Twisted Intramolecular Charge Transfer Mechanism Research Articles

Design and Screening of Fluorescent Probes Based upon Hemicyanine Dyes for Monitoring Mitochondrial Viscosity in Living Cells.

Viscosity, at the subcellular level, plays a crucial role as a physicochemical factor affecting microenvironment homeostasis. Abnormal changes in mitochondrial viscosity often lead to various diseases in the organism. Based on the twisted intramolecular charge transfer mechanism, four hemicyanine dye fluorescent probes (HT-SA, HT-SA-S, HT-Bzh, and HT-NA) were designed and synthesized for viscosity response. The single bond between the nitrogen-containing heterocycle and the carbon-carbon double in the structure of the probe bond served as the viscosity response site. Finally, the probe HT-Bzh was screened as the optimal mitochondrial viscosity probe according to its responsiveness, targeting, and interference resistance. The fluorescence intensity of the probe HT-Bzh increased 22-fold when the viscosity was increased from 13.75 to 811.2 cP. In summary, all four viscosity probes we have developed can be used in different applications depending on the external environment, providing a valuable reference for the design of potential tools to address viscosity monitoring in biological systems.

FRET-based fluorescent probe with favorable water solubility for simultaneous detection of SO2 derivatives and viscosity

The intracellular viscosity is an important parameter of the microenvironment and SO2 is a vital gas signal molecule. At present, some dual-response fluorescence probes for simultaneous measurements of viscosity and SO2 derivatives (HSO3−/SO32−) possessed poor water solubility. In this work, we developed a water-soluble fluorescence probe CIJ (0.0864 g/100 mL of water at 20 °C) for simultaneous measurements of viscosity and SO2 derivatives. CIJ exhibited a sensitive fluorescence enhancement to environmental viscosity from 0.97 to 28.04 cP based on a twisted intramolecular charge transfer mechanism and was applied to effective measurement of viscosity in vitro and in vivo. CIJ could also respond to SO2 derivatives with a low detection limit (44 nM) and a fast response time (5 min) based on the nucleophilic addition reaction. Furthermore, CIJ was applied to monitor SO2 derivatives in ratiometric response manner in living cells.

Fluorescence Modulation by Amines: Mechanistic Insights into Twisted Intramolecular Charge Transfer (TICT) and Beyond

Amine groups are common constituents of organic dyes and play important roles in tuning fluorescence properties. In particular, intensive research works have demonstrated the tendency and capabilities of amines in influencing chromophore brightness. Such properties have been explained by multiple mechanisms spanning from twisted intramolecular charge transfer (TICT) to the energy gap law and beyond, which introduce additional nonradiative energy dissipation pathways. In this review, we aim to provide a focused overview of the mechanistic insights mainly for the TICT mechanism, accompanied by a few other less common or influential fluorescence quenching mechanisms in the amine-containing fluorescent molecules. Various aspects of current scientific findings including the rational design and synthesis of organic chromophores, theoretical calculations, steady-state and time-resolved electronic and vibrational spectroscopies are reviewed. These in-depth understandings of how the amine groups with diverse chemical structures at various atomic sites affect excited-state nonradiative decay pathways will facilitate the strategic and targeted development of fluorophores with desired emission properties as versatile chemosensors for broad applications.

A novel 1,8-naphthalimide-based fluorescent chemosensor for the detection of HSA in living cells.

Human serum albumin (HSA) is an essential protein for maintaining human health. Accurate detection and quantification of HSA are of great significance for disease diagnosis and biochemical research. Here, a new HSA fluorescent probe BNPE based on the 1,8-naphthalimide fluorophore was designed and synthesized. The probe could recognize HSA through a twisted intramolecular charge transfer mechanism, effectively avoid the interference of most substances, and realize HSA fluorescence imaging in living cells.

Vis–NIR Electrochromism and NIR–Green Electroluminochromism in Dual Functional Benzothiadiazole‐Arylamine Mixed‐Valence Compounds

AbstractElectrochromism/electrofluorochromism is the reversible tuning of the absorption/photoluminescence by redox processes. Dual functional electrochromic/electrofluorochromic materials can be exploited in, for example, smart windows, encryption/anticounterfeiting technologies, displays, and biomedical sensors. While electrofluorochromic materials showing tunable fluorescence intensity are relatively widespread, those with tunable color emission are not. Moreover, electrofluorochromic materials with tunable emission in the near‐infrared (NIR) range are seldom encountered. Here, this work shows a new molecular approach to afford both the above functionalities, in addition to red–NIR electrochromism, by combining the properties of highly stable arylamine radical cation mixed valence species, with those of donor–acceptor systems exhibiting the photoinduced twisted intramolecular charge transfer mechanism.

