Randolph P. Thummel

Color Tuning in New Metal‐Free Organic Sensitizers (Bodipys) for Dye‐Sensitized Solar Cells

Fun in the sun! A strategy has been devised for functionalizing and solubilizing boron dipyrromethene (Bodipy) dyes at the central boron atom and changing the color by increasing delocalization on the central core. This approach leads to the formation of stable B-C[triple bond]C and pyrrole--C=C linkages suitable for use in TiO(2)-sensitized devices (see figure).

Enhanced Metal Ion Selectivity of 2,9-Di-(pyrid-2-yl)-1,10-phenanthroline and Its Use as a Fluorescent Sensor for Cadmium(II)

The metal ion complexing properties of the ligand DPP (2,9-di-(pyrid-2-yl)-1,10-phenanthroline) were studied by crystallography, fluorimetry, and UV-visible spectroscopy. Because DPP forms five-membered chelate rings, it will favor complexation with metal ions of an ionic radius close to 1.0 A. Metal ion complexation and accompanying selectivity of DPP is enhanced by the rigidity of the aromatic backbone of the ligand. Cd2+, with an ionic radius of 0.96 A, exhibits a strong CHEF (chelation enhanced fluorescence) effect with 10(-8) M DPP, and Cd2+ concentrations down to 10(-9) M can be detected. Other metal ions that cause a significant CHEF effect with DPP are Ca2+ (10(-3) M) and Na+ (1.0 M), whereas metal ions such as Zn2+, Pb2+, and Hg2+ cause no CHEF effect with DPP. The lack of a CHEF effect for Zn2+ relates to the inability of this small ion to contact all four donor atoms of DPP. The structures of [Cd(DPP)2](ClO4)2 (1), [Pb(DPP)(ClO4)2H2O] (2), and [Hg(DPP)(ClO4)2] (3) are reported. The Cd(II) in 1 is 8-coordinate with the Cd-N bonds to the outer pyridyl groups stretched by steric clashes between the o-hydrogens on these outer pyridyl groups and the central aromatic ring of the second DPP ligand. The 8-coordinate Pb(II) in 2 has two short Pb-N bonds to the two central nitrogens of DPP, with longer bonds to the outer N-donors. The coordination sphere around the Pb(II) is completed by a coordinated water molecule, and two coordinated ClO4(-) ions, with long Pb-O bonds to ClO4(-) oxygens, typical of a sterically active lone pair on Pb(II). The Hg(II) in 3 shows an 8-coordinate structure with the Hg(II) forming short Hg-N bonds to the outer pyridyl groups of DPP, whereas the other Hg-N and Hg-O bonds are rather long. The structures are discussed in terms of the fit of large metal ions to DPP with minimal steric strain. The UV-visible studies of the equilibria involving DPP and metal ions gave formation constants that show that DPP has a higher affinity for metal ions with an ionic radius close to 1.0 A, particularly Cd(II), Gd(III), and Bi(III), and low affinity for small metal ions such as Ni(II) and Zn(II). The complexes of several metal ions, such as Cd(II), Gd(III), and Pb(II), showed an equilibrium involving deprotonation of the complex at remarkably low pH values, which was attributed to deprotonation of coordinated water molecules according to: [M(DPP)(H2O)]n+ <==> [M(DPP)(OH)](n-1)+ + H+. The tendency to deprotonation of these DPP complexes at low pH is discussed in terms of the large hydrophobic surface of the coordinated DPP ligand destabilizing the hydration of coordinated water molecules and the build-up of charge on the metal ion in its DPP complex because of the inability of the coordinated DPP ligand to hydrogen bond with the solvent.

Preparation and study of a family of dinuclear Ru(II) complexes that catalyze the decomposition of water.

An approach is developed for the four-electron oxidation of water to provide dioxygen that involves the juxtaposition of two Ru(II) centers such that a metal-bound water molecule might interact with one or both of the metals. The key element in this approach is an appropriate bridging ligand that will hold the metal assembly intact through the full redox cycle. Various synthetic approaches to such ligands are described with the ultimate preparation of four closely related bis-tridentate polypyridine-type systems in which the bridging and distal portions of the ligand are varied. All of these ligands self-assemble with two Ru(II) centers bridged by a Cl ion in the equatorial plane and four axial monodentate substituted pyridines or N-methylimidazoles to form the well-organized catalyst complexes. These complexes are characterized by their distinctive (1)H NMR spectra as well as an X-ray structure of one representative species. The photophysical and electrochemical features of these complexes are consistent with electronegativity and delocalization effects in the equatorial and axial ligands. Of the 14 complexes studied, all but 2, which each contain four axial N-methylimidazole ligands, catalyze the decomposition of water in the presence of excess Ce(IV) as a sacrificial oxidant at pH = 1. Both the rates of oxygen evolution and the catalyst turnover numbers (TNs) are measured. For the active catalysts, the relative rates vary from 1 to 51 and the TNs measure from 80 to 689. Various analytical methods for making these measurements are discussed, and it is found that there is an approximately linear relationship between the rate and TN. Future work will involve optimization of these systems and studies aimed at a better understanding of the mechanism.

