Supramolecular control over donor-acceptor photoinduced charge separation

A novel donor-bridge-acceptor system has been synthesized by covalently linking a p-phenylene vinylene oligomer (OPV) and a perylene diimid (PERY) at opposite ends of a m-phenylene ethynylene oligomer (FOLD) of twelve phenyl rings, containing nonpolar (S)-3,7-dimethyl-1-octanoxy side chains. For comparison, model compounds have been prepared in which either the donor or acceptor is absent. In chloroform, the oligomeric bridge is in a random coil conformation. Upon addition of an apolar solvent (heptane) the oligomeric bridge first folds into a helical stack and subsequently intermolecular self-assembly of the stacks into columnar architectures occurs. Photoexcitation in the random coil conformation, where the interaction between the donor and acceptor chromophores is small, results only in long-range intramolecular energy transfer in which the OPV singlet-excited state is transformed into the PERY singlet-excited state. In the folded conformation of the bridge, donor and acceptor are closer and their enhanced interaction favors the formation the OPV(*)(+)-FOLD-PERY(*)(-) charge-separated state upon photoexcitation. As a result, the extent of photoinduced charge separation depends on the degree of folding of the bridge between donor and acceptor and therefore on the apolar nature of the medium. As a consequence, and contrary to conventional photoinduced charge separation processes, the formation of the OPV(*)(+)-FOLD-PERY(*)(-) charge-separated state is more favored in apolar media.     

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

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

Stable photoinduced charge separation in heptacene

Heptacene, generated in inert gas matrices by photobisdecarbonylation of a bridged alpha-diketone precursor, undergoes ionization into radical anion and radical cation upon UV irradiation.     

Quasi-solid medium for photoinduced charge separation

New quasi-solid was proposed as a medium for photoinduced charge separation. Photoinduced charge separation and hydrogen production with the new quasi-solid medium using polysaccharide has been investigated. The new medium is a quasi-solid polysaccharide containing a large quantity of water. This medium is tight and elastic solid, obtained by cooling an aqueous solution of a polysaccharide (agarose or κ-carrageenan in this paper) to room temperature after it was dissolved in water by heating. When the quasi-solid (agarose or κ-carrageenan) involving sacrificial electron donor ethylenediaminetetraacetate (EDTA), tris(2,2′-bipyridine)ruthenium complex ([Ru(bpy)₃]²⁺) and methylviologen (MV²⁺) was irradiated with visible light, methylviologen cation radical (MV⁺) was formed. The formation rate of methylviologen cation radical in an agarose solid was faster than that in a κ-carrageenan one. Photoinduced hydrogen production was achieved with this system containing additionally proton reduction catalyst such as platinum black (Pt black), platinum oxide (PtO₂), and ruthenium oxide (RuO₂). The characteristics of this solid medium for photochemical reactions were discussed.     

Pathways for photoinduced charge separation and recombination at donor-acceptor heterojunctions: the case of oligophenylenevinylene-perylene bisimide complexes

Semiempirical Hartree-Fock techniques have been applied to assess the molecular parameters governing the efficiency of photoinduced charge generation and recombination processes in donor/acceptor complexes involving a three-ring oligophenylenevinylene as donor and perylene bisimide as acceptor. The corresponding rates have been estimated in the framework of the Marcus-Levich-Jortner formalism for different geometries of the complexes. The results indicate that dissociation pathways involving the lowest two charge transfer excited states contribute significantly to the dynamics of the whole process. The rates are found to be strongly sensitive to the relative position of the donor and acceptor units and can be rationalized in terms of symmetry arguments applied to relevant electronic levels.     

Morpholine-phthalocyanine (donor-acceptor) construct: photoinduced intramolecular electron transfer and triplet formation from its charge separation state

Silicon phthalocyanine (SiPc) with two axially attached morpholine (MP) units was prepared, and its photophysics was studied by laser flash photolysis, steady state and time-resolved fluorescence methods. Both the fluorescence efficiency and lifetime of SiPc moiety were remarkably quenched, because of the efficient intramolecular photoinduced electron transfer (PET) from morpholine donors to SiPc moiety. The generated charge separation state (CSS), SiPc(?-)-MP(?+), which was observed by transient absorption spectra, showed a lifetime of 4.8 ns. The triplet quantum yield of SiPc unit in the supra-molecule is unexpectedly high, and the predominant spectral signal in microsecond-scale is triplet-triplet (T(1)-T(n)) absorption. This high triplet yield is due to the charge recombination of CSS that generates T(1) in 32% efficiency: SiPc(?-)-MP(?+) → (3)SiPc-MP. The T(1) formation process occurred efficiently because the CSS SiPc(?-)-MP(?+) has a higher energy (1.65 eV) than that of the triplet state (3)SiPc-MP (1.0 eV). Emission from the CSS was also observed: SiPc(?-)-MP(?+) → SiPc-MP + hν'.     

