Back Electron Transfer Research Articles

Photodissociation of van der Waals complexes of iodine X-I2 (X = I2, C2H4) via charge-transfer state: A velocity map imaging investigation.

The photodissociation of van der Waals complexes of iodine X-I2 (X = I2, C2H4) excited via Charge-Transfer (CT) band has been studied with the velocity map imaging technique. Photodissociation of both complexes gives rise to translationally "hot" molecular iodine I2 via channels differing by kinetic energy and angular distribution of the recoil directions. These measured characteristics together with the analysis of the model potential energy surface for these complexes allow us to infer the back-electron-transfer (BET) in the CT state to be a source of observed photodissociation channels and to make conclusions on the location of conical intersections where the BET process takes place. The BET process is concluded to provide an I2 molecule in the electronic ground state with moderate vibrational excitation as well as X molecule in the electronic excited state. In the case of X = I2, the BET process converts anion I2- of the CT state into the neutral I2 in the repulsive excited electronic state which then dissociates promptly giving rise to a pair of I atoms in the fine states 2P1/2. In the case of C2H4-I2, the C2H4 molecules appear in the triplet T1 electronic state. Conical intersection for corresponding BET process becomes energetically accessible after partial twisting of C2H4+ frame in the excited CT state of complex. The C2H4(T)-I2 complex gives rise to triplet ethylene as well as singlet ethylene via the T-S conversion.

Direct Observation of Plasmon-Induced Interfacial Charge Separation in Metal/Semiconductor Hybrid Nanostructures by Measuring Surface Potentials.

Plasmon-induced interfacial charge separation (PICS) is one of the key processes responsible for the improved conversion efficiencies of energy-harvesting devices that incorporate metal nanostructures. In this Letter, we reveal a mechanism of PICS by visualizing (with nanometer-scale resolution) and characterizing plasmon-exciton coupling between p-type poly(pyrrole) (PPy) nanowires (NWs) and Ag nanoparticles (NPs) using light-irradiated Kelvin probe force microscopy (KPFM). Under blue-light irradiation, the Ag NPs are expected to donate electrons to the PPy NWs via a hot electron injection process. However, in this Letter, we observe that under blue-light irradiation the plasmonically and excitonically excited electrons in the semiconductor back-transfer to the metal. The PICS in this system can be explained by comparing it with a similar one where Au NPs are attached to n-type ZnO NWs; we observed a net electron transfer from the Au NPs to the ZnO NWs (an upward band bending is formed at the interface of the two materials, presumably obstructing electron back-transfer). Indeed, energy band matching between the metal and the semiconductor components of hybrid nanostructures influences PICS pathways. These experimental findings and our proposed mechanism consistently explain the PICS occurring in the PPy NW-Ag NP system with important implications on explaining their cooperative optoelectronic activities.

A Triphenylamine–Naphthalenediimide–Fullerene Triad: Synthesis, Photoinduced Charge Separation and Solution‐Processable Bulk Heterojunction Solar Cells

AbstractA new naphthalenediimide (NDI) derivative linked to triphenylamine (TPA) and fullerene (TPA–NDI–C60 triad; S3) was designed, synthesized and characterized. Femtosecond laser transient absorption spectral measurements revealed that fast electron transfer from TPA to the singlet‐excited state of NDI occurred to form a charge‐separated state (TPA.+–NDI.−–C60) with a rate constant of approximately 1012 s−1, followed by the charge‐shift reaction from NDI.− to C60 to produce the charge‐separated state (TPA.+–NDI–C60.−) that decayed by back electron transfer with a rate constant of 4.4×109 s−1 (with a corresponding lifetime of 230 ps). Organic bulk heterojunction solar cells were fabricated using the triad S3 as an n‐type semiconductor along with the conventional donor polymer poly(3‐hexylthiophene). The power conversion efficiency reached 3.03 % before annealing and 4.85 % after annealing in the optimized devices. To rationalize the performance of these organic photovoltaic devices, atomic force microscopy was used to study the morphology of poly(3‐hexylthiophene):S3 blend surfaces, which were found to be well interlaced and free from projections, lumps, or indentations.

