Multicolor Fluorescence Writing Based on Host–Guest Interactions and Force‐Induced Fluorescence‐Color Memory

A new strategy is reported for multicolor fluorescence writing on thin solid films with mechanical forces. This concept is illustrated by the use of a green‐fluorescent pentiptycene derivative 1 , which forms variably colored fluorescent exciplexes: a change from yellow to red was observed with anilines, and fluorescence quenching (a change to black) occurred in the presence of benzoquinone. Mechanical forces, such as grinding and shearing, induced a crystalline‐to‐amorphous phase transition in both the pristine and guest‐adsorbed solids that led to a change in the fluorescence color (mechanofluorochromism) and a memory of the resulting color. Fluorescence drawings of five or more colors were created on glass or paper and could be readily erased by exposure to air and dichloromethane fumes. The structural and mechanistic aspects of the observations are also discussed.     

Mechanically Induced Multicolor Change of Luminescent Materials

Mechanofluorochromic or piezochromic fluorescence chemistry involves the switching and tuning of the luminescent properties of solid‐state materials induced by exogenous forces, such as grinding, shearing, compression, tension, and so forth. Up until now, most reported mechanochromic systems, including liquid crystals, organic molecules, organometallic compounds, polymers, and dye‐doped polymers, have displayed reversible two‐color changes, which arise from either supramolecular or chemical structure transformations. However, fluorescent materials that undergo mechanically induced multicolor changes remain rare; this Minireview is focused on such materials. Topics are categorized according to the different applied forces that are required to induce the multicolor change, including mechanical control of either the supramolecular structures or the chemical structures, and mechanical control of both the supramolecular structures and chemical structures.     

Novel 2,1,3-benzothiadiazole-based red-fluorescent dyes with enhanced two-photon absorption cross-sections

This paper reports the two-photon absorbing and orange-red fluorescence emitting properties of a series of new 2,1,3-benzothiadiazole (BTD)-based D-pi-A-pi-D-type and star-burst-type fluorescent dyes. In the D-pi-A-pi-D-type dyes 1-6, a central BTD core was connected with two terminal N,N-disubstituted amino groups via various pi-conjugated spacers. The star-burst-type dyes 8 and 10 have a three-branched structure composed of a central core (benzene core in 8 and triphenylamine core in 10) and three triphenylamine-containing BTD branches. All the BTD-based dyes displayed intense orange-red color fluorescence in a region of 550-689 nm, which was obtained by single-photon excitation with good fluorescent quantum yield up to 0.98 as well as by two-photon excitation. Large two-photon absorption (TPA) cross-sections (110-800 GM) of these BTD dyes were evaluated by open aperture Z-scan technique with a femtosecond Ti/sapphire laser. The TPA cross-sections of D-pi-A-pi-D-type dyes 2-6 with a benzene, thiophene, ethene, ethyne, and styrene moiety, respectively, as an additional pi-conjugated spacer are about 1.5-2.5 times larger than that of 1c with only a benzene spacer. The TPA cross-sections significantly increased in three-branched star-burst-type BTDs 8 (780 GM) with a benzene core and 10 (800 GM) with a triphenylamine core, which are about 3-5 times larger than those of the corresponding one-dimensional sub-units 9 (170 GM) and 11 (230 GM), respectively. The ratios of sigma/e(pi) between three-branched and one-dimensional dyes were 6.5:3.8 (for 8 and 9) and 6.0:4.0 (for 10 and 11), which are larger than those predicted simply on the basis of the chromophore number density (1:1), according to a cooperative enhancement of the two-photon absorbing nature in the three-branched system.     

