Tandem dye-sensitised solar cells, which contain two dye-sensitized electrodes (one p-type and one n-type), are often cited as a way of improving upon the standard devices, in which only the n-type electrode is photoactive. However, the p-type component of these devices suffers from very poor photocurrents, limiting the overall performance of the tandem cell. This Thesis aims to improve the performance of p-type dye-sensitised solar cells. An emphasis is made on tuning the energy levels of the different components of the device to ensure that the charge transfer processes which dictate the current and voltage are efficient. Chapters 1 & 2: A brief introduction to p-type and tandem dye-sensitised solar cells. The dye characterisation methods and techniques used to test complete devices are also discussed. Chapter 3: Dye synthesis and characterisation methods are presented as well as the apparatus used to characterise complete DSC devices. Material characterisation methods used in Chapter 6 are also discussed. Chapter 4: Novel dye structures were investigated for their use in p-type dye-sensitised solar cells. The optical and electrochemical properties of these dyes were investigated experimentally and by computational methods. It was found that the energies of the frontier orbitals of these dyes had a profound effect on charge transfer processes within p-DSCs. Charge recombination was investigated using small square wave modulated photovoltage and charge extraction measurements, this demonstrated that blocking units can be employed to inhibit this process. Transient absorption and time resolved infrared spectroscopies are employed to investigate the excited and charge separated state lifetimes of a number of these dyes. The dye CAD3 in this Chapter generated a record breaking photocurrent. This dye absorbed towards the red region (> 600 nm) of the visible spectrum, which made it ideal for use in tandem-DSCs in which red light is absorbed at the photocathode. Chapter 5: Three dyes, including CAD3 from Chapter 4 were used on the photocathode in tandem-DSCs. The photocurrent generated was largely dependent on the degree of spectral overlap between these dyes and the dye on the photoanode. CAD3, which absorbed at longer wavelengths than the other dyes, generated a record breaking photocurrent when paired with the literature n-type dye D35. However, these devices suffered from poor photovoltages as result of using an electrolyte solution optimised for p-DSCs. Chapter 6: Mg2+ doping studies were performed on the NiO films used in p-DSCs. It was found that higher concentrations of MgO improved the photovoltage of these devices. However, there was a notable drop in photocurrent with increasing Mg2+. From charge extraction experiments it was revealed that the cause of this was a positive shift in the energy of the valence band. This decreased the driving force for electron transfer from the NiO film to the dye and therefore the photocurrent. The MgO also had a profound effect on the morphology of the films. The larger pore volume in the 150-500 nm range lead to an improvement in dye adsorption, despite the overall surface area being slightly decreased. Chapter 7: Based on work by O’Regan et al., the effect of free iodine on p-DSCs was investigated. In this Chapter it was demonstrated that increased concentration of free iodine reduced the photovoltage obtained by a p-DSC. This was confirmed to be a result of increased charge recombination using small square wave modulated photovoltage and charge extraction measurements. The fact that increasing concentration of free iodine increased the rate of recombination so significantly indicates an alternate main pathway for recombination. This study was then repeated with increasing concentrations of triiodide, which was found to have little or no effect on the performance of the p-DSCs. Chapter 8: A concluding summary of the work presented in this Thesis. Future work is also discussed in this Chapter.

