Water-Soluble Tris(cyclometalated) Iridium(III) Complexes for Aqueous Electron and Energy Transfer Photochemistry

Aggregation-induced emission luminogens for latent fingerprint detection

For over a century, fingerprints have served as a pivotal tool for identification of individuals owing to their enduring characteristics and easily apparent features, particularly in the realm of criminal investigations. Latent fingerprints (LFPs) are "invisible fingerprints" that are most commonly available at crime scenes and require a rapid, selective, sensitive, and convenient method for detection. However, existing fingerprint development techniques harbour limitations, prompting the exploration of novel approaches that prioritize investigator safety and environmental sustainability. Leveraging the unique photophysical properties of aggregation-induced emission luminogens (AIEgens) has emerged as a promising strategy for on-site analysis of LFP visualization. In this highlight, we have presented a comparative analysis of various AIEgens (organic compounds, metal complexes, nanoparticles, and polymers) for the development and detection of LFPs. Through this examination, insights into the efficiency and potential applications of AIE-based fingerprint development techniques are provided. In addition, several strategies have been proposed for circumventing the limitations of existing AIEgens. We hope that this highlight article will encourage more researchers to investigate AIEgens in LFP detection, contributing to forensic science.

Impact of Anchoring Groups on the Photocatalytic Performance of Iridium(III) Complexes and Their Toxicological Analysis

Three different iridium(III) complexes, labelled as Ir1-Ir3, each bearing a unique anchoring moiety (diethyl [2,2'-bipyridine]-4,4'-dicarboxylate, tetraethyl [2,2'-bipyridine]-4,4'-diylbis(phosphonate), or [2,2'-biquinoline]-4,4'-dicarboxylic acid), were synthesized to serve as photosensitizers. Their electrochemical and photophysical characteristics were systematically investigated. ERP measurements were employed to elucidate the impact of the anchoring groups on the photocatalytic hydrogen generation performance of the complexes. The novel iridium(III) complexes were integrated with platinized TiO2 (Pt-TiO2) nanoparticles and tested for their ability to catalyze hydrogen production under visible light. A H2 turnover number (TON) of up to 3670 was obtained upon irradiation for 120 h. The complexes with tetraethyl [2,2'-bipyridine]-4,4'-diylbis(phosphonate) anchoring groups were found to outperform those bearing other moieties, which may be one of the important steps in the development of high-efficiency iridium(III) photosensitizers for hydrogen generation by water splitting. Additionally, toxicological analyses found no significant difference in the toxicity to luminescent bacteria of any of the present iridium(III) complexes compared with that of TiO2, which implies that the complexes investigated in this study do not pose a high risk to the aquatic environment compared to TiO2.

A ratiometric substrate for rapid evaluation of transfer hydrogenation efficiency in solution

A cyclometalated iridium(III) complex bearing a self-immolative quinolinium moiety was developed as a ratiometric substrate for transfer hydrogenation studies. This photoluminescent probe allowed the rapid screening of a variety of Ir catalysts using a microplate reader, offering a convenient method to assess activity using a minimum amount of catalyst sample.

An ALP-responsive, anionic iridium complex for specific recognition of osteosarcoma cells

Poor cellular permeability greatly hampers the utilization of anionic Ir(III) complexes, though efficiently emissive and remarkably stable, in cell-based diagnosis. To overcome this barrier, we present the development of an alkaline phosphatase (ALP)-responsive, anionic, and aggregation-induced emission (AIE)-active Ir(III) complex (Ir1) for specific recognition of osteosarcoma cells. Containing phosphate moieties, Ir1 exhibits a net -1 charge, enabling charge repulsion from the cell membrane and resulting in low cellular uptake and good biocompatibility in normal osteoblast cells. Upon ALP-mediated hydrolysis of phosphate groups, the resulting dephosphorylated product, Ir2, demonstrates a positive charge and increased lipophilicity, promoting cellular uptake and activating its AIE properties for specific recognition of osteosarcoma cells that express elevated levels of ALP. This study elucidates the role of ALP as an ideal trigger for enhancing the cellular permeability of phosphate ester-containing Ir(III) complexes, thus expanding the potential of anionic Ir(III) complexes for biomedical applications.

