Cyclometalated Iridium(III) Complexes Containing 4,4′-π-Conjugated 2,2′-Bipyridine Derivatives as the Ancillary Ligands: Synthesis, Photophysics, and Computational Studies

Synthesis of a New Schiff Base: Nonlinear Optical Properties Investigations

A Schiff base is prepared by the reaction of 7-hydroxy-1-naphthaldehyde with 4-aminoben-zenesulfonamide. The prepared Schiff base L4 compound was characterized by 1H NMR, Mass and FTIR spectroscopies. The optimization of the prepared L4 compound is carried out via DFT-B3LYP and 6-311G (d,p). The nonlinear optical (NLO) properties of the prepared L4 compound are investigated theoretically by the calculation of some quantum chemical descriptors (QCDs). TD-DFT via B3LYP and 6-311G calculations are used to study L4 compound UV-vis. spectrum at the same theoretical method and level. The L4 compound NLO properties are examined via irradiation with a CW laser beam, where the nonlinear index of refraction (NLIR) is calculated via diffraction patterns (DPs) and the Z-scan and as high as 4.579 × 10-11 m2/W of NLRI is obtained. Static (S) and dynamic (D) all optical switching (AOS) are examined using two CW and pulsed laser beams of wavelengths (473 and 532) nm.

Two-photon absorption properties of simple neutral Ir(III) complexes

A series of neutral Ir(2-phenylpyridine)3 derivatives substituted on the para-position of the pyridyl ligands with a π-conjugated substituent possessing different donor abilities has been prepared. Their two-photon absorption properties have been determined using the Z-scan technique. Such simple iridium(III) neutral complexes, which are easy to synthesize, show good two-photon absorption activity, with relevant TPA cross sections (the best is 750 GM), giving rise to multifunctional chromophores, since they present also high second-order NLO properties.

Ligand Dictated Photosensitization of Iridium(III) Dithiocarbamate Complexes for Photodynamic Therapy

Organelle-targeted photosensitizers (PSs) for photodynamic therapy (PDT) are considered as an effective therapeutic strategy for the development of next generation PSs with the least side effects and high therapeutic efficacy. However, multiorganelle targeted PSs eliciting PDT via both type I and type II mechanisms are scarce. Herein, a series of cyclometalated iridium(III) complexes were formulated [Ir(C∧N)2(S∧S)] (C∧N = 2-phenylpyridine (ppy) and 2-(thiophen-2-yl)pyridine (thpy); S∧S = diethyldithiocarbamate (DEDTC), morpholine-N-dithiocarbamate (MORDTC) and methoxycarbonodithioate (MEDTC)) and the newly designed complexes Ir2@DEDTC and Ir1@MEDTC were characterized by single crystal X-ray crystallography. Complexes containing thpy as C∧N ligand exhibit excellent photophysical properties such as red-shifted emission, high singlet oxygen quantum yield (ϕΔ) and longer photoluminescence lifetime when compared with complexes containing ppy ligands. Ir2@DEDTC exhibits the highest ϕΔ and photoluminescence lifetimes among the synthesized complexes. Therefore, Ir2@DEDTC was chosen to evaluate the photosensitizing ability to produce reactive oxygen species (ROS). Upon blue light irradiation (456 nm), it efficiently produces ROS, i.e., hydroxy radical (•OH) and singlet oxygen (1O2), which was confirmed by electron paramagnetic resonance (EPR) spectroscopy. In vitro photocytotoxicity toward HCT116, HeLa, and PC3 cell lines showed that out of all the synthesized complexes, Ir2@DEDTC has the highest photocytotoxic index (PI > 400) value. Ir2@DEDTC is efficiently taken up by the HCT116 cell line and accumulated mainly in the lysosome and mitochondria of the cells, and after PDT treatment, it elicits cell shrinkage, membrane blebbing, and DNA fragmentation. The phototherapeutic efficacy of Ir2@DEDTC has been investigated against 3D spheroids considering its ability to mimic some of the basic features of solid tumors. The morphology was drastically altered in the Ir2@DEDTC treated 3D spheroid after the light irradiation unleashed the potential of the Ir(III) dithiocarbamate complex as a superior PS for PDT. Hence, mitochondria and lysosome targeted photoactive cyclometalated Ir(III) dithiocarbamate complex exerting oxidative stress via both type I and type II PDT can be regarded as a dual-organelle targeted two-pronged approach for enhanced PDT.

