Environment-sensitive behavior of fluorescent molecular rotors
TL;DR: This review presents the pertinent theories of the rotor-solvent interaction on the molecular level and how this interaction leads to the viscosity-sensitive behavior of molecular rotors.
Abstract: Molecular rotors are a group of fluorescent molecules that form twisted intramolecular charge transfer (TICT) states upon photoexcitation. When intramolecular twisting occurs, the molecular rotor returns to the ground state either by emission of a red-shifted emission band or by nonradiative relaxation. The emission properties are strongly solvent-dependent, and the solvent viscosity is the primary determinant of the fluorescent quantum yield from the planar (non-twisted) conformation. This viscosity-sensitive behavior gives rise to applications in, for example, fluid mechanics, polymer chemistry, cell physiology, and the food sciences. However, the relationship between bulk viscosity and the molecular-scale interaction of a molecular rotor with its environment are not fully understood. This review presents the pertinent theories of the rotor-solvent interaction on the molecular level and how this interaction leads to the viscosity-sensitive behavior. Furthermore, current applications of molecular rotors as microviscosity sensors are reviewed, and engineering aspects are presented on how measurement accuracy and precision can be improved.
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TL;DR: A review of the latest developments in TICT research from a materials chemistry point of view can be found in this paper, where the authors present a compact overview of the current state-of-the-art.
Abstract: Twisted intramolecular charge transfer (TICT) is an electron transfer process that occurs upon photoexcitation in molecules that usually consist of a donor and acceptor part linked by a single bond. Following intramolecular twisting, the TICT state returns to the ground state either through red-shifted emission or by nonradiative relaxation. The emission properties are potentially environment-dependent, which makes TICT-based fluorophores ideal sensors for solvents, (micro)viscosity, and chemical species. Recently, several TICT-based materials have been discovered to become fluorescent upon aggregation. Furthermore, various recent studies in organic optoelectronics, non-linear optics and solar energy conversions utilised the concept of TICT to modulate the electronic-state mixing and coupling on charge transfer states. This review presents a compact overview of the latest developments in TICT research, from a materials chemistry point of view.
728 citations
TL;DR: This Review summarizes the existing fluorescent probes that target chemical/biological events within a single organelle and organelle-anchoring strategies are described and emphasized to inspire the design of new generations of fluorescent probes, before concluding with future prospects on the possible further development of chemical biology.
Abstract: Principle has it that even the most advanced super-resolution microscope would be futile in providing biological insight into subcellular matrices without well-designed fluorescent tags/probes. Developments in biology have increasingly been boosted by advances of chemistry, with one prominent example being small-molecule fluorescent probes that not only allow cellular-level imaging, but also subcellular imaging. A majority, if not all, of the chemical/biological events take place inside cellular organelles, and researchers have been shifting their attention towards these substructures with the help of fluorescence techniques. This Review summarizes the existing fluorescent probes that target chemical/biological events within a single organelle. More importantly, organelle-anchoring strategies are described and emphasized to inspire the design of new generations of fluorescent probes, before concluding with future prospects on the possible further development of chemical biology.
581 citations
TL;DR: An emerging method based on fluorescence detection from small synthetic molecules termed 'molecular rotors', which can be used to monitor changing viscosity during dynamic biological processes within cells, such as photoinduced cell death, is described.
Abstract: This article describes an emerging method for quantitative measurement and spatial imaging of microviscosity within individual domains of live cells. The method is based on fluorescence detection from small synthetic molecules termed ‘molecular rotors’, which are characterised by a strong response of fluorescence lifetimes or spectra to the viscosity of their immediate environment. Alongside this new method, two complementary techniques are discussed, which provide further insights into diffusion controlled processes on a microscopic scale in a biological environment. These are time resolved fluorescence anisotropy and imaging of short-lived excited state of molecular oxygen, termed ‘singlet oxygen’. It is possible to utilise all three approaches for the quantitative determination of viscosity in individual organelles of live cells. Finally, it is discussed how the major advantage of molecular rotor imaging, fast signal acquisition, can be used to monitor changing viscosity during dynamic biological processes within cells, such as photoinduced cell death.
378 citations
TL;DR: It has been observed that, although most fluorescent molecules emit from their lowest energy excited state, S1, BODIHY dyes do not, and their fluorescence is enhanced through restricted rotor rotation, which suppresses internal conversion to the dark S1 state, which leads to the proposal that suppression of Kasha's rule is the photophysical mechanism responsible for emission in both viscous solution and the solid state.
