Spectral, electrochemical, and photophysical studies of a magnesium porphyrin-fullerene dyad

A covalently linked magnesium porphyrin-fullerene (MgPo-C60) dyad was synthesized and its spectral, electrochemical, molecular orbital, and photophysical properties were investigated and the results were compared to the earlier reported zinc porphyrin-fullerene (ZnPo-C60) dyad. The ab initio B3LYP/3-21G(*) computed geometry and electronic structure of the dyad predicted that the HOMO and LUMO are mainly localized on the MgP and C60 units, respectively. In o-dichlorobenzene containing 0.1 M (n-Bu)4NClO4, the synthesized dyad exhibited six one-electron reversible redox reactions within the potential window of the solvent. The oxidation and reduction potentials of the MgP and C60 units indicate stabilization of the charge-separated state. The emission, monitored by both steady-state and time-resolved techniques, revealed efficient quenching of the singlet excited state of the MgP and C60 units. The quenching pathway of the singlet excited MgP moiety involved energy transfer to the appended C60 moiety, generating the singlet excited C60 moiety, from which subsequent charge-separation occurred. The charge recombination rates, k(CR), evaluated from nanosecond transient absorption studies, were found to be 2-3 orders of magnitude smaller than the charge separation rate, k(CS). In o-dichlorobenzene, the lifetime of the radical ion-pair, MgPo*+-C60*-, was found to be 520 ns which is longer than that of ZnPo*+-C60*- indicating better charge stabilization in MgPo-C60. Additional prolongation of the lifetime of MgPo*+-C60*- was achieved by coordinating nitrogenous axial ligands. The solvent effect in controlling the rates of forward and reverse electron transfer is also investigated.     

Photophysical and electrochemical properties of meso, meso-linked oligoporphyrin rods with appended fullerene terminals.

The electrochemical and photophysical properties of molecular architectures consisting of oligomeric meso,meso-linked oligoporphyrin rods linked at both extremities to methanofullerene moieties are presented in comparison to those of model systems. Cyclic voltammetry data evidence the presence of a strong intramolecular electronic coupling along the porphyrin oligomers that varies slightly with their length. This interaction affects the redox potentials of both fullerene and porphyrin moieties. The electronic coupling between the two chromophores is confirmed by comparing the redox potentials of porphyrin arrays before and after attachment of the carbon sphere. Electronic absorption, fluorescence, and phosphorescence spectra of the porphyrin oligomers in toluene are reported, which provide the energy of the lowest singlet and triplet electronic excited states. In the fullerene-porphyrin conjugates, ground-state charge-transfer (CT) interactions are evidenced by low-energy absorption features above 750 nm. These systems also exhibit near-infrared (NIR) CT luminescence in toluene with lifetimes shorter than 1000 ps. On increasing the solvent polarity (from toluene to Et2O and THF), CT emissions become progressively weaker, red-shifted, and shorter lived, which reflects the energy-gap law and Marcus inverted region effects. Luminescence is not detected in benzonitrile. Picosecond transient absorption spectroscopy of the porphyrin-fullerene conjugates allows detection of the porphyrin cation as a clear fingerprint for electron transfer. The rate of charge recombination is in agreement with CT luminescence lifetimes, which confirms the occurrence of NIR radiative back-electron transfer.     

Thermodynamic, spectroscopic, and computational studies of lanthanide complexation with diethylenetriaminepentaacetic acid: temperature effect and coordination modes

Stability constants of two DTPA (diethylenetriaminepentaacetic acid) complexes with lanthanides (ML(2-) and MHL(-), where M stands for Nd and Eu and L stands for diethylenetriaminepentaacetate) at 10, 25, 40, 55, and 70 °C were determined by potentiometry, absorption spectrophotometry, and luminescence spectroscopy. The enthalpies of complexation at 25 °C were determined by microcalorimetry. Thermodynamic data show that the complexation of Nd(3+) and Eu(3+) with DTPA is weakened at higher temperatures, a 10-fold decrease in the stability constants of ML(2-) and MHL(-) as the temperature is increased from 10 to 70 °C. The effect of temperature is consistent with the exothermic enthalpy of complexation directly measured by microcalorimetry. Results by luminescence spectroscopy and density functional theory (DFT) calculations suggest that DTPA is octa-dentate in both the EuL(2-) and EuHL(-) complexes and, for the first time, the coordination mode in the EuHL(-) complex was clarified by integration of the experimental data and DFT calculations. In the EuHL(-) complex, the Eu is coordinated by an octa-dentate H(DTPA) ligand and a water molecule, and the protonation occurs on the oxygen of a carboxylate group.     

