charge-transfer state

MQA-P is a multifunctional near-infrared (NIR) fluorescent probe that simultaneously detects ONOO ^-, viscosity, and polarity within mitochondria. MQA-P exhibits significant response to ONOO ^-, λ_em=645 nm; and NIR channel at λ_em>704 nm Medium is highly sensitive to viscosity/polarity. MQA-P possesses excited-state intramolecular charge transfer (ESICT) properties that are highly sensitive to polarity by designing the N,N-dimethylamino group as the electron donor and the quinoline cation unit as the electron acceptor. MQA-P is used for ferroptosis or cancer diagnosis in vitro and in vivo via dual-channel images .

NRERf is an endoplasmic reticulum (ER)-targeted solvatochromic fluorescent probe based on Nile Red (HY-D0718). It possesses ER-targeting capability and can sense changes in local polarity and lipid order of biological membranes. NRERf undergoes excited-state charge transfer, with its emission color changing according to local polarity: the tightly packed, low-polarity liquid-ordered phase (Lo) induces a blue-shifted emission, while the high-polarity liquid-disordered phase (Ld) induces a red-shifted emission. The emission wavelength of NRERf varies with the environment. NRERf can be used to detect changes in lipid order and polarity induced in the ER by cholesterol extraction/enrichment, oxidative stress, and hyperosmotic shock .

NRLD is a lipid droplet-targeting solvatochromic fluorescent probe developed based on Nile Red (HY-D0718), which can target lipid droplets and sense the non-polar lipid microenvironment inside lipid droplets. NRLD exhibits superior lipid droplet-targeting selectivity compared to the parent Nile Red dye in HeLa cell imaging. NRLD achieves detection relying on the solvatochromic effect generated by excited-state charge transfer, and its emission wavelength shifts according to the local polarity changes of the microenvironment it locates in: the more compact and ordered the lipid packing and the lower the environmental polarity, the more blue-shifted the emission; the looser the lipid packing or the stronger the hydration and the higher the environmental polarity, the more red-shifted the emission. NRLD shows blue shift in the non-polar oil core of lipid droplets and red shift in high-polarity environments such as phosphate buffer .

DCA-Mln is a Fluorescent probe for ferric iron (Fe³⁺) detection. The probe works via an internal charge transfer (ICT) process from its diethylamino group to its dicyano group, which produces bright pink fluorescence in its unbound state; when Fe³⁺ is present, two molecules of DCA-Mln chelate with one Fe³⁺ ion, with one cyano group of each DCA-Mln participating in the complexation, and this binding triggers a photoinduced electron transfer (PET) process that provides a nonradiative deactivation pathway, resulting in fluorescence quenching; additionally, the probe exhibits a visible color change from purple to peach pink upon Fe³⁺ binding, enabling dual-channel detection. The probe has an excitation wavelength of 570 nm and an emission wavelength of 670 nm, and it shows a rapid response, with fluorescence stabilizing within 15 seconds of Fe³⁺ addition. DCA-Mln is cell-membrane-permeable, nontoxic at the imaging concentration of 10 μM, and can be used for Fe³⁺ detection in actual water samples, Fe³⁺ imaging in living cells, and as an anti-counterfeiting ink[1].

MBCB is a two-photon Fluorescent probe for dual-detection of mitochondrial SO₂ derivatives and viscosity. For SO₂ derivatives detection, the probe utilizes a Michael addition mechanism: nucleophilic addition of SO₂ derivatives to the C=C bond between the carbazole skeleton and 3-methylbenzothiazolium moiety destroys the strong intramolecular charge transfer (ICT) system between these groups, while enhancing the weak ICT system between the benzothiazole group and carbazole framework; this causes the red emission at 600 nm to decrease and the blue emission at 434 nm to increase, creating a ratiometric response based on the I₄₃₄ₙₘ/I₆₀₀ₙₘ intensity ratio. For viscosity detection, in low-viscosity environments, steric hindrance creates a twisted ICT (TICT) system with weak fluorescence, while in high-viscosity environments, intramolecular rotation is blocked, the TICT state is disrupted, and the strong ICT system is recovered, leading to a strong red emission at 567 nm with negligible change to the short-wavelength emission at 415 nm, creating a ratiometric response based on the I₅₆₇ₙₘ/I₄₁₅ₙₘ intensity ratio that has a logarithmic linear relationship with viscosity. The probe has excitation/emission wavelengths of Ex/Em = 351/434, 600 nm for SO₂ derivatives detection and Ex/Em = 351/567 nm for viscosity detection, with two-photon excitation at 740 nm for bioimaging; it also exhibits good mitochondrial targeting ability with a Pearson's colocalization coefficient of 0.93 when paired with Mito-Tracker Green. The probe shows high sensitivity and selectivity for SO₂ derivatives, has low cell cytotoxicity, and can be applied to detect exogenous/endogenous HSO₃⁻ in living cells and in vivo, as well as visualize mitochondrial viscosity changes induced by nystatin[1].