Hydrogen bonding interfaces in fullerene*TTF ensembles

Novel thermodynamically stable supramolecular donor-acceptor dyads have been synthesized. In particular, we assembled successfully C(60), as an electron acceptor, with the strong electron donor TTF through a complementary guanidinium-carboxylate ion pair. Two strong and well-oriented hydrogen bonds, in combination with ionic interactions, ensure the formation of stable donor-acceptor dyads. The molecular architecture has been fine-tuned by using chemical spacers of different lengths (i.e., phenyl versus biphenyl) and functional groups (i.e., ester versus amide), thus providing meaningful incentives to differentiate between through-bond and through-space electron-transfer scenarios. In electrochemical studies, both the donor and acceptor character of the TTF and C(60) units, respectively, have been clearly identified. Steady-state and time-resolved emission studies, however, show a solvent-dependent fluorescence quenching in C(60)*TTF dyads as well as the formation of the C(60)(*)(-)*TTF(*)(+) radical ion pairs, for which we determined lifetimes that are in the range of hundred of nanoseconds to microseconds. The complex network that connects C(60) with TTF in the dyads and the flexible nature of the spacer result in through-space electron-transfer processes. This first example of electron transfer in C(60)-based dyads, connected by strong hydrogen bonds, demonstrates that this approach can add outstanding benefits to the construction of artificial photosynthetic systems that bear a closer resemblance to the natural one.     

Porphyrin-fullerene linked systems as artificial photosynthetic mimics

We have prepared a variety of porphyrin-fullerene linked systems to mimic photoinduced energy and electron transfer (ET) processes in photosynthesis. Photodynamical studies on porphyrin and analogs-fullerene linked systems have revealed the acceleration of photoinduced electron transfer and charge-shift and the deceleration of charge recombination, which is reasonably explained by the small reorganization energies of electron transfer in fullerenes. In this context, we have proposed two strategies, photoinduced single-step and multi-step electron transfers, for prolonging the lifetime of a charge-separated state in donor-acceptor linked systems. The single-step ET strategy allowed a zinc chlorin-fullerene linked dyad to extend the lifetime up to 120 seconds in frozen PhCN at 123 K, which is the longest value of charge separation ever reported for donor-acceptor linked systems. Unfortunately, however, the quantum yield of formation of the charge-separated state was as low as 12%, probably due to the decay of the precursor exciplex state to the ground state rather than to the favorable complete charge-separated state. In contrast, the multi-step ET strategy has been successfully applied to porphyrin-fullerene linked triads, tetrads, and a pentad. In particular, a ferrocene-porphyrin trimer-fullerene pentad revealed formation of a long-lived charge-separated state (0.53 s in frozen DMF at 163 K) with an extremely high quantum yield (83%), which is comparable to natural bacterial reaction centers. These results not only provide valuable information for a better understanding of photoinduced energy and electron transfer processes in photosynthesis, but also open the door for the development of photoinitiated molecular devices and machines.     

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.     

Energy, charge, and spin transport in molecules and self-assembled nanostructures inspired by photosynthesis

Electron transfer in biological molecules provides both insight and inspiration for developing chemical systems having similar functionality. Photosynthesis is an example of an integrated system in which light harvesting, photoinduced charge separation, and catalysis combine to carry out two thermodynamically demanding processes, the oxidation of water and the reduction of carbon dioxide. The development of artificial photosynthetic systems for solar energy conversion requires a fundamental understanding of electron-transfer reactions between organic molecules. Since these reactions most often involve single-electron transfers, the spin dynamics of photogenerated radical ion pairs provide important information on how the rates and efficiencies of these reactions depend on molecular structure. Given this knowledge, the design and synthesis of large integrated structures to carry out artificial photosynthesis is moving forward. An important approach to achieving this goal is the development of small, functional building blocks, having a minimum number of covalent bonds, which also have the appropriate molecular recognition sites to facilitate self-assembly into a complete, functional artificial photosynthetic system.     

