Guanine quadruplexes (GQs) are four‐stranded DNA/RNA structures exhibiting an important polymorphism. During the past two decades, their study by time‐resolved spectroscopy, from femtoseconds to ...milliseconds, associated to computational methods, shed light on the primary processes occurring when they absorb UV radiation. Quite recently, their utilization in label‐free and dye‐free biosensors was explored by a few groups. In view of such developments, this review discusses the outcomes of the fundamental studies that could contribute to the design of future optoelectronic biosensors using fluorescence or charge carriers stemming directly from GQs, without mediation of other molecules, as it is the currently the case. It explains how the excited state relaxation influences both the fluorescence intensity and the efficiency of low‐energy photoionization, occurring via a complex mechanism. The corresponding quantum yields, determined with excitation at 266/267 nm, fall in the range of (3.0–9.5) × 10−4 and (3.2–9.2) × 10−3, respectively. These values, significantly higher than the corresponding values found for duplexes, depend strongly on certain structural factors (molecularity, metal cations, peripheral bases, number of tetrads …) which intervene in the relaxation process. Accordingly, these features can be tuned to optimize the desired signal.
The primary processes induced in guanine quadruplexes (GQs) by direct UV absorption have been studied by time‐resolved spectroscopy, associated with computational methods. The present review highlights the outcomes of these fundamental studies that could contribute to the development of optoelectronic biosensors using GQ intrinsic signals (label‐free, dye‐free), explored recently by a few groups. It explains how certain structural factors, intervening during the excited state relaxation, affect both fluorescence emission and low‐energy photoionization generating charge carriers. Accordingly, these features can be tuned so that to optimize the desired signal.
Carbon nanodots (CNDs) synthesized from citric acid and formyl derivatives, that is, formamide, urea, or N‐methylformamide, stand out through their broad‐range visible‐light absorbance and ...extraordinary photostability. Despite their potential, their use has thus far been limited to imaging research. This work has now investigated the link between CNDs’ photochemical properties and their chemical structure. Electron‐rich, yellow carbon nanodots (yCNDs) are obtained with in situ addition of NaOH during the synthesis, whereas otherwise electron‐poor, red carbon nanodots (rCNDs) are obtained. These properties originate from the reduced and oxidized dimer of citrazinic acid within the matrix of yCNDs and rCNDs, respectively. Remarkably, yCNDs deposited on TiO2 give a 30% higher photocurrent density of 0.7 mA cm−2 at +0.3 V versus Ag/AgCl under Xe‐lamp irradiation (450 nm long‐pass filter, 100 mW cm−2) than rCNDs. The difference in overall photoelectric performance is due to fundamentally different charge‐transfer mechanisms. These depend on either the electron‐accepting or the electron‐donating nature of the CNDs, as is evident from photoelectrochemical tests with TiO2 and NiO and time‐resolved spectroscopic measurements.
Electron‐rich, yellow carbon nanodots (yCNDs) are obtained by the in situ addition of NaOH during synthesis, while otherwise electron‐poor, red CNDs (rCNDs) are formed. The physicochemical properties, including photoelectric performance, depend on the chemical structure, that is, either the reduced (electron donating) or oxidized (electron accepting) dimer of citrazinic acid in the polymeric citric acid matrix of yCNDs or rCNDs, respectively.
Photochemical hydrogen generation from aqueous solutions can be accomplished with a combination of at least three molecular components: namely, a photosensitizer, a hydrogen-evolving catalyst, and an ...electron donor. A parameter that plays a key role in the light to hydrogen efficiency of such three-component systems is the solution pH. While this evidence has been usually observed in several works aiming at identifying catalysts and optimizing their performances, detailed studies capable of shining light on this issue have been extremely rare. Hence, the pH dependence of a reference three-component system based on Ru(bpy)3 2+ (where bpy = 2,2′-bipyridine) as the sensitizer, a cobaloxime HEC, and ascorbic acid as the sacrificial donor has been studied with care by merging photocatalytic hydrogen evolution kinetic data and detailed time-resolved spectroscopy results. The photocatalytic activity shows a bell-shaped profile as a function of pH which peaks at around pH 5. While at acidic pH (pH <5) the hydrogen-evolving activity is limited by the photogeneration of reduced sensitizer species, at neutral to basic pH (pH >5) the production of hydrogen is hampered by the disfavored protonation of the reduced Co(I) species. In this latter instance, however, hydrogen evolution is mainly slowed down rather than inhibited, as it is instead in the former case. This evidence affects the time scale of the photocatalysis and gives the opportunity to rationalize and correlate different results obtained with the same cobaloxime catalyst but under rather diverse experimental conditions.
