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In this Tutorial series, we aim to provide an accessible introduction to vibrational sum frequency generation (VSFG) spectroscopy, targeted toward people entering the VSFG world without a rigorous formal background in optical physics or nonlinear spectroscopy. In this article, we discuss in detail the processes of collecting and processing VSFG data, and user-friendly processing software (sfgtools) is provided for use by people new to the field. Some discussion of analyzing VSFG spectra is also given, specifically with a discussion of fitting homodyne VSFG spectra, and a discussion of what can be learned (both qualitatively and quantitatively) from VSFG spectra. Published under an exclusive license by the AVS.

The aberrant folding of proteins into amyloid aggregates is associated with over 50 diseases. The amyloid aggregation of α-synuclein (αS), related to Parkinson’s disease, can be catalyzed by lipid-membrane surfaces. Despite the importance of lipid surfaces and several decades of structural studies, the 3D-structure of lipid-membrane bound αS is still not known in detail. In particular, there is little information about the self-assembly and orientation of αS when interacting with lipid surfaces under physiologically relevant conditions. Here, we report interface-specific vibrational sum-frequency generation (VSFG) experiments revealing how monomeric αS binds, folds and orients at anionic lipid membranes. Since VSFG is inherently surface specific, the experiments can be performed at high αS–lipid ratios, far beyond previous structural studies. To interpret the experimental VSFG data, we present an analysis method in which out-of-equilibrium molecular-dynamics simulations are used to generate a large amount of conformations, after which their spectral match is evaluated with excitonic amide-I calculations. Hereby, the previously-determined flat-lying helical structures are corroboratively derived at low αS concentrations, while at higher, physiological concentrations, a transition to interface-protruding αS structures occurs. Such an upright conformation promotes more extensive lateral interactions between monomers and may explain how, at high protein–lipid ratios, lipid membranes can catalyze the formation of αS amyloids.

Spectra and absorption coefficients of the forbidden 765 nm X

Diatoms use peptides based on the protein silaffin to fabricate their silica cell walls. To the interest of material scientists, silaffin peptides can also produce titanium dioxide nanoparticles. Peptide-based synthesis could present an environmentally friendly route to the synthesis of titanium dioxide nanomaterials with potential applications in water splitting and for biocompatible materials design. Two-dimensional nanomaterials have exceptional surface-to-volume ratios and are particularly suited for these applications. We here demonstrate how the silaffin peptide R5 can precipitate free-standing and self-supported sheets of titanium dioxide at the air-water interface, which are stable over tens of micrometers.

We study the primary photolysis dynamics of aqueous lactate induced by photo-excitation atλ= 200 nm. Our calculations indicate that both decarboxylation and dehydroxylation are energetically possible, but decarboxylation is favoured dynamically. UV pump - IR probe transient absorption spectroscopy shows that the photolysis is dominated by decarboxylation, whereas dehydroxylation is not observed. Analysis of the transient IR spectrum suggests that photo-dissociation of lactate primarily produces CO2and CH3CHOH−through the lowest singlet excited state of lactate, which has a lifetime ofτ= 11 ps. UV pump - VIS probe transient absorption spectroscopy of electrons from the dissociating lactate anion indicates that the anionic electron from the CO2˙−fragment is transferred to the CH3CHOH˙ counter radical during the decarboxylation process, and CO2˙−is consequently only observed as a minor photo-product. The photo-dissociation quantum yield after the full decay of the excited state is Φ(100ps) = 38 ± 5%.

The SARS coronavirus 2 (SARS-CoV-2) spike protein is located at the outermost perimeter of the viral envelope and is the first component of the virus to make contact with surrounding interfaces. The stability of the spike protein when in contact with surfaces plays a deciding role for infection pathways and for the viability of the virus after surface contact. While cryo-EM structures of the spike protein have been solved with high resolution and structural studies in solution have provided information about the secondary and tertiary structures, only little is known about the folding when adsorbed to surfaces. We here report on the secondary structure and orientation of the S1 segment of the spike protein, which is often used as a model protein for in vitro studies of SARS-CoV-2, at the air-water interface using surface-sensitive vibrational sum-frequency generation (SFG) spectroscopy. The air-water interface plays an important role for SARS-CoV-2 when suspended in aerosol droplets, and it serves as a model system for hydrophobic surfaces in general. The SFG experiments show that the S1 segment of the spike protein remains folded at the air-water interface and predominantly binds in its monomeric state, while the combination of small-angle X-ray scattering and two-dimensional infrared spectroscopy measurements indicate that it forms hexamers with the same secondary structure in aqueous solution.

