Lifetime Of Charge-separated State Research Articles

Long-Lived Charge Separation and Applications in Artificial Photosynthesis

Researchers have long been interested in replicating the reactivity that occurs in photosynthetic organisms. To mimic the long-lived charge separations characteristic of the reaction center in photosynthesis, researchers have applied the Marcus theory to design synthetic multistep electron-transfer (ET) systems. In this Account, we describe our recent research on the rational design of ET control systems, based on models of the photosynthetic reaction center that rely on the Marcus theory of ET. The key to obtaining a long-lived charge separation is the careful choice of electron donors and acceptors that have small reorganization energies of ET. In these cases, the driving force of back ET is located in the Marcus inverted region, where the lifetime of the charge-separated state lengthens as the driving force of back ET increases. We chose porphyrins as electron donors and fullerenes as electron acceptors, both of which have small ET reorganization energies. By linking electron donor porphyrins and electron acceptor fullerenes at appropriate distances, we achieved charge-separated states with long lifetimes. We could further lengthen the lifetimes of charge-separated states by mixing a variety of components, such as a terminal electron donor, an electron mediator, and an electron acceptor, mimicking both the photosynthetic reaction center and the multistep photoinduced ET that occurs there. However, each step in multistep ET loses a fraction of the initial excitation energy during the long-distance charge separation. To overcome this drawback in multistep ET systems, we used designed new systems where we could finely control the redox potentials and the geometry of simple donor-acceptor dyads. These modifications resulted in a small ET reorganization energy and a high-lying triplet excited state. Our most successful example, 9-mesityl-10-methylacridinium ion (Acr(+)-Mes), can undergo a fast photoinduced ET from the mesityl (Mes) moiety to the singlet excited state of the acridinium ion moiety (Acr(+)) with extremely slow back ET. The high-energy triplet charge-separated state is located deep in the Marcus inverted region, and we have detected the structural changes during the photoinduced ET in this system using X-ray crystallography. To increase the efficiency of both the light-harvesting and photoinduced ET, we assembled the Acr(+)-Mes dyads on gold nanoparticles to bring them in closer proximity to one another. We can also incorporate Acr(+)-Mes molecules within nanosized mesoporous silica-alumina. In contrast to the densely assembled dyads on gold nanoparticles, each Acr(+)-Mes molecule in silica-alumina is isolated in the mesopore, which inhibits the bimolecular back ET and leads to longer lifetimes in solution at room temperature than the natural photosynthetic reaction center. Acr(+)-Mes and related compounds act as excellent organic photocatalysts and facilitate a variety of reactions such as oxygenation, bromination, carbon-carbon bond formation, and hydrogen evolution reactions.

Synthesis and Photophysical and Photoelectrochemical Properties of Diazaporphyrin-Fullerene Linked Dyad