The charge carrier dynamics of epitaxial hematite films is studied by time‐resolved microwave (TRMC) and time‐resolved terahertz conductivity (TRTC). After excitation with above bandgap illumination, ...the TRTC signal decays within 3 ps, consistent with previous reports of charge carrier localization times in hematite. The TRMC measurements probe charge carrier dynamics at longer timescales, exhibiting biexponential decay with characteristic time constants of ≈20–50 ns and 1–2 μs. From the change in photoconductance, the effective carrier mobility is extracted, defined as the product of the charge carrier mobility and photogeneration yield, of differently doped (undoped, Ti, Sn, Zn) hematite films for excitation wavelengths of 355 and 532 nm. It is shown that, unlike in conventional semiconductors, donor doping of hematite dramatically increases the effective mobility of the photogenerated carriers. Furthermore, it is shown that all hematite films possess higher effective mobility for 355 nm excitation than for 532 nm excitation, although the time dependence of the photoconductance decay, or charge carrier lifetime, remains the same. These results provide an explanation for the wavelength dependent photoelectrochemical behavior of hematite photoelectrodes and suggest that an increase in photogeneration yield or charge carrier mobility is responsible for the improved performance at higher excitation energies.
Time‐resolved terahertz microwave and terahertz spectroscopy are used to study the charge carrier dynamics of epitaxial hematite films. A physical basis for the wavelength dependent photoelectrochemical behavior of hematite is provided, demonstrating that an increase in photogeneration yield or charge carrier mobility is responsible for the improved performance at higher excitation energies.
As the properties of a semiconductor material depend on the fate of the excitons, manipulating exciton behavior is the primary objective of nanomaterials. Although nanocrystals exhibit unusual ...excitonic characteristics owing to strong spatial confinement, studying the interactions between excitons in a single nanoparticle remains challenging due to the rapidly vanishing multiexciton species. Here, a platform for exciton tailoring using a straightforward strategy of shape‐tuning of single‐crystalline nanocrystals is presented. Spectroscopic and theoretical studies reveal a systematic transition of exciton confinement orientation from 3D to 2D, which is solely tuned by the geometric shape of material. Such a precise shape‐effect triggers a multiphoton emission in single nanotetrapods with arms longer than the exciton Bohr radius of material. In consequence, the unique interplay between the multiple quantum states allows a geometric modulation of the quantum‐confined Stark effect and nanocrystal memory effect in single nanotetrapods. These results provide a useful metric in designing nanomaterials for future photonic applications.
Single‐crystalline nanotetrapod transits from a single to multiphoton emitter upon tuning its shape, which is revealed by single‐particle spectroscopy. Calculation and time‐resolved spectroscopic results prove that the precise changeover of the exciton confinement dimension is driven by the geometry of the particle shape. Interactions between multiple excitons allow a useful and direct manipulation of single‐nanotetrapod luminescence.
Time-resolved optical measurements of vibrating metal nanoparticles have been used extensively to probe the ultrafast mechanical properties of the nanoparticles and of the surrounding liquid, but ...nearly all investigations so far have been limited to the linear regime. Here, we report the observation of a low-frequency oscillating signal in transient-absorption measurements of nanoparticles with octahedral gold cores and cubic silver shells; the signal appears at the difference of two mechanical vibrational frequencies in the particles, suggesting a nonlinear mixing process. We tentatively attribute this proposed mixing to a nonlinear coupling between a vibrational mode of the nanoparticle and its optical-frequency plasmon resonance. The optimization of this nonlinear transduction may enable high-efficiency opto-mechanical frequency mixing in the GHz–THz frequency regime.
