Sulfated-β-cyclodextrin templated aggregation of a metachromatic dye, Basic Orange 21: A photophysical investigation

Abstract

ABSTRACT The recently discovered molecular rotor and supravital dye, Basic Orange-21 (BO21), has shown improved differential counting of leukocytes through metachromasia (change in colour), which is believed to be due to the aggregation of chromophores. Thus, investigation on the aggregation behaviour of BO21 holds special importance and may help us to delineate its spectroscopic behaviour. In this contribution, we have investigated the photophysical behaviour of BO21 in presence of a negatively charged supramolecular host, sulfated-β-cyclodextrin (SC), using ground-state absorption, steady-state and time-resolved emission spectroscopy including ultrafast fluorescence spectroscopy, and molecular docking calculations. The spectral characteristics of BO21 display extreme modulations in presence of SCD, with features similar to that observed for BO21 during its metachromatic behaviour. These spectroscopic features have been rationalised in terms of aggregation of BO21 molecules in the vicinity of SCD, which are also extremely sensitive to external stimuli, such as, ionic strength and temperature of the medium.