Remotely Modulating the Optical Properties of Organic Charge-Transfer Crystallites via Molecular Packing.

Organic charge-transfer complex (CTC) formation has emerged as an effective molecular engineering strategy for achieving the desired optical properties via intermolecular interactions. By synthesizing organic CTCs with carbazole-based electron donors and a 7,7,8,8-tetracyanoquinodimethane acceptor and adopting a molecular linker located remotely from the charge-transfer interface within the donors, we were able to modulate near-infrared absorptive and short-wavelength infrared emissive properties. Structural characterizations performed by using single-crystal X-ray diffraction confirmed that the unique molecular arrangements induced by the steric hindrance from the remotely located linker significantly influence the electronic interactions between the donor and acceptor molecules, resulting in different photophysical properties. Our findings offer an improved understanding of the interplay between molecular packing and optoelectronic properties, providing a foundation for designing advanced materials for optoelectronic applications.

Excited-State Dynamics of a Bright Fluorescent Dye with Precise Control of Emission Color Using Acid-Base Equilibrium, Intramolecular Charge Transfer, and Host-Guest Chemistry.

A fluorescent dye, a dithiophene-conjugated benzothiazole derivative (DTBz), was prepared to have high fluorescence emission quantum yields (ΦF) across various organic solvents. Its emission color modulation, from bright blue to deep red, was achieved through intramolecular charge transfer (ICT), acid-base equilibrium, and host-guest chemistry. Although it exhibits a weak solvatochromic effect, DTBz exhibited a bright fluorescence emission around 480 nm upon excitation at 390 nm in most solvents. In polar solvents, such as MeOH (methanol), EtOH (ethanol), DMF (N,N-dimethylforamide), and DMSO (dimethyl sulfoxide), an additional ICT emission band emerged around 640 nm, notably intense in DMSO, resulting in a bright greenish-white emission (ΦF = 0.67). The addition of 1,8-diazabicyclo[5,4.0]undec-7-ene (DBU) altered emission characteristics, reducing emission from the local excited (LE) state and enhancing ICT state emission. The degree of emission spectral change saturation with DBU addition varied with the solvent nature. Polar solvents with high dielectric constants, like DMSO and DMF, saw a complete disappearance of LE state emission with 5 equiv of DBU, resulting in a deep red emission (ΦFs of 0.53 and 0.48, respectively). Femtosecond transient absorption spectroscopy and time-resolved photoluminescence measurements elucidated the excited-state dynamics, revealing a long-lived excited state (τ-H = 10.3 ns) at a lower energy emission (640 nm), identified as DTBz-*, supported by transient absorption spectra analysis. Further analysis, including time-resolved fluorescence decay measurements and time-dependent density-functional theory (TD-DFT) calculations, underscored the role of deprotonation of DTBz's hydroxyl group in promoting the ICT process. The CIE coordination plot demonstrated wide linear emission color changes upon successive DBU additions in all solvents, while emission color precision was achieved through host-guest chemistry. Emission changes induced by DBU were reverted to the original state upon beta-cyclodextrin (ß-CD) addition, with the 1H NMR study revealing the competition between acid-base equilibrium and host-guest complex formation as the cause of emission color change.

Identification and Dynamics of Microsecond Long-Lived Charge Carriers for CsPbBr3 Perovskite Quantum Dots, Featuring Ambient Long-Term Stability.

We analyze the stability and photophysical dynamics of CsPbBr3 perovskite quantum dots (PeQDs), fabricated under mild synthetic conditions and embedded in an amorphous silica (SiOx) matrix (CsPbBr3@SiOx), underscoring their sustained performance in ambient conditions for over 300 days with minimal optical degradation. However, this stability comes at the cost of a reduced photoluminescence efficiency. Time-resolved spectroscopic analyses, including flash-photolysis time-resolved microwave conductivity and time-resolved photoluminescence, show that excitons in CsPbBr3@SiOx films decay within 2.5 ns, while charge carriers recombine over approximately 230 ns. This longevity of the charge carriers is due to photoinduced electron transfer to the SiOx matrix, enabling hole retention. The measured hole mobility in these PeQDs is 0.880 cm2 V-1 s-1, underscoring their potential in optoelectronic applications. This study highlights the role of the silica matrix in enhancing the durability of PeQDs in humid environments and modifying exciton dynamics and photoluminescence, providing valuable insights for developing robust optoelectronic materials.

Vacuum-Processed Propylene Urea Additive: A Novel Approach for Controlling the Growth of CH3NH3PbI3 Crystals in All Vacuum-Processed Perovskite Solar Cells.

In this study, we present a novel method for controlling the growth of perovskite crystals in the vacuum thermal evaporation process by utilizing a vacuum-processable additive, propylene urea (PU). By coevaporation of perovskite precursors with PU to form the perovskite layer, PU, acting as a Lewis base additive, retards the direct reaction between the perovskite precursors. This facilitates a larger domain size and reduced defect density. Following the removal of the residual additive, the perovskite layer, exhibiting improved crystallinity, demonstrates reduced charge recombination, as confirmed by a time-resolved microwave conductivity analysis. Consequently, there is a notable enhancement in open-circuit voltage and power conversion efficiency, increasing from 1.05 to 1.15 V and from 17.17 to 18.31%, respectively. The incorporation of a vacuum-processable and removable Lewis base additive into the fabrication of vacuum-processed perovskite solar cells offers new avenues for optimizing these devices.

Cutaneous warmth and hotness thresholds to radiation heat exposure at a distance of 10 cm from 17 body regions.

The purpose of the present study was to evaluate body regional differences in cutaneous warmth and hotness thresholds in relation to radiant heat exposure. Fourteen male subjects participated in this study (age: 25 ± 5 y, height: 176.6 ± 5.5 cm, body weight: 70 ± 5.8 kg). Cutaneous warmth and hotness thresholds were measured on the forehead, neck, chest, abdomen, upper back, lower back, upper arm, forearm, palm, back of hand, front thigh, shin, top of foot, buttock, back thigh, calf, and sole. The forehead (34.8 ± 0.2 °C), lower back (34.1 ± 1.2 °C) and palm (34.3 ± 0.7 °C) had the highest warmth thresholds, whereas the foot (29.8 ± 1.9 °C) and sole (28.0 ± 2.1 °C) had the lowest values among the 17 regions (P<0.001). Higher warmth thresholds were related to higher initial skin temperatures (Tsk) (r=0.972, P<0.001). Increases in Tsk for detecting warmth sensation were smaller for the lower back with a rise of 0.2 ± 0.4 °C and the abdomen (0.3 ± 0.3 °C) than for the buttock (0.9 ± 0.8 °C) and sole (0.8 ± 0.6 °C) (P<0.05). Increases in Tsk for detecting hotness sensation ranged from 0.5 to 1.5 °C. Warmth and hotness thresholds on the abdomen or sole had significant relationships with body mass index, indicating that the overweight are less sensitive to detecting radiant heat on the abdomen or sole. Thermal thresholds from radiant heat exposure of 100 cm2 were lower than the values from conductive heat exposure of 6.25 cm2, which might be explained by the effect of spatial summation.