Thermal Isomerization of Indigo

Abstract

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-

Idiosyncrasies of Physical Vapor Deposition Processes from Various Knudsen Cells for Quinacridone Thin Film Growth on Silicon Dioxide

“… 39 However, only sparse reports exist involving cracking and/or restructuring of chemical bonds with the subsequent formation of new and stable molecules for quinacridone-based and other H-bonded materials. Haucke et al 40 described a smooth and homogeneous transition from indigo to epindolidione in the vapor phase, if the former is heated to 733 K under vacuum conditions. Berg et al 26 observed decreasing mobilities for quinacridone organic field-effect transistors grown in vacuum after repeated temperature gradient sublimation preparation circles.…”

TD-DFT study on electron transfer mobility and intramolecular hydrogen bond of substituted indigo derivatives

“…This shows that the substituent groups are good electron donors of these molecules, and that the increase of electron density may increase the electron transfer mobility of the molecule. [44][45]. Therefore, the results calculated by B3LYP/6-311++g(d) are discussed in detail.…”

Laponite Blue: Dissolving the Insoluble

“…Returning to the absorption spectra, the visible p!p* transition found at 657 nm is strongly red-shifted in comparison to solutions in benzene (595 nm [23] ), CHCl 3 , (602 nm, [24a] 604 nm [24b] ), DMF (610 nm [24a] ), or DMSO (620 nm [25,26] ), which is readily interpreted in terms of the merocyanine effect, that is, a bathochromic shift of the absorption with polarity. The extinction coefficient e for the transition was found to be constant at 10 300 L mol À1 cm À1 in the concentration range between 0.047 10 À3 and 0.177 10 À3 mol L…”