Energy and electron transfer in beta-alkynyl-linked porphyrin-[60]fullerene dyads

Three porphyrin-fullerene dyads, in which a diyne bridge links C(60) with a beta-position on a tetraarylporphyrin, have been synthesized. The free-base dyad was prepared, as well as the corresponding Zn(II) and Ni(II) materials. These represent the first examples of a new class of conjugatively linked electron donor-acceptor systems in which pi-conjugation extends from the porphyrin ring system directly to the fullerene surface. The processes that occur following photoexcitation of these dyads were examined using fluorescence and transient absorption techniques on the femtosecond, picosecond, and nanosecond time scales. In sharp contrast to the photodynamics associated with singlet excited-state decay of reference tetraphenylporphyrins (ZnTPP, NiTPP, and H(2)TPP), the diyne-linked dyads undergo ultrafast (<10 ps) singlet excited-state deactivation in toluene, tetrahydrofuran (THF), and benzonitrile (PhCN). Transient absorption techniques with the ZnP-C(60) dyad clearly show that in toluene intramolecular energy transfer (EnT) to ultimately generate C(60) triplet excited states is the dominant singlet decay mechanism, while intramolecular electron transfer (ET) dominates in THF and PhCN to give the ZnP(*+)/C(60)(*-) charge-separated radical ion pair (CSRP). Electrochemical studies indicate that there is no significant charge transfer in the ground states of these systems. The lifetime of ZnP(*+)/C(60)(*-) in PhCN was approximately 40 ps, determined by two different types of transient absorption measurement in two different laboratories. Thus, in this system, the ratio of the rates for charge separation (k(CS)) to rates for charge recombination (k(CR)), k(CS)/k(CR), is quite small, approximately 7. The fact that charge separation (CS) rates increase with increasing solvent polarity is consistent with this process occurring in the normal region of the Marcus curve, while the slower charge recombination (CR) rates in less polar solvents indicate that the CR process occurs in the Marcus inverted region. While photoinduced ET occurs on a similar time scale in a related dyad 15 in which a diethynyl bridge connects C(60) to the para position of a meso phenyl moiety of a tetrarylporphyrin, CR occurs much more slowly; i.e., k(CS)/k(CR) approximately equal to 7400. Thus, the position at which the conjugative linker is attached to the porphyrin moiety has a dramatic influence on k(CR) but not on k(CS). On the basis of electron density calculations, we tentatively conclude that unfavorable orbital symmetries inhibit charge recombination in 15 vis a vis the beta-linked dyads.     

Dyads and triads containing perylenetetracarboxylic diimide and porphyrin: efficient photoinduced electron transfer elicited via both excited singlet states

Synthesis, characterizations, and photophysical properties of new photoactive dyads and triads containing perylenetetracarboxylic diimide (PIm) and porphyrin (free-base porphyrin (H(2)P) and zinc porphyrin (ZnP)), in which both entities were connected with a short ether bond, were examined with the aim of using these systems for molecular photonics. The porphyrin(P)-PIm systems absorbed strongly across the visible region, which greatly matched the solar spectrum. The geometric and electronic structures of the dyads and triads were probed using density function theory method at the B3LYP/3-21G level. It was revealed that the majority of the highest-occupied molecular orbital was located on the porphyrin entity, while the lowest-unoccupied molecular orbitals were entirely on the PIm entity. The excited-state electron-transfer processes were monitored by both steady-state and time-resolved emission as well as transient-absorption techniques in polar solvent benzonitrile. Upon excitation of the P (H(2)P and ZnP) moieties, efficient fluorescence quenching of the P moiety was observed, suggesting that the main quenching paths involved charge separation from the excited singlet porphyrin ((1)P) to the PIm moiety. Upon excitation of the PIm moiety, fluorescence quenching of the (1)PIm moiety was also observed. The nanosecond transience of spectra in near-IR region revealed the charge separation process from the P moieties to the PIm moiety via their excited singlet states. The lifetimes of the charge-separated states were evaluated to be 7-14 ns, depending on the solvent polarity. Photosensitized electron mediation systems were also revealed in the presence of methyl viologen and sacrificial electron donor.     

