Searching for single-component white-light emitting materials with excellent color rendering index (CRI) is significant to the development of white-light emitting diodes (WLEDs). Herein, we introduce three different cations with increasing steric hindrance to develop novel luminescent low-dimensional organic copper halides: one-dimensional (1D) ETMACu2I3 (ETMA: ethyltrimethylammonium), 1D TEACu2I3 (TEA: tetraethylammonium), and zero-dimensional (0D) TBMACuI2 (TBMA: tributylmethylammonium). The dimensionality of the prepared copper halides decreases with the increase of the steric hindrance of the organic cations. Meanwhile, the photoluminescence spectra become broader. The two 1D compounds exhibit weak single broad emission originating from self-trapped excitons (STEs) and defects. Interestingly, 0D TBMACuI2 emits bright warm white light with dual emission bands. The mechanism of the unique dual emission is unveiled by systematic spectroscopic study including temperature-dependent and femtosecond transient absorption spectroscopies. Two spintriplet STE states contribute to the intriguing dual emission. A WLED based on TBMACuI2 can exhibit an ultra-high CRI of 95.7, which is promising for single-component white-light emitting applications.

Methylammonium (MA) lead halide perovskites have been widely studied as active materials for advanced optoelectronics. As crystalline semiconductor materials, their properties are strongly affected by their crystal structure. Depending on their applications, the size of MA lead halide perovskite crystals varies by several orders of magnitude. The particle size can lead to different structural phase transitions and optoelectronic properties. Herein, we investigate the size effect for phase transition of MA lead bromide (MAPbBr3) by comparing the temperature-dependent neutron powder diffraction patterns of microcrystals and nanocrystals. The orthorhombic-to-tetragonal phase transition occurs in MAPbBr3 microcrystals within the temperature range from 100 to 310 K. However, the phase transition is absent in nanocrystals in this temperature range. In this work, we offer a persuasive and direct evidence of the relationship between the particle size and the phase transition in perovskite crystals.

Access to functionalization of new sites on the triangulenium core structure has been achieved at an early stage by chlorination with N-chlorosuccinimide (NCS), giving rise to two new triangulenium dyes (1 and 2). By introducing the chlorine functionalities in the acridinium precursor, positions complementary to those previously accessed by electrophilic aromatic substitution of the final dyes can be accesed. The chlorination is selective, giving only one regioisomer for both mono- and dichlorination products. For the monochlorinated acridinium compound a highly selective ring-closing reaction was discovered to generate only a single regioisomer of the cationic [4]helicene product. This discovery aspired further investigations into the mechanism of [4]helicene formation and to the first isolation of the previously proposed intermediate of the two-step SNAr reaction, key to all aza-bridged triangulenium and helicenium systems. A late stage functionalization of DAOTA+ with NCS gave rise to a different dichlorinated compound (2). The fully ring closed chlorinated triangulenium dyes 1, 2 and 3 show a redshift in absorption and emission relative to the non-chlorinated analogues, while still maintaining relatively high fluorescence quantum yields of 36%, 26%, and 41%, and long fluorescence lifetimes of 15 ns, 12.5 ns and 16 ns, respectively. Cyclic voltammetry shows that chlorination of the triangulenium dyes significantly lowers reduction potentials and thus allows for efficient tuning of redox and photo-redox properties.

Ultra-bright fluorescentnanoparticles hold great promise for demanding bioimaging applications.Recently, extremely bright molecular crystals of cationic fluorophores wereobtained by hierarchical co-assembly with cyanostar anion-receptor complexes ofassociated counterions. These small-molecule ionic isolation lattices (SMILES)ensure spatial and electronic isolation to prohibit dye aggregation quenching.We report a simple, one-step supramolecular approach to formulate SMILESmaterials into nanoparticles. Rhodamine-based SMILES nanoparticles stabilizedby glycol amphiphiles show high fluorescence quantum yield (30%) and brightnessper volume (5000 M–1 cm–1 / nm3)with 400 dyes packed into 16-nm particles, corresponding to an absorptioncoefficient of 4 × 107 M–1 cm–1. UVexcitation of the cyanostar component leads to highest brightness (>6000 M–1 cm–1 /nm3) by energy transfer to rhodamine emitters. Coated nanoparticlesstain cells and are thus promising for bioimaging.

