Design and Synthesis of Novel Squaraine Dye with Highly Enhanced Far‐Red Fluorescence and its Interaction with a Model Protein

Novel amine‐functionalized symmetrical squaraine (SQ)‐dye and fluorescein isothiocyanate (FITC)–SQ–FITC dye conjugate are designed and successfully synthesized, followed by their structural and photophysical characterizations. Symmetrical SQ dye exhibits complete fluorescence quenching in polar solvents, which is explained by considering photoinduced electron transfer (PET) as a dominant quenching pathway. In contrast, FITC–SQ–FITC dye conjugate results in a decrease in an aggregation of both SQ and FITC units and about 47 times enhancement in the far‐red fluorescence appearing at 674 nm. This profound enhancement in the far‐red fluorescence is explained by the synergistic influence of suppression of aggregation‐induced quenching (ACQ) and PET. This newly designed FITC–SQ–FITC dye conjugate shows complete quenching of the far‐red fluorescence in phosphate buffer saline (PBS) owing to very strong dye aggregation, as confirmed by spectral absorption investigation. In PBS, it strongly interacts with bovine serum albumin (BSA) ( K a = 1.1 × 10 4 m −1 ) used as a model protein breaking the dye aggregation by BSA and increasing far‐red fluorescence associated with SQ as a function of increasing BSA concentration. A profound 117 times increase in fluorescence at 674 nm with 25 times of BSA concentration suggests that the designed FITC–SQ–FITC is an excellent far‐red probe for dye‐protein interaction.