Solvation Effects in Organic Chemistry: A Short Historical Overview

Aminocarbonyl Fluorophores with a Strong Emissive Inverted Solvatochromism

Three aminocarbonyls were synthesized, and their emissive spectral behavior recorded at various solvent polarities showed marked inverted solvatofluorochromism. The emission energy inversion occurs at moderate solvent polarities and was found to be triggered by a change in the solute-solvent interaction responsible for the stabilization of the highly zwitterionic excited state of the dyes, from dipolarity to acidity.

Nuclear Quantum Effects Have a Significant Impact on UV/Vis Absorption Spectra of Chromophores in Water

Despite the broadly acknowledged importance of solvation effects on measured UV/Vis spectra in the context of solvatochromism or chemical reactions in solution, it is still an open challenge to calculate UV/Vis spectra with predictive accuracy. This is particularly true when it comes to the impact of nuclear quantum effects on these experimental observables. In the present work, we calculate the UV/Vis absorption spectrum of indole in aqueous solution with a combination of a correlated wavefunction method for computing electronic excitation energies and enhanced path integral simulations for rigorous sampling of nuclear configurations including the quantum effects in solution. After validating our approach based on gas-phase benchmarking, we demonstrate that the lineshape of the spectrum measured in aqueous solution is quantitatively recovered, without the application of any shifting, scaling, or broadening, only after including nuclear quantum effects in addition to thermal fluctuations and solvation at ambient conditions. Our findings demonstrate that nuclear quantum effects are "visible" in UV/Vis spectra of chromophores measured in solution even at room temperature and, therefore, that they must be considered computationally to achieve predictive accuracy.

A Fluorescent Organic Probe for Naked-Eye Detection of Chloromethanes

An organic fluorescent probe (OFP-TAR) with a propeller-like structure was designed and synthesized. The photoluminescence of OFP-TAR in solution exhibited a significant red shift with the increase of solvent polarity, enabling a transition of fluorescence emission from blue (445 nm) to yellow (540 nm). The organic thin-film materials based on OFP-TAR/PMMA exhibit significant color changes upon exposure to CH2Cl2, CHCl3, and CCl4, with their maximum fluorescence wavelengths measured at 445, 471, and 494 nm respectively. The device facilitates the visual detection of chloromethanes and is capable of enduring more than 7 cycles of testing. These materials can also be prepared as binary-coded microarray data storage devices or applied in the field of anti-counterfeiting. The quantum yields of guest-loaded crystals CH2Cl2@OFP-TAR, CHCl3@OFP-TAR and CCl4@OFP-TAR are observed as 19.13 %, 8.79 %, and 0.83 % respectively, which are consistent with the tendency of OFP-TAR in CH2Cl2 (47.30 %), CHCl3 (34.27 %) and CCl4 (3.10 %). The fluorescence properties of OFP-TAR, OFP-TAR/PMMA, guest-loaded and guest-free crystals provided insights into the special response mechanism of OFP-TAR towards different chloromethanes.

Enantioselective Lewis base catalysed allylation of picoline- and quinaldine-based latent pronucleophiles

Picolines and quinaldines are valuable building blocks and intermediates in the synthesis of natural products and pharmaceuticals. Functionalization of the methyl group in picolines and quinaldines under mild conditions is challenging. We report that the concept of latent pronucleophiles enables Lewis base catalysed allylation of picolines and quinaldines with allylic fluorides starting from silylated picolines and quinaldines. Reactions afford enantioenriched allylation products when chiral Lewis base catalysts are used. The allylation products can be rapidly transformed to quinolizine-4-ones.

