Fulgides of the Indole Series

Heterocyclic fulgides exhibit photochromic properties that are useful for practical applications.86,87 The structures and photochromic transformations of fulgides of the indole series /f-Inl 1 and -InF2, in which the indole substituent is joined to the methylene group at position 3, have been studied.88,89 

H(171) -0(6), 2.72 and H(172)- -C(13), 2.79 A] are essentially shortened compared to the sum of the van der Waals radii of the carbon atoms or the carbon and hydrogen atoms, this being indicative of steric crowding in the molecule. 

The structure of the InF2 molecule is similar to that of InFl. The isopropy-lidene substituent is twisted about the C=C bond by 23.5°. The indolylethylidene substituent is twisted by 19.1° about the C=C double bond and by 26.7° about the C-C single bond. The C=C double bonds are elongated to 1.361(2) A 

The structures of the E-isomers of InF3 and InF4 in which the indole substituent is attached to the methylene group at position 2, are similar to those of E-InFl and E-InF2.90 

In the E-Inl 4 molecule, the indole moiety is disordered in such a manner that 65% of molecules have the N-CH3 group arranged in an anti position with respect to the R1 substituent, and 35% in a syn position. Intramolecular C- C distances between these substituents are shortened to 3.23-3.40 A. Owing to steric 

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The structure of another fulgide of the indole series which contains the bulky adamantylidene group instead of the isopropylidene group, adamant-2-ylidene-(l -methyl-3 -indolylethylidene)succinic anhydride (AdF) (Figure 23), has been investigated.91... 

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