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. 2026 Oct 26. Online ahead of print. doi: 10.1039/d6ra10062g

Anthracene–phenyl-vinyl–quinoline: a molecular rotor with a large Stokes shift and solvatofluorochromism for lipid-droplet imaging

Jayachandran Jayaraman a, Manbit Subhadarsi Panda b, Rafique Sanukhan c, Hirak Chakraborty b,✉, Animesh Samanta c,✉, Goutam Kumar Kole a,✉
PMCID: PMC13699513  PMID: 42936561

Abstract

9-Anthracene–phenyl-vinyl–quinoline (9-AnPhVQ), a π-extended molecular rotor, was synthesized and structurally characterized. It exhibits a very large Stokes shift (12 200 cm−1) and exceptional solvatofluorochromism (156 nm emission shift from hexane to DMSO). Its viscosity-responsive fluorescence due to restricted intramolecular rotation (RIR), as evidenced by fluorescence anisotropy in POPC membranes with varying mol% of cholesterol, has been investigated. Furthermore, it exhibits preferential lipid-droplet imaging and ferroptosis-associated cellular stress. These unique features establish 9-AnPhVQ as a promising molecular rotor probe for fluorescence-based sensing and lipid-droplet imaging.

A π-extended molecular rotor exhibits fluorescence anisotropy, preferential lipid-droplet imaging, and ferroptosis-associated cellular stress.graphic file with name d6ra10062g-ga.webp

Introduction

The structural tunability of organic fluorophores is valuable for diverse applications in optoelectronics,1,2 fluorescence bioimaging,3 various sensing applications,4etc. Recently, π-conjugated donor–acceptor frameworks have emerged as an effective molecular design strategy for constructing intramolecular charge transfer (ICT)-based fluorophores with large Stokes shifts, viscosity response, and pronounced solvatochromism, making them highly promising for bioimaging applications such as viscosity sensing and polarity mapping during cellular dysfunction.5–8 Among π-conjugated fluorophores, anthracene is an attractive molecular scaffold because of its rigid aromatic framework, environment-sensitive fluorescence, and tunable optoelectronic properties.9,10 Unlike anthraquinone derivatives, which typically show significant solvatochromic shifts owing to their carbonyl-based acceptor units, non-quinonoid anthracene fluorophores generally display relatively modest ICT and limited solvatochromic responses.11,12 Developing anthracene-based fluorophores that combine pronounced solvatochromism with high environmental sensitivity therefore remains an important challenge for fluorescent probe design.

Viscosity is a critical physicochemical parameter that regulates molecular diffusion, intermolecular interactions, and structural organization.13 Microviscosity plays a crucial role in regulating membrane fluidity, intracellular transport, enzymatic activity, and signal transduction. Therefore, developing sensitive approaches to monitor viscosity changes at the microscopic and subcellular levels in complex biological environments remains highly desirable.14,15 Fluorescence-based methods are ideal for local viscosity measurements due to their high sensitivity, rapid response, and live cell imaging.16 Unrestricted rotational motion in low-viscosity environments often promotes non-radiative decay, while higher viscosity enforces restricted intramolecular rotation (RIR) and pronounced ICT, resulting in enhanced emission. A systematic progress on the development of probes that have been widely applied for investigating intracellular microenvironments, lipid bilayers, cytoplasmic crowding, etc.17–19 The development of molecular rotors with improved sensitivity, selectivity, and targeting has broadened the scope of biological imaging and functional material characterization applications.20–22 Further, lipid composition in the membrane is a key determinant of membrane physicochemical properties, as cholesterol regulates membrane fluidity, permeability, and lateral heterogeneity by promoting phase coexistence and domain formation. Thus, fluorescent probes with molecular rotors can detect changes in viscosity, making them valuable for studying dynamic biological microenvironments.23–27 Among various intracellular organelles, lipid droplets (LDs) have recently attracted significant interest because they are highly dynamic organelles involved in lipid metabolism, energy homeostasis, and cellular signalling.28,29 Alterations in LD composition, cell death processes such as ferroptosis, apoptosis, and metabolic activity can influence their local microviscosity, especially during cellular stress and metabolic disease progression.30,31 Therefore, viscosity-responsive fluorescent probes with LD-targeting ability are valuable for visualizing intracellular microenvironmental changes during cellular stress.32–35

