In this work, for the first time, the comparative use of P-, As-, and Sb-based ligands in phosphorescent coordination compounds is reported toward new coordination chemical concepts in the design and realization of tailored triplet emitters with nonconventional elements. By means of spectroscopic, X-ray diffractometric, and quantum-chemical methods, we reconstructed the nature of the chemical bonds as well as the influence of the increasingly heavy elements on the photoexcited state properties, which were correlated with the hybridization and polarizability of the pnictogen atoms (Pn). In particular, we elucidated the structural and photophysical properties of a series of homologous Pt(II) complexes with monodentate ancillary ligands based on group 15 elements, namely P, As, and Sb. Six coordination compounds bearing tridentate dianionic 2,6-bis(1H-1,2,4-triazol-5-yl)pyridine luminophoric pincer ligands bearing either CF 3 or t Bu moieties on the triazole rings along with triphenylpnictogens (PnPh 3 ) as monodentate ancillary ligands ([CF 3 /Pn] or [ t Bu/Pn], respectively) have been investigated. The electron donating or withdrawing effect of the peripheral substituent ( t Bu vs. CF 3 , respectively) and its influence on the bonding, crystal packing as well as the excited state energies and lifetimes was assessed in fluid solutions, frozen glassy matrices, amorphous solids, and crystalline phases. A progressively red-shifted phosphorescence was observed with increasing atomic number along with a growing compensation of hybridization defects upon coordination of the Pn atom to the Pt(II) center. The change of molecular geometry of the PnPh 3 unit upon complexation was extrapolated to predict the structural and excited state characteristics of the Bi-based analogues, which according to DFT calculations should be stable species and are the subject of ongoing synthetic efforts. In general, we envisage the use of these ligands for the relativistic enhancement of radiative deactivation rate processes, especially if Bi-based s-orbitals participate on the bond with the metal center, paving the road toward novel coordination compounds using abundant elements with high spin–orbit coupling for sustainable electroluminescent devices.
In a series of Pt(II) complexes [Pt(dba)(L)] containing the very rigid, dianionic, bis-cyclometalating, tridentate C^N^C2‒ heterocyclic ligand dba2– (H2dba = dibenzo[c,h]acridine), the coligand (ancillary ligand) L = dmso, PPh3, CNtBu and Me2Imd (N,N’-dimethylimidazolydene) was varied in order to improve its luminescence properties. Beginning with the previously reported dmso complex, we synthesized the PPh3, CNtBu and Me2Imd derivatives and characterized them by elemental analysis, 1H (and 31P) NMR spectroscopy and MS. Cyclic voltammetry showed partially reversible reduction waves ranging between ‒1.89 and ‒2.10 V and increasing along the series Me2Imd < dmso ≈ PPh3 < CNtBu. With irreversible oxidation waves ranging between 0.55 (L = Me2Imd) and 1.00 V (dmso), the electrochemical gaps range between 2.65 and 2.91 eV while increasing along the series Me2Imd < CNtBu < PPh3 < dmso. All four complexes show in part vibrationally structured long-wavelength absorption bands peaking at around 530 nm. TD-DFT calculated spectra agree quite well with the experimental spectra, with only a slight redshift. The photoluminescence spectra of all four compounds are very similar. In fluid solution at 298 K, they show broad, only partially structured bands, with maxima at around 590 nm, while in frozen glassy matrices at 77 K, slightly blue-shifted (~580 nm) bands with clear vibronic progressions were found. The photoluminescence quantum yields ΦL ranged between 0.04 and 0.24, at 298 K, and between 0.80 and 0.90 at 77 K. The lifetimes τ at 298 K ranged between 60 and 14040 ns in Ar-purged solutions and increased from 17 to 43 µs at 77 K. The TD-DFT calculated emission spectra are in excellent agreement with the experimental findings. In terms of high ΦL and long τ, the dmso and PPh3 complexes outperform the CNtBu and Me2Imd derivatives. This is remarkable in view of the higher ligand strength of Me2Imd, compared with all other coligands, as concluded from the electrochemical data.
The chromium(III) complex [Cr(ddpd) 2 ][BF 4 ] 3 shows two spin-flip emission bands in the near-infrared spectral region. These bands shift bathochromically by À 14.1 and À 7.7 cm À 1 kbar À 1 under hydrostatic pressure (Angew. Chem. Int. Ed. 2018, 57, 11069). The present study elucidates the structural changes of the chromium(III) cations under pressure using density func-tional theory with periodic boundary conditions and the resulting effects on the excited state energies using high-level CASSCF-NEVPT2 calculations. The differences of the bands in pressure sensitivity are traced back to a different orbital occupation of the intraconfigurational excited states.
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