Long-lived Photoinduced Charge Separation Research Articles

Long-lived photoinduced charge separation in new Ru(bipyridine)32+—phenothiazine dyads

Synthesis and photophysical properties of three Ru(bpy)(3)(2+)-Ptz (bpy = 2,2'-bipyridine and Ptz = phenothiazine) dyads, where the number of Ptz groups increased from one to three, are reported. The MLCT absorption bands of these compounds were slightly red shifted compared to Ru(bpy)(3)(2+). The emission, however, was highly quenched and this is attributed to electron transfer from the Ptz moiety to the excited Ru(bpy)(3)(2+) to generate the charge separated state Ru(bpy)(3)(+)-Ptz (+). Observed electron transfer rates (k(et) > 10(8) s(-1)) were much faster than those previously reported (k(et) < 10(7) s(-1)) for linked Ru(bpy)(3)(2+)-Ptz systems. Compared to the previous systems, back electron transfer rates in these systems were about 100 times slower. This has enabled us to observe the charge separated state in nanosecond flash photolysis experiments. Transient absorptions assignable to Ru(bpy)(3)(+) and Ptz (+), having lifetimes in the range of 10-30 ns were observed. In order to explain the fast charge separation and slow charge recombination rates, formation of a folded conformer where the Ptz group attached to one bpy residue comes closer to and associates with another bpy moiety was invoked. A scheme which explains the fast electron transfer and slow recombination in this pre-associated state is proposed.

Long-lived (minutes) photoinduced charge separation in a structured periodic mesoporous titania containing 2,4,6-triphenylpyrylium as guest

2,4,6-Triphenylpyrylium (TP(+)), an electron acceptor, has been included inside the channels of two ordered mesoporous titania (mpTiO(2)). MpTiO(2) contain anatase nanoparticles (3-5 nm) templated by cetyltrimethylammonium and differ by the presence or absence of silica domains as binders of the structure. UV irradiation of TP-mpTiO(2) gives rise to a strong EPR signal. This behaviour was not observed for related materials in which TP(+) was included inside the channels of MCM-41 or the cavities of zeolite Y. Also, transient spectroscopy shows remarkable differences between TP(+) included in mpTiO(2) (charge separation) and in porous silicates (triplet excited state). Based on EPR and laser flash spectroscopic evidence, the occurrence of photo-induced electron transfer from TiO(2) as the donor to TP(+) as the acceptor leading to a long-lived (minutes at room temperature) charge separated state is proposed.

Photoinduced Electron Transfer in a New Bis(C60)−Phthalocyanine Triad

[Structure: see text] A novel covalently linked bis([60]fullerene)-phthalocyanine triad is reported, exhibiting long-lived photoinduced charge separation both in solution and in the solid state. The first demonstration of a working solar cell using triad 1 as the active material is also presented.

Millisecond long-lived charge separated state at room temperature in a flexibly linked diphenylaminopolyene-C60 dyad

Long-lived photoinduced charge separation involving one-step electron transfer is achieved in diphenylaminopolyene based C60-donor dyads with a short, flexible linkage.

Photonic switching of photoinduced electron transfer in a dihydropyrene-porphyrin-fullerene molecular triad.

Photonic control of photoinduced electron transfer has been demonstrated in a dimethyldihydropyrene (DHP) porphyrin (P) fullerene (C(60)) molecular triad. In the DHP-P-C(60) form of the triad, excitation of the porphyrin moiety is followed by photoinduced electron transfer to give a DHP-P(*)(+)-C(60)(*)(-) charge-separated state, which evolves by a charge shift reaction to DHP(*)(+)-P-C(60)(*)(-). This final state has a lifetime of 2 micros and is formed in an overall yield of 94%. Visible (>or=300 nm) irradiation of the triad leads to photoisomerization of the DHP moiety to the cyclophanediene (CPD). Excitation of the porphyrin moiety of CPD-P-C(60) produces a short-lived (<10 ns) CPD-P(*)(+)-C(60)(*)(-) state, but charge shift to the CPD moiety does not occur, due to the relatively high oxidation potential of the CPD group. Long-lived charge separation is not observed. Irradiation of CPD-P-C(60) with UV (254 nm) light converts the triad back to the DHP form. Thermal interconversion of the DHP and CPD forms is very slow, photochemical cycling is facile, and in the absence of oxygen, many cycles may be performed without substantial degradation. Thus, light is used to switch long-lived photoinduced charge separation on or off. The principles demonstrated by the triad may be useful for the design of molecule-based optoelectronic systems.

