New indoline spiropyrans with highly stable merocyanine forms

New spiropyrans with chromene and indoline cores bearing alkoxycarbonyl group and cationic π-acceptor in the chromene moiety were synthesized in one step from 1,2,3,3-tetramethylindolinium and isophthalic dialdehyde derivatives. The appending of a 8′-positioned conjugated cationic fragment causes a significant red-shift of the photoinduced isomers absorption maxima and provides them with extremely long lifetimes up to 75 min. The obtained spiropyrans exhibit both positive and negative photochromism in solutions, demonstrating the properties of ‘photochromic balance’.