Halochromic Molecules. Substituted 6,11‐Dihydrospiro[[1]benzopyrano[4,3‐b]indol‐6,9′‐9′H‐xanthene]‐2′,6′‐diamines and Their Aza Analogues: New Chromogenes for Black Images.
We have synthesized a series of substituted 6,11‐dihydrospiro[[1]benzopyrano[4,3‐b]indo1‐6,9′‐9′H‐xanthene]‐2′,6′‐diamines and their respective aza analogues. These compounds develop a black colour in acidic media. The assumed structures of the analytically pure starting and final products are consistant with the fragmentations in the MS and are also supported by FT 1H‐NMR. The UV/VIS spectra in buffered MeOH/H2O solutions were measured. From the ϵpH* curves, we determined the pK* values. The title compounds can be used in so‐called pressure‐sensitive copying systems.