Yuai Duan

Stimuli-responsive fluorophores with aggregation-induced emission: implication for dual-channel optical data storage

For optical data storage (ODSes), fluorescence may be an ideal computer-readable signal on account of its scalability, correction capability, and potentially enhanced capacity. In this study, we report the synthesis and photoluminescence (PL) behaviour of a class of fluorophores with electron donor and acceptor units. They exhibit AIE properties with long-wavelength emission colour. Owing to the reactive electron withdrawing group, their emission spectra have shown either “on–off” switching or a remarkable blue-shift upon alkali treatment. The changes in their fluorescence are due to TICT–LE transition as confirmed by their absorption spectra and DFT calculation. The fluorescent response can be recovered or not, depending on the chemical stimulant used for encoding. Based on the stimuli-responsive nature of the fluorophores, we have further developed both rewritable and write-once read-many (WORM) ODSes with single or dual-channel outputs. “0” and “1” state of the encoded binary data, corresponding to the TICT and LE emission, can be read easily using a spectrofluorophotometer. In addition, the WORM ODSes enjoy the advantage of long-term stability and the rewritable ODSes can be reused many times without fatigue.

Mechanochromic luminogen with aggregation-induced emission: implications for ink-free rewritable paper with high fatigue resistance and low toxicity

In this study, we report the synthesis and photoluminescence (PL) behaviour of a new luminogen, DHBA, a functionalized Schiff base. The twisted molecular conformation together with the incorporation of electron donor–acceptor pairs endow the luminogen with both aggregation-induced emission and twisted intramolecular charge transfer properties. The crystalline luminogen shows strong green emission at around 514 nm while the emission colour turns orange-red (562 nm) with a big loss in the quantum efficiency after grinding. The colour of the ground sample can be readily converted back into the initial state though recrystallization by either immersing or fuming in organic solvents. The conversion can be implemented for more than 30 cycles, showing good repeatability. This luminogen probably shows the highest fatigue resistance among all the mechanochromic compounds that have been reported. Inspired by the unique mechanochromism, an archetype of ink-free rewritable paper is developed. Letters and patterns can be easily written using pressure and erased by fumigation in handwriting experiments. The luminogen also enjoys the advantages of a one-pot synthesis and low biotoxicity, providing a reliable candidate for green rewritable paper technology to fulfill demands for sustainability and carbon mitigation.

Solvent-assistant self-assembly of an AIE+TICT fluorescent Schiff base for the improved ammonia detection.

Solvent-assistant self-assembly of an AIE+TICT fluorescent Schiff base into one-dimensional nanofilaments has been developed. The orientation of the assemblies can be controlled by a simple dewetting process: the filaments are interweaved when the self-assembly process is performed on a horizontal substrate, while tilting the substrate to a tiny angle results in the formation of highly oriented ones with long-range order as verified by microscopic examination. The compound shows remarkable fluorescent response to ammonia gas based on a TICT-LE transition. The self-assembled film presents higher detection sensitivity compared with the non-assembled test paper: the former enables 4.75 times faster response time and 6.86 times lower detection limit than the latter. Furthermore, the former demonstrates better selectivity toward ammonia gas in the presence of various organic amines. The sensing devices also enjoy the advantage of cyclic utilization. The fluorescence of the fumed devices can be converted back into the original state when they are heated at 100 °C for 5 min, as thermal treatment can desorb the ammonia gas that adsorbed in the sensing devices.

One-pot synthesis of a mechanochromic AIE luminogen: implication for rewritable optical data storage

In this study, we report the synthesis and photoluminescence (PL) properties of a new luminogen, (E)-4-(((2-hydroxynaphthalen-1-yl)methylene)amino)benzoic acid (HNMA), which undergoes both aggregation-induced emission (AIE) and intramolecular charge transfer (TICT) mechanisms. The crystalline powder emits intense green fluorescence under UV illumination, whereas after grinding, the emission decreases dramatically with a red-shift in the emission wavelength. The changes in its fluorescence are due to crystalline–amorphous transition as confirmed by XRD analysis and the microscopic technique. Furthermore, the emission of the amorphous phase can be readily converted back into its original state by a recrystallization process, which can be realized by either immersion or fumigation in common organic solvents. Importantly, the switching between two states can be repeated for over 25 cycles without fatigue. Based on indefatigability intrinsic to mechanochromic luminogens, a new strategy to realize rewritable optical data storage is developed. “0” and “1” states of the binary data, corresponding to the two emission states, can be encoded on a substrate by mechanical force and erased by fumigation treatment, demonstrating good repeatability and potential for bulk storage.