Over the last two decades much effort has been devoted to explore excellent donor-acceptor (D-A) couples for artificial photosynthesis and solar energy conversion.1-6 In this regard fullerenes have been frequently employed as acceptors to exhibit outstanding electron transfer (ET) properties in D-A systems as well as organic photovoltaics. Meanwhile, various donors such as porphyrins, phthalocyanines, and ruthenium bipyridyl complexes have been involved in such systems and devices. However, the examples of superior donors are still limited in terms of light-harvesting and ET properties.5,15-Diazaporphyrins (DAPs) are a class of porphyrins that bear two meso-nitrogen atoms linking two dipyrromethene units. DAPs have received continuing interest in relation to their analogues, porphyrins and phthalocyanines. It has been shown that 5,15-aza-substitution significantly changes the intrinsic properties of the porphyrin p systems that would be comparable to or even better than their analogues. Nevertheless, due to synthetic difficulties of the cyclization and peripheral modification of the core in DAPs, the opportunity of using DAPs as materials has poorly been addressed. As such, DAPs have yet to be used as donors in D-A linked systems to elucidate the intrinsic ET properties.7-9 Here we report, for the first time, the synthesis and photophysical and photoelectrochemical properties of zinc 5,15-diazaporphyrin (ZnDAP)-C60 linked dyad. To evaluate the intrinsic photophysical properties of ZnDAP as a donor accurately, the corresponding zinc porphyrin (ZnP)-C60 linked dyad with the same spacer was also designed (Figure 1). Owing to the electron-withdrawing character of the 5,15-aza-substitution ZnDAP would be more difficult to be oxidized than ZnP. Accordingly, we expected that once a charge-separated state was generated via photoinduced ET, charge recombination (CR) from the charge-separated state to the ground state in ZnDAP-C60 would be slowed down as compared with ZnP-C60, because CR in ZnDAP-C60 is shifted more deeply into the Marcus inverted region. To attain sufficient solubility of the dyads bulky long alkoxy chains were also introduced to the ortho-positions of the meso-phenyl group.In addition to the better matching of ZnDAP with solar energy distribution in the visible region than ZnP, the lifetime of charge-separated state in ZnDAP-C60 was 4 times longer than that in ZnP-C60 due to the high oxidation potential of ZnDAP relative to ZnP. This difference also paralleled the enhanced photocurrent generation of ZnDAP-C60–based device in the visible region. These results unambiguously exemplify the potential utility of DAPs as an excellent donor in artificial photosynthesis and solar energy conversion. In this talk, we will present the synthesis, optical, electrochemical, photophysical, and photoelectro-chemical properties of ZnDAP-C60 in comparison with the corresponding ZnP-C60.[1] H. Imahori and Y. Sakata, Adv. Mater. (Review Article), 9, 537-546 (1997).[2] H. Imahori and Y. Sakata, Eur. J. Org. Chem. (Microreview), 1999, 2445-2457.[3] H. Imahori, Org. Biomol. Chem. (Perspective), 2, 1425-1433 (2004).[4] H. Imahori, Bull. Chem. Soc. Jpn. (Award Accounts), 80, 621-636 (2007).[5] H. Imahori, T. Umeyama, K. Kurotobi, and Y. Takano, Chem. Commun. (Feature Article), 48, 4032-4045 (2012).[6] Photofunctional Hybrid Nanocarbon Materials, T. Umeyama and H. Imahori, J. Phys. Chem. C (Feature Article), 117, 3195-3209 (2013).[7] Y. Matano, T. Shibano, H. Nakano, and H. Imahori, Chem. Eur. J., 18, 6208-6216 (2012).[8] Y. Matano, T. Shibano, H. Nakano, Y. Kimura, and H. Imahori, Inorg. Chem., 51, 12879-12890 (2012).[9] K. Kurotobi, K. Kawamoto, Y. Toude, Y. Fujimori, Y. Kinjo, S. Ito, Y. Matano, and H. Imahori, Chem. Lett., 42, 725-726 (2013).

Gating Charge Recombination Rates through Dynamic Bridges in Tetrathiafulvalene–Fullerene Architectures

