Luminescent Properties of Anthracene-based Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are crystalline materials composed of metal clusters and organic ligands. MOFs that exhibit photoluminescence are promising materials for a broad range of applications. Due to their structural tunability and crystalline nature, luminescent MOFs also provide an excellent platform for studying structure–property relationships of materials.

The photophysical properties of three anthracene-dicarboxylic acids – 1,4-anthracene dicarboxylic acid (1,4-ADCA), 2,6-anthracene dicarboxylic acid (2,6-ADCA) and 9,10-anthracene dicarboxylic acid (9,10-ADCA) – were studied in a series of polar aprotic solvents using steady-state absorption, steady-state emission spectroscopy and time-correlated single photon counting (TCSPC) emission lifetime spectroscopy. The addition of carboxylic acid functional groups on the anthracene ring alters photophysical properties to varying degrees depending on the location and protonation state. Density functional theory (DFT) calculations reveal that the lowest-energy ground-state structures of both 2,6-ADCA and 1,4-ADCA have dihedral angles between the carboxylic acids and aromatic planes of θ = 0°, while the same dihedral angle increases to θ = 56.6° for 9,10-ADCA. Time-dependent DFT calculations suggest that the carboxyl groups of 1,4-ADCA and 2,6-ADCA remain coplanar with the anthracene ring system in the excited state. In contrast, the calculations reveal significant changes between the ground and excited geometries for 9,10-ADCA and puckering of the anthracene moiety of is observed.

The three anthracene dicarboxylic acids were then incorporated into zirconium-based MOFs. The MOF structures were characterized using powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM). The steady-state absorption and emission spectra as well as the fluorescence lifetimes of the MOFs were compared to that of the corresponding ligand in solution. The MOFs comprising 9,10-ADCA and 2,6-ADCA formed highly crystalline octahedral shaped crystals and were found to be isostructural with the well-known UiO-66 and UiO-67 frameworks. However, incorporation of the 1,4-ADCA ligand resulted in large rod-shaped crystals. The absorption spectra of the MOFs are broadened and redshifted compared with that of the corresponding free ligands. The emission spectra of the MOFs constructed from 9,10-ADCA and 1,4-ADCA display emission bands that resemble that of the free ligand in acidic solutions, but are slightly broadened and redshifted in the MOF. Little difference is observed between that of 2,6-ADCA within the MOF and in acidic solution. The broadening and redshift observed in the absorption and emission is indicative of intermolecular interactions between anthracene units and/or with the Zr4+ clusters. The fluorescence lifetimes measured for the anthracene-based MOFs show a long component, comparable to the lifetime of the free ligand, along with shorter component. This may also suggest intermolecular interactions between chromophores in the MOFs.

Altogether, derivatization of anthracene was shown to have specific effects on the photophysical properties of the parent anthracene molecule. These properties are further altered when the ligand is incorporated into a metal organic framework. Such systematic studies can provide a guide in designing luminescent MOFs with the excited-state properties desired for a given application.