Herein, we report a π-extended anthracene fluorophore, 9-anthracene–phenyl-vinyl–quinoline (9-AnPhVQ, Fig. 1), and its exceptional solvent-dependent emission shift of 156 nm from hexane to DMSO and very large Stokes shift. Furthermore, its viscosity-responsive fluorescence, lipid-droplet imaging capabilities, and ferroptosis-associated cellular stress are explored.

Fig. 1. (a) Synthesis of 9-AnPhVQ, and (b) its crystal structure, shown in 50 : 50 ORTEP model.

Fig. 1

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Results and discussion

Synthesis and structure of 9-AnPhVQ

9-AnPhVQ was synthesized via the Pd-catalyzed Suzuki–Miyaura cross-coupling of 9-anthraceneboronic acid and 4-bromophenylvinyl quinoline with a yield of 78% (Fig. 1a). It was characterized using various spectroscopic and analytical tools, such as 1H & 13C-NMR, high-resolution mass spectrometry, and FT-IR (SI, Fig. S4–S7). An unequivocal structure determination by single-crystal X-ray diffraction analysis confirmed the identity of 9-AnPhVQ (Fig. 1b). It crystallizes in the monoclinic P21/n space group with the asymmetric unit containing one independent molecule (Z' = 1), and the unit cell containing four molecules (Z = 4). The overall molecular structure is not planar, having a dihedral angle between the planes of the anthracene and benzene rings of 88.74°, and that between the benzene and quinoline rings is found to be 51.39° (Fig. 1b). The crystal packing is primarily held together by a combination of C–H⋯π, C–H⋯N, and π⋯π interactions (Fig. S1–S3).

Photophysical properties

The UV-vis spectra of the fluorophore in MeCN (c = 1 × 10−5 M) exhibited vibronic absorption bands at 325 to 395 nm, with λmax = 333 nm and ε = 18 200 M−1 cm−1 in MeCN (c = 1 × 10−5 M), attributed to π–π* transitions (Fig. 2a). The absorption profile showed negligible solvatochromism (∼5–10 nm) across solvents ranging from hexane to DMSO, indicating a weak solvent dependence of the ground state (Fig. S9, SI). The vibronic fine structure was better resolved in hexane, whereas increasing solvent polarity led to progressive band broadening and diminished vibronic resolution.36 In contrast, it exhibited a single broad emission band with λmax = 560 nm in MeCN (c = 1 × 10−5 M) with a quantum yield of 0.36, corresponding to an apparent Stokes shift of 12 200 cm−1 (227 nm) (Fig. 2a and Table S2). The emission spectra of 9-AnPhVQ, measured in different solvents of varied polarity, showed substantial redshifts (Table 1). A large bathochromic shift in emission occurred when the solvent was altered from hexane (429 nm) to DMSO (585 nm), with a shift of Δλ = 156 nm (6200 cm−1), which is one of the largest red shifts reported for anthracene-based derivatives (Fig. 2b).37,38 This significant polarity-dependent emission shift was evidenced by a progressive change in fluorescence color from blue in low-polarity solvents to green, yellow, and finally amber in highly polar solvents. This positive solvatochromism arises from preferential stabilization of the more polar ICT excited state by polar solvents, thereby lowering the emission energy and producing red-shifted fluorescence. This was further examined using the correlation of Stokes shifts in different solvents, and the quantitative analysis of positive emission solvatochromism was articulated using the Lippert–Mataga plot, which showed a linear fit (Fig. 2c). Photographs of 9-AnPhVQ in various solvents (c = 1 × 10−5 M) under UV light (365 nm) demonstrate its spectacular solvatochromic emissions (Fig. 2d).