ZnO-based dye-sensitized solar cells: Effects of redox couple and dye aggregation

The performance of ZnO-based dye-sensitized solar cells (DSSCs) has always been lower than that of TiO2-based devices, however, the factors for this difference are still not entirely understood. Here we use current - voltage curves in combination with intensity-modulated photovoltage spectroscopy, charge extraction measurements, and surface photovoltage spectroscopy to gain insight in the photochemical charge separation in ZnO-based DSSCs. Devices were fabricated with electrodeposited nanostructured, mesoporous ZnO films, an organic fluorenyl-thiophene dye (OD-8) as sensitizer, and an electrolyte solution with either the I−/I3− or [Co(2,2′-bipyridyl)3]2+/3+ redox couple. Surface photovoltage measurements and scanning electron microscopy images show that Zn2+-dye aggregates are most likely the cause of a decrease in cell performance with sensitization times longer than 4 h, due to the relatively acidic acrylonitrile bonding moiety of the OD-8 dye. Charge extraction measurements combined with intensity-modulated photovoltage spectroscopy illustrate that a lower electron lifetime for the DSSCs with the [Co(2,2′-bipyridyl)3]2+/3+ redox couple prevents achieving the thermodynamically attainable photovoltage, thus limiting solar cell efficiency. It can be concluded that further improvements in ZnO-based DSSCs are possible by: (i) avoiding the formation of Zn2+-dye aggregates in the mesoporous structure; (ii) preventing back electron transfer from ZnO to the electron acceptor in the electrolyte solution.

Thousandfold Enhancement of Photoreduction Lifetime in Re(bpy)(CO)3 via Spin-Dependent Electron Transfer from a Perylenediimide Radical Anion Donor.

Spin-dependent intramolecular electron transfer is revealed in the ReI(CO)3(py)(bpy-Ph)-perylenediimide radical anion (ReI-bpy-PDI-•) dyad, a prototype model system for artificial photosynthesis. Quantum chemical calculations and ultrafast transient absorption spectroscopy experiments demonstrate that selective photoexcitation of ReI-bpy results in electron transfer from PDI-• to ReI-bpy, forming two distinct charge-shifted states. One is an overall doublet whose return to the ground state is spin-allowed. The other, high-spin quartet state, persists for 67 ns due to spin-forbidden back-electron transfer, constituting a more than thousandfold lifetime improvement compared to the low-spin state. Exploiting this spin dependency holds promise for artificial photosynthetic systems requiring long-lived reduced states to perform multi-electron chemistry.

Charge Transfer Exciton and Spin Flipping at Organic–Transition-Metal Dichalcogenide Interfaces

Two-dimensional transition-metal dichalcogenides (TMD) can be combined with other materials such as organic small molecules to form hybrid van der Waals heterostructures. Because of different properties possessed by these two materials, the hybrid interface can exhibit properties that cannot be found in either of the materials. In this work, the zinc phthalocyanine (ZnPc)-molybdenum disulfide (MoS2) interface is used as a model system to study the charge transfer at these interfaces. It is found that the optically excited singlet exciton in ZnPc transfers its electron to MoS2 in 80 fs after photoexcitation to form a charge transfer exciton. However, back electron transfer occurs on the time scale of ∼1-100 ps, which results in the formation of a triplet exciton in the ZnPc layer. This relatively fast singlet-triplet transition is feasible because of the large singlet-triplet splitting in organic materials and the strong spin-orbit coupling in TMD crystals. The back electron transfer would reduce the yield of free carrier generation at the heterojunction if it is not avoided. On the other hand, the spin-selective back electron transfer could be used to manipulate electron spin in hybrid electronic devices.

Charge Separation and Triplet Exciton Formation Pathways in Small Molecule Solar Cells as Studied by Time-resolved EPR Spectroscopy.

Organic solar cells are a promising renewable energy technology, offering the advantages of mechanical flexibility and solution processability. An understanding of the electronic excited states and charge separation pathways in these systems is crucial if efficiencies are to be further improved. Here we use light induced electron paramagnetic resonance (LEPR) spectroscopy and density functional theory calculations (DFT) to study the electronic excited states, charge transfer (CT) dynamics and triplet exciton formation pathways in blends of the small molecule donors (DTS(FBTTh2)2, DTS(F2BTTh2)2, DTS(PTTh2)2, DTG(FBTTh2)2 and DTG(F2BTTh2)2) with the fullerene derivative PC61BM. Using high frequency EPR the g-tensor of the positive polaron on the donor molecules was determined. The experimental results are compared with DFT calculations which reveal that the spin density of the polaron is distributed over a dimer or trimer. Time-resolved EPR (TR-EPR) spectra attributed to singlet CT states were identified and the polarization patterns revealed similar charge separation dynamics in the four fluorobenzothiadiazole donors, while charge separation in the DTS(PTTh2)2 blend is slower. Using TR-EPR we also investigated the triplet exciton formation pathways in the blend. The polarization patterns reveal that the excitons originate from both intersystem crossing (ISC) and back electron transfer (BET) processes. The DTS(PTTh2)2 blend was found to contain substantially more triplet excitons formed by BET than the fluorobenzothiadiazole blends. The higher BET triplet exciton population in the DTS(PTTh2)2 blend is in accordance with the slower charge separation dynamics observed in this blend.

