We herein propose a new type of efficient neutral photoacid generator. A photoinduced 6 pi-elettrocyclization reaction of photochromic triangle terarylenes triggers subsequent release of a Bronsted acid, which took place from the photocyclized form. A H-atom and its conjugate base were introduced at both sides of a 6 pi-system to form the self-contained photoacid generator. UV irradiation to the 6 pi-system produces a cyclohexa-1,3-diene part with a H-atom and a conjugate base on the se C-atoms at 5- and 6-positions, respectively, which spontaneously release an acid molecule quantitatively forming a polyaromatic compound. A net quantum yield of photoacid generation as high as 0.52 under ambient conditions and a photoinitiated cationic polymerization of an epoxy monomer are demonstrated.