Go with the flow: A novel spectroelectrochemical flow cell with well‐defined mass transport allows time‐resolved electrochemical and in situ FTIR spectroscopy measurements under continuous ...electrolyte flow (e.g. during electrolyte exchange). Its potential for mechanistic and kinetic studies was demonstrated in studies on the electrooxidation of formic acid.
Mixed‐halide lead perovskites have attracted significant attention in the field of photovoltaics and other optoelectronic applications due to their promising bandgap tunability and device ...performance. Here, the changes in photoluminescence and photoconductance of solution‐processed triple‐cation mixed‐halide (Cs0.06MA0.15FA0.79)Pb(Br0.4I0.6)3 perovskite films (MA: methylammonium, FA: formamidinium) are studied under solar‐equivalent illumination. It is found that the illumination leads to localized surface sites of iodide‐rich perovskite intermixed with passivating PbI2 material. Time‐ and spectrally resolved photoluminescence measurements reveal that photoexcited charges efficiently transfer to the passivated iodide‐rich perovskite surface layer, leading to high local carrier densities on these sites. The carriers on this surface layer therefore recombine with a high radiative efficiency, with the photoluminescence quantum efficiency of the film under solar excitation densities increasing from 3% to over 45%. At higher excitation densities, nonradiative Auger recombination starts to dominate due to the extremely high concentration of charges on the surface layer. This work reveals new insight into phase segregation of mixed‐halide mixed‐cation perovskites, as well as routes to highly luminescent films by controlling charge density and transfer in novel device structures.
The changes in photophysical properties of mixed‐halide perovskite films under solar‐equivalent illumination are studied. The illumination generates localized low‐bandgap surface domains, onto which photoexcited charge carriers transfer and recombine with high radiative efficiency. The fraction of radiative and nonradiative (Auger) recombination bandgap can be balanced to achieve extremely high photoluminescence quantum yields at low excitation densities.
Fluorescent probes for detecting the physical properties of cellular structures have become valuable tools in life sciences. The fluorescence lifetime of molecular rotors can be used to report on ...variations in local molecular packing or viscosity. We used a nucleoside linked to a meso‐substituted BODIPY fluorescent molecular rotor (dCbdp) to sense changes in DNA microenvironment both in vitro and in living cells. DNA incorporating dCbdp can respond to interactions with DNA‐binding proteins and lipids by changes in the fluorescence lifetimes in the range 0.5–2.2 ns. We can directly visualize changes in the local environment of exogenous DNA during transfection of living cells. Relatively long fluorescence lifetimes and extensive contrast for detecting changes in the microenvironment together with good photostability and versatility for DNA synthesis make this probe suitable for analysis of DNA‐associated processes, cellular structures, and also DNA‐based nanomaterials.
Rotational probe: A probe comprising a nucleoside linked to a meso‐substituted BODIPY fluorescent molecular rotor (see picture) can be used to sense changes in the DNA microenvironment both in vitro and in living cells. Changes in the fluorescence lifetime of the probe allow detection of interactions with DNA‐binding proteins or lipids.
The light-gated organocatalysis via the release of 4- N , N -dimethylaminopyridine (DMAP) by irradiation of the Ru(bpy) 2 (DMAP) 2 2+ complex with visible light was investigated. As model reaction ...the acetylation of benzyl alcohols with acetic anhydride was chosen. The pre-catalyst releases one DMAP molecule on irradiation at wavelengths longer than 455 nm. The photochemical process was characterized by steady-state irradiation and ultrafast transient absorption spectroscopy. The latter enabled the observation of the 3MLCT state and the spectral features of the penta-coordinated intermediate Ru(bpy) 2 (DMAP) 2+ . The released DMAP catalyzes the acetylation of a wide range of benzyl alcohols with chemical yields of up to 99%. Control experiments revealed unequivocally that it is the released DMAP which takes the role of the catalyst.
