Approaches to the synthesis of chiral and amino-substituted BODIPY dyes

PhD ThesisBoron dipyrromethenes (BODIPYs) have attracted considerable attention due to their fluorescence properties, including high fluorescence quantum yields and absorption coefficients, good photochemical stability and narrow absorption and emission bandwidths. Because of these properties, they have found extensive use for in vivo imaging, analyte sensing, photodynamic therapy, solar cells and light harvesting arrays. This thesis is divided in two parts. Part one covers the synthesis of functionalised chiral BODIPYs whilst part two involves investigations into the synthesis and application of aminoBODIPY via metal catalysed amination reactions. Four approaches to the synthesis of novel chiral BODIPY dyes were investigated. The axially chiral BODIPY dye with C2-symmetry I was synthesised via Suzuki coupling of the corresponding 2,6-dibromoBODIPY with 2-methoxyphenyl boronic acid. The unsymmetrical axially chiral BODIPY II was synthesized via Suzuki coupling of the corresponding 2-bromoBODIPY with phenyl boronic acid. Unfortunately, attempted resolution of these chiral systems by analytical chiral HPLC was unsuccessful. The new helically chiral BODIPYs IIIa-c were synthesised. Resolution of racemic helically chiral BODIPYs IIIa and IIIb was successfully accomplished by preparative chiral HPLC. Abstract ii Electronic circular dichroism (ECD) spectra of both (M) and (P) isomers of IIIa and IIIb were measured by Prof. W. Herrebout (University of Antwerp). Comparison of the measured and computationally predicted ECD spectra allowed the absolute configuration of each of the enantiomeric samples of IIIa-b to be established. The CPL spectra of both (M) and (P) isomers of IIIa and IIIb were recorded by Prof. R. D. Peacock (University of Glasgow). The |glum| of IIIa at 637 nm is 0.0043 and IIIb at 675 nm is 0.0042, which are among the largest so far reported for a simple BODIPY fluorophore in solution. We designed and synthesized the mono-strapped BODIPY V scaffold via sequential regioselective functionalisation of the unsymmetrical BODIPY IV. Resolution of N,N,F,O-BODIPY V by analytical chiral HPLC gave two peaks confirming that single enantiomers of this compound should be accessible. The 8-anisyl-substituted BODIPY VI was synthesized using a standard synthetic route, regioselective iodination with ICl then gave the 2-iodo-substituted BODIPY VII. We unexpectedly found that copper catalysed nucleophilic substitution of the 2-iodoBODIPY VII produced the 3-amino BODIPYs VIIIa-n in good yield. The reactions occurred successfully for a range of primary and secondary alkyl and aryl amines and even with benzamide, albeit in low yield. 2-aminoBODIPYs Xa-e have been prepared successfully via Buchwald–Hartwig amination of the 2-iodoBODIPY IX (fully blocked). The presence of the nitrogen group Abstract iii in the 2-position gives a broadened red-shifted absorption maximum and quenched fluorescence. This palladium catalysed amination was used to prepare the 2-nitroaniline-substituted BODIPY XI. Reduction of the nitro group by hydrogenation gave the novel fluorescencequenched 2-aminoaniline-substituted BODIPY XII. Reaction of this compound with triphosgene gave the corresponding benzimidazolone-substituted BODIPY XIII. The bright yellow fluorescence of the triphosgene reaction product XIII can be easily visualized by the naked eye. The detection limit for phosgene was determined to be 160 nM in solution at room temperature.Taibah Universit

Circularly polarized luminescence from helically chiral N,N,O,O-boron-chelated dipyrromethenes

Helically chiral N,N,O,O-boron chelated dipyrromethenes showed solution-phase circularly polarized luminescence (CPL) in the red region of the visible spectrum (λem(max) from 621 to 663 nm). The parent dipyrromethene is desymmetrised through O chelation of boron by the 3,5-ortho-phenolic substituents, inducing a helical chirality in the fluorophore. The combination of high luminescence dissymmetry factors (|glum| up to 4.7 ×10−3) and fluorescence quantum yields (ΦF up to 0.73) gave exceptionally efficient circularly polarized red emission from these simple small organic fluorophores, enabling future application in CPL-based bioimaging

Synthesis, Optical, and Geometrical Approaches of New Natural Fatty Acids’ Esters/Schiff Base Liquid Crystals