Dimerization and oxidation of tryptophan in UV-A photolysis sensitized by kynurenic acid

Photoinduced generation of radicals in the eye lens may play an important role in the modification of proteins leading to their coloration, aggregation, and insolubilization. The radicals can be formed via the reactions of photoexcited endogenous chromophores of the human lens with lens proteins, in particular with tryptophan residues. In the present work we studied the reactions induced by UV-A (315–400nm) light between kynurenic acid (KNA), an effective photosensitizer present in the human lens, and N-acetyl-L-tryptophan (NTrpH) under aerobic and anaerobic conditions. Our results show that the reaction mechanism strongly depends on the presence of oxygen in solution. Under aerobic conditions, the generation of singlet oxygen is the major channel of the effective NTrpH oxidation. In argon-bubbled solutions, the quenching of triplet KNA by NTrpH results in the formation of KNA•− and NTrp• radicals. Under laser pulse irradiation, when the radical concentration is high, the main pathway of the radical decay is the back electron transfer with the restoration of initial reagents. Other reactions include (i) the radical combination yielding NTrp dimers and (ii) the oxygen atom transfer from KNA•− to NTrp• with the formation of oxidized NTrp species and deoxygenated KNA products. In continuous-wave photolysis, even trace amounts of molecular oxygen are sufficient to oxidize the majority of KNA•− radicals with the rate constant of (2.0 ± 0.2) × 109M−1s−1, leading to the restoration of KNA and the formation of superoxide radical O2•−. The latter reacts with NTrp• via either the radical combination to form oxidized NTrp (minor pathway), or the electron transfer to restore NTrpH in the ground state (major pathway). As the result, the quantum yields of the starting compound decomposition under continuous-wave anaerobic photolysis are rather low: 1.6% for NTrpH and 0.02% for KNA. The photolysis of KNA with alpha-crystallin yields the same deoxygenated KNA products as the photolysis of KNA with NTrpH, indicating the similarity of the photolysis mechanisms. Thus, inside the eye lens KNA can sensitize both protein photooxidation and protein covalent cross-linking with the minor self-degradation. This may play an important role in the lens protein modifications during the normal aging and cataract development.

Back Electron Transfer at TiO2 Nanotube Photoanodes in the Presence of a H2O2 Hole Scavenger.

Adding charge scavengers, which usually are more unstable than water, is an effective method to quantify the quantum efficiency loss of photoelectrode during the charge separation, transfer, and injection processes of the water splitting reaction. Here, we detected, on TiO2 nanotube photoanodes after using hydrogen peroxide (H2O2) as a hole scavenger, a nearly 40% saturated photocurrent decrease in alkaline electrolyte and a negligible saturated photocurrent difference in acid electrolyte. We found that the photoelectrons were trapped in the surface states of TiO2 with nearly the same storage capacity of electrons in a wide range of pH values from 1.0 to 13.6. However, kinetics of a back reaction, H2O2 reduction by the photoelectrons trapped in surface states, is about 10 times higher for that in alkaline electrolyte than in acid electrolyte. As a result, the pH-dependent kinetic difference in H2O2 reduction induced the negative effects on the saturated photocurrent. Our results offer a new insight into understanding the effects of back electron transfer on electrochemical behaviors of surface states and charge scavengers.

Intramolecular Electron Transfer in Frozen Solvents: Charge Transfer and Local Triplet States Population Dynamics Revealed by Dual Phosphorescence.

