Dr. Morris E. Olumba Profile

Dr. Morris E. Olumba

at Air Force Research Lab

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Proceedings Article | 14 March 2023 Presentation + Paper

Triplet migration and reverse-saturable absorption in cyclometalated iridium complexes with pyrene isocyanide ligands

Morris Olumba, Ryan O'Donnell, Thomas Rohrabaugh, Thomas Teets

Proceedings Volume 12418, 1241805 (2023) https://doi.org/10.1117/12.2648467

KEYWORDS: Nonlinear optical materials, Iridium, Absorption, Photoluminescence, Light absorption, Absorption spectrum, Chromophores, Phosphorescence, Visible radiation, Ultraviolet radiation, Energy transfer

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Our group has developed cyclometalated iridium complexes as nonlinear optical materials, focusing especially on reversesaturable absorption (RSA). Cationic cyclometalated iridium complexes with isocyanide ancillary ligands offer several advantages in this context, elaborated in previous studies. This talk describes next-generation complexes of the general formula [Ir(C^N)₂(CNAr)₂]⁺, where C^N is a variable cyclometalating ligand and CNAr is a pyrene-decorated aryl isocyanide. In these compounds the dominant ground-state absorption transitions, especially in the visible range, are controlled by the C^N ligand. However, the lowest-energy triplet excited state (T₁) is typically located on the pyrene moiety, which has two consequences on the spectroscopic properties. First, these compounds exhibit temperaturedependent luminescence profiles. At room temperature, photoluminescence is mostly quenched by triplet energy transfer to the pyrene, and only residual pyrene fluorescence is observed. At low temperature (77 K), phosphorescence from the pyrene is turned on, and bright red luminescence is observed. The pyrene isocyanides also have profound impacts on the transient absorption spectroscopy of these compounds. Following visible excitation, a strongly absorbing, long-lived excited state is rapidly populated, which gives rise to ESA over the entire visible range and is assigned to the pyrene triplet state. The pyrene isocyanide complexes have higher excited-state absorption cross section (i.e. larger ΔOD) relative to first-generation complexes, and the excited-state lifetime increases by as much as an order of magnitude.

Proceedings Article | 5 March 2021 Presentation + Paper

Photophysics and excited-state absorption of bis-cyclometalated iridium complexes with isocyanide ancillary ligands

Morris Olumba, Ryan O'Donnell, Thomas Rohrabaugh, Thomas Teets

Proceedings Volume 11683, 116830S (2021) https://doi.org/10.1117/12.2576139

KEYWORDS: Absorption, Iridium, Spectroscopy, Luminescence, Absorption spectroscopy

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Cyclometalated iridium complexes have long been prominent in electroluminescent applications, and several recent studies have shown that this family of compounds offers several potential advantages for designing materials with reversesaturable absorption (RSA) and other nonlinear optical properties. In this talk we present a comprehensive study of the excited-state properties of three bis-cyclometalated iridium complexes of the general formula [Ir(C^N)₂(CN^dmp)₂]⁺, where C^N is a variable cyclometalating ligand and CN^dmp is 2,6-dimethylisocyanide. The ground-state absorption and photoluminescence (PL) properties are described, with the identity of the cyclometalating ligand having a large effect on the observed PL wavelength. When the cyclometalating ligand is 2-phenylbenzothiazole (pbt), intense yellow PL is observed, whereas the PL with nitro-substituted 9-pyridylphenanthrene or 2-phenylpyridine C^N ligands is red-shifted and much weaker. Transient absorption (TA) spectroscopy was used to evaluate the excited-state absorption of the compounds. TA spectra indicate broad and intense excited-state absorption for all three compounds, with the wavelength profile strongly determined by the cyclometalating ligand. TA lifetimes are consistent with PL lifetimes and strongly oxygendependent, indicating that excited-state absorption that arises from a triplet state. To evaluate the effects of the CN^dmp isocyanide ancillary ligands, we include comparisons to charge-neutral Ir(C^N)₂(acac) (acac = acetylacetonate) complexes with the same C^N ligands. The isocyanide compounds have substantially blue-shifted ground-state absorption, excitedstate absorption, and PL, and in most cases longer lifetimes compared to the acac analogues.

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