Intramolecular electron transfer through the 20-position of a chlorophyll a derivative: an unexpectedly efficient conduit for charge transport

Suzuki cross-coupling reactions have afforded 20-phenyl-substituted Chlorophyll a derivatives (ZCPh) in good yields and significant quantities from readily available Chl a. A series of donor-acceptor dyads was synthesized in which naphthalene-1,8:4,5-bis(dicarboximide) or either of two perylene-3,4:9,10-bis(dicarboximide) electron acceptors is attached to the para position of the 20-phenyl group. Comparisons with the analogous dyads based on a zinc 5,10,15-tri(n-pentyl)-20-phenylporphyrin donor show that, for a given acceptor and solvent, the rates of photoinduced charge separation and recombination as well as the calculated electronic coupling matrix elements, V, for these reactions differ by less than a factor of 2. However, EPR and ENDOR spectroscopy corroborated by DFT calculations show that the highest occupied MO of ZCPh+* has little spin (charge) density at the 20-carbon atom, whereas Z3PnPh+* has significant spin (charge) density there, implying that V, and therefore the electron-transfer rates, should differ significantly for these two macrocyclic donors. DFT calculations on ZCPh+* and Z3PnPh+*, with two -0.5 charges located where the nearest carbonyl oxygen atoms of the acceptor would reside in the donor-acceptor dyads, show that the presence of the negative charges significantly shifts the charge density of both ZCPh+* and Z3PnPh+* from the macrocycle onto the phenyl rings. Thus, the presence of adjacent covalently linked radical anions at a fixed location relative to each of these radical cations results in nearly identical electronic coupling matrix elements for electron transfer and therefore very similar rates.     

Electron transfer dynamics

This paper surveys the current ‘state-of-art’ of the theoretical understanding of electron transfer dynamics in donor-acceptor systems, which provide the conceptual and technical basis for solar energy conversion via optical and optoelectronic molecular devices and for the primary charge separation in photo-synthesis.     

Temperature dependence of charge separation and recombination in porphyrin oligomer-fullerene donor-acceptor systems

Electron-transfer reactions are fundamental to many practical devices, but because of their complexity, it is often very difficult to interpret measurements done on the complete device. Therefore, studies of model systems are crucial. Here the rates of charge separation and recombination in donor-acceptor systems consisting of a series of butadiyne-linked porphyrin oligomers (n = 1-4, 6) appended to C(60) were investigated. At room temperature, excitation of the porphyrin oligomer led to fast (5-25 ps) electron transfer to C(60) followed by slower (200-650 ps) recombination. The temperature dependence of the charge-separation reaction revealed a complex process for the longer oligomers, in which a combination of (i) direct charge separation and (ii) migration of excitation energy along the oligomer followed by charge separation explained the observed fluorescence decay kinetics. The energy migration is controlled by the temperature-dependent conformational dynamics of the longer oligomers and thereby limits the quantum yield for charge separation. Charge recombination was also studied as a function of temperature through measurements of femtosecond transient absorption. The temperature dependence of the electron-transfer reactions could be successfully modeled using the Marcus equation through optimization of the electronic coupling (V) and the reorganization energy (λ). For the charge-separation rate, all of the donor-acceptor systems could be successfully described by a common electronic coupling, supporting a model in which energy migration is followed by charge separation. In this respect, the C(60)-appended porphyrin oligomers are suitable model systems for practical charge-separation devices such as bulk-heterojunction solar cells, where conformational disorder strongly influences the electron-transfer reactions and performance of the device.     

Long-lived charge-transfer states in compact donor-acceptor dyads.

Photoinduced electron transfer is a widely applied method to convert photon energy into a useful (electro)chemical potential, both in nature and in artificial devices. There is a continuing effort to develop molecular systems in which the charge-transfer state, populated by photoinduced electron transfer, survives sufficiently long to tap the energy stored in it. In general this has been found to require the construction of rather complex molecular systems, but more recently a few approaches have been reported that allow the use of much more simple and relatively small electron donor-acceptor dyads for this purpose. The most successful examples of such systems seem to be those that apply "electron spin control" to slow down the spontaneous decay of the charge-transfer state, and these are reviewed in this minireview, with a discussion of the underlying principles and a critical evaluation of some of the claims made with regard to using a pronounced "inverted-region effect" as an alternative method to prolong the lifetime of charge-transfer states.     

Charge transfer in model bioinspired carotene-porphyrin dyads

We present a computational study based on accurate DFT and TD-DFT methods on model bioinspired donor-acceptor dyads, formed by a carotenoid covalently linked to a tetraphenylporphyrin (TPP) at the ortho position of one of the TPP phenyl rings. Dyadic systems can be used in the construction of organic solar cells and development of efficient photocatalytic systems for the solar energy conversion, due to the unique advantages they offer in terms of synthetic feasibility. This study aims to describe the influence of chemical modifications on the absorption spectra, in particular on the lowest energy charge transfer bands. Effects of different metals of biological interest, i.e., Mg, Fe, Ni, and Zn, and of H(2)O and histidine molecules coordinated to the metals in different axial positions are rationalized.     

