be overcome by integrating the dyes into coordination polymers or metal-organic frameworks. Here, we present a structural and photophysical study of dipropyl-9-(4-nitrophenyl)-carbazole-3,6-dicarboxylate, a new “push-pull” organic dye molecule designed as a chromophore linker for NLO-active coordination polymers. Structure determination of a single-crystal showed that it crystallizes in a monoclinic crystal system P 2₁/c. The solvated chromophore exhibits two aromatic absorption bands at 250 nm and 275 nm as well as broad long wavelength band at 350 nm, which we assign to an intramolecular charge transfer state. Photoluminescence measurements in solvents of different polarities revealed two main effects: In nonpolar solvents, the spectrum shows an emission band at 360 nm, whereas in solvents with a higher polarity, the emission maximum broadens and redshifts. Solid-state emission measurement of sample powder exhibits an emission band at 520 nm which is redshifted compared to the measurements in solution, due to excimer formation in the solid-state. The optical as well as solvation-related properties of the investigated pigment render it to be a versatile ligand in coordination polymers.