Potassium ion controlled switching of intra- to intermolecular electron transfer in crown ether appended free-base porphyrin-fullerene donor-acceptor systems

Photoinduced electron transfer in intramolecularly interacting free-base porphyrin bearing one or four 18-crown-6 ether units at different positions of the porphyrin macrocycle periphery and pristine fullerene was investigated in polar benzonitrile and nonpolar o-dichlorobenzene and toluene solvents. Owing to the presence of two modes of binding, stable dyads were obtained in which the binding constants, K, were found to range between 4.2 x 10(3) and 10.4 x 10(3) M(-1) from fluorescence quenching data depending upon the location and number of crown ether entities on the porphyrin macrocycle and the solvent. Computational studies using the B3LYP/3-21G() method were employed to arrive at the geometry and electronic structure of the intramolecular dyads. The energetics of the redox states of the dyads were established from cyclic voltammetric studies. Under the intramolecular conditions, both the steady-state and time-resolved emission studies revealed efficient quenching of the singlet excited free-base porphyrin in these dyads, and the measured rates of charge separation, k(CS), were found to be in the 10(8)-10(9) s(-1) range. Nanosecond transient absorption studies were performed to characterize the electron-transfer products and to evaluate the charge-recombination rates. Shifting of the electron-transfer pathway from the intra- to intermolecular route was achieved by complexing potassium ions to the crown ether cavity(ies) in benzonitrile. This cation complexation weakened the intramolecular interactions between fullerene and the crown ether appended free-base porphyrin supramolecules, and under these conditions, intermolecular type interactions were mainly observed. Reversible inter- to intramolecular electron transfer was also accomplished by extracting the potassium ions of the complex with the addition of 18-crown-6. The present study nicely demonstrates the application of supramolecular methodology to control the excited-state electron-transfer path in donor-acceptor dyads.     

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.     

Exciplex intermediates in photoinduced electron transfer of porphyrin-fullerene dyads

The photoinduced electron transfer in differently linked zinc porphyrin-fullerene dyads and their free-base porphyrin analogues was studied in polar and nonpolar solvents with femto- to nanosecond absorption and emission spectroscopies. A new intermediate state, different from the locally excited (LE) chromophores and the complete charge-separated (CCS) state, was observed. It was identified as an exciplex. The exciplex preceded the CCS state in polar benzonitrile and the excited singlet state of fullerene in nonpolar toluene. The behavior of the dyads was modeled by using a common kinetic scheme involving equilibria between the exciplex and LE chromophores. The scheme is suitable for all the studied porphyrin-fullerene compounds. The rates of reaction steps depended on the type of linkage between the moieties. The scheme and Marcus theory were applied to calculate electronic couplings for sequential reactions, and consistent results were obtained.     

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.     

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.     

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.     

Synthesis and excited-state photodynamics of perylene-porphyrin dyads. 4. Ultrafast charge separation and charge recombination between tightly coupled units in polar media

New perylene-porphyrin dyads that have excellent light-harvesting and energy-utilization capabilities in nonpolar media are found to exhibit efficient, ultrafast and tunable charge-transfer activity in polar media. The dyads consist of a perylene-monoimide dye (PMI) connected to a porphyrin (Por) via an ethynylphenyl (ep) linker. The porphyrin constituent of the PMI-ep-Por arrays is either a zinc or magnesium complex (Por = Zn or Mg) or a free-base form (Por = Fb). Following excitation of the perylene in each array in acetonitrile, PMI* decays in ≤0.4 ps by a combination of energy transfer to the ground-state porphyrin (forming Por*) and hole transfer (forming PMI^-Por⁺). The excited porphyrin formed by energy transfer (or via direct excitation) then undergoes effectively quantitative electron transfer back to the perylene (τ = 1, 1, 700 ps for Por = Mg, Zn, Fb). Subsequently, charge recombination within PMI^- Por⁺ returns each dyad quantitatively to the ground state (τ = 2, 4, 8 ps for Por = Mg, Zn, Fb). The dynamics of the PMI Por* → PMI^-Por⁺ and PMI^- Por⁺ → PMI Por charge-transfer processes can be modulated by altering the type of polar solvent (acetonitrile, benzonitrile, tetrahydrofuran and 2,6-lutidine). The charge-separation times for PMI-ep-Zn are 1, 6, 9 and 22 ps in these solvents, while the charge-recombination times are 4, 24, 38 and 34 ps. The efficient, rapid and tunable nature of the charge-transfer processes in polar media makes the PMI-ep-Por dyads useful units for performing molecular-switching functions. These properties when combined with the excellent light-harvesting and energy-transfer capabilities of the same arrays in nonpolar media afford a robust perylene-porphyrin motif that can be tailored for a variety of functions in molecular optoelectronics.     

