Comparative Sensitivity of Novel Benzothiadiazole Scaffolded Fluorescent Molecular Probes in Sensing the Microenvironment of Aqueous Pluronic Media: A Photophysical Study

Abstract

The photophysical properties of four fluorescent probes based on 4-ethynyl-2,1,3-benzothiadiazole (BTD) are investigated in Pluronic media (F127 and P123). Here, we employ a set of probes with distinct charge-transfer characteristics, and different partitioning behaviour. This can enable validation of localization across different micellar domains. Combined with multiparametric sensing of fluorescence parameters such as emission shift, intensity, lifetime, and anisotropy, this strategy provides a better understanding of polarity, microviscosity, and rotational rigidity than single probes alone. The response and sensitivity of these probes for different fluorescence parameters towards microenvironmental changes in Pluronic micelles, have been studied. Additionally, their sensing potentials are explored during the sol–gel phase transition in these media. These probes are classified by their intramolecular charge transfer (ICT) feature: (a) BTDPhCN and BTDPh with weaker ICT and (b) BTDPhOMe and BTDPhNMe₂ with significant ICT features. Their strong sensitivity to polarity and viscosity change makes them ideal for studying amphiphilic microenvironments. Steady-state and time-resolved fluorescence (TRF) spectroscopy were used to track sol-gel transitions, probe localization, and temperature-induced microenvironmental changes. The results show that these probes are able to sense micellar sol and gel phases through emission shifts (λ_max), fluorescence intensity variations, lifetime changes, and anisotropy changes. BTDPhCN and BTDPh exhibited blue shifts, indicating migration into the micellar core, while BTDPhOMe and BTDPhNMe₂ displayed bathochromic shifts, suggesting expulsion to a more polar interfacial region. Fluorescence lifetime and anisotropy measurements confirmed the differential probe localization, with increased anisotropy in the gel phase indicating restricted molecular motion. Additionally, BTD probes help to elucidate microenvironmental variations associated with change in Pluronic composition.