Anomalous proton transfer of a photoacid HPTS in nonaqueous reverse micelles.

The proton transfer reaction is one of the fundamental chemical reactions where the reaction dynamics strongly depend on solvent properties such as acidity or basicity. A photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) shows a sharp decrease of pKa (7.7 → 0.5) upon photoexcitation, and the excited-state proton transfer (ESPT) occurs with ultrafast time constants of 2.5 and 89 ps in bulk aqueous solution. However, the two-step proton transfers via the contact ion pair formation and the proton diffusion are strongly limited inside the nanopools of reverse micelles (RMs). The confinement in small RMs strongly impeded the proton transfer reactions. In this work, we report the ESPT of HPTS confined in methanol-in-oil RMs by steady-state and time-resolved electronic spectroscopy. Interestingly, HPTS shows substantial deprotonation in the excited state only in small RMs, while the ESPT of HPTS does not occur in bulk methanol solution due to the low basicity of aliphatic alcohols. The kinetic analysis of time-resolved fluorescence and transient absorption measurements will compare the proton transfer dynamics of HPTS in the water-in-oil and methanol-in-oil RMs. The ESPT of photoacids, especially in the nonaqueous RMs, can be crucial in understanding many important chemical reactions involving proton transfer in the confined environments of cells and membranes.

Ultrathin covalent organic overlayers on metal nanocrystals for highly selective plasmonic photocatalysis.

Metal nanoparticle-organic interfaces are common but remain elusive for controlling reactions due to the complex interactions of randomly formed ligand-layers. This paper presents an approach for enhancing the selectivity of catalytic reactions by constructing a skin-like few-nanometre ultrathin crystalline porous covalent organic overlayer on a plasmonic nanoparticle surface. This organic overlayer features a highly ordered layout of pore openings that facilitates molecule entry without any surface poisoning effects and simultaneously endows favourable electronic effects to control molecular adsorption-desorption. Conformal organic overlayers are synthesised through the plasmonic oxidative activation and intermolecular covalent crosslinking of molecular units. We develop a light-operated multicomponent interfaced plasmonic catalytic platform comprising Pd-modified gold nanoparticles inside hollow silica to achieve the highly efficient and selective semihydrogenation of alkynes. This approach demonstrates a way to control molecular adsorption behaviours on metal surfaces, breaking the linear scaling relationship and simultaneously enhancing activity and selectivity.

Highly luminescent dual-phase CsPbBr3/Cs4PbBr6 microcrystals for a wide color gamut for backlight displays.

Cesium lead bromide perovskite nanocrystals (NCs) embedded in Cs4PbBr6 or CsPb2Br5 matrices forming core/shell structures are promising luminescent materials that exhibit remarkable photoluminescence properties meeting the need in a wide range of applications while overcoming stability challenges. Here, we report the large-scale, ligand-free synthesis of dual-phase Cs4PbBr6/CsPbBr3 microcrystals (MCs) using ultrasonication at room temperature, exhibiting a high photoluminescence quantum yield (PLQY) of 82.7% and good stability. High-resolution transmission electron microscopy and X-ray photoelectron characterization confirm that CsPbBr3 NCs are embedded in the Cs4PbBr6 matrix-forming CsPbBr3/Cs4PbBr6 dual-phase structure. The evolution of the luminescence properties with temperature suggests that the strong green emission results from direct exciton recombination in the isolated [PbBr6]4- octahedra, which possess a large exciton binding energy of 283.6 meV. As revealed from their emission intensities, the dual-phase CsPbBr3/Cs4PbBr6 MCs demonstrate excellent stability against ultraviolet irradiation (76%), good moisture resistance (42.7%), and good thermal tolerance (51%). It is understood that such excellent PLQY and stability are due to the surface passivation of the CsPbBr3 NCs attributed to the large bandgap as well as the isolated [PbBr6]4- octahedra separated by Cs+ ions in the Cs4PbBr6 crystal lattice. Finally, the suitability of the green-emitting CsPbBr3/Cs4PbBr6 material for achieving white-light emission and a wide color gamut is evaluated by constructing a prototype white light-emitting diode (w-LED) using CsPbBr3/Cs4PbBr6 and red-emitting K2SiF6:Mn4+ materials taken in different weight ratios and combined with a blue light-emitting InGaN LED chip (λ = 455 nm). The constructed w-LED device exhibits the color coordinates (0.3315, 0.3289), an efficacy of 68 lm W-1, a color rendering index of 87%, a color temperature of 5564 K, and a wide color gamut of ∼118.78% (NTSC) and ∼88.69% (Rec. 2020) with RGB color filters in the CIE 1931 color space. Therefore, based on our present findings, we strongly believe that the dual-phase CsPbBr3/Cs4PbBr6 material is a promising green-emitting phosphor for use in w-LEDs as the backlight of display systems.