Synthesis of new porphyrin/fullerene supramolecular assemblies: a spectroscopic and electrochemical investigation of their coordination equilibrium in solution

Two new fullerene ligands have been designed to provide relatively simple frameworks to build supramolecular systems containing both fullerene and Zn-porphyrin moieties. The coordination of the fullerene ligands to the Zn-porphyrin was supported by UV-vis titration, nuclear magnetic resonance and electrochemical data. The resulting spectrophotometric data were processed both graphically and computationally to yield the stoichiometry, stability constant, and molar absorptivity of the species in equilibrium.     

Synthesis,redox properties,and basicity of substituted 1-aminoanthraquinones: spectroscopic,electrochemical, and computational studies in acetonitrile solutions

1-Amino-, 1-ethylamino-, and 1-(diethylamino)-anthraquinone were characterized by UV–Vis spectroscopy, acid–base titration, electrochemical methods, and quantum-chemical (QM) calculations at the B3LYP/6-31 ++G^** level. Acid–base titration and the relative differences between the free energies of the basic and acidic forms of the studied species show that 1-(diethylamino)anthraquinone is the strongest base in an acetonitrile solution. Moreover, the structural differences between the B3LYP-optimized neutral and protonated anthraquinones, notably the presence or the absence of internal hydrogen bonds, account well for the sequence of the measured/calculated basicity. The basicity of the investigated compounds strongly influences their electrochemical properties in acetonitrile. Indeed, the cyclic voltammograms of 1-aminoanthraquinone and 1-(ethylamino)anthraquinone display two well-resolved reduction waves that indicate a two-step reduction process (EE mechanism). On the other hand, the electroreduction of 1-(diethylamino)anthraquinone becomes complicated by the interaction of its reduced forms with traces of water present in an acetonitrile solution (ECE mechanism). The mechanism of this reaction is proposed, and its possibility to occur is examined based on QM calculations.     

Langmuir-Blodgett films of a cationic zinc porphyrin-imidazole-functionalized fullerene dyad: formation and photoelectrochemical studies

Electron donor-acceptor dyad ensembles of a water-soluble cationic zinc porphyrin (viz., zinc tetrakis(N-methylpyridinium)porphyrin tetrachloride, Zn(TMPyP)) and a C60 derivative that bears an imidazole ligand (viz., 2-(phenylimidazolyl)fulleropyrrolidine, C60im) were assembled during the formation of Langmuir and then Langmuir-Blodgett (LB) films. Surface pressure versus surface area isotherms and surface pressure time profiles, as well as Brewster angle microscopic images documented that the Langmuir films formed were remarkably stable. Subsequently, these Langmuir films were transferred onto different solid substrates, by using the LB technique, for spectroscopic and photoelectrochemical characterization. The UV-vis spectroscopic investigations confirmed that the water-soluble Zn(TMPyP) was, indeed, transferred together with C60im in the LB films. Upon visible light illumination of these LB films, deposited on the ITO transparent conductive supports, a photocurrent generated in the C60im-Zn(TMPyP) system is ascribed to an efficient photoinduced electron transfer from the electron donor, porphyrin singlet excited-state to the electron acceptor, C60. Overall, internal photon-to-current efficiency, IPCE, of the photoanodic current generation (with ascorbate as a sacrificial electron donor) in the ITO/C60im-Zn(TMPyP)/ascorbate/Pt construct is over 5x larger than that of the photocathodic system (with methyl viologen, MV2+, as a sacrificial electron acceptor) in the ITO/Zn(TMPyP)-C60im/MV2+/Pt construct. Highly ordered film stacking favors vectorial electron transfer within the dyad, giving rise to the highest IPCE values of 2.5% determined for a photoanode that was composed of around 20 monolayer films.     