Light harvesting zinc naphthalocyanine-perylenediimide supramolecular dyads: long-lived charge-separated states in nonpolar media

Photoinduced electron-transfer dynamics of self-assembled donor-acceptor dyads formed by axial coordination of zinc naphthalocyanine, ZnNc, and perylenediimide (PDI) bearing either pyridine (py) or imidazole (im) coordinating ligands were investigated. The PDIim unit was functionalized with tert-octylphenoxy groups at the bay positions, which avoid aggregation providing solubility, to examine the effect of the bulky substituents at the bay positions on the rates of electron-transfer reactions. The combination between zinc naphthalocyanine and perylenediimide entities absorbs light over a wide region of the visible and near infrared (NIR) spectrum. The binding constants of the self-assembled ZnNc:PDIpy (1) and ZnNc:PDIim (2) in toluene were found to be 2.40 × 10(4) and 1.10 × 10(5) M(-1), respectively, from the steady-state absorption and emission measurements, indicating formation of moderately stable complexes. The geometric and electronic calculations by using an ab initio B3LYP/6-311G method showed the majority of the highest occupied frontier molecular orbital (HOMO) on the zinc naphthalocyanine entity, while the lowest unoccupied molecular orbital (LUMO) was on the perylenediimide entities, suggesting that the charge-separated states of the supramolecular dyads are ZnNc˙(+):PDI˙(-). The electrochemical results suggest the exothermic charge-separation process via the singlet states of both ZnNc and PDI entities in nonpolar toluene. Upon coordination of perylenediimide to ZnNc, the main quenching pathway involved charge separation via the singlet-excited states of ZnNc and PDIs. Clear evidence of the intramolecular electron transfer from the singlet-excited state of ZnNc to PDI within the supramolecular dyads in toluene was monitored by the femtosecond laser photolysis by observing the characteristic absorption band of the PDI radical anion (PDI˙(-)) and the ZnNc radical cation (ZnNc˙(+)) in the visible and NIR regions. The rate constants of charge-separation (k(CS)) processes of the self-assembled dyads 1 and 2 were determined to be 4.05 × 10(10) and 1.20 × 10(9) s(-1), respectively. The rate constant of charge recombination (k(CR)) and the lifetime of charge-separated states (τ(CS)) of dyad 1 were determined to be 2.34 × 10(8) s(-1) and 4.30 ns, respectively. Interestingly, a slower charge recombination (2.20 × 10(7) s(-1)) and a longer lifetime of the charge separated state (45 ns) were observed in dyad 2 in nonpolar toluene by utilizing the nanosecond transient measurements. The absorption in a wide section of the solar spectrum and the high charge-separation/charge-recombination ratio suggest the usefulness of the self-assembled zinc naphthalocyanine-perylenediimide dyads as good photosynthetic models.     

Photosynthetic Antenna–Reaction Center Mimicry by Using Boron Dipyrromethene Sensitizers

Various molecular and supramolecular systems have been synthesized and characterized recently to mimic the functions of photosynthesis, in which solar energy conversion is achieved. Artificial photosynthesis consists of light‐harvesting and charge‐separation processes together with catalytic units of water oxidation and reduction. Among the organic molecules, derivatives of BF₂‐chelated dipyrromethene (BODIPY), “porphyrin’s little sister”, have been widely used in constructing these artificial photosynthetic models due to their unique properties. In these photosynthetic models, BODIPYs act as not only excellent antenna molecules, but also as electron‐donor and ‐acceptor molecules in both the covalently linked molecular and supramolecular systems formed by axial coordination, hydrogen bonding, or crown ether complexation. The relationships between the structures and photochemical reactivities of these novel molecular and supramolecular systems are discussed in relation to the efficiency of charge separation and charge recombination. Femto‐ and nanosecond transient absorption and photoelectrochemical techniques have been employed in these studies to give clear evidence for the occurrence of energy‐ and electron‐transfer reactions and to determine their rates and efficiencies.     

