A ratiometric thermometer based on a mixed-metal Ln(III) metal-organic framework is reported that has good sensitivity in a wide temperature range from 4 to 290 K and a quantum yield of 22% at room temperature. The sensing mechanism in the europium-doped compound Tb0.95Eu0.05HL (H4L = 5-hydroxy-1,2,4-benzenetricarboxylic acid) is based not only on phonon-assisted energy transfer from Tb(III) to Eu(III) centers, but also on phonon-assisted energy migration between neighboring Tb(III) ions. It shows good performance in a wide temperature range, especially in the range 4-50 K, reaching a sensitivity up to 31% K(-1) at 4 K.

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Eight novel europium(III)‐based coordination compounds with 1,10‐phenanthroline (phen) ligands with a chloro, methoxy, ethoxy, cyano, carboxylic acid, methyl carboxylate, ethyl carboxylate, and amino substituent on the 2‐position have been prepared in yields ranging from 43 to 89 %. Additionally, one lanthanum(III) coordination compound of 2‐amino‐1,10‐phenanthroline has been isolated. All compounds have the general formula [Ln(L)2(NO3)3], except for the compound with the carboxylate ligand, which has the formula [Eu(O2Cphen)3]. Three of the EuIII complexes as well as the LaIII compound crystal structures have been determined, all of which show similar N4O6 coordination spheres for the LnIII ion. Seven compounds exhibit bright luminescence that is characteristic of EuIII upon irradiation with near‐UV radiation, thus indicating efficient ligand‐to‐metal energy transfer. The complex with 2‐amino‐1,10‐phenanthroline is nonluminescent. The solid‐state photoluminescent quantum yields range from 10 to 79 %, and the luminescence lifetimes vary from 0.43 to 1.57 ms. Analysis of the spectral intensities with the Judd–Ofelt theory shows a significant contribution of nonradiative processes that quench the luminescence of the 5D0 level on EuIII.

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