High-spin iron(III) complexes: structural,spectroscopic, and photochemical studies

Reaction of FeCl₃ with one equivalent of acac (acac = pentane-2,4-dionate) and KTp^Me2 (Tp^Me2 = hydrotris(3,5-dimethyl-pyrazol-1-yl)borate) yielded Tp^Me2Fe(acac)Cl (3), which upon reaction with methanolic solution of sodium azide resulted in the formation of a six coordinate compound Tp^Me2Fe(acac)N₃ (4) with a single azide. When the reaction of FeCl₃ and KTp^Me2 was performed with two equivalents of sodium azide and one equivalent of 3,5-dimethylpyrazole (Pz^Me2H), a six coordinate cis azide compound [Tp^Me2Fe(Pz^Me2H)(N₃)₂] (5) was obtained. These compounds were characterized by spectroscopic methods and single crystal X-ray crystallography. Electrochemical studies of 5 show that it can be irreversibly reduced at relatively lower potential than 4. The photolysis of 5 was performed at 77 K at different wavelengths (480, 419, and 330 nm) showing that 5 was photoreduced to a high-spin Fe(II) species instead of photooxidized to Fe(V).     

Synthesis, photophysical, photochemical, and computational studies of coumarin-labeled nicotinamide derivatives

The syntheses and photophysical/photochemical properties of two amide-tethered coumarin-labeled nicotinamides are described. Photochemical studies of 6-bromo-7-hydroxycoumarin-4-ylmethylnicotinamide (BHC-nicotinamide) revealed an unexpected solvent effect. This result is rationalized by computational studies of the different protonation states using TD-DFT with the M06L/6-311+G** method with implicit and explicit solvation models. Molecular orbital energies responsible for the λ(max) excitation show that the functionalization of the coumarin ring results in a strong red-shift from 330 to 370 nm when the pH of solution is increased from 3.06 to 8.07. From this MO analysis, a model for solvent interactions has been proposed. The BHC-nicotinamide proved to be photochemically stable, which is also interpreted in terms of NBO calculations. The results provide a set of principles for the rational design of either photostable labeling reagents or photolabile cage compounds.     

The photoreduction of zinc tetra-2,3-pyridino porphyrazine: A photochemical,electrochemical and spectroscopic study

The behavior of zinc tetra-2,3-porphyrazine (PcZnPy) has been studied by means of spectroscopy (u.v.-visible and in situ Raman diffusion), electrochemistry and photochemistry. In contrast to its phthalocyanin analog, this dye has to undergo a one-electron reduction process prior to being able to photosensitize reduction reactions. The application of this property to perform two-electron transfers is considered.     

Oxidation of nitroxyl radicals: electrochemical and computational studies

We have developed azaadamantane-type nitroxyl radicals (AZADOs) and azabicyclo-type nitroxyl radicals (ABNOs) as highly active alcohol oxidation catalysts. Herein, the electrochemical properties of these nitroxyl radicals were investigated by measuring their formal potentials using cyclic voltammetry (CV). The redox potentials were rationalized with the aid of density functional theory (DFT) calculations. A good correlation between the experimental redox potential and the DFT-computed energy differences (ΔE) between nitroxyl radicals and oxoammonium species was obtained, which shows the effectiveness of DFT in predicting the redox potentials of nitroxyl radicals. Redox potential appears to be an important factor of catalytic activity.     

Structure-reactivity relationship of piperidine nitroxide: electrochemical, ESR and computational studies

We have synthesized several nitroxides with different substituents which vary the steric and electronic environment around the N-O moiety and have systematically investigated the role of substituents on the stability of the radicals. Our results demonstrated the reactivity toward ascorbate correlates with the redox potential of the derivatives. Furthermore, ab initio calculations also indicated a correlation between the reduction rate and the computed singly occupied molecular orbital-lowest unoccupied molecular orbital energy gap, but not with solvent accessible surface area of the N-O moiety, supporting the experimental results and suggesting that the electronic factors largely determine the radicals' stability. Hence, it is possible to perform virtual screening of nitroxides to optimize their stability, which can help to rationally design novel nitroxides for their potential use in vivo.     