Nanosecond time-resolved studies of long-lived photoinduced charge separation in the dyad fluorescein–anthraquinone–methyl ester adsorbed on TiO 2 colloids

A dyad composed of fluorescein and 2-methyl-anthraquinone (FL–AQ) was synthesized and its photophysical properties were examined by absorption, fluorescence spectroscopy, and fluorescence lifetime. The charge-separated state formed by photoinduced intramolecular electron transfer was detected by nanosecond transient absorption spectroscopy for the first time. When FL is excited in solution, the photoinduced electron transfer from FL to AQ proceeds efficiently. The rate constant and the efficiency of intramolecular electron transfer are 3.95×10 9 s −1 and 95%, respectively. Its charge-separated state lifetime is too short to detect by transient absorption spectroscopy. Adding nanometer colloidal TiO 2 to an FL–AQ ethanol solution prolongs the lifetime of the charge-separated state, so its transient absorption signal is recorded significantly. The lifetimes of FL +· at 480 nm and AQ −· at 560 nm in the FL–AQ/TiO 2 colloidal system are 11.1 and 8.93 μs, respectively.

Long‐Lived Photoinduced Charge Separation in (Zinc, Lead) Phosphate Glass–C60 Composites

A group of (zinc, lead) phosphate glass–C60 composites that display long‐lived photoinduced electron transfer from the Pb2+ center to the fullerene upon 230–400 nm UV irradiation has been developed. Both the steady state photoinduced absorption and photoinduced electron spin resonance spectra of the irradiated system support this view. Analysis of the carrier dynamics suggests that the electrons migrate from the hole sites to the fullerene sites in a tunneling mode.

Long-lived photoinduced charge separation in carbazole–pyrene–viologen system incorporated in Langmuir–Blodgett films of substituted diazacrown ethers

Diaza-18-crown-6 ethers appending two pyrenyl (Py) or two carbazolyl (Cz) groups were synthesized. These macrocyclic compounds form 1:1 host–guest complexes with methyl viologen chloride (MV2+), and these complexes were assembled into monolayers by Langmuir–Blodgett technique. The generated assembly involves the general structure of donor–sensitizer–acceptor (Cz–Py–MV2+) in space, although any of the photo- and redox-active components are not covalently bonded. Photoirradiation of the pyrenyl group resulted in the charge-separated pair Cz+–Py–MV+ which survived up to hours in a well anaerobic atmosphere. An electrode was fabricated by transferring the L–B film on an ITO glass. The photoinduced voltage of this electrode was measured with a saturated calomel reference electrode in hydroquinone (H2Q) solution to be ca. 168mV when the light intensity was 218mW/cm2. This electrode was also used as the light electrode to construct a photogalvanic cell with a platinum electrode as the dark electrode. Irradiation of the light electrode with visible light results in anodic photocurrent, and there is no net chemical change associated with the function of the cell which converts light to electricity.

Ultrafast and long-lived photoinduced charge separation in MEH-PPV/nanoporous semiconductor thin film composites

Photoinduced charge separation and recombination dynamics were investigated in composite materials of conjugated polymer deposited on nanocrystalline metal oxide thin film. Using femtosecond IR transient absorption spectroscopy, the electron transfer from the excited state of poly[2-methoxy-5-(2-ethyl-hexyloxy)-(phenylene vinylene)] (MEH-PPV) to SnO 2 and TiO 2 nanoporous thin films is shown to occur with timescales of 800 and <100 fs, respectively. Negligible carrier recombination is observed for MEH-PPV/SnO 2 within 1 ns. Microsecond charge recombination dynamics were measured using step-scan FTIR. Long-lived charge separation is observed in both systems, persisting for microseconds to seconds.

Long-Lived Photoinduced Charge Separation and Redox-Type Photochromism on Mesoporous Oxide Films Sensitized by Molecular Dyads

The photoinduced charge separation in three different assemblies composed of an electron donor D and a chromophore sensitizer S adsorbed on nanocrystalline TiO2 films (D−S|TiO2) was investigated. In all of the systems, the sensitizer was a ruthenium(II) bis-terpyridine complex anchored to the semiconductor surface by a phosphonate group. In two of the assemblies, the donor was a 4-(N,N-di-p-anisylamino) phenyl group linked to the 4‘ position of the terpyridine, either directly (dyad D1−S) or via a benzyl ether interlocking group (dyad D2−S). In the third system, the sensitizer and the donor (3-(4-(N,N-di-p-anisylamino)phenoxy)-propyl-1-phosphonate) were coadsorbed on the surface ((D3+S)|TiO2). Laser flash photolysis showed that the photoinduced charge separation process follows the sequence D−S*|TiO2 D−S+|(e-)TiO2 D+−S|(e-)TiO2 D−S|TiO2 Resonance Raman spectroscopy indicates that in the excited assemblies D2−S*|TiO2 and (D3+S*)|TiO2, one electron is promoted from the metal center to the terpyridine ligand...

