Nonlinear optical (NLO) materials are able to modulate responses of electromagnetic radiation, leading to phenomena critical to modern telecommunications technologies. The last two decades have seen significant advances in the area of molecular nonlinear chromophores, particularly with respect to reverse-saturable absorption (RSA). Here, we introduce a strategy for intense excited-state absorption (ESA) that involves bis-cyclometalated iridium complexes with isocyanide ancillary ligands decorated with pyrene triplet acceptors. Upon excitation, the complexes undergo rapid triplet-triplet energy transfer (TTET) to the acceptor excited states. This report describes five bis-cyclometalated iridium complexes using two different pyrene-substituted isocyanides with the general formula [Ir(C^N)2(CNAr)2]PF6 (C^N = cyclometalating ligand, CNAr = isocyanide ancillary ligand: CNArpyr = 2,6-dimethyl-4-(1-pyrenyl)phenyl isocyanide, CNpyr = 1-pyrenyl isocyanide). The synthesized complexes were thoroughly characterized via 1H and 13C{1H} NMR spectroscopy, Fourier-transform Infrared spectroscopy, and electrospray ionization mass spectrometry. The excited states were evaluated with UV-vis absorption, steady-state and time-resolved photoluminescence, and transient absorption spectroscopy. Phosphorescence is completely quenched at room temperature, but in the solvent glass matrix at 77 K, there is luminescence originating from a π → π* triplet state on the pyrene moiety, abbreviated herein as 3pyrene. All five complexes display intense and long-lived ESA originating from the 3pyrene state. The localization of the ground-state absorption on the cyclometalating ligands and the excited-state absorption on the pyrene moiety allows for independent tuning of ground-state absorption (GSA) and ESA to optimize RSA and other NLO attributes.

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.

In this work, we introduce a series of cyclometalated iridium complexes and evaluate the suitability of this class of compounds in nonlinear optical (NLO) applications, with an emphasis on long-lived, panchromatic reverse-saturable absorption (RSA). The investigated complexes are represented by the general formula [Ir(C^N)2(CNArdmp)2]+, (C^N = cyclometalating ligand, CNArdmp = 2,6-dimethylphenyl isocyanide). Seven such complexes were synthesized and characterized, including in-depth analysis of their photophysical properties (UV-vis absorption, photoluminescence, and transient absorption). This series of compounds contains seven different cyclometalating ligands (2-phenylbenzothiazole (bt) (Ir1), 5-nitro-2-phenylpyridine (ppyNO2) (Ir2), 5-nitro-2-(9-phenanthryl)pyridine (phenNO2) (Ir3), 2-(benzo[b]thiophen-2-yl)quinoline (btq) (Ir4), 6-(benzo[b]thiophen-2'-yl)phenanthridine (btph) (Ir5), 2,4-diphenylquinoline (dpq) (Ir6), and 6-nitro-2,4-diphenylquinoline (NO2dpq) (Ir7)), which have profound effects on their ground-state and excited-state absorption spectra. To evaluate the effects of the isocyanide ancillary ligands, some heteroleptic bis-cyclometalated iridium(III) acetylacetonate (acac) analogue complexes are included as points of comparison. In the ground state, the bis-isocyanide complexes display UV-vis absorption with the characteristic 1LC (π → π*) band at λ < 350 nm and 1MLCT bands at 350-500 nm. Five of the complexes (Ir1, Ir2, Ir4, Ir5, and Ir6) display broad, intense triplet excited-state absorption with no ground-state bleach (GSB) over the spectral window of 400-900 nm, with excited-state lifetimes spanning three orders of magnitude from ∼32 ns to 12 μs. The photophysical data suggests that the isocyanide ancillary ligand blue-shifts the GSB transient into the UV, which is normally found in the middle of the visible region for cyclometalated iridium complexes. This study demonstrates the applicability of cationic cyclometalated iridium(III) bis-isocyanide complexes as candidate RSA materials.

In this Perspective, we highlight many examples of photoluminescent metal complexes supported by isocyanides, with an emphasis on recent developments including several from our own group. Work in this field has shown that the isocyanide can play important structural roles, both as a terminal ligand and as a bridging ligand for polynuclear structures, and can influence the excited-state character and excited-state dynamics. In addition, there are many examples of isocyanide-supported complexes where the isocyanide serves as a chromophoric ligand, meaning the low-energy excited states that are important in the photochemistry are partially or completely localized on the isocyanide. Finally, an emerging trend in the design of luminescent compounds is to use the isocyanide as an electrophilic precursor, converted to an acyclic carbene by nucleophilic addition which imparts certain photophysical advantages. This Perspective aims to show the diverse roles played by isocyanides in the design of luminescent compounds, showcasing the recent developments that have led to a substantial growth in fundamental knowledge, function, and applications related to photoluminescence.

Here, we demonstrate facile [4 + 4] coordination-driven self-assembly of cyclometalated iridium(III) using linear aryldiisocyanide bridging ligands (BLs). A family of nine new [Ir(C^N)2(μ-BL)]44+ coordination cages is described, where C^N is the cyclometalating ligand-2-phenylpyridine (ppy), 2-phenylbenzothiazole (bt), or 1-phenylisoquinoline (piq)-and BL is the diisocyanide BL, with varying spacer lengths between the isocyanide binding sites. These supramolecular coordination compounds are prepared via a one-pot synthesis, with isolated yields of 40-83%. 1H NMR spectroscopy confirms the selective isolation of a single product, which is affirmed to be the M4L4 square by high-resolution mass spectrometry. Detailed photophysical studies were carried out to reveal the nature of the luminescent triplet states in these complexes. In most cases, phosphorescence arises from the [Ir(C^N)2]+ nodes, with the emission color determined by the cyclometalating ligand. However, in two cases, the lowest-energy triplet state resides on the aromatic core of the BL, and weak phosphorescence from that state is observed. This work shows that aromatic diisocyanide ligands enable coordination-driven assembly of inert iridium(III) nodes under mild conditions, producing supramolecular coordination complexes with desirable photophysical properties.

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.