Synthesis and Chemosensory Properties of New Cyanosubstituted 2,2'-Bipyridine Derivatives

A series of novel 6-oxo-1,6-dihydro-[2,2'-bipyridine]-5-carbonitriles has been synthesized and characterized. Their photophysical properties in DMSO solution and aqueous medium as well as fluorescence response to the presence of metal ions have been investigated. The obtained 4-(4-methoxyphenyl)-6-oxo-1,6-dihydro-[2,2'-bipyridine]-5-carbonitrile has been shown as a selective fluorescent "turn-ON" probe for Cd²⁺ ions with LoD 0.359 µM, 1:1 metal-ligand ratio and binding constant of 5.2 × 10⁴ M^-1.

A computational study on second-order nonlinear optical properties based on bis-cyclometalated Ir(iii) complexes: redox and substituent effects

A series of neutral Ir(iii) complexes that possess cyclometalated ligands (C^N) and different ancillary ligands, N-heterocyclic carbene (NHC) and their ionic complexes 1+/−–5+/− have been investigated using density functional theory.

Exploration of the Structural and Photophysical Characteristics of Mono- and Binuclear Ir(III) Cyclometalated Complexes for Optoelectronic Applications

Intrinsic characteristics possessed and exhibited by Ir(III) cyclometalated complexes need to be further examined, understood, and explored for greater value enhancement and potentiation. This work focuses primarily on the comparative studies of the ligand structures, types, and their substituent influence on the photophysical and optoelectronic properties of typical cyclometalated mono- and binuclear iridium(III) complexes in solution or solid states.

Mononuclear Ru(II) Complexes of an Arene and Asymmetrically Substituted 2,2'-Bipyridine Ligands: Photophysics, Computation, and NLO Properties

By using monosubstituted 2,2'-bipyridine asymmetric ancillary ligands with different electron donor moieties and an arene ligand (p-cymene), we successfully designed and synthesized six Ru(II) compounds (RuBPY1-6) that belong to a piano-stool-type system. The NLO properties of the synthesized complexes have been studied in both solution and the solid state. The electronic spectra of these compounds show a broad feature with two absorption bands in the visible window (350-650 nm). RuBPY1-6 complexes exhibit NIR emission spectra in the solution state (at >720 nm), the maxima of which are bathochromically shifted in comparison to those of the concerned ligands. Interestingly, compounds RuBPY1-6 show NIR emission in their solid state too. Title compounds RuBPY1-6 have lifetimes in the range of 0.2 to 0.9 ns. An important feature of this work is the π-association of the p-cymene ligand to Ru(II) in the synthesized complexes; the π complex is formed by breaking the symmetry of p-cymene, found in the starting precursor (Ru₂ dimer). This has been established by NMR spectral studies along with DFT calculations on the ¹H NMR spectra. We could derive the molecular structure of the cationic part of this system by density functional theory (DFT), associated with ¹H NMR spectral studies. The minimum energy structures for RuBPY1 and RuBPY2 have been optimized at DFT/B3LYP along with the LANL2DZ basis set for ruthenium atoms. These optimized structures are further considered to calculate the excited state properties using the TDDFT method. The electrochemical studies of the complexes, investigated in acetonitrile solution, show that this system is associated with a well-defined Ru(III)/Ru(II) reversible couple, rarely observed for a Ru(II) piano-stool-type compound, along with a feature of irreversible ligand oxidation. The absorption cross-section values, obtained from the two-photon absorption studies of title compounds RuBPY1-6, are worth reporting and lie in the range of 3-28 GM (in the femtosecond case).