Exciplex mediated photoinduced electron transfer reactions of phthalocyanine-fullerene dyads

Evidences of an intramolecular exciplex intermediate in a photoinduced electron transfer (ET) reaction of double-linked free-base and zinc phthalocyanine-C60 dyads were found. This was the first time for a dyad with phthalocyanine donor. Excitation of the phthalocyanine moiety of the dyads results in rapid ET from phthalocyanine to fullerene via an exciplex state in both polar and nonpolar solvents. Relaxation of the charge-separated (CS) state Pc(*+)-C60(*-) in a polar solvent occurs directly to the ground state in 30-70 ps. In a nonpolar solvent, roughly 20% of the molecules undergo transition from the CS state to phthalocyanine triplet state (3)Pc*-C60 before relaxation to the ground state. Formation of the CS state was confirmed with electron spin resonance measurements at low temperature in both polar and nonpolar solvent. Reaction schemes for the photoinduced ET reactions of the dyads were completed with rate constants obtained from the time-resolved absorption and emission measurements and with state energies obtained from the fluorescence, phosphorescence, and voltammetric measurements.     

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.     

Preparations and characterizations of bichromophoric systems composed of a ruthenium polypyridine complex connected to a difluoroborazaindacene or a zinc phthalocyanine chromophore

This paper describes the synthesis of a new series of molecules composed of a ruthenium cation liganded by a chloro or a thiocyanato, a 4,4'-(diethoxycarbonyl)-2,2'-bipyridine, and a 2,2':6',2' '-terpyridine substituted in its 4' position by a difluoroborazaindacene or a zinc phthalocyanine. A set of conditions are reported to conveniently synthesize these dyads by a Stille cross-coupling reaction between the trimethyltin derivative of the organic chromophore and the corresponding ruthenium complex with 4'-bromo-2,2':6',2' '-terpyridine and 4,4'-(diethoxycarbonyl)-2,2'-bipyridine. The dyads were studied by UV-visible absorption spectroscopy, steady-state fluorescence, and electrochemistry. The results of these studies indicate strong electronic coupling between the zinc phthalocyanine unit and the ruthenium complex but weakly electronically coupled systems in the case of dyads containing a difluoroborazaindacene unit. The new bichromophoric systems display strong absorbance in the visible spectrum. An efficient quenching of the fluorescence of the organic chromophore by the nearby ruthenium complex was also observed in all of the dyads. In dyads connected to the borazaindacene, excitation spectra indicate efficient photoinduced energy transfer from the borazaindacene to the ruthenium complex.     