Unusual role of oxygen in electron-transfer processes

Molecular oxygen's unique involvement in electron-transfer processes is demonstrated on a series of dyads between porphyrin derivatives and fullerene C60. It has been shown for the first time that oxygen can serve as an inhibitor of back electron transfer by enhancing intersystem crossing of a singlet radical ion pair into its triplet state. The effect is observed only when energy of the charge-separated state is lower than that of the locally excited triplet states. Due to the spin statistics, the reverse intersystem crossing is less efficient, allowing use of oxygen and other paramagnetic species for impeding charge recombination in various electron-transfer systems.     

Directly Linked Zinc Phthalocyanine–Perylenediimide Dyads and a Triad for Ultrafast Charge Separation

Directly linked to promote strong intramolecular interactions, donor–acceptor dyads and a donor–acceptor–donor triad featuring zinc phthalocyanine (ZnPc) as electron donor and perylenediimide (PDI) as electron acceptor have been synthesized and characterized. Owing to complementary absorption features of the entities, improved light absorption was witnessed in these conjugates. The optimized geometry and electronic structures showed the majority of the highest occupied molecular orbital (HOMO) on the ZnPc entity, whereas the lowest unoccupied molecular orbital (LUMO) was on the PDI entity, suggesting that the charge-separated states would be ZnPc ⁺ –PDI ^{. −}. The electrochemical and free-energy calculations suggested exothermic energy and/or electron transfer processes via the singlet states of PDI or ZnPc entities depending on the excitation wavelength of the laser used. The measured rates using femtosecond pump-probe spectroscopy coupled with global analysis of transient data revealed ultrafast energy transfer from ¹PDI* to ZnPc followed by charge separation. However, when ZnPc was selectively excited, only electron transfer was witnessed wherein the time constants for forward and reverse electron transfer processes followed Marcus predictions. The absorption in a wide section of the solar spectrum and the ultrafast charge separation suggest the usefulness of these systems as good photosynthetic models.     

Glassy protein dynamics and gigantic solvent reorganization energy of plastocyanin

We report the results of molecular dynamics simulations of electron-transfer activation parameters of plastocyanin metalloprotein involved as an electron carrier in natural photosynthesis. We have discovered that slow, non-ergodic conformational fluctuations of the protein, coupled to hydrating water, result in a very broad distribution of donor-acceptor energy gaps far exceeding those observed for commonly studied inorganic and organic donor-acceptor complexes. The Stokes shift is not affected by these fluctuations and can be calculated from solvation models in terms of the linear response of the solvent dipolar polarization. The non-ergodic character of large-amplitude protein/water mobility breaks the strong link between the Stokes shift and the reorganization energy characteristic of equilibrium (ergodic) theories of electron transfer. This mechanism might be responsible for fast electronic transitions in natural electron-transfer proteins characterized by low reaction free energy.     

Design, synthesis, and photophysical studies of a porphyrin-fullerene dyad with parachute topology; charge recombination in the marcus inverted region

As part of a continuing investigation of the topological control of intramolecular electron transfer (ET) in donor-acceptor systems, a symmetrical parachute-shaped octaethylporphyrin-fullerene dyad has been synthesized. A symmetrical strap, attached to ortho positions of phenyl groups at opposing meso positions of the porphyrin, was linked to [60]-fullerene in the final step of the synthesis. The dyad structures were confirmed by (1)H, (13)C, and (3)He NMR, and MALDI-TOF mass spectra. The free-base and Zn-containing dyads were subjected to extensive spectroscopic, electrochemical and photophysical studies. UV-vis spectra of the dyads are superimposable on the sum of the spectra of appropriate model systems, indicating that there is no significant ground-state electronic interaction between the component chromophores. Molecular modeling studies reveal that the lowest energy conformation of the dyad is not the C(2)(v)() symmetrical structure, but rather one in which the porphyrin moves over to the side of the fullerene sphere, bringing the two pi-systems into close proximity, which enhances van der Waals attractive forces. To account for the NMR data, it is proposed that the dyad is conformationally mobile at room temperature, with the porphyrin swinging back and forth from one side of the fullerene to the other. The extensive fluorescence quenching in both the free base and Zn dyads is associated with an extremely rapid photoinduced electron-transfer process, k(ET) approximately 10(11) s(-)(1), generating porphyrin radical cations and C(60) radical anions, detected by transient absorption spectroscopy. Back electron transfer (BET) is slower than charge separation by up to 2 orders of magnitude in these systems. The BET rate is slower in nonpolar than in polar solvents, indicating that BET occurs in the Marcus inverted region, where the rate decreases as the thermodynamic driving force for BET increases. Transient absorption and singlet molecular oxygen sensitization data show that fullerene triplets are formed only with the free base dyad in toluene, where triplet formation from the charge-separated state is competitive with decay to the ground state. The photophysical properties of the P-C(60) dyads with parachute topology are very similar to those of structurally related rigid pi-stacked P-C(60) dyads, with the exception that there is no detectable charge-transfer absorption in the parachute systems, attributed to their conformational flexibility. It is concluded that charge separation in these hybrid systems occurs through space in unsymmetrical conformations, where the center-to-center distance between the component pi-systems is minimized. Analysis of the BET data using Marcus theory gives reorganization energies for these systems between 0.6 and 0.8 eV and electronic coupling matrix elements between 4.8 and 5.6 cm(-)(1).     