PREPARATION AND PHOTOPHYSICAL STUDIES OF PORPHYRIN-C60 DYADS

Abstract Porphyrin-C₆₀ dyads in which the two chromophores are linked by a bicyclic bridge have been synthesized using the Diels-Alder reaction. The porphyin singlet lifetimes of both the zinc (Pz_n-C₆₀) and free base (P-C₆₀) dyads, determined by time-resolved fluorescence measurements, are ≦17 ps in toluene. This substantial quenching is due to singlet-singlet energy transfer to C₆₀ The lifetime of Pzn-¹C₆₀ is -5 ps in toluene, whereas the singlet lifetime of an appropriate C₆₀ model compound is 1.2 ns. This quenching is attributed to electron transfer to yield P_zn^bull;+-C₆₀^bull;-. In toluene, P-¹C₆₀ is unquenched; the lack of electron transfer is due to unfavorable thermodynamics. In this solvent, a transient state with an absorption maximum at 700 ran and a lifetime of-10 μs was detected using transient absorption methods. This state was quenched by oxygen, and is assigned to the C₆₀ triplet. In the more polar benzonitrile, P-¹C₆₀ underoes photoinduced electron transfer to give P^•+-C₆₀^bull;-. The electron transfer rate constant is −2 × 10¹¹ s⁻¹.     

Control of electron transfer in supramolecular systems

The fluorescence quantum yield of zinc porphyrin (ZnP) covalently linked to 9,10-bis(phenylethynyl)anthracene (AB) is strongly dependent upon the solvent properties. The bichromophoric system ZnP-AB exhibits 'normal' zinc porphyrin fluorescence in solvents that cannot coordinate to the central zinc atom. In contrast, if a Lewis base, such as pyridine, is added to a sufficiently polar solvent, the fluorescence is significantly quenched. Picosecond transient absorption measurements, in conjunction with fluorescence quenching and cyclic voltammetric measurements, suggest that the quenching mechanism is intramolecular electron transfer from ZnP to AB. The charge separated state. ZnP*+-AB*-, has a lifetime of not more than 220 ps before recombining. If a secondary electron acceptor, iron(III) porphyrin (FeP), is covalently connected to the AB unit, a second electron transfer from AB*- to FeP occurs and the charge separated state, ZnP*+-AB-FeP*-, has a lifetime of at least 5 ns. This demonstrates that electron transfer might be sensitively tuned (switched on) by specific solvent effects.     

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.     

Photoinduced energy and electron-transfer processes in porphyrin-perylene bisimide symmetric triads

The photophysics of two symmetric triads, (ZnP)2PBI and (H2P)2PBI, made of two zinc or free-base porphyrins covalently attached to a central perylene bisimide unit has been investigated in dichloromethane and in toluene. The solvent has been shown to affect not only quantitatively but also qualitatively the photophysical behavior. A variety of intercomponent processes (singlet energy transfer, triplet energy transfer, photoinduced charge separation, and recombination) have been time-resolved using a combination of emission spectroscopy and femtosecond and nanosecond time-resolved absorption techniques yielding a very detailed picture of the photophysics of these systems. The singlet excited state of the lowest energy chromophore (perylene bisimide in the case of (ZnP)2PBI, porphyrin in the case of (H2P)2PBI) is always quantitatively populated, besides by direct light absorption, by ultrafast singlet energy transfer (few picosecond time constant) from the higher energy chromophore. In dichloromethane, the lowest excited singlet state is efficiently quenched by electron transfer leading to a charge-separated state where the porphyrin is oxidized and the perylene bisimide is reduced. The systems then go back to the ground state by charge recombination. The four charge separation and recombination processes observed for (ZnP)2PBI and (H2P)2PBI in dichloromethane take place in the sub-nanosecond time scale. They obey standard free-energy correlations with charge separation lying in the normal regime and charge recombination in the Marcus inverted region. In less polar solvents, such as toluene, the energy of the charge-separated states is substantially lifted leading to sharp changes in photophysical mechanism. With (ZnP)2PBI, the electron-transfer quenching is still fast, but charge recombination takes place now in the nanosecond time scale and to triplet state products rather than to the ground state. Triplet-triplet energy transfer from the porphyrin to the perylene bisimide is also involved in the subsequent deactivation of the triplet manifold to the ground state. With (H2P)2PBI, on the other hand, the driving force for charge separation is too small for electron-transfer quenching, and the deactivation of the porphyrin excited singlet takes place via intersystem crossing to the triplet followed by triplet energy transfer to the perylene bisimide and final decay to the ground state.     

