Photoinduced charge separation in three-layer supramolecular nanohybrids: fullerene-porphyrin-SWCNT

Photoinduced charge separation processes of three-layer supramolecular hybrids, fullerene-porphyrin-SWCNT, which are constructed from semiconducting (7,6)- and (6,5)-enriched SWCNTs and self-assembled via π-π interacting long alkyl chain substituted porphyrins (tetrakis(4-dodecyloxyphenyl)porphyrins; abbreviated as MP(alkyl)(4)) (M = Zn and H(2)), to which imidazole functionalized fullerene[60] (C(60)Im) is coordinated, have been investigated in organic solvents. The intermolecular alkyl-π and π-π interactions between the MP(alkyl)(4) and SWCNTs, in addition, coordination between C(60)Im and Zn ion in the porphyrin cavity are visualized using DFT calculations at the B3LYP/3-21G(*) level, predicting donor-acceptor interactions between them in the ground and excited states. The donor-acceptor nanohybrids thus formed are characterized by TEM imaging, steady-state absorption and fluorescence spectra. The time-resolved fluorescence studies of MP(alkyl)(4) in two-layered nanohybrids (MP(alkyl)(4)/SWCNT) revealed efficient quenching of the singlet excited states of MP(alkyl)(4) ((1)MP*(alkyl)(4)) with the rate constants of charge separation (k(CS)) in the range of (1-9) × 10(9) s(-1). A nanosecond transient absorption technique confirmed the electron transfer products, MP˙(+)(alkyl)(4)/SWCNT˙(-) and/or MP˙(-)(alkyl)(4)/SWCNT˙(+) for the two-layer nanohybrids. Upon further coordination of C(60)Im to ZnP, acceleration of charge separation via(1)ZnP* in C(60)Im→ZnP(alkyl)(4)/SWCNT is observed to form C(60)˙(-)Im→ZnP˙(+)(alkyl)(4)/SWCNT and C(60)˙(-)Im→ZnP(alkyl)(4)/SWCNT˙(+) charge separated states as supported by the transient absorption spectra. These characteristic absorptions decay with rate constants due to charge recombination (k(CR)) in the range of (6-10) × 10(6) s(-1), corresponding to the lifetimes of the radical ion-pairs of 100-170 ns. The electron transfer in the nanohybrids has further been utilized for light-to-electricity conversion by the construction of proof-of-concept photoelectrochemical solar cells.     

Synthesis and photophysical properties of fullerene-phthalocyanine-porphyrin triads and pentads

The synthesis and photophysical properties of several fullerene-phthalocyanine-porphyrin triads (1-3) and pentads (4-6) are described. The three photoactive moieties were covalently connected in an one-step synthesis through 1,3-dipolar cycloaddition to C(60) of the corresponding azomethine ylides generated in situ by condensation reaction of a substituted N-porphyrinylmethylglycine derivative and an appropriated formyl phthalocyanine or a diformyl phthalocyanine derivative, respectively. ZnP-C(60)-ZnPc (3), (ZnP)(2)-ZnPc-(C(60))(2) (6), and (H(2)P)(2)-ZnPc-(C(60))(2) (5) give rise upon excitation of their ZnP or H(2)P components to a sequence of energy and charge-transfer reactions with, however, fundamentally different outcomes. With (ZnP)(2)-ZnPc-(C(60))(2) (6) the major pathway is an highly exothermic charge transfer to afford (ZnP)(ZnP(.+))-ZnPc-(C(60)(.-))(C(60)). The lower singlet excited state energy of H(2)P (i.e., ca. 0.2 eV) and likewise its more anodic oxidation (i.e., ca. 0.2 V) renders the direct charge transfer in (H(2)P)(2)-ZnPc-(C(60))(2) (5) not competitive. Instead, a transduction of singlet excited state energy prevails to form the ZnPc singlet excited state. This triggers then an intramolecular charge transfer reaction to form exclusively (H(2)P)(2)-ZnPc(.+)-(C(60)(.-))(C(60)). A similar sequence is found for ZnP-C(60)-ZnPc (3).     

