Photoinduced Charge Recombination Research Articles

Effect of Cr doping on the photoelectrochemical performance of hematite nanorod photoanodes

Hematite (α-Fe2O3) nanorod arrays modified by surface doping of chromium (III) ions (Cr3+) for photoelectrochemical (PEC) water splitting are fabricated using a general process involving a combination of aqueous chemical growth and spin coating. The PEC activity of Cr-doped α-Fe2O3 nanorod films first increases then decreases with increasing dopant content. At the optimal content of Cr, Cr-doped α-Fe2O3 nanorod films exhibit about 3.5 and 6 times higher PEC activity than the undoped material under solar light (AM 1.5, 100mWcm−2) and visible light (λ>430nm) irradiation, respectively. A comprehensive characterization of the chemical, morphological, PEC, structural, and optical properties of the doped α-Fe2O3 films are presented to assess the mechanisms by which the dopants influence photoelectrode performance. The relationship between dopant content, photoluminescence intensity, and PEC performance suggests Cr doping alters charge transfer in the films under irradiation. At low Cr doping contents, Cr dopants act as electron (or hole) traps and retard photoinduced charge recombination, leading to enhanced PEC activity. Whereas, at high Cr doping contents, Cr dopants act as charge recombination sites and lower the charge separation efficiency, leading to decreased PEC activity. High temperature annealing proves to be effective for further improvement of the PEC activities of doped and undoped hematite films, by Sn diffusion.

Efficient H 2 production by water-splitting using indium–tin-oxide/V-doped TiO 2 multilayer thin film photocatalyst

In order to sensitize TiO 2 in visible light and to reduce photo-induced charge recombination, the multilayer films of Indium–Tin Oxide (ITO)/V-doped TiO 2 were synthesized by radio-frequency magnetron sputtering. V-doped TiO 2 thin films showed red shift in TiO 2 absorption edge with increasing dopant concentration and, most importantly, the dopant energy levels are formed in the TiO 2 band gap due to V 5+/V 4+ ions as confirmed by UV–Visible and XPS spectra. Multilayer films with different numbers of ITO/V-doped TiO 2 (6 at.%) bilayers (namely, 2-, 3-, 4-, 5-, 6- and 7-bilayers) were deposited, in order to reduce the charge recombination rate, by keeping the total thickness of TiO 2 constant in each multilayer film. In multilayer films, when exposed to visible light the photocurrent increases as function of the number of bilayers by reaching the maximum with 6-bilayers of ITO/V-doped TiO 2. The measured enhanced photocurrent is attributed to: 1) ability of V-doped TiO 2 to absorb visible light, 2) number of space-charge layers in form of ITO/TiO 2 interfaces in multilayer films, and 3) generation of photoelectrons just in/or near to the space-charge layer by decreasing the V-doped TiO 2 layer thickness. The reduced charge recombination rate in multilayer films was also confirmed by the photocurrent kinetic curves. The superior photocatalytic efficiency of the 6-bilayers film is also reflected in hydrogen production rate through water-splitting: we obtained indeed 31.2 μmol/h of H 2 production rate.

Photoinduced Charge Carrier Dynamics of Zn−Porphyrin−TiO2 Electrodes: The Key Role of Charge Recombination for Solar Cell Performance

Time resolved absorption spectroscopy has been used to study photoinduced electron injection and charge recombination in Zn-porphyrin sensitized nanostructured TiO(2) electrodes. The electron transfer dynamics is correlated to the performance of dye sensitized solar cells based on the same electrodes. We find that the dye/semiconductor binding can be described with a heterogeneous geometry where the Zn-porphyrin molecules are attached to the TiO(2) surface with a distribution of tilt angles. The binding angle determines the porphyrin-semiconductor electron transfer distance and charge transfer occurs through space, rather than through the bridge connecting the porphyrin to the surface. For short sensitization times (1 h), there is a direct correlation between solar cell efficiency and amplitude of the kinetic component due to long-lived conduction band electrons, once variations in light harvesting (surface coverage) have been taken into account. Long sensitization time (12 h) results in decreased solar cell efficiency because of decreased efficiency of electron injection.