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.     

Testing electron transfer within molecular associates built around anionic C60 and C70 dendrofullerenes and a cationic zinc porphyrin

Mono- and bis-functionalized C(60) and C(70) fullerene derivatives (DF, 1-10) that carry one or two oligoanionic dendritic termini in their malonate addends and an oligocationic octapyridinium zinc porphyrin salt (ZnP) were found to self-assemble in buffered aqueous solution to yield a novel series of 1:1 and/or 1:2 electron transfer hybrid associates. Remarkably high association constants-typically on the order of 10(8) M(-1)-were derived that corroborate stable complex formations. A combination of electrostatic and charge-transfer interactions that are operative between the electron-accepting DF and the electron-donating ZnP is considered to contribute to the uniquely high complex stability. First insight into intracomplex excited state interactions came from steady-state and time-resolved fluorescence quenching experiments that were performed with the molecular ZnP/DF hybrid associates. Excited state quenching processes are, for example, evident in form of a bi-exponential fluorescence decay of ZnP-corresponding to a distribution of associated and non-associated ZnP. Unambiguous evidence for an intracomplex electron transfer quenching, namely, formation of ZnP(.+)/C(60) (.-) and ZnP(.+)/C(70) (.-) radical ion pairs, was gathered in time-resolved transient absorption measurements. Lifetimes of these radical ion-pairs range from nanoseconds to a few microseconds.     

Observation of proton-coupled electron transfer by transient absorption spectroscopy in a hydrogen-bonded, porphyrin donor-acceptor assembly

Proton-coupled electron transfer (PCET) kinetics of a Zn(II) porphyrin donor noncovalently bound to a naphthalene-diimide acceptor through an amidinium-carboxylate interface have been investigated by time-resolved spectroscopy. The S1 singlet excited-state of a Zn(II) 2-amidinium-5,10,15,20-tetramesitylporphyrin chloride (ZnP-beta-AmH+) donor is sufficiently energetic (2.04 eV) to reduce a carboxylate-diimide acceptor (DeltaG degrees = -460 mV, THF). Static quenching of the porphyrin fluorescence is observed and time-resolved measurements reveal more than a 3-fold reduction in the S1 lifetime of the porphyrin upon amidinium-carboxylate formation (THF, 298 K). Picosecond transient absorption spectra of the free ZnP-beta-AmH+ in THF reveal the existence of an excited-state isosbestic point between the S1 and T1 states at lambdaprobe = 650 nm, providing an effective 'zero-kinetics' background on which to observe the formation of PCET photoproducts. Distinct rise and decay kinetics are attributed to the build-up and subsequent loss of intermediates resulting from a forward and reverse PCET reaction, respectively (kPCET(fwd) = 9 x 108 s-1 and kPCET(rev) = 14 x 108 s-1). The forward rate constant is nearly 2 orders of magnitude slower than that measured for covalently linked Zn(II) porphyrin-acceptor dyads of comparable driving force and D-A distance, establishing the importance of a proximal proton network in controlling charge transport.     

Mimicking photosynthetic antenna-reaction-center complexes with a (boron dipyrromethene)3-porphyrin-C60 pentad

A highly efficient functional mimic of the photosynthetic antenna-reaction-center complexes has been designed and synthesized. The model contains a zinc(II) porphyrin (ZnP) core, which is connected to three boron dipyrromethene (BDP) units by click chemistry, and to a C(60) moiety using the Prato procedure. The compound has been characterized using various spectroscopic methods. The intramolecular photoinduced processes of this pentad have also been studied in detail with steady-state and time-resolved absorption and emission spectroscopic methods, both in polar benzonitrile and nonpolar toluene. The BDP units serve as the antennae, which upon excitation undergo singlet-singlet energy transfer to the porphyrin core. This is then followed by an efficient electron transfer to the C(60) moiety, resulting in the formation of the singlet charge-separated state (BDP)(3)-ZnP(·+) -C(60)(·-) , which has a lifetime of 476 and 1000 ps in benzonitrile and toluene, respectively. Interestingly, a slow charge-recombination process (k(CR)(t)=2.6×10(6) s(-1)) and a long-lived triplet charge-separated state (τ(CS)(T)=385 ns) were detected in polar benzonitrile by nanosecond transient measurements.     