Schiff base liquid crystals, known as [4-(hexyloxy)phenylimino)methyl]phenyl palmitate (IA), [4-(hexyloxy)phenylimino)methyl]phenyl oleate (IIA) and [4-(hexyloxy)phenylimino)methyl]phenyl linoleate (IIIA), were synthesized from palmitic, oleic, and linoleic natural fatty acids. The prepared compounds have been investigated for their thermal and optical behavior as well as phase formation using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Molecular structures of all studied compounds were confirmed via elemental analysis, FT-IR, 1H NMR, and 13C NMR. Smectic phase is the observed mesophase for all compounds; however, their type and range depend upon the terminal alkanoate chains attached to the phenyl ring. Computational calculations, Density functional theory (DFT), energy difference of the frontier molecular orbital (FMOs), as well as the thermodynamic parameters of different molecular configurations isomers were discussed. It was found that the mesophase behavior and the geometrical characteristics were affected by the degree of unsaturation of fatty terminal chains. Furthermore, the geometrical structure of the CH=N linkage plays an important role in the thermal stability and optical transition temperature

Effect of Phase Change Materials (PCMs) Integrated into a Concrete Block on Heat Gain Prevention in a Hot Climate

In the current study, a phase change material (PCM) contained in an insulated concrete block is tested in extremely hot weather in the United Arab Emirates (UAE) to evaluate its cooling performance. An insulated chamber is constructed behind the block containing PCM to mimic a scaled down indoor space. The effect of placement of the PCM layer on heat gain indoors is studied at two locations: adjacent to the outer as well as the inner concrete layer. The inclusion of PCM reduced heat gain through concrete blocks compared to blocks without PCM, yielding a drop in cooling load indoors. The placement of PCM and insulation layers adjacent to indoors exhibited better cooling performance compared to that adjacent to the outdoors. In the best case, a temperature drop of 8.5% and a time lag of 2.6 h are achieved in peak indoor temperature, rendering a reduction of 44% in the heat gain. In the tested hot climate, the higher ambient temperature and the lower wind speed hampered heat dissipation and PCM re-solidification by natural ventilation. The findings recommend employing a mechanical ventilation in hot climates to enhance regeneration of the PCM to solid state for its optimal performance

Mesomorphic, Optical and DFT Aspects of Near to Room-Temperature Calamitic Liquid Crystal

A new liquid crystalline, optical material-based Schiff base core with a near to room-temperature mesophase, (4-methoxybenzylideneamino)phenyl oleate (I), was prepared from a natural fatty acid derivative, and its physical and chemical properties investigated by experimental and theoretical approaches. The molecular structure was confirmed by elemental analysis, FT-IR (Fourier-Transform-Infrared Spectroscopy) and NMR (nuclear magnetic resonance) spectroscopy. Optical and mesomorphic activities were characterized by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The results show that compound (I) exhibits an enantiotropic monomorphic phase comprising a smectic A phase within the near to room-temperature range. Ordinary and extraordinary refractive indices as well as birefringence with changeable temperatures were analyzed. Microscopic and macroscopic order parameters were also calculated. Theoretical density functional theory (DFT) calculations were carried out to estimate the geometrical molecular structures of the prepared compounds, and the DFT results were used to illustrate the mesomorphic results and optical characteristics in terms of their predicted data. Three geometrical isomers of the prepared compound were investigated to predict the most stable isomer. Many parameters were affected by the geometrical isomerism such as aspect ratio, planarity, and dipole moment. Thermal parameters of the theoretical calculations revealed that the highest co-planar aromatic core is the most stable conformer

Thermally-Activated, Delayed Fluorescence in O,B,O- and N,B,O-Strapped Boron Dipyrromethene Derivatives

A small series of boron dipyrromethene (BODIPY) dyes has been synthesized whereby the boron atom is constrained in a five-membered ring formed from either <i>o</i>-dihydroxypyridine or <i>o</i>-aminophenol. In the latter case, the amino group has been converted into the corresponding amide derivative so as to curtail the possibility for light-induced charge transfer from strap to BODIPY. These compounds are weakly emissive in fluid solution but cleavage of the strap, by treatment with a photoacid generator, restores strong fluorescence. Surprisingly, the same compounds remain weakly fluorescent in a rigid glass at 80 K where light-induced charge transfer is most unlikely. In fluid solution, the fluorescence quantum yield increases with increasing temperature due to a thermally activated step but does not correlate with the thermodynamics for intramolecular charge transfer. It is proposed that the strap causes rupture of the potential energy surface for the excited state, creating traps that provide new routes by which the wave packet can return to the ground state. Access to the trap from the excited state is reversible, leading to the delayed emission. Analysis of the temperature dependent emission intensities allows estimation of the kinetic parameters associated with entering and leaving the trap