Pathways for photoinduced charge separation in DNA hairpins

By means of correlated quantum-chemical calculations, we explore the chain-length dependence of the electronic coupling for photoinduced charge separation in DNA hairpins associated to conjugated linkers. Pathways for charge transfer from the linker chromophore to a guanine site located at a well-defined distance along the DNA strand are identified. Importantly, these involve not only the frontier molecular orbitals of the interacting donor, bridge, and acceptor units, but also deeper lying orbitals possessing both the appropriate energy and the symmetry to overlap significantly. The relative efficiency of these channels is found to be sensitive to the chemical structure of the linker, leading to falloff parameters for the charge-transfer rates ranging from approximately 0.4 to approximately 1.2 A(-1).     

Pico- and nano-second laser flash photolysis study on photoinduced charge separation in oligothiophene-C60 dyad molecules

Photoinduced charge separation (CS) and charge recombination (CR) processes of octathiophene-C₆₀ and dodecathiophene-C₆₀ dyad molecules (8T-C₆₀ and 12T-C₆₀, respectively) have been investigated by time-resolved absorption spectroscopy in the visible and near-IR regions. In toluene, ¹8T*-C₆₀ and ¹12T*-C₆₀ showed energy transfer to ¹C*-moiety predominantly, while 60 contribution of CS was small. In various polar solvents, on the other hand, CS states were predominantly formed from both singlet-excited oligothiophene and ¹C6*0-moiety because of lower CS level in polar environments. The CR process generating both the triplet state of oligothiophene and the ground state was confirmed in anisole and anisole/toluene mixture within a few nanoseconds. In more polar solvents (dielectric constant (∈_s) > 7), CS states showed two components decay: Slow decay component showed lifetime in the hundred nanosecond-region, while fast component decayed within a few nanoseconds. For the mechanism of the long-living CS state in polar solvents (∈_s > 7), equilibrium between the CS state and the triplet state was proposed. Furthermore, effects of length of oligothiophene on the CS and CR processes were discussed on the basis of the free energy changes.     

High energy and quantum efficiency in photoinduced charge separation

Supramolecular triad assemblies consisting of a central trisbipyridine ruthenium(II) chromophore (C2+) with one or more appended phenothiazine electron donors (D) and a diquat-type electron acceptor (A2+) have been shown to form long-lived photoinduced charge separated states (CSS) with unusually and consistently high quantum efficiency. Up to now, there has been no understanding for why these large efficiencies (often close to unity) are achieved across this entire class of triads when other, seemingly similar systems are often much less efficient. In the present study, using a bimolecular system consisting of a chromophore-acceptor diad (C2+-A2+) and an N-methylphenothiazine donor, we demonstrate that a ground-state association exists between the RuL3(2+) and the phenothiazine prior to photoexcitation. It is this association process that is responsible for the efficient CSS formation in the bimolecular system and, by inference, also must be an essential factor in the fully intramolecular process occurring with the D-C2+-A2+ triad analogues.     

The photoinduced charge separation at the water-porphyrin interface

The photoinduced charge separation and charge transfer at the boundary between a tetraphenylporphine crystal and an aqueous electrode were observed with the porphyrin layers prepared on a gold-coated aluminium foil and on a transparent insulating polymer film.     