In frozen solvents at 77 K, ultrafast (≤250 fs) photoinduced intramolecular electron transfer (ET) in bichromophoric donor-acceptor ([D-A]) diarylmethane lactones produces a covalently linked radical ion pair, 1[D•+-A•-]. Steady state and time-resolved luminescence measurements reveal that 1[D•+-A•-] decays to charge-separated (3[D•+-A•-]) and donor-centered ([3D*-A]) triplets, which display dual phosphorescence. 3[D•+-A•-] and [3D*-A] are formed in parallel via two intersystem crossing mechanisms: spin orbit charge transfer (SOCT) and hyperfine coupling (HFC), with solvent dependent branching ratio. The solvent drives the D-A alignment during the freezing process to adapt to increasing solvent polarity, producing inhomogeneous ground-state population distribution with solvent-dependent D-A exchange interaction, which plays a key role in partitioning into SOCT and HFC mechanisms. In polar glasses, a third phosphorescence band appears due to dissociative back ET in 3[D•+-A•-] resulting in excited open ring biradical.

Self-Assembled Bilayers on Nanocrystalline Metal Oxides: Exploring the Non-Innocent Nature of the Linking Ions.

Self-assembled bilayers on nanocrystalline metal oxide films are an increasingly popular strategy for modulating electron and energy transfer at dye-semiconductor interfaces. A majority of the work to date has relied on ZrII and ZnIV linking ions to assemble the films. In this report, we demonstrate that several different cations (CdII, CuII, FeII, LaIII, MnII, and SnIV) are not only effective in generating the bilayer assemblies but also have a profound influence on the stability and photophysical properties of the films. Bilayer films with ZrIV ions exhibited the highest photostability on both TiO2 and ZrO2. Despite the metal ions having a minimal influence on the absorption/emission energies and oxidation potentials of the dye, bilayers composed of CuII, FeII, and MnII exhibit significant excited-state quenching. The excited-state quenching decreases the electron injection yield but also, for CuII and MnII bilayers, significantly slows the back electron transfer kinetics.

Elucidating the Energy- and Electron-Transfer Dynamics of Photon Upconversion in Self-Assembled Bilayers

Self-assembled bilayers of acceptor (A) and sensitizer (S) molecules on a metal oxide surface is a promising strategy to facilitate photon upconversion via triplet–triplet annihilation (TTA-UC) and extract charge from the upconverted state. The hypothesized mechanism for TTA-UC in a bilayer film includes low energy light absorption, triplet energy transfer, cross-surface energy migration, triplet–triplet annihilation, and electron injection into TiO2. Nonproductive processes can also occur including sensitizer-sensitizer TTA, radiative/nonradiative decay, back-electron transfer, and others. Steady-state and time-resolved emission/absorption spectroscopy were used to determine the rate constants of these processes. The rate constants indicate that S to A triplet energy transfer as well as S and A nonradiative rates are the primary efficiency-limiting processes for TTA-UC at the interface. This information is necessary to guide the design of new self-assembled UC films and is a critical stepping stone towar...

Complexation of bis‐crown stilbene with alkali and alkaline‐earth metal cations. Ultrafast excited state dynamics of the stilbene‐viologen analogue charge transfer complex

AbstractThe complex formation of bis(18‐crown‐6)stilbene (1) and its supramolecular donor‐acceptor complex with N,N′‐bis(ammonioethyl) 1,2‐di(4‐pyridyl)ethylene derivative (2) with alkali and alkaline‐earth metal perchlorates has been studied using absorption, steady‐state fluorescence, and femtosecond transient absorption spectroscopy. The formation of 1∙Mn+ and 1∙(Mn+)2 complexes in acetonitrile was demonstrated. The weak long‐wavelength charge‐transfer absorption band of 1·2 completely vanishes upon complexation with metal cations because of disruption of the pseudocyclic structure. The spectroscopic and luminescence parameters, stability constants, and 2‐stage dissociation constants were calculated. The initial stage of a recoordination process was found in the excited complexes 1∙M+ and 1∙(M+)2 (M = Li, Na). The pronounced fluorescence quenching of 1·2 is explained by very fast back electron transfer (τet = 0.397 ps). The structure of complex 1·2 was studied by X‐ray diffraction; stacked (1·2)m polymer in which the components were connected by hydrogen bonding and stacking was found in the crystal. These compounds can be considered as novel optical molecular sensors for alkali and alkaline‐earth metal cations.

Reducing Interface Recombination through Mixed Nanocrystal Interlayers in PbS Quantum Dot Solar Cells.