Photoinduced Electron Transfer in Ruthenium(II)/Tin(IV) Multiporphyrin Arrays

The photophysical behavior of a series of heterometallic arrays made of a central Sn(IV) porphyrin connected, respectively, to two (SnRu(2)), four (SnRu(4)), or six (SnRu(6)) ruthenium porphyrin units has been studied in dichloromethane. Two different motifs connect the ruthenium porphyrin units to central tin porphyrin core, axial coordination via ditopic bridging ligands and/or coordination to peripheral pyridyl groups of the central porphyrin ring. A remarkable number of electron transfer processes (photoinduced charge separation and recombination processes) have been time-resolved using a combination of emission spectroscopy and fast (nanosecond) and ultrafast (femtosecond) absorption techniques. In these systems both types of molecular components can be selectively populated by light absorption. In all the arrays, the local excited states of these units (the tin porphyrin singlet excited state and the ruthenium porphyrin triplet state) are quenched by electron transfer leading to a charge-separated state where the ruthenium porphyrin unit is oxidized and the tin porphyrin unit is reduced. For each array, the two forward electron transfer processes, as well as the charge recombination process leading back to the ground state, have been kinetically resolved. The rate constants obey standard free-energy correlations with the forward processes lying in the normal free-energy regime and the back reactions in the Marcus inverted region. The comparison between the trimeric (SnRu(2)) and pentameric (SnRu(4)) arrays shows that all the electron transfer processes are faster in the latter than in the former system. This can be rationalized in terms of differences in electronic factors (due to the different connecting motifs) and driving force. In less polar solvents, such as toluene, the energy of the charge-separated states is substantially lifted, leading to a switch (from electron transfer to triplet energy transfer) in the deactivation mechanism of the excited ruthenium triplet.

Structure and dynamics correlations of photoinduced charge separation in rigid conjugated linear donor–acceptor dyads towards photovoltaic applications

The rates of photoinduced charge separation (CS) and charge recombination (CR) of four rigidly linked donor (D)–acceptor (A) dyads that were synthesized as intramolecular p/n junctions were obtained by optical transient absorption spectroscopy in solutions and self-assembled films. The CS processes were much more efficient than CR processes for all dyads in both solutions and films, which is preferable for photovoltaic applications. The rates of CR processes in covalently linked dyad films were slower than those in non-covalently linked D (i.e. oligothiophene (OTH)) and A (i.e.perylenediimide (PDI)) mixtures, demonstrating that the covalent linker may have reduced the probability of geminate recombination of charges due to direct face-to-face packing between D and A. Although both the CS and CR rates of the four dyads were similar with respect to one another in solution, they differed from one another in films, with the slowest CR rate observed in the dyad connected to a bulky alkyl group that hindered packing. After annealing, the CR rates of all films increased non-uniformly, and the extent of increase was correlated to structural changes as evidenced by the grazing-incident X-ray scattering (GIXS).

Photocurrent generation in a CdS nanocrystals/poly[2-methoxy-5-(2′-ethyl-exyloxy)phenylene vinylene] electrochemical cell

In this paper the photoelectrochemical processes occurring in composites formed of organic-capped CdS nanocrystals and low molecular weight poly[2-methoxy-5-(2′-ethyl-exyloxy)phenylene vinylene] conjugated polymer were investigated. High quality colloidal CdS nanoparticles were synthesized by means of thermal decomposition of suitable precursors in non coordinating solvents, using oleic acid as surface capping agent. The absorption and emission properties of the prepared heterojunctions were studied both in solutions and in composite films. The dispersed hybrids were also investigated as photoactive materials, focusing on the photoinduced charge transfer and recombination processes at the interface between the two components. The composites have shown a fundamental role in photoelectrochemical applications due to the presence of a great number of interfaces able to enhance the charge transfer between mixture components. Blend solutions prepared with octylamine capped CdS nanocrystals showed an improvement of the photoconductivity with respect to hybrids containing longer oleate surfactants.