Synthesis and photophysical properties of two dual oligothiophene-fullerene linkage molecules as photoinduced long-distance charge separation systems

To further extend photoinduced charge separation previously observed for oligothiophene-fullerene dyads (nT-C60), we have studied two novel dual oligothiophene-fullerene triads, 8T-4T-C60 and 4T-8T-C60, where quaterthiophene (4T) and octithiophene (8T) are linked by a trimethylene chain and either one is attached to a fullerene (C60). The cyclic voltammograms and electronic absorption spectra of these triad compounds indicated no electronic interactions among the three components. On the other hand, the emission spectra were markedly perturbed by electron transfer and/or energy transfer from the oligothiophene to fullerene. Detailed comparisons between the emission spectra of the triads (8T-4T-C60 and 4T-8T-C60) and the dyads (4T-C60 and 8T-C60) suggest that the additionally attached octithiophene or quaterthiophene in the triads is involved in the photophysical decay mechanism, and the 8T-4T-C60 triad undergoes photoinduced electron transfer leading to long-distance charge separation. This was actually corroborated by observation of the specific bands due to 8T*+-4T-C60*- species in the transient absorption spectra after photoexcitation of the octithiophene. The sandwich device based on the 8T-4T-C60 triad produced a more effective photovoltaic response to visible light owing to the contribution of the additional octithiophene chromophore compared to that using the dyad 4T-C60. On the other hand, the 4T-8T-C60-based device demonstrated a rather poorer photovoltaic performance when compared to the 8T-C60 device.     

Self-assembling porphyrins and phthalocyanines for photoinduced charge separation and charge transport

Large π-conjugated compounds are promising building blocks for organic thin-film electronics such as organic light-emitting diodes, organic field-effect transistors, and organic photovoltaics. Utilization of porphyrins and phthalocyanines for this purpose is highly fascinating because of their excellent electric, photophysical, and electrochemical properties as well as intense self-assembling abilities arising from π-π stacking interactions. This paper focuses on fundamental aspects of self-assembled structures that have been obtained from porphyrin and phthalocyanine building blocks and more complex composites for photoinduced charge separation and charge transport toward the potential applications to organic thin-film electronics.     

Impact of bridging units on the dynamics of photoinduced charge generation and charge recombination in donor-acceptor dyads.

We estimate, at a full quantum-chemical level, the various molecular parameters governing the rate of photoinduced charge generation and charge recombination in model organic structures containing a donor and an acceptor unit in view of the possible use of such systems in organic solar cells. The rate of through-space excitation dissociation, as predicted in the framework of the Marcus-Levich-Jortner theory, is found to be low in comparison to intramolecular decay processes when the donor and acceptor molecules are lying in a head-to-tail arrangement and high when the donor and acceptor molecules are superimposed in a cofacial arrangement. The charge separation rates for side-by-side donor-acceptor dyads are significantly increased by promoting through-bond interactions in covalently linked donor and acceptor units. This has motivated a detailed quantitative analysis of the influence of the nature, size, and conformation of the bridging moiety on the calculated transfer rates.     

A light-harvesting fluorinated fullerene donor-acceptor ensemble; long-lived charge separation

In a first example of a trannulene-based donor-acceptor dyad visible light photoexcitation generates a long-lived (870 ns) charge-separated state.     

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.     

Influence of intermolecular orientation on the photoinduced charge transfer kinetics in self-assembled aggregates of donor-acceptor arrays

The kinetics of photoinduced charge transfer reactions in covalently linked donor-acceptor molecules often undergoes dramatic changes when these molecules self-assemble from a molecular dissolved state into a nanoaggregate. Frequently, the origin of these changes is only partially understood. In this paper, we describe the intermolecular spatial organization of three homologous arrays, consisting of a central perylene bisimide (PERY) acceptor moiety and two oligo(p-phenylene vinylene) (OPV) donor units, in nanoaggregates and identify both face-to-face (H-type) and slipped (J-type) stacking of the OPV and PERY chromophores. For the J-type aggregates, short intermolecular OPV-PERY distances are created that give rise to a charge-transfer absorption band. The proximity of the donor and acceptor groups in the J-type aggregates enables a highly efficient photoinduced charge separation with a rate (k(cs) > 10(12) s(-1)) that significantly exceeds the rate of the intramolecular charge transfer of the same compounds when molecularly dissolved, even in the most polar media. In the H-type aggregates, on the other hand, the intermolecular OPV-PERY distance is not reduced compared to the intramolecular separation, and hence, the rates of the electron transfer reactions are not significantly affected compared to the molecular dissolved state. Similar to the forward electron transfer, the kinetics of the charge recombination in the aggregated state can be understood by considering the different interchromophoric distances that occur in the H- and J-type aggregates. These results provide the first consistent rationalization of the remarkable differences that are observed for photoinduced charge-transfer reactions of donor-acceptor compounds in molecularly dissolved versus aggregated states.     