Alkyl Chain Length- and Polymorph-Dependent Photomechanochromic Fluorescence of Anthracene Photodimerization in Molecular Crystals: Role of the Lattice Stiffness

Anthracene–pentiptycene hybrid systems 1-Cn , where n refers to the number of carbon atoms in the linear alkyl chain, crystallize in three different polymorphs, denoted Y (yellow), G (green), and B (blue) forms in terms of the fluorescence color. While all Y-form crystals show the same yellow-to-blue fluorescence color response to the photomechanical stress generated by the anthracene [4+4] photodimerization reaction, the four G forms exhibit distinct photomechanofluorochromism (PMFC): from green to blue for G-1-C4 , to orange for G-1-C7 , to red for G-1-C8 , and to red then blue for G-1-C9 , and the B forms show no photochromic activity. The intriguing RGB three-color PMFC and abnormal topochemical reactivity of G-1-C9 are attributed to inherent softness of the crystal lattice.     

Mechanofluorochromism of 1‐Alkanoylaminopyrenes

To create a new series of mechanofluorochromic materials and to elucidate the mechanism of the phenomenon of mechanofluorochromism, 1‐alkanoylaminopyrenes including 1‐acetylaminopyrene (AAPy), 1‐octanoylaminopyrene (OAPy), and 1‐stearoylaminopyren (SAPy) were prepared. It was found that these materials exhibited mechanofluorochromism with emission colors in the crystalline samples changing reversibly from bluish purple to yellowish green, which could be induced by mechanical grinding. X‐ray crystal structure analysis, electronic absorption, and fluorescence spectroscopies, as well as fluorescence lifetime analysis and powder X‐ray diffraction analysis of AAPy suggested that the present mechanofluorochromism was caused by developing crystal defects through grinding. Intermolecular hydrogen bonds were suggested to play an important role in the occurrence of mechanofluorochromism, suppressing the face‐to‐face overlapping of pyrene moieties to form excimers in the pristine crystal.     

Polymorph‐Dependent Thermally Activated Delayed Fluorescence Emitters: Understanding TADF from a Perspective of Aggregation State

Current research on thermally activated fluorescence (TADF) emitters is mainly based on the molecular levels, while the aggregation states of TADF emitters are to be explored deeply. Now two multifunctional emitters are reported with simultaneous TADF, aggregation induced emission (AIE), and multicolor mechanochromic luminescence (MCL) features. Both emitters also show polymorph‐dependent TADF emission. Crystal structure analysis reveals that the polymorphism is ascribed to the mutable conformations in different aggregation states. This work brings new insight to TADF emitters from a perspective of aggregation states.     

Designed Benzothiadiazole Fluorophores for Selective Mitochondrial Imaging and Dynamics

A series of new rationale designed 2,1,3‐benzothiadiazole (BTD) fluorescent derivatives has been synthesized and applied for cellular selective staining of cancer cells in cell‐imaging experiments. Four new synthesized BTD derivatives showed only poor or reasonable cellular selection, but with excellent fluorescence intensity and almost no background signal emitting at the blue or green channels. The knowledge gained by analysing their molecular architecture, however, allowed the planning and synthesis of a fluorescent BTD, which was then successfully tested and showed superior mitochondrial selection with outstanding results in bioimaging experiments in living cells. The new marker (named Splendor) was then compared with the commercially available MitoTracker Red (also through co‐staining experiments) and showed far better mitochondrial selection, fluorescence intensity and chemical stability. Mitochondrial imaging and tracking (dynamic changes) was possible using Splendor during the whole cellular division cycle. DFT calculations were performed to offer insights into the origin of the chemical‐ and photostability of BTD derivatives. In addition, molecular docking calculations hint at a potential molecular target for the BTD derivatives in the mitochondrial protein adenine nucleotide translocase, which may explain the mitochondrial selectivity of Splendor versus the other four BTD derivatives.     

A Strongly Fluorescent Molecular Material Responsive to Physical/Chemical Stimuli and their Coupled Impact

Molecular materials with weak but extended and pliable supramolecular interactions are versatile candidates for eliciting stimuli‐sensitive optical responses. A novel diaminodicyanoquinodimethane (DADQ) molecule, 7,7‐bis(2‐(2‐pyridyl)ethylamino)‐8,8‐dicyanoquinodimethane (BPEDQ), has been synthesized and structurally characterized; it exhibits enhanced fluorescence emission in the aggregated and solid states, characteristic of DADQs. The pyridine moieties in the molecule, in addition to the amino and cyano groups of the strongly dipolar fluorophore moiety, induce extensive H‐bonding interactions which can impart structural integrity to the solid material; this enables reversible crystalline‐amorphous transformations triggered by mechanical grinding and solvent fuming. The concomitant fluorescence color and intensity switching are prominent and reversible. Protonation‐deprotonation events induced by acidic and basic vapors also produce stark fluorescence response variations; the chemical stimuli also lead to amorphization of the solid. The full cycle of chemical and physical stimuli, and the consequence of their individual and coupled impact on the fluorescence emission, are illustrated using a BPEDQ‐doped polymer thin film.     

