Synthesis, Luminescence, and Structure of a Polymorphic Polyfluorinated Diiodoplatinum(II) Diimine Complex.

PtI2(5,5'-bis(HCF2CH2OCH2)-2,2'-bpy)], 55-2FH-PtI2, is the first example of a substituted fluorinated diiodoplatinum diimine complex that exhibits polymorphism. The complex, upon recrystallization, forms two different polymorphs, denoted as α and ß forms. The luminescence of the α and ß forms are the same in glassy solution at 77 K; however, in the solid state, they differ significantly. The major difference between them lies in the solid-state packing of the crystalline structure. The α form is a square planar polyfluorinated PtI2-containing complex. Its extended herringbone structure consists of two neighboring stacked bipyridyl planes that do not overlap. The α form emits stronger than its parent molecule, [PtI2bpy], and much stronger than the ß polymorph. The ß form has a slight tetrahedral distortion about the metal center that ultimately changes the geometry of the complex and decreases the d-orbital splitting from square planar. Furthermore, overlapping bipyridine rings in the extended structure of the ß form quench the emission thus resulting in a lower energy emission. Additionally, the ß form shows only one type of C-H···O intermolecular stacking interaction that can cause the moderate distortion of the metal core.

Oxidation Pathways Involving a Sulfide-Endcapped Donor-Acceptor-Donor π-Conjugated Molecule and Antimony(V) Chloride.

The oxidation pathways and products of a discrete, sulfide-endcapped donor-acceptor-donor (D/A/D) molecule, namely, propylenedioxythiophene-benzothiadiazole-propylenedioxythiophene, are investigated. The electrochemical and chemical oxidations proceed by two distinct routes. Specifically, electrochemical oxidation undergoes a sequential two-step, one-electron (1e-) oxidation route with a 117 mV difference between consecutive half-wave potentials. In contrast, chemical oxidation by antimony(V) chloride (SbCl5) causes the generation of four different oxidized species: (a) the 1e- oxidation state, (b) a decomposition product, (c) the 2e- oxidation state, and (d) a chloride adduct of the 2e- oxidation state. The decomposition product is generated by the reaction of the 1e- oxidation state with residual water, resulting in nucleophilic aromatic substitution at the sulfide group terminal positions. This reaction leads to the formation of a 2e- oxidized, oxygen atom (ketone) terminated decomposed molecule. The chloride adduct is determined to be produced by electrophilic chloronium ion (2e-) oxidation by the SbCl4+ complex, which is a product of SbCl5 ligand disproportionation. The formation of the 2e- oxidized chlorine adduct shows to be linearly dependent on the molarity of SbCl5 in dichloromethane, giving new insight into the concentration dependent reactivity of SbCl5 as a 2e- oxidant. The electronic, optical, and magnetic properties and geometric structures of the 1e- and 2e- oxidized hexachloroantimonate salts are fully characterized by a combination of electrochemistry, X-ray crystallography, UV-vis-NIR, electron paramagnetic resonance, NMR spectroscopies, and density functional theory calculations. The aim of this study is to provide a thorough understanding of the redox pathways of a D/A/D π-conjugated organic molecule for potential application in organic electrochromic devices.

Photophysical Properties of Pt(II) Polypyridines with Five- versus Six-Membered Chelate Rings: Trade-Offs in Angle Strain.

This report describes the synthesis and characterization of a series of eight [Pt(NNN)X]+ complexes where the tridentate NNN ligand is (2,2'-bipyrid-6-yl)(pyrid-2-yl)sulfide (btp) or methyl(2,2'-bipyrid-6-yl)(pyrid-2-yl)amine (bmap) and X is OMe, Cl, phenylethynyl (C2Ph), or cyclohexylethynyl (C2Cy). The expectation was that inserting a heteroatom into the backbone of 2,2':6',2″-terpyridine (trpy) would expand the overall intraligand bite angle, introduce ILCT character into the excited states, and improve the photophysical properties. Crystal structures of [Pt(bmap)C2Ph]+ and [Pt(btp)Cl]+ reveal that atom insertion into the trpy backbone successfully expands the bite angle of the ligand by 8-10°. However, the impact on the photophysics is minimal. Indeed, of the eight systems investigated, only the [Pt(bmap)C2Ph]+ and [Pt(btp)C2Ph]+ complexes display appreciable emission in fluid solution, and they exhibit shorter emission lifetimes than [Pt(trpy)C2Ph]+. One reason is that the bond angle preferences of platinum and the inserted heteroatom induce the six-membered rings to deviate from planarity and adopt a boat-like conformation, impairing charge delocalization within the ligand. In addition, angle strain induces the donor atoms about platinum to assume a pseudotetrahedral arrangement, which offsets any benefit due to the increase in overall bite angle by promoting deactivation via d-d excited states. The results reveal that, in order to improve the luminescence of a [Pt(NNN)X]+ system, one must take care to avoid trading one kind of angle strain for another.

