Rhenium(I) Complexes with Alkynylphosphane Ligands: Structural, Photophysical, and Theoretical Studies

Rhenium(I) Block Copolymers Based on Polyvinylpyrrolidone: A Successful Strategy to Water-Solubility and Biocompatibility

A series of diphosphine Re(I) complexes Re1-Re4 have been designed via decoration of the archetypal core {Re(CO)₂(N^N)} through the installations of the phosphines P⁰ and P¹ bearing the terminal double bond, where N^N = 2,2'-bipyridine (N^N1), 4,4'-di-tert-butyl-2,2'-bipyridine (N^N2) or 2,9-dimethyl-1,10-phenanthroline (N^N3) and P⁰ = diphenylvinylphosphine, and P¹ = 4-(diphenylphosphino)styrene. These complexes were copolymerized with the corresponding N-vinylpyrrolidone-based Macro-RAFT agents of different polymer chain lengths to give water-soluble copolymers of low-molecular p(VP-l-Re) and high-molecular p(VP-h-Re) block-copolymers containing rhenium complexes. Compounds Re1-Re4, as well as the copolymers p(VP-l-Re) and p(VP-h-Re), demonstrate phosphorescence from a ³MLCT excited state typical for this type of chromophores. The copolymers p(VP-l-Re#) and p(VP-h-Re#) display weak sensitivity to molecular oxygen in aqueous and buffered media, which becomes almost negligible in the model physiological media. In cell experiments with CHO-K1 cell line, p(VP-l-Re2) and p(VP-h-Re2) displayed significantly reduced toxicity compared to the initial Re2 complex and internalized into cells presumably by endocytic pathways, being eventually accumulated in endosomes. The sensitivity of the copolymers to oxygen examined in CHO-K1 cells via phosphorescence lifetime imaging microscopy (PLIM) proved to be inessential.

Computational Design of Rhenium(I) Carbonyl Complexes for Anticancer Photodynamic Therapy

New Re(I) carbonyl complexes are proposed as candidates for photodynamic therapy after investigating the effects of the pyridocarbazole-type ligand conjugation, addition of substituents to this ligand, and replacement of one CO by phosphines in [Re(pyridocarbazole)(CO)₃(pyridine)] complexes by means of the density functional theory (DFT) and time-dependent DFT. We have found, first, that increasing the conjugation in the bidentate ligand reduces the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gap of the complex, so its absorption wavelength red-shifts. When the enlargement of this ligand is carried out by merging the electron-withdrawing 1H-pyrrole-2,5-dione heterocycle, it enhances even more the stabilization of the LUMO due to its electron-acceptor character. Second, the analysis of the shape and composition of the orbitals involved in the band of interest indicates which substituents of the bidentate ligand and which positions are optimal for reducing the HOMO–LUMO energy gap. The introduction of electron-withdrawing substituents into the pyridine ring of the pyridocarbazole ligand mainly stabilizes the LUMO, whereas the HOMO energy increases primarily when electron-donating substituents are introduced into its indole moiety. Each type of substituents results in a bathochromic shift of the lowest-lying absorption band, which is even larger if they are combined in the same complex. Finally, the removal of the π-backbonding interaction between Re and the CO trans to the monodentate pyridine when it is replaced by phosphines PMe₃, 1,4-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane (DAPTA), and 1,4,7-triaza-9-phosphatricyclo[5.3.2.1]tridecane (CAP) causes another extra bathochromic shift due to the destabilization of the HOMO, which is low with DAPTA, moderate with PMe₃, but especially large with CAP. Through the combination of the PMe₃ or CAP ligands with adequate electron-withdrawing and/or electron-donating substituents at the pyridocarbazole ligand, we have found several complexes with significant absorption at the therapeutic window. In addition, according to our results on the singlet–triplet energy gap, all of them should be able to produce cytotoxic singlet oxygen.

