Photochemical charge accumulation in a heteroleptic copper(i)-anthraquinone molecular dyad via proton-coupled electron transfer

Developing efficient photocatalysts that perform multi electron redox reactions is critical to achieving solar energy conversion. One can reach this goal by developing systems which mimic natural photosynthesis and exploit strategies such as proton-coupled electron transfer (PCET) to achieve photochemical charge accumulation. We report herein a heteroleptic Cu(i)bis(phenanthroline) complex, Cu-AnQ, featuring a fused phenazine-anthraquinone moiety that photochemically accumulates two electrons in the anthraquinone unit via PCET. Full spectroscopic and electrochemical analyses allowed us to identify the reduced species and revealed that up to three electrons can be accumulated in the phenazine-anthraquinone ring system under electrochemical conditions. Continuous photolysis of Cu-AnQ in the presence of sacrificial electron donor produced doubly reduced monoprotonated photoproduct confirmed unambiguously by X-ray crystallography. Formation of this photoproduct indicates that a PCET process occurred during illumination and two electrons were accumulated in the system. The role of the heteroleptic Cu(i)bis(phenanthroline) moiety participating in the photochemical charge accumulation as a light absorber was evidenced by comparing the photolysis of Cu-AnQ and the free AnQ ligand with less reductive triethylamine as a sacrificial electron donor, in which photogenerated doubly reduced species was observed with Cu-AnQ, but not with the free ligand. The thermodynamic properties of Cu-AnQ were examined by DFT which mapped the probable reaction pathway for photochemical charge accumulation and the capacity for solar energy stored in the process. This study presents a unique system built on earth-abundant transition metal complex to store electrons, and tune the storage of solar energy by the degree of protonation of the electron acceptor.

Complexes of transition metal carbonyl clusters with tin(II) phthalocyanine in neutral and radical anion states: methods of synthesis, structures and properties

Coordination of tin(II) phthalocyanine to transition metal carbonyl clusters in neutral {Sn^II(Pc^2-)}⁰ or radical anion {Sn^II(Pc˙^3-)}^- states is reported. Direct interaction of Co₄(CO)₁₂ with {Sn^II(Pc^2-)}⁰ yields a crystalline complex {Co₄(CO)₁₁·Sn^II(Pc^2-)} (1). There is no charge transfer from the cluster to phthalocyanine in 1, which preserves the diamagnetic Pc^2- macrocycle. The Ru₃(CO)₁₂ cluster forms complexes with one or two equivalents of {Sn^II(Pc˙^3-)}^- to yield crystalline {Cryptand[2.2.2](Na⁺)}{Ru₃(CO)₁₁·Sn^II(Pc˙^3-)}^- (2) or {Cryptand[2.2.2](M⁺)}₂{Ru₃(CO)₁₀·[Sn^II(Pc˙^3-)]₂}^2-·4C₆H₄Cl₂ (3) (M⁺ is K or Cs). Paramagnetic {Sn^II(Pc˙^3-)}^- species in 2 are packed in π-stacking [{Sn^II(Pc˙^3-)}^-]₂ dimers, providing strong antiferromagnetic coupling of spins with exchange interaction J/k_B = -19 K. Reduction of Ru₃(CO)₁₂, Os₃(CO)₁₂ and Ir₄(CO)₁₂ clusters by decamethylchromocene (Cp*₂Cr) and subsequent oxidation of the reduced species by {Sn^IVCl₂(Pc^2-)}⁰ yield a series of complexes with high-spin Cp*₂Cr⁺ counter cations (S = 3/2): (Cp*₂Cr⁺){Ru₃(CO)₁₁·Sn^II(Pc˙^3-)}^-·C₆H₄Cl₂ (4), (Cp*₂Cr⁺){Os₃(CO)₁₀Cl·Sn^II(Pc˙^3-)}^-·C₆H₄Cl₂ (5) and (Cp*₂Cr⁺){Ir₄(CO)₁₁·Sn^II(Pc˙^3-)}₂^- (6). It is seen that reduced clusters are oxidized by Sn^IV, which is transferred to Sn^II, whereas the Pc^2- macrocycle is reduced to Pc˙^3-. In the case of Os₃(CO)₁₂, oxidation of the metal atom in the cluster is observed to be accompanied by the formation of Os₃(CO)₁₀Cl with one Os^I center. Rather weak magnetic coupling is observed between paramagnetic Cp*₂Cr⁺ and {Sn^II(Pc˙^3-)}^- species in 4, but this exchange interaction is enhanced in 5 owing to Os₃(CO)₁₀Cl clusters with paramagnetic Os^I (S = 1/2) also being involved in antiferromagnetic coupling of spins. The formation of {Sn^II(Pc˙^3-)}^- with radical trianion Pc˙^3- macrocycles in 2-5 is supported by the appearance of new absorption bands in the NIR spectra and essential N_meso-C bond alternation in Pc (for 3-5). On the whole, this work shows that both diamagnetic {Sn^II(Pc^2-)}⁰ and paramagnetic {Sn^II(Pc˙^3-)}^- ligands substitute carbonyl ligands in the transition metal carbonyl clusters, forming well-soluble paramagnetic solids absorbing light in the visible and NIR ranges.

Dinuclear osmium complexes as mitochondrion-targeting antitumor photothermal agents in vivo

Mitochondrion-targeting dinuclear osmium complexes with extremely high photothermal conversion capability under irradiation of an 808 nm low-power laser without nucleus affinity and photodynamic activity act as antitumor photothermal therapy agents in vivo.

A Dinuclear Osmium(II) Complex Near-Infrared Nanoscopy Probe for Nuclear DNA

With the aim of developing photostable near-infrared cell imaging probes, a convenient route to the synthesis of heteroleptic Os^II complexes containing the Os(TAP)₂ fragment is reported. This method was used to synthesize the dinuclear Os^II complex, [{Os(TAP)₂}₂tpphz]⁴⁺ (where tpphz = tetrapyrido[3,2-a:2',3'-c:3″,2''-h:2‴,3'''-j]phenazine and TAP = 1,4,5,8- tetraazaphenanthrene). Using a combination of resonance Raman and time-resolved absorption spectroscopy, as well as computational studies, the excited state dynamics of the new complex were dissected. These studies revealed that, although the complex has several close lying excited states, its near-infrared, NIR, emission (λ_max = 780 nm) is due to a low-lying Os → TAP based ³MCLT state. Cell-based studies revealed that unlike its Ru^II analogue, the new complex is neither cytotoxic nor photocytotoxic. However, as it is highly photostable as well as live-cell permeant and displays NIR luminescence within the biological optical window, its properties make it an ideal probe for optical microscopy, demonstrated by its use as a super-resolution NIR STED probe for nuclear DNA.

Excited state properties of a series of molecular photocatalysts investigated by time dependent density functional theory

Time dependent density functional theory calculations are applied on a series of molecular photocatalysts of the type [(tbbpy)2M1(tpphz)M2X2]2+ (M1 = Ru, Os; M2 = Pd, Pt; X = Cl, I) in order to provide information concerning the photochemistry occurring upon excitation of the compounds in the visible region. To this aim, the energies, oscillator strengths and orbital characters of the singlet and triplet excited states are investigated. The structural modifications of the complexes have a strong impact on the excited states properties. In particular, it is found that the main differences concern the energies of the charge-separated and metal-centered states. The analysis of these differences provides general trends for the efficiency of population transfers between the states, particularly regarding the charge separation and electron recombination processes.