Open AccessNature of the Ligand-Centered Triplet State in Gd3+β-Diketonate Complexes as Revealed by Time-Resolved EPR Spectroscopy and DFT Calculations
Author(s) -
Silvia Carlotto,
Luca Babetto,
Marco Bortolus,
Alice Carlotto,
Marzio Rancan,
Gregorio Bottaro,
Lidia Armelao,
Donatella Carbonera,
Maurizio Casarin
Publication year - 2021
Publication title -
inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.348
H-Index - 233
eISSN - 1520-510X
pISSN - 0020-1669
DOI - 10.1021/acs.inorgchem.1c01123
Subject(s) - chemistry , phosphorescence , electron paramagnetic resonance , delocalized electron , triplet state , excited state , spectroscopy , population , zero field splitting , ligand (biochemistry) , density functional theory , photochemistry , crystallography , computational chemistry , nuclear magnetic resonance , molecule , electron , fluorescence , atomic physics , spin polarization , physics , demography , organic chemistry , quantum mechanics , sociology , biochemistry , receptor
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.