Computations are crucial for proposing new Re(I) carbonyl complexes with very interesting features that make them promising compounds for photodynamic therapy.

Functionalizing Collagen with Vessel-Penetrating Two-Photon Phosphorescence Probes: A New In Vivo Strategy to Map Oxygen Concentration in Tumor Microenvironment and Tissue Ischemia

The encapsulation and/or surface modification can stabilize and protect the phosphorescence bio-probes but impede their intravenous delivery across biological barriers. Here, a new class of biocompatible rhenium (ReI ) diimine carbonyl complexes is developed, which can efficaciously permeate normal vessel walls and then functionalize the extravascular collagen matrixes as in situ oxygen sensor. Without protective agents, ReI -diimine complex already exhibits excellent emission yield (34%, λem   = 583 nm) and large two-photon absorption cross-sections (σ2   = 300 GM @ 800 nm) in water (pH 7.4). After extravasation, remarkably, the collagen-bound probes further enhanced their excitation efficiency by increasing the deoxygenated lifetime from 4.0 to 7.5 µs, paving a way to visualize tumor hypoxia and tissue ischemia in vivo. The post-extravasation functionalization of extracellular matrixes demonstrates a new methodology for biomaterial-empowered phosphorescence sensing and imaging.

Targeted Synthesis of NIR Luminescent Rhenium Diimine cis,trans-[Re( N N )(CO)2 (L)2 ]n+ Complexes Containing N-Donor Axial Ligands: Photophysical, Electrochemical, and Theoretical Studies

The combined action of ultraviolet irradiation and microwave heating onto acetonitrile solution of [Re( N N )(CO)₃ (NCMe)]OTf ( N N =phenantroline and neocuproine) afforded cis,trans-Re( N N )(CO)₂ (NCMe)₂ ]⁺ acetonitrile derivatives. Substitution of relatively labile NCMe with a series of aromatic N-donor ligands (pyridine, pyrazine, 4,4'-bipyridine, N-methyl-4,4'-bipyridine) gave a novel family of the diimine cis,trans-[Re( N N )(CO)₂ (L)₂ ]⁺ complexes. Photophysical studies of the obtained compounds in solution revealed unusually high absorption across the visible region and NIR phosphorescence with emission band maxima ranging from 711 to 805 nm. The nature of emissive excited states was studied using DFT calculations to show dominant contribution of ³ MLCT (dπ(Re)→π*( N N )) character. Electrochemical (CV and DPV) studies of the monocationic diimine complexes revealed one reduction and one oxidation wave assigned to reduction of the diimine moiety and oxidation of the rhenium center, respectively.

Cross-Conjugated Systems Based On An (E)-Hexa-3-en-1,5-diyne-3,4-diyl Skeleton: Spectroscopic and Spectroelectrochemical Investigations

A series of cross-conjugated compounds based on an (E)-4,4'-(hexa-3-en-1,5-diyne-3,4-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) skeleton (1-6) have been synthesized. The linear optical absorption properties can be tuned by modification of the substituents at the 1 and 5 positions of the hexa-3-en-1,5-diynyl backbone (1: Si(CH(CH3)2)3, 2: C6H4C≡CSi(CH3)3, 3: C6H4COOCH3, 4: C6H4CF3, 5: C6H4C≡N, 6: C6H4C≡CC5H4N), although attempts to introduce electron-donating (C6H4CH3, C6H4OCH3, C6H4Si(CH3)3) substituents at these positions were hampered by the ensuing decreased stability of the compounds. Spectroelectrochemical investigations of selected examples, supported by DFT-based computational studies, have shown that one- and two-electron oxidation of the 1,2-bis(triarylamine)ethene fragment also results in electronic changes to the perpendicular π-system in the hexa-3-en-1,5-diynyl branch of the molecule. These properties suggest that (E)-hexa-3-en-1,5-diynyl-based compounds could have applications in molecular sensing and molecular electronics.