Over the last decade the beneficial properties of hydrogels as artificial cell culture supports have been extensively investigated. Certain synthetic hydrogels have been proposed to be similar in composition and structure to the native extracellular matrix of the stem cell niche, their in vivo cell habitat, which is a powerful component in controlling stem cell fate. The stem cell differentiation pathway taken is influenced by many factors. When culturing cells within or upon hydrogels the choice can be strongly dependent on the underlying 3D hydrogel chemistry which strongly influences hydrogel-cell interactions. The interrelationship between hydrogel chemistry and that of biomolecules in controlling cellular response ideally requires analysis methods to characterise the chemistry without labels and normally in 3D. Time-of-flight secondary ion mass spectrometry (ToF SIMS) has the potential to be utilised for through thickness characterisation of hydrogels. The frozen-hydrated sample format is well suited to minimise changes associated with dehydration or the complication of chemical ‘fixation’. There is however significant challenges associated with this sample format. Frost formation occurs on cold samples in the ambient atmosphere affecting the quality of chemical information acquired from depth profiling frozen hydrogel samples. We develop a simple method to remove this frost by blowing with gas prior to entry into the instrument which is shown to produce remarkably good profiles on a poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogel film where a model protein, lysozyme, is incorporated to demonstrated how biomolecule distribution within hydrogels can be determined. A comparison of lysozyme incorporation is made between the situation where the protein is present in the polymer dip coating solution and lysozyme is a component of the incubation medium. It is shown that protonated water clusters H(H2O)n+ where n=5-11 that are indicative of ice are detected through the entire thickness of the pHEMA and the lysozyme distribution through the pHEMA hydrogel films can be determined using the intensity of characteristic fragment secondary ions. Quantitative TOF-SIMS analysis is highly desirable in biomaterial analysis as the amount and type of molecule in the material analysed may be determined. This has significant interest in hydrogel chemical analysis as cellular development on or within hydrogels may be highly influenced by the concentration and type of soluble molecule. Unfortunately, the matrix effect in SIMS changes the measured signal intensity of the analyte, preventing accurate quantitation. For this reason, we apply X-Ray Photoelectron Spectroscopy (XPS) on the equivalent samples as the ToF-SIMS in an attempt to correlate molecular ion yields to exact elemental concentrations. Similarly to ToF-SIMS the frozen-hydrated format in XPS is however still relatively unexplored. We apply the developed preparatory procedure in 3D XPS analysis of pHEMA/lysozyme hydrogel films in a hydrated state. We show that this format is suitable for alternative high vacuum analysis techniques. Furthermore, we show that lysozyme ingression and concentration can be determined through XPS. This work describes the first example of the characterisation of a hydrogel by ToF-SIMS and XPS in a frozen hydrated format, characterising hydrogels in a format most reflecting its native hydrated state at culture conditions. The described procedure allows for the mapping of biomolecules in a label free manner in 3D, furthermore allowing quantitative determination of biomolecule concentrations in hydrogels.

Gas phase metal-containing complexes provide suitable systems in which to study fundamental binding motifs between a metal ion and molecules in the absence of any solvent, support or competing charge effects. In this thesis, metal-containing species are explored experimentally using infrared resonance enhanced photodissociation (IR-REPD) spectroscopy and velocity map imaging (VMI). The experimental results are further interpreted with the aid of spectral simulations based on density functional theory (DFT). These are the first studies reported using a newly built IR-REPD spectrometer equipped with a purpose-built laser ablation source to allow for the study of single metal ion-molecule complexes. The laser ablation source is shown to efficiently produce various complexes including Rh⁺(CO₂)_n, VO₂⁺(N₂O)_n and Au⁺(CH₄)_n and the IR-REPD spectrometer has been characterised against a well-studied system of V⁺(CO₂)_n complexes. In order to record the IR-REPD spectra for small metal ion-molecule complexes, an argon atom is employed as the inert messenger. A combined IR-REPD spectroscopy and DFT investigation of M⁺(CO₂)_n complexes (where M = Co⁺, Rh⁺ and Ir⁺) reveals a common [M⁺(CO₂)₂] core structure for all three considered metal ions. Additional ligands, which are not directly bound to the central metal ion, experience lower perturbation as evident in the reduced blue-shift for the ligand in the outer coordination shells. A further IR-REPD/DFT study involving CO₂ complexation around NbO₂⁺ and TaO₂⁺ ions reveals a strongly-bound core of four CO₂ ligands around the MO₂⁺ ion (M = Nb, Ta). A significant increase in the intermolecular bond distances for the second coordination sphere ligands coincides with a decrease in the calculated binding energies. Velocity map imaging is employed to explore the rich photodissociation dynamics of VO in the vicinity of C⁴Σ- - X⁴Σ-(v',0) vibronic transitions in VO. The final quantum state distribution was observed to be strongly dependent on the intermediate vibronic state of VO via which the dissociation threshold is reached. This work provides a refined value for the VO dissociation energy of D₀(VO) = 53190 ± 261 cm^-1 in excellent agreement with available literature.

