Accessibility and external versus intercalative binding to DNA as assessed by oxygen-induced quenching of the palladium(II)-containing cationic porphyrins Pd(T4) and Pd(tD4).

Studies reveal that it is possible to design a palladium(II)-containing porphyrin to bind exclusively by intercalation to double-stranded DNA while simultaneously enhancing the ability to sensitize the formation of singlet oxygen. The comparisons revolve around the cations [5,10,15,20-tetra(N-methylpyridinium-4-yl)porphyrin]palladium(II), or Pd(T4), and [5,15-di(N-methylpyridinium-4-yl)porphyrin]palladium(II), or Pd(tD4), in conjunction with A═T and G≡C rich DNA binding sequences. Methods employed include X-ray crystallography of the ligands as well as absorbance, circular dichroism, and emission spectroscopies of the adducts and the emission from singlet oxygen in solution. In the case of the bulky Pd(T4) system, external binding is almost as effective as intercalation in slowing the rate of oxygen-induced quenching of the porphyrin's triplet excited state. The fractional efficiency of quenching by oxygen nevertheless approaches 1 for intercalated forms of Pd(tD4), because of intrinsically long triplet lifetimes. The intensity of the sensitized, steady-state emission signal varies with the system and depends on many factors, but the Pd(tD4) system is impressive. Intercalated forms of Pd(tD4) produce higher sensitized emission yields than Pd(T4) is capable of in the absence of DNA.

Varying substituents and solvents to maximize the luminescence from [Ru(trpy)(bpy)CN]+ derivatives.

Ruthenium(II) in combination with monodentate, bidentate, and tridentate ligands has proven to be a useful design for a variety of applications, but the majority of systems are virtually nonluminescent in solution. The goal of this work has been to design luminescent forms with practicable emission quantum yields, and the focus has been on [Ru(X-T)(dmeb)CN](+) systems, where X-T denotes 2,2':6',2″-terpyridine bearing substituent X at the 4'-position and dmeb denotes [2,2'-bipyridine]-4,4'-dicarboxylic acid, dimethyl ester. Results show that varying the π-electron-donating ability of the 4'-X substituent is an effective way to tune the energy and lifetime of the charge-transfer (CT) emission. The lifetime achieved in a room-temperature, fluid solution is as high as 175 ns, depending on the 4'-substituent and the solvent employed because the excited state is very polar. That represents a 20-fold improvement in lifetime relative to that of the prototype, [Ru(trpy)(bpy)CN](+), one of the earliest examples found to be luminescent in a fluid solution. A simple theoretical model proves to be capable of rationalizing all the experimental lifetimes. It suggests that, with the dmeb ligand available to accept the electron, enhancing the donor ability of the 4'-X substituent lowers the energy of the (3)CT state and reduces the likelihood of thermally activated decay via a higher-energy d-d state. However, direct nonradiative decay to the ground state begins to reduce the excited-state lifetime whenever the emission maximum shifts beyond 750 nm. Within those limits, there is inevitably a maximal attainable lifetime, regardless of the method of tuning.

π donation and its effects on the excited-state lifetimes of luminescent platinum(II) terpyridine complexes in solution.

Introducing electron-donating groups extends the excited-state lifetimes of platinum(II)-terpyridine complexes in fluid solution. Such systems are of interest for a variety of applications, viz., as DNA-binding agents or as components in luminescence-based devices, especially sensors. The complexes investigated here are of the form [Pt(4'-X-T)Y](+), where 4'-X-T denotes a 4'-substituted 2,2':6',2″-terpyridine ligand and Y denotes the coligand. The π-donating abilities of -X and -Y increase systematically in the orders -NHMe < -NMe2 < -(pyrrolidin-1-yl) and -CN < -Cl < -CCPh, respectively. The results presented include crystal structures of two new 4'-NHMe-T complexes of platinum, as well as absorption, emission, and excited-state lifetime data for nine complexes. Excited-state lifetimes obtained in deoxygenated dichloromethane vary by a factor of 100, ranging from 24 µs for [Pt(4'-pyrr-T)CN](+) to 0.24 µs for [Pt(4'-ma-T)Cl](+), where ma-T denotes 4'-(methylamino)-2,2':6',2″-terpyridine and pyrr-T denotes 4'-(pyrrolidin-1-yl)-2,2':6',2″-terpyridine. Analysis of experimental and computational results shows that introducing a simple amine group on the terpyridine and/or a π-donating coligand engenders the emitting state with intraligand charge-transfer (ILCT) and/or ligand-ligand charge-transfer (LLCT) character. The excited-state lifetime increases when the change in orbital parentage lowers the emission energy, suppresses quenching via d-d states, and encourages delocalization of the excitation onto the ligand(s). At some point, however, the energy is low enough that direct vibronic coupling to the ground-state surface becomes important, and the lifetime begins to decrease again.

1,2-Bis{bis-[4-(trifluoro-meth-yl)phen-yl]phosphino}ethane.

