A photochromic triangle terthiophene with DPA attached compound was synthesized conveniently and fully characterized. The photochromic behavior was investigated before and after the addition of Zn2+ to the solution of PT. It was found that the PET effect, induced by DPA, suppressed the photochromic reactivity of PT. Upon the addition of Zn2+, the photochromic reactivity was recovered and light pink color was observed under irradiation of 254nm light. Different metal ions were also added and verify the single chelation towards Zn2+ with PT. Molecular geometric structures were optimized and HOMO/LUMOs were calculated with DMol(3). Two key factors, the distance of reactive carbon atoms and the LUMO distribution of PT-Zn favored it cyclization/cycloreversion triggered by photons.