Covalent ferrocene conjugation as an intramolecular strategy for photostability in fluorescein

Abstract

Photobleaching severely limits the utility and long-term reliability of fluorescence-based measurements. To address this long-standing limitation, we synthesized a conjugate that covalently links fluorescein isothiocyanate (FITC) to ferrocene (Fc), a redox-active metallocene. Our two-step synthesis involved reduction of ferrocene methylene azide followed by formation of a stable thiourea linkage. Photophysical characterization confirmed highly efficient intramolecular quenching, evidenced by an 81.5% reduction in quantum yield (Φ) and a shortened lifetime (τ = 3.2 ns vs. 4.1 ns for FITC). The Fc-FITC conjugate exhibited an 11-fold increase in photobleaching half-life (693 vs. 63 min for FITC), retaining 94% of its initial fluorescence after 60 min of constant 23 mW/cm² irradiation, compared to only 52% for FITC. Direct singlet oxygen (¹O₂​) quantification using Singlet Oxygen Sensor Green (SOSG) confirmed that Fc conjugation reduces the photosensitization rate to only 28% of that of native FITC. Sodium azide (NaN₃​) quenching assays further validated the suppression of reactive oxygen species (ROS), as the Fc-FITC system exhibited negligible quenching (4.6%) compared to the significant response of native FITC (32.5%). This stabilization arises from a Photoinduced Electron Transfer (PET) mechanism that suppresses formation of the destructive triplet state (T¹​). A quantitative Rehm-Weller analysis (ΔG_PET ≈ - 0.76 eV) and direct ROS validation establish a robust mechanistic basis for this photoprotective effect. Together, these findings establish a unique intramolecular photostabilization strategy where signal durability and quantitative precision are prioritized over peak brightness, offering a framework for designing robust hybrid redox-fluorophore probes suited for persistent sensing and long-term quantitative analysis.

Keywords: Ferrocene conjugates; Fluorescein; Intramolecular quenching; Photoinduced electron transfer (PET); Photostability; Redox chemistry.

Fig. 1

Complete synthesis scheme to obtain Fc-FITC conjugate. The diagram illustrates the three-step synthesis. The highlighted area represents the key functional group being transformed, and the percent yield is shown for each reaction step.

Fig. 2

Photophysical characterization and intermolecular interaction of FITC and ferrocene analogs. (a) UV–Vis absorption of FcCH₂OH and FITC in methanol. (b) FITC excitation (red, λ_em​=525 nm, red) and emission (blue, λ_ex​=485 nm) spectra. (c,d) Quenching anomaly: Rising fluorescence emission (c) and its linear 2D plot (d) of FITC upon addition of a dilute solution of FcCH₂OH (1.34 mmol) in methanol. (e) UV–Vis absorption spectra comparing Fc, FcCH₂OH, and FcCH₂NH₂​​ in ethanol. (f) Proposed chemical structure of the O-thiocarbamate intermediate. All spectra recorded at concentration of ~ 2 µM (A < 0.2) to ensure accuracy.

Fig. 3

Comparative photophysical spectra of FITC and Fc-FITC (1:1 v/v, EtOH/H₂O, pH 8 buffer). (a) UV-Vis absorption spectra. (b) Relative emission spectra (λ_ex​=490 nm), showing fluorescence quenching upon conjugation. All spectra recorded at concentration of ~ 2 µM (A < 0.2) to ensure accuracy.

Fig. 4

pH dependence of FITC and Fc-FITC in 1:1 v/v EtOH/H₂O buffer. Panels (a) and (b) show UV-Vis absorption spectra for FITC and Fc-FITC, respectively (pH 2 − 13). Panels (c) and (d) show corresponding fluorescence emission spectra. The bottom panels, (e) absorption at 500 nm and (f) emission at 530 nm vs. pH, were used to plot the pKa​ titration curves for both compounds. All initial spectra recorded at concentration of ~ 2 µM (A < 0.2) to ensure accuracy.

Fig. 5

Comparative electrochemical analysis of ferrocene derivatives. (a) CV and (b) DPV of Fc, FcCH₂OH, FcCH₂NH₂​, and the Fc-FITC conjugate. (anhydrous CH₃CN/0.1 M TBAPF₆, scan at a 100 mV/s from + 1.0 V to − 0.5 V). The working electrode was glassy carbon electrode (GCE), the counter electrode was platinum (Pt) wire, and the reference electrode was Ag/Ag+. The half-wave potential (E_1/2​) represents the potential halfway between the anodic peak (E_p.a.) for oxidation and the cathodic peak (E_pc) for reduction. The separation peak (ΔE_p​) represents the difference between E_p.a. and E_pc.

Fig. 6

DFT predictions of the frontier molecular orbitals for FITC (left) and Fc-FITC (right). The bottom energy levels represent the HOMO, and the top levels represent the LUMO.

Fig. 7

Comparative photostability and fluorescence dynamics of FITC and Fc-FITC. The time-resolved emission spectra collected over 60 min of continuous UV-Vis irradiation (320–500 nm, 23 mW/cm² in 1:1 EtOH/H_2​O pH 8 buffer) show the photobleaching kinetics for (a) FITC and (b) Fc-FITC. (c) Comparative photobleaching curves showing normalized emission intensity at 530 nm versus irradiation time; and (d) Fluorescence decay plot for FITC & Fc-FITC (λ_ex​=247 nm).

Fig. 8

Jablonski diagram of Fc-FITC showing PET to Fc efficiently quenches fluorescence and suppresses ISC to T¹ with BET providing a protective NR return to the GS.