Development of photoluminescent hydrogen-bonded frameworks based on pyromellitic diimide-tethered carboxylic acid hosts and multi-bonding solvent guests

The significance of hydrogen-bonding interactions in improving the chemical and physical properties of functional materials related to sustainable energy, gas absorption, catalysis, and pharmaceuticals has gained considerable research attention. In this report, some unprecedented hydrogen bond motifs between the -COOH group and the solvents capable of forming multiple hydrogen bonds with -COOH are studied. The effects of such diverse motifs on the construction of 3D supramolecular architectures of hydrogen-bonded host-guest frameworks and their optical properties are elucidated. For this purpose, structural studies on seven solvates, namely, 1a (1:2DMF), 1b (1:2pyridine), 1c (1:2quinoline), 2a (1:2DMF), 2b (1:2pyridine), 2c (1:2quinoline), and 2d (1:1quinoline:2piperidine), of two isomeric pyromellitic diimide hosts 1 and 2 were carried out. Single crystal X-ray diffraction (SCXRD) analyses revealed that solvates 1a, 2a, and 2b show 3D non-porous supramolecular host-guest networks, whereas solvates 1b, 1c, 2c, and 2d show 3D supramolecular host-guest channelled architectures accommodating guest solvent molecules within the cavities of different dimensions. Formation of different hydrogen bond motifs, either cyclic/ring (R) or discrete (D) or a combination of both, between the -COOH groups of isomeric hosts and identical guest molecules is analysed through density functional theory (DFT) calculations. Minor differences in the interaction energies of different motifs of isomeric hosts with the same guest suggest that the formation of either motif depends on the steric orientations of hosts and other weak host-guest interactions in the crystal lattices. Solid state fluorescence emission properties of solvates 1a, 2a, and 2b are found to be similar to their respective hosts, whereas those of solvates 1b, 1c, 2c, and 2d are different from their hosts. Along with the diversity of supramolecular synthons, frontier molecular orbital (FMO) analysis of hydrogen-bonded model structures explained well the different emission behaviours of solvates. Thermal analyses for the solvates are in good agreement for the association of the numbers of guest solvent molecules with both the isomeric hosts. Overall, this research is focused on establishing the phenomena for the formation of distinct hydrogen bond patterns between the two similar host-guest binding groups together with the effect of supramolecular states on the photophysical properties of such hydrogen-bonded complexes. Supramolecular aspects of multi-component hydrogen-bonded assemblies constructed by the combinations of pyromellitic diimide-tethered carboxylic acids and some multiple hydrogen-bonded donor/acceptor solvent molecules are studied.