Transition metal complexes are utilized in a broad array of applications such as photoredox catalysis, photodynamic therapy, biological sensing, and as phosphors for organic light-emitting diodes. Furthermore, many transition metals complexes demonstrate two photon absorption and reverse saturable absorption (RSA) characteristics increasing their applications as non-linear optical materials. Iridium is of particular interest to our work because of its strong spin-orbit coupling, which allows for multiple excited states, thereby increasing the compound’s ability to absorb light over a broad spectrum. This work describes the synthesis of a series of triphenylamine and methoxy functionalized 2-phenylbenzo[d]thiazole (pbt) ligands to explore the effects on the photophysical properties of the synthesized Ir(III) cyclometalated chromophores and evaluate their potential application as RSA materials.
Cyclometalated iridium complexes are a specific class of transition metal chromophores (TMCs) widely employed as phosphors in organic light emitting diodes (OLEDs), as photoredox catalysts, and increasingly as non-linear optical (NLO) materials. This research investigated the effect of derivatization on the parent complex [Ir(pbt)2(acac)], where pbt is 2-phenylbenzothiazole and acac is acetylacetonate. Specifically, hole transport groups, either triphenylamines or carbazoles, were installed at different positions along the pbt ligand framework. The chemical syntheses, electrochemistry, and photophysics of these complexes will be discussed in detail with reference to our previous work and the parent complex.
Previous reports of chalcogenopyrylium polymethine dyes establish singlet oxygen yields as high as 12%. Our studies of these systems suggested that the current understanding of the excited state dynamics is incomplete. In fact, we observe no evidence for singlet oxygen generation across a range of experiments. We have carried out steady-state and pulsedlaser kinetic experiments on a family of fourteen dyes, including six novel dyes, selected to vary physical and electronic structure. We have changed the identity of the chalcogen between selenium and tellurium heteroatoms, phenyl, thiophene, tert-butyl substituents, and methine linker length. Excited-state lifetimes were obtained by femtosecond transient absorption spectroscopy. Lifetimes were all sub-300 picoseconds, suggesting rapid relaxation. Furthermore, we observed no evidence of any triplet transient processes; phosphorescence was only observed in samples at 77 K. Variable-temperature NMR experiments implicate rotation of the pyran ring about the methine backbone as a critical determinant of the dynamics of these dyes that distinguishes their photophysics from more rigid analogues.
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.
Transition metal complexes have seen increased application in the fields of photoredox catalysis, photodynamic therapy, biological sensing, and as phosphors for organic light-emitting diodes. Additionally, non-linear optical applications of these materials have increased due to recent reports of two photon absorption and reverse saturable absorption (RSA) characteristics. Recent studies have demonstrated iridium is of particular interest as the transition metal center for organometallic chromophores because of its strong spin-orbit coupling, which allows for multiple excited states, thereby increasing the compound’s ability to absorb light over a broad spectrum. Our work focused on the synthesis of a series of functionalized phenylbenzothiazole (pbt) ligands to explore the effects on the photophysical properties of the synthesized Ir(III) cyclometalated chromophores and evaluate their potential application as RSA materials. The Ir(III) cyclometalated complexes were prepared from the bromo substituted pbt. The intermediate was then subjected to microwave assisted Suzuki reaction conditions to form the derivatized pbt cyclometalated complex.
Recently, cyclometalated iridium (III) complexes have been studied for use as non-linear optical (NLO) materials. Reports of both two photon absorption (2PA) and reverse saturable absorption (RSA) properties for cyclometalated iridium (III) complexes warrant their continued development for enhanced performance. Cyclometalating ligands used with iridium commonly ligate through an anionic carbon on a phenyl ring and a nitrogen on an adjacent ring, such as 2- phenylpyridine (ppy), 1-phenylpyrazole (ppz), 2-phenylbenzoxazole (pbo), and their derivatives. These ligands contain a rotational degree of freedom around the carbon-carbon bond that links the two rings. This flexibility could induce a nonradiative decay pathway in cyclometalated iridium complexes containing these ligands. Benzo[h]quinoline (bzq) was chosen as a more rigid, but structurally analogous, comparison to ppy. The complexes [IrIII(ppy)2(acac)]0 , [IrIII(bzq)2(acac)]0 , [IrIII(ppy)2(bpy)]PF6, and [IrIII(bzq)2(bpy)]PF6, where acac is acetylacetonate and bpy is 2,2’- bipyridine, were synthesized and characterized to assess the impact of enhanced cyclometalating rigidity in both neutral and charged iridium complexes. Through transient absorption spectroscopy, it was found that excited state localization and lifetime was drastically affected by the choice of either the acac or bpy ligand. Furthermore, the more rigid cyclometalating ligand, bzq, exhibited smaller non-radiative decay rates than the more flexible ppy analogue.
Transition metal chromophores (TMCs) are a widely studied class of materials due to their synthetic tunability and photophysical properties. Second- and third-row d6 TMCs, such as RuII or IrIII, are of particular importance due to their large spin-orbit coupling constants and the prevalence of metal-to-ligand charge transfer (MLCT) excited states. TMCs have found broad application in organic light-emitting diodes (OLEDs), photoredox catalysis, photodynamic therapy, and non-linear optics (NLO). Recent photophysical studies on organometallic iridium complexes of the form [IrIII(C^N)2(acac)]0, where C^N is a cyclometalating ligand and acac is acetylacetonate, have demonstrated their potential as reverse saturable absorption (RSA) materials. The photophysical properties, including photoluminescence and transient absorption spectra, are reported for [Ir(pbt)2(acac)], where pbt is 2-phenylbenzothiazole. In an attempt to engender new excited state absorption (ESA) bands, a triphenylamine (TPA) moiety was installed on the pbt ligand via microwave-assisted Suzuki coupling. The spectroscopic properties of the new TMC were compared to the parent [Ir(pbt)2(acac)] complex with particular emphasis on their potential application as RSA materials.
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(CNdmp)2]+, where C^N is a variable cyclometalating ligand and CNdmp 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 CNdmp 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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