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Facilitating photoinduced electron transfer (PET) while minimizing rapid charge-recombination processes to produce a long-lived charge-separated (CS) state represents a primary challenge associated with achieving efficient solar fuel production. Natural photosynthetic systems employ intermolecular interactions to arrange the electron-transfer relay in reaction centers and promote a directional flow of electrons. This work explores a similar tactic through the synthesis and ground- and excited-state characterization of two Cu(I)bis(phenanthroline) chromophores with homoleptic and heteroleptic coordination geometries and which are functionalized with negatively charged sulfonate groups. The addition of sulfonate groups enables solubility in pure water, and it also induces assembly with the dicationic electron acceptor methyl viologen (MV

Two new Re(I)bipyridyltricarbonyl chloride complexes, Re(BB3)(CO)3Cl and Re(BB4)(CO)3Cl, featuring BODIPY groups appended to the 5,5'- or 6,6'-positions of the bipyridine ligand, respectively, were synthesized as structurally isomeric compliments to a previously reported 4,4'-substituted homologue, Re(BB2)(CO)3Cl. X-ray crystal structures of the compounds show that the 4,4'-, 5,5'-, and 6,6'-substitution patterns place the BODIPY groups at progressively shorter distances of 9.43, 8.39, and 5.56 A, respectively, from the complexes' Re centers. The photophysical properties of the isomeric complexes were investigated to ascertain the manner in which the heavy rhenium atom might induce intersystem crossing of the pendant BODIPY moieties positioned at progressively shorter through-space distances. Electronic absorption spectroscopy revealed that the three metal complexes retain the strong visible absorption features characteristic of the bpyBODIPY (BB2-BB4) ligands; however, the fluorescence of the parent borondipyrromethane appended ligands is attenuated by more than an order of magnitude in Re(BB2)(CO)3Cl and Re(BB3)(CO)3Cl and by more than two orders of magnitude in Re(BB4)(CO)3Cl. Furthermore, phosphorescence from Re(BB4)(CO)3Cl is observed under a nitrogen atmosphere, consistent with highly efficient ISC to the triplet-excited state. Singlet oxygen sensitization studies confirm that all three complexes produce singlet oxygen with quantum yields that increase as the distance of the BODIPY groups to the heavy rhenium center is decreased. The trends observed across the series of rhenium complexes with respect to emission and 1O2 sensitization properties can be rationalized in terms of the varied distal separation between the metal center and BODIPY groups in each system.

[Image: see text] Photodynamic therapy (PDT) represents a minimally invasive and highly localized treatment strategy to ablate tumors with few side effects. In PDT, photosensitizers embedded within tumors are activated by light and undergo intersystem crossing, followed by energy transfer to molecular oxygen, resulting in the production of toxic singlet oxygen ((1)O(2)). Previously, we reported a robust, linear tetrapyrrole palladium(II) complex, Pd[DMBil1], characterized by its facile and modular synthesis, broad absorption profile, and efficient (1)O(2) quantum yield of Φ(Δ) = 0.8 in organic media. However, the insolubility of this porphyrinoid derivative in aqueous solution prevents its use under biologically relevant conditions. In this work, we report the synthesis of Pd[DMBil1]-PEG(750), a water-soluble dimethylbiladiene derivative that is appended with a poly(ethylene) glycol (PEG) functionality. Characterization of this complex shows that this PEGylated biladiene architecture maintains the attractive photophysical properties of the parent complex under biologically relevant conditions. More specifically, the absorption profile of Pd[DMBil1]-PEG(750) closely matches that of Pd[DMBil1] and obeys the Beer–Lambert Law, suggesting that the complex does not aggregate under biologically relevant conditions. Additionally, the emission spectrum of Pd[DMBil1]-PEG(750) retains the fluorescence and phosphorescence features characteristic of Pd[DMBil1]. Importantly, the PEGylated photosensitizer generates (1)O(2) with Φ(Δ) = 0.57, which is well within the range to warrant interrogation as a potential PDT agent for treatment of cancer cells. The Pd[DMBil1]-PEG(750) is biologically compatible, as it is taken up by MDA-MB-231 triple negative breast cancer (TNBC) cells and has an effective dose (ED(50)) of only 0.354 μM when exposed to λ(ex) > 500 nm light for 30 min. Impressively, the lethal dose (LD(50)) of Pd[DMBil1]-PEG(750) without light exposure was measured to be 1.87 mM, leading to a remarkably high phototoxicity index of ~5300, which is vastly superior to existing photosensitizers that form the basis for clinical PDT treatments. Finally, through flow cytometry experiments, we show that PDT with Pd[DMBil1]-PEG(750) induces primarily apoptotic cell death in MDA-MB-231 cells. Overall these results demonstrate that Pd[DMBil1]-PEG(750) is an easily prepared, biologically compatible, and well-tolerated photochemotherapeutic agent that can efficiently drive the photoinduced apoptotic death of TNBC cells.

The synthesis, electrochemistry, and photophysical characterization of a 10,10-dimethyl-5,15-bis(pentafluorophenyl)biladiene (DMBil1) linear tetrapyrrole supporting PdII or PtII centers is presented. Both of these nonmacrocyclic tetrapyrrole platforms are robust and easily prepared via modular routes. X-ray diffraction experiments reveal that the Pd[DMBil1] and Pt[DMBil1] complexes adopt similar structures and incorporate a single PdII and PtII center, respectively. Additionally, electrochemical experiments revealed that both Pd[DMBil1] and Pt[DMBil1] can undergo two discrete oxidation and reduction processes. Spectroscopic experiments carried out for Pd[DMBil1] and Pt[DMBil1] provide further understanding of the electronic structure of these systems. Both complexes strongly absorb light in the UV–visible region, especially in the 350–600 nm range. Both Pd[DMBil1] and Pt[DMBil1] are luminescent under a nitrogen atmosphere. Upon photoexcitation of Pd[DMBil1], two emission bands are observed; fluorescence i...