Understanding the photophysical properties of rhenium(I) compounds coordinated to 4,7-diamine-1,10-phenanthroline: synthetic, luminescence and biological studies.

In the present study, the photophysical properties and preliminary time-dependent density functional theory (TD-DFT) data of new rhenium(i) polypyridyl compounds, fac-[Re(L)(Am2phen)(CO)3]0/+, where Am2phen = 4,7-diamine-1,10-phenanthroline and L = Cl and ethyl isonicotinate (et-isonic), provided new insights into excited-state deactivation through an unusual inversion between two metal-to-ligand charge-transfer excited states. In addition, their cellular uptake using breast cancer (MCF-7) and melanoma (SkMel-147 and SkMel-29) cell lines and bioactivity were investigated and their cell-killing mechanism and protein expression were also studied. Preliminary TD-DFT results showed that both compounds exhibited a strong and broad absorption band around 300-400 nm which corresponds to a combination of ILAm2phen and MLCTRe→Am2phen transitions, and a strong contribution of charge transfer transition MLCTRe→et-isonic for fac-[Re(et-isonic)(Am2phen)(CO)3]+ is also observed. In contrast to typical Re(i) polypyridyl complexes, the substitution of Cl with the et-isonic ligand showed a bathochromic shift of the emission maxima, relatively low emission quantum yield and fast lifetime. Photophysical investigation of the fac-[ReCl(et-isonic)2(CO)3] compound provided meaningful information on the excited state manifold of the fac-[Re(L)(Am2phen)(CO)3]0/+ complexes. As shown in the absorption profile, a remarkable inversion of the lowest-lying excited state takes place from the usually observed MLCTRe→Am2phen to the unusual MLCTRe→et-isonic. The lipophilicity of the positive-complex was higher than that of the non-charge compound and the same trend for the activity against cells was observed, in the absence of light. In addition, flow cytometry and Western Blot analyses showed an overexpression of pro-caspase-9, suggesting a caspase proteolytic cascade through an intrinsic-pathway apoptosis mechanism. The photophysical properties of these compounds reported herein provide new fundamental insights into the understanding of substituent groups on polypyridyl ligands which are relevant to practical development.

Photoisomerization of di-nuclear rhenium(i) bpe-based compounds.

Di-nuclear [{(NN)(CO)3Re}2(trans-bpe)](PF6)2 complexes, NN = 4,7-diphenyl-1,10-phenanthroline (ph2phen) or 4,7-dichloro-1,10-phenanthroline (Cl2phen) and trans-bpe = trans-1,2-bis(4-pyridyl)ethylene, were synthesized and characterized by 1H NMR and UV-visible spectroscopy. Irradiation of acetonitrile solutions led to 1H NMR and UV-visible spectral changes ascribed to trans-to-cis photoisomerization processes showing that the presence of another Re(i)-moiety does not preclude the photoisomerization process. Quantum yields for the di-nuclear isomerization of [{(ph2phen)(CO)3Re}2(trans-bpe)]2+ were higher than those for the corresponding mono-nuclear compound process. Additionally, the higher quantum yield obtained with 254 nm irradiation for [{(Cl2phen)(CO)3Re}2(trans-bpe)]2+ showed, for the first time, that the singlet pathway could also be accessed in addition to the usually observed triplet one. The luminescence in fluid solution observed for the [{(NN)(CO)3Re}2(cis-bpe)]2+ complexes is able to efficiently photosensitize the generation of singlet O2.

Photophysical properties of rhenium(i) complexes and photosensitized generation of singlet oxygen.

fac-[Re(ampy)(CO)3(NN)]+ complexes (ampy = 2-aminomethylpyridine and NN = 1,10-phenanthroline (phen), and 2,2'-bipyridine (bpy)) were synthesized, purified and characterized by proton nuclear magnetic resonance (1H NMR), UV-visible and Fourier-transformed infrared (FT-IR) spectroscopies, and their photophysical properties were investigated using steady state and time-resolved emission spectroscopies. The electronic absorption spectra exhibit two main absorption bands: the higher energy band, which was assigned to intraligand transition (IL), and the lower energy band assigned to metal-to-ligand charge transfer (MLCT). Both complexes showed emission at room temperature in a CH3CN solution (λmax = 560 nm, ϕ = 0.091, τ = 560 ns for fac-[Re(ampy)(CO)3(phen)]+; λmax = 568 nm, ϕ = 0.024, τ = 100 ns for fac-[Re(ampy)(CO)3(bpy)]+) and rigid media (λmax = 530 nm, τ = 3300 ns for fac-[Re(ampy)(CO)3(phen)]+; λmax = 530 nm, τ = 853 ns for fac-[Re(ampy)(CO)3(bpy)]+) arising from the lowest lying 3MLCTRe→NN excited state. Both complexes along with fac-[Re(L)(CO)3(NN)]+/0 complexes, L = Cl or pyridine, were capable of efficiently photosensitizing the generation of singlet oxygen with quantum yield in the range of 0.59-0.28. These results highlight the potential application of fac-[Re(L)(CO)3(NN)]+/0 complexes in the development of sensitizers for the generation of singlet oxygen.