Dimethyl sulfoxide , excited-state

Chromium, hexacyano-, 3, 703, 777 hexaamminecobaltate coordination isomerism, 1, 183 ligand field photochemistry, 1, 398 photochemistry excited states, 1, 398 production, 3, 704 Chromium, hexafluoro-, 3, 927 Chromium, hexabalo-, 3, 889 Chromium, hexaiodo-, 3, 766 Chromium, hexakis(dimethyl sulfoxide)-photoanation, 1, 399 Chromium, u-oxalatodi-reduction... 

Kemp and coworkers employed the pulse radiolysis technique to study the radiolysis of liquid dimethyl sulfoxide (DMSO) with several amines as solutes [triphenylamine, and N, A, A, N -tetramethyl-p-phenylenediamine (TMPD)]. The radiolysis led to the formation of transient, intense absorptions closely resembling those of the corresponding amine radical cations. Pulse radiolysis studies determine only the product Ge, where G is the radiolytic yield and e is the molar absorption. Michaelis and coworkers measured e for TMPD as 1.19 X 10 m s and from this a G value of 1.7 is obtained for TMPD in DMSO. The insensitivity of the yield to the addition of electron scavenger (N2O) and excited triplet state scavenger (naphthalene) proved that this absorption spectrum belonged to the cation. 

Local concentration of solvent molecules changes substantially. This can occur in hquid mixtures when the solute groimd and excited states are preferentially solvated by different solvent components. It has been observed in simulations of SD in water-dimethyl sulfoxide (DMSO) -, water-methanoF and n-hexane-methanol mixtures," as well as in om work in progress on SD in benzene-acetonitrile mixtures. ... 

The fluorescence of TPHA 3 is not a mirror image of its absorption spectrum and the emission intensity is sensitive to concentrations greater than 10 M. The excitation profile of 3 also varies with concentration, believed to be due to aggregation of TPHA in solution and only emulates the ultraviolet-visible (UV-Vis) spectrum at concentrations less than 10 M. The Aem decreased from 633 nm in toluene to 619 nm in dimethyl sulfoxide (DMSO), and this is thought to be indicative of a polar ground state and nonpolar excited state <1998JA2989>. 

Whenever possible, the values refer to standard conditions water, 25°C, ionic strength / = 0. To do this, it has been necessary to make corrections for solvents other than water (see footnotes to tables), for temperature (see Section III,F,1), and for ionic strength (see Section III,F,2). In several cases, it has not been possible to apply these corrections due to the lack of data (temperature, ionic strength) or the lack of standard values to establish a linear relationship between values in mixed solvents and values in water. The list of 1000 pK values collected in these tables constitutes a set of the best values found in the literature. Excited state p, s and pK s in dimethyl sulfoxide will be reported in separate tables in subsequent sections. 

Further examination of the excited states reveals a strong dependence of their energy upon solvent polarity that resembles the experimental trends. Single-point calculations on the relaxed structures of the ground and excited states in the simulated solvents hexane (e = 2.023), CCI4 (e = 2.229), benzene (e = 2.274), ether (e = 4.197), chloroform (e = 4.806), methylene chloride (e = 8.930), pyridine (e = 12.40), acetone (e — 20.56), ethanol (e = 24.55), nitrobenzene (e = 43.82), acetonitrile (e = 35.94) and dimethyl sulfoxide (e = 46.45) were performed in order to judge the solvent effects on the relative stabilities of the different states for 9b-d. 

In dimethyl sulfoxide, a single neutral species was detected for both 43 and 44. However, in acetonitrile, an acid-base process between the neutral and an acidic form was detected and characterized through pK values in the ground state and p.A( values in the excited state (44 pA" = 2.0, pAl = 2.3 43 pAT = 2.9, pAl = 4.0). Absorption maxima in the range of 326-304 nm were observed. Fluorescence maxima in the range of 413—469 nm, with relatively high quantum efficiencies (0.3-0.7), were obtained <1995JPH35>. 

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