Electrochemiluminescence of a First-Row d6 Transition Metal Complex

We report the electrochemiluminescence (ECL) of a 3d6 Cr(0) complex ([Cr(LMes)3]; λem=735 nm) with comparable photophysical properties to those of ECL-active complexes of 4d6 or 5d6 precious metal ions. The electrochemical potentials of [Cr(LMes)3] are more negative than those of [Ir(ppy)3] and render the [Cr(LMes)3]* excited state inaccessible through conventional co-reactant ECL with tri-n-propylamine or oxalate. ECL can be obtained, however, through the annihilation route in which potentials sufficient to oxidise and reduce the luminophore are alternately applied. When combined with [Ir(ppy)3] (λem=520 nm), the annihilation ECL of [Cr(LMes)3] was greatly enhanced whereas that of [Ir(ppy)3] was diminished. Under appropriate conditions, the relative intensities of the two spectrally distinct emissions can be controlled through the applied potentials. From this starting point for ECL with 3d6 metal complexes, we discuss some directions for future development.

Aqueous emissive cyclometalated iridium photoreductants: synthesis, computational analysis and the photocatalytic reduction of 4-nitrophenol

Herein, we present luminescent mononuclear iridium complexes [1]3+-[4]3+ using NEt3-appended C^N chelating benzimidazole (L1-L4) and semi-flexible phenanthroline-pyrazine-based (phpy) ligands exhibiting photocatalytic reduction of 4-nitrophenol (4-NP) in the presence of NEt3 in an aqueous medium. The formation of [1]3+-[4]3+ was confirmed by HRMS, 1H-1H COSY, and 13C and 19F NMR spectroscopy. The complex [4]3+ is water soluble, whereas the others ([1]3+-[3]3+) are partially soluble. The complexes are luminescent in both CH3CN and H2O media. The DFT study reveals that the HOMO of [1]3+ resides on the C^N chelating benzimidazole and iridium center. However, it moves to the pyrazine-pyridine of the phpy unit in the case of [2]3+-[4]3+. The LUMOs are localized on the phenanthroline unit of phpy for all the complexes. This suggests an important role of the fluorine atom on electron density distribution. Spin density analysis demonstrates that the emission bands of the complexes arise from 3MLLCT states. The complex [4]3+ displays promising photocatalytic activity towards 4-NP photoreduction, whereas complexes [1]3+-[3]3+ exhibit lower reactivity. The mechanistic study suggests that the reaction proceeds through an oxidative quenching pathway, where 4-NP is reduced by accepting an electron from excited [Ir(III)] and gets oxidized to Ir(IV), which comes back to its original Ir(III) state by accepting an electron from the sacrificial electron donor NEt3.

Acetone Serving as a Solvent and Interaction Partner Promotes the Direct Olefination of N-Tosylhydrazones under Visible Light

The photochemistry of noncovalent interactions to promote organic transformations is an emerging approach to providing fresh opportunities in synthetic chemistry. Generally, the external substance is necessary to add as an interaction partner, thereby sacrificing the atom economy of the reaction. Herein, we describe a catalyst-free and noncovalent interaction-mediated strategy to access the olefination of N-tosylhydrazones using acetone as a solvent and an interaction partner. This protocol also features broad substrate scope, excellent functional group compatibility, and mild reaction conditions without transition metals. Moreover, the gram-scale synthesis of olefins and the generation of pharmaceutical intermediates highlighted its practical applicability. Lastly, mechanistic studies indicate that the reaction was initiated via noncovalent interactions between acetone and N-tosylhydrazone anion, which is also supported by density functional theory calculations.