Porphyrin–Phthalocyanine/Pyridylfullerene Supramolecular Assemblies

The synthesis and photophysical properties of several porphyrin (P)–phthalocyanine (Pc) conjugates (P–Pc; 1 – 3 ) are described, in which the phthalocyanines are directly linked to the β‐pyrrolic position of a meso‐tetraphenylporphyrin. Photoinduced energy‐ and electron‐transfer processes were studied through the preparation of H₂P–ZnPc, ZnP–ZnPc, and PdP–ZnPc conjugates, and their assembly through metal coordination with two different pyridylfulleropyrrolidines ( 4 and 5 ). The resulting electron‐donor–acceptor hybrids, which were formed by axial coordination of compounds 4 and 5 with the corresponding phthalocyanines, mimicked the fundamental processes of photosynthesis; that is, light harvesting, the transduction of excited‐state energy, and unidirectional electron transfer. In particular, photophysical studies confirmed that intramolecular energy‐transfer resulted from the S₂ excited state as well as from the S₁ excited state of the porphyrins to the energetically lower‐lying phthalocyanines, followed by an intramolecular charge‐transfer to yield P–Pc^.+ ? C₆₀^.?. This unique sequence of processes opens the way for solar‐energy‐conversion processes.     

Stepwise charge separation and charge recombination in ferrocene-meso,meso-linked porphyrin dimer-fullerene triad

A meso,meso-linked porphyrin dimer [(ZnP)(2)] as a light-harvesting chromophore has been incorporated into a photosynthetic multistep electron-transfer model for the first time, including ferrocene (Fc), as an electron donor and fullerene (C(60)) as an electron acceptor to construct the ferrocene-meso,meso-linked porphyrin dimer-fullerene system (Fc-(ZnP)(2)-C(60)). Photoirradiation of Fc-(ZnP)(2)-C(60) results in photoinduced electron transfer from the singlet excited state of the porphyrin dimer [(1)(ZnP)(2)] to the C(60) moiety to produce the porphyrin dimer radical cation-C(60) radical anion pair, Fc-(ZnP)(2)(*+)-C(60)(*-). In competition with the back electron transfer from C(60)(*-) to (ZnP)(2)(*+) to the ground state, an electron transfer from Fc to (ZnP)(2)(*+) occurs to give the final charge-separated (CS) state, that is, Fc(+)-(ZnP)(2)-C(60)(*-), which is detected as the transient absorption spectra by the laser flash photolysis. The quantum yield of formation of the final CS state is determined as 0.80 in benzonitrile. The final CS state decays obeying first-order kinetics with a lifetime of 19 micros in benzonitrile at 295 K. The activation energy for the charge recombination (CR) process is determined as 0.15 eV in benzonitrile, which is much larger than the value expected from the direct CR process to the ground state. This value is rather comparable to the energy difference between the initial CS state (Fc-(ZnP)(2)(*+)-C(60)(*-)) and the final CS state (Fc(+)-(ZnP)(2)-C(60)(*-)). This indicates that the back electron transfer to the ground state occurs via the reversed stepwise processes,that is, a rate-limiting electron transfer from (ZnP)(2) to Fc(+) to give the initial CS state (Fc-(ZnP)(2)(*+)-C(60)(*-)), followed by a fast electron transfer from C(60)(*-) to (ZnP)(2)(*+) to regenerate the ground state, Fc-(ZnP)(2)-C(60). This is in sharp contrast with the extremely slow direct CR process of bacteriochlorophyll dimer radical cation-quinone radical anion pair in bacterial reaction centers.     