Second-order nonlinear optical response of phenyl-substituted cationic BIS-cyclometalated iridium(III) complexes: Effect of different position

In this paper, a series of cationic iridium complexes [(2-phenylpyridine)2(2,2[Formula: see text]-bipyridine)Ir][Formula: see text] which substituted phenyl on different ligands position have been systematically investigated by density functional theory (DFT) method. Significantly, the first hyperpolarizability [Formula: see text] values can be enhanced by introducing phenyl on 2-phenylpyridine ligands R1 or R2, whereas substituting phenyl on 2,2[Formula: see text]-bipyridine ligands R3 result in a decreasing [Formula: see text] values. The [Formula: see text] values exhibit obvious connection with the corresponding HOMO and LUMO energy gap. Furthermore, the time-dependent (TD) DFT calculations suggest that the enhanced [Formula: see text] values are related to obvious charge transfer from 2-phenylpyridine ligands to 2,2[Formula: see text]-bipyridine ligands. The investigation of frequency-dependent first hyperpolarizability [Formula: see text] ([Formula: see text]; [Formula: see text], 0) and [Formula: see text] ([Formula: see text]; [Formula: see text], [Formula: see text]) shown less dispersion effect at the low-frequency region for all of the studied complexes. Overall, tuning phenyl on the different ligands position can be seen as an effective strategy to modulate the second-order nonlinear optical response for these iridium complexes, which is benefit to theoretical and experimental further investigation.

Visible-Light-Driven Hydrogen Production and Polymerization using Triarylboron-Functionalized Iridium(III) Complexes

The development of novel iridium(III) complexes has continued as an important area of research owing to their highly tunable photophysical properties and versatile applications. In this report, three heteroleptic dimesitylboron-containing iridium(III) complexes, [Ir(p-B-ppy)₂ (N^N)]⁺ {p-B-ppy=2-(4-dimesitylborylphenyl)pyridine; N^N=dipyrido[3,2-a:2',3'-c]phenazine (dppz) (1), dipyrido[3,2-d:2',3'-f]quinoxaline (dpq) (2), and 1,10-phenanthroline (phen) (3)}, were prepared and fully characterized electrochemically, photophysically, and computationally. Altering the conjugated length of the N^N ligands allowed us to tailor the photophysical properties of these complexes, especially their luminescence wavelength, which could be adjusted from λ=583 to 631 nm in CH₂ Cl₂ . All three complexes were evaluated as visible-light-absorbing sensitizers for the photogeneration of hydrogen from water and as photocatalysts for the photopolymerization of methyl methacrylate. The results showed that all of them were active in both photochemical reactions. High activity for the photosensitizer (over 1158 turnover numbers with 1) was observed, and the system generated hydrogen even after 20 h. Additionally, poly(methyl methacrylate) with a relatively narrow molecular-weight distribution was obtained if an initiator (i.e., ethyl α-bromophenylacetate) was used. The living character of the photoinduced polymerization was confirmed on the basis of successful chain-extension experiments.

Broadband femtosecond nonlinear optical properties of CsPbBr3 perovskite nanocrystals

We report the broadband nonlinear optical (NLO) properties of CsPbBr₃ perovskite films achieved from colloidal nanocrystals prepared following a room temperature and open atmosphere anti-solvent precipitation method. The NLO studies were performed on the films of nanocubes (NCs) and nanorods (NRs) using the Z-scan technique with 1 kHz femtosecond pulses at 600, 700, and 800 nm. Large two-photon absorption cross sections (∼10⁵  GM) were retrieved by fitting the open-aperture Z-scan data. Strong third-order NLO susceptibility (∼10^-10  esu) was observed in these films. At higher peak intensities a switching of sign (in both NCs and NRs) in the real and imaginary parts of the NLO susceptibility was observed from the studies on these CsPbBr₃ nanocrystals. The obtained NLO coefficients clearly suggest that these materials are promising for ultrafast photonic applications.

