Yu-Mo Zhang

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.

AIE (AIEE) and mechanofluorochromic performances of TPE-methoxylates: effects of single molecular conformations

Two methoxy-substituted tetraphenylethylene (TPE) derivatives, tetra(4-methoxyphenyl)ethylene (TMOE) and tetra(3,4-dimethoxyphenyl)ethylene (TDMOE), were synthesized by McMurry reaction in high yields. The nearly centrosymmetric and natural propeller shape of TMOE and TDMOE excluded intermolecular effects, such as H or J-aggregation and π–π stacking, on their AIE (AIEE) and mechanofluorochromic performance. The crystal structures of TMOE and TDMOE, and theoretical calculations proved that their emission colours are determined by single molecular conjugation. These molecules were used to investigate pure conformational effects on molecular emissions. The spectral properties of these molecules in five environments of crystal(s), THF solution, THF–water binary solution, solidified THF and amorphous states, were investigated. The crystalline to amorphous phase transition by grinding resulted in good mechanofluorochromic performances with high quantum yields and distinguishable emission change, which was further explored as anti-counterfeiting inks on banknotes.

Selective recognition of “solvent” molecules in solution and the solid state by 1,4-dimethoxypillar[5]arene driven by attractive forces

Highly selective binding of “solvent” molecules by 1,4-dimethoxypillar[5]arene (DMP[5]A) and their host–guest complexation in solution and the solid state have been investigated by multiple methods. Enthalpy changes and the size-fit effect play an important role in the high selectivity.

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.

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.

Endowing Hydrochromism to Fluorans via Bioinspired Alteration of Molecular Structures and Microenvironments and Expanding Their Potential for Rewritable Paper

Interest and effort toward new materials for rewritable paper have increased dramatically because of the exciting advantages for sustainable development and better nature life cycle. Inspired by how nature works within living systems, herein, we have used fluorans, as a concept verification, to endow original acidochromic, basochromic or photochromic molecules with broader properties, such as switchable with solvent, water, heat, electricity, stress, other force, etc., via simplified methods (i.e., via variation of submolecular structure or microenvironments). The hydrochromic visual change and reversible behavior of selected molecules have been explored, and the primary mechanism at the atomic or subatomic level has been hypothesized. In addition, several newly demonstrated hydrochromic fluorans have been utilized for water-jet rewritable paper (WJRP), which exhibit great photostability, high hydrochromic contrast, and fast responsive rate and which can be reused at least 30 times without significant variation. The water-jet prints have good resolution and various colors and can keep legibility after a few months or years. This improved performance is a major step toward practical applications of WJRP.

A fluorescence molecular switch with high contrast multi-emissions and ON/OFF states

By integrating an AIE active tetraphenylethylene (TPE) with a molecular switch of spiropyran (SP) through a single bond linkage, a single molecular switch with four high contrast states of red, cyan, blue and dark has been achieved by synergistic control of the molecular structures and their packing modes in the solid state.

Tunable RGB luminescence of a single molecule with high quantum yields through a rational design

Organic smart materials with red, green, and blue emissions are important in fabricating full-color display devices and multi-functional sensors. However, achieving RGB emission with high quantum yields using a single-molecule material has proven to be challenging. In this study, a new stimuli-responsive RGB luminescent organic molecule RHBT-G with a high quantum yield was rationally designed based on the combination of an AIEE luminophore and a fluorescent molecular switch. The ESIPT and TBET mechanisms were introduced to realize the RGB emission with single 365 nm excitation light in a solution and a PMMA film. X-ray single crystal diffraction and UV-Vis and photoluminescence (PL) spectroscopic analysis were carried out to prove the working mechanism of the dye. The rational design will accelerate the development of stimuli-responsive materials with excellent optical properties.

A methyl ketone bridged molecule as a multi-stimuli-responsive color switch for electrochromic devices

A new multi-stimuli-responsive molecular switch M5 with a methyl ketone bridge has been developed to fabricate a visible-near infrared absorbing electrochromic device. The basochromic and electrochromic properties of methyl ketone M5 with nitronaphthalene were studied in detail using a spectroelectrochemical method and density functional theory (DFT) calculations. The colorless M5 can be switched to green-yellow with broad visible-near infrared absorption (400–500 nm, 550–800 nm), stimulated by base and electricity. And a multi electron transfer–proton transfer mechanism (electrobase) of electrochromism for M5 has been proved using cyclic voltammetry (CV) and in situ UV-vis spectroscopy. This multi-stimuli-responsive color switch has been successfully applied in an electrochromic device with attractive properties such as short switching time and good reversibility.

Non-invasive fluorescence switch in polymer films based on spiropyran-photoacid modified TPE

Non-invasive fluorescent molecular switch is very challenging to achieve in solid state. Here, a new multi-stimulus-responsive fluorescent molecular switch of SPPATPE was prepared by linking spiropyran-photoacid (SPPA) with tetraphenylethene (TPE). The isomerization of SPPATPE can be triggered with either visible light/dark or acid/base in solution. Both ring-closed form (RCF) and ring-open form (ROF) of SPPATPE exhibit aggregation-induced emission (AIE). The RCF SPPATPE aggregates give out a cyan emission, which is assigned as the local excited emission of TPE moiety, while ROF SPPATPE have two forms of aggregation states, which give out two red emissions from intramolecular charge transfer. By selecting a suitable polymer of polyvinyl pyrrolidone (PVP) as the matrix, which gives the molecules a moderate alkaline environment, SPPATPE can exhibit non-invasive light/heat responsive emission switch in the polymer thin film with both good waterproofness and good processibility, which makes this kind of hybrid material potentially applicable in photo sensing, rewritable display, etc.