Mechanoresponsive Luminescence Research Articles

Unraveling the defect-induced photoluminescence of a donor-acceptor AIE luminogen

This work reveals the synthesis and photophysical properties of a donor-acceptor (D-A) aggregation-induced emission (AIE) compound, (E)-(4-((4-(diethylamino)-2-hydroxybenzylidene)amino)phenyl) (phenyl)methanone (DHPM), and further discloses its unique behavior, i.e., defect-induced photoluminescence (DIPL). The crystalline state of DHPM is non-emissive, whereas it exhibits high emission in the crystal defect points. Semiquantitative analysis demonstrates that mechanical force down to mN level would cause defect points on the crystal surface, further activating the DIPL, which manifests as a turn-on mechanoresponsive luminescence at the macro level. The crystallographic data and quantum chemical calculation suggest that DHPM molecules aggregate into D-A coupling π-dimers in the crystalline state to the disadvantage of luminescence; while the external mechanical stimuli such as pressing and grinding would destroy intermolecular weak interactions to disassemble the dimers, inducing much higher photoluminescence relative to the untreated crystal. In an application example, a mechanoresponsive film was prepared using DHPM, capable of storing handwriting patterns and binary data.

Mechanical Modulation of the Solid-State Luminescence of Tricarbonyl Rhenium(I) Complexes through the Interplay between Two Triplet Excited States.

Mechanoresponsive luminescence (MRL) materials promise smart devices for sensing, optoelectronics and security. We present here the first report on the MRL activity of two ReI complexes, opening up new opportunities for applications in these fields. Both complexes exhibit marked solid-state luminescence enhancement (SLE). Furthermore, the pristine microcrystalline powders emit in the yellow-green region, and grinding led to an amorphous phase with concomitant emission redshift and shrinking of the photoluminescence (PL) quantum yields and lifetimes. Quantum chemical calculations revealed the existence of two low-lying triplet excited states with very similar energy levels, that is, 3 IL and 3 MLCT, having, respectively, almost pure intraligand (IL) and metal-to-ligand charge-transfer (MLCT) character. Transition between these states could be promoted by rotation around the pyridyltriazole-phenylbenzoxazole bond. In the microcrystals, in which rotations are hindered, the 3 IL state induces the prominent PL emission at short wavelengths. Upon grinding, rotation is facilitated and the transition to the 3 MLCT state results in a larger proportion of long-wavelength PL. FTIR and variable-temperature PL spectroscopy showed that the opening of the vibrational modes favours non-radiative deactivation of the triplet states in the amorphous phase. In solution, PL only arises from the 3 MLCT state. The same mechanism accounts for the spectroscopic differences observed when passing from crystals to amorphous powders, and then to solutions, thereby clarifying the link between SLE and MRL for these complexes.

Phenyl-pyta-tricarbonylrhenium(I) complexes: combining a simplified structure and steric hindrance to modulate the photoluminescence properties.

Strongly luminescent tricarbonylrhenium(I) complexes are promising candidates in the field of optical materials. In this study, three new complexes bearing a 3-(2-pyridyl)-1,2,4-triazole (pyta) bidentate ligand with an appended phenyl group were obtained in very good yields owing to an optimized synthetic procedure. The first member of this series, i.e. complex 1, was compared with the previously studied complex RePBO to understand the influence of the fluorescent benzoxazole unit grafted on the phenyl ring. Then, to gauge the effect of steric hindrance on the luminescence properties, the phenyl group of complex 1 was substituted in the para position by a bulky tert-butyl group or an adamantyl moiety, affording complexes 2 and 3, respectively. The results of theoretical calculations indicated that these complexes were quite similar from an electronic point of view, as evidenced by the electrochemical study. In dichloromethane solution, under excitation in the UV range, all the complexes emitted weak phosphorescence in the red region. In the solid state, they could be excited in the blue region of the visible spectrum and they emitted strong yellow light. The photoluminescence quantum yield was markedly increased with raising the size of the substituent, passing from 0.42 for 1 to 0.59 for 3. The latter complex also exhibited clear waveguiding properties, unprecedented for rhenium complexes. From this point of view, these easy-synthesized and spectroscopically attractive complexes constitute a new generation of emitters for use in imaging applications and functional materials. However, the comparison with RePBO showed that the presence of the benzoxazole group leads to unsurpassed mechanoresponsive luminescence (MRL) properties, due to the involvement of a unique photophysical mechanism that takes place only in this type of complex.

