Modulating charge separation and charge recombination dynamics in porphyrin-fullerene linked dyads and triads: Marcus-normal versus inverted region.

Photoinduced charge separation (CS) and charge recombination (CR) processes have been examined in various porphyrin-fullerene linked systems (i.e., dyads and triads) by means of time-resolved transient absorption spectroscopy and fluorescence lifetime measurements. The investigated compounds comprise a homologous series of rigidly linked, linear donor-acceptor arrays with different donor-acceptor separations and diversified donor strength: freebase porphyrin-C60 dyad (H2P-C60), zincporphyrin-C60 dyad (ZnP-C60), ferrocene-zincporphyrin-C60 triad (Fc-ZnP-C60), ferrocene-freebase porphyrin-C60 triad (Fc-H2P-C60), and zincporphyrin-freebase porphyrin-C60 triad (ZnP-H2P-C60). Most importantly, the lowest lying charge-separated state of all the investigated systems, namely, that of ferrocenium ion (Fc+) and the C60 radical anion (C60.-) pair in the Fc-ZnP-C60 triad, has been generated with the highest quantum yields (close to unity) and reveals a lifetime as long as 16 micros. Determination of CS and CR rate constants, together with the one-electron redox potentials of the donor and acceptor moieties in different solvents, has allowed us to examine the driving force dependence (-DeltaG0ET) of the electron-transfer rate constants (kET). Hereby, the semilogarithmic plots (i.e., log kET versus -DeltaG0ET) lead to the evaluation of the reorganization energy (lambda) and the electronic coupling matrix element (V) in light of the Marcus theory of electron-transfer reactions: lambda = 0.66 eV and V = 3.9 cm(-1) for ZnP-C60 dyad and lambda = 1.09 eV and V = 0.019 cm(-1) for Fc-ZnP-C60, Fc-H2P-C60, and ZnP-H2P-C60 triads. Interestingly, the Marcus plot in Fc-ZnP-C60, Fc-H2P-C60, and ZnP-H2P-C60 has provided clear evidence for intramolecular CR located in both the normal and inverted regions of the Marcus parabola. The coefficient for the distance dependence of V (damping factor: betaCR = 0.58 A(-1) is deduced which depends primarily on the nature of the bridging molecule.     

Entropic changes control the charge separation process in triads mimicking photosynthetic charge separation

Laser-induced optoacoustic spectroscopy (LIOAS) measurements with carotene-porphyrin-acceptor "supermolecular" triads (C-P-A, with A = C60, a naphthoquinone NQ, and a naphthoquinone derivative, Q) were carried out with the purpose of analyzing the thermodynamic parameters for the formation and decay of the respective long-lived charge separated state C*+-P-A*-. The novel procedure of inclusion of the benzonitrile solutions of the triads in Triton X-100 micelle nanoreactors suspended in water permitted the separation of the enthalpic and structural volume change contributions to the LIOAS signals, by performing the measurements in the range 4-20 degrees C. Contractions of 4.2, 5.7, and 4.2 mL mol-1 are concomitant with the formation of C*+-P-A*- for A = C60, Q and NQ, respectively. These contractions are mostly attributed to solvent movements and possible conformational changes upon photoinduced electron transfer, due to the attraction of the oppositely charged ends, as a consequence of the giant dipole moment developed in these compounds upon charge separation ( approximately 110 D). The estimations combining the calculated free energies and the LIOAS-derived enthalpy changes indicate that entropy changes, attributed to solvent movements, control the process of electron transfer for the three triads, especially for C-P-C60 and C-P-Q. The heat released during the decay of 1 mol of charge separated state (CS) is much smaller than the respective enthalpy content obtained from the LIOAS measurements for the CS formation. This is attributed to the production of long-lived energy storing species upon CS decay.     

