Compounds of the formula I ##STR1## in which X is O, S, SO, SO.sub.2 or NR.sub.13, R.sub.13 is, for example, methyl, and R.sub.1 to R.sub.12, independently of one another, are, for example, H, C.sub.1 -C.sub.12 alkyl, C.sub.1 14 C.sub.12 alkoxy, halogen or --CN. The compounds are photosensitive and photochromic and as suitable are photosensitizers and simultaneously as colour indicators, and as photoswitchable colour filters.