Photolytic release of carboxylic acids using linked donor-acceptor molecules: direct versus mediated photoinduced electron transfer to N-alkyl-4-picolinium esters

[reaction: see text] Efficient photorelease (Phi = 0.7) of carboxylic acids is achieved with a covalently linked mediator (benzophenone) protecting group (N-alkyl-4-picolinium ester) molecule. The mechanism involves initial photoreduction of the mediator, followed by rapid electron transfer to the protecting group.     

Ultrafast photoinduced charge separation dynamics in polythiophene/SnO2 nanocomposites

We present a study of photoinduced interfacial electron transfer (ET) dynamics of SnO2 nanocrystalline thin films sensitized by polythiophene derivatives (regioregular poly(3-hexylthiophene) (P3HT) and regiorandom poly(3-undecyl-2,2'-bithiophene) (P3UBT)). ET dynamics were measured by following the dynamics of injected electrons in SnO2 and polarons in the conjugated polymer using ultrafast mid-IR transient absorption spectroscopy. The rate of electron transfer from P3HT and P3UBT to SnO2 films was determined to occur on sub-picosecond time scale (120 +/- 20 fs). In P3HT/SnO2 composite, interchain charge transfer was found to compete with and reduce the quantum efficiency of interfacial electron transfer at high polymer loading. This interchain charge separation processes can be reduced in non-regioregular polymer or at low polymer loading levels.     

"Umpolung" photoinduced charge separation in an anion-bound supramolecular complex

A new supramolecular system, consisting of an expanded porphyrin, cyclo[8]pyrrole (C8) and a pyrene carboxylate (Py) is capable not only of electron transfer upon photoexcitation, but results in "umpolung" or a reversal of the predicted charge-separation behavior in that the higher energy radical ion pair, C8.+-Py.- (2.58 eV) is formed as opposed to C8.--Py.+ (1.31 eV). This observation is rationalized in terms of the latter radical ion pair, C8.--Py.+ being in the Marcus inverted region. Transient absorption spectral studies reveal a charge-separated lifetime of 300 micros, which decays to the triplet state of cyclo[8]pyrrole, also an exceedingly long-lived species. The novel features of this noncovalent dyad highlight the potential utility of anion binding in the construction of supramolecular electron transfer systems.     

Energy-gap dependence of photoinduced charge separation and subsequent charge recombination in 1,4-phenylene-bridged zinc--free-base hybrid porphyrins

A series of 1,4-phenylene-bridged ZP-HP hybrid porphyrins (ZP = zinc porphyrin, HP = free-base porphyrin) 1-8 ZH have been prepared in which an electron-donating ZP moiety is kept constant and electron-accepting HP moieties are varied by introducing electron-accepting substituents, so that the energy gap for charge separation, ZP-1HP*--> ZP(+)-HP-, covers a range of about 0.9 eV in DMF. Here selective excitation at the HP moiety was employed to avoid complication in the determination of electron transfer rates derived from energy transfer, 1ZP*-HP --> ZP-1HP*. Definitive evidence for the electron transfer has been obtained in three solvents (benzene, THF, and DMF) through picosecond-femtosecond transient absorption studies, which have allowed the determination of the rates of the photoinduced charge separation, ZP-1HP* --> ZP(+)-HP-, and subsequent thermal charge recombination ZP(+)-HP- --> ZP-HP. Dyad 1ZH in THF exhibits a biphasic fluorescence decay that indicates thermal repopulation of the ZP-1HP* from ZP(+)-HP-; this has been also supported by the transient absorption spectra. On this ground, the energy levels of the ZP(+)-HP- ion pairs have been estimated. Similar biphasic fluorescence decay has been observed for 5 ZH in benzene; this allows furhter estimation of the energy level of the ZP(+)-HP- ion pairs. The free-energy-gap dependence (energy-gap law) has been probed from the normal to the upper limit region for the rate of the charge separation alone, and only the inverted region for the rate of the charge recombination. It was not possible to reproduce both energy-gap dependencies of the charge separation and the charge recombination assuming common parameter values for the reorganization energy and electronic interaction responsible for the electron transfer with the classical Marcus equation. Although both energy-gap dependencies can be approximately reproduced by means of the simplified semiclassical equation, which takes into consideration the effect of the high-frequency vibrations replaced by one mode of averaged frequency, many features, which include the effects of solvent polarity, electron-tunneling matrix element, and so forth on the energy-gap law, are considerably different from those of the previous studied porphyrin-quinone systems with weaker inter-chromophore electronic interactions.