Beyond "Mega": Origin of the "Giga" Stokes Shift for Triazolopyridiniums.

Over the past decade, fluorophores that exhibit "mega" Stokes shifts, defined to be Stokes shifts of greater than 100 nm, have gained considerable attention due to their potential technological applications. A subset of these fluorophores have Stokes shifts of at least 10,000 cm-1, for whom we suggest the moniker "giga" Stokes shift. The majority of "giga" Stokes shifts reported in the literature arise from the twisted intramolecular charge transfer mechanism, but this mechanism does not fit empirical characterization of triazolopyridinium (TOP). This observation inspired a density functional theory (DFT) and time-dependent DFT study of TOP, and several related fluorophores, to elucidate the novel photophysical origin for the "giga" Stokes shift of TOP. The resulting computational models revealed that photoexcitation of TOP yields a zwitterionic excited state that undergoes significant structural relaxation prior to emission. Most notably, TOP has two orthogonal moieties in the ground state that adopt a coplanar geometry in the excited state. According to Hückel's rule, both the heterocycle and phenyl moieties of TOP should be aromatic in an orthogonal ground state. However, according to Baird's rule, these individual moieties should be anti-aromatic in the excited state. By relaxing to a coplanar conformation in the excited state, TOP likely forms a single aromatic system consisting of both the heterocycle and phenyl moieties.

Exploring of blood viscosity in injured liver tissues of hyperlipidemic mice

Exploring the connection between blood viscosity and liver in it have great significance for the detection and treatment of hyperlipidemic. Therefore, we designed a new near-infrared viscosity sensitive fluorescent probe Vis-LW based on the twisted intramolecular charge transfer mechanism. The probe exhibited a strong dependence on viscosity as well as good anti-interference performance. With the aid of Vis-LW, we successfully detected the viscosity changes in chloroquine induced cells and realized the diagnosis of diseases in hyperlipidemic mice by utilizing blood viscosity as a diagnostic indicator for hyperlipidemia. Not only that, the increased liver viscosity of hyperlipidemic mice was found for the first time, the reduced blood and liver viscosity after treatment with lipid-lowering drug fenofibrate was also tracked. This work provides a new method for the diagnosis of hyperlipidemia and offered a new idea on the research and treatment of hyperlipidemia in the future.

Tuning the “critical polarity” of TICT dyes: Construction of polarity-sensitive platform to distinguish duple organelles

Polarity-sensitive dyes have demonstrated potential applications in bioimaging experiments. However, the twisted intramolecular charge transfer (TICT) mechanism to explain the polarity-responsive behavior still requires in-depth exploration. Herein, we proposed a new concept, “critical polarity”, for the in-depth explanation of TICT mechanism. The polarity below “critical polarity” was defined as “low polarity”, in which the fluorophore would present in local excited (LE) states. The polarity above “critical polarity” was defined as “high polarity”, in which the fluorophore would present in TICT states. Both proper “critical polarity” and large emission difference between LE and TICT states are required to construct desirable dual-emissive fluorophores highly sensitive to polarity. Three fluorescent dyes have been fabricated by modifying different electron donors on naphthalene imide, and S-NAT with modest electron donor demonstrated the highest sensitivity to polarity. Theoretical analysis together with the experimental results proved the validation of “critical polarity” mechanism. Finally, two fluorescent probes have been constructed based on the polarity-sensitive fluorophore. Amongst, the S-NAT could distinguish endoplasmic reticulum (ER) and lipid droplets (LDs) in dual emission colors, while S-NAT-M enabled discrimination of lysosomes and LDs in different emission colors. The two fluorescent probes with high quantum yield and low cytotoxicity could be employed to study the interaction between duple organelles, and the “critical polarity” mechanism can serve as a validate strategy to direct the construction of ratiometric dyes for polarity.

Restriction of Twisted Intramolecular Charge Transfer Enables the Aggregation-Induced Emission of 1-(N,N-Dialkylamino)-naphthalene Derivatives.