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.

Cr(III)-HMC (HMC = 5,5,7,12,12,14-Hexamethyl-1,4,8,11-tetraazacyclotetradecane) Alkynyl Complexes: Preparation and Emission Properties.

Presented here is the chemistry of Cr(III) alkynyl complexes based on the rac-HMC and meso-HMC ligands (HMC = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). Thus far, two pairs of cis/trans-[Cr(rac/meso-HMC)(C2R)2]Cl (R = Ph, C2H/C2SiMe3) complexes have been synthesized from reactions between cis/trans-[Cr(rac/meso-HMC)Cl2]Cl and LiC2R. These complexes were characterized using single crystal X-ray diffraction, UV-vis spectroscopy, FT-IR spectroscopy, and fluorimetry. Single crystal X-ray diffraction studies revealed that these complexes adopt a pseudo-octahedral geometry. The electronic spectra of both the cis- and trans-[Cr(rac/meso-HMC)(C4R')2]Cl (R' = H or SiMe3) complexes exhibit d-d bands with pronounced vibronic progression associated with the asymmetric stretch of the Cr-bound C≡C bonds. All of these complexes are phosphorescent and show structured emissions originating from the ligand field excited states.

Engineering Chemically Exfoliated Large-Area Two-Dimensional MoS2 Nanolayers with Porphyrins for Improved Light Harvesting.

Molybdenum disulfide (MoS2 ) is a promising candidate for electronic and optoelectronic applications. However, its application in light harvesting has been limited in part due to crystal defects, often related to small crystallite sizes, which diminish charge separation and transfer. Here we demonstrate a surface-engineering strategy for 2D MoS2 to improve its photoelectrochemical properties. Chemically exfoliated large-area MoS2 thin films were interfaced with eight molecules from three porphyrin families: zinc(II)-, gallium(III)-, iron(III)-centered, and metal-free protoporphyrin IX (ZnPP, GaPP, FePP, H2 PP); metal-free and zinc(II) tetra-(N-methyl-4-pyridyl)porphyrin (H2 T4, ZnT4); and metal-free and zinc(II) tetraphenylporphyrin (H2 TPP, ZnTPP). We found that the photocurrents from MoS2 films under visible-light illumination are strongly dependent on the interfacial molecules and that the photocurrent enhancement is closely correlated with the highest occupied molecular orbital (HOMO) levels of the porphyrins, which suppress the recombination of electron-hole pairs in the photoexcited MoS2 films. A maximum tenfold increase was observed for MoS2 functionalized with ZnPP compared with pristine MoS2 films, whereas ZnT4-functionalized MoS2 demonstrated small increases in photocurrent. The application of bias voltage on MoS2 films can further promote photocurrent enhancements and control current directions. Our results suggest a facile route to render 2D MoS2 films useful for potential high-performance light-harvesting applications.

Tuning a Lanthanide Complex To Be Responsive to the Environment in Solution.

The f-f emissions of lanthanide-ion complexes have predictable emission energies and many practical applications, but the emitting states are generally impervious to the surroundings. This investigation explores ligand- and metal-centered emission processes for a series of mixed-ligand complexes of composition M(X-T)(NO3)3, where the metal ion is europium, gadolinium, terbium, or lutetium, and X-T denotes the tridentate ligand 2,2':6',2″-terpyridine (H-T), 4'-phenyl-2,2':6',2″-terpyridine (Ph-T), or 4'-pyrrolidin-N-yl-2,2':6',2″-terpyridine (pyrr-T). The presence of the pyrrolidinyl substituent imparts intraligand charge-transfer (ILCT) character to the ligand-based excited states and reduces the energy gap between the singlet and the triplet excited states. An enhanced rate of intersystem crossing results in a lutetium complex with a relatively small fluorescence quantum yield (0.15%) and a gadolinium complex with an impressive phosphorescence yield of 9.6% in deaerated solution. The Tb(pyrr-T)(NO3)3 system is unique because the relatively low-energy triplet ILCT state equilibrates with the emissive f-f state. The result is a truly remarkable f-f emission signal that is sensitive to the polarity of the local environment as well as the presence of dioxygen.