Twisted intramolecular charge transfer of nitroaromatic push-pull chromophores.

The structural changes during the intramolecular charge transfer (ICT) of nitroaromatic chromophores, 4-dimethylamino-4'-nitrobiphenyl (DNBP) and 4-dimethylamino-4'-nitrostilbene (DNS) were investigated by femtosecond stimulated Raman spectroscopy (FSRS) with both high spectral and temporal resolutions. The kinetically resolved Raman spectra of DNBP and DNS in the locally-excited and charge-transferred states of the S1 state appear distinct, especially in the skeletal vibrational modes of biphenyl and stilbene including ν8a and νC=C. The ν8a of two phenyls and the νC=C of the central ethylene group (only for stilbene), which are strongly coupled in the planar geometries, are broken with the twist of nitrophenyl group with the ICT. Time-resolved vibrational spectroscopy measurements and the time-dependent density functional theory simulations support the ultrafast ICT dynamics of 220-480 fs with the twist of nitrophenyl group occurring in the S1 state of the nitroaromatic chromophores. While the ICT of DNBP occurs via a barrier-less pathway, the ICT coordinates of DNS are strongly coupled to several low-frequency out-of-phase deformation modes relevant to the twist of the nitrophenyl group.

Intramolecular charge transfer of a push-pull chromophore with restricted internal rotation of an electron donor.

Intramolecular charge transfer (ICT) of 4-(dicyanomethylene)-2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)vinyl]-4H-pyran (LD688) in DMSO solution was investigated by femtosecond stimulated Raman spectroscopy (FSRS) with 403 nm excitation. The molecular structure of LD688 is similar to that of a well-known push-pull chromophore, 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM), except that the internal rotation of the electron-donating dimethylamino group is restricted with the introduction of the julolidine moiety. Upon photo-excitation, LD688 shows an ultrafast (1.0 ps) ICT followed by the vibrational relaxation (3-8 ps) in the charge-transfer (CT) state. Two distinct Raman spectra of LD688 in the locally excited (LE) and CT state of the S1 state were retrieved from FSRS measurements. Based on the time-dependent density functional theory (TDDFT) simulations, a "twisted" julolidine geometry of LD688 was proposed for the ICT state, which was further confirmed in comparison to the spectral changes of several push-pull chromophores with the π-conjugated backbone of stilbene, biphenyl, styrylpyran, styrylpyridinium, and styrene in terms of the skeletal vibrational modes of ν19b,py, νCC,ph, and νCN.

Intramolecular Charge Transfer of 1-Aminoanthraquinone and Ultrafast Solvation Dynamics of Dimethylsulfoxide.

The intramolecular charge transfer (ICT) of 1-aminoanthraquinone (AAQ) in the excited state strongly depends on its solvent properties, and the twisted geometry of its amino group has been recommended for the twisted ICT (TICT) state by recent theoretical works. We report the transient Raman spectra of AAQ in a dimethylsulfoxide (DMSO) solution by femtosecond stimulated Raman spectroscopy to provide clear experimental evidence for the TICT state of AAQ. The ultrafast (~110 fs) TICT dynamics of AAQ were observed from the major vibrational modes of AAQ including the νC-N + δCH and νC=O modes. The coherent oscillations in the vibrational bands of AAQ strongly coupled to the nuclear coordinate for the TICT process have been observed, which showed its anharmonic coupling to the low frequency out of the plane deformation modes. The vibrational mode of solvent DMSO, νS=O showed a decrease in intensity, especially in the hydrogen-bonded species of DMSO, which clearly shows that the solvation dynamics of DMSO, including hydrogen bonding, are crucial to understanding the reaction dynamics of AAQ with the ultrafast structural changes accompanying the TICT.