Synthesis, structure and photochromic properties of indoline spiropyrans with electron-withdrawing substituents

Indoline spiropyrans containing conjugated cationic vinyl-3H-indolium fragment and their analogues with formyl substituent in the 2H-chromene moiety were synthesized by a simple one-pot method. The structure of the obtained compounds was confirmed by H-1 and C-13 NMR (including 2D methods) and IR spectroscopy, as well as by element analysis and HRMS. Molecular structures of 1,3,3,6'-tetramethyl-8'-formyl-spiro[indoline-2,2'-2H-chromene] 8 and its cationic derivative 5 were refined by single crystal X-ray and compared. Notably, it was found that the crystals of formyl-substituted spiropyran contain both enantiomers (R- and S-) of the molecule in the unit cell. Photochromic properties of all the compounds were studied and compared using spectral-photokinetic methods. It was found out that electron-withdrawing substituents (vinyl-3H-indolium fragment and formyl group) located at the position 6' of 2H-chromene moiety stabilize merocyanine isomer more effectively, than at the position 8', leading even to the appearance of negative photochromism for the compound 3 with cationic substituent, which is quite unusual for this class of organic photochromes. Also, it was shown that chlorine atoms as sigma-acceptors at the positions 5 and 5 '' of hetarene moiety and cationic fragment can suppress photochromic behavior. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

Indoline spiropyrans with conjugated cationic vinyl-3H-indolium fragment and their analogues with formyl substituent in the 2H-chromene moiety were synthesized using a simple one-pot method. The structures of the compounds were confirmed using various analytical techniques, including single crystal X-ray. Photochromic properties were studied, showing that electron-withdrawing substituents stabilize merocyanine isomer effectively, while certain substituents can suppress photochromic behavior.