Temperature-Dependent Luminescence of Isostructural [Eu(TPTZ)(NO3)3] and [Tb(TPTZ)(NO3)3] Compounds: Insights into the Ligand–Lanthanide (5D0/5D4 levels) energy gap ΔE

In this study, two isostructural lanthanide compounds, [Eu(TPTZ)(NO 3 ) 3 ] and [Tb(TPTZ)(NO 3 ) 3 ] were synthesized and analyzed to investigate how small electronic differences lead to into distinct optical and thermal responses. Both compounds crystallize in the triclinic system with P-1 space group, as confirmed by PXRD and DFT-Rietveld structural refinement. These compounds share a common decacoordinated environment, 3 N donors from tridentate TPTZ, 6 O donors from 3 bidentate NO 3 including a coordinated methanol molecule. Despite this structural equivalence, their luminescent behaviors differ significantly. The [Eu(TPTZ)(NO 3 ) 3 ] compound displays intense red emission with high quantum efficiency (Φ = 95%) and retains most of its intensity up to 150 °C, demonstrating remarkable thermal stability. In contrast, the [Tb(TPTZ)(NO 3 ) 3 ] compound exhibits green emission with lower efficiency (Φ = 21%) and a pronounced temperature-dependent quenching. The thermal quenching mechanism of the [Tb(TPTZ)(NO 3 ) 3 ] was further analyzed through lifetime-based Arrhenius analysis to evaluate the back-energy transfer (BET) process, yielding an activation energy of 0.32 eV, together with Mott–Seitz modeling of the integrated emission intensity, which provided a comparable value of 0.31 eV. The close agreement between both activation energies indicates that BET is the dominant thermally activated non-radiative pathway in the [Tb(TPTZ)(NO 3 ) 3 ] . These findings show that subtle variations in the electronic coupling between the ligand and the lanthanide, particularly the energy gap (ΔE) between the ligand triplet state and the 5 D 0 / 5 D 4 excited levels, could critically influence radiative stability. This insight provides a rational and comparative basis for designing lanthanide coordination luminescent compounds with tunable thermal stability for optoelectronic applications. • PXRD, Rietveld refinement, and DFT confirm triclinic P-1 space group and coordination number 10 for both [Eu(TPTZ) 3 (NO 3 ) 3 ] and [Tb(TPTZ) 3 (NO 3 ) 3 ]. • Isostructural [Eu(TPTZ) 3 (NO 3 ) 3 ] and [Tb(TPTZ) 3 (NO 3 ) 3 ] show different photophysical and thermal behaviors despite nearly identical crystal structures. • [Eu(TPTZ) 3 (NO 3 ) 3 ] τ = 1.4 ms, Ф = 95%, red emission (x=0.66, y=0.33) • [Tb(TPTZ) 3 (NO 3 ) 3 ] τ = 0.86 ms, Ф = 21%, green emission (x=0.33, y=0.58). • Triplet-lanthanide energy gap (ΔE) governs luminescence efficiency and thermal stability.