Intramolecular Electron Transfer within a Covalent, Fixed-Distance Donor−Acceptor Molecule in an Ionic Liquid

Intramolecular photoinduced charge separation and recombination within the donor-acceptor molecule 4-(N-pyrrolidino)naphthalene-1,8-imide-pyromellitimide, 5ANI-PI, are studied using ultrafast transient absorption spectroscopy in the room-temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide [EMIM][Tf2N]. The rate constants of both photoinduced charge separation and charge recombination for 5ANI-PI in [EMIM][Tf2N] are comparable to those observed in pyridine, which has a static dielectric constant similar to that of [EMIM][Tf2N] but a viscosity that is nearly 2 orders of magnitude lower than that of [EMIM][Tf2N]. The electron-transfer dynamics of 5ANI-PI in [EMIM][Tf2N] are compared to those in pyridine as a function of temperature and are discussed in the context of recently reported ionic liquid solvation studies.

Molecular Wire Behavior in π-Stacked Donor-Bridge-Acceptor Tertiary Arylureas

The synthesis, structure, and electronic properties of a series of tertiary polyarylureas possessing pyrene and dimethylaniline capping groups separated by zero-to-four phenylenediamine bridging groups are described. These molecules adopt folded π-stacked structures. The dynamics of photoinduced charge separation and charge recombination are investigated by femtosecond broadband pump−probe spectroscopy. The structure lacking a bridging group displays ultrafast charge separation (within 2 ps) and charge recombination typical of stacked dyad systems. Structures having one-to-four bridging groups undergo charge separation with decay times of 8−21 ps and charge recombination with decay times of 60−120 ps that are only weakly dependent upon the distance between the pyrene and dimethylaniline capping groups. This behavior is similar to that of wire-like systems having extended conjugated bridges and provides the first example of wire-like behavior in a π-stacked donor-bridge-acceptor system.

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.

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.

Photoinduced Charge Recombination at Dye-Sensitized Individual TiO2 Nanoparticles and Its Application in Probe for the Local Polarity Change around the Nanoparticle in Solution

Application of the Marcus relation to the interfacial charge recombination at the TiO2 film is often limited by interparticle electron transport known as carrier diffusion in the dye-sensitized sintered TiO2 nanocrystalline films, where the nanoparticles are interconnected. Dye-sensitized disconnected individual TiO2 nanoparticles may provide a suitable case for inspecting the Marcus relation in interfacial charge recombination. In this paper we investigated the photoinduced interfacial charge recombination process occurring at all-trans-retinoic acid (ATRA)-sensitized individual TiO2 nanoparticles in hexanol and methanol solutions (containing a small amount of acidified water for suspension of TiO2 nanoparticles) by 355 nm pulsed-laser photolysis. It was found that in the nanosecond to microsecond temporal domain, charge recombination kinetics is monoexponential, and does not depend on the laser excitation power in a range from 0.45 to 6.0 mJ/(cm2 pulse). Temperature dependence study of the observed charge recombination rate shows that it follows the Marcus relation. The experimentally determined corresponding reorganization energy for the photoinduced charge recombination of ATRA/TiO2 in hexanol/H2O (v:v ≈ 40:1) and methanol/H2O (v:v ≈ 9:1) is 0.44 ± 0.10 and 0.48 ± 0.20 eV, respectively. By tuning the relative ratio of water in methanol hence the reorganization energy of the mixed solvent, the observed reorganization energy dependent charge recombination rate also agrees with that predicted from the Marcus relation. On the basis of experimental results, it can be concluded that the injected electron is immobilized on the individual TiO2 nanoparticles and the charge recombination seems more like a germinate pair recombination. This is different from what has been observed for the dye-sensitized TiO2 crystalline film. Furthermore, oscillations in the measured interfacial charge recombination rate have been observed when ATRA/TiO2 hexanolic solution was subjected to prolonged laser irradiation. The oscillation was interpreted as the change of the reorganization energy in response to the photoinduced local polarity change around the TiO2 nanoparticle, which arises from the photoinduced wettability change occurring at the TiO2 nanoparticle surface. This provides a unique example of a molecular probe for detecting the photoinduced wettability change of the TiO2 nanoparticles in solution.

