Synthesis And Spectral Properties Of 6'-Triazolyl-Dihydroxanthene Hemicyanine Fused Near-Infrared Dyes

Fluorescence imaging is an indispensable tool in medical diagnosis as it is rapid,highly sensitive,and non-invasive.Hence,the development of stable fluorescent molecules with excellent photophysical properties is crucial for the further advancement of this field.Fluorescent molecules that absorb and emit in the near-infrared(NIR)spectrum(700-900 nm)are most sought after for imaging and diagnostics applications.This is because,NIR radiation displays unique features that include deep tissue penetration,biocompatibility and less damage to living cells.Dihydroxanthene-hemicyanine(DHX-HC)is a newly developed NIR emitting scaffold that exhibits favourable photophysical properties.However,the synthetic routes to access diverse fluorescent molecules containing this scaffold is still primitive and needs further development.The copper(I)catalyzed azide-alkyne 1,3-dipolar cycloaddition or the so-called "Click Chemistry" is an efficient and rapid method for synthesizing libraries of compounds.Applying click chemistry to the synthesis of fluorescent molecules can improve the properties of fluorescent molecules,such as increasing the brightness and enhancing the photostability.Moreover,libraries of compounds can be efficiently synthesized using click chemistry and enable structure-fluorescence activity relationship(SFR)studies.Keeping this in mind,we designed and synthesized an alkyne functionalized late-stage dihydroxanthene-hemicyanine intermediate and "clicked" it with a panel of azides to obtain a novel series of6'-triazolyl-dihydroxanthene-hemicyanine near-infrared fluorophores.Subsequently,the photophysical properties of these fluorophores were evaluated.The thesis comprises three main parts.The first part reviews the concept of fluorescence,details the structure and properties of different fluorophores that are currently used for imaging applications,introduces NIR fluorophores,their synthetic routes and the current challenges in making dihydroxanthene-hemicyanine based fluorophores.The application of click chemistry to fluorophore synthesis is also discussed.The next part of the thesis details the design and synthesis of6'-triazolyl-dihydroxanthene-hemicyanine(DHX-HC)-based near-infrared dyes.The synthesis of the alkynyl functionalized DHX-HC intermediate(compound 112)was crucial in this study.Alkyne 112 was obtained from commercially available3-bromophenol in 4 steps.Eventually,alkyne 112 was made in over a gram scale.Following this,the click reaction between alkyne 112 and 3-phenyl-azido-propane was optimized.Diverse azides were “clicked” with alkyne 112 to generate a16-member 6'-triazolyl-dihydroxanthene-hemicyanine library in yields ranging from14-81%.The following part of the thesis discusses the photophysical properties of the NIR fluorophores developed.A modest “turn-on” fluorescence(up to 10-fold increase compared to the alkyne precursor)was recorded for a hit triazole compound.Further,the specific molecular absorption signature of both the alkyne and triazole derivatives provide insights on the further development of dark quenchers and turn-on probes for biological imaging.The final part of the thesis furnishes the experimental protocols,along with the fluorescence spectra of the NIR fluorophores and characterization data for the compounds synthesized.