Understanding the mechanisms of aggregation-induced emission (AIE) is essential for the rational design and deployment of AIEgens toward various applications. Such a deep mechanistic understanding demands a thorough investigation of the excited-state behaviors of AIEgens. However, because of considerable complexity and rapid decay, these behaviors are often not experimentally accessible and the mechanistic comprehension of many AIEgens is lacking. Herein, utilizing detailed quantum chemical calculations, we provide insights toward the AIE mechanism of 1-(N,N-dialkylamino)-naphthalene (DAN) derivatives. Our theoretical analysis, corroborated by experimental observations, leads to the discovery that modulating the formation of the twisted intramolecular charge transfer (TICT) state (caused by the rotation of the amino groups) and managing the steric hindrance to minimize solid-state intermolecular interactions provides a plausible explanation for the AIE characteristics of DAN derivatives. These results will inspire the deployment of the TICT mechanism as a useful design strategy toward AIEgen development.

Local Electric Field Controls Fluorescence Quantum Yield of Red and Far-Red Fluorescent Proteins.

Genetically encoded probes with red-shifted absorption and fluorescence are highly desirable for imaging applications because they can report from deeper tissue layers with lower background and because they provide additional colors for multicolor imaging. Unfortunately, red and especially far-red fluorescent proteins have very low quantum yields, which undermines their other advantages. Elucidating the mechanism of nonradiative relaxation in red fluorescent proteins (RFPs) could help developing ones with higher quantum yields. Here we consider two possible mechanisms of fast nonradiative relaxation of electronic excitation in RFPs. The first, known as the energy gap law, predicts a steep exponential drop of fluorescence quantum yield with a systematic red shift of fluorescence frequency. In this case the relaxation of excitation occurs in the chromophore without any significant changes of its geometry. The second mechanism is related to a twisted intramolecular charge transfer in the excited state, followed by an ultrafast internal conversion. The chromophore twisting can strongly depend on the local electric field because the field can affect the activation energy. We present a spectroscopic method of evaluating local electric fields experienced by the chromophore in the protein environment. The method is based on linear and two-photon absorption spectroscopy, as well as on quantum-mechanically calculated parameters of the isolated chromophore. Using this method, which is substantiated by our molecular dynamics simulations, we obtain the components of electric field in the chromophore plane for seven different RFPs with the same chromophore structure. We find that in five of these RFPs, the nonradiative relaxation rate increases with the strength of the field along the chromophore axis directed from the center of imidazolinone ring to the center of phenolate ring. Furthermore, this rate depends on the corresponding electrostatic energy change (calculated from the known fields and charge displacements), in quantitative agreement with the Marcus theory of charge transfer. This result supports the dominant role of the twisted intramolecular charge transfer mechanism over the energy gap law for most of the studied RFPs. It provides important guidelines of how to shift the absorption wavelength of an RFP to the red, while keeping its brightness reasonably high.

Binding Modes of a Phenylpyridinium Styryl Fluorescent Dye with Cucurbiturils.

In order to explore how cucurbituril hosts accommodate an N-phenyl-pyridinium derivative guest, the complexation of the solvatochromic dye, 4-(4-(dimethylamino)styryl)-1-phenylpyridinium iodide (PhSt) with α,α′,δ,δ′-tetramethyl-cucurbit[6]uril (Me4CB6) and cucurbit[7]uril (CB7) was investigated by absorption spectroscopic, fluorescence and NMR experiments. In aqueous solutions, PhSt forms 1:1 complexes with both cucurbiturils, the complex with CB7 has a higher stability constant (Ka = 6.0 × 106 M−1) than the complex with Me4CB6 (Ka = 1.1 × 106 M−1). As revealed by NMR experiments and confirmed by theoretical calculations, CB7 encapsulates the whole phenylpyridinium entity of the PhSt cation guest, whereas the cavity of Me4CB6 includes only the phenyl ring, the pyridinium ring is bound to the carbonyl rim of the host. The binding of PhSt to cucurbiturils is accompanied by a strong enhancement of the fluorescence quantum yield due to the blocking of the deactivation through a twisted intramolecular charge transfer (TICT) state. The TICT mechanism in PhSt was characterized by fluorescence experiments in polyethylene glycol (PEG) solvents of different viscosities. The PhSt-CB7 system was tested as a fluorescence indicator displacement (FID) assay, and it recognized trimethyl-lysine selectively over other lysine derivatives.