Molecular triads composed of ferrocene,C60, and nitroaromatic entities: electrochemical,computational, and photochemical investigations

Synthesis and physicochemical characterization of a series of molecular triads composed of ferrocene, C(60), and nitroaromatic entities are reported. Electrochemical studies revealed multiple redox processes involving all three redox active ferrocene, C(60), and nitrobenzene entities. Up to eight redox couples within the accessible potential window of o-dichlorobenzene containing 0.1 M (TBA)ClO(4) are observed. A comparison between the measured redox potentials with those of the starting compounds revealed absence of any significant electronic interactions between the different redox entities. The geometric and electronic structure of the triads are elucidated by using ab initio B3LYP/3-21G methods. In the energy-optimized structures, as predicted by electrochemical studies, the first HOMO orbitals are found to be located on the ferrocene entity, while the first LUMO orbitals are mainly on the C(60) entity. The coefficients of the subsequent LUMO orbitals track the observed site of electrochemical reductions of the triads. The photochemical events of the triads are probed by both steady-state and time-resolved techniques. The steady-state emission intensities of the triads and the starting dyad, 2-(ferrocenyl)fulleropyrrolidine, are found to be completely quenched compared to fulleropyrrolidine bearing no redox active substituents. The subpicosecond and nanosecond transient absorption spectral studies revealed efficient charge separation (and rapid charge recombination) in the triads, and this has been attributed to the close spacing of the redox entities of the triad to one another.     

First dicyanamide-bridged spin-crossover coordination polymer: synthesis, structural, magnetic, and spectroscopic studies

We report here on the synthesis and characterisation of a first iron(II) spin-crossover coordination polymer with the dca spacer ligand, having the formula [Fe(aqin)2(dca)]ClO4.MeOH (aqin=8-aminoquinoline, dca=dicyanamide), which displays a two-step complete spin transition. Variable-temperature magnetic susceptibility measurements and M?ssbauer spectroscopy have revealed that the two relatively gradual steps are centred at 215 and 186 K and are separated by an inflection point at about 201 K, at which 50 % of the complex molecules undergo a spin transition. The two steps are related to the existence of two crystallographically inequivalent metal sites, as confirmed by the structural and M?ssbauer studies. The crystal structure was resolved at 293 K (HS form) and 130 K (LS form). Both spin-state structures belong to the triclinic P1 space group (Z=2). The complex assumes a linear chain structure, in which the active iron(II) sites are linked to each other by anionic dicyanamide ligands acting as chemical bridges. The Fe-Fe distances through the dca ligand are 8.119(1) and 7.835(1) A in the high-spin and low-spin structures, respectively. The polymeric chains extend along a (1, 0, -1) axis and are packed in sheets, between which the perchlorate anions and methanol molecules are inserted. The complex molecules are linked together by pi-stacking interactions and H-bonding between the H-donor aqin ligands and the perchlorate ions. These structural features provide a basis for cooperative interactions in the crystal lattice. Analysis of the two-step spin-crossover character in this compound suggests that covalent interactions through the spacer ligand do not provide the main mechanism of cooperativity.     

Formation, spectral, electrochemical, and photochemical behavior of zinc N-confused porphyrin coordinated to imidazole functionalized fullerene dyads

Donor-acceptor dyads were constructed using zinc N-confused porphyrin (ZnNCP), a structural isomer of zinc tetraphenylporphyrin, as a donor, and fullerene as an electron acceptor. Two derivatives, pyridine-coordinated zinc N-confused porphyrin (Py:ZnNCP) and the zinc N-confused porphyrin dimer (ZnNCP-dimer) were utilized to form the dyads with an imidazole-appended fulleropyrrolidine (C60Im). These porphyrin isomers formed well-defined 1:1 supramolecular dyads (C60Im:ZnNCP) via axial coordination. The dyads were characterized by optical absorption and emission, ESI-mass, 1H NMR, and electrochemical methods. The binding constant, K, was found to be 2.8 x 10(4) M(-1) for C60Im:ZnNCP. The geometric and electronic structure of C60Im:ZnNCP were probed by using DFT B3LYP/3-21G methods. The HOMO was found to be on the ZnNCP entity, while the LUMO was primarily on the fullerene entity. The electrochemical properties of C60Im:ZnNCP was probed using cyclic voltammetry in o-dichlorobenzene, 0.1 n-Bu4NClO4. The Py:ZnNCP was found to be easier to oxidize by over 340 mV compared to Py:ZnTPP. Upon dyad formation via axial coordination, the first oxidation revealed an anodic shift of nearly 90 mV. Evidence of photoinduced charge separation from the singlet excited ZnNCP to the appended fullerene was established from time-resolved emission and nanosecond transient absorption studies.     

