New lanthanide coordination polymers (Ln-CPs) from 2-hydroxyterephthalate: Excited-state intramolecular proton transfer (ESIPT) and sensing capabilities
来源：X-MOL
Eleven new coordination polymers (Ln-CPs), Ln2(htp)3(DMF)4∙x(DMF), where Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er and htp = 2-hydroxyterephthalate were synthesized, structurally characterized and their photoluminescence properties studied. Time-dependent density-functional theory (TD-DFT) calculations were used to explain key differences and similarities in excitation and emission spectra of Ln2(htp)3(DMF)4∙x(DMF) (Ln = La (x = 2), Gd, Eu, Tb (x = 1.9)) and establish the role of enol-keto tautomerization, excited-state intramolecular proton transfer (ESIPT) and anti-Kasha emission involving htp. The CPs consist of binuclear pseudo-paddlewheel [Ln2(COO)6] secondary building units linked by htp ligands into a three-dimensional pcu framework which is stabilized by extensive hydrogen bonding. In the solid state, a five-step ESIPT on htp, featuring anti-Kasha emission, is active in the Gd(III) analogue. While in Ln2(htp)3(DMF)4∙x(DMF) (Ln = La, Tb and Eu) an enol-keto mixture exists in the ground state with ESIPT and internal conversion leading to emission only from the lowest energy triplet of the keto isomer, K* (T2) in a six-step mechanism. This behaviour is also seen in Gd2(htp)3(DMF)4∙1.9(DMF) when suspended in DMF. Sensitized emission is observed with both Eu2(htp)3(DMF)4∙1.9(DMF) and Tb2(htp)3(DMF)4∙1.9(DMF) but is more efficient in the latter which exhibits potential as a sensor for Fe3+, Al3+, 2,4-dinitrophenol and 4-nitroaniline. This study demonstrates that ESIPT pathways and absorption/emission characteristics of Ln-htp CPs can be tuned by changing lanthanide(III) identity and the local ligand environment including extent of H-bonding.