Photoinduced electron transfer in double-bridged porphyrin-fullerene triads

Electron and energy transfer reactions of porphyrin-porphyrin-fullerene triads (P2P1C) with controllable sandwich-like structures have been studied using spectroscopic and electrochemical methods. The stabile, stacked structure of the molecules was achieved applying a two-linker strategy developed previously for porphyrin-fullerene dyads. Different triad structures with altered linker positions, linker lengths, and center atoms of the porphyrin rings were studied. The final charge-separated (CS) state and the different transient states of the reactions have been identified and energies of the states estimated based on the experimental results. In particular, a complete CS state P2(+) P1C- was achieved in a zinc porphyrin-free-base porphyrin-fullerene triad (ZnP2t9P1C) in both polar (benzonitrile) and nonpolar (toluene) solvents. The lifetime of this state was longer living in the nonpolar solvent. An outstanding feature of the ZnP2t9P1C triad is the extremely fast formation of the final CS state, P2(+) P1C-. This state is formed after primary excitation of either zinc porphyrin or free-base porphyrin chromophores in less than 200 fs. Although the intermediate steps between the locally excited states and the final CS state were not time-resolved for this compound, the process is clearly multistep and the fastest ever observed for porphyrin-based compounds.     

Photochemical charge separation in closely positioned donor-boron dipyrrin-fullerene triads

A series of molecular triads, composed of closely positioned boron dipyrrin-fullerene units, covalently linked to either an electron donor (donor(1)-acceptor(1)-acceptor(2)-type triads) or an energy donor (antenna-donor(1)-acceptor(1)-type triads) was synthesized and photoinduced energy/electron transfer leading to stabilization of the charge-separated state was demonstrated by using femtosecond and nanosecond transient spectroscopic techniques. The structures of the newly synthesized triads were visualized by DFT calculations, whereas the energies of the excited states were determined from spectral and electrochemical studies. In the case of the antenna-donor(1)-acceptor(1)-type triads, excitation of the antenna moiety results in efficient energy transfer to the boron dipyrrin entity. The singlet-excited boron dipyrrin thus generated, undergoes subsequent energy and electron transfer to fullerene to produce a boron dipyrrin radical cation and a fullerene radical anion as charge-separated species. Stabilization of the charge-separated state in these molecular triads was observed to some extent.     

Photoregulated transmembrane charge separation by linked spiropyran-anthraquinone molecules

Amide-linked spiropyran-anthraquinone (SP-AQ) conjugates were shown to mediate ZnTPPS(4-)-photosensitized transmembrane reduction of occluded Co(bpy)3(3+) within unilamellar phosphatidylcholine vesicles by external EDTA. Overall quantum yields for these reactions were dependent upon the isomeric state of the dye; specifically, 30-35% photoconversion of the closed-ring spiropyran (SP) moiety to the open-ring merocyanine (MC) form caused the quantum yield to decrease by 6-fold in the simple conjugate and 3-fold for an analogue containing a lipophilic 4-dodecylphenoxy substituent on the anthraquinone moiety. Transient spectroscopic and fluorescence quenching measurements revealed that two factors contributed to these photoisomerization-induced changes in quantum yields: increased efficiencies of fluorescence quenching of 1ZnTPPS4- by the merocyanine group and lowered transmembrane diffusion rates of the merocyanine-containing redox carriers. Transient spectrophotometry also revealed the sequential formation and decay of two reaction intermediates, identified as 3ZnTPPS4- and a species with the optical properties of a semiquinone radical. Kinetic profiles for Co(bpy)3(3+) reduction under continuous photolysis in the presence and absence of added ionophores indicated that transmembrane redox mediated by SP-AQ was electroneutral, but reaction by the other quinone-containing mediators was electrogenic. The minimal reaction mechanism suggested from the combined studies is oxidative quenching of vesicle-bound 3ZnTPPS4- by the anthraquinone unit, followed by either H+/e- cotransport by transmembrane diffusion of SP-AQH* or, for the other redox mediators, semiquinone anion-quinone electron exchange leading to net transmembrane electron transfer, with subsequent one-electron reduction of the internal Co(bpy)3(3+). Thermal one-electron reduction of Co(bpy)3(3+) by EDTA is energetically unfavorable; the photosensitized reaction therefore occurs with partial conversion of photonic energy to chemical and transmembrane electrochemical potentials.     

The charge distribution in chromium(III) and iron(III) hexafluoride

Semi-empirical molecular orbital calculations were performed for CrF^3?₆ and FeF^3?₆. The method of calculation is derived from the HF SCF equations using approximations appropriate to the highly ionic transition metal halides. The results of these calculations are shown to be in better agreement than previous semis-empirical calculations with X-ray emission line shifts and atom charges and populations estimated from these shifts.     