Non-covalent versus covalent donor-acceptor systems based on near-infrared absorbing azulenocyanines and C60 fullerene derivatives

A novel supramolecular electron donor-acceptor hybrid (2·1) and an electron donor-acceptor conjugate (3), both exhibiting a remarkably shifted Q band in the NIR region of the solar spectrum, were prepared. Irradiation of the supramolecular ensemble 2·1 within the visible range leads to a nanosecond lived radical-ion pair state Zn-azulenocyanine˙(+)-C(60)˙(-).     

Construction and photophysics study of supramolecular complexes composed of three-point binding fullerene-trispyridylporphyrin dyads and zinc porphyrin

A series of novel supramolecular complexes composed of a three-point binding C(60)-trispyridylporphyrin dyad (1) or C(70)-trispyridylporphyrin dyad (2) and zinc tetraphenylporphyrin (ZnP) were constructed by adopting a "covalent-coordinate" bonding approach, composed of three-point binding. The dyads and self-assembled supramolecular triads or pentads formed by coordinating the pyridine groups located on the dyads to ZnP, have been characterized by means of spectral and electrochemical techniques. The formation constants of ZnP-1 and ZnP-2 complexes were calculated as 1.4 × 10(4) M(-1) and 2.0 × 10(4) M(-1), respectively, and the Stern-Volmer quenching constants K(SV) were founded to be 2.9 × 10(4) M(-1) and 5.5 × 10(4) M(-1), respectively, which are much higher than those of other supramolecular complexes such as previously reported ZnP-3 (N-ethyl-2-(4-pyridyl)-3,4-fulleropyrrolidine). The electrochemical investigations of these complexes suggest weak interactions between the constituents in the ground state. The excited states of the complexes were further monitored by time-resolved fluorescence measurements. The results revealed that the presence of the multiple binding point dyads (1 or 2) slightly accelerated the fluorescence decay of ZnP in o-DCB relative to that of the "single-point" bound supramolecular complex ZnP-3. In comparison with 1 and 2, C(70) is suggested as a better electron acceptor relative to C(60). DFT calculations on a model of supramolecular complex ZnP-1 (with one ZnP entity) were performed. The results revealed that the lowest unoccupied molecular orbital (LUMO) is mainly located on the fullerene cage, while the highest occupied molecular orbital (HOMO) is mainly located on the ZnP macrocycle ring, predicting the formation of radical ion pair ZnP(+)˙-H(2)P-C(60)(-)˙ during photo-induced reaction.     

Catalytic effects of dioxygen on intramolecular electron transfer in radical ion pairs of zinc porphyrin-linked fullerenes.

Dioxygen accelerates back electron transfer (BET) processes between a fullerene radical anion (C60) and a radical cation of zinc porphyrin (ZnP) in photolytically generated ZnP.+-C60.- and ZnP.+-H2P-C60.- radical ion pairs. The rate constant of BET increases linearly with increasing oxygen concentration without, however, forming reactive oxygen species, such as singlet oxygen or superoxide anion. When ferrocene (Fc) is used as a terminal electron donor moiety instead of ZnP (i.e., Fc-ZnP-C60), no catalytic effects of dioxygen were, however, observed for the BET in Fc+-ZnP-C60.-, that is, from C60.- to the ferricenium ion. In the case of ZnP-containing C60 systems, the partial coordination of O2 to ZnP.+ facilitates an intermolecular electron transfer (ET) from C60.- to O2. This rate-determining ET step is followed by a rapid intramolecular ET from O2.- to ZnP.+ in the corresponding O2.--ZnP.+ complex and hereby regenerating O2. In summary, O2 acts as a novel catalyst in accelerating the BET of the C60.--ZnP.+ radical ion pairs.     

Strong supramolecular binding of Li(+)@C60 with sulfonated meso-tetraphenylporphyrins and long-lived photoinduced charge separation

A supramolecular binding occurred between lithium ion encapsulated [60]fullerene (Li(+)@C(60)) and sulfonated tetraphenylporphyrins ([MTPPS](4-) M = H(2) and Zn) in a benzonitrile solution. Photoexcitation of Li(+)@C(60)/[MTPPS](4-) results in formation of a long-lived charge-separated state by photoinduced electron transfer.     

