Synthesis and applications of aza-BODIPY derivatives with benzoic acid functional groups

Abstract

With their unique absorption and emission properties, near-infrared (NIR) dyes have emerged as versatile tools in various fields such as biomedical imaging, cancer theranostics, photovoltaics, and sensing applications. Originated by the charge transfer of a metal-ligand complex or a conjugated organic molecule, these dyes exhibit light absorption and emission in the near infrared area of 780-2500 nm. Upon irradiation with light, these dyes can excite their electrons to an elevated energy level, and subsequently electrons return to the ground state through light emission, thus generating fluorescence and becoming significant tools for biotechnological applications. Shifting of absorption to longer wavelengths in the near-infrared (NIR) range provides advantages such as reduced scattering, minimized photodamage, and improved penetration into deep tissues. NIR dyes encompassing cyanine, squaraine, phthalocyanine, porphyrazine and porphyrin dyes have unique chemical structures and spectral properties. Until recently, porphyrins and phthalocyanines were the commonly studied among these dye groups. Despite their strong absorption in the IR and near-IR regions, excellent chemical and thermal stability, and suitable redox properties for activating semiconductors, these macrocyclic tertapyrrole derivatives are being replaced by new generations of dye compounds due to their challenging synthesis methods, difficulties in purification steps, and low yields. To adress the limitations of porphyrin and phthalocyanine dyes, dipyrromethene and aza-dipyrromethenes boron complexes have emerged as a solution. These compounds have tunable optical properties and biocompatibility. Coordination with BF2+ induces a greater degree of planarity and rigidity in the molecular structure, thereby they promote pronounced conjugation along the π-system of the molecule. This extended conjugation facilitates enhanced electronic delocalization and absorption of light across a wider spectrum of wavelengths that results in a red-shift in their absorption maximum. BODIPYs, resembling a half-porphyrine core, are BF2+ complexes of dipyrromethenes. BODIPY compounds possess exceptional spectroscopic features characterized by pronounced absorption within the visible range of the spectrum with an elevated molar absorptivity coefficients and notably high quantum yields. Furthermore, these dyes demonstrate remarkable chemical and photostability. Their inherent amenability to functionalization at all core positions enable modifications. Through these modifications, electron delocalization can be modulated and chromophore's spectral properties can be rendered for generation of intramolecular charge-transfer absorption bands. In contrast to their remarkable high-yield emission maxima, their most unfavorable characteristic appears in their absorption band positioned at wavelengths below 600 nm because it disables deep tissue penetrations and their applications as theranostic agents. Aza-BODIPYs, the direct homologues of BODIPYs, were discovered by Boyer in 1993. In their structure, the carbon meso-atom in BODIPY core is replaced by nitrogen atom to provide better absorption characteristics while retaining the optical properties of their BODIPY analogues. Due to the narrowing of the HOMO-LUMO energy gap, this replacement induces a bathochromic shift in absorption wavelength typically extending approximately to 100 nm, compared to those of BODIPYs. Aza-BODIPYs in their simplest form, which is the 1,3,5,7-tetraphenyl-aza-BODIPY, possess absorption maximums at 650 and 672 nm, respectively, as well as an enhanced molar extinction coefficient in comparison to their BODIPY analogue, namely 1,3,5,7-tetraphenyl-BODIPY. Over the years, various synthetic pathways have been devised for the synthesis of aza-BODIPYs. These methodologies vary mainly in synthesis of aza-dipyrromethene derivative, the precursor of aza-BODIPY. Nonetheless, a unifying feature among the reported synthetic strategies is formation of their BF2+ chelates through the utilization of boron trifluoride diethyletherate in the final step. Among the array of synthetic strategies, the method reported by O'Shea has emerged as the predominant approach fort the preparation of both symmetrical and asymmetrical products. O'Shea method is relatively simpler since it involves less synthetic steps and the final products are obtained with comparatively high yields. The synthetic pathway begins with the aldol condensation between an aryl aldehyde and an aryl ketone to obtain chalcone, a diaryl-α,β-unsaturated ketone. Nitration of the chalcone compound facilitated via Michael addition of nitromethane, is followed by dimerization of the semi-product by using ammonium acetate to yield in aza-dipyrromethene compound. In the final step, the target aza-BODIPY is prepared by chelation of the aza-dipyrromethene derivative with boron trifluoride etherate. In addition to their absorption at around 650-675 nm, aza-BODIPYs posses intense emission at 700-900 nm, the wavelength range which falls within the therapeutic window that allows deep tissue penetration and higher contrast for biological applications. Since their emission wavelength is considerably higher than those of their BODIPY analogues, they have relatively higher singlet oxygen quantum yields. Hence they posses great potential as photosensitizers for biological and therapeutic applications, especially photodynamic therapy and photothermal therapy. In addition to their remarkable spectrochemical features, their biocompatibility makes these complexes more appealing for biological applications. Photodynamic therapy has become more appealing in the current era since it is a non-invasive clinical treatment method of cancer and non-oncological diseases. The cure is based on generation of singlet oxygen, the cytotoxic component responsible for cellular death. Although there are various possible routes for singlet oxygen generation, photosensitized processes are the most favored ones. Photosensitizers are responsible for singlet oxygen formation in the malignant tissue upon irradiation of light. Therefore, the photosensitizer plays a critical role in the success of the therapy. An ideal photosensitizer should be able to accumulate in the cancerous tissue, be easily excreted after the treatment and should exhibit high photoactivity with minimal dark toxicity. Since the discovery of Photofrin® in the early 1990s, extensive research has been conducted to synthesize the ideal photosensitizer. Unfortunately, many of the reported compounds displayed aggregation-caused quenching which reduces reactive oxygen species yield. Consequently, aza-BODIPYs began to gain more attention as alternatives for traditional porphyrin and phtalocyanine dyes used for photodynmaic therapy. In this thesis, it was aimed to synthesize and characterize an aza-BODIPY derivative functionalized with ethynyl benzoic acid groups at the pyrrolic positions of the core structure. For the preparation of the target molecule, tetraphenyl aza-dipyrromethene compound was synthesized in accordance with the reported procedure along with some modifications. Initially, chalcone was synthesized and followed by Michael addition of nitromethane to obtain the corresponding nitrated semi-product. Reacting with ammonium acetate, 1,3,5,7-tetraphenyl aza-dipyrromethene was obtained, which was then brominated in the pyrrolic positions using NBS. Functionalization of the aza-dipyrromethene core was achieved via palladium catalyzed Sonogashira coupling reaction with 4-ethynyl benzoic acid. The target aza-BODIPY was obtained by the complexation of aza dipyrromethene derivative substituted with carboxylic acid groups with boron trifluoride etherate. Characterization of the compounds was performed by UV–Vis, fluorescence, 1H-NMR and mass spectroscopy techniques. In conclusion, the UV absorbance maxima was observed at 616 nm, with a fluorescence emission peak at 680 nm, exhibiting a Stokes shift of 64 nm. Singlet oxygen quantum yield of the final compound was estimated and the cytotoxicity studies were performed.