Isoacridone dyes with parallel reactivity from both singlet and triplet excited states for biphotonic catalysis and upconversion

Metal-based photosensitizers commonly undergo quantitative intersystem crossing into photoactive triplet excited states. In contrast, organic photosensitizers often feature weak spin-orbit coupling and low intersystem crossing efficiencies, leading to photoactive singlet excited states. By modifying the well-known acridinium dyes, we obtained a new family of organic photocatalysts, the isoacridones, in which both singlet- and triplet-excited states are simultaneously photoactive. These new isoacridone dyes are synthetically readily accessible and show intersystem crossing efficiencies of up to 52%, forming microsecond-lived triplet excited states (T1), storing approximately 1.9 eV of energy. Their photoactive singlet excited states (S1) populated in parallel have only nanosecond lifetimes, but store ∼0.4 eV more energy and act as strong oxidants. Consequently, the new isoacridone dyes are well suited for applications requiring parallel triplet-triplet energy transfer and photoinduced electron transfer elementary steps, which have become increasingly important in modern photocatalysis. In proof-of-principle experiments, the isoacridone dyes were employed for Birch-type arene reductions and C-C couplings via sensitization-initiated electron transfer, substituting the commonly used iridium or ruthenium based photocatalysts. Further, in combination with a pyrene-based annihilator, sensitized triplet-triplet annihilation upconversion was achieved in an all-organic system, where the upconversion quantum yield correlated with the intersystem crossing quantum yield of the photosensitizer. This work seems relevant in the greater contexts of developing new applications that utilize biphotonic photophysical and photochemical behavior within metal-free systems.

Visible light-induced iridium(III)-sensitized [2 + 2] and [3 + 2] photocycloadditions of 2-cyanochromone with alkenes

2-Cyanochromone (1) readily undergoes visible light-induced photocycloadditions with diverse alkene partners mediated by (Ir[dF(CF₃)ppy]₂(dtbpy))PF₆ as the photosensitizer. While mono-, di- and trisubstituted styrenes and acrylonitriles as the reactants lead to [2 + 2] cycloadducts with good regiocontrol and high diastereoselectivity, the use of trialkyl-substituted alkenes allows for the isolation of cyclopentenone-fused chromones resulting from a [3 + 2] cycloaddition process in moderate yields.

Metal-Organic Bichromophore Lowers the Upconversion Excitation Power Threshold and Promotes UV Photoreactions

Sensitized triplet-triplet annihilation upconversion is a promising strategy to use visible light for chemical reactions requiring the energy input of UV photons. This strategy avoids unsafe ultraviolet light sources and can mitigate photo-damage and provide access to reactions, for which filter effects hamper direct UV excitation. Here, we report a new approach to make blue-to-UV upconversion more amenable to photochemical applications. The tethering of a naphthalene unit to a cyclometalated iridium(III) complex yields a bichromophore with a high triplet energy (2.68 eV) and a naphthalene-based triplet reservoir featuring a lifetime of 72.1 μs, roughly a factor of 20 longer than the photoactive excited state of the parent iridium(III) complex. In combination with three different annihilators, consistently lower thresholds for the blue-to-UV upconversion to crossover from a quadratic into a linear excitation power dependence regime were observed with the bichromophore compared to the parent iridium(III) complex. The upconversion system composed of the bichromophore and the 2,5-diphenyloxazole annihilator is sufficiently robust under long-term blue irradiation to continuously provide a high-energy singlet-excited state that can drive chemical reactions normally requiring UV light. Both photoredox and energy transfer catalyses were feasible using this concept, including the reductive N-O bond cleavage of Weinreb amides, a C-C coupling reaction based on reductive aryl debromination, and two Paternò-Büchi [2 + 2] cycloaddition reactions. Our work seems relevant in the context of developing new strategies for driving energetically demanding photochemistry with low-energy input light.