A two-component molecular hybrid with enhanced emission characteristics and mechanoresponsive luminescence properties

A new binary charge transfer cocrystal system was successfully fabricated by virtue of donor–acceptor interaction and exhibited enhanced emission and sensitive mechanoresponsive luminescence properties.

Two-step mechanoresponsive luminescence and mechanical stimuli-induced release of small molecules exhibited by a luminescent cyclophane

Both two-step mechanoresponsive luminescence and mechanical stimuli-induced release of guest-molecules are exhibited by the same fluorescent cyclophane.

Acceptor-density engineering of push-pull typed carbazole derivatives for improving luminescent efficiency and mechanoresponsive luminescence

Improving luminous efficiency and mechanoresponsive luminescent (MRL) performance are significant issues in developing functional fluorescent materials. Herein, three push-pull typed carbazole derivatives with similar building blocks but differing in 2-vinylphenylmalononitrile acceptor numbers form mono- (m-VPMCz) to bis- (b-VPMCz) and tetra- (t-VPMCz) were designed and synthesized. Their luminescent properties in solution and aggregate states, as well as MRL performance were investigated. It was demonstrated that t-VPMCz with the highest acceptor-density and a highly twisted configuration achieved the highest luminescent efficiency in both solution and aggregation. b-VPMCz adopting a moderately twisted structure with two acceptor units exhibited the most remarkable MRL performance accompanied with the most distinct phase transition upon grinding. Our results suggest that rational control over the acceptor-density on push-pull typed molecules would be a promising way for realizing advanced functional fluorescent materials.

A flexible topo-optical sensing technology with ultra-high contrast

Elastic folding, a phenomenon widely existing in nature, has attracted great interests to understand the math and physical science behind the topological transition on surface, thus can be used to create frontier engineering solutions. Here, we propose a topo-optical sensing strategy with ultra-high contrast by programming surface folds on targeted area with a thin optical indicator layer. A robust and precise signal generation can be achieved under mechanical compressive strains (>0.4). This approach bridges the gap in current mechano-responsive luminescence mechanism, by utilizing the unwanted oxygen quenching effect of Iridium-III (Ir-III) fluorophores to enable an ultra-high contrast signal. Moreover, this technology hosts a rich set of attractive features such as high strain sensing, encoded logic function, direct visualisation and good adaptivity to the local curvature, from which we hope it will enable new opportunities for designing next generation flexible/wearable devices.

Selective Expression of Chromophores in a Single Molecule: Soft Organic Crystals Exhibiting Full‐Colour Tunability and Dynamic Triplet‐Exciton Behaviours

AbstractSoft luminescent materials are attractive for optoelectronic applications, however, switching dominant chromophores for property enrichment remains a challenge. Herein, we report the first case of a soft organic molecule (DOS) featuring selective expression of chromophores. In response to various external stimuli, different chromophores of DOS can take turns working through conformation changes, exhibiting full‐colour emissions peaking from 469 nm to 583 nm from ten individual single crystals. Dynamic triplet‐exciton behaviours including thermally activated delayed fluorescence (TADF), room‐temperature phosphorescence (RTP), mechanoluminescence (ML), and distinct mechano‐responsive luminescence (MRL) can all be realized. This novel designed DOS molecule provides a multifunctional platform for detection of volatile organic compounds (VOCs), multicolour dynamic displays, sensing, anticounterfeiting, and hopefully many others.

Selective Expression of Chromophores in a Single Molecule: Soft Organic Crystals Exhibiting Full-Colour Tunability and Dynamic Triplet-Exciton Behaviours.

