Surfactant mediated modulation of photophysics and self-assembly process of a small, substituted phenol

Abstract

5-(tert-Butyl)-2-hydroxy-1,3- isophthalaldehyde (5-tBHI) exhibits a large Stokes shifted emission spectrum, irrespective of excitation wavelength in aqueous solution and thus can be useful in bio-imaging. It is imperative to understand the binding of 5-tBHI in micelles as micelle represent the simplest mimic of biological membrane. Hence, the interaction of 5-tBHI with cationic surfactant cetyltrimethylammonium bromide (CTAB), anionic surfactant sodium dodecyl sulfate (SDS), and neutral surfactant Triton X-100 (TX-100) was investigated. This help us elucidate the interplay of hydrophobicity and pH to modulate the ground and excited state intra/inter-molecular proton transfer processes of the probe. The absorption and fluorescence spectra of 5-tBHI have been found to be highly sensitive towards hydrophobicity and electrostatic interactions. However, no particular trend was observed, suggesting an incomplete encapsulation of the probe in the micelles. The observation from the steady state spectra could not be rationalized completely based on the time resolved fluorescence decays in the nanosecond-picosecond regime, suggesting that the processes are ultrafast in nature. Subsequently, analyses of fluorescent transients in femtosecond-picosecond regime provided a better explanation to match well with the steady state phenomena. It was found that pH has a dominant role to guide the direction of the two-state keto-enol tautomerisation. However, the effect of hydrophobicity can not be neglected as it pushes the equilibrium to the enol form. Furthermore, Fluorescence Lifetime Imaging Microscopy (FLIM), supported by SEM images, provide a pictorial idea of how the aggregation of the probe in neat water is affected by the nature of the surfactants. It further shows that SDS, with an acidic local pH favours the aggregation process, over CTAB and TX-100.