Charge transport in an organic semiconductor is strongly dependent on the molecular packing motif, which could be modified by the molecular substitutions and molecular isomerization. We constructed a series of benzodithiophene-based organic semiconductor molecules with different silyethyne substitutions and isomers. The existence of different conformations of these molecules is supported by a low isomerization energy barrier from density functional theory. By using Marcus semiclassical theory calculation, we make a comprehensive assessment for the effect of molecular substitution and isomerization on charge transport. We found that the hole mobility of cis-isomer molecular packing can be enhanced by increasing the length of silylethyne substitutions. We demonstrated that a favorable charge-transport material would possess an identical direction of induced ring currents, stable induced magnetic fields, and dominant π-π stacking interaction in their molecular packing motif to ensure good π-overlap area. Our findings will provide direct guidance for developing organic semiconductor materials.

The authors' previous study demonstrated that miR-128 may exert an inhibitory effect on the osteogenic differentiation of bone marrow-derived mesenchymal stem cells (BM-MSCs), but its downstream mechanisms remain to be elucidated. The aim of the present study was to investigate the microRNA (miRNA/miR) and mRNA profiles of differentiated and undifferentiated BM-MSCs and explore new downstream targets for miR-128. The sequencing datasets of GSE107279 (miRNA) and GSE112318 (mRNA) were downloaded from the Gene Expression Omnibus database. The differentially expressed miRNAs (DEMs) and genes (DEGs) were identified using the DESeq2 method. The target genes of DEMs were predicted by the miRwalk 2.0 database. The hub target genes of miR-128 were screened by constructing the protein-protein interaction (PPI) network and module analysis. The expression levels of miR-128 and crucial target genes were validated by reverse transcription-quantitative (RT-q) PCR before or after transfection of miR-128 mimics to BM-MSCs. The miRNA expression profile analysis identified miR-128 as one of the significantly downregulated DEMs (total 338) in differentiated BM-MSCs compared with the undifferentiated control. A total of 103 predicted target genes of miR-128-3p were overlapped with upregulated DEGs. By calculating the topological properties of each protein in the PPI network, 6 upregulated genes (KIT, NTRK2, YWHAB, GAB1, AXIN1 and RUNX1; fold change was the highest for NTRK2) were considered to be hub genes. Of these, 4 were enriched in module 4 (RUNX1, KIT, GAB1 and AXIN1; RUNX1 was particularly crucial as it can interact with the others), while one was enriched in module 7 (YWHAB). The expression levels of miR-128 and these 6 target genes during the osteogenic differentiation were experimentally confirmed by RT-qPCR. In addition, the expression levels of these 6 genes were significantly reversed after transfection of miR-128-3p mimics into rat BM-MSCs compared with the miR-control group. These findings indicated that miR-128-3p may inhibit the osteoblast differentiation of BM-MSCs by downregulation of these 6 genes, particularly RUNX1, YWHAB and NTRK2.

Carrier multiplication (CM) is an effective mechanism that makes it possible to use hot carriers (HCs) to bypass the Shockley-Queisser limit for solar-cell efficiency. In this paper, we present a detailed study of both CM and HC cooling dynamics in quantum-confined CsPbI3 perovskite nanocrystals (NCs), using femtosecond transient absorption spectroscopy. Our results show that barrierless CM, with an efficiency exceeding 90%, can be achieved in strongly confined NCs on a time scale of ≪200 fs. A low CM efficiency (∼40%), however, is observed in weakly confined NCs. HC cooling dynamics suggests the absence of an intrinsic phonon bottleneck in strongly confined NCs. Furthermore, the biexciton Auger rate increased 4-fold in strongly confined NCs compared to that in weakly confined NCs. These results suggest that the enhanced CM in strongly confined NCs likely originates from enhanced Coulomb coupling and relaxed momentum conservation.