The technology employed by human beings for the generation, storage and usage of energy is presently undergoing the fastest and most profound change since the industrial revolution. The changes in the generation and usage of energy necessitate the development of new methods of energy storage. In these systems, electrochemical energy storage will play a crucial role and to this end new electrolytes need to be explored to complement these changes. One such class of liquids is ionic liquids, a class of salts that are molten at room temperature. These liquids have a broad applicability to batteries and supercapacitors. This thesis details work where molecular dynamics simulations have been used to explore the nanostructure of ionic liquids and their mixtures with various molecular solvents at simplistic electrodes. The thesis has two broad sections. The first is covered in Chapter 3, and explores the nanostructure of ionic liquid propylene carbonate solutions, developing a framework through which these nanostructures can be understood. The section concludes that the increasing dilution of ionic liquids decreases the surface charge at which the characteristic ionic liquid oscillatory interfacial structure gives way to a different structure featuring monotonic charge decay. The behaviour of ionic liquids at interfaces is found to be correlated to ion size and type, as well as concentration. A wide divergence in the observed behaviour is shown at positive and negative electrodes due to the asymmetry of propylene carbonate. The second section, consisting of two chapters, explores the interfacial nanostructure of solvate ionic liquids using two different boundary conditions to model the electrode. This work is the first simulation of solvate ionic liquids at electrified interfaces. This section will explore the effect of electrode model on the behaviour of these ionic liquids at the electrode. Chapter 4 uses a fixed charge electrode, whereas Chapter 5 uses one with a fixed potential. The section concludes that regardless of electrode model, the idealised portrait of a solvate ionic liquid - one where the liquid behaves exactly as an aprotic ionic liquid - is not applicable. In Chapter 4's exploration of fixed charged electrodes, the formation of 2 glyme to lithium complexes contradicts the idealised portrait of the liquid. A different change is observed in Chapter 5's exploration of fixed potential electrodes, with both lithium glyme and lithium anion clusters forming at the interface. The key difference between the two studies is that lithium does not coordinate to the electrode in the fixed charge simulations, while in the fixed potential case it does. At the end of Chapter 5 the results are compared against experimental data, with the efficacy of the two models discussed. The aim of both studies is to look at the nanostructure of ionic liquids, when the symmetry between co-ion and cation repulsion - and related effects - is broken by the presence of a non ionic constituent in the liquid.

The encapsulation of a small molecule inside a fullerene cage through advances in synthetic chemistry have created a new platform to study the dynamics of a freely rotating and translating quantum rotors entrapped inside a symmetric cage potential. These endohedral fullerene complexes are of great interest because the fullerene cages uniquely provide the entrapped molecules a high level of isolation, homogeneity, symmetry and stability. The endohedral fullerene complexes discussed in this thesis are the H2@C60, H2@C70 and H2O@C60. Both variants of small molecules studied in this thesis, H2 and H2O, exhibits spin isomerism, where the spins of both protons in the molecule are able to combine either symmetrically with total spin I=1 (ortho) or anti-symmetrically with total spin I=0 (para). The H2@C60 is the union between the simplest molecule and the most symmetrical molecule in the universe. This thesis discusses the temperature dependence of cold neutron scattering study in this complex to investigate the statistical distribution of the energy states. The H2@C70 is a less symmetric endohedral fullerene which has a prolate ellipsoidal symmetry cage. This thesis discusses the low temperature thermal neutron scattering and the temperature dependence of cold neutron scattering investigations in the complex to study the effect of the ellipsoidal cage on the quantum dynamics of the molecules. H2O@C60 is different to the dihydrogen variant of the small molecule endohedral fullerenes because H2O has a permanent electric dipole moment and is less symmetric than H2. The quantum dynamics of the H2O@C60 is investigated using low temperature thermal neutron scattering, temperature dependence cold neutron scattering and milli-Kelvin NMR. Unlike the dihydrogen endohedral fullerenes, the H2O@C60 also exhibits slow nuclear spin-isomer conversion at low temperatures. This low temperature ortho-H2O to para-H2O conversion process is investigated with both INS and NMR to study the conversion mechanism.

