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Picosecond dynamics of the photoexcited charge-ordered perovskite-type cobalt oxides La$_{{1.5}}$Ca$_{{0.5}}$CoO$_{{4}}$ and La$_{{1.5}}$Sr$_{{0.5}}$CoO$_{{4}}$ were investigated based on time-resolved Raman and reflection spectroscopy. These cobalt oxides show strong Raman scattering at around $700~\mathrm{cm}^{-1}$, originating from the breathing vibration mode of the CoO$_{{6}}$ octahedron coupled with the checkerboard-type charge ordering of Co$^{{2+}}$ and Co$^{{3+}}$. We performed time-resolved Raman and reflection spectroscopy by photoirradiating with picosecond pulses at 400 nm, exciting the charge transfer (CT) transition from the O $2p$ site to the Co $3d$ site. As a result of the excitation, the Raman intensity of the breathing mode was suppressed by $10\%$ on a similar time scale in the reflectance change without changing the frequency of the mode, directly indicating ultrafast suppression of the order parameter of the charge-ordered state by CT transition.

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]