The performance of ZnO/PbS colloidal quantum dot (CQD)-based heterojunction solar cells is hindered by charge carrier recombination at the heterojunction interface. Reducing interfacial recombination can improve charge collection and the photocurrent of the device. Here we report the use of a mixed nanocrystal (MNC) buffer layer comprising zinc oxide nanocrystals and lead sulfide quantum dots at the respective heterojunction interface. Remote trap passivation of the PbS CQDs taking place within this MNC layer reduces interfacial recombination and electron back transfer which in turn improves charge collection efficiency. Upon the addition of the MNC layer, the overall power conversion efficiency increases from 9.11 to 10.16% and Short-circuit current density (JSC) increases from 23.54 to 25.23 mA/cm2. Optoelectronic characterization of the solar cells confirms that the effects underlying device improvement are reduced trap density and improved charge collection efficiency due to the presence of the MNC buffer layer.

Fabrication and characterization of abrupt TiO2–SiOx core-shell nanowires by a simple heat treatment

Three dimensional hierarchical metal oxide nanostructures, like TiO2 nanowire arrays, have attracted great attention for electrochemical energy conversion and storage applications. The functionality of such devices can be further enhanced by adding a nanowire shell with a different stoichiometry or composition compared to the core. Here, we report an approach with a facile heat treatment at 1050 °C, which allows the fabrication of rutile TiO2–SiOx core-shell nanowire arrays on silicon substrates. Our detailed electron microscopic investigation shows that this method is able to cover hydrothermally grown rutile TiO2 nanowires with a uniform shell of several nanometers in thickness. Moreover, the treatment improves the quality of the rutile TiO2 core by removing lattice defects, introduced from the hydrothermal growth. Electron energy loss spectroscopy reveals that the homogeneous shell around the TiO2 core consists of amorphous SiOx and does not form any intermediate phase with TiO2 at the interface. Thus, the properties of the TiO2 core are not affected by the shell, while the shell suppresses undesired electron back transfer. Latter leads to performance losses in many applications, e.g., dye sensitized solar cells, and is the main reason for a fast degradation of devices incorporating organic materials and TiO2.

DSSCs based on aniline derivatives functionalized with a tert-butyldimethylsilyl group and the effect of the π-spacer

We have developed four new dyes for DSSCs with a tert-butyldimethylsilylether in order to test their physical and photovoltaic properties. These dyes consist of a novel donor based on a functionalized N,N-dialkylaniline, a heteroaromatic π-conjugated spacer, such as thiophene or benzothiadiazole ring, and cyanoacetic acid as acceptor group. DFT theoretical studies predict, on the one hand, higher molar extinction coefficient for dyes with thiophene spacers than for those with benzothiadiazole groups while, on the other hand, the use of benzothiadiazole ring would give rise to the reduction of the back electron transfer (BET). The experimental studies have confirmed the predicted higher absorption of thiophene dyes and the better photovoltaic performance of the DSSCs prepared with these dyes. Finally, the stability of the response would support further studies of dyes with anilines functionalized with a tertbutyldimethylsilyl group to perform long-term devices.

Stability, Scale-up, and Performance of Quantum Dot Solar Cells with Carbonate-Treated Titanium Oxide Films.

A novel yet simple approach of carbonate (CBN) treatment of TiO2 films is performed, and quantum dot solar cells (QDSCs) with high power conversion efficiencies (PCEs), reasonably good stabilities, and good fill factors (FFs) are fabricated with TiO2-CBN films. The ability of carbonate groups to passivate defects or oxygen vacancies of TiO2 is confirmed from a nominally enhanced band gap, a lowered defect induced fluorescence intensity, an additional Ti-OH signal obtained after carbonate decomposition, and a more capacitive low frequency electrochemical impedance behavior achieved for TiO2-CBN compared to untreated TiO2. A large area QDSC of 1 cm2 with a TiO2-CBN/CdS/Au@PAA (poly(acrylic acid)) photoanode delivers an enhanced PCE of 4.32% as opposed to 3.03% achieved for its analogous cell with untreated TiO2. Impedance analysis illustrates the role of carbonate treatment in increasing the recombination resistance at the photoanode/electrolyte interfaces and in suppressing back-electron transfer to the electrolyte, thus validating the superior PCE achieved for the cell with carbonate-treated TiO2. QDSCs with the configuration TiO2-CBN/CdS/Au@PAA-polysulfide/SiO2 gel-carbon-fabric/WO3-x and active areas of 0.2-0.3 cm2 yield efficiencies in the range of 5.16 to 6.3%, and the average efficiency of the cells is 5.9%. The champion cell is characterized by the following photovoltaic parameters: JSC (short circuit current density), 11.04 mA cm-2; VOC (open circuit voltage), 0.9 V; FF, 0.63; and PCE, 6.3%. Stability tests performed on this cell show that dark storage has a less deleterious effect on cell performance compared to extended illumination. In dark, the PCE of the cell dropped from 5.69 to 5.52%, and under prolonged continuous irradiance of 5 h, it decreased from 5.91 to 4.83%. A scaled-up QDSC with the same architecture of 4 cm2 size showed a PCE of 1.06%, and the demonstration of the lighting of a LED accomplished using this cell exemplifies that this cell can be used for powering electronic devices that require low power.