The separation of berkelium(III) from cerium(III)

Factors affecting the separation of Bk(III) and Ce(III) in liquid-liquid extraction by tertiary and quaternary amines from neutral solutions of Li, Na, NH₄ and K nitrates have been investigated. It has been found that the maximum separation factor is reached in the extraction from NaNO₃ solutions by solutions of methyl-containing alkylammonium nitrates in xylene.     

Photoinduced intramolecular electron transfer and charge separation in zinc phthalocyanine-viologen linked system

Photoinduced electron transfer and charge separation processes in zinc phthalocya-nine-viologen linked system have been studied and the distance effect of donor/acceptor on electron transfer reaction is discussed. It is indicated that the fluorescence from the zinc phthalocyanine moiety is appreciably quenched and the life-time of singlet excited state is reduced by the pendant viologen. Time-resolved transient absorption spectra measurements show that intramolecular quenching of the triplet state of zinc phthalocyanine by the attached viologen results in charge separation giving reduced viologen radical alive for a rather long period with hundred microsecond duration. The effect of the carbon chain length on the electron transfer rate constant and charge separation efficiency suggests that upon excitation, the zinc phthalocyanine and viologen groups tend to take closer conformation with the increase of the carbon chain examined. The rate constant for the intramolecular electron transfer ket with n = 3     

Unexpected change in charge transfer behavior in a cobalt(II) porphyrin-fullerene conjugate that stabilizes radical ion pair states

Two cobalt(II) porphyrin-C(60) malonate-linked conjugates, the mono-connected Co1 and the bis-connected trans-2 isomer Co3, have been synthesized for the first time either by direct cyclopropanation with the precursor malonate Co4 or by metalation of the bisadduct H(2)3. For the investigation of the interaction between the porphyrin donor and fullerene acceptor within these dyads, electrochemical and photophysical investigations have been carried out. Compared to Zn3 and trans-2 bisadduct 7, the first reduction of the fullerene moiety within Co3 becomes easier (40 mV in dichloromethane and 20 mV in benzonitrile), indicating significant interactions between the pi-system of the fullerene and the d-orbitals of the central Co atom. Compared to the Co complexes 9, Co4, and Co1, the first oxidation of Co3 is considerably shifted to more positive potentials, if benzonitrile instead of dichloromethane is used as solvent. At the same time, the oxidation is no longer centered on the Co(II) center but on the porphyrin macrocycle, as corroborated by spectroelectrochemistry. A similar solvent dependence was observed in transient absorption spectroscopic measurements. In toluene, benzonitrile and anisole photoinduced electron transfer within Co3 leads to the formation of a charge-separated state Co(II)P.+ -C(60).- with a lifetime of 560 +/- 20 ns in benzonitrile, whereas in other solvents such as THF, nitrobenzene, ortho-diclorobenzene, and tert-butylbenzene the formation of a Co(III)P-C(60).- as transient was detected, which is, however, short-lived (860 +/- 40 ps in THF) and exhibits charge recombination dynamics that are in the Marcus inverted region. Particularly important is the fact that the electronic coupling (V) in Co(III)P-C(60).- is 18 cm(-1) substantially smaller than the V value of 313 cm(-1) in ZnP.+ -C(60).- .     

Sequential oxidimetric determination of thallium(I) and (III)

The reaction between thallium(III) and oxalic acid in sulphuric acid medium has been investigated. Spectrophotometric results show that thallium(III) can be quantitatively reduced to thallium(I) with oxalic acid in aqueous medium when heated to near boiling point. Conditions for the estimation of the excess of oxalic acid with cerium(IV) sulphate in the presence of thallium(I) and for the estimation of a mixture of thallium(I) and thallium(III) have been investigated. The method is simpler than many other redox methods reported for the determination of thallium(III) and is free from many interferences encountered in these titrations. The reagents are cheap and quite stable.     

Extraction and separation of bismuth(III)

Separation of bismuth from beryllium, lead, iron(III), indium, scandium, lanthanum, antimony(III), zirconium, titanium, thorium, vanadium(V), molybdenum(VI), uranium (VI) and chromium(VI) is achieved by selective extraction of bismuth from 0.1M sodium salicylate solution (adjusted to pH 7) into mesityl oxide (MeO). The extracted species is Bi (HOC(6)H(4)COO)(3).3MeO. The results are accurate within +/- 0.5%, with a standard deviation of 0.8%. The separation and determination of bismuth takes only 15 min.     

