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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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Publication costs assisted by the U.S. Army Research OfficeQuenching of the direct photochemical trans -cis isomerization of thioindigo dyes by oxygen is attributable to the intermediacy of the triplet state. From a study of the dependence of the quantum yields and photostationary state concentrations on the oxygen concentration for nine such dyes the approximate triplet lifetimes could be estimated. The quantum yields for the cis --+ trans isomerization were found to be unaffected by oxygen, unless the compounds contained chlorine substituents; this indicates the quenching of a common triplet intermediate in the isomerization of the latter. All of the compounds studied also undergo photosensitized isomerization to a common photostationary state the position of which is a function of the oxygen concentration. In addition, the fluorescence quantum yields were determined for the trans isomers. The mechanism of the photoisornerization process and the effect of substituents on the quantum yields and rates of the various excited state processes are discussed. The similarities found between these results and the data from recent reports on the effect of substituenh on the photoisomerization of some thionaphthioindigos and on the photoisomerization and fluorescence of stilbenes suggest that the resonance stabilization of the trans Si state plays an important part in determining the excited state behavior of molecules of this type. IntroductionAlthough the photochemical cis-trans isomerization of olefins is a well-known phenomenon, the mechanism of such reactions has not been fully elucidated to date. Thus the controversy between proponents of the singlet route vs. the triplet mechanism for the direct photoisomerization of stilbene has not been resolved as yet.2 Interestingly enough, evidence has been presented recently to the effect that the photoisomerization of several 4-nitrostilbenes takes place via the triplet state in the trans -+ cis direction, but by way of a singlet mechanism in the opposite dir e~t i o n ,~ attesting to the importance of substituent effects in such reactions.In a recent communication4 we reported preliminary evidence for the intermediacy of the triplet state in the direct photoisomerization of two thioindigo dyes by an oxygen-quenching technique. We now wish to report the results of our investigation of the direct and sensitized photoisomerization of thioindigo and eight of its ringsubstituted derivatives and our conclusions regarding the mechanism of the isomerization of such compounds. In addition, the effect of substituents on the rates and/or quantum yields of the various excited state processes will be discussed and compared to what has been reported in the literature for some related molecules.

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dinitroparaffin resonance forms shows an increased electron density on nitrogen. This would tend to counterbalance the electronegative effect of the second nitro group. At any rate the considerable variation in values for nitroparaffins is almost entirely due to differences in the acid strengths of the nitro forms. forms of the mononitroparaflins because one of the Charlottesville, Virginia