Protonation of 4-diazoisoquinoline-1,3(2 H,4 H)-diones in Bronsted acids gives rise to diazonium cations that can be trapped with arenes to give 4-arylisoquinoline-1,3(2 H,4 H)-diones (homophthalimides). This provides a new, metal-free approach to 4-aryltetrahydroisoquinolines (obtainable from respective homophthalimides by reduction). Similarly, a fluorine atom can be introduced by trapping the diazonium cation with HF. This led to the preparation of the first examples of 4-monofluoro-substituted isoquinoline-1,3-diones (as well as their 4-bromo-4-fluoro and 4-chloro-4-fluoro variants), important carboxylic acid isosteres on their own and precursors of useful 4-fluorotetrahydroisoquinoline building blocks.