AbstractThermally activated delayed fluorescence (TADF) is generally observed in solid‐state organic molecules or metal‐organic complexes. However, TADF in all‐inorganic colloidal nanocrystals (NCs) is rare. Herein, we report the first colloidal synthesis of an air‐stable all‐inorganic lead‐free Cs2ZrCl6 perovskite NCs. The Cs2ZrCl6 NCs exhibit long‐lived triplet excited state (138.2 μs), and feature high photoluminescence (PL) quantum efficiency (QY=60.37 %) due to TADF mechanism. The emission color can be easily tuned from blue to green by synthesizing the mixed‐halide Cs2ZrBrxCl6−x (0≤x≤1.5) NCs. Femtosecond transient absorption and temperature dependent PL measurements are performed to clarify the emission mechanism. In addition, Bi3+ ions are successfully doped into Cs2ZrCl6 NCs, which further extends the PL properties. This work not only develops a new lead‐free halide perovskite NCs for potential optoelectronic applications, but also offers unique strategies for developing new inorganic phosphors.

The synthesis and crystal structure of rhodamine 590 acid phthalate (RhAP) have been reported. This novel solid-state rhodamine derivative not only has a longer fluorescence lifetime compared to rhodamine solid-state matrixes where emission is quenched but also possesses strong nonlinear optical characteristics. The static and dynamic first- and second-order hyperpolarizabilities were calculated using the time-dependent density functional theory at the B3LYP/6-31+G* level. The computed static values of β and γ of RhAP by the X-ray diffraction (XRD) structure were 31.9 × 10-30 and 199.0 × 10-36 esu, respectively. These values were about 62 times larger than the corresponding values in urea, an already well-known nonlinear optical material. The second-order hyperpolarizability of the compound was determined experimentally by measuring the two-photon absorption cross section using intensity-modulated light fields. The reported compound, excitable at near-infrared, exhibited frequency upconversion with the two-photon absorption coefficient enhanced by two orders of magnitude compared to that of the dye solution. Hosting the dye in the solid, at high concentrations, exploits the nonlinearity of the dye itself as well as results in significant excitonic effects including formation of broad exciton band and superradiance.

A series of lead-free double perovskite nanocrystals (NCs) Cs2 AgSb1-y Biy X6 (X: Br, Cl; 0≤y≤1) is synthesized. In particular, the Cs2 AgSbBr6 NCs is a new double perovskite material that has not been reported for the bulk form. Mixed Ag-Sb/Bi NCs exhibit enhanced stability in colloidal solution compared to Ag-Bi or Ag-Sb NCs. Femtosecond transient absorption studies indicate the presence of two prominent fast trapping processes in the charge-carrier relaxation. The two fast trapping processes are dominated by intrinsic self-trapping (ca. 1-2 ps) arising from giant exciton-phonon coupling and surface-defect trapping (ca. 50-100 ps). Slow hot-carrier relaxation is observed at high pump fluence, and the possible mechanisms for the slow hot-carrier relaxation are also discussed.

Lead halide perovskites (LHPs) nanocrystals (NCs), owing to their outstanding photophysical properties, have recently emerged as a promising material not only for solar cells but also for lighting and display applications. The photophysical properties of these materials can be further improved by chemical engineering such as cation exchange. Hot carrier (HC) cooling, as one of the key photophysical processes in LHPs, can strongly influence performance of LHPs NCs based devices. Here, we study HC relaxation dynamics in LHP NCs with cesium (Cs), methylammonium (MA, CH3NH3+), and formamidinium (FA, CH(NH2)2+) cations by using femtosecond transient absorption spectroscopy. The LHP NCs show excitation intensity and excitation energy-dependent HC cooling. We investigate the details of HC cooling in CsPbBr3, MAPbBr3, and FAPbBr3 at three different excitation energies with low excitation intensity. It takes longer time for the HCs at high energy to relax (cool) to the band edge, compared to the HCs generated by low excitation energy. At the same excitation energy (350 nm, 3.54 eV), all the three LHP NCs show fast HC relaxation (<0.4 ps) with the cooling time and rate in the following order: CsPbBr3 (0.39 ps, 2.9 meV/fs) > MAPbBr3 (0.27 ps, 4.6 meV/fs) > FAPbBr3 (0.21 ps, 5.8 meV/fs). The cation dependence can be explained by stronger interaction between the organic cations with the Pb-Br frameworks compared to the Cs. The revealed cation-dependent HC relaxation process is important for providing cation engineering strategies for developing high performance LHP devices.