Fig. 2. (a) UV-vis absorption and emission spectra in acetonitrile (c = 1 × 10−5 M); (b) normalized emission spectra in various solvents; (c) Lippert–Mataga plot and (d) emission colour of 9-AnPhVQ in various solvents under UV light.

Fig. 2

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Table 1. Spectroscopic data of 9-AnPhVQ in various solventsa.

Solvent ε η Δf λ abs (nm) v̄ A (cm−1) λ em (nm) v̄ F (cm−1) Δv̄ (cm−1)
Hexane 1.89 1.374 0.001 329 30 400 429 23 300 7100
Toluene 2.38 1.496 0.013 335 29 800 450 22 200 7600
THF 7.52 1.405 0.210 332 30 100 499 20 000 10 100
EtOAc 6.00 1.372 0.199 331 30 200 487 20 500 9700
DCM 8.93 1.424 0.217 333 30 000 517 19 300 10 700
Acetone 20.70 1.359 0.284 331 30 200 516 19 300 10 800
MeCN 36.64 1.344 0.305 333 30 000 560 17 800 12 200
MeOH 32.60 1.329 0.308 334 29 900 548 18 200 11 700
DMF 36.70 1.430 0.274 335 29 800 548 18 200 11 600
DMSO 46.68 1.479 0.263 350 28 600 585 17 100 11 500
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a

Key: dielectric constant (ε); refractive index (η); orientation polarizability (Δf); absorption wavelength (λabs (nm)); absorption wavenumber (v̄A (cm−1)); emission wavelength (λem (nm)); emission wavenumber (v̄F (cm−1)); Stokes shift (Δv̄ (cm−1)). Solvent: THF (tetrahydrofuran), EtOAc (ethyl acetate), DCM (dichloromethane), MeCN (acetonitrile), MeOH (methanol), DMF (dimethylformamide), DMSO (dimethyl sulfoxide).

Electrochemistry

The redox properties of the compound were investigated using cyclic voltammetry (CV) in MeCN solution at a scan rate of 50 mV s−1, employing 0.1 M [n-Bu4N][PF6] as the supporting electrolyte, Ag/AgCl as the reference electrode, and the ferrocene/ferrocenium (Fc/Fc+) couple as the internal standard. Measurements were carried out from the anodic to the cathodic direction for the oxidation event, followed by an inverse cycle for the reduction event. 9-AnPhVQ exhibited a first quasi-reversible reduction event at E1/2 of −0.78 V, followed by a second irreversible reduction at an Epc of −1.59 V in cyclic voltammetry (Fig. S22).39–41 Successive multiple scans at a rate of 50 mV s−1 are presented in Fig. S23. With each subsequent scan at the same rate, the peak current was observed to increase gradually.

Calculation of HOMO–LUMO and optical bandgap

The optical band gaps were determined by applying the equation Eg = 1240/λonset to establish a correlation between the experimental photophysical behavior and the electronic structure. The LUMO energy levels were ascertained from the cyclic voltammogram by utilizing the reduction onset values and applying the equation ELUMO = – e [Eonsetred + 4.8] V.42,43 The relation Eg = ELUMO − EHOMO was used to estimate the corresponding EHOMO values.44,45 The observed values obtained through experimental data, such as cyclic voltammetry and UV-vis absorption, are presented in Table S3. The results align closely with the estimated values derived from density functional theory (DFT) and will be examined in the subsequent section.

