Synthesis, Spectroscopic, and 1O2 Sensitization Characteristics of Extended Pd(II) 10,10-Dimethylbiladiene Complexes Bearing Alkynyl-Aryl Appendages.

Photodynamic therapy (PDT), which involves the photoinduced sensitization of singlet oxygen, is an attractive treatment for certain types of cancer. The development of new photochemotherapeutic agents remains an important area of research. Macrocyclic tetrapyrrole compounds including porphyrins, phthalocyanines, chlorins, and bacteriochlorins have been pursued as sensitizers of singlet oxygen for PDT applications but historically are difficult to prepare/purify and can also suffer from high nonspecific dark toxicity, poor solubility in biological media, and/or slow clearance from biological tissues. In response to these shortcomings, we have developed a series of novel linear tetrapyrrole architectures complexed to late transition metals as potential PDT agents. We find that these dimethylbiladiene (DMBil1) tetrapyrrole complexes can efficiently photosensitize generation of 1O2 oxygen upon irradiation with visible light. To extend the absorption profile of the DMBil1 platform, alkynyl-aryl groups have been conjugated to the periphery of the tetrapyrrole using Sonogashira methods. Derivatives of this type containing ancillary phenyl (DMBil-PE), naphthyl (DMBil-NE), and anthracenyl (DMBil-AE) groups have been prepared and characterized. In addition to structurally characterizing Pd[DMBil-NE] and Pd[DMBil-AE], we find that extension of the tetrapyrrole conjugation successfully red-shifts the absorption of the DMBil-Ar family of biladienes further into the phototherapeutic window (i.e., 600-900 nm). Photochemical sensitization studies demonstrate that our series of new palladium biladiene complexes (Pd[DMBil-Ar]) can sensitize the formation of 1O2 with quantum yields in the range ΦΔ = 0.59-0.73 upon irradiation with light of λ ≥ 650 nm. The improved absorption properties of the Pd[DMBil-Ar] complexes in the phototherapeutic window, together with their high 1O2 quantum yields, highlight the promise of these compounds as potential agents for PDT.

Synthesis, Electrochemistry, and Photophysics of Pd(II) Biladiene Complexes Bearing Varied Substituents at the sp3-Hybridized 10-Position.

A set of Pd(II) biladiene complexes bearing different combinations of methyl- and phenyl-substituents on the sp3-hybridized meso-carbon (the 10-position of the biladiene framework) was prepared and studied. In addition to a previously described Pd(II) biladiene complex bearing geminal dimethyl substituents a the 10-position (Pd[DMBil]), homologous Pd(II) biladienes bearing geminal methyl and phenyl substituents (Pd[MPBil1]) and geminal diphenyl groups(Pd[DPBil1]) were prepared and structurally characterized. Detailed electrochemical as well as steady-state and time-resolved spectroscopic experiments were undertaken to evaluate the influence of the substituents on the biladiene's tetrahedral meso-carbon. Although all three biladiene homologues are isostructural, Pd[MPBil1] and Pd[DPBil1] display more intense absorption profiles that shift slightly toward lower energies as geminal methyl groups are replaced by phenyl rings. All three biladiene homologues support a triplet photochemistry, and replacement of the geminal dimethyl substituents of Pd[DMBil1] (ΦΔ = 54%) with phenyl groups improves the ability of Pd[MPBil1] (ΦΔ = 76%) and Pd[DPBil1] (ΦΔ = 66%) to sensitize 1O2. Analysis of the excited-state dynamics of the Pd(II) biladienes by transient absorption spectroscopy shows that each complex supports a long-lived triplet excited-state (i.e., τ > 15 µs for each homologue) but that the ISC quantum yields (ΦT) varied as a function of biladiene substitution. The observed trend in ISC efficiency matches that for singlet oxygen sensitization quantum yields (ΦΔ) across the biladiene series considered in this work. The results of this study provide new insights to guide future development of biladiene based agents for PDT and other photochemical applications.

C-Term Faraday Rotation in Low Symmetry tert-Butyl Substituted Polyferroceniums.

Molecular thin films are currently being investigated as candidate materials to replace conventional atomistic inorganic crystal-based Faraday rotators. High symmetry paramagnetic species have been reported to exhibit large Verdet constants via magnetic field splitting of degenerate ground states. However, lower symmetry open-shell species have not been extensively studied. Herein, we report the Faraday rotation of two poly di-tert-butylferroceniums with diphenylsilane and vinylene linkers. Thin films of oxidized poly[(1,1'-di-tert-butylferrocenyl)diphenylsilane] [poly(tBu2fc-SiPh2)] displayed a 30% increase in maximum Verdet constant relative to the previously reported decamethylferrocenium/PMMA composite, with Verdet constants of -4.52 × 104 deg T-1 m-1 at 730 nm and 4.46 × 104 deg T-1 m-1 at 580 nm. When a sp2-type linker was used, as with the oxidized poly(1,1'-di-tert-butyl-ferrocenylene)vinylene [poly(tBu2fc-C═C)], negligible Faraday rotation was observed. Hence, Faraday rotation can be maintained when molecular symmetry is broken, however orbital symmetry breaking in optical transitions of interest leads to a significant loss in magneto-optical activity.