Multi-Stimuli-Induced Mechanical Bending and Reversible Fluorescence Switching in a Single Organic Crystal

Fluorescent single crystals that respond to multiple external stimuli are of great interest in molecular machines, sensors, and displays. The integration of photo- or acid-induced fluorescence enhancement and bending in one organic crystal, however, has not been reported yet. Herein, we report the interesting plastic photomechanical bending and switching on of the fluorescence of an azine crystal in a single-crystal transformation, due to extended π-conjugation and molecular slippage. Moreover, the fluorescent plastic bending driven by multiple volatile acid vapors was firstly observed, and attributed to the synergistic effect of push–pull electronic structure and hydrogen bonding. The single crystal also shows high elasticity under external force. In addition, reversible fluorescence switching can be triggered by grinding and solvent fuming, as well as by the adsorption and desorption of HCl vapor. The integration of plastic, elastic bending and switch-on fluorescence into one single crystal provides a new strategy for next-generation smart materials.     

Mechanochromic luminescence of soft crystals: Recent systematic studies in controlling the molecular packing and mechanoresponsive properties

Soft crystals are a class of smart materials that can switch their photophysical or mechanical properties in response to gentle external stimuli. A representative stimuli-responsive behavior of soft crystals is mechanochromic luminescence (MCL), i.e., a reversible color change of solid-state photoluminescence induced by external mechanical stimuli. Together with the rapid growth in the area of solid-state photoluminescence including fluorescence, room-temperature phosphorescence (RTP), thermally activated delayed fluorescence (TADF), white-light emission (WLE), and circularly polarized luminescence (CPL), a number of soft crystals that exhibit MCL behaviors have been reported during the past decade. In the typical MCL of soft crystals, the emission color switches in the bathochromic direction upon amorphization by grinding and recovers to the original color upon recrystallization by heating or exposure to organic solvents. Relatively few are known to exhibit hypsochromically shifted MCL, two-step MCL, self-recovering MCL, or mechanical-stimuli-induced single-crystal-to-single-crystal (SCSC) transitions. Rational design guidelines to control the mechanoresponsive properties of soft crystals have not yet been established. This review summarizes the systematic studies on the substituent effect to control the MCL properties of soft crystals. Recent studies provide useful insights into the effects of electronic and steric differences of substituents on crystal structure, luminescence properties, and mechanoresponsive behaviors.     

Mechanochromic Luminescence Characteristics of Pyridine‐Terminated Chromophores in the Solid State and in a Poly(vinyl alcohol) Matrix

Mechanoresponsive luminophores containing different substituted pyridine rings at the molecular terminus are synthesized and their photoluminescence properties are investigated. The solid chromophore with a 4‐substituted pyridine ring exhibits a reversible photoluminescent color change, while the 2‐substituted chromophore shows only a small change in luminescence, and the 3‐substituted chromophore displays an irreversible photoluminescent color change with mechanical grinding. A change of the sample color in response to mechanical grinding is also achieved for a dye‐dispersed poly(vinyl alcohol) film. Furthermore, a simultaneous acid and mechanoresponsive photoemission color change is achieved in the dye‐dispersed film.     