Ultrafast excited-state dynamics of strongly coupled porphyrin/core-substituted-naphthalenediimide dyads

The photophysics and excited-state dynamics of two dyads consisting of either a free-base or a zinc-tetraphenylporphyrin linked through a rigid bridge to a core-substituted naphthalenediimide (NDI) have been investigated by femtosecond-resolved spectroscopy. The absorption and fluorescence spectra differ substantially from those of the individual units, pointing to a substantial coupling and to a delocalisation of the excitation over the whole molecule, as confirmed by quantum chemistry calculations. A strong dependence of their excited-state dynamics on the solvent polarity has been observed. In toluene, the fluorescence quantum yield of the dyads is of the order of a few percent and the main decay channel of the emitting state is proposed as intersystem-crossing to the triplet state. However, in a medium polarity solvent like dichloromethane, the emitting state undergoes charge separation from the porphyrin to the NDI unit within 1-3 ps, and the ensuing charge-separated state recombines in about 10-20 ps. This solvent dependence can be explained by the weak driving force for charge separation in polar solvents and the large electronic coupling between the porphyrin and NDI moieties, making charge separation a solvent-controlled adiabatic process.     

A functionalized noncovalent macrocyclic multiporphyrin assembly from a dizinc(II) bis-porphyrin receptor and a free-base dipyridylporphyrin

The bis-porphyrin system ZnP(2), in which two zinc porphyrins are connected by a phenanthroline linker in an oblique fashion, acts as a bifunctional receptor towards the complexation of free-base meso-5,10-bis(4'-pyridyl)-15,20-diphenylporphyrin (4'-cis DPyP). In solution, NMR spectroscopy evidenced quantitative formation of the tris-porphyrin macrocyclic assembly ZnP(2)(4'-cis DPyP), in which the two fragments are held together by two axial 4'-N(pyridyl)-Zn interactions. The remarkable stability of the edifice (an association constant of about 6x10(8) M(-1) was determined by UV/Vis absorption and emission titration experiments in toluene) is due to the almost perfect geometrical match between the two interacting units. The macrocycle was crystallized and studied by X-ray diffraction, which confirmed the excellent complementarity of the two components. Photoinduced energy transfer from the singlet excited state of the zinc porphyrin chromophores to the free-base porphyrin occurs with an efficiency of 98 % (k(en)=2x10(10) s(-1) in toluene, ambient temperature) with a mechanism consistent with a dipole-dipole process with a low orientation factor.     

Corrole-fullerene dyads: formation of long-lived charge-separated states in nonpolar solvents

The first example of covalently linked free-base corrole-fullerene dyads is reported. In the newly synthesized dyads, the free-energy calculations performed by employing the redox and singlet excited-state energy in both polar and nonpolar solvents suggested the possibility of electron transfer from the excited singlet state of corrole to the fullerene entity. Accordingly, steady-state and time-resolved emission studies revealed efficient fluorescence quenching of the corrole entity in the dyads. Further studies involving femtosecond laser flash photolysis and nanosecond transient absorption studies confirmed electron transfer to be the quenching mechanism, in which the electron-transfer product, the fullerene anion radical, was able to be spectrally characterized. The rate of charge separation, kCS, was found to be on the order of 10(10)-10(11) s(-1), suggesting an efficient photoinduced electron-transfer process. Interestingly, the rate of charge recombination, kCR, was slower by 5 orders of magnitude in nonpolar solvents, cyclohexane and toluene, resulting in a radical ion-pair lasting for several microseconds. Careful analysis of the kinetic and thermodynamic data using the Marcus approach revealed that this novel feature is due to appropriately positioning the energy level of the charge-separated state below the triplet states of either of the donor and acceptor entities in both polar and nonpolar solvents, a feature that was not evident in donor-acceptor dyads constructed using symmetric tetrapyrroles as electron donors.     