60 Years of Betaine 30─From Solvatochromic Discovery to Future Frontiers

Betaine-30 (B30) was reported by Karl Dimroth and Christian Reichardt et al. in 1963 as a solvatochromic probe that can be easily synthesized, shows good solubility, and remains stable in various organic solvents and solutions. Its strongly negatively solvatochromic behavior arises from differential solvation between its electronic ground and excited states, making it a valuable tool for assessing solvent polarity using the ET(30) polarity scale, also devised by Dimroth and Reichardt. In addition, advancements in femtosecond laser spectroscopy in the 1990s greatly improved the understanding of B30's relaxation dynamics following photoexcitation. In solvents capable of hydrogen bonding, such as alcohols, intermolecular hydrogen-bond rearrangement contributes to the multiple relaxation components observed. Since the 1990s, the applications of B30 have expanded beyond simple organic solvents to include complex solvent mixtures, such as electrolyte solutions for battery technologies and eutectic solvent mixtures. Given the growing importance of these complex solvent mixtures, B30 is becoming an increasingly valuable tool for studying previously unexplored solvation properties.

A Collection of Dispersion Induction DI, Electrostatic ES, and Hydrogen Bonding α1 and β1 Parameters for 380 Solvents and What They Say on Solvent Effects on Rates, Equilibria, and Spectra

The solvent parameters DI, ES, α1, and β1 are intended for the description of solute-solvent intermolecular forces, i.e., dispersion-induction, electrostatic, hydrogen-bond donation, and hydrogen-bond acceptance, respectively. An up-to-date collection of these parameters is presented for 380 solvents including green solvents, ionic liquids, and deep eutectic solvents. Their determination for additional solvents requires three commercial indicators, betaine dye B(30), 4-F-phenol, and 4-F-anisole (as well as the refractive index). The chemical significance of these parameters is outlined. Their use in the linear solvation energy relationship P = P° + di DI + e ES + a α1 + b β1 for 62 physicochemical properties P (reaction rates, equilibrium constants, and IR, UV, and NMR spectra) yields determination coefficients generally greater than 0.90 and regression coefficients whose sign and relative magnitude provide consistent information on the intermolecular forces acting on these properties.

Catalytic Transformation of Carbohydrates into Renewable Organic Chemicals by Revering the Principles of Green Chemistry

Adherence to the principles of green chemistry in a biorefinery setting ensures energy efficiency, reduces the consumption of materials, simplifies reactor design, and rationalizes the process parameters for synthesizing affordable organic chemicals of desired functional efficacy and ingrained sustainability. The green chemistry metrics facilitate assessing the relative merits and demerits of alternative synthetic pathways for the targeted product(s). This work elaborates on how green chemistry has emerged as a transformative framework and inspired innovations toward the catalytic conversion of biomass-derived carbohydrates into fuels, chemicals, and synthetic polymers. Specific discussions have been incorporated on the judicious selection of feedstock, reaction parameters, reagents (stoichiometric or catalytic), and other synthetic auxiliaries to obtain the targeted product(s) in desired selectivity and yield. The prospects of a carbohydrate-centric biorefinery have been emphasized and research avenues have been proposed to eliminate the remaining roadblocks. The analyses presented in this review will steer to developing superior synthetic strategies and processes for envisaging a sustainable bioeconomy centered on biomass-derived carbohydrates.

Extended Reichardt's Dye-Synthesis and Solvatochromic Properties

Three new pyridinium-phenolate dyes based on the benchmark solvatochromic dye Betaine 30 were synthesised. The dyes contained phenylene spacers between the donor and acceptor groups. Their UV-Vis absorption spectra were measured, with the dyes showing strong negative solvatochromic behaviour comparable to that of Betaine 30. These results stood in contrast to the behaviour of the π-extended dye Betaine 21, originally reported in 1963. This dye was synthesised and found to be significantly more solvatochromic than previously reported but prone to degrade. All π-extended dyes synthesised were found to be unstable in certain solvents. Although the increased distance between donor and acceptor did not enhance solvatochromism to the extent predicted, it was still determined that the reduced planarity caused by a phenylene spacer is not as detrimental as believed.

Kinetics and Thermodynamics of Lactose Mutarotation through Chromatography

The mutarotation kinetics and thermodynamics of the reaction α-lactose ⇌ β-lactose have been measured in dilute solutions using liquid chromatography without any derivatization step, using a C18 column and pure water as the mobile phase. The effect of temperature (0.5-45 °C) of the starting powder composition (α-lactose-rich or β-lactose-rich powder) and of the solvent composition (water with up to 35% weight fraction of seven organic solvents) has been experimentally investigated. Increasing the temperature leads to faster kinetics, following an Arrhenius model, and to slightly decreasing concentration-based equilibrium ratio. Conversely, increasing the weight fraction of organic solvent at 25 °C resulted in slower kinetics and smaller concentration-based equilibrium ratio. The starting powder composition is shown not to influence the kinetics or thermodynamics of the process. The corresponding parameter estimation problem is thoroughly discussed, taking into account the small difference in response factors of the lactose diastereomers.

