Monocationic Iridium(III) Complexes with Far‐Red Charge‐Transfer Absorption and Near‐IR Emission: Synthesis, Photophysics, and Reverse Saturable Absorption

Abstract

The synthesis, photophysics, and reverse saturable absorption (RSA) of three monocationic iridium(III) complexes (Ir1–Ir3) bearing diimine (N^N) ligand with different degrees of π‐conjugation are reported. Spectroscopic methods, including UV/Vis absorption, emission, and transient absorption spectroscopy, and density functional theory calculations were carried out to understand the nature of the singlet and triplet optical transitions and the influence of N^N ligand π‐conjugation on the photophysical properties of the complexes. All complexes possessed predominant ligand–centered, 1π,π* absorption bands at 280–420 nm and weak charge‐transfer absorption bands at 420–610 nm for Ir1, 420–680 nm for Ir2, and 420–730 nm for Ir3. The extended π‐conjugation of the N^N ligand red–shifted the charge‐transfer absorption bands and increased the molar extinction coefficients of all of the absorption bands. All complexes were emissive in the far‐red to the near‐infrared regions, with the emission energies gradually decreasing when the π‐conjugation of the N^N ligand increased. However, the emission lifetime of Ir3 increased when its emitting state energy decreased. This was caused by the different nature of the T1 state in Ir3, which was the predominant N^N ligand–localized 3π,π* state compared to the charge‐transfer T1 states in Ir1 and Ir2. The different parentage of the T1 state in Ir3 also gave rise to a much broader and stronger triplet excited‐state absorption in the 420–800 nm regions. The varied ground‐state and triplet excited‐state absorption characteristics led to a different strength of the reverse saturable absorption (RSA) for ns laser pulses at 532 nm, with the RSA strength following the trend of Ir3 > Ir1 ≥ Ir2. Complex Ir3 is particularly attractive for its potential as a broadband reverse saturable absorber in view of its broader ground‐ and excited‐state absorption in the regions of 420–730 nm and longer–lived triplet excited state.

Mono-/Bimetallic Neutral Iridium(III) Complexes Bearing Diketopyrrolopyrrole-Substituted N-Heterocyclic Carbene Ligands: Synthesis and Photophysics

“…Overall, this approach has shown good agreement with experimental spectra for many heteroleptic Ir(III) complexes. 14,47,48 However, vibrational fine structures that are clearly seen in the experimental absorption spectra for the lowest-energy bands of Ir-1− Ir-4 give an indication of the high degree of Frank Condon (FC) factorsthe overlap between the vibrational states of two electronic states. 69 Thus, the vibronic features associated with FC overlaps cannot be neglected for the considered complexes in order to reproduce an accurate shape of their first absorption band.…”

“…4 Efforts in red-shifting the absorption of the Ir(III) complexes to far-red/NIR spectral regions have been quite limited. 14,17,20,21,24 To date, two of the reported red/NIR-absorbing Ir(III) complexes bear the styryl-BODIPY (boron dipyrromethene difluoride) or cyanine motif. 20,21 Although the absorption of the styryl-BODIPY-containing complexes was red-shifted to 606−729 nm and their T 1 states were long-lived (∼31−157 μs), the in vitro PDT effects of these complexes were quite weak, with high dark toxicity (inhibitory concentration to reduce the cell viability to 50%, IC 50 = 8.16−16.70 μM) and minimal phototherapeutic indices (PIs < 4) upon 635 nm excitation.…”

“…Developing molecules that strongly absorb and/or emit in the far-red to near-infrared (NIR) regions has aroused significant attention due to their wide applications on night-vision devices, optoelectronics, heat shielding, information security displays, phototherapy, bioimaging, and so forth. − Particularly, far-red/NIR-absorbing molecules are desirable photosensitizers for photodynamic and/or photothermal therapy because light in these spectral regions can penetrate tissue deeper. − For the same reason, far-red/NIR-emitting molecules are attractive for bioimaging applications. ,, Far-red/NIR-absorbing molecules are also desired for dye-sensitized solar cells (DSSCs) or triplet–triplet annihilation upconversion (TTA-UC) applications because solar light has significant far-red/NIR components. To date, most of the far-red/NIR-absorbing molecules are organic molecules with an extended π-conjugation. ,,− , Studies on far-red/NIR-absorbing transition-metal complexes have been rare. − …”

Zn Coordination and the Identity of the Halide Ancillary Ligand Dramatically Influence the Excited-State Dynamics and Bimolecular Reactions of 2,3-Di(pyridin-2-yl)benzo[g]quinoxaline

“…Coordination complexes of 2,3-di­(pyridin-2-yl)­pyrazine (dpp), 2,3-di­(pyridin-2-yl)­quinoxaline (dpq), and 2,3-di­(pyridin-2-yl)­benzo­[ g ]­quinoxaline ( dpb , shown in Figure ) with several first-row transition metals were first reported in the late 1950s and late 1960s. Interest in the photophysical properties of coordination compounds featuring dpp, dpq, and dpb ligands was stimulated in the late 1980s by a number of reports investigating the photophysical and redox properties of transition-metal complexes containing dpp − and dpq. − In the 3 decades that have followed, metal complexes utilizing dpp, dpq, and dpb as diimine-type ligands have been extensively studied and have found application in a multitude of research areas, including photodynamic therapy (PDT), − DNA sensing, − catalysis, − electron-transfer systems, − near-IR light absorbers, − biologically active complexes, − and electroluminescent devices. − Additionally, modified versions of these ligands have been studied for use as monomer units in specialty polymers, , and starburst-shaped analogues have shown promise as electron-transport materials in organic light-emitting diodes (OLEDs) and other optoelectronic devices because of their ability to form stable amorphous molecular glasses. − …”

Ultrasensitive Nucleic Acid Assay Based on Cyclometalated Iridium(III) Complex with High Electrochemiluminescence Efficiency

“…Cyclometalated iridium­(III) complexes, as one of the most promising ECL emitters, have been investigated in depth . Due to their high ECL efficiency, long luminescent lifetimes, and excellent tunability with an emission range from 410 to 1180 nm via ligand changing, − cyclometalated iridium­(III) complexes have been successfully applied in ECL systems for the detection of small biomolecules and microRNAs, as well as the construction of ECL devices including ECL gels and multicolor ECL platform . Kim’s group first reported two neutral cyclometalated iridium­(III) complexes of (pq) 2 Ir­(acac) and (pq) 2 Ir­(tmd) with ECL efficiencies higher than [Ru­(bpy) 3 ] 2+ , (pq = 2-phenylquinoline anion, acac = acetylacetonate anion, tmd = 2,2′,6,6′-tetramethylhepta-3,5-dione anion).…”