Experimental and theoretical charge density distribution in two ternary cobalt(III) complexes of aromatic amino acids

The experimental charge density distributions in two optically active isomers of a Co complex have been determined. The complexes are Delta-alpha-[Co(R,R-picchxn)(R-trp)](ClO4)2.H2O) (1) and Lambda-beta1-[Co(R,R-picchxn)(R-trp)](CF3SO3)2) (2), where picchxn is N,N'-bis(2-picolyl-1,2-diaminocyclohexane) and R-trp is the R-tryptophane anion. The molecular geometries of 1 and 2 are distinguished by the presence in complex 1 of intramolecular pi...pi stacking interactions and the presence in complex 2 of intramolecular hydrogen bonding. This pair of isomers therefore serves as an excellent model for studying noncovalent interactions and their effects on structure and electron density and the transferability of electron density properties between closely related molecules. For complex 2, a combination of X-ray and neutron diffraction data created the basis for a X-N charge density refinement. A topological analysis of the resulting density distribution using the atoms in molecules methodology is presented along with d-orbital populations, showing that the metal-ligand bonds are relatively unaltered by the geometry changes between 1 and 2. The experimental density has been supplemented by quantum chemical calculations on the cobalt complex cations: close agreement between theory and experiment is found in all cases. The energetics of the weak interactions are analyzed using both theory and experiment showing excellent quantitative agreement. In particular it is found that both methods correctly predict the stability of 2 over 1. The transferability between isomers of the charge density and derived parameters is investigated and found to be invalid for these structurally related systems.     

Covalently linked heterofullerene-porphyrin conjugates; new model systems for long-lived intramolecular charge separation

Attaching tetraphenyl porphyrins, with peripheral acetyl or malonate groups, to C₅₉N leads to the first covalently linked heterofullerene-porphyrin conjugates that exhibit long-lived intramolecular charge separation.     

Gold nanoparticle enhanced charge transfer in thin film assemblies of porphyrin-fullerene dyads

Photoinduced vectorial electron transfer in a molecularly organized porphyrin-fullerene (PF) dyad film is enhanced by the interlayer charge transfer from the porphyrin moiety of the dyad to an octanethiol protected (dcore approximately 2 nm) gold nanoparticle (AuNP) film. By using the time-resolved Maxwell displacement charge (TRMDC) method, the charge separation distance was found to increase by 5 times in a multilayer film structure where the gold nanoparticles face the porphyrin moiety of the dyad, that is, AuNP|PF, compared to the case of the PF layer alone. Films were assembled by the Langmuir-Blodgett (LB) method using octadecylamine (ODA) as the matrix compound. Atomic force microscopy (AFM) images of the monolayers revealed that AuNPs are arranged into continuous, islandlike structures and PF dyads form clusters. The porphyrin reference layer was assembled with the AuNP layer to gain insight on the interaction mechanism between porphyrin and gold nanoparticles. Interlayer electron transfer was also observed between the AuNPs and porphyrin reference, but the efficiency is lower than that in the AuNP|PF film. Fluorescence emission of the reference porphyrin is slightly quenched, and fluorescence decay becomes faster in the presence of AuNPs. The proposed mechanism for the electron transfer in the AuNP|PF film is thus the primary electron transfer from the porphyrin to the fullerene followed by a secondary hole transfer from the porphyrin to the AuNPs, resulting in an increased charge separation distance and enhanced photovoltage.     

From ruthenium(II) to iridium(III): 15 years of triads based on bis-terpyridine complexes

In order to mimic the photosynthetic reaction centre and better understand photoinduced electron transfer processes, a family of compounds has been studied for the past 15 years. These are transition metal complexes, M(tpy)(2) where tpy is a 2,2':6',2" terpyridine based ligand, bearing on one side a donor group and on the other side an acceptor group. The resulting triad molecules or their two-component reference compounds (donor-M(tpy)(2) and M(tpy)(2-acceptor) can contain Ru, Os, Rh or Ir as the metal centre and both visible-light non absorbing groups and porphyrins as donor and acceptor groups. This tutorial review will briefly present the different systems studied and the reasons that led to the preparation of new systems with improved performances.     