Inverting the Solvatochromism of Pyridinium-N-phenolate Dyes by the Addition of a Second Pyridinium Unit

Three bipyridinium phenolates were synthesized, and their spectral behavior was recorded at various solvent polarities and compared to a classic pyridinium phenolate dye possessing only one pyridinium acceptor ring in its structure. The addition of a second pyridinium unit to the classic solvatochromic core results in an unexpected change in the spectral behavior from negative solvatochromism (displacement of the absorption band to shorter wavelengths) to inverted solvatochromism, characterized by the transition from negative to positive solvatochromism (displacement of the absorption band to longer wavelengths) at moderate solvent polarities.

Thermally Activated Geometrical Regioselective E→Z Isomerization-Enabled Cascade Sequences of Conjugated Dienals: Experimental and DFT Studies

The geometrical regioselective E→Z isomerization of a conjugated alkene under thermal activation pose a challenge due to microscopic reversibility. Herein we report that such reversibility issues can be circumvented by integrating E→Z isomerization with subsequent cyclization cascade, particularly in the absence of commonly employed light, acids, or metal-catalysts. Thus, linearly conjugated dienals in a mixture of toluene-alcohol (2 : 1) solvents or only with alcohol at 60-70 °C can be converted to γ-alkoxybutenolides in moderate to good yields. The intermediary 2Z,4E-isomer can be isolated, which includes the first example of isolating the regioselective isomerization product under thermal conditions. Density functional theory (DFT) studies have been employed to shed light on the feasibility of geometrical alkene isomerization and ensuing cascade sequences. It has been observed that the regioselective 2E,4E→2Z,4E isomerization of dienal is a thermodynamically facile (ΔG <0) process. Structural elucidation further reveals that the presence of a certain charge transfer and a non-covalent interaction may be the primary reasons for the enhanced stability of the 2Z,4E-isomer. The thermodynamic plausibility of the subsequent cascade reaction from the Z-isomer to the anticipated product in the presence of a polar protic solvent (here MeOH) is also explicated. Out of the two probable pathways, the "hemiacetal pathway" involving a relay proton transfer is kinetically more feasible due to the diminished activation barrier than the "conjugate addition pathway".

Further Understanding the Roles of Solvent, Brønsted Acids, and Hydrogen Bonding in Iron Porphyrin-Mediated Carbon Dioxide Reduction

Improving our understanding of how molecules and materials mediate the electrochemical reduction of carbon dioxide (CO2) to upgraded products is of great interest as a means to address climate change. A leading class of molecules that can facilitate the electrochemical conversion of CO2 to carbon monoxide (CO) is iron porphyrins. These molecules can have high rate constants for CO2-to-CO conversion; they are robust, and they rely on abundant and inexpensive synthetic building blocks. Important foundational work has been conducted using chloroiron 5,10,15,20-tetraphenylporphyrin (FeTPPCl) in N,N-dimethylformamide (DMF) solvent. A related and recent report points out that the corresponding perchlorate complex, FeTPPClO4, can have superior function due to its solubility in other organic solvents. However, the importance of hydrogen bonding and solvent effects was not discussed. Herein, we present a detailed kinetic study of the triflate (CF3SO3-) complex of FeTPP in DMF and in MeCN using a range of phenol Brønsted acid additives. We also detected the formation of Fe(III)TPP-phenolate complexes using cyclic voltammetry experiments. Importantly, our new analysis of apparent rate constants with different added phenols allows for a modification to the established mechanistic model for CO2-to-CO conversion. Critically, our improved model accounts for hydrogen bonding and solvent effects by using simple hydrogen bond acidity and basicity descriptors. We use this augmented model to rationalize function in other reported porphyrin systems and to make predictions about operational conditions that can enhance the CO2 reduction chemistry.

