How Polyoxometalate Promote the Aggregation of a Cyanine Dye in the Aqueous Solution of Non-Ionic Copolymers of Varying Hydrophilic-Lipophilic Balance?

The nonradiative pathway leading to the photoisomerization of a cyanine dye is well-established information. However, the modulations induced in the photoisomerization pathway by a Keggin-type polyoxometalate in a confined media is new. Our study reveals that, in the presence of pluronic block copolymers F-108 and P-123, phosphomolybdic acid hydrate (PMA) promotes the aggregation of 3,3'-diethylthiadicarbocyanine iodide (DTDCI). The absorption spectra show a clear indication of a red-shifted trimer band in F-108 and P-123, whereas it is absent in F-127 and P-84. Fluorescence emission studies suggest that, in the presence of PMA, the rate of photoisomerization is accelerated in F-108, P-123, and P-84 micelles, whereas it is retarded in F-127 micelles. Time-resolved studies in the presence of PMA indicate the preference of F-108, P-84, and P-123 toward the trapped conformer of DTDCI, whereas F-127 favors the formation of photoisomer of DTDCI. Our findings imply the importance of the interplay between the hydrophobic and electrostatic interactions between the DTDCI cations and the PMA anions in nonionic micelles of varying hydrophilic-lipophilic balance (HLB). Dynamic light scattering (DLS) data suggest a modulation by PMA in the intermicellar electrostatic repulsions of a hydrophilic copolymer micelle, whereas its unaffected in a hydrophobic copolymeric micelle.

Solute dynamics of a hydrophobic molecule in a menthol-thymol based type-V deep eutectic solvent: effect of composition of the components.

Type-V deep eutectic solvents (DESs) are a newly emerging unique class of solvents obtained by physical mixing and heating of non-ionic components. These solvents show deviation from the thermodynamic ideality. Compared to type-I to IV DESs, type-V DESs are less explored and their physical chemistry is in its nascent stage. In this work, we have chosen a type-V DES based on menthol-thymol (MT) for our working media. Solvent and rotational dynamics were studied with varying temperature using a well-known solvatochromic probe, Coumarin 153 (C153). We prepared the MT-based DES using a reported procedure at three molar ratios: 1 : 1 (M1T1), 1 : 1.5 (M1T1.5), and 2 : 1 (M2T1) of menthol (M) and thymol (T). Time-resolved emission spectra (TRES) were constructed with varying temperature. Utilizing TRES, the decay of the solvent correlation function (C(t)) was plotted. We have correlated the solvent relaxation time in these DESs as a function of viscosity. The time-resolved anisotropy decays were also collected to perceive the rotational relaxation dynamics of C153 as a function of temperature. The decay of solvent relaxation was found to be bi-exponential, and the average solvation time (〈τs〉) in M2T1 was found to be longer than those of M1T1.5 and M1T1. The rotational reorientation times (〈τrot〉) also follow the same trend. We have analysed the rotational dynamics of C153 in type-V DESs employing the Stokes-Einstein-Debye (SED) hydrodynamic model. The rotational dynamics in DESs demonstrate a good correlation with the SED model with a little deviation. In MT-based DESs, the solute's rotational relaxation times approach hydrodynamic stick boundary condition at low viscosity (or at high temperatures) for all molar compositions. Using the Arrhenius-type equations, we have correlated the activation energies for the rotational motion of C153, along with the viscous flow and non-radiative pathways for all the DESs.

Co-Solutes Induced Changes in the Properties of Polymeric Solution and Water Dynamics.

In this paper we are addressing the co-solute-induced changes in the properties of an aqueous solution of a block copolymer. Due to the preferential interaction of different co-solute with different regions of the block copolymer, the changes were observed in both the physical properties and water dynamics. The modulation of both the physical properties and water dynamics was monitored using different spectroscopic techniques. Different co-solutes affect micellar properties of copolymer to a different extent signifying their interactions with different regions within the copolymer. The solvent relaxation dynamics were also modulated with the additions of different co-solutes. The change in free-energy (ΔGbf) and rate constant for bound to free water interconversion (kbf) in a copolymeric micelle was calculated which gets affected by the addition of co-solutes. The calculated kbf suggests that betaine, sarcosine, TMAO, and GnHCl favor the ordering of water molecules around the micelle and are excluded from the micellar surface whereas, urea favors the formation of free-water molecules rather than the structurally ordered bound water molecules around the micelle by accumulating at the micellar surface. Among the added methylamines trimethylamine N-oxide affected the water dynamics and its kinetics most profoundly. The protective property of GnHCl was revealed.

Unraveling the Photoluminescence Properties of a Boron Nitride Nanosheet Dispersed in Different Solvents and Its Application to Generate White Light.