Conversion of light into chemical energy is a process that nature has optimized over eons in photosynthetic organisms, such as bacteria, plants, and algae. However, the search for non-natural systems that mimic the complex overall process of photosynthesis has remained a challenge. In particular, the key step of the initial light-induced charge separation between electron donors and acceptors is hampered by its inherent microscopic reversibility, that is, competing charge recombination. Well-defined molecular model systems typically comprise a donor (D) and an acceptor (A) covalently linked by a bridge (B). In the resulting D–B–A structures, the role of the bridge is ideally to facilitate the desired initial photoinduced charge separation, yet, to slow down the undesired charge recombination. Among the many combinations of donors and acceptors that have been explored, those consisting of proaromatic tetrathiafulvalene (TTF) and fullerene derivatives, such as C60, have shown outstanding results. The exceptional electron donating and accepting properties originate from the aromatic stabilization of the formed TTF radical cation and from C60 s unique three dimensional delocalized p-electron system, respectively. This last feature leads to low reorganization energies upon the reduction to the C60 radical anion and allows for the uptake of up to six additional electrons. The photophysical properties of various TTF–C60 conjugates featuring different p-conjugated molecular bridges have been investigated and charge-separated states with lifetimes ranging from a few nanoseconds up to hundreds of microseconds have been realized. Of particular interest are conjugates with p-extended TTF derivatives, in which a conjugated p-quinoid anthracene moiety is placed between the TTF s two 1,3-dithiole rings. Nevertheless, the design of such D–B–A architectures features inherent drawbacks. For example, even with optimized donors and acceptors, the bridge needs to play two opposing roles. On the one hand, it should enhance the coupling between D and A to facilitate the initial charge separation. On the other hand, once the charge-separated state has been formed it should prevent charge recombination by decoupling DC and AC . Clearly, conventional, static bridges have to be a compromise of these two demands. However, if D and A are connected by a dynamic bridge, which can be switched between a coupled and a decoupled form, prolonged charge-separated state lifetimes could potentially be attained without compromising the initial charge separation. Such improved molecular design requires a switch entity that adopts two electronically distinct forms and allows for precise timing of the switching, that is, when the bridge is being coupled or decoupled. Dithienylethenes (DTE) are ideal candidates as switchable bridges as they reversibly interconvert between their ring-open (decoupled) and ring-closed (p-conjugated) forms upon irradiation with light of specific wavelengths. Adopting this new strategy, we prepared four novel D–DTE–A structures connecting either TTF or exTTF acting as D and with C60 functioning as A by photochromic dithienylperhydrocyclopentene or perfluorocyclopentene bridges (Scheme 1). Some researchers have used photochromic units as lightresponsive electronic traps that allow or prevent intramolecular electron transfer from D to A depending on the adopted isomeric form. There are also some examples, in which the electron transfer kinetics are clearly altered by the structural modification of the bridging units by chemical inputs (chelation) or, in mechanically interlocked D and A units, by topological changes. However, herein, we show for the first time that in (ex)TTF–DTE–C60 architectures, the lifetime of the charge-separated state can be significantly shortened or prolonged by performing light-induced structural changes in the bridging unit. [*] Dr. S. Castellanos, Dr. J. Moreno, Prof. Dr. S. Hecht Department of Chemistry Humboldt-Universit t zu Berlin Brook-Taylor-Strasse 2, 12489 Berlin (Germany) E-mail: sh@chemie.hu-berlin.de

Tuning the photodriven electron transport within the columnar perylenediimide stacks by changing the π-extent of the electron donors

Photodriven electron-transport properties of the self-assemblies of N,N'-di(2-(trimethylammoniumiodide)ethylene)perylenediimide stacks (TAIPDI)(n) with three electron donors, disodium 4,4'-bis(2-sulfonatostyryl)biphenyl (BSSBP, stilbene-420), sodium 9,10-dimethoxyanthracene-2-sulfonate (DANS) and disodium 6-amino-1,3-naphthalenedisulfonate (ANADS) have been studied in water. These electron donors vary in their π-extent to adjust the electronic coupling and the distance with the PDI stacks. Possessing the largest π-extent, BSSBP has strong π-π interactions as well as ionic interactions with (TAIPDI)(n). Instead of π-stacking with TAIPDI planes, DANS and ANADS, with a relatively small π-extent, are embedded in the side chains of TAIPDIs via ionic interactions, resulting in a distance increment from the aromatic TAIPDI cores. After excitation, the BSSBP-(TAIPDI)(n) system exhibits fast charge separation (0.70 ps) and relatively slow charge recombination (485 ps) due to intermolecular electron delocalization along the TAIPDI stacks. On the other hand, charge separation in DANS-(TAIPDI)(n) and ANADS-(TAIPDI)(n) occurs within 1.5 and 1.6 ns, respectively, calculated from the quenching of singlet excited states. The lifetimes of charge-separated states are determined to be 44 and 96 μs, at least 10(5) times slower than that of BSSBP-(TAIPDI)(n) due to remarkably improved electron transport throughout the (TAIPDI)(n).