Soft luminescent materials are attractive for optoelectronic applications, however, switching dominant chromophores for property enrichment remains a challenge. Herein, we report the first case of a soft organic molecule (DOS) featuring selective expression of chromophores. In response to various external stimuli, different chromophores of DOS can take turns working through conformation changes, exhibiting full-colour emissions peaking from 469 nm to 583 nm from ten individual single crystals. Dynamic triplet-exciton behaviours including thermally activated delayed fluorescence (TADF), room-temperature phosphorescence (RTP), mechanoluminescence (ML), and distinct mechano-responsive luminescence (MRL) can all be realized. This novel designed DOS molecule provides a multifunctional platform for detection of volatile organic compounds (VOCs), multicolour dynamic displays, sensing, anticounterfeiting, and hopefully many others.

Naphthalimide-containing coordination polymer with mechanoresponsive luminescence and excellent metal ion sensing properties.

Mechanoresponsive luminescent materials coupled with other functionalities are of particular interest due to their multiple external stimuli responsive properties. In this paper, a new sensitive mechanoresponsive luminescent coordination polymer, [Cd(INI)(DMF)2·DMF] (1) (H2INI = N-(5-isophthalic acid)-1,8-naphthalimide), has been successfully designed and synthesized. Complex 1 exhibits interesting mechanoresponsive and grinding-enhanced luminescence properties, and its luminescence colour changed from weak blue-green to bright blue upon grinding owing to the external pressure-induced destruction of ππ stacked arrangements in local defective areas. Moreover, the luminescence properties and uncoordinated carbonyl groups of well-ground g-1 endow it with excellent sensing ability for Cr3+ ions. This work will provide a new perspective to rationally design multifunctional coordination polymers that can serve as practical multi-responsive sensors to pressure and chemicals.

A new silver cluster that emits bright-blue phosphorescence.

A new hexanuclear silver(i) cluster (NC-Ag6) that emits bright-blue phosphoresce with a lifetime of 22.4 μs and a quantum yield (QY) of 22% under ambient conditions was synthesized. A film combining NC-Ag6 with yellow cerium(iii)-doped yttrium aluminum garnet phosphors (YAG:Ce3+) displays "chameleon"-like emission (including white), which is related to excitation-dependent Förster resonance energy transfer (FRET) between dual phosphors. The mechanoresponsive luminescence colour of this silver cluster ranges from blue to green virtually without loss of QY.

Switchable mechanoresponsive luminescence from traditional triphenylamine-thiophene carbaldehyde luminogens

Two donor-π-acceptor molecules (TPA-TC and 2Br-TPA-TC) with the common backbone of triphenylamine-thiophene carbaldehyde are used to explore the mechanoresponsive luminescence (MRL) properties. Both of them show remarkable solvatochromic effects, and their emission colors in solution could be changed from blue to yellow depending on the solvent polarity. Density function theory calculations indicate that the energy band gap of 2Br-TPA-TC is slightly larger than that of TPA-TC, which could explain their UV/Vis and fluorescence behaviors. It is noted that the green emission of TPA-TC and 2Br-TPA-TC is turned into deeper yellow-green after grinding and recovered via heating, and the two-color emission cycles are reversible. The MRL behaviors associated with the morphology transformation between crystalline and amorphous states could be monitored by powder X-ray diffraction spetra. Single-crystal structural analyses for TPA-TC and 2Br-TPA-TC help to understand the MRL mechanisms, in which the twisted molecules are loosely packed with the affixation of multiple H-bonding interactions and they are susceptible to grinding-induced photoluminescence changes.

Fluorophore-Labeling Tetraphenylethene Dyes Ranging from Visible to Near-Infrared Region: AIE Behavior, Performance in Solid State, and Bioimaging in Living Cells.

Functional molecules with aggregation-induced emission (AIE) have lately received the most attention due to their versatile functions in many fields. In this work, tetraphenylethene (TPE) as one of the most discussed AIE system was installed on different fluorophores including dansyl, naphthalimide, 4-nitro-1,2,3-benzoxadiazole (NBD), borondipyrromethene, and hemicyanine to afford a series of new fluorescent dyes, whose spectra covered the fluorescence region of the visible to near-infrared emission. Especially, these tetraphenylethene-containing compounds all exhibited AIE behavior. In solid state, they presented multiple colors including blue, orange, red, crimson, and NIR (near-infrared emission). It was worth mentioning that the dansyl- and NBD-coated TPEs exhibited obvious mechanoresponsive luminescence phenomena. Moreover, these AIE-based TPEs displayed a good bioimaging performance in living cells. This work will be helpful for designing functional AIE dyes with different fluorescence emission.