This thesis presents a theoretical and experimental investigation of the interaction of electronically excited Rydberg hydrogen atoms with metal surfaces and the associated charge-transfer process. As a Rydberg atom approaches a metal surface, the energies of the Rydberg states are perturbed by the surface potential generated by the image charges of the Rydberg electron and core. At small atom-surface separations, the Rydberg atom may be ionised by resonant charge transfer of the Rydberg electron to the continuum of delocalised unoccupied metal states, with which the Rydberg electron is degenerate in energy. Typically, this ‘surface ionisation’ can be measured by extracting the remaining positively charged ion-cores with externally applied electric fields. By applying various levels of theory, from classical to fully time-dependent quantum calculations, this thesis explores various experimentally relevant effects on the charge-transfer process, such as the magnitude and direction of the externally applied electric field, the atom collisional velocity, the presence of local surface stray fields and electronically structured surfaces. The theoretical results give insight into the previous experimental work carried out for the xenon atom and hydrogen molecule, and point out some of the fundamental differences from the hydrogen atom system. Experiments involving Rydberg hydrogen atoms incident on an atomically flat gold surface, a rough machined aluminium surface and a single crystal copper (100) surface are presented, providing for the first time the opportunity to make a quantitative comparison of theory and experiments. The ability to control the critical distance at which charge-transfer occurs is demonstrated by using Rydberg states of varying dimensions and collisional velocities. By changing the collisional angle of the incident Rydberg beam, the effect of Rydberg trajectory is also investigated. By manipulating the polarisation of the Rydberg electron with electric fields, genuine control over the orientation of the electron density distribution in the charge-transfer process is demonstrated. This property was predicted by the theory and should be unique to the hydrogen atom due to its intrinsic symmetry. By reversing the direction of the electric field with respect to the metal surface, electrons rather than positive ions are detected, with ionisation dynamics that appear to be very different, as predicted by quantum calculations. Experiments involving the single crystal Cu(100) surface also suggests possible resonance effects from image states embedded in the projected bandgap which are shown from quantum calculations to play an important role in the surface charge transfer of electronically structured metal substrates. The experimental technique developed in this work provides some exciting future applications to study quantum confinement effects with thin films, nanoparticles and other bandgap surfaces. The ability to control the Rydberg orbital size, electronic energy, collisional velocity and orientation in the charge-transfer process will provide novel ways of probing the surface’s electronic and physical structure, as well as being a valuable feature in offering new opportunities for controlling reactive processes at metallic surfaces.

The measurement of radical species in the atmosphere has far reaching implications. For example, it is necessary to both understand and improve our knowledge of radicals in the atmosphere to better inform the models which in many cases are the best way of predicting future air quality and climate change. Although many of these models are often not fully representative of all the processes occurring, they are the current best estimate based on the knowledge available, and can be useful in informing and directing future policy. The numerous, varied and interlinked cycles in the atmosphere are complex and only by obtaining data on specific species can accurate concentrations be retrieved and fed back into the models to improve their accuracy. This work is concerned with the development and application of an ultrasensitive absorption spectroscopy technique to the problem of detection of the peroxy radical, HO₂. Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS) combines cavity enhancement techniques (in order to increase the path length) with frequency and wavelength modulation techniques (in order to reduce the noise). Following a discussion of the current detection methods used by atmospheric scientists to accurately measure and quantitative concentrations, some preliminary work on the detection of ammonia by a simple cavity enhanced absorption setup is presented. Pressure broadening and shift results were obtained for a number of ammonia transitions in the near infrared region, broadened by He, Ne, Ar, Xe, O₂ and N₂. The bulk of the work concentrates on the implementation of the NICE-OHMS technique, presenting the first results with the use of an external cavity diode laser and a ring shaped cavity. A sensitivity of 4 x 10⁻¹¹ cm⁻¹ Hz⁻^1/2 is obtained on an individual rovibrational transition of methane at 6610.063 cm⁻¹, along with a selection of other data from the atmospherically important molecules methane, nitrogen dioxide and carbon dioxide, highlighting the broad wavelength range over which the instrument can operate. Finally, the NICE-OHMS technique is used to probe HO₂ radicals formed through the photolysis of a Cl₂/CH₃OH/O₂ mixture. Following the creation and detection of HO₂ radicals in the cavity, and based on the optimum sensitivity outlined above, a minimum concentration of 1 x 10⁹ molecules cm⁻³ has been demonstrated.