Supramolecular triad and pentad composed of zinc-porphyrin(s), oxoporphyrinogen, and fullerene(s): design and electron-transfer studies

By adopting a "covalent-coordinate" bonding approach, novel supramolecular pentad and triad molecules composed of zinc-porphyrin(s), fullerene(s), and oxoporphyrinogen redox-/photoactive entities have been constructed, and also characterized by means of spectral and electrochemical techniques. The geometry and electronic structures of the pentad and the triad were deduced by means of DFT calculations. Free-energy calculations suggested that the photoinduced electron/energy transfer from the zinc-porphyrin (ZnP) singlet-excited state to the imidazole modified fullerene (ImC(60)) acceptor and oxoporphyrinogen (OxP) entities is feasible for both the triad and the pentad. The charge-separation rates (k(CS)) determined from picosecond time-resolved emission studies were higher for pentad (C(60)Im:ZnP)(2)-OxP than for the corresponding triad, C(60)Im:ZnP-OxP. A comparison of the k(CS) values previously reported for the covalently linked bis(zinc-porphyrin)-oxoporphyrinogen triad suggests that employing a fullerene acceptor improves the electron-transfer rates. Nanosecond transient absorption studies provide evidence for the occurrence of electron-transfer processes. Lifetimes of the radical ion pairs (tau(RIP)) are in the range of hundreds of nanoseconds, which indicates that there is charge stabilization in the supramolecular systems.     

Interactions in single wall carbon nanotubes/pyrene/porphyrin nanohybrids

This work provides an in-depth look at a range of physicochemical aspects of (i) single wall carbon nanotubes (SWNT), (ii) pyrene derivatives (pyrene(+)), (iii) porphyrin derivatives (ZnP(8)()(-)() and H(2)()P(8)()(-)()), (iv) poly(sodium 4-styrenesulfonate), and (v) their combinations. Implicit in their supramolecular combinations is the hierarchical integration of SWNT (as electron acceptors), together with ZnP(8)()(-)() or H(2)()P(8)()(-)() (as electron donors), in an aqueous environment mediated through pyrene(+). This supramolecular approach yields novel electron donor-acceptor nanohybrids (SWNT/pyrene(+)/ZnP(8)()(-)() or SWNT/pyrene(+)/H(2)()P(8)()(-)()). In particular, we report on electrochemical and photophysical investigations that as a whole suggest sizeable and appreciable interactions between the individual components. The key step to form SWNT/pyrene(+)()/ZnP(8)()(-)() or SWNT/pyrene(+)()/H(2)()P(8)()(-)() hybrids is pi-pi interactions between SWNT and pyrene(+), for which we have developed for the first time a sensitive marker. The marker is the monomeric pyrene fluorescence, which although quenched is (i) only present in SWNT/pyrene(+) and (ii) completely lacking in just pyrene(+). Electrostatic interactions help to immobilize ZnP(8)()(-)() or H(2)()P(8)()(-)() onto SWNT/pyrene(+) to yield the final electron donor-acceptor nanohybrids. A series of photochemical experiments confirm that long-lived radical ion pairs are formed as a product of a rapid excited-state deactivation of ZnP(8)()(-)() or H(2)()P(8)()(-)(). This formation is fully rationalized on the basis of the properties of the individual moieties. Additional modeling shows that the data are likely to be relevant to the SWNTs present in the sample, which possess wider diameters.     