Binding Studies of G-Quadruplex DNA and Porphyrins: Cu(T4) vs Sterically Friendly Cu(tD4).

This investigation explores binding interactions involving G-quadruplex DNA and two copper(II)-containing porphyrins (5,10,15,20-tetra(N-methylpyridinium-4-yl)porphyrinato)copper(II) and the sterically friendlier analogue (trans-5,15-di(N-methylpyridinium-4-yl)porphyrinato)copper(II), or Cu(T4) and Cu(tD4), respectively. The study incorporates five different DNA sequences that support the formation of unimolecular and bimolecular G-quadruplex hosts capable of exhibiting at least nine different structures in toto. Absorbance and emission results establish that G-quadruplex DNA is more adept at sequestering Cu(tD4) compared with the bulkier Cu(T4) ligand, even though the predominant mode of uptake is by end-capping, irrespective of the porphyrin or DNA sequence employed. One of the more impressive observations is that the emission intensities exhibited by Cu(tD4) bound to G-quadruplex DNA are many-fold higher than corresponding signals obtained with single- or double-stranded DNA hosts. With human sequence DNA the Cu(tD4) system is also unusual in that it preferentially binds to structures containing antiparallel strands. Refining the binding properties of porphyrin ligands is of interest because work from many laboratories has established that stabilizing G-quadruplex structure is an effective way to inhibit telomerase, a key enzyme involved in the immortalization of most types of cancer cells.

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-binding studies of a tetraalkyl-substituted porphyrin and the mutually adaptive distortion principle.

This investigation explores DNA-binding interactions of various forms of an alkyl-substituted cationic porphyrin, H2TC3 (5,10,15,20-tetra[3-(3'-methylimidazolium-1'-yl)]porphyrin). The motivating idea is that incorporating alkyl rather than aryl substituents in the meso positions will enhance the prospects for intercalative as well as external binding to DNA hosts. The ligands may also be applicable for photodynamic and/or anticancer therapy. Methods employed include absorbance, circular dichroism, and emission spectroscopies, as well as viscometry and X-ray crystallography. By comparison with the classical H2T4 system, H2TC3 exhibits a higher molar extinction coefficient but is more prone to self-association. Findings of note include that the copper(II)-containing form Cu(TC3) is adept at internalizing into single-stranded as well as B-form DNA, regardless of the base composition. Surprisingly, however, external binding of H2TC3 occurs within domains that are rich in adenine-thymine base pairs. The difference in the deformability of H2TC3 versus Cu(TC3) probably accounts for the reactivity difference. Finally, Zn(TC3) binds externally, as the metal center remains five-coordinate.

Internal versus external binding of cationic porphyrins to single-stranded DNA.

Absorbance, induced circular dichroism, and emission studies establish that the tetrasubstituted cationic porphyrin Cu(T4) preferentially binds externally to single-stranded (ss) DNA sequences, except in a purine-rich system like 5'-(dA)10-3' where a degree of internalization occurs. On the other hand, the sterically friendly, disubstituted Cu(tD4) system exclusively binds to ss DNA by internalization, that is, pseudointercalation. By and large the results show that double-stranded DNA hosts decisively outcompete more flexible ss hosts for the uptake of a porphyrin, regardless of the binding motif. The findings are relevant because ss domains of DNA appear during replication, in different types of DNA-secondary structure, and as products of the disassembly of multistranded forms.

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.

Structure and emission studies of dimorphic crystals of [PtBr2(5,5'-bis(CF3CH2OCH2)-2,2'-bpy)] (1).

The yellow (1y) and orange (1o) crystalline polymorphs of [PtBr2(5,5'-bis(CF3CH2OCH2)-2,2'-bipyridine)] exhibit surprisingly short nearest neighbour Pt···Pt separations of 3.526 Å and 3.590 Å, respectively, at 295 K. Both distances are much shorter than those found in structures of the unsubstituted [PtBr2(2,2'-bipyridine)] analogue. Consistent with a linear chain structure in 1o and dimer formation in 1y, both solids exhibit emission spectra shifted to much longer wavelengths than that exhibited by the monomer in a low-temperature glass. Furthermore, the emission spectra of 1o and 1y shift to even longer wavelengths as the temperature decreases and the Pt···Pt separations contract. Till now delocalized emission of this type has been considered to be restricted to [PtCl2(diimine)] systems and implausible in PtBr2-containing analogues for steric reasons. Ironically, in the system at hand the bulky 5,5'-substituents apparently promote delocalization of the emission by forming a network of hydrogen-bonding-like C-H···F-C interactions that help shape the packing.

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.

