Unprecedented high fluorescence anisotropy in protic solvents: Hydrogen bond induced solvent caging?

Excitation wavelength-dependent fluorescence anisotropy of 3-hydroxyflavone: revisiting the solvation processes and high-energy state excitation in ESIPT-active compounds

To gain a more comprehensive understanding of the phenomenon of high-fluorescence anisotropy of the normal form emission of ESIPT-active compounds in protic solvents, excitation wavelength dependence of emission anisotropy was investigated for 3-hydroxyflavone (3HF) using steady-state spectroscopic technique and quantum chemical calculations. It was shown for the first time that the anisotropy of 3HF normal form emission is characterized by significant dependence on excitation energy. Experimental results indicate that the fluorescence anisotropy of 3HF in methanol (at 20 °C) changes abruptly from about 0.18 to about 0.10 with a decrease in excitation wavelength. This spectroscopic phenomenon can be explained by two factors: (1) breaking of intermolecular solute-solvent hydrogen bonds upon photoexcitation and (2) excitation of ESIPT-active fluorophores to the second singlet state (S2). The results of quantum chemical calculations clearly indicate that specific hydrogen bonding solvation interactions can lead to the formation of 3HF-methanol complexes with larger molecular volumes than the volume of free 3HF molecule. High excitation energy can reform and break solute-solvent bonds, which leads to a decrease in molecular system volume. This results in a decrease in rotational correlation time and fluorescence anisotropy. As is known, the fluorescence lifetime of small-sized molecules is closely correlated with the conformational changes in the excited state, and in the case of ESIPT-active compounds, the lifetime of normal form emission is almost fully determined by the ultrafast ESIPT process. Therefore, although in general, fluorescence lifetime is considered independent of excitation energy, but because the timescale of ESIPT processes is of the same order as the timescale of internal conversion, in the case of ESIPT-active compounds, fluorescence lifetime changes caused by high-energy state excitation cannot be neglected. The emission anisotropy of the normal form of an ESIPT-active compound will decrease with an increase in fluorescence lifetime caused by an increase in excitation energy sufficient to excite molecules to higher electronic states. In this work, both hypotheses are discussed and verified using experimental data and quantum chemical calculations for 3HF in methanol.

Probing the heterogeneity of molecular level organization of ionic liquids: a comparative study using neutral Nile red and cationic Nile blue sulfate as fluorescent probes for butyrolactam-based protic ionic liquids

Ionic liquids (ILs) are liquid salts composed of cations and anions, known for their significant local heterogeneity at the molecular level. To understand the microheterogeneity with regard to their local polarity and local viscosity, we have used two structurally similar but chemically distinguishable fluorescent probes: Nile red (NR), a neutral molecule, and Nile blue sulfate (NBS), a charged molecule. A comparative study of the response of the two probes to the molecular level heterogeneity of ILs is expected to provide a better clarity of understanding regarding the charged polar domain and the uncharged hydrophobic domain of ILs. Towards this, we synthesized two butyrolactam-based protic ionic liquids (PILs), i.e., BTF and BTD, with the same ionic headgroup ([BT]+) and different alkyl tails ([RCOO]-), where {R = H, C11H23}. BTF has no significant hydrophobic domain, whereas BTD has a larger hydrophobic domain. Temperature-dependent fluorescence parameters such as fluorescence intensity, lifetime, and anisotropy were measured for both NR and NBS molecules. The use of a pair of structurally similar but ionically different probes enables differential estimation of parameters like the microviscosity of a domain using the fluorescence anisotropy parameter (r). The absorption and emission spectra of both probe molecules are observed to be blue shifted upon going from BTF to BTD. NR showed a significant blue shift in absorption and emission band maxima. Conversely, NBS exhibited a small wavelength shift, possibly influenced by the preferred location of their charged head group domain. Temperature-dependent rotational relaxation time (θ) of NR in BTD is smaller than that of NBS by 60-70%, indicating that stronger charge-charge interactions exist between the polar domain of BTD and NBS. Moreover, it is observed that the local viscosity of the BTF IL around both probes is similar, whereas there is a considerable difference for the BTD IL. These results are an indication that NBS being charged prefers to locate itself in the charged head group region of the IL, whereas NR being neutral tends to reside both in the hydrophobic domain and in the head group but is predominantly located in the hydrophobic domain.

Impact of cationic surfactant-induced DNA compaction on the characteristics of a minor groove bound flavonol

3-Hydroxyflavone (3-HF), which binds to the minor groove of DNA, is a strong antioxidant and hence a potent therapeutic and diagnostic agent. A special photo-property, called excited state intramolecular proton transfer (ESIPT), makes the 3-HF derivatives sensitive to the cellular hydrophobic microenvironment. The present study depicts the various changes in the ESIPT of 3-HF due to cationic surfactant-induced compaction of DNA.

Enhanced Hydrodynamic Radius of AOT/n-heptane/Water Reverse Micellar System Through Altered Electrostatic Interactions and Molecular Self-Assemblies

We have demonstrated a unique approach to alter the aqueous pool size of an AOT/n-heptane/water reverse micellar system. A positively charged dye Rhodamine B (RhB) and negatively charged Rose Bengal (RB) were incorporated in the reverse micellar pool to investigate the effect of electrostatic interactions and stacking effects among the dye molecules on the AOT/n-heptane/water interface. Dynamic light scattering revealed increase in reverse micellar pool size in presence of positively charged dye aggregates at the oil-water interface. However, less expansion was observed in presence of negatively charged dye aggregates (RB). This confirms the role of electrostatic interaction in modulating the hydrodynamic radius. A head-to-tail type of stacking of RhB molecules at the interface favors this expansion. The differences in stacking of the two dyes inside the reverse micelles and their torsional mobility indicated the role of the reverse micellar interface and H-bonding ability of the microenvironment on dye aggregation. Conductivity measurements demonstrated a significant drop in percolation temperature of the reverse micellar system in presence of dye aggregates. This confirms the effect of dye aggregation and electrostatic interaction on such expansion. This strategy can be exploited for solubilizing greater amounts and a wider variety of drug molecules in microemulsions.