Long-lived photoinduced charge separation for solar cell applications in supramolecular complexes of multi-metalloporphyrins and fullerenes

Monomers, dimers, trimers, dendrimers and oligomers of metalloporphyrins form supramolecular complexes with fullerene derivatives via electrostatic interactions, π-π interactions and coordination bonds. Photoexcitation of the supramolecular complexes resulted in photoinduced electron transfer from the porphyrin moiety to the fullerene moiety to produce the charge-separated states as revealed by laser flash photolysis measurements. The rate constants of photoinduced charge separation and charge recombination in supramolecular complexes of multi-metalloporphyrins and fullerenes were also determined by laser flash photolysis measurements and the results depending on the number of porphyrins in the supramolecular complexes are discussed in terms of efficiency of photoinduced energy transfer and charge separation as well as the lifetimes of charge-separated states. The photoelectrochemical performances of solar cells composed of supramolecular complexes of monomers, dimers, dendrimers and oligomers of metalloporphyrins with fullerenes are compared in relation to the rate constants of photoinduced charge separation and charge recombination.

Hydrogen-Bonding Effects on the Formation and Lifetimes of Charge-Separated States in Molecular Triads

Photoinduced electron transfer in two molecular triads comprised of a triarylamine donor, a d(6) metal diimine photosensitizer, and a 9,10-anthraquinone acceptor was investigated with particular focus on the influence of hydrogen-bonding solvents on the electron transfer kinetics. Photoexcitation of the ruthenium(II) and osmium(II) sensitizers of these triads leads to charge-separated states containing an oxidized triarylamine unit and a reduced anthraquinone moiety. The kinetics for formation of these charge-separated states were explored by using femtosecond transient absorption spectroscopy. Strong hydrogen bond donors such as hexafluoroisopropanol or trifluoroethanol cause a thermodynamic and kinetic stabilization of these charge-separated states that is attributed to hydrogen bonding between alcoholic solvent and reduced anthraquinone. In the ruthenium triad this effect leads to a lengthening of the lifetime of the charge-separated state from ~750 ns in dichloromethane to ~3000 ns in hexafluoroisopropanol while in the osmium triad the respective lifetime increases from ~50 to ~2000 ns between the same two solvents. In both triads the lifetime of the charge-separated state correlates with the hydrogen bond donor strength of the solvent but not with the solvent dielectric constant. These findings are relevant in the greater context of solar energy conversion in which one is interested in storing light energy in charge-separated states that are as long-lived as possible. Furthermore they are relevant for understanding proton-coupled electron transfer (PCET) reactivity of electronically excited states at a fundamental level because changes in hydrogen-bonding strength accompanying changes in redox states may be regarded as an attenuated form of PCET.

Synthesis, characterization, and photophysical properties of covalent-linked ferrocene–porphyrin–single-walled carbon nanotube triad hybrid

The ferrocene–porphyrin–single-walled carbon nanotube (Fc–H2P–SWCNT) triad hybrid was prepared by amidation reaction between carboxylated SWCNT and aminoporphyrin bearing an appended ferrocenyl substituent. The hybrid described here was fully characterized by a combination of analytical techniques such as Fourier transform infrared spectroscopy, Raman, absorption and emission spectroscopy, atomic force and scanning electron microscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy. The steady emission characteristics revealed the existence of the effective photoinduced electron transfer among ferrocene, excited porphyrin moiety and SWCNT, which was further confirmed by the results of time-resolved transient absorption spectra. The final lifetime of charge-separation state was observed to be 62.9μs in N,N-dimethylformamide, which was significant increased compared to the reference nanohybrid porphyrin–SWCNT and the reported ferrocene–porphyrin–fullerene triad. Therefore, Fc–H2P–SWCNT triad hybrid constructed by amidation is rationally expected to be an improved photon-to-electron conversion system.

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.

Two Different Lifetimes of Charge Separated States: a Porphyrin–Quinone System in Artificial Photosynthesis

We investigated the charge separation process of an artificial photosynthesis molecule, zinc porphyrin–benzoquinone (ZnP–BQ) using the ab initio symmetry adapted cluster configuration interaction method and density functional theory. The energy of the charge separated (CS) state of the monomer indicates that electron transfer “via through bond” is inhibited. We used a dimer model of ZnP–BQ as an example for electron transfer “via through space” and qualitatively reproduced the experimentally determined energy relation. The calculated oscillator strengths show a difference in lifetimes between the CSLOW and CSHIGH states.