The presence of molecular oxygen in organic materials designed for use in photoresponsive devices (e.g., solar cells, photon up-converters) can adversely influence the performance of the device in several ways. The most important of these is arguably through reactions that oxygenate and/or oxidize the organic components and thereby change properties relevant for a functioning device. The ground electronic state of molecular oxygen is a spin triplet, O2(X3Σg−). As such, it behaves as a biradical in its chemical reactions, trapping adventitious organic free radicals to yield reactive peroxyl radicals. The lowest excited electronic state of molecular oxygen is a spin singlet, O2(a1Δg), and can be formed in appreciable yield by energy transfer from a photoexcited organic molecule to O2(X3Σg−). Functional groups common to molecules used in photoresponsive materials (e.g., double bonds, sulfides) can react with O2(a1Δg) to form peroxides, which likewise are reactive and propagate disruption. The quenching of an excited-state organic component by O2(X3Σg−) also points to other ways in which oxygen can influence device performance. For example, the oxygen-mediated deactivation of comparatively long-lived triplet states can adversely influence the energy fusion process essential for some up-conversion devices. Likewise, electron transfer from an excited-state organic molecule to O2(X3Σg−) and/or O2(a1Δg) can not only interfere with desired charge movement (e.g., in a photovoltaic device), but it will also produce the superoxide radical ion that, in turn, can contribute to oxygenation reactions. Thus, to exploit fully the functional capabilities of photoresponsive organic materials and to prolong device longevity, and in lieu of completely excluding oxygen, it is necessary to monitor, understand, and ultimately control the behavior of oxygen in such systems.

Interfaces between bulk media are often where critical molecular processes occur that can dictate the chemistry of an entire macroscopic system. Optical spectroscopy such as IR or Raman spectroscopy is often challenging to apply to interfaces due to contributions from bulk phases that dominate the spectra, masking any detail about the interfacial layer. Vibrational sum frequency generation (VSFG) spectroscopy is a nonlinear spectroscopy that allows vibrational spectra of molecules at interfaces to be directly measured. This Tutorial series is aimed at people entering the VSFG world without a rigorous formal background in optical physics or nonlinear spectroscopy. In this article, we present the fundamental theory of VSFG spectroscopy, with a focus on qualitative, intuitive explanation of the relevant physical phenomena, with minimal mathematics, to enable a newcomer to VSFG spectroscopy to quickly become conversant in the language and fundamental physics of the technique.

Photochemical and mechanical stability are critical in the production and application of organic solar cells. While these factors can individually be improved using different additives, there is no example of studies on the combined effects of such additive-assisted stabilization. In this study, the properties of PTB7:[70]PCBM organic solar cells are studied upon implementation of two additives: the carotenoid astaxanthin (AX) for photochemical stability and the silicone polydimethylsiloxane (PDMS) for improved mechanical properties. A newly designed additive, AXcPDMS, based on astaxanthin covalently bonded to PDMS was also examined. Lifetime tests, produced in ISOS-L-2 conditions, reveal an improvement in the accumulated power generation (APG) of 10% with pure AX, of 90% when AX is paired with PDMS, and of 140% when AXcPDMS is added in the active layer blend, as compared to the control devices. Singlet oxygen phosphorescence measurements are utilized to study the ability of AX and AXcPDMS to quench singlet oxygen and its precursors in the films. The data are consistent with the strong stabilization effect of the carotenoids. While AX and AXcPDMS are both efficient photochemical stabilizers, the improvement in device stability observed in the presence of AXcPDMS is likely due to a more favorable localization of the stabilizer within the blend. The mechanical properties of the active layers were investigated by tensile testing and cohesive fracture measurements, showing a joint improvement of the photooxidative stability and the mechanical properties, thus yielding organic solar cell devices that are promising for flexible photovoltaic applications.

The fabrication of inorganic nanomaterials is important for a wide range of disciplines. While many purely inorganic synthetic routes have enabled a manifold of nanostructures under well-controlled conditions, organisms have the ability to synthesize structures under ambient conditions. For example, magnetotactic bacteria, can synthesize tiny ‘compass needles’ of magnetite (Fe3O4). Here, we demonstrate the bio-inspired synthesis of extended, self-supporting, nanometer-thin sheets of iron oxide at the water–air interface through self-assembly using small histidine-rich peptides.