2,2']paracyclophane-based π-conjugated molecular wires reveal molecular-junction behavior

The electronic coupling as well as the attenuation factor (β), which depends primarily on the nature of the molecular bridge and is used as a benchmark to test the molecular wire behavior, have been determined in a systematic study carried out on a series of ZnP/C(60) conjugates connected through a [2,2']paracyclophane-oligophenylenevinylene (pCp-oPPV). The convergent synthesis involves a series of Horner-Emmons olefination reactions or double palladium-catalized Heck-type reactions. ZnP-pCp-C(60) conjugates were finally obtained by the 1,3-dipolar cycloaddition reaction of the in situ-generated azomethyne ylide containing the ZnP-pCp moiety to the [60]fullerene using Prato conditions. Experimental (UV-vis, fluorescence, transient absorption spectroscopy, and solution electrochemistry) and theoretical studies revealed that the pCps act as molecular junctions. If hole transfer is assumed to be the dominant charge transfer (CT) mechanism, CT is facilitated in one direction (from C(60) to ZnP via pCp) but disfavored in the other direction (from ZnP to C(60) via pCp).     

Supramolecular pattern of fullerene on 2D bimolecular "chessboard" consisting of bottom-up assembly of porphyrin and phthalocyanine molecules

Two-component adlayers consisting of zinc(II) phthalocyanine (ZnPc) and a metalloporphyrin, such as zinc(II) octaethylporphyrin (ZnOEP) or zinc(II) tetraphenylporphyrin (ZnTPP), were prepared by immersing either an Au(111) or Au(100) substrate in a benzene solution containing those molecules. The bimolecular adlayers thus prepared were investigated in 0.1 M HClO4 by cyclic voltammetry (CV) and electrochemical scanning tunneling microscopy (EC-STM). A supramolecularly organized "chessboard" structure was formed for the ZnPc and ZnOEP bimolecular array on Au(111), while characteristic nanohexagons were found in the ZnTPP and ZnOEP bimolecular adlayer. EC-STM revealed that the surface mobility and the molecular re-organization of ZnPc and ZnOEP on Au(111) were tunable by manipulating the electrode potential, whereas the ZnTPP and ZnOEP bimolecular array was independent of the electrode potential. A "bottom-up" hybrid assembly of fullerene molecules was formed successfully on an alternate array of bimolecular ZnPc and ZnOEP molecules. The bimolecular "chessboard" served as a template to form the supramolecular assembly of C60 by selective trapping in the open spaces. A supramolecular organization of ZnPc and ZnOEP was also found on the reconstructed Au(100)-(hex) surface. A highly ordered, compositionally disordered but alternate array of ZnPc and ZnOEP was formed on the reconstructed Au(100)-(hex) surface, indicating that the bimolecular adlayer structure is dependent on the atomic arrangement of underlying Au in the formation of supramolecular nanostructures composed of those molecules. On the bimolecular array consisting of ZnPc and ZnOEP on the Au(100)-(hex), no highly ordered supramolecular assembly of C60 was found, suggesting that the supramolecular assembly of C60 molecules is strongly dependent upon the bimolecular packing arrangement of ZnPc and ZnOEP.     

Synthesis and photoinduced electron-transfer properties of phthalocyanine-[60]fullerene conjugates

A series of three novel ZnPc-C60 conjugates (Pc=phthalocyanine) 1 a-c bearing different spacers (single, double, and triple bond) between the two electroactive moieties was synthesized and compared to that of ZnPc-C60 conjugate 2, in which the two electroactive moieties are linked directly. The synthetic strategy- towards the preparation of 1 a-c- involved palladium-catalyzed cross-coupling reactions over a monoiodophthalocyanine precursor 4 to introduce the corresponding spacer, and subsequent dipolar cycloaddition reaction to C60. Detailed photophysical investigations of 1 a-c and 2 prompted an intramolecular electron transfer that evolves from the photoexcited ZnPc to the electron-accepting C60. In particular, with the help of femtosecond laser photolysis charge separation was indeed confirmed as the major deactivation channel. Complementary time-dependent density functional calculations supported the spectral assignment, namely, the spectral identity of the ZnPc(*+) radical cation and the C60 (*-) radical anion as seen in the differential absorption spectra. The lifetimes of the correspondingly formed radical ion-pair states depend markedly on the solvent polarity: they increase as polarity decreases. Similarly, although to a lesser extent, the nature of the linker impacts the lifetime of the radical ion-pair states. In general, the lifetimes of these states tend to be shortest in the system that lacks any spacer at all (2), whereas the longest lifetimes were found in the system that carries the triple-bond spacer (1 a).     