Supramolecular Zinc Phthalocyanine–Imidazolyl Perylenediimide Dyad and Triad: Synthesis, Complexation, and Photophysical Studies

Two new supramolecular architectures based on zinc phthalocyanine (Pc) and imidazolyl-substituted perylenediimide (PDI), ZnPc/DImPDI/ZnPc 1 and ZnPc/ImPDI 2, have been prepared. A strong electron-donor, ZnPc-8, which contained eight tert-octylphenoxy groups was synthesized to ensure high solubility, thereby reducing aggregation in solution and providing σ-donor features while avoiding regioisomeric mixtures. Also, PDI units were functionalized with tert-octylphenoxy groups at the bay positions, which provide solubility to avoid aggregation in solution, together with one and two imidazole moieties in the amide position, PDI-6 and PDI-4, respectively, to be able to strongly coordinate with the ZnPc complex. Supramolecular complexation studies by (1)H NMR spectroscopy and ESI-MS demonstrate a high coordinative binding constant between imidazole-substituted PDI-4 or PDI-6 and ZnPc-8. The same results were confirmed by UV/Vis and fluorescence titration studies. UV/Vis titration studies revealed the formation of a 1:1 complex ZnPc/ImPDI 2 for the systems ZnPc-8 and PDI-6 and a 2:1 complex ZnPc/DImPDI/ZnPc 1 for the interaction of ZnPc-8 and PDI-4. The binding constant in both cases was determined to be on the order of 10(5) M(-1). Femtosecond laser flash photolysis measurements provided a direct proof of the charge-separated state within both supramolecular assemblies by observing the transient absorption band at 820 nm due to the zinc phthalocyanine radical cation. The lifetimes of charge-separated states are (9.8±3) ns for triad 1 and (3±1) ns for dyad 2. As far as we know, this is the first time that a radical ion pair has been detected in a supramolecular assembled ZnPc-PDI system and has obtained the longest lifetime of a charge-separated state published for ZnPc-PDI assemblies.

Balancing binding strength and charge transfer lifetime in supramolecular associates of fullerenes

A macrocyclic exTTF host for fullerenes offers control over the electronic coupling between an electron donor and an acceptor, and stabilizes the charge separated state lifetimes into the range of 500 ps.

Synthesis, photophysical and photovoltaic investigations of acceptor-functionalized perylene monoimide dyes for nickel oxide p-type dye-sensitized solar cells

We report on the synthesis, electrochemical, photophysical, and photovoltaic properties of a series of three organic dyads comprising a perylene monoimide (PMI) dye connected to a naphthalene diimide (NDI) or a fullerene (C60) for application in dye-sensitized solar cells (DSCs) with nanocrystalline NiO electrodes. It was found that the secondary electron acceptor (NDI or C60) in all the three dyads extends the charge separated state lifetime by about five orders of magnitude compared to the respective parent PMI dye. Nanosecond pump–probe experiments of the NiO/dyads in the presence of the electrolyte show that the reduction of triiodide by the secondary electron acceptor is slow in all the dyads, which we ascribe to a weak driving force for this reaction. This reaction is significantly faster with the cobalt electrolyte (tris(4,4′-di-tert-butyl-2,2′-bipyridine)cobalt(II/III)), whose driving force is larger; however, its reaction with the reduced dyads is still rather slow. We demonstrate that the larger photovoltage observed with the cobalt electrolyte (VOC = 285 mV) relative to the iodide electrolyte (VOC = 120 mV) is due to a decrease in the dark current for the former owing to slower interfacial electron transfer of the reduced mediator with the injected holes into the NiO electrode. In terms of photovoltaic performances, the most efficient dyad is the system in which the NDI is directly connected to the PMI (η = 0.14% under AM 1.5 with the cobalt electrolyte), but the dyad containing the fullerene acceptor exhibits the highest IPCE and the highest short circuit current density (IPCE = 57%, JSC = 1.88 mA cm−2) with the iodide electrolyte. The latter performances are attributed to the slightly stronger reducing power of C60 relative to NDI, which favours the reduction of the mediator in the electrolyte.