Probing the efficiency of electron transfer through porphyrin-based molecular wires

Electron transfer over long distances is important for many future applications in molecular electronics and solar energy harvesting. In these contexts, it is of great interest to find molecular systems that are able to efficiently mediate electrons in a controlled manner over nanometer distances, that is, structures that function as molecular wires. Here we investigate a series of butadiyne-linked porphyrin oligomers with ferrocene and fullerene (C60) terminals separated by one, two, or four porphyrin units (Pn, n = 1, 2, or 4). When the porphyrin oligomer bridges are photoexcited, long-range charge separated states are formed through a series of electron-transfer steps and the rates of photoinduced charge separation and charge recombination in these systems were elucidated using time-resolved absorption and emission measurements. The rates of long-range charge recombination, through these conjugated porphyrin oligomers, are remarkably fast (kCR2 = 15 - 1.3 x 108 s-1) and exhibit very weak distance dependence, particularly comparing the systems with n = 2 and n = 4. The observation that the porphyrin tetramer mediates fast long-range charge transfer, over 65 A, is significant for the application of these structures as molecular wires.     

Molecular approaches to solar energy conversion with coordination compounds anchored to semiconductor surfaces

Strategies toward the realization of molecular control of interfacial charge transfer at nanocrystalline semiconductor interfaces are described. Light excitation of coordination compounds, based on (dpi)6 transition metals, anchored to wide band-gap semiconductors, such as TiO2, can initiate electron-transfer processes that ultimately reduce the semiconductor. Such photoinduced charge-separation processes are a key step for solar energy conversion. The thermodynamics and kinetic rate constants for three different interfacial charge separation mechanisms are discussed. Tuning the energetic position of the semiconductor conduction band relative to the molecular sensitizer has provided new insights into interfacial charge transfer. Supramolecular compounds that efficiently absorb light, promote interfacial electron transfer, and feature additional functions such as intramolecular electron transfer when bound to semiconductor surfaces have also been studied. New approaches for enhancing charge-separation lifetimes for solar energy conversion are presented.     

Supramolecular electron transfer by anion binding

Anion binding has emerged as an attractive strategy to construct supramolecular electron donor-acceptor complexes. In recent years, the level of sophistication in the design of these systems has advanced to the point where it is possible to create ensembles that mimic key aspects of the photoinduced electron-transfer events operative in the photosynthetic reaction centre. Although anion binding is a reversible process, kinetic studies on anion binding and dissociation processes, as well as photoinduced electron-transfer and back electron-transfer reactions in supramolecular electron donor-acceptor complexes formed by anion binding, have revealed that photoinduced electron transfer and back electron transfer occur at time scales much faster than those associated with anion binding and dissociation. This difference in rates ensures that the linkage between electron donor and acceptor moieties is maintained over the course of most forward and back electron-transfer processes. A particular example of this principle is illustrated by electron-transfer ensembles based on tetrathiafulvalene calix[4]pyrroles (TTF-C4Ps). In these ensembles, the TTF-C4Ps act as donors, transferring electrons to various electron acceptors after anion binding. Competition with non-redox active substrates is also observed. Anion binding to the pyrrole amine groups of an oxoporphyrinogen unit within various supramolecular complexes formed with fullerenes also results in acceleration of the photoinduced electron-transfer process but deceleration of the back electron transfer; again, this is ascribed to favourable structural and electronic changes. Anion binding also plays a role in stabilizing supramolecular complexes between sulphonated tetraphenylporphyrin anions ([MTPPS](4-): M = H(2) and Zn) and a lithium ion encapsulated C(60) (Li(+)@C(60)); the resulting ensemble produces long-lived charge-separated states upon photoexcitation of the porphyrins.