Electron Transfer Kinetics between an Electron-Accepting Ionic Liquid and Coumarin Dyes

Design, synthesis, and spectroscopic profiling of novel coumarin dyes: Investigating solvent sensitivity and photophysical properties

Coumarin dyes are highly versatile and widely employed as fluorescent chemosensors in a variety of fields, including molecular imaging, bioorganic chemistry, analytical chemistry, materials chemistry, biology, and medical science. Thanks to their outstanding photostability and high quantum yield, they represent an ideal choice for developing sensitive and selective sensing platforms. In this study, we successfully designed and synthesized four new dyes based on the coumarin dye molecular skeleton, investigating their solvent sensitivity and spectroscopic properties. Our novel coumarin dyes were synthesized by a straightforward approach, reacting coumarin-3-carboxylic acid succinimidyl ester derivatives with corresponding amines in 1,4-dioxane as a solvent. We carefully monitored the completion of the reactions using thin-layer chromatography (TLC) and characterized these dyes using spectral and elemental analyses. We further investigated the UV, fluorescence, time-correlated single photon counting (TCSPC) technique and time-resolved spectroscopy (TRES) of these dyes in different solvents and on polymer film poly(methyl methacrylate) (PMMA). The quantum yield of the synthesized dyes was determined, with values observed to range between 0.55 and 0.94. Most of the dye-solvent and dye-polymer combinations exhibited single exponential decay, with lifetimes ranging from 2.3 to 3 ns. Minor deviations from single exponential behavior were observed for most of the dyes in toluene, while significant deviations were observed for coumarin dyes with piperazine moiety. We have provided a rationalization of these results in terms of the chemical functionalities of the various dyes. Furthermore, we investigated the effect of interactions between 7-methoxy-2-oxo-N-(2-(piperazin-1-yl)ethyl)-2H-chromene-3-carboxamide and silica nanoparticles (Ludox) on the spectroscopic properties of these dyes, with charge transfer being one possible mechanism contributing to the behavior of the dyes. Additionally, we explored the effect of trifluoroacetic acid (TFA) on the dyes' emission intensity and fluorescence decay. Based on our UV and fluorescence measurements of the dyes in different solvents, we have concluded that these dyes can create excellent donor-acceptor pairs for our upcoming fluorescence resonance energy transfer (FRET) experiments.

Effect of Lithium-Ion on the Structural Organization of Monocationic and Dicationic Ionic Liquids

In recent times, ionic liquid-based (ILs) electrolytic system has emerged as suitable alternative to the conventional organic solvent-based electrolytic system. However, since, anion of ILs is known to form aggregates in the presence of lithium-ions (Li⁺), and this can influence the transport properties of Li⁺ ion in a significant manner, it is, therefore, important to understand how lithium-ions influence the structure and dynamics of ILs. With this objective, in the present study, intermolecular interaction, structural organization, and dynamics of monocationic ILs (MILs) and dicationic IL (DIL) have been studied in the absence and presence of lithium salt. Specifically, for this purpose, two MILs, 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide ([C₃C₁im][NTf₂]), 1-hexyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide ([C₆C₁im][NTf₂]), and a DIL, 1,6-bis(3-methylimidazolium-1-yl)hexane bis(trifluoromethylsulfonyl)amide ([C₆(mim)₂][NTf₂]₂) have been chosen in such a way that either the alkyl chain of MILs becomes equal or half of the spacer chain length of DIL. To understand the effect of the addition of lithium-ion on the structural organization of MILs and DIL, steady-state absorption and fluorescence spectroscopies, time-resolved fluorescence anisotropy and nuclear magnetic resonance (NMR) techniques have been used. Structural organization in the apolar and polar domains of ILs has been probed by following the rotational diffusion of suitably chosen solute in the concerned media through time-resolved fluorescence anisotropy (TRFA) measurements. TRFA studies have revealed that with the addition of Li⁺ ion, coordination between the Li⁺ ions and anions of MILs and DILs takes place in the ionic region leading to a change in the structural organization of the apolar regions of the respective medium. In fact, upon adding lithium-ions, a reduction in the packing of alkyl chains has also been observed for the MILs. However, not much change in the structural organization of the apolar region of the DIL has been observed when Li⁺ ion is added to it. In the presence of Li⁺ ions, a similar trend in the change of structural organization of polar regions for both MILs and DIL has been observed. Further, measurements of the self-diffusion coefficient through NMR have also supported the observation that Li⁺ ion also perturbs the nanostructural organization of the MIL in a significant manner than that it does for the DIL. The behavior of DIL in the presence of Li⁺ ion, as revealed by the present study, has been rationalized by considering the folded arrangement of DIL in the fluid-structure. Essentially, all of these investigations have suggested that the addition of lithium-ion significantly alters the microscopic behavior of MILs in comparison to that of DIL. The outcome of this study is expected to be helpful in realizing the potentials of these media as electrolytes in battery applications.