Electronic 2D spectroscopy allows nontrivial quantum effects in chemistry and biology to be explored in unprecedented detail. Here, we apply recently developed fluorescence detected coherent 2D spectroscopy to study the light harvesting antenna 2 (LH2) of photosynthetic purple bacteria. The method utilizes the destructive interference between two signal components thereby uncovering cross peaks which are not visible in conventional photon-echo based 2D and transient absorption measurements. Analyses of signal generating quantum pathways leads to the conclusion that, contrary to the currently prevailing physical picture, the two weakly-coupled pigment rings of LH2 share the initial electronic excitation leading to quantum mechanical correlation between the two clearly separate bands. These results are general and have consequences for the interpretation of excited states not only in photosynthesis but in all light absorbing systems. The initial delocalization could be the key for enhancing the light harvesting efficiency via biased motion towards the energy funnel.

All-inorganic CsPbI3 perovskite quantum dots (QDs) have attracted intense attention for their successful application in photovoltaics (PVs) and optoelectronics that are enabled by their superior absorption capability and great photoluminescence (PL) properties. However, their photostability remains a practical bottleneck and further optimization is highly desirable. Here, we studied the photostability of as-obtained colloidal CsPbI3 QDs suspended in hexane. We found that light illumination does induce photodegradation of CsPbI3 QDs. Steady-state spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, and transient absorption spectroscopy verified that light illumination leads to detachment of the capping agent, collapse of the CsPbI3 QD surface, and finally aggregation of surface Pb0. Both dangling bonds containing surface and Pb0 serve as trap states causing PL quenching with a dramatic decrease of PL quantum yield. Our work provides a detailed insight about the correlation between the structural and photophysical consequences of the photodegradation process in CsPbI3 QDs and may lead to the optimization of such QDs toward device applications.

All-inorganic CsPbBr3 perovskite colloidal quantum dots have recently emerged as a promising material for a variety of optoelectronic applications, among others for multiphoton-pumped lasing. Nevertheless, high irradiance levels are generally required for such multiphoton processes. One strategy to enhance the multiphoton absorption is taking advantage of high local light intensities using photonic nanostructures. Here, we investigate two-photon-excited photoluminescence of CsPbBr3 perovskite quantum dots on a silicon photonic crystal slab. By systematic excitation of optical resonances using a pulsed near-infrared laser beam, we observe an enhancement of two-photon-pumped photoluminescence by more than 1 order of magnitude when comparing to using a bulk silicon film. Experimental and numerical analyses allow relating these findings to near-field enhancement effects on the nanostructured silicon surface. The results reveal a promising approach for significantly decreasing the required irradiance levels for multiphoton processes being of advantage in applications such as biomedical imaging, lighting, and solar energy.

The sensing mechanism of a fluoride‐anion probe BODIPY‐amidothiourea (1c) has been elucidated through the density functional theory (DFT) and time‐dependent density functional theory (TDDFT) calculations. The theoretical study indicates that in the DMSO/water mixtures the fluorescent sensing has been regulated by the fluoride complex that formed between the probe 1c/two water molecules and the fluoride anion, and the excited‐state intermolecular hydrogen bond (H‐B) plays an important role in the fluoride sensing mechanism. In the first excited state, the H‐Bs of the fluoride complex 1cFH2 are overall strengthened, which induces the weak fluorescence emission. In addition, molecular orbital analysis demonstrates that 1cFH2 has more obvious intramolecular charge transfer (ICT) character in the S1 state than 1cH2 formed between the probe 1c and two water molecules, which also gives reason to the weaker fluorescence intensity of 1cFH2. Further, our calculated UV‐vis absorbance and fluorescence spectra are in accordance with the experimental measurements. © 2018 Wiley Periodicals, Inc.