DFT calculations

DFT simulations revealed further information about the energy levels and population of electron densities on the frontier molecular orbitals of 9-AnPhVQ. The optimized geometries of the ground state (S0) and the first excited singlet state (S1) in the gas phase were then used in TD-DFT computations to obtain electronic transition information. The computed absorption maxima, oscillator strengths, main electronic transitions, and dipole moments are presented in Table S4 in the SI. The optimized geometrical parameters closely match the single-crystal XRD data (Table S5). The dipole moment behavior remains unchanged in both the ground and excited states (Fig. S27). TD-DFT computations at the B3LYP and CAM-B3LYP/6-31+G(d) levels revealed a consistent agreement between simulated and observed absorption and emission spectra (Fig. S24–S26). DFT calculations at the B3LYP and CAM-B3LYP levels with the 6-31+G(d) basis set in the gas phase46,47 revealed that the HOMO is mainly localized on the electron-rich anthracene unit, while the LUMO is mainly distributed over the phenyl-vinyl–quinoline side arm as the acceptor, indicating strong intramolecular charge transfer (ICT) character (Fig. 3). The computed energy gap of 3.28 eV between the HOMO (–5.54 eV) and LUMO (–2.26 eV) aligns closely with the experimental optical bandgap (Eg = 3.02 eV).48

Fig. 3. DFT-computed FMOs and HOMO–LUMO energy gap of 9-AnPhVQ.

Fig. 3

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Viscosity-dependent fluorescence anisotropy

The fluorescence intensity of 9-AnPhVQ, measured in methanol–glycerol mixtures with varying glycerol content, increases significantly with increasing glycerol content in the mixture (Fig. S17). This enhancement of fluorescence in a viscous medium suggests a molecular rotor property.49 Furthermore, we measured the fluorescence anisotropy in methanol–glycerol mixtures by plotting it as a function of viscosity. The fluorescence anisotropy vs. viscosity plot fits a hyperbolic function perfectly (Fig. 4), which can be used as a standard curve to determine the microviscosity of the membrane environment.

Fig. 4. Plot of steady-state fluorescence anisotropy of 9-AnPhVQ probe with increasing viscosity of a solvent mixture of methanol and glycerol. The viscosities of solutions with different methanol and glycerol mole fractions have been taken from the standard literature. The 9-AnPhVQ concentration was maintained at 2 µM, and measurements were performed at 25 °C. The excitation wavelength was 340 nm for anisotropy measurements. Data points shown are the mean ± S.E. of at least three independent measurements. See the Materials and Methods section for more details.

Fig. 4

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As cholesterol is an important constituent of the biological membrane, and its content depends on different pathophysiological conditions, we measured fluorescence anisotropy in POPC membranes with varying mol% of cholesterol. The fluorescence anisotropy value increases with increasing concentration of cholesterol (Fig. 5a) as the membrane becomes more ordered in the presence of cholesterol.50 Using the standard curve (Fig. 4), we calculated the microviscosity of the POPC membranes in the presence of different mol% of cholesterol and plotted it in Fig. 5b. The microviscosity of the POPC membranes increases almost two-fold in the presence of 30 mol% of cholesterol. Therefore, the molecular rotor property of 9-AnPhVQ can be successfully utilized to determine the microviscosity of membranes containing different mol% of cholesterol.

Fig. 5. Plots of (a) anisotropy of 9-AnPhVQ and (b) microviscosity in POPC/cholesterol membranes containing different mol% of cholesterol at 25 °C. The concentration of 9-AnPhVQ was maintained at 2 µM, and measurements were performed in 10 mM phosphate buffer at pH 7.4. Data points shown are the mean ± S.E. of at least three independent measurements.

Fig. 5

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Solvent-dependent fluorescence lifetime

The solvent-dependent time-resolved fluorescence decay profiles of 9-AnPhVQ (10 µM), recorded in solvents with different polarities (hexane, DCM, and DMSO; λex = 320 nm; Fig. S14), were fitted well using a mono-exponential model, yielding average fluorescence lifetimes that increased progressively with solvent polarity: 2.35 ns in hexane, 4.75 ns in DCM, and 5.06 ns in DMSO (hexane < DCM < DMSO). This indicates that the excited state of 9-AnPhVQ stabilizes the charge-separated excited state, reduces nonradiative relaxation, and is sensitive to its microenvironment. Such a lifetime response is consistent with the observed solvatochromic emission behavior and provides a mechanistic explanation for the polarity-dependent color changes. In nonpolar hexane, a relatively shorter lifetime likely reflects the behavior of 9-AnPhVQ in hydrophobic microenvironments such as lipid droplets. Thus, an LD-specific blue-fluorescent probe is anticipated, offering a rarely explored imaging window with minimal fluorescence in other channels and enabling DAPI channel LD imaging during dynamic cellular processes, including ferroptosis.