Can a light harvesting material be always common in photocatalytic and photovoltaic applications?

Nanomaterials and nanohybrids hold promising potency to enhance the performance of photovoltaic as well as photocatalytic efficiency by improving both light trapping and photo-carrier collection. In the present study, we have synthesized Protoporphyrin IX-Titanium dioxide (PP-TiO2) nanohybrid as a model light harvesting nanohybrid for potential applications in photovoltaics and photocatalytic devices. We observed that the light harvesting nanohybrid shows efficient photocatalytic activity when copper (II) ion is centrally located within the porphyrin moiety. In contrast, presence of copper (II) ion within the porphyrin moiety decreases photovoltaic efficiencies. High-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and steady-state absorption and emission spectroscopies have been used to analyze the structural details and optical properties of this nanohybrid. Time-resolved fluorescence technique has been applied to study the ultrafast dynamics which is key to photocatalytic and photovoltaic activities. The reason behind the outstanding photocatalytic performance of the nanohybrid after copper metalation, is found to have additional stability against photobleaching while enhanced back electron transfer after copper metalation decreases its photovoltaic efficiency.

Alcohol‐Based Sensitizer–Semiconductor Linkages Towards Improved Interfacial Electron Transfer Kinetics

AbstractA series of four ruthenium complexes with alcohol binding groups of the general form [Ru(dtb)2(LL)](PF6)2 (where dtb is 4,4′‐(tert‐butyl)2‐2,2′‐bipyridine and LL is an alcohol‐containing ligand) were anchored to mesoporous TiO2 thin films. The complexes were found to sensitize light‐driven electron‐transfer reactivity of TiO2 to the visible region and the complexes were hence termed sensitizers. The kinetics and yields of interfacial electron transfer were quantified on the nanosecond and longer timescales. The stability under oxidizing and light excitation conditions was assessed by spectroscopic methods. Comparative analysis with the more commonly utilized dicarboxylic acid containing ligand revealed that the sensitizer bearing two terminal CH2OH groups was found to have more efficient electron injection and slower back electron transfer.

Inosine Can Increase DNA's Susceptibility to Photo-oxidation by a RuII Complex due to Structural Change in the Minor Groove.

Key to the development of DNA-targeting phototherapeutic drugs is determining the interplay between the photoactivity of the drug and its binding preference for a target sequence. For the photo-oxidising lambda-[Ru(TAP)2 (dppz)]2+ (Λ-1) (dppz=dipyridophenazine) complex bound to either d{T1 C2 G3 G4 C5 G6 C7 C8 G9 A10 }2 (G9) or d{TCGGCGCCIA}2 (I9), the X-ray crystal structures show the dppz intercalated at the terminal T1 C2 ;G9 A10 step or T1 C2 ;I9 A10 step. Thus substitution of the G9 nucleobase by inosine does not affect intercalation in the solid state although with I9 the dppz is more deeply inserted. In solution it is found that the extent of guanine photo-oxidation, and the rate of back electron-transfer, as determined by pico- and nanosecond time-resolved infrared and transient visible absorption spectroscopy, is enhanced in I9, despite it containing the less oxidisable inosine. This is attributed to the nature of the binding in the minor groove due to the absence of an NH2 group. Similar behaviour and the same binding site in the crystal are found for d{TTGGCGCCAA}2 (A9). In solution, we propose that intercalation occurs at the C2 G3 ;C8 I9 or T2 G3 ;C8 A9 steps, respectively, with G3 the likely target for photo-oxidation. This demonstrates how changes in the minor groove (in this case removal of an NH2 group) can facilitate binding of RuII dppz complexes and hence influence any sensitised reactions occurring at these sites. No similar enhancement of photooxidation on binding to I9 is found for the delta enantiomer.