An official website of the United States government
The design of deep-red to near-infrared (DR-NIR) phosphorescent compounds with high photoluminescence quantum yields ( Φ PL ) is a significant fundamental challenge that impacts applications including optoelectronic devices, imaging, and sensing. Here we show that bis-cyclometalated iridium complexes with electron-rich ancillary ligands can have exceptional quantum yields for DR-NIR phosphorescence (peak λ > 700 nm). Six bis-cyclometalated iridium( iii ) complexes with DR-NIR phosphorescence are described in this work, pairing highly conjugated cyclometalating ligands with electron-rich and sterically encumbered β-ketoiminate (acNac), β-diketiminate (NacNac), and N , N ′-diisopropylbenzamidinate (dipba) ancillary ligands. The photoluminescence spectra and quantum yields are solvent-dependent, consistent with significant charge-transfer character in the emissive excited state. The ancillary ligands perturb the excited-state kinetics relative to closely related compounds, which can lead to enhanced Φ PL values in the DR-NIR region, particularly in toluene solution and in doped polymer films.
more »
« less
β-Diketiminate-supported iridium photosensitizers with increased excited-state reducing power
A series of bis-cyclometalated iridium complexes were prepared which combine triazole or NHC-based cyclometalating ligands with substituted β-diketiminate (NacNac) ancillary ligands. The HOMO is localized on the NacNac ligand and its energy and associated redox potential are determined by the NacNac substitution pattern. The effect of the cyclometalating ligand, relative to the more common 2-phenylpyridine derivatives, is to destabilize the LUMO and increase the triplet excited-state energy ( E T1 ). These results are supported by DFT calculations, which show HOMOs and LUMOs that are respectively localized on the NacNac and cyclometalating ligands. With this new design, we observe more negative excited-state reduction potentials, E (Ir IV /*Ir III ), with two members of the series standing out as the most potent visible-light iridium photoreductants ever reported. Stern–Volmer quenching experiments with ketone acceptors (benzophenone and acetophenone) show that the increased thermodynamic driving force for photoinduced electron-transfer correlates with faster rates relative to fac -Ir(ppy) 3 and previous generations of NacNac-supported iridium complexes. A small selection of photoredox transformations is shown, demonstrating that these new photoreductants are capable of activating challenging organohalide substrates, albeit with modest conversion.
more »
« less
- Award ID(s):
- 1846831
- PAR ID:
- 10317822
- Date Published:
- Journal Name:
- Inorganic Chemistry Frontiers
- Volume:
- 8
- Issue:
- 13
- ISSN:
- 2052-1553
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The design of molecular phosphors with near-unity photoluminescence quantum yields in the low-energy regions of the spectrum, red to near-infrared, is a long-standing challenge. Because of the energy gap law and the quantum mechanical dependence of radiative decay rate on the excited-state energy, compounds which luminesce in this region of the spectrum typically suffer from low quantum yields. In this article, we highlight our group's advances in the design of top-performing cyclometalated iridium complexes which phosphoresce in red to near-infrared regions. The compounds we have introduced in this body of work have the general formula Ir(C^N) 2 (L^X), where C^N is a cyclometalating ligand that controls the photoluminescence color and L^X is a monoanionic chelating ancillary ligand. The Ir(C^N) 2 (L^X) structure type is among the most widely studied and technologically successful classes of molecular phosphors, particularly when L^X = acetylacetonate (acac). In our work we have pioneered the use of electron-rich, nitrogen containing ancillary (L^X) ligands as a means of controlling the excited-state dynamics and optimizing them to give record-breaking phosphorescence quantum yields. This paper progresses through our work in three distinct regions of the spectrum – red, deep-red, and near-infrared – and summarizes the many insights we have gained on the relationships between molecular structure, frontier orbital energies, and excited-state dynamics.more » « less
-
In this work, we describe bis-cyclometalated iridium complexes with efficient deep-red luminescence. Two different cyclometalating (C^N) ligands-1-phenylisoquinoline (piq) and 2-(2-pyridyl)benzothiophene (btp)-are used with five strong π-donating ancillary ligands (L^X) to furnish a suite of nine new complexes with the general formula Ir(C^N) 2 (L^X). Improvements in deep-red photoluminescence quantum yields were accomplished by the incorporation of sterically encumbering substituents onto the ancillary ligand, which can enhance the radiative rate constant ( k r ) and/or reduce the non-radiative rate constant ( k nr ). Five of the complexes were characterized by X-ray crystallography, and all of them were investigated by in-depth spectroscopic and electrochemical measurements.more » « less
-
Cyclometalated iridium complexes have emerged as top-performing emitters in organic light-emitting diodes (OLEDs) and other optoelectronic devices. A persistent challenge has been the development of cyclometalated iridium complexes with deep blue luminescence that have the requisite color purity, efficiency, and stability to function in color displays. In this work we report a new class of cyclometalated iridium complexes with saturated blue luminescence. These complexes have the general structure Ir(C^C: NHC ) 2 (C^C: ADC ), where C^C: NHC is an N-heterocyclic carbene (NHC) derived cyclometalating ligand and C^C: ADC is a different type of cyclometalating ligand featuring an acyclic diaminocarbene (ADC). The complexes are prepared by a cascade reaction that involves nucleophilic addition of propylamine to an isocyanide precursor followed by base-assisted cyclometalation of the ADC intermediate. All three emit deep blue light with good quantum efficiencies ( Φ PL = 0.13–0.48) and color profiles very close to the ideal primary blue standards for color displays.more » « less
-
Molecules undergo a structural change to minimize the energy of excited states generated via external stimuli such as light. This is particularly problematic for Cu(I) coordination complexes which are an intriguing alternative to the rare and expensive transition metal containing complexes (e.g., Pt, Ir, Ru, etc.) but suffer from short excited state lifetimes due to D2d to D2 distortion and solvent coordination. Here we investigate strategic surface binding as an approach to hinder this distortion and increase the excited state lifetime of Cu(I) polypyridyl complexes. Using transient absorption spectroscopy, we observe a more than 20-fold increase in excited state lifetime, relative to solution, for a Cu(I) complex that can coordinate to the ZrO2 via both carboxylated ligands. In contrast, the Cu(I) complex that coordinates via only one ligand has a less pronounced enhancement upon surface binding and exhibits greater sensitivity to coordinating solvents. A combination of ATR-IR and polarized visible ATR measurements as well as theoretical calculations suggest that the increased lifetime is due to surface binding which decreases the degrees of freedom for molecular distortion (e.g., D2d to D2), with the doubly bound complex exhibiting the most pronounced enhancement.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D1QI00382H