Understanding the effects of X‐rays on halide perovskite thin films is critical for accurate and reliable characterization of this class of materials, as well as their use in detection systems. In ...this study, advanced optical imaging techniques are employed, both spectrally and temporally resolved, coupled with chemical characterizations to obtain a comprehensive picture of the degradation mechanism occurring in the material during photoemission spectroscopy measurements. Two main degradation pathways are identified through the use of local correlative physico‐chemical analysis. The first one, at low X‐Ray fluence, shows minor changes of the surface chemistry and composition associated with the formation of electronic defects. Moreover, a second degradation route occurring at higher fluence leads to the evaporation of the organic cations and the formation of an iodine‐poor perovskite. Based on the local variation of the optoelectronic properties, a kinetic model describing the different mechanisms is proposed. These findings provide valuable insight on the impact of X‐rays on the perovskite layers during investigations using X‐ray based techniques. More generally, a deep understanding of the interaction mechanism of X‐rays with perovskite thin films is essential for the development of perovskite‐based X‐ray detectors and solar for space applications.
Mixed halide perovskites are exposed to X‐rays during X‐ray photoelectron spectroscopy (XPS) for different exposure times. Degradation patterns are analyzed via advanced photoluminescence (PL) imaging techniques allowing for the localization of defects and evidence of an emissive I‐poor phase. Correlation between surface chemical analysis and bulk/surface luminescence observation provide insightful knowledge on the degradation of perovskite layers under X‐ray radiation.
The relationship between exposure to ultraviolet (UV) radiation and skin cancer urges the need for extra photoprotection, which is presently provided by widespread commercially available sunscreen ...lotions. Apart from having a large absorption cross section in the UVA and UVB regions of the electromagnetic spectrum, the chemical absorbers in these photoprotective products should also be able to dissipate the excess energy in a safe way, i.e. without releasing photoproducts or inducing any further, harmful, photochemistry. While sunscreens are tested for both their photoprotective capability and dermatological compatibility, phenomena occurring at the molecular level upon absorption of UV radiation are largely overlooked. To date, there is only a limited amount of information regarding the photochemistry and photophysics of these sunscreen molecules. However, a thorough understanding of the intrinsic mechanisms by which popular sunscreen molecular constituents dissipate excess energy has the potential to aid in the design of more efficient, safer sunscreens. In this review, we explore the potential of using gas-phase frequency- and time-resolved spectroscopies in an effort to better understand the photoinduced excited-state dynamics, or photodynamics, of sunscreen molecules. Complementary computational studies are also briefly discussed. Finally, the future outlook of expanding these gas-phase studies into the solution phase is considered.
Intermolecular bond formations among Pt atoms in oligomers of PtII complexes are induced by photo‐irradiation. Femtosecond time‐resolved absorption measurements for aqueous solutions of K2Pt(CN)4 ...recorded clear oscillations of transient absorption in the first few picoseconds with complex dynamics. The oscillations arise from the Pt−Pt stretch motions of the S1 trimer and S1 tetramer. Analysis of the oscillations provides clear assignments of the oligomers, as described by M. Iwamura, T. Tahara, and co‐workers in their Research Article on page 23154.
Thin films of 5,11‐dicyano‐6,12‐diphenyltetracene (TcCN) have been studied for their ability to undergo singlet exciton fission (SF). Functionalization of tetracene with cyano substituents yields a ...more stable chromophore with favorable energetics for exoergic SF (2E(T1)−E(S1)=−0.17 eV), where S1 and T1 are singlet and triplet excitons, respectively. As a result of tuning the triplet‐state energy, SF is faster in TcCN relative to the corresponding endoergic process in tetracene. SF proceeds with two time constants in the film samples (τ=0.8±0.2 ps and τ=23±3 ps), which is attributed to structural disorder within the film giving rise to one population with a favorable interchromophore geometry, which undergoes rapid SF, and a second population in which the initially formed singlet exciton must diffuse to a site at which this favorable geometry exists. A triplet yield analysis using transient absorption spectra indicates the formation of 1.6±0.3 triplets per initial excited state.
Divide and conquer: Transient absorption measurements reveal sub‐picosecond singlet exciton fission in thin films of a cyano‐substituted diaryltetracene. A triplet yield analysis of the transient absorption data set indicates the formation of 1.6±0.3 triplet excitons per singlet exciton, as a result of rapid and efficient singlet fission.
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