Intracellular colocalization studies

To elucidate the intracellular localization of 9-AnPhVQ, MTT assays were first performed to confirm its good biocompatibility (>80% up to 50 µM, Fig. S21), followed by live-cell colocalization experiments using the organelle-selective markers Nile Red (lipid droplets) and LysoTracker Deep Red (lysosomes), given the observed punctate fluorescence pattern. Cells were incubated with 9-AnPhVQ and subsequently co-stained with either Nile Red or Lyso-Deep Red, followed by confocal fluorescence imaging conducted in the DAPI (λex = 405 nm) and TRITC (λex = 560 nm) channels for Nile Red, and the Cy5 channel (λex = 640 nm) for LysoTracker Deep Red. Merged DAPI/TRITC overlays were generated, and pixel-intensity scatter plots were analyzed to quantify the extent of signal overlap. It displayed a discrete, punctate distribution that co-localized strongly with Nile Red, with Pearson's colocalization coefficient (PCC) of 0.81, consistent with preferential accumulation of 9-AnPhVQ within lipid droplets (Fig. 6). Conversely, a negligible spatial overlap was observed between 9-AnPhVQ and Lyso-Deep Red (PCC = 0.21), indicating minimal lysosomal accumulation. Collectively, these results establish lipid droplets, rather than lysosomes, as the principal subcellular target of 9-AnPhVQ in live cells.

Fig. 6. Co-staining of 9-AnPhVQ with Nile Red (top row) and Lyso-Deep Red (bottom row) in live cells, and imaged in the DAPI (405 nm) and TRITC (560 nm) channels, with DAPI/TRITC merged and scatter plots quantifying colocalization. Scale bars = 10 µm.

Fig. 6

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Lipid droplet biogenesis

To evaluate the responsiveness of 9-AnPhVQ toward lipid droplet biogenesis, live HeLa cells were treated with oleic acid (500 µM, 6 h) to induce lipid droplet accumulation, while untreated cells served as controls. Both control and oleic acid-treated HeLa cells were incubated with 9-AnPhVQ (10 µM, 30 min) and imaged live by confocal microscopy in the DAPI (405 nm) and bright field (BF) channels, with merged overlays generated to visualize probe localization relative to cellular morphology. Untreated cells displayed a relatively weak, diffuse punctate 9-AnPhVQ fluorescence signal, whereas oleic acid-treated cells exhibited a markedly enhanced and more abundant punctate fluorescence pattern consistent with increased lipid droplet formation (Fig. 7). Quantitative analysis of mean fluorescence intensity confirmed a significant increase in 9-AnPhVQ signal in oleic acid-treated cells relative to untreated controls. These results demonstrate that 9-AnPhVQ sensitively and selectively reports oleic acid-induced lipid droplet accumulation in live cells.

Fig. 7. Fluorescence response of 9-AnPhVQ in HeLa cells treated with ferroptosis and apoptosis inducers. Live cells were treated with RSL3, erastin, or staurosporine (STS), alongside untreated controls. Confocal images were acquired in the DAPI (405 nm), transmitted light (TD), and merged DAPI/TD channels for each treatment condition. Bar graphs show quantification of mean 9-AnPhVQ fluorescence intensity in control versus RSL3-, erastin-, and STS-treated cells, respectively. Data are presented as mean ± SD. Scale bars = 10 µm.