Genetically encodable proteins that photosensitize the production of singlet oxygen, O2(a1Δg), will play an increasingly important role in elucidating mechanisms of cellular processes modulated by reactive oxygen species, ROS, and changes in redox balance. In the development of such tools, it is essential to characterize the oxygen-dependent photophysics of the protein-encased chromophore. Of the O2(a1Δg)-photosensitizing systems recently developed, a protein-bound derivative of Malachite Green has several desirable features: (1) it absorbs light at wavelengths longer than those typically absorbed by endogenous molecules, and (2) the chromophore becomes a viable sensitizer only when bound to the activating protein. However, we now demonstrate that the photophysics of this Malachite Green system is not simple. Our data indicate that, with an increase in the concentration of ground-state oxygen, O2(X3Σg−), the yield of O2(a1Δg) does not increase in a proportional manner. Moreover, the lifetime of O2(a1Δg) decreases as the O2(X3Σg−) concentration is increased. One mechanism that could account for our observations involves the concomitant photo-initiated formation of O2(a1Δg) and the superoxide radical anion. We propose that the superoxide ion acts as a dynamic diffusion-dependent quencher to influence the O2(a1Δg) lifetime and as a static quencher within the protein enclosure to influence the measured O2(a1Δg) yield. Thus, in the least, caution should be exercised when using this Malachite Green system to probe mechanisms of ROS-mediated processes. Our results contribute to a better understanding of the general photophysics of protein-bound O2(a1Δg) sensitizers which, in turn, facilitates the further development of these useful mechanistic tools.

Ice-nucleation active (INA) bacteria can promote the growth of ice more effectively than any other known material. Using specialized ice-nucleating proteins (INPs), they obtain nutrients from plants by inducing frost damage and, when airborne in the atmosphere, they drive ice nucleation within clouds, which may affect global precipitation patterns. Despite their evident environmental importance, the molecular mechanisms behind INP-induced freezing have remained largely elusive. We investigate the structural basis for the interactions between water and the ice-nucleating protein InaZ from the INA bacterium Pseudomonas syringae. Using vibrational sum-frequency generation (SFG) and two-dimensional infrared spectroscopy, we demonstrate that the ice-active repeats of InaZ adopt a β-helical structure in solution and at water surfaces. In this configuration, interaction between INPs and water molecules imposes structural ordering on the adjacent water network. The observed order of water increases as the interface is cooled to temperatures close to the melting point of water. Experimental SFG data combined with molecular-dynamics simulations and spectral calculations show that InaZ reorients at lower temperatures. This reorientation can enhance water interactions, and thereby the effectiveness of ice nucleation., Ice-nucleating proteins promote ice formation at high sub-zero temperatures, but the mechanism is still unclear. The authors investigate a model ice-nucleating protein at the air-water interface using vibrational sum frequency generation spectroscopy and simulations, revealing its reorientation at low temperatures, which increases contact with water molecules and promotes their ordering.

Fluorescein, and derivatives of fluorescein, are often used as fluorescent probes and sensors. In systems where pH is a variable, protonation/deprotonation of the molecule can influence the pertinent photophysics. Fluorination of the xanthene moiety can alter the molecule&rsquo, s pKa such as to render a probe whose photophysics remains invariant over a wide pH range. Di-fluorination is often sufficient to accomplish this goal, as has been demonstrated with compounds such as Oregon Green in which the xanthene moiety is symmetrically difluorinated. In this work, we synthesized a non-symmetrical difluorinated analog of Oregon Green which we call Athens Green. We ascertained that the photophysics and photochemistry of Athens Green, including the oxygen-dependent photophysics that results in the sensitized production of singlet oxygen, O2(a1&Delta, g), can differ appreciably from the photophysics of Oregon Green. Our data indicate that Athens Green will be a more benign fluorescent probe in systems that involve the production and removal of O2(a1&Delta, g). These results expand the available options in the toolbox of fluorescein-based fluorophores.