Photoinduced Electron Transfer and Excitation Energy Transfer in Directly Linked Zinc Porphyrin/Zinc Phthalocyanine Composite

Photoinduced electron transfer (ET) and excitation energy transfer (ENT) reactions in monomer and slipped-cofacial dimer systems of a directly linked Zn porphyrin (Por)-Zn phthalocyanine (Pc) heterodyad, ZnPc-ZnPor, were investigated by means of the picosecond and femtosecond transient absorption spectroscopies. In the dimer dyad system of two heterodyads connected through the coordination bond between two imidazolyl-substituted ZnPor bearing ZnPc, ZnPc-ZnPor(D), the rapid ENT from the ZnPor to ZnPc in the subpicosecond time region was followed by photoinduced charge separation (CS) and charge recombination (CR) with time constants of 47 and 510 ps, respectively. On the other hand in the monomer dyad system, no clear charge-separated state was observed although the CS with a time constant of 200 ps and CR with < or =70 ps were estimated. These results indicated that the dimer slipped-cofacial arrangement of pair porphyrins is advantageous for the effective production of the CS state. This advantage was discussed from the viewpoint of a decrease in the reorganization energy of the dimer relative to that of the monomer system. In addition, the electrochemical measurements indicated that the strong interaction between ZnPc and ZnPor moieties also contributed to the fast CS process despite the marginal driving force for the CS process. The dimer dyad of ZnPc-ZnPor provides full advantages in efficiencies of the light harvesting and the CS state production.

Dynamics of Photoinduced Electron Transfer in a Molecular Donor−Acceptor Quartet

The electronic structures and dynamics of photoinduced charge separation and recombination in a new donor/acceptor quartet molecule with bis-oligothiophene (BOTH) and bis-perylenediimide (BPDI) blocks attached to a benzene ring were described. Detailed transient spectroscopic studies were carried out on this compound and reference compounds at isolated molecular levels in solution. Two different dynamics of charge separation and recombination associated with two types of donor/acceptor pair conformations in solution were observed. These results were discussed based on Marcus theory and ascribed to both through-bond and through-space electron-transfer processes associated with two different orientations of the acceptors relative to the donor group. This molecular system exhibits a more efficient charge separation than charge recombination processes in both polar and nonpolar organic solvents, indicating that the material is an interesting candidate for photovoltaic studies in solid state.

Time-resolved EPR investigation of charge recombination to a triplet state in a carotene-diporphyrin triad

Photoinduced charge separation and recombination were investigated in a triad consisting of a carotenoid (C), a tetraarylporphyrin (P) and a tris(heptafluoropropyl)porphyrin (PF), C–P–PF, by means of time-resolved electron paramagnetic resonance. The electron transfer process was studied in a glass of 2-methyltetrahydrofuran at 10 K, in the crystalline phase at 150 K and in the liquid nematic phase of the uniaxial LC E-7 at 295 K, and in the nematic phase of the LC ZLI-1167 at 300 K. In all the different media and in the different phases, the molecular triad undergoes two-step photoinduced electron transfer, with the generation of a long-lived charge-separated state (C•+–P–), and charge recombination to the triplet state, localized in the carotene moiety 3C–P–PF. Low-temperature charge separation and triplet recombination are common features of both fullerene-based and diporphyrin molecular triads, proving that the large delocalized π-electron system of the porphyrin electron acceptor leads to low total reorganization energy and low sensitivity to solvent stabilization of the radical ions in a similar way as for fullerene systems.

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.

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.

Novel Nanoscale Organic Materials for Optimal Photovoltaic Functions

ABSTRACTCovalently linked electron donor (D) and electron acceptor (A) with conjugated organic building blocks are novel materials for potential solar cell applications, because these molecular p-n junctions can minimize the exciton diffusion and transform the charge separation from interdomain to intramolecular processes. Hence, the bottleneck of the exciton diffusion in many bulk heterojuction materials can be eliminated. Meanwhile, these planar conjugated assemblies, such as supermolecules, multiblock oligomers and polymers, have strong tendency to π-π stacking to form continuous channels for charge carriers to hop/diffuse to respective electrodes. A quartet D-A assembly has been synthesized with bis-oligothiophene (BOTH) and bis-perylenediimide (BPDI) derivatives attached to a benzo template. The electronic structures and dynamics of photoinduced charge separation and recombination of this quartet molecule and reference compounds in solutions and films were studied at isolated the molecular level in solutions as well as at the molecular assembly level in films with stacked structures. Two different dynamics of charge separation and recombination associated with two types of donor/acceptor pair conformations in solution were observed. This molecular system exhibits a more efficient charge separation than charge recombination processes in both polar and non-polar organic solvents, as well as films. More efficient charge separation and slower charge recombination due to the covalent linkage indicating that the material is a potential candidate for photovoltaic studies in solid-state.

