Ultrafast Intramolecular Charge Transfer of Formyl Perylene Observed Using Femtosecond Transient Absorption Spectroscopy

The excited-state photophysics of formylperylene (FPe) have been investigated in a series of nonpolar, polar aprotic, and polar protic solvents. A variety of experimental and theoretical methods were employed including femtosecond transient absorption (fs-TA) spectroscopy with 130 fs temporal resolution. We report that the ultrafast intramolecular charge transfer from the perylene unit to the formyl (CHO) group can be facilitated drastically by hydrogen-bonding interactions between the carbonyl group oxygen of FPe and hydrogen-donating solvents in the electronically excited state. The excited-state absorption of FPe in methanol (MeOH) is close to the reported perylene radical cation produced by bimolecular quenching by an electron acceptor. This is a strong indication for a substantial charge transfer in the S1 state in protic solvents. The larger increase of the dipole moment change in the protic solvents than that in aprotic ones strongly supports this observation. Relaxation mechanisms including vibrational cooling and solvation coupled to the charge-transfer state are also discussed

Modifying reaction rates and stimulus-responsive behavior of polymer-coated catalysts using aprotic solvents

The impact of solvent composition on the reaction rates and apparent activation barriers for the reduction of nitrobenzene on Pd has been investigated by changing the solvent from pure water to mixtures with increasing concentrations of 1-methyl-2-pyrrolidone (NMP). When using pure NMP as the solvent, the activity was negligible and a high activation energy barrier was observed. Surprisingly, switching to water led to faster reaction rates and lower apparent barriers. Considering that previous research has demonstrated that water molecules near the catalyst surface facilitate the hydrogen insertion on R-NO* and R-HNO* surface species via proton-electron transfer, it is possible to link the herein observed trends in activity for the nitrobenzene hydrogenation to the ability of the reaction media to shuttle protons during the reaction. Furthermore, the polymer-induced solvation effects were investigated using thermo-responsive Pd/SiO2-p-NIPAM catalyst. Here, we observed that the utilization of NMP inhibits the thermo-responsive behaviour of poly N-isopropylacrylamide (p-NIPAM). This explains the constant particle size of Pd/SiO2-p-NIPAM catalyst observed at different temperatures during dynamic light scatting characterization (DLS). We speculate that this non-responsive behaviour of the p-NIPAM in the presence of NMP is the cause of the constant activation energy barrier at temperatures above and below the lower critical solution temperature (LCST) of the polymer (32 °C). When the reaction was conducted in pure water, the polymer-coated catalyst showed significant changes in both the apparent enthalpy and entropy of activation for temperatures below and above the LCST. This suggests that the microenvironment induced by the polymer can significantly influence the reaction rate.</p

The solvent effect on the electrocatalytic cleavage of carbon-halogen bonds on Ag and Au

In recent years it has been shown in detail how the electrocatalytic cleavage of carbonhalogen bonds is modulated by (a) the stepwise or concerted nature of the dissociative electrontransfer mechanism, which is influenced by the nature of the electrode surface, the type of halogen atom and the molecular structure of RX as a whole, and (b) the double-layer structure (as a function of the nature and bulkiness of the supporting electrolyte ions). In order to both complete and support the interpretative scheme thus developed, this work is focused on the solvent role. When one compares aprotic with protic organic solvents after appropriate intersolvental normalization, interesting peculiarities emerge, especially concerning protic media. Solvent proticity deeply affects both the reaction mechanism (on both non-catalytic and catalytic electrodes) and the extent of the catalytic effects. These items are discussed on the basis of a complete investigation carried out with a carefully controlled experimental protocol on two chloride and bromide couples, one aromatic and one aliphatic, representative of stepwise and concerted mechanisms, respectively, in four aprotic and four protic solvents, on both non-catalytic GC and catalytic Ag and Au electrodes. The results are discussed in the framework of a recently developed interpretative scheme of the carbon-halogen cleavage mechanism