Photoinitiated multistep charge separation in ferrocene–zinc porphyrin–diiron hydrogenase model complex triads

Two covalently linked linear electron donor–acceptor triads Fc-ZnTPP-[NMI-FeI-FeI-S2(CO)6] (1) and Fc-Ph-ZnTPP-[NMI-FeI-FeI-S2(CO)6] (2) consisting of a zinc meso-tetraphenylporphyrin (ZnTPP) chromophore, a naphthalene monoimide diiron hydrogenase active site model [NMI-FeI-FeI-S2(CO)6], and a ferrocene (Fc) secondary electron donor have been synthesized along with their corresponding dyad reference molecules ZnTPP-[NMI-FeI-FeI-S2(CO)6] (3), Fc-ZnTPP (4), and Fc-Ph-ZnTPP (5). Time-resolved transient absorption and emission studies in CH2Cl2 show that selective photoexcitation of ZnTPP in triads 1 and 2 results in two competing quenching pathways for 1*ZnTPP: electron transfer from 1*ZnTPP to [NMI-FeI-FeI-S2(CO)6] and energy transfer from 1*ZnTPP to low-lying Fc excited states. Our studies on reference dyads 4 and 5 show that the majority of 1*ZnTPP produced by the laser pulse decays rapidly by energy transfer to Fc in triad 1 (τ < 10 ps), while electron transfer to [NMI-FeI-FeI-S2(CO)6] dominates in triad 2, allowing the second rapid electron transfer step from Fc to ZnTPP+˙ to proceed. Quantum yields of the fully charge separated states Fc+-ZnTPP-[NMI-Fe0-FeI-S2(CO)6] and Fc+-Ph-ZnTPP-[NMI-Fe0-FeI-S2(CO)6] are 0.13 and 0.71, respectively. Charge recombination in Fc+-ZnTPP-[NMI-Fe0-FeI-S2(CO)6] occurs with τCR = 9 ± 1 ns and τCR = 67 ± 2 ns for Fc+-Ph-ZnTPP-[NMI-Fe0-FeI-S2(CO)6]. By incorporating a secondary electron donor, the lifetime of the reduced diironhydrogenase mimic was extended by a factor of >450. Studies of photochemical hydrogen evolution using 1 and 2 reveal that the hydrogen generation efficiency depends on the lifetime of the final charge separated state. The ability to execute a multi-electron proton-coupled electron transfer mechanism in a stepwise manner will allow us to investigate the structural and electronic requirements for each step aiding in overall system optimization. Thus, it is possible to use the same multi-step electron transfer strategy that has been employed to extend the lifetime of charge-separated states in photodriven donor–acceptor systems to extend the lifetime of the reduced states of metal complexes of potential use in catalytic proton reduction.

Meso-meso linked diporphyrin functionalized single-walled carbon nanotubes

Meso-meso linked diporphyrins ([H 2Por] 2) covalently functionalized soluble single-walled carbon nanotubes ([H 2Por] 2-SWNTs) have been successfully prepared. As a light-harvesting chromophore, meso-meso linked diporphyrins have been incorporated into a photosynthetic electron-transfer model with SWNTs as an electron-acceptor. The steady emission characteristics revealed the existence of the effective energy and electron transfer between the excited porphyrin moiety and SWNTs, and the electron transfer process was confirmed by electrochemical study. The charge separation quenching process was supported by the results of time-resolved transient absorption spectra, and the lifetime of charge-separation state was observed to be 145 ns, which was 2.5 times of that of monoporphyrin modified SWNTs (TPP-SWNTs).