Exploitation of Long-Lived 3IL Excited States for Metal–Organic Photodynamic Therapy: Verification in a Metastatic Melanoma Model

Members of a family of Ru(II)-appended pyrenylethynylene dyads were synthesized, characterized according to their photophysical and photobiological properties, and evaluated for their collective potential as photosensitizers for metal-organic photodynamic therapy. The dyads in this series possess lowest-lying (3)IL-based excited states with lifetimes that can be tuned from 22 to 270 μs in fluid solution and from 44 to 3440 μs in glass at 77 K. To our knowledge, these excited-state lifetimes are the longest reported for Ru(II)-based dyads containing only one organic chromophore and lacking terminal diimine groups. These excited states proved to be extremely sensitive to trace amounts of oxygen, owing to their long lifetimes and very low radiative rates. Herein, we demonstrate that (3)IL states of this nature are potent photodynamic agents, exhibiting the largest photocytotoxicity indices reported to date with nanomolar light cytotoxicities at very short drug-to-light intervals. Importantly, these new agents are robust enough to maintain submicromolar PDT in pigmented metastatic melanoma cells, where the presence of melanin in combination with low oxygen tension is known to compromise PDT. This activity underscores the potential of metal-organic PDT as an alternate treatment strategy for challenging environments such as malignant melanoma.

The photophysical behavior of d6 complexes having nearly isoenergetic MLCT and ligand localized excited states

Abstract A large number of second- and third-row d6 transition metal complexes with N-heterocyclic ligands have been prepared in which the lowest energy excited state is a metal-to-ligand charge transfer (MLCT) state. In some of these complexes ligand localized (IL) excited states exist which are of the same spin multiplicity as the MLCT state and are energetically accessible from the 1MLCT state. Several complexes have been examined which exhibit luminescence from two states in frozen matrices. In fluid solutions, however, only a few examples exist of complexes having two non-equilibrated triplet excited states. This article provides a brief overview of the experimental methods commonly used in the analysis of triplet excited state formation and relaxation in transition metal complexes having coexisting triplet excited states. In addition, descriptive examples are given of complexes for which both 3IL and 3MLCT states can be populated following excitation into the 1MLCT state.

Effects of a proximal base on water oxidation and proton reduction catalyzed by geometric isomers of [Ru(tpy)(pynap)(OH2)]2+.

Basic difference: The importance of a pendent base in promoting proton-coupled electron-transfer reactions with low activation barriers has been discussed for H(+) reduction or H(2) oxidation in acetonitrile. Investigation of the interaction between a base positioned in the second coordination sphere of a complex and a water ligand in water oxidation reactions using geometric isomers of [Ru(tpy)(pynap)(OH(2))](2+) (see picture) gave intriguing results.

Ruthenium(II) complexes of tetra-2-pyridyl-1,4-diazine

Abstract Tetra-2-pyridyl-1,4-diazine (TPD) reacts with one equivalent of Ru(tpy)Cl3 to provide (tpy)Ru(TPD)2+ which when treated with a second equivalent of Ru(tpy)Cl3 under more forcing conditions provides the TPD bridged binuclear complex, (tpy)Ru(TPD)Ru(tpy)4+. The structure of both complexes, particularly with regard to planarity of the pendant pyridine rings is analyzed by high field NMR. The electronic spectrum of the binuclear complex shows pronounced bathochromic shift, while the redox potentials for (tpy)Ru(TPD)2+. indicate a diminished HOMO-LUMO energy gap.

Photobiological Activity of Ru(II) Dyads Based on (Pyren-1-yl)ethynyl Derivatives of 1,10-Phenanthroline

Several mononuclear Ru(II) dyads possessing 1,10-phenanthroline-appended pyrenylethynylene ligands were synthesized, characterized, and evaluated for their potential in photobiological applications such as photodynamic therapy (PDT). These complexes interact with DNA via intercalation and photocleave DNA in vitro at submicromolar concentrations when irradiated with visible light (lambda(irr) > or = 400 nm). Such properties are remarkably sensitive to the position of the ethynylpyrenyl substituent on the 1,10-phenanthroline ring, with 3-substitution showing the strongest binding under all conditions and causing the most deleterious DNA damage. Both dyads photocleave DNA under hypoxic conditions, and this photoactivity translates well to cytotoxicity and photocytotoxicity models using human leukemia cells, where the 5- and 3-substituted dyads show photocytotoxicity at 5-10 microM and 10-20 microM, respectively, with minimal, or essentially no, dark toxicity at these concentrations. This lack of dark cytotoxicity at concentrations where significant photoactivity is observed emphasizes that agents with strong intercalating units, previously thought to be too toxic for phototherapeutic applications, should not be excluded from the arsenal of potential photochemotherapeutic agents under investigation.

Proton transfer with a twist? Femtosecond Dynamics of 7‐(2‐pyridyl)indole in Condensed Phase and in Supersonic Jets

The results of spectral and photophysical studies strongly suggest that photoinduced proton transfer in 7-(2-pyridyl)indole (1) is accompanied by mutual twisting of the pyridyl and indole moieties. This conclusion is supported by the unusual finding that the photoreaction is faster in a cold, supersonicjet-isolated molecule than in solutions at room temperature, and by the ultrafast repopulation of the ground state substrate. The twisting and the presence of S1–S0 conical intersection (CI) are also predicted by calculations. The phenomenon may be quite general for several classes of organic molecules with intramolecular hydrogen bonds. Recent experimental and theoretical developments demonstrate that proton transfer—a fundamental chemical reaction—is by no means a one-dimensional process. [1] In particular, for tautomerization occurring along a hydrogen bond, coupling with modes that modulate the hydrogen bond strength may be crucial. [2] Much less explored is the possibility of large conformational changes accompanying proton/hydrogen transfer, although the role of torsional motion in the deactivation of phototautomerization products has been discussed in some depth. [3–9] Compound 1 is a member of a series of three isomeric 7pyridylindoles (Scheme 1), which were studied previously in solution. [10] Compounds 1 and 2 can exist in syn and anti

Bridged dibenzimidazolinylidenes as new derivatives of tetraaminoethylene

Abstract The deprotonation of N,N′-polymethylene bridged bis-benzimidazolium salts in the absence of air provides the corresponding bridged dibenzimidazolinylidenes which undergo a spontaneous chemiluminescent reaction with dioxygen to afford ureaphanes.