Sequential titration of iron(II), antimony(III) and arsenic(III) in binary or ternary mixture

A simple titrimetric procedure for the determination of iron(II), antimony(III) and arsenic-(III) in a mixture, with cerium(IV) sulphate as titrant, is proposed. The end-points can all be detected with ferroin or potentiometrically. Phosphoric acid medium is used for titration of the iron(II), then acetic acid medium for the titration of antimony(III), and finally the arsenic(III) is titrated in presence of iodine as catalyst. The procedure utilizes a single portion of test solution; it can be adopted for the analysis of binary mixtures.     

Light intensity effects on photoinduced charge separation parameters in a molecular triad based on an Iridium(III) bis(terpyridine) unit.

The effect of photon flux on the yield and lifetime of charge separation over the extreme components of a D-Ir-A triad, where D is a triphenyl amine electron donor, A is a naphthalene bis(imide) electron acceptor and Ir is an Ir(III) bis(terpyridine) complex, has been investigated. In usual laboratory conditions, with nanosecond and picosecond laser pulses in the 4-8 mJ range, biphotonic processes take place. Biphotonic products and their evolution can introduce complications in reaction mechanisms and their interpretation but can also drastically reduce the yield and the lifetime of the charge-separated state. In the present case, after discussion of several possible mechanisms, the process detrimental to charge separation is ascribed to absorption of a photon by the photogenerated charge-separated state D(+)-Ir-A(-).     

Strong effects of the bridge configuration on photoinduced charge separation in rigidly linked donor-acceptor systems

Photoinduced electron-transfer rates are reported for two pairs of rigid bichromophoric molecules 1(6)/2(6) and 1(8)/2(8). In the first pair electron donor and acceptor are separated by six, in the second pair by eight, carbon—carbon σ bonds. While these σ bonds provide an all-trans coupling path in 1(6) and 1(8), that path contains s-cis elements in 2(6) and 2(8), which - as shown by X-ray structure data and by spectroscopic evidence - leads to a slight decrease in the effective, spatial donor-acceptor separation. Nevertheless, photoinduced electron transfer in each of the “stretched” compounds is about one order of magnitude faster than in the corresponding “bent” compound. This remarkable effect is interpreted as resulting from the unique ability of an all-trans array of σ bonds to mediate electronic through-bond interaction (TBI). Interestingly the solvent dependence of the rate of photoinduced electron transfer is significantly larger in the “bent” systems, thus indicating that superexchange via solvent molecules becomes competitive with TBI if an all-trans array is not available.     

Porphyrinic dyads and triads assembled around iridium(III) bis-terpyridine: photoinduced electron transfer processes

Multicomponent arrays based on a central iridium(III) bis-terpyridine complex (Ir) used as assembling metal and free-base, zinc(II) or gold(III) tetraaryl-porphyrins (PH(2), PZn, PAu) have been designed to generate intramolecular photoinduced charge separation. The rigid dyads PH(2)-Ir, PZn-Ir, PAu-Ir, and the rigid and linear triads PH(2)-Ir-PAu, PZn-Ir-PAu, as well as the individual components Ir, PH(2), PZn, PAu have been synthesized and characterized by various techniques including electrochemistry. Their photophysical properties either in acetonitrile or in dichloromethane and toluene have been determined by steady-state and time-resolved methods. In acetonitrile, excitation of the triad PH(2)-Ir-PAu leads to a charge separation with an efficiency of 0.5 and a resulting charge-separated (CS) state with a lifetime of 3.5 ns. A low-lying triplet localized on PH(2) and the presence of the heavy Ir(III) ion offer the CS state an alternative deactivation path through the triplet state. The behavior of the triad PZn-Ir-PAu in dichloromethane is rather different from that of PH(2)-Ir-PAu in acetonitrile since the primary electron transfer to yield PZn(+)()-Ir(-)-PAu is not followed by a secondary electron transfer. In this solvent, both unfavorable thermodynamic and electronic parameters contribute to the inefficiency of the second electron-transfer reaction. In contrast, in toluene solutions, the triad PZn-Ir-PAu attains a CS state with a unitary yield and a lifetime of 450 ns. These differences can be understood in terms of ground-state charge-transfer interactions as well as different stabilization of the intermediate and final CS states by solvent.