Bimodal Electrochemiluminescence Microscopy of Single Cells

Electrochemiluminescence (ECL) microscopy is an emerging technique with new applications such as imaging of single entities and cells. Herein, we have developed a bimodal and bicolor approach to record both positive ECL (PECL: light-emitting object on dark background) and shadow label-free ECL (SECL: nonemissive object shadowing the background luminescence) images of single cells. This bimodal approach is the result of the simultaneous emissions of [Ru(bpy)₃]²⁺ used to label the cellular membrane (PECL) and [Ir(sppy)₃]^3- dissolved in solution (SECL). By spectrally resolving the ECL emission wavelengths, we recorded the images of the same cells in both PECL and SECL modes using the [Ru(bpy)₃]²⁺ (λ_max = 620 nm) and [Ir(sppy)₃]^3- (λ_max = 515 nm) luminescence, respectively. PECL shows the distribution of the [Ru(bpy)₃]²⁺ labels attached to the cellular membrane, whereas SECL reflects the local diffusional hindrance of the ECL reagents by each cell. The high sensitivity and surface-confined features of the reported approach are demonstrated by imaging cell-cell contacts during the mitosis process. Furthermore, the comparison of PECL and SECL images demonstrates the differential diffusion of tri-n-propylamine and [Ir(sppy)₃]^3- through the permeabilized cell membranes. Consequently, this dual approach enables the imaging of the morphology of the cell adhering on the surface and can significantly contribute to multimodal ECL imaging and bioassays with different luminescent systems.

Photochemically Induced Cyclometalations at Simple Platinum(II) Precursors

Photochemical cycloplatinations of 2-arylpyridines and related C∧N ligands, as well as terdentate heteroaromatic N∧N∧C, N∧C∧N, and N∧C∧C compounds, are demonstrated using (Bu₄N)₂[Pt₂Cl₆] or [PtCl₂(NCPh)₂] as precursors at room temperature. Mono- or bis-cyclometalated Pt(II) complexes with C∧N ligands are obtained depending on excitation wavelength and precursor. Monitoring experiments show that photoexcitation enables both the N-coordination and the subsequent C-H metalation. Photochemical synthetic protocols have been developed, which are advantageous with respect to the established thermal procedures and have allowed the synthesis of the first Pt(II) complexes with N∧C∧C ligands.

Luminescence and Palladium: The Odd Couple

The synthesis, photophysical properties, and applications of highly fluorescent and phosphorescent palladium complexes are reviewed, covering the period 2018–2022. Despite the fact that the Pd atom appears closely related with an efficient quenching of the fluorescence of different molecules, different synthetic strategies have been recently optimized to achieve the preservation and even the amplification of the luminescent properties of several fluorophores after Pd incorporation. Beyond classical methodologies such as orthopalladation or the use of highly emissive ligands as porphyrins and related systems (for instance, biladiene), new concepts such as AIE (Aggregation Induced Emission) in metallacages or in coordination-driven supramolecular compounds (CDS) by restriction of intramolecular motions (RIM), or complexes showing TADF (Thermally Activated Delayed Fluorescence), are here described and analysed. Without pretending to be comprehensive, selected examples of applications in areas such as the fabrication of lighting devices, biological markers, photodynamic therapy, or oxygen sensing are also here reported.

Red light-driven electron sacrificial agents-free photoreduction of inert aryl halides via triplet-triplet annihilation

Selective photoactivation of inert aryl halides is a fundamental challenge in organic synthesis. Specially, the long-wavelength red light is more desirable than the widely-applied blue light as the excitation source for photoredox catalysis, due to its superior penetration depth. However, the long-wavelength red light-driven photoactivation of inert aryl halides remains a challenge, mainly because of the low energy of the single long-wavelength red photon. Herein, we report the photoreduction of aryl bromides/chlorides with 656 nm LED via triplet-triplet annihilation (TTA) strategy. This method is based on our discovery that the commonly used chromophore of perylene can serve as an efficient and metal-free photocatalyst to enable the photoreduction of inert aryl halides without the conventional need for electronic sacrificial agents. By introducing a red light-absorbing photosensitizer to this perylene system, we accomplish the long-wavelength red light-driven photoreduction of aryl halides via sensitized TTA mechanism. Moreover, the performance of such a TTA-mediated photoreduction can be significantly enhanced when restricting the rotation freedom of phenyl moiety for perylene derivatives to suppress their triplet nonradiative transition, in both small and large-scale reaction settings.

