Unraveling the Role of Polyoxometalates-Based Superchaotropes on the Photophysics of Organic Molecules and Modulation of Water Dynamics in a Hydrophilic Block Copolymer Solution

Polyoxometalates (POMs) are composed of nanometric metal-oxide anions and have rich solution chemistry. In this class, Keggin POMs have been identified as the most influential inorganic additives for aqueous nonionic soft matter systems. POMs being at the borderline of classical ions and charged colloids possess fascinating solution properties; the present work aims to delve deeper into the interactions between nanoions and nonionic soft matters from a spectroscopic point of view. Our studies reveal that although of the same structural makeup, silicotungstic acid hydrate (SiW) and phosphotungstic acid hydrate (PW) affect the photophysics of Coumarin-480 (C-480) in poly(ethylene oxide)-<i>block</i>-poly(propylene oxide)-<i>block</i>-poly(ethylene oxide) ((PEO)₇₆-(PPO)₃₀-(PEO)₇₆, F-68) copolymeric media in a dissimilar manner. From time-resolved studies, we find a preference for SiW toward the intramolecular charge transfer state of C-480, whereas PW favors the locally emissive state of the probe. Further, from rotational relaxation studies, it appears that SiW renders a rigid environment around the probe molecule, while PW relaxes the copolymeric environment. Finally, the dynamic quenching mechanism of the added nanoions was unraveled, which showed a straightforward Förster mechanism for SiW but a short-range interaction was operative for PW. From Fourier transform infrared and ¹H NMR, it can be concluded that both the nanoions interacted with the PPO moiety of the copolymer; yet, their contrasting effect on the photophysics has been rationalized as a consequence of charge density on the ions.