Ultrafast dynamics of proflavine bound to poly (methacrylic acid) in aqueous solution

Abstract

The excited-state dynamics of fluorescent probes attached to polymer segments under un-entangled conditions unveil the internal dynamics and interior polarity of both synthetic and natural polymers. Synthetic polyelectrolytes have been considered as a simple model system to circumvent the structural and dynamic complexity of bio-macromolecules on interaction with solvent molecules in the vicinity. In this study, femtosecond dynamics of proflavine cation, a molecular probe with well-established excited state dynamics have been explored by steady-state and time-resolved fluorescence to uncover structural dynamics of polymethacrylic acid at the sub-picosecond to the picosecond time domain. The femtosecond fluorescence transients display a bi-exponential decay. The shorter decay time of 1–5 ps represents the reorientation of the water molecules during the excited state of the fluorophore. The increase in the lifetime of this component from 1.0 ps to 5.0 ps on increasing the pH of the solution from 2.0 to 5.0 is ascribed to the reorientation of water molecules due to the compact and hydrophobic nature of the macromolecule. The second decay component of proflavine bound to PMAA increases from 70 ps to 180 ps on increasing the pH of the solution from 2.5 to 5.0. The hydrophobic part of the polyelectrolyte present under acidic conditions restricts the solvent reorganization during the solvation of the covalently bound dye molecule and the stretched linear chains of the polyelectrolyte do not influence the solvation and the observed decay time is identical to that of free dye in aqueous solution.