Triplet-Triplet Energy Transfer in Bis-Cyclometalated Iridium Complexes with Pyrene-Substituted Isocyanides.

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.

Panchromatic Excited-State Absorption in Bis-Cyclometalated Iridium Isocyanide Complexes.

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.

The diverse functions of isocyanides in phosphorescent metal complexes.

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.

Coordination-Driven Self-Assembly of Cyclometalated Iridium Squares Using Linear Aromatic Diisocyanides.

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.