Fig. 7

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To assess whether 9-AnPhVQ fluorescence is influenced by live HeLa cells, the cells were treated with the ferroptosis inducers, RSL3 and erastin, or the apoptosis inducer staurosporine (STS), alongside untreated cells as controls.51–54 Following treatment, cells were incubated with 9-AnPhVQ and imaged live by confocal microscopy in the DAPI (405 nm) and BF channels, with merged DAPI/BF overlays generated for each condition. Cells treated with RSL3 displayed a visibly enhanced punctate 9-AnPhVQ fluorescence signal relative to untreated controls, whereas erastin- and STS-treated cells showed only modest increases in fluorescence intensity compared to their respective controls. Quantitative analysis of mean fluorescence intensity corroborated these observations, revealing a more pronounced increase in 9-AnPhVQ signal upon RSL3 treatment relative to smaller increments observed with erastin and STS. These results indicate that 9-AnPhVQ fluorescence is more responsive to RSL3-induced ferroptosis than to erastin-induced ferroptosis or STS-induced apoptosis.

Conclusion

We report 9-AnPhVQ, a π-extended anthracene-based molecular rotor that integrates features of pronounced solvatochromism, large Stokes shifts, viscosity-responsive fluorescence, and lipid-droplet targeting within a single fluorophore. Structural and computational analyses establish a distinct donor–acceptor electronic architecture, with the anthracene acting as the electron-rich component and the phenyl-vinyl–quinoline unit acting as the electron-deficient component, supporting a pronounced ICT character. It displays an exceptional emission shift of 156 nm from hexane to DMSO, as well as a marked viscosity-dependent fluorescence response arising from restricted intramolecular rotation (RIR), as evidenced by measuring fluorescence anisotropy in POPC membranes with varying mol% of cholesterol. Importantly, these intrinsic photophysical properties translate into biological functionality, with 9-AnPhVQ showing preferential localization in lipid droplets and enhanced fluorescence upon lipid-droplet accumulation and ferroptosis-associated cellular stress. The combination of structural tunability, exceptional environmental sensitivity, and intracellular applicability establishes 9-AnPhVQ as a promising molecular rotor probe for fluorescence-based sensing and lipid-droplet imaging.

Conflicts of interest

There are no conflicts to declare.

Supplementary Material

RA-OLF-D6RA10062G-s001

Acknowledgments

J. J. and G. K. K. thank the SRM Institute of Science and Technology for the financial support and the Department of Chemistry, DST-FIST, the Nanotechnology Research Centre, and the High-Performance Computing Centre, SRMIST, for research infrastructure. R.S. is grateful to Shiv Nadar Institution of Eminence, Delhi NCR, and the Shiv Nadar Foundation for the research fellowship. A. S. acknowledges the Shiv Nadar Foundation (Collab0007) for funding. H. C. and M. S. P. acknowledge the University Grants Commission for the UGC Assistant Professor position and the Odisha State Higher Education Council, Higher Education Department, Government of Odisha, for the Mukhyamantri Research and Innovation Fellowship Program (MRIP) fellowship, respectively.

Data availability

CCDC 2580170 contains the supplementary crystallographic data for this paper.55

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: experimental details, additional figures, spectroscopic characterization data, additional experimental data, crystallographic data in CIF format, and checkCIF reports. See DOI: https://doi.org/10.1039/d6ra10062g.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Citations

  1. CCDC 2580170: Experimental Crystal Structure Determination, 2026, 10.5517/ccdc.csd.cc2slw9l [DOI]

Supplementary Materials

RA-OLF-D6RA10062G-s001
RA-OLF-D6RA10062G-s001.pdf (5.3MB, pdf)

Data Availability Statement

CCDC 2580170 contains the supplementary crystallographic data for this paper.55

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: experimental details, additional figures, spectroscopic characterization data, additional experimental data, crystallographic data in CIF format, and checkCIF reports. See DOI: https://doi.org/10.1039/d6ra10062g.

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