Highly efficient and recyclable chiral phosphine-functionalized polyether ionic liquids for asymmetric hydrogenation of β-keto esters

Herein, based on the concept of integration of phosphine ligands and ionic liquids (ILs), a class of chiral phosphine-functionalized polyether ionic liquids (CPF-PILs) were synthesized by ion-exchange reaction between polyether imidazolium ILs and a phenyl-sulfonated (S)-(-)-2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (BINAP) chiral diphosphine ligand, and employed in the Ru-catalyzed homogeneous asymmetric hydrogenation of β-keto esters. The resulting CPF-PILs combined the dual functions of the chiral phosphine ligand and ILs, allowing efficient recovery and recycling of the chiral catalysts using only a catalytic amount of CPF-PILs. The effects of various factors, including the chiral catalyst structure, solvent properties, reaction temperature, hydrogen pressure, and hydrobromic acid dosage, on catalytic performance were thoroughly investigated, as well as the cycling stability and universality of the chiral catalysts were examined. The findings of the present study demonstrated that, under optimal reaction conditions, the model substrate methyl acetoacetate underwent quantitative conversion to methyl β-hydroxybutyrate with a 97% enantiomeric excess (ee). The chiral catalyst used in this process can be recycled up to 12 times and showed good applicability to structurally various β-keto esters. The present study presents a novel approach for using ILs in asymmetric hydrogenation reactions in an environmentally friendly manner.

Estimation of Nanoparticle's Surface Electrostatic Potential in Solution Using Acid-Base Molecular Probes I: In Silico Implementation for Surfactant Micelles

Surface electrostatic potential Ψ is a key characteristic of colloid particles. Since the surface of the particles adsorbs various compounds and facilitates chemical reactions between them, Ψ largely affects the properties of adsorbed reactants and governs the flow of chemical reactions occurring between them. One of the most popular methods for estimating Ψ in hydrophilic colloids, such as micellar surfactant solutions and related systems, is the application of molecular probes, predominantly acid-base indicator dyes. The Ψ value is calculated from the difference of the probe's indices of the apparent acidity constant between the examined colloid solution and, usually, some other colloid solution with noncharged particles. Here, we show how to implement this method in silico using alchemical free energy calculations within the framework of molecular dynamics simulations. The proposed implementation is tested on surfactant micelles and is shown to predict experimental Ψ values with quantitative accuracy depending on the kind of surfactant. The sources of errors in the method are discussed, and recommendations for its application are given.

The Global Polarity of Alcoholic Solvents and Water - Importance of the Collectively Acting Factors Density, Refractive Index and Hydrogen Bonding Forces

The DHBD quantity represents the hydroxyl group density of alcoholic solvents or water. DHBD is purely physically defined by the product of molar concentration of the solvent (N) and the factor Σn=n×f which reflects the number n and position (f-factor) of the alcoholic OH groups per molecule. Whether the hydroxyl group is either primary, secondary or tertiary is taken into account by f. Σn is clearly linearly correlated with the physical density or the refractive index of the alcohol derivative. Relationships of solvent-dependent UV/Vis absorption energies as ET (30) values, 129 Xe NMR shifts and kinetic data of 2-chloro-2-methylpropane solvolysis with DHBD are demonstrated. It can be shown that the ET (30) solvent parameter reflects the global polarity of the hydrogen bond network rather than specific H-bond acidity. Significant correlations of the log k1 rate constants of the solvolysis reaction of 2-chloro-2-methylpropane with DHBD show the physical reasoning of the approach.

