Os(II) Phosphors with Near-Infrared Emission Induced by Ligand-to-Ligand Charge Transfer Transition

Abstract

Heating of Os3(CO)12 with 6 equiv of 2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl) pyridine (fptzH) in refluxing diethylene glycol monomethyl ether, followed by sequential treatment with stoichiometric Me3NO and addition of PPhMe2, afforded two isomeric mixtures of red-emitting [Os(fptz)2(PPhMe2)2] (1T and 1C), for which the notations T and C stand for the trans and cis-oriented fptz chelates, respectively. Alternatively, preparation of Os(II) complex using a 1:1 mixture of 5,5'-di(trifluoromethyl)-3,3'-di-1,2,4-triazole (dttzH2) and 2,2'-bipyridine (bpy), instead of fptzH, gave isolation of a mononuclear Os(II) complex [Os(bpy)(dttz)(CO)2] (2) in moderate yield. Replacement of CO with PPhMe2 on 2 afforded near-infrared (NIR)-emitting Os(II) complex [Os(bpy)(dttz)(PPhMe2)2] (3). The single-crystal X-ray structural analyses were executed on 1C, 2, and 3 to reveal the structural influence imposed by the various chelates. The photophysical and electrochemical properties were measured and discussed using the results of density functional theory (DFT) and time-dependent DFT calculations. Complex 3 is selected as the dopant to probe its electroluminescent properties by fabrication of the NIR emitting organic light-emitting diodes.

Boosting Efficiency of Near‐Infrared Organic Light‐Emitting Diodes with Os(II)‐Based Pyrazinyl Azolate Emitters

“…Although these values are lower than that of the 4 ‐based device, they still exceed those for all other Os(II)‐, Ir(III)‐, and TADF‐based NIR emitters with similar emission wavelength reported in the literature (c.f. Figure ) . More importantly, thanks to both the increased radiative and suppressed nonradiative decay processes, the devices showed relatively small efficiency roll‐off with small EQEs drop of 15–30% at a current density of 100 mA cm −2 , which is unmatched by all NIR‐emitting TADF OLEDs with similar emission peak wavelengths documented in literature …”

“…Four functional pyrazinyl azolate chelates, including 2‐(3‐(trifluoromethyl)‐1 H ‐pyrazol‐5‐yl)pyrazine (fprpzH), 2‐methyl‐5‐(3‐(trifluoromethyl)‐1 H ‐pyrazol‐5‐yl)pyrazine (fprmpzH), 2‐(3‐(trifluoromethyl)‐1 H ‐1,2,4‐triazol‐5‐yl)pyrazine (ftrpzH), and 2‐methyl‐5‐(3‐(trifluoromethyl)‐1 H ‐1,2,4‐triazol‐5‐yl)pyrazine (ftrmpzH), were prepared according to the literature methods . The target Os(II) complexes, Os(fprpz) 2 (PPhMe 2 ) 2 ( 1 ), Os(fprmpz) 2 (PPhMe 2 ) 2 ( 2 ), Os(ftrpz) 2 (PPhMe 2 ) 2 ( 3 ), and Os(ftrmpz) 2 (PPhMe 2 ) 2 ( 4 ) ( Figure ), were next synthesized via a one‐pot and two‐step procedure to which the functional azolate chelate was first reacted with Os 3 (CO) 12 in refluxing diethylene glycol monomethyl ether (DGME) solution to form the dicarbonyl intermediate, followed by treatment with Me 3 NO with an attempt to oxidized the coordinated carbonyl ligands, and addition of PPhMe 2 and heating in sequence. These four Os(II) metal complexes were fully characterized by mass analysis, NMR spectroscopy and elemental analysis, to which the detailed synthetic procedures and characterization data are described in Supporting Information.…”

“…In this work, we designed and synthesized a series of trans ‐disposed Os(II) metal complexes by combining pyrazinyl azolate chelates and dimethyl(phenyl)phosphane (PPhMe 2 ) ancillaries. In our previous work, we found that the trans ‐substituted pyridyl triazolate Os(II) complex ( 1T ) exhibited similar luminescence efficiency, higher synthesis yield and red‐shifted emission wavelength as compared to the corresponding cis isomer ( 1C ) . Moreover, previous works have proved that replacing the pyridine with a pyrazine fragment can effectively reduce the band gap of chelate, due to the stronger electron deficient nature of pyrazine .…”

“…In comparison to the Pt(II) complexes, Os(II) complexes usually have shorter excition lifetime, attributed to the lower oxidation potential of the Os(II) metal d π orbitals which significantly increases the metal‐to‐ligand charge transfer (MLCT) contribution . In addition, their octahedral configuration can effectively reduce intermolecular interaction . Considering the advantages of these aspects, Os(II) complexes are considered to be a promising candidate in realizing high efficiency and low roll‐off NIR‐emitting OLEDs.…”

Photophysical and Cellular Imaging Studies of Brightly Luminescent Osmium(II) Pyridyltriazole Complexes

“…32,33 More commonly reported are charge-neutral complexes with anionic 1,2,4-triazolate or pyrazolate ligand systems which are frequently combined with phosphine, arsine or carbonyl donors. [34][35][36][37][38] We have recently reported a series of tris-bidentate triazole-based osmium(II) complexes and their application in light-emitting electrochemical cell devices 39,40 and the bis-tridentate complex [Os(btzpy) 2 ] 2+ (btzpy = 2,6-bis(1-phenyl-1,2,3triazol-4-yl)pyridine) which exhibits phosphorescence at 595 nm. This latter complex was shown to be readily taken up by HeLa and U2OS cancer cell lines and displays a high degree of mitochondrial localisation.…”