Arylated versus cyclometalated platinum(II) polypyridines: photoluminescence from Pt(4'-R-trpy)Ph+ systems (R = NMe2, N-pyrrolidinyl, or 1-pyrenyl).

Many platinum(II) polypyridine complexes are good luminophores, an enigmatic exception being Pt(trpy)Ph(+), where trpy denotes 2,2':6',2"-terpyridine. A new analysis suggests the complex is nonemissive due to (3)SBLCT (sigma-bond-to-ligand charge transfer) character in the lowest energy excited state. Bases for two distinct strategies for inducing emission from aryl derivatives become clear. The standard approach of incorporating a phenyl group into a (N(/\)N(/\)C) cyclometalating ligand relies in part on the rigidity of the ligand framework. An alternative strategy, which involves expanding the chromophore and altering the orbital parentage of the emitting state, is capable of suppressing radiationless decay even further. Indeed, the Pt(4'-pyren-1-yl-trpy)Ph(+) system emits from a low-lying (3)π-π*(pyrene) excited state that has a lifetime of 45 µs in fluid solution.

Cationic copper(II) porphyrins intercalate into domains of double-stranded RNA.

A cationic, copper(II)-containing ligand, derived from bulky 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin, Cu(T4), and two sterically friendlier forms, [trans-5,15-di(N-methylpyridinium-4-yl)porphyrinato]copper(II), Cu(tD4), and [cis-5,10-di(N-methylpyridinium-4-yl)porphyrinato]copper(II), Cu(cD4), bind to DNA and RNA hosts. Six hairpin-forming RNA 18-mer sequences and two previously studied DNA analogues serve as convenient binding platforms of programmable base composition. A crystal structure shows that the copper center of Cu(tD4) is four-coordinate, establishing compatibility with intercalative binding as well as susceptibility to solvent-induced emission quenching. From the hypochromic responses and the induced emission intensities obtained with all three porphyrins, it is clear that internalization into the RNA host occurs, irrespective of the base pair composition. Further analysis reveals that the porphyrins intercalate into the double-stranded stem domains. Subtle geometric and/or electronic aspects of the binding account for the signs of induced circular dichroic signals and splitting of the Soret band of Cu(tD4).

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.

Regiospecific quenching of a photoexcited platinum(II) complex at acidic and basic sites.

The carbometalated complex Pt(dppzφ*)Cl, where dppzφ* denotes the 6-(4-tert-butylphenyl)-dipyrido[3,2-a:2',3'-c]phenazine ligand, exhibits emission in a dichloromethane solution at room temperature with a concentration-dependent excited-state lifetime. Extrapolation to zero Pt(dppzφ*)Cl concentration yields a limiting lifetime of 11.0 µs in the absence of dioxygen along with an impressive emission quantum yield of 0.17. The visible absorption of Pt(dppzφ*)Cl has intraligand charge-transfer as well as metal-to-ligand charge-transfer character, but the oscillator strength may derive, in part, from π-π* excitation within the phenazine moiety. An intriguing aspect of the Pt(dppzφ*)Cl system is that its reactive excited state is subject to regiospecific quenching by Lewis bases and hydrogen-bonding Lewis acids. Base-induced quenching involves an attack at the platinum center. The rate constant increases with the donor strength of the quencher and reaches the order of 10(8) M(-1) s(-1) with a relatively strong base like dimethyl sulfoxide. The orbital parentage of the excited state probably influences the quenching rates by affecting the charge density at platinum, as well as at the phenazine nitrogen atoms, where attack by Lewis acids occurs. With mildly acidic alcohols like 1,1,1,3,3,3-hexafluoropropan-2-ol and 2,2,2-trifluoroethanol, high concentrations of the quencher are necessary to suppress the emission. Carboxylic acids are stronger quenchers, and the quenching constant increases with the acid strength according to tabulated pK(a) values. Cyanoacetic acid exhibits the highest measured quenching rate constant (2.6 × 10(9) M(-1) s(-1)), which only decreases 30% when the acid is in the (NC)CH(2)CO(2)D form. A weaker acid, CH(3)CO(2)H, exhibits an even smaller kinetic isotope effect. Literature comparisons suggest that acid-induced quenching probably involves hydrogen-bond formation as opposed to net proton transfer.

First emission studies of Tc2X8(2-) systems (X = Cl, Br).

The emission spectra of the solids [n-Bu(4)N](2)Tc(2)X(8) (X = Cl, Br) have been investigated at room temperature and 77 K. In each case, the emission originates in the (1)δ-δ* excited state, as with the rhenium homologues, but has a shorter lifetime.