Hexagonal boron nitride (h-BN) is an influential 2D nanomaterial; however, its practical optoelectronic applications rely primarily on controlling the structural defects. The photoluminescence depends explicitly on the developed vacancies and substitutional defects. The present work utilizes the concept of facile liquid-phase exfoliation of hexagonal (h) boron nitride (BN) powder in common organic solvents and cosolvent mixtures to obtain a layered boron nitride nanosheet (BNNS). Although the literature concerning the layered structure of BNNS obtained by different methods is substantial, what is lacking is a detailed photoluminescence study of the layered structure obtained by changing the solvent and cosolvent mixtures, and here lies the novelty of our work. The obtained layered structure was subjected to a detailed photoluminescence study by varying the temperature. We tried to correlate how the defects originating upon changing the solvent and cosolvent affected the photoluminescence of the layered BNNS. The obtained layered structure is suitably supported by optical and electron microscopy images. High-resolution transmission electron microscopy confirm the presence of a few layers, and X-ray photoelectron spectroscopy studies give an idea of the atomic composition of the obtained BNNS. The photoluminescence properties of the obtained BNNS in water were modulated by the addition of two different classes of block copolymers, e.g., Pluronic (F-68, P-407, and P-123) and Tetronic (T-904, T-908, and T-90R4) copolymers. As an application, we were successful in constructing a nanocomposite material made up of a BNNS-copolymer-organic fluorophore to check the possibilities of generating white light.

Photoluminescence Properties of Graphene Oxide in Non-Aqueous Solvents.

Detailed attention to the interaction between graphene oxide (GO) and various organic fluorophores has been documented in literature as a result of which the impact of GO on the photophysical properties of the fluorophores is well known to the scientific community. However, the photoluminescence (PL) properties of GO in polar aprotic solvents are yet to be established. In this article, the PL properties of GO in polar aprotic solvents using various spectroscopic techniques have been reported. n-π* transition due to the C=O bonds in the sp3 hybrid regions and π-π* transition due to C=C bonds in the sp2 hybrid are prominent in GO. The presence of quasi-molecules within sp2 -sp3 domains acts as PL centers located in the sp3 matrixes of GO are responsible for the PL properties. This study showcases the presence of multiple emissive states of GO in polar aprotic solvents and conveys the fact that the PL properties of GO are very much wavelength-dependent.

Glucose-induced self-assembly and phase separation in hydrophilic triblock copolymers and the governing mechanism.

Poly(ethylene oxide, EO)-poly(propylene oxide, PO)-poly(ethylene oxide, EO)-based triblock copolymers (BCPs) with 80% hydrophilicity stay molecularly dissolved as Gaussian chains at ambient temperature, even at fairly high concentrations (>5 %w/v). This study presents the plausible micellization behaviour of such very-hydrophilic Pluronics® - F38, F68, F88, F98, and F108 - incited upon the addition of glucose at low concentrations and temperatures. The outcomes obtained from phase behaviour and scattering studies are described. At temperatures near to ambient temperature, these BCPs form micelles with a central core made of a PO block, surrounded by a corona of highly hydrated EO chains. The phase transitions in these hydrophilic Pluronics® in the presence of glucose are demonstrated via the dehydration of the copolymer coil, leading to a decrease in the I1/I3 ratio, as determined using fluorescence spectroscopy. The temperature-dependent cloud point (CP) showed a marked decrease with an increase in the PO molecular weight and also in the presence of glucose. The change in solution relative viscosity (ηrel) caused by glucose is due to the enhanced dehydration of the EO block of the BCP amphiphile. Dynamic light scattering (DLS) and small-angle neutron scattering (SANS) investigations suggested that the dimensions of the hydrophobic core increase during the dehydration of the EO-PO blocks upon a temperature increase or after adding varying concentrations of glucose, thereby resulting in a micellar shape transition. It has been observed that added glucose influences the phase behaviour of BCPs in an analogous way to the influence of temperature. Also, plausible interactions between the EO-PO blocks and glucose were suggested based on the evaluated optimized descriptors obtained from a computational simulation approach. In addition, the core-shell blended micelles obtained using these BCPs are successfully utilized for drug (curcumin, Cur) solubilization based on the observed peak intensities from UV-visible spectroscopy. The loading of Cur into glucose-containing and glucose-free hydrophilic Pluronic® micelles shows how the radius of the micellar core (Rc) increases in the presence of glucose, thereby indicating Cur solubility enhancement for the Pluronic® micelles. Various kinetics models were employed, demonstrating a drug release profile that enables this approach to be used as an ideal platform for drug delivery.

How Kosmotropic and Chaotropic Osmolytes Perturb the Properties of an Aqueous Solution of a Pluronic Block Copolymer?

Poloxamer 407 (P-407) composed of a poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG) unit has two distinct microenvironments: the interior core formed by the PPG unit and the exterior shell formed by the PEG unit. In this work, we have used two fluorescent molecules coumarin-153 and 8-anilino-1-naphthalene sulfonic acid (ANS) of contrasting natures to characterize and probe the water dynamics in the core and corona regions of the copolymer by means of spectroscopic techniques, namely, absorption, fluorescence, and time-resolved fluorescence emission spectroscopy and Fourier transform infrared (FTIR) spectroscopy. Changes in the surface morphologies were characterized by using microscopic techniques. Further, two classes of osmolytes kosmotropic (betaine and sarcosine) and chaotropic (urea) known to perturb the water structure were added to aqueous solutions of P-407. Our studies reveal that the addition of kosmotropes decreases the critical micelle temperature (CMT) of the copolymer, whereas the chaotropic osmolyte increases the CMT. Steady-state studies reveal that the addition of the osmolytes to the copolymer increases the polarity of the micelle formed and hence results in the red shift in the ANS absorbance maximum. FTIR spectroscopy reveals that kosmotropes interact with the PEG moiety of the copolymer, whereas the chaotrope interacts with both the PEG and PPG moieties of the copolymer. Solvent relaxation studies produced less changes upon the addition of the kosmotropes, whereas a greater change in the relaxation time was observed in the presence of the chaotrope.