Anion-Complexation-Induced Stabilization of Charge Separation

A supramolecular oligochromophoric system possessing exclusive binding sites for both a guest electron acceptor and an anionic cofactor species is developed, and anion-binding-induced stabilization of the charge-separated (CS) state is demonstrated. Toward this, intramolecular and intermolecular photochemical processes of a supramolecular complex of a bis-porphyrinyl-substituted oxoporphyrinogen with a bis(4-pyridyl)-substituted fullerene were investigated by using femtosecond and nanosecond laser flash photolysis measurements. Transient absorption spectra of the supramolecular complex obtained by femtosecond laser flash photolysis indicate that efficient electron transfer occurs from the porphyrin moiety to the fullerene moiety, followed by faster back electron transfer to the ground state. Binding of several different anionic species at the pyrrole amine groups of an oxoporphyrinogen unit within the supramolecular complex was found to improve the rate of the photoinduced electron transfer due to the favorable structural change. The anion binding also improves persistence of the photoinduced CS state between the anion-bound oxoporphyrinogen and fullerene moieties, which is produced by intermolecular electron transfer from the triplet excited state of free porphyrin molecules to free fullerene molecules, as indicated by the nanosecond laser flash photolysis measurements. In the case of fluoride anion binding, anion-complexation-induced stabilization of charge separation gave a 90-fold elongation of the CS state lifetime from 163 ns to 14 micros. Complexation with other anions (acetate or dihydrogen phosphate) also resulted in stabilization of the CS state, whereas weakly bound perchlorate anions gave no improvement. Complexation of anions to the oxoporphyrinogen center lowers its oxidation potential by nearly 600 mV, creating an intermediate energy state for charge migration from the ZnP(*+) to the oxoporphyrinogen:anion complex. An increase in reorganizational energy of electron transfer combined with the decrease in charge recombination driving force caused by anion binding results in an increase in the lifetime of the CS state.

Synthesis and Photoinduced Intramolecular Processes of Light‐Harvesting Silicon Phthalocyanine–Naphthalenediimide–Fullerene Connected Systems

Photoinduced intramolecular processes of a newly synthesized pentad composed of silicon phthalocyanine (SiPc) that is connected with two units of naphthalenediimide (NDI) and fullerene C(60) to form SiPc-(NDI)(2)-(C(60))(2) have been studied and the results are compared with the reference compounds, namely, the SiPc-(NDI)(2) triad and NDI-C(60) dyad. Upon photoexcitation, the main quenching pathway in polar solvents involved electron transfer via the singlet excited states of SiPc-(NDI)(2)-(C(60))(2) and SiPc-(NDI)(2), but not NDI-C(60) for which the energy transfer is dominant. The occurrence of electron-transfer processes of SiPc-(NDI)(2)-(C(60))(2) and SiPc-(NDI)(2) were studied by time-resolved emission and transient absorption techniques and confirmed by redox measurements and molecular orbital calculations with ab initio B3 LYP/3-21G(*) methods. Fast and efficient charge-separation processes via the singlet excited states of NDI and SiPc were monitored, followed by charge recombination to populate the C(60) and SiPc triplet states. The lifetimes of charge-separated states were estimated as 1000 and 250 ps for SiPc-(NDI)(2)-(C(60))(2) and SiPc-(NDI)(2), respectively.

Photophysics of Fluorene Copolymers: Control of Fluorescence and Charge Separation by the Presence of Carbazole, Oxadiazole, or Biphenyl Units

A set of fluorene copolymers designed to introduce electron donor/acceptor structures in the polymer chain that could eventually produce more efficient charge separation with long lifetimes has been prepared. To validate our approach, herein we report photoluminescence and laser flash photolysis measurements of the series of alternating specially functionalized fluorene copolymers. The copolymers contain 9,9-dioctylfluorene and N-octylcarbazole, oxadiazole, and oxadiazole−octafluorobiphenyl−oxadiazole units. It appears that the copolymer with the longest charge separated state lifetime and the highest charge separation quantum yield of the series is an alternating copolymer of a N-octylcarbazole and trifluorene units in which two fluorenes were attached with hydroxyhexyl side groups and the central fluorene with octyl side group at C-9 positions, respectively. These photophysical data suggest that this polymer a good candidate for photovoltaic cell and polymer light-emitting diode applications.