Synthesis and photophysical properties of light-harvesting arrays comprised of a porphyrin bearing multiple perylene-monoimide accessory pigments

We present the synthesis and characterization of new light-harvesting arrays containing two, four, or eight perylene-monoimide accessory pigments attached to a zinc porphyrin. Each perylene is substituted with one or three 4-tert-butylphenoxy substituents. A 4,3'- or 4,2'-diarylethyne linker joins the perylene N-imide position and the porphyrin meso-position, affording divergent or convergent architectures, respectively. The architectures are designed to provide high solubility in organic media and facile perylene-to-porphyrin energy transfer, while avoiding charge-transfer quenching of the excited porphyrin product. For the array containing four perylenes per porphyrin in both nonpolar (toluene) and polar (benzonitrile) media and for the array containing eight perylenes per porphyrin in toluene, the photoexcited perylene-monoimide dye (PMI) decays rapidly ( approximately 3.5 ps) and predominantly (>or=90%) by energy transfer to the zinc porphyrin to form the excited zinc porphyrin (Zn), which has excited-state characteristics (lifetime, fluorescence yield) comparable (within approximately 10%) to those of the isolated chromophore. For the array containing eight perylenes in benzonitrile, PMI decays approximately 80% by energy transfer (forming Zn) and approximately 20% by hole transfer (forming PMI- Zn+); Zn subsequently decays approximately 20% by electron transfer (also forming PMI- Zn+) and approximately 80% by the normal routes open to the porphyrin monomer (intersystem crossing, internal conversion, fluorescence). In addition to rapid and efficient perylene-to-porphyrin energy transfer, the broad blue-green to yellow absorption of the perylene dyes complements the blue absorption of the porphyrin, resulting in excellent light harvesting across a significant spectral region. Collectively, the work described herein identifies multiperylene-porphyrin arrays that exhibit suitable photochemical properties for use as motifs in larger light-harvesting systems.     

Charge-transfer interactions in face-to-face porphyrin-fullerene systems: solvent-dependent luminescence in the infrared spectral region

The cyclophane-type molecular dyads 1 x 2H and 1 x Zn, in which a doubly bridged porphyrin donor adopts a close, tangential orientation relative to the surface of a fullerene acceptor, were prepared by Bingel macrocylization. The porphyrin derivatives 2 x 2H and 2 x Zn with two appended, singly linked C60 moieties were also formed as side products. NMR investigations revealed that the latter compounds strongly prefer conformations with one of the carbon spheres nesting on the porphyrin surface, thereby taking a similar orientation to that of the fullerene moiety in the doubly bridged systems. Cyclic voltammetric measurements showed that the mutual electronic effects exerted by the fullerene on the porphyrin and vice versa are only small in all four dyads, despite the close proximity of the donor and acceptor components. The steady-state and time-resolved absorption and luminescence properties of 1 x Zn and 2 x Zn were investigated in toluene solution and it was shown that, upon light excitation, both the porphyrin- and the fullerene-centered excited states are deactivated to a lower-lying CT state, emitting in the IR spectral region (lambda max = 890 and 800 nm at 298 and 77 K, respectively). In the more polar solvent benzonitrile, this CT state is still detected but, owing to its very low energy (below 1.4 eV), is not luminescent and shorter-lived than in toluene. The remarkable observation of similar photophysical behavior of 1 x Zn and 2 x Zn suggests that a tight donor-acceptor distance cannot only be established in doubly bridged cyclophane-type structures but also in singly bridged dyads, by taking advantage of favourable fullerene-porphyrin ground-state interactions.     

Single-step electron transfer on the nanometer scale: ultra-fast charge shift in strongly coupled zinc porphyrin-gold porphyrin dyads

