Synthesis and Photophysical Properties of Novel BODIPY and BOPHY Dyes

Abstract

This thesis describes the synthesis and characterisation of novel fluorescent dyes based on BODIPY and BOPHY cores. Pyrrolylquinoline-BF2 2.15 and pyrrolylquinoline-BPh2 2.20 were synthesised in good yield by a simple, three-step sequence of Suzuki cross-coupling reaction of N-Boc 2-pyrroleboronic acid with 8-bromoquinoline, followed by thermolytic tert-butyloxycarbonyl deprotection and subsequent boron chelation. The absorption spectra of these chelates are quite similar to each other. While the emission spectrum of the BF2-chelate 2.15 resembles a normal BODIPY emission in appearance, the corresponding pyrrolylquinoline-BPh2 2.20 displays a very significantly red-shifted and broadened emission spectrum, which may be due to emission from a dimer in solution. Both 2.15 and 2.20 have a low quantum yield. The 8-benzoic acid-substituted BODIPY 3.33 was synthesised in good yield by a simple, four-step sequence of condensation reaction of methyl 4-formylbenzoate with pyrrole, followed by oxidation using p-chloranil, subsequent boron chelation and final demethylation of the aryl ester with lithium iodide. The helically chiral N,N,O,O-boron-chelated BODIPY carboxylic acid 3.45 was synthesised in good yield by a Suzuki cross-coupling reaction of Boc-protected pyrroleboronic acid with 2-bromophenol, followed by subsequent condensation, oxidation, and chelation to afford the ester-substituted N,N,O,O-BODIPY which was hydrolysed by base to form the required carboxylic acid 3.45. These 8-benzoic acid-substituted BODIPYs (3.33 and 3.45) were coupled with aminopropyltriethoxysilane to form silylated BODIPYs (3.34 and 3.46 which were successfully used ii to synthesise new periodic mesoporous organosilica (PMO) materials by both co-condensation and post-condensation methods at dye concentrations of 1% molar ratios versus the silica precursor template (P123). The successful formation of mesoporous silicas was confirmed by TEM. The photophysical properties of the fluorophore-functionalised silica were investigated using optical spectroscopy. The structural integrity of the dyes (co- and post condensation) appears to be maintained after immobilisation in silica and the excitation maxima fall within a small range, similar to the free dyes. The strapped BODIPYs (3.45) (free dye in solution and the silica samples loaded with the same dye) showed three different excitation peaks at 325 nm, 448 nm and 632 nm, and the usual shoulder peak at 584 nm which is typical of BODIPY. As expected, the combination of extending the conjugation and bonding of phenolic oxygens with boron, does lead to a significant red shift of both the absorption and emission spectra in comparison to the simple F2-BODIPY (3.33) and thus the optical properties can easily be fine-tuned towards the red region of the spectrum. BODIPY dyes carrying covalently linked electron donor and electron acceptor at the β-positions represent an extended conjugated system that should enhance charge transfer between donor and acceptor and redshift the absorption and emission. Synthesis of such species which were also suitably functionalised for attachment to silica was investigated. iii Synthesis of the 2-aryl BODIPY 4.5 by Suzuki-Miyaura coupling of a 2-bromo BODIPY with 4- carboxyphenylboronic acid was successful but low yielding. Consequently, 4-hydroxymethylphenyl boronic acid was used, resulting in an acceptable yield of the β-benzyl alcohol-substituted BODIPY 4.6. An aldehyde functional group was introduced into the other β-position to produce BODIPY 4.7 carrying benzyl alcohol and formyl substituents. Knoevenagel reaction of this aldehyde 4.7 with malononitrile gave the corresponding dicyanovinyl-functionalised BODIPY 4.14. These benzyl alcohol-substituted BODIPYs (4.6, 4.7 and 4.14) were reacted with triethoxy(3- isocyanato propyl)silane to form silylated BODIPYs (4.18, 4.19 and 4.20) which were successfully used to synthesise new PMO materials by both co-condensation and post-condensation methods at dye concentrations of 1% molar ratios versus the silica precursor template (P123). The successful formation of fluorophore-functionalised mesoporous silicas was confirmed by TEM and XRD. iv The bis(propargyl alcohol) functionalised BOPHY 5.2 was synthesised by Sonogashira coupling of 2,7-diiodoBOPHY with propargyl alcohol. This propargyl alcohol-substituted BOPHY was reacted with triethoxy(3-isocyanatopropyl)silane to form mono- (5.4) and bis-silylated (5.3) BOPHYs. Both dyes were incorporated into MCM-41 type silica at varying concentrations to up to 5% molar ratios versus the silica precursor (TEOS). Catechol adducts 6.13 and 6.14 were synthesised by a two-step condensation reaction of 3,5- dimethylpyrrole 2-carboxaldehyde with hydrazine hydrate to give the BOPHY ligand. Formation of the chiral boron complex 6.13 was achieved in good yield by a one-pot reaction of the ligand with boron tribromide followed by the highly diastereoselective addition of 3-methoxycarbonyl-1,2- catechol. The bis(di-tert-butylcatechol) adduct 6.14 was obtained in an analogous way, but proceeded in a much lower diastereoselectivity (3:1 racemic:meso). Chiral BOPHYs 6.13 and 6.14 were successfully resolved by preparative chiral HPLC. In both cases, the enantiomers produced mirror-image ECD spectra but attempts to measure CPL were unsuccessful. v O,C,N,N-Boron chelated BOPHY 7.26 was synthesised by chelation of the ligand 7.25 with phenylboronic acid. The two enantiomers of this half-strapped BOPHY were easily separated by chiral HPLC, but the resolved enantiomers were found to racemise on a timescale of hours, and so the characterisation of the chiroptical properties was not possible. N.B. Chemical diagrams are not included in this abstract.