In recent years there has been much progress in the field of cold and ultracold molecular physics and a variety of experimental techniques for producing cold matter now exist. In particular, the generation of trapped molecular ions at mK temperatures has been achieved by sympathetic-cooling with laser-cooled atomic ions. By implementing schemes to selectively prepare and control the internal quantum state of molecular ions, and developing detection techniques, it will be increasingly possible to study cold state-selected chemical collisions in an ion-trap. Most molecular species produced in a selected rovibrational state have a lifetime of a few seconds, before the population is redistributed across numerous rovibrational states by interaction with the ambient blackbody radiation (BBR). Consequently, the investigation of state-selected reaction dynamics at low temperatures in experiments where long time scales (minutes to hours) are required, is hindered. This thesis looks into developing strategies that maintain state selection in molecular ions, allowing one to observe state-selected reactions in cold environments, in particular the state-selected reaction between C₂H⁺₂ and ND₃. Examining reactive ion molecule collisions under cold conditions provides insight into fundamental reaction dynamics, which are thermally averaged out at higher temperatures. A theoretical model is used to investigate laser-driven, blackbody-mediated, rotational cooling schemes for several ¹Σ and ²Π diatomic species. The rotational cooling is particularly effective for DCl⁺ and HCl⁺, for which 92% and >99% (respectively) of the population can be driven into the rovibrational ground state. For the other systems a broadband optical pumping source is found to enhance the population that can be accumulated in the rovibrational ground state by up to 29% more than that achieved when exciting a single transition. The influence of the rotational constant, dipole moments and electronic state of the diatomics on the achievable rotational cooling is also studied. This approach is extended to consider the BBR interaction and rotational cooling of a linear polyatomic ion, C₂H⁺₂, which has a ²Π electronic ground state. The (1-0) band of the ν₅ cis-bending mode is infrared active and strongly overlaps the 300 K blackbody spectrum. Hence the lifetimes of state-selected rotational levels are found to be short compared to the typical timescale of ion trapping experiments. Laser cooling schemes are proposed that could be experimentally viable, which involves simultaneous pumping of a set of closely spaced Q-branch transitions on the ²Δ_5/2-²Π_3/2 band together with two ²Σ⁺– ²Π_1/2 lines. It is shown that this should lead to >70% of total population in the lowest rotational level at 300 K and over 99% at 77 K. In order to identify states of the acetylene ion that could be trapped sufficiently long enough for state-selected reactions in the ion trap with decelerated ND3, the theoretical work has been complemented by experimental investigations into the production of C₂H⁺₂ in selected states, and ion trapping of the same using sinusoidal and digital trapping voltages. Appropriate (2+1) REMPI (Resonance Enhanced Multiphoton Ionization) schemes are used to produce C₂H⁺₂ in different quantum states, with (1+1) Resonance Enhanced Multiphoton Dissociation (REMPD) employed to detect the ion thus produced. The concept of digital ion trapping for ejection onto MCPs is introduced. A comprehensive comparison between sinusoidal and digital trapping fields has been performed with respect to trap depth and stability regions. Programs have been developed to calculate the stability regions for different ions under varying experimental conditions. The trap depth has been derived for both digital and sinusoidal trapping fields. It is observed that as τ increases, the trap depth of a digital trap increases. For τ = 0.293, the trap depth and stability diagram for both sinusoidal and digital trapping fields would be equivalent. The trap depth at which the sinusoidal trap operates experimentally in our research group is ~1.36 eV. In contrast, the experimental parameters at which the digital trap operates generates a trap depth of 1.21 eV. Ca⁺ Coulomb crystals have been formed, stably trapped and stored for extended periods of time in both sinusoidally and digitally time-varying trapping fields. The sympathetic cooling of a diverse range of ions into Ca⁺ Coulomb crystals is demonstrated, again using both sinusoidal and digital trapping fields. Mass spectrometric detection of ionic reaction products using a novel ejection scheme has been developed, where ejection is achieved by switching off the trapping voltage and converting the quadrupole trap into an extractor-repeller pair by providing the ion trap electrodes with appropriate ejection pulses. This technique is developed using a digital trapping voltage rather than the sinusoidal trapping voltage, as ejection with sinusoidal trapping voltages is not clean (resonance circuitry used in the electronics induces ringing after switching off the trapping voltage). Coulomb crystals, both pure Ca⁺ and multi-component crystals, are ejected from the ion trap and the TOF trace obtained is recorded on an oscilloscope. When the integrated, base-line subtracted TOF peak is plotted against the number of ions in a Ca+ crystal and sympathetically-cooled Ca⁺ – CaF⁺ crystal, a linear relationship is obtained. This technique is found to be well mass-resolved, with the signal arising from CaOH⁺ (57 amu) and CaOD⁺ (58 amu) resolvable on the TOF trace. This technique would enable one to monitor a reaction in a Coulomb crystal where the reactant and product species are both either lighter or heavier than calcium, such as the reaction between C₂H⁺₂ and ND₃, something which has not been previously possible. It is, also, potentially a very important technique for reactions with many product channels.