Photoexcited state properties of H2-porphyrin/C60-based rotaxanes as studied by time-resolved electron paramagnetic resonance spectroscopy

Light-driven intramolecular electron transfer (ET) and energy transfer (EnT) processes in two rotaxanes, the first containing two free base porphyrins and C(60) fullerene moieties incorporated around a Cu(I)bisphenanthroline core ((H(2)P)(2)-Cu(I)(phen)(2)-C(60)) and a second lacking the fullerene moiety ((H(2)P)(2)-Cu(I)(phen)(2)), were studied by X-band (9.5 GHz) time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The experiments were performed in frozen toluene and ethanol and different phases of the nematic liquid crystal (E-7). It is demonstrated that the ET and EnT processes in the (H(2)P)(2)-Cu(I)(phen)(2)-C(60) rotaxane in different media result in the formation of the same charge-separated state, namely (H(2)P)(2)(?+)-Cu(I)(phen)(2)(?-)-C(60), while photoexcitation of the (H(2)P)(2)-Cu(I)(phen)(2) rotaxane does not induce noticeable transfer processes in these matrices. The results are discussed in terms of the high conformational mobility of the rotaxanes, which enables changes in the molecular topography and resultant modification of the rates and routes of photoinduced processes occurring in these systems. The parameters of the transfer processes are compared with those obtained in our previous study of (ZnP)(2)-Cu(I)(phen)(2)-C(60) and (ZnP)(2)-Cu(I)(phen)(2) rotaxanes under the same experimental conditions.     

Effect of the N-terminal basic residue on facile Cα-C bond cleavages of aromatic-containing peptide radical cations

Fragmentation of radical cationic peptides [R(G)(n-2)X(G)(7-n)]˙(+) and [R(G)(m-2)XG]˙(+) (X = Phe or Tyr; m = 2-5; n = 2-7) leads selectively to a(n)(+) product ions through in situ C(α)-C peptide backbone cleavage at the aromatic amino acid residues. In contrast, substituting the arginine residue with a less-basic lysine residue, forming [K(G)(n-2)X(G)(7-n)]˙(+) (X = Phe or Tyr; n = 2-7) analogs, generates abundant b-y product ions; no site-selective C(α)-C peptide bond cleavage was observed. Studying the prototypical radical cationic tripeptides [RFG]˙(+) and [KFG]˙(+) using low-energy collision-induced dissociation and density functional theory, we have examined the influence of the basicity of the N-terminal amino acid residue on the competition between the isomerization and dissociation channels, particularly the selective C(α)-C bond cleavage viaβ-hydrogen atom migration. The dissociation barriers for the formation of a(2)(+) ions from [RFG]˙(+) and [KFG]˙(+)via their β-radical isomers are comparable (33.1 and 35.0 kcal mol(-1), respectively); the dissociation barrier for the charge-induced formation of the [b(2)- H]˙(+) radical cation from [RFG]˙(+)via its α-radical isomer (39.8 kcal mol(-1)) was considerably higher than that from [KFG]˙(+) (27.2 kcal mol(-1)). Thus, the basic arginine residue sequesters the mobile proton to promote the charge-remote selective C(α)-C bond cleavage by energetically hindering the competing charge-induced pathways.     