The synthesis, electrochemical properties, and photoinduced electron transfer processes of a series of three novel zinc(II)-gold(III) bisporphyrin dyads (ZnP--S--AuP(+)) are described. The systems studied consist of two trisaryl porphyrins connected directly in the meso position via an alkyne unit to tert-(phenylenethynylene) or penta(phenylenethynylene) spacers. In these dyads, the estimated center to center interporphyrin separation distance varies from 32 to 45 A. The absorption, emission, and electrochemical data indicate that there are strong electronic interactions between the linked elements, thanks to the direct attachment of the spacer on the porphyrin ring through the alkyne unit. At room temperature in toluene, light excitation of the zinc porphyrin results in almost quantitative formation of the charge shifted state (.+)ZnP--S--AuP(.), whose lifetime is in the order of hundreds of picoseconds. In this solvent, the charge-separated state decays to the ground state through the intermediate population of the zinc porphyrin triplet excited state. Excitation of the gold porphyrin leads instead to rapid energy transfer to the triplet ZnP. In dichloromethane the charge shift reactions are even faster, with time constants down to 2 ps, and may be induced also by excitation of the gold porphyrin. In this latter solvent, the longest charge-shifted lifetime (tau=2.3 ns) was obtained with the penta-(phenylenethynylene) spacer. The charge shift reactions are discussed in terms of bridge-mediated super-exchange mechanisms as electron or hole transfer. These new bis-porphyrin arrays, with strong electronic coupling, represent interesting molecular systems in which extremely fast and efficient long-range photoinduced charge shift occurs over a long distance. The rate constants are two to three orders of magnitude larger than for corresponding ZnP--AuP(+) dyads linked via meso-phenyl groups to oligo-phenyleneethynylene spacers. This study demonstrates the critical impact of the attachment position of the spacer on the porphyrin on the electron transfer rate, and this strategy can represent a useful approach to develop molecular photonic devices for long-range charge separations.     

Azobenzene-linked porphyrin-fullerene dyads

A new group of porphyrin-fullerene dyads with an azobenzene linker was synthesized, and the photochemical and photophysical properties of these materials were investigated using steady-state and time-resolved spectroscopic methods. The electrochemical properties of these compounds were also studied in detail. The synthesis involved oxidative heterocoupling of free base tris-aryl-p-aminophenyl porphyrins with a p-aminophenylacetal, followed by deprotection to give the aldehyde, and finally Prato 1,3-dipolar azomethineylide cycloaddition to C60. The corresponding Zn(II)-porphyrin (ZnP) dyads were made by treating the free base dyads with zinc acetate. The final dyads were characterized by their 1H NMR, mass, and UV-vis spectra. 3He NMR was used to determine if the products are a mixture of cis and trans stereoisomers, or a single isomer. The data are most consistent with the isolation of only a single configurational isomer, assigned to the trans (E) configuration. The ground-state UV-vis spectra are virtually a superimposition of the spectral features of the individual components, indicating there is no interaction of the fullerene (F) and porphyrin (H2P/ZnP) moieties in the ground state. This conclusion is supported by the electrochemical data. The steady-state and time-resolved fluorescence spectra indicate that the porphyrin fluorescence in the dyads is very strongly quenched at room temperature in the three solvents studied: toluene, tetrahydrofuran (THF), and benzonitrile (BzCN). The fluorescence lifetimes of the dyads in all solvents are sharply reduced compared to those of H2P and ZnP standards. In toluene, the lifetimes of the free base dyads are 600-790 ps compared to 10.1 ns for the standard, while in THF and BzCN the dyad lifetimes are less than 100 ps. For the ZnP dyads, the fluorescence lifetimes were 10-170 ps vs 2.1-2.2 ns for the ZnP references. The mechanism of the fluorescence quenching was established using time-resolved transient absorption spectroscopy. In toluene, the quenching process is singlet-singlet energy transfer (k approximately 10(11) s-1) to give C60 singlet excited states which decay with a lifetime of 1.2 ns to give very long-lived C60 triplet states. In THF and BzCN, quenching of porphyrin singlet states occurs at a similar rate, but now by electron transfer, to give charge-separated radical pair (CSRP) states, which show transient absorption spectra very similar to those reported for other H2P-C60 and ZnP-C60 dyad systems. The lifetimes of the CSRP states are in the range 145-435 ns in THF, much shorter than for related systems with amide, alkyne, silyl, and hydrogen-bonded linkers. Thus, both forward and back electron transfer is facilitated by the azobenzene linker. Nonetheless, the charge recombination is 3-4 orders of magnitude slower than charge separation, demonstrating that for these types of donor-acceptor systems back electron transfer is occurring in the Marcus inverted region.     

