Minjie Li

Hydrochromic molecular switches for water-jet rewritable paper

The days of rewritable paper are coming, printers of the future will use water-jet paper. Although several kinds of rewritable paper have been reported, practical usage of them is rare. Herein, a new rewritable paper for ink-free printing is proposed and demonstrated successfully by using water as the sole trigger to switch hydrochromic dyes on solid media. Water-jet prints with various colours are achieved with a commercial desktop printer based on these hydrochromic rewritable papers. The prints can be erased and rewritten dozens of times with no significant loss in colour quality. This rewritable paper is promising in that it can serve an eco-friendly information display to meet the increasing global needs for environmental protection.

Dynamic Behavior of Molecular Switches in Crystal under Pressure and Its Reflection on Tactile Sensing

Molecular switches have attracted increasing interest in the past decades, due to their broad applications in data storage, optical gating, smart windows, and so on. However, up till now, most of the molecular switches are operated in solutions or polymer blends with the stimuli of light, heat, and electric fields. Herein, we demonstrate the first pressure-controllable molecular switch of a benzo[1,3]oxazine OX-1 in crystal. Distinct from the light-triggered tautomerization between two optical states, applying hydrostatic pressure on the OX-1 crystal results in large-scale and continuous states across the whole visible light range (from ∼430 to ∼700 nm), which has not been achieved with other stimuli. Based on detailed and systematic control experiments and theoretical calculation, the preliminary requirements and mechanism of pressure-dependent tautomerization are fully discussed. The contributions of molecular tautomerization to the large-scale optical modulation are also stressed. Finally, the importance of studying pressure-responsive materials on understanding tactile sensing is also discussed and a possible mechanotransduction mode is proposed.

Directing the Growth of Semiconductor Nanocrystals in Aqueous Solution : Role of Electrostatics

In this study, we demonstrate a new insight into the growth stage of aqueous semiconductor nanocrystals (NCs); namely, that the experimental variable-dependent growth rate and photoluminescence quantum yields (PLQYs) are understandable according to electrostatics. In this context, the aqueous NCs possess (from core outwards) an inorganic core, ligand layer, adsorbed layer, and a diffuse layer. The presence of an electric double-layer not only makes the NCs dispersible in the colloidal solution, but also governs the migration of monomers and monomer adsorption on the NC surface. To maintain NC growth, monomers need to migrate through the double-layer. Consequently, the nature of the diffuse layer influences the ability of monomer diffusion and hence the growth rate of NCs. Systematic studies reveal that the experimental variables, including precursor concentrations, pH of the solution, additional NaCl concentrations, ratio of Cd to ligand, and the nature of the ligands significantly govern the nature of the NC electric double-layer. The experimental variables, which reduce the thickness of the diffuse layer, benefit from monomer diffusion and a rapid growth of NCs. However, on the other hand, the diffuse layer also presents a charge-selective transfer of Cd monomers. The neutral monomers, such as the complex of Cd(2+) and 3-mercaptopropionic acid (MPA) with 1:1 molar ratio [Cd(MPA)], migrate through the diffuse layer more easily than the charged ones [Cd(MPA)(2) (2-) or Cd(MPA)(3) (4-)], thus facilitating the growth of NCs. The nature of the adsorbed layer inside the diffuse layer, defined as the assumed interface of solid NCs and the liquid environment, also affects the growth rate and especially the PLQYs of NCs through the adsorption and coalescence of monomers on this interface. Strong interaction between the adsorbed layer and Cd monomers provides the opportunity to accelerate NC growth and to obtain NCs with high PLQYs.

Full-color tunable mechanofluorochromism and excitation-dependent emissions of single-arm extended tetraphenylethylenes

We propose a new single-arm extension strategy on traditional tetraphenylethylene and successfully develop a new series of full-color (from ∼450 nm to ∼740 nm) tunable mechanofluorochromic materials. These materials exhibit efficient solid-state emission (quantum yield Φf > 10%) and high mechanofluorochromic contrast (wavelength shift from ∼50 nm to ∼100 nm). More importantly, we discover an unexpected excitation-dependent emission phenomenon of mechanofluorochromic materials and propose to utilize this new excitation-dependent emission behavior of materials to evaluate their mechanical-responsive performances more comprehensively. Finally, the unique feature of abundant emissions of mechanofluorochromic materials by changing the excitation light has shown application potential in dual channel anti-counterfeiting.