Ice-nucleation active (INA) bacteria can promote the growth of ice more effectively than any other known material. Utilizing specialized ice-nucleating proteins (INPros), they obtain nutrients from plants by inducing frost damage and, when airborne in the atmosphere, they drive ice nucleation within clouds and may affect global precipitation patterns. Despite their evident environmental importance, the molecular mechanisms behind INPro-induced freezing have remained largely elusive. In the present study, we investigated the folding and the structural basis for interactions between water and the ice-nucleating protein InaZ from the INA bacteriumPseudomonas syringaestrain R10.79. Using vibrational sum-frequency generation and two-dimensional infrared spectroscopy, we demonstrate that the ice-active repeats of InaZ adopt a β-helical structure in solution and at water surfaces. In this configuration, hydrogen bonding between INPros and water molecules imposes structural ordering on the adjacent water network. The observed order of water increases as the interface is cooled to temperatures close to the melting point of water. Experimental SFG data combined with spectral calculations and molecular-dynamics simulations shows that the INPro reorients at lower temperatures. We suggest that the reorientation can enhance order-inducing water interactions and, thereby, the effectiveness of ice nucleation by InaZ.

The production of reactive oxygen species, ROS, plays an important role in light-mediated oxidative stress. As a tool to study general mechanisms of stress and eustress, and associated cellular signaling pathways, light allows for spatial and temporal control of ROS production in the heterogeneous environment of a living cell. Light is particularly suited for the selective initial production of singlet molecular oxygen at the expense of the superoxide radical anion and vice versa. With fluorescent probes, light is also a valuable tool used to monitor the space- and time-dependent effects of stress on cell physiology, as well as selected reactive intermediates pertinent to oxidative stress.

Fluorescein, and derivatives of fluorescein, are often used as fluorescent probes and sensors. In systems where pH is a variable, protonation/deprotonation of the molecule can influence the pertinent photophysics. Fluorination of the xanthene moiety can alter the molecule’s pKa such as to render a probe whose photophysics remains invariant over a wide pH range. Di-fluorination is often sufficient to accomplish this goal, as has been demonstrated with compounds such as Oregon Green in which the xanthene moiety is symmetrically difluorinated. In this work, we synthesized a non-symmetrical difluorinated analog of Oregon Green which we call Athens Green. We ascertained that the photophysics and photochemistry of Athens Green, including the oxygen-dependent photophysics that results in the sensitized production of singlet oxygen, O2(a1∆g), can differ appreciably from the photophysics of Oregon Green. Our data indicate that Athens Green will be a more benign fluorescent probe in systems that involve the production and removal of O2(a1∆g). These results expand the available options in the toolbox of fluorescein-based fluorophores. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.

In this Tutorial series, we aim to provide an accessible introduction to vibrational sum frequency generation (VSFG) spectroscopy, targeted toward people entering the VSFG world without a rigorous formal background in optical physics or nonlinear spectroscopy. In this article, we describe in depth how a broadband VSFG spectrometer is designed and constructed, using the instrument in SurfLab, Aarhus University, as an illustrative case. Detailed information about specific instrumentation (together with reasons why things are the way they are) is given throughout. This information is often omitted in other descriptions of such instrumentation and so will be invaluable to people new to the field.

At several points in this article, we referred to an incorrect laser power. Specifically, for our work with cells, we stated the laser power delivered to the microscope (700 mW) instead of the more relevant number of the power measured at the sample on the microscope stage (90 mW). Thus 700 mW should be replaced by 90 mW at the following places in the text: 1. page 5425, line 12 of the right-hand column 2. page 5426, caption of Figure 4a,b 3. page 5427, line 40 of the right-hand column 4. page S14 of the Supporting Information, just after equation S8 This difference in power translates to a steady-state O 2 (a 1Δ g) concentration of 9.5 nM and a total of ∼5 × 10 11 O 2 (a 1Δ g) molecules produced over the 30 min irradiation period (page 5427, line 42 of the right-hand column). This correction does not influence the thrust and conclusions of our paper.

Uric acid and/or its monoanion has long been used as chemical-trapping agents to demonstrate the presence of singlet oxygen, O2(a1Δg), in aqueous systems. “Oxidative bleaching” of uric acid, generally monitored through changes in the uric acid absorption spectrum, is often used in support of claims for the intermediacy of O2(a1Δg). The bleaching of uric acid has also been used to quantify photosensitized O2(a1Δg) yields in selected systems. Unfortunately, experiments performed to these ends often neglect processes and phenomena that can influence the results obtained. For the present study, we experimentally examined the behavior of uric acid under a variety of conditions relevant to the photoinitiated creation and subsequent removal of O2(a1Δg). Although the oxidative destruction of uric acid can indeed be a useful tool in some cases, we conclude that caution must be exercised such as not to incorrectly interpret the data obtained.

