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Degradation of organic materials is responsible for the short operation lifetimes of organic light-emitting devices, but the mechanism by which such degradation is initiated has yet to be fully established. Here we report a new mechanism for degradation of emitting layers in blue-phosphorescent devices. We investigate binary mixtures of a wide bandgap host and a series of novel Ir(III) complex dopants having N-heterocyclocarbenic ligands. Our mechanistic study reveals the charge-neutral generation of polaron pairs (radical ion pairs) by electron transfer from the dopant to host excitons. Annihilation of the radical ion pair occurs by charge recombination, with such annihilation competing with bond scission. Device lifetime correlates linearly with the rate constant for the annihilation of the radical ion pair. Our findings demonstrate the importance of controlling exciton-induced electron transfer, and provide novel strategies to design materials for long-lifetime blue electrophosphorescence devices.

Nitrophenolates (NPs) are molecular anions that can undergo charge-transfer (CT) transitions determined by the degree of electron delocalization between the phenolate oxygen (donor group) and the nitro group (acceptor). Here we have studied four different NPs: 4’-nitro-[1,1’-biphenyl]-4-olate (1), 7-nitro-9H -carbazol-2-olate (NH linker, 2), 7-nitrodibenzo[b,d]furan-3-olate (oxygen linker, 3), and 7-nitrodibenzo[b,d]thiophen-3-olate (sulphur linker, 4), and recorded their electronic absorption spectra when isolated in vacuo to determine the effect of locking the biphenyl spacer group between
the donor and acceptor on transition energies. Absorption was identified from ion dissociation (action spectroscopy) using a homebuilt setup (sector mass spectrometer combined with pulsed laser). We find that the absorption is broad in the visible region for all four NPs with significant vibronic features. The lowest energy peak is at 601 ± 4 nm, 606 ± 4 nm, 615 ± 4 nm, and 620 ± 4 nm, for 3, 4, 2, and 1, respectively. NP 1 is flexible, and its lowest
energy structure is nonplanar while the other three NPs are planar according to density functional theory calculations. Hence in the case of 1 the electronic transition has a higher degree of CT than for the other three, accounting for its absorption furthest to the red. Our work demonstrates that oxygen and sulphur are best at conveying the electronic coupling between the donor and acceptor sites as 3 and 4 absorb furthest to the blue (i.e., the degree of CT is lowest for these two NPs). Based on the average spacing between the peaks in the vibrational progressions, coupling occurs to skeleton vibrational modes with frequencies of 649 ± 69 cm−1 (3), 655 ± 49 cm−1 (4), and 697 ± 52 cm−1 (2).

In the recent years, there is an extensive effort concentrated towards the development of nanoparticles with near-infrared emission within the so called second or third biological windows induced by excitation outside 800–1000 nm range corresponding to the traditional Nd (800 nm) and Yb (980 nm) sensitizers. Here, we present a first report on the near-infrared (900–1700 nm) emission of significant member of cubic sesquioxides, Er-Lu₂O₃ nanoparticles, measured under both near-infrared up-conversion and low energy X-ray excitations. The nanoparticle compositions are optimized by varying Er concentration and Li addition. It is found that, under ca. 1500 nm up-conversion excitation, the emission is almost monochromatic (>93%) and centered at 980 nm while over 80% of the X-ray induced emission is concentrated around 1500 nm. The mechanisms responsible for the up-conversion emission of Er - Lu₂O₃ are identified by help of the up-conversion emission and excitation spectra as well as emission decays considering multiple excitation/emission transitions across visible to near-infrared ranges. Comparison between the emission properties of Er-Lu₂O₃ and Er-Y₂O₃ induced by optical and X-ray excitation is also presented. Our results suggest that the further optimized Er-doped cubic sesquioxides represent promising candidates for bioimaging and photovoltaic applications.