Organic conducting wire formation on a TiO2 nanocrystalline structure: towards long-lived charge separated systems

Conjugated polymer monolayers have successfully been fabricated on a TiO(2) nanocrystalline electrode by a lateral intermonomer charge transfer process, extending the photo-induced charge separated state lifetime by a factor of 50-70.

Long-lived charge-separated states of simple electron donor-acceptor dyads using porphyrins and phthalocyanines

Control of electron-transfer processes is described for a number of electron donor-acceptor dyads containing porphyrins or phthalocyanines as models for the photosynthetic reaction center. The rates for intramolecular electron transfer in the dyads are controlled by the driving force and reorganization energy of electron transfer. The small reorganization energy of electron transfer reactions and large driving force of charge recombination are required to form long-lived charge-separated states. A directly linked zinc chlorin-fullerene dyad, especially, has the longest lifetime of charge-separated state at 120 s at -150 °C, which is a much longer lifetime and higher energy than those of natural photosynthetic reaction centers. On the other hand, the charge-separated states of the phthalocyanine-based donor-acceptor dyads (silicon phthalocyanine-fullerene, and zinc phthalocyanine-perylenebisimide) are short-lived since charge recombination forms the low-lying triplet excited state of the chromophore. The energy of the charge-separated state of a zinc phthalocyanine-perylenebisimide dyad is decreased by binding of metal ions to the radical anion moiety in order to be lower than the triplet excited state. This results in formation of a long-lived charge-separated state. The mechanistic viability of formation of long-lived charge-separated states is demonstrated by a variety of examples based on the Marcus theory of electron transfer.

Ultrafast Photoinduced Charge Separation in Naphthalene Diimide Based Multichromophoric Systems in Liquid Solutions and in a Lipid Membrane

The photophysical properties of multichromophoric systems consisting of eight red or blue naphthalene diimides (NDIs) covalently attached to a p-octiphenyl scaffold, as well as a blue bichromophoric system with a biphenyl scaffold, have been investigated in detail using femtosecond time-resolved spectroscopy. The blue octachromophoric systems have been recently shown to self-assemble as supramolecular tetramers in lipid bilayer membranes and to enable generation of a transmembrane proton gradient upon photoexcitation ( Bhosale, S. ; Sisson, A. L. ; Talukdar, P. ; Fürstenberg, A. ; Banerji, N. ; Vauthey, E. ; Bollot, G. ; Mareda, J. ; Röger, C. ; Würthner, F. ; Sakai, N. ; Matile, S. Science 2006, 313, 84 ). A strong reduction of the fluorescence quantum yield was observed when going from the single NDI units to the multichromophoric systems in methanol, the effect being even stronger in a vesicular lipid membrane. Fluorescence up-conversion measurements reveal ultrafast self-quenching in the multichromophoric systems, whereas the formation of the NDI radical anion, evidenced by transient absorption measurements, points to the occurrence of photoinduced charge separation. The location of the positive charge could not be established unambiguously from the transient absorption measurements, but energetic considerations indicate that charge separation should occur between two NDI units in the blue systems, whereas both an NDI unit and the p-octiphenyl scaffold could act as electron donor in the red system. The lifetime of the charge-separated state was found to increase from 22 to 45 ps by going from the bi- to the octachromophoric blue systems in methanol, while a 400 ps decay component was observed in the lipid membrane. This lifetime lengthening is explained in terms of charge migration that is most efficient when the octachromophoric systems are assembled as supramolecular tetramers in the lipid membrane. Furthermore, the average charge-separated state lifetime of the red system in methanol is even larger and amounts to 750 ps. This effect cannot be simply explained in terms of Marcus inverted regime as the driving force for charge recombination in the red system is only slightly larger than in the blue one. A better spatial separation of the charges in the red system stemming from the localization of the hole on the p-octiphenyl scaffold could additionally contribute to the slowing down of charge recombination.