The localized surface plasmon of a metal nanoparticle can influence the optical properties of a molecule in the plasmon field. In a previous study of molecular oxygen adjacent to nanodisks on a flat substrate, we showed that a plasmon field can increase the probability of the O2(a1Δg) → O2(X3Σg–) radiative transition at 1275 nm. For the present study, we set out to ascertain if metal nanoparticles suspended in a liquid solvent could likewise induce measurable plasmonic effects on optical transitions in oxygen. Metal nanoparticles were prepared with the intent of selectively perturbing the 765 nm O2(X3Σg–) → O2(b1Σg+) absorption transition. Because O2(b1Σg+) efficiently decays to O2(a1Δg), we used the spectrally distinct O2(a1Δg) → O2(X3Σg–) phosphorescent transition at 1275 nm to probe the potential plasmon effects at 765 nm. Although we indeed observed nanoparticle-mediated effects on the O2(X3Σg–) → O2(b1Σg+) transition, our present data are readily explained in terms of a nanoparticle-dependent change ...

We present a simple and inexpensive liquid surface height controller that can monitor and maintain the height of a liquid surface in a surface-sensitive experiment. The system is based on a commercial laser pointer, universal serial bus webcam, syringe pump, and homemade control software. The system can sense changes in the height of the surface of ±1 µm, and the maximum range of the device without readjustment is around 2.5 mm. The intended use of the device is to maintain the height of a sample at the air-water interface in a sum-frequency generation spectroscopy measurement, which constantly changes due to water evaporation. A demonstration of the system maintaining the height of a water surface to a tolerance of ±5 µm over a period of 8 h is shown to illustrate the stability of a system controlled by this device.

ConspectusSinglet oxygen, O2(a1Δg), the lowest excited electronic state of molecular oxygen, is an omnipresent part of life on earth. It is readily formed through a variety of chemical and photochemical processes, and its unique reactions are important not just as a tool in chemical syntheses but also in processes that range from polymer degradation to signaling in biological cells. For these reasons, O2(a1Δg) has been the subject of intense activity in a broad distribution of scientific fields for the past ∼50 years.The characteristic reactions of O2(a1Δg) kinetically compete with processes that deactivate this excited state to the ground state of oxygen, O2(X3Σg–). Moreover, O2(a1Δg) is ideally monitored using one of these deactivation channels: O2(a1Δg) → O2(X3Σg–) phosphorescence at 1270 nm. Thus, there is ample justification to study and control these competing processes, including those mediated by solvents, and the chemistry community has likewise actively tackled this issue.In themselves, the solv...

The oxidation of lipids is an important phenomenon with ramifications for disciplines that range from food science to cell biology. The development and characterization of tools and techniques to monitor lipid oxidation are thus relevant. Of particular significance in this regard are tools that facilitate the study of oxidations at interfaces in heterogeneous samples (e.g., oil-in-water emulsions, cell membranes). In this article, we establish a proof-of-principle for methods to initiate and then monitor such oxidations with high spatial resolution. The experiments were performed using oil-in-water emulsions of polyunsaturated fatty acids (PUFAs) prepared from cod liver oil. We produced singlet oxygen at a point near the oil-water interface of a given PUFA droplet in a spatially localized two-photon photosensitized process. We then followed the oxidation reactions initiated by this process with the fluorescence-based imaging technique of structured illumination microscopy (SIM). We conclude that the approach reported herein has attributes well-suited to the study of lipid oxidation in heterogeneous samples.

Recent efficiency records of organic photovoltaics (OPV) highlight stability as a limiting weakness. Incorporation of stabilizers is a desirable approach for inhibiting degradation-it is inexpensive and readily up-scalable. However, to date, such additives have had limited success. We show that β-carotene (BC), an inexpensive and green, naturally occurring antioxidant, dramatically improves OPV stability. When compared to nonstabilized reference devices, the accumulated power generation of PTB7:[70]PCBM devices in the presence of BC increases by an impressive factor of 6, due to stabilization of both the burn-in and the lifetime, and by a factor of 21 for P3HT:[60]PCBM devices, owing to a longer lifetime. Using electron spin resonance and time-resolved near-IR emission spectroscopies, we probed radical and singlet oxygen concentrations. We demonstrate that singlet oxygen sensitized by [70]PCBM causes the "burn-in" of PTB7:[70]PCBM devices and that BC effectively mitigates it. Our results provide an effective solution to the problem that currently limits widespread use of OPV.