Comparison of excited-state energy transfer in arrays of hydroporphyrins (chlorins, oxochlorins) versus porphyrins: rates, mechanisms, and design criteria

A set of chlorin-chlorin and oxochlorin-oxochlorin dyads has been prepared with components in the same or different metalation states. In each case a 4,4'-diphenylethyne linker spans the respective 10-position of each macrocycle. The dyads have been studied using static and time-resolved absorption and emission spectroscopy, resonance Raman spectroscopy, and electrochemical techniques. Excited-state energy transfer from a zinc chlorin to a free-base (Fb) chlorin occurs with a rate constant of (110 ps)(-1) and an efficiency of 93%; similar values of (140 ps)(-1) and 83% are found for the corresponding oxochlorin dyad. Energy transfer in both dyads is slower and less efficient than found previously for the analogous porphyrin dyad, which displays a rate of (24 ps)(-1) and a yield of 99%. The slower rates and diminished efficiencies in the ZnFb chlorin and oxochlorin dyads versus the ZnFb porphyrin dyad are attributed to substantially weaker linker-mediated through-bond (TB) electron-exchange coupling (as indicated by resonance Raman data). Although the through-space (TS, i.e., dipole-dipole) coupling in the ZnFb-chlorin and -oxochlorin dyads is enhanced relative to the ZnFb porphyrin dyad (as indicated by F?rster calculations), this enhancement is insufficient to compensate for the greatly diminished TB coupling. Taken together, the chlorin and oxochlorin dyads examined herein serve as benchmarks for elucidating the energy-transfer, electrochemical, and other properties of light-harvesting arrays containing multiple chlorins or oxochlorins.     

Synthesis of novel N-linked porphyrin-phthalocyanine dyads

[structure: see text] Two types of covalently NH-linked porphyrin-phthalocyanine dyads, connected either through the meso phenyl group or the beta-pyrrolic position of the porphyrin, have been synthesized following statistical condensation methodologies for phthalocyanine preparation and palladium-catalyzed amination methods. Photophysical studies have revealed that energy transfer from the porphyrin to the phthalocyanine prevails regardless of linkage.     

Promising fast energy transfer system via an easy synthesis: Bodipy-porphyrin dyads connected via a cyanuric chloride bridge, their synthesis, and electrochemical and photophysical investigations

The boron dipyrrin (Bodipy) chromophore was combined with either a free-base or a Zn porphyrin moiety (H(2)P and ZnP respectively), via an easy synthesis involving a cyanuric chloride bridging unit, yielding dyads Bodipy-H(2)P (4) and Bodipy-ZnP (5). The photophysical properties of Bodipy-H(2)P (4) and Bodipy-ZnP (5) were investigated by UV-Vis absorption and emission spectroscopy, cyclic voltammetry, and femtosecond transient absorption spectroscopy. The comparison of the absorption spectra and cyclic voltammograms of dyads Bodipy-H(2)P (4) and Bodipy-ZnP (5) with those of their model compounds Bodipy, H(2)P, and ZnP shows that the spectroscopic and electrochemical properties of the constituent chromophores are essentially retained in the dyads indicating negligible interaction between them in the ground state. In addition, luminescence and transient absorption experiments show that excitation of the Bodipy unit in Bodipy-H(2)P (4) and Bodipy-ZnP (5) into its first singlet excited state results in rapid Bodipy to porphyrin energy transfer-k(4) = 2.9 × 10(10) s(-1) and k(5) = 2.2 × 10(10) s(-1) for Bodipy-H(2)P (4) and Bodipy-ZnP (5), respectively-generating the first porphyrin-based singlet excited state. The porphyrin-based singlet excited states give rise to fluorescence or undergo intersystem crossing to the corresponding triplet excited states. The title complexes could also be used as precursors for further substitution on the third chlorine atom on the cyanuric acid moiety.