Re(1,2-diimine)(CO)3Cl complexes with the aliphatic bidentate ligands diiminosuccinodinitrile (DISN), benzilbisanile (BEAN), bisacetylbisanile (BABA), and benzildiimine (BEDIM) are reported. The compounds show Re(I) to π*(diimine) metal-to-ligand charge transfer (MLCT) absorptions in the visible spectral region. The energy of these MLCT-transitions decreases in the series BABA > BEDIM > BEAN > DISN, depending on the π* acceptor properties of the coordinated diimine ligands. The maxima of these CT bands undergo a solvent-dependent shift (negative solvatochromism), which indicates a partial charge redistribution in the excited state. The compounds are not photoluminescent at room temperature and 77 K.
In this work we present a rational design of the active part of third generation photosensitizers for photodynamic therapy based on phthalocyanine and an azaphthalocyanine core. The preferred zinc complexes of the AAAB type that contain bulky tert-butylsulfanyl substituents (A) and one carboxy group (B) have been synthesized by statistical condensation and fully characterized. The tetramerization was performed using magnesium(ii) butoxide followed by demetalation and insertion of ZnII. Compound 1 synthesized from 4,5-bis(tert-butylsulfanyl)phthalonitrile (A) and 2,3-dicyanoquinoxaline-6-carboxylic acid (B) exerted very promising photophysical properties (Q-band absorption at 726 nm, ϵ = 140000 M–1 cm–1), which allowed strong absorption of light at long wavelengths where the penetration of the light through human tissues is deeper. The very high singlet oxygen quantum yield of 1 (ΦΔ = 0.80) assures efficient photosensitization. As a result of bulky peripheral substituents, compound 1 shows good solubility in organic solvents with a low degree of aggregation, which makes it potentially viable for non-complicated modification. One carboxy group in the final structure of 1 allows simple binding to possible carriers. This compound is suitable for binding to targeting moieties to form the highly active part of a third-generation photosensitizer.