White-light emission nanofibers obtained from assembling aqueous single-colored CdTe NCs into a PPV precursor and PVA matrix

Nanofibers with white-light emission were directly prepared from aqueous single-colored CdTe nanocrystals (NCs), poly[p-xylene-α-(dialkyl-sulfonium halide)] [namely, the sulfonium polyelectrolyte precursor of poly(p-phenylene-vinylene) (PPV)], and poly(vinyl alcohol) (PVA) by an electrospinning technique. An interesting result was that the nanofibers gave white-light emission without further thermal transformation of the precursor into PPV and hence the photoluminescence (PL) of CdTe NCs was well maintained. The procurement of white-light emission was due to careful tuning of the interactions between CdTe NCs, PVA and the PPV precursor. Herein, PVA was an important matrix because it not only improved the blue-light emission significantly by avoiding the π–π stack quenching from the PPV precursor, but also increased compatibility between the NCs and the PPV precursor. In addition, the single-colored CdTe NCs avoided the Forster resonance energy transfer (FRET) between NCs, resulting in strong white-light emission. The use of the electrospinning technique kept the emission color of solid film almost the same as that of the liquid and as a result, nanofibers with bright white-light emission were finally obtained.

Time-resolved monitoring of dynamic self-assembly of Au(I)-thiolate coordination polymers

Research interest in dynamic assemblies of coordination polymers (CPs) has been rising in recent years for the similarity with life systems in their self-adaptable morphologies and properties. However, monitoring of the assembly process and elucidating the nature for the morphological transformation are very challenging. Here, UV-Vis spectroscopy has been explored as a time-resolved method for monitoring the self-assembly of Au(I)–thiolate CPs in situ. Both step-wise and synergetic effects of the weak interactions in Au(I)–3-mercaptopropionic acid (MPA) CPs, such as H-bonding, coordination bonding, Au(I)–Au(I) interactions and static interactions have been found from the spectral fingerprints, which elucidated the driving forces for the unique morphological transformations from strings to lamellar structures. This work represents a breakthrough in that dynamic self-assembly behaviours can be explained by molecular interactions from molecular level evidences. Based on the spectral fingerprint–structure relationship the reversible and dynamic assembly of Au(I)–MPA CPs can be easily probed.

A new class of “electro-acid/base”-induced reversible methyl ketone colour switches

Methyl ketone has been designed as a switching unit for electrically addressable molecular colour switches. A newly proposed mechanism of “electro-acid/base” (radical ions)-induced intermolecular proton transfer for the colour switch is proven clearly by cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR) and in situ UV-Vis spectroscopy. A dramatic spectral absorption shift (about 291 nm) is observed during the switching, and blue, yellow and green colours are obtained by adjusting the substituents on the methyl ketone-bridged unit. The in situ “electro-acid/base” is far more convenient than the conventional chemical stimulus of acids or bases for the manipulation of the molecular switching properties. This new switching method and molecular structure manipulation will inspire and accelerate the further development of broad switching materials and applications in ultrathin flexible displays, etc.

Manipulation of semiconductor nanocrystal growth in polymer soft solids

Polymers are demonstrated as an efficient medium to manipulate the size and fluorescence evolution of semiconductor nanocrystals (NCs). By tuning the nature of the polymers and the interactions between NCs and polymers, the growth rate of NCs could be controlled. This finding provides a new protocol for the design and fabrication of functional nanodevices.

Cationic Ligand Protection: A Novel Strategy for One-Pot Preparation of Narrow-Dispersed Aqueous CdS Spheres

In order to prepare the building blocks with middle refractive index for fabricating colloidal crystals, a new strategy of cationic ligand protection (CLP) was developed for one-pot preparation of narrow-dispersed CdS spheres with tunable sizes (94-303 nm) and good aqueous dispersibility. The key of CLP strategy is controlling the aggregation behavior of small CdS nanocrystals (NCs) via automatic control of the ligand modification condition at different reaction stages, which is realized by selecting gradual decomposition of thioacetamide (TAA) as the anionic precursor and cetyltrimethylammonium bromide (CTAB) cations as the ligands for anions. Accordingly, at the initial stage of the reaction when TAA decomposes incompletely, the surface atoms of the CdS NCs are mainly Cd cations, leaving less anionic sites for the coordination of ligands. The poor ligand modification condition of CdS NCs still leads to their aggregation. Along with the decomposition of TAA, S anions and cationic ligands gradually dominate the surface of CdS aggregates, providing the aggregates sufficient protection, and thus, they are stably dispersed in water. The current CLP strategy simultaneously involves the processes of NC formation and NC assembly and therefore promotes the one-pot preparation of NC aggregates. The as-prepared CdS spheres possess uniform size, good water-dispersibility, and relative higher refractive index and thus can be applied for fabricating colloidal crystals.

“One-pot” synthesis and shape control of ZnSe semiconductor nanocrystals in liquid paraffin

In this paper, we demonstrated a “one-pot” strategy for synthesizing ZnSe nanocrystals (NCs) in liquid paraffin. All materials, including Zn source, Se source, and ligand, were mixed in liquid paraffin beforehand, which avoided the injection of Se source at high temperature. The resultant ZnSe NCs possessed high photoluminescence quantum yields and narrow size distribution. Moreover, the size, shape, and crystal phase of NCs were controllable by altering the experimental variables, such as precursor concentration, Zn:Se molar ratio, and heating rate. Because the raw materials used here were low-cost and environmentally friendly, this “one-pot” synthetic protocol would facilitate the commercial scale synthesis of high-quality ZnSe NCs.