A new photochromic 8 pi-system based on an azaheptatriene-tetrahydroazepinoisoquinoline electrocyclization

A new photochromic family of tetrahydroazepinoisoquinolines (THAI) 4a-i has been prepared. It undergoes light-induced ring opening from spiro compound s 4 to betaines 3 that decolorize in a very fast cyclizing reaction. Depend ing on substitution of the precursors 1 and 6, the photochromic styryltetra hydroindolizines (THI) 5k-q are formed in a periselective way. The conforma tion and configuration of the new photochromic THAI 4 and THI 5 were invest igated by NMR and the structure of both compounds was proven by X-ray analy sis. The photochromic properties were studied by laser flash photolysis, wh ich afforded the lifetime of the colored form 3 and 3' in the micro- or nan osecond range.