Hydroxyl Group-Directed Solvation of Excited-State Intramolecular Proton Transfer Probes in Water: A Demonstration from the Fluorescence Anisotropy of Hydroxyflavones

Formation of a probe-solvent network resulting in unusually high fluorescence anisotropy (FA) of an excited-state intramolecular proton transfer (ESIPT) probe, 3-hydroxyflavone (3HF), in water prompted us to explore the solvation patterns on its 7-hydroxy (7HF) and 6-hydroxy (6HF) positional analogues. In the present study, it was observed that 7HF exhibits a lower FA than 3HF does in water, implying that the volume of the 7HF-water cluster is less than that of the 3HF-water cluster. Experimental and computational results led us to propose that 7HF forms its water cluster at the molecular periphery in contrast to the projected-out structure in case of the 3HF-water cluster. Density functional theory (DFT)-based quantum chemical calculations provide an approach for the differential solvation patterns of 3HF and 7HF. 6HF, a non-ESIPT probe, exhibits very low FA in water compared with both 3HF and 7HF. This study demonstrates that proper positioning of the hydroxyl group and its participation in the extended π-conjugation within the molecule dictate the formation of the solvated cluster endorsing directed solvation.

Origin of Unusually High Fluorescence Anisotropy of 3-Hydroxyflavone in Water: Formation of Probe-Solvent Cage-like Cluster

Based on the unusually high fluorescence anisotropy (FA) of 3-hydroxyflavone (3HF) in water medium in contrast to the very low FA of its methoxy counterpart (3MF), our proposition invoked formation of an intermolecular hydrogen-bonded cage-like probe-solvent cluster of 3HF in water. In the present work, ab-initio DFT-based quantum chemical calculations have been exploited to provide a foundation for our interpretation. Ground-state optimization of 3HF with varying numbers of water molecules leads to the formation of a cage-like or loop-like probe-water cluster. Our calculations reveal that the structures with four to five water molecules are stabilized to the maximum extent. Classical molecular dynamics simulations reveal that the rotational dynamics of 3HF is much slower in water compared to that in alkane medium, which also goes in favor of the probe-solvent cluster formation in water medium. Apart from the theoretical studies, an indirect experimental approach has been adopted to substantiate formation of the probe-water cluster. The atypical observation of reduced FA of 3HF entrapped in micelles relative to that of the fluorophore in water implies disruption of the probe-water cluster with the addition of micelles, corroborating our original proposition of formation of an intermolecularly hydrogen-bonded 3HF-water cluster.

Aggregation of Nile Red in Water: Prevention through Encapsulation in β-Cyclodextrin

The present work, based on various spectroscopic investigations, vividly demonstrates the self-association of Nile red (NR) in aqueous medium. The rapid decrease in the absorbance as well as emission of NR in water bears the signature of the aggregation process. Appearance of a new blue-shifted absorption band in addition to the original one and a drastic decrease in the emission intensity imply that the aggregation is of H-type. Poor solubility of NR in water, hydrophobic interaction, and the planar structure of the dye are ascribed to favor the formation of the aggregate in the aqueous medium. Absorption-based kinetic studies reveal the aggregation process to be second order, thereby establishing the aggregate to be a dimer. Similar kinetic profiles of the absorbance of NR in the presence and absence of light confirm that the aggregation process is not photoassisted. The presence of an isosbestic point in the absorbance spectra and an isoemissive point in the time-resolved area normalized emission spectra bears the evidence of equilibrium between the dimeric and the monomeric species of NR in the ground state as well as in the photoexcited state. Encapsulation of the monomer of NR within the hydrophobic cavity of β-cyclodextrin is demonstrated to prevent the aggregation process.

Role of solute-solvent hydrogen bonds on the ground state and the excited state proton transfer in 3-hydroxyflavone. A systematic spectrophotometric study

A detailed account on the photophysics of 3-hydroxyflavone (3HF) in 27 organic solvents is reported. Dual fluorescence of neutral 3HF was observed in protic, polar, and weakly polar solvents, endowed with sufficiently high hydrogen bond accepting and/or donating capabilities. Ground-state solvent-induced 3HF deprotonation was reported in 14 cases. 3HF anion photophysics was investigated, and the deprotonation constant Kdep calculated. Previously reported models (based on solute-solvent intermolecular hydrogen bonds) to explain solvent effects on Excited-State Intramolecular Proton Transfer (ESIPT) and on solvent-induced deprotonation have been re-examined and improved in order to rationalize the observed photophysical behaviour in all the studied solvents. Hydrogen bond donor acidity and hydrogen bond acceptor basicity are shown to be key parameters. The results are discussed in the framework of the use of 3HF as an environment-sensitive fluorescent sensor in several research fields, and as a model system in the study of ESIPT reactions. Solvent effects on 3HF reactivity are also discussed, as the role of the surrounding media on the chemistry of flavonols is an emerging topic in natural product research.

Mimicking multivalent protein–carbohydrate interactions for monitoring the glucosamine level in biological fluids and pharmaceutical tablets

Easily synthesizable ESIPT probe for dual mode sensing of glucosamine in pure water.