Increasing Solvent Polarity (increasing + solvent_polarity)

Distribution by Scientific Domains

Selected Abstracts

Intramolecular electronic communication in a dimethylaminoazobenzene,fullerene C60 dyad: An experimental and TD-DFT study

K. Senthil Kumar
Abstract An electronically push,pull type dimethylaminoazobenzene,fullerene C60 hybrid was designed and synthesized by tailoring N,N -dimethylaniline as an electron donating auxochrome that intensified charge density on the ,-azonitrogen, and on N -methylfulleropyrrolidine (NMFP) as an electron acceptor at the 4 and 4, positions of the azobenzene moiety, respectively. The absorption and charge transfer behavior of the hybrid donor-bridge-acceptor dyad were studied experimentally and by performing TD-DFT calculations. The TD-DFT predicted charge transfer interactions of the dyad ranging from 747 to 601 nm were experimentally observed in the UV-vis spectra at 721 nm in toluene and dichloromethane. A 149 mV anodic shift in the first reduction potential of the NN group of the dyad in comparison with the model aminoazobenzene derivative further supported the phenomenon. Analysis of the charge transfer band through the orbital picture revealed charge displacement from the n(NN) (nonbonding) and , (NN) type orbitals centered on the donor part to the purely fullerene centered LUMOs and LUMO+n orbitals, delocalized over the entire molecule. The imposed electronic perturbations on the aminoazobenzene moiety upon coupling it with C60 were analyzed by comparing the TD-DFT predicted and experimentally observed electronic transition energies of the dyad with the model compounds, NMFP and (E)-N,N -dimethyl-4-(p-tolyldiazenyl)aniline (AZNME). The n(NN) , ,*(NN) and ,(NN) , ,*(NN) transitions of the dyad were bathochromically shifted with a significant charge transfer character. The shifting of ,(NN) , ,*(NN) excitation energy closer to the n , ,*(NN) in comparison with the model aminoazobenzene emphasized the predominant existence of charge separated quinonoid-like ground state electronic structure. Increasing solvent polarity introduced hyperchromic effect in the ,(NN) , ,*(NN) electronic transition at the expense of transitions involved with benzenic states, and the extent of intensity borrowing was quantified adopting the Gaussian deconvolution method. On a comparative scale, the predicted excitation energies were in reasonable agreement with the observed values, demonstrating the efficiency of TD-DFT in predicting the localized and the charge transfer nature of transitions involved with large electronically asymmetric molecules with HOMO and LUMO centered on different parts of the molecular framework. 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 [source]

Effect of the Media on the Quantum Yield of Singlet Oxygen (O2(1,g)) Production by 9H -Fluoren-9-one: Solvents and Solvent Mixtures

We have investigated the effect of a series of 18 solvents and mixtures of solvents on the production of singlet molecular oxygen (O2(1,g), denoted as 1O2) by 9H -fluoren-9-one (FLU). The normalized empirical parameter E derived from ET(30) has been chosen as a measure of solvent polarity using Reichardt's betaine dyes. Quantum yields of 1O2 production (,,) decrease with increasing solvent polarity and protic character as a consequence of the decrease of the quantum yield of intersystem crossing (,ISC). Values of ,, of unity have been found in alkanes. In nonprotic solvents of increasing polarity, ,ISC and, therefore, ,, decrease due to solvent-induced changes in the energy levels of singlet and triplet excited states of FLU. This compound is a poor 1O2 sensitizer in protic solvents, because hydrogen bonding considerably increases the rate of internal conversion from the singlet excited state, thus diminishing ,, to values much lower than those in nonprotic solvents of similar polarity. In mixtures of cyclohexane and alcohols, preferential solvation of FLU by the protic solvent leads to a fast decrease of ,, upon addition of increasing amounts of the latter. [source]