吩噻嗪-Corrole 镓(III)配合物的合成、荧光及其光断裂DNA性质

合成了吩噻嗪(PTZ)-corrole二元体1-3及其镓(III)配合物4-6.采用稳态吸收与稳态发射及时间分辨的瞬态光谱技术研究了这几种化合物的光物理特性.结合荧光量子产率和荧光寿命计算得到它们的辐射和无辐射速率常数.稳态吸收光谱表明:几种二元体中,corrole镓(III)单元表现出更强的Soret带和Q带.化合物1-3的荧光量子产率分别是0.156、0.134和0.139,辐射速率常数分别为4.02′107、3.47′107和2.89′107s-1.化合物4-6的荧光量子产率分别是0.502、0.443和0.494,辐射速率常数分别为20.90′107、16.78′107和21.11′107s-1.可见,化合物4-6的荧光量子产率和辐射速率常数均高于化合物1-3.然而,化合物4-6的荧光寿命分别是2.40、2.64和2.34ns,低于自由corrole1-3.琼脂糖凝胶电泳实验表明:在光照的条件下,这些吩噻嗪-corrole镓(III)二元体化合物能够把超螺旋DNA(formI)切割成缺刻型DNA(formII).     

Ultrafast charge separation in a photoreactive rhenium-appended porphyrin assembly monitored by picosecond transient infrared spectroscopy

Rhenium(bipyridine)(tricarbonyl)(picoline) units have been linked covalently to tetraphenylmetalloporphyrins of magnesium and zinc via an amide bond between the bipyridine and one phenyl substituent of the porphyrin. The resulting complexes, abbreviated as [Re(CO)(3)(Pic)Bpy-MgTPP][OTf] and [Re(CO)(3)(Pic)Bpy-ZnTPP][OTf], exhibit no signs of electronic interaction between the Re(CO)(3)(bpy) units and the metalloporphyrin units in their ground states. However, emission spectroscopy reveals solvent-dependent quenching of porphyrin emission on irradiation into the long-wavelength absorption bands localized on the porphyrin. The characteristics of the excited states have been probed by picosecond time-resolved absorption (TRVIS) spectroscopy and time-resolved infrared (TRIR) spectroscopy in nitrile solvents. The presence of the charge-separated state involving electron transfer from MgTPP or ZnTPP to Re(bpy) is signaled in the TRIR spectra by a low-frequency shift in the nu(CO) bands of the Re(CO)(3) moiety similar to that observed by spectroelectrochemical reduction. Long-wavelength excitation of [Re(CO)(3)(Pic)Bpy-MTPP][OTf] results in characteristic TRVIS spectra of the S(1) state of the porphyrin that decay with a time constant of 17 ps (M = Mg) or 24 ps (M = Zn). The IR bands of the CS state appear on a time scale of less than 1 ps (Mg) or ca. 5 ps (Zn) and decay giving way to a vibrationally excited (i.e., hot) ground state via back electron transfer. The IR bands of the precursors recover with a time constant of 35 ps (Mg) or 55 ps (Zn). The short lifetimes of the charge-transfer states carry implications for the mechanism of reaction in the presence of triethylamine.     

