Palladium/norbornene-catalyzed C-H bond activation and annulation to construct polycyclic aromatic hydrocarbon-based fluorescent materials

Abstract

Reported herein is the first example of NBE-CO₂Me-mediated palladium-catalyzed C-H bond activation and annulation of bromo(hetero)aromatics to construct structurally diverse polycyclic aromatic hydrocarbon (PAH)-based fluorescent materials. The approach shows a broad substrate scope and provides straightforward access to screening high-performance fluorescent materials. A novel organic single-molecule white-light material with anti-Kasha dual-emission characteristics has been developed herein. Furthermore, the anti-Kasha dual-emission material was fabricated as water-dispersed nanoparticles (NPs) to target the mitochondria of living cells. The corresponding NPs could be further applied in two-channel emission intensity ratio imaging to observe the cellular local imaging information and the intercellular structure.

Scheme 1. Polycyclic aromatic hydrocarbon (PAH)-based fluorescent materials and synthetic routes.

Scheme 2. Substrate scope of bromides. Reaction conditions: 1 (0.1 mmol), 2 (0.2 mmol), 3a (0.15 mmol), Pd(OAc)₂ (10 mol%), P(2-MeC₆H₄)₃ (25 mol%), NBE-CO₂Me (1.5 equiv.), K₂CO₃ (4.5 equiv.), toluene (0.1 M), N₂, 130 °C, 72 h. ^aPhDavePhos (25 mol%). ^bPd(OAc)₂ (20 mol%), P(2-MeC₆H₄)₃ (40 mol%). Test conditions for absorption and emission: absorption maximum in CH₂Cl₂ (1 × 10⁻⁶ M). Emission maximum in CH₂Cl₂ (5.0 × 10⁻⁵ M). Excitation slit: 2.5 nm, emission slit: 1.0 nm. λ_ex = 370 nm, PMT voltage = 700 V. Test parameters for quantum yield: scan slit: 0.55, fixed/offset slit: 2.5, detector: PMT-900.

Scheme 3. Substrate scope of alkynes. Reaction conditions: 1 (0.1 mmol), 2 (0.2 mmol), 3 (0.15 mmol), Pd(OAc)₂ (10 mol%), P(2-MeC₆H₄)₃ (25 mol%), NBE-CO₂Me (1.5 equiv.), K₂CO₃ (4.5 equiv.), toluene (0.1 M), N₂, 130 °C, 72 h. ^a0.05 mmol. Test conditions for absorption and emission: absorption maximum in CH₂Cl₂ (1 × 10⁻⁶ M). Emission maximum in CH₂Cl₂ (5.0 × 10⁻⁵ M). Excitation slit: 2.5 nm, emission slit: 1.0 nm. λ_ex = 370 nm, PMT voltage = 700 V. Test parameters for quantum yield: scan slit: 0.55 nm, fixed/offset slit: 2.5 nm, detector: PMT-900.

Scheme 4. Proposed reaction mechanism.

Fig. 1. Anti-Kasha dual-emission characteristics of 6u and 6w. (a), (d) Excitation-wavelength-dependent fluorescence spectra of 6u (0.25 μM in DCM) and 6w (2.4 μM in toluene). (b) and (e) Molecular orbitals of the S₀, S₁, S₃, or S₄ states of 6u and 6w; (c) and (f) Jablonski diagram illustrating the anti-Kasha dual-emission mechanism of 6u and 6w.

Fig. 2. Fluorescence microscopy images of HeLa cells incubated with 6w NPs (10 μM) for 2 h at 37 °C. (a) Fluorescence microscope image from channel 1 at 425–500 nm (excitation 405 nm). (b) Fluorescence microscope image from channel 2 at 500–700 nm (excitation 405 nm). (c) Fluorescence microscope image from channel 2 at 508–700 nm (excitation 488 nm). (d) The emission intensity ratio of (a) and (c) of HeLa cells (image generation by Image J software). Elliptic: mark out the intercellular structure and cellular local imaging information. The scale bar is 25.0 μm.

Fig. 3. The co-staining experiments. (a) Fluorescent image of HeLa cells cultured with 6w NPs (10 μM) (λ_ex = 552 nm, λ_em = 575–700 nm). (b) Fluorescent image of HeLa cells with Mito-Tracker Green (λ_ex = 488 nm, λ_em = 508–540 nm). (c) Merged image of (a) and (b). (d) The Pearson correlation coefficient r = 0.87. (e) Fluorescent image of HeLa cells cultured with 6w NPs (5 μM) (λ_ex = 552 nm, λ_em = 570–600 nm). (f) Fluorescent image of HeLa cells with Mito-Tracker Red (λ_ex = 633 nm, λ_em = 650–700 nm). (g) Merged image of (e) and (f). (h) The Pearson correlation coefficient r = 0.85; the scale bar is 25.0 μm.