Blue-to-UVB Upconversion, Solvent Sensitization and Challenging Bond Activation Enabled by a Benzene-Based Annihilator

Several energy-demanding photoreactions require harsh UV light from inefficient light sources. The conversion of low-energy visible light to high-energy singlet states via triplet-triplet annihilation upconversion (TTA-UC) could offer a solution for driving such reactions under mild conditions. We present the first annihilator with an emission maximum in the UVB region that, combined with an organic sensitizer, is suitable for blue-to-UVB upconversion. The annihilator singlet was successfully employed as an energy donor in subsequent FRET activations of aliphatic carbonyls. This hitherto unreported UC-FRET reaction sequence was directly monitored using laser spectroscopy and applied to mechanistic irradiation experiments demonstrating the feasibility of Norrish chemistry. Our results provide clear evidence for a novel blue light-driven substrate or solvent activation strategy, which is important in the context of developing more sustainable light-to-chemical energy conversion systems.

Photocatalytic Regeneration of a Nicotinamide Adenine Nucleotide Mimic with Water-Soluble Iridium(III) Complexes

Nicotinamide adenine nucleotide (NADH) is involved in many biologically relevant redox reactions, and the photochemical regeneration of its oxidized form (NAD⁺) under physiological conditions is of interest for combined photo- and biocatalysis. Here, we demonstrate that tri-anionic, water-soluble variants of typically very lipophilic iridium(III) complexes can photo-catalyze the reduction of an NAD⁺ mimic in a comparatively efficient manner. In combination with a well-known rhodium co-catalyst to facilitate regioselective reactions, these iridium(III) photo-reductants outcompete the commonly used [Ru(bpy)₃]²⁺ (bpy = 2,2'-bipyridine) photosensitizer in water by up to 1 order of magnitude in turnover frequency. This improved reactivity is attributable to the strong excited-state electron donor properties and the good chemical robustness of the tri-anionic iridium(III) sensitizers, combined with their favorable Coulombic interaction with the di-cationic rhodium co-catalyst. Our findings seem relevant in the greater context of photobiocatalysis, for which access to strong, efficient, and robust photoreductants with good water solubility can be essential.

Unravelling the Effect of Water Addition in Consecutive Photocatalysis with Naphthalene Diimide

We describe the effect of water addition in the catalyst performance for a C-H functionalization of benzene. Improved yields and selectivity were achieved in a consecutive photoredox catalysis in contrast to the reaction without water. Mechanistic analysis demonstrated this is due to a better catalyst stability and faster kinetic rather than a change in the different steps of the mechanism. The addition of water constitutes a convenient approach to improve catalyst performance, and it was also observed with other catalysts.

Cyclometalated Spirobifluorene Imidazolylidene Platinum(II) Complexes with Predominant 3LC Emissive Character and High Photoluminescence Quantum Yields

Two novel bidentate C^C*_spiro cyclometalated platinum(II) complexes comprising a spiro-conjugated bifluorene ligand and different β-diketonate auxiliary ligands are synthesized and characterized. Their preparation employs a robust and elaborate synthetic protocol commencing with an N-heterocyclic carbene precursor. Structural characterization by means of NMR techniques and solid-state structures validate the proposed and herein presented molecular scaffolds. Photophysical studies, including laser flash photolysis methods, reveal an almost exclusively ligand-centered triplet state, governed by the C^C*_spiro-NHC ligand. The high triplet energies and the long triplet lifetimes in the order of 30 μs in solution make the complexes good candidates for light-emitting diode-driven photocatalysis, as initial energy transfer experiments reveal. In-depth time-dependent density functional theory investigations are in excellent accordance with our spectroscopic findings. The title compounds are highly emissive in the bluish-green color region with quantum yields of up to 87% in solid-state measurements.