Photochromic properties of terarylene derivatives having a pi-conjugation unit on central aromatic ring

A series of terarylenes based on 4,5-bisbenzothienylthiazole have been synthesized, and their photochromic properties are studied both in solution and in the crystalline state. The substitution effect of the central aromatic ring on the photochromic reactivity were also studied. The introduction and expansion of the pi-conjugated system at the 2-position of the thiazole ring led to a red-shift of the absorption peaks for both open- and closed-ring isomers of terarylenes. 2-Hydro and 2-phenyl derivatives of 4,5-bisbenzothienylthiazoles displayed a photochromic reaction, even in the single-crystal state. X-Ray crystal analysis showed their molecular structure to have quasi-C-2 symmetry, suggesting significant CH center dot center dot center dot N and CH center dot center dot center dot S interactions. Although the 2-hydro and 2-phenyl derivatives exhibited high photocyclization reaction quantum yields over 0.5, the 2-phenylthienyl derivative showed a considerably smaller reactivity, even though it is supposed to have a similar conformation to those of the other two derivatives. Meanwhile, these molecules showed similar photocycloreversion quantum yields as high as 0.3. The relationship between their electronic structures and photochromic reactivity is also discussed by means of quantum chemical calculations.