Photo-generated charge carrier dynamics in Ruddlesden–Popper 2D perovskites with linear (n-BA) and branched (iso-BA) butylamine as spacing cations have been studied by using transient absorption and time-resolved photoluminescence spectroscopies.

Lead-free perovskite nanocrystals (NCs) were obtained mainly by substituting a Pb2+ cation with a divalent cation or substituting three Pb2+ cations with two trivalent cations. The substitution of two Pb2+ cations with one monovalent Ag+ and one trivalent Bi3+ cations was used to synthesize Cs2AgBiX6 (X=Cl, Br, I) double perovskite NCs. Using femtosecond transient absorption spectroscopy, the charge carrier relaxation mechanism was elucidated in the double perovskite NCs. The Cs2AgBiBr6 NCs exhibit ultrafast hot-carrier cooling (<1ps), which competes with the carrier trapping processes (mainly originate from the surface defects). Notably, the photoluminescence can be increased by 100 times with surfactant (oleic acid) added to passivate the defects in Cs2AgBiCl6 NCs. These results suggest that the double perovskite NCs could be potential materials for optoelectronic applications by better controlling the surface defects.

We fabricate photodetectors based on solution-processed single CH3NH3PbBr3 microcrystals (MCs) and map the two-photon absorption (TPA) excited photocurrent (PC) with spatial resolution of 1 mu m. We find that the charge carrier transport length in the MCs depends on the applied electric field, and increases from 5.7 mu m for 0.02 V bias (dominated by carrier diffusion) to 23.2 mu m for 2 V bias (dominated by carrier drift). Furthermore, PC shows strong spatial variations. Combining the PC mapping results with time-resolved photoluminescence microscopy, we demonstrate that the spatial distribution of PC mainly originates from the inhomogeneous distribution of trap-states across perovskite MCs. This suggests that there is still large margin for improvement of perovskite single crystal devices by better controlling of the traps.

White-light supercontinuum generation can be readily observed when gold nanostructures are irradiated with short pulses of light. It is believed that the nanostructures enhance the optical fields, which facilitates the supercontinuum white-light generation from the surrounding environment or the substrate. Here, we investigate the different nonlinear processes that contribute to the generation of the supercontinuum from plasmonic nanostructures themselves using a technique that isolates the different nonlinear contributions. By exciting a gold nanofilm with a pair of frequency shifted optical frequency combs, we demonstrate multiple modulation frequencies in the supercontinuum. Their dependence on the excitation intensity reveals that the supercontinuum originates from different orders of nonlinear light-matter interactions. This contrasts with the supercontinuum generation by a cascaded third-order optical nonlinear response in traditional dielectric-based white-light sources. The while-light emission from the gold nanofilm is efficient even under relatively weak excitation indicating that nonlocal effects in nanostructures may facilitate the supercontinuum generation by adding new pathways in the nonlinear interactions. The results provide experimental basis for the understanding of the collective nonlinear response of free-electrons in the metallic nanostructures and associated nonlinear processes, which are crucial in development of nonlinear metasurfaces and nanophotonic devices.

Efficient organic photosensitizers are attractive for cancer cell ablation in photodynamic therapy. Bright fluorescent photosensitizers are highly desirable for simultaneous imaging and therapy. However, due to fundamental competition between emission and singlet oxygen generation, design attempts to increase singlet oxygen generation almost always leads to the loss of fluorescence. Herein, it is shown for the first time that nanocrystallization enables a simultaneous and significant increase in the brightness and singlet oxygen generation of an organic photosensitizer. Spectroscopic studies show simultaneous enhancement in the visible light absorption and fluorescence after nanocrystallization. The enhanced absorption of visible light in nanocrystals is found to translate directly to the enhanced singlet oxygen production, which shows a higher ability to kill HeLa cells as compared to their amorphous counterpart.