Experimental, DFT and quantum Monte Carlo studies of a series of peptide-based metal–organic frameworks: synthesis, structures and properties

A series of bio-analogous peptide-based metal–organic frameworks (Mn(1), Fe(2), Co(3), Cu(4), Ag(5) and Pb(6)) based on one cyclic dipeptide (2,5-piperazinedione-1,4-diacetic acid, H2PODC) were obtained and the relationships between properties (luminescence and magnetism) and structure was investigated. Crystal structure analysis shows that (1) 1–3 feature a three-dimensional isomorphic framework; (2) 4 shows one two-dimensional plane structure; (3) 5 shows a three-dimensional framework with one alternate Ag–Ag chain (dAg–Ag = 2.7918 and 2.9346 A); (4) 6 indicates a three-dimensional structure with one dimensional 7.0 × 7.0 A2 channel. Combination of magnetic measures and quantum Monte Carlo (QMC) studies reveals that 1 has anti-ferromagnetic properties with a J of −0.5 cm−1 and 4 exhibits ferromagnetic properties with a J of 2.23 cm−1, while both 2 and 3 show ferrimagnetic properties. Spin polarized density functional theory (DFT) calculations uncover that antiferromagnetism of 1 is attributed to the coupling of paramagnetic Mn(II) ions by −/−/+ spin nets of linking carboxylate, while in 4 ferromagnetic superexchange of Cu(II) ions is derived from the spin polarization effect through the carboxylate bridge in the +/+/+ spin nets of linking carboxylates. Furthermore, DFT calculation results show that two absorption peaks of 5 should be attributed to the transitions of the valence band (VB)→ the second empty band and VB→ the conduction band (CB), and one emission peak is the result of the CB→VB transition, where the electrons transfer from the Ag–Ag metal bond to localized 4d orbitals of Ag atoms.