Principles of carbopeptoid folding: a molecular dynamics simulation study

Riccardo Baron
Abstract The conformational spaces of five oligomers of tetrahydrofuran-based carbopeptoids in chloroform and dimethyl sulfoxide were investigated through nine molecular dynamics simulations. Prompted by nuclear magnetic resonance experiments that indicated various stable folds for some but not all of these carbopeptoids, their folding behaviour was investigated as a function of stereochemistry, chain length and solvent. The conformational distributions of these molecules were analysed in terms of occurrence of hydrogen bonds, backbone torsional-angle distributions, conformational clustering and solute configurational entropy. While a cis -linkage across the tetrahydrofuran ring favours right-handed helical structures, a trans -linkage results in a larger conformational variability. Intra-solute hydrogen bonding is reduced with increasing chain length and with increasing solvent polarity. Solute configurational entropies confirm the picture obtained: they are smaller for cis - than for trans -linked peptides, for chloroform than for dimethyl sulfoxide as solvent and for shorter peptide chains. The simulations provide an atomic picture of molecular conformational variability that is consistent with the available experimental data. Copyright 2004 European Peptide Society and John Wiley & Sons, Ltd. [source]

Fluorescence and photoisomerization studies of p -nitrophenyl-substituted ethenylindoles

Anil K. Singh
Abstract The synthesis, electronic absorption, fluorescence (,f, ,ex, ,f, ,f) and photoisomerization (,t,c, photostationary state composition) properties of 3-(4-nitrophenylethenyl- E)-NH-indole (1), 3-(4-nitrophenylethenyl- E)- N -ethylindole (2) and 3-(4-nitrophenyl ethenyl- E)- N -benzenesulfonylindole (3) in organic solvents of varying polarity are reported. The absorption maximum of these compounds undergoes a moderate red shift with increasing solvent polarity. However, the fluorescence maximum becomes highly red shifted with increasing solvent polarity. Whereas 1 and 2 show broad fluorescence bands, 3 exhibits dual fluorescence. Further, 1 and 2 fluoresce much more efficiently than 3. Correlation of the Stokes shift with solvent polarity parameters such as ,f and ET(30) and excited-state dipole moment indicate a highly polar excited state for 1,3. Time-resolved fluorescence studies show that the fluorescence decays are single- and multi-exponential type, depending on the solvent polarity. Further, 1 and 2 do not show photoisomerization on irradiation. However, 3 is photoactive and shows efficient photoisomerization in non-polar heptane. The sensitivity (,) of the photoreaction is determined in various solvent in terms of the Hammett plot, which showed that the excited states involved are electron deficient in nature and consequently stabilized more by an electron sufficient polar solvent and electron donating substituent. These results led us to suggest the existence of three types of excited states, namely the locally excited state, the intramolecular charge-transfer excited state and the conformationally relaxed intramolecular charge-transfer excited state in the photoprocesses of these compounds. Copyright 2005 John Wiley & Sons, Ltd. [source]

Ultrafast Photoisomerization of Photoactive Yellow Protein Chromophore Analogues in Solution: Influence of the Protonation State

CHEMPHYSCHEM, Issue 8 2006
Agathe Espagne Dr.
Abstract We investigate solvent viscosity and polarity effects on the photoisomerization of the protonated and deprotonated forms of two analogues of the photoactive yellow protein (PYP) chromophore. These are trans- p -hydroxybenzylidene acetone and trans- p -hydroxyphenyl cinnamate, studied in solutions of different polarity and viscosity at room temperature, by means of femtosecond fluorescence up-conversion. The fluorescence lifetimes of the protonated forms are found to be barely sensitive to solvent viscosity, and to increase with increasing solvent polarity. In contrast, the fluorescence decays of the deprotonated forms are significantly slowed down in viscous media and accelerated in polar solvents. These results elucidate the dramatic influence of the protonation state of the PYP chromophore analogues on their photoinduced dynamics. The viscosity and polarity effects are, respectively, interpreted in terms of different isomerization coordinates and charge redistribution in S1. A trans-to-cis isomerization mechanism involving mainly the ethylenic double-bond torsion and/or solvation is proposed for the anionic forms, whereas "concerted" intramolecular motions are proposed for the neutral forms. [source]