Triarylboron/Triarylamine-Functionalized 2,2'-Bipyridine Ligands and Their Copper(I) Complexes.

We report herein three new either -BMes2 (Mes = mesityl) or -NPh2 group-functionalized 2,2'-bipyridine ligands-4,4'-(p-BMes2-phenyl-C≡C)2-2,2'-bpy (BB-bpy), 4-(p-BMes2-phenyl-C≡C)-4'-(p-NPh2-phenyl-C≡C)-2,2'-bpy (BN-bpy), and 4,4'-(p- NPh2-phenyl-C≡C)2-2,2'-bpy (NN-bpy)-along with their heteroleptic copper(I) complexes Cu(L)(PPh3)2(BF4) (L = BB-bpy, BN-bpy, and NN-bpy, respectively). The electron-donor and -acceptor units are connected to the bipyridine core via acetylene linkers. The incorporation of acetylene linkers decreases the dihedral angle between the bridging phenyl and pyridine rings, resulting in a nearly coplanar geometry of the ligands. All free ligands display temperature-dependent luminescence changes, which may be explained by the twisted intramolecular charge-transfer mechanism. Binding with Cu(I) results in significantly red-shifted emission maxima for the heteroleptic complexes Cu(NNbpy)(PPh3)2(BF4) [Cu(NNbpy)] and Cu(BNbpy)(PPh3)2(BF4) [Cu(BNbpy)] relative to those of the free ligands. The electronic and photophysical properties of all compounds were investigated by electrochemical, absorption, and emission spectroscopic analyses as well as density functional theory calculations.

Enhancement of Aggregation-Induced Emission by Introducing Multiple o-Carborane Substitutions into Triphenylamine.

The enhancement of aggregation-induced emission (AIE) is presented on the basis of the strategy for improving solid-state luminescence by employing multiple o-carborane substituents. We synthesized the modified triphenylamines with various numbers of o-carborane units and compared their optical properties. From the optical measurements, the emission bands from the twisted intramolecular charge transfer (TICT) state were obtained from the modified triphenylamines. It was notable that emission efficiencies of the multi-substituted triphenylamines including two or three o-carborane units were enhanced 6- to 8-fold compared to those of the mono-substituted triphenylamine. According to mechanistic studies, it was proposed that the single o-carborane substitution can load the AIE property via the TICT mechanism. It was revealed that the additional o-carborane units contribute to improving solid-state emission by suppressing aggregation-caused quenching (ACQ). Subsequently, intense AIEs were obtained. This paper presents a new role of the o-carborane substituent in the enhancement of AIEs.

Fluorescent Polystyrene Films for the Detection of Volatile Organic Compounds Using the Twisted Intramolecular Charge Transfer Mechanism.

Thin films of styrene copolymers containing fluorescent molecular rotors were demonstrated to be strongly sensitive to volatile organic compounds (VOCs). Styrene copolymers of 2-[4-vinyl(1,1′-biphenyl)-4′-yl]-cyanovinyljulolidine (JCBF) were prepared with different P(STY-co-JCBF)(m) compositions (m% = 0.10–1.00) and molecular weights of about 12,000 g/mol. Methanol solutions of JCBF were not emissive due to the formation of the typical twisted intramolecular charge transfer (TICT) state at low viscosity regime, which formation was effectively hampered by adding progressive amounts of glycerol. The sensing performances of the spin-coated copolymer films (thickness of about 4 µm) demonstrated significant vapochromism when exposed to VOCs characterized by high vapour pressure and favourable interaction with the polymer matrix such as tetrahydrofurane (THF), CHCl3 and CH2Cl2. The vapochromic response was also reversible and reproducible after successive exposure cycles, whereas the fluorescence variation scaled linearly with VOC concentration, thus suggesting future applications as VOC optical sensors.

