Unlocking Arene Phosphorescence in Bismuth-Organic Materials

Abstract

Three novel bismuth-organic compounds, with the general formula [Bi₂(HPDC)₂(PDC)₂]·(arene)·2H₂O (H₂PDC = 2,6-pyridinedicarboxylic acid; arene = pyrene, naphthalene, and azulene), that consist of neutral dinuclear Bi-pyridinedicarboxylate complexes and outer coordination sphere arene molecules were synthesized and structurally characterized. The structures of all three phases exhibit strong π-π stacking interactions between the Bi-bound PDC/HPDC and outer sphere organic molecules; these interactions effectively sandwich the arene molecules between bismuth complexes and thereby prevent molecular vibrations. Upon UV irradiation, the compounds containing pyrene and naphthalene displayed red and green emission, respectively, with quantum yields of 1.3(2) and 30.8(4)%. The emission was found to originate from the T₁ → S₀ transition of the corresponding arene and result in phosphorescence characteristic of the arene employed. By comparison, the azulene-containing compound displayed very weak blue-purple phosphorescence of unknown origin and is a rare example of T₂ → S₀ emission from azulene. The pyrene- and naphthalene-containing compounds both display radioluminescence, with intensities of 11 and 38% relative to bismuth germanate, respectively. Collectively, these results provide further insights into the structure-property relationships that underpin luminescence from Bi-based materials and highlight the utility of Bi-organic molecules in the realization of organic emission.

Figure 1

Polyhedral representation of 1. The structure consists of Bi₂(HPDC)₂(PDC)₂ dimers, with pyrene in the outer coordination sphere. Strong π–π stacking interactions exist between the bridging PDC and the pyrene. Purple = bismuth, blue = nitrogen, red = oxygen, and black = carbon atoms. Hydrogen atoms and lattice water molecules have been omitted for clarity.

Figure 2

Ball and stick representation of 1 showing the 1-D chains formed by PDC···pyrene···PDC π–π stacking interactions (blue dotted lines, depicting centroid···centroid interactions). Purple = bismuth, blue = nitrogen, red = oxygen, and black = carbon atoms. Hydrogen atoms and lattice water molecules have been omitted for clarity.

Figure 3

Polyhedral representation of 2. The structure is built from the same Bi-PDC dimer as that observed in 1; however, naphthalene is present in the outer coordination sphere and exhibits strong π–π stacking interactions with the bridging PDC. Purple = bismuth, blue = nitrogen, red = oxygen, and black = carbon atoms. Hydrogen atoms and lattice water molecules have been omitted for clarity.

Figure 4

Ball and stick representation of 2 showing the 1-D chains that extend down the [100] by PDC···naphthalene···PDC π–π stacking interactions (blue dotted lines). Purple = bismuth, blue = nitrogen, red = oxygen, and black = carbon atoms. Hydrogen atoms and lattice water molecules have been omitted for clarity.

Figure 5

Polyhedral representation of 3. Outer sphere azulene displays strong π–π stacking interactions with the bridging PDC. Purple = bismuth, blue = nitrogen, red = oxygen, and black = carbon atoms. Hydrogen atoms, lattice water molecules, and disorder of the azulene have been omitted for clarity.

Figure 6

Ball and stick representation of 3 highlighting the 1-D chains formed via PDC···azulene···PDC π–π interactions (blue dotted lines). Purple = bismuth, blue = nitrogen, red = oxygen, and black = carbon atoms. Hydrogen atoms, lattice water molecules, and disorder of the azulene are not shown for clarity.

Figure 7

Normalized excitation (dashed lines) and emission (solid lines) spectra for pyrene (1; red), naphthalene (2; green), and azulene (3; blue).

Figure 8

Radioluminescence spectra for 1 (blue), 2 (red), and BGO (black). The spectra were integrated and the relative intensities were compared to that of BGO.