Carbazole- and Fluorene-Fused Aza-BODIPYs: NIR Fluorophores with High Brightness and Photostability

Abstract

Three new aza-BODIPY dyes incorporating fused fluorene or carbazole moieties have been prepared. The dyes show significant enhancement of photophysical properties compared to the parent 1,3,5,7-tetraphenyl aza-BODIPY (TPAB): a bathochromic shift of the absorption maximum (up to 2700 cm^-1 ) and emission maximum (up to 2270 cm^-1 ); an almost threefold increase in molar absorption coefficients (to ca. 230 000 M^-1 cm^-1 ) and a significant increase in the fluorescence quantum yield to 49-66 %. Owing to the combination of these properties, the new aza-BODIPY dyes belong to the brightest NIR dyes reported. The dyes also show excellent photostability. Due to their outstanding properties, the new dyes represent a promising platform for further exploration in biomedical research. A pH indicator containing only one fused carbazole unit was also prepared and shows absorption and emission spectra that are bathochromically shifted by about 110 and 100 nm, respectively, compared to the indicator dye based on the TPAB chromophore.

Keywords: dyes; fluorescence; near-infrared; pH; sensing.

Figure 1

Structure of the BODIPY core, tetraphenyl aza‐BODPY (TPAB) and aza‐BODIPYs aza‐FL, aza‐CZ and aza‐OHCl (reported in this work).

Scheme 1

Synthesis of the aza‐BODIPY dyes.

Figure 2

Normalized absorption (solid lines) and emission (dashed lines) spectra of dyes TPAB (blue), aza‐FL (red), aza‐OHCl (yellow), and aza‐CZ (turquoise) in THF.

Figure 3

A) Absorption spectra of aza‐FL, aza‐CZ and aza‐OHCl compared to IR‐125 (the gray line indicates the excitation wavelength corresponding to the LED peak, 730 nm) B) Photodegradation of aza‐FL, aza‐CZ, aza‐OHCl (in THF) and IR‐125 (in EtOH) upon irradiation of the solutions with the far‐red light of a high‐power LED array (λ=730 nm, photon flux: ∼15 000 μmol s⁻¹ m⁻²). The absorption spectra measured over the course of the photobleaching experiment are provided in Figures S3‐S6.

Figure 4

pH‐sensing properties of B)‐D) TPAB‐OHCl and E)‐G) aza‐OHCl. A) Principle of pH sensing by PET sensor. B) and E) pH dependency of the absorption spectra. C) pH dependency of the emission spectra of TPAB‐OHCl (λ _ex=630 nm). F) pH dependency of the emission spectra of aza‐OHCl (λ _ex=725 nm). D) and G) Corresponding response curves with fit using a Boltzmann sigmoid. All the spectra were acquired in THF/EtOH/aqueous buffer (1 : 1 : 1, v/v/v) at room temperature.

Figure 5

Security ink based on aza‐CZ immobilized into RL‐100 nanoparticles. A) Aqueous dispersion of nanoparticles under ambient light (C=0.15 mg/mL of beads). B) Filter paper with invisible writing under ambient light. C) Fluorescence image of the filter paper obtained with a NIR camera.