A novel turn-off fluorescent probe based on TICT for the detection of NO2 and nitramines with high sensitivity and selectivity

Two new fluorescent probes based on a coumarin core have been developed for the detection of nitramines (RDX and HMX) via responding to their photo-degradated nitrogen dioxide (NO2). Although both probes contain the same fluorophore and recognition site, they display different fluorescent responses to NO2 and nitramines. 8-Oxime-7-dimethylamino-coumarin (8-ODC) emits traditional turn-on fluorescence toward NO2, which is based on the CN isomerization. In contrast, 3-Oxime-7-dimethylamino-coumarin (3-ODC) exhibits a unique turn-off response with strong fluorescence quenching (about 98%) as a result of a twisted intramolecular charge transfer mechanism (TICT). In addition, 3-ODC offers a highly selective and sensitive determination of nitramines via detecting NO2 in solution. The limit of detection of 3-ODC for NO2, HMX and RDX was 90 ppb, 60nM and 90nM, respectively. Notably, main interferents such as TNT, PETN, NO, NO2+ and H2O2 showed no obvious interference to the detection of NO2 and nitramines. Therefore, a novel fluorescent probe (3-ODC) based on the TICT mechanism was firstly developed to detect NO2 and nitramines by fluorescence quenching with high selectivity and sensitivity.

Photochemical behaviour of a new 1,2,3,4-tetrahydroxanthylium fluorescent dye with “rhodamine-like” structure in liquid media and adsorbed onto a TiO2 photo-responsive substrate

A new “rhodamine-like” fluorescent NIR dye based on the 9-(2′-carboxyphenyl)-6-(N,N-diethylamino)-1,2,3,4-tetrahydroxanthylium system was synthesized. Its spectral-luminescent properties were investigated both in solution and adsorbed onto a TiO2 photoactive solid substrate. Regarding liquid samples, the highest fluorescent quantum yield and fluorescence lifetime were obtained in dichloromethane (øF = 0.28, τF = 2.67 ns) while in ethanol and acetonitrile we observed the largest Stokes Shift, indicating a twisted intramolecular charge transfer mechanism. Additionally, the behaviour of the dye adsorbed onto TiO2 revealed some interesting features regarding the formation of the radical derived from the dye cation, following the electron injection in the conduction band of the semiconductor. Laser flash photolysis and ground state diffuse reflectance were used to evaluate and discriminate the mechanism of photoexcitation presented, as well as to characterize the photostability of the molecule. In this case, two different scenarios could be recognized: the self-photosensitization pathway and the photocatalytic mechanism.

Long-living optical gain induced by solvent viscosity in a push-pull molecule.

The combination of continuum and ultrafast pump-probe spectroscopy with DFT and TDDFT calculations, in viscous and non-viscous environments, is effective in unraveling important features of the twisted intramolecular charge transfer mechanism in a new push-pull molecule that possesses aggregation induced emission properties. Long-living optical gain is found when this mechanism is inhibited, highlighting the importance of the environment rigidity in the design of materials for photonic applications.

A two-step electron-driven proton transfer mechanism for the solution-phase nonradiative decay process of squaraine molecule

The strong solvent polarity dependence observed in the nonradiative decay rates of Bis[4-(dimethylamino)phenyl]squaraine (SQ-DMA) had previously been explained by the twisted intramolecular charge transfer mechanism. However, our recent theoretical study did not support this mechanism. We theoretically study the possibility of electron-driven proton transfer (EDPT) processes between an alcohol solvent molecule and the CO− groups of SQ-DMA using a complex model. The results actually support the EDPT mechanism that proceeds via two nonadiabatic transitions: the first from the locally excited (π→π*) state of SQ-DMA to the charge transfer (CT) state, and the second from the CT to S0 states.

A novel fluorescent pH probe with valuable pKabased on a twisted intramolecular charge transfer mechanism, and its applications in cell imaging

A novel TICT-mechanism based pH probe with valuable pKaof 5.32 has been developed. It is a promising visualizing agent for lysosomes in intracellular application.

Diarylethene Photoswitches Featuring Tetrathiafulvalene-Containing Aryl Units

The modulation of the electron-donor properties of tetrathiafulvalene (TTF) with light is attempted by two molecular designs: the integration of the TTF core as aryl unit(s) into the hexatriene system of diarylethene (DAE) derivatives and the attachment of terminal TTF unit(s) to a conventional 1,2-dithienylcyclopentene chromophore via conjugated ethynylene spacers. While photochromism is inhibited in the first approach, presumably by a twisted intramolecular charge transfer mechanism, the second approach yields photoswitchable TTF derivatives. Remarkably, the extension of the conjugated π-system upon ring closure affects the first oxidation potential values of the TTF–DAEs only slightly. Nevertheless, the electronic changes in the photochromic core are foreseen to play a crucial role in the intramolecular electron transfer process in donor–DAE–acceptor architectures.