Hydrated singly charged magnesium ions Mg⁺(H₂O)_n, n ≤ 5, in the gas phase are ideal model systems to study photochemical hydrogen evolution since atomic hydrogen is formed over a wide range of wavelengths, with a strong cluster size dependence. Mass selected clusters are stored in the cell of an Fourier transform ion cyclotron resonance mass spectrometer at a temperature of 130 K for several seconds, which allows thermal equilibration via blackbody radiation. Tunable laser light is used for photodissociation. Strong transitions to D_1–3 states (correlating with the 3s-3p_x,y,z transitions of Mg⁺) are observed for all cluster sizes, as well as a second absorption band at 4–5 eV for n = 3-5. Due to the lifted degeneracy of the 3p_x,y,z energy levels of Mg⁺, the absorptions are broad and red shifted with increasing coordination number of the Mg⁺ center, from 4.5 eV for n = 1 to 1.8 eV for n = 5. In all cases, H atom formation is the dominant photochemical reaction channel. Quantum chemical calculations using the full range of methods for excited state calculations reproduce the experimental spectra and explain all observed features. In particular, they show that H atom formation occurs in excited states, where the potential energy surface becomes repulsive along the O⋯H coordinate at relatively small distances. The loss of H₂O, although thermochemically favorable, is a minor channel because, at least for the clusters n = 1-3, the conical intersection through which the system could relax to the electronic ground state is too high in energy. In some absorption bands, sequential absorption of multiple photons is required for photodissociation. For n = 1, these multiphoton spectra can be modeled on the basis of quantum chemical calculations.

Although marine aerosols undergo extensive photochemical processing in the troposphere, a molecular level understanding of the elementary steps involved in these complex reaction sequences is still missing. As a defined laboratory model system, the photodissociation of sea salt clusters doped with glyoxylate, [Na_nCl_n−2(C₂HO₃)]⁺, n = 5–11, is studied by a combination of mass spectrometry, laser spectroscopy and ab initio calculations. Glyoxylate acts as a chromophore, absorbing light below 400 nm via two absorption bands centered at about 346 and 231 nm. Cluster fragmentation dominates, which corresponds to internal conversion of the excited state energy into vibrational modes of the electronic ground state and subsequent unimolecular dissociation. Photochemical dissociation pathways in electronically excited states include CO and HCO elimination, leading to [Na_n−xCl_n−x−2HCOO]⁺ and [Na_nCl_n−2COO˙]⁺ with typical quantum yields in the range of 1–3% and 5–10%, respectively, for n = 5. The latter species contains CO₂˙⁻ stabilized by the salt environment. The comparison of different cluster sizes shows that the fragments containing a carbon dioxide radical anion appear in a broad spectral region of 310–380 nm. This suggests that the elusive CO₂˙⁻ species may be formed by natural processes in the troposphere. Based on the photochemical cross sections obtained here, the photolysis lifetime of glyoxylate in a dry marine aerosol is estimated as 10 h. Quantum chemical calculations show that dissociation along the C–C bond in glyoxylic acid as well as glyoxylate embedded in the salt cluster occurs after reaching the S₁/S₀ conical intersection, while this conical intersection is absent in free glyoxylate ions.

Marine aerosols consist of a variety of compounds and play an important role in many atmospheric processes. In the present study, sodium iodide clusters with their simple isotope pattern serve as model systems for laboratory studies to investigate the role of iodide in the photochemical processing of sea-salt aerosols. Salt clusters doped with camphor, formate and pyruvate are studied in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) coupled to a tunable laser system in both UV and IR range. The analysis is supported by ab initio calculations of absorption spectra and energetics of dissociative channels. We provide quantitative analysis of IRMPD measurements by reconstructing one-photon spectra and comparing them with the calculated ones. While neutral camphor is adsorbed on the cluster surface, the formate and pyruvate ions replace an iodide ion. The photodissociation spectra revealed several wavelength-specific fragmentation pathways, including the carbon dioxide radical anion formed by photolysis of pyruvate. Camphor and pyruvate doped clusters absorb in the spectral region above 290 nm, which is relevant for tropospheric photochemistry, leading to internal conversion followed by intramolecular vibrational redistribution, which leads to decomposition of the cluster. Potential photodissociation products of pyruvate in the actinic region may be formed with a cross section of <2×10⁻²⁰ cm², determined by the experimental noise level.

