Search

Photoinduced metal-to-ligand (or ligand-to-metal) charge-transfer (CT) states in metal complexes have been extensively studied toward the development of luminescent materials. However, previous studies have mainly focused on CT transitions between d- and π-orbitals. Herein, we report the demonstration of CT emission from 4f- to π-orbitals using a trivalent europium (Eu(III)) complex, supported by both experimental and theoretical analyses. The Eu(III) complex exhibits an eight-coordination structure, comprising three anionic nitrates and two neutral electron-donating ligands containing a carbazole unit. The diffuse reflectance spectrum of the complex displays an absorption band at 440 nm and time-resolved emission analyses reveal a characteristic emission band at 550 nm. Comparative studies employing a trivalent gadolinium (Gd(III)) complex, alongside quantum chemical analyses, confirm that the observed absorption and emission bands are associated with CT transitions between π- and 4f-orbitals. The observation of CT emission based on the 4f-orbital offers novel insights into the field of molecular luminescence science and technology. Photoinduced metal-to-ligand (or ligand-to-metal) charge-transfer (CT) states in metal complexes have been extensively studied, but primarily limited to CT transitions between d- and π-orbitals. Here, CT emission from 4f- to π-orbitals is demonstrated using a trivalent europium complex. [ABSTRACT FROM AUTHOR]

Peripherally unsubstituted dioxadiaza[8]circulene, as the first example of structurally identified pristine hetero[8]circulene, was synthesized by the substituent detachment reactions. The solid-state structures and photophysical properties were analysed to elucidate intermolecular interactions. Herzberg–Teller type emission was considered to explain the optical behavior. [ABSTRACT FROM AUTHOR]

Photomechanical crystals have applications in light‐fueled actuators and soft robots. Herein, light‐responsive, versatile, anthraquinone dye crystal oscillators actuated via natural vibrations that are resonated by a photothermal effect are described. A black needle‐shaped crystal cantilever oscillates at 70 Hz in the first mode of natural vibration upon irradiation by broad‐wavelength light ranging from the ultraviolet through the visible to the near‐infrared, and also under continuous‐wavelength light (400–2000 nm). The second and third natural vibration modes are induced at higher frequencies (530 and 1350 Hz) and evidence complex flagellum‐like motions. The frequency can be readily tuned by moving the support of a crystal piece; this is analogous to playing a guitar. The crystal exhibits high durability (more than 10 000 cycles): the high elasticity prevents deterioration. Oscillatory motions can be designed via simulations using finite element analysis. This work will facilitate the use of photomechanical crystals in light‐fueled soft robots. [ABSTRACT FROM AUTHOR]

Femtosecond resolution electron scattering techniques are applied to resolve the first atomic-scale steps following absorption of a photon in the prototypical hybrid perovskite methylammonium lead iodide. Following above-gap photoexcitation, we directly resolve the transfer of energy from hot carriers to the lattice by recording changes in the mean square atomic displacements on 10-ps time scales. Measurements of the time-dependent pair distribution function show an unexpected broadening of the iodine-iodine correlation function while preserving the Pb-I distance. This indicates the formation of a rotationally disordered halide octahedral structure developing on picosecond time scales. This work shows the important role of light-induced structural deformations within the inorganic sublattice in elucidating the unique optoelectronic functionality exhibited by hybrid perovskites and provides new understanding of hot carrier-lattice interactions, which fundamentally determine solar cell efficiencies.

The singlet fission characteristics of aqueous nanoparticles, self‐assembled from ion pairs of tetracene dicarboxylic acid and various amines with or without chirality, are thoroughly investigated. The structure of the ammonium molecule, the counterion, is found to play a decisive role in determining the molecular orientation of the ion pairs and its regularity, spectroscopic properties, the strength of the intermolecular coupling between the tetracene chromophores, and the consequent singlet fission process. Using chiral amines has led to the formation of crystalline nanosheets and efficient singlet fission with a triplet quantum yield as high as 133% ±20% and a rate constant of 6.99 × 109 s−1. The chiral ion pairs also provide a separation channel to free triplets with yields as high as 33% ±10%. In contrast, nanoparticles with achiral counterions do not show singlet fission, which gave low or high fluorescence quantum yields depending on the size of the counterions. The racemic ion pair produces a correlated triplet pair intermediate by singlet fission, but no decorrelation into two free triplets is observed, as triplet‐triplet annihilation dominates. The introduction of chirality enables higher control over orientation and singlet fission in self‐assembled chromophores. It provides new design guidelines for singlet fission materials. [ABSTRACT FROM AUTHOR]