The ammonolysis of esters in liquid ammonia

The rates of ammonolysis of alkyl benzoate and phenylacetate esters in liquid ammonia increase with the acidity of the leaving group alcohol and show relatively large Brønsted βlg values of −1.18 and −1.34, respectively, when plotted against the aqueous pKa of the alcohol. The Brønsted βlg obtained using the pKa of the leaving group alcohol in liquid ammonia is significantly reduced to ~ −0.7, which indicates that the rate-limiting step involves a reaction of the tetrahedral intermediate with little C–OR bond fission in the transition state. The solvolysis reaction is subject to significant catalysis by ammonium ion, which, surprisingly, generates a similar Brønsted βlg indicating little interaction between the ammonium ion and the leaving group. It is concluded that the rate-limiting step for the ammonium-ion-catalysed solvolysis of alkyl esters in liquid ammonia is the diffusion-controlled protonation of the zwitterionic tetrahedral intermediate T+- to give T+, which is rapidly deprotonated to give T0 which is compatible with the rate-limiting step for the uncatalysed reaction being the formation of the neutral T0 by a ‘proton switc

Structure and photophysics of 2-(2'-pyridyl)benzindoles: The role of intermolecular hydrogen bonds

The photophysical properties of two isomeric 2-(2′-pyridyl) benzindoles depend on the environment. Strong fluorescence is detected in nonpolar and polar aprotic solvents. In the presence of alcohols, the emission reveals an unusual behavior. Upon titration of n-hexane solutions with ethanol, the fluorescence intensity goes through a minimum and then increases with rising alcohol concentration. Transient absorption and time-resolved emission studies combined with ground- and excited-state geometry optimizations lead to the conclusion that two rotameric forms, syn and anti, coexist in alcohols, whereas in nonpolar and aprotic polar media, only the syn conformation is present. The latter can form cyclic complexes with alcohols, which are rapidly depopulated in the excited state. In the presence of excess alcohol, syn → anti rotamerization occurs in the ground state, promoted by the cooperative action of nonspecific and specific effects such as solvent polarity increase and the formation of hydrogen bonds to both donor and acceptor sites of the bifunctional compounds. © 2007 American Chemical Society

Kinetics of alkoxysilanes hydrolysis: An empirical approach

Alkoxysilanes and organoalkoxysilanes are primary materials in several industries, e.g. coating, anti-corrosion treatment, fabrication of stationary phase for chromatography, and coupling agents. The hydrolytic polycondensation reactions and final product can be controlled by adjusting the hydrolysis reaction, which was investigated under a variety of conditions, such as different alkoxysilanes, solvents, and catalysts by using gas chromatography. The hydrolysis rate of alkoxysilanes shows a dependence on the alkoxysilane structure (especially the organic attachments), solvent properties, and the catalyst dissociation constant and solubility. Some of the alkoxysilanes exhibit intramolecular catalysis. Hydrogen bonding plays an important role in the enhancement of the hydrolysis reaction, as well as the dipole moment of the alkoxysilanes, especially in acetonitrile. There is a relationship between the experimentally calculated polarity by the Taft equation and the reactivity, but it shows different responses depending on the solvent. It was found that negative and positive charges are respectively accumulated in the transition state in alkaline and acidic media. The reaction mechanisms are somewhat different from those previously suggested. Finally, it was found that enthalpy–entropy compensation (EEC) effect and isokinetic relationships (IKR) are exhibited during the hydrolysis of CTES in different solvents and catalysts; therefore, the reaction has a linear free energy relationship (LFER).The publication of this article was funded by the Qatar National Library.Scopu

Investigation on Tautomeric Equilibrium of Schiff Base in Mixed Binary Solvent

Enol imine- keto inamine tautomeric equilibrium has been studied in bis-salicilidene schiff base (sal-en-Sal) with a spacer unit in aqueous-organic binary solvent mixture. Organic solvent favors the E-form (enol form) while aqueous solvent favors both the E-form and K-form (keto form). The binary solvent mixture has a transition at almost 70% volume of organic solvent. i.e. the non-aqueous solvent molecules remain within water clusters upto a volume percentage of almost 70, beyond which the water clusters remain in the oil framework. The tautomeric equilibrium is very sensitive to water

Preferential Solvation of a Dipolar Solute in Mixed Binary Solvent. A Study by UV-Visible Spectroscopy

Solvation characteristics of p-nitroaniline have been studied in completely miscible binary solvent mixtures, namely, ethanol + dichloromethane (I), ethanol + hexane (II) and butanol + dioxane (III), by monitoring the solvatochromic absorption band. The maximum energy of absorption (E) of the solute in a binary solvent mixture differs significantly from the mole fraction average of the E values in the component solvents. The results have been explained in terms of preferential solvation using a two phase model of solvation. In I and II negative deviation is observed while in III both positive and negative deviation is observed. The local mole fraction of the component solvents has been calculated explained in terms of specific solute-solvent and solvent-solvent interaction