Excited State Chemistry of lndigoid Dyes 831 dissociation-recombination mechanism rather than a stepwise process. The mechanism appears to be a reverse of the forrnation of the endo adduct followed by recombination contrary to the Alder rule.ll From the earlier kinetic data of Baldwin and Roberts15 on the thermal isomerization of endo to exo isomer in decalin we calculate the activation energy for the reverse process, ErS, as 35 kcal/ mol. This value is in satisfactory agreement with the activation energy, Ef$ (37.5), derived from our thermochemical data.As the accumulation of thermochemical data lags, it bas become necessary to develop empirical methods for the estimation of thermodynamic parameters. While eq 1 is satisfactory in dealing with this particular D-A reaction we have explored another approach for estimation of the gas-phase reaction heat which is somewhat simplier and equally satisfactory. From the gas-phase heats of formation of ethane (-20.24 kcal/mol)l6 and succinic anhydride I -128 kcal/mol) rt appears that the replacement of two hydrogens by an anhydride group changes the heat of formation by -108 kcal/mol. Combining this value with the heat of formation of norbornene (20.6 kcal/mol)l gives .LHf(adduct) = -88 kcal/mol. With this value and the heats of formation oE cyclopentadiene (31.95 kcal/moI) and maleic anhydride (-95.2 kcal/mol), the gas-phase heat of reaction is calculated as AHr(g) = -25 kcal/mol compared to the experimental value of -26.6 (endo) kcal/ mol. Thus, this simple group approach also yields good results.The heat of formation of the endo adduct calculated from the heats of formation of the reactants and AH,(g) is -89.8 kcal/mol. The strain energy in a molecule is generally assigned as the difference between this experimental value and the heat of formation predicted on the basis of some empirical scheme. We have adopted the olefinic and carbonyl increments of Bensonl7 and the "single conformation" values of the saturated CH and CH2 groups from Schleyer.18 The predicted formation heat and strain energy are -113.57 and 23.7 kcal/mol, respectively. The strain energy of norbornene itself is 23 kcai/mol which implies a small contribution from the succinic anhydride moiety. Benson estimates the strain is this ring system at 1.1 kcal /moL17The absorption spectra of the cis and trans isomers of thioindigo, 6,6'-diethoxythioindigo, selenoindigo, and 3,3'-dioxo-4,4,4,'4'-tetramethyl-2,2'-bithiolanylidene were determined by a combination of absorption and fluorescence spectroscopy in the visible region. Quantum yields of fluorescence (for the trans isomers) and for the photochemical cistrans and transcis isomerizations were measured. The effect of structural parameters on the absorption spectra and on the rates of the excited state processes (fluorescence, twisting, and nonradiative decay) of indigoid dyes is discussed.
lntroductionIn conjunction with studies on the quenching of the fluorescence and the photochemical cis-trans isomerization of some indigoid dyes,2 it was necessary to deter...
The ultraviolet and visible absorption spectra of indigo and of six halogen-substituted indigo dyes were determined in chloroform solution. In contrast with the behavior of thioindigo dyes, no evidence was found for photochemical isomerization. This was believed to be attributable to the preferential stabilization of the trans configuration by hydrogen-bonding.The discovery of phototropism in , '-diacetylmdigo and a comparison of the infrared absorption spectra of indigo and thioindigo provided evidence for this hypothesis. In addition, the effect of halogen substitution on the ultraviolet and visible spectra was considered.
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