In recent years, much attention has been paid to the development of thermally activated delayed fluorescence (TADF) materials with short delayed-fluorescence lifetimes to improve the device performances of OLEDs. In principle, by reducing the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) overlap, while the energy difference between S1–T1 (ΔEST) and activation energy (Ea) can be reduced, and the reverse intersystem crossing rate constant (kRISC) can be accelerated, a decrease in the radiative rate constant happens, necessitating an advanced molecular design. Furthermore, a molecule based on heptazine as a parent skeleton has recently been found to have a peculiar temperature dependence of luminescence decay, suggesting a negative gap (NG) material. In this report, we show that 9-[1,4]benzoxaborino[2,3,4-kl]phenoxaborine-7-yl-1,3,6,8-tetramethyl-9H-carbazole (TMCz-BO), a donor–acceptor linked TADF molecule with a very short delay lifetime of 750 ns, exhibits a peculiar thermal behavior similar to that of NG materials based on the temperature dependence of its luminescence decay in solution. [ABSTRACT FROM AUTHOR]

It is well known that triplet–triplet annihilation upconversion (TTU) can enhance electroluminescence (EL) efficiency by converting two low‐energy triplet excitons into one high‐energy singlet exciton. However, conventional intermolecular TTU requires high doping concentration to activate, limiting device architecture. Here, six dimer molecules comprised of two anthracene units connected by a spacer unit are investigated, which have a possibility of the intramolecular TTU process. As a result, two dimer molecules show high TTU efficiencies close to the theoretical maximum even in dispersed, diluted conditions. By comparing these molecular conformations, a correlation is found between the dimer structures of two anthracene units and the TTU activity in the diluted condition, i.e., the 3D molecular structure with the exciton utilizing efficiency. [ABSTRACT FROM AUTHOR]

We synthesized a (1‐propylpyridinium)2[ReN(CN)4]‐type organic–inorganic hybrid exhibiting water‐vapor‐induced drastic structural changes of the [ReN(CN)4]2− assemblies. Specifically, upon exposure to water vapor, dehydrated nitrido‐bridged chains were converted to hydrated cyanido‐bridged tetranuclear clusters via rearrangements of large molecular building units in the crystals. These switchable assembly forms display substantially different photo‐physical properties, although in both cases the emission is caused by a metal‐centered d–d transition. The nitrido‐bridged chain exhibited a near‐infrared (749 nm) emission, which blue‐shifted as the temperature increased, while a visible (561 nm) emission and its red shift was demonstrated by the cyanido‐bridged cluster. [ABSTRACT FROM AUTHOR]

Lead halide perovskites are promising materials for optoelectronic applications because of their exceptional performances in carrier lifetime and diffusion length; however, the microscopic origins of their unique characteristics remain elusive. The organic–inorganic hybrid perovskites show unique dielectric functions, i.e., ferroelectric-like phonon responses in the 0.1–10 THz region and liquid-like rotational relaxation in the 1–100 GHz range. To reveal the role of the dielectric responses is of primal importance because the dielectric screening is a key to understanding the optoelectronic properties governed by polarons in the perovskites. Here, we conducted comparative studies of broadband dielectric spectroscopy on both all-inorganic CsPbBr3 and organic–inorganic hybrid (CH3NH3)PbBr3 single crystals to uncover the origin of the liquid-like dielectric relaxation in the 1–100 GHz range. We confirmed the absence of the dielectric response in the range of 106–1010 Hz in CsPbBr3, which was clearly present in the hybrid (CH3NH3)PbBr3. This suggests that the response is almost purely due to the rotational motions of the organic dipoles in the hybrid perovskites. We evaluated the lifetimes of the polarons using surface-free transient photoluminescence. The lifetime in CsPbBr3 was up to 1.6 µs, while the lifetime in (CH3NH3)PbBr3 was 18 µs. The lifetime in the hybrid (CH3NH3)PbBr3 was significantly longer than in CsPbBr3, also confirmed by transient infrared spectroscopy. We concluded that the liquid-like dielectric response inhibits polaron recombination due to the efficient separation of opposite charges by the additional dynamic disorder. [ABSTRACT FROM AUTHOR]