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Indigo does not undergo thermal or photochemical trans-cis isomerization in contrast, for example, to thioindigo."] Dry distillation of indigo produces and irradiation in the presence of dissolved oxygen. isatin.13] However, if indigo ( I ) is heated to 450 C in vacuum in the vapor phase it isomerizes to dibenzonaphthyridindione (2) in a suprisingly smooth reaction. 2The UViVis spectra of this isomerization reaction are shown in Figure I . The occurrence of an isosbestic point demonstrates the homogeneity of this reaction (yield about 80% ; in addition to a carbonized residue the only other product detected is aniline). Compound 2 was synthesized for the first time in 1934 in a relatively complicated multistep reaction.r41 It is extraordinarily thermally stable and can be sublimed at about 1000°C without degradation. To obtain a better understanding of the structure and the spectroscopic properties of both isomers, and also the course of the reaction, quantum chemical calculations using the MNDO-PM3 procedurer5' were carried out.For both I and 2 a nonplanar structure (pyramidalization at the N atom) was calculated. According to the calculations, the energy difference between this and a planar structure was 13.4 kJmol-I for 1 and 2.3 kJmol-' for 2. In contrast, in the electronically excited S, state, an almost planar structure resulted for both molecules. The free energy of reaction at 460°C was calculated to be AGR = -62.8 kJmol-'. To explain the mechanism of the isomerization. saddle point calculations were made. A structure for a tranzition state was found and is illustrated in Figure 2. The mechanism of the isomerization can thus be conceived as follows: The bond between TS 2If the orbital symmetry is considered, this process can also be regarded as a dyotrope rearrangement (intramolecular symmetrical transfer of two (CJ bonds).rb1 Such a [D? + 0 2 ] process is forbidden by symmetry. However, by including the n-electron system of 1 in the four-membered transition state. the restriction can be relaxed. The very high value of the activation energy (610 kJmol-') for the calculated transition state TS instead favors a stepwise process. An analogous isomerization was also observed for 5S'-dibromo-. 4,5,4'.5'-tetrabromo-, and 5.7S.7'-tetrabromoindigo. Thioindigo and thionaphthenindigo (2.2') do not isomerize. Heating 1 in a solvent results in practically no isomerization (in DMSO traces of 2 were detected by its very intense fluorescence).The spectroscopic properties (UV/Vis absorption and fluorescence) of I have been frequently investigated and d i s c~s s e d ,~'~ also with respect to the differences in absorption behavior between 1 and 2.[*] A comparison of the absorption and fluores-

Spektroskopie und Photochemie indigoider Verbindungen. VI. Direkte trans → cis Photoisomerisierung indigoider Farbstoffe Wir haben die Fluoreszenz und die photochemische trans → cis Isomerisierung einiger indigoider Farbstoffe in flüssigen Lösungen bei 300 K und festen Gläsern bei 77 K, die Temperaturabhängigkeit der Fluoreszenzquantenausbeute und die Löschung der trans → cis Isomerisierung durch Elektronendonatoren in Benzen und Iodbenzen und durch O2 in Benzen untersucht. Diese und andere Ergebnisse werden vorgestellt und diskutiert. Es wird angenommen, daß die direkte trans → cis Photoisomerisierung — zumindest im ersten Schritt — über einen Singulett‐Zustand verläuft.

Fluorescence J Light, Absorption Light, Emission 1 Spectroscopy, Visible / TICT Absorption and fluorescence of donor-substituted pyrylium compounds are studied in order to get information of the x-and y-chromophore system and of the role of structural and solvent effects. The experimental results are in accordance with the assumption that the absorption and fluorescence behaviour can be rationalized in terms of intramolecular radiative and non-radiative charge transfer (charge shift)processes.

A wide variety of substituted flavylium salts and bridged derivatives are compared with respect to their fluorescence quantum yields and lifetimes. The dependence of the nonradiative decay rate constant on substitution pattern, on twisting possibility and on solvent polarity could be explained by the formation of non‐fluorescent Twisted Intramolecular Charge Transfer (TICT) states. In some dyes, two different TICT channels compete. Molecules‐in‐molecules (MIM) calculations are used to analyze and predict different TICT formation possibilities.