SPM, optical microscopy, and electrochemical techniques are used to study a range of electrochemical deposition processes on carbon electrodes, particularly those associated with diffusion-controlled multiple nucleation. Anodic stripping voltammetry for analytical measurements using solid electrodes is addressed in the light of limitations arising from electrode heterogeneity, electrode morphology, inhibited electrodeposition, and incomplete stripping of deposited metal. It is shown, using direct imaging of electrode surfaces, that each of the preceding factors may produce significant deviations from ideal electrode behaviour. The electrochemical nucleation of silver on BDD is examined. Data are obtained for the nucleation rate by interpretation of the deposition voltammetry, and by inspection of in-situ optical microscopic images. The particle distributions are analyzed and a stochastic model of nucleation developed. A model for the potentiostatic nucleation and three dimensional growth of deposits on an electrode surface under hydrodynamic conditions is examined. A wall-tube and stirred cell are used to generate conditions in which the diffusion layer thickness is in the range 10 – 40 μm. It is shown that the model provides excellent fits to the experimental data. A previously unrecognised correlation between the morphology of the PbO2 deposits and their electrocatalytic activity is established. The morphology of the films are observed as a function of time and potential using in-situ AFM. Nanotrench arrays are fabricated on HOPG surfaces. Cyclic voltammetry in simple redox couples is used to provide experimental evidence that the voltammetric response of a graphite electrode is solely due to the edge plane sites, with the basal plane sites having no measurable contribution. Nanotrenches are used as templates in a simple method for generating random assemblies of metal nanobands. This method is shown to be effective for generating gold, silver and copper nanowires. The electrochemical properties of the array are investigated via cyclic voltammetry.

The research described in this thesis employs a variety of Electron Paramagnetic Resonance (EPR) techniques for the study of the electronic and structural properties of artificial supramolecular porphyrin systems and of protein complexes of biological relevance. The electron delocalisation in the cationic radical and photoexcited triplet states of linear and cyclic Π-conjugated multiporphyrin arrays was investigated. In the radical cations, information on the extent of delocalisation can be inferred from the measurement of hyperfine couplings, either indirectly from the continuous wave EPR spectrum or directly using pulsed hyperfine EPR techniques. The results of room temperature EPR experiments showed complete delocalisation of the electron on the timescale of the EPR experiments, but frozen solution EPR measurements revealed localisation onto mainly two to three porphyrin units in the larger porphyrin systems. Information on the delocalisation of the triplet state in the same porphyrin systems is contained both in the hyperfine couplings and in the zero-field splitting (ZFS) interaction. The results outlined in this thesis show that the hyperfine couplings provide a much more accurate estimate of the extent of delocalisation. The trends in proton and nitrogen hyperfine couplings with the size of the porphyrin systems indicate uneven spin density distributions over the linear arrays, but complete delocalisation in the cyclic systems. Time-resolved EPR and magnetophotoselection experiments have shown a reorientation of the zero-field splitting tensor between a single porphyrin unit and longer linear arrays, resulting in alignment of the main optical transition moment and the Z axis of the ZFS tensor. Continuous wave and pulsed dipolar EPR techniques were employed for the determination of the structure of two different protein complexes, the homomultimeric twin-arginine translocase A (TatA) protein channel and the ferredoxin-P450 complex of the electron transport chain in Novosphingobium aromaticivorans. The interaction between nitroxide spin labels introduced at different positions of the TatA monomer was investigated in the complex reconstituted in detergent micelles by analysing the dipolar broadening of the EPR spectra and the results of three- and four-pulse Double Electron-Electron Resonance (DEER) measurements. In combination with results from NMR and molecular dynamics calculations, a structure for the channel complex was proposed. The structure of the ferredoxin-cytochrome P450 complex was investigated by orientation-selective DEER between nitroxide labels introduced on the cytochrome P450 protein and the iron-sulfur cluster of the ferredoxin. The distance and orientation information contained in the experimental DEER data was interpreted in terms of a structural model of the protein complex by orientation-selective DEER simulations combined with a modelling approach based on protein-protein docking.