Crossover from Superexchange to Hopping as the Mechanism for Photoinduced Charge Transfer in DNA Hairpin Conjugates

The mechanism and dynamics of photoinduced charge separation and charge recombination have been investigated in synthetic DNA hairpins possessing donor and acceptor stilbenes separated by one to seven A:T base pairs. The application of femtosecond broadband pump-probe spectroscopy, nanosecond transient absorption spectroscopy, and picosecond fluorescence decay measurements permits detailed analysis of the formation and decay of the stilbene acceptor singlet state and of the charge-separated intermediates. When the donor and acceptor are separated by a single A:T base pair, charge separation occurs via a single-step superexchange mechanism. However, when the donor and acceptor are separated by two or more A:T base pairs, charge separation occurs via a multistep process consisting of hole injection, hole transport, and hole trapping. In such cases, hole arrival at the electron donor is slower than hole injection into the bridging A-tract. Rate constants for charge separation (hole arrival) and charge recombination are dependent upon the donor-acceptor distance; however, the rate constant for hole injection is independent of the donor-acceptor distance. The observation of crossover from a superexchange to a hopping mechanism provides a "missing link" in the analysis of DNA electron transfer and requires reevaluation of the existing literature for photoinduced electron transfer in DNA.

Ultrafast Photoinduced Intramolecular Charge Separation and Recombination Processes in the Oligothiophene-Substituted Benzene Dyads with an Amide Spacer

Photoinduced intramolecular charge separation (CS) and recombination (CR) processes of the tetrathiophene-substituted benzene dyads with an amide spacer (4T-PhR, R = 4-H (1), 4-CN (2), 3,4-(CN)2 (3), 4-NO2 (4), 3,5-(NO2)2 (5)) in solvents of different polarities were investigated using various fast spectroscopies. It was revealed that the CS rates depend on the ability of the acceptor and solvent polarity. Ultrafast CS with the rate of 5 x 10(12) s(-1) was revealed for 5 in PhCN and MeCN. The ultrafast CS can be attributed to the large electronic coupling matrix element between the donor and the acceptor despite the relative long donor-acceptor distance. The existence of the state with large electron density on the spacer between 14T*-PhR and LUMO should facilitate the CS process in the present dyad system. It was also revealed that the CR rates in these dyads were rather fast because of the enhanced superexchange interaction through the amide spacer.

Electronic Coupling Effects on Photoinduced Electron Transfer in Carotene−Porphyrin−Fullerene Triads Detected by Time-Resolved EPR

Photoinduced charge separation and recombination in a carotenoid-porphyrin-fullerene triad C-P-C60 (Bahr et al., 2000) have been followed by time-resolved electron paramagnetic resonance. The electron-transfer process has been characterized in a glass of 2-methyltetrahydrofuran and in the nematic phase of two uniaxial liquid crystals (E-7 and ZLI-1167). In all the different media, the molecular triad undergoes two-step photoinduced electron transfer, with the generation of a long-lived charge-separated state (C*+-P-C60*-), and charge recombination to the triplet state, localized in the carotene moiety, mimicking different aspects of the photosynthetic electron-transfer process. The magnetic interaction parameters have been evaluated by simulation of the spin-polarized radical pair spectrum. The weak exchange interaction parameter (J = +1.7 +/- 0.1 G) provides a direct measure of the dominant electronic coupling matrix element V between the C*+-P-C60*- radical pair state and the recombination triplet state 3C-P-C60. Comparison of the estimated values of V for this triad and a structurally related triad differing only in the porphyrin bridge (octaalkylporphyrin vs tetraarylporphyrin) explains in terms of an electronic coupling effect the approximately 6-fold variation of the recombination rate induced by the modification of the porphyrin bridge as derived by kinetic experiments (Bahr et al., 2000).