Water-soluble cyclometalated platinum(ii) and iridium(iii) complexes: synthesis, tuning of the photophysical properties, and in vitro and in vivo phosphorescence lifetime imaging

This paper presents synthesis and photophysical investigation of cyclometalated water-soluble Pt(ii) and Ir(iii) complexes containing auxiliary sulfonated diphosphine (bis(diphenylphosphino)benzene (dppb), P^P*) ligand. The complexes demonstrate considerable variations in excitation (extending up to 450 nm) and emission bands (with maxima ranging from ca. 450 to ca. 650 nm), as well as in the sensitivity of excited state lifetimes to molecular oxygen (from almost negligible to more than 4-fold increase in degassed solution). Moreover, all the complexes possess high two-photon absorption cross sections (400–500 GM for Pt complexes, and 600–700 GM for Ir complexes). Despite their negative net charge, all the complexes demonstrate good uptake by HeLa cells and low cytotoxicity within the concentration and time ranges suitable for two-photon phosphorescence lifetime (PLIM) microscopy. The most promising complex, [(ppy)₂Ir(sulfo-dppb)] (Ir1*), upon incubation in HeLa cells demonstrates two-fold lifetime variations under normal and nitrogen atmosphere, correspondingly. Moreover, its in vivo evaluation in athymic nude mice bearing HeLa tumors did not reveal acute toxicity upon both intravenous and topical injections. Finally, Ir1* demonstrated statistically significant difference in lifetimes between normal tissue (muscle) and tumor in macroscopic in vivo PLIM imaging.

Novel water-soluble iridium complexes with sulfonated diphosphine allow in vitro and in vivo lifetime hypoxia imaging.

Photophysical dynamics of the efficient emission and photosensitization of [Ir(pqi)2(NN)]+complexes

Rational photophysical investigation through experimental and theoretical analyses reveals the photophysical dynamics of the highly-emissive [Ir(pqi)₂(NN)]⁺complex series, with remarkable emission quantum yields and efficient generation of singlet oxygen.

A rare example of a compact heteroleptic cyclometalated iridium(iii) complex demonstrating well-separated dual emission

Cationic heteroleptic Ir(iii) complexes [Ir(C^N)₂(NN)][PF₆] exhibit unique singlet–triplet dual emission in solution with two well separated emission bands.

Using highly emissive and environmentally sensitive o-carborane-functionalized metallophosphors to monitor mitochondrial polarity

Mitochondria as vital intracellular organelles play critical roles in multiple physiological processes, and their polarity is a crucial characteristic that can reveal the intracellular environment and impact cellular events. In this work, we designed and synthesized a novel series of highly emissive and environmentally sensitive phosphorescent iridium(iii) complexes (2a-2e, 3a-3e and 4) functionalized by o-carborane. These complexes showed high emission quantum yields both in solution and in solid state (up to ΦPL = 0.82), long emission lifetime and tunable emission wavelength over 74 nm by introduction of a carboranyl motif in their ligands. Importantly, all the complexes have shown significant solvatochromic effects in contrast to the carborane-free control complex. Among them, complex 2d shows the highest sensitivity to polarity of solvents with a MPPS (maximum peak phosphorescence shift) value of 42 nm and clear dependence of phosphorescence lifetime on solvent polarity. Interestingly, complex 2d can easily penetrate into cells and preferentially distribute in mitochondria. To utilize these properties, the first phosphorescent imaging of mitochondrial polarity has been realized by photoluminescence lifetime imaging microscopy (PLIM), which can monitor mitochondria-relevant cellular processes such as cell apoptosis and distinguish cancer cells from normal cells. Compared to intensity-based sensing, lifetime-based detection is independent of the probe concentration, excitation power and photobleaching of probes, which can show high accuracy and reproducibility.