Abstract Direct photolysis of 5-chloro-1, 4-dihydro-9-methyl-naphthalen-1, 4-imine in cyclohexane gave a mixture of 1-chloronaphthalene and isomeric dihydrocyclobut [b] indoles 5 and 6. Dihydrocyclobut [b] indoles 5 and 6 thermally rearranged to give 1-