Synthesis of Fluorescent, Dumbbell-Shaped Polyurethane Homo- and Heterodendrimers and Their Photophysical Properties

Abstract

Fluorescent dendrimers have wide applications in biomedical and materials science. Here, we report the synthesis of fluorescent polyurethane homodendrimers and Janus dendrimers, which often pose challenges due to the inherent reactivity of isocyanates. Polyurethane dendrons (G1-G3) were synthesized via a convergent method using a one-pot multicomponent Curtius reaction as a crucial step to establish urethane linkages. The alkyne periphery of the G1-G3 dendrons was modified by a copper catalyzed azide-alkyne click reaction (CuAAC) to form fluorescent dendrons. In the reaction of the surfaces functionalized two different dendrons with a difunctional core, a mixture of three dendrimers consisting of two homodendrimers and a Janus dendrimer were obtained. The Janus dendrimer accounted for a higher proportion in the products' distribution, being as high as 93% for G3. The photophysical properties of Janus dendrimers showed the fluorescence resonance energy transfer (FRET) from one to the other fluorophore of the dendrimer. The FRET observation accompanied by a large Stokes shift make these dendrimers potential candidates for the detection and tracking of interactions between the biomolecules, as well as potential candidates for fluorescence imaging.

Keywords: Janus dendrimers; absorption; click reaction; emission; fluorescence; heterodendrimers; homodendrimers; late-stage modification; polyurethane dendrimers.

Scheme 1

General methods of synthesis of Janus dendrimers.

Scheme 2

Synthesis of first-generation dendron 3 via one-pot multicomponent Curtius reaction.

Scheme 3

Late-stage modification of G1 dendron 3 forming fluorescent dendrons 6 and 7.

Scheme 4

Synthesis of first-generation homo- and Janus dendrimers 9–11.

Scheme 5

Growth of dendrons to the third generation.

Scheme 6

Late-stage modification of G2 and G3 dendrons 18–21.

Scheme 7

Synthesis of the second-generation homo- and Janus dendrimers 22–24.

Scheme 8

Synthesis of the third-generation homo- and Janus dendrimers 25–27.

Figure 1

Representative NMR spectrum of a Janus dendrimer showing G2 dendrimer, 23. The integration shows only a half-portion of compound structure (top left). Representative NMR of a late-stage-modified dendron by CuAAC showing G3 dendron, 20 (bottom left) (the integration involves the full compound structure). MALDI-TOF-MS spectra of different dendrons (11 and 20, top right) and dendrimer (26, bottom right).

Figure 2

UV–vis spectra of G1–G3 dendrimers (A,C,E) and fluorescence spectra of homo− and Janus dendrimers (B,D,F). The UV–vis spectra were recorded 50 mM concentration using 1:1 mixture of DCM and methanol, whereas the emission spectra were recorded at 50 $\mathsf{\mu }$m concentration for blue–fluorescent dendrimers but 1 mM for naphthalimide-containing dendrimers. The Janus dendrimers were shown to cause fluorescence resonance energy transfer (FRET) as demonstrated by longer emission wavelengths for those dendrimers.

Figure 3

Representative Stokes shifts of G2 homo− and Janus dendrimers 22–24 (A–C) in 1:1 solution of DCM and methanol. The shifts are shown both in wavelength (nm) and wavenumbers (cm⁻¹) in the adjoining figure. The dimmed black curve represents absorption whereas intense black or blue curve shows fluorescence emission of the dendrimer. The bottom right figure (D) shows FRET phenomenon observed in G2 Janus dendrimer 23. Abs = Absorption, Em = Emission.