Multistimuli-Responsive Luminescence of Naphthalazine Based on Aggregation-Induced Emission

Stimuli-responsive luminescent materials, which are dependent on changes in physical molecular packing modes, have attracted more and more interest over the past ten years. In this study, 2,2-dihydroxy-1,1-naphthalazine was synthesized and shown to exhibit different fluorescence emission in solution and solid states with characteristic aggregation-induced emission (AIE) properties. A remarkable change in the fluorescence of 2,2-dihydroxy-1,1-naphthalazine occurred upon mechanical grinding, heating, or exposure to solvents. According to the characterization by solid-state fluorescence spectroscopy, X-ray crystallography, differential scanning calorimetry, and X-ray powder diffraction, the fluorescence change could be attributed to transitions between two structurally different polymorphs. These significant properties could also give 2,2-dihydroxy-1,1-naphthalazine more potential applications as a multifunctional material.     

Mechano- and Photoresponsive Behavior of a Bis(cyanostyryl)benzene Fluorophore

The mechanoresponsive behavior and photochemical response of a new bis(cyanostyryl)benzene fluorophore (CSB-5) were investigated. Green fluorescence with λ_em,max of 507 nm was found for CSB-5 in chloroform solution, mirroring the behavior of a previously reported similar dye (CSB-6). Alternatively, crystalline samples of CSB-5 exhibited orange fluorescence with λ_em,max of 620 nm, attributable to excimer emission. Although the emission color change was not clearly noticeable by naked eye, CSB-5 exhibited mechanochromic luminescence, due to transformation into the amorphous state upon grinding the crystalline powder. Interestingly, rubbed films of CSB-5 prepared on glass substrates exhibited a pronounced emission color change from orange to green when exposed to UV light. This response is the result of a photochemical reaction that occurs in the amorphous state and which causes a decrease of the excimer emission sites so that the emission color changes from excimer to monomer. The crystalline material did not display such a photoinduced emission color change and the difference in photochemical reactivity between crystalline and amorphous states was exploited to pattern the emission color of rubbed films.     

共晶工程的颜色和形态调控

共晶工程作为化学材料的重要研究手段之一,在光电学、药学等多个领域取得了不同的研究进展与成果。与传统合成方法相比,共晶可通过简便、低成本的操作实现共晶组件的多功能性,同时通过微观调控实现共晶形态、结构乃至颜色的调节。本综述首先介绍了共晶的分类和制备,然后介绍了有机电荷转移共晶发光材料、机械变色发光材料的荧光颜色调控以及药物共晶、有机光电功能共晶的颜色调控;其次,介绍了外部内部因素对晶体形态调控的影响;最后,介绍了共晶晶体多态性和荧光色彩之间的关系以及共晶材料的未来发展。希望本文可以为共晶材料颜色和形态方面的优化与改善提供一定的思路,也为其他化学材料的质量优化提供借鉴意义。     

Multicolor super-resolution fluorescence imaging via multi-parameter fluorophore detection

Understanding the complexity of the cellular environment will benefit from the ability to unambiguously resolve multiple cellular components, simultaneously and with nanometer-scale spatial resolution. Multicolor super-resolution fluorescence microscopy techniques have been developed to achieve this goal, yet challenges remain in terms of the number of targets that can be simultaneously imaged and the crosstalk between color channels. Herein, we demonstrate multicolor stochastic optical reconstruction microscopy (STORM) based on a multi-parameter detection strategy, which uses both the fluorescence activation wavelength and the emission color to discriminate between photo-activatable fluorescent probes. First, we obtained two-color super-resolution images using the near-infrared cyanine dye Alexa 750 in conjunction with a red cyanine dye Alexa 647, and quantified color crosstalk levels and image registration accuracy. Combinatorial pairing of these two switchable dyes with fluorophores which enhance photo-activation enabled multi-parameter detection of six different probes. Using this approach, we obtained six-color super-resolution fluorescence images of a model sample. The combination of multiple fluorescence detection parameters for improved fluorophore discrimination promises to substantially enhance our ability to visualize multiple cellular targets with sub-diffraction-limit resolution.     