Chlorin–Bacteriochlorin Energy‐transfer Dyads as Prototypes for Near‐infrared Molecular Imaging Probes: Controlling Charge‐transfer and Fluorescence Properties in Polar Media

The photophysical properties of two energy‐transfer dyads that are potential candidates for near‐infrared (NIR) imaging probes are investigated as a function of solvent polarity. The dyads ( FbC‐FbB and ZnC‐FbB ) contain either a free base (Fb) or zinc (Zn) chlorin (C) as the energy donor and a free base bacteriochlorin (B) as the energy acceptor. The dyads were studied in toluene, chlorobenzene, 1,2‐dichlorobenzene, acetone, acetonitrile and dimethylsulfoxide (DMSO). In both dyads, energy transfer from the chlorin to bacteriochlorin occurs with a rate constant of ～(5–10 ps)^?1 and a yield of >99% in nonpolar and polar media. In toluene, the fluorescence yields (Φ _f = 0.19) and singlet excited‐state lifetimes (τ～5.5 ns) are comparable to those of the benchmark bacteriochlorin. The fluorescence yield and excited‐state lifetime decrease as the solvent polarity increases, with quenching by intramolecular electron (or hole) transfer being greater for FbC‐FbB than for ZnC‐FbB in a given solvent. For example, the Φ _f and τ values for FbC‐FbB in acetone are 0.055 and 1.5 ns and in DMSO are 0.019 and 0.28 ns, whereas those for ZnC‐FbB in acetone are 0.12 and 4.5 ns and in DMSO are 0.072 and 2.4 ns. The difference in fluorescence properties of the two dyads in a given polar solvent is due to the relative energies of the lowest energy charge‐transfer states, as assessed by ground‐state redox potentials and supported by molecular‐orbital energies derived from density functional theory calculations. Controlling the extent of excited‐state quenching in polar media will allow the favorable photophysical properties of the chlorin–bacteriochlorin dyads to be exploited in vivo. These properties include very large Stokes shifts (85 nm for FbC‐FbB , 110 nm for ZnC‐FbB ) between the red‐region absorption of the chlorin and the NIR fluorescence of the bacteriochlorin (λ _f = 760 nm), long bacteriochlorin excited‐state lifetime (～5.5 ns), and narrow (≤20 nm) absorption and fluorescence bands. The latter will facilitate selective excitation/detection and multiprobe applications using both intensity‐ and lifetime‐imaging techniques.     

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).- .     

Synthesis and photoinduced electron-transfer process of a novel triphenylamine-substituted polyfluorene-C60 triad

The photoinduced electron-transfer process of a newly prepared, soluble, pi-conjugated poly[9,9-bis(4-diphenylaminophenyl)-2,7-fluorene] (PDPAF), covalently bridged, C60 triad (C60-PDPAF-C60) is described. The molecular orbital calculations revealed that the majority of the highest occupied molecular orbital (HOMO) is located on the polyfluorene entity, while the lowest unoccupied molecular orbitals (LUMO) are found to be entirely on the C60 entity. The excited-state electron-transfer processes were monitored by both steady-state and time-resolved emission as well as by transient absorption techniques in toluene and benzonitrile. By excitation of the polyfluorene moiety, fluorescence quenching of the singlet excited state of polyfluorene moiety was observed. The nanosecond transient spectra in near-IR region revealed the charge-separation process from the polyfluorene moieties to the C60 moiety through the excited singlet states of polyfluorene. The lifetimes of the charge separated states were evaluated to be 20-50 ns, depending on the solvent polarity.     

Synthesis and photoinduced electron transfer processes of rotaxanes bearing [60]fullerene and zinc porphyrin: effects of interlocked structure and length of axle with porphyrins

Three rotaxanes, with axles with two zinc porphyrins (ZnPs) at both ends penetrating into a necklace pending a C60 moiety, were synthesized with varying interlocked structures and axle lengths. The intra-rotaxane photoinduced electron transfer processes between the spatially positioned C60 and ZnP in rotaxanes were investigated. Charge-separated (CS) states (ZnP*+, C60*-)rotaxane are formed via the excited singlet state of ZnP (1ZnP*) to the C60 moiety in solvents such as benzonitrile, THF, and toluene. The rate constants and quantum yields of charge separation via 1ZnP decrease with axle length, but they are insensitive to solvent polarity. When the axle becomes long, charge separation takes place via the excited triplet state of ZnP (3ZnP*). The lifetime of the CS state increases with axle length from 180 to 650 ns at room temperature. The small activation energies of charge recombination were evaluated by temperature dependence of electron-transfer rate constants, probably reflecting through-space electron transfer in the rotaxane structures.     

