Nature of the Ligand-Centered Triplet State in Gd³⁺β-Diketonate Complexes as Revealed by Time-Resolved EPR Spectroscopy and DFT Calculations

Abstract

A series of Gd 3+ complexes ( Gd1 – Gd3 ) with the general formula GdL 3 (EtOH) 2 , where L is a β-diketone ligand with polycyclic aromatic hydrocarbon substituents of increasing size ( 1 – 3 ), was studied by combining time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy and DFT calculations to rationalize the anomalous spectroscopic behavior of the bulkiest complex ( Gd3 ) through the series. Its faint phosphorescence band is observed only at 80 K and it is strongly red-shifted (∼200 nm) from the intense fluorescence band. Moreover, the TR-EPR spectral analysis found that triplet levels of 3 / Gd3 are effectively populated and have smaller | D | values than those of the other compounds. The combined use of zero-field splitting and spin density delocalization calculations, together with spin population analysis, allows us to explain both the large red shift and the low intensity of the phosphorescence band observed for Gd3 . The large red shift is determined by the higher delocalization degree of the wavefunction, which implies a larger energy gap between the excited S 1 and T 1 states. The low intensity of the phosphorescence is due to the presence of C–H groups which favor non-radiative decay. These groups are present in all complexes; nevertheless, they have a relevant spin density only in Gd3 . The spin population analysis on NaL models, in which Na + is coordinated to a deprotonated ligand, mimicking the coordinative environment of the complex, confirms the outcomes on the free ligands.

Complexing Eu3+/Tb3+ in a Nanoscale Postmodified Zr-MOF toward Temperature-Modulated Multispectrum Chromism

“… Experimentally, the triplet energy levels of the ligand ctpy were resolved by synthesizing the Gd-mtpy complex (mtpy = Ester form of the ctpy ligand; see the experimental section in Supporting Information, Figure S36) and thereby recording the low-temperature phosphorescence spectrum of the complex excited at 330 nm. This result revealed the triplet energy level of ctpy to be 25,314 cm –1 , which is in good accordance with the value of 25,123 cm –1 obtained from the DFT calculation. ,, Furthermore, the energy transfer mechanism from the ligand ctpy to Ln 3+ was calculated based on the experimental results. Due to the energy mismatch between the ligand and Ln 3+ ions, the electron–phonon coupling between the Ln 3+ ions and the surrounding lattice environment assists in transferring the energy from the ligand to the Ln 3+ ion. ,, The electron–phonon coupling of Zr-ctpy-NMOF@Tb and Zr-ctpy-NMOF@Eu was found to be 2.57 and 1.43, respectively (Figures S37a,b and S38a,b and Table S5; see the Supporting Information for more details).…”

“…This result revealed the triplet energy level of ctpy to be 25,314 cm −1 , which is in good accordance with the value of 25,123 cm −1 obtained from the DFT calculation. 13,19,65 Furthermore, the energy transfer mechanism from the ligand ctpy to Ln 3+ was calculated based on the experimental results. Due to the energy mismatch between the ligand and Ln 3+ ions, the electron−phonon coupling between the Ln 3+ ions and the surrounding lattice environment assists in transferring the energy from the ligand to the Ln 3+ ion.…”

A Multifunctional CaII–EuIII Heterometallic Organic Framework with Sensing and Selective Adsorption in Water

“…The S 1 of Ca II –CAM 3– is evaluated to be 31 646 cm –1 (316 nm) by referring to the absorbance edges of the UV–Vis spectra . To gain a molecular level insight into the UV–vis spectra of CaEuCAM , the proton saturated Ca II –CAM 3– extracted from CaEuCAM is used as a calculation model for TD-DFT calculations at the B3LYP/DGDZVP (d, f) level. , The calculations are carried out for the optimized geometry of the antenna in the ground ( S 0 ) and first excited ( S 1) singlets . The electronic transitions, oscillator strengths of the main absorption bands, and contributions are shown in Table S4.…”

Dynamics of the Ligand Excited States Relaxation in Novel β-Diketonates of Non-Luminescent Trivalent Metal Ions

“…Notably, there is currently no general theory describing how photophysical properties of a ligand change upon coordination to different metal ions. Several empirically derived guidelines enable a prediction of the energy change in the first excited singlet state [22], the triplet state [23], or the luminescence quantum yield [24]. According to these rules, we can estimate the luminescence efficiency of coordination compounds.…”