Molecular Packing and Solid-State Photophysical Properties of 1,3,6,8-Tetraalkylpyrenes

The relationship between the photophysical properties and molecular orientation of 1,3,6,8-tetraalkylpyrenes in the solid state is described herein. The introduction of alkyl groups with different chain structures (in terms of length and branching) did not affect the photophysical properties in solution, but significantly shifted the emission wavelengths and fluorescence quantum yields in the solid state for some samples. Pyrenes bearing ethyl, isobutyl, or neopentyl groups at the 1-, 3-, 6-, and 8-positions showed similar emission profiles in both the solution and solid states. In contrast, pyrenes bearing other alkyl groups exhibited an excimer emission in the solid state, similar to that of the parent pyrene. On studying the photophysical properties in the solid state with respect to the obtained crystal structures, the observed solid-state photophysical properties were found to depend on the relative position of the pyrene chromophores. The solid-state photophysical properties can be controlled by the alkyl groups, which provide changing crystal packing. Among the pyrenes tested, 1,3,6,8-tetraethylpyrene showed the highest fluorescence quantum yield of 0.88 in the solid state.     

2-Aza-1,3-butadiene derivatives featuring an anthracene or pyrene unit: highly selective colorimetric and fluorescent signaling of Cu2+ cation

[Structure: see text] A new probe based on an anthryl derivative bearing an azadiene side chain selectively senses Cu2+ in acetonitrile through two different channels: the yellow-to-orange color change and a remarkable enhancement of the fluorescence, whereas the pyrenyl analogous behaves as a fluorescent sensor for Cu2+ and Hg2+ in aqueous environment.     

Multiple‐Stimuli Responsive and Tunable Luminescent Supramolecular Assembly by Oligo(p‐phenylvinylene) and Surfactant

Achieving multicolor photoluminescence under multiple stimuli response based on a single fluorescent compound remains a great challenge. Herein, we report a novel multicolor fluorescent supramolecular assembly, which was constructed from surfactant sodium dodecyl sulfate (SLS) and fluorescent compound 1 bearing a rigid symmetrical acceptor–donor–acceptor structure. The luminescence property of 1 /SLS assembly showed the multiple stimuli response towards temperature, cyclodextrin complexation and UV light irradiation, exhibiting the tunable emission wavelengths from 490 nm to 590 nm and the multicolor photoluminescence including cyan, green, yellow and orange. Furthermore, this assembly could be used in light writing owing to the fast fluorescence change within 15 s. These results could provide a convenient and useful method for fabricating smart tunable photoluminescent materials.     

Mechanochromic Luminescence of Fluorenyl‐Substituted Ethylenes

It has been reported several times that some organic luminogens with aggregation‐induced emission (AIE) characteristics exhibit the abnormal phenomenon of crystallization‐induced blueshift fluorescence, which makes them suitable for utilization as luminescence color‐switching materials. Because of the attractive application potential and the numerous underlying structure–property relationships in such materials, we investigated a series of fluorenyl‐containing tetrasubstituted ethylenes for their novel optical properties and structural features. The dyes show morphology‐dependent luminescence. Their emission color can be switched between green and blue by means of mechanical grinding and solvent fuming. The transformation between crystalline and amorphous accounts for the luminescence changing. Through single‐crystal and X‐ray diffraction (XRD) analysis, the twisted molecular geometries and loose packing motifs in the crystalline samples are believed to be the intrinsic origin of the external‐stimuli‐induced structural transformation.     

Development of a far-red to near-infrared fluorescence probe for calcium ion and its application to multicolor neuronal imaging

To improve optical imaging of Ca(2+) and to make available a distinct color window for multicolor imaging, we designed and synthesized CaSiR-1, a far-red to near-infrared fluorescence probe for Ca(2+), using Si-rhodamine (SiR) as the fluorophore and the well-known Ca(2+) chelator BAPTA. This wavelength region is advantageous, affording higher tissue penetration, lower background autofluorescence, and lower phototoxicity in comparison with the UV to visible range. CaSiR-1 has a high fluorescence off/on ratio of over 1000. We demonstrate its usefulness for multicolor fluorescence imaging of action potentials (visualized as increases in intracellular Ca(2+)) in brain slices loaded with sulforhodamine 101 (red color; specific for astrocytes) that were prepared from transgenic mice in which some neurons expressed green fluorescent protein.