Synthesis of clickable fluorophores for incorporation into nanoscale materials

Abstract

The incorporation of fluorophores into nanoscale materials has attracted considerable attention due to their excellent fluorescence properties, including exceptional brightness, potential for aggregation induced emission, large Stokes shift, long fluorescence lifetime and high photostability and sensitivity. This thesis reports the synthesis of novel fluorophores carrying clickable functions based on the BOPHY or BODIPY backbone and subsequent investigation of their incorporation into both nanoscale (polymeric micelles and silica nanoparticles) and macroscale (thin films) materials. The 2,7-difunctionalised BOPHY diacrylate I and dimethacrylate II were successfully synthesised by a Sonogashira reaction of the corresponding 2,7-diiodo BOPHY IIIa (R1, R2 = I) and propargyl alcohol followed by esterification. Similarly, the 2-substituted BOPHY monomethacrylate IV was prepared from the 2-iodo BOPHY IIIb (R1 = H, R2 = I). Suzuki coupling between the diiodide IIIa (R1 = R2 = I) and 4- (hydroxymethyl)phenylboronic acid followed by acylation produced the 2,7-diaryl substituted BOPHY dimethacrylate V. These palladium catalysed couplings were found to be very sensitive to steric hindrance, the use of arylalkynes in place of propargyl alcohol was not successful. In a similar way, Suzuki coupling of the meso-unsubstituted 2,6-diiodo BODIPY VIa (R1 = R2 = I), 2,6-dibromo BODIPY VIb (R1 = R2 = Br) or the 2-bromo BODIPY VIc (R1 = H, R2 = Br) enabled preparation of the acrylate and methacrylate VII a-b and VIII. Similar routes were applied to the synthesis of meso-substituted BODIPY acrylates IX, X and XI. The BODIPY mono-acrylate VIII was shown to successfully undergo photoinitiated thiolene reaction with 2-mercaptoethanol to give the corresponding addition product. Measurements of the photophysical properties of the BOPHY and BODIPY dyes revealed that they are bright, strongly emissive fluorophores with high quantum yields in most cases. Addition of alkyne or aryl substituents leads to the anticipated red-shift in absorption and emission maxima and the spectra are relatively unaffected by subsequent esterification of pendant hydroxyl groups to form the acrylate esters. RAFT polymerisation of PEGMA was used to produce a macroRAFT initiator. This was then used together with acetylthiohexyl acrylate (ATA) and methacrylate (ATM) to produce diblock copolymers PI and PII. Aminolysis of the thioester groups was then employed to produce the corresponding thiolfunctionalised block copolymers (R2 = H). The methacrylate copolymer PII and the derived thiol both self-assembled in water to form polymeric micelles which were characterised by dynamic light scattering (DLS). Attempts to covalently attach acrylate functionalised BODIPY dyes to the thiol-containing block copolymers using the thiol-ene reaction were rather disappointing. In all cases, the level of BODIPY incorporation into the final polymers was very low, making unambiguous characterisation difficult. Similarly, attempted incorporation of the BODIPY monoacrylate VIII into the main chain of block copolymers by RAFT polymerisation together with the macroRAFT initiator and ATM or ATA led to very low levels of dye incorporation. Despite the difficulty in synthesis and characterisation of these polymers, the final materials and the derived polymeric micelles formed by self-assembly in water, were all emissive thus indicating some level of dye incorporation. Thin films were prepared by simple dissolution of all six acrylate-functionalised BODIPY and BOPHY dyes into a poly(methyl methacrylate) (PMMA) matrix. For comparison, all six dyes were covalently incorporated into polymers by thermally initiated copolymerisation with methyl methacrylate, followed by thin film casting. The fluorescence lifetimes of the dyes were found to increase on dissolution into PMMA and a further increase was observed on covalent incorporation into the polymer. This finding was rationalised on the basis of a reduction in conformational freedom of the substituents on the periphery of the dyes and the associated reduction in non-radiative pathways. Thiol-functionalised silica nanoparticles were produced by the Stober method from TEOS followed by reaction with (3-mercaptopropyl)trimethoxysilane. All six acrylatefunctionalised BODIPY and BOPHY dyes were then attached to the surface of these nanoparticles by thiol-ene click reaction to produce a set of fluorescent nanoparticles which were characterised by TEM, DLS, excitation, fluorescence emission and lifetime measurements. In addition, luminescence spectral images were obtained by confocal Raman microscopy.