Recent advances on the construction of long-wavelength emissive supramolecular coordination complexes for photo-diagnosis and therapy

Recently, metal-based drugs have attracted relentless interest in the biomedical field. However, their short excitation/emission wavelengths and unsatisfactory therapeutic efficiency limit their biological applications in vivo. Currently, the second near-infrared window (NIR-II, 1000-1700 nm) provides more accurate imaging and therapeutic options. Thus, there has been a constant focus on developing multifunctional NIR metal agents for imaging and therapy that have deeper tissue penetration. Fortunately, supramolecular coordination complexes (SCCs) formed by the coordination-driven self-assembly of NIR-II emissive ligands can address the above issues. Importantly, metal receptors with chemotherapeutic properties in SCCs can bind to luminescent ligands, thus becoming a versatile therapeutic platform for chemotherapy, imaging and phototherapy. In this context, we systematically summarize the evolution of NIR-II emissive SCCs for biomedical applications and discuss future challenges and prospects.

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.

Nanoscale integration of porphyrin in GroEL protein cage: Photophysical and photochemical investigation

In this paper, we introduce a new type of functional, supramolecular porphyrin conjugate created using the bacterial GroEL protein cage based on non-specific hydrophobic interaction. The synthesis, structure and property of the porphyrin conjugate were characterized by dynamic light scattering, UV-vis spectroscopy and fluorescence spectroscopy. We observed that the model zinc-tetraphenylporphyrin (Zn-TPP) with high hydrophobicity can be well-dispersed in aqueous solutions with the aid of GroEL open chamber, which is known to be a favorable nanocompartment for aggregation-prone molecules. The maximal encapsulation efficiency of Zn-TPP in GroEL was determined to be ~98%. It is further seen that the constructed double Zn-TPP-GroEL complex exhibited good photocatalytic activity in the model reactions of the production of singlet oxygen and the reduction of methyl viologen under illumination with visible light. Moreover, we found that GroEL can significantly improve the photostability of Zn-TPP molecules as a result of nanoscale assembly within its hydrophobic chamber. Hence enhanced water solubility and photostability of Zn-TPP, which are considered as the first two hurdles for the wide usage of porphyrins, were achieved simultaneously by the development of GroEL cage as a building block. Supramolecular nanostructures formed from porphyrins (or related molecules) and GroEL for photocatalysis would greatly simplify applications of such structures.

Iridium(III)porphyrin arrays with tuneable photophysical properties

The photophysical properties of iridium(III) porphyrins complexes with two different axial ligands (Cl(CO) and bipyridine (bpy)) in solution and in cellulose acetate polymer matrix were investigated. The axial ligands substitution was made aiming to evaluate the photophysical properties and the solubility in different solvents. Therefore, dissimilar from the free porphyrin, non-polar solvents (as toluene) favours the quantum yield of iridium(III)porphyrins and ligands with a more extended π-conjugated compound as bpy results in higher yields. Moreover, despite all the porphyrins reveals a negative solvatochromism, the substitution of Cl(CO) ligand by bpy ligand exhibits similar solubility either on non-polar or polar solvents. The observed photoluminescence (PL) at room temperature appears at NIR region in contrast to the previously reported iridium(III) porphyrins. Comparing with free porphyrin H₂TTP, the red/NIR PL spectra of the iridium(III)porphyrins (either in solution and in the polymer matrix) reveals remarkable changes. Particularly, a more significative decrease of the red/NIR intensity ratio was detected for [Ir(ttp)(bpy)₂] 2 where the maxima of the NIR emission can be adjusted under suitable excitation wavelength.

Positional Isomers of Isocyanoazulenes as Axial Ligands Coordinated to Ruthenium(II) Tetraphenylporphyrin: Fine-Tuning Redox and Optical Profiles

Two isomeric ruthenium(II)/5,10,15,20-tetraphenylporphyrin complexes featuring axially coordinated redox-active, low-optical gap 2- or 6-isocyanoazulene ligands have been isolated and characterized by NMR, UV-vis, and magnetic circular dichroism (MCD) spectroscopic methods, high-resolution mass spectrometry, and single-crystal X-ray crystallography. The UV-vis and MCD spectra support the presence of the low-energy, azulene-centered transitions in the Q band region of the porphyrin chromophore. The first coordination sphere in new L₂RuTPP complexes reflects compressed tetragonal geometry. The redox properties of the new compounds were assessed by electrochemical and spectroelectrochemical means and correlated with the electronic structures predicted by density functional theory and CASSCF calculations. Both experimental and theoretical data are consistent with the first two reduction processes involving the axial azulenic ligands, whereas the oxidation profile (in the direction of increasing potential) is exerted by the ruthenium ion, the porphyrin core, and the axial azulenic moieties.