Abstract: Although there are some proposed explanations for aggregation-induced emission, a phenomenon with applications that range from biosensors to organic light-emitting diodes, current understanding of the quantum-mechanical origin of this photophysical behaviour is limited. To address this issue, we assessed the emission properties of a series of BF2-hydrazone-based dyes as a function of solvent viscosity. These molecules turned out to be highly efficient fluorescent molecular rotors. This property, in addition to them being aggregation-induced emission luminogens, enabled us to probe deeper into their emission mechanism. Time-dependent density functional theory calculations and experimental results showed that the emission is not from the S1 state, as predicted from Kasha's rule, but from a higher energy (>S1) state. Furthermore, we found that suppression of internal conversion to the dark S1 state by restricting the rotor rotation enhances fluorescence, which leads to the proposal that suppression of Kasha's rule is the photophysical mechanism responsible for emission in both viscous solution and the solid state.
273 citations
Cites background from "Environment-sensitive behavior of f..."
...Although the field of molecular rotors has seen much development of late, there are still limitations that need to be addressed, for example, (1) a high polarity sensitivity that leads to inaccurate microviscosity measurements15, (2) small Stokes shifts that limit biochemical applications16, such as multimodal cell imaging17, and (3) a low viscosity sensitivity that limits the imaging contrast18....
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TL;DR: A visual sensing approach based on fluorescent imaging is presented, by assembling aggregation-induced-emission (AIE)-active molecular rotors into a moisture-captured network; the resulting AIE humidity sensors are compatible with diverse applications, having tunable geometries and desirable architectures.
Abstract: Building humidity sensors possessing the features of diverse-configuration compatibility, and capability of measurement of spatial and temporal humidity gradients is of great interest for highly integrated electronics and wearable monitoring systems. Herein, a visual sensing approach based on fluorescent imaging is presented, by assembling aggregation-induced-emission (AIE)-active molecular rotors into a moisture-captured network; the resulting AIE humidity sensors are compatible with diverse applications, having tunable geometries and desirable architectures. The invisible information of relative humidity (RH) is transformed into different fluorescence colors that enable direct observation by the naked eyes based on the twisted intramolecular charge-transfer effect of the AIE-active molecular rotors. The resulting AIE humidity sensors show excellent performance in terms of good sensitivity, precise quantitative measurement, high spatial-temporal resolution, and fast response/recovery time. Their multiscale applications, such as regional environmental RH detection, internal humidity mapping, and sensitive human-body humidity sensing are demonstrated. The proposed humidity visualization strategy may provide a new insight to develop humidity sensors for various applications.
177 citations
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6,510 citations
"Environment-sensitive behavior of f..." refers background in this paper
...The free volume is the temperaturedependent factor, and for glass-forming liquids, the free volume reaches a minimum at the glass transition temperature [66]....
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TL;DR: The Rehybridization of the Acceptor (RICT) and Planarization ofThe Molecule (PICT) III is presented, with a comparison of the effects on yield and radiationless deactivation processes.
Abstract: 6. Rehybridization of the Acceptor (RICT) 3908 7. Planarization of the Molecule (PICT) 3909 III. Fluorescence Spectroscopy 3909 A. Solvent Effects and the Model Compounds 3909 1. Solvent Effects on the Spectra 3909 2. Steric Effects and Model Compounds 3911 3. Bandwidths 3913 4. Isoemissive Points 3914 B. Dipole Moments 3915 C. Radiative Rates and Transition Moments 3916 1. Quantum Yields and Radiationless Deactivation Processes 3916
2,924 citations
TL;DR: The theoretical basis and some practical guidelines for simple, rigorous analysis of FPR experiments are presented and some model experiments on aqueous solutions of rhodamine 6G are described.
2,594 citations
"Environment-sensitive behavior of f..." refers methods in this paper
...FRAP is governed by lateral diffusion of a fluorophore into a region where the dye has been destroyed by intense light....
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...Following the same line of investigation, Nipper et al. [45] demonstrated that microviscosity, determined through molecular rotor fluorescence, correlates highly with the viscosity determined through fluorescence recovery after photobleaching (FRAP)....
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...In fact, fluorescence emission of molecular rotors can be used to determine the microviscosity of the environment with the same level of rigor as two established methods, FRAP [46] and fluorescence anisotropy [47]....
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...FRAP is a microscopy method where fluorophore diffusivity in a phospholipid membrane can be determined, thus allowing to estimate microviscosity....
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