Red-emitting Mn⁴⁺-doped fluorides are a promising class of materials to improve the color rendering and luminous efficacy of white light-emitting diodes (w-LEDs). For w-LEDs, the luminescence quenching temperature is very important, but surprisingly no systematic research has been conducted to understand the mechanism for thermal quenching in Mn⁴⁺-doped fluorides. Furthermore, concentration quenching of the Mn⁴⁺ luminescence can be an issue but detailed investigations are lacking. In this work, we study thermal quenching and concentration quenching in Mn⁴⁺-doped fluorides by measuring luminescence spectra and decay curves of K₂TiF₆:Mn⁴⁺ between 4 and 600 K and for Mn⁴⁺ concentrations from 0.01% to 15.7%. Temperature-dependent measurements on K₂TiF₆:Mn⁴⁺ and other Mn⁴⁺-doped phosphors show that quenching occurs through thermally activated crossover between the ⁴T₂ excited state and ⁴A₂ ground state. The quenching temperature can be optimized by designing host lattices in which Mn⁴⁺ has a high ⁴T₂ state energy. Concentration-dependent studies reveal that concentration quenching effects are limited in K₂TiF₆:Mn⁴⁺ up to 5% Mn⁴⁺. This is important, as high Mn⁴⁺ concentrations are required for sufficient absorption of blue LED light in the parity-forbidden Mn⁴⁺ d–d transitions. At even higher Mn⁴⁺ concentrations (>10%), the quantum efficiency decreases, mostly due to direct energy transfer to quenching sites (defects and impurity ions). Optimization of the synthesis to reduce quenchers is crucial for developing more efficient highly absorbing Mn⁴⁺ phosphors. The present systematic study provides detailed insights into temperature and concentration quenching of Mn⁴⁺ emission and can be used to realize superior narrow-band red Mn⁴⁺ phosphors for w-LEDs.

Solid-state dye-sensitized solar cells currently suffer from issues such as inadequate nanopore filling, low conductivity and crystallization of hole-transport materials infiltrated in the mesoscopic TiO₂ scaffolds, leading to low performances. Here we report a record 11% stable solid-state dye-sensitized solar cell under standard air mass 1.5 global using a hole-transport material composed of a blend of [Cu (4,4′,6,6′-tetramethyl-2,2′-bipyridine)₂](bis(trifluoromethylsulfonyl)imide)₂ and [Cu (4,4′,6,6′-tetramethyl-2,2′-bipyridine)₂](bis(trifluoromethylsulfonyl)imide). The amorphous Cu(II/I) conductors that conduct holes by rapid hopping infiltrated in a 6.5 μm-thick mesoscopic TiO₂ scaffold are crucial for achieving such high efficiency. Using time-resolved laser photolysis, we determine the time constants for electron injection from the photoexcited sensitizers Y123 into the TiO₂ and regeneration of the Y123 by Cu(I) to be 25 ps and 3.2 μs, respectively. Our work will foster the development of low-cost solid-state photovoltaic based on transition metal complexes as hole conductors.

Solar cells that operate efficiently under indoor lighting are of great practical interest as they can serve as electric power sources for portable electronics and devices for wireless sensor networks or the Internet of Things. Here, we demonstrate a dye-sensitized solar cell (DSC) that achieves very high power-conversion efficiencies (PCEs) under ambient light conditions. Our photosystem combines two judiciously designed sensitizers, coded D35 and XY1, with the copper complex Cu(II/I)(tmby) as a redox shuttle (tmby, 4,4′,6,6′-tetramethyl-2,2′-bipyridine), and features a high open-circuit photovoltage of 1.1 V. The DSC achieves an external quantum efficiency for photocurrent generation that exceeds 90% across the whole visible domain from 400 to 650 nm, and achieves power outputs of 15.6 and 88.5 μW cm^–2 at 200 and 1,000 lux, respectively, under illumination from a model Osram 930 warm-white fluorescent light tube. This translates into a PCE of 28.9%.

Intrinsic optical properties of several rhodamine cations were probed by measuring their dispersed fluorescence spectra in vacuo. Three different rhodamine structures were investigated, each with four different chalcogen heteroatoms. Fluorescence band maxima were blue-shifted by between 0.15 and 0.20 eV (1200-1600 cm⁴) relative to previous solution-phase measurements. Trends in emission wavelengths and fluorescence quantum yields previously measured in solution are generally reproduced in the gas phase, confirming the intrinsic nature of these effects. One important exception is gas-phase brightness of the Texas Red analogues, which is significantly higher than the other rhodamine structures studied, despite having similar fluorescence quantum yields in solution. These results expand the library of fluorophores for which gas-phase photophysical data is available, and will aid in the design of experiments utilizing gas-phase structural biology methods such as Forster resonance energy transfer.