Femtosecond optical spectroscopy and DFT calculations are used by Hiroko Tokoro, Shin‐ichi Ohkoshi, Eric Collet, and co‐workers in their Research Article on page 23267 to investigate photomagnetism ...in the RbMnFe material. Two photoswitching pathways with different dynamics are found. The initial electronic excitation may launch first the reverse Jahn–Teller distortion or the intervalence Fe–Mn charge transfer.
Coherent multidimensional electronic spectroscopy can be employed to unravel various channels in molecular chemical reactions. This approach is thus not limited to analysis of energy transfer or ...charge transfer (i.e. processes from photophysics), but can also be employed in situations where the investigated system undergoes permanent structural changes (i.e. in photochemistry). Photochemical model reactions are discussed by using the example of merocyanine/spiropyran‐based molecular switches, which show a rich variety of reaction channels, in particular ring opening and ring closing, cis–trans isomerization, coherent vibrational wave‐packet motion, radical ion formation, and population relaxation. Using pump‐probe, pump‐repump‐probe, coherent two‐dimensional and three‐dimensional, triggered‐exchange 2D, and quantum‐control spectroscopy, we gain intuitive pictures on which product emerges from which reactant and which reactive molecular modes are associated.
Two dimensions provide more insight than one: Coherent electronic multidimensional spectroscopy in different variations allows the separation of photochemical reaction channels. This is demonstrated exemplarily for a molecular switch capable of light‐induced ring‐opening and ring‐closure reactions, cis–trans isomerization, vibrational wave‐packet dynamics, radical cation formation, and population relaxation.
Singlet fission in pentacene dimers Zirzlmeier, Johannes; Lehnherr, Dan; Coto, Pedro B. ...
Proceedings of the National Academy of Sciences - PNAS,
04/2015, Letnik:
112, Številka:
17
Journal Article
Recenzirano
Odprti dostop
Significance In the present work, we show compelling evidence for the unprecedented intramolecular singlet fission at room temperature and in dilute solutions within a set of three different ...regioisomeric pentacene dimers. Pump–probe experiments, which were complemented by theoretical calculations using high-level ab initio multireference perturbation theory methods, corroborate triplet quantum yields as high as 156 ± 5%. To this end, electronic couplings between the two pentacenes in the dimers, by virtue of through-bond or through-space interactions, are decisive in tuning the rates of singlet fission.
Singlet fission (SF) has the potential to supersede the traditional solar energy conversion scheme by means of boosting the photon-to-current conversion efficiencies beyond the 30% Shockley–Queisser limit. Here, we show unambiguous and compelling evidence for unprecedented intramolecular SF within regioisomeric pentacene dimers in room-temperature solutions, with observed triplet quantum yields reaching as high as 156 ± 5%. Whereas previous studies have shown that the collision of a photoexcited chromophore with a ground-state chromophore can give rise to SF, here we demonstrate that the proximity and sufficient coupling through bond or space in pentacene dimers is enough to induce intramolecular SF where two triplets are generated on one molecule.
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.
Zirconium‐containing metal‐organic framework (MOF) with UiO‐66 topology is an extremely versatile material, which finds applications beyond gas separation and catalysis. However, after more than 10 ...years after the first reports introducing this MOF, understanding of the molecular‐level mechanism of its nucleation and growth is still lacking. By means of in situ time‐resolved high‐resolution mass spectrometry, Zr K‐edge X‐ray absorption spectroscopy, magic‐angle spinning nuclear magnetic resonance spectroscopy, and X‐ray diffraction it is showed that the nucleation of UiO‐66 occurs via a solution‐mediated hydrolysis of zirconium chloroterephthalates, whose formation appears to be autocatalytic. Zirconium‐oxo nodes form directly and rapidly during the synthesis, the formation of pre‐formed clusters and stable non‐stoichiometric intermediates are not observed. The nuclei of UiO‐66 possess identical to the crystals local environment, however, they lack long‐range order, which is gained during the crystallization. Crystal growth is the rate‐determining step, while fast nucleation controls the formation of the small crystals of UiO‐66 with a narrow size distribution of about 200 nanometers.
A novel insight into the chemical transformations taking place during the synthesis of UiO‐66 metal‐organic framework from induction period through nucleation to the crystal growth is reported.
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.
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