A turn-on type mechanochromic fluorescent material based on defect-induced emission: implication for pressure sensing and mechanical printing

In principle, mechanochromic fluorescent (MCF) materials can be classified into two categories: turn-off and turn-on types. The latter is superior to the former as it is more sensitive and less likely to induce false-positive signals. In this study, we report the synthesis and photoluminescence (PL) behaviour of a biplane molecule, 2-amino-3-((E)-((2-hydroxynaphthalen-1-yl)methylene)amino)maleonitrile (AHM), which consists of an electron donor (D) plane and an acceptor (A) plane. AHM undergoes both aggregation-enhanced emission (AEE) and intramolecular charge transfer (ICT) mechanisms. It is weakly emissive in the crystalline phase but shows a remarkable emission enhancement with a large bathochromic-shift (67 nm) upon applying mechanical force. As confirmed by both spectroscopic methods and fluorescence microscopy, MCF arises from crystal defects, where the molecules would twist their conformation to break D–A coupling, inducing strong fluorescence. This defect-induced emission (DIE) endows the material with ultra-high sensitivity toward pressure. The detection limit (DL) of an AHM-based sensing film is as low as 1.1 N (0.62 MPa). Additionally, the DIE phenomenon of AHM further enables a new mechanical printing technique. Handwriting and imprinted letters exhibit a distinct emission change which is readily detected by the naked eyes. We believe that the unique turn-on MCF and high sensitivity make this material well-suited to tackle the challenges faced by conventional MCF materials.

Solid-supported synergistic twain probes with aggregation-induced emission: A sensing platform for fingerprinting volatile amines

Two solid emitters DMA and HBA were synthetized, showing aggregation-induced emission (AIE) phenomenon. Incorporation of electron donor-acceptor (D-A) pairs endows the solid emitters with both charge transfer (CT) state emission and distinct solvatochromic effect. The compounds undergo a rapid interaction with amines followed by the resulting weakened electron withdrawing ability to yield florescent products with either hypochromatic shift or decrease in the emission intensity. DFT calculations on HOMO and LUMO electronic cloud distribution and energy levels have demonstrated that the transition from CT state to locally excited (LE) state is mainly responsible for the spectral changes. The AIE compounds were thus developed into solid-supported amine sensor and show nice linear relationship. The detection limits of DMA- and HBA-based sensing film to ammonia gas are 2.61Pa and 2.63Pa, respectively. Time-dependent emission spectra upon treating with a variety of amines exhibited differential responses, in which two factors were defined accordingly, including maximum emission wavelength and quenching efficiency. Two-dimensional coordinate system was then built on the basis of the two factors, creating a fingerprint data base for the involved amines. The fingerprint map shows a clear differentiation of the tested amines, making DMA and HBA synergistic twain probes for identification of diverse amines.

Effect of substituent position on aggregation-induced emission, customized self-assembly, and amine detection of donor-acceptor isomers: Implication for meat spoilage monitoring

We synthesized a class of positional isomers by attaching electron donor and acceptor units in different sites of a conjugated core. These isomers exhibit both aggregation-induced emission (AIE) and intramolecular charge transfer (ICT) effects, which are proved by adequate spectroscopic analysis. Their structure-property relationships were systematically studied. We found that relocation of the D/A units would have remarkable impact on the intermolecular dipole-dipole interaction, further controlling the shape and color of the self-assembled architectures. With D/A units shifting to different sites, four types of the structures appear sequentially, including quadrate microsheets, microrods, nanofilaments and nanowires. Furthermore, the A unit (benzoic acid moiety) of the AIE isomers is easy to adsorb amines, leading to changes in both emission wavelength and intensity. Then a portable sensor is prepared on solid support based on the self-assembled architecture of HMBA-4, which has been proved to be the most sensitive to amines. It affords fast spectral responses as well as a low detection limit of 186 Pa (vapour pressure). The sensing mechanism was revealed by density functional theory (DFT) calculation, which indicates that the spectral responses stem from the weakened ICT effect. The sensor is able to detect amine vapours generated by meat, and thus succeeds in detecting the spoiled pork samples, offering high potential for meat spoilage monitoring in real-world applications.