Tetraphenyethylene-Fused Coumarin Compound Showing Highly Switchable Solid-State Luminescence

This paper explores high-contrast mechanoresponsive luminescence (MRL) turn-on materials by combining the features of the coumarin and typical aggregation-induced emission enhancement (AIEE) tetrap...

A TADF compound with high-contrast mechano-responsive fluorescence on/off switching for both sequential and combinational logic gates

Both sequential and combinational type logic gates can be achieved within a TADF luminogen with high-contrast mechano-responsive luminescence.

Utilizing the heterocyclic effect towards high contrast ratios of mechanoresponsive luminescence based on aromatic aldehydes

Utilizing heterocycles with different-membered rings and additional heteroatoms would be an effective way to increase MRL contrast ratios by tuning different types of noncovalent interactions in the solid state.

Construction of a Layered Hydrogen-Bonded Organic Framework Showing High-Contrast Mechanoresponsive Luminescence Turn-On

This paper probes the mechanoresponsive luminescence and aggregation-induced emission (AIE) properties of N-phenylcarbazole-based thiophene/dicyanovinyl (T–DCV) derivatives. In bis-T–DCV planar mol...

Twisted Donor−π–Acceptor Carbazole Luminophores with Substituent-Dependent Properties of Aggregated Behavior (Aggregation-Caused Quenching to Aggregation-Enhanced Emission) and Mechanoresponsive Luminescence

To get insight on how the donor (D)/acceptor (A) substitutions and their substitution pattern affect optical properties in solution and in the solid state, four novel carbazole derivatives C1–C4 wi...

Highly sensitive switching of solid-state luminescence by controlling intersystem crossing

The development of intelligent materials, in particular those showing the highly sensitive mechanoresponsive luminescence (MRL), is desirable but challenging. Here we report a design strategy for constructing high performance On–Off MRL materials by introducing nitrophenyl groups to molecules with aggregation-induced emission (AIE) characteristic. The on–off methodology employed is based on the control of the intersystem crossing (ISC) process. Experimental and theoretical investigations reveal that the nitrophenyl group effectively opens the nonradiative ISC channel to impart the high sensitivity and contrast On–Off behavior. On the other hand, the twisted AIE luminogen core endows enhanced reversibility and reduces the pressure required for the luminescence switching. Thin films can be readily fabricated from the designed materials to allow versatile applications in optical information recording and haptic sensing. The proposed design strategy thus provides a big step to expand the scope of the unique On–Off MRL family.

Distinct Mechanoresponsive Luminescence, Thermochromism, Vapochromism, and Chlorine Gas Sensing by a Solid-State Organic Emitter.

In this study, we report a synthetically simple donor–acceptor (D–A)-type organic solid-state emitter 1 that displays unique fluorescence switching under mechanical stimuli. Orange and yellow emissive crystals of 1 (1O, 1Y) exhibit an unusual “back and forth” fluorescence response to mechanical force. Gentle crushing (mild pressure) of the orange or yellow emissive crystal results in hypsochromic shift to cyan emissive fragments (λem = 498–501 nm) with a large wavelength shift Δλem = −71 to −96 nm, while further grinding results in bathochromic swing to green emissive powder λem = 540–550 nm, Δλem = +40 to 58 nm. Single-crystal X-ray diffraction study reveals that molecules are packed by weak interactions, such as C–H···π, C–H···N, and C–H···F, which facilitate intermolecular charge transfer in the crystal. With the aid of structural, spectroscopic, and morphological studies, we established the interplay between intermolecular and intramolecular charge-transfer interaction that is responsible for this elusive mechanochromic luminescence. Moreover, we have also demonstrated the application of this organic material for chlorine gas sensing in solid state.