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.     

Long-lived charge-separated state generated in a ferrocene-meso,meso-linked porphyrin trimer-fullerene pentad with a high quantum yield

A meso,meso-linked porphyrin trimer, (ZnP)3, as a light-harvesting chromophore, has been incorporated for the first time into a photosynthetic multistep electron-transfer model including ferrocene (Fc) as an electron donor and fullerene (C60) as an electron acceptor, to construct the ferrocene-meso,meso-linked porphyrin trimer-fullerene system Fc-(ZnP)3-C60. Photoirradiation of Fc-(ZnP)3-C60 results in photoinduced electron transfer from both the singlet and triplet excited states of the porphyrin trimer, 1(ZnP)3* and 3(ZnP)3*, to the C60 moiety to produce the porphyrin trimer radical cation-C60 radical anion pair, Fc-(ZnP)3*+-C60*-. Subsequent formation of the final charge-separated state Fc+-(ZnP)3-C60*- was confirmed by the transient absorption spectra observed by pico- and nanosecond time-resolved laser flash photolysis. The final charge-separated state decays, obeying first-order kinetics, with a long lifetime (0.53 s in DMF at 163 K) that is comparable with that of the natural bacterial photosynthetic reaction center. More importantly, the quantum yield of formation of the final charge-separated state (0.83 in benzonitrile) remains high, despite the large separation distance between the Fc+ and C60*- moieties. Such a high quantum yield results from efficient charge separation through the porphyrin trimer, whereas a slow charge recombination is associated with the localized porphyrin radical cation in the porphyrin trimer. The light-harvesting efficiency in the visible region has also been much improved in Fc-(ZnP)3-C60 because of exciton coupling in the porphyrin trimer as well as an increase in the number of porphyrins.     

Photoswitched singlet energy transfer in a porphyrin-spiropyran dyad.

A photochromic nitrospiropyran moiety (Sp) has been covalently linked to a zinc (PZn) and to a free-base (P(H2)) porphyrin. In the resulting dyads (P(Zn)-Sp(c) and P(H2)-Sp(c)), the porphyrin first excited singlet states are unperturbed by the closed form of the attached spiropyran. Excitation of the spiropyran moiety of either dyad in the near-UV region results in ring opening to a merocyanine form (P-Sp(o)) that absorbs at 600 nm. The open form re-closes thermally in 2-methyltetrahydrofuran with a time constant of 20 s, or following irradiation into the 600 nm band. Excitation of the zinc porphyrin moiety in the merocyanine form of the dyad yields 1PZn-Sp(o). The lifetime of the zinc porphyrin excited state is reduced from its usual value of 1.8 ns to 130 ps by singlet-singlet energy transfer to the merocyanine moiety to give PZn-1Sp(o). The quantum yield of energy transfer is 0.93. Quenching is also observed in the free base dyad, where 1P(H2)-Sp(o) and P(H2)-1Sp(o) exchange singlet excitation energy. This photoswitchable quenching phenomenon provides light-activated control of the porphyrin excited states, and consequently control of any subsequent energy or electron-transfer processes that might be initiated by these excited states in more complex molecular photonic or optoelectronic devices.     

Efficient charge separation in porphyrin-fullerene-ligand complexes

Photoprocesses associated with the complexation of a pyridine-functionalized C60 fullerene derivative to ruthenium- and zinc-tetraphenylporphyrins (tpp) have been studied by time-resolved optical and transient EPR spectroscopies. It has been found that upon irradiation in toluene, a highly efficient triplet-triplet energy transfer governs the deactivation of the photoexcited [Ru(tpp)], while electron transfer (ET) from the porphyrin to the fullerene prevails in polar solvents. Complexation of [Zn(tpp)] by the fullerene derivative is reversible and, following excitation of the [Zn(tpp)], gives rise to very efficient charge separation. In fluid polar solvents such as THF and benzonitrile, radical-ion pairs (RPs) are generated both by intramolecular ET inside the complex and by intermolecular ET in the uncomplexed form. Charge-separated states have lifetimes of about 10 micros in THF and several hundred of microseconds in benzonitrile at room temperature.