Mr4511 from Methylobacterium radiotolerans is a 164 amino acid protein built of a flavin mononucleotide (FMN) binding, blue-light responsive LOV (Light, Oxygen, Voltage) core domain plus flanking regions. In contrast to the majority of LOV domains, Mr4511 lacks a tryptophan residue that was previously identified as a major quencher for the FMN triplet state in photosensitizers for singlet oxygen (SO) engineered from these photoreceptors. Here we show that for Mr4511 it is sufficient to only mutate the reactive cysteine responsible for the photocycle (Cys71) in the native protein to generate an efficient SO photosensitizer: both C71S and C71G variants exhibit SO quantum yields of formation, ΦΔ, around 0.2 in air-saturated solutions. Under oxygen saturated conditions, ΦΔ reaches ∼0.5 in deuterated buffer. The introduction of Trp112 in the canonical position for LOV domains dramatically lowers ΦΔ to values comparable to miniSOG, one of the early FMN binding proteins touted as a SO sensitizer. Besides its SO properties, Mr4511 is also exceedingly robust against denaturation with urea and is more photostable than free FMN.

Photo-initiated, oxygen-mediated degradation of the molecules in the active layer of organic photovoltaic, OPV, devices currently limits advances in the development of solar cells. To address this problem systematically and at a molecular level, it is informative to quantify the kinetics of the pertinent processes, both in solution phase and in solid films. To this end, we examined the oxygendependent photophysics and photochemistry of selected functionalized fullerenes, thiophenederivatives, and a subphthalocyanine commonly used in OPV devices. We find that the photosensitized production of singlet molecular oxygen, O2(a1 Δg), by these molecules is a key step in the degradation process. We demonstrate that the addition of either β-carotene or astaxanthin as antioxidants can inhibit degradation by a combination of three processes: (a) deactivation of O2(a1 Δg)to the oxygen ground state, O2(X3 Σg−), (b) quenching of the O2(a1 Δg) precursor, and (c)sacrificial reactions of the carotenoid with free radicals formed in the photo-initiated reactions. For OPV systems in which reaction with O2(a1Δg) contributes to the degradation, the first two of these processes are desired and should have appreciable impact in prolonging the longevity of OPV devicesbecause they do not result in a chemical change of the system.

Aeration of common liquid solvents (e.g., cyclohexane, benzene, or water) gives rise to a unique absorption band associated with a transition from a ground state oxygen-solvent collision complex to an oxygen-solvent charge-transfer (CT) state. Although the absorption band is solvent-dependent, the energies involved invariably correspond to a one-photon transition in UV. We now show that this CT state can also be produced in a nonlinear two-photon process using visible light. Moreover, the CT state decays to produce an appreciable amount of singlet oxygen, O

The photophysics of selected tungsten iodide clusters was examined with respect to their role as a photosensitizer for the production of singlet oxygen, O 2 (a 1 Δ g ). We examined all-iodo octahedral clusters, [W 6 I 8 (I 6 )] 2- , and ligand-substituted octahedral clusters, [W 6 I 8 (L 6 )] 2- , in which the ligand, L, occupies the outer apical positions surrounding the cluster core. We also examined a square-pyramidal cluster, [W 5 I 8 (I 5 )] ? , in which the tungsten core was presumably more accessible to diffusional encounter with ground state oxygen, O 2 (X 3 Σ g - ). For the compounds examined, we find pronounced cluster-dependent changes in the yield of photosensitized O 2 (a 1 Δ g ) production. In particular, although the iodine-encased octahedral cluster, [W 6 I 8 (I 6 )] 2- , is an efficient O 2 (a 1 Δ g ) sensitizer, the pyramidal cluster, [W 5 I 8 (I 5 )] ? , does not make O 2 (a 1 Δ g ) at all. The latter provides fundamental insight into the important case where the sensitizer triplet state is nearly degenerate with the O 2 (X 3 Σ g - )-O 2 (a 1 Δ g ) transition energy at 1 eV. Our data indicate that even with near resonance, energy transfer to form O 2 (a 1 Δ g ) will not occur within the 3 sensitizer-O 2 (X 3 Σ g - ) encounter pair if other more efficient channels for energy dissipation are available.