Non-aqueous solvation of n-octanol and ethanol: spectroscopic and computational studies

Raman spectroscopy was used to examine the interactions of the free O-H bonds in n-octanol and ethanol with the organic solvents carbon tetrachloride (CCl(4)), cyclohexane, and benzene. These spectra reveal that the solvents CCl(4) and cyclohexane have a small effect on the free O-H peak of alcohols, whereas benzene as a solvent significantly red-shifts the free O-H band. Calculated spectra were generated via MP2/6-31G* calculations and the B3LYP/6-31+G**//MP2/6-31G*-derived Boltzmann populations of each ethanol complex and are consistent with the experimental results. Additional spectra were calculated using Boltzmann populations derived from single-point energies at the polarizable continuum model (PCM) level with the B3LYP/6-31+G** level of theory to take overall solvent effects into account, and these simulated spectra are also largely consistent with the experimental results. Analysis of the computational results reveals a lengthening of the O-H bond from the O-H interaction with the delocalized electronic structure of benzene as well as a bimodal distribution of the free O-H peak of the alcohol/benzene mixtures due to two distinctly different types of alcohol/benzene complexes.     

Hexadecapropyloxy-substituted diphthalocyanine complexes of rare-earth elements: synthesis,spectroscopic and electrochemical studies

Hexadecapropyloxy-substituted diphthalocyanine complexes of rare-earth elements (REE = Lu, Tm, Sm) were synthesized. The new symmetrically substituted diphthalocyanine complexes prepared starting from 4,5-dipropyloxyphthalodinitrile (phthalogen) are characterized by better solubilities compared to the known hexadecamethyl-substituted diphthalocyanine complexes of the same REE. Spectral and electrochemical characteristics of the complexes were studied. The compounds can be used as materials for high-contrast electrochromic devices.     

Nitro-functionalized oligothiophenes as a novel type of electroactive molecular material: spectroscopic,electrochemical, and computational study

A novel series of terthiophenes bearing electron-donor and electron-acceptor groups at the end alpha-positions has been prepared. The analysis of the UV-vis, infrared, and Raman spectra, performed with the aid of density functional theory calculations, shows that the asymmetrically substituted nitro compounds PhT(3)NO(2) and BrT(3)NO(2) behave as push-pull systems and present an intense photoinduced charge transfer in the visible spectrum. The symmetrically substituted dinitro compound NO(2)T(3)NO(2) displays a highly delocalized structure with a low single-double bond length alternation and also displays a low-energy absorption band in the visible region. The novel nitroterthiophenes possess attractive electrochemical properties since they generate stable species both upon oxidation and reduction. Oxidation mainly involves changes in the oligothiophene backbone and leads to the formation of stable cations even for NO(2)T(3)NO(2). Reduction is mainly nitro-centered but also affects the conjugated structure. Radical anions and dianions are formed for PhT(3)NO(2) and BrT(3)NO(2). Dianions, not radical anions, and trianions are obtained for NO(2)T(3)NO(2). Nitro-functionalized terthiophenes are shown to be very promising as electroactive molecular materials since they behave as push-pull systems, present a very intense photoinduced charge transfer in the visible region, and could act as both n- and p-channel conductors in organic electronic transistors.     

Synthetic, spectroscopic, computational and structural studies of some 13-vertex ruthenacarboranes