Ultrafast excitation transfer and charge stabilization in a newly assembled photosynthetic antenna-reaction center mimic composed of boron dipyrrin, zinc porphyrin and fullerene

A self-assembled supramolecular triad as a model to mimic the light-induced events of the photosynthetic antenna-reaction center, that is, ultrafast excitation transfer followed by electron transfer ultimately generating a long-lived charge-separated state, has been accomplished. Boron dipyrrin (BDP), zinc porphyrin (ZnP) and fullerene (C(60)), respectively, constitute the energy donor, electron donor and electron acceptor segments of the antenna-reaction center imitation. Unlike in the previous models, the BDP entity was placed between the electron donor, ZnP and electron acceptor, C(60) entities. For the construction, benzo-18-crown-6 functionalized BDP was synthesized and subsequently reacted with 3,4-dihydroxyphenyl functionalized ZnP through the central boron atom to form the crown-BDP-ZnP dyad. Next, an alkyl ammonium functionalized fullerene was used to self-assemble the crown ether entity of the dyad via ion-dipole interactions. The newly formed supramolecular triad was fully characterized by spectroscopic, computational and electrochemical methods. Steady-state fluorescence and excitation studies revealed the occurrence of energy transfer upon selective excitation of the BDP in the dyad. Further studies involving the pump-probe technique revealed excitation transfer from the (1)BDP* to ZnP to occur in about 7 ps, much faster than that reported for other systems in this series of triads, as a consequence of shorter distance between the entities. Upon forming the supramolecular triad by self-assembling fullerene, the (1)ZnP(*) produced by direct excitation or by energy transfer mechanism resulted in an initial electron transfer to the BDP entity. The charge recombination resulted in the population of the triplet excited state of C(60), from where additional electron transfer occurred to produce C(60)(?-):crown-BDP-ZnP(?+) ion pair as the final charge-separated species. Nanosecond transient absorption studies revealed the lifetime of the charge-separated state to be ~100 μs, the longest ever reported for this type of antenna-reaction center mimics, indicating better charge stabilization as a result of the different disposition of the entities of the supramolecular triad.     

Control of photoinduced energy- and electron-transfer steps in zinc porphyrin-oligothiophene-fullerene linked triads with solvent polarity

The dramatic changes of the lifetimes of the charge-separated (CS) states were confirmed in zinc porphyrin (ZnP)-oligothiophene (nT)-fullerene (C(60)) linked triads (ZnP-nT-C(60)) with the solvent polarity. After the selective excitation of the ZnP moiety of ZnP-nT-C(60), an energy transfer took place from the (1)ZnP moiety to the C(60) moiety, generating ZnP-nT-(1)C(60). In polar solvents, the CS process also took place directly via the (1)ZnP moiety, generating ZnP(*+)-nT-C(60)(*-), as well as the energy transfer to the C(60) moiety. After this energy transfer, an indirect CS process took place from the (1)C(60) moiety. In the less polar solvent anisole, the radical cation (hole) of ZnP(*+)-nT-C(60)(*-) shifted to the nT moiety; thus, the nT moiety behaves as a cation trapper, and the rates of the hole shift were evaluated to be in the order of 10(8) s(-1); then, the final CS states ZnP-nT(*+)-C(60)(*-) were lasting for 6-7 mus. In the medium polar solvent o-dichlorobenzene (o-DCB), ZnP-nT(*+)-C(60)(*-) and ZnP(*+)-nT-C(60)(*-) were present as an equilibrium, because both states have almost the same thermodynamic stability. This equilibrium resulted in quite long lifetimes of the CS states (450-910 mus) in o-DCB. In the more polar benzonitrile, the generation of ZnP-nT(*+)-C(60)(*-) was confirmed with apparent short lifetimes (0.6-0.8 mus), which can be explained by the fast hole shift to more stable ZnP(*+)-nT-C(60)(*-) followed by the faster charge recombination. It was revealed that the relation between the energy levels of two CS states, which strongly depend on the solvent polarity, causes dramatic changes of the lifetimes of the CS states in ZnP-nT-C(60); that is, the most appropriate solvents for the long-lived CS state are intermediately polar solvents such as o-DCB. Compared with our previous data for H(2)P-nT-C(60), in which H(2)P is free-base porphyrin, the lifetimes of the CS states of ZnP-nT-C(60) are approximately 30 times longer than those in o-DCB.     