Over the years, singlet oxygen has traditionally (and often with great success) been produced in a photosensitized process (see Chap. 1). The very fact that the singlet oxygen precursor is an excited electronic state of a molecule allows for a great deal of control over when and where singlet oxygen is formed. This is particularly relevant if one wants to study biological systems where focused lasers and microscopes may be used to, for example, illuminate and perturb a specific sensitizer-incubated organelle of a cell.

The non-radiative deactivation of O2(a1∆g) and O2(\({\text{b}}^{1}\Sigma _{\text{g}}^{{^{ + } }}\)) by solvent molecules has been a topic of intense and extensive research for almost half a century [1, 2, 3, 4, 5, 6]. The seminal interest derived in part from an unusually large effect of solvent and solvent deuteration on the lifetime of O2(a1∆g) [3, 7].

Metal nanoparticles are an omnipresent source of interesting optical phenomena [1]. It has been known since before the Middle Ages that gold nanoparticle suspensions in glass can color the windows of a cathedral in almost all the colors of the rainbow [2]. Faraday was the first to scientifically investigate this phenomenon in the 18th century [3], but a deeper understanding of the optical behavior of nano-particles was first achieved when Mie solved Maxwell’s equations for a spherical particle in 1908 [4].

In contrast to the O2(b1\(\Sigma_{\text{g}}^{ + }\)) → O2(X3\(\Sigma_{\text{g}}^{ - }\)) and O2(a1∆g) → O2(X3\(\Sigma_{\text{g}}^{ - }\)) transitions, the O2(b1\(\Sigma_{\text{g}}^{ + }\)) → O2(a1∆g) transition is not forbidden by the selection rule for spin, only those regarding parity, symmetry, and angular momentum [1]. As such, this transition is presumably stronger and more readily detected in a spectroscopic experiment. Unfortunately, the transition falls in a spectral region (~1920 nm, ~5200 cm−1), where fast photomultiplier tubes generally do not function, and we have to rely on slow and insensitive semiconductor devices to detect the desired signal. Therefore, the inherently short lifetime of O2(b1\(\Sigma_{\text{g}}^{ + }\)) in solution limits the range of systems where O2(b1\(\Sigma_{\text{g}}^{ + }\)) → O2(a1∆g) fluorescence can be detected with time-resolution [2].

Our main light source is a femtosecond laser system described extensively in the literature. The main oscillator is a Ti:Sapphire laser (Spectra Physics Tsunami 3941) pumped by a continuous wave Nd:YVO4 laser (Spectra Physics Millenia V). The oscillator delivers ≈ 100 fs pulses at a repetition rate of 80 MHz, tunable over the range 730–900 nm. The pulse energy of the Tsunami is amplified by a factor of ≈10 in a regenerative amplifier (Spectra Physics Spitfire) pumped by a Q-switched Nd:YLF laser (Spectra Physics Evolution). The amplification process reduces the repetition rate to 1 kHz, and limits the tunable range to 765–840 nm while stretching the pulses to 100–150 fs. When other wavelengths are required, the Spitfire output can either be frequency-doubled in a BBO crystal or be used to pump an optical parametric amplifier (Spectra Physics OPA-900CF-1). This extends the spectral range to 300–3000 nm when combined with second or fourth harmonic generation of the signal or the idler beam.

The effect of 16 liquid solvents on both the spectrum and molar absorption coefficient of the X3Σg- → b1Σg+ transition in molecular oxygen has been examined. The ability to monitor this weak transition using air or oxygen saturated samples at atmospheric pressure was facilitated by the rapid and efficient O2(b1Σg+) → O2(a1Δg) transition, which allowed the use of O2(a1Δg) phosphorescence as a sensitive probe of O2(b1Σg+) production. The results of these O2(a1Δg) phosphorescence experiments are consistent with the results of independent experiments in which the O2(a1Δg) thus produced was "trapped" via a chemical reaction. The data recorded were used to calculate rate constants for the O2(b1Σg+) → O2(X3Σg-) radiative transition, a parameter that is otherwise difficult to directly obtain from such a wide range of solvents using O2(b1Σg+) → O2(X3Σg-) phosphorescence. The data show that the response of the O2(b1Σg+) → O2(X3Σg-) radiative transition to solvent is not the same as that of the O2(b1Σg+) → O2(a1Δg) and O2(a1Δg) → O2(X3Σg-) radiative transitions, both of which have been extensively examined over the years. However, our data are consistent with a theoretical model proposed by Minaev for the effect of solvent on radiative transitions in oxygen and, as such, arguably provide one of the final chapters in describing a system that has challenged the scientific community for years.