Reduction of 1,2-closo-C2B10H12 followed by treatment with [RuCl2(p-cymene)]2(p-cymene = C6H4MeiPr-1,4) affords the 13-vertex ruthenacarborane 4-(p-cymene)-4,1,6-closo-RuC2B10H12, characterised both spectroscopically and, in two crystalline forms, crystallographically. Although asymmetric in the solid state, having a docosahedral cage architecture with cage C atoms at vertices 1 and 6, this species clearly has Cs symmetry on the NMR timescale at room temperature. However, the fluctional process in operation can be arrested at low temperature, and an activation energy of 43.1 kJ mol(-1) is estimated. A computational study of the related species 4-(eta-C6H6)-4,1,6-closo-RuC2B10H12 reveals that the fluctionality is due to a double diamond-square-diamond process, first suggested by Hawthorne et al for the analogous CpCo species. These calculations yield an activation energy of 40.4 kJ mol(-1), in excellent agreement with that derived from experiment. Reduction of 1,2-Ph(2)-1,2-closo-C2B10H10 followed by treatment with [RuCl2(eta-C6H6)]2 or [RuCl2(p-cymene)]2 yields the analogous species 1,6-Ph2-4-(eta-C6H6)-4,1,6-closo-RuC2B10H10 and 1,6-Ph2-4-(p-cymene)-4,1,6-closo-RuC2B10H10, respectively. These C,C-diphenyl compounds were again studied spectroscopically and crystallographically, the p-cymene species again showing two crystalline modifications. In contrast to their CpCo and Cp*Co analogues all three ruthenacarboranes do not undergo isomerisation in refluxing toluene.     

Ruthenium complexes with lumazine derivatives: structural,electrochemical, computational and radical scavenging studies

In this research study, the formation and characterization of new ruthenium(II) and (III) complexes encompassing multidentate ligands derived from 6-acetyl-1,3,7-trimethyllumazine (almz) are reported. The 1:1 molar coordination reactions of trans-[RuCl₂(PPh₃)₃] with N-1-[1,3,7-trimethyllumazine]benzohydride (bzlmz) and 6-(N-methyloxime)-1,3,7-trimethyllumazine (ohlmz) formed a diamagnetic ruthenium(II) complex, cis-[RuCl₂(bzlmz)(PPh₃)] (1), and paramagnetic complex, cis-[Ru^IIICl₂(olmz)(PPh₃)] (2) [Holmz = 6-(N-hydroxy-N′-methylamino)-1,3,7-trimethyllumazine], respectively. These ruthenium complexes were characterized via physico-chemical and spectroscopic methods. Structural elucidations of the metal complexes were confirmed using single crystal X-ray analysis. The redox properties of the metal complexes were investigated via cyclic voltammetry. Electron spin resonance spectroscopy confirmed the presence of a paramagnetic metal centre in 2. The radical scavenging activities of the metal complexes were explored towards the DPPH and NO radicals. Quantum calculations at the density functional theory level provided insight into the interpretation of the IR and UV–Vis experimental spectra of 1.     

The electronic influence of ring substituents and ansa bridges in zirconocene complexes as probed by infrared spectroscopic,electrochemical, and computational studies

The electronic influence of unbridged and ansa-bridged ring substituents on a zirconocene center has been studied by means of IR spectroscopic, electrochemical, and computational methods. With respect to IR spectroscopy, the average of the symmetric and asymmetric stretches (nu(CO(av))) of a large series of dicarbonyl complexes (Cp(R))(2)Zr(CO)(2) has been used as a probe of the electronic influence of a cyclopentadienyl ring substituent. For unbridged substituents (Me, Et, Pr(i), Bu(t), SiMe(3)), nu(CO(av)) on a per substituent basis correlates well with Hammett sigma(meta) parameters, thereby indicating that the influence of these substituents is via a simple inductive effect. In contrast, the reduction potentials (E degrees ) of the corresponding dichloride complexes (Cp(R))(2)ZrCl(2) do not correlate well with Hammett sigma(meta) parameters, thereby suggesting that factors other than the substituent inductive effect also influence E degrees. Ansa bridges with single-atom linkers, for example [Me(2)C] and [Me(2)Si], exert a net electron-withdrawing effect, but the effect is diminished upon increasing the length of the bridge. Indeed, with a linker comprising a three-carbon chain, the [CH(2)CH(2)CH(2)] ansa bridge becomes electron-donating. In contrast to the electron-withdrawing effect observed for a single [Me(2)Si] ansa bridge, a pair of vicinal [Me(2)Si] ansa bridges exerts an electron-donating effect relative to that from the single bridge. DFT calculations demonstrate that the electron-withdrawing effect of the [Me(2)C] and [Me(2)Si] ansa-bridges is due to stabilization of the cyclopentadienyl ligand acceptor orbital, which subsequently enhances back-donation from the metal. The calculations also indicate that the electron-donating effect of two vicinal [Me(2)Si] ansa bridges, relative to that of a single bridge, is a result of it enforcing a ligand conformation that reduces back-donation from the metal.