Thermally activated delayed fluorescence-assisted thermosensing and thermochromic behaviors of bimetallic lanthanide(III) complexes based on heteroditopic phenanthroline-terpyridine ancillary ligand

Abstract

Syntheses and characterization of a new array of four homobimetallic Ln^III-complexes of type {(Ln₂(tta)₆(phen-Hbzim-tpy)} have been carried out in this work, where Ln = La^III (1), Eu^III (2), Sm^III (3), and Tb^III (4); tta = 2-thenoyltrifluoroacetonate and phen-Hbzim-tpy = 2-[4-(2,6-dipyridin-2-ylpyridin-4-yl)phenyl]-1H-imidazole[4,5-f] [1,10] phenanthroline. Detailed room temperature photophysical behaviors and temperature-dependent luminescence spectral characteristics of the complexes, including free phen-Hbzim-tpy ligand, have been thoroughly explored. The ancillary ligand appears to be thermally activated delayed fluorescence (TADF)-active, supported by delayed luminescence measurements as well as DFT study. All the four complexes are found to exhibit excellent thermo-responsive luminescence attributes as indicated by their S_m values (1.01 for complex 1; 3.2 and 11.9 for 2; 7.2 and 4.6 for 3; and 2.16 for 4). Complex 1 cannot sustain the TADF event, whereas the rest of the complexes show remarkable thermochromism owing to this TADF as well as because of their dual emission characteristics. To the best of our knowledge, this is the first report of TADF-assisted thermosensing and thermochromism from Ln^III-based discrete molecules. Attempts have also been made to elucidate how the low-lying T₁ of tta prevents the TADF event in La-complex or compels the Tb-complex to undergo back-energy-transfer via plausible energy transfer pathways, and at the same time, how three of them can show off TADF-assisted thermochromism phenomena.