Crystals of the title compound, C(30)H(20)F(12)P(2) or R(2)PCH(2)CH(2)PR(2) (R = 4-C(6)H(4)CF(3)), were inadvertently prepared while attempting to recrystallize a crude sample of trans-Re(Cl)(N(2))(R(2)PCH(2)CH(2)PR(2))(2) from diethyl ether. The molecule lies on a center of inversion. One of the rings lies approximately in the P-C-C-P plane; the dihedral angle is 174.53°.The other ring is not quite perpendicular; the dihedral angle is 71.1°. The compound is isostructural with the R = Ph, 4-C(6)H(4)CH(3) and 4-C(6)H(4)CH(2)CH(3) analogues. It is well known that the basicity of phosphines and diphosphines can be altered by changing the electron-donating ability of R; however, the structural parameters for the title compound do not significantly differ from those of the aforementioned substituted-phenyl compounds.

Photodynamic inactivation of methicillin-resistant Staphylococcus aureus and Escherichia coli: A metalloporphyrin comparison.

Increasing rates of antibiotic resistance coupled with the lack of novel antibiotics threatens proper clinical treatment and jeopardizes their use in prevention. A photodynamic approach appears to be an innovative treatment option, even for multi-drug resistant strains of bacteria. Three components are utilized in photodynamic inactivation: a photosensitizer, light source, and oxygen. Variations in photosensitizers strongly influence microbial binding and bactericidal activity. In this study, four different cationic metalloporphyrins (Cu2+, Fe2+, Pd2+, Zn2+) were compared to the free-base ligand 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin regarding their electronic properties and generation of reactive oxygen species upon subsequent 405nm violet-blue irradiation. Staphylococcus aureus and Escherichia coli were used as representatives of Gram-positive and -negative, respectively, to assess bactericidal effects by the photodynamic process. Bacterial cultures were pre-incubated with porphyrins and exposed to varying doses of 405nm irradiation (0-30J/cm2). Metalloporphyrins containing Cu2+ and Fe2+ demonstrated minimal effects on viability. Pronounced bactericidal activity was evident with free-base ligand, Zn2+, and Pd2+; though significantly stronger effects were apparent with Pd2+. Photodynamic killing was directly proportional to reactive oxygen species production post-illumination. These data provide new insight into the influence of metal chelation on photosensitizer activity on bactericidal singlet oxygen production. The strong anti-microbial photodynamic action through the use of a portable light-emitting diode over short time intervals (seconds) provides support for its potential use in self-treatment.

DNA binding of Pd(TC3), a conformable cationic porphyrin with a long-lived triplet state.

The goal of this work has been to synthesize and investigate Pd(TC3), an intercalating porphyrin that has conformable substituents capable of groove binding to B-form DNA. (TC3 denotes the doubly deprotonated form of 5,10,15,20-tetra[3-(3'-methylimidazolium-1'-yl)prop-1-yl]porphyrin.) Palladium(ii) is an apt choice for the central metal ion because it remains strictly four-coordinate and provides for a luminescent triplet excited state with a long lifetime. The DNA hosts are hairpin-forming sequences programmed to differ in base composition. Luminescence, absorbance, and circular dichroism results are consistent with the idea that congruent structural reorganization takes place at the host and ligand during uptake. Photoexcitation of DNA-bound Pd(TC3) generates a comparatively modest steady state concentration of singlet oxygen, due to a relatively slow reaction with molecular oxygen in solution. The sheer size of the substituent groups disfavors quenching, but groove-binding interactions compound the problem by inhibiting mobility. The results show how ligand design affects adduct structure as well as function.

Regeneration of Light-Harvesting Complexes via Dynamic Replacement of Photodegraded Chromophores.

All-synthetic molecular donor-acceptor complexes are designed, which are capable of counteracting the effect of photoinduced degradation of donor chromophores. Anionic gallium protoporphyrin IX (GaPP) and semiconducting carbon nanotube (CNT) are used as a model donor-acceptor complex, which is assembled using DNA oligonucleotides. The GaPP-DNA-CNT complex produces an anodic photocurrent in a photoelectrochemical cell, which steadily decays due to photo-oxidation. By modulating the chemical environment, we showed that the photodegraded chromophores may be dissociated from the complex, whereas the DNA-coated carbon nanotube acceptors are kept intact. Reassociation with fresh porphyrins leads to the full recovery of GaPP absorption and photocurrents. This strategy could form a basis for improving the light-harvesting performance of molecular donor-acceptor complexes and extending their operation lifetime.

DNA oligonucleotide templated nanohybrids using electronic type sorted carbon nanotubes for light harvesting.

Light harvesting nanostructure hybrids have been designed and demonstrated using single-wall carbon nanotubes (SWCNTs) and porphyrin chromophores. DNA oligonucleotides are used to conjugate SWCNTs with light-absorbing chromophores for transparent films which generate photocurrents. High-purity semiconducting SWCNTs demonstrate significant enhancement in the photocurrent compared to metallic or unsorted tubes.