Vibrational Dephasing along the Reaction Coordinate of an Electron Transfer Reaction

The role of molecular vibration in photoinduced electron transfer (ET) reactions has been extensively debated in recent years. In this study, we investigated vibrational wavepacket dynamics in a model ET system consisting of an organic dye molecule as an electron acceptor dissolved in various electron donating solvents. By using broad band pump-probe (BBPP) spectroscopy with visible laser pulses of sub-10 fs duration, coherent vibrational wavepackets of naphthacene dye with frequencies spanning 170-1600 cm-1 were observed in the time domain. The coherence properties of 11 vibrational modes were analyzed by an inverse Fourier filtering procedure, and we discovered that the dephasing times of some vibrational coherences are reduced with increasing ET rates. Density functional theory calculations indicated that the corresponding vibrational modes have a large Huang-Rhys factor between the reactant and the product states, supporting the hypothesis that the loss of phase coherence along certain vibrational modes elucidates that those vibrations are coupled to the reaction coordinate of an ET reaction.

Synergistic dynamics of photoionization and photoinduced electron transfer probed by laser flash photolysis and ultrafast fluorescence spectroscopy

Photoionization (PI) and photoinduced electron transfer (PET) dynamics of coumarin 450 (C450) in micelles were investigated in the time domains of micro to femtoseconds using steady-state and time-resolved absorption and fluorescence spectroscopy. The PI of C450 occurs inside the micelles leads to the formation of C450 cation radical (CR) and hydrated electron, which is characterized by the respective transient absorption. The PI of C450 is monophotonic in nature and the yield is dependent on the charge of the micelles. The observation of amine CR in the transient absorption confirms the PET from amine to the excited state of C450 in micelles, which results in the quenching of both fluorescence intensity and lifetime. The decrease in femtosecond fluorescent decay of C450 and the absence of transient C450 radical anion in the presence of amine implies that the concerted ultrafast PET promoted PI and PET to the C450 CR-electron pair. The decrease in the time constant for the formation of relaxed state in the presence of amines is due to the ultrafast PET to the C450 CR-electron pair, which prevents the formation of a relaxed state through recombination at a longer time scale. In the present investigation, the recombination dynamics of the CR-electron pair is justified as one of the origins of the slow solvation in micelles. The influence of amine concentration on the decay of C450 CR indicates ET reaction between C450 CR and amine, which is further confirmed by the bleach recovery of C450 ground state in the presence of amine.

Binary mixtures of ionic liquids: Ideal, non-ideal, or quasi-ideal?

The mixing of ILs provides an opportunity for fine tuning the physiochemical properties of ILs for various applications. However, a suitable mixture having desired properties can only be designed when the physiochemical properties of the mixtures of ILs along with their spectroscopic properties are well understood. With an aim to achieve this objective, three different mixtures with a common anion, namely, [C₂C₁im][C₄C₁im][NTf₂], [C₃C₁pyr][C₄C₁pyr][NTf₂], and [C₃C₁im][C₃C₁pyr][NTf₂], have been investigated in the current study. Investigations have been carried out at the macroscopic level by observing the thermophysical properties, such as molar volume and thermal expansion coefficient, and at the microscopic level with time-resolved fluorescence measurements and the pulse field gradient nuclear magnetic resonance (NMR) technique. The results obtained from the thermophysical study have indicated that excess molar volume for imidazolium-based IL-IL mixtures may be linked to the free volume created by the alkyl chain of the imidazolium cation whereas for the mixture of pyrrolidinium ILs, lowering of density can give rise to free volume. Analysis of time-resolved fluorescence anisotropy data has provided clear evidence in favor of the presence of free volume in the binary mixture of ILs. NMR studies have also supported the fluorescence anisotropy data. The outcome of the present investigation reveals that the mixtures show appreciable deviation from ideal behavior and the deviation from the ideal behavior is caused due to the generation of free volume in the resultant mixture, describing these IL mixtures as quasi-ideal rather than ideal or non-ideal.