Solvation Effects on Quantum Tunneling Reactions

A decisive factor for obtaining high yields and selectivities in organic synthesis is the choice of the proper solvent. Solvent selection is often guided by the intuitive understanding of transition state-solvent interactions. However, quantum-mechanical tunneling can significantly contribute to chemical reactions, circumventing the transition state and thus depriving chemists of their intuitive handle on the reaction kinetics. In this Account, we aim to provide rationales for the effects of solvation on tunneling reactions derived from experiments performed in cryogenic matrices.The tunneling reactions analyzed here cover a broad range of prototypical organic transformations that are subject to strong solvation effects. Examples are the hydrogen tunneling probability for the cis-trans isomerization of formic acid which is strongly reduced upon formation of hydrogen-bonded complexes and the [1,2]H-shift in methylhydroxycarbene where a change in product selectivity is predicted upon interaction with hydrogen bond acceptors.Not only hydrogen but also heavy atom tunneling can exhibit strong solvent effects. The direction of the nearly degenerate valence tautomerization between benzene oxide and oxepin was found to reverse upon formation of a halogen or hydrogen bond with ICF₃ or H₂O. But even in the absence of strong noncovalent interactions such as hydrogen or halogen bonding, solvation can have a decisive effect on tunneling as evidenced by the Cope rearrangement of semibullvalenes via heavy-atom tunneling. Can quantum tunneling be catalyzed? The acceleration of the ring expansion of 1H-bicyclo[3.1.0.]-hexa-3,5-dien-2-one by complexation with Lewis acids provides a proof-of-concept for tunneling catalysis.Two concepts are central for the explanation and prediction of solvation effects on tunneling phenomena: a simple approach expands the Born-Oppenheimer approximation by separating nuclear degrees of freedom into intra- and intermolecular degrees. Intermolecular movements represent the slowest motions within molecular aggregates, thus effectively freezing the position of the solvent in relation to the reactant during the tunneling process. Another useful approach is to treat reactants and products by separate single-well potentials, where the intersection represents the transition state. Thus, stabilization of the reactants via solvation should result in an increase in barrier heights and widths which in turn lowers tunneling probabilities. These simple models can predict trends in tunneling kinetics and provide a rational basis for controlling tunneling reactions via solvation.

Dip-In Photoresist for Photoinhibited Two-Photon Lithography to Realize High-Precision Direct Laser Writing on Wafer

For decades, photoinhibited two-photon lithography (PI-TPL) has been continually developed and applied into versatile nanofabrication. However, ultrahigh precision fabrication on wafer by PI-TPL remains challenging, due to the lack of a refractive index (n) matched photoresist (Rim-P) with effective photoinhibition capacity for dip-in mode. In this paper, various Rim-P are developed and then screened for their applications in PI-TPL. In addition, different lithography methods (in terms of oil-mode and dip-in mode) are analyzed by use of optical simulations combined with experiments. Remarkably, one type of Rim-P (n = 1.518) shows effective photoinhibition capacity, which represents an outstanding breakthrough in the field of PI-TPL. In contrast to photoresist with an unsuitable refractive index, optical aberrations are almost completely eliminated in the dip-in mode by using the Rim-P. Consequently, features with a minimum critical dimension as small as 39 nm are successfully achieved on wafer by dip-in PI-TPL, which paves the way for subdiffraction silicon-based chip manufacturing by PI-TPL. Moreover, through a combination of the Rim-P and dip-in mode, the ability to achieve tall and high-precision three-dimensional nanostructures is no longer problematic.

SN2 versus E2 reactions in a complex microsolvated environment: theoretical analysis of the equilibrium and activation steps of a nucleophilic fluorination

The reactivity of the fluoride ion towards alkyl halides is highly dependent on the solvating environment. In polar aprotic solvents with large counter-ions is highly reactive and produces substantial E2 product, whereas in polar protic solvents leads to slow kinetics and high selectivity for S_N2 reactions. The use of a more complex environment with stoichiometric addition of tert-butanol to acetonitrile solvent is able to module the reactivity and selectivity of tetrabutylammonium fluoride (TBAF). In the present work, we have performed a detailed theoretical analysis of this complex reaction system by density functional theory, continuum solvation model, and including explicit tert-butanol molecules. A kinetic model based on the free energy profile was also used to predict the reactivity and selectivity. The results indicated that the TBAF(tert-butanol) complex plays the key role to increase the S_N2 selectivity, whereas higher aggregates are not relevant. The E2 product is formed exclusively via free TBAF, because the solvating tert-butanol in the TBAF(tert-butanol) complex inhibits the E2 pathway. Our analysis suggests that diols or tetraols could produce an improved selectivity.