Structure elucidation by sparkle/RM1, effect of lanthanide contraction and photophysical properties of lanthanide(III) trifluoroacetylacetonate complexes with 1,10-phenanthroline

• [Ln(tfaa)3phen] complexes were synthesized with low hydrogen content (2.52–2.78%).
• Sparkle/RM1 structures were supported by FT-IR, ESI–MS and NMR.
• Visible luminescence of ternary complexes was studied in solution.
• Theoretical and experimental photophysical properties of Eu complex are in agreement.
• Ligand-to-metal energy transfer is dominated by the exchange mechanism.

A series of luminescent anhydrous eight-coordinated anhydrous complexes of the general formula [Ln(tfaa)3phen] [Ln = La(1), Pr(2), Nd(3), Eu(4), Tb(5), Dy(6), Ho(7), Er(8), Tm(9), Yb(10) and Lu(11); tfaa = 1,1,1-trifluoro-2,4-pentanedione and phen = 1,10-phenanthroline] have been synthesized and fully characterized by elemental analysis, ESI–MS, thermogarvimetric analysis (TGA), FT-IR and 1H NMR spectroscopy. The complexes retain their identity in solution, phen and β-diketonate (methine and methyl) protons resonances are shifted in opposite directions and lanthanide induced shifts (LISs) is dipolar in nature. The unknown geometry of the complexes was determined from semi-empirical Sparkle/RM1 method. A distorted square antiprism geometry is proposed for [Eu(tfaa)3phen] which is further attested by NMR spectroscopy. The hypersensitive transitions of Nd(III), Ho(III) and Er(III) complexes are sensitive to the environment (solvent), which is reflected by the oscillator strength and band shape of the transitions. The room-temperature PL spectra of Pr(III), Eu(III), Tb(III), Dy(III) and Tm(III) complexes in solution produce their characteristic red, brilliant red, green, yellow and blue emissions, respectively. The replacement of water molecule from the inner-coordination sphere by phen ligand and forming highly protective coordination environment (LnO6N2) by the tfaa− and phen around the Ln(III) ion, leads to increase in the photoluminescence quantum yield (PLQy) and the luminescence lifetime. Theoretical Judd-ofelt and PL parameters of [Eu(tfaa)3phen] complex including PLQy predicted by the Sparkle/RM1 method are in excellent agreement with the experimental values, reflecting the efficacy of this method. The energy transfer processes show that energy transfer occurs via both T → 5D1 (6.98 × 106) and T→ 5D0 (7.39 × 106) levels. Furthermore, Eu and Tb complex display bright red and green luminescence and therefore interesting photonics applications could be foreseen.

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