Metal ion-dependent fluorescent dynamics of photoexcited zinc-porphyrin and zinc-myoglobin modified with ethylenediaminetetraacetic acid

The reconstituted zinc-myoglobin (ZnMb) dyads, ZnMb-[M(II)(edta)], have been prepared by incorporating a zinc-porphyrin (ZnP) cofactor modified with ethylenediaminetetraacetic acid (H(4)edta) into apo-Mb. In case of the monomeric ZnP(edta) cofactor coordinated by one pyridine molecule, ZnP(py)(edta), a spontaneous 1:1 complex with a transient metal ion was formed in an aqueous solvent, and the photoexcited singlet state of ZnP, (1)(ZnP)*, was quenched by the [Cu(II)(edta)] moiety through intramolecular photoinduced electron-transfer (ET) reaction. The rate constant for the intramolecular quenching ET (k(q)) at 25 degrees C was successfully obtained as k(q) = 5.1 x 10(9) s(-1). In the case of Co(2+), Ni(2+), and Mn(2+), intersystem crossing by paramagnetic effect was mainly considered between (1)(ZnP)* and the [M(II)(edta)] complex. For the ZnMb-[M(II)(edta)] systems, the intramolecular ET reaction between the excited singlet state of (1)(ZnMb)* and the [Cu(II)(edta)] moieties provided the slower quenching rate constant, k(q) = 2.1 x 10(8) s(-1), compared with that of the ZnP(py)(edta) one. Kinetic studies also presented the efficient fluorescence quenching of the (1)(ZnMb)*-[Co(II)(edta)] dyad. Our study clearly demonstrates that wrapping of the ZnP cofactor by the apoprotein matrix and synthetic manipulation at the Mb surface ensure metal ion-sensitive fluorescent dynamics of ZnMb and provides valuable information to elucidate the complicated mechanism of the biological photoinduced ET reactions of hemoproteins.     

Multiple photosynthetic reaction centres composed of supramolecular assemblies of zinc porphyrin dendrimers with a fullerene acceptor

Multiple photosynthetic reaction centres have successfully been constructed using supramolecular complexes of zinc porphyrin dendrimers [D(ZnP)(n): n = 4, 8, 16] with fulleropyrrolidine bearing a pyridine ligand (C(60)py). Efficient energy migration occurs completely between the ZnP units of dendrimers prior to the electron transfer with increasing the generation of dendrimers to attain an extremely long charge-separation lifetime.     

Proton coupled electron transfer of ubiquinone Q2 incorporated in a self-assembled monolayer

We present a complete study of the reduction of ubiquinone Q(2) (UQ(2)) in simpler aqueous medium, over a pH range of 2.5 to 12.5. The short isoprenic chain ubiquinones (UQ(2)) were incorporated in a self-assembled monolayer. Under these conditions, the global 2e(-) electrochemical reaction can be described on the basis of a nine-member square scheme. The thermodynamic constants of the system were determined. The global 2e(-) process is controlled by the uptake of the second electron. The elementary electrochemical rate constants obtained by fitting of the experimental rate constant were k(s4) = 1.5 s(-1) for QH˙(+)(2)? QH(2), k(s5) = 1.5 s(-1) for QH˙? QH(-) and k(s6) = 1 s(-1) for Q˙(-)? Q(2-). The three electrochemical reactions QH˙(+)(2)? QH(2), QH˙? QH(-) and Q˙(-)? Q(2-) are successively involved when increasing the pH. Protonations can occur or not, before or after the electron uptake and the reaction paths are, from low to high pH: e(-), H(+)e(-), e(-)H(+), H(+)e(-)H(+), H(+)e(-) and e(-)H(+).