Increasing the Yield of Photoinduced Charge Separation through Parallel Electron Transfer Pathways

A strategy for increasing the yield of long-lived photoinduced charge separation in artificial photosynthetic reaction centers which is based on multiple electron transfer pathways operating in parallel has been investigated. Excitation of the porphyrin moiety of a carotenoid ( C )–porphyrin ( P )–naphthoquinone (Q) molecular triad leads to the formation of a charge-separated state C ·+– P – Q ·− with an overall quantum yield of 0.044 in benzonitrile solution. Photoinduced electron transfer from the porphyrin first excited singlet state gives C – P ·+– Q ·− with a quantum yield of ~1.0. However, electron transfer from the carotenoid to the porphyrin radical cation to form the final state does not compete well with charge recombination of C – P ·+– Q ·−, reducing the yield. The related pentad C 3– P – Q features carotenoid, porphyrin and quinone moieties closely related to those in the triad. Excitation of this molecule gives a C ·+– P ( C 2)– Q ·− state with a quantum yield of 0.073. The enhanced yield is ascribed to the fact that three electron donation pathways operating in parallel compete with charge recombination. The yield does not increase by the statistically predicted factor of three owing to small differences in thermodynamic driving force between the two compounds.

Construction of One‐Dimensional Multicomponent Molecular Arrays: Control of Electronic and Molecular Motions

Transition metals are powerful three-dimensional templates, which can gather various functionalized ligands and orient them in a precise fashion so that complex multicomponent molecular systems can be obtained without constructing covalently-assembled edifices. The compounds thus prepared exhibit precise chemical or physical functions, which are governed by the design of the system. The construction of one-dimensional molecules around ruthenium(II) or osmium(II), using rigid ligands attached to the desired electroactive species, leads to systems that are able to undergo charge separation after photonic excitation. In other related compounds, a ruthenium(II)-based chromophore is, for example, connected to an osmium(II) complex by means of rod-like bridging ligand, thereby ensuring strict control over the Ru···Os distance. By tuning the length and the electronic properties of the bridge, one can control the efficiency of the electronic energy transfer between the two chromophores. In particular, the use of a bis-cyclometallating ligand is very conducive to energy transfer and allows the observation of this process up to a Ru…Os distance of ≈20 A. By combining the building blocks of these inorganic systems with appropriate porphyrins, long-range (centre-to-centre distance between the donor and the acceptor porphyrins ≈30 A) and relatively long-lived photoinduced charge separation has been demonstrated. Finally, with copper(I) as the template, compounds of the rotaxane family are obtained, which consist of a coordinating ring threaded by a string-like component. If this acyclic fragment is end-functionalized by two bulky stoppering groups and if it incorporates two different coordination sites (a bi- and a terdentate chelate site), novel dynamic properties are observed. The movement of a given fragment of the molecule is triggered by changing the metal oxidation state. This one-dimensional motion of the ring along the string on which it is threaded is controlled by redox manipulation, resulting in a primitive molecular machine.

Long-lived photoinduced charge separation in a bridged C 60-porphyrin dyad

Long-range photoinduced energy and electron transfer have been investigated in a novel dyad containing a zinc tetraarylporphyrin (P Zn) donor and a C 60 acceptor separated by a saturated norbornylogous bridge nine sigma bonds in length (P Zn-9σ-C 60). In non-polar solvents singlet-singlet energy transfer from P Zn to C 60 is observed but in benzonitrile efficient (> 90%) charge separation occurs with a rate constant of 1 × 10 10 sec −1 to yield P Zn .+-9σ-C 60 .−. The charge separated state exhibits a remarkably long lifetime for a dyad of 420 ns. This behaviour is discussed with reference to the energetics, dynamics and orbital symmetry properties of the states involved.

Long-Lived Photoinduced Charge Separation across Nanocrystalline TiO2 Interfaces

Long-Lived Photoinduced Charge Separation across Nanocrystalline TiO2 Interfaces

Long-lived photoinduced charge separation in a redox system trapped in a sol–gel glass

A KEY feature in the mechanism of photosynthesis is the initial storage of a substantial fraction of the light energy in the form of a long-lived radical pair1. In attempts to mimic this process in artificial systems2–4, impressive progress has been made in increasing the efficiency of the charge-separation reaction between an excited photosensitizer and an appropriate electron acceptor, but prevention of the energy-wasting back-reaction to neutral species still constitutes a major challenge. The back-reaction limits the length of time during which charge separation (and thus energy storage) can be maintained. Here we report exceedingly long-lived (up to a few hours) photoinduced charge separation in an artificial photosynthetic system that does not require a secondary substrate to react with the charged species. Our system uses pyrene (Py*) as the photosensitized electron donor and N,N'-dimethy 1-4,4'-bipyridinium (methyl viologen, MV2+) as the electron acceptor, both immobilized in a porous sol–gel silica glass5. The redox reaction is carried out by the mediation of a third mobile charge carrier in the intrapore space6. The spatial separation between the donor and acceptor inhibits the back-reaction to produce the long lifetimes of the charge-separated pair.